JP2007103114A - Operation method of fuel cell system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To restrain degradation of a fuel cell due to radicals generated at decomposition of hydrogen peroxide produced in the fuel cell. <P>SOLUTION: A control device 23 carries out at least either an operation of raising moisture concentration in the fuel cell 10 or an operation of lowering oxygen concentration in an oxidant electrode 10b, in case a fuel cell voltage detected by a fuel cell voltage detection means 15 exceeds a given value, and that, a fuel battery temperature detected by a cell temperature detection means 16 exceeds a given value. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a method for operating a fuel cell system including a polymer electrolyte fuel cell.

The fuel cell is expected to be used as a clean and highly efficient energy conversion device, but has a problem that the battery performance deteriorates when operated for a long time. One of the factors is the generation of hydrogen peroxide in the fuel cell. This is because the generated hydrogen peroxide is decomposed into radicals in the presence of metal ions, and the electrolyte membrane as a constituent material is damaged. Several attempts have been made to suppress performance degradation due to this phenomenon and realize stable operation for a long time. For example, according to Patent Document 1, the concentration of hydrogen peroxide contained in at least one of exhaust gas and exhaust water from a fuel cell is measured, and when the hydrogen peroxide concentration exceeds a preset upper limit value, A degradation suppression method is disclosed in which operating conditions are controlled so that the production of hydrogen peroxide is suppressed.
Japanese Patent Laying-Open No. 2004-273209 (5th page, FIG. 1)

  However, since the apparatus disclosed in Patent Document 1 determines the progress of deterioration by measuring hydrogen peroxide, the hydrogen peroxide generated inside the fuel cell is exhausted to the outside of the fuel cell. Judgment delay occurs due to time delay or response delay of hydrogen peroxide measurement. Therefore, during this determination delay, there has been a problem that the operation condition operation for suppressing the performance deterioration cannot be performed and a sufficient deterioration suppressing effect cannot be obtained.

  In addition, the inventor not only has the performance degradation due to the above mechanism in the operation region where the output current density is low (battery voltage is high) but also when the temperature of the fuel cell is high or the oxygen concentration of the oxidizer electrode is low. It was found that when it was high, it was significantly accelerated (FIGS. 10 and 11). The present invention has been made on the basis of the problems and knowledge of the above-described prior art. A fuel cell operation method and a control system thereof are provided in order to suppress performance deterioration in the fuel cell and realize stable operation for a long time. The issue is to provide.

  In order to solve the above problems, the present invention provides a fuel electrode supplied with a fuel gas containing hydrogen, an oxidant electrode supplied with an oxidant gas containing oxygen, the fuel electrode, and the oxidant electrode. A fuel cell comprising a plurality of electrode assemblies made of an electrolyte sandwiched between the separators via a separator, a fuel cell voltage detecting means, and a fuel cell temperature detecting means. If the detected value of the fuel cell voltage exceeds the predetermined value and the detected value of the fuel cell temperature exceeds the predetermined value, the moisture concentration in the fuel cell is The gist is to perform at least one of the operation of increasing and the operation of decreasing the oxygen concentration in the oxidant electrode of the fuel cell.

  The present invention provides an operation for increasing the water concentration in the fuel cell or an operation for decreasing the oxygen concentration in the oxidant electrode of the fuel cell under operating conditions where the battery voltage is higher than the predetermined value and the battery temperature is higher than the predetermined value. The operation is performed. By increasing the water concentration in the fuel cell, the water content in the electrolyte membrane is also increased, and the production of hydrogen peroxide can be suppressed. In addition, by reducing the oxygen concentration of the oxidant electrode, the amount of oxygen that passes through the electrolyte membrane and moves from the oxidant electrode to the fuel electrode is reduced, and the production of hydrogen peroxide at the fuel electrode can be suppressed.

  Therefore, according to the present invention, it is possible to extremely effectively suppress the deterioration of the battery performance accompanying the generation of hydrogen peroxide. Furthermore, various adverse effects caused by changing the operating conditions from the optimally set conditions by executing the above-described driving operations only under conditions where the battery voltage at which the performance deterioration is remarkable and the battery temperature is high are limited. For example, a decrease in system efficiency can be minimized.

  Embodiments of the present invention will be described below with reference to the drawings. Note that the fuel cell system and the operation method thereof according to the present invention are not limited to the embodiments described later, and can be implemented with modifications and improvements without departing from the gist of the present invention.

  Next, the configuration of a fuel cell system to which Example 1 of the operation method of the fuel cell system according to the present invention is applied will be described with reference to the system configuration diagram of FIG. In FIG. 1, a fuel cell 10 is a fuel in which a plurality of membrane electrode assemblies in which a hydrogen ion conductive solid polymer electrolyte membrane (not shown) is sandwiched between a fuel electrode 10a and an oxidant electrode 10b via a separator. It is a battery stack. As the type of the solid polymer electrolyte, for example, a perfluorocarbon sulfonic acid membrane such as “Nafion” (registered trademark, manufactured by DuPont), an aromatic hydrocarbon-based solid polymer electrolyte membrane, or the like can be used.

  The fuel electrode 10a of the fuel cell 10 is supplied with hydrogen as a fuel gas whose flow rate is adjusted by the fuel gas flow rate control means 12 from a fuel gas supply means 11 such as a high-pressure hydrogen tank. The oxidant electrode 10b is supplied with an oxidant gas whose flow rate is adjusted by an oxidant gas flow rate supply unit 14 from an oxidant gas supply unit 13 using an air compressor or the like.

  In addition, the fuel cell 10 is provided with fuel cell voltage detection means 15 for detecting the voltage of the entire fuel cell stack or each cell constituting the fuel cell stack, and the detected value is transmitted to the control device 23.

  The battery temperature detecting means 16 detects the temperature of the fuel cell 10 itself or the temperature of the fuel cell cooling water, and the cell resistance detecting means detects the resistance between the fuel electrode and the oxidant electrode of a specific cell of the fuel cell 10. 17 (for example, high-frequency impedance measuring means), and each detected value is transmitted to the control device 23. In addition, the electrical output terminal of the fuel cell 10 is connected to a load device (not shown) and is connected to a power storage device 22 via a DCDC converter 21 that can convert a DC voltage bidirectionally. Further, the power storage device 22 includes a charge amount estimation unit 24, and the detected value is transmitted to the control device 23. The power storage device 22 is a capacitor using a secondary battery such as a lithium ion battery or an electric double layer capacitor, and is charged when the power generation output of the fuel cell 10 has a margin, and the power generation output of the fuel cell 10 is insufficient. When it does, it discharges and assists the electric power supply to a load apparatus.

  The control device 23 is a control device that implements the operation method of the fuel cell system of the present invention, and includes, for example, a microprocessor including a CPU, a program ROM, a working RAM, and an input / output interface. .

  Next, with reference to the control flowchart of FIG. 2A, an operation method of the fuel cell system in the present embodiment, which is executed by the control device 23, will be described. First, in step (hereinafter, step is abbreviated as S) 10, the temperature T of the fuel cell 10 is detected by the cell temperature detection means 16. Next, in S12, it is determined whether or not the detected value T of the fuel cell temperature exceeds a predetermined value Tmax. If it exceeds, the process proceeds to S14, and if not, the process returns to S10.

  The predetermined value Tmax for determination in S12 varies depending on the material, manufacturing method, thickness, etc. of the solid polymer electrolyte membrane used in the fuel cell 10, but generally derived from hydrogen peroxide when the fuel cell temperature exceeds a certain temperature. The temperature is set so that the deterioration of the solid polymer electrolyte membrane is accelerated by the action of radicals. As an example of the predetermined value Tmax setting method, a characteristic graph of the voltage drop rate per unit operation time with respect to the battery temperature as shown in FIG. 9 is obtained by an experiment with a real fuel cell device or a computer simulation. There is a method of setting, as Tmax, the temperature at which the rate of change of the maximum value is maximum.

  In S14, the fuel cell voltage detecting means 15 detects the voltage V of the fuel cell 10. Here, if the fuel cell voltage detecting means 15 detects the voltage of the fuel cell stack, the detection value V may be left as it is, but if the fuel cell voltage detecting means 15 detects the cell voltage of the fuel cell 10. For example, the highest cell voltage among the detected cell voltages is preferably set to V. Next, in S16, it is determined whether or not the fuel cell voltage V exceeds a predetermined value Vmax. If exceeded, the process proceeds to S18, and if not exceeded, the process returns to S10.

  As a setting method of the predetermined value Vmax for determination in S16, for example, a characteristic graph of the voltage drop rate per unit operation time with respect to the fuel cell voltage is obtained by an experiment with a fuel cell actual machine or a computer simulation, and during the life of the fuel cell There is a method of setting, as Vmax, a voltage at which the decrease in the fuel cell voltage within the desired range due to the high fuel cell voltage operation (low load operation) in FIG.

  Next, in S18, the cell resistance detection means 17 detects the cell resistance R of a specific cell of the fuel cell 10 as a value related to the moisture concentration in the fuel cell. The higher the water content of the solid polymer electrolyte membrane of the fuel cell, the lower the DC resistance value and AC impedance between the fuel electrode and oxidant electrode of the cell. Therefore, even if the fuel cell 10 is generating power, if the AC impedance of the fuel cell is measured, the moisture content (moisture concentration) of the electrolyte membrane of the cell can be estimated. In this embodiment, the cell resistance detecting means 17 measures the AC impedance of the cell to estimate the moisture concentration.

  Next, in S20, the control target voltage Vo of the fuel cell voltage is calculated. The control target voltage Vo is internal data A stored in advance in the control device 23 as a function with respect to the fuel cell temperature or a control map.

  Next, in S22, the control target current Io of the load current taken out from the fuel cell 10 is calculated. The control target current Io can be obtained, for example, by searching a control map of a load current control target value with respect to the fuel cell temperature and the cell resistance as shown in FIG.

  Next, in S24, it is determined whether or not the power storage device 22 can be charged based on the charge amount estimating means 24. If the charging is impossible, the process returns to S10. If charging is possible in the determination of S24, the amount of charge to the power storage device 22 is increased so that the load current of the fuel cell 10 becomes the control target Io in S26. As the load current increases, the generated water in the fuel cell 10 increases and the oxygen concentration in the oxidant electrode 10b decreases.

  According to the present embodiment described above, the generation of hydrogen peroxide is suppressed due to an increase in generated water accompanying an increase in load current under operating conditions where the battery voltage is higher than a predetermined value and the battery temperature is higher than a predetermined value. In addition, the hydrogen peroxide contained in the electrolyte membrane and the electrode catalyst layer is washed with the produced water, so that the battery performance can be prevented from deteriorating.

  In addition, according to the present embodiment, by providing the power storage device, even when the required value of the generated power to the fuel cell is small, it is possible to perform an operation to increase the load current without reducing the efficiency of the fuel cell system. There is an effect of becoming.

  Furthermore, according to the present embodiment, the battery voltage is set to a preset voltage value with respect to the battery temperature and the estimated value of the moisture concentration in the operating condition where the battery voltage is higher than the predetermined value and the battery temperature is higher than the predetermined value. Since the load current of the fuel cell is increased so that it becomes less than (lower as the battery temperature is higher and lower as the moisture concentration is lower), the power storage device is charged with surplus power. Since the deterioration in performance tends to be accelerated as the water concentration in the fuel cell is lower, the operation for increasing the load current of the fuel cell can be executed more accurately and with a minimum.

  Next, the configuration of the fuel cell system to which the second embodiment of the method for operating the fuel cell system according to the present invention is applied will be described with reference to the system configuration diagram of FIG. The fuel cell system according to this embodiment is provided with a fuel gas circulation pump 18 and a fuel circulation path 25 in order to recirculate the fuel gas to the fuel electrode 10a of the fuel cell 10 as shown in FIG. 1 different from the fuel cell system. Since other configurations are the same as those in FIG. 1, the same components are denoted by the same reference numerals and redundant description is omitted.

  Next, with reference to the control flowchart of FIG. 4A, the operation method of the fuel cell system in the present embodiment, which is executed by the control device 23, will be described. Steps S10 to S18 are the same as those in the first embodiment shown in FIG. 2A. The detected value of the fuel cell temperature exceeds the predetermined value Tmax and the detected value of the fuel cell voltage V exceeds the predetermined value Vmax. If it is determined, the cell resistance R is detected by the cell resistance detection means 17.

  Next, in S30, the control device 23 calculates a control target Ra of the fuel gas recirculation amount with respect to the fuel cell temperature T and the cell resistance R. The control target Ra of the fuel gas recirculation amount is, for example, a control map as shown in FIG. 4B and is stored in advance as internal data C of the control device 23. Next, in S32, the control device 23 increases the fuel gas recirculation amount to the control target Ra by controlling the discharge amount of the fuel gas circulation pump 18. This increase in the amount of recirculation of the fuel gas can increase the humidity of the fuel gas at the inlet of the fuel electrode 10a, and hence the water concentration inside the fuel cell can also be increased, thereby suppressing the production of hydrogen peroxide. Can do.

  According to the present embodiment, the operation for increasing the amount of circulation of the fuel gas can be performed more accurately and with minimization, the performance deterioration due to the generation of hydrogen peroxide is suppressed, and the amount of circulation of the fuel gas is increased. Along with minimizing the accompanying loss (increasing power consumption of the fuel gas circulation pump) is possible.

  Next, the configuration of the fuel cell system to which the third embodiment of the operation method of the fuel cell system according to the present invention is applied will be described with reference to the system configuration diagram of FIG. The fuel cell system according to this embodiment includes an oxidant gas circulation pump 19 and an oxidant circulation path 26 for recirculating oxidant gas to the oxidant electrode 10b of the fuel cell 10 as shown in FIG. It is different from the fuel cell system of Example 1 shown. Since other configurations are the same as those in FIG. 1, the same components are denoted by the same reference numerals and redundant description is omitted.

  Next, with reference to the control flowchart of FIG. 6A, the operation method of the fuel cell system in the present embodiment executed by the control device 23 will be described. Steps S10 to S18 are the same as those in the first embodiment shown in FIG. 2A. The detected value of the fuel cell temperature exceeds the predetermined value Tmax and the detected value of the fuel cell voltage V exceeds the predetermined value Vmax. If it is determined, the cell resistance R is detected by the cell resistance detection means 17.

  Next, in S40, the control device 23 refers to the internal data D and calculates the control target Rc of the oxidant gas recirculation amount with respect to the fuel cell temperature T and the cell resistance R. In S42, the oxidant gas circulation pump 19 is calculated. By increasing the discharge amount, the oxidant gas recirculation amount is increased to the control target Rc. By increasing the amount of oxidant gas recirculation, the humidity of the oxidant gas at the inlet of the oxidant electrode 10b can be increased and the oxygen concentration can be reduced. Therefore, the water concentration inside the fuel cell is also increased and the oxygen concentration is reduced. As a result, the production of hydrogen peroxide can be suppressed.

  According to the present embodiment, the operation for increasing the circulation amount of the oxidant gas can be performed more appropriately and with minimization, and it is possible to suppress the performance deterioration due to the generation of hydrogen peroxide, and to circulate the oxidant gas circulation amount. It is possible to achieve both minimization of the increase in power consumption of the oxidant gas circulation pump with the increase of the oxidant gas.

  Next, a fourth embodiment of the operation method of the fuel cell system according to the present invention will be described. The configuration of the fuel cell system to which this embodiment is applied is the same as the configuration of Embodiment 1 shown in FIG.

  Next, an operation method of the fuel cell system executed by the control device 23 will be described with reference to the control flowchart of FIG. The operation method of the present embodiment is an operation method in which S50 is added between S12 and S14 in the operation method of the first embodiment shown in FIG.

  In FIG. 7A, first, in S10, the temperature T of the fuel cell 10 is detected by the cell temperature detecting means 16. Next, in S12, it is determined whether or not the detected value T of the fuel cell temperature exceeds a predetermined value Tmax. If it exceeds, the process proceeds to S50, and if not, the process returns to S10. The predetermined value Tmax for determination in S12 is as described in the first embodiment.

In S50, by reducing the control target (upper limit value) of the charging rate of the power storage device 22 from SOCo to SOC _L , a charging margin for the power storage device 22 is secured, and the process proceeds to S14. In S14, the fuel cell voltage detecting means 15 detects the voltage V of the fuel cell 10. Here, if the fuel cell voltage detecting means 15 detects the voltage of the fuel cell stack, the detection value V may be left as it is, but if the fuel cell voltage detecting means 15 detects the cell voltage of the fuel cell 10. For example, the highest cell voltage among the detected cell voltages is preferably set to V.

  Next, in S16, it is determined whether or not the fuel cell voltage V exceeds a predetermined value Vmax. If exceeded, the process proceeds to S18, and if not exceeded, the process returns to S10. The setting method of the predetermined value Vmax for determination in S16 is as described in the first embodiment.

  Next, in S18, the cell resistance detection means 17 detects the cell resistance R of a specific cell of the fuel cell 10 as a value related to the moisture concentration in the fuel cell.

  Next, in S20, the control target voltage Vo of the fuel cell voltage is calculated. The control target voltage Vo is internal data A stored in advance in the control device 23 as a function with respect to the fuel cell temperature or a control map.

  Next, in S22, the control target current Io of the load current taken out from the fuel cell 10 is calculated. The control target current Io can be obtained, for example, by searching a control map of the load current control target value with respect to the fuel cell temperature and the cell resistance as shown in FIG.

  Next, in S24, it is determined whether or not the power storage device 22 can be charged based on the charge amount estimating means 24. If the charging is impossible, the process returns to S10. If charging is possible in the determination in S24, the charge amount to the power storage device 22 is increased so that the load current of the fuel cell becomes the control target Io in S26.

  According to the present embodiment, when the temperature of the fuel cell becomes higher than a predetermined value, a sufficient charge margin for the power storage device is ensured, so that the deterioration suppressing operation by increasing the load current of the fuel cell is surely executed. can do.

  Next, a fifth embodiment of the operation method of the fuel cell system according to the present invention will be described. The configuration of the fuel cell system to which this embodiment is applied is the same as the configuration of Embodiment 3 shown in FIG.

  With reference to the control flowchart of FIG. 8 and the graph showing the internal data of FIG. 9, the operation method of the fuel cell system in the present embodiment executed by the control device 23 will be described. The operation method of this embodiment is an operation method in which S40 and S42 are added to the operation method of the embodiment 4 shown in FIG. 7A, and other steps are the same as those of the embodiment 4.

  In FIG. 8, it is determined whether or not the power storage device 22 can be charged in S24. In this determination, if the charging rate estimated by the charge amount estimating unit 24 of the power storage device 22 does not exceed the control target upper limit value, it is determined that charging is possible. If the determination in S24 is chargeable, the charge amount to the power storage device 22 is increased so that the load current of the fuel cell 10 becomes Io in S26. If the determination of S24 is impossible, the control target Rc of the oxidant gas recirculation amount is calculated for the fuel cell temperature T and the cell resistance R in S40, and the discharge amount of the oxidant gas circulation pump 19 is calculated in S42. Is increased to the control target Rc.

  According to the present embodiment, when the power storage device 22 can be charged, the deterioration is suppressed by increasing the load current, and when the power storage device 22 cannot be charged, the amount of oxidant gas recirculation is increased. By doing so, it is possible to achieve both a reliable deterioration suppression and a reduction in power consumption.

1 is a system configuration diagram of Embodiment 1. FIG. 2 is a control flowchart illustrating an operation method according to the first embodiment. FIG. 6 is a system configuration diagram of a second embodiment. 7 is a control flowchart illustrating an operation method according to the second embodiment. FIG. 10 is a system configuration diagram of a third embodiment. 6 is a control flowchart for explaining an operation method according to a third embodiment. 6 is a control flowchart illustrating an operation method according to a fourth embodiment. 10 is a control flowchart for explaining an operation method according to a fifth embodiment. It is a figure which shows the example of the internal data referred with the control flowchart of FIG. It is a figure explaining the voltage fall rate with respect to battery temperature at the time of low load operation. It is a figure explaining the voltage fall rate with respect to oxidant gas oxygen partial pressure at the time of low load operation.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 ... Fuel cell 10a ... Fuel electrode 10b ... Oxidant electrode 11 ... Fuel gas supply means 12 ... Fuel gas flow control means 13 ... Oxidant gas supply means 14 ... Oxidant gas flow control means 15 ... Fuel cell voltage detection means 16 ... Battery temperature detection means 17 ... cell resistance detection means 18 ... fuel gas circulation pump 19 ... oxidant gas circulation pump 20 ... fuel gas purge valve 21 ... DCDC converter 22 ... power storage device 23 ... control device 24 ... power storage device charge amount estimation means

Claims (9)

An electrode joint comprising a fuel electrode supplied with a fuel gas containing hydrogen, an oxidant electrode supplied with an oxidant gas containing oxygen, and an electrolyte sandwiched between the fuel electrode and the oxidant electrode A fuel cell system operating method comprising: a fuel cell comprising a plurality of stacked bodies with separators interposed therebetween; a fuel cell voltage detecting means; and a fuel cell temperature detecting means.
When the detected value of the fuel cell voltage exceeds the predetermined value and the detected value of the fuel cell temperature exceeds the predetermined value, an operation for increasing the water concentration in the fuel cell and the oxidant electrode of the fuel cell A method for operating a fuel cell system, comprising performing at least one of an operation for reducing the oxygen concentration inside the fuel cell system. The fuel cell system includes a power storage device that temporarily stores power generated by the fuel cell,
When the detected value of the fuel cell voltage exceeds the predetermined value and the detected value of the fuel cell temperature exceeds the predetermined value, the predetermined value assigned to the detected value of the fuel cell temperature. 2. The method of operating a fuel cell system according to claim 1, wherein the load current of the fuel cell is increased so that the surplus power is charged in the power storage device as described below. The fuel cell system includes water concentration estimation means for estimating the water concentration in the fuel cell,
When the detected value of the fuel cell voltage exceeds the predetermined value and the detected value of the fuel cell temperature exceeds the predetermined value, the fuel cell voltage is determined based on the detected value of the fuel cell temperature and the moisture concentration in the fuel cell. 3. The fuel cell according to claim 2, wherein the load current of the fuel cell is increased so as to be equal to or less than a predetermined value assigned to the combination with the estimated value, and surplus power is charged in the electric means. How to operate the system. The fuel cell system includes a fuel circulation device for recirculating fuel gas to the fuel electrode,
When the detected value of the fuel cell voltage exceeds the predetermined value and the detected value of the fuel cell temperature exceeds the predetermined value, the control value of the recirculation amount of the fuel gas to the fuel electrode is set to the value of the fuel cell temperature. 2. The method of operating a fuel cell system according to claim 1, wherein when the detection value is assigned, the fuel cell system is increased to a predetermined value. The fuel cell system includes water concentration estimation means for estimating the water concentration in the fuel cell,
When the detected value of the fuel cell voltage exceeds the predetermined value and the detected value of the fuel cell temperature exceeds the predetermined value, the control value of the recirculation amount of the fuel gas to the fuel electrode is set to the value of the fuel cell temperature. 5. The method of operating a fuel cell system according to claim 4, wherein the operating value is increased to a predetermined value assigned to the combination of the detected value and the estimated value of the moisture concentration in the fuel cell. The fuel cell system includes an oxidant circulation device for recirculating oxidant gas to the oxidant electrode,
When the detected value of the fuel cell voltage exceeds the predetermined value and the detected value of the fuel cell temperature exceeds the predetermined value, the control value of the recirculation amount of the oxidant gas to the oxidant electrode is 2. The method of operating a fuel cell system according to claim 1, wherein the fuel cell system is increased to a predetermined value after being assigned to the detected temperature value. The fuel cell system includes water concentration estimation means for estimating the water concentration in the fuel cell,
When the detected value of the fuel cell voltage exceeds the predetermined value and the detected value of the fuel cell temperature exceeds the predetermined value, the control value of the recirculation amount of the oxidant gas to the oxidant electrode is 7. The method of operating a fuel cell system according to claim 6, wherein the temperature is increased to a predetermined value assigned to the combination of the detected value of the temperature and the estimated value of the moisture concentration in the fuel cell.   When the detected value of the fuel cell temperature exceeds a predetermined value, the control target (upper limit value) of the charging rate of the power storage device is reduced with respect to the normal control target value (upper limit value). Item 3. A method for operating a fuel cell system according to Item 2. The fuel cell system includes at least one of a fuel circulation device that recirculates fuel gas to the fuel electrode and an oxidant circulation device that recirculates oxidant gas to the oxidant electrode,
When the detected value of the fuel cell temperature exceeds a predetermined value, and the charging rate of the power storage device is equal to or higher than the control target value (upper limit value) and the power storage device cannot be charged, the recirculation amount of fuel gas or oxidant gas The method of operating a fuel cell system according to claim 2, wherein at least one of the recirculation amount of the fuel cell is increased.

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