Fuel cell system and fuel cell control method

Abstract

This prevents the voltage of the fuel cell unit from dropping or even becoming negative due to the presence of liquid water in the fuel cell stack, which can lead to deterioration of the catalyst layer. [Solution] Fuel gas is supplied to a fuel cell stack having an electrolyte membrane, a catalyst layer, etc., and at least a portion of the gas discharged from the fuel cell stack is circulated through the fuel cell stack, and the stack temperature, which is the temperature of the fuel cell stack, and the circulation system temperature, which is the temperature of the fuel gas circulation system, are acquired. When the temperature difference between the stack temperature and the circulation system temperature exceeds a predetermined specified temperature difference, the proportion of fuel gas supplied to the fuel cell units is increased compared to when the temperature difference is equal to or less than the specified temperature difference.

Description

[Technical Field]

[0001] The present disclosure relates to a fuel cell system and a method for controlling a fuel cell. [Background technology]

[0002] In a fuel cell system that generates power using a fuel cell stack that uses a solid electrolyte membrane, if liquid water enters the fuel cell stack, the liquid water may clog the fuel gas flow path in the fuel cell. If the fuel gas flow path is clogged, the fuel gas will be insufficient, the voltage between the electrodes of the fuel cell will become negative, and the catalyst arranged in the fuel cell will deteriorate. For this reason, various measures have been taken to prevent the intrusion of liquid water. For example, in the following cited document 1, a porous filter is placed upstream of the fuel gas flow path of the fuel cell to prevent liquid water from entering the fuel cell. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 2018-60717 Summary of the Invention [Problem to be solved by the invention]

[0004] However, the behavior of liquid water within a fuel cell stack varies widely, and even if a filter is placed in the gas flow path of the fuel cell, it is practically difficult to completely prevent liquid water from seeping in. As a result, the presence of liquid water can cause the voltage of the fuel cell to drop or, in some cases, become negative, which can lead to deterioration of the catalyst layer. [Means for solving the problem]

[0005] The present disclosure can be realized in the following forms or application examples.

[0006] (1) One aspect of the present disclosure is a fuel cell system. The fuel cell system includes a fuel cell stack including a plurality of fuel cells, each having a gas flow path outside an electrolyte membrane, a catalyst layer, and a gas diffusion layer, stacked with separators interposed therebetween; a fuel gas circulation system that circulates at least a portion of the gas discharged from the fuel cell stack into the fuel gas supplied to the fuel cell stack; a first temperature acquisition unit that acquires a stack temperature, which is the temperature of the fuel cell stack; a second temperature acquisition unit that acquires a circulation system temperature, which is the temperature of the fuel gas circulation system; and a fuel gas control unit that, when the temperature difference between the stack temperature and the circulation system temperature exceeds a predetermined temperature difference, performs fuel gas control to increase the proportion of fuel gas supplied to the fuel cell cells compared to when the temperature difference is equal to or less than the predetermined temperature difference. This increases the proportion of fuel gas supplied to the fuel cell stack when liquid water is likely to accumulate in the fuel cell cells of the fuel cell stack, thereby preventing or avoiding situations such as a drop in the voltage of the fuel cell cells or the generation of negative voltage. This also prevents or avoids deterioration of the catalyst layers of the fuel cell cells. Hydrogen gas is a typical fuel gas, but any fuel gas that generates electricity through the exchange of protons, such as a mixed gas of hydrogen gas and methane, may be used. (2) The above configuration may further include a gas partial pressure acquisition unit that acquires the fuel gas partial pressure in the fuel gas circulation system, and an exhaust valve that is provided in the fuel gas circulation system and adjusts the discharge of the fuel gas from the fuel gas circulation system, wherein the fuel gas control unit controls the exhaust valve to adjust the fuel gas partial pressure so that it does not fall below a first reference gas partial pressure set as a reference gas partial pressure, and sets the reference gas partial pressure to a second reference gas partial pressure higher than the first reference gas partial pressure, thereby performing the fuel gas control. This makes it possible to easily increase the proportion of fuel gas in the fuel cell. (3) In each of the above configurations, a voltage detection unit may be further provided that detects the output voltage of the fuel cell stack, and when the control unit estimates the inflow of liquid water into the fuel cell units from the stack temperature and the output voltage, the control unit may set the fuel gas partial pressure to a third reference gas partial pressure higher than the second reference gas partial pressure and purge the fuel cell units. In this way, if a situation occurs in which liquid water continuously inflows into the fuel cell units and the power generation voltage of the fuel cell stack drops, the liquid water can be purged from the fuel cell units, making it easier to restore the fuel cell stack to a normal state. (4) In each of the above configurations, the fuel gas control unit may further adjust the stoichiometric ratio of the fuel gas supplied to the fuel cell stack, and perform stoichiometric control to increase the stoichiometric ratio of the fuel gas when the temperature difference between the stack temperature and the circulation system temperature exceeds a predetermined specified temperature difference. In this way, the proportion of fuel gas supplied to the fuel cell units can be easily increased through stoichiometric control, and even if liquid water is present in the fuel cell units, negative voltage of the fuel cell units can be suppressed or avoided, and catalyst degradation can also be suppressed or avoided. (5) In each of the above configurations, a liquid water sensor may be further provided to detect the presence of liquid water in the fuel gas circulation system, and the control unit may perform the fuel gas control even when the liquid water sensor detects the presence of liquid water in the fuel gas circulation system. Liquid water present in the fuel gas circulation system is likely to be carried to the fuel cell stack by the circulation of fuel gas. Therefore, this configuration can increase the proportion of fuel gas in the fuel cell before the effects of liquid water in the fuel cell occur, thereby making it possible to prevent the intrusion of liquid water into the fuel cell. (6) Another aspect of the present disclosure relates to a method for controlling a fuel cell. This fuel cell control method supplies fuel gas to a fuel cell stack, which is composed of multiple fuel cells stacked via separators, each having a gas flow path outside an electrolyte membrane, a catalyst layer, and a gas diffusion layer. At the same time, at least a portion of the gas discharged from the fuel cell stack is circulated through the fuel cell stack. A stack temperature, which is the temperature of the fuel cell stack, and a circulation system temperature, which is the temperature of the fuel gas circulation system, are acquired. When the temperature difference between the stack temperature and the circulation system temperature exceeds a predetermined temperature difference, fuel gas control is performed to increase the proportion of fuel gas supplied to the fuel cell compared to when the temperature difference is equal to or less than the predetermined temperature difference. This increases the proportion of fuel gas supplied to the fuel cell stack when liquid water is likely to accumulate in the fuel cell of the fuel cell stack, thereby suppressing or avoiding situations such as a drop in the voltage of the fuel cell or the generation of negative voltage. This also suppresses or avoids deterioration of the catalyst layer of the fuel cell. [Brief explanation of the drawings]

[0007] [Figure 1] FIG. 1 is a schematic diagram of a fuel cell system according to a first embodiment. [Figure 2] FIG. 2 is an explanatory diagram illustrating the configuration of a fuel cell. [Figure 3] FIG. 2 is an explanatory diagram showing the connections of sensors, actuators, etc., centered around the ECU. [Figure 4] 4 is a flowchart showing a fuel cell control processing routine according to the first embodiment. [Figure 5] 1 is a graph showing the relationship between hydrogen partial pressure and temperature difference. [Figure 6] FIG. 10 is a schematic diagram of a fuel cell system according to a second embodiment. [Figure 7] 10 is a flowchart showing a fuel cell control processing routine according to a second embodiment. [Figure 8] 6 is a graph showing the relationship between the stoichiometric ratio and the temperature difference. [Figure 9] FIG. 10 is a schematic diagram of a fuel cell system according to a third embodiment. [Figure 10]10 is a flowchart showing a fuel cell control processing routine according to a third embodiment. [Figure 11] 10 is a flowchart showing a fuel cell control processing routine according to a fourth embodiment. [Figure 12] FIG. 10 is an explanatory diagram schematically showing a modified example of the fuel cell of each embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0008] A. First embodiment: (A1) Hardware configuration: 1A shows a schematic configuration of a fuel cell system 10 according to a first embodiment. As shown in the figure, the fuel cell system 10 includes a fuel cell stack 20 formed by stacking a plurality of fuel cell cells 25, an oxygen supply system 30 that supplies air, which is an oxygen-containing gas, to the fuel cell stack 20, a fuel gas supply system 40 that supplies hydrogen gas, which is a fuel gas, to the fuel cell stack 20, a power processing system 60 that drives various electrical components based on a DC voltage output from the fuel cell stack 20, and an ECU 100 that functions as a fuel gas control unit that receives signals from various sensors provided in the fuel cell system 10 and drives valves, inverters, etc., which will be described later.

[0009] The fuel cell stack 20 has a plurality of fuel cell cells 25 stacked in the Y direction in the figure, and end plates 21, 22 at both ends. The end plates 21, 22 have current collector plates on their inner sides, but these are not shown individually and are shown collectively as the end plates 21, 22. Two hydrogen gas through holes, two air through holes, and two cooling water through holes (not shown) are provided in the surface of each fuel cell 25. Of these, the hydrogen gas through holes form a hydrogen gas supply manifold 23 and a hydrogen gas discharge manifold 24 when the fuel cell cells 25 are stacked. Air supply and discharge manifolds and cooling water supply and discharge manifolds are also formed by stacking the fuel cell cells 25, but these are not shown in the figure.

[0010] The fuel cell stack 20 is provided with a first temperature acquisition unit that measures a stack temperature TS, which is the temperature of the fuel cell stack 20. In the figure, this first temperature acquisition unit is shown as a first temperature sensor 81 provided in the fuel cell stack 20, but in reality, a sensor is provided at the inlet of the hydrogen gas supply manifold 23, and the average temperature TS of the fuel cell stack 20 is determined taking into account the temperature rise calculated from the amount of power generated by the fuel cell stack 20. Of course, the stack temperature TS may be detected directly by a sensor provided inside the fuel cell stack 20, or it may be determined by measuring the inlet and outlet temperatures of the fuel cell stack 20 and averaging them.

[0011] An oxygen supply system 30 supplies air, an oxygen-containing gas, to the fuel cell stack 20 and is provided with a pump 31, which supplies air drawn in through an air filter (not shown) to an air supply manifold of the fuel cell stack 20. A fuel gas supply system 40 supplies hydrogen gas, a fuel gas, to the fuel cell stack 20 and is provided with a gas tank 41 for storing high-pressure hydrogen gas, a main stop valve 43 attached to the nozzle of the gas tank 41, a pressure regulating valve 44 for reducing and regulating the pressure of the hydrogen gas supplied from the gas tank 41 via the main stop valve 43, an injector 45 for injecting the pressure-regulated hydrogen gas into the fuel cell stack 20, a fuel gas circulation system 50 for circulating used fuel gas discharged from the fuel cell stack 20 to the supply side, an ejector 52 provided in the fuel gas circulation system 50, a gas-liquid separator 46 provided in the fuel gas circulation system 50, and a drainage / exhaust valve 47 for discharging the separated water and exhaust gas to the outside.

[0012] When the main stop valve 43 opens, high-pressure hydrogen gas stored in the gas tank 41 is supplied from the injector 45 via the pressure regulator valve 44 to the hydrogen gas supply manifold 23 of the fuel cell stack 20. The hydrogen gas supplied to the hydrogen gas supply manifold 23 is then supplied to each of the stacked fuel cell cells 25. The supplied hydrogen is used to generate electricity in the fuel cell cells 25, but the remaining hydrogen gas is sent to the fuel gas circulation system 50 via the hydrogen gas discharge manifold 24 and returned to the hydrogen gas supply pipe 48 by the ejector 52. Separated water and exhaust gas are discharged to the outside from the gas-liquid separator 46 provided in the fuel gas circulation system 50. The fuel gas circulation system 50 is provided with a pressure sensor 71 corresponding to a gas partial pressure acquisition unit that measures the fuel gas partial pressure PH, and a pressure sensor 72 that measures the supply gas pressure IP is provided in the hydrogen gas supply pipe 48 between the injector 45 and the inlet of the hydrogen gas supply manifold 23. Furthermore, the fuel gas circulation system 50 is provided with a second temperature sensor 82 corresponding to a second temperature acquisition unit that measures the circulation system temperature TH, which is the temperature at which the fuel gas circulation system 50 is heated. The second temperature sensor 82 may be provided anywhere in the fuel gas circulation system 50, but it is preferable that the second temperature sensor 82 measures the temperature at a location where water is likely to be generated by condensation. Multiple second temperature sensors 82 may be provided in the fuel gas circulation system 50, and the lowest temperature among them may be used as the circulation system temperature TH. Alternatively, the average value of multiple measurement results may be used.

[0013] The power processing system 60 includes an inverter 61 that operates by receiving DC power from the fuel cell stack 20, a DC / DC converter 63 that converts the high-voltage DC voltage from the fuel cell stack 20 to a low-voltage DC voltage, and a battery and DC auxiliary equipment 65 connected to the DC / DC converter 63. The power processing system 60 is also provided with a voltage detector 67 that measures the output voltage Vs of the fuel cell stack 20. The fuel cell system 10 is mounted on a vehicle, and an on-board power motor 70 is driven by the AC output from the inverter 61. The DC auxiliary equipment 65 also includes on-off valves such as the main stop valve 43 and the drain / exhaust valve 47, actuators such as the injector 45 and the ejector 52, and a pump 31. The power processing system 60 may have any configuration, and the DC auxiliary equipment 65 may be an AC-driven device. Although not specifically shown, a high-voltage battery capable of directly storing the high-voltage DC power generated by the fuel cell stack 20 may also be provided.

[0014] Next, a brief description will be given of the structure of the fuel cell 25 that constitutes the fuel cell stack 20. FIG. 2 is an explanatory diagram that schematically shows the structure of 25. The fuel cell 25 is configured such that a membrane electrode assembly that generates electricity is sandwiched between an oxidant gas separator 231 and a fuel gas separator 232. When the fuel cell cells 25 are stacked, the oxidant gas separator 231 and the fuel gas separator 232 ensure electrical conductivity between adjacent fuel cell cells 25 and are impermeable to fuel gas and the like. For this reason, the oxidant gas separator 231 and the fuel gas separator 232 are formed from a gas-impermeable, electrically conductive material such as titanium or sintered carbon.

[0015] The power generation body is configured with a solid polymer electrolyte membrane 200 at its center, and catalyst layers 201, 202, which are electrodes supporting a catalyst such as platinum, are provided on both sides of the solid polymer electrolyte membrane 200. The solid polymer electrolyte membrane 200 and the outer catalyst layers 201, 202 are formed integrally and are also called a membrane electrode assembly (MEA). Diffusion layers 211, 212, which are gas diffusion layers, are provided on the outer side of the membrane electrode assembly. An oxidant gas channel 221 and a fuel gas channel 222 are provided further outside the diffusion layers 211, 212, respectively. Although described here as separate components, the oxidant gas channel 221 is actually formed integrally with an oxidant gas separator 231, and the fuel gas channel 222 is formed integrally with the fuel gas separator 232. The oxidizing gas flow path 221 communicates with the air supply manifold described above, and the fuel gas flow path 222 communicates with the hydrogen gas supply manifold 23 .

[0016] (A2) ECU configuration and fuel cell control: The ECU 100 controls the entire fuel cell system 10. As shown in FIG. 3 , the ECU 100 mainly comprises a known CPU 110 and memory 120, and includes an input interface 130 that receives signals from sensors and an output interface 140 that outputs control signals to actuators. The input interface 130 is connected to the voltage detector 67, the pressure sensor 71 for measuring the fuel gas partial pressure PH, the pressure sensor 72 for measuring the supply gas pressure IP, the first temperature sensor 81, and the second temperature sensor 82, all of which are already described. The output interface 140 is connected to the air supply pump 31, the main stop valve 43, the injector 45, the drain / exhaust valve 47, the ejector 52, the inverter 61, the converter 63, the battery, and the DC auxiliary equipment 65, all of which are already described. The figure also shows liquid water sensors 91 and 92 connected to the input interface 130 of the ECU 100, but these will be described in the third embodiment.

[0017] Next, the fuel cell control process executed by the ECU 100 will be described with reference to Fig. 4. The figure is a flowchart showing an extracted portion of the process executed by the ECU 100 for controlling the fuel cell system 10, which is related to measures against liquid water intrusion into the fuel cell cells 25 of this embodiment. The process shown in the figure is executed when the fuel cell system 10 is started up, and in particular, the following process is executed as a measure against liquid water intrusion into the fuel cell cells 25.

[0018] When the process starts, the CPU 110 first acquires the stack temperature TS of the fuel cell stack 20 from the first temperature sensor 81 and the circulation system temperature TH of the fuel gas circulation system 50 from the second temperature sensor 82 (step S101). Next, the CPU 110 calculates the temperature difference ΔT between the acquired temperatures TS and TH using the following equation (1) (step S111). ΔT = TS - TH … (1) Then, it is determined whether or not the temperature difference ΔT is greater than a predetermined specified temperature difference Tc (step S121).

[0019] If the temperature difference ΔT is not greater than the specified temperature difference Tc (step S121: "NO"), a predetermined first reference gas partial pressure Ph1 is set as the reference gas partial pressure Pth for determining the minimum value of the fuel gas partial pressure PH (described later) (step S131). On the other hand, if the temperature difference ΔT is greater than the specified temperature difference Tc (step S121: "YES"), a predetermined second reference gas partial pressure Ph2 greater than the first reference gas partial pressure Ph1 is set as the reference gas partial pressure Pth for determining the minimum value of the fuel gas partial pressure PH (step S132).

[0020] After setting the reference gas partial pressure Pth to either the first reference gas partial pressure Ph1 or a second reference gas partial pressure Ph2 higher than the first reference gas partial pressure Ph1, a fuel cell operation process is executed (step S150), which includes a fuel gas control process for adjusting the fuel gas partial pressure PH to be equal to or higher than the reference gas partial pressure Pth. Specifically, this process acquires the fuel gas partial pressure PH using the pressure sensor 71, and when the fuel gas partial pressure PH becomes lower than the reference gas partial pressure Pth, the drain / exhaust valve 47 is opened to increase the fuel gas partial pressure PH, thereby controlling the amount of hydrogen gas supplied to the fuel cell stack 20. Here, the reference gas partial pressure Pth is set to the first reference gas partial pressure Ph1 when ΔT≦Tc, depending on the temperature difference ΔT between the stack temperature TS of the fuel cell stack 20 and the circulation system temperature TH of the fuel gas circulation system 50. When ΔT≦Tc, the reference gas partial pressure Pth is set to the first reference gas partial pressure Ph1. When the temperature difference ΔT>Tc, the second reference gas partial pressure Ph2 higher than the first reference gas partial pressure Ph1 is set.

[0021] 5 shows an example of how the fuel gas partial pressure PH is controlled in this case. If the fuel gas partial pressure PH is higher than the reference gas partial pressure Pth, the CPU 110 closes the drainage / exhaust valve 47, and the fuel gas partial pressure PH gradually decreases as power is generated in the fuel cell stack 20, that is, as hydrogen gas, the fuel gas, is consumed. When the fuel gas partial pressure PH falls below the reference gas partial pressure Pth, the CPU 110 opens the drainage / exhaust valve 47 to discharge the exhaust gas circulating through the fuel gas circulation system 50, that is, the exhaust gas whose fuel gas partial pressure PH has decreased, and to supply fresh fuel gas via the injector 45 to compensate for this. As a result, the fuel gas partial pressure PH increases.

[0022] In this control, if the temperature difference ΔT is not greater than the specified temperature difference Tc, the reference gas partial pressure Pth is set to the first reference gas partial pressure Ph1. If the temperature difference ΔT is greater than the specified temperature difference Tc, the reference gas partial pressure Pth is set to the second reference gas partial pressure Ph2, which is higher than the first reference gas partial pressure Ph1. Therefore, as shown in the figure, the minimum value of the fuel gas partial pressure PH is controlled depending on the relationship of the temperature difference ΔT to the specified temperature difference Tc. Therefore, under conditions where the temperature difference ΔT is greater than the specified temperature difference Tc, in which liquid water is likely to penetrate the fuel cell cells 25 of the fuel cell stack 20, the fuel gas partial pressure PH is increased to increase the proportion of fuel gas supplied to the fuel cell cells 25, making it less likely that negative voltage will occur in the fuel cell cells 25. If negative voltage does not occur, deterioration of the catalyst in the fuel cell cells 25 is less likely to occur.

[0023] After performing the fuel cell operation process (step S150), which includes fuel gas control processing for adjusting the fuel gas partial pressure PH to equal to or higher than the reference gas partial pressure Pth, it is determined whether to continue power generation in the fuel cell stack 20 (step S161), and if it is to continue, the process returns to step S101 and repeats the above process. On the other hand, if power generation is not to be continued, for example, if an instruction to stop operation of the fuel cell system 10 is given, a process for stopping operation of the fuel cell stack 20 is performed (step S171), and this processing routine is terminated.

[0024] In the fuel cell system 10 of the first embodiment described above, when the temperature difference ΔT between the stack temperature TS of the fuel cell stack 20 and the circulation system temperature TH of the fuel gas circulation system 50 becomes larger than the predetermined temperature difference Tc, that is, when conditions are favorable for the presence of liquid water in the fuel cell 25, the fuel gas partial pressure PH is controlled to a second reference gas partial pressure Ph2 that is higher than the previous first reference gas partial pressure Ph1. When the temperature difference ΔT between the stack temperature TS and the circulation system temperature TH becomes larger than the predetermined temperature difference Tc, some of the water vapor contained in the circulating fuel gas condenses into water in the fuel gas circulation system 50, increasing the likelihood that this water will infiltrate the fuel cell 25 of the fuel cell stack 20. Therefore, although the likelihood of liquid water infiltrating the fuel cell 25 or the presence of water generated inside the fuel cell 25 increases, the fuel cell 25 will still have a sufficient amount of hydrogen as fuel gas even if the flow of fuel gas through the fuel gas flow path 222 of the fuel cell 25 is obstructed. In this state, it was confirmed through experiments that the potential difference between both sides of the fuel cell 25 is unlikely to decrease and is unlikely to become negative voltage.

[0025] For example, if the fuel gas partial pressure PH is high and the so-called hydrogen stoichiometric ratio is 1.2 or higher, even if a portion of the fuel gas flow path 222 is blocked, the presence of abundant hydrogen gas prevents the fuel gas from flowing through a region blocked by liquid water. Even if the voltage does drop, it remains within a range of 0.3 to 0.8 volts, preventing negative voltage from occurring. Even if the fuel gas cannot reach a region due to the presence of liquid water, the voltage drop is limited, preventing negative voltage from occurring. This reduces the risk of deterioration of the catalyst layer 202 due to negative voltage. Furthermore, when the temperature difference ΔT between the stack temperature TS and the circulation system temperature TH is equal to or less than the specified temperature difference Tc, the fuel gas partial pressure PH is set to the first reference gas partial pressure Ph1, which is lower than the second reference gas partial pressure Ph2 when the temperature difference ΔT is greater than the specified temperature difference Tc. Therefore, if the temperature difference is small, excessive fuel gas is not supplied to the fuel cell stack 20. This also reduces fuel efficiency.

[0026] B. Second embodiment: Next, the configuration and operation of a fuel cell system 10B of a second embodiment will be described. FIG. 6 is a schematic diagram of the fuel cell system 10B of the second embodiment. As shown in the figure, the hardware configuration of the fuel cell system 10B of the second embodiment is the same as that of the first embodiment, except that the fuel gas circulation system 50 is provided with a fuel pump 54 for circulating hydrogen gas instead of the ejector 52. The fuel cell control processing routine shown in FIG. 7 is the same as that of the first embodiment, except that steps S133 and S134 are executed instead of steps S131 and S132, and step S152 is executed instead of step S150.

[0027] In the fuel cell system 10B of the second embodiment, the calculation of the temperature difference ΔT between the stack temperature TS and the circulation system temperature TH (steps S101 to S111) and the comparison of the temperature difference ΔT with the specified temperature difference Tc (step S121) are performed in the same manner as in the first embodiment. If the temperature difference ΔT is not greater than the specified temperature difference Tc (step S131: "NO"), the target stoichiometric ratio Ust of the fuel cell stack 20 is set to a predetermined first stoichiometric ratio u1 (step S133), as stoichiometric control, and if the temperature difference ΔT is greater than the specified temperature difference Tc (step S131: "YES"), the target stoichiometric ratio Ust of the fuel cell stack 20 is set to a second stoichiometric ratio u2 that is higher than the predetermined first stoichiometric ratio u1 (step S134).

[0028] In the second embodiment, a fuel cell operation process is then executed, including a process for adjusting the stoichiometric ratio U of the fuel cell stack 20 to a target stoichiometric ratio Ust (step S152). The manner in which the stoichiometric ratio of hydrogen gas is controlled in this case is illustrated in FIG. 7. In the second embodiment, when the temperature difference ΔT is equal to or less than the specified temperature difference Tc, the hydrogen stoichiometric ratio of the fuel cell stack 20 is controlled to a first stoichiometric ratio u1. However, when the temperature difference ΔT becomes greater than the specified temperature difference Tc, the hydrogen stoichiometric ratio is controlled to a second stoichiometric ratio u2, which is greater than the first stoichiometric ratio u1. As a result, similar to the first embodiment, the proportion of hydrogen, which is the fuel gas supplied to the fuel cell units 25 of the fuel cell stack 20, is increased, resulting in a state in which a sufficient amount of fuel gas is present. Therefore, even if liquid water enters the fuel cell units 25 or purified water is present, a negative voltage in the fuel cell units 25 is suppressed or avoided. The remaining effects are the same as those of the first embodiment.

[0029] C. Third embodiment: Next, a third embodiment will be described. As shown in Fig. 9, a fuel cell system 10C of the third embodiment differs from the second embodiment in that a fuel gas circulation system 50C is provided with liquid water sensors 91 and 92, but the hardware configuration is otherwise the same as that of the second embodiment. The liquid water sensors 91 and 92 provided in the fuel gas circulation system 50C can directly detect whether liquid water is present in the fuel gas circulation system 50C by measuring the impedance of an optical sensor or a pipe line.

[0030] Assuming the presence of the liquid water sensors 91, 92, in the third embodiment, the ECU 100 executes a fuel cell control processing routine shown in Fig. 10. The processing shown in the figure is similar to the processing of the first embodiment (Fig. 4) except for the inclusion of step S125. Note that if step S125 is included, the processing of the second embodiment (Fig. 7) may also be performed.

[0031] In the third embodiment, assuming the presence of liquid water sensors 91, 92, it is determined in step S125 whether these sensors have detected liquid water. If liquid water is detected (step S125: "YES"), even if the temperature difference ΔT is equal to or less than the specified temperature difference Tc (step S121: "NO"), step S132 is executed instead of step S131, and the reference gas partial pressure Pth is set to the second reference gas partial pressure Ph2 (step S132). Therefore, in addition to the effects similar to those of the first embodiment, the fuel cell system 10C of the third embodiment sets the fuel gas partial pressure PH to the second reference gas partial pressure Ph2, which is higher than the normal first reference gas partial pressure Ph1, if liquid water is detected in the fuel gas circulation system 50C, even if the temperature difference ΔT is equal to or less than the specified temperature difference Tc. As a result, the fuel cell system 10C of the third embodiment not only has the same effects as the first embodiment, but also increases the fuel gas partial pressure PH due to the presence of liquid water in the fuel gas circulation system 50C, thereby more reliably suppressing or avoiding deterioration of the fuel cell 25 due to the presence of liquid water.

[0032] D. Fourth embodiment: Next, a fuel cell system 10 according to a fourth embodiment will be described. The fuel cell system 10 according to the fourth embodiment has the same hardware configuration as the first embodiment, and differs only in the fuel cell control processing routine executed by the ECU 100. The processing according to the fourth embodiment is shown in FIG. 11. As shown in the figure, the fuel cell system 10 according to the fourth embodiment differs from the first embodiment in three respects: it detects the stack voltage Vs, which is the voltage generated by the fuel cell stack 20, using the voltage detector 67 shown in FIG. 1 (step S117); it detects liquid water using this stack voltage Vs (step S119); and, if liquid water is detected, it sets the reference gas partial pressure Pth to a third reference gas partial pressure Ph3. The operation of the fuel cell system 10 according to the fourth embodiment will be described below, focusing on these points.

[0033] In the fuel cell system 10 of the fourth embodiment, the stack temperature TS and the circulation system temperature TH are detected and the temperature difference ΔT is calculated (steps S101 and S111), and then the stack voltage Vs is detected by the voltage detector 67 (step S117). The order of detection and the order of calculation of the temperature difference ΔT may be any order. Then, based on the stack temperature TS and the stack voltage Vs, it is determined whether liquid water is present in the fuel cell cells 25 of the fuel cell stack 20 (step S119). If a sufficient amount of fuel gas is supplied to the fuel cell stack 20 and the stack temperature TS is at a temperature at which electricity can be generated, but the stack voltage Vs does not reach a predetermined voltage, it can be estimated that there is a high possibility that liquid water is accumulating in the fuel gas flow path 222 of the fuel cell cells 25.

[0034] Therefore, if accumulation of liquid water is estimated from the stack temperature TS and stack voltage Vs (step S119: "YES"), a third reference gas partial pressure Ph3 higher than the second reference gas partial pressure Ph2 is set as the reference gas partial pressure Pth (step S137). If it is determined that the conditions for inferring accumulation of liquid water from the stack temperature TS and stack voltage Vs have not been established, the magnitude relationship between the temperature difference ΔT and the specified temperature difference Tc is determined (step S121), and the reference gas partial pressure Pth is set to the first reference gas partial pressure Ph1 or the second reference gas partial pressure Ph2 (step S131 or S132), as in the first embodiment.

[0035] As a result of the above processing, by performing the processing of step S150, i.e., the fuel cell operation processing including the processing of adjusting the fuel gas partial pressure PH to equal to or higher than the reference gas partial pressure Pth, if it is estimated that liquid water is accumulated such that the stack voltage Vs, which is the output voltage of the fuel cell stack 20, will decrease, the fuel gas partial pressure PH is increased to the highest third reference gas partial pressure and the fuel cell is operated, thereby making it possible to purge liquid water that is likely to be present in the fuel cell cells 25. As a result, it becomes easier to restore the operation of the fuel cell stack 20 to a normal state. Therefore, deterioration of the catalyst layer 202 due to the inflow or accumulation of liquid water can be suppressed or avoided. Even if accumulation of liquid water in the fuel cell 25 is not estimated from the relationship between the stack temperature TS and the stack voltage Vs, the reference gas partial pressure Pth is set according to the temperature difference ΔT between the stack temperature TS and the circulation system temperature TH, as in the first embodiment, and therefore, even if the flow of fuel gas in the fuel gas flow path 222 of the fuel cell 25 is obstructed due to the intrusion of liquid water into the fuel cell 25 or the presence of water generated inside the fuel cell 25, a state will be reached in which sufficient hydrogen, which is the fuel gas, is present in the fuel cell 25. In this state, the potential difference across both sides of the fuel cell 25 is unlikely to decrease, and a negative voltage is unlikely to occur, as in the first embodiment.

[0036] E. Variations: In the above embodiment, as shown in FIG. 2, the fuel cell 25 has a catalyst layer 202 provided on the front surface on the anode side of the membrane electrode assembly to which the fuel gas is introduced. A modified example of the configuration of such a fuel cell is shown in FIG. 12. In this modified fuel cell 25E, the catalyst layer 202 is not provided in a predetermined range at the lower end in the Z direction. This range is called a catalyst deficient region 202E. In other words, the fuel gas flow path 222 at the lower end of the fuel cell 25E in the gravity direction (Z direction) does not have a corresponding catalyst layer.

[0037] When liquid water infiltrates the fuel cell 25E along with the fuel gas, the liquid water tends to accumulate in the lower part of the fuel cell 25E, and is therefore likely to collect in the lower part of the fuel gas flow path 222 of the fuel cell 25E. In this embodiment, a catalyst deficient region 202E is formed in part of the lower part of the fuel cell 25E, so even if liquid water accumulates there, it is unlikely that the power generation surface of the membrane electrode assembly will become negative voltage. This prevents problems such as the power generation surface becoming negative voltage and the catalyst in the solid electrolyte membrane becoming deteriorated. The width of this catalyst deficient region 202E in the Z direction is 10% or more, preferably 15% or more, of the surface area of ​​the membrane electrode assembly, i.e., the area where the catalyst layer would normally be present. In this modified example, the catalyst deficient region 202E does not have a catalyst layer itself, but it may be provided as a catalyst layer with a reduced catalyst amount, i.e., a reduced catalyst amount region.

[0038] By providing the catalyst deficiency region 202E of the above-mentioned modified example in the fuel cell that is the subject of the control of the first to fourth embodiments, in combination with controls such as increasing the hydrogen partial pressure and the hydrogen stoichiometric ratio that are performed in response to conditions under which liquid water is likely to penetrate into the fuel cell 25E, it is possible to make it even more difficult for the potential difference between the two surfaces of the fuel cell 25E to decrease under conditions under which liquid water is likely to penetrate into the fuel cell 25E, and further make it even more difficult for a negative voltage to occur.

[0039] In each of the above embodiments, some of the configurations realized by hardware may be replaced with software. At least a portion of the configurations realized by software may also be realized by a discrete circuit configuration. Furthermore, when some or all of the functions of the present disclosure are realized by software, the software (computer program) may be provided in a form stored on a computer-readable recording medium. The term "computer-readable recording medium" is not limited to portable recording media such as floppy disks and CD-ROMs, but also includes internal storage devices within a computer, such as various RAMs and ROMs, and external storage devices fixed to a computer, such as a hard disk. In other words, the term "computer-readable recording medium" has a broad meaning, including any recording medium on which data packets can be fixed, not just temporarily.

[0040] The present disclosure is not limited to the above-described embodiments and can be realized in various configurations without departing from the spirit thereof. For example, the technical features in the embodiments corresponding to the technical features in each aspect described in the Summary of the Invention section can be appropriately replaced or combined to solve some or all of the above-described problems or achieve some or all of the above-described effects. Furthermore, if a technical feature is not described as essential in this specification, it can be appropriately deleted. [Explanation of symbols]

[0041] 10, 10B, 10C... fuel cell system, 20... fuel cell stack, 21, 22... end plate, 23... hydrogen gas supply manifold, 24... hydrogen gas discharge manifold, 25... fuel cell, 30... oxygen supply system, 31... pump, 40... fuel gas supply system, 41... gas tank, 43... main stop valve, 44... pressure regulating valve, 45... injector, 46... gas-liquid separator, 47... drain and exhaust valve, 48... hydrogen gas supply pipe, 50, 50C... fuel gas circulation system, 52... ejector, 54... fuel pump, 60... power processing system, 61... inverter, 63... DC / DC converter, 65... DC auxiliary equipment, 67... voltage detector, 70... power motor, 71, 72... pressure sensor, 81... first temperature sensor, 82... second temperature sensor, 91, 92... liquid-water separator sensor, 100...ECU, 110...CPU, 120...memory, 130...input interface, 140...output interface, 200...solid polymer electrolyte membrane, 201...catalyst layer, 202...catalyst layer, 202E...catalyst depleted region, 211...diffusion layer, 221...oxidant gas flow path, 222...fuel gas flow path, 231...oxidant gas separator, 232...fuel gas separator, IP...supply gas pressure, PH...fuel gas partial pressure, Ph1...first reference gas partial pressure, Ph2...second reference gas partial pressure, Ph3...third reference gas partial pressure, Pth...reference gas partial pressure, T...temperature difference Δ, TH...circulation system temperature, TS...stack temperature, Tc...specified temperature difference, U...stoichiometric ratio, Ust...target stoichiometric ratio, Vs...stack voltage, u1...first stoichiometric ratio, u2...second stoichiometric ratio

Claims

1. a fuel cell stack in which a plurality of fuel cells, each having a gas flow path on the outside of an electrolyte membrane, a catalyst layer, and a gas diffusion layer, are stacked with separators interposed therebetween; a fuel gas circulation system that circulates at least a portion of the gas discharged from the fuel cell stack into the fuel gas supplied to the fuel cell stack; a first temperature acquisition unit that acquires a stack temperature, which is the temperature of the fuel cell stack; a second temperature acquisition unit that acquires a circulation system temperature, which is the temperature of the fuel gas circulation system; a fuel gas control unit that, when a temperature difference between the stack temperature and the circulation system temperature exceeds a predetermined temperature difference, performs fuel gas control to increase the proportion of fuel gas supplied to the fuel cell compared to when the temperature difference is equal to or less than the predetermined temperature difference; A fuel cell system comprising:

2. 10. The fuel cell system according to claim 1, further comprising: a gas partial pressure acquisition unit that acquires a fuel gas partial pressure in the fuel gas circulation system; an exhaust valve provided in the fuel gas circulation system to adjust the discharge of the fuel gas from the fuel gas circulation system, The fuel gas control unit The exhaust valve is controlled to adjust the fuel gas partial pressure so that it does not fall below a first reference gas partial pressure that is set as a reference gas partial pressure, and The fuel gas control is performed by setting the reference gas partial pressure to a second reference gas partial pressure higher than the first reference gas partial pressure. Fuel cell system.

3. a voltage detection unit that detects an output voltage of the fuel cell stack; 3. The fuel cell system according to claim 2, wherein when the control unit estimates an inflow of liquid water into the fuel cell from the stack temperature and the output voltage, the control unit sets the fuel gas partial pressure to a third reference gas partial pressure higher than the second reference gas partial pressure, and purges the fuel cell.

4. 10. The fuel cell system according to claim 1, further comprising: The fuel gas control unit adjusting the stoichiometric ratio of the fuel gas supplied to the fuel cell stack; When the temperature difference between the stack temperature and the circulation system temperature exceeds a predetermined temperature difference, stoichiometric control is performed to increase the stoichiometric ratio of the fuel gas. Fuel cell system.

5. a liquid water sensor for detecting the presence of liquid water in the fuel gas circulation system; 5. The fuel cell system according to claim 1, wherein the control unit performs the fuel gas control even when the liquid water sensor detects the presence of the liquid water in the fuel gas circulation system.

6. a fuel cell stack including a plurality of fuel cells stacked with separators interposed therebetween, each of which has a gas flow path outside an electrolyte membrane, a catalyst layer, and a gas diffusion layer; and a fuel gas supply system for supplying fuel to the fuel cell stack, and circulating at least a portion of the gas discharged from the fuel cell stack to the fuel cell stack; a stack temperature that is the temperature of the fuel cell stack and a circulation system temperature that is the temperature of the fuel gas circulation system are acquired; When the temperature difference between the stack temperature and the circulation system temperature exceeds a predetermined specified temperature difference, fuel gas control is performed to increase the proportion of fuel gas supplied to the fuel cell compared to when the temperature difference is equal to or less than the specified temperature difference. A method for controlling a fuel cell.

Images