Patent Publication Number: US-2022223883-A1

Title: Fuel cell system

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2021-004172 filed on Jan. 14, 2021, the contents of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a fuel cell system that is provided in a moving body and generates electric power during operation of the moving body. 
     Description of the Related Art 
     In a fuel cell system mounted on a moving body such as a fuel cell vehicle, when the temperature of the surrounding environment becomes low and the inside of the system is frozen during operation stop (power generation stop), power generation immediately after operation start is delayed. For this reason, as shown in JP 2017-147022 A, the present applicant has proposed a technology for performing freezing suppression control in a case where freezing is predicted by monitoring the temperature in the system during stop of operation. In the freezing suppression control, the fuel cell system operates an air pump that supplies cathode gas, and opens a stop valve of a cathode path. As a result, the cathode gas flows into the cathode path, and water in the cathode path is discharged to the outside. 
     SUMMARY OF THE INVENTION 
     There are cases where the fuel cell system may receive a request to stop power generation of the fuel cell stack even during operation of the moving body. If the cathode gas is caused to flow through the fuel cell stack each time a request to stop power generation is received during operation of the moving body, deterioration of the electrolyte membrane of the power generation cell inside the fuel cell stack progresses. As a result, the durability of the fuel cell stack deteriorates early in the long term. 
     On the other hand, if the power generation of the fuel cell stack is stopped by closing the stop valve of the cathode path every time a power generation stop request is received, there is a possibility that the stop valve is frozen while being closed when the temperature of the surrounding environment becomes low. 
     An object of the present invention is to provide a fuel cell system capable of appropriately preventing freezing in the system and suppressing deterioration of an electrolyte membrane even when power generation is stopped during operation of a moving body. 
     According to an aspect of the present invention, there is provided a fuel cell system provided in a moving body, the fuel cell system including: a fuel cell stack; a cathode supply path configured to supply cathode gas to the fuel cell stack; a cathode discharge path through which cathode off-gas is discharged from the fuel cell stack; a bypass passage connecting the cathode supply path and the cathode discharge path so as to bypass the fuel cell stack; and an air pump configured to supply the cathode gas to the cathode supply path; one or more stop valves provided between a connection point of the bypass passage on the cathode supply path or the cathode discharge path and the fuel cell stack; a bypass valve provided in the bypass passage; and a control device configured to control operations of the air pump, the one or more stop valves, and the bypass valve, wherein the control device is configured to: acquire temperature information related to a temperature of the fuel cell stack, from a temperature detection unit provided in the fuel cell system; determine whether or not the acquired temperature information exceeds a predetermined temperature value when a signal related to stoppage of power generation of the fuel cell stack is received during operation of the moving body; perform a first control of stopping power generation of the fuel cell stack by closing the one or more stop valves and opening the bypass valve if it is determined that the temperature information exceeds the predetermined temperature value; and perform a second control of generating, with the fuel cell stack, electric power smaller than electric power consumed by the air pump, by operating the air pump, if it is determined that the temperature information is equal to or lower than the predetermined temperature value. 
     The above-described fuel cell system can appropriately prevent freezing in the system and suppress deterioration of the electrolyte membrane even when power generation is stopped during operation of the moving body. 
     The above and other objects, features and advantages of the present invention will become more apparent from the following description when taken in conjunction with the accompanying drawings in which a preferred embodiment of the present invention is shown by way of illustrative example. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an explanatory diagram schematically showing an overall configuration of a fuel cell system according to an embodiment of the present invention; 
         FIG. 2  is a block diagram of an ECU of the fuel cell system; 
         FIG. 3A  is an explanatory diagram schematically illustrating a flow state of cathode gas in a stop control, and 
         FIG. 3B  is a timing chart illustrating an operation of each component in the stop control; 
         FIG. 4A  is an explanatory diagram schematically showing a flow state of the cathode gas in an idle control, and  FIG. 4B  is a timing chart illustrating an operation of each component in the idle control; 
         FIG. 5  is a flowchart illustrating a process of the ECU when a power generation stop signal is received during operation of the moving body; 
         FIG. 6  is a flowchart illustrating processing of the ECU during a stop control; 
         FIG. 7  is a flowchart illustrating processing of the ECU in the idle control; and 
         FIG. 8A  is a flowchart illustrating processing of the ECU during stop control according to a modification, and  FIG. 8B  is a flowchart illustrating processing of the ECU in idle control according to another modification. 
     
    
    
     DESCRIPTION OF THE INVENTION 
     As shown in  FIG. 1 , a fuel cell system  10  according to an embodiment of the present invention includes a fuel cell stack  12 , an anode system apparatus  14 , a cathode system apparatus  16 , and a cooling apparatus  18 . The fuel cell system  10  is mounted on a moving body  11  such as a fuel cell vehicle. The fuel cell system  10  supplies electric power generated by the fuel cell stack  12  to a battery Bt, a traction motor Mt, and the like. Note that the moving body  11  on which the fuel cell system  10  is mounted is not limited to a fuel cell vehicle, and may be another vehicle, a ship, an aircraft, a robot, or the like. 
     In the fuel cell stack  12 , a stack body  21  in which a plurality of power generation cells  20  are stacked is housed in a stack case (not shown). Each power generation cell  20  generates power by an electrochemical reaction between an anode gas (a fuel gas such as hydrogen) and a cathode gas (an oxygen-containing gas such as air). 
     Each power generation cell  20  includes a membrane electrode assembly  22  (hereinafter referred to as a “MEA  22 ”) and a pair of separators  24  ( 24   a ,  24   b ) sandwiching the MEA  22 . The MEA  22  includes an electrolyte membrane  26 , an anode  28  provided on one surface of the electrolyte membrane  26 , and a cathode  30  provided on the other surface of the electrolyte membrane  26 . The electrolyte membrane  26  is, for example, a solid polymer electrolyte membrane (cation exchange membrane). In the separator  24   a , an anode gas flow field  32  through which the anode gas flows is formed on one surface of the MEA  22 . The separator  24   b  forms a cathode gas flow field  34  through which the cathode gas flows on the other surface of the MEA  22 . In addition, by stacking the plurality of power generation cells  20 , a coolant flow field  36  through which a coolant flows is formed between the surfaces of the separator  24   a  and the separator  24   b  facing each other. 
     Further, each power generation cell  20  includes a plurality of passages (an anode gas passage, a cathode gas passage, and a coolant passage) (not shown) through which the anode gas, the cathode gas, and the coolant flow along the stacking direction of the stack body  21 . The anode gas passage communicates with the anode gas flow field  32 , the cathode gas passage communicates with the cathode gas flow field  34 , and the coolant passage communicates with the coolant flow field  36 . 
     The fuel cell stack  12  is supplied with anode gas by the anode system apparatus  14 . In the fuel cell stack  12 , the anode gas flows through the anode gas passage (anode gas supply passage) into the anode gas flow field  32 . The anode gas is used for power generation in the anode  28 . Anode off-gas used for power generation flows out from the anode gas flow field  32  to the anode gas passage (anode gas discharge passage) and is discharged from the fuel cell stack  12  to the anode system apparatus  14 . The anode off-gas contains unreacted hydrogen. 
     Cathode gas is supplied to the fuel cell stack  12  by the cathode system apparatus  16 . In the fuel cell stack  12 , the cathode gas flows through the cathode gas passage (cathode gas supply passage) into the cathode gas flow field  34 . The cathode gas is used for power generation in the cathode  30 . The cathode off-gas used for power generation flows out from the cathode gas flow field  34  to the cathode gas passage (cathode gas discharge passage) and is discharged from the fuel cell stack  12  to the cathode system apparatus  16 . 
     Further, the fuel cell stack  12  is supplied with a coolant by the cooling apparatus  18 . In the fuel cell stack  12 , coolant flows through the coolant passage (coolant supply passage) into the coolant flow field  36 . The coolant cools the power generation cell  20 . The coolant that has cooled the power generation cells  20  flows out from the coolant flow field  36  to the coolant passage (coolant discharge passage) and is discharged from the fuel cell stack  12  to the cooling apparatus  18 . 
     The anode system apparatus  14  of the fuel cell system  10  has an anode path  38 . The anode path  38  includes an anode supply path  40  that supplies anode gas to the fuel cell stack  12  and an anode discharge path  42  that discharges anode off-gas from the fuel cell stack  12 . The anode path  38  has an anode circulation path  44  for returning unreacted hydrogen contained in the anode off-gas of the anode discharge path  42  to the anode supply path  40 . The anode circulation path  44  is connected to a bleed path  46  through which part of the anode off-gas flows from the circulation circuit of the anode system apparatus  14  to the cathode system apparatus  16 . 
     A tank  47  that stores anode gas is provided upstream of the anode supply path  40 . Further, in the anode supply path  40 , an injector  48  and an ejector  50  are provided in this order toward the downstream side in the flow direction of the anode gas. The injector  48  is opened and closed during the operation of the fuel cell system  10  to discharge the anode gas whose pressure has been reduced to less than the pressure on the tank  47  side, downstream. The ejector  50  supplies the anode gas discharged from the injector  48  to the fuel cell stack  12 . The ejector  50  suctions the anode off-gas from the anode circulation path  44  and supplies the suctioned anode off-gas to the fuel cell stack  12 . The ejector  50  suctions the anode off-gas by a negative pressure generated by the flow of the anode gas discharged from the injector  48 . 
     A gas-liquid separator  52  is provided in the anode discharge path  42 . The gas-liquid separator  52  separates liquid water (water generated during power generation) contained in the anode off-gas from the anode off-gas. The anode circulation path  44  is connected to an upper portion of the gas-liquid separator  52 . Thus, the anode off-gas (gas), from the gas-liquid separator  52 , that does not contain water flows to the anode circulation path  44 . One end of a drain path  54  for discharging separated water is connected to a bottom portion of the gas-liquid separator  52 . A drain valve  56  for opening and closing the flow path is provided in the drain path  54 . The bleed path  46  is provided with a bleed valve  58  that opens and closes a flow passage in the bleed path  46 . 
     The cathode system apparatus  16  of the fuel cell system  10  has a cathode path  60 . The cathode path  60  includes a cathode supply path  62  that supplies cathode gas to the fuel cell stack  12 , and a cathode discharge path  64  that discharges cathode off-gas from the fuel cell stack  12 . A cathode bypass passage  66  is connected between the cathode supply path  62  and the cathode discharge path  64 . As a result, the cathode gas in the cathode supply path  62  flows directly to the cathode discharge path  64  without passing through the fuel cell stack  12 . 
     An air pump  68  (air compressor) that supplies cathode gas to the fuel cell stack  12  is provided in the cathode supply path  62 . During rotation of a fan (not shown), the air pump  68  compresses air (outside air) of the upstream side of the air pump  68  and supplies the compressed air to the cathode supply path  62  on the downstream side. Further, the air pump  68  according to the present embodiment is a shaft-floating type air pump that separates the fan from a peripheral wall surrounding the fan during rotation of the fan. 
     The cathode supply path  62  includes a supply-side stop valve  70  on the downstream side of the air pump  68  and the cathode bypass passage  66 . The cathode supply path  62  includes a humidifier  72  between the supply-side stop valve  70  and the fuel cell stack  12 . Although not illustrated, an auxiliary device such as an intercooler for cooling the cathode gas may be provided in the cathode supply path  62 . The bleed path  46  is connected to the cathode supply path  62  at the downstream side of the humidifier  72 . A gas-liquid separator (not shown) is preferably provided at a connection portion between the cathode supply path  62  and the bleed path  46 . 
     The humidifier  72  is provided so as to straddle the cathode supply path  62  and the cathode discharge path  64 . The humidifier  72  humidifies the cathode gas flowing through the cathode supply path  62  with moisture (such as water generated during power generation) contained in the cathode off-gas discharged from the fuel cell stack  12  to the cathode discharge path  64 . 
     The cathode discharge path  64  includes a discharge-side stop valve  74  between the humidifier  72  and the cathode bypass passage  66 . Further, the drain path  54  of the anode system apparatus  14  is connected to the cathode discharge path  64  on the downstream side of the cathode bypass passage  66 . The cathode bypass passage  66  is provided with a bypass valve  76  for adjusting the flow rate of cathode gas bypassing the fuel cell stack  12 . 
     A stop valve  69  includes a supply-side stop valve  70  and a discharge-side stop valve  74  that open and close the cathode path  60 . One or more stop valves  69  are provided. In the present embodiment, a butterfly valve whose opening degree can be linearly adjusted is applied as the stop valve. Similarly, as the bypass valve  76 , a butterfly valve whose opening degree can be linearly adjusted is used. Note that the supply-side stop valve  70  and the discharge-side stop valve  74  may be valves that switch between ON (opening degree 100%) and OFF (opening degree 0%), such as solenoid valves. Further, the fuel cell system  10  is not limited to including both the supply-side stop valve  70  and the discharge-side stop valve  74 , and may include at least one of these valves. 
     The cooling apparatus  18  of the fuel cell system  10  has a coolant path  78  through which coolant flows. The coolant path  78  includes a coolant supply path  80  for supplying coolant to the fuel cell stack  12  and a coolant discharge path  82  for discharging coolant from the fuel cell stack  12 . The coolant supply path  80  and the coolant discharge path  82  are connected to a radiator  84  that cools the coolant. A coolant pump  86  is provided in the coolant supply path  80 . The coolant pump  86  circulates the coolant through the coolant circulation circuit (between the fuel cell stack  12 , the coolant supply path  80 , the coolant discharge path  82 , and the radiator  84 ). 
     Further, the fuel cell system  10  includes a plurality of temperature detection units  90  for detecting the temperature of the fuel cell system  10 . The temperature detection units  90  include a coolant outlet temperature sensor  90   a  provided upstream of the coolant discharge path  82  (on the fuel cell stack  12  side), a cathode temperature sensor  90   b  provided in the cathode supply path  62 , and the like. The coolant outlet temperature sensor  90   a  are provided near the coolant discharge port of the fuel cell stack  12  to approximately detect the temperature of the fuel cell stack  12 . The cathode temperature sensor  90   b  is provided on the upstream side of the cathode bypass passage  66  (and on the downstream side of the intercooler) to approximately detect the ambient temperature of the cathode system apparatus  16 . The ambient temperature of the cathode system apparatus  16  also correlates with the temperature of the fuel cell stack  12 . In this sense, the detection of the ambient temperature of the cathode system apparatus  16  also detects temperature information related to the temperature of the fuel cell stack  12 . 
     Further, the fuel cell system  10  includes a plurality of pressure detection units  92  in order to obtain a differential pressure (pressure difference) between electrodes of the fuel cell stack  12 . The pressure detection unit  92  includes an anode pressure sensor  92   a  that detects the pressure in the anode supply path  40  downstream of the ejector  50 , and a cathode pressure sensor  92   b  that detects the pressure in the cathode supply path  62  downstream of the air pump  68 . The anode pressure sensor  92   a  detects the pressure in the circulation circuit of the anode path  38  to approximately detect the pressure at the anode  28  in the fuel cell stack  12 . The cathode pressure sensor  92   b  detects the pressure in the cathode supply path  62  to approximately detect the pressure at the cathode  30  in the fuel cell stack  12 . 
     The above-described fuel cell system  10  includes an ECU  100  (Electronic Control Unit: a control device) that controls operation of each component of the fuel cell system  10 . The ECU  100  is constituted by a computer having one or more processors, memories, input/output interfaces, and electronic circuits. The ECU  100  controls operations of the air pump  68 , the stop valves  69 , the bypass valve  76 , and the like by one or more processors executing programs (not shown) stored in a memory. In addition, the ECU  100  according to the present embodiment performs a process of stopping power generation of the fuel cell stack  12  (hereinafter referred to as a power generation stop process) during operation of the moving body  11 . Note that the operation of the moving body  11  includes situations such as moving and stopping of the moving body  11 . 
     In order to perform the power generation stop process, as shown in  FIG. 2 , the ECU  100  contains therein a power generation request acquisition unit  102 , a temperature acquisition unit  104 , a pressure acquisition unit  106 , a valve state acquisition unit  108 , a stop operation determination unit  110 , and an operation control unit  112 . 
     The power generation request acquisition unit  102  receives a power generation request signal transmitted from another ECU during operation of the moving body  11 . Examples of the other ECUs include a travel control ECU that controls the traction motor Mt and a battery ECU that monitors the remaining battery level of the battery Bt. Note that the ECU  100  itself may have the functions of the travel control ECU and the battery ECU, and may calculate the power generation request based on signals from sensors (an accelerator opening sensor, a vehicle speed sensor, and the like). Upon receiving a signal (power generation stop signal) indicating that the power generation request is zero during operation, the power generation request acquisition unit  102  gives the stop operation determination unit  110  and the operation control unit  112  a command for performing a power generation stop process. 
     The temperature acquisition unit  104  acquires the temperature detected by the temperature detection unit  90  at an appropriate timing (for example, every predetermined period) during the operation of the moving body  11  and stores the acquired temperature in the memory. The pressure acquisition unit  106  acquires the detected pressure of the pressure detection unit  92  at an appropriate timing (for example, every predetermined period) during the operation of the moving body  11  and stores the detected pressure in the memory. 
     The valve state acquisition unit  108  acquires state information (normal or abnormal) of the supply-side stop valve  70 , the discharge-side stop valve  74 , and the like from the abnormality detection unit  114 . The abnormality detection unit  114  monitors whether each component of the fuel cell system  10  is normal or abnormal, by an appropriate detection determination method. When there is an abnormality, the abnormality detection unit  114  stores an abnormality code in a memory (status register). For example, the abnormality detection unit  114  detects an abnormality such as a closing abnormality in which the supply-side stop valve  70 , the discharge-side stop valve  74 , and the bypass valve  76  are not switched from open to closed, or an opening abnormality in which the supply-side stop valve  70 , the discharge-side stop valve  74 , and the bypass valve  76  are not switched from closed to open, and stores an abnormality code corresponding to the abnormality content. 
     Upon receiving an instruction to stop power generation (power generation request is zero) from the power generation request acquisition unit  102 , the stop operation determination unit  110  determines the processing content of the power generation stop process, based on the temperature information of the temperature acquisition unit  104 . Here, in the power generation stop process, the fuel cell system  10  according to the present embodiment performs stop control for stopping power generation of the fuel cell stack  12  as first control, and idle control for slightly performing power generation of the fuel cell stack  12  as second control. Therefore, the operation control unit  112  includes therein a stop control processing unit  116  that performs the stop control and an idle control processing unit  118  that performs the idle control. 
     As shown in  FIGS. 3A and 3B , the stop control processing unit  116  stops the supply of cathode gas to the fuel cell stack  12  in the stop control. As a result, the power generation amount of the fuel cell stack  12  becomes 0. Specifically, in the cathode system apparatus  16 , the stop control processing unit  116  closes both or at least one of the supply-side stop valve  70  and the discharge-side stop valve  74  while opening the bypass valve  76 . At this time, the ECU  100  fully closes the flow path of the cathode supply path  62  (i.e., opening degree 0%) in the supply-side stop valve  70 , and fully closes the flow path of the cathode discharge path  64  (i.e., opening degree 0%) in the discharge-side stop valve  74 . On the other hand, the ECU  100  fully opens the flow path of the cathode bypass passage  66  in the bypass valve  76  (i.e., opening degree 100%). Accordingly, the cathode gas supplied to the downstream side of the air pump  68  does not flow toward the fuel cell stack  12 , but flows from the cathode supply path  62  to the cathode discharge path  64  through the cathode bypass passage  66 . 
     Further, the ECU  100  closes both the supply-side stop valve  70  and the discharge-side stop valve  74 . At the same time, by continuing the flow of the anode gas from anode system apparatus  14 , the residual oxygen (residual cathode gas) inside the fuel cell stack  12  and in the pipes is consumed. That is, in the fuel cell stack  12 , the anode gas and the cathode gas react with each other, whereby residual oxygen in the cathode gas is consumed. By continuing the supply of the anode gas, it is possible to avoid insufficient supply of the anode gas to the fuel cell stack  12  when the power generation returns to normal power generation after the stop control has been performed. 
     In addition, the ECU  100  operates the air pump  68  by supplying electric power lower than electric power during normal traveling (normal power generation), from the battery Bt to the air pump  68 . As a result, the fan of the air pump  68  rotates at a constant rotational speed, and air corresponding to the rotational speed is supplied to the cathode supply path  62 . In the stop control, the ECU  100  may adjust the electric power supplied from the battery Bt to the air pump  68 , based on a waste electric power request value acquired from the battery ECU or the like. 
     Note that, in the fuel cell system  10  to which the non-shaft-floating type air pump  68  is applied, the ECU  100  may stop the rotation of the air pump  68  during execution of the stop control (see the two dot-chain line of the air pump  68  in  FIG. 3B ). This reduces the power consumption of the battery Bt by the air pump  68 . 
     By the stop control described above, in the fuel cell system  10 , electric current or electric power output from the fuel cell stack  12  to the traction motor Mt and the battery Bt becomes 0. The fuel cell system  10  supplies electric power of the battery Bt to various electrical devices. By performing such stop control, for example, even when regenerative power is generated in a state where the SOC of the battery Bt is high, the regenerative power can be appropriately discharged. Further, in the fuel cell stack  12 , membrane deterioration of the electrolyte membrane  26  due to permeation of the cathode gas in the cathode gas flow field  34  through the membrane electrode assembly  22  is reduced. 
     As shown in  FIGS. 4A and 4B , in the idle control, the idle control processing unit  118  reduces the supply amount of the cathode gas in the power generation stop process to the fuel cell stack  12  to be smaller than the supply amount of the cathode gas during the normal traveling. As a result, the power generation amount of the fuel cell stack  12  decreases. Specifically, the idle control processing unit  118  opens both the supply-side stop valve  70  and the discharge-side stop valve  74 , and also opens the bypass valve  76 . Thus, the cathode gas flowing out to the downstream side of the air pump  68  is divided into a first flow flowing from the cathode supply path  62  toward the fuel cell stack  12  and a second flow flowing through the cathode bypass passage  66  toward the cathode discharge path  64 . 
     Therefore, in the fuel cell system  10 , the fuel cell stack  12  generates electric power while suppressing the generated electric power, and the generated electric power is supplied to each electrical device including the air pump  68 . In addition, the fuel cell system  10  supplies electric power of the battery Bt to each electrical device including the air pump  68  as necessary. 
     For example, in the idle control, the ECU  100  supplies the air pump  68  with power greater than or equal to the power generated by the fuel cell stack  12 . That is, when the generated power of the fuel cell stack  12  is defined as A (W), the power consumption of the air pump  68  is A+B (W) (B is a positive number and smaller than A). As a result, electric power generated by the fuel cell stack  12  is consumed (wasted) by the air pump  68 . 
     The air pump  68  supplies the cathode gas to the cathode supply path  62  by rotating the fan at a rotational speed corresponding to the electric power. Further, the ECU  100  adjusts opening and closing (opening degree) of the bypass valve  76  in accordance with a change in the generated power (or current) output from the fuel cell stack  12 . For example, the ECU  100  decreases the opening degree of the bypass valve  76  if the current value output from the fuel cell stack  12  increases. On the other hand, the ECU  100  performs control to increase the opening degree of the bypass valve  76  if the current value output from the fuel cell stack  12  decreases. 
     Further, in the idle control, the ECU  100  fully opens both the supply-side stop valve  70  and the discharge-side stop valve  74  (i.e., opening degree 100%). Thus, the cathode gas smoothly flows to the fuel cell stack  12 . The ECU  100  may adjust the opening degree of the stop valve  69  (the supply-side stop valve  70  and the discharge-side stop valve  74 ) in accordance with the adjustment of the opening degree of the bypass valve  76 . By adjusting the opening degree of the stop valve  69 , the generated electric power of the fuel cell stack  12  can be adjusted more accurately. 
     By performing the idle control described above, a large potential fluctuation of the power generated by the fuel cell stack  12  is suppressed. Therefore, deterioration of the pair of separators  24 , the anode  28 , and the cathode  30  caused by potential fluctuation is suppressed. 
     Returning to  FIG. 2 , when all of the following conditions (a) to (c) are satisfied, the stop operation determination unit  110  determines to perform the stop control. Therefore, the stop operation determination unit  110  determines to perform the idle control if even one of the conditions (a) to (c) is not satisfied. 
     (a) The temperature of the fuel cell stack  12  and/or the ambient temperature of the cathode system apparatus  16  exceeds a predetermined temperature value (determination temperature threshold Tt).
 
(b) Both the supply-side stop valve  70  and the discharge-side stop valve  74  are not in the closing abnormality.
 
(c) The electrode differential pressure between the pressure at the anode  28  and the pressure at the cathode  30  is equal to or less than a predetermined pressure value (determination differential pressure threshold Tp).
 
     The condition (a) is a condition for determining the possibility of freezing of one or more stop valves  69 , the cathode path  60 , or the like due to water generated during power generation of the fuel cell stack  12  in the fuel cell system  10 . If any one of the one or more stop valves  69  is frozen in a closed state during the stop control, the start of power generation of the fuel cell stack  12  is delayed or power generation cannot be performed. 
     The stop operation determination unit  110  has in advance the determination temperature threshold Tt for determining the possibility of freezing. There is often a large difference between the temperature of the fuel cell stack  12  and the outside air temperature during traveling of the moving body  11 . Therefore, the determination temperature threshold Tt is preferably set in consideration of an elapsed time after the start-up, a measurement error, and the like. 
     As the temperature information of the fuel cell stack  12  to be compared with the determination temperature threshold Tt, the temperature detected by the coolant outlet temperature sensor  90   a  provided in the coolant discharge path  82  is used. Further, the fuel cell system  10  may use, as the temperature information of the fuel cell stack  12 , a temperature detected by a cathode temperature sensor  90   b  provided in the cathode supply path  62 . In the present embodiment, the stop operation determination unit  110  performs the determination using, as the temperature information, both the temperature detected by the coolant outlet temperature sensor  90   a  and the temperature detected by the cathode temperature sensor  90   b . The temperature around the cathode system apparatus  16  (i.e., the temperature at the cathode temperature sensor  90   b ) correlates with the temperature of the fuel cell stack  12 . Therefore, the stop operation determination unit  110  does not necessarily need to use the ambient temperature of the cathode system apparatus  16  as the temperature information for the determination. 
     Alternatively, the stop operation determination unit  110  may use, as the temperature information, a temperature detected by an outside air temperature sensor  90   c  (see  FIG. 1 ) that detects the outside air temperature. For example, the stop operation determination unit  110  may estimate the possibility of freezing of the stop valve  69  (the supply-side stop valve  70  and the discharge-side stop valve  74 ) using the temperature detected by the coolant outlet temperature sensor  90   a  or the cathode temperature sensor  90   b  and the temperature detected by the outside air temperature sensor  90   c.    
     Immediately after the start of operation of the moving body  11  or the like, there is a large difference between the temperature of the fuel cell stack  12  and the ambient temperature of the cathode system apparatus  16 . Therefore, the determination temperature threshold Tt may include two different values, i.e., a value for determining the temperature of the fuel cell stack  12  and another value for monitoring the ambient temperature of the cathode system apparatus  16 . For example, when starting up in a cryogenic environment, there is a possibility that a large difference occurs between the temperature of the fuel cell stack  12  and the ambient temperature of the cathode system apparatus  16 . Therefore, at the start-up under the cryogenic environment, the determination temperature threshold Tt for determining the temperature of the fuel cell stack  12  and the determination temperature threshold Tt for determining the ambient temperature of the cathode system apparatus  16  may be each set to a value at which it can be ensured that the corresponding component is in a thawed state. 
     When the temperature of the fuel cell stack  12  exceeds the determination temperature threshold Tt, the stop operation determination unit  110  determines that the condition (a) is satisfied. Conversely, when the temperature of the fuel cell stack  12  is equal to or lower than the determination temperature threshold Tt, the stop operation determination unit  110  determines that the above condition (a) is not satisfied. 
     When any of the one or more stop valves  69  is suffering from the closing abnormality, an inconvenience occurs in the execution of the stop control. The condition (b) excludes the closing abnormality of each valve. The stop operation determination unit  110  monitors the states of the valves (the supply-side stop valve  70  and the discharge-side stop valve  74 ) acquired by the valve state acquisition unit  108 . Then, the stop operation determination unit  110  determines that the condition (b) is satisfied when all of the valves are normal, and determines that the condition (b) is not satisfied when there is a closing abnormality in any of the valves. 
     If the stop valve  69  is closed when the electrode differential pressure between the anode  28  and the cathode  30  in the fuel cell stack  12  is large, there is a possibility that the fuel cell stack  12  may be damaged. The condition (c) excludes a state where the electrode differential pressure is large. The stop operation determination unit  110  calculates the absolute value of the electrode differential pressure, based on the pressure of the anode pressure sensor  92   a  and the pressure of the cathode pressure sensor  92   b  acquired by the pressure acquisition unit  106 . Then, the stop operation determination unit  110  determines that the condition (c) is satisfied when the calculated electrode differential pressure is equal to or less than the determination differential pressure threshold Tp, and determines that the condition (c) is not satisfied when the calculated electrode differential pressure exceeds the determination differential pressure threshold Tp. 
     In addition, the stop operation determination unit  110  continuously monitors the above-described conditions (a) to (c) even while the stop control is being performed. For example, even when the stop control is performed, if the temperature of the fuel cell stack  12  becomes equal to or lower than the determination temperature threshold Tt, the stop operation determination unit  110  shifts from the stop control to the idle control. Alternatively, even when the idle control is performed based on the condition (a) not being satisfied, if the temperature of the fuel cell stack  12  exceeds the determination temperature threshold Tt, the stop operation determination unit  110  shifts from the idle control to the stop control. Further, for example, in a case where the idle control is performed based on the condition (c) not being satisfied while the conditions (a) and (b) are satisfied, the stop operation determination unit  110  may shift from the idle control to the stop control if the subsequent electrode differential pressure becomes equal to or less than the determination differential pressure threshold Tp. 
     The stop operation determination unit  110  may determine at least whether the temperature of the fuel cell stack  12  exceeds the determination temperature threshold Tt (whether the condition of (a) is satisfied or not) without determining all of the conditions of (a) to (c). When there is no possibility of freezing of the fuel cell system  10 , the stop control is performed to give priority to suppression of deterioration of the electrolyte membrane  26 , so that the durability of the fuel cell stack  12  can be significantly increased. 
     The fuel cell system  10  according to the present embodiment is basically configured as described above. The operation will be described below. 
     During driving (during operation based on turning on an ignition or a starter switch), the moving body  11  travels based on a driving operation of a user or automatic driving of a control device of the moving body  11 . Even if the temperature of the surrounding environment is low at the time of start-up, the fuel cell stack  12 , the anode system apparatus  14 , and the cathode system apparatus  16  are not frozen by being warmed up at the time of start-up. Therefore, if the fuel cell stack  12  does not stop power generation while the moving body  11  is traveling, the one or more stop valves  69  and the bypass valve  76  are opened and closed to allow the cathode gas to flow therethrough. 
     The fuel cell system  10  operates the anode system apparatus  14  and the cathode system apparatus  16  during normal traveling of the moving body  11 . The anode system apparatus  14  supplies an anode gas to the fuel cell stack  12 . The cathode system apparatus  16  supplies a cathode gas to the fuel cell stack  12 . As a result, the fuel cell stack  12  generates electric power, and the generated power is supplied to the traction motor Mt, the battery Bt, and the like. 
     When receiving a power generation request from the travel control ECU or the battery ECU (not shown) during operation (during travel or during travel stop), the ECU  100  starts the power generation stop process according to the process flow shown in  FIG. 5 . 
     Specifically, the ECU  100  receives a power generation stop signal through the power generation request acquisition unit  102  during operation (step S 1 ). Upon receiving the instruction to stop power generation, from the power generation request acquisition unit  102 , the stop operation determination unit  110  determines whether or not the temperature of the fuel cell stack  12  (coolant outlet temperature sensor  90   a ) acquired by the temperature acquisition unit  104  at that time exceeds the determination temperature threshold Tt (step S 2 ). 
     When the temperature of the fuel cell stack  12  is equal to or lower than the determination temperature threshold Tt (step S 2 : NO), the process proceeds to step S 3 , and the stop operation determination unit  110  determines execution of the idle control. In the idle control, the operation control unit  112  opens the supply side stop valve  70 , the discharge-side stop valve  74 , and the bypass valve  76 , of the cathode system apparatus  16 . In addition, the operation control unit  112  consumes power generated by the fuel cell stack  12 , by supplying power larger than the power generated by the fuel cell stack  12 , from the fuel cell stack  12  and the battery Bt to the air pump  68 . Part of the cathode gas supplied from the air pump  68  to the cathode supply path  62  is discharged to the cathode discharge path  64  via the cathode bypass passage  66 . Thus, the flow rate of the cathode gas flowing toward the fuel cell stack  12  is adjusted to an amount corresponding to the electric power generated by the fuel cell stack  12 . 
     On the other hand, when the temperature of the fuel cell stack  12  exceeds the determination temperature threshold Tt (step S 2 : YES), the stop operation determination unit  110  proceeds to step S 4 . In step S 4 , the stop operation determination unit  110  determines whether or not the ambient temperature of the cathode system apparatus  16  (the temperature of the cathode temperature sensor  90   b ) acquired by the temperature acquisition unit  104  at the time of the instruction to stop power generation exceeds the determination temperature threshold Tt. When the ambient temperature of the cathode system apparatus  16  is equal to or lower than the determination temperature threshold Tt (step S 4 : NO), the stop operation determination unit  110  proceeds to step S 3 . On the other hand, when the ambient temperature of the cathode system apparatus  16  exceeds the determination temperature threshold Tt (step S 4 : YES), the stop operation determination unit  110  proceeds to step S 5 . 
     In step S 5 , the stop operation determination unit  110  determines whether each of states of the supply-side stop valve  70  and the discharge-side stop valve  74  acquired via the valve state acquisition unit  108  is normal or abnormal. When any one of the supply-side stop valve  70  and the discharge-side stop valve  74  has a closing abnormality (step S 5 : NO), the stop operation determination unit  110  proceeds to step S 3 . On the other hand, when both the supply-side stop valve  70  and the discharge-side stop valve  74  are normal (step S 5 : YES), the stop operation determination unit  110  proceeds to step S 6 . 
     In step S 6 , the stop operation determination unit  110  calculates the electrode differential pressure, based on the pressures of the anode pressure sensor  92   a  and the cathode pressure sensor  92   b  acquired via the pressure acquisition unit  106 . The stop operation determination unit  110  determines whether or not the electrode differential pressure is equal to or less than a determination differential pressure threshold Tp (i.e., whether the electrode differential pressure the threshold). When the electrode differential pressure exceeds the determination differential pressure threshold Tp (step S 6 : NO), the stop operation determination unit  110  proceeds to step S 7 . 
     In step S 7 , the stop operation determination unit  110  waits until the power generated by the fuel cell stack  12  becomes equal to or lower than a predetermined power. When the standby in step S 7  ends, the stop operation determination unit  110  proceeds to step S 8 . The processing of step S 7  is not limited to the above, and may wait until the electrode differential pressure becomes equal to or less than the determination differential pressure threshold Tp. 
     Alternatively, the stop operation determination unit  110  may perform step S 3  when the differential pressure exceeds the determination differential pressure threshold Tp without performing step S 7 . 
     On the other hand, when the electrode differential pressure is equal to or less than the determination differential pressure threshold Tp in step S 6  (step S 6 : YES) or after step S 7  has been performed, the stop operation determination unit  110  determines to perform the stop control (step S 8 ). In the stop control, the operation control unit  112  fully closes the supply-side stop valve  70  and the discharge-side stop valve  74  of the cathode system apparatus  16 , and fully opens the bypass valve  76 . Further, the operation control unit  112  drives the air pump  68  to supply the cathode gas from the air pump  68  to the cathode supply path  62 . Since the supply-side stop valve  70  and the discharge-side stop valve  74  are fully closed, the cathode gas is discharged to the cathode discharge path  64  via the cathode bypass passage  66 . As a result, the cathode gas does not flow into the fuel cell stack  12 , so that the fuel cell stack  12  stops power generation after consuming the remaining cathode gas. 
     Further, the ECU  100  performs the processing flow illustrated in  FIG. 6  during the execution of the stop control. That is, the stop operation determination unit  110  determines whether or not the temperature of the fuel cell stack  12  (coolant outlet temperature sensor  90   a ) after the start of the stop control exceeds the determination temperature threshold Tt (step S 11 ). When the temperature of the fuel cell stack  12  becomes equal to or lower than the determination temperature threshold Tt (step S 11 : NO), the stop operation determination unit  110  proceeds to step S 12  and switches from the stop control to the idle control. On the other hand, when the temperature of the fuel cell stack  12  exceeds the determination temperature threshold Tt (step S 11 : YES), the stop operation determination unit  110  proceeds to step S 13 . 
     In step S 13 , the stop operation determination unit  110  determines whether or not the ambient temperature of the cathode system apparatus  16  after the start of the stop control (the temperature of the cathode temperature sensor  90   b ) exceeds the determination temperature threshold Tt. When the ambient temperature of the cathode system apparatus  16  is equal to or lower than the determination temperature threshold Tt (step S 13 : NO), the stop operation determination unit  110  proceeds to step S 12 . On the other hand, when the ambient temperature of the cathode system apparatus  16  exceeds the determination temperature threshold Tt (step S 13 : YES), the stop operation determination unit  110  proceeds to step S 14 . 
     In step S 14 , the stop operation determination unit  110  determines whether each of states of the supply-side stop valve  70  and the discharge-side stop valve  74  after the start of the stop control is normal or abnormal. When any one of the supply-side stop valve  70  and the discharge-side stop valve  74  has a closing abnormality (step S 14 : NO), the stop operation determination unit  110  proceeds to step S 12 . On the other hand, when all of the supply-side stop valve  70  and the discharge-side stop valve  74  are normal (step S 14 : YES), the stop operation determination unit  110  proceeds to step S 15 . 
     In step S 15 , the stop operation determination unit  110  determines continuation of the stop control. As described above, in the stop control, the ECU  100  can shift from the stop control to the idle control by repeating the processing flow of steps S 11  to S 15  while performing the stop control by the operation control unit  112 . In the fuel cell system  10 , when the stop control is continued, there is a possibility that one or more stop valves  69  are frozen due to the influence of the outside air temperature or the like. However, the fuel cell system  10  can avoid freezing, by shifting from the stop control to the idle control. 
     The ECU  100  performs the processing flow shown in  FIG. 7  during execution of the idle control. That is, the stop operation determination unit  110  determines whether or not the temperature of the fuel cell stack  12  (coolant outlet temperature sensor  90   a ) after the start of the idle control is equal to or lower than the determination temperature threshold Tt (i.e., whether the temperature of the fuel cell stack the threshold) (step S 21 ). When the temperature of the fuel cell stack  12  is maintained at or below the determination temperature threshold Tt (step S 21 : YES), the stop operation determination unit  110  proceeds to step S 22  and continues the idle control. On the other hand, when the temperature of the fuel cell stack  12  exceeds the determination temperature threshold Tt (step S 21 : NO), the stop operation determination unit  110  proceeds to step S 23 . 
     In step S 23 , the stop operation determination unit  110  determines whether or not the ambient temperature of the cathode system apparatus  16  after the start of the idle control (the temperature of the cathode temperature sensor  90   b ) exceeds the determination temperature threshold Tt. When the ambient temperature of the cathode system apparatus  16  is equal to or lower than the determination temperature threshold Tt (step S 23 : NO), the stop operation determination unit  110  proceeds to step S 22 . On the other hand, when the ambient temperature of the cathode system apparatus  16  exceeds the determination temperature threshold Tt (step S 23 : YES), the stop operation determination unit  110  proceeds to step S 24 . 
     In step S 24 , the stop operation determination unit  110  switches from the idle control to the stop control. In this manner, the ECU  100  can shift from the idle control to the stop control by repeating the processing flow of steps S 21  to S 24  while performing the idle control by the operation control unit  112 . 
     When the idle control is performed based on occurrence of the closing abnormality of the stop valve  69  acquired from the abnormality detection unit  114 , the ECU  100  prohibits the stop control from being performed and continues the idle control. That is, the processing flow of steps S 21  to S 24  is not performed. As a result, the ECU  100  can prevent inadvertent switching to the stop control when the idle control based on occurrence of the closing abnormality of the stop valve  69  is being performed. 
     The present invention is not limited to the above-described embodiment, and various modifications can be made along the gist of the invention. For example, the cathode discharge path  64  may be provided with a back pressure valve (not shown) in addition to the discharge-side stop valve  74 . In this case, the back pressure valve may perform the same opening/closing operation as the discharge-side stop valve  74  during the stop control and the idle control. 
     Further, during execution of the stop control and/or during execution of the idle control, the determination temperature threshold Tt may be set to a value different from the determination temperature threshold Tt used at the start of the power generation stop process. Hereinafter, a processing flow in the stop control and the idle control of the fuel cell system  10  according to modifications will be exemplified with reference to  FIGS. 8A and 8B . 
     As shown in  FIG. 8A , in the stop control, the stop operation determination unit  110  determines whether or not the temperature of the fuel cell stack  12  (coolant outlet temperature sensor  90   a ) after the start of the stop control exceeds a value obtained by adding a first margin tx to the determination temperature threshold Tt (step S 31 ). The first margin tx is determined by obtaining in advance a change in the temperature of the fuel cell stack  12  with respect to the temperature of the surrounding environment when the stop control is performed, by an experiment or the like. When the temperature of the fuel cell stack  12  becomes equal to or lower than Tt+tx (step S 31 : NO), the stop operation determination unit  110  proceeds to step S 32  and switches from the stop control to the idle control. On the other hand, when the temperature of the fuel cell stack  12  exceeds Tt+tx (step S 31 : YES), the process proceeds to step S 33 . 
     In step S 33 , the stop operation determination unit  110  determines whether or not the ambient temperature of the cathode system apparatus  16  after the start of the stop control (the temperature of the cathode temperature sensor  90   b ) exceeds a value obtained by adding a second margin ty to the determination temperature threshold Tt. The second margin ty is determined by obtaining in advance a change in the ambient temperature of the cathode system apparatus  16  with respect to the temperature of the surrounding environment when the stop control is performed, by an experiment or the like. When the ambient temperature of the cathode system apparatus  16  is equal to or lower than Tt+ty (step S 33 : NO), the stop operation determination unit  110  proceeds to step S 32 . When the ambient temperature of the cathode system apparatus  16  exceeds Tt+ty (step S 33 : YES), the stop operation determination unit  110  proceeds to step S 34 . 
     In step S 34 , the stop operation determination unit  110  determines whether each of states of the supply-side stop valve  70  and the discharge-side stop valve  74  after the start of the stop control is normal or abnormal. When any one of the supply-side stop valve  70  and the discharge-side stop valve  74  has a closing abnormality (step S 34 : NO), the stop operation determination unit  110  proceeds to step S 32 . When both the supply-side stop valve  70  and the discharge-side stop valve  74  are normal (step S 34 : YES), the stop operation determination unit  110  proceeds to step S 35 . In step S 35 , the stop operation determination unit  110  determines continuation of the stop control. 
     In ECU  100 , as shown in  FIG. 8B , in the idle control, the stop operation determination unit  110  determines whether or not the temperature of the fuel cell stack  12  (coolant outlet temperature sensor  90   a ) after the start of the idle control is equal to or lower than a determination temperature threshold Ttf that is a fixed value (i.e., whether the temperature of the fuel cell≤the threshold Ttf) (step S 41 ). The determination temperature threshold Ttf is preferably set to an appropriate value in consideration of temperature dependence of chemical deterioration of the fuel cell stack  12 . 
     When the temperature of the fuel cell stack  12  is maintained at or below the determination temperature threshold Ttf (step S 41 : YES), the stop operation determination unit  110  proceeds to step S 42  and continues the idle control. On the other hand, when the temperature of the fuel cell stack  12  exceeds the determination temperature threshold Ttf (step S 41 : NO), the stop operation determination unit  110  proceeds to step S 43 , and the operation control unit  112  shifts to the stop control. As described above, by using the determination temperature threshold Ttf that is higher than the determination temperature threshold Tt, the fuel cell system  10  can stably shift from the idle control to the stop control. 
     The technical concept and effects grasped from the above embodiment will be described below. 
     According to an aspect of the present invention, there is provided a fuel cell system  10  provided in a moving body  11 , the fuel cell system including: a fuel cell stack  12 ; a cathode supply path  62  configured to supply cathode gas to the fuel cell stack  12 ; a cathode discharge path  64  through which cathode off-gas is discharged from the fuel cell stack  12 ; a bypass passage (cathode bypass passage  66 ) connecting the cathode supply path  62  and the cathode discharge path  64  so as to bypass the fuel cell stack  12 ; and an air pump  68  configured to supply the cathode gas to the cathode supply path  62 ; one or more stop valves  69  provided between a connection point of the bypass passage on the cathode supply path  62  or the cathode discharge path  64  and the fuel cell stack  12 ; a bypass valve  76  provided in the bypass passage; and a control device (ECU  100 ) configured to control operations of the air pump  68 , the one or more stop valves  69 , and the bypass valve  76 , wherein the control device is configured to: acquire temperature information related to a temperature of the fuel cell stack  12 , from a temperature detection unit  90  provided in the fuel cell system  10 ; determine whether or not the acquired temperature information exceeds a predetermined temperature value (a determination temperature threshold Tt) when a signal related to stoppage of power generation of the fuel cell stack  12  is received during operation of the moving body  11 ; perform a first control (stop control) of stopping power generation of the fuel cell stack  12  by closing the one or more stop valves  69  and opening the bypass valve  76  if it is determined that the temperature information exceeds the predetermined temperature value; and perform a second control (idle control) of generating, with the fuel cell stack, electric power smaller than electric power consumed by the air pump  68 , by operating the air pump  68 , if it is determined that the temperature information is equal to or lower than the predetermined temperature value. 
     With the above configuration, the fuel cell system  10  stops the supply of the cathode gas to the fuel cell stack  12 , as the first control (stop control), when the temperature information exceeds the predetermined temperature value (determination temperature threshold Tt) during operation of the moving body  11 . As a result, the fuel cell system  10  can stop power generation of the fuel cell stack  12  while suppressing power consumption, and can suppress deterioration of the electrolyte membrane  26  caused by cathode gas. On the other hand, when the temperature of the fuel cell stack  12  is equal to or lower than the predetermined temperature value, there is a possibility that the one or more stop valves  69  are frozen and cannot be closed. Therefore, the fuel cell system  10  performs the second control in which the generated power of the fuel cell stack  12  is consumed by the air pump  68 , so that the output of power can be substantially eliminated in the entire system. 
     In addition, the control device (ECU  100 ) performs the second control when it is determined that the temperature information is equal to or less than a predetermined temperature value (determination temperature threshold Tt) during traveling of the moving body  11 . Accordingly, the fuel cell system  10  can perform the power generation stop process in consideration of the possibility that the one or more stop valves  69  are frozen and cannot move, during traveling of the moving body  11 , and the output of electric power can be substantially eliminated in the entire fuel cell system  10 . 
     Further, the control device (ECU  100 ) opens all of the one or more stop valves  69  in the second control. As a result, the fuel cell system  10  can reliably and stably flow the cathode gas through the fuel cell stack  12  in the second control. 
     Further, the control device (ECU  100 ) stops the rotation of the air pump  68  in the first control. Accordingly, the fuel cell system  10  can suppress a decrease in the remaining battery level of the battery Bt during the execution of the first control. 
     When the temperature information becomes equal to or lower than a predetermined temperature value (determination temperature threshold Tt) during execution of the first control, the control device (ECU  100 ) stops the first control and switches the control to the second control. Here, in the fuel cell system  10 , when the power generation of the fuel cell stack  12  continues to be stopped by the first control, there is a possibility that the one or more stop valves  69  are frozen due to the influence of the ambient temperature. Therefore, the ECU  100  continues to monitor the temperature information even during the first control, and switches from the first control to the second control when the temperature information becomes equal to or lower than the predetermined temperature value. Thus, the one or more stop valves  69  can be heated by heat generated by the fuel cell stack  12 . As a result, freezing of the one or more stop valves  69  is avoided. 
     When the temperature information exceeds the predetermined temperature value (determination temperature threshold Tt) during the execution of the second control, the control device (ECU  100 ) stops the second control and switches to the first control. In the fuel cell system  10 , when the fuel cell stack  12  continues to generate power in the second control, the temperature of the stop valves  69  increases due to the heat of the power generation, and the stop valves  69  are less likely to freeze. Therefore, the ECU  100  can suppress the deterioration of the electrolyte membrane  26  by switching from the second control to the first control when the temperature information exceeds the predetermined temperature value. 
     The control device (ECU  100 ) further includes an abnormality detection unit  114  that detects whether the one or more stop valves  69  operate normally or abnormally. The control device prohibits the first control and performs the second control when the control device acquires a closing abnormality in which any of the one or more stop valves  69  cannot be closed, from the abnormality detection unit  114 . In the fuel cell system  10 , in the case of a closing abnormality of the one or more stop valves  69 , the first control cannot be executed. Therefore, by performing the second control, the output of electric power can be substantially eliminated as the entire fuel cell system  10 . 
     The control device (ECU  100 ) acquires pressure information regarding the pressure at the anode  28  and the pressure at the cathode  30  of the fuel cell stack  12 , from the pressure detection unit  92  provided in the fuel cell system  10 ; determines whether the electrode differential pressure between the pressures at the anode  28  and the cathode  30  exceeds a predetermined pressure value (determination differential pressure threshold Tp); and at least temporarily prohibits the first control when the electrode differential pressure exceeds the predetermined pressure value. In the fuel cell stack  12 , if the one or more stop valves  69  are closed when the electrode differential pressure is large, there is a possibility that the pair of separators  24  and the fuel cell stack  12  are damaged. Therefore, the fuel cell system  10  can prevent damage to the pair of separators  24  and the fuel cell stack  12 , by temporarily prohibiting the first control. 
     In a case where the control device (ECU  100 ) prohibits the first control based on the electrode differential pressure exceeding the predetermined pressure value (determination differential pressure threshold Tp) while determining that the temperature information exceeds the predetermined temperature value, the control device waits until the electric power of the fuel cell stack  12  decreases to the predetermined value or until the electrode differential pressure becomes equal to or lower than the predetermined pressure value, and performs the first control after the waiting. As a result, the fuel cell system  10  can perform the first control after the electrode differential pressure decreases, and can suppress deterioration of the electrolyte membrane  26  while preventing damage to the pair of separators  24  and the fuel cell stack  12 . 
     Further, the control device (ECU  100 ) fully opens the bypass valve  76  in the first control, and adjusts the opening degree of the bypass valve in accordance with the power generation amount of the fuel cell stack  12  in the second control. Thus, the fuel cell system  10  can smoothly discharge the cathode gas from the cathode supply path  62  in the first control. Further, in the second control, an appropriate amount of cathode gas can be supplied to the fuel cell stack  12 . 
     Further, the control device (ECU  100 ) continues flowing of the anode gas to the fuel cell stack  12  during the execution of the first control. As a result, when the fuel cell system  10  returns to normal power generation after the first control has been performed, the fuel cell system  10  can avoid insufficient supply of the anode gas to the fuel cell stack  12  (i.e., it can ensure a sufficient stoichiometry).