JP2006179199A - Fuel cell system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a fuel cell system in which a humid state of an electrolyte film and a catalyst layer is maintained, in which a dry and wet history is inhibited to be added to the electrolyte film and the catalyst layer, in which deterioration of the electrolyte film and the catalyst layer is little and which is high in durability when a cell voltage is higher than a prescribed one. <P>SOLUTION: When the cell voltage detected by an output voltage detecting means 18 is higher than the prescribed voltage, a control device 22 switches a three-way valve 11 or 13 to a humidifying means 9 or 10 side, supplies a humidified fuel gas or an oxidizer gas to the fuel cell stack 1, and increases the amount of water contained in the electrolyte film and the catalyst layer. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a fuel cell system.

  In a fuel cell, a fuel gas such as hydrogen gas and an oxidizing gas containing oxygen are electrochemically reacted through an electrolyte, and electric energy is directly taken out between electrodes provided on both surfaces of the electrolyte. In particular, a polymer electrolyte fuel cell using a polymer electrolyte has attracted attention as a power source for electric vehicles because of its low operating temperature and easy handling. That is, a fuel cell vehicle is equipped with a hydrogen storage device such as a high-pressure hydrogen tank, a liquid hydrogen tank, or a hydrogen storage alloy tank in the vehicle, and reacts by supplying hydrogen supplied therefrom and air containing oxygen to the fuel cell. This is the ultimate clean vehicle that drives the motor connected to the drive wheels with the electric energy extracted from the fuel cell, and the only exhaust material is water.

  By the way, many solid polymer electrolyte membranes do not exhibit good hydrogen ion conductivity unless they are sufficiently humidified. For this purpose, there are some equipped with a humidifier that humidifies hydrogen gas or air supplied to the polymer electrolyte fuel cell.

Also, in order to prevent the solid polymer electrolyte membrane from being dried during the shutdown of the fuel cell system, the air flow is closed to the air supply line to the air electrode and the air discharge line from the air electrode when the fuel cell operation is stopped. There is one provided with an opening / closing valve (for example, Patent Document 1).
JP 2002-216823 A (page 3, FIG. 1)

  However, the above prior art seems to be effective for preventing the electrolyte membrane from being dried during the shutdown of the fuel cell system, but mentions the idling or no-load time when supplying the minimum power from the fuel cell. Not. Even in such a low load or no load, the amount of water generated with power generation is small or not, and in addition to the diffusion of water from the electrolyte membrane and the catalyst layer to the gas flowing through the flow path, the electrolyte membrane And drying of the catalyst layer proceeds. Therefore, the wet and dry history is added to the electrolyte membrane along with the load fluctuation during the fuel cell operation, which causes the problem that the electrolyte membrane deteriorates more than the rated operation and the life of the fuel cell is shortened. .

  In order to solve the above problems, the present invention provides a fuel cell in which unit cells each having a membrane electrode assembly in which a catalyst layer and a gas diffusion electrode of a fuel electrode and an oxidant electrode are disposed on both surfaces of an electrolyte membrane are laminated. The fuel cell system includes a stack, and the gist is to increase the water content of the electrolyte membrane and the catalyst layer when a cell voltage, which is a voltage of the unit cell, is higher than a predetermined voltage.

  According to the present invention, when the cell voltage of the fuel cell stack is higher than a predetermined voltage, the moisture content of the electrolyte membrane and the catalyst layer at high voltage is increased by increasing the water content of the electrolyte membrane and the catalyst layer. This prevents the wet and dry history from being applied to the electrolyte membrane and the catalyst layer, and can provide a highly durable fuel cell system that is less deteriorated due to drying of the electrolyte membrane and the catalyst layer. There is.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a system configuration diagram showing a configuration example of a fuel cell system common to the embodiments described below. As shown in FIG. 1, the fuel cell system includes a solid polymer electrolyte type fuel cell stack 1 that obtains electric power by utilizing an electrochemical reaction between hydrogen and oxygen.

  The fuel cell stack 1 includes an output voltage detection means 18 for detecting the output voltage of each cell constituting the fuel cell stack 1, a load detection means 19 for detecting the size of the load, and an electrolyte membrane resistance of each cell. And a temperature sensor 21 for detecting the fuel cell stack temperature, and these are connected to the control device 22.

  The electrolyte membrane resistance detection means 20 applies, for example, an AC voltage between the fuel electrode and the oxidant electrode of the detection target cell, detects an AC current flowing through the cell, and uses the AC voltage and the AC current as an electrolyte. The resistance of the film can be detected. Alternatively, a third electrode for measuring electrolyte membrane resistance may be formed in the detection target cell to detect the resistance between the fuel electrode and the third electrode or between the oxidant electrode and the third electrode.

  The fuel cell system is provided with a fuel supply pipe 5 for supplying hydrogen gas to the fuel electrode of the fuel cell stack 1 and an oxidant supply pipe 7 for supplying air (oxygen) to the oxidant electrode. Yes. The fuel supply pipe 5 is provided with a fuel supply amount adjustment valve 3 for adjusting the supply amount of hydrogen gas. The oxidant supply pipe 7 is provided with an oxidant blower 4 for supplying air.

  In order to advance the electrochemical reaction during power generation, it is necessary to keep the electrolyte membrane in the fuel cell stack moist. When the supply of moisture is required, for example, the three-way valve 11 is switched, and the hydrogen gas supplied from the fuel tank 2 is passed through the fuel bypass passage 12 and the fuel humidifying means 9 to supply the humidified hydrogen gas to the fuel. It can be supplied to the battery stack 1. Also on the oxidant side, the air supplied from the oxidant blower 4 is switched by the three-way valve 13 and passed through the oxidant bypass passage 14 and the oxidant humidification means 10, whereby the humidified air can be supplied to the fuel cell stack 1. .

  In addition, humidification in the fuel cell system is performed by collecting generated water accompanying power generation with a condenser (not shown) provided in the oxidant exhaust pipe 8 and supplying the collected water to the fuel humidification unit 9 or the oxidant humidification unit 10. May be performed.

  Furthermore, a porous member can be provided in the cooling water passage inside the fuel cell stack 1, and the gas, the electrolyte membrane, or the gas diffusion layer inside the fuel cell stack can be humidified via the porous member.

  The fuel cell stack 1 generates heat during power generation. For this reason, the fuel cell system has cooling systems 15 to 17 for cooling the fuel cell stack 1 so that the temperature is suitable for the progress of the electrochemical reaction (about 80 ° C.). This cooling system includes a cooling water path 17 for circulating cooling water through the fuel cell stack 1, a cooling water pump 16 for circulating cooling water, and a cooling water tank 15 for storing cooling water.

  Hereinafter, an embodiment of the present invention will be described taking the fuel cell system shown in FIG. 1 as an example.

  The fuel cell system according to Embodiment 1 of the present invention provides an electrolyte membrane by supplying a highly humidified gas to at least one of the fuel electrode and the oxidant electrode when the cell voltage is higher than a predetermined voltage. In addition, deterioration due to drying of the catalyst layer is avoided.

  FIG. 2 is a timing chart showing a control method of the fuel cell system according to Embodiment 1 of the present invention. In FIG. 2, the load of the fuel cell starts to decrease from time t1, and accordingly, the voltage of the fuel cell stack and each cell voltage increase. Then, it is assumed that the cell voltage reaches the predetermined voltage V1 at time t2.

  When the cell voltage of the fuel cell stack 1 is higher than a predetermined voltage, the generation amount of water accompanying power generation is small, the diffusion of water due to evaporation from the electrolyte membrane to the gas in the flow path, and the like. Drying proceeds.

  In this embodiment, when the fuel cell voltage detected by the output voltage detection means 18 becomes the high voltage V1 at time t2, the control device 22 causes at least one of the three-way valve 11 or 13 to be humidified means (9 Alternatively, by switching to the 10) side and introducing at least one of water or a highly humidified gas into at least one of the electrodes, the water content of the electrolyte membrane and the catalyst layer is increased and drying is suppressed. The high voltage V1 is preferably 0.85 [V] or more, in which the amount of generated water accompanying power generation is small and drying of the electrolyte membrane is large.

  FIG. 12 is a control flowchart of the control device 22 according to the first embodiment. This flowchart is called at predetermined time intervals (for example, 100 [ms]) and executed by the control device 22.

  First, in step (hereinafter, step is abbreviated as S) 10, each cell voltage of the fuel cell stack 1 is detected by the output voltage detection means 18. Next, in S12, it is determined whether or not the representative value (for example, average value, mode value, and median value) of the cell voltage is 0.85 [V] or more. If it is determined in S12 that the cell voltage is 0.85 [V] or more, the process proceeds to S14, and the water content increasing process of the electrolyte membrane and the catalyst layer is performed. This moisture content increasing process is performed by switching at least one of the three-way valve 11 or 13 to the humidifying means (9 or 10) side by the control device 22 and introducing at least one of water or a highly humidified gas into at least one electrode. is there.

  If it is determined in S12 that the cell voltage is less than 0.85 [V], the process proceeds to S16, and the normal operation, that is, the three-way valves 11 and 13 select the fuel supply pipe 5 and the oxidant supply pipe 7, respectively. The fuel cell system is operated in a state where the fuel gas and the oxidant gas that are not humidified are supplied to the fuel cell stack 1.

  According to this example, by increasing the water content of the electrolyte membrane and the catalyst layer at a cell voltage of 0.85 [V] or more with a small amount of generated water accompanying power generation, the voltage is higher than 0.85 [V]. It is possible to suppress the drying of the electrolyte membrane and the catalyst layer that sometimes progress, and to prevent the wet and dry history from being applied to the electrolyte membrane and the catalyst layer.

  FIG. 3 is a timing chart showing a control method of the fuel cell system according to a modification of the first embodiment of the present invention. In this modification, with respect to the control method of the fuel cell system shown in the first embodiment, at least one of water and highly humidified gas is supplied only to the fuel electrode.

  In FIG. 3, the load of the fuel cell starts to decrease from time t1, and accordingly, the voltage of the fuel cell stack and each cell voltage increase. Then, it is assumed that the fuel cell voltage detected by the output voltage detector 18 becomes the high voltage V1 at time t2. When the control device 22 recognizes that the cell voltage has reached the high voltage V1, the three-way valve 11 is switched to the fuel humidifying means 9 side, and at least one of water or highly humidified gas is supplied only to the fuel electrode, The water content of the electrolyte membrane and the catalyst layer can be increased, and drying can be suppressed. At the same time, flooding at the oxidizer electrode when power generation is resumed can be suppressed.

  The fuel cell system according to Embodiment 2 of the present invention supplies a highly humidified gas to at least one of the electrodes during idling or no load when the power (or current) extracted from the fuel cell stack is minimum. In addition, deterioration due to drying of the electrolyte membrane and the catalyst layer is avoided.

  FIG. 4 is a timing chart showing a control method of the fuel cell system according to Embodiment 2 of the present invention. In FIG. 4, the load of the fuel cell starts to decrease from time t1, and accordingly, the voltage of the fuel cell stack and each cell voltage increase. Then, it is assumed that the fuel cell load detected by the load detection means 19 reaches the minimum load current I1 at time t2.

  During idling or no load when the power (or current) extracted from the fuel cell stack is minimal, the amount of water generated during power generation is small, and water is diffused by evaporation from the electrolyte membrane to the gas in the flow path. Etc., the drying of the electrolyte membrane and the catalyst layer proceeds.

  In this embodiment, it is determined that the fuel cell load detected by the load detection means 19 is in an idling state when the load current I1 reaches the minimum load current I1, and at least one of the three-way valve 11 or 13 is humidified by the control device 22. By switching to the means (9 or 10) side and introducing at least one of water or a highly humidified gas into at least one of the electrodes, the water content of the electrolyte membrane and the catalyst layer is increased and drying is suppressed. The minimum load current I1 is preferably within 10 [A].

  FIG. 12 is a control flowchart of the control device 22 according to the second embodiment. This flowchart is called at predetermined time intervals (for example, 100 [ms]) and executed by the control device 22.

  First, in step (hereinafter, step is abbreviated as S) 20, the operating state of the fuel cell is detected. In the present embodiment, the load current of the fuel cell stack 1 is detected by the load detection means 19 as the operating state of the fuel cell.

  Next, in S22, it is determined whether or not the operating state of the fuel cell is in a state where it is necessary to increase the water content of the electrolyte membrane and the catalyst layer. In this embodiment, it is determined whether or not the load current of the fuel cell stack is equal to or less than the minimum value I1.

  If it is determined in S22 that the load current is equal to or less than the minimum value I1, the process proceeds to S24, and the water content increase processing of the electrolyte membrane and the catalyst layer is performed. This moisture content increasing process is performed by switching at least one of the three-way valve 11 or 13 to the humidifying means (9 or 10) side by the control device 22 and introducing at least one of water or a highly humidified gas into at least one electrode. is there.

  If it is determined in S22 that the load current exceeds the minimum value I1, the process proceeds to S26, and the normal operation, that is, the three-way valves 11 and 13 select the fuel supply pipe 5 and the oxidant supply pipe 7, respectively, and humidify them. The fuel cell system is operated in a state where the fuel gas and the oxidant gas that are not supplied are supplied to the fuel cell stack 1.

  FIG. 5 is a timing chart showing a control method of the fuel cell system according to Embodiment 3 of the present invention. In FIG. 5, the load of the fuel cell starts to decrease from time t1, and accordingly, the voltage of the fuel cell stack and the voltage of each cell increase. Then, it is assumed that at time t2, the cell voltage reaches the predetermined voltage V1, and the state where the cell voltage is at a high voltage continues for a time T1 from time t2 to time t3.

  When the state in which the cell voltage of the fuel cell stack is maintained at a high voltage continues for a long time, the drying of the electrolyte membrane and the catalyst layer proceeds with time. In the present embodiment, when the fuel cell voltage detected by the output voltage detection means 18 is at the high voltage V1, when the predetermined time T1 has elapsed, at least one of the three-way valve 11 or 13 is humidified by the control device 22. By switching to the means (9 or 10) side and introducing at least one of water or a highly humidified gas into at least one of the electrodes, the water content of the electrolyte membrane and the catalyst layer is increased and drying is suppressed.

  The control flowchart of the control device 22 in the present embodiment is the same as the control flowchart of the second embodiment shown in FIG. However, the operating state of the fuel cell detected in S20 is that the cell voltage continues to be in a high voltage state, and whether or not the water content increasing process in S22 is necessary is determined by whether the high voltage of the cell voltage is It is whether or not the predetermined time T1 is continued. This predetermined time T1 is, for example, based on a result of investigating how long the state of the cell voltage of 0.85 [V] continues due to the actual device of the fuel cell system and causes deterioration on the fuel cell stack 1, The value to be set.

  There is a correlation between the electrolyte membrane resistance and the dry state of the electrolyte membrane in a polymer electrolyte fuel cell, and the dry state (wet state) of the electrolyte membrane is indirectly accurately detected by detecting the electrolyte membrane resistance. can do.

  FIG. 6 is a timing chart showing a control method of the fuel cell system according to Embodiment 4 of the present invention. In FIG. 6, the load of the fuel cell starts decreasing from time t1, and accompanying this, the resistance value of the electrolyte membrane of the fuel cell stack increases. At time t2, it is assumed that the electrolyte membrane resistance value detected by the electrolyte membrane resistance detection means 20 becomes a predetermined value or more.

  In this embodiment, when the electrolyte membrane resistance becomes equal to or greater than a predetermined value, the control device 22 determines that the electrolyte membrane is in a dry state, and at least one of the three-way valve 11 or 13 is humidified (9 or 10). By switching to the side and introducing at least one of water or a highly humidified gas into at least one of the electrodes, the water content of the electrolyte membrane and the catalyst layer is increased and drying is suppressed.

  The control flowchart of the control device 22 in the present embodiment is the same as the control flowchart of the second embodiment shown in FIG. However, the operating state of the fuel cell detected in S20 is the resistance of the electrolyte membrane, and whether or not the water content increasing process is necessary in S22 is determined whether or not the electrolyte membrane resistance value is equal to or higher than a predetermined value. It is. The predetermined value is set based on, for example, a result of investigating the relationship between the electrolyte membrane resistance value and the deterioration of the fuel cell stack 1 using an actual fuel cell system.

  7 and 8 are timing charts showing a control method of the fuel cell system according to Embodiment 5 of the present invention. When the fuel cell voltage is high, the electrolyte membrane resistance is detected by the electrolyte membrane resistance detector 20. The dry state of the electrolyte membrane is detected from the difference between the resistance value and the predetermined value at this time, but the first predetermined value R1 and larger than this are used as the predetermined value in order to effectively supply water according to the dryness. A predetermined value R2 is set.

  When the resistance value R detected by the electrolyte membrane resistance detection means 20 is R1 ≦ R ≦ R2 during the period from time t1 to time t2 in FIGS. 7 and 8, at least one of the three-way valves 11 or 13 is humidified ( Switch to the 9 or 10) side and introduce a highly humidified gas into at least one of the electrodes. Still, drying proceeds, and it is assumed that the resistance value R detected by the electrolyte membrane resistance detection means 20 exceeds R2 at time t3. Since the progress of drying is large when the detected resistance value R is R2 <R, rapid water supply is performed by supplying water and a highly humidified gas simultaneously. Furthermore, in any of the above cases, the moisture is supplied until the electrolyte membrane resistance falls within a predetermined range, thereby controlling the water content of the electrolyte membrane and the catalyst layer and effectively suppressing drying. For example, the water is supplied to the fuel cell stack 1 by further spraying water onto the gas after humidification in the fuel humidifying means 9 or the oxidant humidifying means 10.

  FIG. 9 is a timing chart showing a control method of the fuel cell system according to Embodiment 6 of the present invention. In the state where the fuel cell voltage is high, when moisture is supplied by judging the dry state from the electrolyte membrane resistance value detected by the electrolyte membrane resistance detecting means 20, in some cases, excessive moisture is supplied. There is a possibility that it will end. If the water content of the electrolyte membrane is too high, flooding is likely to occur when power generation is resumed, leading to a significant performance degradation of the system.

  In the present embodiment, in addition to the predetermined resistance values R1 and R2 shown in the fifth embodiment, a minimum resistance value R3 for keeping the electrolyte membrane in an appropriate wet state is provided. When <R3, at least one gas having a humidity lower than that used for supplying moisture is supplied to at least one electrode to remove excess moisture, and the electrolyte membrane resistance R is set to R3 ≦ R ≦ R1. Control within range. As a result, the water content of the electrolyte membrane and the catalyst layer can be maintained, and at the same time, flooding when power generation is resumed can be suppressed.

  The fuel cell system according to the seventh embodiment lowers the stack temperature by controlling the flow rate of the cooling water when the cell voltage is high, and the water from the electrolyte membrane to the remaining gas in the stack gas flow path. The amount of evaporation is reduced, and drying of the electrolyte membrane and the catalyst layer is suppressed.

  FIG. 10 is a timing chart showing a control method of the fuel cell system according to Embodiment 7 of the present invention. When the fuel cell voltage is high and the gas is not flowed in order to improve fuel efficiency, the electrolyte membrane is dried due to the absence of water generation and the diffusion of water from the electrolyte membrane to the residual gas in the flow path. Progresses.

  In the present embodiment, the dew point is lowered by increasing the flow rate of the cooling water by the cooling water pump 16 to lower the temperature of the fuel cell stack 1, and the evaporation of water from the electrolyte membrane and the catalyst layer is suppressed and drying is performed. Can be prevented.

  FIG. 11 is a timing chart showing a control method of the fuel cell system according to Embodiment 8 of the present invention. Since there is a correlation between the electrolyte membrane resistance and the dry state of the electrolyte membrane, the dry state (wet state) of the electrolyte membrane is indirectly accurately detected by detecting the electrolyte membrane resistance by the electrolyte membrane resistance detecting means 20. Can be detected.

  In this embodiment, when the electrolyte membrane resistance becomes larger than a predetermined value, it is determined that the electrolyte membrane is in a dry state, and the temperature of the fuel cell stack 1 is lowered by increasing the flow rate of the cooling water, thereby By controlling the resistance value to be within a predetermined range, it is possible to prevent excessive cooling of the fuel cell stack, to suppress a decrease in system efficiency associated therewith, and to prevent flooding when power generation is resumed.

1 is a system configuration diagram showing a configuration example of a fuel cell system according to the present invention. 3 is a time chart for explaining operation control in the fuel cell system according to the first embodiment. 6 is a time chart illustrating operation control in a fuel cell system according to a modification of Example 1. 6 is a time chart for explaining operation control in the fuel cell system of Example 2. 6 is a time chart illustrating operation control in the fuel cell system of Example 3. 6 is a time chart illustrating operation control in the fuel cell system of Example 4. 10 is a time chart illustrating operation control in the fuel cell system of Example 5. 10 is a time chart illustrating operation control in the fuel cell system of Example 5. 10 is a time chart illustrating operation control in the fuel cell system of Example 6. 12 is a time chart illustrating operation control in the fuel cell system of Example 7. 10 is a time chart illustrating operation control in the fuel cell system of Example 8. 3 is a flowchart for explaining operation control in the fuel cell system according to the first embodiment. 6 is a flowchart for explaining operation control in the fuel cell systems of Examples 2 to 4.

Explanation of symbols

1: Fuel cell stack 2: Fuel tank 3: Fuel supply amount adjustment valve 4: Oxidant blower 5: Fuel supply pipe 6: Fuel exhaust pipe 7: Oxidant supply pipe 8: Oxidant exhaust pipe 9: Fuel humidification means 10: Oxidant humidification means 11: three-way valve 12: fuel bypass passage 13: three-way valve 14: oxidant bypass passage 15: cooling water tank 16: cooling water pump 17: cooling water path 18: output voltage detection means 19: load detection means 20 : Electrolyte membrane resistance detection means 21: Temperature sensor 22: Control device

Claims (13)

A fuel cell system including a fuel cell stack in which unit cells each having a membrane electrode assembly in which a catalyst layer of a fuel electrode and an oxidant electrode and gas diffusion electrodes are arranged on both surfaces of an electrolyte membrane are stacked,
The fuel cell system, wherein the water content of the electrolyte membrane and the catalyst layer is increased when a cell voltage, which is a voltage of the unit cell, becomes equal to or higher than a predetermined voltage. Voltage detecting means for detecting a cell voltage of each unit cell in the fuel cell stack;
2. The fuel cell system according to claim 1, wherein the predetermined voltage is 0.85 [V]. When the current or power taken out from the fuel cell stack becomes a predetermined current or a predetermined power or less,
The fuel cell system according to claim 1 or 2, wherein the water content of the electrolyte membrane and the catalyst layer is increased.   4. The water content of the electrolyte membrane and the catalyst layer is increased when a state in which the cell voltage is at a high voltage elapses for a predetermined time or longer. 5. Fuel cell system.   5. The water content is increased by supplying at least one of water and a highly humidified gas to at least one of the fuel electrode and the oxidant electrode. The fuel cell system according to any one of the above.   6. The fuel cell system according to claim 5, wherein at least one of the electrodes is a fuel electrode. An electrolyte membrane resistance detecting means for detecting a resistance value of the electrolyte membrane of any cell in the fuel cell stack;
6. The method according to claim 5, wherein at least one of water and a highly humidified gas is supplied when an electrolyte membrane resistance value detected by the electrolyte membrane resistance detection means becomes equal to or greater than a first predetermined resistance value. Item 7. The fuel cell system according to Item 6.   8. The fuel cell system according to claim 7, wherein the amount of supplied water is adjusted according to the difference between the electrolyte membrane resistance value and the first predetermined resistance value. 9.   9. The fuel cell system according to claim 8, wherein water and a highly humidified gas are supplied when the electrolyte membrane resistance value is equal to or greater than a second predetermined resistance value that is greater than the first predetermined resistance value.   The fuel cell system according to any one of claims 7 to 9, wherein moisture is supplied until the electrolyte membrane resistance value falls within a predetermined range.   The gas having a lower humidity than the gas is supplied to at least one of the electrodes when the electrolyte membrane resistance value is less than a first predetermined resistance value. The fuel cell system described in 1.   The fuel cell system according to any one of claims 1 to 4, wherein the water content is increased by lowering a temperature of the fuel cell stack.   13. The fuel cell system includes an electrolyte membrane resistance detection unit, and the temperature is controlled so that the electrolyte membrane resistance detected by the electrolyte membrane resistance detection unit falls within a predetermined range. Fuel cell system.

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