Patent Publication Number: US-2022216498-A1

Title: Fuel cell stack

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of U.S. Ser. No. 16/357,701, filed Mar. 19, 2019, which is a divisional of U.S. Ser. No. 14/668,052, filed Mar. 25, 2015, which is based upon and claims the benefit of priority from Japanese Patent Applications No. 2014-069568 filed on Mar. 28, 2014, No. 2014-082929 filed on Apr. 14, 2014, and No. 2014-175623 filed on Aug. 29, 2014. The entire contents of all of these parent and priority applications are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a fuel cell stack including a plurality of fuel cells for generating electrical energy by electrochemical reactions of a fuel gas and an oxygen-containing gas. The fuel cells are stacked together in a stacking direction, and end plates are provided at both ends of the fuel cell stack in the stacking direction. 
     Description of the Related Art 
     For example, a solid polymer electrolyte fuel cell employs a polymer ion exchange membrane as an electrolyte membrane, and the polymer electrolyte membrane is interposed between an anode and a cathode to form a membrane electrode assembly (MEA). The membrane electrode assembly and a pair of separators sandwiching the membrane electrode assembly make up a power generation cell for generating electricity. In use, typically, a predetermined number of the power generation cells are stacked together to form a fuel cell stack, e.g., mounted in a fuel cell vehicle (fuel cell electric automobile, etc.). 
     In the fuel cell, a fuel gas flow field for supplying a fuel gas to the anode and an oxygen-containing gas flow field for supplying an oxygen-containing gas to the cathode are provided in the surfaces of the separators. Further, a coolant flow field for supplying a coolant is provided between the adjacent separators along surfaces of the adjacent separators. 
     In the fuel cell, internal manifold structure has been adopted. In the internal manifold structure, fuel gas passages for allowing the fuel gas to flow through the fuel cell, oxygen-containing gas passages for allowing the oxygen-containing gas to flow therethrough, and coolant passages for allowing the coolant to flow therethrough extend through the fuel cells in the stacking direction. The fuel gas passages are a fuel gas supply passage and a fuel gas discharge passage. The oxygen-containing gas passages are an oxygen-containing gas supply passage and an oxygen-containing gas discharge passage. The coolant passages are a coolant supply passage and a coolant discharge passage. 
     In the fuel cell, at least one of the end plates is equipped with a fluid manifold connected to each passage for supplying or discharging fluid (fuel gas, oxygen-containing gas, or coolant). Further, a fluid supply pipe and a fluid discharge pipe are connected to the fluid manifold. 
     In this regard, a reactant gas as one of the oxygen-containing gas and the fuel gas is humidified beforehand, and the humidified reactant gas is then supplied to the fuel cell. Further, in the fuel cell, water tends to be produced at the cathode by electrochemical reaction, and back diffusion of the produced water toward the anode tends to occur. Consequently, water vapor may be retained in the fluid manifold, and the water vapor may be condensed to produce liquid water (condensed water). Under the circumstances, the fuel cell may be undesirably connected electrically to external equipment, etc. due to connection through the liquid water (i.e., liquid junction may occur). 
     As a fuel cell aimed to prevent production of water droplets in the reactant gas, for example, a solid polymer electrolyte fuel cell disclosed in Japanese Laid-Open Patent Publication No. 10-012262 is known. The fuel cell has a pressing plate for pressing a stack body of the fuel cell in a stacking direction. The pressing plate has a heating section at a position where a pipe connector is provided, for heating at least one of the oxygen-containing gas and the fuel gas. 
     The heating section has a cylindrical outer shape having substantially the same thickness as a body portion of the pressing plate. A cylindrical hollow area is provided in the heating section. The heating section has a gas conduction section for sealing the hollow area in an air-tight manner from the inside. At the center of the gas conduction section, a through hole as a passage of the oxygen-containing gas is formed. Further, according to the disclosure, since a heating medium heated by cooling the stack body is supplied to the hollow area, the oxygen-containing gas flowing through the gas conduction section is heated by the heating medium, and it is possible to suppress production of liquid water. 
     Moreover, in the fuel cell, a pair of coolant supply passages and a pair of coolant discharge passages may be arranged separately at both sides (in one of two pairs of opposite sides) of the separator. The coolant supply passages and the coolant discharge passages extend through the fuel cell in the stacking direction for allowing the coolant to flow through the fuel cell. In this regard, the fuel cell adopts a structure where the pair of coolant supply passages are connected together by a single coolant manifold, and the pair of coolant discharge passages are connected together by a single coolant manifold. 
     For example, in a fuel cell stack disclosed in Japanese Patent No. 5054080, electrolyte electrode assemblies and separators are stacked together, and rectangular end plates are provided at both ends of the fuel cell stack in the stacking direction. On two long opposite sides of the fuel cell stack, a pair of coolant supply passages are arranged oppositely at one end side of the long sides, and a pair of coolant discharge passages are arranged oppositely at the other end side thereof. 
     Further, a pair of manifold sections are provided at one of the end plates. The manifold sections are connected to at least the pair of coolant supply passages or the pair of coolant discharge passages. Moreover, a coupling section is provided for coupling the pair of manifold sections together. The width of the coupling section along the long side is smaller than the dimension of the pair of manifold sections. 
     As described above, since the pair of manifold sections are coupled by the coupling section having a narrow width, the manifold does not have a rectangular shape as a whole. According to the disclosure, increase in the pressure loss of the coolant flowing into the manifold is suppressed effectively, and the coolant can be supplied smoothly and uniformly to the fuel cell. 
     SUMMARY OF THE INVENTION 
     However, in Japanese Laid-Open Patent Publication No. 10-012262, the heating section and the gas conduction section are provided for heating the reactant gas such as the oxygen-containing gas. Therefore, the structure is complicated, and uneconomical. 
     Further, in the fuel cell, in addition to the manifolds for the reactant gases, the coolant manifold as a passage of the coolant is provided. The coolant manifold tends to be electrically connected to the inside of the fuel cell through the coolant, and liquid junction between the fuel cell and external equipment may occur through the coolant. However, in the above fuel cell, it is not possible to suppress liquid junction between the fuel cell and the external equipment. 
     The present invention has been made to solve the problems of this type, and an object of the present invention is to provide a fuel cell stack having simple and economical structure in which it is possible to suitably achieve a desired electrical insulating performance between fluid manifolds and end plates. 
     Further, an object of the present invention is to provide a fuel cell stack having simple and economical structure in which a coolant can flow smoothly and uniformly inside a coolant manifold. 
     A fuel cell stack according to an aspect of the present invention includes a stack body formed by stacking a plurality of fuel cells together in a stacking direction for generating electrical energy by electrochemical reactions of a fuel gas and an oxygen-containing gas. A fluid passage extends through the stack body in the stacking direction for allowing a fluid, which is a coolant, the fuel gas, or the oxygen-containing gas, to flow through the fuel cells. 
     End plates are provided at both ends of the stack body in the stacking direction. At least one of the end plates has a fluid manifold member connected to the fluid passage. An insulating plate is provided between the one of the end plates and an attachment surface of the fluid manifold member. 
     Further, a fuel cell stack according to another aspect of the present invention includes a plurality of fuel cells stacked together in a stacking direction and end plates provided at both ends of the fuel cells in the stacking direction. Each of the fuel cells is formed by stacking a membrane electrode assembly and separators. The membrane electrode assembly includes a pair of electrodes and an electrolyte membrane interposed between the electrodes. A coolant flow field is formed between adjacent ones of the separators for allowing a coolant to flow along separator surfaces. 
     A pair of coolant supply passages are provided at an inlet side of the coolant flow field and arranged respectively on both sides of the coolant flow field in a flow field width direction. A pair of coolant discharge passages are provided at an outlet side of the coolant flow field and arranged respectively on both sides of the coolant flow field in the flow field width direction. A coolant manifold connected to the pair of coolant supply passages or the pair of coolant discharge passages is provided on one of the end plates. A pipe section as a coolant supply port or a passage discharge port is provided at a central portion of the coolant manifold in the flow field width direction. A protrusion bulging toward the pipe section is provided on a manifold inner surface facing the pipe section. 
     Further, in a fuel cell stack according to another aspect of the present invention, a pipe section as a coolant supply port or a passage discharge port is provided at a central portion of the coolant manifold in the flow field width direction. Protrusions bulging toward an inside of the coolant manifold are provided respectively on both sides of the pipe section. 
     In the present invention, the insulating plate is provided between the fluid manifold member and the end plate. Therefore, with the simple and economical structure, a desired electrical insulating performance between the fluid manifold member and the end plate is achieved suitably. Accordingly, it is possible to suitably suppress electrical connection between the fuel cell and the external equipment through liquid water. 
     Further, in the present invention, a pipe section is provided on the coolant manifold, and a protrusion bulging toward the pipe section is provided on a manifold inner surface facing the pipe section. In the structure, for example, the coolant supplied from the single coolant supply port into the coolant manifold is distributed toward each coolant supply passage by the guiding action of the protrusion. Further, by the guiding action of the protrusion, the coolant discharged from each coolant discharge passage to the coolant manifold flows toward the single coolant discharge port. 
     Thus, with the simple and economical structure, the coolant supplied into the coolant manifold can smoothly and uniformly flow toward the pair of coolant supply passages. Further, the coolant can flow from the pair of coolant discharge passages to the coolant manifold smoothly and uniformly. Accordingly, improvement in the cooling performance in each fuel cell is achieved suitably. 
     Further, in the present invention, a pipe section is provided on the coolant manifold, and protrusions bulging toward the inside of the coolant manifold are provided respectively on both sides of the pipe section. In the structure, for example, the coolant supplied from the single coolant supply port into the coolant manifold is distributed toward each coolant supply passage by the guiding action of the protrusions. Further, by the guiding action of the protrusions, the coolant discharged from each coolant discharge passage to the coolant manifold flows toward the single coolant discharge port. 
     Thus, with the simple and economical structure, the coolant supplied into the coolant manifold can smoothly and uniformly flow toward the pair of coolant supply passages. Further, the coolant can flow from the pair of coolant discharge passages to the coolant manifold smoothly and uniformly. Accordingly, improvement in the cooling performance in each fuel cell is achieved suitably. 
     The above and other objects, features and advantages of the present invention will become more apparent from the following description when taken in conjunction with the accompanying drawings in which preferred embodiments of the present invention are shown by way of illustrative example. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a fuel cell stack according to a first embodiment of the present invention, as viewed from a coolant manifold member side; 
         FIG. 2  is a partial exploded perspective view showing the fuel cell stack; 
         FIG. 3  is an exploded perspective view showing main components of a fuel cell of the fuel cell stack; 
         FIG. 4  is a cross sectional view showing the fuel cell stack, taken along a line IV-IV in  FIG. 1 ; 
         FIG. 5  is an exploded perspective view showing the coolant supply manifold member and an insulating plate of the fuel cell stack, as viewed from one side; 
         FIG. 6  is an exploded perspective view showing the coolant supply manifold member and the insulating plate, as viewed from the other side; 
         FIG. 7  is a perspective view of a fuel cells stack according to a second embodiment of the present invention, as viewed from a coolant manifold member side; 
         FIG. 8  is a front view of the fuel cell stack, as viewed from the coolant manifold member side; 
         FIG. 9  is a front view showing of a fuel cell stack according to a third embodiment of the present invention, as viewed from a coolant manifold member side; 
         FIG. 10  is a perspective view of a fuel cell stack according to a fourth embodiment of the present invention, as viewed from a coolant manifold member side; 
         FIG. 11  is a front view of the fuel cell stack, as viewed from the coolant manifold member side; and 
         FIG. 12  is a front view of a fuel cell stack according to a fifth embodiment of the present invention, as viewed from a coolant manifold member side. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A fuel cell stack  10  according to a first embodiment of the present invention shown in  FIGS. 1 and 2  is mounted, e.g., in a fuel cell electric vehicle (not shown). The fuel cell stack  10  includes a stack body  12   as  formed by stacking a plurality of fuel cells  12  in a horizontal direction indicated by an arrow B such that electrode surfaces of the fuel cells  12  are oriented upright. It should be noted the fuel cell stack  10  may be formed by stacking a plurality of fuel cells  12  in the direction of gravity. 
     As shown in  FIG. 2 , at one end of the fuel cells  12  in a stacking direction (one end of the stack body  12   as ), a first terminal plate  14   a  is provided. A first insulating plate  16   a  is provided outside the first terminal plate  14   a , and a first end plate  18   a  is provided outside the first insulating plate  16   a . At the other end of the fuel cells  12  in the stacking direction (the other end of the stack body  12   as ), a second terminal plate  14   b  is provided. A second insulating plate  16   b  is provided outside the second terminal plate  14   b , and a second end plate  18   b  is provided outside the second insulating plate  16   b.    
     A first power output terminal  20   a  extends outward from a substantially central position of the first end plate  18   a  having a laterally elongated shape (rectangular shape). The first power output terminal  20   a  may extend from a position deviated from the central position of the first end plate  18   a . The first power output terminal  20   a  is connected to the first terminal plate  14   a . A second power output terminal  20   b  extends outward from a substantially central position of the second end plate  18   b  having a laterally elongated shape (rectangular shape). The second power output terminal  20   b  is connected to the second terminal plate  14   b.    
     Coupling bars  22  each having a constant length are provided between the first end plate  18   a  and the second end plate  18   b  at substantially central positions of respective sides of the first end plate  18   a  and the second end plate  18   b . Both ends of each of the coupling bars  22  are fixed respectively to the first end plate  18   a  and the second end plate  18   b  using screws  24 , whereby a tightening load is applied to the stack body  12   as  in the direction indicated by the arrow B. 
     The fuel cell stack  10  includes a casing  26  as necessary. Two sides (surfaces) of the casing  26  at both ends in a vehicle width direction indicated by an arrow B are the first end plate  18   a  and the second end plate  18   b . Two sides (surfaces) of the casing  26  at both ends in a vehicle length direction indicated by an arrow A are a front side panel  28   a  and a rear side panel  28   b . The front side panel  28   a  and the rear side panel  28   b  are laterally elongated plates. Two sides (surfaces) of the casing  26  at both ends in a vehicle height direction indicated by an arrow C are an upper side panel  30   a  and a lower side panel  30   b . The upper side panel  30   a  and the lower side panel  30   b  are laterally elongated plates. 
     Each side of the first end plate  18   a  and the second end plate  18   b  has screw holes  32 . The front side panel  28   a , the rear side panel  28   b , the upper side panel  30   a , and the lower side panel  30   b  have holes  34  at positions facing the respective screw holes  32 . Screws  36  inserted through the holes  34  are screwed into the screw holes  32  to fix the components of the casing  26  together. 
     As shown in  FIG. 3 , the fuel cell  12  includes a membrane electrode assembly  40 , and a first metal separator (cathode separator)  42  and a second metal separator (anode separator)  44  sandwiching the membrane electrode assembly  40 . 
     The first metal separator  42  and the second metal separator  44  are made of metal plates such as steel plates, stainless steel plates, aluminum plates, plated steel sheets, or metal plates having anti-corrosive surfaces by surface treatment. Each of the first metal separator  42  and the second metal separator  44  has a rectangular planar surface, and is formed by corrugating a thin metal plate by press forming to have ridges and recesses in cross section and a wavy or serpentine shape on the surface. Instead of the first metal separator  42  and the second metal separator  44 , for example, carbon separators may be used. 
     Each of the first metal separator  42  and the second metal separator  44  has a laterally elongated shape. The long sides of the first metal separator  42  and the second metal separator  44  extend in the horizontal direction indicated by the arrow A, and the short sides of the first metal separator  42  and the second metal separator  44  extend in the direction of gravity indicated by the arrow C. Alternatively, the short sides may extend in the horizontal direction and the long sides may extend in the direction of gravity. 
     At one end of the fuel cell  12  in a long-side direction indicated by the arrow A, an oxygen-containing gas supply passage (fluid passage)  46   a  and a fuel gas supply passage (fluid passage)  48   a  are provided. The oxygen-containing gas supply passage  46   a  and the fuel gas supply passage  48   a  extend through the fuel cell  12  in the direction indicated by the arrow B. The oxygen-containing gas is supplied through the oxygen-containing gas supply passage  46   a . A fuel gas such as a hydrogen-containing gas is supplied through the fuel gas supply passage  48   a.    
     At the other end of the fuel cell  12  in the long-side direction, a fuel gas discharge passage (fluid passage)  48   b  for discharging the fuel gas and an oxygen-containing gas discharge passage (fluid passage)  46   b  for discharging the oxygen-containing gas are provided. The fuel gas discharge passage  48   b  and the oxygen-containing gas discharge passage  46   b  extend through the fuel cell  12  in the direction indicated by the arrow B. 
     At opposite ends of the fuel cell  12  in the short-side direction indicated by the arrow C, two pairs of coolant supply passages (fluid passages)  50   a  for supplying a coolant are oppositely arranged on one side (i.e., on one end side in the horizontal direction) i.e., on a side closer to the oxygen-containing gas supply passage  46   a  and the fuel gas supply passage  48   a . The two pairs of coolant supply passages  50   a  extend through the fuel cell  12  in the direction indicated by the arrow B for supplying the coolant. The two pairs of coolant supply passages  50   a  are provided respectively on upper and lower opposite sides. 
     The two coolant supply passages  50   a  provided at the upper positions of the fuel cell  12  are separated from each other in the horizontal direction as independent passages of the coolant. The two coolant supply passages  50   a  provided at the lower positions of the fuel cell  12  are separated from each other in the horizontal direction as independent passages of the coolant. 
     At opposite ends of the fuel cell  12  in the short-side direction, two pairs of coolant discharge passages (fluid passages)  50   b  for discharging the coolant are oppositely arranged on the other side (i.e., on the other end side in the horizontal direction), i.e., on a side closer to the fuel gas discharge passage  48   b  and the oxygen-containing gas discharge passage  46   b . The two pairs of coolant discharge passages  50   b  extend through the fuel cell  12  in the direction indicated by the arrow B for discharging the coolant. The coolant discharge passages  50   b  are provided respectively on upper and lower opposite sides. The two coolant discharge passages  50   b  provided at the upper positions of the fuel cell  12  are separated from each other in the horizontal direction as independent passages of the coolant, and the two coolant discharge passages  50   b  provided at the lower positions of the fuel cell  12  are separated from each other in the horizontal direction as independent passages of the coolant. 
     The membrane electrode assembly  40  includes a cathode  54  and an anode  56 , and a solid polymer electrolyte membrane  52  interposed between the cathode  54  and the anode  56 . The solid polymer electrolyte membrane  52  is formed by impregnating a thin membrane of perfluorosulfonic acid with water, for example. 
     Each of the cathode  54  and the anode  56  has a gas diffusion layer (not shown) such as a carbon paper. Porous carbon particles supporting platinum alloy on a surface thereof are deposited uniformly on the surface of the gas diffusion layer, to thereby form an electrode catalyst layer (not shown). The electrode catalyst layer of the cathode  54  and the electrode catalyst layer of the anode  56  are fixed to both surfaces of the solid polymer electrolyte membrane  52 , respectively. 
     The first metal separator  42  has an oxygen-containing gas flow field  58  on its surface  42   a  facing the membrane electrode assembly  40 . The oxygen-containing gas flow field  58  is connected to the oxygen-containing gas supply passage  46   a  and the oxygen-containing gas discharge passage  46   b . The oxygen-containing gas flow field  58  includes a plurality of wavy flow grooves (or straight flow grooves) extending in the direction indicated by the arrow A. 
     The second metal separator  44  has a fuel gas flow field  60  on its surface  44   a  facing the membrane electrode assembly  40 . The fuel gas flow field  60  is connected to the fuel gas supply passage  48   a  and the fuel gas discharge passage  48   b . The fuel gas flow field  60  includes a plurality of wavy flow grooves (or straight flow grooves) extending in the direction indicated by the arrow A. 
     A coolant flow field  62  is formed between the adjacent first and second metal separators  42 ,  44 , more specifically, between a surface  42   b  of the first metal separator  42  and a surface  44   b  of the second metal separator  44 . The coolant flow field  62  is connected to the coolant supply passages  50   a  and the coolant discharge passages  50   b . The coolant flow field  62  extends in the horizontal direction, and in the coolant flow field  62 , the coolant flows over the electrode area of the membrane electrode assembly  40 . 
     A first seal member  64  is formed integrally with the surfaces  42   a ,  42   b  of the first metal separator  42 , around the outer circumferential end of the first metal separator  42 . A second seal member  66  is formed integrally with the surfaces  44   a ,  44   b  of the second metal separator  44 , around the outer circumferential end of the second metal separator  44 . 
     Each of the first seal member  64  and the second seal member  66  is an elastic seal member which is made of seal material, cushion material, packing material, or the like, such as an EPDM (ethylene propylene diene monomer), an NBR (nitrile butadiene rubber), a fluoro rubber, a silicone rubber, a fluorosilicone rubber, a Butyl rubber, a natural rubber, a styrene rubber, a chloroprene rubber, an acrylic rubber, or the like. 
     As shown in  FIG. 2 , an oxygen-containing gas supply manifold member  68   a , an oxygen-containing gas discharge manifold member  68   b , a fuel gas supply manifold member  70   a , and a fuel gas discharge manifold member  70   b  are attached to the first end plate  18   a . The oxygen-containing gas supply manifold member  68   a , the oxygen-containing gas discharge manifold member  68   b , the fuel gas supply manifold member  70   a , and the fuel gas discharge manifold member  70   b  are made of electrically insulating resin. 
     The oxygen-containing gas supply manifold member (fluid manifold member)  68   a  and the oxygen-containing gas discharge manifold member (fluid manifold member)  68   b  are connected to the oxygen-containing gas supply passage  46   a  and the oxygen-containing gas discharge passage  46   b , respectively. The fuel gas supply manifold member (fluid manifold member)  70   a  and the fuel gas discharge manifold member (fluid manifold member)  70   b  are connected to the fuel gas supply passage  48   a  and the fuel gas discharge passage  48   b , respectively. 
     As shown in  FIG. 1 , a resin coolant supply manifold member (fluid manifold member)  72   a  formed by injection molding is provided at the second end plate (one of end plates)  18   b . The coolant supply manifold member  72   a  is connected to the upper and lower pairs of coolant supply passages  50   a . A resin coolant discharge manifold member (fluid manifold member)  72   b  formed by injection molding is provided at the second end plate  18   b . The coolant discharge manifold member  72   b  is connected to the upper and lower pairs of coolant discharge passages  50   b . Preferably, the coolant supply manifold member  72   a  and the coolant discharge manifold member  72   b  have electrically insulating property. 
     As shown in  FIGS. 4 to 6 , the coolant supply manifold member  72   a  is fixed to the second end plate  18   b  such that an insulating plate  74   a  made of electrically insulating resin or the like is interposed between the coolant supply manifold member  72   a  and the second end plate  18   b . The insulating plate  74   a  is a substantially flat plate, and has a coolant inlet port  76   a  connected to the two separate coolant supply passages  50   a  at the upper positions and a coolant inlet port  76   a  connected to the two separate coolant supply passages  50   a  at the lower positions. 
     As shown in  FIGS. 4 and 5 , the insulating plate  74   a  has a contact surface  74   as  which contacts the second end plate  18   b . A first recess  78   a  is formed in the contact surface  74   as , excluding portions  77   a  thereof that surround the upper and lower pairs of coolant supply passages  50   a  and portions  77   b  connecting both ends of the surrounding portions  77   a  (see  FIG. 5 ). The first recess  78   a  has a substantially rectangular shape, and the first recess  78   a  is formed at the central portion of the contact surface  74   as.    
     As shown in  FIGS. 4 and 6 , a second recess  80   a  is formed on a surface of the insulating plate  74   a  that contacts the coolant supply manifold member  72   a . The second recess  80   a  is connected to an internal space  72   ac  of the coolant supply manifold member  72   a . The second recess  80   a  has a substantially rectangular shape, and for example, the size of the opening of the second recess  80   a  is substantially equal to the size of the opening of the first recess  78   a.    
     A plurality of holes  82   a  are formed in the outer circumferential edge portion of the insulating plate  74   a . As shown in  FIG. 4 , screws (bolts)  84   a  inserted into the respective holes  82   a  are screwed into screw holes  85   a  of the second end plate  18   b  to thereby fix the insulating plate  74   a  to the second end plate  18   b . A plurality of screw holes  86   a  are formed on a surface  74   af  of the insulating plate  74   a  facing the coolant supply manifold member  72   a , around the second recess  80   a  and the coolant inlet ports  76   a  (see  FIG. 6 ). 
     The coolant supply manifold member  72   a  has a flange  88   a  around the internal space  72   ac . The flange  88   a  has a plurality of holes  90   a  corresponding to the screw holes  86   a . Screws  92   a  inserted through the holes  90   a  are screwed into the screw holes  86   a  to thereby fix the coolant supply manifold member  72   a  to the insulating plate  74   a . It should be noted that screw holes may be formed in the second end plate  18   b  for allowing the screws  92   a  to be inserted into the screw holes, whereby the coolant supply manifold member  72   a  and the insulating plate  74   a  can be tightened together. 
     An inlet pipe section  94   a  is provided at an intermediate position of the coolant supply manifold member  72   a  in the direction indicated by the arrow C (center of the coolant flow field  62  in the flow field width direction). The inlet pipe section  94   a  is provided along the horizontal direction, or inclined from the horizontal direction. 
     As shown in  FIG. 4 , a first gap  96   a  is formed between a surface of the second end plate  18   b  and the contact surface  74   as  of the insulating plate  74   a  through the first recess  78   a . A second gap  98   a  is formed between the surface  74   af  of the insulating plate  74   a  and an attachment surface  72   as  of the coolant supply manifold member  72   a  through the second recess  80   a . It should be noted that only at least one of the first gap  96   a  and the second gap  98   a  may be provided. Further, seal members (not shown) are formed between the coolant supply manifold member  72   a  and the insulating plate  74   a , and between the insulating plate  74   a  and the second end plate  18   b , around the area where coolant flows. 
     As shown in  FIG. 1 , the coolant discharge manifold member  72   b  is fixed to the second end plate  18   b  through an insulating plate  74   b  made of electrically insulating resin, etc. The constituent elements of the coolant discharge manifold member  72   b  that are identical to those of the coolant supply manifold member  72   a  are labeled with the same reference numerals (with suffix b instead of a), and detailed description thereof is omitted. An outlet pipe section  94   b  is provided at an intermediate position of the coolant discharge manifold member  72   b  in the direction indicated by the arrow C as a coolant discharge port. The outlet pipe section  94   b  is provided along the horizontal direction, or inclined from the horizontal direction. 
     Operation of the fuel cell stack  10  will be described below. 
     Firstly, as shown in  FIG. 2 , an oxygen-containing gas is supplied from the oxygen-containing gas supply manifold member  68   a  at the first end plate  18   a  to the oxygen-containing gas supply passage  46   a . A fuel gas such as a hydrogen-containing gas is supplied from the fuel gas supply manifold member  70   a  at the first end plate  18   a  to the fuel gas supply passage  48   a.    
     Further, as shown in  FIG. 1 , a coolant such as pure water, ethylene glycol, oil, or the like is supplied from the inlet pipe section  94   a  to the internal space  72   ac  of the coolant supply manifold member  72   a  at the second end plate  18   b . The coolant is distributed to the upper pair of coolant supply passages  50   a  and the lower pair of coolant supply passages  50   a  connected to the internal space  72   ac.    
     Thus, as shown in  FIG. 3 , the oxygen-containing gas flows from the oxygen-containing gas supply passage  46   a  into the oxygen-containing gas flow field  58  of the first metal separator  42 . The oxygen-containing gas flows along the oxygen-containing gas flow field  58  in the direction indicated by the arrow A, and the oxygen-containing gas is supplied to the cathode  54  of the membrane electrode assembly  40  for inducing an electrochemical reaction at the cathode  54 . 
     In the meanwhile, the fuel gas is supplied from the fuel gas supply passage  48   a  to the fuel gas flow field  60  of the second metal separator  44 . The fuel gas flows along the fuel gas flow field  60  in the direction indicated by the arrow A, and the fuel gas is supplied to the anode  56  of the membrane electrode assembly  40  for inducing an electrochemical reaction at the anode  56 . 
     Thus, in the membrane electrode assembly  40 , the oxygen-containing gas supplied to the cathode  54  and the fuel gas supplied to the anode  56  are consumed in the electrochemical reactions at the electrode catalyst layers of the cathode  54  and the anode  56  for generating electricity. 
     Then, the oxygen-containing gas consumed at the cathode  54  of the membrane electrode assembly  40  is discharged along the oxygen-containing gas discharge passage  46   b  in the direction indicated by the arrow B. In the meanwhile, the fuel gas consumed at the anode  56  of the membrane electrode assembly  40  is discharged along the fuel gas discharge passage  48   b  in the direction indicated by the arrow B. 
     Further, the coolant supplied to the upper pair of coolant supply passages  50   a  and the lower pair of coolant supply passages  50   a  flows into the coolant flow field  62  between the first metal separator  42  and the second metal separator  44 . After the coolant temporarily flows inward in the direction indicated by the arrow C such that the coolant from the upper pair of coolant supply passages  50   a  and the coolant from the lower pair of coolant supply passages  50   a  move closer to each other, the coolant moves in the direction indicated by the arrow A to cool the membrane electrode assembly  40 . Then, the coolant diverges to flow away from each other in the direction indicated by the arrow C, and the coolant is discharged along the upper pair of coolant discharge passages  50   b  and the lower pair of coolant discharge passages  50   b  in the direction indicated by the arrow B. 
     As shown in  FIG. 1 , the coolant is discharged from the upper pair coolant discharge passages  50   b  and the lower pair of coolant discharge passages  50   b  into an internal space  72   bc  of the coolant discharge manifold member  72   b . After the coolant flows toward the center of the internal space  72   bc , the coolant is discharged to the outside from the outlet pipe section  94   b.    
     In the first embodiment, as shown in  FIG. 1 , the insulating plate  74   a  is provided between the coolant supply manifold member  72   a  and the second end plate  18   b . Further, the insulating plate  74   b  is provided between the coolant discharge manifold member  72   b  and the second end plate  18   b.    
     Thus, with the simple and economical structure, a desired electrical insulation between the coolant supply manifold member  72   a  and the second end plate  18   b , and between the coolant discharge manifold member  72   b  and the second end plate  18   b  is achieved suitably. 
     Further, as shown in  FIGS. 4 and 5 , in the insulating plate  74   a  stacked on the coolant supply manifold member  72   a , the first recess  78   a  is formed at the contact surface  74   as  thereof excluding the portions of the contact surface  74   as  that surround the upper pair of coolant supply passages  50   a  and the lower pair of coolant supply passages  50   a . In the structure, the first gap  96   a  is formed between the surface of the second end plate  18   b  and the contact surface  74   as  of the insulating plate  74   a  through the first recess  78   a , and electrical resistance between the second end plate  18   b  and the insulating plate  74   a  is thus increased. The coolant discharge manifold member  72   b  functions in the same manner as the coolant supply manifold member  72   a.    
     Accordingly, it becomes possible to suitably suppress electrical connection between the fuel cell stack  10  and external equipment (not shown) through the coolant flowing through the coolant supply manifold member  72   a  and the coolant discharge manifold member  72   b.    
     Further, as shown in  FIGS. 4 and 6 , the second recess  80   a  is formed in the insulating plate  74   a , and the second recess  80   a  is connected to the internal space  72   ac  of the coolant supply manifold member  72   a . Moreover, the second gap  98   a  is provided between the surface  74   af  of the insulating plate  74   a  and the attachment surface  72   as  of the coolant supply manifold member  72   a  through the second recess  80   a.    
     In the structure, electrical resistance of the insulating plate  74   a  becomes large, the volume of the internal space  72   ac  is increased, and it is possible to effectively achieve size reduction of the coolant supply manifold member  72   a . Further, since the shape of the internal space  72   ac  is simplified, forming is performed easily. Moreover, the same advantages are obtained also on the part of the coolant discharge manifold member  72   b.    
     In the first embodiment, though the coolant supply manifold member  72   a  and the coolant discharge manifold member  72   b  are used as fluid manifold members, the present invention is not limited in this respect. For example, the present invention may be applicable to the fluid manifold member forming passages of the fuel gas and the oxygen-containing gas. 
     As shown in  FIG. 7 , a fuel cell stack  100  according to a second embodiment of the present invention is mounted, e.g., in a fuel cell electrical vehicle (not shown). The constituent elements that are identical to those of the fuel cell stack  10  according to the first embodiment are labeled with the same reference numerals and detailed description thereof will be omitted. Also in the third embodiment described later, the constituent elements that are identical to those of the fuel cell stack  10  according to the first embodiment are labeled with the same reference numerals and detailed description thereof will be omitted. 
     As shown in  FIGS. 7 and 8 , a resin coolant supply manifold member (coolant manifold)  102   a  is attached to the second end plate  18   b . The coolant supply manifold member  102   a  is connected to two pairs of coolant supply passages  50   a  (one pair of two coolant supply passages  50   a  at upper positions and the other pair of two coolant supply passages  50   a  at lower positions) arranged respectively on the opposite long sides of the second end plate  18   b . A resin coolant discharge manifold member (coolant manifold)  102   b  is attached to the second end plate  18   b . The coolant discharge manifold member  102   b  is connected to two pairs of coolant discharge passages  50   b  (one pair of two coolant discharge passages  50   b  at upper positions and the other pair of two coolant discharge passages  50   b  at lower positions) arranged respectively on the opposite long sides of the second end plate  18   b.    
     The coolant supply manifold member  102   a  includes upper and lower flanges  104   a  connected respectively to the upper and lower pairs of coolant supply passages  50   a . The flanges  104   a  are formed integrally with a substantially rectangular cylindrical supply body section  106   a . An inlet pipe section  108   a  as a coolant supply port is provided at an intermediary position of the supply body section  106   a  (at the central portion of the coolant flow field  62  in the flow field width direction). 
     A protrusion  110   a  bulging toward the inlet pipe section  108   a  is provided on a manifold inner surface of the supply body section  106   a  facing the inlet pipe section  108   a , at substantially the center between the upper and lower coolant supply passages  50   a . The protrusion  110   a  is formed by recessing an outer wall surface of the supply body section  106   a  toward the inlet pipe section  108   a  to have a smooth curved surface, e.g., circular arc surface bulging into the interior of the manifold. The protrusion  110   a  has a vertically symmetrical shape. It should be noted that the protrusion  110   a  may have a vertically asymmetrical shape. In this case, as shown by a two dot chain line in  FIG. 8 , preferably, the slope on the upper side is steep, and the slope on the lower side is gentle in comparison with the upper side. Each of the flanges  104   a  is fixed to the second end plate  18   b  using a plurality of bolts  84   a.    
     The coolant discharge manifold member  102   b  includes upper and lower flanges  104   b  connected respectively to upper and lower pairs of coolant discharge passages  50   b . The flanges  104   b  are formed integrally with a substantially rectangular cylindrical discharge body section  106   b . An outlet pipe section  108   b  as a coolant discharge port is provided at an intermediary position of the discharge body section  106   b.    
     A protrusion  110   b  bulging toward the outlet pipe section  108   b  is provided on a manifold inner surface of the discharge body section  106   b  facing the outlet pipe section  108   b , at substantially the center of the upper and lower coolant discharge passages  50   b . The protrusion  110   b  is formed by recessing an outer wall surface of the discharge body section  106   b  toward the outlet pipe section  108   b  to have a smooth curved surface, e.g., circular arc surface bulging into the interior of the manifold. It is noted that the protrusion  110   b  is provided on the discharge body section  106   b  as necessary, and the protrusion  110   b  may not be provided. Each of the flanges  104   b  is fixed to the second end plate  18   b  using a plurality of bolts  84   b.    
     In the second embodiment, the coolant supply manifold member  102   a  and the coolant discharge manifold member  102   b  are provided on the second end plate  18   b . In the coolant supply manifold member  102   a , the protrusion  110   a  bulging toward the inlet pipe section  108   a  is provided on the manifold inner surface of the supply body section  106   a  facing the inlet pipe section  108   a.    
     In the structure, as shown in  FIG. 8 , the coolant supplied from the inlet pipe section  108   a  into the supply body section  106   a  (into the manifold) flows toward the protrusion  110   a  facing the inlet pipe section  108   a . Therefore, since the coolant is blown onto the protrusion  110   a , by the guiding action of the protrusion  110   a , the coolant bifurcates so as to flow toward the upper side in the vertical direction (direction indicated by an arrow C1) and toward the lower side in the vertical direction (direction indicated by an arrow C2). 
     Thus, since the coolant is suitably distributed and supplied in the direction indicated by the arrow C1 and in the direction indicated by the arrow C2, bad distribution (instability of distribution) of the coolant is suppressed reliably. Accordingly, the coolant is reliably supplied to the upper two coolant supply passages  50   a  and the lower two coolant supply passages  50   a.    
     In the second embodiment, with the simple and economical structure, the coolant supplied into the coolant supply manifold member  102   a  flows toward the upper and lower pairs of coolant supply passages  50   a  smoothly and uniformly. Accordingly, improvement in the cooling performance of each fuel cell  12  is achieved suitably. 
     In the coolant discharge manifold member  102   b , the protrusion  110   b  bulging toward the outlet pipe section  108   b  is provided on the manifold inner surface of the discharge body section  106   b  facing the outlet pipe section  108   b.    
     In the structure, the coolant introduced from the upper two coolant discharge passages  50   b  and the lower two coolant discharge passages  50   b  into the discharge body section  106   b  flows toward the protrusion  110   b  facing the outlet pipe section  108   b . Thus, by the guiding action of the protrusion  110   b , the coolant flows from the vertically downward direction to the horizontal direction, or from the vertically upward direction to the horizontal direction. Accordingly, the coolant is suitably discharged from the outlet pipe section  108   b  facing the protrusion  110   b.    
     Therefore, with the simple economical structure, the coolant flows smoothly and uniformly from the upper and lower pairs of the coolant discharge passages  50   b  into the coolant discharge manifold member  102   b , and the coolant is discharged into the outlet pipe section  108   b . Accordingly, the cooling performance of each fuel cell  12  is improved suitably. 
       FIG. 9  is a front view showing a fuel cell stack  120  according to a third embodiment of the present invention. 
     In the fuel cell stack  120 , a resin coolant supply manifold member (coolant manifold)  122  and a resin coolant discharge manifold member  102   b  are attached to the second end plate  18   b . An inlet pipe section  124  as a coolant supply port is provided on the coolant supply manifold member  122  at a position closer to the lower coolant supply passages  50   a  of the supply body section  106   a.    
     The inlet pipe section  124  is inclined downwardly at an angle α° relative to the flow direction of the coolant in the coolant flow field  62  indicated by an arrow B. A protrusion  126  bulging toward the inlet pipe section  124  is provided on the manifold inner surface of the supply body section  106   a  facing the inlet pipe section  124 . The center of the protrusion  126  is situated at a position closer to the upper coolant supply passages  50   a . The protrusion  126  is formed by recessing an outer wall surface of the supply body section  106   a  toward the inlet pipe section  124  (i.e., forming a slope on the outer wall surface of the supply body section  106   a ) to have a smooth curved surface, e.g., circular arc surface bulging into the manifold. In the protrusion  126 , the slope on the upper side is steep in comparison with the lower side. 
     It should be noted that, in the case where the inlet pipe section  124  is provided at a position closer to the upper coolant supply passages  50   a  in the coolant supply manifold member  122 , the angle of the inlet pipe section  124  and the angle of the protrusion  126  are set in a manner opposite to the angle described above (see two dot chain line in  FIG. 9 ). 
     In the third embodiment, the coolant supplied obliquely upward from the inlet pipe section  124  to the inside of the supply body section  106   a  (into the manifold) at the angle α° flows toward the protrusion  126  facing the inlet pipe section  124 . Accordingly, the coolant is blown onto the protrusion  126 , and thus, by the guiding action of the protrusion  126 , the coolant is distributed so as to flow in the vertically upward direction indicated by the arrow C1 and in the vertically downward direction indicated by the arrow C2. 
     Thus, since the coolant is suitably distributed and supplied in the direction indicated by the arrow C1 and in the direction indicated by the arrow C2, bad distribution (instability of distribution) of the coolant is suppressed reliably. In the structure, the coolant is reliably supplied to the upper two coolant supply passages  50   a  and the lower two coolant supply passages  50   a . Accordingly, the same advantages as in the case of the second embodiment are obtained. It should be noted that the coolant discharge manifold member  102   b  may have the same structure as the above described coolant supply manifold member  122 . 
     As shown in  FIG. 10 , a fuel cell stack  130  according to a fourth embodiment of the present invention is mounted, e.g., in a fuel cell electrical vehicle (not shown). The constituent elements that are identical to those of the fuel cell stack  100  according to the second embodiment are labeled with the same reference numerals and detailed description thereof will be omitted. Also in the fifth embodiment described later, the constituent elements that are identical to those of the fuel cell stack  100  according to the second embodiment are labeled with the same reference numerals and detailed description thereof will be omitted. 
     As shown in  FIGS. 10 and 11 , a resin coolant supply manifold member (coolant manifold)  132   a  is attached to the second end plate  18   b . The coolant supply manifold member  132   a  is connected to a pair of upper and lower coolant supply passages  50   a  arranged respectively on the opposite long sides of the second end plate  18   b . A resin coolant discharge manifold member (coolant manifold)  132   b  is attached to the second end plate  18   b . The coolant discharge manifold member  132   b  is connected to a pair of upper and lower coolant discharge passages  50   b  arranged respectively on the opposite long sides of the second end plate  18   b . Alternatively, as with in the first and second embodiments, two coolant supply passages  50   a  may be arranged on each of the opposite long sides, and two coolant discharge passages  50   b  may be arranged on each of the opposite long sides. Further, in the first and second embodiment, one coolant supply passage  50   a  may be arranged on each of the opposite long sides, and one coolant discharge passage  50   b  may be arranged on each of the opposite long sides. 
     The coolant supply manifold member  132   a  includes a supply body section  106   a . A protrusion  110   a  bulging toward an inlet pipe section  108   a  is provided on a manifold inner surface  132   as  of the supply body section  106   a  facing the inlet pipe section  108   a , at substantially the center between the upper and lower coolant supply passages  50   a . Protrusions  134   a  bulging toward an inside  132   a   in  of the manifold are provided respectively on both sides of the inlet pipe section  108   a  of the supply body section  106   a . Each of the protrusions  134   a  is formed on the manifold inner surface  132   as  to have a smooth curved surface, e.g., circular arc surface. 
     The coolant discharge manifold member  132   b  includes a discharge body section  106   b . A protrusion  110   b  bulging toward an outlet pipe section  108   b  is provided on a manifold inner surface  132   bs  of the discharge body section  106   b  facing the outlet pipe section  108   b , at substantially the center between the upper and lower coolant discharge passages  50   b . Protrusions  134   b  bulging toward an inside  132   b   in  of the manifold are provided respectively on both sides of the outlet pipe section  108   b  of the discharge body section  106   b . Each of the protrusions  134   b  is formed on the manifold inner surface  132   bs  to have a smooth curved surface, e.g., circular arc surface. 
     In this case, as shown in  FIGS. 10 and 11 , in the fourth embodiment, the coolant supply manifold member  132   a  and the coolant discharge manifold member  132   b  are provided on the second end plate  18   b . In the coolant supply manifold member  132   a , the protrusions  134   a  bulging toward the manifold inside  132   a   in  are provided respectively on both sides (upper and lower sides) of the inlet pipe section  108   a  of the supply body section  106   a.    
     Thus, as shown in  FIG. 11 , coolant supplied from the inlet pipe section  108   a  into the supply body section  106   a  (manifold inside  132   a   in ) flows along the shape of the protrusions  134   a  arranged respectively on both sides (upper and lower sides) of the inlet pipe section  108   a . Accordingly, by the guiding action of the protrusions  134   a , the coolant is distributed so as to flow in the vertically upward direction (indicated by an arrow C1) and in the vertically downward direction (indicated by an arrow C2). 
     Owing thereto, the coolant is suitably and smoothly distributed and supplied in the direction indicated by the arrow C1 and in the direction indicated by the arrow C2, and bad distribution (instability of distribution) of the coolant is thus suppressed reliably. Accordingly, the coolant is reliably supplied to the upper coolant supply passage  50   a  and the lower coolant supply passage  50   a.    
     In the fourth embodiment, with the simple and economical structure, the coolant supplied into the coolant supply manifold member  132   a  flows toward the pair of upper and lower coolant supply passages  50   a  smoothly and uniformly. Accordingly, improvement in the cooling performance of each fuel cell  12  is achieved suitably. 
     Meanwhile, in the coolant discharge manifold member  132   b , the protrusions  134   b  bulging toward the manifold inside  132   b   in  are provided respectively on both sides (upper and lower sides) of the outlet pipe section  108   b  of the discharge body section  106   b.    
     In the structure, as shown in  FIG. 11 , the coolant introduced from the upper coolant discharge passage  50   b  and the lower coolant discharge passage  50   b  into the discharge body section  106   b  flows along the shape of the protrusions  134   b . Thus, by the guiding action of the protrusions  134   b , the coolant flows from the vertically downward direction to the horizontal direction, or from the vertically upward direction to the horizontal direction. Accordingly, the coolant is suitably discharged from the outlet pipe section  108   b  facing the protrusion  110   b.    
     Therefore, with the simple economical structure, the coolant flows smoothly and uniformly from the pair of upper and lower coolant discharge passages  50   b  into the coolant discharge manifold member  132   b , and the coolant is discharged into the outlet pipe section  108   b . Accordingly, the cooling performance of each fuel cell  12  is improved suitably. 
       FIG. 12  is a front view showing a fuel cell stack  140  according to a fifth embodiment of the present invention, as viewed from a coolant manifold member side. The constituent elements that are identical to those of the fuel cell stack  130  according to the fourth embodiment are labeled with the same reference numerals and detailed description thereof will be omitted. 
     In the fuel cell stack  140 , a resin coolant supply manifold member (coolant manifold)  142  and a resin coolant discharge manifold member  132   b  are attached to the second end plate  18   b . An inlet pipe section  144  as a coolant supply port is provided on the coolant supply manifold member  142  at a position closer to the lower coolant supply passage  50   a  of the supply body section  106   a.    
     The inlet pipe section  144  is inclined downwardly at an angle α1° relative to the flow direction of the coolant in the coolant flow field  62  indicated by an arrow A. A protrusion  146  bulging toward the inlet pipe section  144  is provided on a manifold inner surface  142   s  of the supply body section  106   a  facing the inlet pipe section  144 . The center of the protrusion  146  is situated at a position closer to the upper coolant supply passage  50   a . The protrusion  146  is formed by recessing an outer wall surface of the supply body section  106   a  toward the inlet pipe section  144  (i.e., forming a slope on the outer wall surface of the supply body section  106   a ) to have a smooth curved surface, e.g., circular arc surface bulging toward a manifold inside  142   in . In the protrusion  146 , the slope on the upper side is steep in comparison with the lower side. 
     Protrusions  148 ,  150  bulging toward the manifold inside  142   in  are provided respectively on both sides of the inlet pipe section  144  of the supply body section  106   a . Each of the protrusions  148 ,  150  is formed on the manifold inner surface  142   s  to have a smooth curved surface, e.g., circular arc surface. 
     It should be noted that, in the case where the inlet pipe section  144  is provided at a position closer to the upper coolant supply passage  50   a  in the coolant supply manifold member  142 , the angle of the inlet pipe section  144 , the angle of the protrusion  146 , and the angles of the protrusions  148 ,  150  are set in a manner opposite to the angles described above (see two dot chain line in  FIG. 12 ). 
     In the fifth embodiment, the coolant supplied obliquely upward from the inlet pipe section  144  to the inside of the supply body section  106   a  (to the manifold inside  142   in ) at the angle α1° flows along the shape of the protrusions  148 ,  150 . Accordingly, by the guiding action of the protrusions  148 ,  150 , the coolant is distributed so as to flow in the vertically upward direction indicated by the arrow C1 and in the vertically downward direction indicated by the arrow C2. 
     Thus, since the coolant is suitably distributed and supplied in the direction indicated by the arrow C1 and in the direction indicated by the arrow C2, bad distribution (instability of distribution) of the coolant is suppressed reliably. In the structure, the coolant is reliably supplied to the upper coolant supply passage  50   a  and the lower coolant supply passage  50   a . Accordingly, the same advantages as in the case of the fourth embodiment are obtained. It should be noted that the coolant discharge manifold member  132   b  may have the same structure as the above described coolant supply manifold member  142 . 
     While the invention has been particularly shown and described with a 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 scope of the invention as defined by the appended claims.