JP2005150020A - Fuel cell system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To start up the fuel cell at temperatures below freezing point and to suppress the amount of use of the reactant gas and power consumption amount to the minimum necessary. <P>SOLUTION: The memory 28 memorizes the changes in resistance and temperature of the fuel cell 1 from the previous system stop to the starting time and the control part 29 judges precisely the level of water droplet adhered on the surface of the catalyst layer and freezing status of water inside and on the surface of the electrolyte membrane in accordance with the changes in the resistance and the temperature of the fuel cell 1 memorized in the memory 28, and based on the judgement result, determines the starting method of the fuel cell 1. Therefore, the fuel cell 1 can be started up certainly at the temperature below freezing point and the amount of use of the reactant gas and power consumption amount can be suppressed to the minimum necessary. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a fuel cell system, and more specifically, to a technology that makes it possible to accurately grasp the amount of water and ice in a catalyst layer surface and an electrolyte membrane when generating power at a temperature below the freezing point. Involved.

  In general, in a polymer electrolyte fuel cell that generates electricity by an electrochemical reaction using hydrogen gas and oxygen gas, if power is generated at a temperature below the freezing point, the water near the electrode and the surface of the electrolyte membrane freezes. The reaction gas may be prevented from reaching the catalytic reaction section, and even if the reaction gas is supplied, the electrochemical reaction may not proceed and the fuel cell may not be activated.

  From such a background, in conventional polymer electrolyte fuel cells, when generating power at a temperature below freezing point, the fuel cell stack is heated by heating a heater attached to an end plate or the like. Various measures have been taken to reliably start the fuel cell, such as increasing the flow rate and pressure of the reaction gas to be supplied, and devising how to extract current and voltage.

  However, when power generation is performed at a temperature below the freezing point due to the measures described above, a large amount of power and reaction gas is consumed, and power generation may become impossible during the startup of the fuel cell. Therefore, recently, in order to reliably start the fuel cell at a temperature below the freezing point and at the same time to minimize the amount of reaction gas used and the power consumption, the catalyst surface and the electrolyte membrane in the fuel cell stack are required. A method of starting a fuel cell according to the amount of moisture in the inside and the presence or absence of freezing has been studied.

By the way, as one of the methods for grasping the moisture content in the electrolyte membrane, the cell resistance is measured as a real component of a high frequency impedance of about 1 [kHz], and the moisture content in the electrolyte membrane is grasped by the measured cell resistance. There is a known method (see, for example, Patent Document 1).
JP 2000-243418 A (paragraph [0054], FIG. 4)

  However, in general, the cell resistance only corresponds to the amount of water in the electrolyte membrane, so whether the water in the electrolyte membrane or the water on the catalyst surface is frozen, and how much the moisture content on the catalyst surface is It cannot be determined from the cell resistance. Therefore, the cell resistance is insufficient as an index for starting the fuel cell more reliably below the freezing point and at the same time minimizing the amount of reaction gas used and the power consumption.

  In addition, the water does not always freeze at 0 [° C.], and the water in the electrolyte membrane or on the catalyst surface interacts with the supercooling phenomenon, the sulfonic acid group that is a component of the electrolyte membrane, or in the electrolyte membrane. It is conceivable that the freezing point drops due to an increase in surface tension due to the small water cluster size. In addition, if hydrogen remains in the cell when power generation is stopped, oxygen in the air is mixed while the cell is left standing, so that water is generated in the catalyst layer or water vapor confined in the cell. It is conceivable that water condenses as the temperature decreases and water droplets adhere to the catalyst layer. Therefore, accurately grasping the amount of water and ice in the catalyst layer and electrolyte membrane ensures that the fuel cell is started below the freezing point, and at the same time, the amount of reaction gas used and the power consumption are minimized. This is a very important task.

  The present invention has been made to solve the above-described problems, and its object is to accurately grasp the amount of water and ice on the surface of the catalyst layer and the electrolyte membrane, so that the fuel cell can be operated at a temperature below the freezing point. It is an object of the present invention to provide a fuel cell system that can reliably start up and simultaneously reduce the amount of reaction gas used and power consumption.

  In order to solve the above-described problems, a fuel cell system according to the present invention uses a fuel cell startup method according to the relationship between the resistance and temperature change of the fuel cell between the previous system shutdown and the startup. decide.

  According to the fuel cell system of the present invention, the amount of water and ice in the catalyst layer and the electrolyte membrane can be accurately grasped, so that the fuel cell is reliably started at a temperature below the freezing point, and at the same time, the reaction gas Usage and power consumption can be minimized.

The fuel cell system according to the present invention can be applied to a process of supplying reaction gas to a fuel cell having a configuration as shown in FIG. Here, the fuel battery cell 1 shown in FIG. 1 includes an electrolyte membrane 2 formed of a solid polymer material such as perfluorosulfonic acid polymer or Nafion, and an anode (see FIG. 1) disposed so as to sandwich the electrolyte membrane 2. Hydrogen electrode) 3 and cathode (air electrode) 4, separators 5 a and 5 b and end plates 6 a and 6 b disposed outside anode 3 and cathode 4 as main components, and these components are bolts It is fixed by 7a, 7b. Each of the anode 3 and the cathode 4 has a catalyst layer 8a and a gas diffusion layer 9a, and a catalyst layer 8b and a gas diffusion layer 9b. The catalyst layers 8a and 8b are formed of a carbon-supported platinum catalyst and Nafion. Yes. The gas diffusion layers 9a and 9b are formed of carbon paper, and the thickness thereof is adjusted by the silicon seals 10a and 10b. The separators 5a and 5b are respectively formed with a hydrogen gas passage 11 for supplying hydrogen to the gas diffusion layer 9a and an air gas passage 12 for supplying air to the gas diffusion layer 9b. In addition, heaters 13a and 13b for heating the fuel cell 1 are disposed outside the end plates 6a and 6b. In this embodiment, the MEA (Membrane and Electrode Assembly) composed of the electrolyte membrane 2, the catalyst layers 8a and 8b, and the gas diffusion layers 9a and 9b has an area of 25 [cm ² ]. did. Moreover, in this embodiment, although the fuel cell was comprised with the fuel battery cell 1, you may comprise a fuel battery by laminating | stacking the fuel battery cell 1 in multiple numbers. In the fuel cell 1 having such a configuration, when hydrogen is supplied to the anode 3 through the hydrogen gas flow path 11, the catalytic reaction by platinum of the catalyst layer 9 a shown in the following reaction formula is performed at the anode 3. Arise.

Protons (H ⁺ ) generated by the catalytic reaction move from the anode 3 toward the cathode 4 side. The proton that has moved to the cathode 4 side causes a catalytic reaction by air (O ₂ ) supplied to the cathode 4 through the air gas flow path 12 and platinum in the catalyst layer 9b shown in the following reaction formula, and water (H _2). O). Thereby, the fuel cell 1 generates an electromotive force.

  Hereinafter, the configuration and operation of a fuel cell system according to an embodiment of the present invention configured by the fuel cell 1 will be described with reference to the drawings.

[Configuration of fuel cell system]
First, the configuration of the fuel cell system according to the embodiment of the present invention will be described with reference to FIG.

  As shown in FIG. 2, a fuel cell system 20 according to an embodiment of the present invention includes a fuel cell 1 and mass flow controllers (MFC) 21 a and 21 b that control the amount of hydrogen and air supplied to the fuel cell 1. A heater power source 22 for supplying power to the heater 13 provided in the fuel cell 1, a constant current control circuit 23 for controlling the amount of current taken out from the fuel cell 1, and a direct current output from the constant current control circuit 23 And an inverter 25 that converts the current into an alternating current and supplies power to the load 24.

  The fuel cell system 20 includes a thermometer 26 for measuring the temperature of the fuel cell 1 (hereinafter abbreviated as cell temperature), and a cell resistance for measuring the resistance of the fuel cell 1 (hereinafter abbreviated as cell resistance). It has a measuring device 27, a memory 28 that stores the cell temperature and cell resistance measured by the thermometer 26 and the cell resistance measuring device 27, and a control unit 29 that controls the operation of the fuel cell system 20. In this embodiment, the cell temperature is measured by forming a groove in the separators 5a and 5b constituting the fuel cell 1 and inserting a thermocouple into the groove. The cell resistance is measured by measuring the resistance between the anode 3 and the cathode 4 constituting the fuel cell 1 as a real part component of a high frequency impedance of 1 [kHz]. The thermometer 26 and the cell resistance measuring device 27 function as measuring means according to the present invention. The memory 28 functions as storage means according to the present invention.

[Operation of fuel cell system]
In the fuel cell system 20 having such a configuration, the control unit 29 refers to the change in cell temperature and cell resistance stored in the memory 28 from the previous system stop to the start-up. And the logarithm of the resistance component (membrane resistance component) of the electrolyte membrane 2 are created, and the fuel cell 1 is activated by executing the following startup process according to the created Arrhenius plot.

The membrane resistance component can be calculated by subtracting the contact resistance between the catalyst layer, the gas diffusion layer, and the separator, which are measured in advance from the cell resistance and have a low temperature dependency. Further, as shown in FIG. 3, the Arrhenius plot has a characteristic that the magnitude of the slope (differential value) changes in a temperature range of about 0 to −10 [° C.] when the outside air temperature is below freezing point. The relationship between cell temperature and cell resistance is generally defined by the following formula (RM: membrane resistance component, R0: sum of contact resistance of gas diffusion layer and catalyst layer and resistance of separator or GDL, λ: electrolyte Moisture content of the membrane).

  Hereinafter, with reference to the flowcharts shown in FIGS. 4 and 5, the operation of the control unit 29 when executing the startup process according to the first and second embodiments of the present invention will be described.

  First, the operation of the control unit 29 when executing the startup process according to the first embodiment of the present invention will be described with reference to the flowchart shown in FIG.

  The flowchart shown in FIG. 4 starts when an activation command for the fuel cell 1 is input to the control unit 29, and the activation process proceeds to step S1. The control unit 29 periodically measures the cell temperature and the cell resistance while the fuel cell 1 is left after the previous system stop, and stores the measured values in the memory 28. .

  In the process of step S1, the control unit 29 refers to the information stored in the memory 28 and calculates an Arrhenius plot in which the reciprocal of the cell temperature and the logarithm of the membrane resistance component are plotted. In the process of step S1, the control unit 29 may use the logarithm of the product of the film resistance component and the temperature instead of the logarithm of the film resistance component. Thereby, the process of step S1 is completed, and the activation process proceeds from the process of step S1 to the process of step S2.

  In the process of step S2, the control unit 29 determines whether there is a change point of the slope (differential value) in the Arrhenius plot calculated by the process of step S1. If the result of determination is that there is no slope change point in the Arrhenius plot, the control unit 29 starts the startup process from the process of step S2 to the process of step 6 in order to start the fuel cell 1 in the normal startup mode. Proceed with the process. On the other hand, when there is a change point of the slope in the Arrhenius plot, the control unit 29 advances the startup process from the process of step S2 to the process of step S3 in order to start the fuel cell 1 in the startup mode at the low temperature.

  In the process of step S <b> 3, the control unit 29 controls the heater power source 22 to heat the heaters 13 a and 13 b provided on the end plates 6 a and 6 b, thereby heating the fuel cell 1. Thereby, the process of step S3 is completed, and the activation process proceeds from the process of step S3 to the process of step S4.

  In the process of step S <b> 4, the controller 29 measures the temperature of the fuel cell 1 by controlling the thermometer 26. Thereby, the process of step S4 is completed, and the activation process proceeds from the process of step S4 to the process of step S5.

  In the process of step S5, the control unit 29 refers to the temperature of the fuel cell 1 and determines whether or not the temperature of the fuel cell 1 has become equal to or higher than a predetermined temperature. And according to the temperature of the fuel cell 1 becoming more than predetermined temperature, the control part 29 advances a starting process from the process of step S5 to the process of step S6. On the other hand, when the temperature of the fuel cell 1 is not equal to or higher than the predetermined temperature, the control unit 29 waits for a predetermined time, and then returns the activation process from the process of step S5 to the process of step S4.

  In this embodiment, the predetermined temperature is 0 [° C.]. However, the present invention is not limited to this, and as long as it is a temperature at which the fuel cell 1 can sustain power generation, the predetermined temperature is not limited to this. Any value may be used. Further, the predetermined temperature may be set high when the temperature at the change point of the slope is high, and low when the temperature at the change point is low.

  In the process of step S6, the control part 29 starts supply of the hydrogen and air to the fuel cell 1 by controlling MFC21a, 21b, and starts the fuel cell 1. FIG. Thereby, the process of step S6 is completed, and the activation process proceeds from the process of step S6 to the process of step S7.

  In the process of step S <b> 7, the control unit 29 controls the constant current control circuit 23 to start taking out a direct current from the fuel cell 1. As a result, the series of startup processes ends.

  As is apparent from the above description, according to the start-up process according to the first embodiment of the present invention, the memory 28 has the resistance and temperature of the fuel cell 1 between the previous system stop and the start-up. The change is stored, and the control unit 29 determines how much water droplets are attached to the surface of the catalyst layer according to the change in resistance and temperature of the fuel cell 1 stored in the memory 28, Whether the water on the surface is frozen is accurately determined, and the starting method of the fuel cell 1 is determined according to the determination result. Therefore, the fuel cell 1 is reliably started at a temperature below the freezing point, and at the same time, the reaction gas Usage and power consumption can be minimized.

  Further, according to the start-up process according to the first embodiment of the present invention, the control unit 29 creates an Arrhenius plot in which the reciprocal absolute temperature is plotted as the x axis and the logarithm of the membrane resistance component is plotted as the y axis. By determining whether there is a slope change point in the Arrhenius plot, it is determined whether water is frozen inside or on the surface of the electrolyte membrane, so the fuel cell can be more reliably operated at a temperature below the freezing point. Simultaneously with the start-up, the amount of reaction gas used and the power consumption can be minimized.

  Further, according to the start-up process according to the first embodiment of the present invention, when there is a change point of the slope in the Arrhenius plot, the control unit 29 controls the heater power supply 22 as a start-up mode at a low temperature to end. The heaters 13a and 13b provided on the plates 6a and 6b are heated, and the fuel cell 1 is started after being heated until the fuel cell 1 reaches a predetermined temperature. Therefore, useless power is consumed even at a temperature below the freezing point. Thus, the fuel cell 1 can be reliably started without doing so.

  Next, with reference to the flowchart shown in FIG. 5, the operation of the control unit 29 when executing the startup process according to the second embodiment of the present invention will be described. Note that the flowchart shown in FIG. 5 is a process after it is determined that there is a slope change point in the Arrhenius plot in the process of step S2 shown in FIG. 4 (after the process of step S3). Moreover, since the process of step S11-step S13 shown in FIG. 5 is the same as the process of step S3-step S5 shown in FIG. 4, it starts from the process of step S14 which becomes the 2nd Embodiment of this invention below. Processing will be described.

  In the process of step S14, the control unit 29 determines whether or not the magnitude of the slope of the Arrhenius plot is equal to or less than a predetermined value. If the magnitude of the slope of the Arrhenius plot is not less than or equal to the predetermined value as a result of the determination, the control unit 29 controls the MFCs 21a and 21b and starts supplying hydrogen and air to the fuel cell 1 as the process of step S15. After that, the startup process proceeds to the process of step S18.

On the other hand, when the magnitude of the slope of the Arrhenius plot is not more than a predetermined value, the control unit 29
The hydrogen remaining in the fuel cell 1 reacts with oxygen that has entered from the outside to produce water, and the temperature decreases, or the water vapor in the fuel cell 1 condenses as the temperature decreases, resulting in a temperature below the freezing point. It is judged that the amount of water remaining in the catalyst layer at the time of start-up is large. Then, the control unit 29 controls the MFCs 21a and 21b as the process of step S16 to increase the flow rates of hydrogen and air from the normal time and supplies them to the fuel cell 1 and waits for a predetermined time as the process of step S17. Then, the startup process proceeds to the process of step S18. Thereby, supply failure of hydrogen and air due to ice existing in the catalyst layer can be suppressed, and an increase in diffusion polarization (overvoltage) can be suppressed.

  In the process of step S18, the control unit 29 determines the amount of current to be extracted from the fuel cell 1 and the speed at which the current is increased when extracting the current from the slope of the Arrhenius plot and the magnitude of the membrane resistance component at the time of startup (time differentiation). To decide. Specifically, when the slope of the Arrhenius plot is small, it is considered that there is a lot of ice in the catalyst layer and the diffusion polarization is large. Therefore, the control unit 29 reduces the amount of current to be extracted. In addition, when the slope of the Arrhenius plot is small and the membrane resistance component is a predetermined value or more, in addition to the presence of a large amount of ice in the catalyst layer, the polarization due to the resistance of the electrolyte membrane is large due to the drying of the electrolyte membrane. Therefore, the control unit 29 slows the rate at which the current is increased, so that the water generated by the power generation while the current is being increased is back-diffused into the electrolyte membrane to reduce the membrane resistance component. Thereby, the process of step S18 is completed, and the activation process proceeds from the process of step S18 to the process of step S19.

  In the process of step S19, the control unit 29 controls the constant current control circuit 23 so as to extract the current from the fuel cell 1 at a rate of increasing the extraction current amount and the current determined in the process of step S18. Thereby, a series of start-up processes are completed.

  As is clear from the above description, according to the start-up process according to the second embodiment of the present invention, the control unit 29 plots the Arrhenius plotting the reciprocal absolute temperature as the x axis and the logarithm of the membrane resistance component as the y axis. Qualitatively grasp the amount of water and ice on the catalyst layer and electrolyte membrane surface by creating a plot and determining whether the slope of the created Arrhenius plot is less than a predetermined value, The fuel cell 1 can be reliably started at a temperature below the freezing point, and at the same time, the amount of reaction gas used and the power consumption can be minimized.

  Further, according to the starting process according to the second embodiment of the present invention, when the magnitude of the slope of the Arrhenius plot is equal to or smaller than a predetermined value, the control unit 29 controls the MFCs 21a and 21b to flow the hydrogen and air. Is increased and supplied to the fuel cell 1 from the normal start-up, so that the ice in the catalyst layer is removed and the voltage drop of the fuel cell 1 due to diffusion polarization can be prevented.

  Further, according to the start-up process according to the second embodiment of the present invention, the control unit 29 controls the MFCs 21a and 21b to increase the flow rates of hydrogen and air from the normal start-up and supply them to the fuel cell 1. Then, control is performed so that a current is taken out from the fuel cell 1 after a predetermined time has elapsed, so that it is possible to prevent the fuel cell 1 from starting power generation before the reaction gas sufficiently reaches the catalyst layer.

  Furthermore, according to the start-up process according to the second embodiment of the present invention, the control unit 29 extracts the amount of current and current extracted from the fuel cell 1 according to the slope of the Arrhenius plot and the magnitude of the membrane resistance component. In this case, the speed at which the current is increased is controlled, so that when the fuel cell 1 is started at a temperature below the freezing point, the fuel cell 1 can be prevented from being unable to generate power, and the power generation efficiency can be improved. .

  As mentioned above, although the embodiment to which the invention made by the present inventor is applied has been described, the present invention is not limited by the description and the drawings that form part of the disclosure of the present invention according to this embodiment. That is, it should be added that other embodiments, examples, operation techniques, and the like made by those skilled in the art based on the above embodiments are all included in the scope of the present invention.

It is sectional drawing which shows the structure of the polymer electrolyte fuel cell used as one Embodiment of this invention. It is a block diagram which shows the structure of the fuel cell system used as one Embodiment of this invention. It is a figure which shows an example of an Arrhenius plot. It is a flowchart figure which shows the flow of the starting process used as the 1st Embodiment of this invention. It is a flowchart figure which shows the flow of the starting process used as the 1st Embodiment of this invention.

Explanation of symbols

1: Fuel cell 13: Heater 21a, 21b: Mass flow controller (MFC)
22: Heater power supply 23: Constant current control circuit 24: Load 25: Inverter 26: Thermometer 27: Cell resistance measuring device 28: Memory 29: Control unit

Claims (10)

A fuel cell system including a fuel cell that generates electric power by electrochemically reacting a reaction gas in a membrane electrode assembly,
A fuel cell system, wherein a fuel cell activation method is controlled in accordance with a relationship between a change in resistance and temperature of the fuel cell between a previous system stop and a system activation. A fuel cell system including a fuel cell that generates electric power by electrochemically reacting a reaction gas in a membrane electrode assembly,
Measuring means for measuring the resistance and temperature of the fuel cell;
Storage means for storing changes in resistance and temperature of the fuel cell between the previous system shutdown and system startup;
A fuel cell system comprising: control means for controlling a starting method of the fuel cell according to the relationship between the resistance of the fuel cell stored in the storage means and a change in temperature.   The control means calculates the resistance component of the electrolyte membrane from the resistance of the fuel cell, and calculates the logarithm of the membrane resistance component with respect to the reciprocal of the absolute temperature, or the logarithm of the product of the membrane resistance component and temperature with respect to the reciprocal of the absolute temperature. 3. The fuel cell system according to claim 2, wherein a starting method of the fuel cell is determined based on presence or absence of a change in the differential value.   The control means calculates the resistance component of the electrolyte membrane from the resistance of the fuel cell, and calculates the logarithm of the membrane resistance component with respect to the reciprocal of the absolute temperature, or the logarithm of the product of the membrane resistance component and temperature with respect to the reciprocal of the absolute temperature. The fuel cell system according to claim 2 or 3, wherein a starting method of the fuel cell is determined based on a magnitude of the differential value. Comprising heating means for heating the fuel cell,
The control means heats the fuel cell by controlling the heating means when there is a change in the logarithm or differential value, and the fuel battery cell is changed in response to the temperature of the fuel battery cell exceeding a predetermined temperature. The fuel cell system according to claim 3 or 4, wherein the fuel cell system is started. Adjusting means for adjusting at least one of the flow rate and pressure of the reaction gas;
When the magnitude of the logarithmic value or the differential value is equal to or less than a predetermined value, the control means controls the adjustment means so that at least one of the flow rate and pressure of the reaction gas supplied to the fuel cell is normally activated. 6. The fuel cell system according to claim 4 or 5, wherein the fuel cell system is increased from the time.   When the magnitude of the logarithm or the differential value is equal to or less than a predetermined value, the control means makes the time from the supply of the reaction gas to the fuel battery cell to the start of current extraction longer than the normal startup time. The fuel cell system according to any one of claims 4 to 6, wherein:   The control means controls the amount of current extracted from the fuel cell and the speed at which the current is increased according to the magnitude of the logarithm or differential value and the magnitude of the membrane resistance component. The fuel cell system according to claim 7.   The said control means makes the magnitude | size of the electric current value taken out from the said fuel battery cell smaller than the time of normal starting, when the magnitude | size of the said logarithm or a differential value is below a predetermined value, It is characterized by the above-mentioned. Fuel cell system.   When the magnitude of the logarithm or differential value is not more than a predetermined value and the magnitude of the membrane resistance component is not less than a predetermined value, the control means makes the speed of increasing the current slower than normal startup. The fuel cell system according to claim 8 or 9, characterized in that

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