JP4884663B2 - Starting the fuel cell - Google Patents

Description

  The present invention relates to a method for starting a fuel cell when a fuel cell including a fuel cell stack formed by stacking a plurality of cells is started at a temperature below freezing point.

  In recent years, a fuel cell vehicle provided with a fuel cell as a vehicle drive source has been proposed. As this type of fuel cell, one having a structure in which a predetermined number of cells having a solid polymer electrolyte membrane interposed between an anode and a cathode is laminated. Then, hydrogen (fuel gas) is introduced into the anode and air (oxidant gas) is introduced into the cathode, thereby generating electric power through an electrochemical reaction between hydrogen and oxygen.

In general, this type of fuel cell has an optimum temperature range of 70 to 80 ° C., but depending on the usage environment, it may take a long time to reach the temperature after startup. It is necessary to take.
In particular, in the case of a fuel cell vehicle as a moving means, quick startability is required even at low temperatures, so the low temperature startability of the fuel cell is extremely important.

For example, Patent Document 1 proposes a technique for decreasing the power generation efficiency and increasing the heat generation amount by periodically changing the generated current.
As another method, there is also known a method of increasing the amount of heat generated by power generation by increasing the output current of the fuel cell.
JP 2002-313388 A

However, the conventional techniques have the following problems.
In other words, in the technique of periodically changing the generated current, the power generation efficiency is lowered even though the amount of heat generation is increased, and the fuel cell is forced to operate inefficiently. It is not preferable.

  In addition, when starting a fuel cell below freezing point, residual water remaining in the fuel cell stack may freeze and the reaction gas cannot be sufficiently supplied to the electrodes (anode and cathode), and power generation may not be performed normally. is there. In any of the conventional techniques, if an excessive amount of current flows through the fuel cell stack, if the power generation cannot be performed normally as described above, the catalyst component of the electrode causes an electrochemical reaction and corrodes. (Hereinafter referred to as electric corrosion), and there is a risk that the performance and life may be reduced.

  Accordingly, the present invention provides a fuel cell startup method capable of extending the life of the fuel cell by preventing electrode corrosion and suppressing performance degradation when starting at a temperature below freezing point. With the goal.

The invention according to claim 1 is a method for starting a fuel cell including a fuel cell stack (for example, the fuel cell stack 4 in the embodiment) formed by stacking a plurality of cells (for example, the cell 3 in the embodiment). A rate of change measurement step of measuring a rate of change of voltage with respect to the current value by increasing a value of current flowing through the fuel cell stack when the fuel cell stack is started at a temperature at least below freezing; in change rate measuring step, the rate of change, decreasing the magnitude of the current flowing through said fuel cell stack when the fuel cell stack is below a predetermined value which can determine whether it is warm, the rate of change, the fuel The current value is increased when a predetermined value that can determine whether the battery stack is warmed is exceeded.
The invention according to claim 2, prior to the change rate measuring step, has a target current value setting step of the fuel cell stack sets the target current value is the minimum current value or a value which can be free-standing hot air, said change The rate measuring step is characterized in that the current value passed through the fuel cell stack is increased toward the target current value .
According to a third aspect of the present invention, after the target current value setting step and before the change rate measurement step, if the current value flowing through the fuel cell stack is equal to or greater than the target current value, The present invention is characterized in that the value of current flowing through the fuel cell stack is reduced.

  According to the present invention, when the rate of change of the voltage with respect to the current value flowing through the fuel cell stack in a sub-freezing environment is 0 or less, the electrode surface area decreases due to freezing of residual water on the electrode surface of the fuel cell stack. End up. For this reason, it can be estimated that the current density of the electrode is excessive with respect to the power generation conditions. Therefore, by reducing the current value, the current density of the electrode is reduced, and an excessive current flows to the fuel cell electrode. Can prevent electrode corrosion. Therefore, performance degradation can be suppressed and the life of the fuel cell can be extended.

According to the present invention, when the fuel cell is started below freezing point, it is possible to prevent electrode corrosion, so that the life of the fuel cell can be extended by suppressing the performance degradation.

A fuel cell activation method according to an embodiment of the present invention will be described below with reference to the drawings. In the present embodiment, a fuel cell system when the fuel cell is mounted on a vehicle will be described.
FIG. 1 is a schematic configuration diagram of a fuel cell system to which a fuel cell activation method according to an embodiment of the present invention is applied.

The fuel cell 2 shown in the figure has a configuration in which a stack 4 formed by stacking a plurality of cells 3... 3 is sandwiched between a pair of plates 5 and 5. Each cell 3 is formed by sandwiching a solid polymer electrolyte membrane made of, for example, a solid polymer ion exchange membrane between the anode and the cathode from both sides. When hydrogen as a fuel is supplied to the anode of each cell and air containing oxygen as an oxidant is supplied to the cathode, hydrogen ions generated by a catalytic reaction at the anode move to the cathode through the electrolyte membrane, and the cathode It generates electricity by causing an electrochemical reaction with oxygen.
During power generation, water is generated on the cathode side, and part of the generated water generated on the cathode side is diffused back to the anode side through the electrolyte membrane, so that generated water is also present on the anode side.

  Hydrogen supplied from the hydrogen tank 6 is supplied to the anode of the fuel cell 2 through the hydrogen supply flow path 8 via the shutoff valve 7 and a regulator (not shown). Then, the unreacted hydrogen off-gas that has not been consumed by the power generation is discharged from the anode to the hydrogen off-gas circulation passage 9 together with residual water such as produced water on the anode side, and joins the hydrogen supply passage 8 via the ejector 10. To do.

  That is, the hydrogen off-gas discharged from the fuel cell 2 merges with fresh hydrogen supplied from the hydrogen tank 6 and is supplied again to the anode of the fuel cell 2. Further, the hydrogen off-gas discharge channel 11 branched from the hydrogen off-gas circulation channel 9 is connected to a dilution box (not shown). A hydrogen purge valve 12 is provided in the hydrogen off gas discharge channel 11, and the used hydrogen off gas is discharged from the hydrogen off gas discharge channel 11 to the dilution box by opening the hydrogen purge valve 12.

  On the other hand, the air is pumped to the air supply channel 14 by the compressor 13 and supplied to the cathode of the fuel cell 2. After the air supplied to the cathode of the fuel cell 2 is used for power generation, it is discharged from the fuel cell 2 to the air off-gas discharge channel 15 as off-gas along with residual water such as generated water on the cathode side.

  The air off gas discharge channel 15 is connected to the above-described dilution box (not shown), and the air off gas discharged from the air off gas discharge channel 15 is mixed with the hydrogen off gas in the dilution box. Thereby, the hydrogen off gas discharged from the hydrogen off gas discharge channel 11 is diluted to a predetermined concentration or less by the dilution box.

Further, the fuel cell 2 includes a cooling water flow path (not shown) provided with a circulation pump for circulating the cooling water. By circulating the cooling water when the fuel cell 2 is operated, the fuel cell 2 is controlled to a temperature suitable for an electrochemical reaction (for example, 80 ° C.).
The fuel cell 2 is connected to a load 16 such as a vehicle driving motor via an electric wire 17, and supplies the electric power obtained by the power generation of the fuel cell 2 to the load 16 via the electric wire 17.

Moreover, in this Embodiment, in order to grasp | ascertain the internal temperature of the fuel cell 2, the temperature sensors 19-21 are provided in several places. That is, the temperature sensor 19 near the outlet of the stack 4 in the air off gas discharge passage 15, the temperature sensor 20 near the outlet of the stack 4 in the hydrogen off gas discharge passage 11, and the temperature sensor 21 on the plate 5 of the fuel cell 2, respectively. Provided. Furthermore, a temperature sensor is also provided in a refrigerant passage (not shown) that circulates through the fuel cell 2. In addition, the structure provided with not only the above-mentioned temperature sensor but at least 1 temperature sensor may be sufficient.
In order to measure the current I of the fuel cell 2, a current sensor 22 is provided on the electric wire 17 connected to the load 16. Furthermore, a voltage sensor 23 for measuring the voltage of the fuel cell 2 and a voltage sensor (not shown) for detecting the voltage of each cell 3 are also provided.

  The fuel cell system 1 is provided with a control unit (control unit) 24 for controlling the system 1. An ignition switch (IG SW) is connected to the control unit 24. The controller 24 receives ignition ON / OFF (IG-ON, IG-OFF) signals from the ignition switch and detection values from the temperature sensors 19 to 21, the current sensor 22, and the voltage sensor 23. And the control part 24 outputs the signal which drives the cutoff valve 7, the compressor 13, and the hydrogen purge valve 12 based on these input detection values and signals.

FIG. 2 is a flowchart showing the processing contents of the starting method of the fuel cell system.
First, in step S10, when the ignition switch is turned on (IG-ON) and it is detected that the vehicle has been started, the shutoff valve 7 is opened and the compressor 13 is driven, so that the anode and cathode of the fuel cell 2 are opened. The control for supplying the reaction gas (hydrogen, air) to each is performed, and the starting operation is started.

  Next, in step S12, the internal temperature of the fuel cell 2 is grasped based on the temperature detected by the temperature sensors 19 to 21 and the temperature sensor provided in the refrigerant passage. In step S14, it is determined whether or not the internal temperature of the fuel cell 2 is greater than 0 degrees. If the determination result is YES, it can be determined that the engine has been sufficiently warmed up, and thus the process proceeds to step S32, shifts to normal power generation control, and the process of this flowchart ends. This is because it can be determined that the residual water in the fuel cell 2 is not frozen in this case.

  On the other hand, when the determination result of step S14 is NO (when it is determined that the low temperature is started), the process proceeds to step S16. In step S16, the current value I is grasped from the current value detected by the current sensor 22. In step S18, a target current is set (see FIG. 5). Here, the target current is an appropriate current value that can prevent electrolytic corrosion at the electrodes (anode, cathode) of the fuel cell 2 when the residual water is not frozen on the electrode surface of the stack 4, The value is set to a value equal to or higher than the minimum current value at which the fuel cell 2 can be warmed up independently by power generation. More specifically, the minimum current value is the amount of heat generated by power generation of the fuel cell stack 4 rather than the sum of the amount of heat released from the outer periphery of the fuel cell stack 4 and the amount of heat absorbed by the refrigerant circulating in the refrigerant flow path. The minimum current value is calculated so as to exceed, and the lowest current value among the current values that does not freeze the generated water generated during power generation while melting the frozen residual water.

  In step S20, it is determined whether the target value is larger than the current value. If the determination result is YES, the process proceeds to step S22, and if the determination result is NO, the process proceeds to step S28. In step S28, control for reducing the current of the fuel cell 2 is performed. Then, it returns to step S12 and performs the above-mentioned process again.

  In step S22, a process of increasing the current with a predetermined gradient is performed. FIG. 5 is a graph showing current increase processing with respect to time in the present embodiment. As shown in the figure, by performing control to increase the current value in proportion to time, it is possible to easily and reliably grasp the amount of change in voltage value with respect to the amount of increase in current value. Note that the current increasing process is not limited to the above-described direct proportion, but may be increased in a quadratic curve, for example, and the current value may be kept constant in a partial section.

In step S24, it is determined whether or not the rate of change of the voltage with respect to the current value flowing through the fuel cell stack 4 is greater than 0 (dV / dI> 0). If the determination result is YES, the process proceeds to step S26, and if the determination result is NO, the process proceeds to step S28.
In step S26, it is determined whether or not the internal temperature of the fuel cell 2 is greater than 0 degrees. If the determination result is YES, it can be determined that the engine has been sufficiently warmed up, and thus the process proceeds to step S32, shifts to normal power generation control, and the process of this flowchart ends.
If the determination result of step S26 is NO, the process proceeds to step S30. In step S30, it is determined whether or not the current current value is equal to the target value. If this determination result is YES, it returns to step S12 and performs the above-mentioned processing. Moreover, if the determination result of step S30 is NO, it will return to step S22 and will perform the above-mentioned process again.

  On the other hand, when the determination result of step S24 is NO, that is, when the rate of change of the voltage with respect to the current value is 0 or less, the process proceeds to step S28 to perform control to reduce the current. The reason why the control is performed in this way is that if the rate of change of the voltage with respect to the current value flowing through the fuel cell stack 4 is 0 or less, it can be estimated that there is a portion where the power generation of the fuel cell stack 4 is not normal but is frozen. It is. This will be described with reference to FIGS.

  FIG. 3 is a graph showing the relationship between the current and voltage of the fuel cell stack when power generation at the time of starting below freezing is normal. As shown in the figure, when the power generation is normal at the time of starting below freezing, when the current of the fuel cell stack 4 increases, the voltage increases accordingly. That is, the voltage change rate dV / dI with respect to the current value flowing through the fuel cell stack 4 is greater than zero. This is because the increase in voltage due to the temperature rise is larger than the decrease in voltage due to the increase in current. More specifically, when the temperature is constant, the voltage decreases as the current increases.However, when power generation is normally performed at the time of starting below freezing, the temperature rises because heat is generated by power generation. Along with this, the voltage increases more than the above decrease. As a result, when power generation is normal at the time of starting below freezing point, the rate of change of the voltage with respect to the current value flowing through the fuel cell stack 4 exhibits the characteristics as shown in FIG.

  FIG. 4 is a graph showing the relationship between the current and voltage of the fuel cell stack when the power generation at the time of starting below freezing is abnormal. As shown in the figure, when power generation is abnormal, when the current of the fuel cell stack 4 increases, the voltage decreases accordingly. That is, the voltage change rate dV / dI with respect to the current value flowing through the fuel cell stack 4 is 0 or less. This is because the increase in voltage due to temperature rise is smaller than the decrease in voltage due to increase in current. More specifically, when power generation is abnormal, the temperature hardly increases and the voltage hardly increases because heat generation by power generation is not sufficient. As described above, the voltage decreases as the current increases when the temperature is constant. Insufficient supply of reactants occurs. Since the influence of the voltage reduction caused by this is larger, the rate of change of the voltage with respect to the current value exhibits characteristics as shown in FIG.

  In the present embodiment, when it can be estimated that power generation is not normal in the determination in step S24, the process proceeds to step S28, and a process of decreasing the current value is performed. Thereby, since the current density of the power generation unit can be reduced by reducing the current value, it is possible to prevent an excessive current from flowing through the fuel cell stack 4.

As described above, according to the method for starting the fuel cell in the present embodiment, when the fuel cell 2 is started below freezing point, it is possible to prevent electrode corrosion, so that the performance of the fuel cell 2 is reduced. By suppressing it, the life can be extended.
Of course, the contents of the present invention are not limited to the above-described embodiments. For example, in the embodiment, the case where the fuel cell is mounted on the vehicle has been described, but the present invention may be applied to a fuel cell system other than the vehicle. Further, the threshold value of step S14 may not be 0 ° C., and may be a predetermined value for determining whether the cell 3 of the fuel cell 2 is warmed up.

1 is an overall configuration diagram of a fuel cell system in an embodiment of the present invention. It is a flowchart which shows the processing content of the starting method of a fuel cell system. It is a graph which shows the relationship between the electric current and voltage of a fuel cell stack in case electric power generation is normal. It is a graph which shows the relationship between the electric current and voltage of a fuel cell stack in case power generation is abnormal. It is a graph which shows the increase process of the electric current with respect to time in this Embodiment.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Fuel cell system 2 ... Fuel cell 3 ... Cell 4 ... Stack 19-21 ... Temperature sensor 22 ... Current sensor 23 ... Voltage sensor 24 ... Control unit

Claims (3)

A method for starting a fuel cell including a fuel cell stack formed by stacking a plurality of cells,
When starting the fuel cell stack at a temperature of at least below freezing,
A rate of change measurement step of increasing a current value flowing through the fuel cell stack and measuring a rate of change of voltage with respect to the current value;
In the differential rate measuring step, the rate of change, decreasing the magnitude of the current flowing through said fuel cell stack when the fuel cell stack is below a predetermined value which can determine whether it is warm, the rate of change, the A method for starting a fuel cell, comprising: increasing the current value when a predetermined value that can determine whether the fuel cell stack is warmed is exceeded. Prior to the change rate measuring step, has a target current value setting step of the fuel cell stack sets the target current value is the minimum current value or a value that can be self warming,
2. The method of starting a fuel cell according to claim 1, wherein in the change rate measuring step, a current value flowing through the fuel cell stack is increased toward the target current value . Even after the target current value setting step, prior to said change rate measuring step, when the current value is flowing to the fuel cell stack is equal to or greater than the target current value, the current value flowing to the fuel cell stack 3. The method for starting a fuel cell according to claim 2, wherein the starting method is decreased.

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