Abstract:
A fuel cell system, including: a fuel cell having at least one combination of an electrolyte membrane and a cathode-side catalyst layer and an anode-side catalyst layer that have a plurality of pores; a control unit that operates the fuel cell such that an output current determined in accordance with an external load is output from the fuel cell; and an output current acquisition unit that acquires an output current of the fuel cell; wherein, when the control unit determines that an anode in-flowing water amount, which flows to the anode-side catalyst layer when the fuel cell continues power generation at a first output current acquired at a prescribed timing, exceeds a prescribed anode-side allowable water amount, the control unit performs current limitation control to operate the fuel cell at a second output current that is higher than the first output current, regardless of a requirement of the external load.

Description:
INCORPORATION BY REFERENCE 
     The disclosure of Japanese Patent Application No. 2014-105048 filed on May 21, 2014 including the specification, drawings and abstract is incorporated herein by reference in its entirety. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a fuel cell system. 
     2. Description of Related Art 
     An available fuel cell has a structure in which an electrolyte membrane is sandwiched between catalyst layers of both polarities (cathode and anode), and is further sandwiched between gas diffusion layers of both polarities. When this fuel cell is started up below freezing point, there is a risk that the water generated during power generation will freeze, and the catalyst layers will separate from the electrolyte membrane. Therefore, technology has been proposed for controlling the operation of the fuel cell in such a manner that the amount of water flowing into the cathode-side catalyst layer does not exceed an allowable amount when starting up a fuel cell below freezing point (see, for example, Japanese Patent Application Publication No. 2011-113774 (JP 2011-113774 A)). 
     In an available fuel cell system, reaction gas is humidified before being supplied to the fuel cell, in order to maintain a good water-containing state in the electrolyte membrane of the fuel cell. On the other hand, in response to demands for simplification of the fuel cell system, the removal of the humidifier which humidifies the reaction gas has been investigated. Since there is a risk of the electrolyte membrane becoming dry during high-temperature operation, when the reaction gas is not humidified, then it is desirable to make the thickness of the electrolyte membrane smaller than in the related art, in order to maintain a satisfactory water-containing state of the electrolyte membrane. This is because it is considered that, when the electrolyte membrane is made thin, a large amount of water moves from the cathode side toward the anode side (this water is called “reversely diffused water” below), thereby preventing the drying of the electrolyte membrane. However, when there is a large amount of reversely diffused water, there is a risk of the catalyst layer separating from the electrolyte membrane due to freezing of the water on the anode side, when the cell is started up below freezing point. 
     Therefore, technology for adjusting the amount of water flowing into the anode-side catalyst layer is required. This technology is demanded in order to achieve appropriate operation of the fuel cell, and not only for start-up below freezing point. Apart from this, there have been demands for reduced cost, reduced resource usage, simplified manufacturing, and improved performance, and the like, in available fuel cell systems. 
     SUMMARY OF THE INVENTION 
     A first aspect of this invention relates to a fuel cell system, including: a fuel cell having at least one combination of an electrolyte membrane and a cathode-side catalyst layer and an anode-side catalyst layer that are disposed in contact with the electrolyte membrane and have a plurality of pores; an output current acquisition unit which acquires an output current of the fuel cell; and a control unit which operates the fuel cell such that an output current determined in accordance with an external load is output from the fuel cell. The output current acquisition unit acquires a first output current of the fuel cell at a prescribed timing. When the control unit determines that an anode in-flowing water amount, which flows to the anode-side catalyst layer when the fuel cell continues power generation at a first output current acquired at a prescribed timing, exceeds a prescribed anode-side allowable water amount, the control unit performs current limitation control to operate the fuel cell at a second output current that is higher than the first output current, regardless of a requirement of the external load. 
     According to this fuel cell system, when it is determined that the anode in-flowing water amount exceeds the anode-side allowable water amount, then the fuel cell is operated at a higher output current than the output current upon the determination is made, regardless of the requirement of the external load. In this fuel cell system, irrespective of the timing at which an increase or decrease in the output current is required by the external load, the output current is increased when it is determined that the anode in-flowing water amount exceeds the anode-side allowable water amount. In other words, even if the requirement of the external load is for a uniform output current, or a reduced output current, the output current is increased when it is determined that the anode in-flowing water amount exceeds the anode-side allowable water amount. The range of the increase in the output current may be changed in accordance with the requirement of the external load. Since the anode in-flowing water amount becomes greater when the output current is low, then it is possible to suppress the occurrence of an excessive anode in-flowing water amount, by raising the output current. Therefore, it is possible to suppress insufficiency in the fuel gas due to excessive anode in-flowing water, and decline in the fuel cell performance can be suppressed. Furthermore, it is also possible to suppress separation of the anode catalyst layer from the electrolyte membrane due to freezing of the anode in-flowing water. 
     The control unit may implement the current limitation control based on the output current, when the acquired temperature is lower than a prescribed temperature. By adopting this configuration, it is possible to implement current limitation control when starting up the fuel cell at low temperature, and so on. 
     The control unit may determine whether or not the anode in-flowing water amount exceeds the prescribed anode-side allowable water amount, by using the anode in-flowing water amount during a prescribed period set in accordance with the temperature of the fuel cell. By adopting this configuration, a suitable determination can be made on the basis of the temperature of the fuel cell. 
     A time until the anode in-flowing water amount reaches the prescribed anode-side allowable water amount, when the fuel cell continues power generation at a prescribed output current, may be set as an anode-side allowable time; the control unit may include an anode allowable time table indicating a relationship between the prescribed output current and the anode-side allowable time; and the prescribed output current, at which the anode allowable time and the prescribed period are equal each other, may be set as a lower limit of the second output current. By adopting this configuration, it is possible to implement appropriate current limitation control in accordance with the output current. 
     The prescribed temperature may be a freezing point. By implementing current limitation control when the temperature of the fuel cell is lower than freezing point, it is possible to supress separation of the anode catalyst layer from the electrolyte membrane due to freezing of the anode in-flowing water. 
     The prescribed period may be a period until the temperature of the fuel cell becomes equal to or higher than the freezing point. By adopting this configuration, since the anode in-flowing amount is adjusted so as not to exceed the anode-side allowable water amount, during the period until the temperature of the fuel cell becomes equal to or higher than freezing point, then it is possible to more appropriately suppress separation of the anode catalyst layer from the electrolyte membrane due to freezing of the anode in-flowing water. 
     The control unit, during the current limitation control, may operate the fuel cell at a third output current which is lower than the first output current, regardless of the requirement of the external load, when the control unit determines that a cathode in-flowing water amount which flows to the cathode-side catalyst layer when the fuel cell continues power generation at the first output current exceeds a prescribed cathode-side allowable water amount. By adopting this configuration, it is possible to control the operation of the fuel cell system appropriately in such a manner that, on the cathode side also, the cathode in-flowing water amount does not exceed the cathode-side allowable water amount, in accordance with the output current. 
     The control unit may determine whether or not the cathode in-flowing water amount exceeds the prescribed cathode-side allowable water amount, by using the cathode in-flowing water amount during a prescribed period set in accordance with the temperature of the fuel cell. By adopting this configuration, a suitable determination can be made on the basis of the temperature of the fuel cell. 
     The time until the cathode in-flowing water amount reaches the prescribed cathode-side allowable water amount, when the fuel cell continues power generation at a prescribed output current, may be set as a cathode-side allowable time; the control unit may include a cathode allowable time table indicating a relationship between the prescribed output current and the cathode-side allowable time; and the prescribed output current, at which the cathode allowable time and the prescribed period are equal each other, may be set as an upper limit of the third output current. By adopting this configuration, it is possible to implement appropriate current limitation control in accordance with the output current. 
     A second aspect of this invention relates to a control method for a fuel cell system having a fuel cell having at least one combination of an electrolyte membrane and a cathode-side catalyst layer and an anode-side catalyst layer that are disposed in contact with the electrolyte membrane and have a plurality of pores; the control method including: (a) a step of acquiring an output current of the fuel cell; (b) a step of determining whether or not an anode in-flowing water amount, which flows to the anode-side catalyst layer when the fuel cell continues power generation at the acquired output current, exceeds a prescribed anode-side allowable water amount; and (c) a step of operating the fuel cell at an output current that is higher than the acquired output current, regardless of the output current determined in accordance with a requirement of an external load, when determined is made in the step (b) that the anode in-flowing water amount, which flows to the anode-side catalyst layer when the fuel cell continues power generation at the acquired output current, exceeds the prescribed anode-side allowable water amount. 
     According to this control method, when it is determined that the anode in-flowing water amount exceeds the anode-side allowable water amount, then the fuel cell is operated at a higher output current than the output current upon the determination is made, regardless of the requirement of the external load. Since the anode in-flowing water amount becomes greater when the output current is low, then it is possible to suppress the occurrence of an excessive anode in-flowing water amount, by raising the output current. Therefore, it is possible to suppress insufficiency in the fuel gas due to excessive anode in-flowing water, and decline in the fuel cell performance can be suppressed. Furthermore, it is also possible to suppress separation of the anode catalyst layer from the electrolyte membrane due to freezing of the anode in-flowing water. 
     This invention can be implemented in various modes. For example, the invention can be implemented as a moving body in which a fuel cell system is installed, a computer program for realizing the functions of a control method for a fuel cell system, a recording medium on which the computer program is recorded, and so on. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Features, advantages, and technical and industrial significance of exemplary embodiments of the invention will be described below with reference to the accompanying drawings, in which like numerals denote like elements, and wherein: 
         FIG. 1  is an illustrative diagram showing a general configuration of a fuel cell system which is one embodiment of this invention; 
         FIG. 2  is a diagram showing a configuration of a fuel cell; 
         FIG. 3  is an illustrative diagram showing an allowable time table which is stored in a control unit; 
         FIG. 4  is a flowchart showing a procedure of a current limitation process which is carried out in a fuel cell system; 
         FIG. 5  is a flowchart showing a procedure of a current limitation process which is carried out in a fuel cell system; 
         FIG. 6  is an illustrative diagram showing change in the output current of a fuel cell stack when a power limitation process is implemented; and 
         FIG. 7  is a flowchart showing a procedure of a current limitation process which is carried out in a fuel cell system according to a modification. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     A. First Embodiment 
     (A1) Configuration of fuel cell system:  FIG. 1  is an illustrative diagram showing the approximate configuration of a fuel cell system which is one embodiment of this invention. In this embodiment, the fuel cell system  100  is mounted in and used in an electric automobile as a system for supplying drive power. The fuel cell system  100  is provided with: a fuel cell stack  110 , a hydrogen tank  40 , an electromagnetic valve  41 , a circulation pump  42 , a combustion gas supply path  93 , a combustion gas discharge path  94 , an air compressor  50 , an oxidant gas supply path  91 , an oxidant gas discharge path  92 , a current measurement unit  71 , a temperature measurement unit  74  and a control unit  20 . 
     The fuel cell stack  110  is provided with a plurality of fuel cells  10 , which are stacked together, two terminal plates  111 , and two end plates  112 . The two terminal plates  111  are both electrodes (an anode and a cathode) for the whole fuel cell stack  110 , which are disposed so as to sandwich the stacked fuel cells  10  therebetween. The two terminal plates  111  are electrically connected to a drive motor  62  which forms a load. The two end plates  112  are disposed so as to sandwich the two terminal plates  111  therebetween. The two end plates  112  are connected to each other by tension plates (not illustrated), and the fuel cells  10  are fastened with a predetermined compression force in the stacking direction. 
       FIG. 2  is a cross-sectional diagram showing the configuration of a fuel cell  10  shown in  FIG. 1 . As shown in the upper part of  FIG. 2 , the fuel cell  10  is provided with an electrolyte membrane  12 , an anode-side catalyst layer  13   a , an anode-side gas dispersion layer  14   a , an anode-side separator  15   a , a cathode-side catalyst layer  13   c , a cathode-side gas dispersion layer  14   c , and a cathode-side separator  15   c . The lower part of  FIG. 2  shows an enlarged view of the region X in the upper part of  FIG. 2 . These enlarged views are described below. 
     The electrolyte membrane  12  is a fluorine resin-type ion exchange membrane containing a sulfonic acid group, and may use Flemion (registered trademark), Aciplex (registered trademark), or the like. The electrolyte membrane  12  is not limited to one containing a sulfonic acid group, and may use a membrane containing another ion exchange group, such as a phosphoric acid group or carboxylic acid group, or the like. In this embodiment, the thickness of the electrolyte membrane  12  is 10 μm. The thickness of the electrolyte membrane  12  is not limited to this embodiment and can be set, as appropriate, to 20 μm, 30 μm, etc. 
     The anode-side catalyst layer  13   a  is disposed in contact with the electrolyte membrane  12 . This anode-side catalyst layer  13   a  is constituted of a member in which a catalyst is carried in electroconductive particles, and an ionomer, which is a proton conductor. For the catalyst, it is possible to use platinum, for example, or an alloy of platinum and a metal such as ruthenium, iron, or the like. For the electroconductive particles, it is possible to use carbon particles, such as carbon black, or carbon fibers, or the like, for example. For the ionomer, it is possible to employ fluorine resin containing a sulfonic ion group. The ionomer bonds together the electroconductive particles which carry the catalyst. The anode-side catalyst layer  13   a  has a plurality of fine holes, which are not illustrated. These fine holes are voids which are formed between secondary particles of the electroconductive particles which carry the catalyst (cluster particles formed by collection of a plurality of the primary particles). In this embodiment, the quantity of pores in the anode-side catalyst layer  13   a  is approximately ⅓ of the cathode-side catalyst layer  13   c . The ratio between the pore quantity in the anode-side catalyst layer and the cathode-side catalyst layer is not limited to that of this embodiment. The pore quantity in the anode-side catalyst layer and the cathode-side catalyst layer may be the same, or the ratio thereof can be set, as appropriate, to 1/2, 1/4, 3/2, or the like. 
     The anode-side gas dispersion layer  14   a  is constituted of a porous member in order to disperse hydrogen, which is the reaction gas, and to discharge water generated by the electro-chemical reaction, and the like. More specifically, for instance, the anode-side gas dispersion layer  14   a  is constituted of a carbon porous material, such as carbon paper, carbon cloth, or the like, or a metal porous material, such as a metal mesh or foamed metal. 
     The anode-side separator  15   a  can be constituted of a conductive member which retains gas, for example, dense carbon formed by compressing carbon so as to retain gas, or a press-molded metal plate. The anode-side separator  15   a  has a concavoconvex shape, and a fuel gas flow path  17   a  is formed between the anode-side separator  15   a  and the anode-side gas dispersion layer  14   a , due to contact between the anode-side separator  15   a  and the anode-side gas dispersion layer  14   a . This fuel gas flow path  17   a  guides hydrogen gas supplied by the hydrogen tank  40  and the circulation pump  42 , to the anode-side gas dispersion layer  14   a , and discharges gas (excess hydrogen gas) emitted from the anode-side gas dispersion layer  14   a , to the combustion gas discharge path  94 . 
     The configuration of the cathode side is similar to the configuration of the anode side. In other words, the cathode-side catalyst layer  13   c  has the same configuration as the anode-side catalyst layer  13   a . Furthermore, the cathode-side gas dispersion layer  14   c  has the same configuration as the anode-side gas dispersion layer  14   a , and the cathode-side separator  15   c  has the same configuration as the anode-side separator  15   a . The oxidant gas flow path  17   c  which is formed between the cathode-side separator  15   c  and the cathode-side dispersion layer  14   c  guides air supplied from the air compressor  50 , to the cathode-side gas dispersion layer  14   c , and also discharges gas emitted from the cathode-side gas dispersion layer  14   c  (excess air and water vapor) to outside the fuel cell  10 . 
     The water generated in the cathode-side catalyst layer  13   c  due to the generation of power in the fuel cell  10  moves to the anode-side catalyst layer  13   a  via the electrolyte membrane  12 , due to the difference in water concentration between the anode-side catalyst layer  13   a  and the cathode-side catalyst layer  13   c . The water which moves from the cathode-side catalyst layer  13   c  to the anode-side catalyst layer  13   a  is called “reversely diffused water W 1 ”. On the other hand, when the fuel cell  10  is generating power, as the protons generated in the anode-side catalyst layer  13   a  move to the cathode-side catalyst layer  13   c  via the electrolyte membrane  12 , water moves from the anode-side catalyst layer  13   a  to the cathode-side catalyst layer  13   c , via the electrolyte membrane  12 . The water which moves from the anode-side catalyst layer  13   a  to the cathode-side catalyst layer  13   c  is called “accompanying water W 2 ”. The directions of movement of the reversely diffused water W 1  and the accompanying water W 2  are shown in the enlarged diameter in the lower part of  FIG. 2 . In this embodiment, the electrolyte membrane  12  has a small thickness of approximately 10 μm, and the water moves readily from the cathode-side catalyst layer  13   c  to the anode-side catalyst layer  13   a , and therefore the amount of reversely diffused water W 1  becomes large. 
     The hydrogen tank  40  shown in  FIG. 1  stores high-pressure hydrogen gas, and supplies hydrogen gas to the fuel cell stack  110  via the combustion gas supply path  93 . The electromagnetic valve  41  is disposed in the combustion gas supply path  93 , and adjusts the amount of hydrogen gas which is supplied to the fuel cell stack  110 . The circulation pump  42  is disposed in the combustion gas discharge path  94  and circulates the hydrogen gas discharged from the fuel cell stack  110 , from the combustion gas discharge path  94  to the combustion gas supply path  93 . The air compressor  50  supplies compressed air (oxidant gas) to the fuel cell stack  110  via the oxidant gas supply path  91 . The air discharged from the fuel cell stack  110  is discharged into the atmosphere via the oxidant gas discharge path  92 . 
     The current measurement unit  71  is disposed between the fuel cell stack  110  and the motor  62 , and measures the output current of the fuel cell stack  110  and sends the measurement value to the control unit  20 . The temperature measurement unit  74  is disposed in the oxidant gas discharge path  92  and measures the temperature of the oxidant gas (air) that is discharged, and sends the measurement value to the control unit  20 . In the fuel cell system  100 , the temperature of the discharge gas is used as the internal temperature of the fuel cell stack  110 . 
     The control unit  20  is configured by a micro-computer provided with a central processing unit and a main storage device. The control unit  20  derives a required output power corresponding to a depression amount of an accelerator pedal  30 , on the basis of a detection value acquired from an accelerator position sensor  32 , and controls the respective constituent parts of the fuel cell system  100  described above in accordance with the required output power, so as to cause the fuel cell stack  110  to generate electric power. 
     Furthermore, as described in detail below, when starting up the fuel cell system  100  below freezing point, the control unit  20  implements a current limitation process to control the operation of the fuel cell stack  110  in such a manner that the amount of water flowing into to the electrode catalyst layers (the anode-side catalyst layer  13   a  and the cathode-side catalyst later  13   c ) of the fuel cells  10  which constitute the fuel call stack does not exceed an allowable amount. The control unit  20  previously stores an allowable time table  24  to determine the operating conditions of the fuel cell stack  110  in this current limiting process. 
     (A2) Allowable time table:  FIG. 3  is an illustrative diagram showing an allowable time table stored in the control unit. The allowable time table  24  shown in  FIG. 3  indicates a relationship between the current density and the time taken until the amount of water flowing into each electrode catalyst layer reaches an allowable amount of water (called “allowable amount of water below”). In this embodiment, the allowable time is calculated as indicated below. 
     The in-flowing water amount to each electrode catalyst layer and the allowable water amount in each electrode catalyst layer are defined as indicated in (Expression 1) to (Expression 4) below, and the time taken until in-flowing water amount to the electrode catalyst layers becomes equal to the allowable water amount in the electrode catalyst layers is taken to be the allowable time. In this embodiment, the reversely diffused water amount, the accompanying water amount, the water content in the electrolyte membrane  12 , and the pore quantity in the catalyst layers, each indicate amounts per unit surface area.
 
(In-flowing water amount to anode-side catalyst layer 13 a )=(Reversely diffused water amount)−(Accompanying water amount)  (Expression 1)
 
(Allowable water amount in anode-side catalyst layer 13 a )=(Water content in electrolyte membrane 12)+(Pore quantity in anode-side catalyst layer 13 a )  (Expression 2)
 
(In-flowing water amount to cathode-side catalyst layer 13 c )=(Accompanying water amount)−(Reversely diffused water amount)  (Expression 3)
 
(Allowable water amount in cathode-side catalyst layer 13 c )=(Water content in electrolyte membrane 12)+(Pore quantity in cathode-side catalyst layer 13 c )  (Expression 4)
 
In this embodiment, as described above, the pore quantity in the anode-side catalyst layer  13   a  is approximately ⅓ of the cathode-side catalyst layer  13   c , and therefore the allowable water amount in the anode-side catalyst layer  13   a  and the allowable water amount in the cathode-side catalyst layer  13   c  are different values. In (Expression 1), when the in-flowing water amount to the anode-side catalyst layer  13   a  has become a negative value, there is no movement of water to the anode-side catalyst layer  13   a . Similarly, in (Expression 2), when the in-flowing water amount to the cathode-side catalyst layer  13   c  becomes a negative value, there is no movement of water to the cathode-side catalyst layer  13   c.  
 
     The reversely diffused water amount and the accompanying water amount are respectively obtained by multiplying the reversely diffused water flux and the accompanying water flux by time. Taking the reversely diffused water flux to be Jg and the accompanying water flux to be Jz, the reversely diffused water flux and the accompanying water flux are calculated by the following equations.
 
 Jg=D ×( Cwc−Cwa )/ t   (Expression 5)
 
In (Expression 5) given above, D indicates a diffusion coefficient, Cwc indicates a water concentration Cw in the cathode-side catalyst layer  13   c , Cwa indicates a water concentration Cw in the anode-side catalyst layer  13   a , and  t  indicates the thickness of the electrolyte membrane  12 . In other words, the reversely diffused water flux Jg is inversely proportional to the film thickness t of the electrolyte membrane. The water concentration Cw in the cathode-side catalyst layer  13   c  and the anode-side catalyst layer  13   a  is calculated by the following equation.
 
 Cw=ε×ρλ/EW   (Expression 6)
 
In (Expression 6) described above, ε indicates the volume ratio of ionomer, ρ indicates the ionomer density, λ indicates the water content in the electrode catalyst layer (the number of water molecules per sulfonic acid molecule), and EW indicates the acid group density. ε, ρ, EW are physical values and are constants. In this embodiment, scavenging is performing when the operation of the fuel cell system  100  is stopped, and presuming that the electrode catalyst layer does not contain any water, the water content λa of the anode-side catalyst layer  13   a  is taken to be 0. On the other hand, the cathode-side catalyst layer  13   c  is taken to have a water content λc of 14, considering that the water content becomes a maximum immediately due to water generated by power generation.
 
 Jz=i/F×Da   (Expression 7)
 
In (Expression 7) described above, i indicates the current density, F indicates Faraday&#39;s constant, and Da indicates the water production coefficient. In other words, the accompanying water flux is proportional to the current density.
 
     The reversely diffused water flux and the accompanying water flux are calculated on the basis of (Expression 1) to (Expression 7) above, and the allowable time corresponding to the current density is calculated with respect to the anode-side catalyst layer  13   a  and the cathode-side catalyst layer  13   c . In this embodiment, the reversely diffused water amount, the accompanying water amount, the water content of the electrolyte membrane  12  and the pore quantity of the catalyst layer are calculated as weights (mg/cm 2 ) per unit surface area, but may also be calculated as volumes. The pore quantity of the catalyst layer is calculated as a weight by assuming that water has entered into the pores. In this embodiment, the allowable time table  24  which indicates the allowable time with respect to the current density is represented as a curve (assuming that continuous values are set), but this table may also be defined in terms of discrete values. Furthermore, the table may be set by carrying out prior experimentation. Moreover, the reversely diffused water amount and the accompanying water amount may be calculated by the method described in JP 2011-113774 A. 
     (A3) Current limitation process:  FIGS. 4 and 5  are flowcharts indicating the procedure of a current limitation process which is carried out in the fuel cell system  100 . When the ignition of the electric automobile (not illustrated) is switched on, a current limitation process is started in the fuel cell system  100 . 
     As shown in  FIG. 4 , the control unit  20  sets a target current density on the basis of the required output power corresponding to the detection value acquired from the accelerator position sensor  32  (step S 12 ). The target current density is set on the basis of the W-I characteristics relating to the fuel cell stack  110  which are stored previously in the control unit  20 . The control unit  20  acquires the temperature of the fuel cell stack  110  from the temperature measurement unit  74  (step S 14 ), and determines whether or not the temperature is lower than freezing point (step S 15 ). In this embodiment, the freezing point is taken to be 0° C., but the freezing point may be set in accordance with the conditions (air pressure) where the fuel cell stack  110  is used. In step S 15 , when the temperature of the fuel cell stack  110  is determined to be equal to or higher than freezing point, then the control unit  20  terminates the current limitation process. 
     When the control unit  20  determines in step S 15  that the temperature of the fuel cell stack  110  is lower than freezing point, then the time taken until the temperature of the fuel cell stack  110  becomes equal to or higher than freezing point is calculated (step S 16 ). In this embodiment, a situation where the temperature of the fuel cell stack  110  is equal to or higher than freezing point is called “freezing point exceed”. The time until the temperature of the fuel cell stack  110  exceeds the freezing point (below, this is also called “freezing point exceed time”) is calculated on the basis of the temperature of the fuel cell stack  110  acquired in step S 14 , the thermal capacity of the fuel cell stack  110 , and the total calorific value of the fuel cell stack  110  when generating power at the target current density. In this embodiment, the freezing point exceed time is calculated, but it is also possible to provide a table which indicates the correspondences between the temperature of the fuel cell stack  110 , the target current density and the freezing point exceed time, in advance, and to obtain the freezing point exceed time by referring to the table. 
     The control unit  20  sets upper and lower limit values of the target current density in such a manner that the freezing point exceed time becomes equal to or lower than the allowable time, on the basis of the freezing point exceed time calculated in step S 16  and the allowable time table  24  ( FIG. 3 ) (step S 17 ). More specifically, as shown in  FIG. 3 , the current density at which the freezing point exceed time and the anode-side allowable time are the same is set as the lower limit value, and the current density at which the freezing point exceed time and the cathode-side allowable time are the same is set as the upper limit value. In this embodiment, the heat capacity and the calorific value of the fuel cell  10  are designed in such a manner that the freezing point exceed time is below the intersection of the anode and cathode curves in  FIG. 3 . 
     As shown in  FIG. 5 , the control unit  20  determines whether or not the freezing point exceed time calculated in step S 16  is equal to or less than the allowable time for the cathode (at the target current density set in step S 12 ) (step S 18 ). In step S 18 , the control unit  20  controls the operation of the fuel cell stack  110  in such a manner that power is output at the target current density set in step S 12 , when the freezing point exceed time is determined to be equal to or less than the allowable time for the cathode (step S 20 ). On the other hand, when the freezing point exceed time is determined to be longer than the allowable time for the cathode (NO at step S 18 ), then the control unit  20  lowers the target current density on the basis of the allowable time table and sets the target current density to the upper limit value set in step S 17 . In other words, the control unit  20  resets the target current density, regardless of the required power (step S 24 ). The control unit  20  controls the operation of the fuel cell stack  110  so as to output power at the target current density that has been reset (step S 20 ). In this embodiment, in step S 24 , the target current density is set to the upper limit value set in step S 17 , but the invention is not limited to this. For example, a configuration may be adopted in which a decrease amount Δ1 when resetting the target current density is set in advance, and resetting is carried out repeatedly until the target current density becomes a value lower than the upper limit value set in step S 17 , whereby the target current density is reset to a value lower than the upper limit value. Furthermore, when the upper and lower limit values of the target current density are set (step S 17 ), the target current density may also be set appropriately on the basis of the upper and lower limit values, for instance, to the median value of the upper and lower limit values, or a value obtained by subtracting a predetermined value from the upper limit value. Furthermore, in this embodiment, in step S 18 , it is only determined whether or not the freezing point exceed time is equal to or less than the allowable time for the cathode, but it may also be determined whether or not the freezing point exceed time is equal to or less than the allowable times for both the cathode and the anode. Normally, the target current density set in step S 12  is higher than the upper limit value of the target current density, which means that the exceed time will be equal to or less than the allowable time for the anode, provided that the exceed time is equal to or less than the allowable time for the cathode, and therefore it is sufficient to determine whether or not the exceed time is equal to or less than the allowable time for the cathode. 
     As shown in  FIG. 5 , the control unit  20  determines whether or not the current density has declined, on the basis of the measurement value of the current measurement unit  71  (step S 22 ). This is because, when power is generated in the fuel cell stack  110  at or below freezing point, then the water generated by power generation freezes, thereby impeding the diffusion of the reaction gas, and the current density declines. In step S 22 , when it is determined that the current density has not declined, then the control unit  20  acquires the temperature of the fuel cell stack  110  from the temperature measurement unit  74  and determines whether or not the temperature has exceeded freezing point (step S 30 ). The control unit  20  terminates the current limitation process when it is determined that the temperature has exceeded freezing point in step S 30 . 
     In step S 22 , when the current density is determined to have declined, the control unit  20  refers to the allowable time table  24 , and determines whether or not the freezing point exceed time is equal to or less than the allowable time for the anode (at the declined current density) (step S 26 ). This is because, when the current density is low, then the amount of reversely diffused water increases, and therefore the amount of water flowing into the anode-side catalyst layer  13   a  increases, and the allowable time becomes shorter. In step S 26 , when it is determined that the freezing point exceed time is equal to or less than the anode allowable time, then the control unit  20  advances to step S 30 . In step S 26 , when the freezing point exceed time is determined to be longer than the anode allowable time (NO at step S 26 ), then the control unit  20  increases the target current density on the basis of the allowable time table  24 , sets the target current density to the lower limit value set in step S 17  (resets the target current density), and controls the operation of the fuel cell stack  110  so as to achieve the reset target current density (step S 28 ). When the temperature of the fuel cell stack  110  has exceeded freezing point, the control unit  20  terminates the current limitation process and implements normal control of the operation of the fuel cell stack  110  on the basis of the required power. In this embodiment, in step S 28 , the target current density is set as the lower limit value set in step S 17 , but the invention is not limited to this. For example, the target current density may also be set to the upper limit value set in step S 17 . By adopting this configuration, it is possible to obtain a relatively large output current, while adjusting the in-flowing water amount to the anode-side catalyst layer  13   a . Furthermore, it is also possible to set an increase range, similarly to step S 24 , or to set the value in step S 17 . 
     In step S 30 , when the temperature of the fuel cell stack  110  is less than freezing point, then the control unit  20  returns to step S 22 . In other words, there is a possibility of steps S 22 , S 26 , S 28 , S 30  being repeated until the temperature of the fuel cell stack  110  exceeds freezing point. In step S 28 , the target current density is increased within the range of the lower and upper limit values of the target current density which are set in step S 17 . Therefore, the target current density does not increase endlessly. 
     Even though it has been determined in step S 18  whether or not the freezing point exceed time is equal to or less than the allowable time for the anode, it is desirable to determine again, in step S 26 , whether or not the freezing point exceed time is equal to or less than the allowable time for the anode. The reason for this is that, when the power generating surface area is reduced due to freezing of the water flowing into the electrode catalyst layer, and the output current density becomes lower than the target current density, then even though the exceed time was equal to or less than the allowable time for the anode in step S 18 , there is a possibility that the exceed time will not be equal to or less than the allowable time at the reduced output current density. 
     In the current limitation process, the control unit  20  controls the electromagnetic valve  41 , the circulation pump  42 , the air compressor  50 , and the like, so as to increase and decrease the output current by adjusting the flow volume of the reaction gas, the back pressure during supply of reaction gas, the output voltage, and the like. The fuel cell system  100  according to this embodiment is provided with a secondary cell (not illustrated) which functions as a subsidiary power source of the fuel cell  10 , and as described above, when the fuel cell stack  110  is caused to generate power at an output current lower than the output current corresponding to the output required by the load, then the insufficiency of the output with respect to the output requirement is compensated for by the secondary cell. The secondary cell may be constituted of a rechargeable lithium ion cell, for example. 
       FIG. 6  is an illustrative diagram showing temporal change in the output current of the fuel cell stack when a power limitation process is implemented. As shown in  FIG. 6 , in the fuel cell system  100  according to this embodiment, the output current of the fuel cell stack  110  is controlled so as to come between the upper and lower limit values of the target current, until the temperature of the fuel cell stack  110  becomes equal to or higher than freezing point. 
     (A4) Effects of embodiments: In the fuel cell system  100  according to this embodiment, the target current density is adjusted in such a manner that the in-flowing water amount to the anode-side catalyst layer  13   a  does not exceed the allowable amount. Consequently, the water flowing into the anode-side catalyst layer  13   a  is prevented from overflowing and collecting between the electrolyte membrane  12  and the anode-side catalyst layer  13   a . As a result of this, when the fuel cell is started up below freezing point, it is possible to suppress the separation of the anode-side catalyst layer  13   a  from the electrolyte membrane  12 , due to a phenomenon (such as frost heaving) which accompanies the freezing of collected water between the electrolyte membrane  12  and the anode-side catalyst layer  13   a . Since the fuel cell system  100  according to this embodiment is formed without including a humidifier to humidify the reaction gas, and since the electrolyte membrane  12  has a small thickness, then the amount of reversely diffused water is large and collected water is liable to occur on the anode side. Therefore, it is considered that a significant effect in suppressing breakage of the anode-side catalyst layer  13   a  is obtained by carrying out the current limitation process described above. 
     Furthermore, in the fuel cell system  100  according to this embodiment, the target current density is set by taking account of the cathode-side in-flowing water amount as well. More specifically, the target current density is adjusted in such a manner that the amount of water flowing into the cathode-side catalyst layer  13   c  does not exceed the allowable amount. Therefore, when the fuel cell is started up below freezing point, it is possible to suppress separation of the cathode-side catalyst layer  3   c  from the electrolyte membrane  12 , similarly to the anode side. 
     B: Modifications: This invention is not limited to the embodiments described above, and can be implemented in various modes without departing from the essence of the invention; for example, modifications such as the following are also possible. 
     (B1) First Modification: In the embodiment described above, the target current density is set by taking account of both the anode-side in-flowing water amount and the cathode-side in-flowing water amount, but the target current density may be set by taking account of the anode-side in-flowing water amount only. In other words, the target current density may be set on the basis of the allowable time table for the anode side only. Even when this configuration is adopted, it is possible to suppress separation of the anode-side catalyst layer  13   a  from the electrolyte membrane  12 . 
     (B2) Second Modification: In the embodiment described above, the output current is limited, when the temperature of the fuel cell stack  110  is less than freezing point, but the temperature of the fuel cell stack  110  is not limited to freezing point, and can be set, as appropriate, to −5° C., 5° C., 10° C., and the like. Furthermore, the output current may also be limited, regardless of the temperature of the fuel cell stack  110 . Even when this configuration is adopted, by keeping the in-flowing water amount to the electrode catalyst layer, to a suitable amount, it is possible to suppress deterioration of the electrode catalyst layer and/or decline in the power generation performance. It is desirable to implement limitation of the output current when the temperature of the fuel cell stack  110  is low. 
     (B3) Third Modification: In the embodiment described above, the allowable water amount is set on the basis of the maximum water content of the electrolyte membrane and the pore quantity of the electrode catalyst layer, but the invention is not limited to this. For instance, the allowable water amount may also be set by experimentation. Furthermore, a humidifier which humidifies the reaction gas may be provided in the fuel cell system, and the amount of water produced by the humidified reaction gas may also be taken into consideration. 
     (B4) Fourth Modification: In the embodiment described above, the upper and lower limit values of the target current density are set on the basis of the time taken until the temperature of the fuel cell stack  110  becomes equal to or higher than freezing point, but the invention is not limited to this. For example, the time taken until the temperature becomes 5° C. or above may be used, or a time may be set regardless of the temperature. 
     Below, one example of a case where the output current is limited regardless of the temperature of the fuel cell stack is described below with reference to  FIG. 7 .  FIG. 7  is a flowchart showing a procedure of a current limitation process which is carried out in the fuel cell system according to this modification. The fuel cell system according to the modification has the same configuration as the embodiment described above, apart from the fact that the current limitation process carried out by the control unit is different to that of the embodiment described above. As shown in  FIG. 7 , the control unit  20  sets a target current density on the basis of the required output power corresponding to the detection value acquired from the accelerator position sensor  32 , similarly to the embodiment described above (step T 12 ). Next, the control unit  20  sets upper and lower limit values for the target current density (step T 17 ). In this modification, a map (called a “water clogging time map” below), which is previously determined by experimentation and which indicates a relationship between the current density and the time taken until adequate power generation cannot be continued due to the occurrence of water clogging in the electrode catalyst layer (called “water clogging time” below), is provided in the control unit  20 . The control unit  20  sets the upper and lower limit values for the target current density on the basis of the allowable time table and the water clogging time map. 
     Thereupon, the control unit  20  determines whether or not the water clogging time is equal to or less than the allowable time for the cathode (at the target current density set in step T 12 ), on the basis of the allowable time table and the water clogging time map (step T 18 ). In step T 18 , the control unit  20  controls the operation of the fuel cell stack  110  in such a manner that power is output at the target current density set in step T 12 , when the water clogging time is determined to be equal to or less than the allowable time for the cathode (step T 20 ). On the other hand, when the water clogging time is determined to be longer than the allowable time for the cathode (NO at step T 18 ), then the control unit  20  lowers the target current density on the basis of the allowable time table and sets the target current density to the upper limit value set in step T 17 . In other words, the control unit  20  resets the target current density, regardless of the required power (step T 24 ). The control unit  20  controls the operation of the fuel cell stack  110  so as to output power at the target current density that has been reset (step T 20 ). 
     The control unit  20  determines whether or not the current density has declined, on the basis of the measurement value of the current measurement unit  71  (step T 22 ). When it is determined in step T 22  that the current density has not declined, then the control unit  20  determines whether or not a previously determined prescribed time has elapsed (step T 30 ). The control unit  20  terminates the current limitation process when it is determined that the prescribed time has elapsed in step T 30 . In this modification, the current limitation process terminates when a prescribed time has elapsed from the start (the start of counting when the ignition is switched on). The prescribed time is determined in advance as the time required for the start-up process, for instance. 
     In step T 22 , when the current density is determined to have declined, the control unit  20  refers to the allowable time table and the water clogging map, and determines whether or not the water clogging time is equal to or less than the allowable time for the anode (at the declined current density) (step T 26 ). In step T 26 , when it is determined that the water clogging time is equal to or less than the anode allowable time, then the control unit  20  advances to step T 30 . In step T 26 , when the water clogging time is determined to be longer than the anode allowable time (NO at step T 26 ), then the control unit  20  increases the target current density on the basis of the allowable time table  24 , sets the target current density to the lower limit value set in step T 17  (resets the target current density), and controls the operation of the fuel cell stack  110  so as to achieve the reset target current density (step T 28 ). When the prescribed time has elapsed (YES in step T 30 ), the control unit  20  terminates the current limitation process and implements normal operation control of the fuel cell stack  110  on the basis of the required power. 
     Even when this configuration is adopted, it is possible to adjust the amount of water flowing into both electrode catalyst layers, and to suppress decline in the fuel cell performance due to water clogging.