JP2008262822A - Starting and stopping method of fuel cell, and starting and stopping system of fuel cell - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method as well as a system for starting and stopping a fuel cell capable of suppressing the closure of fine pores of a catalyst layer which is apt to occur at the starting of the fuel cell in a cold environment, and moreover, capable of shortening the scavenging time in a fuel battery cell prepared for starting in the cold environment. <P>SOLUTION: This is the method for starting and stopping the fuel cell equipped with an electrolyte membrane, a membrane electrode assembly including a catalyst layer and a gas diffusion layer from the electrolyte membrane side and having a pair of electrodes installed by pinching the electrolyte membrane, and a compression mechanism. The gas diffusion layer is what to have a higher rigidity than that of the electrolyte membrane, and the method for starting and stopping the fuel cell includes the following processes: (1) a process for compressing the electrolyte membrane in the laminating direction of the membrane electrode assembly by the compression mechanism when operation of the fuel cell is stopped, (2) a process for maintaining compression of the electrolyte membrane with a thickness smaller than that of the swollen size of the electrolyte membrane until the next starting time, and (3) a process for eliminating the pressure of the electrolyte membrane by the compression mechanism at the starting of the fuel cell. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a method for starting and stopping a fuel cell, and a system used for starting and stopping a fuel cell, and more particularly to a technique for removing water generated when the fuel cell is started.

  A fuel cell directly converts chemical energy into electrical energy by supplying fuel and an oxidant to two electrically connected electrodes and causing the fuel to be oxidized electrochemically. Unlike thermal power generation, fuel cells are not subject to the Carnot cycle and thus exhibit high energy conversion efficiency. Solid polymer electrolyte fuel cells using a solid polymer electrolyte as an electrolyte have advantages such as easy miniaturization and operation at a low temperature. Has been.

A solid polymer electrolyte fuel cell includes a membrane electrode assembly (MEA) in which electrodes are stacked on both surfaces of an electrolyte membrane. A pair of electrodes (that is, a fuel electrode and an oxidizer electrode) provided on both surfaces of the electrolyte membrane generally have a structure in which a catalyst layer and a gas diffusion layer are laminated in order from the electrolyte membrane side. Many.
The catalyst layer usually includes a catalyst component that promotes a reaction in each electrode, a polymer electrolyte that forms a proton conduction path, and a conductive material that forms an electron conduction path, and is a portion that serves as a field for each electrode reaction. As the polymer electrolyte contained in the catalyst layer, the same polymer electrolyte as that for forming the electrolyte membrane is often used.
On the other hand, the gas diffusion layer is made of a conductive porous body and is provided for the purpose of improving the diffusion of the reaction gas into the catalyst layer and the electronic conductivity.

In the solid polymer electrolyte fuel cell, when hydrogen is used as the fuel, the reaction of the formula (1) proceeds at the anode (fuel electrode).
H ₂ → 2H ⁺ + 2e ⁻ (1)
The electrons generated by the equation (1) reach the cathode after working with an external load via an external circuit. Then, the proton generated in the formula (1) moves by electroosmosis from the anode side to the cathode side in the solid polymer electrolyte membrane in a state of being hydrated with water. For this reason, the solid polymer electrolyte membrane is generally humidified in order to improve ionic conductivity.

Further, when oxygen is used as the oxidizing agent, the reaction of the formula (2) proceeds at the cathode (oxidant electrode).
2H ⁺ + (1/2) O ₂ + 2e ⁻ → H ₂ O (2)
The water produced at the cathode mainly passes through the gas diffusion layer and is discharged to the outside.

For electric vehicle applications, the fuel cell is required to generate electrical energy immediately after startup, that is, to have good startability (startability).
However, as described above, since a power generation system using a fuel cell always includes water, in a cold environment where the ambient temperature is below freezing, water generated by power generation when the fuel cell is stopped The humidified water may freeze, or the water generated by the restart when the fuel cell is restarted may freeze in the cathode. As a result of such freezing of water, it is difficult to obtain good startability, and there are concerns about physical failure of separators, electrodes, etc., clogging of gas flow paths, and start-up failure due to poor gas diffusion.
It is inevitable that some moisture remains in various places in the fuel cell and partially freezes in the cold environment, but the startability of the fuel cell in the cold environment (hereinafter, “ It is desired to improve the efficiency of the fuel cell by improving “low temperature startability (low temperature startability)”.

Conventionally, in order to ensure the low temperature startability of the fuel cell, it has been proposed to stop the operation of the fuel cell after scavenging at a high temperature of 60 to 80 ° C. For example, Patent Document 1 includes a humidifier that humidifies a reaction gas supplied to the fuel cell and a dehumidifier that dehumidifies the reaction gas supplied to the fuel cell, and provides a supply path for the reaction gas when the fuel cell is stopped. A fuel cell gas supply device provided with a gas flow path switching means for switching to the dehumidifier side is disclosed, and the inside of the fuel cell is appropriately dried in a short time by flowing the dehumidified gas when the fuel cell is stopped. I can do it.
In Patent Document 2, when the fuel cell is stopped, the supply of the fuel gas to the anode is cut off, the scavenging gas is supplied to the anode electrode, and the scavenging gas is supplied to the cathode electrode via a humidifier. Discloses a fuel cell system provided with a control means for controlling the scavenging gas supply means, and can be dried while preventing the occurrence of uneven humidity in the electrode.
However, residual moisture in the fuel cell is particularly problematic during low-temperature start-up, with the catalyst layer being a thin film and an electrolyte membrane adjacent to the catalyst layer, and the catalyst layer and the electrolyte membrane adjacent to the catalyst layer. When the water remaining in the catalyst freezes, the gas permeation path in the catalyst layer is blocked, making it difficult to start at a low temperature. On the other hand, there are few problems because a space in which the reaction gas can be sufficiently circulated remains even if moisture remaining partially in the gas flow path or the porous gas diffusion layer is frozen.
In Patent Document 1 and Patent Document 2, scavenging of the entire fuel cell is performed in order to reduce the residual moisture content. However, even if gas is supplied to the fuel cell and dried to a predetermined humidity, After the drying is finished at a low ambient temperature, the fuel cell is exposed to low temperature (naturally left) and left to cool, or even if the ambient temperature at the end of drying is not low, When the temperature of the fuel cell decreases, such as when the temperature cools down at night, the relative humidity in the cell rises, and the water in the cell once vaporized by drying condenses. It is easily absorbed by the membrane. Furthermore, when the fuel cell is started in a state where the ambient temperature of the fuel cell is lowered, water is generated in the catalyst layer by an electrochemical reaction. However, the electrolyte membrane has already absorbed water to a considerable extent, Since the ability to sufficiently absorb the water generated by the electrochemical reaction is insufficient, the generated water freezes at the catalyst layer and the electrolyte membrane adjacent to the catalyst layer, hinders the gas flow in the catalyst layer, and low temperature startability. Is damaged.
Conventionally, the inside of the fuel cell has been sufficiently dried over time in anticipation that the electrolyte membrane absorbs moisture generated by dew condensation when the operation is stopped in such a cold environment. When this is mounted on an electric vehicle, it takes a long time to stop the electric vehicle.

  On the other hand, Patent Document 3 includes one or more fuel cells for the purpose of providing a fuel cell system capable of efficiently suppressing a deterioration reaction of an oxidant electrode such as a partial cell reaction, and further includes at least a gas diffusion layer as a membrane electrode. A fuel cell system is disclosed that includes a compression unit that compresses the joined body in the stacking direction, and a control unit that controls compression and decompression by the compression unit. According to this system, the gas diffusion layer can be compressed by the compression means to reduce the amount of gas contained in the gas diffusion layer, and the contact between the catalyst and the gas can be suppressed. Although it is possible to suppress a reaction between the counter electrodes when a region where hydrogen is present and a region where oxygen is present in the diffusion layer, a cloth-like material made of carbon fiber is used for the gas diffusion layer. I use it. Since carbon cloth and carbon paper are less rigid than the electrolyte membranes usually used, when pressure is applied from the outside, the gas diffusion layer with low rigidity is compressed and the electrolyte has higher rigidity than the gas diffusion layer. The amount of compression and moisture content of the membrane cannot be controlled.

JP 2002-313394 A JP-A-2005-251441 JP 2005-166596 A

  The present invention has been accomplished in view of the above circumstances, and can prevent clogging of the pores of the catalyst layer, which is likely to occur at the start of the fuel cell in a cold environment, and prepare for the start-up in the cold environment. It is an object of the present invention to provide a fuel cell start and stop method and a fuel cell start and stop system that can reduce the scavenging time in the fuel cell when the operation is stopped.

To achieve the above object, the present invention provides an electrolyte membrane, and a membrane electrode assembly including a catalyst layer and a gas diffusion layer from the electrolyte membrane side and having a pair of electrodes provided with the electrolyte membrane interposed therebetween, And a method of starting and stopping a fuel cell including a compression mechanism, wherein the gas diffusion layer is a gas diffusion layer having higher rigidity than the electrolyte membrane, and includes the following steps. provide.
(1) a step of compressing the electrolyte membrane in the stacking direction of the membrane electrode assembly by the compression mechanism when the operation of the fuel cell is stopped;
(2) maintaining the compression of the electrolyte membrane at a thickness smaller than the swelling dimension thickness of the electrolyte membrane until the next startup, and (3) at the startup of the fuel cell, the compression mechanism A process of releasing pressure.

  In the present invention, in order to further shorten the scavenging time, it is preferable to dry the electrolyte membrane in the step (1).

  In the step (1), the ratio of the difference between the thickness before compression and the thickness after compression of the electrolyte membrane to the difference between the swelling dimension thickness and the dry dimension thickness of the electrolyte membrane is expressed as a percentage of the electrolyte membrane. It is desirable to compress the electrolyte membrane until the compression rate becomes 20 to 80%.

  From the viewpoint of durability because the membrane electrode assembly is compressed in the stacking direction of the membrane electrode assembly, the gas diffusion layer is made of a rigid body having a volume reduction rate of 30% or less at a pressure (compression pressure) of 7 MPa or more. Is preferred.

The present invention also includes an electrolyte membrane, a membrane electrode assembly including a catalyst layer and a gas diffusion layer from the electrolyte membrane side, and having a pair of electrodes provided with the electrolyte membrane interposed therebetween, and a compression mechanism. A fuel cell start and stop system comprising:
The gas diffusion layer is a gas diffusion layer having higher rigidity than the electrolyte membrane;
The start-up of the fuel cell, wherein the compression mechanism includes compression means for compressing the electrolyte membrane in the stacking direction of the membrane electrode assembly, and control means for controlling compression and decompression by the compression means. Provide a stop system.

  Further, the system includes detection means for detecting a start and stop command signal of the fuel cell and transmitting the signal to the control means.

  According to the fuel cell start and stop method and the fuel cell start and stop system according to the present invention, in order to compress the electrolyte membrane when the fuel cell is stopped and depressurize the electrolyte membrane when the fuel cell is started, The electrolyte membrane that has been depressurized can absorb the generated water at the start-up of the fuel cell in a cold environment, and the freezing of the generated water in the catalyst layer can be prevented and the pores of the catalyst layer can be blocked. Therefore, it is possible to realize a fuel cell excellent in low-temperature startability. Furthermore, since the electrolyte membrane is compressed and maintained until the next start-up when the fuel cell is stopped, the scavenging time in the fuel cell at the time of the stop in preparation for the start-up in the cold environment can be shortened.

A fuel cell start and stop method provided by the present invention includes an electrolyte membrane, and a membrane electrode including a catalyst layer and a gas diffusion layer from the electrolyte membrane side, and a pair of electrodes provided with the electrolyte membrane interposed therebetween. A method for starting and stopping a fuel cell including a joined body and a compression mechanism, wherein the gas diffusion layer is a gas diffusion layer having higher rigidity than the electrolyte membrane, and includes the following steps: Is.
(1) a step of compressing the electrolyte membrane in the stacking direction of the membrane electrode assembly by the compression mechanism when the operation of the fuel cell is stopped;
(2) maintaining the compression of the electrolyte membrane at a thickness smaller than the swelling dimension thickness of the electrolyte membrane until the next startup, and (3) at the startup of the fuel cell, the compression mechanism A process of releasing pressure.

  In the present invention, the electrolyte membrane is compressed when the fuel cell is stopped, and the electrolyte membrane is decompressed when the fuel cell is started. It can be absorbed, and the generated water in the catalyst layer can be prevented from freezing and the pores of the catalyst layer can be blocked. Therefore, it is possible to realize a fuel cell excellent in low-temperature startability. Furthermore, since the electrolyte membrane is compressed and maintained until the next start-up when the fuel cell is stopped, the scavenging time in the fuel cell at the time of the stop in preparation for the start-up in the cold environment can be shortened.

The fuel cell start and stop system provided by the present invention used in the fuel cell start and stop method includes an electrolyte membrane, and a catalyst layer and a gas diffusion layer from the electrolyte membrane side. A membrane electrode assembly having a pair of electrodes provided sandwiched between, and a fuel cell start and stop system including a compression mechanism,
The gas diffusion layer is a gas diffusion layer having higher rigidity than the electrolyte membrane;
The compression mechanism includes compression means for compressing the electrolyte membrane in the stacking direction of the membrane electrode assembly, and control means for controlling compression and decompression by the compression means.

Hereinafter, the components of the fuel cell start and stop system according to the present invention will be described in detail with reference to FIGS. 1 and 2. In addition, this invention is not limited to the laminated structure shown in FIG.1 and FIG.2, and the following embodiment.
<Membrane electrode assembly>
FIG. 1 is a cross-sectional view showing an example of a single cell (single cell 100) in a fuel cell. As shown in FIG. 1, the electrolyte membrane 1 is provided with a cathode (oxidant electrode) 4 a on one surface and an anode (fuel electrode) 4 b on the other surface to form a membrane electrode assembly 5. In this example, both the electrodes 4 (cathode 4a, anode 4b) are respectively composed of a cathode side catalyst layer 2a and a cathode side gas diffusion layer 3a, an anode side catalyst layer 2b and an anode side gas diffusion layer 3b in this order from the electrolyte membrane side. It has a laminated structure.

The membrane electrode assembly 5 is sandwiched between a cathode side separator 6 a and an anode side separator 6 b to constitute a single cell 100. The separator 6 defines a flow path 7 (7a, 7b) for supplying a reaction gas (fuel gas, oxidant gas) to each electrode 4 and gas-seals between each single cell, and also functions as a current collector. To do. The cathode 4a is supplied with an oxidant gas (a gas containing oxygen or generating oxygen, usually air) from the flow path 7a, and the anode 4b is a fuel gas (containing hydrogen or generating hydrogen) from the flow path 7b. Gas, usually hydrogen gas).
(Electrolyte membrane)
As the electrolyte membrane 1 in the present invention, a polymer electrolyte for an electrolyte membrane conventionally used in a solid polymer electrolyte fuel cell, for example, an electrolyte membrane containing a fluorine-based polymer electrolyte and a hydrocarbon-based polymer electrolyte is used. be able to.

  Examples of the fluorine-based polymer electrolyte include fluorine-based ion exchange resins such as perfluorocarbon sulfonic acid. Examples of the perfluorocarbon sulfonic acid membrane include commercially available products such as Nafion manufactured by DuPont of the United States and Flemion manufactured by Asahi Glass.

On the other hand, a hydrocarbon-based polymer electrolyte has a polymer main chain composed of carbon and hydrogen and an ion exchange group, and typically has a proton dissociable polar group as an ion exchange group. A proton-conducting hydrocarbon polymer electrolyte is used. Examples of proton dissociative polar groups include sulfonic acid groups, carboxylic acid groups, boronic acid groups, phosphonic acid groups, phosphoric acid groups, and hydroxyl groups.
An electrolyte membrane made of a hydrocarbon-based polymer electrolyte is economically advantageous because it is cheaper than that made of a fluorine-based polymer electrolyte, and is also advantageous in terms of durability because of its high heat resistance.

  The hydrocarbon polymer electrolyte is a fluorine polymer electrolyte in which the main chain and side chain hydrogen represented by perfluorocarbon sulfonic acid resin membranes such as Nafion (trade name, manufactured by DuPont) are all substituted with fluorine. Not included. The hydrocarbon-based polymer electrolyte typically does not contain any fluorine. However, when a polymer electrolyte having a relatively high glass transition temperature is used, the effect according to the present invention can be sufficiently obtained, and therefore, partially substituted with fluorine or contains hetero atoms other than fluorine. Even if the glass transition temperature is high to some extent, it is included in the hydrocarbon polymer electrolyte.

  The hydrocarbon polymer electrolyte having a proton-dissociating polar group (hereinafter sometimes simply referred to as a hydrocarbon polymer electrolyte) includes an aromatic ring in the main chain and / or side chain, and proton dissociation. A high molecular resin containing a polar group can be used. Specifically, the proton-dissociating properties described above are applied to engineering plastics such as polyether ether ketone, polyether ketone, polyether sulfone, polyphenylene sulfide, and polyphenylene ether, and general-purpose plastics such as polystyrene, ABS resin, and AS resin. Examples thereof include a polar group introduced and covalently bonded. Further, it is composed of a complex of a basic polymer and a strong acid, such as polybenzimidazole, polypyrimidine, and polybenzoxazole, which are disclosed in JP-A-11-503262. Examples also include polymer electrolytes such as solid polymer electrolytes.

  Since it is excellent in heat resistance and can be suitably used for a fuel cell operated under high temperature conditions, the hydrocarbon polymer electrolyte has a Tg (glass transition temperature) of 110 ° C. or higher, particularly 130 ° C. or higher. Furthermore, it is preferable that it is 160 degreeC or more. Hydrocarbon polymer electrolytes having an aromatic ring structure in the main chain skeleton of the polymer are excellent in heat resistance and many have a Tg in the above range, and can be suitably used.

The electrolyte membrane 1 is obtained by dissolving or dispersing the above polymer electrolyte in a solvent appropriately combined with alcohols such as methanol, ethanol, propanol, or water, or in a polar organic solvent such as dimethyl sulfoxide or dimethylformamide. The electrolyte solution can be prepared by casting and drying the obtained solution on the surface or mold of a substrate or the like. It can also be produced by a method of extruding a polymer electrolyte at a temperature above the glass transition point.
The electrolyte membrane 1 may contain only one type of polymer electrolyte as described above, or may contain two or more types. Moreover, the proton dissociative polar group to be introduced may be one type or two or more types. Moreover, the other component may be included as needed.

The film thickness of the electrolyte membrane 1 may usually be about 10 to 100 μm. The electrolyte membrane 1 is preferably thin from the viewpoint of improving proton conductivity. However, if it is too thin, the function of isolating gas is reduced, the permeation amount of non-proton hydrogen is increased, and cross leakage occurs in a severe case. To do.
The electrolyte membrane 1 can be reinforced with a perfluorocarbon polymer in the form of a fibril, a fabric, a non-fabric, or a porous sheet, or by coating the membrane surface with an inorganic oxide or a metal.

  As the electrolyte membrane 1 in the present invention, the volume is reduced by drying (drainage) and the volume is reduced, the swelling material is swollen by water absorption and the volume is increased, and the volume is reduced by compression (drainage). A rubber elastic / swellable material that absorbs water while increasing the volume by opening (removing pressure) can be used. The electrolyte membrane 1 is preferably a material having swelling property or swelling property and rubber elasticity that can be restored to a volume of about 90% or more before compression when completely released from pressure. Actually, most of the above-described electrolyte membranes have poor rubber elasticity, and the volume changes due to water absorption and drainage.

In the swellable material, simply compressing does not reduce the volume of the electrolyte membrane 1 and does not shrink unless moisture is removed. Therefore, it is necessary to dry with compression. In this case, it is not always necessary to dry. However, even in the case of rubber elastic and swellable materials, the electrolyte membrane may be damaged by repeated compression for dehydration, so the drying time of the electrolyte membrane can be shortened by using drying together with compression. It is possible.
Regardless of which material is used, the electrolyte membrane 1 of the present invention is forced to compress at the time of operation stop and maintain until the next start-up to prevent re-expansion, and has the remaining capacity to absorb new product water at the start-up. leave. Therefore, it is possible to shorten the scavenging time compared with the conventional case.
The electrolyte membrane 1 may be dried while the electrolyte membrane 1 is compressed as long as the environmental temperature of the fuel cell 101 is lowered to the same level as the temperature of the cold environment. The electrolyte membrane 1 may be compressed or dried after being compressed.
Examples of the drying means include dehumidification and flowing dry gas. Drying can be performed under conditions of a humidity comparable to the atmosphere and a temperature (60 to 80 ° C.) when the fuel cell is stopped.

In the present embodiment, a perfluorocarbon sulfonic acid membrane, which is one of solid polymer electrolyte membranes, which is a kind of proton conducting membrane, is described as an electrolyte membrane. However, the electrolyte membrane used in the fuel cell of the present invention is It is not particularly limited, and may be proton-conductive or other ion-conductive such as hydroxide ion or oxide ion (O ²⁻ ).

(Catalyst layer)
The catalyst layer 2 usually contains a conductive material in addition to the catalyst and the polymer electrolyte for the catalyst layer (electrode polymer electrolyte) as a proton conductive substance, and further contains a water-repellent polymer or a binder as necessary. Other components or materials such as an adhesive may be included.
The catalyst layer 2 can be formed using a catalyst ink containing a catalyst, a polymer electrolyte for electrodes, a solvent, and other components as necessary.
As the catalyst, supported catalyst particles in which a catalyst component is supported on a conductive material such as a carbon material such as carbonaceous particles and carbonaceous fibers are preferably used. The catalyst component is not particularly limited as long as it has a catalytic action on the hydrogen oxidation reaction at the anode and the oxygen reduction reaction at the cathode. For example, platinum, ruthenium, iridium, rhodium, palladium, osnium, tungsten, It can be selected from metals such as lead, iron, chromium, cobalt, nickel, manganese, vanadium, molybdenum, gallium, aluminum, or alloys thereof. Pt and an alloy made of Pt and another metal such as Ru are preferable.

  The polymer electrolyte for electrodes is not particularly limited, and a polymer electrolyte used when forming the above-described electrolyte membrane can be used. The polymer electrolyte for electrodes and the polymer electrolyte for electrolyte membrane may be different, but usually the same one is preferably used.

The catalyst ink is obtained by dissolving or dispersing the above catalyst component and electrode polymer electrolyte in a solvent. The solvent of the catalyst ink may be appropriately selected according to the polymer electrolyte for the electrode to be used. For example, in addition to alcohols such as methanol, ethanol and propanol, dimethyl sulfoxide (DMSO), N-methyl-2-pyrrolidone (NMP) Dimethylacetamide (DMAC), dichloromethane, chloroform, 1,1-dichloroethane, 1,2-dichloroethane, 1,1,1-trichloroethane, tetrahydrofuran, diethyl ether, acetone, methyl ethyl ketone, dimethylformamide, or a mixture of these organic solvents, Mixtures of these organic solvents and water can be used.
The solvent is generally added in an amount of about 10 to 20 parts by weight per 1 part by weight of the catalyst component.

A method for applying the catalyst ink to the electrolyte membrane 1 is not particularly limited, and examples thereof include a spray method, a screen printing method, a doctor blade method, a gravure printing method, and a die coating method. After the application, the catalyst layer is formed by natural drying or heat drying such as a hot plate, an oven, or warm air.
Although the film thickness of the catalyst layer 2 is not specifically limited, What is necessary is just to be about 5-50 micrometers.

(Gas diffusion layer)
A gas diffusion layer 3 is further laminated on the catalyst layer 2. The gas diffusion layer 3 has gas diffusivity, conductivity, and strength required as a material constituting the gas diffusion layer so that gas can be efficiently supplied to the catalyst layer 2. Uses a gas diffusion layer having higher rigidity than the electrolyte membrane 1 described above. Higher rigidity than the electrolyte membrane 1 means that the compressive deformation rate when compressed in the stacking direction of the membrane electrode assembly 5 in a stacked state is smaller than that of the electrolyte membrane. The gas diffusion layer 3 is preferably composed of a rigid body having a pressure (compression pressure) of 7 MPa or more and a volume reduction rate of 30% or less. When the gas diffusion layer is out of the above range, it is difficult to compress it while maintaining gas diffusibility. The volume reduction rate can be obtained (measured) by “(1−Diffusion layer thickness during compression) / Diffusion layer thickness during non-compression”.
Examples of the gas diffusion layer 3 include titanium, aluminum, copper, nickel, nickel-chromium alloy, copper and alloys thereof, silver, aluminum alloy, zinc alloy, lead alloy, titanium, niobium, tantalum, iron, stainless steel, and gold. It can be formed using a metal mesh composed of a metal such as platinum, a conductive porous body such as a metal porous body, or a gas diffusion layer sheet composed of a foam metal. The carbon cloth or carbon paper that is usually used for the gas diffusion layer has low rigidity, and when the membrane / electrode assembly is compressed in the stacking direction, it is crushed before the electrolyte membrane is compressed. It is not suitable as a diffusion layer.

The gas diffusion layer 3 may be composed of a single layer of the conductive porous body as described above, but a water repellent layer may be provided on the side facing the catalyst layer. The water repellent layer may be a layer impregnated in the conductive porous body.
The water-repellent layer usually has a porous structure containing conductive particles such as carbon particles and carbon fibers, a water-repellent resin such as polytetrafluoroethylene, and the like. The water-repellent layer is not always necessary, but it can improve the drainage of the gas diffusion layer while maintaining an appropriate amount of water in the catalyst layer and the electrolyte membrane. There is an advantage that electrical contact can be improved.
The gas diffusion layer 3 can be bonded by thermocompression bonding a gas diffusion layer sheet provided with a water-repellent layer as necessary to the surface of the catalyst layer by a technique such as hot pressing.
The thickness of the gas diffusion layer 3 is preferably about 15 to 100 μm.

In this way, a membrane electrode assembly 5 is obtained in which the electrodes 4a and 4b are joined to both surfaces of the electrolyte membrane 1. This membrane electrode assembly 5 is further sandwiched between separators 6a and 6b provided with gas flow paths 7a and 7b to form a single cell 100.
As the separator 6, for example, a carbon separator containing a high concentration of carbon fiber and made of a composite material with a resin, a metal separator using a metal material, or the like can be used. Examples of the metal separator include those made of a metal material excellent in corrosion resistance, and those coated with a coating that enhances the corrosion resistance by coating the surface with carbon or a metal material excellent in corrosion resistance. . Any material that is normally used as a separator can be used without any problem without causing damage in the compression of the present invention.

When a fuel cell having such a single cell 100 is used as an energy source for a mobile object, for example, an automobile, start and stop are frequently repeated. While the fuel cell is stopped in a cold environment, the supply of hydrogen and air to the single cell 100 is stopped, and the humidity in the cell is kept within a certain range by supplying and exhausting dry gas as necessary. Reduce the freezing of moisture in the cell.
However, even if gas is supplied to the fuel cell 100 and dried to a predetermined humidity, after the drying is finished at a low atmospheric temperature, the fuel cell is exposed to a low temperature (naturally left to stand). ) In the case where the temperature of the fuel cell is lowered, such as when the temperature is lowered from the daytime to the nighttime even when the drying is finished or the atmosphere temperature is not low, When the relative humidity increases, water in the cell once evaporated by vapor condensation forms, and the generated water is absorbed by the electrolyte membrane 1. If the moisture content of the electrolyte membrane 1 increases at the time of stoppage even though the drying is once completed to a predetermined humidity, the amount that the electrolyte membrane 1 can absorb water after restarting is reduced. When the fuel cell in such a state is started in a cold environment, the electrolyte membrane 1 cannot fully absorb the water generated in the catalyst layer 2 by the electrochemical reaction, and the generated water has pores as compared with the gas diffusion layer 3. The catalyst layer 2 and the electrolyte membrane 1 adjacent to the catalyst layer are frozen to clog the pores in the catalyst layer 2, which may impair the startability of the fuel cell.
In the present invention, by providing a compression mechanism in the fuel cell, the electrolyte membrane 1 can be compressed and maintained when the fuel cell is stopped, and the electrolyte membrane 1 can absorb water generated in the catalyst layer 2 at the time of low temperature startup. Give extra energy.

<Compression mechanism>
FIG. 2 shows an example of a solid polymer electrolyte fuel cell. The compression mechanism 10 includes a compression unit 8 that compresses the electrolyte membrane 1 in the stacking direction of the membrane electrode assembly 5, and a control unit 9 that controls compression and decompression by the compression unit 8.
As shown in FIG. 2, a plurality of unit cells 100 each including the membrane electrode assembly 5 having the above-described configuration are used, stacked in the stacking direction of the membrane electrode assembly 5, and electrically stacked to form a stack. The fuel cell 101 is disposed so that the compression means 8 of the compression mechanism 10 faces the separator 6 at one end of the single cell 100 and is sandwiched and fixed by a pair of end plates 11a and 11b from the outside of the stack. Form. The end plates 11a and 11b are fixed by the fastening rod 12 so that the stacking direction of the membrane electrode assembly 5 has a constant thickness.
Although not shown in FIG. 2, the gas flow paths 7a and 7b of the single cell 100 communicate with gas manifolds that introduce reaction gas into the cells, respectively. Further, in order to prevent gas leakage from the outer edge portion of the laminated surface, the outer edge portions of the catalyst layers 2a and 2b and the gas diffusion layers 3a and 3b are sealed with a gasket, centering on the electrolyte membrane 1. In addition, excess water in the cell is collected and discharged to the outside.

(Compression means)
The compression means 8 in the present invention is not particularly limited, and examples thereof include a hydraulic jack, a pneumatic compressor, and a mechanical compressor that applies pressure by a screw fastening force. It is preferable that pressure can be applied to the entire electrolyte membrane 1 substantially uniformly.

(Control means)
The control means 9 in the present invention causes the compression means 8 to compress the electrolyte membrane 1 when the fuel cell 101 is stopped, and the compression of the electrolyte membrane 1 is started next with a thickness smaller than the swelling dimension thickness of the electrolyte membrane 1. After maintaining for a while, when the fuel cell 101 is started, the compression means 8 is made to depressurize the electrolyte membrane 1. In addition, since compression in the present invention is performed on the single cell 100 including the membrane electrode assembly 5, pressure is applied to the catalyst layer 2, the gas diffusion layer 3, and the separator 6 in addition to the electrolyte membrane 1. However, since a rigid material is used for the gas diffusion layer 3 and the separator 6 and the catalyst layer 2 is a very thin film, only the electrolyte membrane 1 is substantially compressed.

The compression of the electrolyte membrane 1 performed by the compression mechanism 10 is such that a gas permeation path can be secured in the electrolyte membrane 1 and the electrode 4 of the single cell 100 in order to prepare for the start of the fuel cell 101 in a cold environment. The electrolyte membrane 1 is compressed so that the thickness is smaller than the swelling dimension thickness. Here, the swelling dimension thickness is the thickness when the electrolyte membrane absorbs most water, and means the thickness when the electrolyte membrane is immersed in hot water at 80 ° C. for 60 minutes. On the other hand, the thickness when water is most removed from the electrolyte membrane is called dry dimension thickness, which means the thickness when dried in a dry oven at 80 ° C. for 24 hours.
Specifically, the ratio of the difference between the thickness before compression of the electrolyte membrane 1 and the thickness after compression to the difference between the swelling dimension thickness and the dry dimension thickness of the electrolyte membrane 1 is expressed as the compressibility of the electrolyte membrane 1. It is preferable to compress the electrolyte membrane until the compression ratio is 20 to 80%, preferably 40 to 60%. When the compressibility of the electrolyte membrane 1 is out of the above range, there is a possibility that problems such as insufficient generation of water generated at the time of start-up or breakage of the electrolyte membrane may occur.
The electrolyte membrane 1 may be compressed until it becomes thinner than the film thickness having the predetermined compression ratio, and then the pressure is removed to restore the film thickness so that the compression ratio falls within the predetermined range. .
The fuel cell start and stop system according to the present invention preferably includes a film thickness measuring means (not shown) for measuring the thickness of the electrolyte membrane 1 and transmitting the measured value to the control means 9.

The fuel cell start and stop system according to the present invention preferably further includes input detection means (not shown) for detecting a start and stop command signal for the fuel cell 101 and transmitting the signal to the control means 9. .
At the start of the fuel cell 101, the electrolyte membrane 1 is depressurized in the present invention. Therefore, the electrolyte membrane 1 that has depressurized the generated water at the start-up of the fuel cell 101 in a cold environment absorbs it, prevents the generated water from freezing in the catalyst layer 2, and suppresses the blockage of the gas permeation path.

Next, a fuel cell start and stop method in such a fuel cell start and stop system will be described.
(1) The step of compressing the electrolyte membrane in the stacking direction of the membrane electrode assembly by the compression mechanism when the operation of the fuel cell is stopped The stacking of the electrolyte membrane 1 by the compression mechanism 10 is performed by the compression mechanism 10 when the operation of the fuel cell 101 is stopped. Compress in the direction. Here, “when the fuel cell operation is stopped” means that the supply of the reaction gas is stopped immediately before the supply of the reaction gas is stopped until the electrochemical reaction is almost finished, and then the electrochemical reaction is almost finished, It means a period in which the electrolyte membrane 1 is in an unfrozen state so that the electrolyte membrane 1 can be compressed to a thickness smaller than the predetermined swelling dimension thickness. In a cold environment, when the temperature of the electrolyte membrane is below freezing point and the water in the electrolyte membrane is frozen, it becomes difficult to compress the electrolyte membrane.
When the fuel cell 101 is mounted on a moving body or a vehicle, the supply of the reaction gas is normally stopped after the ignition is turned off. In this case, compression can be started immediately before the ignition is turned off.
Further, the compression in this step (1) can be performed immediately before stopping the operation of the fuel cell or while stopping the operation of the fuel cell. The compression pressure is preferably 7 MPa or more, particularly 7 to 10 MPa. When the compression pressure is 7 MPa or more, the compression rate of the electrolyte membrane 1 described above can be in the above range. When the compression pressure is lower than the above range, it is difficult to open the electrolyte membrane 1 while being sufficiently controlled when the electrolyte membrane 1 expands due to water absorption.
The compression in this step serves to prevent the volume from increasing by suppressing the state where the volume has once decreased in the electrolyte membrane 1.
As described above, since many of the electrolyte membranes of the present invention are electrolyte membranes that swell and shrink when water enters and exits, drying is usually performed in parallel with compression.

  FIG. 3 shows a flowchart when the operation of the fuel cell is stopped. When the ignition is turned off and the operation of the vehicle, that is, the fuel cell 101 is stopped, the input detection means detects a stop signal and transmits it to the control means 9 in step S01. In step S02, the control means 9 issues a command to the compression means 8 to start compression of the electrolyte membrane 1, and in step S03, the compression means 8 places the electrolyte membrane 1 in the stacking direction of the membrane electrode assembly 5. Start compressing. In step S <b> 04, the film thickness measuring unit measures the thickness of the electrolyte membrane 1 every predetermined time, and transmits the measured value to the control unit 9. In step S05, the control unit 9 determines whether the measured value has reached the predetermined electrolyte membrane thickness as described above. If the determination result is NO, the compression unit 8 continues the compression. The compression means 8 further performs compression in step S03. On the other hand, if the determination result is YES, in step S06, the control means 9 issues a command to the compression means 8 to stop the compression of the electrolyte membrane 1, and in step S07, the compression means 8 causes the electrolyte membrane 1 to stop. Stop compression.

(2) Step of maintaining the compression of the electrolyte membrane with a thickness smaller than the swelling dimension thickness of the electrolyte membrane until the next start-up Following the above step (1), the compression of the electrolyte membrane 1 is performed with the swelling dimension thickness of the electrolyte membrane 1 Maintain a thinner thickness until the next start-up. That is, after compressing the electrolyte membrane 1 in the step (1), it is left in a state in which the pressure on the electrolyte membrane 1 in step S06 is maintained without performing pressure reduction. Thus, since the compressed state is maintained, the water content of the electrolyte membrane 1 hardly increases during the period from the stop of the operation of the fuel cell to the start of the fuel cell. Therefore, as described above, the scavenging time is shortened compared to the conventional method in which scavenging is performed for a long time in anticipation that the electrolyte membrane absorbs water generated by condensation due to an increase in relative humidity when stopped at low temperatures. can do.
Normally, a polymer electrolyte fuel cell is operated at a temperature close to room temperature of less than 100 ° C. However, when the operation of the fuel cell is stopped in a cold environment, or after the operation is stopped, the ambient temperature of the fuel cell has decreased to below freezing point. In this case, the temperature in the fuel cell 101 changes so as to be approximately the same as the ambient temperature from the time when the operation is stopped until the next activation. For example, after completion of compression below freezing point, the fuel cell 101 is exposed to a low temperature (naturally left) and allowed to cool, and the temperature of the fuel cell 101 becomes approximately the same as the ambient temperature. When the compression is completed, the ambient temperature is not low in the daytime. However, when the air temperature cools down at night, the temperature of the fuel cell 101 decreases according to the ambient temperature. Below freezing, water that remains without being discharged out of the fuel cell freezes.

(3) Step of Depressurizing the Electrolyte Membrane by the Compression Mechanism when Starting the Fuel Cell After the step (2), the pressure of the electrolyte membrane 1 is removed by the compression mechanism 10 when starting the fuel cell 101. Here, “when the fuel cell is activated” means a period from the start of the supply of the reaction gas to the start of the supply of the reaction gas until the warm-up is almost completed. “Warming up” means about 3 minutes from the start of the reaction gas supply in a state where the fuel cell 101 is still frozen.
When the fuel cell 101 is mounted on a moving body or a vehicle, the supply of the reaction gas is usually started after the ignition is turned on. In this case, the decompression can be started from the time when the ignition is turned on.
Further, the depressurization in this step (3) means that the pressure is decreased until the pressure applied in step (1) becomes zero, and can be performed immediately before starting the fuel cell or while starting the fuel cell. it can. The depressurization can be performed at once or gradually. However, if the depressurization is suddenly performed, the restoration of the electrolyte membrane 1 is biased, and the membrane may be distorted, resulting in poor contact in the stack. It is preferable that the pressure is gradually reduced while maintaining the contact pressure with the compression mechanism 10 (compression means), and the volume is increased while the electrolyte membrane 1 absorbs generated water.
The decompression in this step plays a role of preventing the increase in volume in the electrolyte membrane 1. When the fuel cell is started, a reaction gas is supplied and an electrochemical reaction starts. As a result, water is generated in the catalyst layer 2. As described above, in order to restore the volume while the electrolyte membrane 1 of the swellable material absorbs the generated water, or the electrolyte membrane 1 of the rubber elastic / swellable material restores the volume by releasing the pressure, Since it absorbs, it can avoid freezing the water produced | generated at the time of starting in the catalyst layer 2 or the electrolyte membrane 1 adjacent to this catalyst layer, and can improve low temperature starting property.

  FIG. 4 shows a flowchart when starting the fuel cell. In step S08, the ignition is turned on, and the input detection means detects the vehicle, that is, the operation start signal of the fuel cell 101, and transmits it to the control means 9. In step S09, the control unit 9 issues a command to the compression unit 8 to start depressurization of the electrolyte membrane 1, and in step S10, the compression unit 8 starts depressurization of the electrolyte membrane 1. The pressure is released all at once or gradually so that the pressure applied in step S07 becomes zero. In step S11, the pressure removal ends when the pressure applied in step S07 becomes zero.

  According to the fuel cell starting and stopping method and the fuel cell starting and stopping system according to the present invention described above, the electrolyte membrane is compressed when the fuel cell is stopped, and the electrolyte membrane is depressurized when the fuel cell is started. Therefore, the electrolyte membrane that has been depressurized can absorb the generated water at the start-up of the fuel cell in a cold environment, preventing the freezing of the generated water in the catalyst layer and suppressing the clogging of the pores of the catalyst layer. it can. Therefore, it is possible to realize a fuel cell excellent in low-temperature startability. Further, since the electrolyte membrane is compressed and maintained until the next start-up when the fuel cell is stopped, the scavenging time in the fuel cell at the time of the stop in preparation for the start-up in the cold environment can be shortened.

Example 1
Prepare an Nafion (trade name, manufactured by DuPont) with a water content of 30% as an electrolyte membrane, and maintain a pressure of 7 MPa using a creep tester, etc., in an environment with a relative humidity of 100% (60 ° C.). And the increase rate of the thickness of the electrolyte membrane was measured.

(Comparative Example 1)
The increase rate of the thickness of the electrolyte membrane was measured in the same manner as in Example 1 except that the pressure was 1 MPa.

(Comparative Example 2)
The increase rate of the thickness of the electrolyte membrane was measured in the same manner as in Example 1 except that the pressure was 2.5 MPa.

In FIG. 5, the graph showing the relationship between the elapsed time in Example 1 and Comparative Examples 1-2 and a film thickness increase rate is shown. The rate of film thickness increase represents the rate of increase of each measured film thickness as a percentage based on the dry dimension thickness of the electrolyte membrane. In addition, the film thickness of the electrolyte membrane before compression is in a state in which it is increased by 25% of the dry dimension thickness.
From the graph, it can be seen that in Example 1, the pressure is appropriate because it is restored only to about half the thickness of the electrolyte membrane before compression. In Comparative Examples 1 and 2, it is understood that the applied pressure is too small and the electrolyte membrane increases the film thickness while absorbing water.

(Example 2)
Using Nafion (trade name, manufactured by DuPont) as an electrolyte membrane, a carbon material (E-TEK) supporting 20 wt% Pt as a catalyst, and TGPH-060 (trade name, manufactured by Toray) as a gas diffusion layer, A membrane electrode assembly was produced. Next, the membrane electrode assembly was compressed using a compression holder capable of controlling the surface pressure as the compression means so that the compression rate of the electrolyte membrane was 60%, and the membrane electrode assembly was placed at −15 ° C. and left for 12 hours.
Subsequently, while releasing the compression pressure and opening the membrane electrode assembly, H ₂ and air were passed through the membrane electrode assembly in both electrodes, and power generation was continued while maintaining a constant voltage at 0.1 V. The change with time was measured.

(Comparative Example 3)
The change with time of the current density of the membrane electrode assembly was measured in the same manner as in Example 2 except that the compression means was not used.

FIG. 6 shows the measurement results of Example 2 and Comparative Example 3. In the prior art shown in Comparative Example 3, the water generated by power generation freezes at the catalyst layer and the electrolyte membrane adjacent to the catalyst layer and clogs the pores in the catalyst layer. I have.
On the other hand, in Example 2, since the produced | generated water was rapidly absorbed into the electrolyte membrane released from the compression pressure, it was possible to suppress the produced water from staying in and blocking the pores in the catalyst layer.

It is typical sectional drawing of an example of the single cell of the fuel cell in this invention. It is typical sectional drawing of an example of the fuel cell in this invention. It is a flowchart figure at the time of the stop of the fuel cell of this invention. It is a flowchart figure at the time of starting of the fuel cell of this invention. It is a graph which shows the measurement result of the Example of this invention. It is a graph which shows the measurement result of the Example of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Electrolyte membrane 2 ... Catalyst layer (2a: Cathode side catalyst layer, 2b: Anode side catalyst layer)
3. Gas diffusion layer (3a: cathode side gas diffusion layer, 3b: anode side gas diffusion layer)
4 ... Electrode (4a: cathode, 4b: anode)
5 ... Membrane electrode assembly 6 ... Separator (6a: cathode side separator, 6b: anode side separator)
7: Gas flow path (7a, 7b)
DESCRIPTION OF SYMBOLS 8 ... Compression means 9 ... Control means 10 ... Compression mechanism 11 ... End plate (11a, 11b)
12 ... Fastening rod 100 ... Single cell 101 ... Fuel cell

Claims (6)

Starting and stopping of an electrolyte membrane, a membrane electrode assembly including a catalyst layer and a gas diffusion layer from the electrolyte membrane side and having a pair of electrodes provided with the electrolyte membrane interposed therebetween, and a fuel cell including a compression mechanism A method for starting and stopping a fuel cell, comprising the following steps, wherein the gas diffusion layer is a gas diffusion layer having higher rigidity than the electrolyte membrane.
(1) a step of compressing the electrolyte membrane in the stacking direction of the membrane electrode assembly by the compression mechanism when the operation of the fuel cell is stopped;
(2) maintaining the compression of the electrolyte membrane at a thickness smaller than the swelling dimension thickness of the electrolyte membrane until the next startup, and (3) at the startup of the fuel cell, the compression mechanism A process of releasing pressure.   The method of starting and stopping a fuel cell according to claim 1, wherein the electrolyte membrane is dried in the step (1).   In the step (1), the ratio of the difference between the thickness before compression of the electrolyte membrane and the thickness after compression with respect to the difference between the swelling dimension thickness and the dry dimension thickness of the electrolyte membrane is defined as the compressibility of the electrolyte membrane. 3. The method of starting and stopping the fuel cell according to claim 1, wherein the electrolyte membrane is compressed until the compression rate becomes 20 to 80%.   4. The method for starting and stopping a fuel cell according to claim 1, wherein the gas diffusion layer is made of a rigid body having a pressure (compression pressure) of 7 MPa or more and a volume reduction rate of 30% or less. Starting and stopping of an electrolyte membrane, a membrane electrode assembly including a catalyst layer and a gas diffusion layer from the electrolyte membrane side and having a pair of electrodes provided with the electrolyte membrane interposed therebetween, and a fuel cell including a compression mechanism A system,
The gas diffusion layer is a gas diffusion layer having higher rigidity than the electrolyte membrane;
The start-up of the fuel cell, wherein the compression mechanism includes compression means for compressing the electrolyte membrane in the stacking direction of the membrane electrode assembly, and control means for controlling compression and decompression by the compression means. Stop system.   6. The fuel cell start and stop system according to claim 5, further comprising detection means for detecting a start and stop command signal of the fuel cell and transmitting the signal to the control means.

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