Fuel cell system

Abstract

To provide a fuel cell system capable of suppressing the degradation of a catalyst used in a fuel cell stack.SOLUTION: The fuel cell system includes: a fuel cell stack in which multiple fuel cells are laminated and arranged; a battery that is electrically connected to the fuel cell stack and charges power generated by the fuel cell stack; and a control device that controls the power generation by the fuel cell stack. The control device is configured so as to, when the fuel cell system is started, be able to execute a low-voltage operation prior to shifting to a normal operation. In the low-voltage operation, the power generation by the fuel cell stack is controlled so that an output voltage of the fuel cell is maintained to be equal to or less than a first voltage value, and in the normal operation, the power generation by the fuel cell stack is controlled so that the output voltage of the fuel cell is maintained to be equal to or more than a second voltage value which is higher than the first voltage value.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The technology disclosed in this specification relates to a fuel cell system including a plurality of fuel cells. [Background technology]

[0002] Patent Document 1 describes a fuel cell system. This fuel cell system includes a fuel cell stack having a plurality of fuel cell units, and a control device that controls power generation by the fuel cell stack. The control device is configured to be able to perform voltage control operation when the fuel cell system is started up. In voltage control operation, after start-up, the output voltage of the fuel cell units is increased to a first voltage value, and then controlled to maintain the first voltage value for a predetermined period of time. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2007-287674 Summary of the Invention [Problem to be solved by the invention]

[0004] Generally, fuel cells have a current-voltage characteristic (so-called IV characteristic) in which the output voltage decreases as the output current increases. Therefore, when a fuel cell stack generates power, suppressing the output current and increasing the output voltage of the fuel cell (hereinafter also referred to as cell voltage) can improve the power generation efficiency of the fuel cell. On the other hand, the higher the cell voltage during power generation, the more likely it is that the catalyst (e.g., platinum-based fine particles) contained in the fuel cell will dissolve and coarsen, accelerating catalyst degradation.

[0005] In view of the above, the present specification provides a novel and useful technique for suppressing catalyst degradation in fuel cells. [Means for solving the problem]

[0006] The technology disclosed in this specification is embodied in a fuel cell system. The fuel cell system includes a fuel cell stack in which multiple fuel cells are stacked, a battery electrically connected to the fuel cell stack and charged with power generated by the fuel cell stack, and a control device that controls power generation by the fuel cell stack. The control device is configured to be able to perform low-voltage operation when the fuel cell system starts up and before transitioning to normal operation. In low-voltage operation, power generation by the fuel cell stack is controlled so that the output voltage of the fuel cell cells is maintained at or below a first voltage value, and in normal operation, power generation by the fuel cell stack is controlled so that the output voltage of the fuel cell cells is maintained at or above a second voltage value that is higher than the first voltage value.

[0007] With the above configuration, when the fuel cell system is started, low-voltage operation can be performed prior to the start of normal operation. In low-voltage operation, the output voltage (cell voltage) of the fuel cell unit is maintained at a level lower than that of normal operation, while the fuel cell stack generates electricity. As a result, an oxide film is formed on the surface of the catalyst in each fuel cell unit. As a result, even if the output voltage of the fuel cell unit is maintained high during subsequent normal operation, the oxide film formed on the catalyst surface suppresses catalyst elution and coarsening. In other words, catalyst degradation is suppressed.

[0008] In one embodiment of the present technology, when the fuel cell system is started, the control device may omit execution of low voltage operation if the battery's charging rate exceeds a first charging rate. In low voltage operation, the cell voltage is maintained at a relatively low level, so the current output from the fuel cell stack becomes relatively large. Therefore, when the battery's charging rate is relatively high, execution of low voltage operation may be omitted, thereby preventing the battery from being overcharged.

[0009] In one embodiment of the present technology, the low voltage operation may include a first low voltage operation in which power generation by the fuel cell stack is controlled so that the output voltage of the fuel cell is maintained at or below a third voltage value lower than the first voltage value, and a second low voltage operation in which power generation by the fuel cell stack is controlled so that the output voltage of the fuel cell is maintained between the third voltage value and the first voltage value.

[0010] According to the above configuration, the cell voltage can be changed in at least two stages during low-voltage operation. In particular, in the early stage of low-voltage operation, the cell voltage can be maintained lower to promote the formation of an oxide film. Then, in the later stage of low-voltage operation, the cell voltage can be increased to suppress the power generated by the fuel cell stack while promoting further growth of the oxide film. Furthermore, increasing the cell voltage and reducing the power generated by the fuel cell stack can also ease the load on the battery.

[0011] In one embodiment of the present technology, when the fuel cell system is started, the control device may perform the first low voltage operation and the second low voltage operation in that order when the battery charging rate falls below a second charging rate that is lower than the first charging rate, and may omit the first low voltage operation and perform the second low voltage operation when the battery charging rate is between the second charging rate and the first charging rate.

[0012] According to the above configuration, both the first and second low voltage operations or only the second low voltage operation can be performed depending on the charging rate of the battery. That is, the low voltage operation can be performed in an appropriate manner depending on the charging rate of the battery.

[0013] In one embodiment of the present technology, the control device may transition to the second low-voltage operation when the charging rate of the battery reaches the first charging rate after starting the first low-voltage operation. With this configuration, it is possible to perform the low-voltage operation in an appropriate manner according to the charging rate of the battery while preventing the battery from being overcharged. [Brief explanation of the drawings]

[0014] [Figure 1] FIG. 2 is a block diagram showing the configuration of a fuel cell system 2. [Figure 2] 3 is a flowchart showing a series of processes executed by a control device 30 in a fuel cell system 2 in the first embodiment. [Figure 3] 1 is a graph showing the change over time in the output voltage of a fuel cell 12 in a fuel cell system 2 in Example 1. The graph also shows the process of forming an oxide film (PtO) on a catalyst (Pt). [Figure 4] 10 is a flowchart showing a series of processes executed by a control device 30 in a fuel cell system 2 in a second embodiment. [Figure 5] 10 is a graph showing the change over time in the output voltage of the fuel cell 12 in the fuel cell system 2 of Example 2. The graph also shows the process of forming an oxide film (PtO) on the catalyst (Pt). [Figure 6] 10 is a flowchart showing a series of processes executed by a control device 30 in a fuel cell system 2 according to a third embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0015] (First Example) A fuel cell system 2 according to a first embodiment will be described with reference to the drawings. As an example, the fuel cell system 2 according to the first embodiment can be used in a fuel cell electric vehicle (FCEV). The configuration described in this embodiment is not limited to electric vehicles, but can also be used as a power source for other types of devices and equipment that require electric power.

[0016] As shown in FIG. 1, the fuel cell system 2 includes a fuel cell stack 10, a boost converter 20, a control device 30, a battery 40, and an electrical load 50. The fuel cell stack 10 is configured by stacking a plurality of fuel cell units 12. The fuel cell system 2 also includes a hydrogen supply system 14 and an air supply system 16. The hydrogen supply system 14 supplies a fuel gas containing hydrogen (hereinafter referred to as hydrogen gas) to the fuel cell stack 10. The air supply system 16 supplies air containing oxygen to the plurality of fuel cell units 12. In the fuel cell stack 10, each fuel cell unit 12 generates electricity by reacting hydrogen supplied from the hydrogen supply system 14 with oxygen in compressed air supplied from the air supply system 16. The electricity generated by the fuel cell stack 10 is supplied to the battery 40 or the electrical load 50 via the boost converter 20.

[0017] The boost converter 20 is electrically connected to the fuel cell stack 10. The boost converter 20 is configured to boost the output power from the fuel cell stack 10. The specific configuration of the boost converter 20 is not particularly limited. As an example, the boost converter 20 may be a non-insulated converter that uses a switching element and a coil.

[0018] The control device 30 controls the operation of the hydrogen supply system 14, the air supply system 16, and the boost converter 20. That is, the control device 30 controls the flow rate and pressure of hydrogen gas supplied to the fuel cell stack 10 by controlling the operation of the hydrogen supply system 14. The control device 30 also controls the operation of the air supply system 16, thereby controlling the flow rate and pressure of air supplied to the fuel cell stack 10. The control device 30 is connected to the hydrogen supply system 14, the air supply system 16, and the boost converter 20 via signal lines. The control device 30 is also connected to the fuel cell stack 10 via signal lines, and monitors the temperature of the fuel cell stack 10 and the output voltage (cell voltage) of each fuel cell 12. The control device 30 is equipped with a processor and memory, and controls the operation of the hydrogen supply system 14, the air supply system 16, and the boost converter 20 by the processor executing various programs stored in the memory.

[0019] The battery 40 is a high-voltage battery. Here, the high voltage means an operating voltage exceeding 60 V DC. The battery 40 incorporates a plurality of secondary battery cells and is configured to be chargeable and dischargeable. The secondary battery cells are not particularly limited and may be, for example, lithium-ion batteries or full individual batteries.

[0020] The electrical load 50 is a device driven by the electric power supplied from the fuel cell stack 10 or the battery 40. The specific configuration of the electrical load 50 is not particularly limited. As an example, the electrical load 50 may be a drive device of a fuel cell vehicle and may include an electric motor that drives the wheels and an inverter that controls the power supplied to the electric motor.

[0021] Referring to FIGS. 2 and 3, a series of processes executed by the control device 30 in the fuel cell system 2 of the first embodiment will be described. The control device 30 is configured to start a series of processes shown in FIG. 2 when the fuel cell system 2 is activated. The fuel cell system 2 is activated, for example, in response to a user's operation. When the fuel cell system 2 is activated (S10), the control device 30 determines whether the charge rate (SOC: State of Charge) of the battery 40 is less than the first charge rate (SOC1) (S12). If the charge rate of the battery 40 is less than the first charge rate (SOC < SOC1) (S12: Yes), the control device 30 starts low-voltage operation (S14). As shown in FIG. 3, in the low-voltage operation (period X in the figure), the power generation of the fuel cell stack 10 is controlled so that the cell voltage of the fuel cell 12 is maintained at or below the first voltage value. The control device 30 controls the power generation (including the cell voltage) of the fuel cell stack 10 by controlling the operations of the hydrogen supply system 14, the air supply system 16, and the boost converter 20. The power generated by the fuel cell stack 10 is charged to the battery 40 via the boost converter 20. As an example, the first voltage value may be a value such as 600 millivolts, 700 millivolts, or 800 millivolts.

[0022] The low voltage operation is performed for a predetermined time (S16). As an example, the predetermined time may be 10 seconds, 15 seconds, 20 seconds, or 60 seconds. When the predetermined time has elapsed (S16: YES), the control device 30 ends the low voltage operation and transitions to normal operation (S18). In normal operation, power generation by the fuel cell stack 10 is controlled so that the cell voltage of the fuel cell 12 is maintained at or above a second voltage value. Here, the second voltage value is higher than the first voltage value. As an example, the second voltage value may be 800 millivolts, 850 millivolts, or 900 millivolts. Thereafter, when the operation of the fuel cell system 2 is stopped, the series of processing steps shown in FIG. 2 is terminated (END).

[0023] On the other hand, if the charging rate of the battery 40 exceeds the first charging rate in step S12 described above (S12: NO), the control device 30 omits execution of the low voltage operation and starts normal operation (S18). In low voltage operation, the current output from the fuel cell stack 10 needs to be relatively large in order to maintain the cell voltage at a relatively low level. As a result, the charging current supplied to the battery 40 also increases, so in order to execute low voltage operation, the charging rate of the battery 40 needs to have a sufficient margin. Therefore, when the charging rate of the battery 40 is relatively high, it is advisable to omit execution of the low voltage operation, thereby preventing the battery 40 from being overcharged.

[0024] As described above, the fuel cell system 2 of this embodiment can perform low-voltage operation prior to starting normal operation. As shown in FIG. 3, during low-voltage operation (period X in FIG. 3), power generation is performed in the fuel cell stack 10 while the output voltage (cell voltage) of the fuel cell 12 is maintained at a level lower than that of the subsequent normal operation. As a result, an oxide film (PtO) is formed on the surface of the catalyst (Pt) in each fuel cell 12. As a result, even if the output voltage of the fuel cell 12 is maintained high during the subsequent normal operation, the oxide film formed on the catalyst surface suppresses catalyst elution and coarsening. In other words, catalyst degradation is suppressed.

[0025] (Second Embodiment) Referring to FIGS. 4 and 5, the fuel cell system of the second embodiment will be described. The fuel cell system of this embodiment has the content of the processing executed by the control device 30 changed as compared with the fuel cell system 2 of the first embodiment. That is, the control device 30 in this embodiment is configured to execute a series of processes shown in FIG. 4 instead of the series of processes shown in FIG. 2. In other respects, the fuel cell system of this embodiment has the same configuration as the fuel cell system 2 of the first embodiment shown in FIG. 1. That is, the fuel cell system of this embodiment also includes a fuel cell stack 10, a boost converter 20, a control device 30, a battery 40, an electrical load 50, a hydrogen supply system 14, and an air supply system 16 (see FIG. 1). The configurations and functions of these are as described in the first embodiment, and duplication of the description will be avoided here.

[0026] Also in the fuel cell system of this embodiment, the control device 30 is configured to start a series of processes shown in FIG. 4 when the fuel cell system is started. When the fuel cell system is started (S20), the control device 30 determines whether the charge rate (SOC) of the battery 40 is less than the second charge rate (SOC2) (S22). The second charge rate is set to be lower than the first charge rate described in the first embodiment (that is, SOC2 < SOC1). When the charge rate of the battery 40 is not less than the second charge rate (S22: NO), the control device 30 starts normal operation (S34) without performing the following first low voltage operation (S24) and second low voltage operation (S30). Similar to the first embodiment, in normal operation, the power generation by the fuel cell stack 10 is controlled so that the output voltage (cell voltage) of the fuel cell 12 is maintained at or higher than the second voltage value.

[0027] On the one hand, when the charge rate of the battery 40 is less than the second charge rate (SOC < SOC2) (S22: YES), the control device 30 starts the first low-voltage operation (S24). As shown in FIG. 5, in the first low-voltage operation (period Y in the figure), the power generation of the fuel cell stack 10 is controlled so that the cell voltage of the fuel cell 12 is maintained at or below the third voltage value. Although not particularly limited, the third voltage value is lower than the first voltage value described in the first embodiment (that is, the third voltage value < the first voltage value). As an example, the third voltage value may be a value such as 500 millivolts, 600 millivolts, or 700 millivolts.

[0028] The first low-voltage operation is executed for a first predetermined time (S26). As an example, the first predetermined time may be a value such as 10 seconds, 15 seconds, 20 seconds, 60 seconds, or 90 seconds. When the first predetermined time has elapsed (S26: YES), the control device 30 ends the first low-voltage operation and shifts to the second low-voltage operation (S30). As shown in FIG. 5, in the second low-voltage operation (period Z in the figure), the power generation by the fuel cell stack 10 is controlled so that the cell voltage of the fuel cell 12 is maintained at or above the first voltage value. As described above, the first voltage value may be a value such as 800 millivolts, 850 millivolts, or 900 millivolts.

[0029] The second low-voltage operation is executed for a second predetermined time (S32). As an example, the second predetermined time may be a value such as 10 seconds, 15 seconds, 20 seconds, 60 seconds, or 90 seconds. When the second predetermined time has elapsed (S32: YES), the control device 30 ends the second low-voltage operation and shifts to the normal operation (S34). In the normal operation, the power generation by the fuel cell stack 10 is controlled so that the cell voltage of the fuel cell 12 is maintained at or above the second voltage value. As described above, the second voltage value is higher than the first voltage value. The second voltage value may be a value such as 800 millivolts, 850 millivolts, or 900 millivolts. After that, when the operation of the fuel cell system 2 is stopped, the series of processing procedures shown in FIG. 4 ends (end).

[0030] As described above, the fuel cell system of the second embodiment can perform first and second low-voltage operations prior to starting normal operation. As shown in FIG. 5 , during the first low-voltage operation (period Y) and the second low-voltage operation (period Z), the cell voltage of the fuel cell 12 is maintained at a lower level than during the subsequent normal operation, while the fuel cell stack 10 generates electricity. Additionally, during the series of low-voltage operations (periods Y and Z), the cell voltage is changed in two stages. In particular, during the first low-voltage operation in the early stage, the cell voltage is maintained lower, thereby activating the oxidation reaction of the catalyst (Pt) and promoting the formation of an oxide film (PtO). During the second low-voltage operation in the later stage, the cell voltage is increased, thereby suppressing the power generated by the fuel cell stack 10 and promoting further growth of the oxide film (PtO). As a result, even when the cell voltage of the fuel cell 12 is maintained high during the subsequent normal operation, the oxide film formed on the catalyst surface suppresses catalyst elution and coarsening. Furthermore, by increasing the cell voltage and reducing the power generated by the fuel cell stack 10, the load on the battery 40 can also be alleviated.

[0031] (Third Example) A fuel cell system of the third embodiment will be described with reference to FIG. 6. The fuel cell system of the third embodiment is different from the fuel cell system 2 of the first embodiment in that the process executed by the control device 30 is modified. That is, the control device 30 of the present embodiment is configured to execute the series of processes shown in FIG. 6 instead of the series of processes shown in FIG. 2. In other respects, the fuel cell system of the present embodiment has the same configuration as the fuel cell system 2 of the first embodiment shown in FIG. 1. That is, the fuel cell system of the present embodiment also includes a fuel cell stack 10, a boost converter 20, a control device 30, a battery 40, an electrical load 50, a hydrogen supply system 14, and an air supply system 16 (see FIG. 1). The configurations and functions of these components are the same as those described in the first embodiment, and therefore will not be described again here.

[0032] In the fuel cell system of this embodiment as well, when the fuel cell system is started up, the control device 30 starts a series of processes shown in FIG. 6. The series of processes shown in FIG. 6 is different from the series of processes of the second embodiment shown in FIG. 4 in that the process of determining the state of charge of the battery 40 (S22; S22A, S22B) is changed. When the fuel cell system is started up (S20), the control device 30 first determines whether the state of charge (SOC) of the battery 40 is less than the first state of charge (SOC1) (S22A). If the state of charge of the battery 40 is not less than the first state of charge (S22A: NO), the control device 30 starts normal operation (S34) without performing the first low-voltage operation (S24) and the second low-voltage operation (S30).

[0033] If the state of charge of the battery 40 is less than the first state of charge (SOC < SOC1) (S22A: YES), the control device 30 further determines whether the state of charge of the battery 40 is less than the second state of charge (SOC2) (S22B). As described above, the second state of charge is lower than the first state of charge (i.e., SOC2 < SOC1). If the state of charge of the battery 40 is less than the second state of charge (SOC < SOC2) (S22B: YES), the control device 30 performs the first low-voltage operation (S24) and the second low-voltage operation (S30) in that order. On the other hand, if the state of charge of the battery 40 is not less than the second state of charge (S22B: NO), the control device 30 omits the execution of the first low-voltage operation (S24) and only performs the second low-voltage operation (S30). Since the first low-voltage operation and the second low-voltage operation are as described in the second embodiment, duplicate explanations are omitted here. After that, the control device 30 shifts to normal operation (S34).

[0034] As described above, the fuel cell system of the third embodiment can perform both the first and second low voltage operations, or can perform only the second low voltage operation, depending on the charging rate of the battery 40. That is, the low voltage operation can be performed in an appropriate manner depending on the charging rate of the battery 40. As an example, after starting the first low voltage operation, the control device 30 may transition to the second low voltage operation when the charging rate of the battery 40 reaches the first charging rate. With this configuration, if the charging rate of the battery 40 increases more than expected after starting the first low voltage operation, the first low voltage operation can be stopped and the operation can be transitioned to the second low voltage operation at that point. The low voltage operation can be performed in an appropriate manner depending on the charging rate of the battery 40 while preventing the battery 40 from being overcharged.

[0035] Although specific examples of the technology disclosed in this specification have been described in detail above, these are merely examples and do not limit the scope of the claims. The technology described in the claims includes various modifications and variations of the specific examples exemplified above. The technical elements described in this specification or drawings exhibit technical utility either alone or in various combinations, and are not limited to the combinations described in the claims at the time of filing. Furthermore, the technology exemplified in this specification or drawings can achieve multiple objectives simultaneously, and achieving one of these objectives itself has technical utility. [Explanation of symbols]

[0036] 2: fuel cell system, 10: fuel cell stack, 12: fuel cell cell, 14: hydrogen supply system, 16: air supply system, 20: boost converter, 30: control device, 40: battery, 50: electrical load,

Claims

1. 1. A fuel cell system, comprising: a fuel cell stack in which a plurality of fuel cell units are stacked; a battery electrically connected to the fuel cell stack and configured to charge the battery with the electricity generated by the fuel cell stack; a control device for controlling power generation by the fuel cell stack; Equipped with the control device is configured to be able to perform low-voltage operation prior to transitioning to normal operation when the fuel cell system is started up, In the low voltage operation, power generation by the fuel cell stack is controlled so that the output voltage of the fuel cell unit is maintained at a first voltage value or less; During the normal operation, power generation by the fuel cell stack is controlled so that the output voltage of the fuel cell unit is maintained at or above a second voltage value that is higher than the first voltage value. Fuel cell system.

2. 2. The fuel cell system according to claim 1, wherein the control device omits execution of the low voltage operation when the charging rate of the battery exceeds a first charging rate when the fuel cell system is started up.

3. The low voltage operation is a first low voltage operation in which power generation by the fuel cell stack is controlled so that the output voltage of the fuel cell is maintained at or below a third voltage value that is lower than the first voltage value; a second low voltage operation in which power generation by the fuel cell stack is controlled so that the output voltage of the fuel cell unit is maintained between the third voltage value and the first voltage value; 3. The fuel cell system of claim 2, comprising:

4. When the fuel cell system is started, the control device When the charging rate of the battery falls below a second charging rate that is lower than the first charging rate, the first low voltage operation and the second low voltage operation are performed in that order; 4. The fuel cell system according to claim 3, wherein when the charging rate of the battery is between the second charging rate and the first charging rate, the first low voltage operation is omitted and the second low voltage operation is performed.

5. 5. The fuel cell system according to claim 3, wherein the control device transitions to the second low voltage operation when the charging rate of the battery reaches the first charging rate after the first low voltage operation is started.

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