WO2006069070A2 - Fuel cell system - Google Patents

Fuel cell system Download PDF

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Publication number
WO2006069070A2
WO2006069070A2 PCT/US2005/046173 US2005046173W WO2006069070A2 WO 2006069070 A2 WO2006069070 A2 WO 2006069070A2 US 2005046173 W US2005046173 W US 2005046173W WO 2006069070 A2 WO2006069070 A2 WO 2006069070A2
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WO
WIPO (PCT)
Prior art keywords
fuel cell
catalytic layer
predetermined
cell stack
value
Prior art date
Application number
PCT/US2005/046173
Other languages
French (fr)
Other versions
WO2006069070A3 (en
Inventor
Yoshitaka Ono
Ryoichi Shimoi
Hiroyuki Tanaka
Original Assignee
Nissan Motor Co., Ltd.
Nissan North America
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
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Publication date
Application filed by Nissan Motor Co., Ltd., Nissan North America filed Critical Nissan Motor Co., Ltd.
Publication of WO2006069070A2 publication Critical patent/WO2006069070A2/en
Publication of WO2006069070A3 publication Critical patent/WO2006069070A3/en

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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M8/00—Fuel cells; Manufacture thereof
    • H01M8/24—Grouping of fuel cells, e.g. stacking of fuel cells
    • H01M8/2457—Grouping of fuel cells, e.g. stacking of fuel cells with both reactants being gaseous or vaporised
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M8/00—Fuel cells; Manufacture thereof
    • H01M8/04—Auxiliary arrangements, e.g. for control of pressure or for circulation of fluids
    • H01M8/04082—Arrangements for control of reactant parameters, e.g. pressure or concentration
    • H01M8/04089—Arrangements for control of reactant parameters, e.g. pressure or concentration of gaseous reactants
    • H01M8/04119—Arrangements for control of reactant parameters, e.g. pressure or concentration of gaseous reactants with simultaneous supply or evacuation of electrolyte; Humidifying or dehumidifying
    • H01M8/04126—Humidifying
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M16/00—Structural combinations of different types of electrochemical generators
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M16/00—Structural combinations of different types of electrochemical generators
    • H01M16/003—Structural combinations of different types of electrochemical generators of fuel cells with other electrochemical devices, e.g. capacitors, electrolysers
    • H01M16/006—Structural combinations of different types of electrochemical generators of fuel cells with other electrochemical devices, e.g. capacitors, electrolysers of fuel cells with rechargeable batteries
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M8/00—Fuel cells; Manufacture thereof
    • H01M8/04—Auxiliary arrangements, e.g. for control of pressure or for circulation of fluids
    • H01M8/04082—Arrangements for control of reactant parameters, e.g. pressure or concentration
    • H01M8/04089—Arrangements for control of reactant parameters, e.g. pressure or concentration of gaseous reactants
    • H01M8/04119—Arrangements for control of reactant parameters, e.g. pressure or concentration of gaseous reactants with simultaneous supply or evacuation of electrolyte; Humidifying or dehumidifying
    • H01M8/04156—Arrangements for control of reactant parameters, e.g. pressure or concentration of gaseous reactants with simultaneous supply or evacuation of electrolyte; Humidifying or dehumidifying with product water removal
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M8/00—Fuel cells; Manufacture thereof
    • H01M8/04—Auxiliary arrangements, e.g. for control of pressure or for circulation of fluids
    • H01M8/04082—Arrangements for control of reactant parameters, e.g. pressure or concentration
    • H01M8/04089—Arrangements for control of reactant parameters, e.g. pressure or concentration of gaseous reactants
    • H01M8/04119—Arrangements for control of reactant parameters, e.g. pressure or concentration of gaseous reactants with simultaneous supply or evacuation of electrolyte; Humidifying or dehumidifying
    • H01M8/04156—Arrangements for control of reactant parameters, e.g. pressure or concentration of gaseous reactants with simultaneous supply or evacuation of electrolyte; Humidifying or dehumidifying with product water removal
    • H01M8/04179—Arrangements for control of reactant parameters, e.g. pressure or concentration of gaseous reactants with simultaneous supply or evacuation of electrolyte; Humidifying or dehumidifying with product water removal by purging or increasing flow or pressure of reactants
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M8/00—Fuel cells; Manufacture thereof
    • H01M8/04—Auxiliary arrangements, e.g. for control of pressure or for circulation of fluids
    • H01M8/04223—Auxiliary arrangements, e.g. for control of pressure or for circulation of fluids during start-up or shut-down; Depolarisation or activation, e.g. purging; Means for short-circuiting defective fuel cells
    • H01M8/04225—Auxiliary arrangements, e.g. for control of pressure or for circulation of fluids during start-up or shut-down; Depolarisation or activation, e.g. purging; Means for short-circuiting defective fuel cells during start-up
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M8/00—Fuel cells; Manufacture thereof
    • H01M8/04—Auxiliary arrangements, e.g. for control of pressure or for circulation of fluids
    • H01M8/04223—Auxiliary arrangements, e.g. for control of pressure or for circulation of fluids during start-up or shut-down; Depolarisation or activation, e.g. purging; Means for short-circuiting defective fuel cells
    • H01M8/04228—Auxiliary arrangements, e.g. for control of pressure or for circulation of fluids during start-up or shut-down; Depolarisation or activation, e.g. purging; Means for short-circuiting defective fuel cells during shut-down
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M8/00—Fuel cells; Manufacture thereof
    • H01M8/04—Auxiliary arrangements, e.g. for control of pressure or for circulation of fluids
    • H01M8/04298—Processes for controlling fuel cells or fuel cell systems
    • H01M8/04313—Processes for controlling fuel cells or fuel cell systems characterised by the detection or assessment of variables; characterised by the detection or assessment of failure or abnormal function
    • H01M8/0438—Pressure; Ambient pressure; Flow
    • H01M8/04388—Pressure; Ambient pressure; Flow of anode reactants at the inlet or inside the fuel cell
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M8/00—Fuel cells; Manufacture thereof
    • H01M8/04—Auxiliary arrangements, e.g. for control of pressure or for circulation of fluids
    • H01M8/04298—Processes for controlling fuel cells or fuel cell systems
    • H01M8/04313—Processes for controlling fuel cells or fuel cell systems characterised by the detection or assessment of variables; characterised by the detection or assessment of failure or abnormal function
    • H01M8/0438—Pressure; Ambient pressure; Flow
    • H01M8/04395—Pressure; Ambient pressure; Flow of cathode reactants at the inlet or inside the fuel cell
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M8/00—Fuel cells; Manufacture thereof
    • H01M8/04—Auxiliary arrangements, e.g. for control of pressure or for circulation of fluids
    • H01M8/04298—Processes for controlling fuel cells or fuel cell systems
    • H01M8/04313—Processes for controlling fuel cells or fuel cell systems characterised by the detection or assessment of variables; characterised by the detection or assessment of failure or abnormal function
    • H01M8/04537—Electric variables
    • H01M8/04544—Voltage
    • H01M8/04552—Voltage of the individual fuel cell
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M8/00—Fuel cells; Manufacture thereof
    • H01M8/04—Auxiliary arrangements, e.g. for control of pressure or for circulation of fluids
    • H01M8/04298—Processes for controlling fuel cells or fuel cell systems
    • H01M8/04313—Processes for controlling fuel cells or fuel cell systems characterised by the detection or assessment of variables; characterised by the detection or assessment of failure or abnormal function
    • H01M8/04537—Electric variables
    • H01M8/04544—Voltage
    • H01M8/04559—Voltage of fuel cell stacks
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M8/00—Fuel cells; Manufacture thereof
    • H01M8/04—Auxiliary arrangements, e.g. for control of pressure or for circulation of fluids
    • H01M8/04298—Processes for controlling fuel cells or fuel cell systems
    • H01M8/04694—Processes for controlling fuel cells or fuel cell systems characterised by variables to be controlled
    • H01M8/04746—Pressure; Flow
    • H01M8/04753—Pressure; Flow of fuel cell reactants
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M8/00—Fuel cells; Manufacture thereof
    • H01M8/04—Auxiliary arrangements, e.g. for control of pressure or for circulation of fluids
    • H01M8/04298—Processes for controlling fuel cells or fuel cell systems
    • H01M8/04694—Processes for controlling fuel cells or fuel cell systems characterised by variables to be controlled
    • H01M8/04828—Humidity; Water content
    • H01M8/04835—Humidity; Water content of fuel cell reactants
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M8/00—Fuel cells; Manufacture thereof
    • H01M8/24—Grouping of fuel cells, e.g. stacking of fuel cells
    • H01M8/241—Grouping of fuel cells, e.g. stacking of fuel cells with solid or matrix-supported electrolytes
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M8/00—Fuel cells; Manufacture thereof
    • H01M8/04—Auxiliary arrangements, e.g. for control of pressure or for circulation of fluids
    • H01M8/04298—Processes for controlling fuel cells or fuel cell systems
    • H01M8/04313—Processes for controlling fuel cells or fuel cell systems characterised by the detection or assessment of variables; characterised by the detection or assessment of failure or abnormal function
    • H01M8/04537—Electric variables
    • H01M8/04634—Other electric variables, e.g. resistance or impedance
    • H01M8/04641—Other electric variables, e.g. resistance or impedance of the individual fuel cell
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
    • Y02E60/10—Energy storage using batteries
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
    • Y02E60/30—Hydrogen technology
    • Y02E60/50—Fuel cells

Abstract

Techniques are described for substantially reducing deterioration of a fuel cell stack within a fuel cell system. More specifically, the techniques substantially reduce deterioration of the fuel cell stack by modifying moisture content of an oxidant electrode catalytic layer of the fuel cell stack. A fuel cell system may include fuel cell stack with a membrane electrode assembly that includes a fuel electrode and an oxidant electrode placed on either side of an electrolytic membrane supported by a plurality of unit cells. The fuel cell system may also includes a controller capable of decreasing the moisture content of the oxidant electrode catalytic layer in response to detecting a loading state of the fuel cell stack reaches a predetermined loading value and a voltage of the unit cells reaches a predetermined voltage value. In this way, the techniques may prevent the deterioration of the oxidant electrode catalytic layer.

Description

FUEL CELL SYSTEM
[0001] This application claims priority from Japanese Patent Application No. 2004- 368111, filed December 20, 2004, and from Japanese Patent Application No. 2005- 213459, filed July 22, 2005, the entire contents of each of which are incorporated herein by reference.
TECHNICAL FIELD
[0002] The invention relates to fuel cell systems.
BACKGROUND
[0003] A fuel cell is electrochemically reacted with a fuel gas, such as hydrogen gas, and an oxidizing gas, which contains oxygen, through an electrolyte. The fuel cell directly extracts electric energy from electrodes placed on both sides of the electrolyte. For example, a fuel cell vehicle loaded with a hydrogen storing device (e.g., a high pressure hydrogen tank, a liquid hydrogen tank, or a hydrogen storage alloy tank) reacts by feeding hydrogen from the hydrogen storing device and air containing oxygen to the fuel cell, and powers the electric motor connected to the drive wheels with electric energy extracted from the fuel cell. In this way, the fuel cell vehicle is a "clean" vehicle as the exhaust materials are only water.
[0004] In particular, a solid polymer-type fuel cell stands out as an electric power supply for an electric motor vehicle since its operative temperature is low and the handling is easy. However, in the solid polymer-type fuel cell, when the fuel cell is activated and inactivated repeatedly the fuel cell tends to deteriorate faster than when the fuel cell continues running. Furthermore, in the case where a loading cycle is repeated along with the fuel cell activation and inactivation, the deterioration of the fuel cell is advanced compared with the case where the fuel cell is repeatable activated and inactivated with the same load.
SUMMARY
[0005] In general, the invention is directed to techniques for substantially reducing deterioration of a fuel cell stack within a fuel cell system. More specifically, the techniques substantially reduce deterioration of the fuel cell stack by modifying moisture content of an oxidant electrode catalytic layer of the fuel cell stack. Therefore, the techniques described herein may provide a fuel cell stack with high durability to prevent deterioration of the oxidant electrode catalytic layer.
[0006] A fuel cell system may include a membrane electrode assembly with catalytic layers, i.e., a fuel electrode and an oxidant electrode, and a gas diffusion electrode placed on both sides of an electrolytic membrane. Ih addition, the fuel cell system may include a fuel cell stack with a plurality of unit cells that support the membrane electrode assembly with a separator equipped with a gas flow path through which fuel gas is supplied to the fuel electrode and oxidant gas is supplied to the oxidant electrode. [0007] The fuel cell system may also includes a controller capable of decreasing the moisture content of the oxidant electrode catalytic layer in response to detecting that a loading state of the fuel cell stack increases or decreases to a predetermined loading value and a voltage of the unit cells of the fuel cell stack increases to a predetermined voltage value. By decreasing the moisture content of the oxidant electrode catalytic layer to a predetermined moisture value or lower, the techniques described herein may prevent the deterioration of the oxidant electrode catalytic layer.
[0008] In one embodiment, a fuel cell system comprises a membrane electrode assembly including a fuel electrode catalytic layer and an oxidant electrode catalytic layer placed on either side of an electrolytic membrane, and a fuel cell stack including a plurality of unit cells that support the membrane electrode assembly with a separator that includes a gas flow path through which fuel gas is supplied to the fuel electrode catalytic layer of the membrane electrode assembly and oxidant gas is supplied to the oxidant electrode catalytic layer of the membrane electrode assembly. The fuel cell system also comprises a controller that decreases moisture content of the oxidant electrode catalytic layer corresponding to a loading state of the fuel cell stack and a voltage of the plurality of unit cells of the fuel cell stack.
[0009] In another embodiment, a method comprises detecting a loading state of a fuel cell stack that includes a plurality of unit cells and detecting a voltage of the plurality of unit cells of the fuel cell stack, wherein the plurality of unit cells support a membrane electrode assembly including a fuel electrode catalytic layer and an oxidant electrode catalytic layer placed on either side of an electrolytic membrane with a separator that includes a gas flow path through which fuel gas is supplied to the fuel electrode catalytic layer of the membrane electrode assembly and oxidant gas is supplied to the oxidant electrode catalytic layer of the membrane electrode assembly. The method also comprises decreasing moisture content of the oxidant electrode catalytic layer corresponding to the loading state of the fuel cell stack and the voltage of the plurality of unit cells of the fuel cell stack.
[0010] In another embodiment, a system comprises means for generating electric energy including a fuel electrode catalytic layer and an oxidant electrode catalytic layer formed on an electrolytic membrane, and means for supporting the generating means with a separator that includes a gas flow path through which fuel gas is supplied to the fuel electrode catalytic layer and oxidant gas is supplied to the oxidant electrode catalytic layer. The system also comprises means for decreasing moisture content of the oxidant electrode catalytic layer corresponding to a loading state of the supporting means and a voltage of the supporting means.
[0011] The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.
BRIEF DESCRIPTION OF DRAWINGS
[0012] FIG. 1 illustrates a fuel cell system in accordance with a first embodiment of the invention.
[0013] FIG. 2 is a flowchart illustrating an operation of the first embodiment.
[0014] FIG. 3 is a time chart illustrating an operation of a first case of the first embodiment.
[0015] FIG. 4 is a plot illustrating the method of determining the predetermined value of the moisture content of the oxidant electrode catalytic layer.
[0016] FIG. 5 is a time chart illustrating an operation of a second case of the first embodiment.
[0017] FIG. 6 illustrates a fuel cell system in accordance with a second embodiment of the invention.
[0018] FIG. 7 is a flowchart illustrating an operation of the second embodiment.
[0019] FIG. 8 is a time chart illustrating an operation of a first case of the second embodiment.
[0020] FIG. 9 is a time chart illustrating an operation of a second case of the second embodiment. [0021] FIG. 10 illustrates a fuel cell system in accordance with third embodiment of the invention.
[0022] FIG. 11 is a flowchart illustrating an operation of the third embodiment.
[0023] FIG. 12 is a time chart illustrating an operation of a first case of the third embodiment.
[0024] FIG. 13 is a time chart illustrating an operation of a second case of the third embodiment.
[0025] FIG. 14 is a time chart illustrating an operation of a first case of a fourth embodiment of the invention.
[0026] FIG. 15 is a time chart illustrating an operation of a second case of the fourth embodiment of the invention.
[0027] FIG. 16 is a time chart illustrating an operation of a fifth embodiment of the invention.
[0028] FIG. 17 is a time chart illustrating an operation of a sixth embodiment of the invention.
[0029] FIG. 18 illustrates a fuel cell system of a seventh embodiment of the invention.
[0030] FIG. 19 is a time chart illustrating an operation of a first case of the seventh embodiment of the invention.
[0031] FIG. 20 is a time chart illustrating an operation of a second case of the seventh embodiment of the invention.
[0032] FIG. 21 illustrates a fuel cell system of an eighth embodiment of the invention.
[0033] FIG. 22 is a time chart illustrating an operation of the eighth embodiment of the invention.
DETAILED DESCRIPTION
[0034] FIG. 1 illustrates a fuel cell system in accordance with a first embodiment of the invention. In FIG. 1, the fuel cell system includes a fuel cell stack 1 that may comprise a solid polymer electrolyte-type fuel cell stack that includes catalytic agents such as platinum. The fuel cell system also includes a loading device 2 connected to anode (i.e., fuel electrode) Ia and cathode (i.e., oxidant electrode) Ib of fuel cell stack 1 through conductors 3. Embodiments of the invention will be described herein primarily for use with fuel cell systems that operate as electric power sources for a vehicle. However, the embodiments of the invention are not limited in this respect and may refer to fuel cell systems used for a variety of reasons. [0035] Fuel cell stack 1 includes a membrane electrode assembly with catalytic layers (i.e., a fuel electrode and an oxidant electrode), and a gas diffusion electrode placed on both sides of an electrolytic membrane. Fuel cell stack 1 also includes a plurality of unit cells that support the membrane electrode assembly with a separator equipped with a gas flow path through which fuel gas is supplied to fuel electrode Ia and oxidant gas is supplied to oxidant electrode Ib.
[0036] The fuel cell system has a fuel gas tank 4 that stores high-pressure hydrogen as the fuel gas, a fuel supply path 8 that carries the fuel gas to fuel cell stack 1 through humidifier 7a, which humidifies the fuel gas, a fuel bypass pipe 10 that bypasses humidifier 7a to supply non-humidified fuel gas to fuel cell stack 1, and three-way valves 6a and 6b used to switch between fuel supply path 8 and fuel bypass pipe 10. In addition, the fuel cell system has an oxidant blower 5 that supplies air as the oxidant gas, an oxidant supply path 9 that carries the oxidant gas to fuel cell stack 1 through humidifier 7b, which humidifies the oxidant gas, an oxidant bypass pipe 11 that bypasses humidifier 7b to supply non-humidified fuel gas to fuel cell stack 1, and three-way valves 6c and 6d used to switch between oxidant supply path 9 and oxidant bypass pipe 11. The fuel cell system also includes a fuel flow rate control device 12 that controls the flow rate of the fuel gas supplied from fuel gas tank 4 and an oxidant flow rate control device 13 that controls the flow rate of the oxidant gas supplied from oxidant blower 5. In other embodiments, the fuel cell system may include a humidifier for only of fuel supply path 8 or oxidant supply path 9.
[0037] As shown in FIG. 1, the fuel cell system also includes a controller, which may include a programmable control unit or other device, for performing the functions described herein. Conventionally, when a load of the fuel cell stack decreases, moisture content of the oxidant electrode catalytic layer gradually decreases due to the decrease of the water used at the time of electric power generation. This occurs because the fuel gas flow rate or oxidant gas flow rate decreases at the same time the load decreases. Furthermore, a rate of spontaneous evaporation of the gas that remains in the gas flow path controls the rate of the decrease of the moisture content of the oxidant electrode catalytic layer. Therefore, when the load of the fuel cell stack decreases and the voltage of the plurality of unit cells of the fuel cell stack becomes high (e.g., approximately 0.95 V) the moisture content of the oxidant electrode catalytic layer cannot decrease enough and deterioration of the oxidant electrode catalytic layer is accelerated. [0038] One reason for this deterioration is a decrease in the dimensions where the electrochemical reaction occurs. As the moisture content of the oxidant electrode catalytic layer increases, deterioration of the oxidant electrode catalytic layer is accelerated. The oxidant electrode has a larger potential change than the fuel electrode when the load of the fuel cell is changed and, since the oxidant electrode has a higher electrode potential, the deterioration of the oxidant electrode tends to be more conspicuous. In addition, deterioration of the fuel cell stack may be more advanced when the loading state of the fuel cell stack changes.
[0039] The techniques described herein substantially reduce deterioration of the fuel cell stack by modifying moisture content of an oxidant electrode catalytic layer of the fuel cell stack. When a vehicle is driven, the fuel cell system humidifies the fuel gas or oxidant gas supplied to fuel cell stack 1 to a preset relative humidity by passing the fuel gas and the oxidant gas through humidifiers 7a and 7b, respectively. In the illustrated embodiment, the controller passes at least one of the fuel gas and the oxidant gas through fuel bypass pipe 10 and oxidant bypass pipe 11, respectively, by the switching the three- way valves 6a-6d. In this way, fuel cell stack receives at least one of the fuel gas and the oxidant gas that is not humidified in order to substantially reduce deterioration of the fuel cell stack.
[0040] FIG. 2 is a flowchart illustrating an operation of the first embodiment and FIG. 3 is a time chart illustrating an operation of a first case of the first embodiment. FIG. 3 indicates variations of a load of fuel cell stack 1, a voltage of the plurality of unit cells of fuel cell stack 1, a supply gas flow rate when the fuel gas and the oxidant gas are passed through humidifiers 7a and 7b, a supply gas flow rate when the fuel gas and the oxidant gas are passed through bypass supply paths 10 and 11, and a moisture content of the oxidant electrode catalytic layer. The fuel cell system decreases the moisture content of the oxidant electrode catalytic layer when the loading state of fuel cell stack 1 decreases from a high level to a low level and the voltage of the plurality of unit cells of fuel cell stack 1 exceeds a criteria. The loading state of fuel cell stack 1 is at a high level when the voltage of the plurality of unit cells is approximately 0.75 V (v.s. RHE) or lower. [0041] The voltage of the plurality of unit cells of fuel cell stack 1 may be the voltage of a specific unit cell. Alternatively, a voltage of each of the plurality of the unit cells inside fuel cell stack 1 may be detected and a representative voltage value may be calculated from the plurality of voltage values. For example, the representative voltage value may comprise an average value, a mean value, or a median value. In addition, by dividing the output voltage of fuel cell stack 1 by the number of layers of unit cells, an average voltage of the plurality of unit cells may be obtained.
[0042] In FIG. 2, in step SlO it is determined whether or not the control of the moisture content of the oxidant electrode catalytic layer is started based on the loading state of the fuel cell stack 1 and the voltage of the plurality of unit cells of the fuel cell stack 1. In step SlO, when the loading state of the fuel cell stack 1 decreases to a predetermined loading value or lower (e.g., in the case where the electric potential of the oxidant electrode is 0.90 V or higher) and the voltage of the plurality of unit cells of fuel cell stack 1 increases to a predetermined voltage value or higher, the controller determines that the control of the moisture content should be started and the controller moves on to step S 12. If the controller determines that the control of the moisture control should not be started, no action is taken and the control operation ends. [0043] In step S 12, the fuel gas and the oxidant gas are supplied to fuel cell stack 1 without passing through humidifiers 7a and 7b by switching three-way valves 6a-6d in the direction of bypass pipes 10 and 11. Next, in step S 14, the controller determines whether or not the moisture content of the oxidant electrode catalytic layer has decreased to a predetermined moisture value or lower. If the moisture content has not reached the predetermined value or lower, step S 12 is repeated. If the moisture content has reached the predetermined value or lower, the controller moves on to step S 16. In step S 16, three- way valves 6a-6d are switched in the direction of humidifiers 7a and 7b and the control operation is finished.
[0044] FIG. 4 is a plot illustrating the method of determining the predetermined value of the moisture content of the oxidant electrode catalytic layer. In FIG. 4, the relative humidity of the oxidant electrode during the operation of shifting the loading state of fuel cell stack 1 is used as the x-parameter on the plot. FIG. 4 shows the electrochemical reaction area decreasing rate of the oxidant electrode when the loading state of fuel cell stack 1 changes a predetermined number of times. As the relative humidity of the oxidant electrode increases, the electrochemical reaction area of the oxidant electrode further decreases.
[0045] There is a close relationship between the relative humidity of the supply gas and the moisture content of the catalytic layer. The relationship has a specific predisposition due to the materials (e.g., a high polymer membrane, a catalytic agent, a carrier carbon, and an electrolyte inside the catalytic layer). Furthermore, the relationship has a specific predisposition due to the amount of electrolyte which exists inside the catalytic layer. [0046] Before the moisture content is determined, the plot shown in FIG. 4 may be created to study a maximum relative humidity of the supply gas that does not cause a major decrease in the electrochemical reaction area of the catalytic layer. Then, a desired moisture content of the catalytic layer is calculated based on the relationship between the relative humidity of the supply gas, which is specific to the materials, and the moisture content of the catalytic layer. For example, in order to shift to the desired moisture content of the catalytic layer for a short period of time, it is possible to control the moisture content of the catalytic layer by a relative humidity of the supply gas which is different from that of the above described relationship.
[0047] FIG. 5 is a time chart illustrating an operation of a second case of the first embodiment. In FIG. 5, a case is described where the controller decreases the moisture content of the oxidant electrode catalytic layer to a predetermined moisture value or lower when the voltage of the unit cells exceed a predetermined voltage value and the loading state of fuel cell stack 1 is increased. FIG. 5 indicates variation of each value of a load of fuel cell stack 1, a voltage of the plurality of unit cells of fuel cell stack 1, a supply gas flow rate when the fuel gas and the oxidant gas pass through humidifiers 7a and 7b, a supply gas flow rate when the fuel gas and the oxidant gas pass through bypass supply paths 10 and 11, and a moisture content of the oxidant electrode catalytic layer. [0048] In FIG. 2, step SlO determines whether or not the control of the moisture content of the oxidant electrode catalytic layer is started based on the loading state of the fuel cell stack 1 and the voltage of the plurality of unit cells of the fuel cell stack 1. In step SlO, when the loading state of the fuel cell stack 1 increases to a predetermined loading value or higher and the voltage of the plurality of unit cells of fuel cell stack 1 increases to a predetermined voltage value or higher, the controller determines that the control of the moisture content should be started and the controller moves on to step S12. If the controller determines that the control of the moisture control should not be started, no action is taken and the control operation ends.
[0049] In step S 12, the fuel gas and the oxidant gas are supplied to fuel cell stack 1 without passing through humidifiers 7a and 7b by switching three-way valves 6a-6d in the direction of bypass pipes 10 and 11. When the loading state of fuel cell stack 1 is increased to a predetermined loading value or higher and the voltage of the plurality of unit cells of fuel cell stack 1 is decreased to a predetermined voltage value or lower, controller moves on to step S 16. In step S 16, three-way valves 6a-6d are switched in the direction of humidifiers 7a and 7b and the control operation is finished. [0050] Generally, immediately after the fuel cell stack 1 generates electric power with a high load, the moisture content of the oxidant electrode catalytic layer is high due to the generation of water. However, it is possible to decrease the moisture content of the oxidant electrode catalytic layer to a predetermined moisture value or lower by using the above described control operation.
[0051] In this case, by supplying a low humidified gas to decrease the moisture content of the oxidant electrode catalytic layer to a predetermined moisture value or lower, the controller may prevent deterioration of the catalytic layer when the loading state of fuel cell stack 1 decreases and the electric potential of the oxidant electrode increases. Therefore, the techniques described herein can provide fuel cell stack 1 with high durability that may prevent deterioration of the oxidant electrode catalytic layer caused by the loading cycle.
[0052] FIG. 6 illustrates a fuel cell system in accordance with a second embodiment of the invention. In FIG. 6, the fuel cell system includes of a fuel cell stack 1 that may comprise a solid polymer electrolyte-type fuel cell stack that includes catalytic agents such as platinum. The fuel cell system also includes a loading device 2 connected to anode (i.e., fuel electrode) Ia and cathode (i.e., oxidant electrode) Ib of fuel cell stack 1 through conductors 3.
[0053] Fuel cell stack 1 includes a membrane electrode assembly with catalytic layers (i.e., a fuel electrode and an oxidant electrode), and a gas diffusion electrode placed on both sides of an electrolytic membrane. Fuel cell stack 1 also includes a plurality of unit cells that support the membrane electrode assembly with a separator equipped with a gas flow path through which fuel gas is supplied to fuel electrode Ia and oxidant gas is supplied to oxidant electrode Ib.
[0054] The fuel cell system has a fuel gas tank 4 that stores high-pressure hydrogen as the fuel gas and a fuel supply path 8 that carries the fuel gas to fuel cell stack 1 through humidifier 7a, which humidifies the fuel gas. In addition, the fuel cell system has an oxidant blower 5 that supplies air as the oxidant gas and an oxidant supply path 9 that carries the oxidant gas to fuel cell stack 1 through humidifier 7b, which humidifies the oxidant gas. The fuel cell system also includes a fuel flow rate control device 12 that controls the flow rate of the fuel gas supplied from fuel gas tank 4 and an oxidant flow rate control device 13 that controls the flow rate of the oxidant gas supplied from oxidant blower 5. In other embodiments, the fuel cell system may include a humidifier for only of fuel supply path 8 or oxidant supply path 9. The fuel gas or oxidant gas supplied to fuel cell stack 1 is humidified to a preset relative humidity by passing through humidifiers 7a and 7b, respectively. As shown in FIG. 6, the fuel cell system includes a controller, which may include a programmable control unit or other device, for performing the functions described herein.
[0055] FIG. 7 is a flowchart illustrating an operation of the second embodiment and FIG. 8 is a time chart illustrating an operation of a first case of the second embodiment. FIG. 8 indicates variations of a load of fuel cell stack 1, a voltage of the plurality of unit cells of fuel cell stack 1 , a supply gas flow rate when the fuel gas and the oxidant gas are passed through humidifiers 7a and 7b, and a moisture content of the oxidant electrode catalytic layer. The controller decreases the moisture content of the oxidant electrode catalytic layer when the loading state of fuel cell stack 1 decreases from a high level to a low level and the voltage of the plurality of unit cells of fuel cell stack 1 exceeds a criteria. [0056] In FIG. 7, step S20 determines whether or not the control of the moisture content of the oxidant electrode catalytic layer is started based on the loading state of the fuel cell stack 1 and the voltage of the plurality of unit cells of the fuel cell stack 1. In S20, when the loading state of the fuel cell decreases to a predetermined loading value or lower and the voltage of the plurality of unit cells of fuel cell stack 1 increases to a predetermined value or higher, the controller determines that the control operation of the moisture content should be started and the controller moves on to step S22. If the controller determines that the control of the moisture control should not be started, no action is taken and the control operation ends.
[0057] In step S22, the supply gas flow rate of the fuel gas or the oxidant gas is increased and supplied to fuel cell stack 1. Next, in step S24, the controller determines whether or not the moisture content of the oxidant electrode catalytic layer has decreased to a predetermined moisture value or lower. If the moisture content has not reached the predetermined value or lower, step S22 is repeated. If the moisture content has reached the predetermined value or lower, the control operation is finished. [0058] FIG. 9 is a time chart illustrating an operation of a second case of the second embodiment. In FIG. 9, a case is described where the controller decreases the moisture content of the oxidant electrode catalytic layer to a predetermined moisture value or lower when the voltage of the unit cells exceed a predetermined voltage value and the loading state of fuel cell stack 1 is increased. FIG. 9 indicates variation of each value of a load of fuel cell stack 1, a voltage of the plurality of unit cells of fuel cell stack 1, a supply gas flow rate when the fuel gas and the oxidant gas pass through humidifiers 7a and 7b, a supply gas flow rate when the fuel gas and the oxidant gas pass through bypass supply paths 10 and 11, and a moisture content of the oxidant electrode catalytic layer. [0059] In FIG. 7, step S20 determines whether or not the control of the moisture content of the oxidant electrode catalytic layer is started based on the loading state of the fuel cell stack 1 and the voltage of the plurality of unit cells of the fuel cell stack 1. In step S20, when the loading state of the fuel cell stack 1 increases to a predetermined loading value or higher and the voltage of the plurality of unit cells of fuel cell stack 1 increases to a predetermined voltage value or higher, the controller determines that the control of the moisture content should be started and the controller moves on to step S22. If the controller determines that the control of the moisture control should not be started, no action is taken and the control operation ends.
[0060] In step S22, the supply gas flow rate of the fuel gas or the oxidant gas is increased and supplied to fuel cell stack 1 before the loading state of fuel cell stack 1 increases to a predetermined value or higher and the voltage of the plurality of unit cells of the fuel cell stack 1 increases to a predetermined value or higher. Next, in step S24, the controller determines whether or not the moisture content of the oxidant electrode catalytic layer has decreased to a predetermined moisture value or lower. If the moisture content has not reached the predetermined value or lower, step S22 is repeated. If the moisture content has reached the predetermined value or lower, the control operation is finished. [0061] Generally, immediately after fuel cell stack 1 generates electric power with a high load, the moisture content of the oxidant electrode catalytic layer is high due to the generation of water. However, it is possible to decrease the moisture content of the oxidant electrode catalytic layer to a predetermined moisture value or lower by using the above described control operation.
[0062] In this case, by increasing the supply gas flow rate to fuel cell stack 1 to decrease the moisture content of the oxidant electrode catalytic layer to a predetermined value or lower, the controller may prevent deterioration of the catalytic layer when the loading state of fuel cell stack 1 is decreased and the electric potential of the oxidant electrode increases. Therefore, the techniques described herein may provide fuel cell stack 1 with a high durability that may prevent deterioration of the oxidant electrode catalytic layer caused by the loading cycle.
[0063] Figure 10 illustrates a fuel cell system in accordance with third embodiment of the invention. In FIG. 10, the fuel cell system includes of a fuel cell stack 1 that may comprise a solid polymer electrolyte-type fuel cell stack that includes catalytic agents such as platinum. The fuel cell system also includes a loading device 2 connected to anode (i.e., fuel electrode) Ia and cathode (i.e., oxidant electrode) Ib of fuel cell stack 1 through conductors 3.
[0064] Fuel cell stack 1 includes a membrane electrode assembly with catalytic layers (i.e., a fuel electrode and an oxidant electrode), and a gas diffusion electrode placed on both sides of an electrolytic membrane. Fuel cell stack 1 also includes a plurality of unit cells that support the membrane electrode assembly with a separator equipped with a gas flow path through which fuel gas is supplied to fuel electrode Ia and oxidant gas is supplied to oxidant electrode Ib.
[0065] The fuel cell system has a fuel gas tank 4 that stores high-pressure hydrogen as the fuel gas and a fuel supply path 8 that carries the fuel gas to fuel cell stack 1 through humidifier 7a, which humidifies the fuel gas. In addition, the fuel cell system has an oxidant blower 5 that supplies air as the oxidant gas and an oxidant supply path 9 that carries the oxidant gas to fuel cell stack 1 through humidifier 7b, which humidifies the oxidant gas. The fuel cell system also includes a fuel flow rate control device 12 that controls the flow rate of the fuel gas supplied from fuel gas tank 4 and an oxidant flow rate control device 13 that controls the flow rate of the oxidant gas supplied from oxidant blower 5. In other embodiments, the fuel cell system may include a humidifier for only of fuel supply path 8 or oxidant supply path 9. As shown in FIG. 10, the fuel cell system includes a controller, which may include a programmable control unit or other device, for performing the functions described herein.
[0066] In the illustrated embodiment, the fuel cell system includes a switch 14 that opens and closes a connection between fuel cell stack 1 and an electric storage means 15, e.g., a battery. Electric storage means 15 may store electric power generated by fuel cell stack 1. Switch 14 may be a mechanical relay or a semiconductor switch, such as MOS-FET and IGBT, within increased operation speed, durability and safety. The fuel cell system also includes a demanded load detecting means 16 that detects a demanded load amount of fuel cell stack 1 and a switch control means 17 that controls the opening and closing of switch 14.
[0067] Fuel cell stack 1 couples to electric storage means 15 through switch 14. Switch control means 17 controls switch 14 to store the electric power generated by fuel cell stack 1 in electric storage means 15 based on demanded load detecting means 16. In this case, when demanded load detecting means 16 detects that the demanded load amount changes from a high loading level to a low loading level and the amount of the electric power stored by electric storage means 15 decreases, switch control means 17 closes switch 14 and fuel cell stack 1 stores electric power in electric storage means 15. [0068] When the amount of the electric power in electric storage means 15 reaches a predetermined loading value, switch control means 17 opens switch 14 and extracts any remaining electric power from fuel cell stack 1. When the controller determines that the voltage of the plurality of unit cells of fuel cell stack 1 reaches a predetermined voltage value or higher based on the demanded load amount detected by demanded load detecting means 16, the moisture content of the oxidant electrode catalytic layer decreases to a predetermined moisture value or lower.
[0069] FIG. 11 is a flowchart illustrating an operation of the third embodiment and FIG. 12 is a time chart illustrating an operation of a first case of the third embodiment. FIG. 12 indicates variations of a demanded load amount of fuel cell stack 1, a load of fuel cell stack 1, a voltage of the plurality of unit cells of fuel cell stack 1, an amount of electric power stored by electric storage means 15, a supply gas flow rate when the fuel gas and the oxidant gas are passed through humidifiers 7a and 7b, and a moisture content of the oxidant electrode catalytic layer. The controller decreases the moisture content of the oxidant electrode catalytic layer when the loading state of fuel cell stack 1 decreases from a high level to a low level and the voltage of the plurality of unit cells of fuel cell stack 1 exceeds a criteria.
[0070] In step S30 of Figure 11, the demanded load amount for fuel cell stack 1 is detected by demanded load detecting means 16. In step S32, the controller determines whether or not the control of the moisture content of the oxidant electrode catalytic layer is started based on the loading state of the fuel cell stack 1 and the voltage of the plurality of unit cells of the fuel cell stack 1. In S32, when the loading state of the fuel cell decreases to a predetermined loading value or lower and the voltage of the plurality of unit cells of fuel cell stack 1 increases to a predetermined value or higher, the controller determines that the control operation of the moisture content should be started and the controller moves on to step S34. If the controller determines that the control of the moisture control should not be started, no action is taken and the control operation ends. [0071] In step S34 switch control means 17 closes switch 14 and the electric power generated by fuel cell stack 1 is stored in electric storage means 15. Next, in step S36, the controller determines whether or not the amount of the electric power stored by electric storage means 15 is a predetermined loading value or higher. If it is less than the predetermined loading value, step S34 is repeated and fuel cell stack 1 continues to store the electric power in electric storage means 15. In step S36, if the amount of the electric power reaches the predetermined loading value or higher, the controller moves on to step S38 where the demanded load is detected by demanded load detecting means 16. [0072] Next, in step S40, the fuel control system determines whether or not the voltage of the plurality of unit cells of fuel cell stack 1 is a predetermined voltage value or higher. In step S40, if the voltage of the plurality of unit cells reaches the predetermined voltage value or higher, the controller moves on to step S42. In step S42 the moisture content of the oxidant electrode catalytic layer is decreased to a predetermined moisture value or lower. In step S44, switch control means 17 opens switch 14 and the operation of storing the electric power in electric storage means 15 is finished and the control operation is finished.
[0073] FIG. 13 is a time chart illustrating an operation of a second case of the third embodiment. In FIG. 13, a case is described where the controller decreases the moisture content of the oxidant electrode catalytic layer to a predetermined moisture value or lower when the voltage of the unit cells exceed a predetermined voltage value and the loading state of fuel cell stack 1 is increased. FIG. 13 indicates variations of a demanded load amount of fuel cell stack 1, a load of fuel cell stack 1, a voltage of the plurality of unit cells of fuel cell stack 1, an amount of electric power stored by electric storage means 15, a supply gas flow rate when the fuel gas and the oxidant gas are passed through humidifiers 7a and 7b, and a moisture content of the oxidant electrode catalytic layer. [0074] When it is detected by demanded load detecting means 16 that a demand for increasing the load from a low level (e.g., a zero loading state) to a high level and the amount of electric power stored in electric storage means 15 is a predetermined loading value or lower, the moisture content of the oxidant electrode catalytic layer decreases to a predetermined moisture value or lower. Switch control means 17 closes switch 14 and fuel cell stack 1 generates electric power with a predetermined high load regardless of the demanded load. Fuel cell stack 1 then stores the difference of the generated electric power and the demanded load in electric storage means 15. When the amount of electric power stored in electric storage means 15 reaches a predetermined high loading value, switch control means 17 opens switch 14 thereby ending extraction of the electric power from fuel cell stack 1.
[0075] In step S30 of Figure 11, the demanded load amount for fuel cell stack 1 is detected by demanded load detecting means 16. In step S32, the controller determines whether or not the control of the moisture content of the oxidant electrode catalytic layer is started based on the loading state of the fuel cell stack 1 and the voltage of the plurality of unit cells of the fuel cell stack 1. In S32, when the loading state of the fuel cell increases to a predetermined loading value or higher and the voltage of the plurality of unit cells of fuel cell stack 1 increases to a predetermined value or higher, the controller determines that the control operation of the moisture content should be started and the controller moves on to step S34. If the controller determines that the control of the moisture control should not be started, no action is taken and the control operation ends. [0076] In step S34 switch control means 17 closes switch 14 and the electric power generated by fuel cell stack 1 is stored in electric storage means 15. Next, in step S36, the controller determines whether or not the amount of the electric power stored by electric storage means 15 is a predetermined loading value or higher. If it is less than the predetermined loading value, step S34 is repeated and fuel cell stack 1 continues to store the electric power in electric storage means 15. In step S36, if the amount of the electric power reaches the predetermined loading value or higher, the controller moves on to step S38 where the demanded load is detected by demanded load detecting means 16. [0077] Next, in step S40, the fuel control system determines whether or not the voltage of the plurality of unit cells of fuel cell stack 1 is a predetermined voltage value or higher. In step S40, if the voltage of the plurality of unit cells reaches the predetermined voltage value or higher, the controller moves on to step S42. In step S42 the moisture content of the oxidant electrode catalytic layer is decreased to a predetermined moisture value or lower. In step S44, switch control means 17 opens switch 14 and the operation of storing the electric power in electric storage means 15 is finished and the control operation is finished.
[0078] In this way, electric storage means 15 enables the controller to determine a starting time for decreasing the moisture content of the oxidant electrode catalytic layer based on the amount of electric power stored in the electric storage means. In addition, electric storage means 15 also enables fuel cell stack 1 to store the difference between the demanded load and the amount of the generated electric power. Therefore, the techniques described herein may provide fuel cell stack 1 with a high durability that may prevent deterioration of the oxidant electrode catalytic layer caused by the loading cycle. [0079] FIG. 14 is a time chart illustrating an operation of a first case of a fourth embodiment of the invention. The fourth embodiment of the invention may be applied to any of the fuel cell systems from FIGS. 1, 6, and 10. FIG. 14 indicates variations of a load of fuel cell stack 1, a voltage of the plurality of unit cells of fuel cell stack 1, a supply gas flow rate when the fuel gas and the oxidant gas are passed through humidifiers 7a and 7b, and a moisture content of the oxidant electrode catalytic layer. The controller decreases the moisture content of the oxidant electrode catalytic layer when the loading state of fuel cell stack 1 decreases from a high level to a low level and the voltage of the plurality of unit cells of fuel cell stack 1 exceeds a criteria.
[0080] When the load of fuel cell stack 1 decreases, as shown in the hatching portion (i.e., time tl to t2) of FIG. 14, fuel flow rate control device 12 and oxidant flow rate control device 13 supply the fuel gas and the oxidant gas, respectively, to fuel cell stack 1 at a continuous flow rate for a predetermined period of time. The predetermined period of time expires when the moisture content of the oxidant electrode catalytic layer reaches a predetermined moisture value or lower. Generally, immediately after fuel cell stack 1 generates the electric power with a high load, the moisture content of the oxidant electrode catalytic layer is high due to the generating water. However, it is possible to decrease the moisture content of the oxidant electrode catalytic layer to a predetermined value or lower by using the above described control operation. In this case, both the fuel gas and oxidant gas may be supplied at a continuous flow rate to fuel cell stack 1. In other cases, only one of the fuel gas and the oxidant gas may be supplied to fuel cell stack 1 at a continuous flow rate.
[0081] FIG. 15 is a time chart illustrating an operation of a second case of the fourth embodiment of the invention. The fourth embodiment of the invention may be applied to any of the fuel cell systems from FIGS. 1, 6, and 10. FIG. 15 indicates variations of a load of fuel cell stack 1, a voltage of the plurality of unit cells of fuel cell stack 1, a supply gas flow rate when the fuel gas and the oxidant gas are passed through humidifiers 7a and 7b, and a moisture content of the oxidant electrode catalytic layer. The controller decreases the moisture content of the oxidant electrode catalytic layer when the loading state of fuel cell stack 1 increases from a low level to a high level and the voltage of the plurality of unit cells of fuel cell stack 1 exceeds a criteria.
[0082] When the load of fuel cell stack 1 increases, as shown in the hatching portion (i.e., time tl to t2) of FIG. 15, fuel flow rate control device 12 and oxidant flow rate control device 13 supply the fuel gas and the oxidant gas, respectively, to fuel cell stack 1 at a continuous flow rate for a predetermined period of time. The predetermined period of time expires when the moisture content of the oxidant electrode catalytic layer reaches a predetermined moisture value or lower. In this case, both the fuel gas and oxidant gas may be supplied at a continuous flow rate to fuel cell stack 1. In other cases, only one of the fuel gas and the oxidant gas may be supplied to fuel cell stack 1 at a continuous flow rate.
[0083] In the illustrated embodiment, when the load of fuel cell stack 1 is increased, it is possible to decrease the moisture content of the oxidant electrode catalytic layer to a predetermined moisture value or lower without any special devices by supplying at least one of the fuel gas and the oxidant gas to fuel cell stack 1 at a continuous flow rate. Therefore, the techniques may provide fuel cell stack 1 with a high durability that may prevent deterioration of the oxidant electrode catalytic layer caused by the loading cycle. [0084] FIG. 16 is a time chart illustrating an operation of a fifth embodiment of the invention. The fifth embodiment of the invention may be applied to the fuel cell system from FIG. 1. FIG. 16 indicates variations of a load of fuel cell stack 1, a voltage of the plurality of unit cells of fuel cell stack 1, a supply gas flow rate when the fuel gas and the oxidant gas are passed through humidifiers 7a and 7b, a supply gas flow rate when the fuel gas and the oxidant gas pass through fuel bypass path 10 and oxidant bypass path 11, an amount of water supply of fuel cell stack 1, and a moisture content of the oxidant electrode catalytic layer. The controller decreases the moisture content of the oxidant electrode catalytic layer when the loading state of fuel cell stack 1 decreases from a high level to a low level and the voltage of the plurality of unit cells of fuel cell stack 1 exceeds a criteria.
[0085] In the illustrated embodiment, after the moisture content of the oxidant electrode catalytic layer decreases to a predetermined moisture value or lower, the voltage of the plurality of unit cells of fuel cell stack 1 is high, e.g., approximately 0.95 V, and the moisture content of the catalytic layer reaches a first predetermined value or lower. When the moisture content of the catalytic layer remains at the first predetermined value for a predetermined period of time, a high-humidified gas or water may be supplied to the fuel electrode until the moisture content of the oxidant electrode catalytic layer increases from the first predetermined value to a second predetermined value. The high-humidified gas or water may be supplied to either the oxidant electrode alone or both the fuel electrode and the oxidant electrode. The second predetermined value may be a certain moisture content, or more preferably, when fuel cell stack 1 becomes saturated with the high-humidity gas, taking into consideration the deterioration that occurs when fuel cell stack 1 is kept at a high voltage for a long period of time.
[0086] In this case, after the control operation of decreasing the moisture content of the oxidant electrode catalytic layer, it is possible to prevent deterioration of the catalytic layer or the electrolytic membrane by increasing the moisture content of the oxidant electrode catalytic layer to a predetermined value or higher. Therefore, the techniques may prevent deterioration of the catalytic layer or the electrolytic membrane caused by the continuation of a state of low moisture content in the oxidant electrode catalytic layer. [0087] FIG. 17 is a time chart illustrating an operation of a sixth embodiment of the invention. The sixth embodiment of the invention may be applied to the fuel cell system from FIG. 10. FIG. 17 indicates variations of a demanded load amount of fuel cell stack 1, a load of fuel cell stack 1, a voltage of the plurality of unit cells of fuel cell stack 1, an amount of the electric power stored by electric storage means 15, a supply gas flow rate when the fuel gas and the oxidant gas are passed through humidifiers 7a and 7b, and a moisture content of the oxidant electrode catalytic layer. The controller decreases the moisture content of the oxidant electrode catalytic layer when the loading state of fuel cell stack 1 decreases from a high level to a low level and the voltage of the plurality of unit cells of fuel cell stack 1 exceeds a criteria.
[0088] In the illustrated embodiment, after the control operation of decreasing the moisture content of the oxidant electrode catalytic layer to a predetermined moisture value or lower, the voltage of the plurality of unit cells of fuel cell stack 1 is high, e.g., approximately 0.95 V, and the moisture content of the catalytic layer reaches a first predetermined value or lower. When the moisture content of the catalytic layer remains at the first predetermined value for a predetermined period of time, fuel cell stack 1 continues generating the electric power with the high load until the moisture content increases from the first predetermined value to a second predetermined value. [0089] In this case, after the control operation of decreasing the moisture content of the oxidant electrode catalytic layer, it is possible to prevent deterioration of the catalytic layer or the electrolytic membrane by increasing the moisture content of the oxidant electrode catalytic layer to a predetermined value or higher. As a result, the techniques described herein may prevent deterioration of the catalytic layer or the electrolytic membrane caused by the continuation of a state of low moisture content in the oxidant electrode catalytic layer
[0090] FIG. 18 illustrates a fuel cell system of a seventh embodiment of the invention. The fuel cell system illustrated in FIG. 18 is substantially similar to the fuel cell system from FIG. 6. In the illustrated embodiment, the fuel cell system includes a load changing time detecting means 18. As shown in FIG. 18, the fuel cell system includes a controller, which may include a programmable control unit or other device, for performing the functions described herein.
[0091] FIG. 19 is a time chart illustrating an operation of a first case of the seventh embodiment of the invention. FIG. 19 indicates variations of a load of fuel cell stack 1, a voltage of the plurality of unit cells of fuel cell stack 1, a supply gas flow rate when the fuel gas and the oxidant gas are passed through humidifiers 7a and 7b, and a moisture content of the oxidant electrode catalytic layer. The controller decreases the moisture content of the oxidant electrode catalytic layer when the loading state of fuel cell stack 1 decreases from a high level to a low level and the voltage of the plurality of unit cells of fuel cell stack 1 exceeds a criteria. As shown in Figure 19, load changing time detecting means 18 calculates a time derivative value D, D = dL/dt where dL is time for decreasing the load and dt is time.
[0092] When the value of D is a predetermined time value or lower, i.e., when the time for decreasing the load is shorter than a predetermined time, and when the load of fuel cell stack 1 decreases and the voltage of the plurality of unit cells of the fuel cell stack 1 reaches a predetermined voltage value or higher, the moisture content of the oxidant electrode catalytic layer decreases to a predetermined moisture value or lower. [0093] FIG. 20 is a time chart illustrating an operation of a second case of the seventh embodiment of the invention. FIG. 20 indicates variations of a load of fuel cell stack 1, a voltage of the plurality of unit cells of fuel cell stack 1, a supply gas flow rate when the fuel gas and the oxidant gas are passed through humidifiers 7a and 7b, and a moisture content of the oxidant electrode catalytic layer. The controller decreases the moisture content of the oxidant electrode catalytic layer when the loading state of fuel cell stack 1 increases from a low level to a high level and the voltage of the plurality of unit cells of fuel cell stack 1 exceeds a criteria.
[0094] As shown in FIG. 20, load changing time detecting means 18 calculates a value of D. When the value of D is small, i.e., when the load increasing time is shorter than a predetermined time, and when the loading state of fuel cell stack 1 increases from a low level to a high level and the voltage of the plurality of unit cells of fuel cell stack 1 reaches a predetermined value or higher, the moisture content of the oxidant electrode catalytic layer decreases to a predetermined moisture value or lower. [0095] When the loading state of fuel cell stack 1 increases from a low level to a high level, if the moisture content of the oxidant electrode catalytic layer is a predetermined value or higher, fuel flow rate control device 12 and oxidant flow rate control device 13 temporarily supply the fuel gas and the oxidant gas to fuel cell stack 1 at a flow rate of the high loading state during the low loading state. In this way, the techniques may instantaneously decrease the moisture content of the oxidant electrode catalytic layer. [0096] In the illustrated embodiment, as the time in which the load of fuel cell stack 1 decreases from a high level to a low level shortens, the momentary oxidized electric current value increases when the oxidant electrode potential becomes high. Therefore, the techniques described herein may control the moisture content of the oxidant electrode catalytic layer only when deterioration of the catalytic layer is more likely to occur. [0097] FIG. 21 illustrates a fuel cell system of an eighth embodiment of the invention. The fuel cell system illustrated in FIG. 21 is substantially similar to the fuel cell system from FIG. 6. In the illustrated embodiment, the fuel cell system includes a cell resistance value detecting means 19. As shown in FIG. 21, the fuel cell system includes a controller, which may include a programmable control unit or other device, for performing the functions described herein.
[0098] FIG. 22 is a time chart illustrating an operation of the eighth embodiment of the invention. FIG. 22 indicates variations of a load of fuel cell stack 1, a voltage of the plurality of unit cells of fuel cell stack 1, a supply gas flow rate when the fuel gas and the oxidant gas are passed through humidifiers 7a and 7b, a cell resistance value of the fuel cell stack 1, and a moisture content of the oxidant electrode catalytic layer. The controller decreases the moisture content of the oxidant electrode catalytic layer when the loading state of fuel cell stack 1 decreases from a high level to a low level and the voltage of the plurality of unit cells of fuel cell stack 1 exceeds a criteria. [0099] In the illustrated embodiment, the cell resistance value detected by cell resistance value detecting means 19 is used as the criteria for the moisture content of the oxidant electrode catalytic layer by empirically investigating the correlation between the change in the moisture content of the oxidant electrode catalytic layer and the change in the cell resistance value. The cell resistance value may be a representation of a certain unit cell or a cell resistance value in a part of fuel cell stack 1. Cell resistance value detecting means 19 may comprise an alternating current ohm meter that applies alternating voltage between the anode and the cathode of a certain unit cell of fuel cell stack 1 to obtain the cell resistance.
[0100] In this case, it is possible to determine the control time for controlling the moisture content of the oxidant electrode catalytic layer based on the cell resistance value detected by cell resistance value detecting means 19. Therefore, the techniques described herein may prevent deterioration of the catalytic layer or the electrolytic membrane by using the method described above.
[0101] Various embodiments of the invention have been described. However, the present disclosure is not limited to the embodiments described herein. These and other embodiments are within the scope of the following claims. Embodiments including modifications or changes are applicable to the extent of operation and description of the disclosure.

Claims

CLAIMS:
1. A fuel cell system comprising: a membrane electrode assembly including a fuel electrode catalytic layer and an oxidant electrode catalytic layer placed on a side of an electrolytic membrane, a fuel cell stack including a plurality of cells that support the membrane electrode assembly with a separator that includes a gas flow path through which fuel gas is supplied to the fuel electrode catalytic layer and oxidant gas is supplied to the oxidant electrode catalytic layer; and a controller that decreases moisture content of the oxidant electrode catalytic layer corresponding to a loading state of the fuel cell stack.
2. The fuel cell system of claim 1, wherein the controller decreases the moisture content of the oxidant electrode catalytic layer to at least a predetermined moisture value in response to detecting that the loading state of the fuel cell stack decreases to at least a predetermined loading value and that a voltage of the plurality of unit cells of the fuel cell stack increases to at least a predetermined voltage value.
3. The fuel cell system of claim 1, wherein the controller decreases the moisture content of the oxidant electrode catalytic layer to at least a predetermined moisture value in response to detecting that the loading state of the fuel cell stack increases to at least a predetermined loading value and that a voltage of the plurality of unit cells of the fuel cell stack increases to at least a predetermined voltage value.
4. The fuel cell system of claim 1, wherein the controller decreases the moisture content of the oxidant electrode catalytic layer by supplying a low-humidity gas to at least one of the fuel electrode catalytic layer and the oxidant electrode catalytic layer.
5. The fuel cell system of claim 4, further comprising at least one humidifier along the gas flow path and a gas bypass path that avoids the at least one humidifier, wherein the low-humidity gas is supplied to the at least one of the fuel electrode catalytic layer and the oxidant electrode catalytic layer through the gas bypass path.
6. The fuel cell system of claim 5, wherein the gas flow path comprises a fuel supply path and an oxidant supply path, and wherein the gas bypass flow path comprises a fuel bypass path and an oxidant bypass path.
7. The fuel cell system of claim 1, further comprising: an electric storage means that stores electric power generated by the fuel cell stack; and a demanded load detecting means that detects a demanded load amount in the fuel cell stack, wherein, when the demanded load amount decreases to at least a predetermined loading value, the fuel cell stack generates electric power that exceeds the predetermined loading value and stores the electric power with the electric storage means.
8. The fuel cell system of claim 7, wherein the controller decreases the moisture content of the oxidant electrode catalytic layer to at least a predetermined moisture value when the electric power stored by the electric storage means reaches the predetermined loading value and the voltage of the plurality of unit cells of the fuel cell stack increases to at least a predetermined voltage value based on the demanded load amount.
9. The fuel cell system of claim 8, wherein the controller continues to decrease the moisture content of the oxidant electrode catalytic layer when the demanded load detecting means detects that the demanded load amount increases to at least the predetermined loading value before the electric power stored by the electric storage means reaches the predetermined voltage value and the moisture content of the oxidant electrode catalytic layer decreases to at least the predetermined moisture level.
10. The fuel cell system of claim 1, wherein the controller decreases the moisture content of the oxidant electrode catalytic layer to at least a predetermined moisture value when a gas flow rate through the gas flow path to at least one of the fuel electrode catalytic layer and the oxidant electrode catalytic layer increases.
11. The fuel cell system of claim 1, further comprising a supply gas flow rate control means that supplies a gas flow through the gas flow path to at least one of the fuel electrode catalytic layer and the oxidant electrode catalytic layer at a gas flow rate determined based on a load of the fuel cell stack.
12. The fuel cell system of claim 11, wherein the controller decreases the moisture content of the oxidant electrode catalytic layer to at least a predetermined moisture value when the supply gas flow rate control means supplies the gas flow at a continuous gas flow rate for a predetermined period of time.
13. The fuel cell system of claim 1, wherein the loading state of the fuel cell stack reaches a high level when a voltage of the plurality of unit cells of the fuel cell stack is approximately 0.75 volts or lower.
14. The fuel cell system of claim 1 , wherein the controller increases the moisture content of the oxidant electrode catalytic layer from a first predetermined moisture value to a second predetermined moisture value when the moisture content of the oxidant electrode catalytic layer reaches the first predetermined moisture value and the voltage of the plurality unit cells of the fuel cell stack remains high for a predetermined period of time.
15. The fuel cell system of claim 1, further comprising a load changing time detecting means that detects a changing time when a predetermined loading value decreases, wherein the controller decreases the moisture content of the oxidant electrode catalytic layer to at least a predetermined moisture value when the changing time detected by the load changing time detecting means reaches a predetermined time value.
16. The fuel cell system of claim 1, further comprising a load changing time detecting means that detects a changing time when a predetermined loading value increases, wherein the controller decreases the moisture content of the oxidant electrode catalytic layer to at least a predetermined moisture value when the changing time detected by the load changing time detecting means reaches a predetermined time value.
17. The fuel cell system of claim 1 , further comprising a cell resistance value detecting means that detects a resistance value of the plurality of unit cells of the fuel cell stack, wherein the controller ceases decreasing the moisture content of the oxidant electrode catalytic layer when the resistance value reaches a predetermined resistance value or higher.
18. A method comprising: detecting a loading state of a fuel cell stack that includes a plurality of unit cells; detecting a voltage of the plurality of unit cells of the fuel cell stack, wherein the plurality of unit cells support a membrane electrode assembly including a fuel electrode catalytic layer and an oxidant electrode catalytic layer placed on either side of an electrolytic membrane with a separator that includes a gas flow path through which fuel gas is supplied to the fuel electrode catalytic layer of the membrane electrode assembly and oxidant gas is supplied to the oxidant electrode catalytic layer of the membrane electrode assembly; and decreasing moisture content of the oxidant electrode catalytic layer corresponding to a loading state of the fuel cell stack.
19. The method of claim 18, wherein decreasing the moisture content of the oxidant electrode catalytic layer comprises decreasing the moisture content to at least a predetermined moisture value in response to detecting that the loading state of the fuel cell stack decreases to at least a predetermined loading value and that a voltage of the plurality of unit cells of the fuel cell stack increases to at least a predetermined voltage value.
20. The method of claim 18, wherein decreasing the moisture content of the oxidant electrode catalytic layer comprises decreasing the moisture content to at least a predetermined moisture value in response to detecting that the loading state of the fuel cell stack increases to at least a predetermined loading value and that a voltage of the plurality of unit cells of the fuel cell stack increases to at least a predetermined voltage value.
21. The method of claim 18, wherein decreasing the moisture content of the oxidant electrode catalytic layer comprises supplying a low-humidity gas to at least one of the fuel electrode catalytic layer and the oxidant electrode catalytic layer.
22. The method of claim 21, wherein the gas flow path includes at least one humidifier and a gas bypass path avoids the at least one humidifier, wherein supplying the low-humidity gas comprises supplying the low-humidity gas through the gas bypass path.
23. The method of claim 18, further comprising: storing electric power generated by the fuel cell stack with an electric storage means; detecting a demanded load amount in the fuel cell stack with a demanded load detecting means; and when the demanded load amount decreases to at least a predetermined loading value, generating electric power with the fuel cell stack that exceeds the predetermined loading value.
24. The method of claim 23, wherein decreasing the moisture content of the oxidant electrode catalytic layer comprises decreasing the moisture content to at least a predetermined moisture value when the electric power stored by the electric storage means reaches the predetermined loading value and the voltage of the plurality of unit cells of the fuel cell stack increases to at least a predetermined voltage value based on the demanded load amount.
25. The method of claim 24, further comprising continuing to decrease the moisture content of the oxidant electrode catalytic layer when the demanded load amount increases to at least the predetermined loading value before the electric power stored by the electric storage means reaches the predetermined voltage value and the moisture content of the oxidant electrode catalytic layer decreases to at least the predetermined moisture level.
26. The method of claim 18, wherein decreasing the moisture content of the oxidant electrode catalytic layer comprises decreasing the moisture content to at least a predetermined moisture value when a gas flow rate through the gas flow path to at least one of the fuel electrode catalytic layer and the oxidant electrode catalytic layer increases.
27. The method of claim 18, further comprising supplying a gas flow through the gas flow path with supply gas flow rate control means to at least one of the fuel electrode catalytic layer and the oxidant electrode catalytic layer at a gas flow rate determined based on a load of the fuel cell stack.
28. The method of claim 27, wherein decreasing the moisture content of the oxidant electrode catalytic layer comprises supplying the gas flow with the supply gas flow rate control means at a continuous gas flow rate for a predetermined period of time.
29. The method of claim 18, further comprising increasing the moisture content of the oxidant electrode catalytic layer from a first predetermined moisture value to a second predetermined moisture value when the moisture content of the oxidant electrode catalytic layer reaches the first predetermined moisture value and the voltage of the plurality of unit cells of the fuel cell stack remains high for a predetermined period of time.
30. The method of claim 18, further comprising detecting a changing time with a load changing time detecting means when a predetermined loading value decreases, wherein decreasing the moisture content of the oxidant electrode catalytic layer comprises decreasing the moisture content to at least a predetermined moisture value when the changing time detected by the load changing time detecting means reaches a predetermined time value.
31. The method of claim 18, further comprising detecting a changing time with a load changing time detecting means when a predetermined loading value increases, wherein decreasing the moisture content of the oxidant electrode catalytic layer comprises decreasing the moisture content to at least a predetermined moisture value when the changing time detected by the load changing time detecting means reaches a predetermined time value or lower.
32. The method of claim 18, further comprising: detecting a resistance value of the plurality of unit cells of the fuel cell stack with a cell resistance value detecting means; and ceasing to decrease the moisture content of the oxidant electrode catalytic layer when the resistance value reaches a predetermined resistance value or higher.
33. A system comprising: means for generating electric energy including a fuel electrode catalytic layer and an oxidant electrode catalytic layer formed on an electrolytic membrane; means for supporting the generating means with a separator that includes a gas flow path through which fuel gas is supplied to the fuel electrode catalytic layer and oxidant gas is supplied to the oxidant electrode catalytic layer; and means for decreasing moisture content of the oxidant electrode catalytic layer corresponding to a loading state of the supporting means.
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