JP5151293B2 - Operation method of fuel cell - Google Patents

Description

  The present invention relates to a method for operating a fuel cell, and more particularly to a method for operating a fuel cell using platinum as an electrode catalyst.

  In recent years, fuel cells have attracted attention in response to social demands and trends against the background of environmental problems. In particular, a polymer electrolyte fuel cell (hereinafter referred to as “PEFC”) using an ion conductive polymer electrolyte membrane can be operated at a low temperature of 100 ° C. or lower. It is expected as a power source and is being developed for practical use. In order for the PEFC to be put into practical use as a vehicle drive source or a stationary power source, it is necessary to have durability over a long period of time.

  The PEFC generally has a structure in which a membrane-electrode assembly (hereinafter referred to as “MEA”) is sandwiched between separators. The MEA generally has a structure in which a gas diffusion layer, a cathode catalyst layer, a solid polymer electrolyte layer, and an anode catalyst layer are sequentially laminated. The cell reaction proceeds at least in a catalyst layer comprising a catalyst, a carrier supporting the catalyst, and an ion conductive polymer. For this reason, suppressing deterioration of the catalyst layer is an important issue in enhancing the durability of PEFC.

A conventional technique for suppressing the deterioration of the catalyst is described in Patent Document 1 below. Patent Document 1 describes a method for producing a catalyst in which a platinum alloy catalyst supported on carbon black is subjected to a heat treatment, then contacted with carbon monoxide, and again subjected to a heat treatment in an inert gas atmosphere. That is, a method for producing a catalyst having excellent catalytic activity and durability by reducing and stabilizing the distance between platinum and platinum atoms has been proposed.
JP 2001-52718 A

  However, even the technique described in Patent Document 1 cannot prevent the platinum in the cathode catalyst layer from eluting due to a change in the voltage of the fuel cell accompanying a periodic change in the load on the fuel cell.

  During operation of the fuel cell, a phenomenon in which the output voltage of the fuel cell changes periodically or aperiodically (hereinafter referred to as “duty cycle”) occurs in accordance with a change in load on the fuel cell. Since the occurrence of the duty cycle is accompanied by a change in the voltage of the fuel cell, the oxidation and reduction reactions of the catalyst constituting the catalyst layer of the fuel cell are frequently repeated.

  A platinum catalyst is stable even if it is held at a constant potential even at a high potential, and the catalyst elution is slight. However, when a duty cycle involving oxidation / reduction of the catalyst occurs, the potential of the catalyst changes greatly due to a change in the voltage of the fuel cell. Thereby, the elution of the catalyst becomes remarkable, the catalyst is deteriorated, and the power generation performance of the fuel cell is lowered, which causes a decrease in the durability of the fuel cell.

  When the rate of change in the voltage of the fuel cell accompanying the change in the load on the fuel cell from the high side to the low side (hereinafter referred to as “cell voltage change rate”) exceeds a certain value, It is intended to improve the durability and reliability of fuel cells by finding that the rate of elution of platinum is rapidly increased and controlling the cell voltage change rate to a certain value or less. is there.

  In order to achieve the above object, a fuel cell operation method according to the present invention is a fuel cell operation method using platinum as an electrode catalyst, and a change in load of electric power to the fuel cell is changed from a high side to a low side. A cell that is a rate of change in the voltage of each fuel cell that constitutes the fuel cell when it is detected that the change in load is a change from a high side to a low side. And a step of controlling the voltage change rate to 200 mV / s or less.

  According to the fuel cell operating method of the present invention, by controlling the cell voltage change rate to be a certain value or less, elution of the catalyst can be prevented, and the durability and reliability of the fuel cell can be improved.

  Hereinafter, a control method for a fuel cell according to the present invention will be described in detail with reference to the drawings, divided into first to third embodiments.

  Before describing these embodiments, the overall configuration of the fuel cell stack will be briefly described in order to facilitate understanding of the present invention. FIG. 1 is a perspective view showing the overall configuration of the fuel cell stack, and FIG. 2 is an enlarged cross-sectional view of the main part showing the cell structure of the fuel cell stack.

  As shown in FIG. 1, the fuel cell stack 1 includes a unit battery cell (hereinafter simply referred to as “cell”) that generates an electromotive force by a reaction between an anode reaction gas (in this specification, hydrogen) and a cathode reaction gas (in this specification, oxygen). 2) is laminated to form a laminated body 3. A current collecting plate 4, an insulating plate 5 and an end plate 6 are disposed at both ends of the laminated body 3, and penetrated into the laminated body 3. The tie rod 7 is passed through a through hole (not shown), and a nut (not shown) is screwed into the end of the tie rod 7.

  In this fuel cell stack 1, an anode reaction gas, a cathode reaction gas, and a liquid medium (specifically, cooling water or hot water) are circulated in flow channel grooves formed in separators (not shown) of each cell 2, respectively. An anode reaction gas supply port 8, an anode reaction gas discharge port 9, a cathode reaction gas supply port 10, a cathode reaction gas discharge port 11, a medium supply port 12 and a medium discharge port 13 are formed in one end plate 6. ing.

  The anode reaction gas is supplied from the anode reaction gas supply port 8, flows through the anode reaction gas supply channel groove formed in the separator, and is discharged from the anode reaction gas discharge port 9. The cathode reaction gas is supplied from the cathode reaction gas supply port 10, flows through the cathode reaction gas supply channel groove formed in the separator, and is discharged from the cathode reaction gas discharge port 11. The liquid medium is supplied from the medium supply port 12, flows through a medium supply channel groove formed in the separator, and is discharged from the medium discharge port 13.

  As shown in FIG. 2, the cell 2 is composed of a membrane electrode assembly (hereinafter also referred to as MEA (membrane electrode assembly)) 14 and separators 15 disposed on both surfaces of the MEA 14. Hereinafter, the separator 15 disposed on the anode side of the MEA 14 is referred to as an anode separator 15A, and the separator 15 disposed on the cathode side is referred to as a cathode separator 15B.

  The MEA 14 includes, for example, a solid polymer electrolyte membrane 141 that is a polymer electrolyte membrane that allows hydrogen ions to pass through, an anode electrode 144A that is an anode composed of an anode catalyst layer 142A and a gas diffusion layer 143A, and a cathode that is a catalyst metal that is an electrode catalyst. It consists of a cathode electrode 144B as a cathode comprising a catalyst layer 142B and a gas diffusion layer 143B. The MEA 14 has a laminated structure in which a solid polymer electrolyte membrane 141 is sandwiched from both sides by an anode electrode 144A and a cathode electrode 144B.

  The separator 15 is formed by forming a thin conductive metal plate into a predetermined shape using a mold. As shown in the figure, the separator 15 has an uneven shape (so-called corrugated shape) in which convex portions 16 and concave portions 17 are alternately formed in an active region that contributes to power generation (region of the central portion in contact with the MEA 14). doing.

The convex portion 16A and the concave portion 17A of the anode separator 15A arranged in contact with the anode electrode 144A side of the MEA 14 serve as a channel groove for flowing an anode reaction gas (hydrogen; H ₂ ) between the MEA 14 and the anode reaction gas channel. (Anode channel) 18 is formed. On the other hand, the convex portion 16B and the concave portion 17B of the cathode separator 15B disposed in contact with the cathode electrode 144B side of the MEA 14 serve as a channel groove for allowing the cathode reactive gas (oxygen; O ₂ ) to flow between the MEA 14 and the cathode reactive gas. A flow path (cathode flow path) 19 is formed.

  When hydrogen is passed through the anode reaction gas flow path 18 and oxygen is passed through the cathode reaction gas flow path 19, the hydrogen is converted into hydrogen ions by the catalytic action of the anode catalyst layer 142A to release electrons. The hydrogen ions that have released the electrons pass through the solid polymer electrolyte membrane 141. In the cathode catalyst layer 142B, hydrogen passing through the solid polymer electrolyte membrane 141 and electrons passing through an external circuit (not shown) react with oxygen to generate water. As a result, the anode electrode 144A becomes negative and the cathode electrode 144B becomes positive, and a DC voltage is generated between the anode electrode 144A and the cathode electrode 144B as shown in FIG.

  In this specification, a voltage appearing between the anode electrode 144A and the cathode electrode 144B (that is, the potential of the cathode electrode with respect to the potential of the anode electrode) is referred to as a cell voltage. The cell voltage can be obtained by conversion based on known data from the output power of the fuel cell stack. However, the cell voltage may be detected by the cell voltmeter 20 functioning as a cell voltage detection means. In this case, the cell voltage may be detected from all the cells constituting the fuel cell stack 1 or may be detected only from a plurality of representative cells among the cells constituting the fuel cell stack 1. Anyway.

Hereinafter, the fuel cell control method and the control device according to the present invention will be described in detail from the first embodiment to the third embodiment.
[First Embodiment]
FIG. 3 is a block diagram showing a schematic configuration of a fuel cell system for carrying out the fuel cell control method according to the first embodiment of the present invention. FIG. 4 is a view showing a flowchart of the method of operating the fuel cell according to the first embodiment of the present invention.

  As shown in FIG. 3, the fuel cell system for carrying out the fuel cell control method according to the first embodiment is a fuel cell stack that generates electricity by supplying an anode reaction gas (hydrogen) and a cathode reaction gas (oxygen). 300, a flow rate regulator 310 that regulates the flow rate of hydrogen as an anode reaction gas, a compressor 330 that regulates the flow rate of air containing oxygen as a cathode reaction gas, a secondary battery 370 that functions as an auxiliary power source for the fuel cell stack 300, A discharge circuit 390 having a variable resistance for short-circuiting the electrodes of the fuel cell stack or the electrodes of the cells may be provided.

  The fuel cell stack 300 uses a catalytic metal as an electrode catalyst. Specifically, platinum is used for the cathode catalyst layer 142B (see FIG. 2) of the cathode electrode 144B.

  The flow controller 310 may have a function of supplying hydrogen to the anode electrode of the fuel cell stack 300 and may have a function of adjusting the hydrogen supply flow rate based on control by the control unit.

  The compressor 330 has a function of supplying external air to the cathode electrode of the fuel cell. The flow rate of air supplied to the fuel cell can be adjusted by the control unit 360 controlling the compressor 330.

  When there is a difference between the power required for the fuel cell system and the power output from the fuel cell stack 300, the secondary battery 370 is charged when the power output from the fuel cell stack is larger. When the power output by the stack is smaller, it is discharged. That is, the secondary battery 370 functions as an auxiliary power source for the fuel cell stack.

  The discharge circuit 390 is connected between the electrodes of the fuel cell stack or between the cathode electrode and the anode electrode of the cell, and can function as a means for short-circuiting these electrodes based on control by the control unit 360. The discharge circuit 390 includes a variable resistor and a switch element, and the resistance value of the variable resistor and the connection between the electrodes can be controlled by the control unit 360. Here, the short circuit includes that the electrodes are electrically connected by a resistor having a certain resistance value.

  The control unit 360 includes a control device, an arithmetic device, a storage device, an input / output device, and a timer, and may have a function of controlling the flow rate regulator 310, the compressor 330, the secondary battery 370, and the discharge circuit 390. Further, it may have a function of detecting the flow rate of hydrogen from the flow rate regulator 310, the flow rate of air from the compressor 430, and the charged amount from the secondary battery. Then, the power required for the fuel cell stack 300 is detected, the cell voltage change rate is obtained, and the power load on the fuel cell stack can be controlled based on this.

  The hydrogen gas supplied to the fuel cell stack 300 is filled in the hydrogen cylinder 320 at a high pressure, and the flow rate of the hydrogen gas supplied to the fuel cell stack 300 is adjusted by the flow regulator 310. The flow rate of air supplied to the fuel cell stack 300 is adjusted by the compressor 330 and is pumped. The air that has passed through the fuel cell stack 300 is directly discharged into the atmosphere.

  The operation method of the fuel cell according to the first embodiment will be described in detail according to the flowchart shown in FIG. First, during operation of the fuel cell, a change in power load on the fuel cell stack (hereinafter simply referred to as “power load change”) is detected (S400). The change in power load may be detected by the power required for the fuel cell stack calculated based on the accelerator opening. Further, a voltmeter that detects the cell voltage and an ammeter that detects the current output from the fuel cell stack may be provided in the fuel cell stack, and detection may be performed from the electric power obtained by the voltmeter and the ammeter.

  Next, it is judged in a control part whether the electric power load change decreased (S401). If it is determined that the power load on the fuel cell stack has not decreased, the process returns to step S400, and the detection of the power load change and the determination of whether the power load has decreased are continued.

  If it is determined that the power load change for the fuel cell stack has decreased, the cell voltage change rate is calculated in the control unit (S402). The cell voltage change rate can be calculated by the following method. That is, when the power load on the fuel cell is large, the cell voltage is small, and when the power load on the fuel cell is small, the cell voltage is large. Therefore, the relationship between the power load on the fuel cell stack and the cell voltage is measured in advance and stored in the storage device of the control unit (hereinafter referred to as “mapping”). The detected power load change is converted into a cell voltage change rate using the relationship between the mapped power load and the cell voltage. By measuring and mapping the relationship between the power load on the fuel cell fuel cell and the cell voltage in advance, the cell voltage change rate can be controlled by the output of the fuel cell, so it is necessary to add new elements to the existing system. The existing system can be used effectively. However, when a voltmeter for detecting the voltage of each fuel cell is provided in the fuel cell stack, the cell voltage change rate may be calculated using the cell voltage detected by the voltmeter.

  Next, the control unit determines whether the cell voltage change rate is greater than a predetermined value (S403). If the cell voltage change rate is not greater than the predetermined value, the flowchart ends. However, this flowchart can always be operated while the fuel cell stack is operating.

  If the cell voltage change rate is greater than the predetermined value, the process proceeds to the next step S404. That the cell voltage change rate is greater than a predetermined value means that the rate at which the power load on the fuel cell stack changes from a high load to a low load exceeds a predetermined value. In this case, the rate at which platinum as a catalyst elutes from the cathode catalyst layer increases rapidly. Therefore, in the first embodiment, control is performed to make the rate of change in the voltage of the cell smaller than a predetermined value by changing the ratio of the power load that the secondary battery and the fuel cell stack respectively bear. The predetermined value may be 200 mV / s. More desirably, it may be 100 mV / s. A detailed description of the phenomenon that when the cell voltage change rate exceeds the predetermined value, the rate of platinum elution from the cathode further increases will be described later. Control of the cell voltage change rate that accompanies a change in the load of power on the fuel cell stack from a high load to a low load is controlled to a predetermined value or less. And reliability can be improved.

  In step S404, the control unit determines whether the output power of the secondary battery can be reduced in consideration of the total power required for the fuel cell stack and the secondary battery. That is, it is possible to change the load ratio of the power that should be borne by the fuel cell stack and the secondary battery, reduce the ratio of the power that should be borne by the secondary battery, and increase the ratio of the power that the fuel cell stack should bear. Judge whether or not. If it is determined that the output power of the secondary battery can be reduced, the output power of the secondary battery is reduced until the cell voltage change rate becomes smaller than a predetermined value (S405). By reducing the output power of the secondary battery, the power load on the fuel cell increases because the ratio of the power load borne by the secondary battery out of the total power required for the fuel cell stack and the secondary battery decreases. When the power load on the fuel cell increases, the current density of the cell increases, so the overvoltage due to polarization increases and the cell voltage decreases. Here, the overvoltage refers to the difference between the electrode potential and the equilibrium potential of platinum elution / deposition. Therefore, the cell voltage change rate can be reduced. As described above, increasing the power load on the fuel cell may be achieved by changing the ratio of the power load borne by the fuel cell and the secondary battery. The machine may be driven by a fuel cell. That is, in addition to driving a vehicle on which a fuel cell is mounted, the power load on the fuel cell may be increased by driving an auxiliary device mounted on the vehicle. The reason why the cell voltage decreases as the cell current density increases will be described later.

  If it is determined in step S404 that the output power of the secondary battery cannot be reduced, the control unit determines whether the secondary battery can be charged (S406). If it is determined that the secondary battery can be charged, the secondary battery is charged until the cell voltage change rate becomes lower than a predetermined value (S407). Since the power for charging the secondary battery is supplied from the fuel cell stack, the power load on the fuel cell becomes large. As the power load on the fuel cell increases, the current density of the cell increases, so the polarization increases and the cell voltage decreases. Therefore, the cell voltage change rate can be reduced. Since excess power generated by controlling the cell voltage change rate to a predetermined value or less is utilized, fuel waste can be saved and fuel consumption can be improved.

  If it is determined that the secondary battery cannot be charged, the process proceeds to a redeposition operation subroutine (S408). Here, the reprecipitation operation subroutine will be described in detail with reference to FIG.

  FIG. 5 is a flowchart of the redeposition operation subroutine. The reprecipitation operation is performed to reprecipitate platinum eluted from the cathode by increasing the overvoltage of the cell and to prevent progression of platinum elution. The overvoltage can be increased by increasing the polarization.

  First, a discharge circuit is connected between the anode electrode and the cathode electrode of the cell (S500). The discharge circuit may be a circuit having switching elements at both ends of the variable resistor. That is, the control unit can short-circuit the cell electrodes by connecting the variable resistance between the cell electrodes by controlling the opening and closing of the switching element. Further, the control unit can change the resistance value of the variable resistor. The discharge circuit may be connected between the cathode electrode and the anode electrode of each cell, or may be connected between the electrodes of the fuel cell stack. By connecting a discharge circuit composed of a resistor between the anode electrode and the cathode electrode of the cell, a current flowing through the discharge circuit is generated, so that the current density of the cell increases. As the cell current density increases, the cell overvoltage increases, so that the cell voltage can be reduced.

  Next, the cell voltage decreased by connecting the discharge circuit between the cathode electrode and the anode electrode of the cell is detected (S501). The cell voltage can be detected by converting the power load on the fuel cell stack with the mapped data. However, when a voltmeter for detecting the voltage of each fuel cell is provided in the fuel cell stack, the cell voltage may be detected by the voltmeter.

  The control unit determines whether the detected cell voltage is smaller than a predetermined value (S502). If it is determined that the detected cell voltage is not smaller than the predetermined value, the overvoltage is further increased by reducing the resistance value of the variable resistance of the discharge circuit (S503). Then, the cell voltage is detected again (S501), and it is determined whether the detected cell voltage is smaller than a predetermined value (S502). If it is determined that the detected cell voltage is not yet lower than the predetermined value, the resistance value of the discharge circuit can be decreased until the cell voltage becomes lower than the predetermined value. Here, the predetermined value of the cell voltage may be 0.7V or less. Moreover, it may be 0.5 V or less. The reason why the predetermined value of the cell voltage is such a value will be described later.

If it is determined that the detected cell voltage is smaller than the predetermined value, time recording is started (S504). Based on the recording, the control unit determines whether or not a predetermined time has elapsed (S505). That is, the time when the cell voltage is lower than the predetermined value is measured by the loop consisting of step numbers S504 and S505, and the state where the cell voltage is lower than the predetermined value is maintained until the predetermined time elapses. To do. The time can be recorded by a timer and a storage device provided in the control unit. The predetermined time, the cathode catalyst layer, platinum ionized can the time required for diffuses through the ionomer t _D is greater than time. By setting the predetermined time in this way, platinum ions in the ionomer can be reprecipitated.

The predetermined time _{t D} is given by Equation (1). If normal cells, as can be seen from equation (1), the predetermined time t _D is the value of a few seconds. Therefore, usually, the predetermined time _{t D} may be less than 10 seconds. However, the predetermined time t _D can be appropriately selected depending on the thickness of the catalyst layer of the cell and the effective diffusion coefficient in the ionomer of platinum ions. Since the time required for reprecipitation of platinum in the reprecipitation operation is in seconds, the power consumption associated with reprecipitation of platinum is not large, and the durability and reliability of the fuel cell stack can be improved efficiently and economically. it can. The redeposition operation subroutine ends when a predetermined time elapses when the cell voltage is lower than the predetermined value.

tD = d / D (1)
d: catalyst layer thickness [m], D: platinum ion effective diffusion coefficient in ionomer [m ² / s]
Since the reprecipitation of platinum can be promoted by the reprecipitation operation, the progress of platinum elution can be suppressed, and the durability and reliability of the fuel cell stack can be improved. In addition, since the reduction of the effective electrochemical surface area of platinum can be mitigated by performing the reprecipitation operation, it is possible to prevent deterioration of the platinum function as a catalyst, and to improve the durability and reliability of the fuel cell stack. Can do.

  With the completion of the reprecipitation operation subroutine, the flowchart of the operation method of the fuel cell according to the first embodiment of the present invention shown in FIG. 4 is also ended. However, this flowchart can always be operated while the fuel cell stack is operating.

  FIG. 6A shows the relationship between the cell voltage (that is, the predetermined value) in the reprecipitation operation and the platinum elution rate after the reprecipitation operation. As shown in this figure, the platinum elution rate can be considerably suppressed by performing the reprecipitation operation with a predetermined value of 0.7 V or less. Moreover, the platinum elution rate can be further suppressed by setting the predetermined value to 0.5 V or less. Therefore, in the first embodiment, platinum is re-deposited at a cell voltage of 0.7 V, preferably 0.5 V, thereby suppressing the progress of subsequent platinum elution, and the durability and reliability of the fuel cell stack. Can be improved.

  FIG. 6B shows the change in cell voltage of the effective electrochemical surface area of platinum when reprecipitation operation is performed (with reprecipitation operation) and when reprecipitation operation is not performed (without reprecipitation operation). It shows sex. By performing the reprecipitation operation, it is possible to reduce the degree of decrease in the effective electrochemical surface area of platinum accompanying the change cycle of the cell voltage. If the state in which the effective electrochemical surface area of platinum is large is maintained, the function of platinum as a catalyst remains high. Therefore, in the first embodiment, by performing the reprecipitation operation, it is possible to prevent deterioration of the function of platinum as a catalyst and improve the durability and reliability of the fuel cell stack.

  Here, the phenomenon described in step S403 in FIG. 4 in which the rate of platinum elution from the cathode increases when the cell voltage change rate exceeds a predetermined value will be described in detail.

  FIG. 7A is a diagram showing the relationship between the cell voltage change rate from the low voltage to the high voltage and the platinum elution rate in the cathode catalyst layer. FIG. 7B shows the cell voltage change rate and the platinum elution rate. It is a figure which shows the magnitude relationship in the relationship between time and a cell potential.

  FIG. 7A is a plot of data measured using an electrochemical quartz crystal nanobalance (hereinafter referred to as “EQCN”) apparatus. When platinum elutes, the electrode becomes lighter and the resonance frequency increases. Therefore, the change in the weight of the electrode can be calculated from the change in the resonance frequency. The EQCN apparatus can measure a change in weight with high accuracy based on such a principle. Therefore, after applying various cell voltage cycles, the elution rate of platinum was measured using an EQCN apparatus. As a result, as shown in FIG. 7 (A), it was found that when the cell voltage change rate was greater than 200 mV / s, the platinum elution rate increased rapidly. On the other hand, it was found that when the cell voltage change rate was smaller than 200 mV / s, the rate of increase in the platinum elution rate due to the increase in the cell voltage rate was small.

  This phenomenon is caused by the principle described below. That is, if the cathode potential is kept high, platinum is oxidized, and platinum on the surface of the catalyst layer becomes stable platinum oxide. On the other hand, if the cathode potential is kept low, platinum oxide is reduced, and platinum is exposed on the surface of the catalyst layer. However, if the time for changing from a low potential to a high potential is short, platinum does not become platinum oxide, and the rate of ionization and elution as platinum ions increases. Therefore, the rate of platinum elution increases as the cell voltage change rate increases. The present invention suppresses the progress of platinum elution by controlling the cell voltage speed to be lower than 200 mV / s. That is, in order to operate the fuel cell in the region where the platinum elution rate is low as shown in FIG. 7B, control is performed to reduce the cell voltage change rate.

  Next, the reason why the cell voltage decreases as the current density of the cell increases as described in step S405 in FIG. 4 will be described in detail.

  FIG. 8A is a diagram showing the relationship between the cell current density and the cell voltage (solid line) and the relationship between the cell current density and the cell output power (dotted line). In the relationship between the cell current density and the cell voltage, the cell voltage decreases as the cell current density increases because the cell overvoltage increases as the cell current density increases. Cell overvoltage occurs due to activation polarization, resistance polarization, and diffusion polarization. Here, activation polarization refers to energy consumption due to hydrogen and oxygen being once activated in a chemical reaction between hydrogen oxidation at the anode electrode and oxygen reduction at the cathode. This is the loss due to the part being used. Resistance polarization refers to a loss due to the use of a part of the electromotive force of the cell for energy consumption due to the current flowing through the internal resistance in the cell. Diffusion polarization refers to a loss due to the use of a part of the electromotive force of a cell to consume energy for diffusion transfer based on a concentration difference generated by a chemical reaction on an electrode. These polarizations increase with increasing cell current density. Therefore, as the cell current density increases, the overvoltage increases and the cell voltage decreases.

  Regarding the relationship between the cell current density and the cell output power, when the cell current density is relatively small, the cell output power increases as the cell current density increases. However, when the output power of the cell reaches a peak and the current density of the cell is further increased, the output power of the cell decreases as the current density of the cell increases. In the region where the current density of the cell is relatively large, the output power of the cell decreases as the cell current density increases. As the cell current density increases, the overvoltage due to polarization increases and the cell voltage decreases. It is because it falls. In principle, in the fuel cell operating method according to the first embodiment, the fuel cell can be operated in the region indicated as the normal operation region in FIGS. 8A and 8B. However, in the reprecipitation operation, the fuel cell can be operated in a region other than the region shown as the normal operation region in FIGS.

As mentioned above, this 1st Embodiment implements invention of the operating method of the fuel cell concerning Claims 1-3 and 5-8. Step numbers S400 and S401 correspond to the step of detecting that the load of power on the fuel cell changes from a high load to a low load according to the present invention. Step numbers S402 to S407 correspond to the step of controlling the cell voltage change rate, which is the rate of change of the voltage between the electrodes of each fuel cell constituting the fuel cell, to 200 mV / s or less. . Step numbers S500 to S505, when it is determined that the detected cell voltage change rate of the present invention cannot be controlled to 200 mV / s or less , the diffusion coefficient of platinum ions in the layer constituting the electrode catalyst, This corresponds to the step of lowering the cell voltage to 0.7 V or less for a time longer than the diffusion time constant of platinum ions determined by the thickness of the layer constituting the electrode catalyst.

The effects of the fuel cell operation method according to the first embodiment of the present invention will be described below.
・ Because control is performed so that the cell voltage change rate that accompanies a change in the load of power to the fuel cell stack from a high load to a low load is reduced to a predetermined value or less, the progress of platinum elution can be suppressed. Durability and reliability can be improved.
・ By measuring the relationship between the power load and the cell voltage on the fuel cell fuel cell in advance and mapping it, the cell voltage change rate can be controlled by the output of the fuel cell, so it is necessary to add new elements to the existing system The existing system can be used effectively.
-Since excess power generated by controlling the cell voltage change rate to a predetermined value or less is utilized, fuel waste can be saved and fuel consumption can be improved.
-Since the reprecipitation of platinum can be promoted by the reprecipitation operation, the progress of platinum elution can be suppressed, and the durability and reliability of the fuel cell stack can be improved.
-Since the time required for reprecipitation of platinum in the reprecipitation operation is in units of seconds, the power consumption associated with reprecipitation of platinum is not large, and the durability and reliability of the fuel cell stack should be improved efficiently and economically. Can do.
・ By reducing the effective electrochemical surface area of platinum by performing reprecipitation operation, it is possible to prevent deterioration of the platinum catalyst function and improve the durability and reliability of the fuel cell stack. it can.
[Second Embodiment]
The fuel cell operating method according to the second embodiment of the present invention is obtained by adding control for setting the cell voltage within a predetermined range to the fuel cell control method according to the first embodiment. Since the structure of the fuel cell system for implementing 2nd Embodiment is the same as that of 1st Embodiment (refer FIG. 3), description is abbreviate | omitted. FIG. 9 is a diagram showing a flowchart of the operation method of the fuel cell according to the second embodiment.

  The operation method of the fuel cell according to the second embodiment will be described in detail according to the flowchart shown in FIG. However, since the second embodiment includes the same part as the first embodiment, the description of the part is omitted or simplified. First, as in the first embodiment, a change in power load on the fuel cell stack is detected (S600). Next, it is judged by a control part whether the electric power load change decreased (S601). If it is determined that the power load change for the fuel cell stack has not decreased, the process returns to step S600, and the detection of the power load change for the fuel cell stack and the determination of whether the power load has decreased are continued. If it is determined that the power load change for the fuel cell stack has decreased, the cell voltage change rate is obtained by calculation (S602).

  Next, the control unit determines whether the detected cell voltage is smaller than the first reference voltage (S603). If it is determined that the detected cell voltage is not smaller than the first reference voltage, it is determined whether or not the output power of the secondary battery can be reduced (S605). That is, it is possible to reduce the proportion of power that the secondary battery should bear by changing the proportion of power that the fuel cell stack and the secondary battery should bear, and to increase the proportion of power that the fuel cell stack should bear. Determine if it is possible. If it is determined that the output power of the secondary battery can be reduced, the secondary battery is operated until the cell voltage is lower than the second reference voltage and the rate of change of the cell voltage is lower than a predetermined value. Output power is reduced (S606). Here, the predetermined value is 200 mV / s, more preferably 100 mV / s, as in the first embodiment. The first reference voltage may be 0.9V, and the second reference voltage may be 0.6V. The reason why the first reference voltage and the second reference voltage are set to such values is that platinum is promoted by the redox cycle, and the greater the maximum cell voltage, the more intense the platinum elution. This is to limit the cell voltage range and further suppress the elution rate of platinum. That is, by controlling the cell voltage to be in the range of 0.6 V to 0.9 V, the elution rate of platinum is further suppressed, and the durability and reliability of the fuel cell stack are improved.

  If it is determined that the cell voltage is smaller than the first reference voltage, the control unit determines whether the rate of voltage change of the cell is greater than a predetermined value (S604). When the rate of voltage change of the cell is greater than a predetermined value, that is, when the rate of change from high load to low load of the power load on the fuel cell stack exceeds the predetermined value, platinum as a catalyst from the cathode catalyst layer Increases the rate of elution. Therefore, when the cell voltage change rate is higher than a predetermined value, the cell voltage change rate is made lower than the predetermined value by changing the ratio of the power load borne by the secondary battery and the fuel cell stack. .

  When the speed of the cell voltage change is not greater than the predetermined value, this flowchart ends. However, this flowchart can always be operated while the fuel cell stack is operating.

  If the cell voltage change rate is greater than the predetermined value, the control unit determines whether the output power of the secondary battery can be reduced in consideration of the total power required for the fuel cell stack and the secondary battery. (S605). If it is determined that the output power of the secondary battery can be reduced, the secondary battery is operated until the cell voltage is lower than the second reference voltage and the rate of change of the cell voltage is lower than a predetermined value. Output power is reduced (S606).

  If it is determined that the output power of the secondary battery cannot be reduced, the control unit determines whether the secondary battery can be charged (S607). If it is determined that the secondary battery can be charged, the secondary battery is charged until the cell voltage is lower than the second reference voltage and the voltage change rate of the cell is lower than a predetermined value (S407). ).

  If it is determined that the secondary battery cannot be charged, the process proceeds to a redeposition operation subroutine (S609).

  As mentioned above, this 2nd Embodiment implements invention of the operating method of the fuel cell which concerns on Claim 4, 5-8. Step numbers S606 and 608 correspond to the step of controlling the cell voltage to a voltage between 0.6V and 0.8V in the present invention.

The fuel cell operating method according to the second embodiment of the present invention has the following effects in addition to the effects of the first embodiment.
-By restrict | limiting the change range of a cell voltage further with respect to 1st Embodiment, the elution rate of platinum can further be suppressed and durability and reliability of a fuel cell stack can be improved.
[Third Embodiment]
The fuel cell operating method according to the third embodiment of the present invention is a cathode reaction gas (oxygen) supplied to the cathode electrode instead of the reprecipitation operation in the fuel cell control method according to the first and second embodiments. The reprecipitation operation is performed by reducing the stoichiometric ratio (hereinafter referred to as “SR”). SR refers to the ratio of the amount of reaction gas supplied to the fuel cell to the required amount of reaction gas, with the minimum amount of reaction gas calculated based on the required power generation amount as the required reaction gas amount. Since the configuration of the fuel cell system for implementing the third embodiment is the same as that of the first embodiment (see FIG. 3), the description thereof is omitted. Moreover, the flowchart of the operating method of the fuel cell according to the third embodiment other than the reprecipitation operation subroutine is the same as that of either the first embodiment or the second embodiment. Therefore, only the reprecipitation operation subroutine of the fuel cell operation method according to the third embodiment will be described here.

  FIG. 10 is a diagram showing a flow of a redeposition operation subroutine of the fuel cell operating method according to the third embodiment. As in the first and second embodiments, the reprecipitation operation is performed to reprecipitate platinum eluted from the cathode by ionization by increasing the overvoltage of the fuel cell, and to prevent progression of platinum elution. Do.

  First, the cell voltage is detected (S700). The cell voltage can be detected by converting the power load required for the fuel cell stack according to the mapped data. However, when a voltmeter for detecting the voltage of each fuel cell is provided in the fuel cell stack, the cell voltage may be detected by the voltmeter.

  The control unit determines whether the detected cell voltage is smaller than a predetermined value (S701). If it is determined that the detected cell voltage is not smaller than the predetermined value, the diffusion overvoltage is increased by lowering SR (S702). Then, the cell voltage is detected again (S700), and it is determined whether or not the detected cell voltage is smaller than a predetermined value (S502). If it is determined that the detected cell voltage is not yet lower than the predetermined value, the SR can be decreased until the cell voltage becomes lower than the predetermined value. Here, SR can be lowered to a value smaller than 1.2. Further, the predetermined value of the cell voltage can be 0.7 V or less, as in the first and second embodiments. Moreover, it may be 0.5 V or less. The reprecipitation operation is performed by reducing SR and does not require new parts, so that it has a cost reduction effect.

If it is determined that the detected cell voltage is lower than the predetermined value, the time during which the cell voltage is lower than the predetermined value is measured by the loop consisting of step numbers S703 and S704, and the cell voltage is lower than the predetermined value. The reduced state is maintained until a predetermined time has elapsed. The predetermined time can be the same value as in the first embodiment and the second embodiment. That is, in the cathode catalyst layer, platinum ionized can the time required for diffuses through the ionomer t _D is greater than time. By setting the predetermined time in this way, platinum ions can be reprecipitated.

  FIG. 11 is a diagram showing the relationship between SR of air supplied to the cathode and diffusion overvoltage of the cathode. By setting SR to 1.2 or less, the diffusion overvoltage of the cathode can be increased rapidly. That is, by setting the SR to 1.2 or less, the cell voltage can be easily lowered to a predetermined value of 0.7 V or less, preferably 0.5 V or less, and the reprecipitation of the eluted platinum is promoted. Can do.

  As described above, the third embodiment is an embodiment of the fuel cell operating method according to the ninth aspect. Step numbers S700 to 702 correspond to the step of setting the stoichiometric ratio of the cathode reaction gas to 1.2 or less according to the present invention.

The operation method of the fuel cell according to the third embodiment of the present invention has the following effects in addition to the effects of the first embodiment and the second embodiment.
-Since a new part is not required for reprecipitation operation, it has a cost reduction effect with respect to the first embodiment and the second embodiment.

  Although the fuel cell operation method according to the present invention has been described above, the scope of the present invention is not limited to the above-described embodiment, and any person who has ordinary knowledge in the technical field to which the present invention belongs can use the present invention. The present invention may be modified or changed without departing from the spirit and spirit of the invention.

  The fuel cell operation method according to the present invention can be used mainly as a fuel cell operation method used as a vehicle drive source.

It is a perspective view which shows the whole structure of a fuel cell stack. It is a principal part expanded sectional view which shows the cell structure of a fuel cell stack. 1 is a block diagram showing a schematic configuration of a fuel cell system for carrying out a fuel cell operation method according to a first embodiment of the present invention. FIG. It is a figure which shows the flowchart of the operating method of the fuel cell which concerns on 1st Embodiment of this invention. It is a figure which shows the reprecipitation operation subroutine of the flowchart of the operating method of the fuel cell which concerns on 1st Embodiment of this invention. It is a figure (A) which shows the relationship between the cell voltage in reprecipitation operation, and the platinum elution rate after implementation of reprecipitation operation, and the figure (B) which shows the change cycle number dependence of the cell voltage of the effective electrochemical surface area of platinum. A diagram showing the relationship between the cell voltage change rate from low voltage to high voltage and the platinum elution rate (A), and a diagram showing the relationship between the cell voltage change rate and platinum elution rate in terms of time and cell potential (B) It is. FIG. 4A is a diagram showing a relationship between a cell current density and a cell voltage, a relationship between a cell current density and a cell output power, and a diagram showing a relationship between the cell voltage and the cell output power. It is a figure which shows the flowchart of the operating method of the fuel cell which concerns on 2nd Embodiment of this invention. It is a figure which shows the reprecipitation operation subroutine of the flowchart of the operating method of the fuel cell which concerns on 3rd Embodiment of this invention. It is a figure which shows the relationship between SR of the air supplied to a cathode, and the diffusion overvoltage of a cathode.

Explanation of symbols

1 Fuel cell stack,
2 fuel cells,
300 fuel cell stack,
310 flow regulator,
320 hydrogen cylinder,
330 compressor,
360 control unit,
370 secondary battery,
390 Discharge circuit.

Claims (9)

A method of operating a fuel cell using platinum as an electrode catalyst,
Detecting that a change in load of power to the fuel cell is a change from a high side to a low side;
When it is detected that the load change is a change from the high side to the low side, the cell voltage change rate, which is the change rate of the voltage between the electrodes of each fuel cell constituting the fuel cell, is set to 200 mV / controlling to s or less,
A method for operating a fuel cell, comprising: The step of controlling the cell voltage change rate to 200 mV / s or less includes:
Converting the detected change in power load to the fuel cell into the cell voltage change rate according to the relationship between the power load and the cell voltage obtained in advance.
2. The fuel cell according to claim 1, wherein the cell voltage change rate is controlled to 200 mV / s or less by controlling electric power output from the fuel cell based on the converted cell voltage change rate. Driving method. The step of controlling the cell voltage change rate to 200 mV / s or less includes:
Further, any one of driving an automobile mounted with the fuel cell, driving an auxiliary machine mounted on the automobile, and charging a secondary battery provided to supplement the output of the fuel cell. 3. The method of operating a fuel cell according to claim 1, further comprising a step of increasing the power output from the fuel cell by performing one or more of the steps. The step of controlling the cell voltage change rate to 200 mV / s or less includes:
And controlling the cell voltage to a voltage between 0.6V and 0.8V;
The method for operating a fuel cell according to claim 1, comprising: When it is determined that the cell voltage change rate cannot be controlled to 200 mV / s or less, the diffusion of platinum ions determined by the diffusion coefficient of platinum ions in the layer forming the electrode catalyst and the thickness of the layer forming the electrode catalyst Lowering the cell voltage to 0.7V or less for a time longer than a time constant;
The method of operating a fuel cell according to any one of claims 1 to 4, wherein: The step of lowering the cell voltage to 0.7V or less includes
The method of operating a fuel cell according to claim 5, further comprising increasing an overvoltage of the fuel cell. The step of increasing the overvoltage of the fuel cell is,
The method of operating a fuel cell according to claim 6, further comprising increasing a load on the fuel cell. The step of increasing the overvoltage of the fuel cell is,
The method for operating a fuel cell according to claim 6, further comprising a step of short-circuiting between electrodes of the fuel cell or between an anode electrode and a cathode electrode of the fuel cell. The step of increasing the overvoltage of the fuel cell is,
The method of operating a fuel cell according to claim 6, further comprising a step of setting the stoichiometric ratio of the cathode reaction gas to 1.2 or less.

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