WO2006085211A1

WO2006085211A1 - Fuel cell system

Info

Publication number: WO2006085211A1
Application number: PCT/IB2006/000281
Authority: WO
Inventors: Koji Matsuda
Original assignee: Nissan Motor Co., Ltd.; Nissan Technical Center North America, Inc.
Priority date: 2005-02-10
Filing date: 2006-02-10
Publication date: 2006-08-17
Also published as: JP2006221986A

Abstract

To control the fluctuation in the starting voltage due to the voltage-lowering process performed when starting a plurality of fuel cells and prevent a deterioration in performance. The difference in the concentration of oxygen in cathodes (3) of each of fuel cells (1) is determined based on the concentration of oxygen in cathodes (3) of each of fuel cells 1 detected by oxygen concentration sensors (14-17) and if the difference in the concentration of oxygen is determined to be larger than a predetermined threshold value, the inside of cathodes (3) of each of fuel cells (1) is replaced with air prior to starting the system and when the difference in the concentration of oxygen becomes less than the threshold value, the load from variable resistances (18 and 19) is connected to each of fuel cells (1) and a voltage lowering process is performed.

Description

FUEL CELL SYSTEM

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority under 35 U.S. C. §119 of Japanese Application No. 2005-034960, filed on February 10, 2005, the entire content of which is expressly incorporated by reference herein.

FIELD

The present invention pertains to a fuel cell system that performs starting control based on the concentration of oxygen in the cathodes when starting the plurality of fuel cells.

BACKGROUND

An example of known technology for eliminating the water condensation and residual combustible gas inside of a fuel cell system is described in Unexamined Japanese Patent Application No. 2002-231293. For the technology described in Unexamined Japanese Patent Application No. 2002-231293, air is used to perform a purge operation and completely eliminate the water condensation and residual combustible gas inside of the system after the first purge using vapor is performed when performing a purge operation on the fuel reformer, fuel cell unit and fuel supply system.

On the other hand, an example of known technology for monitoring the deficiency in phosphoric acid that occurs when the amount of phosphoric acid retained by the matrix of each single-cell battery of phosphoric acid type fuel cells decreases from dispersion that takes place during power generation is described in Unexamined Patent Application Publication No. H03 -101061. For the technology described in Unexamined Patent Application Publication No. H03-101061, an oxygen concentration sensor is provided for detecting the concentration of oxygen in the off- gas exhausted from the outlet of the anodes and cathodes and when the concentration of oxygen or the rate of increase in the concentration detected by this oxygen concentration sensor exceeds a prescribed level it reports the decrease in the amount of phosphoric acid.

SUMMARY When simultaneously starting a plurality of fuel cells, if the fuel cells were started with different concentrations of oxygen in the cathodes of each respective fuel cell, a malfunction would occur in the processing performed to lower the voltage when the starting took place. In other words, due to the difference in the concentration of oxygen in the cathode of each fuel cell, there were fuel cells in which a reaction between the fuel gas and the hydrogen occurred easily and fuel cells in which this reaction did not occur easily, causing a difference in the starting voltage of each fuel cell and a possible risk that a portion of fuel cells would have negative voltage when started. This was problematic because repetition of such a state during the start-up would cause deterioration of the catalyst layer of the fuel cell.

However, as described in the aforementioned conventional technology, the malfunction mentioned above could not be solved by merely purging the combustible gas and water condensation or by detecting the concentration of oxygen in the off-gas.

Therefore, the present invention was contrived in light of the aforementioned problem and its purpose is to provide a fuel cell system that controls the fluctuation in the starting voltage when processing is performed to lower the voltage when starting the plurality of fuel cells and prevent a decrease in performance.

In order to achieve the aforementioned purpose, the means for solving the problem pertaining to the present invention is a fuel cell system that is provided with a plurality of fuel cells that generate power due to an electrochemical reaction between the fuel gas supplied to the anode and the oxidizer gas supplied to the cathode, wherein said fuel cell system is comprised of a detecting means for detecting the concentration of oxygen in the cathode of each of said fuel cells, a determining means for determining the difference in the concentration of oxygen in the cathode of each fuel cell based on the concentration of oxygen detected by said detecting means and a control means for deciding and executing the starting method of said fuel cell system based on the difference in the concentration of oxygen determined by said determining means.

BRIEF DESCRIPTION OF DRAWINGS

Figure 1 shows the configuration for the fuel cell system pertaining to Embodiment 1 of the present invention.

Figure 2 is a flowchart showing the operating procedure for the system pertaining to Embodiment 1 of the present invention. Figure 3 is a flowchart showing the operating procedure for the system pertaining to Embodiment 2 of the present invention.

Figure 4 is a flowchart showing the operating procedure for the system pertaining to Embodiment 3 of the present invention.

Figure 5 is a flowchart showing the operating procedure for the system pertaining to Embodiment 4 of the present invention.

DETAILED DESCRIPTION

According to the present invention, by deciding the starting method for the fuel cell system based on the difference in the concentration of oxygen of the cathode of each fuel cell, the fluctuation in the starting voltage of each fuel cell that occurs when the system is started can be prevented.

The most favorable embodiment for enforcing the present invention is explained below with reference to the Drawings.

Figure 1 shows the configuration of the fuel cell system that pertains to Embodiment 1 of the present invention. The system for Embodiment 1 shown in Figure 1 is provided with two fuel cells 1 and fuel gas and oxidizer gas are provided in parallel to anode 2 and cathode

3 of these two fuel cells 1. In other words, fuel gas, such as hydrogen gas, stored in fuel tank

4 passes through fuel supply passage 6, serially provided to each of fuel cells 1, via fuel supply valve 10, which controls the supply of fuel as well as fuel cell 1 and the external air by selectively cutting them off, after which it is separated into two channels, and is then supplied to anode 2 of each of fuel cell 1. The fuel off-gas exhausted from each of fuel cells 1 merges, after which it is diluted via fuel exhaust passage 7, provided in parallel to each of fuel cells 1, and fuel exhaust one-way valve 11 and then exhausted.

On the other hand, the air, which serves as the oxidizer gas, passes through oxidizer blower 5, which compresses and then supplies the air, oxidizer supply passage 8 provided parallel to each of fuel cells 1, and oxidizer supply valve 12 that not only controls the supply of oxidizer but also controls fuel cell 1 and the external air by selectively cutting them off, after which it is separated into two channels, and is then supplied to cathode 3 of each of fuel cells 1. The oxidizer off-gas exhausted from each of fuel cells 1 merges and is then exhausted via oxidizer exhaust passage 9, provided parallel to each of fuel cells 1, and oxidizer exhaust one-way valve 13. At the connections for oxidizer supply passage 8 and oxidizer exhaust passage 9 and cathodes 3 are provided oxidizer concentration sensors 14-17, which are the means for detecting the concentration of oxygen. In other words, oxidizer concentration sensor 14 detects the concentration of oxygen at the inlet side of the cathode for one of fuel cells 1, oxidizer concentration sensor 15 detects the concentration of oxygen at the outlet side of the cathode for one of fuel cells 1, oxidizer concentration sensor 16 detects the concentration of oxygen at the inlet side of the cathode for the other one of fuel cells 1 and oxidizer concentration sensor 17 detects the concentration of oxygen at the outlet side of the cathode for the other one of fuel cells 1.

The length and diameter of oxidizer supply passage 8 from the inlet of cathode 3 of each of fuel cells 1 to the merging point of oxidizer supply passage 8 is the same and the length and diameter of oxidizer exhaust passage 9 from the outlet of cathode 3 of each of fuel cells 1 to merging point 23 of oxidizer exhaust passage 9 is the same and oxidizer exhaust one-way valve 13 prevents external air from entering fuel cells 1 from the downstream portion of merging point 23 on oxidizer exhaust passage 9.

Connected between the output terminals that put out the power obtained from the power generated by one of fuel cells 1 are variable resistance 18 and the switching element, relay 20, which are serially connected, and in the same manner, connected between the output terminals that put out power obtained from the power generated by the other fuel cell 1 are serially connected variable resistance 19 and the switching element, relay 21. The portion between the output terminals of each of fuel cells 1 is selectively short-circuited via these variable resistances 18 and 19 in order to execute the process for lowering the voltage of each of fuel cells 1 during the start-up. The resistance values of variable resistances 18 and 19 are changed due to the control of controller 22. Relays 20 and 21 are controlled by the switching of controller 22.

Controller 22 functions as the control hub for controlling the operation of this system and in order to achieve this it is equipped with resources, such as a CPU, a memory device and an input/output device, which are required in a computer, such as a microcomputer, that controls the process for each operation in accordance with a program. Controller 22 reads the signals from each of the sensors (not shown in the Figure), including oxygen concentration sensors 14-17, sends the commands to each component of the system, including relays 20 and 21 based on each signal that is read and a control logic (program) that is preprogrammed into the system and performs general administration to control all of the operations required to operate/stop the system including the start operation, which is explained below.

Figure 2 is a flowchart showing the procedure for the start control process for the fuel cell system of Embodiment 1. When the fuel cell system is started and fuel gas is introduced to anode 2 at state in which the concentration of oxygen differs inside of cathodes 3 of plurality of fuel cells 1, the starting voltage for the cathode 3 that has a low concentration of oxygen does not rise, while the starting voltage for the cathode 3 that has a high concentration of oxygen does rise. In such a case, when processing is performed to lower the voltage by short-circuiting the portion between the output terminals by means of the load from plurality of fuel cells 1, a difference in starting voltage occurs in each of plurality of fuel cells 1. hi order to prevent this, start-up control of the fuel cell system is performed by means of the procedure shown in Figure 2.

For Figure 2, first, when a command is made to start the fuel cell system, before the system is started, the concentration of oxygen in cathodes 3 of each of fuel cells 1 is detected by oxygen concentration sensors 14-17. When a difference is detected in both concentrations of oxygen between the oxygen concentration at the outlet side and inlet side of cathodes 3, the higher concentration is set as the oxygen concentration for cathodes 3 of fuel cell 1. Then, the difference in the oxygen concentration of cathodes 3 of fuel cell 1 is calculated at controller 22 and it is determined whether or not this difference is more than a first predetermined threshold value. When the concentration of oxygen is expressed as mol % for the first determined value, it should be about 6%. (Step S21).

When two or more fuel cells are provided, the difference in the concentration of oxygen between all of the combined fuel cells is calculated.

Based on the result, if the difference is more than the determined value, oxidizer blower 5 is operated and air is supplied to each of fuel cells 1 as the oxidizer gas and the inside of cathodes 3 of fuel cells 1 is completely replaced with air so that the concentration of oxygen inside of cathodes 3 is all the same (Step S22). Aforementioned Steps S21 and S22 are repeated until the difference in the concentration of oxygen between the fuel cells is less than the determined value and when the difference becomes less than the determined value, relays 20 and 21 are turned on by controller 22, the portion between the output terminals of each of fuel cells 1 is short-circuited via variable resistances 18 and 19, the voltage-lowering process is performed and the system is started (Step S23). In this manner, according to aforementioned Embodiment 1, for the fuel cell system equipped with a plurality of fuel cells 1 that generate power due to a electrochemical reaction, by making the concentration of oxygen in cathodes 3 of both of fuel cells 1 approximately the same in accordance with the oxygen concentrations detected by oxygen concentration sensors 14-17 placed at the inlet and outlet sides of cathodes 3 inside of both of fuel cells 1, the fluctuation in the starting voltage of each of fuel cells 1 that occurs when the voltage- lowering processing is performed during system start-up can be prevented.

And, by providing oxygen concentration sensors 14-17 at the connections between oxidizer supply passage 8 and the oxidizer gas inlets of fuel cells 1 and at the connections between oxidizer exhaust passage 9 and oxidizer off-gas outlets of fuel cells 1 , the oxygen concentration inside of cathodes 3, which cannot be directly measured, can be detected with high precision.

Oxidizer supply passage 8 of plurality of fuel cells 1 diverges and the length and diameter of oxidizer supply passage 8 is the same until it is distributed to cathodes 3 of both of fuel cells 1 and the length and diameter of oxidizer exhaust passage 9 of plurality of fuel cells 1 is also the same from cathodes 3 until the point at which it merges so that even if external air enters either of fuel cells 1 via the passage, the concentration of oxygen inside of cathodes 3 of both of fuel cells 1 can easily be maintained at the same level so that when air is used to repeatedly purge the oxidizer supply system, the concentration of oxygen inside of cathodes 3 of each of fuel cells 1 can generally be maintained at the same state.

Oxidizer supply valve 12 is provided on oxidizer supply passage 8, oxidizer exhaust one-way valve 13 is placed on oxidizer exhaust passage 9, and similarly, fuel supply valve 10 is provided on fuel supply passage 6 and fuel exhaust one-way valve 11 is provided on fuel exhaust passage 7 so that by closing each of the valves, air that enters cathodes 3 from the exterior is cut off and the concentration of oxygen inside of each of cathodes 3 can be maintained at a low level.

Oxygen concentration sensors 14-17 detect the concentration of oxygen inside of oxidizer supply passage 8 and oxidizer exhaust passage 9 of plurality of fuel cells 1 in order to get an accurate grasp of the difference in concentrations of oxygen between the plurality of fuel cells 1.

The inside of cathodes 3 of plurality of fuel cells 1 are replaced with air before starting the fuel cell system only when the difference in the concentration of oxygen inside of cathodes 3 of plurality of fuel cells 1 is more than the prescribed determined value so that the concentration of oxygen inside of each of cathodes 3 can be at the same level.

Figure 3 is a flowchart showing the procedure for the start control process for the fuel cell system application of Embodiment 2 and its configuration is the same as that shown in Figure 1 described above. In Figure 3, first, when the command is made to start the fuel cell system, before the system is started, the concentration of oxygen in the cathodes of each of fuel cells 1 is detected by oxygen concentration sensors 14-17. When a difference is detected in both concentrations of oxygen between the oxygen concentration at the outlet side and inlet side of cathodes 3, the higher concentration is set as the oxygen concentration for cathode 3 of that fuel cell 1. Then, the difference in the oxygen concentration of the cathodes of fuel cell 1 is calculated at controller 22 and it is determined whether or not this difference is more than the aforementioned first predetermined threshold value (Step S31).

Based on the result, if the difference is more than the determined value, the level of oxygen concentration for each of fuel cells 1 is recognized (Step S32), and the voltage lowering process is performed in accordance with each of the levels of oxygen concentration (Step S33). In other words, at controller 22, the resistance value is adjusted and set for variable resistances 18 and 19 that correspond with either of fuel cells 1 according to the level of oxygen concentration and these variable resistances 18 and 19, which have been adjusted and set, are used to perform the voltage lowering process, just as was described in Embodiment 1. The relationship between the oxygen concentration levels of cathodes 3 and the load (the resistance value of variable resistances 18 and 19) is obtained in advance through experimentation or theoretical analysis and is formatted into a table or the like and stored in controller 22.

On the other hand, if the difference in the concentration of oxygen is detected to be less than the determined value in Step S31 , variable resistances 18 and 19, which have pre-set resistance values, are used to perform the voltage lowering process, just as was the case for Embodiment 1, and the system is started (Step S34).

In this manner, not only can the same effects be achieved for Embodiment 2 as were achieved for Embodiment 1, but the starting voltage of the plurality of fuel cells 1 can generally be maintained at the same state when starting the fuel cell system.

Figure 4 is a flowchart showing the procedure for the start control process for the fuel cell system application of Embodiment 3 and its configuration is the same as that for Embodiment 1 shown in Figure 1 described above. In Figure 4, first, when the command is made to start the fuel cell system, before the system is started, the concentration of oxygen in cathodes 3 of each of fuel cells 1 is detected by oxygen concentration sensors 14-17. When a difference is detected in both concentrations of oxygen between the oxygen concentration at the inlet side and outlet side of cathodes 3, the higher concentration is set as the oxygen concentration for cathode 3 of that fuel cell 1. Then, the difference in the oxygen concentration of cathodes 3 of fuel cell 1 is calculated at controller 22 and it is determined whether or not this difference is more than the aforementioned first predetermined threshold value (Step S41).

Based on the result, if the difference is more than the determined value, the fuel cell with the lowest concentration of oxygen of fuel cells 1 is specified (Step S42). Next, in the same manner, the voltage-lowering process is performed for all of fuel cells 1 in accordance with the lowest level of oxygen concentration (Step S43). In other words, at controller 22, the resistance value is adjusted and set for variable resistances 18 and 19 that correspond with either of fuel cells 1 in accordance with the lowest level of oxygen concentration and these variable resistances 18 and 19, which have been adjusted and set, perform the voltage lowering process, just as was described in Embodiment 1. At this point, even if the resistance values of variable resistances 18 and 19 are used to perform the voltage-lowering process at fuel cell 1 that has the lowest concentration of oxygen, the output voltage of fuel cell 1 is selected so that it does not become negative voltage, The relationship between the oxygen concentration level of cathodes 3 and the load (the resistance values of variable resistances 18 and 19) is obtained in advance through experimentation or theoretical analysis and is formatted into a table or the like and stored in controller 22.

On the other hand, if the difference in the concentration of oxygen is detected to be less than the determined value in Step S41, variable resistances 18 and 19, which have pre-set resistance values, are used to perform the voltage lowering process, just as was the case for Embodiment 1, and the system is started (Step S44). hi this manner, not only can the same effects be achieved for Embodiment 3 as were achieved for Embodiment 1, but the starting voltage of both of fuel cells 1 can be prevented from becoming negative voltage.

Figure 5 is a flowchart showing the procedure for the start control process for the fuel cell system application of Embodiment 4 and its configuration is the same as for Embodiment 1 shown in Figure 1 described above, hi Figure 5, first, when the command is made to start the fuel cell system, before the system is started, the concentration of oxygen in cathodes 3 of each of fuel cells 1 is detected by oxygen concentration sensors 14-17. When a difference is detected in both concentrations of oxygen between the oxygen concentration at the inlet side and outlet side of cathodes 3, the higher concentration is set as the oxygen concentration for cathode 3 of that fuel cell 1. Then, it is determined whether or not all of the concentrations of oxygen of cathodes 3 of fuel cells 1 are less than a second predetermined threshold value. The second predetermined threshold value is preferably equal to or smaller than the first predetermined threshold value, and for the present embodiment, it is about the same as the first determined value, which is approximately 6%. (Step S51).

Based on the result, if all of the concentrations of oxygen for cathodes 3 of fuel cells 1 are less than the determined value, the process for starting up the fuel cell system is begun without executing the voltage-lowering process (Step S52). Due to this, when performing the voltage-lowering process when starting the fuel cell system, the starting voltage of the fuel cell 1 with the lowest concentration of oxygen is prevented from becoming negative voltage.

On the other hand, based on the results achieved in aforementioned Step S 51, if at least one of the concentrations of oxygen is more than the determined value when the fuel system is started, variable resistances 18 and 19, which have a pre-set resistance value, are used to perform the voltage-lowering process, as was done in aforementioned Embodiment 1, and then the system is started (Step S53).

In this manner, for Embodiment 4, the load to fuel cells 1 due to the voltage-lowering process being performed when the concentration of oxygen is small and the starting voltage does not rise can be avoided. In addition, since the voltage lowering process is selectively not performed when starting the system, the amount of time required to control the start-up of the system can be reduced.

For aforementioned Embodiments 1-4, an explanation was provided for two fuel cells, but the present invention can be enforced in the same manner using more than two fuel cells and the same effects can be achieved. In addition, aforementioned Embodiments 1-4 can also be properly combined in order to enforce the invention.

Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that any arrangement, which is calculated to achieve the same purpose, may be substituted for the specific embodiment shown. This application is intended to cover any adaptations or variations of the present invention. Therefore, it is manifestly intended that this invention be limited only by the claims and the equivalents thereof.

Claims

What is claimed is:

1. A fuel cell system provided with a plurality of fuel cells that generate power by means of electrochemical reaction between fuel gas supplied to an anode and oxidizer gas supplied to a cathode, the fuel cell system comprising: a plurality of sensors, each for detecting the concentration of oxygen of the cathode for one of said fuel cells; a controller to determine the difference in the concentration of oxygen of the cathode for each fuel cell based on the concentration of oxygen detected by each of said sensors, and to decide on and execute a starting method of said fuel cell system based on the determined difference in the concentration of oxygen.

2. The fuel cell system of claim 1, and further comprising: an oxidizer supply passage for supplying oxidizer gas to the cathode of each of said fuel cells; and an oxidizer exhaust passage for exhausting oxidizer off-gas from the cathode of each of said fuel cells, wherein each of the plurality of sensors comprises a first oxygen concentration sensor that detects the concentration of oxygen inside of said oxidizer supply passage and a second oxygen concentration sensor that detects the concentration of oxygen inside of said oxidizer exhaust passage.

3. The fuel cell system of claim 2, wherein a length and diameter of the oxidizer supply passage is the same between the area from a diverging portion of said oxidizer supply passage to where the oxidizer supply passage is distributed to each cathode, and wherein a length and diameter of the oxidizer exhaust passage is the same between an area from each of the cathodes to a point where the exhaust passage merges to one exhaust pipe.

4. The fuel cell system of claim 2, and further comprising: a valve provided on said oxidizer supply passage for selectively cutting off the cathode and external air; a one-way valve provided on said oxidizer exhaust passage which is always open in the direction in which oxidizer off-gas is distributed to an exterior by said oxidizer exhaust passage; a fuel supply passage for supplying fuel gas to the anode for each of said fuel cells; a valve provided on said fuel supply passage for selectively cutting off the anode and the external air; a fuel exhaust passage for exhausting fuel off-gas from the anode of each of said fuel cells; and a one-way valve provided on said fuel exhaust passage which is always open in the direction in which the fuel off-gas is distributed to the exterior by said fuel exhaust passage.

5. The fuel cell system of claim 3, and further comprising: a valve provided on said oxidizer supply passage for selectively cutting off the cathode and external air; a one-way valve provided on said oxidizer exhaust passage which is always open in the direction in which oxidizer off-gas is distributed to an exterior by said oxidizer exhaust passage; a fuel supply passage for supplying fuel gas to the anode for each of said fuel cells; a valve provided on said fuel supply passage for selectively cutting off the anode and the external air; a fuel exhaust passage for exhausting fuel off-gas from the anode of each of said fuel cells; and a one-way valve provided on said fuel exhaust passage which is always open in the direction in which the fuel off-gas is distributed to the exterior by said fuel exhaust passage.

6. The fuel cell system of claim 2, wherein the concentration of oxygen in the cathode of each of the fuel cells is set based on the concentration of oxygen detected by the first oxygen concentration sensor and the concentration of oxygen detected by the second oxygen concentration sensor, wherein the controller determines the difference in the concentration of oxygen of the cathode for each fuel cell based on said concentration of oxygen that is set.

7. The fuel cell system of claim 3, wherein the concentration of oxygen in the cathode of each of the fuel cells is set based on the concentration of oxygen detected by the first oxygen concentration sensor and the concentration of oxygen detected by the second oxygen concentration sensor, wherein the controller determines the difference in the concentration of oxygen of the cathode for each fuel cell based on said concentration of oxygen that is set.

8. The fuel cell system of claim 4, wherein the concentration of oxygen in the cathode of each of the fuel cells is set based on the concentration of oxygen detected by the first oxygen concentration sensor and the concentration of oxygen detected by the second oxygen concentration sensor, wherein the controller determines the difference in the concentration of oxygen of the cathode for each fuel cell based on said concentration of oxygen that is set.

9. The fuel cell system of claim 5, wherein the concentration of oxygen in the cathode of each of the fuel cells is set based on the concentration of oxygen detected by the first oxygen concentration sensor and the concentration of oxygen detected by the second oxygen concentration sensor, wherein the controller determines the difference in the concentration of oxygen of the cathode for each fuel cell based on said concentration of oxygen that is set.

10. The fuel cell system of claim 1 , wherein the controller performs processing to supply air to the cathode of each fuel cell prior to starting said fuel cell system when the difference in the concentration of oxygen of the cathode of each fuel cell is determined to be larger than a first predetermined threshold value.

11. The fuel cell system of claim 1 , wherein the controller performs processing to lower the voltage of each fuel cell using a load corresponding to the concentration of oxygen in each respective cathode of each of said fuel cells that is detected by said sensors prior to the starting of said fuel cell system when the difference in the concentration of oxygen of the cathode of each fuel cell is determined to be larger than a first predetermined threshold value.

12. The fuel cell system of claim 11 , wherein the controller compares the concentration of oxygen in the cathode of each fuel cell detected by said sensors with a second predetermined threshold value that is smaller than the first predetermined threshold value, and based on the result of the comparison made by the controller, the controller does not perform processing to lower the voltage of each of said fuel cells and begins starting said fuel cell system when all of the concentrations of oxygen in the cathode of each fuel cell are smaller than said second predetermined threshold value.

13. The fuel cell system of claim 1 , wherein the controller performs processing to lower the voltage of each fuel cell using a load corresponding to the lowest concentration of oxygen of the concentration of oxygen in each respective cathode of each of said fuel cells detected by said sensors prior to the starting of said fuel cell system when the difference in the concentration of oxygen of the cathode of each fuel cell is determined to be larger than a first predetermined threshold value.

14. The fuel cell system of claim 13, wherein the controller compares the concentration of oxygen in the cathode of each fuel cell detected by said sensors with a second predetermined threshold value that is smaller than the first predetermined threshold value, and based on the result of the comparison made by the control, the controller does not perform processing to lower the voltage of each of said fuel cells and begins starting said fuel cell system when all of the concentrations of oxygen in the cathode of each fuel cell is smaller than said second predetermined threshold value.

15. A fuel cell system provided with a plurality of fuel cells that generate power by means of electrochemical reaction between fuel gas supplied to an anode and oxidizer gas supplied to a cathode, the fuel cell system comprising: a plurality of detecting means, each for detecting the concentration of oxygen of the cathode for one of said fuel cells; a determining means for determining the difference in the concentration of oxygen of the cathode for each fuel cell based on the concentration of oxygen detected by each of said detecting means; and a control means for deciding and executing a starting method of said fuel cell system based on the difference in the concentration of oxygen determined by said determining means.

16. A method of operating a fuel cell system provided with a plurality of fuel cells that generate power by means of electrochemical reaction between fuel gas supplied to an anode and oxidizer gas supplied to a cathode, comprising: detecting the concentration of oxygen of the cathode for one of said fuel cells; determining the difference in the concentration of oxygen of the cathode for each fuel cell based on the concentration of oxygen detected by each of said detecting means; and deciding and executing a starting method of said fuel cell system based on the difference in the concentration of oxygen determined by said determining means.