WO2006033390A1

WO2006033390A1 - Gas diffusion layer for fuel cell

Info

Publication number: WO2006033390A1
Application number: PCT/JP2005/017481
Authority: WO
Inventors: Kazuhiko Kudo
Original assignee: National University Corporation Hokkaido University
Priority date: 2004-09-22
Filing date: 2005-09-22
Publication date: 2006-03-30

Abstract

This invention provides a gas diffusion layer for a fuel cell, having a multilayer structure comprising a hydrophilic material-containing layer and a layer consisting essentially of moisture transpiration electrically conductive fibers. There is also provided a gas diffusion layer for a fuel cell, having a multilayer structure comprising a mixed layer comprising a hydrophilic material and moisture transpiration electrically conductive fibers and a layer consisting essentially of moisture transpiration electrically conductive fibers. Further, there is provided a fuel cell comprising the above gas diffusion layer held between a cathode catalyst layer and a separator. In a solid polymer electrolyte fuel cell, the gas diffusion layer can ensure a path for diffusing an oxidizing agent gas to the catalyst layer and can regulate the concentration of water in an electrolyte film in a given range, without being affected by a variation in water production amount caused by a load variation.

Description

Specification

Gas diffusion layer for fuel cells

Technical field

TECHNICAL FIELD [0001] The present invention relates to a gas diffusion layer for a polymer electrolyte fuel cell capable of maintaining a constant water concentration in an electrolyte membrane, and a fuel cell using the same.

Background art

In recent years, fuel cells, which are power generation devices with low emissions, high energy efficiency, and low burden on the environment, have attracted attention again due to the rise in global environmental protection, and great efforts have been made to develop them. Yes.

[0003] Fuel cells are classified into alkali type, phosphoric acid type, molten carbonate type, solid electrolyte type, solid polymer type, and the like depending on the type of electrolyte used. Among these, solid polymer fuel cells have advantages such as being usable at room temperature, short start-up time and high output, and high output density. Small-scale distributed generation facilities, such as electric vehicle power supplies, have been developed for household power supplies!

[0004] The basics of a fuel cell are configured as a unit of a cell in which an anode electrode, a force sword electrode, and an electrolyte membrane serving as an ion conductor between the anode and the force sword are sandwiched by separators. Here, the electrode is composed of an electrode substrate (also called a current collector) that promotes gas diffusion and collects (supply) electricity, and an electrode catalyst layer that actually becomes an electrochemical reaction field.

[0005] In the case of a polymer electrolyte fuel cell, fuel (hydrogen gas) is supplied to the anode side catalyst layer and an oxidant (oxygen-containing gas) is supplied to the power sword side catalyst layer through the grooves of the grooved separator. A chemical reaction takes place, and the flow of electrons generated through the membrane electrode body at this time is taken out to the outside as electrical energy.

That is, on the anode electrode side, the fuel gas reacts on the catalyst surface to generate protons and electrons, and the electrons are conducted to the electrode base material, and the protons are conducted to the electrolyte membrane. On the other hand, in the force sword electrode, protons conducted from the electrolyte membrane and electrons conducted from the electrode base material react with the acid gas on the surface of the catalyst layer to generate water.

[0007] Under the above principle, the anode electrode has gas diffusivity, electron conductivity, ion conductivity. The force sword electrode, which requires the characteristics of good quality, is effective in draining the water generated in the catalyst layer in addition to the powerful characteristics, and at the same time, the moisture in the solid electrolyte membrane is accompanied by proton transfer. In order to make up for the lack of water in the membrane caused by the discharge to the force sword side, it is necessary to devise a device that retains moderate moisture. This is because the movement of protons in the electrolyte membrane requires the moisture inside the membrane to be kept above a certain level.

[0008] In response to the demand for draining and holding moisture on the force sword electrode side, conventionally, moisture supplied to the anode side is humidified to supply moisture from the anode side, or is supplied to the force sword side. A method has been adopted in which the oxygen-containing gas is humidified and the reverse diffusion of water into the anode is used. However, the humidification control of the supply gas has complicated the fuel cell itself, and various attempts have been made within the electrode.

[0009] For example, a technique using carbon paper as a gas diffusion layer provided on a force sword electrode has been reported (Patent Document 1, JP-A-50-25808). There is also a method of suppressing the transfer of moisture from the catalyst layer to the oxidizing gas flow through the gas diffusion layer by sandwiching a porous membrane adsorbed with a hydrophobic substance between the catalyst layer and the gas diffusion layer. It is reported! (Patent Document 2 · Special 2004-134134).

[0010] However, the carbon paper has lower conductivity in the thickness direction than in the in-plane direction, and is mechanically rigid, but is relatively brittle and poor in elasticity, or in the surface direction. Problems such as inferiority have been pointed out. In addition, the insertion of a porous layer that suppresses the diffusion of moisture may cause flooding (flooding) near the gas outlet at high loads, that is, under conditions where a large amount of moisture is generated. It is necessary to add a layer having a gradient characteristic that reduces the ability to suppress moisture diffusion in the flow direction. In addition, there is a possibility that the water concentration in the electrolyte membrane may change with the fluctuation of water production due to battery output fluctuation.

From the above, it has been desired to develop a means for keeping the water concentration in the electrolyte membrane constant without being affected by changes in the amount of water produced due to fluctuations in the power generation load.

Patent Document 1: Japanese Patent Laid-Open No. 50-25808

Patent Document 2: JP 2004-134134 A

Disclosure of the invention [0013] The present invention provides a layer containing a hydrophilic material capable of holding a certain amount of moisture and moisture quickly in response to the demand for draining and holding moisture in a force sword electrode of a polymer electrolyte fuel cell. Absorbs into the oxidative gas stream that flows outside the fiber and transpirations! It efficiently drains moisture generated on the side of the force sword electrode and has a layer consisting essentially of moisture-vaporizable conductive fibers that have the function of tanning and diffusing, and moderately drains the solid electrolyte membrane. A gas diffusion layer that can be retained and a fuel cell having the diffusion layer are provided. Such a gas diffusion layer and a fuel cell can be described as follows.

[0014] 1) A gas diffusion layer for a fuel cell, having a multilayer structure of a layer containing a hydrophilic material and a layer substantially made of moisture-vaporizable conductive fibers.

[0015] 2) The gas diffusion layer for a fuel cell according to 1), wherein the layer containing a hydrophilic material is a mixed layer containing a hydrophilic material and moisture-conductive conductive fibers.

[0016] 3) The fuel cell gas according to 2), which has a two-layer structure of a mixed layer containing a hydrophilic material and moisture-evaporating conductive fibers and a layer consisting of substantially high moisture-evaporating conductive fibers. Diffusion layer.

[0017] 4) A fuel cell according to 3), wherein a mixed layer containing a hydrophilic material and highly moisture-evaporating conductive fibers and a layer substantially consisting of highly moisture-evaporating conductive fibers are pressure-bonded. Gas diffusion layer.

[0018] 5) The ratio of the thickness of the mixed layer containing the hydrophilic material and the high moisture transpiration conductive fiber to the entire gas diffusion layer is 30% to 80%, 2) to 4) Gas diffusion layer for fuel cells.

[0019] 6) A fuel cell comprising the gas diffusion layer according to any one of 1) to 5) sandwiched between a force sword catalyst layer and a separator.

[0020] 7) The fuel cell according to 6), wherein the layer containing a hydrophilic material is in contact with the force sword catalyst layer.

[0021] 8) The fuel cell according to 6) or 7), wherein the maximum value of the moisture evaporation rate of the gas diffusion layer is equal to or higher than the maximum moisture generation rate of the force sword catalyst layer.

Brief Description of Drawings

[0022] [Fig. 1] Fig. 1 is a schematic view of an apparatus prepared for evaluating the performance of the gas diffusion layer of the present invention.

FIG. 2 shows the relationship between the amount of water supply in the gas diffusion layer of Example 1 and the moisture content inside the gas diffusion layer. FIG. 3 shows the relationship between the amount of water supply in the gas diffusion layer of Example 2 and the moisture content inside the gas diffusion layer.

BEST MODE FOR CARRYING OUT THE INVENTION

[0023] The hydrophilic material in the present invention refers to a granular or fibrous resin or inorganic substance, such as polyacryl-tolyl resin, carbon fiber, ceramics, quartz glass, etc. in a chemical oxidation treatment or in an electrolyte. It means a material that has been rendered hydrophilic by a method such as electrolytic acid treatment. Here, the term “hydrophilic” means the property and the degree that the material can be covered with a water film. Specifically, the contact angle between water and processed material is defined as 30 degrees or less.

[0024] Further, it is particularly preferable that the hydrophilic material thus cured has conductivity.

Examples thereof include PAN-based carbon fibers obtained by carbonizing a polyacrylonitrile precursor, and pitch-based carbon fibers obtained by carbonizing a pitch precursor. These have excellent conductivity. Here, “conductivity” for a hydrophilic material is defined as 10 ⁻¹ Ωcm or less as the specific resistance value of the hydrophilic material. Preferred for hydrophilic materials, conductivity below 10_ ² Omega cm, particularly preferred U, conductivity is below 10_ ³ Omega cm

[0025] Although the following is an inference, in the present invention, the layer containing the hydrophilic material contains granular or fibrous resin or inorganic substance in the entire layer, so that a large number of particles penetrating between the particles or between the fibers are used. It is considered that gas diffusion to the catalyst layer is rapidly performed through the voids and that the water content of the electrolyte membrane is kept constant by equilibrium with the water retained on the surface of the hydrophilic material. However, the present invention is not restricted to strong reasoning.

[0026] The moisture transpirationable conductive fiber in the present invention means a fiber developed so as to have a high moisture transpiration ability, typically a fiber having an atypical cross section and imparted conductivity. Examples of fibers having an irregular cross section include JP 2000-282323 A, JP 2004-019046 A, JP 2003-301322 A, JP 10-280232 A, or JP 2002- The quick-drying fiber etc. which were indicated by the 146626 gazette etc. can be mentioned.

[0027] Such a fiber has an atypical or non-circular shape (for example, a star shape) in the cross section of the fiber. Therefore, the surface area of the fiber is larger than that of the columnar fiber. As a result, it has a property of absorbing water that is in contact with the fiber due to capillary force, and that the absorbed water is evaporated and diffused over a wide area on the fiber surface.

[0028] The "moisture transpiration" in the present invention means the water absorbability and moisture diffusibility inherently retained by such a fiber. If this is expressed by appropriate numerical values, characteristics that can be 60 ° C, to absorb transpiration of water woven lcm ² per 0. 012Ml within one minute in an atmosphere of a relative humidity of 10% is taken as the void ratio of 70% to 90% of the fabric fibers Can be understood as

[0029] In the present invention, such a moisture transpirationable fiber further imparted with conductivity is used. For example, a fiber with an unusual cross section made of polyacrylic nitrile or the like that can be converted into carbon fiber is produced by a dry spinning method, which is flame-resistant and carbonized according to the preparation method of PAN-based carbon fiber, Carbon fiber can be used. The conductivity mentioned here can be defined as the specific resistance value of the fiber after being processed is 10 ^_1 Ωcm or less. Preferred conductive for water evaporation conductive fibers below 10_ ² Ω cm, and particularly preferably V, the conductivity is less than 5 X 10-3 Ω cm.

[0030] The gas diffusion layer of the present invention comprises a thin layer obtained by mixing the above-mentioned hydrophilic material and an appropriate binder such as polyvinyl alcohol and atta-tolyl-based short fibers, and a water-evaporating conductive property. It can be produced by adhering a thin layer obtained by mixing fibers and a suitable binder. The bonding may be a pressure bonding. The pressure bonding may be performed by applying a pressure of 10 to 30 kgZcm ² which is a surface pressure at the time of assembly of a normal polymer electrolyte fuel cell.

[0031] In the gas diffusion layer of the present invention, it is preferable that the layer containing the hydrophilic material is a mixed layer containing a hydrophilic material and moisture-vaporizable conductive fibers.

[0032] The mixed layer containing the hydrophilic material and the water-evaporating conductive fibers is composed of a fibrous resin or an inorganic material such as polyacryl-tolyl-based resin, carbon fiber, ceramics, quartz glass, and the like. It can be prepared by mixing a transpirationable conductive fiber into a thin layer and imparting hydrophilicity by chemical oxidation treatment or electrolytic oxidation in an electrolyte. For example, carbon short fibers and polyacrylonitrile-based carbon fibers (moisture-conducting conductive fibers) are mixed together with a binder such as polyvinyl alcohol or acrylonitrile-based short fibers to make paper. The surface of a thin layer dried and baked by the method can be prepared by oxidation treatment in a 60% nitric acid aqueous solution.

[0033] Further, a binder such as Teflon (registered trademark) powder is added to a granular powder or inorganic substance such as carbon powder or acid zirconium powder, and the mixture is suspended and mixed with water. It can be produced by mixing transpirationable conductive fibers, drying and firing, and forming a thin film.

[0034] The gas diffusion layer of the present invention can be produced by laminating the mixed layer thus prepared and a layer substantially composed of moisture-evaporating conductive fibers. The bonding may be a pressure bonding. Crimping should be performed with a pressure of 10 to 30 kgZ cm ² , which is the surface pressure when assembling a normal polymer electrolyte fuel cell!

[0035] Alternatively, granular or fibrous resin or inorganic material such as carbon fiber short fibers and moisture-vaporizable conductive fibers are mixed with a binder to form a thin layer by a papermaking method. It is also possible to make a mixture of conductive short fibers and a binder by a papermaking method, and then dry and calcinate this to make it hydrophilic. By doing so, it is possible to dispose the water-vaporizable conductive fibers so as to penetrate the gas diffusion layer and to cut through the gas diffusion layer, thereby allowing the water diffusion in the gas diffusion layer to be performed more efficiently.

[0036] The gas diffusion layer thus prepared can be used as it is in the production of a fuel cell, replacing the conventional gas diffusion layer used in a fuel cell. When the gas diffusion layer is made by laminating two layers as described above, the force sword electrode is placed below the anode electrode so that water that also generates force sword electrode force easily penetrates into the gas diffusion layer by the action of gravity. It is preferable to arrange them. In addition, the gas diffusion layer of the present invention is disposed so that the layer containing the hydrophilic material is in contact with the force sword electrode. At this time, the moisture transpiration can be further improved by projecting the moisture transpiration conductive fiber into the force sword side separator groove from the surface of the layer substantially composed of the moisture transpiration conductive fiber.

[0037] In general, the water production rate of a force cathode catalyst layer of a fuel cell is proportional to the output, when change in the output density force ^ ~ 0. 8AZcm ^2, 0~0. It is known that changes 006gZcm ² Zmin ing. In order to suck up the water generated as the reaction proceeds from the catalyst layer and evaporate it into the acidic gas stream without causing flooding, the water is generated in the gas diffusion layer. Moisture transpiration capacity of min production rate (0 ～ 0.006g / cmVmin) or more is required.

[0038] The gas diffusion layer of the present invention has a moisture transpiration capacity of 0.008 gZcm ² Zmin by adopting the above-described configuration, so that the problem of flooding does not actually occur. Although not restricted by the following reasoning, in the fuel cell using the gas diffusion layer of the present invention, a large amount of fuel cell was generated during the high power reaction of the fuel cell and filled in the void of the layer containing the hydrophilic material. Excess water is absorbed into the water-vaporizable conductive fibers and quickly removed from the voids, and is evaporated into the oxidant gas flowing through the separator flow path. It is assumed that a diffusion path to the layer can be secured.

[0039] Hereinafter, the present invention will be described in detail with reference to examples. However, the present invention is not limited to these examples and should not be understood.

Example 1

[0040] A quick-drying fiber (trade name Swift, thickness 0.5mm, width 10cm, length 10cm) made of a modified cross-section acrylic fiber manufactured by Mitsubishi Rayon Co., Ltd. was applied to the method of Ogawa et al. studies on tolyl system to increase productivity and functionality of carbon fibers "follow Heisei _June _1995), 30 minutes after heating at a temperature of 270 ° C from 240 ° C under 50% oxygen atmosphere, further nitrogen The temperature was raised to 1300 ° C in a gas to form carbon fibers, and a thin layer of moisture transpiration fibers and conductive fibers was obtained. Next, the surface of the moisture-vaporizable conductive fiber was rendered hydrophilic by nitric acid oxidation according to the method of Fitzer et al. (Carbon, Vol. 18, pages 389-393, 1980). Thereto, Ja Pango § Tex Co. hydrophilic Kabonpeno (trade name CARBEL, thickness 0. 3 mm, horizontal 10 cm, vertical 10 cm) superposed, and pressed at a surface pressure of 10 to 30 kg / cm ^2, A gas diffusion layer of the present invention was obtained.

Example 2

[0041] A quick-drying fiber (trade name Swift) made of a modified cross-section acrylic fiber manufactured by Mitsubishi Rayon Co., Ltd., Ogawa et al. (Hokkaido University degree thesis, "Research on productivity and functionality improvement of polyacrylonitrile carbon fiber" ”, June 1995), heated for 30 minutes at a temperature of 240 ° C to 270 ° C in a 50% oxygen atmosphere, and further heated to 1300 ° C in nitrogen gas to convert to carbon fiber. The fiber was cut to obtain short fibers of moisture-vaporizable conductive fibers having an average fiber length of 3 to 20 mm. [0042] This short fiber and a carbon fiber having a circular cross section (diameter 3 μm to 20 μm) are mixed with a short fiber (average fiber length 3 to 20 mm) at a weight ratio of 1: 1, and further shortened. For each 1 part by weight of the fiber mixture, 0.3 part by weight of polybulualcohol was added and stirred in water at a concentration of 1% by weight and mixed homogeneously.

[0043] Using 0.16 g of the mixture consisting of this moisture transpirationable conductive fiber, carbon fiber and binder powder, it was made into a sheet of length 10 cm x width 10 cm x thickness 0.06 mm by the paper making method, and water transpiration on this. A mixture of short fibers of conductive conductive fibers and polybulal alcohol (weight ratio 1: 1) 0.6 4 g was overlaid and finally formed into a sheet having a thickness of 0.3 mm. This sheet is dried, pre-carbonized at 200 ° C to 800 ° C in an inert atmosphere, and further carbonized at 1500 ° C and 3000 ° C, followed by chemical oxidation with an aqueous solution containing 60 wt% nitric acid. (Boiled with 60% nitric acid for 5 hours) to make it hydrophilic and gas diffusion layer A (gas diffusion where the ratio of the thickness of the mixed layer of hydrophilic material and water-vaporizable conductive fiber to the entire gas diffusion layer is 20% Layer).

[0044] Further, 0.24 g of a mixture comprising moisture-vaporizable conductive fibers, carbon fibers and a binder fiber is used to make a sheet of 10 cm in length, 10 cm in width, and 0.09 mm in thickness by a papermaking method. A mixture of short transpirationable conductive fibers and polybulal alcohol (weight ratio 1: 1) 0.5 6 g was overlaid and finally formed into a sheet having a thickness of 0.3 mm. This sheet is dried, pre-carbonized at 200 ° C to 800 ° C in an inert atmosphere, and further carbonized at 1500 ° C and 3000 ° C, followed by chemical oxidation with an aqueous solution containing 60 wt% nitric acid. (Boiled with 60% nitric acid for 5 hours) to hydrophilize the gas diffusion layer B (the ratio of the thickness of the mixed layer of hydrophilic material and moisture-vaporizable conductive fiber to the entire gas diffusion layer is 30%) Layer).

[0045] Furthermore, 0.64 g of a mixture consisting of moisture-vaporizable conductive fibers, carbon fibers and a binder fiber was taken into a sheet form of 10 cm in length × 10 cm in width × 0.2 mm in thickness by the papermaking method, A mixture of short fibers of transpirationable conductive fibers and polybulal alcohol (weight ratio 1: 1) was overlaid with 0.16 g, and finally formed into a sheet with a thickness of 0.3 mm. After drying this sheet, it was pre-carbonized at 200 ° C and 800 ° C in an inert atmosphere, and further carbonized at 1500 ° C to 3000 ° C, and then chemical oxidation with an aqueous solution containing 60 wt% nitric acid ( Boiled with 60% nitric acid for 5 hours to make it hydrophilic, and gas diffusion layer C (a gas with a ratio of the thickness of the mixed layer of hydrophilic material and water-vaporizable conductive fiber to the entire gas diffusion layer is 80%) Diffusion layer) was obtained. [0046] Further, 0.8 g of a mixture composed of moisture-vaporizable conductive fibers, carbon fibers and a binder cake was used to make a sheet of 10 cm in length, 10 cm in width, and 0.3 mm in thickness by a papermaking method. After drying this sheet, After pre-carbonization at 200 ° C and 800 ° C in an inert atmosphere, and further carbonization from 1500 ° C to 300 ° C, chemical oxidation with an aqueous solution containing 60% by weight nitric acid (5% with 60% nitric acid) The mixture was hydrophilized by boiling (time boiling) to obtain a gas diffusion layer D (a gas diffusion layer, a comparative example in which only a mixed layer of a hydrophilic material and a water-vaporizable conductive fiber was used).

<0047> <Test example>

1) Moisture content evaluation device

A moisture content evaluation apparatus schematically shown in Fig. 1 was prepared, and using this, a gas diffusion layer made of carbon paper subjected to conventional water-repellent treatment or hydrophilic treatment, and the present invention created in the examples were used. In the gas diffusion layer, the moisture content change in the layer was measured when the water supply was changed in response to the output of the fuel cell.

[0048] The moisture content evaluation apparatus in FIG. 1 includes a simulated anode tank having a water tank, a blower, and a heater. The water tank is connected to the lower part of the anode tank via a constant flow pump. Further, the upper side surface of the anode tank has an open port, and the blower supplies an air flow passing through the open side of the anode tank in the lateral direction. Furthermore, the anode tank has a repellent made of a polypropylene sintered filter plate manufactured by Filaen Co., Ltd. between the opening through which the air flow passes and the air flow passage and the water filled in the lower part of the anode tank. Separated by aqueous porous layer.

[0049] The operation of the present apparatus is as follows. That is, the water supplied to the lower part of the simulated anode tank by the constant flow pump is also heated to a temperature higher than the normal temperature (about 60 ° C) by the heater, so that the constant flow pump It is leached to the upper part of the same layer through the porous layer that separates the simulated anode tank, and is evaporated by the air flow heated to the same temperature as water. is there. Therefore, the porous layer surface is on the anode side catalyst layer surface of the fuel cell, the leaching water is the water generated on the anode side catalyst layer surface by the chemical reaction of the fuel cell, and the flow rate of the constant flow pump is proportional to the fuel cell output. The amount of water generated in the anode side catalyst layer corresponds to the oxygen-containing gas flow in which the air flow is supplied to the force sword side catalyst layer. [0050] In this apparatus, by installing a gas diffusion layer to be tested on this porous layer, the drainage efficiency of the water leached on the upper surface of the porous layer is measured. is there. The drainage efficiency is obtained by measuring the moisture content inside the gas diffusion layer by measuring the mass.

[0051] 2) Measurement results for the gas diffusion layer produced in Example 1

The gas diffusion layer of the present invention produced in Example 1, a carbon-treated carbon paper (trade name CARBEL) manufactured by Japan Gore-Tex Co., Ltd., and a water-repellent carbon paper (both thickness 0.3 mm, length 10 cm) (10cm in width), each set on the porous layer of the device of 1), and the result of measuring the moisture content change in the diffusion layer when the water supply amount of the constant flow pump force was changed, Figure 2 shows. Since the gas diffusion layer of Example 1 uses the action of gravity for the permeation of the water generated in the anode side catalyst layer in the gas diffusion layer, the result of FIG. 2 shows the result of FIG. It was obtained in an experiment in which the tank was turned upside down.

[0052] In the case of the conventional carbon paper-only gas diffusion layer, the water content curve (dotted line and broken line) in the diffusion layer increases almost linearly due to an increase in the amount of water supplied as the battery output increases. . In addition, it can be seen that the inclination decreases when the treatment of the fiber surface of this carbon paper changes from water repellency (dotted line) to hydrophilicity (dashed line). This is because, in the case of hydrophilicity, a water film is formed on the surface of a fiber having a large surface area, so that the transpiration rate into the air stream is increased compared to the case of water-repellent fiber.

[0053] On the other hand, in the present invention (solid line), the water content in the gas diffusion layer is kept constant at a water supply amount of about 0.5 to 6.5 m (X 10 ^_2 g / min / cm ² ). I understand that. On the other hand, since the moisture transpirationable conductive fiber layer is on the outside of the hydrophilic conductive fiber layer, the moisture directly from the water film on the fiber surface in the hydrophilic conductive fiber layer into the air stream In order to suppress transpiration of water, the moisture content in the diffusion layer rapidly rises to a certain value even at low water supply.

[0054] Since the free water collected in the fiber gaps of the hydrophilic conductive fiber layer is absorbed by the moisture-vaporizable conductive fiber layer and evaporated in the air, moisture absorption and evaporation by the moisture-vaporizable conductive fiber layer are performed. Until the limit water supply amount of action is reached, almost no water accumulates on the porous layer surface, that is, the gap between the anode catalyst surface and the hydrophilic conductive fiber layer, and at the same time, the moisture content in the diffusion layer is So that it is kept at a substantially constant value.

That is, when the gas diffusion layer according to the present invention is used, the gas diffusion layer has a wide output range. It is possible to keep the moisture content of the liquid almost constant. In addition, the gas diffusion layer of the present invention keeps the moisture content in the anode catalyst layer and the electrolyte membrane in contact with the surface by keeping the moisture content in the same layer constant regardless of the amount of water supply (= evaporation amount). Can do.

[0056] Further, in the conventional carbon paper, moisture transpiration from the gas diffusion layer is intense, and in order to eliminate the difference in the amount of moisture transpiration between the vicinity of the oxidizing gas supply section and the vicinity of the gas discharge section with low transpiration. In addition, it is necessary to make the water content of the gas diffusion layer uniform in the plane by increasing the water repellency of the carbon paper near the supply section and at the same time increasing the hydrophilicity near the gas discharge section. However, no intensive treatment is required, and the moisture content distribution in the plane of the diffusion layer can be kept uniform.

[0057] In addition, by distributing the water repellent / hydrophilicity of the carbon paper in this way, even if the water content is uniform, the change in the water content accompanying the change in the current output can be suppressed. However, with this method, it is possible to reduce the water content accompanying output changes.

[0058] 3) Measurement results on the gas diffusion layer produced in Example 2

Gas diffusion layers A to D produced in Example 2 and carbon paper (trade name CARBEL) manufactured by Japan Gore-Tex made hydrophilic by chemical oxidation treatment (comparative example (hydrophilic)) and repellent Prepare water-based carbon paper (comparative example (water repellency)) (all thickness 0.3 mm, length 10 cm, width 10 cm) and install them on the upper part of the porous layer of the device 1). Fig. 2 shows the results of measuring the change in moisture content in the diffusion layer when the amount of water supply from the source changes. The experimental conditions are a water temperature of 60 ° C and an air flow of 11.5 liters Zmin.

[0059] In the case of a conventional carbon paper-only gas diffusion layer, in the case of water repellency (comparative example (water repellency)), the water supply amount becomes 0.005 g / cmVmin (current of 0.7 AZcm ^{2 as} the battery output increases). It can be seen that the moisture content curve in the diffusion layer suddenly increases to a value close to 0. This shows that the water-repellent carbon paper has very little moisture in the carbon paper before flooding, and flooding occurs when the water supply exceeds 0.005 g / cmVmin.

[0060] In addition, the hydrophilic carbon paper (comparative example (hydrophilic)) has a small amount of water supply! / In some cases, the water content is about 40%, and the hydrophilic material can retain water in the diffusion layer. Show. However, as the amount of water supply increases, the flatness is lower than that of water-repellent carbon paper. You can see that it causes ding.

On the other hand, it can be seen that the gas diffusion layers A to C all have better characteristics than the other comparative examples with respect to the flooding limit and the moisture content of the gas diffusion layer. However, when paying attention to the water retention by the hydrophilic material, the ratio of the thickness of the mixed layer of the hydrophilic material and the water-vaporizable conductive fiber to the entire gas diffusion layer is preferably 30% or more. It can be said. Also, when focusing on the flooding limit characteristics due to moisture-evaporating conductive fibers, the ratio of the thickness of the mixed layer of hydrophilic material and moisture-evaporating conductive fibers to the entire gas diffusion layer is 80% or less. It can be said that it is preferable. Industrial applicability

[0062] The gas diffusion layer of the present invention secures a diffusion path to the catalyst layer of the oxidant gas without being affected by fluctuations in the amount of water generated due to load fluctuations in the polymer electrolyte fuel cell. It becomes possible to control the moisture concentration in the membrane to a certain range.

[0063] When the gas diffusion layer according to the present invention is used, it is possible to keep the moisture content in the gas diffusion layer almost constant over a wide output range. In addition, the gas diffusion layer of the present invention keeps the water content in the same layer constant regardless of the amount of water supply (= evaporation amount), so that the surface of the anode catalyst layer in contact with the layer and the water in the electrolyte belly are also kept constant. be able to.

[0064] Further, in the conventional carbon paper, moisture transpiration from the gas diffusion layer is intense, so as to eliminate the difference in the amount of moisture transpiration between the vicinity of the oxidizing gas supply section and the vicinity of the gas discharge section with low transpiration. In addition, it is necessary to make the water content of the gas diffusion layer uniform in the plane by increasing the water repellency of the carbon paper near the supply section and at the same time increasing the hydrophilicity near the gas discharge section. However, no intensive treatment is required, and the moisture content distribution in the plane of the diffusion layer can be kept uniform. In the present invention, it is also possible to suppress fluctuations in the moisture content accompanying output changes.

Claims

The scope of the claims

[1] A gas diffusion layer for a fuel cell, which has a multilayer structure of a layer containing a hydrophilic material and a layer substantially made of water-vaporizable conductive fibers.

[2] The layer containing a hydrophilic material is a mixed layer containing a hydrophilic material and moisture-conducting conductive fibers

The gas diffusion layer for a fuel cell according to claim 1.

[3] The gas diffusion for a fuel cell according to claim 2, having a two-layer structure of a mixed layer containing a hydrophilic material and moisture-evaporating conductive fibers and a layer substantially having a high moisture-evaporating conductive fiber force. layer.

[4] The fuel cell according to claim 3, wherein a mixed layer containing a hydrophilic material and highly moisture-evaporating conductive fibers and a layer substantially consisting of highly moisture-evaporable conductive fibers are pressure-bonded. Gas diffusion layer

[5] The fuel cell according to any one of claims 2 to 4, wherein the ratio of the thickness of the mixed layer containing the hydrophilic material and the highly water-vaporizable conductive fiber to the entire gas diffusion layer is 30% to 80%. Gas diffusion layer.

[6] A fuel cell comprising the gas diffusion layer according to any one of claims 1 to 5 sandwiched between a force sword catalyst layer and a separator.

7. The fuel cell according to claim 6, wherein the layer containing a hydrophilic material is in contact with the force sword catalyst layer.

[8] The fuel cell according to claim 6 or 7, wherein the maximum value of the moisture evaporation rate of the gas diffusion layer is equal to or greater than the maximum moisture generation rate of the force sword catalyst layer.