JP4839596B2 - Stopping storage method of fuel cell system - Google Patents

Description

  The present invention relates to a method for stopping and storing a fuel cell system, and more particularly to a method for stopping and storing a fuel cell system that suppresses carbon oxidation degradation of the fuel cell.

  A fuel cell system is a power generation system that directly generates electricity by electrochemically reacting hydrogen obtained by reforming a fuel such as natural gas and oxygen in the air, and effectively uses the chemical energy of the fuel. Because it is capable of being environmentally friendly, technological development is in full swing for practical application.

  It is known that hydrogen gas remains in the fuel electrode immediately after the operation of the fuel cell is stopped. If left in this state, air is mixed from the outside and hydrogen gas and oxygen gas are mixed. When hydrogen gas and oxygen gas coexist, the catalyst on the air electrode side is deteriorated.

Conventionally, a technique for controlling the gas concentration in the fuel cell during stop storage of the system is known (see, for example, Patent Documents 1 and 2). Patent Document 1 discloses a fuel cell system that includes a gas suction device that sucks gas from a gas discharge port of a fuel electrode, detects the gas concentration in the fuel cell, and generates power generation timing and gas based on the gas concentration. By controlling the discharge, mixing of hydrogen gas and oxygen gas is prevented, and deterioration of the catalyst is prevented. Patent Document 2 discloses a technique for holding 4% or less of hydrogen gas while the fuel cell system is stopped and stored.
JP 2003-109630 A US Patent Application Publication No. 2002/0182456

  However, in the fuel cell system described in Patent Document 1, it is noted that the reason for selecting the concentration is due to the deterioration of the catalyst on the air electrode side simply by shifting to the next state when a predetermined concentration is reached. It has not been.

  Further, in the fuel cell system described in Patent Document 2, hydrogen gas is defined as 4% or less from the viewpoint of safety, and no reference is made to the concentration that affects the deterioration of the catalyst on the air electrode side.

Features of the present invention, the fuel electrode and the oxidant electrode disposed on either side of the electrolyte membrane, and the unit fuel cells adjacent to the fuel electrode and the oxidant electrode comprising a membrane electrode assembly formed by providing a gas diffusion layer, respectively And a method for stopping and storing a fuel cell system having means for measuring an electromotive force of a unit fuel cell, wherein the supply of fuel gas and oxidant gas to the unit fuel cell is stopped when the fuel cell system is stopped later, the electromotive force of the unit fuel cell is measured, compared with the range of the electromotive force and the oxidizing electrode of the unit fuel cell is oxidized as a result of this comparison, the electromotive force of the unit fuel cell oxidant electrode is oxidized When the current is taken out from the unit fuel cell when included in the range, and after the electromotive force of the unit fuel cell falls below the lower limit of the range in which the oxidizer electrode oxidizes, the current extraction from the unit fuel cell is stopped, and the comparison As a result, the unit fuel cell The gist is that the unit fuel cell waits for re-generation when the electric power is higher than the upper limit of the range in which the oxidant electrode is oxidized .

  According to the present invention, based on the electromotive force when oxygen and hydrogen coexist in the fuel electrode or the concentration ratio of hydrogen to oxygen, current is taken out from the unit fuel cell, or the unit fuel cell waits for re-generation. Accordingly, it is possible to provide a method for stopping and storing the fuel cell system that efficiently suppresses carbon oxidation deterioration of the fuel cell, particularly the oxidant electrode, during stop storage of the fuel cell system.

Embodiments and reference examples of the present invention will be described below with reference to the drawings. In the description of the drawings, the same or similar parts are denoted by the same or similar reference numerals.

(First embodiment)
[Constitution]
As shown in FIG. 1, the fuel cell system according to the first embodiment of the present invention is a fuel cell 1 that generates electricity by electrochemically reacting hydrogen gas as a fuel gas and air as an oxidant gas. A hydrogen cylinder 9 for storing hydrogen gas supplied to the fuel cell 1, a hydrogen supply line 4 connecting the hydrogen cylinder 9 and the hydrogen inlet of the fuel cell 1, and a hydrogen outlet of the fuel cell 1. A hydrogen discharge line 5, a hydrogen circulation line 10 connecting the hydrogen discharge line 5 and the hydrogen supply line 4, an ejector 30 disposed at a location where the hydrogen circulation line 10 intersects the hydrogen supply line 4, and air Compressor 8 supplied to the air inlet of fuel cell 1, air supply line 6 connecting compressor 8 and the air inlet of fuel cell 1, and air discharge line 7 connected to the air outlet of fuel cell 1 A first valve 3a disposed on the hydrogen supply line 4 in the vicinity of the hydrogen cylinder 9, and a second valve 3b disposed on the hydrogen discharge line 5 downstream from the connection point with the hydrogen circulation line 10, The voltage sensor 31 connected to both ends of the fuel cell 1, the switch 24 and the fixed resistor 34 connected in series to both ends of the fuel cell 1, the compressor 8, the first and second valves 3a, 3b, and the switch 24 And a control unit 12 for controlling the operation.

  The fuel cell 1 has a side-by-side stacked structure of unit fuel cells (hereinafter referred to as “unit cells”). The unit cell includes a membrane electrode assembly in which a fuel electrode (hydrogen electrode) and an oxidant electrode (air electrode) are arranged on both sides of an electrolyte membrane, and a gas diffusion layer is provided adjacent to the hydrogen electrode and the air electrode, respectively. .

  Hydrogen gas led out from the hydrogen cylinder 9 is supplied to the fuel cell 1 through the hydrogen supply line 4. The pressure of the hydrogen gas at this time is adjusted by the first valve 3 a and mixed with the exhausted hydrogen gas from the hydrogen circulation line 10 through the ejector 30. The hydrogen gas discharged from the fuel cell 1 is returned to the hydrogen supply line 4 via the hydrogen circulation line 10 or discharged via the hydrogen discharge line 5. The ejector 30 circulates the hydrogen gas discharged from the hydrogen discharge port of the fuel cell 1 to the hydrogen supply line 4 via the hydrogen circulation line 10.

  The second valve 3b is closed during normal power generation or when the fuel cell system is stopped. The second valve 3b is opened when discharging the hydrogen remaining in the hydrogen supply line 4 and the hydrogen circulation line 10 and the nitrogen crossing over from the air electrode side. On the other hand, the air supplied from the compressor 8 is supplied to the air electrode system of the fuel cell 1 through the air supply line 6 and discharged from the air discharge line 7. The control unit 12 receives the voltage measurement value (E2-E1) signal of the fuel cell 1 from the voltage sensor 31, and opens and closes the first valve 3a and the second valve 3b, the rotation speed of the compressor 8, and the switch 24. Control on / off.

  Although FIG. 1 shows a case where a fixed resistor 34 is connected in series with the switch 24 at both ends of the fuel cell 1, the present invention is not limited to this. Instead of the fixed resistor 34, means for consuming any form of power, such as a capacitor (capacitor), a secondary battery, an electronic control load device, a heat generating device (heater), another fuel cell (acting as a capacitor), etc. You may use.

[Control method]
With reference to FIG. 2, a control method when the fuel cell system of FIG. 1 is stopped and stored will be described.

  Regardless of the first embodiment, the stop storage in the present invention includes a temporary stop such as during idle stop. That is, the stop storage method in the present invention includes a temporary stop method.

  The stop storage method shown in FIG. 2 is particularly effective when the vehicle is temporarily stopped when the fuel cell vehicle is stopped at a convenience store or the like for a short time.

  (A) First, in step S01, the supply of hydrogen and air to the fuel cell 1 is stopped. Specifically, the control unit 12 stops the operation of the compressor 8 and closes the second valve 3a.

  (B) In step S02, the voltage sensor 31 is used to detect the generated voltage (E2-E1) of the entire fuel cell 1, and the average generated voltage of the unit cell divided by the total number of unit cells is estimated, and the average unit cell voltage Is monitored to be 0.3 V / cell or more and 0.9 V / cell or less. When the average unit cell voltage is in the range of 0.3 V / cell to 0.9 V / cell (YES in step S02), the process proceeds to step S03. Thus, when the fuel cell system is stopped, in the situation where hydrogen and oxygen are mixed on the hydrogen electrode side, the electromotive force of the fuel cell 1 is measured, and the range in which the electromotive force oxidizes the air electrode (0.3 V / cell It is determined whether or not it is included in the range of 0.9 V / cell or less. When the average unit cell voltage falls within the range of 0.3 V / cell to 0.9 V / cell, the hydrogen electrode of the fuel cell 1 is in a mixed state of hydrogen and air (including a crossover phenomenon).

  (C) In step S03, the control unit 12 connects the switch 24, that is, turns it on, and extracts current from the fuel cell 1. A current flows through the fixed resistor 34, hydrogen on the hydrogen electrode side and a small amount of oxygen, and oxygen on the air electrode side are consumed, and the voltage of the fuel cell 1 decreases.

  (D) After a predetermined time, in step S04, the voltage sensor 31 is used to monitor that the generated voltage of the unit cell in a part of the fuel cell 1 is less than 0.3 V per unit cell. If the voltage is less than 0.3 V per unit cell (YES in step S04), the process proceeds to step S05.

  (E) In step S05, the control unit 12 turns off the switch 24, that is, turns it off, and stops taking out the current from the fuel cell 1 by the fixed resistor 34. No current flows through the fixed resistor 34, and part of the hydrogen remains in the fuel cell 1. Although the electromotive force of the fuel cell 1 may rise again after the switch 24 is disconnected, in this case, the process may return to the step S01 and repeat the steps S01 to S05.

[Experimental example]
Next, in the control method of FIG. 2, an example of experiments conducted by the inventors for quantifying the range of the electromotive force of the fuel cell 1 and the concentration ratio of hydrogen to oxygen will be described.

  As shown in FIG. 3, the fuel cell system according to the experimental example includes a fuel cell 1, an air supply line 6 for supplying air to the fuel cell 1, an air discharge line 7 for discharging air from the fuel cell 1, A hydrogen supply line 4 that supplies a mixture of hydrogen gas and air to the fuel cell 1, a hydrogen discharge line 5 that discharges hydrogen gas from the fuel cell 1, and an air mass flow that controls the flow rate of air flowing through the air supply line 6. The controller 15a, the hydrogen mass flow controller 16 for controlling the flow rate of the hydrogen gas flowing through the hydrogen supply line 4, the air mass flow controller 15b for controlling the flow rate of the air flowing through the hydrogen supply line 4, and the hydrogen gas and air mixed to form hydrogen A mixer 19 for supplying to the supply line 4, an air-side humidifier 17 disposed on the air supply line 6, and a hydrogen supply line 4. A hydrogen-side humidifier 18, a hydrogen-side mass analyzer 21 connected to the hydrogen supply line 4 near the hydrogen inlet of the fuel cell 1 via the analysis port 23 a, and an air discharge line near the air outlet of the fuel cell 1 7 includes an air-side mass analyzer 20 connected to the fuel cell 1 via the analysis port 23b, and a thermocouple 22 that measures the temperature on the air electrode side of the fuel cell 1.

  The fuel cell 1 includes an electrolyte membrane 25, a membrane electrode assembly including a hydrogen electrode 28 and an air electrode 26 disposed on both sides of the electrolyte membrane 25, a hydrogen side separator 29 and an air side separator 27 that sandwich the membrane electrode composite, Is provided. The hydrogen supply line 4 and the hydrogen discharge line 5 are connected to a hydrogen side separator 29, and the air supply line 6 and the air discharge line 7 are connected to an air side separator 27.

  In the fuel gas system, hydrogen gas and air are mixed in the mixer 19 via the mass flow controllers 16 and 15b, respectively, and then supplied to the hydrogen supply line 4 and pass through the hydrogen-side humidifier 18 to hydrogen of the fuel cell 1. It is supplied to the side separator 29. The hydrogen side mass spectrometer 21 can measure the gas composition at the hydrogen inlet of the fuel cell 1. On the other hand, in the oxidant gas system, air is supplied to the air supply line 6 via the air mass flow controller 15 a and is supplied to the air side separator 27 of the fuel cell 1 through the air side humidifier 17. The air side mass analyzer 20 can measure the gas composition at the air outlet of the fuel cell 1.

  Using the experimental apparatus shown in FIG. 3, while introducing constant air to the hydrogen electrode side and the air electrode side, hydrogen gas whose flow rate was controlled by the hydrogen mass flow controller 16 was gradually added to the hydrogen electrode side, The change in the generated voltage of the fuel cell 1 and the amount of carbon dioxide discharged from the air outlet of the fuel cell 1 were measured using the air-side mass analyzer 20. The mixing ratio of hydrogen and oxygen in the fuel gas supplied to the hydrogen-side separator 29 is confirmed by combining the analysis results obtained by the hydrogen-side mass analyzer 21 in addition to the flow rate ratios displayed on the hydrogen mass flow controller 16 and the air mass flow controller 15b. did. Note that the temperature of the fuel cell 1 is arbitrarily controlled using a thermocouple 22 inserted into the air-side separator 27.

FIG. 4 shows a typical experimental result when the control temperature of the fuel cell 1 by the thermocouple 22 of FIG. 3 is set to 70.degree. When the hydrogen concentration was gradually increased from zero over time, the hydrogen generation ratio (H ₂ / O ₂ ) to oxygen became approximately 9.5%, and then the power generation voltage of the unit cell was 200 mV. Rose to a degree. Thereafter, until the concentration ratio of hydrogen to oxygen (H ₂ / O ₂ ) is less than 70%, the power generation voltage of the unit cell rises gradually from 200 mV, but carbon dioxide (CO ₂ ) emissions from the air electrode Not detected. However, when the concentration ratio (H ₂ / O ₂ ) reached 70% or more, the generated voltage suddenly increased from 300 mV and reached around 750 mV. At the same time, when the concentration ratio (H ₂ / O ₂ ) reached 70% or more, carbon dioxide (CO ₂ ) began to be rapidly discharged from the air electrode and continued to be discharged as it was. As the concentration ratio (H ₂ / O ₂ ) increases, the generated voltage gradually increases. When the concentration ratio (H ₂ / O ₂ ) reaches 400%, the generated voltage was about 900 mV. In addition, when the concentration ratio (H ₂ / O ₂ ) reached 400%, the carbon dioxide emission rate rapidly decreased and decreased to the vicinity of the detection limit. After this, when the supply of air on the hydrogen electrode side was stopped (O ₂ concentration = 0%) and the concentration ratio (H ₂ / O ₂ ) was set to ∞ (infinity), the carbon dioxide detection rate was almost detected. I settled near the limit.

The experimental results shown in FIG. 4 are the results when the concentration ratio of hydrogen to oxygen (H ₂ / O ₂ ) is shifted from low to high, but conversely when it is shifted from low to high. The inventors have experimentally confirmed that the concentration ratio falls within the above-mentioned concentration ratio (H ₂ / O ₂ ) range and the generated voltage range. It has also been confirmed that the experimental results in FIG. 4 have reproducibility regardless of the temperature of the fuel cell 1 or the like.

Furthermore, it has been confirmed not only by experiments but also by simulation that the generated voltage of the unit cell changes in approximately two steps depending on the concentration ratio of hydrogen to oxygen (H ₂ / O ₂ ). A simulation result close to the experimental result shown in FIG. The generated voltage of the unit cell is determined by the Butler-Volmer equation mainly by parameters such as the partial pressure and temperature of hydrogen and oxygen.

  Further, the theory that carbon dioxide is discharged from the air discharge port of the air electrode is as shown in US Patent Application Publication No. 2002/0076582, and oxidation caused by changes in the electrolyte potential due to the coexistence of hydrogen and oxygen. This is an increase in drug potential.

  The inventors carried out the experiment example shown in FIG. 4 several times at intervals of 10 ° C. from 40 ° C. to 90 ° C. FIG. 5 shows the result of temperature dependence regarding the carbon dioxide emission rate from the air electrode at this time. The vertical axis represents the logarithmic value of the carbon dioxide emission ratio, and the vertical axis represents the reciprocal of the temperature (absolute temperature) of the air-side separator 27. It was found that the logarithmic value of the carbon dioxide emission amount is a linear function with respect to the reciprocal of the temperature of the thermocouple 22 provided in the fuel cell 1. This tends to be consistent with Arrhenius's theoretical formula related to the generally known reaction rate and temperature. Therefore, it can be said that the carbon dioxide emission increases exponentially as the temperature increases.

  In the experimental example, the measurement was conducted focusing on the corrosion of the carbon used as the catalyst support, but other supports (graphitized carbon, metal, silicon carbide, etc.) and metal catalysts (platinum, cobalt, iridium, nickel) , Ruthenium, and their alloys) are generally known to tend to undergo oxidative degradation as the potential increases. Since the carbon used in the experimental example is likely to be oxidized at a relatively low potential compared to the other carriers and metal catalysts described above, if the predetermined range obtained in the experimental result of the experimental example is reflected in the control, the air Oxidative degradation as a pole can be mostly suppressed.

[Action]
The operation of the fuel cell system stop storage method shown in FIG. 2 based on the above experimental results will be described. It is known that hydrogen gas remains on the hydrogen electrode 28 side hydrogen electrode of the fuel cell 1 immediately after the operation of the fuel cell 1 is stopped. If left in this state, air enters from the outside and hydrogen gas and oxygen gas coexist, including the cross leak phenomenon in which the gas of the hydrogen electrode 28 and the air electrode 26 are mixed through the electrolyte membrane 25.

When the concentration ratio of hydrogen to oxygen in the air (H ₂ / O ₂ ) falls within the “predetermined concentration ratio range”, the electrolyte potential in the oxygen-rich region is higher than the electrolyte potential in the hydrogen-rich region on the hydrogen electrode side. Therefore, the oxygen-rich region has a higher potential of the oxidizer electrode with respect to the electrolyte potential than the hydrogen-rich region. Therefore, the air electrode (carbon carrier, platinum, etc.) in the oxygen-rich region is oxidatively corroded. In the above experimental example, the corrosion of the carbon support was measured by a mass analyzer. However, since the potential becomes high, oxidation degradation of not only the carbon support but also a metal catalyst such as platinum occurs.

Therefore, by newly performing control to avoid the “predetermined concentration range”, the oxidative corrosion of the air electrode as described above can be suppressed. Further, as the experimental results show, when the concentration ratio of hydrogen to oxygen (H ₂ / O ₂ ) is 70% or more and 400% or less, the oxidative deterioration of the air electrode is accelerated. Therefore, the oxidative deterioration of the air electrode can be suppressed by controlling the concentration ratio of hydrogen to oxygen (H ₂ / O ₂ ) in the range of 70% to 400% as the “predetermined concentration range”.

In the stop storage method of the fuel cell system shown in FIG. 2, when the hydrogen concentration ratio (H ₂ / O ₂ ) to oxygen in the air falls within the “predetermined concentration ratio range”, the fuel cell is determined according to the hydrogen / oxygen concentration ratio. Since the electromotive force of 1 changes, by using the electromotive force as a determination means, it is possible to control gas replacement or the like with a simple system without requiring a gas concentration detection means.

  Moreover, as the experimental result of FIG. 4 shows, when the electromotive force per unit cell is 0.3 V or more and 0.9 V or less, hydrogen and oxygen are mixed in the vicinity of the hydrogen electrode, and a potential difference occurs on the air electrode side. There is a possibility that the oxidative deterioration of the air electrode is accelerated, so that the oxidative deterioration can be suppressed by turning on the switch 24 and passing a current through the fixed resistor 34.

  In addition, when the electromotive force per unit cell becomes less than 0.3 V, the current is stopped by turning off the switch, so that consumption of residual hydrogen at the hydrogen electrode can be stopped while suppressing the oxidative deterioration of the air electrode. Therefore, unlike the conventional control method in which the fixed resistor 34 is connected at the time of stopping, excess hydrogen consumption can be suppressed.

[effect]
According to the first embodiment of the present invention, control is performed based on the electromotive force of the main body of the fuel cell 1, so that a fuel cell system that achieves labor saving and simplification and is excellent in durability can be obtained. .

(Second Embodiment)
The fuel cell system according to the second embodiment is the same as the fuel cell system shown in FIG.

  With reference to FIG. 6, a control method when the fuel cell system according to the second embodiment is stopped and stored will be described. The control method shown in FIG. 6 is particularly effective at an idle stop, which is an extremely short temporary stop, not a long-term stopped storage.

  (A) First, in step S11, the vehicle on which the fuel cell 1 is mounted is set in an idle stop state.

  (B) In step S12, the voltage sensor 31 is used to detect the power generation voltage (E2-E1) of the entire fuel cell 1, and the average power generation voltage of the unit cell divided by the total number of unit cells is estimated. Is determined to be higher than 0.9 V / cell. If the average unit cell voltage is higher than 0.9 V / cell (YES in step S12), the process proceeds to step S13.

  (C) In step S13, the fuel cell system always stands by for re-generation. When power generation is resumed (YES in step S13), the process proceeds to step S14, where hydrogen gas and air are supplied to the fuel cell 1 and power generation is resumed. On the other hand, if power generation is not resumed (NO in step S13), the process returns to step S12.

  (D) On the other hand, when the average unit cell voltage is 0.9 V / cell or less in step S12 (NO in step S12), the process proceeds to step S02 shown in FIG. 2, and steps S02 to S05 are performed. That is, when the average unit cell voltage is 0.3 V / cell or more and 0.9 V / cell or less, the voltage of the fuel cell 1 is lowered by taking out the current by the fixed resistor 34, and the average unit cell voltage is 0.3 V / cell. When it becomes less than the cell, the current extraction by the fixed resistor 34 is stopped.

  When the average unit cell voltage of the fuel cell is higher than 0.9 V / cell, the current is not taken out by the power consuming means such as the fixed resistor 34 because the degree of oxidative corrosion of the air electrode is smaller than the experimental result of FIG. As a result, the consumption of the residual hydrogen in the vicinity of the fuel electrode in the fuel cell 1 can be stopped, so that excess hydrogen consumption can be suppressed. Therefore, according to the second embodiment, it is possible to obtain a fuel cell system that is excellent in durability, improved in fuel efficiency, and excellent in economy.

(Third embodiment)
[Constitution]
As shown in FIG. 7, the fuel cell system according to the third embodiment of the present invention differs from the fuel cell system shown in FIG. 1 in the following points. That is, the fuel cell system shown in FIG. 7 includes a hydrogen circulation pump 11 disposed on the hydrogen circulation line 10 instead of the ejector 30 shown in FIG. The fuel cell system shown in FIG. 7 includes a secondary battery 35 instead of the fixed resistor 34 of FIG. The fuel cell system shown in FIG. 7 further includes a temperature sensor 32 a disposed immediately after the fuel cell 1 in the air discharge line 7.

  The hydrogen circulation pump 11 circulates a part of the exhausted hydrogen discharged from the fuel cell 1 to the hydrogen discharge line 5 to the hydrogen supply line 4. The temperature sensor 32a detects or estimates the temperature of the air immediately after the fuel cell 1. The temperature sensor 32a may be of any type such as a thermocouple or a thermistor. The control unit 12 receives the signal of the potential difference (E2-E1) in the fuel cell 1 from the voltage sensor 31 and the temperature signal from the temperature sensor 32a, and opens the opening of the first and second valves 3a and 3b, the compressor The number of rotations of 8 and the opening / closing (on / off) of the switch 24 are controlled.

  Other configurations are the same as those of the fuel cell system shown in FIG.

  In FIG. 7, the secondary battery 35 is connected to both ends of the fuel cell 1, but it may be at least a part of unit cell units instead of the both ends of the fuel cell 1. 7 shows the case where the secondary battery 35 is connected in series with the switch 24 at both ends of the fuel cell 1, the present invention is not limited to this. Instead of the secondary battery 35, for example, a capacitor (capacitor), a fixed resistor, an electronic control load device, a heat generator (heater), another fuel cell (acting as a capacitor), or any other means for consuming electric power in any form. You may use. Further, although the temperature sensor 32a is disposed immediately after the fuel cell 1 in the air discharge line 7, it is preferable to measure the temperature of the air electrode that directly affects the oxidative corrosion reaction in the air electrode. If this is difficult, it may be substituted by measuring or estimating the surrounding temperature. That is, for example, the temperature inside or on the surface of the fuel cell 1, the temperature on the hydrogen electrode side, the measurement of the supply line / discharge line, etc. can be substituted. A plurality of temperature sensors 32a may be provided.

[Control method]
With reference to FIG. 8, a control method for stopping and storing the fuel cell system of FIG. 7 will be described. The control method of FIG. 8 is particularly effective when a fuel cell vehicle is stopped for a short time at a convenience store or the like.

  (A) First, in step S21, supply of hydrogen and air to the fuel cell 1 is stopped. Specifically, the control unit 12 stops the operation of the compressor 8 and closes the first valve 3a.

  (B) In step S22, the voltage sensor 31 is used to detect the power generation voltage (E2-E1) of the entire fuel cell 1, and the average power generation voltage of the unit cell divided by the total number of unit cells is estimated. Is monitored to be 0.3 V / cell or more and 0.9 V / cell or less. When the average unit cell voltage is in the range of 0.3 V / cell to 0.9 V / cell (YES in step S22), the process proceeds to step S23. Thus, when the fuel cell system is stopped, in the situation where hydrogen and oxygen are mixed on the hydrogen electrode side, the electromotive force of the fuel cell 1 is measured, and the range in which the electromotive force oxidizes the air electrode (0.3 V / cell It is determined whether or not it is included in the range of 0.9 V / cell or less.

  (C) In step S23, the temperature of the air immediately after the fuel cell 1 is measured using the temperature sensor 32a, and it is determined whether or not the temperature is 50 ° C. or higher. If the temperature is 50 ° C. or higher (YES in step S23), the process proceeds to step S24. If the temperature is not 50 ° C. or higher (NO in step S23), the process returns to step S22.

  (D) In step S24, the control unit 12 connects the switch 24, that is, turns it on. A current flows through the secondary battery 35 and is extracted from the fuel cell 1. Thereby, the secondary battery 35 is charged. Hydrogen on the hydrogen electrode side and a small amount of oxygen and oxygen on the air electrode side are consumed, and the voltage of the fuel cell 1 decreases.

  (E) After a predetermined time, in step S25, the voltage sensor 31 is used to monitor that the generated voltage of the unit cell in a part of the fuel cell 1 is less than 0.3 V per unit cell. If it is less than 0.3 V per unit cell (YES in step S25), the process proceeds to step S26.

  (F) In step S26, the control unit 12 stops the extraction of current from the fuel cell 1 by the secondary battery 35 with the switch 24 disconnected, that is, turned off. No current flows through the secondary battery 35, and a part of hydrogen remains in the fuel cell 1. Although the electromotive force of the fuel cell 1 may increase again after the switch 24 is disconnected, in this case, the process may return to the step S21 and repeat the steps S21 to S26. In step S23, 50 ° C. is an example, and the reference temperature may be set to a temperature other than 50 ° C.

[Action]
The effect | action which the stop storage method of the fuel cell system shown in FIG. 8 show | plays is demonstrated. It is known that hydrogen gas remains on the hydrogen electrode 28 side hydrogen electrode of the fuel cell 1 immediately after the operation of the fuel cell 1 is stopped. If left in this state, air enters from the outside and hydrogen gas and oxygen gas coexist, including the cross leak phenomenon in which the gas of the hydrogen electrode 28 and the air electrode 26 are mixed through the electrolyte membrane 25.

When the concentration ratio of hydrogen to oxygen in the air (H ₂ / O ₂ ) falls within the “predetermined concentration ratio range”, the electrolyte potential in the oxygen-rich region is higher than the electrolyte potential in the hydrogen-rich region on the hydrogen electrode side. Therefore, the oxygen-rich region has a higher potential of the oxidizer electrode with respect to the electrolyte potential than the hydrogen-rich region. Therefore, the air electrode (carbon carrier, platinum, etc.) in the oxygen-rich region is oxidatively corroded. In the above experimental example, the corrosion of the carbon support was measured by a mass analyzer. However, since the potential becomes high, oxidation degradation of not only the carbon support but also a metal catalyst such as platinum occurs.

In the stopped storage method of the fuel cell system shown in FIG. 8, when the hydrogen concentration ratio (H ₂ / O ₂ ) with respect to oxygen in the air falls within the “predetermined concentration ratio range”, the fuel cell system uses the concentration ratio of hydrogen and oxygen. Since the electromotive force of 1 changes, by using the electromotive force as a determination means, it is possible to control gas replacement or the like with a simple system without requiring a gas concentration detection means.

  Moreover, as the experimental result of FIG. 4 shows, when the electromotive force per unit cell is 0.3 V or more and 0.9 V or less, hydrogen and oxygen are mixed in the vicinity of the hydrogen electrode, and a potential difference occurs on the air electrode side. There is a possibility. Further, as shown in FIG. 5, the oxidative degradation of the air electrode is accelerated exponentially as the temperature of the fuel cell 1 or the air in the fuel cell 1 increases. The oxidation degradation can be suppressed by flowing the current through the fixed resistor 34 with the switch 24 turned on when the temperature of the fuel cell 1 or the air in the fuel cell 1 is within a predetermined temperature (50 ° C. or more).

  In addition, when the electromotive force per unit cell becomes less than 0.3 V, the current is stopped by turning off the switch, so that consumption of residual hydrogen at the hydrogen electrode can be stopped while suppressing the oxidative deterioration of the air electrode. Therefore, unlike the conventional control method in which the fixed resistor 34 is connected at the time of stopping, excess hydrogen consumption can be suppressed. Therefore, the occurrence of energy conversion loss during charging of the secondary battery 35 can be suppressed.

[effect]
According to the third embodiment of the present invention, since control is performed based on the electromotive force of the fuel cell 1 body, it is possible to obtain a fuel cell system with improved fuel efficiency, excellent economy, and excellent durability. it can.

( First reference example )
[Constitution]
As shown in FIG. 9, the fuel cell system according to the first reference example of the present invention is different from the fuel cell system shown in FIG. 1 in the following points. That is, the fuel cell system shown in FIG. 9 includes a hydrogen circulation pump 11 disposed on the hydrogen circulation line 10 instead of the ejector 30 shown in FIG. Further, in the fuel cell system shown in FIG. 7, the fuel on the first gas concentration sensor 2a and the hydrogen discharge line 5 disposed immediately before the fuel cell 1 on the hydrogen supply line 4 is used instead of the voltage sensor 31 in FIG. A second gas concentration sensor 2b disposed immediately after the battery 1 is provided, and a capacitor 36 is provided instead of the fixed resistor 34 of FIG. The fuel cell system shown in FIG. 7 further includes a temperature sensor 32b disposed at the end of the fuel cell 1 on the gas supply side.

  The temperature sensor 32b detects or estimates the temperature of the fuel cell 1. The temperature sensor 32b may be of any type such as a thermocouple or a thermistor. In addition, the control unit 12 receives the concentration signal obtained by estimating the concentration of oxygen relative to hydrogen in the first and second gas concentration sensors 2a and 2b and the temperature signal detected or estimated by the temperature sensor 32b. The switch 24 is turned on / off (connected / not connected).

  Other configurations are the same as those of the fuel cell system shown in FIG.

  In FIG. 9, the capacitor 36 is connected to both ends of the fuel cell 1, but it may be at least a part of unit cell units instead of both ends of the fuel cell 1. FIG. 9 shows the case where the capacitor 36 is connected in series with the switch 24 at both ends of the fuel cell 1, but the present invention is not limited to this. Instead of the capacitor 36, any form of power consuming means such as a secondary battery, a fixed resistor, an electronic control load device, a heat generation device (heater), another fuel cell (acting as a capacitor) may be used. I do not care. Further, although the temperature sensor 32b is disposed at the end of the fuel cell 1 on the gas supply side, it is preferable to measure the temperature of the air electrode that directly affects the oxidative corrosion reaction in the air electrode. If this is difficult, it may be substituted by measuring or estimating the surrounding temperature. That is, for example, the temperature inside or on the surface of the fuel cell 1, the temperature on the hydrogen electrode side, the measurement of the supply line / discharge line, etc. can be substituted. A plurality of temperature sensors 32b may be provided.

[Control method]
With reference to FIG. 10, a control method when the fuel cell system of FIG. 9 is stopped and stored will be described.

  (A) First, in step S31, supply of hydrogen and air to the fuel cell 1 is stopped. Specifically, the control unit 12 stops the operation of the compressor 8 and closes the first valve 3a.

  (B) In step S32, at least one of the first gas concentration sensor 2a and the second gas concentration sensor 2b detects that the concentration ratio of hydrogen to oxygen in the hydrogen electrode is 70% or more and 400% or less. Monitor that. When it falls within the range of 70 to 400% (YES in step S32), the process proceeds to step S33. As described above, when the fuel cell system is stopped, the concentration ratio of hydrogen to oxygen in the hydrogen electrode is measured, and the concentration ratio of hydrogen to oxygen is included in the range in which the air electrode is oxidized (70 to 400% range). Determine whether or not.

  (C) In step S33, the temperature of the end of the fuel cell 1 on the gas supply side is measured using the temperature sensor 32b, and it is determined whether or not the temperature is 30 ° C. or higher. If the temperature is 30 ° C. or higher (YES in step S33), the process proceeds to step S34. If the temperature is not 30 ° C. or higher (NO in step S33), the process returns to step S32.

  (D) In step S34, the control unit 12 connects the switch 24, that is, turns it on. A current flows through the capacitor 36 and is extracted from the fuel cell 1. As a result, the capacitor 36 is charged. Hydrogen on the hydrogen electrode side and a small amount of oxygen and oxygen on the air electrode side are consumed, and the voltage of the fuel cell 1 decreases.

  (E) In step S35, it is monitored that both the first and second gas concentration sensors 2a and 2b detect that the concentration ratio of hydrogen to oxygen is less than 70%. If it is less than 70% (YES in step S35), the process proceeds to step S36.

  (F) In step S36, the control unit 12 turns off the switch 24, that is, turns it off, and stops taking out the current from the fuel cell 1 by the capacitor 36. No current flows through the capacitor 36, and a part of hydrogen remains in the fuel cell 1. Although the electromotive force of the fuel cell 1 may increase again after the switch 24 is disconnected, in this case, the process may return to the step S31 and repeat the steps S31 to S36. In step S33, 30 ° C. is an example, and the reference temperature may be set to a temperature other than 30 ° C.

[Action]
The operation of the fuel cell system stop storage method shown in FIG. 10 will be described. It is known that hydrogen gas remains in the hydrogen electrode immediately after the operation of the fuel cell 1 is stopped. If left in this state, air is mixed from the outside and hydrogen gas and oxygen gas are mixed. When the concentration ratio of hydrogen to oxygen in the air (H ₂ / O ₂ ) falls within the “predetermined concentration ratio range”, the electrolyte potential in the oxygen-rich region is higher than the electrolyte potential in the hydrogen-rich region on the hydrogen electrode side. Therefore, the oxygen-rich region has a higher potential of the oxidizer electrode with respect to the electrolyte potential than the hydrogen-rich region. Therefore, the air electrode (carbon carrier, platinum, etc.) in the oxygen-rich region is oxidatively corroded. In the above experimental example, the corrosion of the carbon support was measured by a mass analyzer. However, since the potential becomes high, oxidation degradation of not only the carbon support but also a metal catalyst such as platinum occurs.

On the other hand, in the stopped storage method of the fuel cell system shown in FIG. 10, the oxidative corrosion of the air electrode as described above can be suppressed by newly performing control to avoid the “predetermined concentration range”. Further, as shown in the experimental results of FIG. 4, when the concentration ratio of hydrogen to oxygen (H ₂ / O ₂ ) is 70% or more and 400% or less, the oxidative deterioration of the air electrode is accelerated. Therefore, when the concentration ratio of hydrogen to oxygen (H ₂ / O ₂ ) falls within the range of 70% or more and 400% or less, oxidative deterioration of the air electrode can be suppressed by flowing current through the capacitor 36.

  Further, as shown in the experimental results of FIG. 5, the oxidative corrosion of the air electrode (in this case, the carbon support) is temperature-dependent, and the higher the temperature, the faster the oxidation, so the temperature of the fuel cell 1 is 30 ° C. By passing a current through the capacitor 36 when the temperature exceeds a predetermined temperature, deterioration can be effectively and economically suppressed.

  Further, when the concentration ratio of hydrogen to oxygen is less than 70%, the current is stopped by turning off the switch 24, so that the consumption of residual hydrogen in the hydrogen electrode can be stopped while suppressing the oxidative deterioration of the air electrode. Therefore, unlike the conventional control method in which a fixed resistance is connected at the time of stopping, excess hydrogen consumption can be suppressed. Therefore, the occurrence of energy conversion loss when charging the capacitor 36 can be suppressed.

[effect]
According to the first reference example , it is possible to obtain a fuel cell system that is excellent in durability, improved in fuel efficiency, and excellent in economy.

( Second reference example )
Since the fuel cell system according to the second reference example is the same as the fuel cell system shown in FIG. 9, illustration and description thereof are omitted.

With reference to FIG. 11, a control method for stopping and storing the fuel cell system according to the second reference example will be described. The control method shown in FIG. 11 is particularly effective at the time of idling stop, which is at the time of temporary stop for a very short time, not at the time of stop storage for a long time.

  (A) First, in step S41, the vehicle on which the fuel cell 1 is mounted is set in an idle stop state.

  (B) Whether or not at least one of the first gas concentration sensor 2a and the second gas concentration sensor 2b has detected that the concentration ratio of hydrogen to oxygen in the hydrogen electrode exceeds 400% in step S42. Determine whether. If it exceeds 400% (YES in step S42), the process proceeds to step S43.

  (C) In step S43, the fuel cell system always stands by for re-generation. When power generation is resumed (YES in step S43), the process proceeds to step S44, where hydrogen gas and air are supplied to the fuel cell 1 and power generation is resumed. On the other hand, when power generation is not resumed (NO in step S43), the process returns to step S42.

  (D) On the other hand, if it does not exceed 400% in step S42 (NO in step S42), the process proceeds to step S32 shown in FIG. 10, and steps S32 to S36 are performed. That is, when the concentration ratio of hydrogen to oxygen in the hydrogen electrode is not less than 70% and not more than 400%, the voltage of the fuel cell 1 is lowered by taking out the current through the capacitor 36, and the concentration ratio of hydrogen to oxygen in the hydrogen electrode is reduced. When the current becomes less than 70%, the current extraction by the capacitor 36 is stopped.

When the concentration ratio of hydrogen to oxygen exceeds 400%, since the degree of oxidative corrosion of the air electrode is smaller than the experimental result of FIG. 4, by not taking out the current by the power consuming means such as the capacitor 36, Since the consumption of residual hydrogen in the vicinity of the fuel electrode inside the fuel cell 1 can be stopped, excess hydrogen consumption can be suppressed. Therefore, according to the second reference example , it is possible to obtain a fuel cell system that is excellent in durability, improved in fuel efficiency, and excellent in economy.

(Other embodiments)
As described above, the present invention has been described according to the first to third embodiments. However, it should not be understood that the description and drawings constituting a part of this disclosure limit the present invention. From this disclosure, various alternative embodiments, examples and operational techniques will be apparent to those skilled in the art. That is, it should be understood that the present invention includes various embodiments not described herein. Therefore, the present invention is limited only by the invention specifying matters according to the scope of claims reasonable from this disclosure.

1 is a block diagram showing a fuel cell system according to a first embodiment of the present invention. 2 is a flowchart illustrating a control method when stopping the fuel cell system of FIG. 1. It is a block diagram which shows the fuel cell system concerning an experiment example. When the control temperature of the fuel cell by the thermocouple of FIG. 3 is set to 70 ° C., the hydrogen concentration ratio to oxygen (H 2 / O 2), the power generation voltage of the unit cell, and the carbon dioxide (CO 2) emission rate from the air electrode It is a graph which shows a time change. It is a graph which shows the result of the temperature dependence regarding the carbon dioxide discharge | emission amount from an air electrode when the temperature of a fuel cell is implemented several times in 10 degree intervals from 40 degreeC to 90 degreeC, respectively. It is a flowchart which shows the stop storage method of the fuel cell system concerning the 2nd Embodiment of this invention. It is a block diagram which shows the fuel cell system concerning the 3rd Embodiment of this invention. It is a flowchart which shows the control method at the time of stopping the fuel cell system of FIG. It is a block diagram which shows the fuel cell system concerning the 1st reference example of this invention. 10 is a flowchart illustrating a control method when stopping the fuel cell system of FIG. 9. It is a flowchart which shows the stop storage method method of the fuel cell system concerning the 2nd reference example of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Fuel cell 2a ... 1st gas concentration sensor 2b ... 2nd gas concentration sensor 3a ... 1st valve 3b ... 2nd valve 4 ... Hydrogen supply line 5 ... Hydrogen discharge line 6 ... Air supply line 7 ... Air Discharge line 8 ... Compressor 9 ... Hydrogen cylinder 10 ... Hydrogen circulation line 11 ... Hydrogen circulation pump 12 ... Control unit 15a, 15b ... Air mass flow controller 16 ... Hydrogen mass flow controller 17 ... Air side humidifier 18 ... Hydrogen side humidifier 19 ... Mixing 20: Air side mass analyzer 21 ... Hydrogen side mass analyzer 22 ... Thermocouple 23a, 23b ... Analysis port 24 ... Switch 25 ... Electrolyte membrane 26 ... Air electrode 27 ... Air side separator 28 ... Hydrogen electrode 29 ... Hydrogen side separator DESCRIPTION OF SYMBOLS 30 ... Ejector 31 ... Voltage sensor 32a, 32b ... Temperature sensor 34 ... Fixed resistance 35 ... Secondary electricity 36 ... capacitor

Claims (3)

A unit fuel cell including a membrane electrode assembly in which a fuel electrode and an oxidant electrode are disposed on both sides of an electrolyte membrane, and a gas diffusion layer is provided adjacent to the fuel electrode and the oxidant electrode; and the unit fuel cell A method for stopping and storing a fuel cell system comprising means for measuring the electromotive force of
When stopping the fuel cell system, after stopping the supply of fuel gas and oxidant gas to the unit fuel cell, measure the electromotive force,
Compare the electromotive force with the range where the oxidant electrode is oxidized ,
As a result of the comparison, when the electromotive force is included in the range, a current is extracted from the unit fuel cell,
After the electromotive force falls below the lower limit of the range due to this current extraction, the extraction of the current from the unit fuel cell is stopped ,
As a result of the comparison, when the electromotive force is higher than the upper limit of the range, the unit fuel cell waits for re-power generation . Wherein a range of the oxidant electrode is oxidized, storage of stopping the fuel cell system according to claim 1, wherein the electromotive force is in a range equal to or less than 0.9V or 0.3V per unit fuel cells Method. 2. The fuel cell according to claim 1 , wherein whether or not to extract current from the unit fuel cell is determined by a function including at least one of a temperature of the unit fuel cell and a gas temperature in the unit fuel cell. How to stop and save the system.