Abstract:
A fuel cell wherein reactive gas or coolant flow directions within single-cell internal passages may be changed by switching the positions of flow control means such as electromagnetic valves in order to alter flow directions and flow velocities in accordance with fuel cell operating states. Altering flow to different directions within the internal passages, such as to a second flow direction perpendicular to a first flow direction, permits control of gas flow to combat fuel cell flooding, wherein the altered reactant gas flow causes accumulated water to be discharged from the cell. Such flow control further permits control of gas and coolant temperatures to optimize cell moisture distribution and control.

Description:
INCORPORATION BY REFERENCE 
     The disclosure of Japanese Patent Application No. 2001-336013 filed on Nov. 1, 2001, including the specification, drawings, and abstract is incorporated herein by reference in its entirety. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a fuel cell and a method of controlling the same. 
     2. Description of the Related Art 
     A fuel cell constructed by laminating a plurality of single cells each having a membrane electrode assembly (hereinafter referred to as the MEA) interposed between two separators has been known. The MEA is composed of an electrolytic membrane and gas diffusion electrodes. Platinum as catalytic electrodes is applied to both surfaces of the electrolytic membrane, which is interposed between the gas diffusion electrodes. The catalytic electrode and the gas diffusion electrode formed on one surface of the MEA constitutes an anode, and the catalytic electrode and the gas diffusion electrode formed on the other surface of the MEA constitute a cathode. A fuel gas passage in which hydrogen gas as fuel gas to flows through a single cell of the fuel cell is formed in a separator facing the anode. An oxidative gas passage in which air as oxidative gas to flows through the single cell is formed in a separator facing the cathode. 
     This electrolytic membrane usually functions as a good proton-conductive electrolyte in a wet state and thus is held wet by supplying fuel gas and oxidative gas that have been humidified in advance. However, for various reasons, flooding (a state of being too wet) may be caused. For example, if flooding occurs, water is produced in the gas passage and constitutes a flow resistance against gas, which may make it impossible to supply the gas diffusion electrodes with a sufficient amount of gas. In consideration of this drawback, Japanese Laid-Open Patent Application No. 7-2353234 or the like discloses a fuel cell wherein a dynamic pressure of gas is temporarily increased in response to the occurrence of flooding and wherein produced water in a gas passage is blown off and removed by the dynamic pressure. 
     However, the fuel cell of the aforementioned publication is designed to cause gas to constantly flow in a certain direction. Hence, if there is some factor (e.g., an obstacle) preventing produced water from moving in the direction, the produced water cannot be easily removed even in the case of an increase in the dynamic pressure of reactive gas. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide a fuel cell capable of efficiently removing an unwanted substance such as produced water in a gas passage. It is another object of the invention to provide a method of controlling such a fuel cell. 
     A first aspect of the invention relates to a fuel cell. This fuel cell comprises a single-cell internal gas passage in which reactive gas flows through a single cell of the fuel cell, a switching device that switches a flow direction of gas in the single-cell internal gas passage from a first direction to a second direction intersecting with the first direction, and a controller that switches the flow direction of gas in the single-cell internal gas passage from the first direction to the second direction in accordance with an operational state of the fuel cell by means of the switching device. 
     In this fuel cell, the flow direction of gas in the single-cell internal gas passage is switched from the first direction to the second direction intersecting with the first direction in accordance with an operational state of the fuel cell. Hence, even if there is some factor preventing an unwanted substance (e.g., produced water) existing in the single-cell internal gas passage in a certain operational state from moving in the first direction, the flow direction of gas is switched to the second direction intersecting with the first direction, whereby the factor is made less influential. As a result, the unwanted substance can be removed by gas. Accordingly, the unwanted substance in the gas passage can be efficiently removed. 
     It is to be noted herein that “reactive gas” means a gas used for an electrochemical reaction in a fuel cell and that “the operational state of the fuel cell” is not specifically limited and may be a parameter regarding operational control such as an output voltage or an impedance of the fuel cell, may be a humidity, a temperature, or an amount of supply of reactive gas used in the fuel cell, or may be a temperature or an amount of supply of coolant for the fuel cell. 
     A second aspect of the invention relates to a method of controlling a fuel cell capable of changing a flow direction of gas in a single-cell internal gas passage in which reactive gas flows through a single cell of the fuel cell. This method is designed to switch a flow direction of gas in the single-cell internal gas passage to a second direction intersecting with a first direction in accordance with an operational state of the fuel cell. In this control method, the flow direction of gas in the single-cell internal gas passage is switched from the first direction to the second direction intersecting with the first direction in accordance with an operational state of the fuel cell. Hence, even if there is some factor preventing an unwanted substance (e.g., produced water) existing in the single-cell internal gas passage in a certain operational state from moving in the first direction, the flow direction of gas is switched to the second direction intersecting with the first direction, whereby the factor is made less influential. As a result, the unwanted substance can be removed by gas. Accordingly, the unwanted substance in the gas passage can be efficiently removed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing and further objects, features and advantages of the invention will become apparent from the following description of preferred embodiments with reference to the accompanying drawings, wherein like numerals are used to represent like elements and wherein: 
         FIG. 1  is a perspective view of the overall construction of a fuel cell in accordance with a first embodiment of the invention; 
         FIG. 2  is a partial cross-sectional view of a stack of the fuel cell; 
         FIG. 3  is an exploded perspective view of a cell module; 
         FIG. 4  is an exploded perspective view of the cell module which is viewed from an angle indicated by “A” (a blank arrow) in  FIG. 3 ; 
         FIG. 5  is a schematic block diagram showing how components of the fuel cell are electrically connected; 
         FIG. 6  is a flowchart of switching control for switching the flow direction of oxidative gas; 
         FIGS. 7A and 7B  are plan views of a central separator which is viewed from the side of a first single-cell internal gas passage and is also an explanatory view showing two flow directions of oxidative gas; 
         FIGS. 8A and 8B  are partially enlarged views of the first single-cell internal gas passage of the central separator; 
         FIG. 9  is a flowchart of control for switching the flow direction of oxidative gas in accordance with a modification example of the first embodiment of the invention; 
         FIGS. 10A and 10B  are explanatory views of the construction of the central separator when the flow directions of both oxidative gas and fuel gas are switched; 
         FIGS. 11A and 11B  are explanatory views of the flow direction of oxidative gas in accordance with a second embodiment of the invention; 
         FIG. 12  is a flowchart of switching control for switching the flow direction of oxidative gas in accordance with the second embodiment; 
         FIG. 13  is an explanatory view of a method of causing oxidative gas to flow in accordance with a modification example of the second embodiment; and 
         FIG. 14A and 14B  are explanatory views of the flow direction of coolant in accordance with a third embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     In order to further clarify the invention, preferred embodiments of the invention will be described hereinafter with reference to the drawings. 
     [First Embodiment] 
     A fuel cell  10  in accordance with the present embodiment shown in  FIG. 1  is a polymer electrolyte fuel cell and mainly comprises a stack  40 , an oxidative gas supply pipe  41 , an oxidative gas discharge pipe  42 , an oxidative gas supply/discharge device  410  (see  FIG. 5 ), a fuel gas supply pipe  43 , a fuel gas discharge pipe  44 , a fuel gas supply/discharge device  430  (see  FIG. 5 ), a coolant introduction pipe  45 , a coolant exhaust pipe  46 , a coolant supply/discharge device  450  (see  FIG. 5 ), and a control device  50  (see  FIG. 5 ). The stack  40  is composed of a plurality of laminated cell modules  20 . The oxidative gas supply pipe  41 , the oxidative gas discharge pipe  42 , and the oxidative gas supply/discharge device  410  are designed to supply oxidative gas to and discharge oxidative gas from the stack  40 . The fuel gas supply pipe  43 , the fuel gas discharge pipe  44 , and the fuel gas supply/discharge device  430  are designed to supply fuel gas to and discharge fuel gas from the stack  40 . The coolant introduction pipe  45 , the coolant exhaust pipe  46 , and the coolant supply/discharge device  450  are designed to supply coolant to and discharge coolant from the stack  40 . The control device  50  is designed to perform switching control for switching the flow direction of oxidative gas or the like. 
     As shown in  FIG. 2 , each of the cell modules  20  is constructed by laminating a cooling separator  60 , an MEA  30 , a central separator  70 , an MEA  30 , and an end separator  80  in this order. The cooling separator  60 , the MEA  30 , and the central separator  70  constitute a single cell  21 . The central separator  70 , the MEA  30 , and the end separator  80  constitute a single cell  22 . That is, the single cell  21  and the single cell  22  have the central separator  70  as a common member. 
     The MEA  30  is a anode-electrolyte-cathode assembly having an electrolytic membrane  31  interposed between an anode  32  and a cathode  33 . It is to be noted herein that the electrolytic membrane  31  is a proton-conductive ion-exchange membrane (e.g., a Nafion® membrane manufactured by DuPont®) made of a polymer electrolyte material such as fluororesin and exhibits high electric conductivity in a wet state. Platinum or an alloy composed of platinum and another metal is applied to both surfaces of the electrolytic membrane  31 , whereby catalytic electrodes  34 ,  35  are formed. Gas diffusion electrodes  36 ,  37 , which are formed of carbon cloth woven from carbon fiber threads, are disposed outside the catalytic electrodes  34 ,  35  respectively. The catalytic electrode  34  and the gas diffusion electrode  36  constitute the anode  32 . The catalytic electrode  35  and the gas diffusion electrode  37  constitute the cathode  33 . It is not absolutely required that the gas diffusion electrodes  36 ,  37  be formed of carbon cloth. The gas diffusion electrodes  36 ,  37  may also be formed of carbon paper or carbon felt made of carbon fiber and are only required to exhibit sufficient gas diffusibility and sufficient conductivity. 
     Each of the separators  60 ,  70 ,  80  is formed of a conductive member impervious to gas, for example, is formed so as to be impervious to gas by compressing carbon. A second single-cell internal gas passage  62  through which fuel gas flows is formed on one surface of the cooling separator  60  which faces the anode  32  of the MEA  30  of the single cell  21 . A coolant passage  63  through which coolant flows is formed on the other face of the cooling separator  60 . A first single-cell internal gas passage  71  through which oxidative gas flows is formed on one surface of the central separator  70  which faces the cathode  33  of the MEA  30  of the single cell  21 . A second single-cell internal gas passage  72  through which fuel gas flows is formed on the other surface of the central separator  70  which faces the anode  32  of the MEA  32  of the single cell  22 . A first single-cell internal gas passage  81  is formed on one surface of the end separator  80  which faces the cathode  33  of the MEA  30  of the single cell  22 . 
     A sealing member  38  is disposed in a gap between the cooling separator  60  and the central separator  70 . A sealing member  39  is disposed in a gap between the central separator  70  and the end separator  80 . The sealing members  38 ,  39  play roles of preventing fuel gas and oxidative gas from being mixed in those portions and preventing the gases from leaking out to the outside. 
     As shown in  FIGS. 3 and 4 , each of long holes  601  to  604  is formed through the cooling separator  60  in such a manner as to extend along a corresponding one of four sides thereof, and each of square holes  605  to  608  is formed through the cooling separator  60  at a corresponding one of four angles thereof. As shown in  FIG. 4 , a concave portion  610  having a generally rectangular outer periphery is formed in one surface of the cooling separator  60  which faces the anode  32  of the MEA  30  of the single cell  21 . The concave portion  610  communicates with the square holes  605 ,  606 . Fuel gas is supplied from the square hole  605 , flows through the concave portion  610  of the single cell  21 , that is, through the second single-cell internal gas passage  62 , and is discharged from the square hole  606 . As shown in  FIG. 3 , a concave portion  611  having a generally rectangular outer periphery is formed on the other surface of the cooling separator  60  as well. The concave portion  611  communicates with the square holes  607 ,  608 . Coolant is supplied from the square hole  607 , flows through the concave portion  611 , that is, through the coolant passage  63 , and is discharged from the square hole  608 . 
     Each of long holes  701  to  704  is formed through the central separator  70  in such a manner as to extend along a corresponding one of four sides thereof, and each of square holes  705  to  708  is formed through the central separator  70  at a corresponding one of four corners thereof. A concave portion  711  having a generally rectangular outer periphery is formed in one surface of the central separator  70  which faces the cathode  33  of the MEA  30  of the single cell  21 . The concave portion  711  communicates with the long holes  701  to  704 . Oxidative gas is supplied from the long hole  701  to the single cell  21 , flows through the concave portion  711 , that is, through the first single-cell internal gas passage  71 , and is discharged from the long hole  703 . Alternatively, oxidative gas is supplied from the long hole  702  to the single cell  21 , flows through the concave portion  711 , that is, through the first single-cell internal gas passage  71 , and is discharged from the long hole  704 . As shown in  FIG. 4 , a concave portion  710  having a generally rectangular outer periphery is formed also on the other surface of the central separator  70  which faces the anode  32  of the MEA  30  of the single cell  22 . The concave portion  710  communicates with the square holes  705 ,  706 . Fuel gas is supplied from the square hole  705  to the single cell  22 , flows through the concave portion  710 , that is, through the second single-cell internal gas passage  72 , and is discharged from the square hole  706 . 
     Each of long holes  801  to  804  is formed through the end separator  80  in such a manner as to extend along a corresponding one of four sides thereof, and each of square holes  805  to  808  is formed through the end separator  80  at a corresponding one of four corners thereof. A concave portion  811  having a generally rectangular outer periphery is formed in one surface of the end separator  80  which faces the cathode  33  of the MEA  30  of the single cell  22 . The concave portion  811  communicates with the long holes  801  to  804 . Oxidative gas is supplied from the long hole  801  to the single cell  22 , flows through the concave portion  811 , that is, through the first single-cell internal gas passage  81 , and is discharged from the long hole  803 . Alternatively, oxidative gas is supplied from the long hole  802  to the single cell  22 , flows through the concave portion  811 , that is, through the first single-cell internal gas passage  81 , and is discharged from the long hole  804 . The other surface of the end separator  80  is formed as a flat surface. 
     Each of the concave portions  610 ,  611 ,  710 ,  711 ,  811  has a plurality of small protrusions. For example, small protrusions  720  generally in the shape of a cube or a rectangular parallelepiped as shown in a circle in  FIG. 3  are formed in the concave portion  711  of the central separator  70  like a grid. The small protrusions  720  are formed such that their leading edge portions are in contact with the cathode  33  of the MEA  30  of the single cell  21 . Areas in which the leading edge portions are in contact with the cathode  33  ensure conductivity. Oxidative gas flowing through the concave portion  711 , that is, through the first single-cell internal gas passage  71  collides with lateral surfaces of the small protrusions  720  and is thereby diffused in the first single-cell internal gas passage  71 . Furthermore, since the small protrusions  720  are disposed like a grid, oxidative gas can also flow in a direction from the long hole  701  to the long hole  703  or in a direction substantially perpendicular thereto, that is, in a direction from the long hole  702  to the long hole  704 . 
     The stack  40  is composed of a plurality of the laminated cell modules  20  and is completed by sequentially disposing a current collector plate  11 , an insulating plate  13 , and an end plate  15  on one end of the cell module  20  and a current collector plate  12 , an insulating plate  14 , and an end plate  16  on the other end of the cell module  20 . The current collector plates  11 ,  12  are formed of a conductive member impervious to gas, such as compact carbon, a copper plate, or the like. The insulating plates  13 ,  14  are formed of an insulative member such as rubber, resin, or the like. The end plates  15 ,  16  are formed of a metal such as rigid steel or the like. The current collector plates  11 ,  12  have output terminals  11   a ,  12   a  respectively, so that an electromotive force generated in the fuel cell  10  can be output. The end plates  15 ,  16  pressurize the stack  40  in the direction of lamination by means of a pressurizing device (not shown) and thus hold the stack  40 . 
     In the stack  40 , the long holes  601 ,  701 ,  801  are aligned in the direction of lamination, whereby a first oxidative gas supply manifold M 1  is formed. The long holes  602 ,  702 ,  802  are aligned in the direction of lamination, whereby a second oxidative gas supply manifold M 2  is formed. The long holes  603 ,  703 ,  803  are aligned in the direction of lamination, whereby a first oxidative gas discharge manifold M 3  is formed. The long holes  604 ,  704 ,  804  are aligned in the direction of lamination, whereby a second oxidative gas discharge manifold M 4  is formed. Oxidative gas that has been supplied to the first oxidative gas supply manifold M 1  or the second oxidative gas supply manifold M 2  flows through the first single-cell internal gas passages  71 ,  81  and then is collected in and discharged from the first oxidative gas discharge manifold M 3  or the second oxidative gas discharge manifold M 4 . The square holes  605 ,  705 ,  805  are aligned in the direction of lamination, whereby a fuel gas supply manifold M 5  is formed. The square holes  606 ,  706 ,  806  are aligned in the direction of lamination, whereby a fuel gas discharge manifold M 6  is formed. Fuel gas that has been supplied to the fuel gas supply manifold M 5  flows through the second single-cell internal gas passages  62 ,  72  and then is collected in and discharged from the fuel gas discharge manifold M 6 . Furthermore, the square holes  607 ,  707 ,  807  are aligned in the direction of lamination, whereby a coolant supply manifold M 7  is formed. The square holes  608 ,  708 ,  808  are aligned in the direction of lamination, whereby a coolant discharge manifold M 8  is formed. Coolant that has been supplied to the coolant supply manifold M 7  flows through the coolant passage  63  and then is collected in and discharged from the coolant discharge manifold M 8 . 
     The oxidative gas supply pipe  41  branches off into two branch pipes from the oxidative gas supply/discharge device  410  for supply and discharging oxidative gas (compressed air in this case), and is connected to the first oxidative gas supply manifold M 1  and the second oxidative gas supply manifold M 2 . A first electromagnetic valve B 1  is installed in the branch pipe extending toward the first oxidative gas supply manifold M 1 . A second electromagnetic valve B 2  is installed in the branch pipe extending toward the second oxidative gas supply manifold M 2 . On the other hand, branch pipes extending from the first oxidative gas discharge manifold M 3  and the second oxidative gas discharge manifold M 4  are assembled into the oxidative gas discharge pipe  42 , which is connected to the oxidative gas supply/discharge device  410 . A third electromagnetic valve B 3  is installed in the branch pipe extending from the first oxidative gas discharge manifold M 3 . A fourth electromagnetic valve B 4  is installed in the branch pipe extending from the second oxidative gas discharge manifold M 4 . 
     The fuel gas supply pipe  43  is connected to the fuel gas supply manifold M 5  from the fuel gas supply/discharge device  430  for supplying and discharging fuel gas (hydrogen gas in this case). The fuel gas discharge pipe  44  is connected to the fuel gas supply/discharge device  430  from the fuel gas discharge manifold M 6 . 
     The coolant introduction pipe  45  is connected to the coolant supply manifold M 7  from the coolant supply/discharge device  450  for supplying and discharging coolant (cooling water in this case). The coolant exhaust pipe  46  is connected to the coolant supply/discharge device  450  from the coolant exhaust manifold M 8 . 
     The control device  50  shown in  FIG. 5  is constructed of a microcomputer having a known CPU, a known ROM, a known RAM, and the like. As shown in  FIG. 5 , the control device  50  is connected such that a detection signal from a voltmeter  49  for detecting an output voltage of the stack  40  is input to the control device  50 , that control signals are output to the first electromagnetic valve B 1  and the second electromagnetic valve B 2  installed in the branch pipes of the oxidative gas supply pipe  41 , and that control signals are output to the third electromagnetic valve B 3  and the fourth electromagnetic valve B 4  installed in the branch pipes of the oxidative gas discharge pipe  42 . 
     Next, switching control for switching the flow direction of oxidative gas will be described with reference to a flowchart shown in  FIG. 6 . This control is performed by the control device  50  of the present embodiment at intervals of a predetermined period (e.g., several milliseconds). If this switching control is started, the control device  50  first reads an output voltage detected by the voltmeter  49  (step S 100 ), and determines on the basis of the output voltage whether or not flooding has occurred, that is, whether or not the first single-cell internal gas passages  71 ,  81  have become too wet (step S 110 ). For example, in the case where flooding has occurred, it is known from experience that the fluctuation of the output voltage may exceed a predetermined fluctuation range. Hence, if the fluctuation of the output voltage has exceeded the predetermined fluctuation range, it is appropriate to estimate that flooding has occurred. In this case, it is appropriate that the fluctuation of the output voltage be a difference between a current output voltage and a last output voltage or that the fluctuation of the output voltage be a difference between a maximum value and a minimum value in a time width that precedes the current moment by a predetermined period. If flooding has not occurred, the present program is immediately terminated. If flooding has occurred, control signals are output to the electromagnetic valves B 1  to B 4  (step S 120 ) so as to switch the flow direction of oxidative gas from a current direction to a direction that is substantially perpendicular thereto. Then, the present program is terminated. 
     The switching of the flow direction of oxidative gas will be described with reference to  FIGS. 7A and 7B .  FIGS. 7A and 7B  are plan views of the central separator  70  which is viewed from the side of the first single-cell internal gas passage  71 .  FIG. 7A  shows a first gas flow direction.  FIG. 7B  shows a second gas flow direction. First of all, it is assumed that the first electromagnetic valve B 1  and the third electromagnetic valve B 3  have been closed and that the second electromagnetic valve B 2  and the fourth electromagnetic valve B 4  have been opened (see  FIG. 7A ). In this case, oxidative gas horizontally flows from the long hole  702  through the first single-cell internal gas passage  71  and is discharged from the long hole  704 . If it is assumed herein that the flow direction of gas is switched in step S 120 , the control device  50  outputs control signals to the electromagnetic valves B 1  to B 4  so as to open the first electromagnetic valve B 1  and the third electromagnetic valve B 3  and close the second electromagnetic valve B 2  and the fourth electromagnetic valve B 4  (see  FIG. 7B ). Thereby oxidative gas vertically flows from the long hole  701  through the first single-cell internal gas passage  71  and is discharged from the long hole  703 . That is, the flow direction of oxidative gas is switched from the horizontal direction to the vertical direction, which is substantially perpendicular to the horizontal direction. 
     A state of the first single-cell internal gas passage  71  in the case of flooding will now be described with reference to  FIGS. 8A and 8B .  FIGS. 8A and 8B  are partially enlarged views of the first single-cell internal gas passage  71  of the central separator  70 . Because the small protrusions  720  rise from the surface of the first single-cell internal gas passage  71  like a grid, water produced in the passage  71  tends to accumulate among the small protrusions  720 . For example, when oxidative gas horizontally flows, produced water W 1  accumulating among the small protrusions  720  that are vertically disposed is removed along with the flow of oxidative gas as shown in  FIG. 8A . However, produced water W 2  accumulating among the small protrusions  720  that are horizontally disposed is prevented by the surrounding small protrusions  720  from moving along with the flow of oxidative gas and thus remains without being removed. This is a state in which flooding has occurred. If flooding as mentioned herein occurs, an increase in the fluctuation of the output voltage or the like is caused. In this case, the fluctuation of the output voltage may exceed a predetermined fluctuation range. 
     In the present embodiment, as has been described with reference to  FIGS. 7A and 7B , if flooding has occurred, the flow direction of oxidative gas is switched to a direction that is substantially perpendicular thereto. Hence, as shown in  FIG. 8B , the produced water W 2  remaining without being removed in  FIG. 8A  is removed along with the switched flow of oxidative gas. Accordingly, flooding is eliminated. Even if the opening and closing of the electromagnetic valves B 1  to B 4  has been switched, it may take a while until flooding is eliminated. Thus, the control device  50  may start switching control next time in consideration of a period required for the elimination of flooding. 
     It will now be clarified how the components of the present embodiment correspond to the components of the invention. The first to fourth electromagnetic valves B 1  to B 4  of the present embodiment can be regarded as switching device of the invention. The control device  50  of the present embodiment can be regarded as a controller of the invention. 
     According to the fuel cell  10  of the present embodiment that has been described hitherto in detail, if flooding has occurred in the first single-cell internal gas passages  71 ,  81 , the flow direction of oxidative gas in the first single-cell internal gas passages  71 ,  81  is suitably switched from a current flow direction to another flow direction that is substantially perpendicular thereto. Hence, even if the small protrusions  720  prevent the produced water W 2  from moving in the current flow direction as shown in  FIG. 8A , the flow direction of oxidative gas is switched to another flow direction extending substantially perpendicular to the current flow direction in  FIG. 8B , whereby it becomes possible to easily remove the produced water W 2  along with the switched flow of gas. Accordingly, the produced water in the first single-cell internal gas passages  71 ,  81  can be efficiently removed. 
     In the aforementioned embodiment, it is also appropriate that oxidative gas be normally caused to flow downwards as shown in  FIG. 7B , namely, in a direction allowing produced water to be easily discharged by gravity, and that the flow direction of oxidative gas be temporarily switched to the horizontal direction as shown in  FIG. 7A  only in response to the occurrence of flooding. Thus, even if a large amount of water has been produced in the cathode  33  as in the case of a high output of the fuel cell, the produced water is normally discharged by gravity and flooding is eliminated by switching the flow direction of oxidative gas to the horizontal direction only in response to the occurrence of flooding. Therefore, the produced water can be efficiently removed. 
     In the aforementioned embodiment, it is determined whether or not flooding has occurred, depending on whether or not the fluctuation of the output voltage has exceeded the predetermined fluctuation range. However, it is also appropriate to determine whether or not flooding has occurred, on the basis of a result of a comparison between the output voltage of the stack  40  and a predetermined voltage set in advance or a result of a comparison between an impedance of the stack  40  and a predetermined impedance set in advance. It is also appropriate that this determination be made on the basis of a suitable combination of the results. 
     If the operating temperature of the fuel cell  10  is equal to or lower than a predetermined temperature, the amount of saturated aqueous vapor contained in reactive gas is small and thus condensate tends to be generated in the single-cell internal gas passages. Hence, even if condensate has been generated, it may be removed by switching the flow direction of gas regularly or irregularly. For example, the control device  50  may perform switching control shown in  FIG. 9  at intervals of a predetermined period (e.g., several milliseconds). If this switching control is started, the control device  50  first reads an operating temperature (step S 200 ) and determines whether or not the operating temperature is equal to or lower than a predetermined temperature (e.g., 60° C.) (step S 210 ). If the operating temperature is equal to or lower than the predetermined temperature, the control device  50  determines whether or not a low-temperature flag has been set as “1” (step S 220 ). If the low-temperature flag has been set as “0”, it is set as “1” and a predetermined period tc is set for a timer so as to start a count-down operation (step S 230 ). Then, the present program is terminated. It is to be noted herein that the low-temperature flag is designed to indicate that the operating temperature is equal to or lower than the predetermined temperature. That is, if the low-temperature flag has been set as “1”, it indicates that the operating temperature is equal to or lower than the predetermined temperature. If the low-temperature flag has been set as “1” in step S 220 , the control device  50  determines whether or not the timer indicates zero (step S 240 ). If the timer does not indicate zero, the present program is terminated. If the timer indicates zero, it is concluded that the operating temperature has remained equal to or lower than the predetermined temperature for the predetermined period tc, which implies a possibility of condensate being generated. Hence, the flow direction of gas is switched (step S 250 ). After the flow direction of gas has been switched or if the operating temperature is not equal to or lower than the predetermined temperature, the low-temperature flag and the timer are reset (step S 260 ). Then, the present program is terminated. As a result, the flow direction of gas is regularly switched every time the predetermined period tc elapses with the operating temperature being equal to or lower than the predetermined temperature. Thus, even if condensate has been generated, it can be removed. It is appropriate that the operating temperature be read for example from a temperature detection sensor installed at a predetermined position of the stack  40  or be obtained by detecting a temperature of coolant for cooling the fuel cell by means of a temperature detection sensor. 
     Furthermore, in the aforementioned embodiment, the flow direction of oxidative gas is switched. However, it is also appropriate to switch the flow direction of fuel gas instead of the flow direction of oxidative gas. For example, in the aforementioned embodiment, the construction of the supply/discharge paths for oxidative gas and the construction of the supply/discharge paths for fuel gas may be interchanged. In this case, produced water in the single-cell internal gas passages for fuel gas can be efficiently removed. 
     Alternatively, it is also appropriate that both the flow direction of oxidative gas and the flow direction of fuel gas be switched. In an exemplary case of the central separator  70 , two long holes  701 ,  731 , two long holes  702 ,  732 , two long holes  703 ,  733 , and two long holes  704 ,  734  are each formed in a corresponding one of sides of the central separator  70  as shown in  FIGS. 10A and 10B , which show one surface and the other surface of the central separator  70  respectively. In this exemplary case, the flow direction of oxidative gas may be switched between the flow direction of gas from the long hole  701  through the concave portion  711  (the first single-cell internal gas passage  71 ) to the long hole  703  and the flow direction of gas from the long hole  702  through the concave portion  711  (the first single-cell internal gas passage  71 ) to the long hole  704  by controlling the electromagnetic valves B 1  to B 4  by means of the control device  50 . In this exemplary case, the flow direction of fuel gas may be switched between the flow direction of gas from the long hole  731  through the concave portion  710  (the second single-cell internal gas passage  72 ) to the long hole  733  and the flow direction of gas from the long hole  732  through the concave portion  710  (the second single-cell internal gas passage  72 ) to the long hole  734  by controlling the electromagnetic valves B 31  to B 34  by means of the control device  50 . In this case, produced water in the single-cell internal gas passages for both the gases can be efficiently removed. It is also appropriate herein that switching control be performed such that the flow directions of oxidative gas and fuel gas constantly establish a counterflow relationship (i.e., that oxidative gas and fuel gas flow in opposite directions). Thus, the distribution of water in the plane of the MEA  30  can be substantially homogenized. 
     [Second Embodiment] 
     The present embodiment is substantially identical to the first embodiment except that the first single-cell internal gas passages  71 ,  81  through which oxidative gas flows are differently constructed. An exemplary case of the first single-cell internal gas passage  71  will be described herein with reference to  FIGS. 11A and 11B  and the description will be omitted in other respects. 
     As shown in  FIG. 11 , each of long holes  741  to  744  is formed through the central separator  70  as a component of the cell module  20  along a corresponding one of four sides thereof. A concave portion  745  having a generally oblong outer periphery is formed in one surface of the central separator  70  which faces the cathode of the single cell. The concave portion  745  communicates with the long holes  741  to  744 . Although not shown, small protrusions substantially identical to those of the first embodiment rise from the surface of the concave portion  745  like a grid. Oxidative gas is supplied to the single cell either by horizontally flowing from the long hole  742  through the concave portion  745 , that is, through the first single-cell internal gas passage  71  and being discharged from the long hole  744  (see  FIG. 11A ) or by vertically flowing from the long hole  741  through the concave portion  745 , that is, through the first single-cell internal gas passage  71  and being discharged from the long hole  743  (see  FIG. 11B ) If a comparison is made herein between the former flow direction of gas and the latter flow direction of gas, the first single-cell internal gas passage  71  is longer and narrower in the case of the former flow direction of gas than in the case of the latter flow direction of gas. This is because the concave portion  745  is formed with a horizontal length L 1  that is longer than a vertical length L 2 . It is to be noted herein that the central separator  70  of the present embodiment has the square holes  705 ,  706  through which fuel gas flows and the square holes  707 ,  708  through which coolant flows as in the case of the first embodiment. 
     Next, switching control for switching the flow direction of oxidative gas in accordance with the present embodiment will be described with reference to a flowchart shown in  FIG. 12 . This switching control is performed by the control device  50  at intervals of a predetermined period. If this switching control is started, the control device  50  first determines whether the current output of the fuel cell is a low output or a high output (step S 300 ). In the case of a low output, the first electromagnetic valve B 1  and the third electromagnetic valve B 3  are closed and the second electromagnetic valve B 2  and the fourth electromagnetic valve B 4  are opened, whereby oxidative gas is caused to flow through the first single-cell internal gas passage  71  in the flow direction shown in  FIG. 11A  (i.e., in the horizontal direction) (step S 310 ). On the other hand, in the case of a high output, the first electromagnetic valve B 1  and the third electromagnetic valve B 3  are opened and the second electromagnetic valve B 2  and the fourth electromagnetic valve B 4  are closed, whereby oxidative gas is caused to flow through the first single-cell internal gas passage  71  in the flow direction shown in  FIG. 11B  (i.e., in the vertical direction) (step S 320 ). After one of the flow directions of gas has been determined in step S 310  or step S 320 , the control device  50  determines substantially in the same manner as in the first embodiment whether or not flooding has occurred (step S 330 ). If flooding has not occurred, the present program is immediately terminated. On the other hand, if flooding has occurred, the control device  50  switches the flow direction of oxidative gas to the other flow direction (step S 340 ). The control device  50  then determines whether or not flooding has been eliminated (step S 350 ). If flooding has not been eliminated, the control device  50  waits without performing any further processing. If flooding has been eliminated, the control device  50  returns the flow direction of oxidative gas to the original flow direction (step S 360 ), whereby the present program is terminated. 
     According to the present embodiment that has been described hitherto, one of the flow directions of gas that corresponds to the longer and narrower passage is selected in the case of a low output. This brings about an increase in flow rate, gas diffusibility, and gas utilization ratio. Further, one of the flow directions of gas that corresponds to the shorter and wider passage is selected in the case of a high output. This brings about a decrease in gas pressure loss. Furthermore, if flooding has occurred, a current flow direction of gas is temporarily switched to another flow direction that is substantially perpendicular thereto. Hence, produced water in the first single-cell internal gas passage  71  is efficiently removed. 
     As shown in  FIG. 13 , it is also appropriate in the case of a low output that the first electromagnetic valve B 1  be slightly open instead of being closed. That is, it is also appropriate to perform supply/discharge control such that oxidative gas is supplied to the first single-cell internal gas passage  71  from both the long hole  742  serving as a gas supply port during the horizontal flow of gas and the long hole  741  serving as a gas supply port during the vertical flow of gas and that oxidative gas is discharged from the long hole  744  serving as a gas discharge port during the horizontal flow of gas. Thus, the flow rate of gas supplied to each of the gas supply ports can be reduced in comparison with a case where oxidative gas is supplied from a single gas supply port. As a result, the upstream side of the first single-cell internal gas passage  71  becomes unlikely to be dry. Further, oxidative gas that has been supplied from the long hole  741  merges on the downstream side of the first single-cell internal gas passage  71 . Thus, the gas concentration can be prevented from being reduced on the downstream side. This supply/discharge control may also be adopted in a construction in which length and width of a passage of the first flow direction of gas are length and width of a passage of the second flow direction of the gas as in the case of the first embodiment. In this case as well, the same effect can be substantially achieved. 
     [Third Embodiment] 
     The third embodiment is realized by adopting another exemplary coolant passage in the second embodiment.  FIGS. 14A ,  14 B are plan views of the cooling separator  60  which is viewed from the side of the coolant passage.  FIG. 14A  shows a first flow direction of coolant.  FIG. 14B  shows a second flow direction of coolant. As shown in  FIGS. 14A ,  14 B, the square holes  607 ,  608  through which coolant flows are not formed in the cooling separator  60 . Instead, each of long holes  651  to  654  for supplying and discharging coolant is formed through the coolant separator  60  along a corresponding one of four sides thereof. Similarly, each of long holes  641  to  644  is formed through the cooling separator  60  along a corresponding one of four sides thereof. These long holes communicate with the long holes  741  to  744  (already mentioned) of the central separator  70  in the direction of lamination. A concave portion  655  having a generally oblong outer periphery is formed in one surface of the cooling separator  60  which does not face the anode of the single cell. The concave portion  655  communicates with the long holes  651  to  654 . Coolant is supplied to the cell module  20  either by horizontally flowing from the long hole  652  through the concave portion  655 , that is, through the coolant passage  63  and being discharged from the long hole  654  (see  FIG. 14A ) or by vertically flowing from the long hole  651  through the concave portion  655 , that is, through the coolant passage  63  and being discharged from the long hole  653  (see  FIG. 14B ). 
     The coolant introduction pipe  45  diverges into two branch pipes that are connected to the long holes  651 ,  652  respectively. Electromagnetic valves B 51 , B 52  are installed in the branch pipes respectively. On the other hand, branch pipes extending from the long holes  653 ,  654  are assembled into the coolant exhaust pipe  46  as a single pipe. Electromagnetic valves B 53 , B 54  are installed in the branch pipes respectively. 
     Switching control, which is performed by the control device  50  in accordance with the present embodiment at intervals of a predetermined period so as to switch the flow direction of oxidative gas, is substantially the same as in the case of the second embodiment. This switching control is represented by the flowchart shown in  FIG. 12 . However, according to the present embodiment, the control is further performed such that the flow directions of oxidative gas and coolant coincide with each other. That is, when the flow direction of oxidative gas is set as the horizontal direction shown in  FIG. 11A  in step S 310 , the flow direction of coolant is also set as the horizontal direction shown in  FIG. 14A . When the flow direction of oxidative gas is set as the vertical direction shown in  FIG. 11B  in step S 320 , the flow direction of coolant is also set as the vertical direction shown in  FIG. 14B . When the flow direction of gas is changed in step S 340  or step S 360  as well, the flow direction of coolant is synchronously switched so as to coincide with the changed direction. It is to be noted in the present embodiment that the electromagnetic valves B 51  to B 54  can be regarded as a third switching device of the invention. 
     According to the present embodiment that has been described hitherto, the upstream side of oxidative gas is cooled by coolant that is at a low temperature, and the downstream side of oxidative gas is cooled by coolant that has absorbed heat and thus reached a slightly high temperature. Hence, the upstream side of oxidative gas undergoes a more substantial drop in temperature than the downstream side of oxidative gas. As a rule, the upstream side of gas tends to be dry and the downstream side of gas tends to be wet. In this case, however, since the dew point is lowered by reducing the temperature on the upstream side of gas, the upstream side of gas is unlikely to be dry. Also, since the dew point is raised by increasing the temperature on the downstream side of gas, the downstream side of gas is unlikely to be wet. 
     In the aforementioned embodiment, coolant horizontally flows in the case of a low output of the fuel cell and vertically flows in the case of a high output. However, it is also appropriate that heat transmissibility be enhanced by adopting vertical flow of coolant corresponding to the shorter length of the coolant passage in the case of a low output and thus reducing the pressure loss of coolant and by adopting horizontal flow of coolant corresponding to the longer length of the coolant passage in the case of a high output and thus increasing the flow rate of coolant. 
     It is indisputably obvious that the invention is not limited to the aforementioned embodiments at all and that the invention can be implemented in various modes as long as they belong to the technical scope of the invention.