Patent Publication Number: US-2012045705-A1

Title: Fuel cell system

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a fuel cell structured to generate electric power through chemical reactions of a fuel gas and an oxidizing gas. 
     2. Description of the Related Art 
     In the fuel cell structured to generate electric power through the chemical reactions of the fuel gas and the oxidizing gas, even after an operation stop, the power generation continues with the remaining fuel gas and the remaining oxidizing gas to generate an open circuit voltage. The open circuit voltage causes catalyst deterioration, such as corrosion of carbon included in a catalyst layer of the fuel cell or elution of a catalyst metal. Diverse techniques have been proposed to lower the open circuit voltage. The catalyst deterioration also occurs when the fuel cell has a negative voltage. In a fuel gas deficient condition of some fuel cell included in a fuel cell stack, an increase in anode potential in the fuel cell causes a negative voltage. In this state, even the fuel cell having deficiency of the fuel gas tries to make a flow of electric current at the same level as that in the other fuel cells. This may lead to oxidation of carbon included in the catalyst. 
     On the occurrence of oxidation of carbon by the negative voltage of the fuel cell, an effective catalyst area is narrowed to lower the power generation efficiency. None of the proposed techniques has sufficiently prevented such catalyst deterioration in the condition of the fuel cell having a negative voltage. 
     SUMMARY 
     By taking into account at least part of the issue discussed above, there is a requirement for preventing or at least restricting catalyst deterioration in a condition of a fuel cell having a negative voltage. 
     In order to address at least part of the requirement described above, the present invention provides various aspects and applications described below. 
     [Aspect 1] According to an aspect of the present invention, a fuel cell system is provided. The fuel cell system comprises: a first power generation module includes a power generation assembly including a catalyst, an anode-side non-power generation structure placed in contact with the power generation assembly, and a cathode-side non-power generation structure placed opposite to the power generation assembly across the power generation assembly; a second power generation module placed adjacent to the first power generation module; a current regulation circuit connected in parallel to each of the power generation modules; and a current controller configured to, when a cell voltage representing a voltage between the anode-side non-power generation structure and the cathode-side non-power generation structure is not higher than a preset first voltage that is a negative voltage, control the current regulator connected in parallel to the first power generation module to increase an amount of electric current flowing in a first direction from the anode-side non-power generation structure to the cathode-side non-power generation structure. 
     In the fuel cell system according to the aspect 1 of the invention, when the cell voltage of the first power generation module is not higher than the preset first voltage, the current regulation circuit is controlled to increase the amount of electric current flowing in the first direction. Electrons can thus be supplied to the adjacent second power generation module via the current regulation circuit. In a hydrogen deficient condition of the first power generation module, such supply of electrons to the second power generation module inhibits chemical reactions accompanied with generation of electrons at an anode of the first power generation module. This arrangement effectively prevents or at least restricts oxidation of carbon at the anode, thus preventing or at least restricting reduction of the power generation efficiency. 
     [Aspect 2] In the fuel cell system described in the aspect 1, the current controller includes an electric current direction detector configured to detect a direction of electric current flowing in the current regulation circuit connected in parallel to the first power generation module, and the current controller determines whether the cell voltage is not higher than the preset first voltage, based on the direction of electric current detected by the electric current direction detector. 
     The fuel cell system of the aspect 2 assures accurate detection of a state in which chemical reactions in a normal power generation condition are inhibited by, for example, deficiency of the fuel gas, in the power generation module but the chemical reactions accompanied with generation of electrons tend to proceed at the anode. 
     [Aspect 3] In the fuel cell system described in either one of aspect 1 and 2, when the cell voltage is higher than the preset first voltage and is higher than a preset second voltage that causes elution of the catalyst, the current controller controls the current regulation circuit connected in parallel to the first power generation module to increase an amount of electric current flowing in a second direction opposite to the first direction. 
     In the fuel cell system of the aspect 3, when the cell voltage of the first power generation module is higher than the preset second voltage, electrons generated through the chemical reactions in the normal power generation condition can be supplied from the cathode to the anode of the first power generation module via the current regulation circuit. An electromotive force induced in the first power generation module is accordingly consumed as heat in the current regulation circuit. In an OC (Open Circuit) state of a fuel cell stack, such heat consumption lowers the cell voltage of the first power generation module and thereby prevents or at least restricts elution of the catalyst caused by a high voltage. 
     [Aspect 4] In the fuel cell system described in any one of aspects 1 through 3, the current regulation circuit and the current controller include a diode. 
     In the fuel cell system of the aspect 4, on a condition that (an absolute value of) the preset first voltage is greater than (an absolute value of) a forward voltage drop of the diode, the current regulation circuit is controlled to increase the amount of electric current flowing in the first direction in the current regulation circuit when the cell voltage of the first power generation module is not higher than the preset first voltage, while being controlled to decrease the amount of electric current flowing in the first direction in the current regulation circuit when the cell voltage of the first power generation module is higher than the preset first voltage. This arrangement does not require any additional functional block to determine whether the cell voltage is not higher than the preset first voltage and simplifies the structures of the current regulation circuit and the current controller, thus saving the manufacturing cost of the fuel cell system. 
     [Aspect 5] In the fuel cell system described in the aspect 4, each of the power generation modules has a pair of separators and a membrane electrode assembly interposed between the pair of separators, and the current regulation circuit and the current controller are placed on a circumference of the membrane electrode assembly and are interposed between the pair of separators. 
     In the fuel cell system of the aspect 5, the current regulation circuit and the current controller are incorporated in each of the power generation modules. This preferably simplifies the structure of the fuel cell stack and enables size reduction of the overall fuel cell system. The current regulation circuit and the current controller are produced simultaneously in the manufacturing process of the membrane electrode assemblies. This simplifies the manufacturing procedure of the fuel cell stack. 
     [Aspect 6] In the fuel cell system described in either one of the aspect 4 and 5, the diode is a silicon diode or a Schottky barrier diode. The fuel cell system of the aspect 6 uses the diode having a relatively low forward voltage drop. The amount of electric current flowing in the first direction can thus be increased in a relatively short time period since a decrease of the inter-power generation module voltage to a negative voltage. Compared with structures using other diodes, the structure of this aspect more effectively prevents or at least restricts catalyst deterioration. Especially the silicon diode has a high resistance to a voltage in a reverse direction, so that there is substantially no electric current flowing in the second direction in the normal power generation condition of the first power generation module. Namely this arrangement prevents the electric current from flowing in the current regulation circuit in the normal power generation condition to consume the electromotive force induced in the first power generation module as heat in the current regulation circuit, thus preventing or at least restricting reduction of the power generation efficiency. 
     The technique of the invention is not restricted to the fuel cell system having any of the configurations and arrangements discussed above but may be actualized by diversity of other aspects, for example, a method of controlling electric current, a computer program for controlling electric current, and a recording medium in which such a computer program is recorded. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an explanatory diagrammatic representation of the general configuration of a fuel cell system according to one embodiment of the invention; 
         FIG. 2  is an explanatory diagrammatic representation of the detailed structure of the unit cells and the current regulator shown in  FIG. 1 ; 
         FIG. 3  is an explanatory diagrammatic representation of a variation of voltages of unit cells; 
         FIG. 4  is a flowchart showing a processing flow of current regulation process performed in the first embodiment; 
         FIG. 5  is an explanatory diagrammatic representation of electric current flowing in the variable resistance in the normal condition; 
         FIG. 6  is an explanatory diagrammatic representation of electric current flowing in the variable resistance in the hydrogen deficient condition; 
         FIG. 7  is an explanatory diagrammatic representation of the detailed structure of unit cells and a current regulator according to a second embodiment; 
         FIG. 8  is an explanatory diagrammatic representation of a processing flow of current regulation process performed in the second embodiment; 
         FIG. 9  is an explanatory diagrammatic representation of the general configuration of a fuel cell system according to a third embodiment; 
         FIG. 10  is an explanatory diagrammatic representation of the detailed structure of unit cells and the current regulator in the third embodiment; 
         FIG. 11  is an explanatory diagrammatic representation of electric current flowing in the diode in the normal condition of the third embodiment; 
         FIG. 12  is an explanatory diagrammatic representation of electric current flowing in the diode in the hydrogen deficient condition of the third embodiment; 
         FIG. 13  is an explanatory diagrammatic representation of the detailed structure of unit cells and a current regulator according to a fourth embodiment; 
         FIG. 14  is an explanatory diagrammatic representation of a processing flow of current regulation process performed in the fourth embodiment; 
         FIG. 15  is an explanatory diagrammatic representation of the detailed structure of unit cells and a current regulator according to a fifth embodiment; and 
         FIG. 16  is an explanatory diagrammatic representation of the detailed structure of unit cells and a current regulator according to a sixth embodiment. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A. First Embodiment 
     A. System Configuration 
       FIG. 1  is an explanatory diagrammatic representation of the general configuration of a fuel cell system according to one embodiment of the invention. In this embodiment, a fuel cell system  100  is mounted, as a driving power supply system, on an electric vehicle. The fuel cell system  100  includes a fuel cell stack  10 , a hydrogen tank  61 , an air compressor  62 , a shutoff valve  71 , a pressure regulator  72 , a fuel gas supply path  81 , a fuel gas discharge path  82 , a bypass flow path  83 , a circulation pump  63 , an oxidizing gas supply path  84 , an oxidizing gas discharge path  85 , a current regulator  30 , a resistance adjuster  50 , and a control unit  90 . 
     The fuel cell stack  10  is constructed as a stack of multiple unit cells  20  (power generation modules), which are relatively small-sized polymer electrolyte fuel cells with excellent power generation efficiency. The fuel cell stack  10  induces an electromotive force through electrochemical reactions of pure hydrogen as a fuel gas with oxygen in the air as an oxidizing gas proceeding at respective electrodes. 
     The hydrogen tank  61  stores high-pressure hydrogen gas. The hydrogen tank  61  may be, for example, a tank filled with a hydrogen-absorbing alloy to absorb and thereby store hydrogen. The air compressor  62  is located in the oxidizing gas supply path  84  to compress the externally intake air and supply the compressed air to the fuel cell stack  10 . 
     The shutoff valve  71  is located at a hydrogen gas outlet (not shown) of the hydrogen tank  61  to start and stop a supply of hydrogen gas. The pressure regulator  72  is located in the fuel gas supply path  81  to lower the pressure of the high-pressure hydrogen gas supplied from the hydrogen tank  61  to a preset pressure level. 
     The fuel gas supply path  81  is arranged as a flow passage to connect the hydrogen tank  61  to the fuel cell stack  10  and introduce the hydrogen gas supplied from the hydrogen tank  61  to the fuel cell stack  10 . The fuel gas discharge path  82  is arranged as a flow passage to discharge excess hydrogen gas (anode off-gas) from the fuel cell stack  10 . The bypass flow path  83  is arranged to connect the fuel gas supply path  81  to the fuel gas discharge path  82  and introduce the excess hydrogen gas discharged from the fuel gas discharge path  82  to the fuel gas supply path  81  in a normal operating condition. The circulation pump  63  is provided on the bypass flow path  83  to recirculate the excess hydrogen gas (anode off-gas) from the fuel gas discharge path  82  to the fuel gas supply path  81 . 
     The oxidizing gas supply path  84  is arranged as a flow passage to connect the air compressor  62  to the fuel cell stack  10  and introduce the compressed air supplied from the air compressor  62  to the fuel cell stack  10 . The oxidizing gas discharge path  85  is arranged as a flow passage to discharge the excess air (cathode off-gas) and generated water from the fuel cell stack  10  to the outside (atmosphere). 
     The current regulator  30  is connected to the respective unit cells  20 . The detailed structure of the current regulator  30  is explained later. The resistance adjuster  50  is connected to the current regulator  30  to adjust a value of a resistance (not shown) included in the current regulator  30 . 
     The control unit  90  is electrically connected with the circulation pump  63 , the air compressor  62 , the shutoff valve  71 , the pressure regulator  72 , and the resistance adjuster  50  to control these components. The control unit  90  is also connected with a current sensor (not shown) included in the current regulator  30  to receive signals representing current values notified by the current sensor. 
     The control unit  90  includes a CPU (Central Processing Unit)  91 , a ROM (Read Only Memory)  92 , and a RAM (Random Access Memory)  93 . The ROM  92  stores therein a control program (not shown) executed to control the operations of the fuel cell system  100 . The CPU  91  loads and executes the control program on the RAM  93  to serve as a controller  91   a . The controller  91   a  performs a current regulation process described later. 
       FIG. 2  is an explanatory diagrammatic representation of the detailed structure of the unit cells and the current regulator shown in  FIG. 1 . The unit cell  20  includes a membrane electrode assembly  25 , a cathode-side separator  26   c , and an anode-side separator  26   a . The membrane electrode assembly  25  includes an electrolyte membrane  21 , a cathode catalyst layer  22   c , a cathode gas diffusion layer  23   c , an anode catalyst layer  22   a , and an anode gas diffusion layer  23   a.    
     The electrolyte membrane  21  is implemented by a sulfonate group-containing fluororesin ion exchange membrane and may be made of, for example, Flemion (registered trademark) or Aciplex (registered trademark). Alternatively the electrolyte membrane  21  may be implemented by another ion exchange group-containing membrane, such as a phosphate group or a carboxylic group instead of sulfonate group. 
     The cathode catalyst layer  22   c  is placed in contact with the electrolyte membrane  21 . The cathode catalyst layer  22   c  is composed of a a material of catalyst-supported conductive particles and an ionomer as a proton conductor. Typical examples of the catalyst include platinum and platinum alloys, such as platinum-ruthenium and platinum-iron. Typical examples of the conductive particles include carbon particles, such as carbon black, and carbon fibers. A typical example of the ionomer is a sulfonate group-containing fluororesin. 
     The cathode gas diffusion layer  23   c  is composed of a porous material to diffuse the air used as a reaction gas and discharge the water produced through the electrochemical reactions or by any other reasons. More specifically, the cathode gas diffusion layer  23   c  may be made of a carbon porous material, such as carbon paper or carbon cloth, or a metal porous material, such as metal mesh or foamed metal. 
     The cathode-side separator  26   c  is made of a gas-impermeable conductive material, for example, gas-impermeable dense carbon formed by compression of carbon or a press-formed metal plate. The cathode-side separator  26   c  is formed to have concavities and convexes, such that an oxidizing gas flow path  27   c  is formed between the cathode-side separator  26   c  and the cathode gas diffusion layer  23   c  that is placed in contact with the cathode-side separator  26   c . The oxidizing gas flow path  27   c  serves to introduce the air supplied from the air compressor  62  to the cathode gas diffusion layer  23   c  and discharge off-gases (the excess air and water vapor) from the cathode diffusion layer  23   c  to the outside of the unit cell  20 . 
     The anode-side structure is similar to the cathode-side structure described above. The anode catalyst layer  22   a  has the same structure as that of the cathode catalyst layer  22   c . The anode gas diffusion layer  23   a  and the anode-side separator  26   a  respectively have the same structures as those of the cathode gas diffusion layer  23   c  and the cathode-side separator  26   c . A fuel gas flow path  27   a  formed between the anode-side separator  26   a  and the anode gas diffusion layer  23   a  serves to introduce the hydrogen gas supplied from the hydrogen tank  62  or by the circulation pump  63  to the anode gas diffusion layer  23   a  and discharge an off-gas (excess hydrogen gas) from the anode gas diffusion layer  23   a  to the outside of the unit cell  20 . 
     The current regulator  30  includes a plurality of variable resistances  31  connected in parallel to the respective unit cells  20  and a plurality of current sensors  32  connected in series with the respective variable resistances  31 . The resistance value of each of the variable resistances  31  is adjusted by the resistance adjuster  50 . Each of the current sensors  32  detects a value of electric current flowing in the corresponding variable resistance  31  and notifies the control unit  90  of the detected current value. The control unit  90  (controller  91   a ) identifies a direction of electric current flowing in the variable resistance  31 , based on the sign (positive or negative) of the received current value. 
     A specific structure discussed below may be adopted for the current regulator  30  and the resistance adjuster  50 . Each of the variable resistances  31  may be designed to include a plurality of resistances having a plurality of resistance values and being connectable in parallel to the unit cell  20 . The resistance adjuster  50  is implemented as switches connected to the respective resistances in series. The resistance value of the variable resistance  31  is adjustable by varying the number of the resistances connected (in parallel) to the unit cell  20 . 
     The unit cell  20  described above corresponds to the power generation module in the claims of the invention. The membrane electrode assembly  25 , the anode-side separator  26   a , the cathode-side separator  26   c , and the variable resistance  31  respectively correspond to the power generation assembly, the anode-side non-power generation structure, the cathode-side non-power generation structure, and the current regulation circuit in the claims of the invention. The combination of the resistance adjuster  50 , the current sensor  32 , and the controller  91   a  is equivalent to the current controller in the claims of the invention. The current sensor  32  corresponds to the electric current direction detector in the claims of the invention. In another aspect of the invention, the electrolyte membrane  21 , the anode catalyst layer  22   a , and the cathode catalyst layer  22   c  of the membrane electrode assembly  25  correspond to the power generation assembly in the claims of the invention. The anode-side separator  26   a , the cathode-side separator  26   c , the anode gas diffusion layer  23   a , and the cathode gas diffusion layer  23   c  correspond to the non-power generation structure in the claims of the invention. 
       FIG. 3  is an explanatory diagrammatic representation of a variation of voltages of unit cells. The upper drawing of  FIG. 3  shows voltages of the unit cells  20  in a normal condition, and the lower drawing of  FIG. 3  shows voltages of the unit cells  20  in a hydrogen deficient condition. For the convenience of explanation, the fuel cell stack  10  is illustrated in  FIG. 3  as a stack of three unit cells  20  ( 20   a ,  20   b , and  20   c ). Each of the unit cells  20   a ,  20   b , and  20   c  is defined by a combination of an electrolyte membrane, a cathode, an anode, and separators. More specifically, the unit cell  20   a  is defined by part of a separator s 0 , an anode a 1 , an electrolyte membrane m 1 , a cathode c 0 , and part of a separator s 1 . The separator s 0  represents the anode-side separator  26   a  and the cathode-side separator  26   c  shown in  FIG. 2 . The anode a 1  represents the anode gas diffusion layer  23   a  and the anode catalyst layer  22   a  shown in  FIG. 2 . The electrolyte membrane m 1  represents the electrolyte membrane  21  shown in  FIG. 2 . The cathode c 1  represents the cathode catalyst layer  22   c  and the cathode gas diffusion layer  23   c  shown in  FIG. 2 . The separator s 1  represents the anode-side separator  26   a  and the cathode-side separator  26   c  shown in  FIG. 2 . 
     Similarly, the unit cell  20   b  is defined by part of the separator s 1 , an anode a 2 , an electrolyte membrane m 2 , a cathode c 2 , and part of a separator s 2 . The unit cell  20   c  is defined by part of the separator s 2 , an anode a 3 , an electrolyte membrane m 3 , a cathode c 3 , and part of a separator s 3 . As shown in the upper drawing of  FIG. 3 , in the normal condition that the respective unit cells  20   a ,  20   b , and  20   c  receive sufficient supplies of hydrogen gas and the air and generate electric power, a reaction expressed by Equation 1 given below proceeds on the anode, while a reaction expressed by Equation 2 given below proceeds on the cathode. In this condition, each of the unit cells  20   a ,  20   b , and  20   c  has a voltage (inter-separator voltage in each unit cell: cell voltage Vc) approximately equal to +1.0 V. The reactions of Equations 1 and 2 proceed when the voltages of the respective unit cells  20   a ,  20   b , and  20   c  are higher than 0 V. 
       2H 2 →4H + +4 e   −   (1)
 
       O 2 +4H + +4 e   − →2H 2 O  (2)
 
     The cell voltage Vc is defined by Equation 3. In Equation 3, Vc, Ec, and Ea respectively denote a cell voltage, a cathode potential, and an anode potential and IR represents a voltage drop due to a resistance of the unit cell (e.g., a resistance of the electrolyte membrane and a contact resistance of wiring). The cell voltage Vc means an inter-unit cell voltage. For example, the cell voltage Vc of the unit cell  20   b  means a voltage between the unit cell  20   a  (cathode c 1 ) and the unit cell  20   c  (anode a 3 ). 
         Vc=Ec−Ea −IR  (3)
 
     In the condition that a supply of hydrogen gas to a unit cell is less than an amount required for power generation (in the hydrogen deficient condition), an increase in anode potential Ea may make the cell voltage Vc negative in Equation 3 given above. The hydrogen deficiency may occur when water produced through the electrochemical reactions (produced water) is accumulated in the fuel gas flow path  27   a  to increase a pressure loss in the fuel gas flow passage or when the produced water accumulated in, for example, the anode gas diffusion layer  23   a  is frozen in a sub-zero environment to deteriorate the gas diffusion performance. 
     In the state shown in the lower drawing of  FIG. 3 , the unit cell  20   b  has hydrogen deficiency and a negative value of the cell voltage Vc. While the other unit cells  20   a  and  20   c  normally generate electric power, the unit cell  20   b  tries to generate electric current (i.e., tries to transmit electrons). Due to the hydrogen deficiency, however, the reactions of Equations 1 and 2 given above are inhibited in the unit cell  20   b , but reactions expressed by Equations 4 and 5 proceed at the anode a 2 . The reaction of Equation 4 proceeds when the cell voltage Vc is not higher than approximately −0.8 V. The reaction of Equation 5 proceeds when the cell voltage Vc is not higher than approximately −1.5 V. The reaction of Equation 6 given below proceeds at the cathode c 2 . 
       2H 2 O→O 2 +4H + +4 e   −   (4)
 
       C+2H 2 O→CO 2 +4H + +4 e   −   (5)
 
     The reaction of Equation 5 implies oxidation of carbon included in the catalyst of the anode a 2 . Namely there is a possibility of catalyst deterioration at the anode a 2  due to the reaction of Equation 5 in the hydrogen deficient condition. The fuel cell system  100  of this embodiment performs a current regulation process described below to prevent or at least restrict catalyst deterioration accompanied with the hydrogen deficiency. 
     A2. Current Regulation Process 
       FIG. 4  is a flowchart showing a processing flow of current regulation process performed in the first embodiment. In the fuel cell system  100 , the current regulation process is triggered by a start of the fuel cell system  100 . The controller  91   a  shown in  FIG. 1  identifies whether the direction of electric current in the variable resistance  31  is a first direction, based on the current value of each unit cell  20  notified by the corresponding current sensor  32  (step S 105 ). 
     When it is identified that the direction of electric current is not the first direction (but a second direction opposite to the first direction), the controller  91   a  controls the resistance adjuster  50  to set a predetermined large value to the resistance value of the variable resistance  31  corresponding to the unit cell  20  (step S 110 ) and returns the processing flow to step S 105 . With it is identified that the direction of electric current is the first direction, on the other hand, the controller  91   a  controls the resistance adjuster  50  to set a predetermined small value to the resistance value of the variable resistance  31  corresponding to the unit cell  20  (step S 115 ) and returns the processing flow to step S 105 . 
       FIG. 5  is an explanatory diagrammatic representation of electric current flowing in the variable resistance in the normal condition.  FIG. 6  is an explanatory diagrammatic representation of electric current flowing in the variable resistance in the hydrogen deficient condition. The respective unit cells  20   a ,  20   b , and  20   c  shown in  FIGS. 5 and 6  are identical with the unit cells  20   a ,  20   b , and  20   c  explained above with reference to  FIG. 3 . For the convenience of explanation, only the variable resistance  31  corresponding to the unit cell  20   b  is illustrated in  FIGS. 5 and 6 . 
     In the normal condition (having the cell voltage Vc of higher than 0 V), the reaction of Equation 1 given above proceeds at the anode a 2  of the respective unit cells  20   a ,  20   b , and  20   c , while the reaction of Equation 2 given above proceeds at the cathode c 2 . In this state, electrons flow in the variable resistance  31  in a direction from the separator s 1  (the anode-side separator  26   a  of the unit cell  20   b ) to the separator s 2  (the cathode-side separator  26   c  of the unit cell  20   b ) as shown in  FIG. 5 . Namely the electric current flows in the variable resistance  31  in a direction from the separator s 2  (the cathode-side separator  26   c  of the unit cell  20   b ) to the separator s 1  (the anode-side separator  26   a  of the unit cell  20   b ). In this embodiment, this direction (from the separator s 2  to the separator s 1 ) is specified as the second direction, and the opposite direction (from the separator s 1  to the separator s 2 ) is specified as the first direction. In the normal condition, the direction of electric current in the variable resistance  31  is the second direction, so that the processing flow goes to step S 110  to set a large value to the resistance value of the variable resistance  31 . Such setting restricts the amount of electric current flowing in the variable resistance  31  in the second direction. 
     Setting a large value to the resistance value of the variable resistance  31  in the normal condition is ascribed to the following reason. An increase of the amount of electric current flowing in the variable resistance  31  increases the amount of electric power consumed as heat in the variable resistance  31 , out of the electromotive force induced in the unit cell  20 . This lowers the power generation efficiency of the unit cell  20 . The fuel cell system  100  sets a large value to the resistance value of the variable resistance  31  in the normal condition to decrease the amount of electric current flowing in the second direction and thereby reduce the amount of electric power consumed as heat in the variable resistance  31 , thus enhancing the power generation efficiency of the unit cell  20 . 
     In the hydrogen deficient condition of the unit cell  20   b , the cell voltage Vc of the unit cell  20   b  decreases to a negative voltage, so that the reaction of Equation 1 given above is inhibited in the unit cell  20   b . There is accordingly no supply of electrons from the anode a 2  of the unit cell  20   b  to the cathode c 1  of the unit cell  20   a . In this state, electrons generated through the reaction of Equation 1 at the anode a 3  of the unit cell  20   c  are supplied to the cathode c 1  of the unit cell  20   a  via the variable resistance  31  as shown in  FIG. 6 . In the hydrogen deficient condition, the direction of electric current in the variable resistance  31  is accordingly the first direction, so that the processing flow goes to step S 115  to set a small value to the resistance value of the variable resistance  31 , so as to increase the amount of electric current flowing in the variable resistance  31 . This leads to supply of electrons to the cathode c 1  of the unit cell  20   a . Such electron supply inhibits the reactions of Equations 4 and 5 in the unit cell  20   b  in the hydrogen deficient condition. This prevents or at least restricts catalyst deterioration at the anode a 2 . 
     As described above, when the cell voltage Vc is higher than 0 V, the reactions of Equations 1 and 2 given above proceed in each unit cell  20  to make the electric current flow in the second direction. The voltage ‘0 V’ of this embodiment corresponds to the preset first voltage in the claims of the invention. The unit cell  20   b , the unit cell  20   a , and the unit cell  20   c  respectively correspond to the first power generation module, the second power generation module, and the third power generation module in the claims of the invention. 
     As described above, when the direction of electric current in the variable resistance  31  is the first direction (i.e., when the cell voltage Vc of the unit cell  20  is a negative voltage), the fuel cell system  100  of the first embodiment lowers the resistance value of the variable resistance  31  connected in parallel to the unit cell  20 . The lowered resistance value increases the amount of electric current flowing in the variable resistance  31  and prevents or at least restricts oxidation of carbon in the unit cell  20 . This prevents or at least restricts catalyst deterioration in the unit cell  20  having a negative voltage. When the cell voltage Vc is not a negative voltage, the fuel cell system  100  of the first embodiment increases the resistance value of the variable resistance  31  to decrease the amount of electric current flowing in the variable resistance  31 , thus enhancing the power generation efficiency of the unit cell  20 . 
     The setting of the resistance value of the variable resistance  31  is changed over, based on the identified direction of electric current flowing in the variable resistance  31  (step S 105 ). This arrangement assures accurate detection of the state in which the chemical reactions in the normal condition (i.e., the reactions of Equations 1 and 2 given above) are inhibited in the unit cell  20   b  but the chemical reactions accompanied with generation of electrons tend to proceed at the anode a 2 . 
     B. Second Embodiment 
       FIG. 7  is an explanatory diagrammatic representation of the detailed structure of unit cells and a current regulator according to a second embodiment. Unlike the fuel cell system  100  of the first embodiment, a fuel cell system of the second embodiment has a current regulator  30   a  including voltage sensors  33 , in place of the current sensors  32 , and uses the cell voltage Vc as the criterion for changing over the setting of the resistance value of the variable resistance  31 . Otherwise the configuration of the fuel cell system of the second embodiment is similar to the configuration of the fuel cell system  100  of the first embodiment. The unit cells  20  of the second embodiment have the same structure as that of the first embodiment. 
     As shown in  FIG. 7 , the current regulator  30   a  of the second embodiment has the voltage sensors  33  connected in parallel to the respective unit cells  20 . The voltage sensor  33  measures the cell voltage Vc and notifies the control unit  90  (controller  91   a ) of the measured cell voltage Vc. 
       FIG. 8  is an explanatory diagrammatic representation of a processing flow of current regulation process performed in the second embodiment. The controller  91   a  determines whether the cell voltage Vc notified by the voltage sensor  33  is a negative voltage (step S 205 ). Upon determination that the cell voltage Vc is a negative voltage (step S 205 : Yes), the controller  91   a  performs the processing of step S 115  explained above (i.e., setting a small value to the resistance value of the variable resistance  31 ) and returns to step S 205 . 
     Upon determination that the cell voltage Vc is not a negative voltage at step S 205 , the controller  91   a  subsequently determines whether the cell voltage Vc is equal to or higher than a catalyst-eluting voltage Vd (step S 210 ). The catalyst-eluting voltage Vd is, for example, 0.85V for platinum (Pt) as the catalyst. In a state where platinum is used as the catalyst and the cell voltage Vc reaches or exceeds 0.85V (e.g., in an OC (Open Circuit) state after an operation stop of the fuel cell system  100 ), platinum ionization occurs to cause catalyst deterioration. The catalyst-eluting voltage Vd corresponds to the preset second voltage in the claims of the invention. 
     Upon determination that the cell voltage Vc is lower than the catalyst-eluting voltage Vd (step S 210 : No), the controller  91   a  performs the processing of step S 110  explained above (i.e., setting a large value to the resistance value of the variable resistance  31 ) and returns to step S 205 . Upon determination that the cell voltage Vc is equal to or higher than the catalyst-eluting voltage Vd (step S 210 : Yes), on the other hand, the controller  91   a  performs the processing of step S 115  explained above (i.e., setting a small value to the resistance value of the variable resistance  31 ) and returns to step S 205 . 
     The fuel cell system of the second embodiment with the arrangement described above has the similar effects to those of the fuel cell system  100  of the first embodiment. When the cell voltage Vc reaches or exceeds the catalyst-eluting voltage Vd, the fuel cell system of the second embodiment sets a small value to the resistance value of the variable resistance  31  so as to increase the amount of electric current flowing in the variable resistance  31 . The increased amount of electric current enables a greater amount of the electromotive force induced in the unit cell  20  to be consumed as heat in the variable resistance  31 . This lowers the voltage of the unit cell  20  and thereby prevents or at least restricts elution of the catalyst at the anode. 
     C. Third Embodiment 
       FIG. 9  is an explanatory diagrammatic representation of the general configuration of a fuel cell system according to a third embodiment. A fuel cell system  100   a  of the third embodiment has a similar configuration to that of the fuel cell system  100  of the first embodiment, except omission of the resistance adjuster  50  and the controller  91   a  and replacement of the current regulator  30  with a current regulator  30   b.    
       FIG. 10  is an explanatory diagrammatic representation of the detailed structure of unit cells and the current regulator in the third embodiment. Unlike the current regulator  30  of the first embodiment, the current regulator  30   b  of the third embodiment has diodes  40  connected to the respective unit cells  20 , in place of the variable resistances  31  and the current sensors  32 . The unit cells  20  of the third embodiment have the same structure as that of the first embodiment. 
     As shown in  FIG. 10 , the current regulator  30   b  of the third embodiment has the diodes  40  connected in parallel to the respective unit cells  20 . The diode  40  has a P-side connected to the anode of the unit cell  20  and an N-side connected to the cathode of the unit cell  20 . Silicon diodes are used for the diodes  40  of the third embodiment. The silicon diode has a forward voltage drop Vf of about 0.6 V and a large resistance to a reverse voltage. 
       FIG. 11  is an explanatory diagrammatic representation of electric current flowing in the diode in the normal condition of the third embodiment. In the normal condition of the first embodiment, electrons flow in the direction from the anode a 2  of the unit cell  20   b  to the cathode c 2  of the unit cell  20   b , so that there is an electric current in the second direction. In the normal condition of the third embodiment, however, there is substantially no electric current flowing in the diode  40 , since the electric current in the second direction corresponds to the electric current in a reverse direction in the diode  40 . The state of  FIG. 11  occurs in the normal condition (where the cell voltage Vc is not lower than 0 V) and in a condition that the cell voltage Vc of the unit cell  20   b  is a negative voltage of higher than −Vf (for example, −0.6 V). 
       FIG. 12  is an explanatory diagrammatic representation of electric current flowing in the diode in the hydrogen deficient condition of the third embodiment. The state of  FIG. 12  occurs, when the unit cell  20   b  has hydrogen deficiency and the cell voltage Vc of not higher than −Vf. 
     In the state where the unit cell  20   b  has hydrogen deficiency and the cell voltage Vc of the unit cell  20   b  is a negative voltage of not higher than −Vf, there is an electric current in a forward direction in the diode  40 , i.e., the electric current in the first direction. As in the state of  FIG. 6  explained in the first embodiment, electrons generated through the reaction of Equation 1 given above at the anode a 3  of the unit cell  20   c  are supplied to the cathode c 1  of the unit cell  20   a  via the variable resistance  31 . This prevents or at least restricts catalyst deterioration at the anode a 2 . 
     As described above, in the configuration of this embodiment, there is substantially no electric current either in the first direction or in the second direction, when the cell voltage Vc of the unit cell  20  is higher than −Vc. There is an electric current in the first direction, when the cell voltage Vc of the unit cell  20  is not higher than −Vc. The voltage ‘−Vf’ of this embodiment corresponds to the preset first voltage in the claims of the invention. 
     The fuel cell system  100   a  of the third embodiment with the arrangement described above has the similar effects to those of the fuel cell system  100  of the first embodiment. The omission of the resistance adjuster  50  and the controller  91   a  and the simplified structure of the current regulator  30   a  preferably save the manufacturing cost of the fuel cell system  100   a.    
     D. Fourth Embodiment 
       FIG. 13  is an explanatory diagrammatic representation of the detailed structure of unit cells and a current regulator according to a fourth embodiment. Unlike the fuel cell system  100  of the first embodiment, a fuel cell system of the fourth embodiment has a current regulator  30   c  including first diodes  40   a  in place of the current sensors  32  and additionally including second diodes  40   b  and voltage sensors  34  and uses the cell voltage Vc as the criterion for changing over the setting of the resistance value of the variable resistance  31 . Otherwise the configuration of the fuel cell system of the fourth embodiment is similar to the configuration of the fuel cell system  100  of the first embodiment. The unit cells  20  of the fourth embodiment have the same structure as that of the first embodiment. 
     As shown in  FIG. 13 , the current regulator  30   c  of the fourth embodiment has the first diodes  40   a  connected in series with the variable resistances  31  corresponding to the respective unit cells  20 . The first diode  40   a  has a P-side connected to the variable resistance  31  and an N-side connected to the anode of the unit cell  20 . The current regulator  30   c  also has the second diodes  40   b  connected in parallel to the respective unit cells  20 . The second diode  40   b  has a P-side connected to the anode of the unit cell  20  and an N-side connected to the cathode of the unit cell  20 . Like the third embodiment, silicon diodes are used for both the first diodes  40   a  and the second diodes  40   b . The current regulator  30   c  further has the voltage sensors  34  connected in parallel to the respective unit cells  20 . 
       FIG. 14  is an explanatory diagrammatic representation of a processing flow of current regulation process performed in the fourth embodiment. The controller  91   a  determines whether the cell voltage Vc notified by the voltage sensor  34  is equal to or higher than the catalyst-eluting voltage Vd (step S 210 ). Upon determination that the cell voltage Vc is lower than the catalyst-eluting voltage Vd (step S 210 : No), the controller  91   a  performs the processing of step S 110  explained above (i.e., setting a large value to the resistance value of the variable resistance  31 ) and returns to step S 210 . Upon determination that the cell voltage Vc is equal to or higher than the catalyst-eluting voltage Vd (step S 210 : Yes), on the other hand, the controller  91   a  performs the processing of step S 115  explained above (i.e., setting a small value to the resistance value of the variable resistance  31 ) and returns to step S 210 . 
     As in the third embodiment, in the hydrogen deficient condition of the fourth embodiment, the electric current flows in the forward direction in the second diode  40   b . This prevents or at least restricts catalyst deterioration at the anode of each unit cell  20 . When the cell voltage Vc is lower than the catalyst-eluting voltage Vd in the normal condition, a large value is set to the resistance value of the variable resistance  31 . In this state, there is substantially no electric current flowing in the first diode  40   a , as well as in the second diode  40   b . This prevents or at least restricts the reduction of the power generation efficiency due to the flow of electric current in the first diode  40   a  and in the second diode  40   b . When the cell voltage Vc is not lower than the catalyst-eluting voltage Vd in the normal condition, a small value is set to the resistance value of the variable resistance  31 . In this state, the electric current flows in the first diode  40   a , while there is substantially no electric current flowing in the second diode  40   b . This enables a greater amount of the electromotive force induced in the unit cell  20  to be consumed as heat in the variable resistance  31  and lowers the voltage of the unit cell  20 , thus preventing or at least restricting elution of the catalyst at the anode. 
     The fuel cell system of the fourth embodiment with the arrangement described above has the similar effects to those of the fuel cell system  100  of the first embodiment. When the cell voltage Vc reaches or exceeds the catalyst-eluting voltage Vd, the fuel cell system of the fourth embodiment sets a small value to the resistance value of the variable resistance  31  so as to increase the amount of electric current flowing in the variable resistance  31 . The increased amount of electric current enables a greater amount of the electromotive force induced in the unit cell  20  to be consumed as heat in the variable resistance  31 . This lowers the voltage of the unit cell  20  and thereby prevents or at least restricts elution of the catalyst at the anode. The fuel cell system of the fourth embodiment excludes the decision step of determining whether the cell voltage Vc is a negative voltage (step S 205 ) and the processing flow to change over the setting of the resistance of the variable resistance  31  (step S 115 ) in the state where the cell voltage Vc is a negative voltage. This preferably simplifies the processing flow of current regulation process in the fuel cell system of the fourth embodiment, compared with the fuel cell system of the second embodiment, and saves the resources, such as the CPU  91  and the RAM  93 . 
     E. Fifth Embodiment 
       FIG. 15  is an explanatory diagrammatic representation of the detailed structure of unit cells and a current regulator according to a fifth embodiment. A fuel cell system of the fifth embodiment has a similar configuration to that of the fuel cell system  100   a  of the third embodiment shown in  FIGS. 9 to 12 , except that current regulator  30   b  is incorporated in the respective unit cells  20 . 
     In the configuration of the third embodiment, the current regulator  30  (diodes  40 ) is placed outside the respective unit cells. In the configuration of the fifth embodiment, however, the current regulator  30   c  (diodes  40 ) is placed inside the respective unit cells  20  as shown in  FIG. 15 . 
     As shown in  FIG. 15 , the current regulator  30   b  consists of only diodes  40  placed on the outer circumferences of the respective membrane electrode assemblies  25 . The diode  40  includes P-type silicon  40   p  connected to the anode-side separator  26   a  and N-type silicon  40   n  connected to the cathode-side separator  26   c . This structure may be achieved by implementing portions of seal member made of, for example, a resin and placed on the outer circumferences of the respective membrane electrode assemblies  25  as the diodes  40 . 
     The fuel cell system of the fifth embodiment with the arrangement described above has the similar effects to those of the fuel cell system  100   a  of the third embodiment. In the fuel cell system of the fifth embodiment, the current regulator  30   b  (diodes  40 ) is incorporated in the respective unit cells  20 . This preferably simplifies the structure of the fuel cell stack  10  and enables size reduction of the overall fuel cell system. The current regulator  30   b  provided on the outer circumferences of the respective membrane electrode assemblies  25  is produced simultaneously in the manufacturing process of the membrane electrode assemblies  25 . This simplifies the manufacturing procedure of the fuel cell stack  10 . 
     F. Sixth Embodiment 
       FIG. 16  is an explanatory diagrammatic representation of the detailed structure of unit cells and a current regulator according to a sixth embodiment. A fuel cell system of the sixth embodiment has a similar configuration to that of the fuel cell system of the fourth embodiment shown in  FIGS. 13 and 14 , except that the first diodes  40   a  and the second diodes  40   b  are incorporated in the respective unit cells  20  as in the fuel cell system of the fifth embodiment. 
     In the configuration of the fourth embodiment, the first diodes  40   a , the second diodes  40   b , and the variable resistances  31  are placed outside the respective unit cells. In the configuration of the sixth embodiment, however, the first diodes  40   a , the second diodes  40   b , and the variable resistances  31  are placed inside the respective unit cells  20  as shown in  FIG. 16 . 
     As shown in  FIG. 16 , the first diode  40   a  includes P-type silicon  41   p  connected to the variable resistance  31  and N-type silicon  41   n  connected to the anode-side separator  26   a . The variable resistance  31  is placed in contact with the cathode-side separator  26   c.    
     The second diode  40   b  includes P-type silicon  42   p  connected to the anode-side separator  26   a  and N-type silicon  42   n  connected to the cathode-side silicon  26   c.    
     As in the fifth embodiment, this structure of the sixth embodiment may be achieved by implementing portions of seal member made of, for example, a resin and placed on the outer circumferences of the respective membrane electrode assemblies  25  as the first diodes  40   a , the second diodes  40   b , and the variable resistances  31 . 
     The fuel cell system of the sixth embodiment with the arrangement described above has the similar effects to those of the fuel cell system of the fourth embodiment. In the fuel cell system of the sixth embodiment, the first diodes  40   a , the second diodes  40   b , and the variable resistances  31  are incorporated in the respective unit cells  20 . This preferably simplifies the structure of the fuel cell stack and enables size reduction of the overall fuel cell system. The current regulator  30   b  provided on the outer circumferences of the respective membrane electrode assemblies  25  is produced simultaneously in the manufacturing process of the membrane electrode assemblies  25 . This simplifies the manufacturing procedure of the fuel cell stack  10 . 
     G. Modifications 
     Among the various constituents and components included in the respective embodiments discussed above, those other than the constituents and components disclosed in independent claims are additional and supplementary elements and may be omitted according to the requirements. 
     The invention is not limited to any of the embodiments and their aspects discussed above but may be actualized in diversity of other embodiments and aspects within the scope of the invention. Some examples of possible modification are given below. 
     G1. Modification 1: 
     The silicon diodes are used as the diodes in the third through the sixth embodiments. The technique of the invention is, however, not restricted to the silicon diodes but may be applicable to diversity of other diodes, such as germanium diodes and Schottky barrier diodes. There is a lower level of the forward voltage drop Vf in the application with germanium diodes or Schottky barrier diodes. The amount of electric current flowing in the first direction can thus be increased in a relatively short time period since a decrease of the cell voltage Vc of the unit cell  20  to a negative voltage. This arrangement more effectively prevents or at least restricts catalyst deterioration. In the application with Schottky barrier diodes, each Schottky barrier diode may be provided as a joint body of a separator and a semiconductor layer. For example, the Schottky barrier diode may be manufactured by forming a semiconductor layer on a separator by the sputtering method. 
     G2. Modification 2: 
     The first embodiment uses only the direction of electric current flowing in the variable resistance  31  as the criterion for changing over the setting of the resistance value of the variable resistance  31 . The second embodiment uses only the cell voltage Vc as the criterion for changing over the setting of the resistance value of the variable resistance  31 . The present invention is, however, not restricted to these configurations. One modification may use the direction of electric current flowing in the variable resistance  31  as a criterion for changing over the resistance value of the variable resistance  31  from a small value to a large value and use the cell voltage Vc as a criterion for changing over the resistance value of the variable resistance  31  from a large value to a small value. A modified procedure of current regulation process first determines whether the cell voltage Vc is not lower than 0 V. When the cell voltage Vc is a negative voltage (lower than 0 V), the modified procedure sets a small value to the resistance value of the variable resistance  31 . After setting the small value to the resistance value of the variable resistance  31 , the modified procedure monitors the direction of electric current flowing in the variable resistance  31  and, when the hydrogen deficiency is eliminated to change the direction of electric current to the second direction, sets a large value to the resistance value of the variable resistance  31 . When the cell voltage Vc is not lower than 0 V, the modified procedure monitors a variation in cell voltage Vc and, when the cell voltage Vc reaches or exceeds the catalyst-eluting voltage Vd, changes over the setting of the resistance value of the variable resistance  31  from the large value to the small value. 
     G3. Modification 3: 
     In the second embodiment, ‘0 V’ is used as the criterion voltage at step S 205  in the current regulation process. The criterion voltage is, however, not restricted to ‘0 V’ but may be any other suitable value, for example, −0.8 V or −1.5 V. In the application with the criterion voltage of −0.8 V or −1.5 V, the reaction of Equation 5 is inhibited, while the reaction of Equation 4 proceeds. This arrangement also prevents or at least restricts oxidation of carbon at the anode, thus preventing or at least restricting the reduction of the power generation efficiency of the unit cell  20 . The voltage ‘−0.8 V’ or ‘−1.5 V’ of this modification corresponds to the preset first voltage in the claims of the invention. 
     G4. Modification 4: 
     In the respective embodiments discussed above, the membrane electrode assembly  25  has the MEGA (Membrane Electrode and Gas Diffusion Layer Assembly) structure including the anode gas diffusion layer  23   a  and the cathode gas diffusion layer  23   c . The membrane electrode assembly  25  may have the standard MEA (Membrane Electrode Assembly) structure excluding the anode gas diffusion layer  23   a  and the cathode gas diffusion layer  23   c . In this modified structure, the anode gas diffusion layer  23   a  and the cathode gas diffusion layer  23   c  may be placed outside the MEA (i.e., on the side of the anode-side separator  26   a  and on the side of the cathode-side separator  26   c ). 
     G5. Modification 5: 
     In the first embodiment, the controller  91   a  identifies the direction of electric current flowing in the variable resistance  31 , based on the current value notified by the current sensor  32 . In one modification, the current sensor  32  itself may identify the direction of electric current based on the measured current value. The controller  91   a  may make the decision of step S 105  based on the direction of electric current notified by the current sensor  32 . 
     G6. Modification 6: 
     In the above description with reference to  FIGS. 3 ,  5 , and  6 , the fuel cell stack  10  is formed as a stack of three unit cells  20 . The number of stacked unit cells is, however, not restricted to three, but the fuel cell stack  10  may be provided as a stack of two or any greater number of unit cells  20 . For example, in the structures of  FIGS. 3 ,  5 , and  6 , the two unit cells  20   a  and  20   b  may be placed in contact with each other with omission of the unit cell  20   c . In this modified structure, electrons generated at the anode a 1  of the unit cell  20   a  are supplied to the separator s 2  (i.e., the cathode-side separator  26   c  of the unit cell  20   b ) via the separator S 0  and the load  200 . When the unit cell  20   b  has hydrogen deficiency, a small value is set to the resistance value of the variable resistance  31  as in the first embodiment. The electrons supplied to the separator s 2  are then supplied to the cathode c 1  of the unit cell  20   a  via the variable resistance  31  and the separator s 1 . 
     G7. Modification 7: 
     In the respective embodiments discussed above, the fuel cell system is mounted on the electric vehicle. The fuel cell system may be mounted on diversity of other moving bodies, such as hybrid vehicles, boats and ships, and robots. In other applications, the fuel cell stack  10  may be used as a stationary power source, and the fuel cell system may be incorporated in constructions, such as buildings and houses. 
     G8. Modification 8: 
     Part of the structure implemented by the software configuration in the above embodiments may be replaced with the hardware configuration. Part of the structure actualized by the hardware configuration in the above embodiments may be replaced with the software configuration.