JP2006164939A - Fuel cell system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a fuel cell system in which the deterioration of a fuel cell stack is suppressed when the fuel cell system starts or stops. <P>SOLUTION: In the case of starting or stopping the fuel cell system, the activity of a catalyst is weakened and the deterioration of an oxidizing agent pole is suppressed by applying a voltage to the fuel cell stack 1 by a battery 5 and forming an oxidation coating on the catalyst surface of a catalyst layer 32 of the oxidizing agent pole. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a fuel cell system, and particularly to suppression of catalyst deterioration in a fuel cell using a solid polymer electrolyte membrane as an electrolyte membrane.

In a fuel cell system, an oxidant gas such as air is present on the oxidant electrode side, and a region where an oxidant gas such as air is present on the fuel electrode side and a region where a fuel gas such as hydrogen is present are formed. There is a case. For example, when the system is started from a state where air is mixed in both the fuel electrode and the oxidizer electrode, hydrogen is not contained in the fuel gas channel at the initial stage when the supply of hydrogen to the fuel gas channel on the fuel electrode side is started. An existing region and a non-existing region are formed. In a region where hydrogen is present in the fuel electrode, a reaction similar to that in a normal operation state occurs, and the oxidant electrode side is exposed to a high potential of 1 V or more. On the other hand, in the region where hydrogen does not exist in the fuel electrode, the oxidant electrode is opposed to this region.
_{C + 2H 2 O → CO 2} + 4H + + 4e - formula (1)
This reaction occurs. As a result, the carbon carrier carrying the catalyst such as Pt is corroded, and the function of the electrode catalyst of the oxidant electrode is greatly deteriorated, which becomes a factor of lowering the performance of the fuel cell thereafter. At this time, in the region where the air on the fuel electrode side exists,
O ₂ + 4H ⁺ + 4e ⁻ → 2H ₂ O Formula (2)
The reaction called occurs and water is produced.

  In the conventional fuel cell system, in order to prevent the deterioration of the oxidizer electrode due to this phenomenon, the boundary between the region where the hydrogen exists in the fuel electrode and the region where the hydrogen does not exist (hereinafter referred to as hydrogen) in a short time (1 second or less). Patent Document 1 describes that hydrogen is supplied so as to pass through the fuel gas flow path.

In addition to this, as an example in which the oxidizer electrode is exposed to a high potential, there is a low output power generation operation (hereinafter referred to as idling). The fuel cell has a high voltage when the load is low, that is, when the current density required for the fuel cell is small, and the highest potential is obtained when the open-circuit voltage is applied. In such a high potential state, for example, when platinum is used as a catalyst,
Pt → Pt ²⁺ + 2e ^- Eq. (3)
It is known that the dissolution of platinum (ionization of platinum) proceeds by this reaction, and the power generation performance of the fuel cell is significantly reduced.

In the conventional fuel cell system, a method has been proposed in which the potential difference is eliminated by short-circuiting the fuel electrode and the oxidant electrode, and the dissolution of platinum shown in Formula (3) is suppressed. In addition, Patent Document 2 discloses a method for suppressing deterioration by controlling the potential at the time of stopping between 0.6V and 0.8V.
US Patent Application Publication No. 2002/0076582 JP 2004-172105 A

  However, in the above invention, in order to pass the hydrogen / air front in a short time, depending on the design of the flow path of the fuel cell, a compressor or the like is provided in the middle of the piping flow path for supplying hydrogen to the fuel electrode gas flow path. Therefore, it is necessary to take measures to increase the flow velocity in the fuel gas flow path by reducing the cross-sectional area of the fuel gas flow path. In the former, an additional device is required, which increases costs and increases the size of the fuel cell system. In the latter, there is a problem that the reaction surface is narrowed and the reaction efficiency is significantly reduced.

  Further, deterioration can be suppressed if the potential at the time of stopping can be controlled from 0.6 V to 0.8 V. However, in practice, the potential may increase excessively depending on the concentration of the oxidant gas, and there is a problem that it is difficult to perform such potential control accurately.

  The present invention has been invented to solve such problems, and has an object to efficiently suppress deterioration of a fuel cell, particularly deterioration of an oxidizer electrode, with a simple configuration.

  According to the present invention, in a fuel cell system comprising a fuel cell comprising a fuel electrode having a catalyst layer sandwiching an electrolyte membrane and carrying a catalyst between the electrolyte membrane and an oxidizer electrode, the fuel cell is started or stopped Occasionally there is provided an oxide film generating means for oxidizing the surface of the catalyst and forming an oxide film that reduces the activity of the catalyst.

  According to the present invention, an oxide film is formed on the catalyst surface of the catalyst layer when the fuel cell system is started or stopped, for example, to reduce the active state of the catalyst. Thus, for example, when hydrogen is supplied to the fuel electrode in which air is mixed, deterioration of the oxidizer electrode that occurs when hydrogen and air are mixed in the fuel electrode can be suppressed.

  A unit cell 30 constituting the fuel cell stack 1 used in the first embodiment of the present invention will be described with reference to FIG. FIG. 1 is a schematic configuration diagram of the unit cell 30.

  The unit cell 30 includes a polymer electrolyte membrane (hereinafter referred to as an electrolyte membrane) 31 having proton conductivity, an oxidant electrode layer 32 and a fuel electrode layer 33 sandwiching the electrolyte membrane 31. Further, an oxidant gas separator 34 and a fuel gas separator 35 are provided on the outside thereof.

  The oxidant electrode layer 32 and the fuel electrode layer 33 include gas diffusion layers 32a and 33a made of a porous material such as carbon fiber, and catalyst layers 32b and 33b made of a carbon carrier carrying platinum as a catalyst.

  Further, an oxidant gas channel 36 and a fuel gas channel 37 are provided between the oxidant electrode layer 32, the fuel electrode layer 33 and the oxidant separator 34, and the fuel separator 35, respectively. The oxidant gas flow path 36 and the fuel gas flow path 37 are provided so that the oxidant gas flowing through the oxidant gas flow path 36 and the fuel gas flowing through the fuel gas flow path 37 flow in substantially the same direction. Note that air is used as the oxidant gas supplied to the oxidant gas flow path 36, and hydrogen gas is used as the fuel gas. As a result, the following reactions occur at the fuel electrode and oxidant electrode of the unit cell 30 during normal operation.

Fuel electrode ^{_{side: H 2 → 2H + + 2e}} - Formula (4)
Oxidant electrode side: 2H ⁺ + 1 / 2O ₂ + 2e ⁻ → H ₂ O Formula (5)
On the fuel electrode side, hydrogen in the fuel gas is separated into protons and electrons as shown in equation (4). Protons diffuse inside the electrolyte membrane 31 and reach the oxidant electrode layer 33 side, and electrons flow through an external circuit (not shown) and are taken out as an output. On the other hand, on the oxidant electrode side, the equation (5) shows on the three-phase interface formed by protons diffusing in the electrolyte membrane 31, electrons moving through an external circuit (not shown), and oxygen in the air. A reaction occurs.

  When the fuel cell stack 1 is used as a power source for a moving body, for example, an automobile, start / stop is frequently repeated. When the operation of the fuel cell stack 1 is stopped, the supply of hydrogen and air to the fuel cell stack 1 is left in a stopped state. More air may have entered and may be present. When the fuel cell system is started from a state in which air is mixed in the fuel gas flow path 37, the fuel cell stack 1 is in a state as shown in FIG.

  In the state of FIG. 2, the oxidant gas passage 36 is filled with air in all regions, but the fuel gas passage 37 has a region where hydrogen is present (region A) and air is present. A region (region C) is formed. Further, a hydrogen / air interface B is formed between the region A and the region C (hereinafter, this interface is referred to as a hydrogen / air front B).

  In region A, reactions during normal power generation of equations (4) and (5) occur, and the oxidant electrode has a high potential of 1 V or higher. On the other hand, in the region C with the hydrogen / air front B as a boundary, the reaction of the formula (1) occurs in the oxidant electrode layer 32 and the reaction of the formula (2) occurs in the fuel electrode layer 33. That is, the oxidant electrode layer 32 corrodes the carbon of the catalyst layer 32b carrying platinum. Note that the reaction of the formula (1) occurs on the surface of platinum and in contact with carbon. As a result, the oxidant electrode layer 32 deteriorates and the fuel cell stack 1 deteriorates. The present invention suppresses the deterioration of the oxidant electrode 32.

  Next, a schematic diagram of the fuel cell system according to the first embodiment of the present invention will be described with reference to FIG. This embodiment includes a fuel cell stack 1, a compressor (air supply means) 2 that supplies air to the oxidant gas passage 36, a hydrogen cylinder 3 that supplies hydrogen to the fuel gas passage 37, and the fuel cell stack 1. The load 4 that consumes the electric power generated by the battery, the battery (voltage applying means) 5 that is a secondary battery that stores the electric power generated by the fuel cell stack 1, the generated voltage of the fuel cell stack 1, or the applied voltage by the battery 5 A voltage sensor (voltage detection means) 17 for detection.

  An air filter 16 that removes impurities in the air is provided upstream of the compressor 2, and is supplied from the compressor 2 to the fuel cell stack 1. Exhaust gas (exhaust air) containing oxygen that has not been used in the fuel cell stack 1 is supplied to a hydrogen consuming device 15 to be described later, and then discharged to the atmosphere.

  The hydrogen supplied from the hydrogen cylinder 3 to the fuel gas flow path 37 is depressurized by the pressure reducing valve 6 provided in the hydrogen supply path 9, and the flow rate is controlled by the flow rate controller 7. Further, hydrogen that has not been used in the fuel cell stack 1 is supplied again to the fuel cell stack 1 through the three-way valve 12 and the hydrogen supply passage 9 by the recycle compressor 11 provided in the hydrogen circulation passage 10. When a large amount of nitrogen in the air is mixed into the hydrogen discharged from the fuel cell stack 1 from the oxidizer electrode side through the electrolyte membrane 30, the hydrogen discharged from the fuel cell stack 1 is hydrogenated by the three-way valve 13. The hydrogen passes through a hydrogen discharge passage 14 branched from the circulation passage 10 and is supplied to a hydrogen consumption device 15 that consumes hydrogen.

  The hydrogen consuming device 15 has a catalyst that consumes hydrogen and the like. The exhaust gas discharged from the oxidant electrode side of the fuel cell stack 1 is supplied, hydrogen is consumed by this exhaust gas, and then no hydrogen is contained. The gas is discharged outside the fuel cell system.

  The load 4 is electrically connected to the oxidant side electrode plate 20 and the fuel side electrode plate 21 of the fuel cell stack 1 and consumes the electric power generated by the fuel cell stack 1. A switch 22 for switching ON / OFF of the electrical connection between the fuel cell stack 1 and the load 4 is provided between the fuel cell stack 1 and the load 4.

  The battery 5 is provided in parallel with the fuel cell stack 1 and the load 4 and is electrically connected to the fuel cell stack 1 and the load 4, and a part of the power generated by the fuel cell stack 1 (surplus power). Store. It is also possible to supply power to the fuel cell stack 1 or the load 4. Note that a switch 23 for switching ON / OFF of the electrical connection with the fuel cell stack 1 or the load 4 is provided. That is, the fuel cell stack 1, the load 4, and the battery 5 can be electrically connected and disconnected by the switches 22 and 23, respectively.

  Further, the compressor 2, the flow controller 6, the three-way valves 12 and 13, the recycle compressor 11, and the controller 40 that controls the switches 22 and 23 are provided.

  Next, the operation control when starting the fuel cell system of this embodiment will be described with reference to the flowchart of FIG. Note that the switches 22 and 23 are both OFF when the fuel cell system is stopped. That is, the fuel cell stack 1, the load 4, and the battery 5 are not electrically connected. The compressor 2 and the recycle compressor 11 are both stopped.

  When the controller 40 detects the start command of the fuel cell system, in step S100, the switches 22 and 23 are turned ON to electrically connect the battery 5 and the fuel cell stack 1 so that the oxidant electrode is positive. A predetermined voltage (first predetermined voltage) V <b> 1 is applied to the stack 1. When air is mixed in the fuel electrode and the oxidant electrode, both the fuel electrode and the oxidant electrode of the unit cell 30 are at a potential of 1.2 V before being applied by the battery 5. The potential when hydrogen is present in the fuel electrode is 0V. Then, by applying a predetermined voltage V1 to the fuel cell stack 1, the potential of the catalyst layer 32b is increased, and an oxide film is formed on the surface of the platinum of the catalyst layer 32b. In this embodiment, the predetermined voltage V1 is expressed as a voltage applied to the unit cell 30, that is, a voltage per unit cell 30, and hereinafter, the predetermined voltage indicates a voltage per unit cell 30. The setting of the predetermined voltage V1 will be described in detail later (step S100 constitutes an oxide film generation unit).

Here, the oxide film formed on the surface of the platinum particles of the catalyst layer 32b will be described. When the fuel cell stack 1 is applied by the battery 5, platinum as catalyst particles is
^{_{Pt + xH 2 O → PtOx +}} 2xH + + 2xe - Equation (6)
As shown in FIG. 5 (a), the state changes from a state in which no oxide film is formed on the surface of platinum to a state in which an oxide film is formed on the surface of platinum as shown in FIG. 5 (b). To do. When an oxide film is formed on the surface of platinum, platinum becomes inactive or the active state decreases.

  When air is present in the fuel electrode at the start of the fuel cell system, the reaction expressed by the formula (1) occurs on the surface of platinum and in contact with carbon at the oxidizer electrode. By forming the oxide film, the activity of the catalyst is lowered and the reaction of the formula (1) is suppressed. Thereby, the corrosion deterioration of carbon can be suppressed.

  In step S101, it is determined whether a predetermined time T1 has elapsed since the switches 22 and 23 were turned on. When the predetermined time T1 has elapsed, the process proceeds to step S102. The predetermined time T1 is a preset time, an oxide film is formed on the surface of platinum, and even when hydrogen is supplied to the fuel electrode in step S103 described later, the reaction of the formula (1) is suppressed, and carbon corrosion This is the time when no reaction occurs. The predetermined time T1 varies depending on the material of the catalyst and the material of the catalyst layers 32 and 33. The longer the time T1, the more the amount of the oxide film increases. There is no limit.

  In step S102, the switch 23 is turned off, and the application from the battery 5 to the fuel cell stack 1 is terminated.

  In step S103, supply of hydrogen from the hydrogen cylinder 3 to the fuel electrode is started. When hydrogen is supplied to the fuel electrode, the hydrogen / air front B is formed at the fuel electrode as described above, but at the oxidizer electrode, an oxide film is formed on the platinum surface of the catalyst layer 32b. The carbon corrosion reaction of the catalyst layer 32b represented by the formula (1) can be suppressed. When the hydrogen / air front B moves downstream and reaches the hydrogen, the fuel cell stack 1 undergoes the normal power generation reaction of formulas (4) and (5) at that location.

  Thereafter, in step S104, supply of air from the compressor 2 to the oxidant electrode is started, and power generation required by the fuel cell system is started.

  With the above control, the hydrogen / air front B is purged from the fuel electrode by forming an oxide film on the catalyst surface of the catalyst layer 32b of the oxidant electrode before starting the supply of hydrogen and then supplying hydrogen. Until then, the activity of the catalyst is reduced, and the fuel cell stack 1 can be started up while suppressing the deterioration of the catalyst layer 32b due to the corrosion of carbon.

  In addition, although it may be formed even when the voltage is not applied by the battery 5 on the surface of the platinum of the fuel electrode and the oxidant electrode, the voltage of the oxidant electrode can be formed by applying the voltage by the battery 5 in step S100. The thickness of the oxide film is increased and the active state is further lowered. In addition, the oxide film on the surface of platinum gradually deviates due to normal operation of the fuel cell stack 1, and platinum returns from the inactive state to the original state, and the potential of each fuel electrode and oxidant electrode decreases to 0.8V. The inactive state of platinum is completely restored. That is, even when an oxide film is formed on platinum of the fuel electrode, the oxide film of the fuel electrode is separated by supplying hydrogen.

  Here, the predetermined voltage V1 in step S100 described above will be described with reference to FIG. FIG. 6 is a map showing a change in voltage of the unit cell 30 when hydrogen is supplied to the fuel electrode, air is supplied to the oxidant electrode, and the unit cell 30 is held in an unloaded state (open state) for 45 seconds. At this time, the open circuit voltage is about 0.95V. The open end voltage represents the activity state of the catalyst, and the higher the open end voltage, the higher the activity of the catalyst. If the voltage applied to the unit cell 30 by the battery 5 is 0 V or higher, an oxide film is formed on the catalyst particles. Here, after 50 seconds have elapsed, different voltages of 1.2 V, 1.1 V, and 1.0 V are applied from the external power supply, and then the external power supply is removed to detect the open end voltage. As a result, the open circuit voltage becomes the lowest voltage when 1.2V is applied, and then becomes the lower voltage in the order in which 1.1V and 1.0V are applied. This indicates that the higher the applied voltage, the lower the open end voltage of the unit cell 30 thereafter, that is, the oxide film formed on the catalyst is formed in a wider range, and the activity of the catalyst is reduced. In the case of an applied voltage of 1.2 V where the decrease of the open end voltage is the largest, the open end voltage is lowered to 0.89 V, which is 0.06 V lower than before the oxide film is formed.

  As described above, the higher the voltage applied to the fuel cell stack 1 by the battery 5, the lower the catalytic activity. However, when the applied voltage is too high, depending on the temperature of the fuel cell stack 1, if the applied voltage is increased to about 1.5 V to 2.0 V per unit cell 30, the carbon of the catalyst layer 32 is corroded by the applied voltage. to degrade. Therefore, in this embodiment, the predetermined voltage V1 that is the voltage applied by the battery 5 is 1.2V. Although depending on the material (molecular structure) of the electrolyte membrane 30, if moisture is lost, the resistance of the electrolyte membrane 30 increases due to drying, and the electrolyte membrane 30 may be deteriorated. Therefore, the predetermined voltage V1 is set to a voltage lower than the voltage at which water is electrolyzed. The predetermined voltage V1 is not limited to 1.2V, and may be any voltage that forms an oxide film and does not cause deterioration of the fuel cell stack 1.

  Next, operation control when the fuel cell system of this embodiment is stopped will be described with reference to the flowchart of FIG. During operation of the fuel cell system, the switch 22 is turned on, the switch 23 is turned on when charging from the fuel cell stack 1, and is turned off otherwise. The compressor 2 and the recycle compressor 11 are both in operation.

  When the operation stop signal of the fuel cell system is detected, the compressor 2 is stopped in step S200.

  In step S201, the recycle compressor 11 is stopped and the supply of hydrogen from the hydrogen cylinder 3 is stopped.

  In step S202, the switch 23 is turned ON, and the fuel cell stack 1 is applied at a predetermined voltage V1 by the battery 5. If the supply of hydrogen is stopped in step S201, air may enter the fuel electrode from the outside. When air enters the fuel electrode, a hydrogen / air front B is formed at the fuel electrode in the same manner as when the fuel cell system is started. In step S202, the battery 5 applies a predetermined voltage V1 to the fuel cell stack 1, thereby forming an oxide film on the platinum surface in the catalyst layer 32b of the oxidant electrode (step S202 constitutes an oxide film generation unit). .

  In step S203, the switch 23 is turned on, and it is determined whether or not the time applied by the battery 5 has passed the predetermined time T1. And when predetermined time T1 passes, it will progress to step S204.

  In step S204, the switch 23 is turned OFF, and the application from the battery 5 to the fuel cell stack 1 is terminated.

  By the above control, an oxide film is formed on the platinum surface of the catalyst layer 32b when the fuel cell system is stopped, thereby suppressing the corrosion of carbon in the oxidizer electrode even when air is mixed into the fuel electrode. Can do.

Next, the relationship between the decrease in the open end voltage of the fuel cell stack 1 and the surface area of the catalyst covered with the oxide film will be described with reference to FIG. FIG. 8 is a map showing the current density and voltage characteristics of the unit cell 30 of the fuel cell stack 1 used in this embodiment. The logarithmic value of the current density and the voltage are in a proportional relationship, and the slope is 90 (mV / decade). That is, the voltage drop ΔE when the current density changes from i1 to i2 is
ΔE = 0.09 × log (i2 / i1) Equation (7)
It can be expressed as.

When the applied voltage is 1.2 V and the open-circuit voltage decreases by 0.06 V, the effective surface area that indicates the active state of the catalyst before being applied by the external power source is A1 (cm ² ). The effective surface area of the catalyst after the formation of the active state is defined as A2 (cm ² ). Also in this case, a minute current flows, and if the current before application is I1 (A) and the current after application is I2 (A), current density i1 and current density i2 are
i1 = I1 / A1 Formula (8)
i2 = I2 / A2 Formula (9)
The relationship holds. Here, the current value does not change before and after voltage application,
I1 = I2 Formula (10)
That means
A2 / A1 = i1 / i2 Formula (11)
It becomes. Here, from the equation (7), A2 / A1 = i1 / i2 = 10−0.06 / 0.09 = 0.22 Equation (12)
It becomes.

  As a result, when applied to the fuel cell stack 1 at an applied voltage of 1.2 V, the voltage decreased by 0.06 V, and 78% of the effective surface area where the catalyst was active was lost. That is, about 80% of the catalyst was in an inactive state. Indicates that Therefore, even when hydrogen is supplied to the fuel electrode when air is present in the fuel electrode, the corrosion deterioration of carbon can be suppressed at the oxidant electrode. The above calculation is an example of this embodiment, and is not limited to the numerical values used here.

  The effect of 1st Embodiment of this invention is demonstrated.

  In this embodiment, the fuel cell stack 1 is applied by the battery 5 at the start of the fuel cell system, whereby an oxide film is formed on the platinum of the catalyst layer 32 of the oxidant electrode, thereby bringing the platinum into an inactive state. As a result, even when air is mixed into the fuel electrode at the time of stopping and hydrogen / air front B is generated at the fuel electrode by supplying hydrogen to the fuel electrode at the time of starting, an oxide film is formed on the platinum of the catalyst layer 32 of the oxidant electrode. Since it is formed, the carbon corrosion reaction at the oxidizer electrode can be suppressed.

  Further, even when the fuel cell system is stopped, the battery 5 forms an oxide film on the platinum of the catalyst layer 32 of the oxidant electrode, thereby suppressing the carbon corrosion reaction of the oxidant electrode even when air is mixed in the fuel electrode. can do.

  By applying the fuel cell stack 1 by the battery 5, an oxide film can be quickly formed on platinum of the catalyst layer 32 of the oxidant electrode, and the carbon corrosion reaction of the oxidant electrode can be suppressed and the startup time of the fuel cell can be reduced. Can be shortened. Moreover, by making the voltage applied by the battery 5 lower than the electrolysis of water, drying of the electrolyte membrane 3 can be prevented, an increase in resistance due to the electrolyte membrane 3 can be suppressed, and deterioration of the electrolyte membrane 3 can be prevented. it can.

  By using the battery 5 that stores the surplus power of the fuel cell stack 1 as means for applying the fuel cell stack 1, the fuel cell system can be downsized.

  Next, a second embodiment of the present invention will be described with reference to FIG. In this embodiment, a three-way valve 26 is provided downstream of the compressor 2. With this configuration, air can be supplied from the compressor 2 to the hydrogen supply path 9. The three-way valve 26 communicates the oxidant electrode of the fuel cell stack 1 from the compressor 2 during normal operation of the fuel cell system. Since other configurations are the same as those in the first embodiment, description thereof is omitted here. Further, since the control operation at the time of starting the fuel cell system is the same as that of the first embodiment, description thereof is omitted here.

  The control operation when the fuel cell system is stopped will be described with reference to the flowchart of FIG.

  Since the control from step 300 to step S303 is the same as the control from step S200 to step S203 of the first embodiment, a description thereof is omitted here.

  In step S304, the switch 23 is turned OFF and the application from the battery 5 to the fuel cell stack 1 is terminated.

  In step S305, the three-way valve 26 is switched to connect the compressor 2 and the hydrogen supply path 9. As a result, air can be supplied from the compressor 2 to the fuel electrode of the fuel cell stack 1 through the hydrogen supply path 9. Further, the fuel cell stack 1 and the hydrogen discharge passage 14 are communicated with each other by the three-way valve 13 so that the fuel electrode communicates with the outside.

  In step S306, the compressor 2 is started to purge hydrogen remaining in the fuel electrode. The purged hydrogen is consumed by the hydrogen consuming device 15. In step S303, an oxide film is formed on platinum in the catalyst layer 32b of the oxidant electrode. However, if hydrogen remains and left for a long time, the hydrogen / air front B at the fuel electrode exists for a long time. In this case, although the platinum oxide film dissociates over time, after forming the oxide film on the platinum of the oxidizer electrode, the hydrogen of the fuel electrode is purged by the compressor 2 and the hydrogen / air front B is moved from the fuel electrode to the outside. By doing so, it is possible to further suppress the deterioration of the fuel cell stack 1 when the fuel cell system is stopped for a long time.

  In step S307, it is determined whether or not the time for purging the fuel electrode by starting the compressor 2 has passed a predetermined time T2. When the predetermined time T2 has elapsed, the process proceeds to step S308. The predetermined time T2 is a time for purging the fuel electrode hydrogen from the fuel electrode, and is 10 seconds here, but is not limited to this time, and is a time during which hydrogen can be purged from the fuel electrode.

  In step S308, the compressor 2 is stopped and the three-way valve 26 is switched so that the compressor 2 and the oxidizer electrode communicate with each other, or the three-way valve 26 is fully closed and the fuel cell system stop control is ended.

  By the above control, an oxide film is formed and hydrogen in the fuel electrode is purged, whereby deterioration of the fuel cell stack 1 when the fuel cell system is stopped for a long time can be further suppressed.

  The effect of 2nd Embodiment of this invention is demonstrated.

  In this embodiment, in addition to the effects of the first embodiment, after forming an oxide film on platinum of the oxidizer electrode when the fuel cell system is stopped, air is supplied from the compressor 2 to the fuel electrode, and the hydrogen remaining in the fuel electrode When the fuel cell system is stopped for a long time, the carbon corrosion reaction of the oxidant electrode can be further suppressed and deterioration of the fuel cell stack 1 can be suppressed.

  Next, a third embodiment of the present invention will be described. Since the configuration of the fuel cell system of this embodiment is the same as that of the first embodiment, description thereof is omitted here. The control of this embodiment will be described using the flowchart of FIG. The flowchart of FIG. 11 shows the control operation when the output required for the fuel cell stack 1 becomes small.

  In step S400, the voltage sensor 17 detects the voltage V of the fuel cell stack 1, that is, the output required for the fuel cell stack 1.

  In step S401, the voltage V detected in step S400 is compared with a predetermined voltage (second predetermined voltage) V2. The predetermined voltage V2 is a voltage at which platinum of the catalyst layer 32b is dissolved by the reaction shown in Formula (3), which will be described later in detail, and is 0.88 V here. If the voltage V is higher than the predetermined voltage V2, the process proceeds to step S402. If the voltage V is lower than the predetermined voltage V2, power generation is performed so as to achieve the output required for the fuel cell stack 1.

  For example, when the fuel cell system is idling and the voltage V of the fuel cell stack 1 is higher than the predetermined voltage V2, there is a possibility that the platinum shown in the equation (3) is dissolved. Therefore, in step S402, the switch 23 is turned on, and a voltage is applied stepwise by the battery 5 so that the fuel cell stack 1 becomes the predetermined voltage V1, and instantaneously onto the platinum surface of the catalyst layer 32b of the oxidant electrode. An oxide film is formed. The predetermined voltage V1 is a voltage for forming an oxide film on the platinum surface of the catalyst layer 32b, and is set to 1.2V. By forming an oxide film on the platinum surface of the catalyst layer 32b, dissolution of the catalyst layer 32b platinum can be suppressed even when the oxidant electrode has a high potential.

  When the voltage is gradually raised and applied to the fuel cell stack 1, platinum dissolution represented by the formula (3) occurs in the catalyst layer 32b until an oxide film is formed on the platinum surface. However, when the predetermined voltage V1 is applied to the fuel cell stack 1 by the battery 5, by applying the predetermined voltage V1 in steps, an oxide film is instantaneously formed on the platinum surface of the catalyst layer 32b. Can be dissolved. This is because once the oxide film is formed on the surface of platinum, dissolution of platinum shown in Formula (3) does not occur.

  In step S403, the time during which the predetermined voltage V1 is applied to the fuel cell stack 1 is calculated. When the calculated time T has passed the predetermined time T3, the process proceeds to step S404. The predetermined time T3 is a time for forming an oxide film on the platinum surface of the catalyst layer 32b, and is 0.5 seconds here.

  In step S404, the switch 23 is turned OFF, and the application of voltage to the fuel cell stack 1 by the battery 5 is terminated.

  In step S405, it is determined whether or not a fuel cell system operation stop signal has been received. If the fuel cell system is to be stopped, the process proceeds to step S406. If the fuel cell system is not to be stopped, the fuel cell stack 1 is required. Continue power generation according to the output.

  In step S406, the recycle compressor 11 is stopped, the supply of hydrogen from the hydrogen cylinder 3 is stopped, and the supply of air by the compressor 2 is stopped. Even when the fuel cell system is stopped, since the oxide film is formed on the platinum surface of the catalyst layer 32b, the dissolution of platinum in the catalyst layer 32b, which becomes a high potential when the fuel cell system is stopped, is further suppressed. Carbon corrosion that may occur when the system is shut down can be suppressed.

  By the above control, when the output required for the fuel cell stack 1 becomes small, that is, when the voltage of the fuel cell stack 1 becomes high, by applying a voltage to the fuel cell stack 1 by the battery 5, An oxide film can be formed on the surface of platinum of the catalyst layer 32b that is at an electric potential to suppress the dissolution of platinum.

  Here, a method of setting the predetermined voltage V2 will be described. FIG. 12 is a map showing the voltage change with respect to time when the voltage of the unit cell 30 is changed stepwise from 0.5V to 1.0V. Such voltage change is from 0.5V to 0.85V, 0.5V to 0.88V, 0.5V to 0.91V, 0.5V to 0.95V, 0.5V to 1.0V, 0.5V Perform in the case of 1.2V. That is, the unit cell 30 is changed in six patterns from high output to low output. This was repeated 3000 cycles. FIG. 13 shows the relationship between the changed voltage and the amount of power generation voltage drop of the unit cell 30 per 1000 cycles.

  According to this, when the voltage of the unit cell 30 increases, that is, when the unit cell 30 is changed from a high output to a low output, when the voltage exceeds 0.88 V, the amount of decrease in the generated voltage increases. In the case of 92V, the amount of power generation voltage drop of the unit cell 30 is the maximum, that is, the deterioration is the largest. In addition, when the voltage of the unit cell 30 is higher than 0.92 V, the deterioration of the unit cell 30 is reduced. This is because, when the voltage of the unit cell 30 is changed stepwise, an oxide film is instantaneously formed on the platinum surface of the catalyst layer 32b of the unit cell 30, and particularly when the voltage becomes higher than 0.92V, This is because the amount of the oxide film is increased and the dissolution of platinum represented by the formula (3) is suppressed.

  From the above results, it is desirable that the predetermined voltage V1 is 0.93 V or higher in this embodiment, and the predetermined voltage V1 is 1.2 V in this embodiment. The predetermined voltage V2 is set to 0.88V.

  In addition, you may use control of this embodiment for the fuel cell system of 2nd Embodiment.

  The effect of the third embodiment of the present invention will be described.

  In this embodiment, for example, when the output required for the fuel cell stack 1 during idling becomes small, that is, when the voltage of the fuel cell stack 1 becomes high, a voltage is applied to the fuel cell stack 1 by the battery 5. As a result, an oxide film is formed on the platinum surface of the catalyst layer 32b having a high potential. By forming an oxide film on platinum of the catalyst layer 32b, even when the catalyst layer 32b has a high potential, dissolution of platinum can be suppressed and deterioration of the fuel cell stack 1 can be suppressed.

  Since voltage application by the battery 5 is performed in steps, an oxide film can be instantaneously formed on the platinum surface of the catalyst layer 32b, and even when the catalyst layer 32b is at a high potential, the platinum layer of the catalyst layer 32b Dissolution can be suppressed.

  Further, when the fuel cell system is stopped thereafter, the formation of an oxide film on the platinum surface of the catalyst layer 32b can suppress the corrosion of carbon that occurs during the stop of the fuel cell system.

  Next, a fourth embodiment of the present invention will be described. Since the configuration of the fuel cell system of this embodiment is the same as that of the first embodiment, description thereof is omitted here. The control of this embodiment will be described using the flowchart of FIG. The flowchart of FIG. 14 shows the control operation when the output required for the fuel cell stack 1 becomes small.

  In step S500, the voltage sensor 17 detects the voltage V of the fuel cell stack 1, that is, the output required for the fuel cell stack 1.

  In step S501, the voltage V detected in step S500 is compared with the predetermined voltage V2. When the voltage V is higher than the predetermined voltage V2, the process proceeds to step S502. When the voltage V is lower than the predetermined voltage V2, that is, when the output required for the fuel cell stack 1 is relatively large, the fuel cell stack. Power generation is performed to achieve the output required for 1.

  In step S502, the voltage V detected in step S500 is compared with a predetermined voltage (third predetermined voltage) V3. If the voltage V is smaller than the predetermined voltage V3, the process proceeds to step S506, where the voltage V is predetermined. If it is higher than the voltage V3, the process proceeds to step S503. As shown in FIG. 13, when the voltage of the unit cell 30 is between 0.88V and 0.92V, the power generation voltage drop amount of the unit cell 30 increases rapidly. That is, when the unit cell 30 is at a high potential between 0.88V and 0.92V, the platinum shown in the formula (3) in the catalyst layer 32b is dissolved more and the deterioration of the unit cell 30 becomes very large. Therefore, in this embodiment, the predetermined voltage V3 is set to 0.90V, and when the voltage V is higher than the predetermined voltage V3, it is determined that a large amount of platinum is dissolved as shown in the equation (3).

  In step S503, electric power is extracted from the fuel cell stack 1 by the load 4, and the voltage of the fuel cell stack 1 is set to a predetermined voltage V4 lower than the predetermined voltage V3. Note that the battery 5 may be charged with electric power extracted from the fuel cell stack 1.

Here, FIG. 15 shows a cyclic voltammogram obtained by sweeping the potential of the oxidant electrode with respect to the relative hydrogen electrode from 0.04 V to 0.9 V with hydrogen supplied to the fuel electrode of the unit cell 30 and nitrogen to the oxidant electrode. Show. According to this, when the potential is lowered from 0.9V,
Pt ²⁺ 2e ⁻ → Pt Formula (13)
The reduction reaction of platinum ions shown in FIG. In the reaction of the formula (13), the reduction current density increases around 0.8V. That is, by setting the voltage of the fuel cell stack 1 to 0.8 V in step S503, the platinum ions generated by, for example, dissolution of platinum according to the formula (3) are reduced by the platinum ion reduction reaction of the formula (13). In addition, platinum can be generated, and deterioration of the fuel cell stack 1 can be suppressed. Therefore, here, the predetermined voltage V4 is set to 0.8V. Thereby, the platinum ion which melt | dissolved when the electric potential of the catalyst layer 32b became high can be reduced, and platinum can be produced | generated.

  In step S504, when the time T when the voltage of the fuel cell stack 1 is set to the predetermined voltage V4 has passed the predetermined time T4, the process proceeds to step S505. The predetermined time T4 is a time for reducing the platinum ions to generate platinum. By increasing the time, the platinum ions can be reduced to platinum. However, if the predetermined time T4 is set longer, the efficiency of the fuel cell system is increased. Since it decreases, it is assumed here to be 1 second.

  In step S505, the extraction of electric power from the fuel cell stack 1 by the load 4 is terminated (steps S503 to S505 constitute a catalyst reduction unit).

  Since steps S506 to S510 are the same control as steps S402 to S406 of the third embodiment, the description thereof is omitted here.

  With the above control, when the output required for the fuel cell stack 1 decreases and the voltage becomes higher than the predetermined voltage V3, the voltage of the unit cell 10 is temporarily decreased. As a result, the catalyst layer 32b is at a high potential and platinum ions are dissolved to reduce platinum ions, and the platinum ions are returned to the platinum, thereby suppressing the deterioration of the catalyst layer 32b and the deterioration of the fuel cell stack 1. Can be suppressed.

  Note that the output of the fuel cell stack 1 is monitored by the voltage sensor 17, and when the output of the fuel cell stack 1 is small for a long time, that is, when the voltage of the fuel cell stack 1 is high, the process proceeds from step S503 to step S505. In the catalyst layer 32b, platinum ions may be reduced and platinum ions may be returned to platinum. Thereby, deterioration of the catalyst layer 32b can be suppressed.

  The effect of 4th Embodiment of this invention is demonstrated.

  In this embodiment, when the output required for the fuel cell stack 1 becomes small, that is, when the voltage of the fuel cell stack 1 becomes higher than the predetermined voltage V3, for example, the voltage of the fuel cell stack 1 by the load 4 or the like. Is reduced to a predetermined voltage V4. Thereby, the platinum ion produced when the catalyst layer 32b becomes a high potential can be reduced, and the platinum ion can be returned to platinum. Thereby, deterioration of the catalyst layer 32b can be suppressed.

  Furthermore, since an oxide film is formed on platinum after returning platinum ions to platinum, dissolution of platinum can be suppressed by the oxide film.

  It goes without saying that the present invention is not limited to the above-described embodiments, and includes various modifications and improvements that can be made within the scope of the technical idea.

  It can be used for a fuel cell vehicle equipped with a fuel cell.

It is a schematic explanatory drawing of the unit cell of the fuel cell of 1st Embodiment of this invention. It is a figure explaining the state of a fuel cell when air mixes in a fuel electrode. 1 is a schematic diagram of a fuel cell system according to a first embodiment of the present invention. It is a flowchart which shows the operation control at the time of the fuel cell system starting of 1st Embodiment of this invention. It is a figure which shows the catalyst layer before applying to a fuel cell stack. It is a figure which shows the catalyst layer after applying to a fuel cell stack. It is a map which shows the voltage change of a fuel cell stack. It is a flowchart which shows the control at the time of the fuel cell system stop of 1st Embodiment of this invention. It is a map which shows the current density and voltage characteristic of a unit cell. It is a fuel cell system schematic diagram of a 2nd embodiment of the present invention. It is a flowchart which shows the control at the time of the fuel cell system stop of 2nd Embodiment of this invention. It is a flowchart which shows control of the fuel cell system of 3rd Embodiment of this invention. It is a map which shows the voltage change of the unit cell with respect to time. It is a map which shows the generated voltage fall amount with respect to the voltage change of a unit cell. It is a flowchart which shows control of the fuel cell system of 4th Embodiment of this invention. It is a cyclic voltammogram of a unit cell.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Fuel cell stack 2 Compressor 3 Hydrogen cylinder 4 Load 5 Battery (voltage application means)
17 Voltage sensor (voltage detection means)
20 Oxidant Side Electrode Plate 21 Fuel Side Electrode Plate 22 Switch 23 Switch 30 Unit Cell 31 Electrolyte Membrane 32b, 33b Catalyst Layer 40 Controller

Claims (13)

In a fuel cell system comprising a fuel cell composed of a fuel electrode and an oxidant electrode having a catalyst layer sandwiching an electrolyte membrane and supporting a catalyst between the electrolyte membrane,
A fuel cell system comprising an oxide film generation means for forming an oxide film on the catalyst when the fuel cell is started or stopped.   The said oxide film production | generation means is a voltage application means which applies the 1st predetermined voltage to the said fuel cell by making the said oxidant electrode into positive, and forms an oxide film in the said catalyst, The Claim 1 characterized by the above-mentioned. Fuel cell system.   The oxide film generation means applies the first predetermined voltage with the oxidizer electrode being positive before supplying hydrogen to the fuel electrode when the fuel cell is started, and applies an oxide film to the catalyst. The fuel cell system according to claim 2, wherein the fuel cell system is formed. Air supply means for supplying air to the fuel electrode;
4. The fuel according to claim 2, wherein when the fuel cell is stopped, the fuel electrode is purged with the air by the air supply unit after applying the first predetermined voltage to the fuel cell. 5. Battery system.   The fuel cell system according to any one of claims 2 to 4, wherein the first predetermined voltage is lower than a voltage at which water causes electrolysis.   The fuel cell system according to any one of claims 2 to 5, wherein the first predetermined voltage is higher than an oxidation voltage of the catalyst. Voltage detecting means for detecting the voltage of the fuel cell;
The oxide film forming means forms an oxide film on the catalyst when the voltage detected by the voltage detecting means is higher than a second predetermined voltage. The fuel cell system described in 1.   When the voltage detected by the voltage detecting means is higher than a third predetermined voltage that is higher than the second predetermined voltage, the voltage of the fuel cell is reduced to a voltage that reduces the catalyst to the catalyst. The fuel cell system according to claim 7, further comprising catalyst reduction means for reducing the ionized catalyst to a catalyst.   9. The fuel cell system according to claim 8, wherein the oxide film forming unit applies a voltage to the fuel cell after reducing the ionized catalyst to the catalyst by the catalyst reducing unit.   The fuel cell system according to claim 8 or 9, wherein the third predetermined voltage is higher than a voltage at which the catalyst is ionized.   The fuel cell system according to any one of claims 7 to 10, wherein the second predetermined voltage is 0.88V or more.   The fuel cell system according to any one of claims 2 to 11, wherein the voltage applying unit applies a voltage in a stepped manner.   14. The fuel cell system according to claim 2, wherein the voltage application unit is a secondary battery that stores a part of electric power generated in the fuel cell.

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