JP2004172105A - Operation method of fuel cell system and fuel cell system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an operation method of a fuel cell system capable of avoiding deterioration of the fuel cell even if the fuel cell repeats stop and start, and to provide a fuel cell system for performing the method. <P>SOLUTION: The fuel cell system comprises a fuel cell having at least one cell comprising an electrolyte sandwiched between an anode and a cathode each having a platinum based metallic catalyst, and a pair of separators having a gas passage for supplying fuel gas to the anode and for supplying oxidant gas to the cathode. The operation method of the fuel cell system comprises stopping the supply of oxidant gas to the cathode with the anode exposed to the fuel gas when the fuel cell stops power generation (STEP 101), and applying a predetermined voltage between the anode and cathode using an outside power source(STEP 102) so that the potential of the cathode is controlled within a range from 0.6V to 0.8V to the standard hydrogen electrode. <P>COPYRIGHT: (C)2004,JPO

Description

  The present invention relates to a method of operating a fuel cell system including a fuel cell using a polymer electrolyte, and more particularly, to a method of operating a fuel cell system capable of suppressing deterioration of the fuel cell caused by starting and stopping the fuel cell, and The present invention relates to a fuel cell system for performing the method.

  A fuel cell using a polymer electrolyte generates electric power and heat simultaneously by electrochemically reacting a fuel gas containing hydrogen and an oxidizing gas such as air containing oxygen. FIG. 1 is a cross-sectional view schematically showing a configuration of an MEA (electrolyte membrane electrode assembly) provided in a unit cell (cell) of a solid polymer electrolyte fuel cell. As shown in FIG. 1, a catalyst obtained by supporting a platinum (Pt) -based metal catalyst on carbon powder on both surfaces of a polymer electrolyte membrane 11 for selectively transporting hydrogen ions, and a hydrogen ion conductive polymer A catalyst layer 12 composed of a mixture with an electrolyte is provided.

  At present, as the polymer electrolyte membrane 11, a polymer electrolyte membrane made of perfluorocarbon sulfonic acid (for example, a Nafion membrane manufactured by DuPont, USA) is generally used.

  On the outer surface of the catalyst layer 12, a gas diffusion layer 13 made of, for example, carbon paper subjected to a water-repellent treatment and having both air permeability and electron conductivity is formed. The catalyst layer 12 and the gas diffusion layer 13 are collectively called an electrode 14. Hereinafter, the electrode 14 to which the fuel gas is supplied is referred to as an anode, and the electrode 14 to which the oxidizing gas is supplied is referred to as a cathode.

  In order to prevent the fuel gas and the oxidizing gas supplied to the polymer electrolyte membrane 11 from leaking to the outside, and to prevent the two types of gases from being mixed, a high area is provided around the electrode 14. A gas seal material and a gasket are arranged with the molecular electrolyte membrane 11 interposed therebetween. The sealing material and the gasket are integrated with the electrode 14 and the polymer electrolyte membrane 11, and a combination thereof is called an MEA 15.

  FIG. 2 is a cross-sectional view schematically showing a configuration of a cell including the MEA shown in FIG. As shown in FIG. 2, a conductive separator plate 16 for mechanically fixing the MEA 15 is disposed outside the MEA 15. On the surface of the separator plate 16 on the side in contact with the MEA 15, a gas flow path 17 for supplying a reaction gas to the electrode 14 and carrying away generated gas and excess gas is formed. Although the gas flow path can be provided separately from the separator plate, a method of forming a gas flow path by providing a groove on the surface of the separator plate is generally used.

As described above, the MEA 15 is fixed by the pair of separator plates 16, the fuel gas is supplied to one gas passage 17, and the oxidizing gas is supplied to the other gas passage 17, so that several tens to several hundred mA are supplied. When a practical current density of / cm ² is applied, an electromotive force of about 0.7 to 0.8 V can be generated in one cell. However, when a fuel cell is used as a power source, a voltage of several volts to several hundreds of volts is usually required. Therefore, in practice, the required number of cells are connected in series.

  In order to supply the gas to the gas flow path, the pipe through which the gas supplied from the outside flows is branched into a number corresponding to the number of the separator plates 16, and the branch destination is directly formed in the groove on the separator plate 16. A jig for connecting piping is required. This jig is referred to as a manifold, and a manifold of a type in which the pipe for supplying gas as described above is directly connected to the groove of the separator plate 16 is referred to as an external manifold. Further, there is a type of this manifold called an internal manifold which has a simpler structure. The internal manifold is one in which a through hole is provided in a separator plate in which a gas flow path is formed, an inlet / outlet of the gas flow path is passed to this hole, and gas is directly supplied from the hole to the gas flow path.

  Next, the functions of the gas diffusion layer 13 and the catalyst layer 12 will be described. The gas diffusion layer 13 mainly has the following three functions. The first function is to diffuse the reaction gas in order to uniformly supply the reaction gas such as a fuel gas or an oxidizing gas to the catalyst in the catalyst layer 12 from the gas flow path located on the outer surface of the gas diffusion layer 13. It is a function to make it. The second function is a function of quickly discharging water generated by the reaction in the catalyst layer 12 to a gas flow path formed in the separator plate 16. The third function is to conduct electrons required for the reaction or generated electrons. Therefore, the gas diffusion layer 13 needs to have high reaction gas permeability, water discharge property, and electron conductivity.

  On the other hand, the catalyst layer 12 mainly has the following four functions. The first function is to supply a reaction gas such as a fuel gas or an oxidizing gas supplied from the gas diffusion layer 13 to a reaction site of the catalyst layer 12. The second function is to conduct hydrogen ions necessary for the reaction on the catalyst or generated hydrogen ions. The third function is to conduct electrons necessary for the reaction on the catalyst or generated electrons. The fourth function is to promote electrode reaction by having high catalytic performance. Therefore, the catalyst layer 12 requires high reaction gas permeability, hydrogen ion conductivity, electron conductivity, and catalytic performance.

  FIG. 3 is a cross-sectional view schematically showing a detailed configuration of the MEA. The gas diffusion layer 13 is generally configured using a conductive porous substrate such as carbon fine powder having a developed structure, a pore former, carbon paper, and carbon cloth. Thereby, the gas diffusion layer 13 has a porous structure, and as a result, high reaction gas permeability is obtained.

  It is common practice to disperse a water-repellent polymer represented by a fluororesin in the gas diffusion layer 13. As a result, a high water discharging property can be obtained.

  Further, the gas diffusion layer 13 is generally formed using an electron conductive material such as carbon fiber 105, metal fiber, and carbon fine powder. Thereby, high electron conductivity is obtained.

  On the other hand, as the catalyst carrier 104 in the catalyst layer 12, carbon fine powder having a developed structure and a pore former are generally used. Thereby, the catalyst layer 12 has a porous structure, and the gas channel 107 is formed, so that high reaction gas permeability can be obtained.

  Further, it is common practice to form a hydrogen ion network 108 by dispersing the polymer electrolyte 102 in the vicinity of the catalyst 103 in the catalyst layer 12. Thereby, high hydrogen ion conductivity is obtained.

  Further, it is general to use an electron conductive material such as carbon fine powder or carbon fiber for the catalyst carrier 104 in the catalyst layer 12. Thereby, the electron channel 106 is formed, so that high electron conductivity is obtained.

  Further, as the catalyst 103, a metal catalyst having a high reaction activity represented by Pt was used, and this metal catalyst was supported on carbon fine powder as very fine particles having a particle size of several nm. Is highly dispersed in the catalyst layer 12. Thereby, the catalytic performance of the catalyst layer 12 can be improved.

  In order for the power generation reaction to be stably performed over a long period of time in the fuel cell configured as described above, it is necessary that the interface between the polymer electrolyte membrane 11 and the electrode 14 is stably maintained for a long period of time. . In addition, since the polymer electrolyte membrane 11 has hydrogen ion conductivity when it is hydrated, it is necessary that the polymer electrolyte membrane 11 maintain a water retention state for a long time. In recent years, in a fuel cell, when a reaction gas is used in a relatively low humidification state, a phenomenon that the polymer electrolyte membrane is decomposed has been a problem. Therefore, it has been found that it is necessary to maintain the water retention state of the polymer electrolyte membrane in order to suppress such decomposition of the polymer electrolyte membrane.

  In order to solve such a problem, for example, an operation of a fuel cell system in which a fuel gas and an oxidizing gas remaining in a fuel cell are replaced with water or a humidified inert gas and the device is stopped in a sealed state is performed. A method has been proposed (for example, see Patent Document 1). By operating the fuel cell system in this manner, the water retention state of the polymer electrolyte membrane is maintained, so that when power is restarted, power generation can be started promptly. It is possible to do.

  In addition, the fuel cell in which power generation of the fuel cell is stopped with the supply of the oxidant gas stopped, oxygen of the cathode is consumed, and the fuel gas remaining on the anode is replaced with an inert gas such as nitrogen to stop the power generation of the fuel cell. An operation method of a battery system has been proposed (for example, see Patent Document 2). Thereby, not only can oxidation of the cathode be suppressed, but also hydrogen diffused from the anode through the polymer electrolyte can reduce and remove impurities attached to the cathode.

  By the way, when the cathode is in a high potential state due to the fact that the fuel cell holds a very high voltage close to an open circuit state exceeding 0.9 V, the elution of the Pt catalyst on the cathode and the It has been found that the ring (expansion of Pt particles) causes a problem that the reaction area of the Pt catalyst decreases.

  Similarly, when the fuel cell holds a very high voltage close to the open circuit state, a problem that the polymer electrolyte is decomposed also occurs. This is thought to be due to the following reasons.

The open circuit voltage of a fuel cell using hydrogen and oxygen as reactive species is theoretically 1.23V. However, the actual open circuit voltage depends on the mixed potential of impurities and adsorbed species at the respective anode and cathode poles, and is about 0.93 V to 1.1 V. In addition, the open circuit voltage becomes lower than the theoretical value due to the slight diffusion of hydrogen and oxygen in the polymer electrolyte membrane. Assuming that there is no dissolution of impurities such as extreme metal species, the potential of the anode is greatly affected by the adsorbed species of the cathode, and as shown in Reaction Formulas 1 to 5 as described in Non-Patent Document 1, Is considered to be a hybrid potential of various chemical reactions. In addition, the voltage shown corresponding to a reaction formula has shown the standard electrode electric potential with respect to SHE when the reaction shown by the said reaction formula occurred. In this case the anode potential is higher, radical hydroxide (OH ·), superoxide (O 2 _- ^·), and hydrogen radical (H ·) is a state that occurs in a high concentration, these radicals polyelectrolyte It is considered that the highly reactive part of the polymer is attacked and the polymer electrolyte is decomposed.
(Formula 1)
O ₂ + 4H ⁺ + 4e ⁻ = 2H ₂ O 1.23V
(Formula 2)
PtO ₂ + 2H ⁺ + 2e ⁻ = Pt (OH) ₂ 1.11 V
(Formula 3)
Pt (OH) ₂ + 2H ⁺ + 2e ⁻ = Pt + 2H ₂ O 0.98 V
(Formula 4)
PtO + 2H ⁺ + 2e ⁻ = Pt + H ₂ O 0.88 V
(Formula 5)
O ₂ + 2H ⁺ + 2e ⁻ = H ₂ O ₂ 0.68 V
In order to avoid the above-described problem that occurs when the fuel cell has an open circuit voltage, some operating methods of the fuel cell system have been conventionally proposed.

  For example, a power consuming means for consuming power in the fuel cell system separately from the external load is provided, and the fuel cell is connected between the fuel cell and the external load after the fuel cell starts generating power. And the power consuming means are connected to each other, so that the power generated in the fuel cell is consumed by the power consuming means, and as a result, the fuel cell can be prevented from becoming an open circuit voltage. (For example, see Patent Document 1).

Further, a method of operating a fuel cell system has been proposed in which a discharge unit for suppressing an open circuit voltage is provided in a fuel cell system, thereby preventing the fuel cell from becoming an open circuit voltage (eg, And Patent Document 2).
With these fuel cell system operating methods, the elution of the Pt catalyst at the cathode and the reduction of the reaction area of the catalyst due to sintering as described above can be avoided. Moreover, the situation where the polymer electrolyte is decomposed due to the generation of radicals can be avoided.
JP-A-6-251788 JP-A-2002-93448 JP-A-5-251101 JP-A-8-222258 H. Wroblowa, et al., J. Electroanal.Chem., 15, p139-150 (1967), "Adosorption and Kinetics at Platinum Electrodes in The Presence of Oxygen at Zero Net Current"

  However, in the case of the above-described operation method of the fuel cell system which purges with an inert gas such as nitrogen, a gas cylinder of the inert gas is required, which not only increases the size of the fuel cell system but also reduces maintenance such as cylinder replacement. Since the cost is high, there is a problem that the cost is increased.

  Further, in the above-described operation method of the fuel cell system in which the fuel cell is purged with water or a humidified inert gas, the temperature of the fuel cell decreases when the power generation of the fuel cell is stopped. Reduction occurs. Therefore, since the inside of the fuel cell has a negative pressure, there is a problem that oxygen may flow in from the outside, or the polymer electrolyte membrane may be damaged and the electrodes may be short-circuited.

  Further, as described above, when the cell is powered while the supply of the oxidizing gas is stopped, and the anode is purged with an inert gas after consuming the oxygen of the cathode, the remaining oxygen that cannot be completely consumed by the cathode, and Since the Pt catalyst of the cathode is oxidized by air mixed due to diffusion and leakage, the cathode is deteriorated. In addition, since the power is generated and the oxygen is forcibly consumed, the potential of the cathode is not uniform, and the activation state of the cathode is different each time the power generation of the fuel cell is stopped. Was.

  Further, in the case of the above-described method of operating the fuel cell system in which the fuel cell is prevented from reaching an open circuit voltage, the fuel cell is always in a state of generating power. However, in the case of a household fuel cell system that uses a source gas such as city gas containing methane as a main component, power generation is stopped during periods of low electricity consumption, and electricity consumption is reduced, in order to reduce energy costs. It is desirable to control the operation of the fuel cell so as to generate power in a large time zone. For example, a DSS (Daily Start-up & Shut-down) operation in which power is generated during the daytime and power generation is stopped at midnight can avoid an increase in utility costs. Therefore, it is desirable that the operation of the fuel cell be controlled so that the power generation state and the non-power generation state are repeated, and even in such an operation pattern, the fuel cell system can avoid the open circuit voltage of the fuel cell. The driving method is desirable.

  The present invention has been made in view of such circumstances, and has as its object to control oxidation and reduction of a cathode by controlling the potential of the cathode within a certain range when the fuel cell stops generating power. It is an object of the present invention to provide a fuel cell system operating method capable of suppressing deterioration due to fuel cell and improving durability, and a fuel cell power generation system for implementing the method.

  FIG. 4 is a cyclic voltammogram of Pt. In FIG. 4, the vertical axis indicates the current value, and the horizontal axis indicates the potential with respect to the standard hydrogen electrode (SHE). As shown in FIG. 4, the oxidation of Pt starts at a potential of about 0.7 V with respect to SHE and peaks at a potential of about 0.8 V with respect to SHE. Here, when the potential is further increased, the oxidation of Pt proceeds and the oxidation from divalent to tetravalent proceeds.

  On the other hand, the reduction of the Pt oxide peaks at a potential of about 0.7 V with respect to SHE, and progresses to a potential of about 0.5 V with respect to SHE.

  In the case of an ordinary stationary type or the like, the operating voltage of the fuel cell is around 0.7 to 0.75V. Here, since the potential of the anode when the fuel cell is generating power is a value close to SHE, the potential of the cathode is substantially the same as the operating voltage of the fuel cell. Therefore, as can be seen from the cyclic voltammogram shown in FIG. 4, when the fuel cell is generating power, it is considered that the surface of the cathode Pt is in an oxidized state.

  When the fuel gas and the oxidizing gas are supplied to the fuel cell to stop the current, the potential of the cathode rises to about 1 V, the oxidation further proceeds to the inside of Pt, and the catalytic activity decreases. On the other hand, when the potential of the cathode is maintained at a low potential in order to restore the characteristics of the oxidized Pt catalyst, the surface of Pt is reduced and the catalytic activity is restored.

  However, as described above, when redox is repeated on the surface of Pt, fluctuations occur on the surface of Pt, and expansion / contraction of the surface, rearrangement of atoms, and the like occur, and as a result, the catalytic activity gradually decreases. It is going down.

  Therefore, even when the fuel cell repeats the power generation state and the non-power generation state, it is necessary to prevent the oxidation-reduction from being repeated on the surface of the cathode Pt.

  Therefore, the operating method of the fuel cell system according to the present invention comprises an electrolyte, an anode and a cathode having a platinum-based metal catalyst sandwiching the electrolyte, supplying a fuel gas to the anode, and supplying an oxidizing gas to the cathode. In a method of operating a fuel cell system including a fuel cell having at least one cell including a pair of separator plates having a gas flow path formed therein, when the fuel cell has stopped generating power, The potential of the cathode is controlled within a range of 0.6 to 0.8 V with respect to a standard hydrogen electrode.

  Thereby, even if the fuel cell repeatedly switches between the power generation state and the non-power generation state, it is possible to prevent the cathode Pt from being repeatedly oxidized and reduced, thereby preventing deterioration of the fuel cell.

  Further, in the operating method of the fuel cell system according to the present invention, in the state where the anode is exposed to the fuel gas, the supply of the oxidizing gas to the cathode is stopped, and an external power source is used to connect the anode and the cathode. It is preferable to control the potential of the cathode within a range of 0.6 to 0.8 V with respect to a standard hydrogen electrode by applying a predetermined voltage to the cathode.

  By using the external power supply while the anode is exposed to the fuel gas, the potential of the cathode can be easily controlled.

  Further, in the operating method of the fuel cell system according to the present invention, the fuel cell is a fuel cell stack in which a plurality of cells are stacked, and the cathode of each cell is exposed while the anode of each cell is exposed to a fuel gas. By stopping the supply of the oxidizing gas to the cell, and applying a predetermined voltage between the anode and the cathode of each cell using an external power supply, the potential of the cathode of each cell with respect to a standard hydrogen electrode It is preferable to control within the range of 0.6 to 0.8 V.

  Further, in the operating method of the fuel cell system according to the present invention, the potential of the cathode is changed with respect to the standard water cable electrode until the battery temperature of the fuel cell drops to 50 ° C. or less after the fuel cell stops generating power. It is preferable to control within the range of 0.6 to 0.8 V.

  Further, in the operating method of the fuel cell system according to the present invention, when the fuel cell stops generating power, and when the battery temperature of the fuel cell drops to 50 ° C. or less, the fuel cell Preferably, a purge with air is performed.

  In the method for operating a fuel cell system according to the present invention, it is preferable that the purging is performed using dry air.

  Further, the fuel cell system of the present invention provides an electrolyte, an anode and a cathode having a platinum-based gold delivery catalyst, sandwiching the electrolyte, supplying a fuel gas to the anode, and supplying an oxidizing agent to the cathode. A fuel cell having at least one cell including a pair of separator plates having a gas flow path formed therein, and a control device that controls supply of fuel gas to the anode and supply of oxidant gas to the cathode. A fuel cell system comprising: an external power supply for applying a predetermined voltage between the anode and the cathode; wherein the control device controls the anode to emit a fuel gas when the fuel cell stops generating power. While the supply of oxidant gas to the cathode is stopped, and a predetermined voltage is applied between the anode and the cathode. By controlling the operation of the external power supply so as to apply the voltage, the potential of the cathode is controlled to be in a range of 0.6 to 0.8 V with respect to a standard hydrogen electrode. I do.

  Further, in the fuel cell system according to the present invention, the fuel cell is a fuel cell stack in which a plurality of cells are stacked, and the control device controls each cell when the fuel cell stack stops generating power. While the anode is exposed to the fuel gas, the supply of the oxidizing gas to the cathode of each cell is stopped, and the external power supply is applied so that a predetermined voltage is applied between the anode and the cathode of each cell. By controlling the operation described above, it is preferable to control the potential of the cathode of each cell within a range of 0.6 to 0.8 V with respect to a standard hydrogen electrode.

  In the fuel cell system according to the present invention, the fuel cell system further includes a temperature sensor for measuring a cell temperature of the fuel cell, wherein the control device controls the temperature of the fuel cell measured by the temperature sensor to be 50 ° C. It is determined whether or not the temperature has dropped to below 50 ° C., and as a result, when it is determined that the cell temperature of the fuel cell has dropped to 50 ° C. or less, the application of the voltage between the anode and the cathode is stopped so that the external voltage is stopped. It is preferable to control the operation of the power supply.

  Further, in the fuel cell system according to the present invention, the control device determines whether or not the battery temperature of the fuel cell measured by the temperature sensor has decreased to 50 ° C. or less, and as a result, the battery of the fuel cell When it is determined that the temperature has dropped to 50 ° C. or lower, it is preferable to perform air purging on the cathode and the anode.

  Further, in the fuel cell system according to the present invention, it is preferable that the purging is performed using dry air.

  The operating method of the fuel cell system of the present invention and the fuel cell system for implementing the method can avoid the deterioration of the fuel cell even when the fuel cell repeats the power generation state and the non-power generation state. .

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

(Embodiment 1)
FIG. 5 is a block diagram showing an example of a configuration of the fuel cell system according to Embodiment 1 of the present invention. In FIG. 5, reference numeral 301 denotes a fuel cell stack. The fuel cell stack 301 is configured by stacking a plurality of cells 31, 31,... Each cell 31 includes a pair of electrodes, an anode 32 and a cathode 33, and is connected in series.

  The configuration of the fuel cell stack 301 is the same as that of a normal polymer electrolyte fuel cell stack. Therefore, a polymer electrolyte membrane is disposed between the anode 32 and the cathode 33. The anode 32 and the cathode 33 are composed of a gas diffusion layer and a catalyst layer, and the catalyst layer has a Pt catalyst.

  The fuel cell stack 301 is connected to a load 306 and a cell voltage detecting device 304 for detecting a voltage of each cell 31. Further, the fuel cell stack 301 is connected to an external power supply 307 used to control the potential of the cathode 33 as described later.

  The anode 32 of each cell 31 is connected to a fuel gas control device 302 for controlling the supply of fuel gas. On the other hand, the cathode 33 of each cell 31 is connected to an oxidizing gas control device 303 for controlling the supply of the oxidizing gas.

  The above-described fuel gas control device 302, oxidizing gas control device 303, cell voltage detection device 304, load 306, and external power supply 307 are connected to the control device 305.

  The control device 305 controls the operation of the fuel gas control device 302 in order to start / stop the supply of the fuel gas at an appropriate timing. Similarly, the control device 305 controls the operation of the oxidizing gas control device 303 in order to start / stop the supply of the oxidizing gas at an appropriate timing.

  Further, the control device 305 receives a signal indicating the voltage of each cell 31 detected by the cell voltage detection device 304 from the cell voltage detection device 304, and based on the voltage of each cell 31 indicated in the received signal, The operation of the external power supply 307 is controlled as described later.

  Further, the control device 305 monitors the state of the load 306, and controls the operation of devices such as the fuel gas control device 302 and the oxidizing gas control device 303 according to the state of the load 306.

  The above system configuration is an example of controlling the voltage of the entire fuel cell stack 301 when controlling the potential of the cathode 33 as described later. However, the voltage of each cell 31 can be controlled as follows. The system configuration may be simple.

  FIG. 6 is a block diagram showing another example of the configuration of the fuel cell system according to Embodiment 1 of the present invention. As shown in FIG. 6, the cell voltage detecting device 304 is connected to each cell 31 constituting the fuel cell stack 301. Also, the external power supplies 307 are provided in a number corresponding to each cell 31, and each external power supply 307 and each cell 31 are connected.

  As shown in FIG. 5, in the case of a system configuration for controlling the voltage of the entire fuel cell stack 301, the cell voltage detecting device 304 does not need to be connected to each cell 31, and one external power supply 307 is sufficient. This has the advantage that the system configuration can be simplified. However, when the resistance value between the cells 31 varies, there is a disadvantage that the voltage of each cell 31 cannot be accurately controlled.

  On the other hand, as shown in FIG. 6, in the case of a system configuration in which the voltage of each cell 31 can be controlled, the cell voltage detecting device 304 must be connected to each cell 31 and the external power supply 307 is connected to There is a drawback that the system configuration becomes complicated, such as a need to provide a plurality. However, since the voltage of each cell 31 can be controlled with higher accuracy, the improvement of the durability of each cell 31 described later can be further expected.

  These system configurations are appropriately selected according to the required durability, cost, and the like.

  Next, a procedure when the fuel cell stack 301 stops generating power in the fuel cell system of the present embodiment configured as described above will be described.

  The control device 305 included in the fuel cell system according to the present embodiment monitors the state of the load 306 as described above. When it is detected that the load 306 has stopped, the following processing is executed to stop the power generation of the fuel cell stack 301.

  FIG. 7 is a flowchart illustrating a processing procedure of the control device 305 included in the fuel cell system according to Embodiment 1 of the present invention when the fuel cell stack stops generating power. First, the control device 305 controls the operation of the oxidizing gas control device 303 so as to stop the supply of the oxidizing gas to the cathode 33 (S101). As a result, the oxidant gas is not supplied to the cathode 33 of each cell 31.

  In this state, the control device 305 controls the fuel gas control device 302 to supply the fuel gas to the anode 32 in the same manner as when the fuel cell stack 301 is generating power. As a result, the potential of the anode 32 when the fuel cell stack 301 is generating power is maintained.

As described above, the potential of the anode when the fuel cell is generating power has a value close to SHE. Specifically, for example, when a current value of 0.2 A / cm ² flows in the fuel cell, the anode is polarized by about 10 to 20 mV with respect to SHE. Therefore, even when the power generation of the fuel cell stack 301 is stopped as described above, by supplying the fuel gas to the anode 32, the potential of the anode 32 can be maintained at a value close to SHE.

  When a Pt-Ru (ruthenium) alloy is used as a catalyst in the anode 32, when the potential of the anode 32 increases, Ru is oxidized and eluted from the Pt-Ru alloy, and the fuel cell stack 301 is deteriorated. However, in this embodiment, since the potential of the anode 32 is kept at a low value, such deterioration can be avoided.

  In the present embodiment, the control device 305 controls the operation of the fuel gas control device 302 so as to continuously supply the fuel gas to the anode 32, but the state in which the anode 32 is exposed to the fuel gas For example, a valve that can be opened and closed by the control device 305 is provided on a pipe connecting the anode 32 and the fuel gas control device 302, and the control device 305 opens the valve at an appropriate timing. By closing, a state in which the fuel gas is sealed may be realized.

  In order to prevent the potential of the anode 32 from fluctuating due to a change in the gas composition in the anode 32, it is desirable to continue to supply the fuel gas to the anode 32, but in a state where the fuel cell stack 301 is not generating power. Supplying the fuel gas to the anode 32 wastes the fuel gas. When the fuel gas is sealed as described above, the hydrogen leaked to the cathode 33 is returned to the anode by application of a voltage from an external power supply, so that the change in the gas composition in the anode 32 is relatively small.

  Next, the control device 305 controls the external power supply 307 so as to apply a voltage of 0.6 V or more and 0.8 V or less between the anode 32 and the cathode 33 (S102). As a result, the potential of the cathode 33 of each cell 31 becomes about 0.6 V or more and 0.8 V or less with respect to SHE. At this time, since the supply of the oxidizing gas to the cathode 33 is stopped, the current flowing to maintain the potential of the cathode 33 becomes small after the oxygen in the cathode 33 is consumed.

  The control device 305 sets the potential of the cathode 33 of each cell 31 to about 0.6 V or more with respect to SHE based on the signal output from the cell voltage detection device 304 until the fuel cell stack 301 starts power generation. The operation of the external power supply 307 is controlled so as to keep the state of 0.8 V or less.

  The control unit 305 executes the above-described processing, so that the potential of the cathode 33 is maintained at about 0.6 V or more and 0.8 V or less with respect to SHE while the fuel cell stack 301 stops generating power. can do. As a result, it is possible to avoid the situation where the Pt of the cathode 33 repeats the oxidation-reduction, so that the deterioration of the Pt catalyst of the cathode 33 can be prevented.

[Example 1]
Embodiment 1 described below will be described with reference to FIGS. 5 and 6 as appropriate.

  As Example 1, a fuel cell stack 301 was manufactured as follows.

  Acetylene black (denka black manufactured by Denki Kagaku Kogyo Co., Ltd., particle size 35 nm), which is a carbon powder, is mixed with an aqueous dispersion of polytetrafluoroethylene (PTFE) (D1 manufactured by Daikin Industries, Ltd.) and the dry weight A water-repellent ink containing 20% by weight of PTFE was prepared. This ink is applied and impregnated on carbon paper (TGPH060H manufactured by Toray Industries, Inc.) as a base material of the gas diffusion layer, and is heat-treated at 300 ° C. using a hot air drier to obtain a gas having a thickness of about 200 μm. A diffusion layer was formed.

  On the other hand, 66 parts by weight of a catalyst body (50% by weight of Pt) obtained by supporting a Pt catalyst on Ketjen black (Ketjen B1ack EC, particle size 30 nm, manufactured by Ketjen Black International Co., Ltd.) which is a carbon powder was used. , 33 parts by weight (polymer dry weight) of a perfluorocarbon sulfonic acid ionomer (5% by weight Nafion dispersion manufactured by Aldrich, USA) as a hydrogen ion conductive material and a binder, and the resulting mixture was molded. A catalyst layer having a thickness of 10 to 20 μm was formed.

  By joining the gas diffusion layer and the catalyst layer obtained as described above to both sides of a polymer electrolyte membrane (Nafion 112 membrane manufactured by DuPont, USA), an MEA having the same configuration as that shown in FIG. 1 is obtained. Produced. Here, an electrode composed of a gas diffusion layer and a catalyst layer disposed on one side of the polymer electrolyte membrane serves as an anode 32, and an electrode composed of a gas diffusion layer and a catalyst layer disposed on the other side serves as a cathode 33. .

  Next, a rubber gasket plate was joined to the outer peripheral portion of the polymer electrolyte membrane of the MEA manufactured as described above, and a manifold hole for passing cooling water, fuel gas, and oxidizing gas was formed. .

  In addition, a conductive plate made of a graphite plate impregnated with a phenol resin and having a gas channel and a cooling water channel having an outer dimension of 20 cm × 32 cm × 1.3 mm and a depth of 0.5 mm is formed. A separator plate was prepared. By using two separator plates, a separator plate having an oxidizing gas channel formed on one surface of the MEA and a separator plate having a fuel gas channel formed on the other surface are stacked. Got.

On the surface of the separator plate opposite to the MEA side, a groove serving as a flow path of the cooling water is formed. By stacking two cells, the cooling water flows between the MEAs. Was obtained. By repeating this pattern, a fuel cell stack 301 in which 50 cells were stacked was produced. At this time, a current collector plate made of stainless steel, an insulating plate made of an electrically insulating material, and an end plate were arranged at both ends of the fuel cell stack 301, and the whole was fixed with a fastening rod. The fastening pressure at this time was 10 kgf / cm ² per area of the separator plate.

  The following evaluation test was performed using the fuel cell stack 301 manufactured as described above.

  First, the fuel gas control device 302 supplies the fuel gas obtained by reforming the 13A gas, which is the raw material gas, with the reformer to the anode 32, and the oxidizing gas control device 303 Air is supplied to the cathode 33. Then, a discharge test was performed under the conditions that the cell temperature of the fuel cell stack 301 was 70 ° C., the fuel gas utilization rate (Uf) was 75%, and the air utilization rate (Uo) was 40%. The fuel gas and the air are humidified so as to have dew points of 65 ° C. and 70 ° C., respectively.

A state in which the fuel cell stack 301 is continuously generated with a continuous load of a current density of 200 mA / cm ² in a state where air and fuel gas are continuously supplied, and a non-power generation state are repeated, before and after the start and stop of power generation. The voltage of each cell 31 of the fuel cell stack 301 was measured.

  FIG. 8 is a flowchart illustrating a flow of the operation of the fuel cell system in the evaluation test of the first embodiment. First, the fuel cell stack 301 was made to generate power, and power was supplied to the load 306 for one hour, and then the load 306 was stopped (S201). Next, the control device 305 controlled the operation of the oxidizing gas control device 303 so as to stop the supply of air to the cathode 33 of each cell 31 (S202). At this time, the supply of the fuel gas to the anode 32 is continuously performed.

  Next, a predetermined voltage was applied between the anode 32 and the cathode 33 using the external power supply 307 so that the potential difference between the anode 32 and the cathode 33 of each cell 31 became a predetermined value (S203).

  As described above, the state where the potential difference between the anode 32 and the cathode 33 becomes a predetermined value was maintained for one hour. As a result, the cell temperature of the fuel cell stack 301 dropped to 40 ° C.

  Thereafter, the controller 305 raises the battery temperature of the fuel cell stack 301 (S204), and controls the oxidizing gas controller 303 to supply air to the cathode 33 when the battery temperature reaches 65 ° C. The operation is controlled (S205), and the power generation of the fuel cell stack 301 is started (S206).

  By repeatedly executing the above-described steps S201 to S206, the start / stop of the fuel cell stack 301 was repeated.

  FIG. 9 is a graph for explaining the time when the voltage of the cell 31 is measured. In FIG. 9, the vertical axis represents the voltage of the cell 31, and the horizontal axis represents the elapsed time. As described above, the power generation is stopped one hour after the power generation of the fuel cell stack 301 is started, and then the power generation is started when the battery temperature of the fuel cell stack 301 reaches 65 ° C. Then, as shown in FIG. 9, the voltage of the cell 31 is measured 30 minutes after the start of the power generation.

  In FIG. 9, A indicates a voltage drop amount which is a difference between the voltage of the cell 31 at the time of the previous measurement and the time of the current measurement. Here, the amount of voltage decrease per start / stop of the fuel cell stack 301 is defined as a voltage deterioration rate.

  In the above-described step S203, the voltage applied between the anode 32 and the cathode 33, in other words, the voltage held by the cell 31 is changed to 0.2V, 0.4V, 0.6V, 0.7V, 0.8V, The voltage was set to 0.9 V, and steps S201 to S206 were repeated 200 times. Table 1 shows the obtained relationship between the holding voltage of the cell 31 and the voltage deterioration rate.

  As is clear from Table 1, when the holding voltage of the cell 31 exceeds 0.8 V, the voltage drops significantly before and after the start and stop of the fuel cell stack 301. In this case, since the reduction of Pt does not occur at the cathode 33, the recovery of the catalytic activity does not occur. Therefore, as the fuel cell stack 301 repeatedly starts and stops, the voltage of the cells 31 deteriorates.

  On the other hand, when the holding voltage of the cell 31 becomes 0.7 V or less, the recovery of the catalyst activity due to the reduction of the Pt of the cathode 33 occurs before and after the start and stop of the fuel cell stack 301, so that the fuel cell stack 301 starts power generation. Immediately after, the voltage of the cell 31 increases. When the holding voltage of the cell 31 is 0.6 V or less, the recovery of the same catalytic activity becomes more remarkable.

  However, it is confirmed that when the fuel cell stack 301 is repeatedly started and stopped, the degree of recovery gradually decreases. This is considered to be due to deterioration of the Pt catalyst due to repeated oxidation reduction of Pt. Therefore, when the holding voltage of the cell 31 is smaller than 0.6 V, the voltage of the cell 31 deteriorates.

  In the case of the present embodiment, as described above, when the fuel cell stack 301 stops generating power, the external power supply 307 is used to apply a voltage of 0.6 V to 0.8 V between the anode 32 and the cathode 33. Apply voltage. In this case, although the recovery of the catalyst activity as described above is smaller than when the holding voltage of the cell 31 is smaller than 0.6 V, the voltage deterioration rate becomes as low as 10 μV / times or less. It is considered that the stack 301 is least deteriorated.

(Embodiment 2)
The fuel cell power generation system according to the second embodiment controls the potential of the cathode for a predetermined period of time when the fuel cell stops generating power, and then performs air purging on the anode and the cathode. In addition, when purging with an inert gas such as nitrogen as in the related art, a gas cylinder of the inert gas is required, so there is a problem that the fuel cell system becomes large, but in the present embodiment, Such a problem does not occur because the air is purged.

  FIG. 10 is a block diagram showing an example of the configuration of the fuel cell system according to Embodiment 2 of the present invention. In FIG. 10, reference numeral 401 denotes a temperature sensor for detecting the cell temperature of the fuel cell stack 301. The temperature sensor 401 is connected to the control device 305. The control device 305 receives a signal indicating the cell temperature of the fuel cell stack 301 output from the temperature sensor 401, and based on the received signal, stops processing for controlling the potential of the cathode as described later, and , An air purge for the anode 32 and the cathode 33 is performed.

  The system configuration shown in FIG. 10 is an example in which the voltage of the entire fuel cell stack 301 is controlled when controlling the potential of the cathode 33, but the voltage of each cell 31 is controlled in the same manner as in the first embodiment. It is needless to say that a controllable system configuration may be used.

  Note that other configurations of the fuel cell system according to the present embodiment are the same as those in the first embodiment, and a description thereof will not be repeated.

  In the fuel cell system of the present embodiment configured as described above, after the fuel cell stack 301 stops generating power, the fuel gas is supplied to the anode 32 as in the case of the first embodiment. A predetermined voltage is applied between the anode 32 and the cathode 33 using the external power supply 307. Thereafter, in the fuel cell system of the present embodiment, when the battery temperature of the fuel cell stack 301 decreases to a predetermined temperature, the air purge is performed on the anode 32 and the force sword 33 of the cell 31.

  FIG. 11 is a flowchart illustrating a processing procedure of the control device 305 included in the fuel cell system according to Embodiment 2 of the present invention when the fuel cell stack stops generating power. First, as in the first embodiment, the control device 305 controls the operation of the oxidizing gas control device 303 so as to stop the supply of the oxidizing gas to the cathode 33 (S301). The operation of the external power supply 307 is controlled so that a voltage of 0.6 V or more and 0.8 V or less is applied to the cathode 33 (S302). As a result, the potential of the cathode 33 of each cell 31 becomes about 0.6 V or more and 0.8 V or less with respect to SHE.

  Next, the control device 305 specifies the battery temperature of the fuel cell stack 301 based on a signal output from the temperature sensor 401 at an appropriate timing, and determines whether the battery temperature is equal to or lower than a predetermined threshold. (S303).

  If it is determined in step S303 that the battery temperature of fuel cell stack 301 is higher than the predetermined threshold (NO in S303), control device 305 again performs step S303 based on the signal output from temperature sensor 401. Execute. On the other hand, when it is determined that the battery temperature of the fuel cell stack 301 is equal to or lower than the predetermined threshold (YES in S303), the control device 305 operates the fuel gas control device 302 to stop the supply of the fuel gas to the anode 32. Is controlled (S304), and the operation of the external power supply 307 is controlled so as to stop the application of the voltage between the anode 32 and the cathode 33 (S305).

  Next, the control device 305 performs an air purge with dry air on the anode 32 and the cathode 33 (S306). By performing the air purge on the anode 32 and the cathode 33 in this manner, it is possible to avoid a situation in which oxygen and hydrogen react with each other and the inside of the fuel cell stack 301 becomes negative pressure.

  Hereinafter, how much the above-described threshold value should be set will be described. As described above, by controlling the potential of the force source 33 within the range of 0.6 V or more and 0.8 V or less with respect to SHE, it is possible to avoid the deterioration of Pt of the cathode 33. It is preferable that the period for controlling the potential of 33 is longer.

  However, since the potential of the cathode 33 is controlled by the external power supply 307, the longer the period of controlling the potential of the force source 33, the greater the energy loss.

  In addition, the battery temperature of the twisted-charge battery stack 301 decreases in accordance with the period for controlling the potential of the cathode 33. However, the lower the battery temperature of the fuel cell stack 301, the smaller the amount of carbon monoxide contained in the fuel gas. The anode 31 is easily poisoned.

  Therefore, in the case of the fuel cell system according to the present embodiment, the threshold value is set to 50 ° C., and the potential of the cathode 33 is maintained until the battery temperature of the fuel cell stack 301 decreases to 50 ° C. after the fuel cell stack 301 stops generating power. Control is performed so as to be within a certain range, and thereafter, the control is stopped, and air purge is performed on the anode 32 and the cathode 33. This makes it possible to avoid the deterioration of the fuel cell stack 301 while suppressing the energy loss due to the external power supply 307.

[Example 2]
Using the fuel cell stack 301 manufactured in the same manner as in Example 1, the following evaluation test was performed.

  First, the fuel gas control device 302 supplies the fuel gas obtained by reforming the 13A gas, which is the raw material gas, with the reformer to the anode 32, and the oxidizing gas control device 303 Air is supplied to the cathode 33. Then, a discharge test was performed under the conditions that the cell temperature of the fuel cell stack 301 was 70 ° C., the fuel gas utilization rate (Uf) was 75%, and the air utilization rate (Uo) was 40%. The fuel gas and the air are humidified so as to have dew points of 65 ° C. and 70 ° C., respectively.

A state in which the fuel cell stack 301 is continuously generated with a continuous load of a current density of 200 mA / cm ² in a state where air and fuel gas are continuously supplied, and a non-power generation state are repeated, before and after the start and stop of power generation. The voltage of each cell 31 of the fuel cell stack 301 was measured.

  FIG. 12 is a flowchart illustrating a flow of operation of the fuel cell system in the evaluation test of the second embodiment. First, the fuel cell stack 301 was caused to generate electric power, and power was supplied to the load 306 for one hour, and then the load 306 was stopped (S401). Next, the control device 305 controlled the operation of the oxidizing gas control device 303 so that the supply of air to the cathode 33 of each cell 31 was stopped (S402). At this time, the supply of the fuel gas to the anode 32 is continuously performed.

  Next, a predetermined voltage was applied between the anode 32 and the cathode 33 using the external power supply 307 so that the potential difference between the anode 32 and the cathode 33 of each cell 31 became a predetermined value (S403). Thereby, the state where the potential difference between the anode 32 and the cathode 33 becomes a predetermined value was continued.

  Next, based on a signal output from the temperature sensor 401, the control device 305 determines whether or not the battery temperature of the fuel cell stack 301 is equal to or lower than a predetermined threshold (S404). Is determined to be equal to or smaller than the predetermined threshold (YES in S404), step S404 was repeatedly executed.

  When it is determined that the battery temperature of the fuel cell stack 301 is equal to or lower than the predetermined threshold (YES in S404), the control device 305 causes the operation of the fuel gas control device 302 to stop supplying the fuel gas to the anode 32. The operation of the external power supply 307 was controlled so as to stop the application of the voltage between the anode 32 and the cathode 33 (S405) (S406). Thereafter, an air purge with dry air was performed on the anode 32 and the cathode 33 for 30 minutes (S407).

  Next, the control device 305 controls the operations of the fuel gas control device 302 and the oxidizing gas control device 303 so as to supply the fuel gas to the anode 32 and the air to the cathode 33, respectively (S408). The power generation of the fuel cell stack 301 was started (S409). In step S404 described above, the threshold values were set to 30 ° C., 40 ° C., 50 ° C., 60 ° C., and 70 ° C., and steps S401 to S408 were repeated 200 times. Here, as in the case of Example 1, the voltage of the cell 31 was measured 30 minutes after the fuel cell stack 301 started power generation. Table 2 shows the relationship between the resulting threshold and the voltage deterioration rate.

  As shown in Table 2, the lower the threshold value, the lower the voltage deterioration rate. However, as described above, the lower the threshold value, the greater the energy loss due to the external power supply 307. Therefore, by setting the threshold to 50 ° C. as in the present embodiment, it is possible to prevent the fuel cell stack 301 from deteriorating while suppressing energy loss due to the external power supply 307.

[Example 3]
In the second embodiment, the anode 32 and the cathode 33 are purged with dry air. In Example 3, the following evaluation test was performed in order to compare the case where the air purge for the anode 32 and the cathode 33 was performed with humidified air and the case where the air purge was performed with dry air.

  As in the case of the second embodiment, after the load 306 is stopped and the supply of air to the cathode 33 of each cell 31 is stopped, the potential difference between the anode 32 and the cathode 33 of each cell 31 becomes a predetermined value. Then, a predetermined voltage was applied between the anode 32 and the cathode 33 using the external power supply 307. Thereby, the state where the potential difference between the anode 32 and the cathode 33 becomes a predetermined value was continued.

  Then, when the battery temperature of the fuel cell stack 301 dropped to 40 ° C., the supply of the fuel gas to the anode 32 and the application of the voltage between the anode 32 and the cathode 33 were stopped. Next, the anode 32 and the cathode 33 were air-purged with humidified air and dry operation air, and were left for 120 hours. Thereafter, the fuel cell stack 301 was restarted to start power generation, and the cycle of measuring the voltage of the cells 31 was performed for 10 cycles.

  Table 3 shows the results of the above evaluation tests.

  As shown in Table 3, the voltage deterioration rate was lower when air purging was performed with dry air than when air purging was performed with humidified air. It is considered that this is because when the air purge was performed with dry air, the amount of water in the catalyst layers of the anode 32 and the cathode 33 was reduced, so that the oxidation reaction of Pt was suppressed.

  From the above results, it is understood that when air purging is performed on the anode 32 and the cathode 33 in the present embodiment, it is more preferable to use dry air than to use humidified air.

  The operation method and the fuel cell system of the fuel cell system according to the present invention can avoid deterioration of the fuel cell even when the fuel cell repeats the power generation state and the non-power generation state. The present invention is useful for a fuel cell system for an automobile that is performed, a fuel cell system that is started and stopped every day, and the like.

FIG. 2 is a cross-sectional view schematically illustrating a configuration of an MEA (electrolyte membrane electrode assembly) provided in a unit cell (cell) of a solid polymer electrolyte fuel cell. FIG. 2 is a cross-sectional view schematically illustrating a configuration of a cell including the MEA illustrated in FIG. 1. It is sectional drawing which shows the detailed structure of MEA typically. It is a cyclic voltammogram of Pt. FIG. 1 is a block diagram illustrating an example of a configuration of a fuel cell system according to Embodiment 1 of the present invention. FIG. 2 is a block diagram showing another example of the configuration of the fuel cell system according to Embodiment 1 of the present invention. 6 is a flowchart illustrating a processing procedure of a control device provided in the fuel cell system according to Embodiment 1 of the present invention when the fuel cell stack stops generating power. 4 is a flowchart illustrating a flow of operation of the fuel cell system in an evaluation test according to the first embodiment. 6 is a graph for explaining a time when a cell voltage is measured. FIG. 7 is a block diagram illustrating an example of a configuration of a fuel cell system according to Embodiment 2 of the present invention. 9 is a flowchart illustrating a processing procedure of a control device provided in the fuel cell system according to Embodiment 2 of the present invention when the fuel cell stack stops generating power. 9 is a flowchart illustrating a flow of operation of the fuel cell system in an evaluation test according to the second embodiment.

Explanation of reference numerals

Reference Signs List 11 polymer electrolyte membrane 12 catalyst layer 13 gas diffusion layer 14 electrode 15 MEA
Reference Signs List 16 separator plate 17 gas flow path 18 cooling water flow path 31 cell 32 anode 33 cathode 301 fuel cell stack 302 fuel gas control device 303 oxidizing gas control device 304 cell voltage detection device 305 control device 306 load 307 external power supply 401 temperature sensor

Claims (11)

An electrolyte, an anode and a cathode having a platinum-based metal catalyst sandwiching the electrolyte, and a pair of separators formed with a gas flow path for supplying a fuel gas to the anode and supplying an oxidizing gas to the cathode; A method of operating a fuel cell system comprising a fuel cell having at least one cell comprising
A method for operating a fuel cell system, comprising: controlling a potential of the cathode within a range of 0.6 to 0.8 V with respect to a standard hydrogen electrode when the fuel cell stops generating power.   While the anode is exposed to the fuel gas, the supply of the oxidizing gas to the cathode is stopped, and a predetermined voltage is applied between the anode and the cathode by using an external power supply, whereby the potential of the cathode is reduced. 2. The operating method of the fuel cell system according to claim 1, wherein is controlled within a range of 0.6 to 0.8 V with respect to a standard hydrogen electrode.   The fuel cell is a fuel cell stack in which a plurality of cells are stacked, and in a state where the anode of each cell is exposed to a fuel gas, the supply of the oxidizing gas to the cathode of each cell is stopped, and an external power supply is used. By applying a predetermined voltage between the anode and the cathode of each cell, the potential of the cathode of each cell is controlled within a range of 0.6 to 0.8 V with respect to a standard hydrogen electrode. The method for operating the fuel cell system according to claim 1.   The potential of the cathode is controlled within the range of 0.6 to 0.8 V with respect to the standard water cable electrode until the cell temperature of the fuel cell drops to 50 ° C. or less after the fuel cell stops generating power. The method for operating the fuel cell system according to any one of claims 1 to 3.   5. The fuel according to claim 4, wherein the fuel cell stops power generation, and when the temperature of the fuel cell drops to 50 ° C. or lower, the cathode and the anode are purged with air. How to operate the battery system.   The method according to claim 5, wherein the purging is performed using dry air. A pair of electrolytes, an anode and a cathode having a platinum-based gold delivery catalyst sandwiching the electrolyte, and a gas flow path for supplying a fuel gas to the anode and supplying an oxidizing agent to the cathode; A fuel cell including at least one cell having a separator plate, and a control device that controls supply of fuel gas to the anode and supply of oxidant gas to the cathode,
An external power supply for applying a predetermined voltage between the anode and the cathode,
The control device, when the fuel cell has stopped generating power, stops supplying the oxidizing gas to the cathode while the anode is exposed to the fuel gas, and sets a gap between the anode and the cathode. Controlling the operation of the external power supply so as to apply a predetermined voltage to the cathode to control the potential of the cathode within a range of 0.6 to 0.8 V with respect to a standard hydrogen electrode. A fuel cell system. The fuel cell is a fuel cell stack in which a plurality of cells are stacked,
The control device, when the fuel cell stack is stopping power generation, while stopping the supply of the oxidizing gas to the cathode of each cell while exposing the anode of each cell to fuel gas, By controlling the operation of the external power supply so as to apply a predetermined voltage between the anode and the cathode, the potential of the cathode of each cell is set to 0.6 to 0.8 V with respect to a standard hydrogen electrode. The fuel cell system according to claim 7, wherein control is performed within the following range. The fuel cell further includes a temperature sensor for measuring a cell temperature,
The control device determines whether the battery temperature of the fuel cell determined by the temperature sensor has decreased to 50 ° C. or less, and determines that the battery temperature of the fuel cell has decreased to 50 ° C. or less as a result. 9. The fuel cell system according to claim 7, wherein the operation of the external power supply is controlled so as to stop applying a voltage between the anode and the cathode.   The control device determines whether the battery temperature of the fuel cell measured by the temperature sensor has decreased to 50 ° C. or less, and as a result, has determined that the battery temperature of the fuel cell has decreased to 50 ° C. or less. The fuel cell system according to claim 9, wherein, in the case, the cathode and the anode are purged with air.   The fuel cell system according to claim 10, wherein the purging is performed using dry air.

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