JP2013020842A - Porous electrode substrate - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a porous electrode substrate in which a short circuit current and cross leak of a reactant gas hardly occur when used in a fuel cell.SOLUTION: The porous electrode is made by binding carbon short fibers with net-like carbon fibers and/or a resin carbide, where a bulk density of a range from a surface of the porous electrode substrate to 30 μm of the thickness on at least one face thereof is 0.30 g/cmor more. A short circuit current density of the porous electrode substrate is 0.1 mA/cmor less.

Description

  The present invention relates to a porous electrode substrate used for a polymer electrolyte fuel cell.

  A porous electrode substrate using carbon fibers and carbides such as carbon fiber paper, carbon fiber cloth, and carbon fiber felt is generally used for the gas diffusion layer of the polymer electrolyte fuel cell. These porous electrode base materials are not only highly conductive due to short carbon fibers and carbides, but are porous materials, so they are suitable for gas diffusion layers because of high permeability of liquids such as fuel gas and generated water. Material.

  However, the porous electrode base material used as the gas diffusion layer is made of carbon fiber fluff or carbon short due to friction or compression generated in the gas diffusion layer and electrolyte membrane joining process or stack fastening process in manufacturing a fuel cell. There is a risk that the carbide binding the fibers and short carbon fibers may fall off or break. Since these dropped and broken carbon short fibers and carbides are more rigid than the electrolyte membrane, they may pierce the electrolyte membrane. By piercing the electrolyte membrane, a short circuit current is generated between the anode electrode and the cathode electrode, and the hydrogen gas on the anode electrode side and / or the oxygen gas on the cathode electrode side cause a cross leak, resulting in a fuel cell. There was a tendency for the electromotive force and durability of the steel to be significantly impaired.

  As a method for reducing the damage caused by the stab of short carbon fibers and carbon material on the electrolyte membrane, for example, Patent Document 1 discloses in advance the contact pressure applied to the gas diffusion layer in the joining step with the electrolyte membrane and the catalyst layer and the stack fastening step. A method is disclosed in which carbon short fibers and carbides, which are applied to the gas diffusion layer and weakly bonded, are removed in advance and removed. Patent Document 2 discloses a method of removing short carbon fibers having a weak binding during the production of a gas diffusion layer by blowing and simultaneously sucking gas into the gas diffusion layer. Furthermore, Patent Document 3 discloses a method of removing short carbon fibers having weak binding by applying ultrasonic treatment to a gas diffusion layer.

JP 2009-190951 A JP 2010-70433 A JP 2010-61964 A

  However, in the method described in Patent Document 1, it is possible to remove in advance the carbon powder generated in the subsequent process by pressing. However, since the pressure applied during pressing and the removal treatment after pressing are insufficient, the generated carbon powder is If it is not sufficiently removed, a short-circuit current may occur when used in a fuel cell.

  In the method described in Patent Document 2, the surface of the gas diffusion layer can be cleaned to some extent, but carbon powder is newly generated when the gas diffusion layer is compressed, such as a bonding step with the electrolyte membrane, However, there is a problem in that a large short-circuit current is generated by being stuck into the electrolyte membrane.

Moreover, even if it used the method of patent document 3, there existed a problem that carbon powder will generate | occur | produce similarly by compression of a post process.
An object of the present invention is to overcome the above-described problems and to provide a porous electrode base material that is less likely to cause short-circuit current and cross-leakage of reaction gas when used in a fuel cell.

  The above problems are solved by the following inventions [1] to [2].

[1] A porous electrode base material in which short carbon fibers are bound with reticulated carbon fibers and / or resin carbides, and the bulk from the surface to a thickness of 30 μm on at least one surface of the porous electrode base material. A porous electrode substrate having a density of 0.30 g / cm ³ or more.

[2] A porous electrode substrate having a short-circuit current density of 0.1 mA / cm ² or less measured under the following experimental conditions.
<Experimental conditions>
(1) A catalyst layer comprising catalyst-supported carbon (catalyst: Pt, catalyst support amount: 50% by mass) on both sides of a perfluorosulfonic acid polymer electrolyte membrane (Nafion NR211: DuPont Co., Ltd .: film thickness: 25 μm) A laminate formed with (catalyst layer area: 25 cm ² , Pt adhesion amount: 0.3 mg / cm ² ) is sandwiched between porous electrode base materials for cathode and anode, and these are joined to form a membrane for a fuel cell -Obtaining an electrode assembly (MEA) MEA;

  (2) The MEA is sandwiched between two carbon separators having a bellows-like gas flow path attached to a JARI standard cell and fastened so that the surface pressure becomes 1.0 MPa. A solid polymer fuel cell (single cell) ).

(3) Terminals are connected to both electrodes of the obtained single cell, and the potential difference between the electrodes is set to 1.0 V using a digital multimeter TR6487 (manufactured by Advantest), and the short-circuit current density is measured.
(Nafion is a registered trademark of EI DuPont de Nemours and Company)

  When used in a fuel cell, it is possible to obtain a porous electrode base material having a sufficiently small amount of resin carbide that is less likely to cause short-circuit current and reactive gas cross-leakage.

Hereinafter, the present invention will be described in detail.

<Porous electrode substrate>
The porous electrode substrate of the present invention is a structure in which short carbon fibers (A) dispersed in a structure are joined by network carbon fibers (B), or short carbon fibers dispersed in the structure. (A) are joined together by reticulated carbon fibers (B), and between carbon short fibers (A) and between carbon short fibers (A) and reticulated carbon fibers (B) are bonded by resin carbide (C). It consists of an attached structure or a structure in which short carbon fibers (A) are bound by resin carbide (C).

The porous electrode substrate can take a sheet shape, a spiral shape, or the like. When formed into a sheet, the basis weight of the porous electrode substrate is preferably about 15 to 100 g / m ² , the porosity is preferably about 50 to 90%, and the thickness is preferably 20 μm to 400 μm, more preferably 50 μm or more and 300 μm or less.

The gas air permeability of the porous electrode substrate is preferably 500 to 30000 ml / hr / cm ² / mmAq. Moreover, it is preferable that the electrical resistance (penetration direction resistance) of the thickness direction of a porous electrode base material is 50 m (ohm) * cm < ² > or less. In addition, the measuring method of the gas permeability of a porous electrode base material and penetration direction resistance is mentioned later.

<Short carbon fiber (A)>
The short carbon fiber can be used regardless of the raw material, but one or more selected from polyacrylonitrile (hereinafter abbreviated as PAN) carbon fiber, pitch carbon fiber, rayon carbon fiber, and phenolic carbon fiber. It is preferable that the carbon fiber is included, and it is more preferable that the PAN-based carbon fiber or the pitch-based carbon fiber is included. The average diameter of the short carbon fibers is preferably about 3 to 30 μm, more preferably 4 to 20 μm, and still more preferably 4 to 12 μm. Within this range, the surface smoothness and conductivity as a porous electrode substrate are good.

  The average length of the short carbon fibers is preferably about 2 to 12 mm. Within this range, the dispersibility during sheet formation and the mechanical strength as a porous electrode substrate are increased.

  The polyacrylonitrile-based carbon fiber is manufactured using a polymer mainly composed of acrylonitrile as a raw material. Specifically, a spinning process for spinning PAN-based fibers, a flameproofing process in which the fibers are heated and fired in an air atmosphere at 200 to 400 ° C. to convert them into oxidized fibers, and in an inert atmosphere such as nitrogen, argon, and helium In addition, the carbon fiber can be obtained through a carbonization step of carbonizing by heating to 300 to 2500 ° C., and is suitably used as a composite material reinforcing fiber. Therefore, it is possible to form a carbon sheet having a higher strength and a higher mechanical strength than other carbon fibers.

<Reticulated carbon fiber (B)>
Reticulated carbon fiber is a fiber that joins short carbon fibers (A) to each other and exists in a bent or curved state at the joint, each forming a network structure, and precursor fibers ( Reticulated carbon fibers obtained by carbonizing b) by heating can be used.

  The content of the reticulated carbon fiber (B) in the porous electrode substrate is preferably 10 to 90% by mass. In order to maintain sufficient electrical conductivity and mechanical strength of the porous electrode substrate, the content of the reticulated carbon fiber (B) is more preferably 15 to 80% by mass.

<Precursor fiber (b)>
As the precursor fiber (b), one or both of the carbon fiber precursor short fiber (b1) and the fibrillar carbon precursor fiber (b2) can be used.

<Carbon fiber precursor short fiber (b1)>
The carbon fiber precursor short fiber (b1) can be obtained by cutting a long fiber-like carbon fiber precursor fiber produced using a polymer (for example, an acrylic polymer) described later into an appropriate length. The average fiber length of the carbon fiber precursor short fibers (b1) is preferably 2 mm or more and 20 mm or less from the viewpoint of dispersibility. The average fiber length can be measured with an optical microscope and an electron microscope. Although the cross-sectional shape of the carbon fiber precursor short fiber (b1) is not particularly limited, a high roundness is preferable from the viewpoint of mechanical strength after carbonization and production cost. In addition, the average diameter of the carbon fiber precursor short fibers (b1) is preferably 5 μm or less from the viewpoint of suppressing breakage due to shrinkage during carbonization. The average fiber diameter (diameter) can be measured with an optical microscope and an electron microscope.

  As the carbon fiber precursor short fiber (b1), it is preferable to use a polymer having a remaining mass of 20% by mass or more in the step of carbonization treatment. Examples of such polymers include acrylic polymers, cellulose polymers, and phenolic polymers. When considering the spinnability and the short carbon fibers (A) from low temperature to high temperature, the remaining mass at the time of carbonization is large, and the fiber elasticity and fiber strength when performing the entanglement treatment described below, It is preferable to use an acrylic polymer containing 50% by mass or more of acrylonitrile units.

  The carbon fiber precursor short fibers (b1) may be one type, or a plurality of types having different fiber diameters and polymer types. Depending on the type of these carbon fiber precursor short fibers (b1) and the fibrillar carbon precursor fibers (b2) to be described later, the mixing ratio with the carbon short fibers (A), and the presence or absence of oxidation treatment at 200 ° C. or more and 300 ° C. or less The proportion of the carbon fiber (B) remaining in the finally obtained porous electrode substrate can be adjusted.

<Acrylic polymer used for carbon fiber precursor short fiber (b1)>
The acrylic polymer may be a homopolymer of acrylonitrile or a copolymer of acrylonitrile and other monomers. The monomer copolymerized with acrylonitrile is not particularly limited as long as it is an unsaturated monomer constituting a general acrylic fiber. For example, methyl acrylate, ethyl acrylate, isopropyl acrylate, n-butyl acrylate, Acrylic esters represented by 2-ethylhexyl acrylate, 2-hydroxyethyl acrylate, hydroxypropyl acrylate, etc .; methyl methacrylate, ethyl methacrylate, isopropyl methacrylate, n-butyl methacrylate, isobutyl methacrylate, methacrylic acid Typical examples include t-butyl acid, n-hexyl methacrylate, cyclohexyl methacrylate, lauryl methacrylate, 2-hydroxyethyl methacrylate, hydroxypropyl methacrylate, diethylaminoethyl methacrylate and the like. Methacrylic acid esters; acrylic acid, methacrylic acid, maleic acid, itaconic acid, acrylamide, N-methylol acrylamide, diacetone acrylamide, styrene, vinyl toluene, vinyl acetate, vinyl chloride, vinylidene chloride, vinylidene bromide, vinyl fluoride, And vinylidene fluoride.

  The weight average molecular weight of the acrylic polymer is not particularly limited, but is preferably from 50,000 to 1,000,000. When the weight average molecular weight of the acrylic polymer is 50,000 or more, the spinnability is improved and the yarn quality of the fiber tends to be good. When the weight average molecular weight of the acrylic polymer is 1,000,000 or less, the polymer concentration that gives the optimum viscosity of the spinning dope increases, and the productivity tends to improve.

<Fibrous carbon precursor fiber (b2)>
As the fibrillar carbon precursor fiber (b2), for example, the following can be used. Carbon precursor fiber (b2-1) having a structure in which a large number of fibrils having a diameter of several μm or less (for example, 0.1 to 3 μm) are branched from a fibrous trunk having a diameter of 100 μm or less, or a carbon precursor fibrillated by beating Short body fibers (b2-2) can be used. Hereinafter, the two fibrillar carbon precursor fibers (b2) may be referred to as fibers (b2-1) and fibers (b2-2), respectively.

  By using these fibrillar carbon precursor fibers (b2), the short carbon fibers (A) and the fibrillar carbon precursor fibers (b2) are entangled well in the precursor sheet, and handling properties and mechanical strength are excellent. It becomes easy to obtain a precursor sheet. Although the freeness of the fibrillar carbon precursor fiber (b2) is not particularly limited, the use of fibrillar fibers having a low freeness generally improves the mechanical strength of the precursor sheet. There is a tendency that the gas permeability of gas decreases.

  As the fibrillar carbon precursor fiber (b2), one type of fiber (b2-1) or one type of fiber (b2-2) may be used, and the freeness, fiber diameter, polymer type, etc. are different. A plurality of these fibers may be used in combination.

  Hereinafter, the two fibrillar carbon precursor fibers (b2) will be described in detail.

<Fiber (b2-1)>
The polymer used for the fiber (b2-1) preferably has a residual mass of 20% by mass or more in the carbonization treatment step. Examples of such polymers include acrylic polymers, cellulose polymers, and phenolic polymers. Considering the spinnability, the short carbon fibers (A) can be bonded to each other from low temperature to high temperature, the remaining mass at the time of carbonization is large, and the entanglement with the short carbon fibers (A) and the sheet strength are taken into consideration. It is preferable to use an acrylic polymer containing 50% by mass or more of units. As the acrylic polymer, the same polymer as the carbon fiber precursor short fiber (b1) can be used.

  Although the manufacturing method of a fiber (b2-1) is not specifically limited, It is preferable to manufacture using the injection coagulation method with easy control of drainage. The fiber (b2-1) by the jet coagulation method can be manufactured by the following method, for example.

  First, a spinning stock solution is prepared by dissolving an acrylonitrile-based copolymer in a solvent. As this solvent, for example, dimethylamide, dimethylformamide, dimethylsulfoxide and the like can be used. Then, the spinning solution is passed through the spinning outlet and discharged into the mixing cell. At the same time, water vapor is ejected into the mixing cell at an angle of 0 degree or more and less than 90 degrees with respect to the discharging line direction of the spinning solution. The acrylonitrile copolymer is solidified under a shear flow rate. A fiber (b2-1) is obtained by discharging the formed solidified body together with the solvent and water vapor from the mixing cell into the solidified liquid. As the coagulation liquid, water or a mixed liquid of water and the solvent can be used.

  The fiber (b2-1) thus obtained has a fibril part in which fibers having a small fiber diameter gather and a core part (stem) having a thick fiber diameter solidified without much contact with water vapor. The fibril part of the fiber (b2-1) has good entanglement with the short fiber A and the fibril part of the fiber (b2-1), and the core part of the fiber (b2-1) expresses the strength as a binder. Can do.

  The fiber diameter of the fibril part of the fiber (b2-1) is preferably 2 μm or less in order to improve the entanglement with the short carbon fiber to be mixed.

  The core part preferably has a diameter of 100 μm or less from the viewpoint of homogenization of the porous electrode substrate. By setting the diameter to 100 μm or less, the uneven distribution of the fibers (b2-1) can be easily suppressed, and the short carbon fibers A can be easily bound by a relatively small amount of fibers (b2-1). Moreover, it is preferable that the diameter of a core part is 10 micrometers or more from a viewpoint of expressing intensity | strength.

  From the viewpoint of the function that the fiber (b2-1) is entangled with the short carbon fiber A, it is preferable that a plurality of fibril parts of the fiber (b2-1) exist for one core part, and the fibrils for one core part. It is considered that the more parts, the better.

  In one fiber (b2-1), it is preferable that the thickness of the core part is constant or changes steplessly. By using such a fiber (b2-1), it is possible to easily prevent the stepped portion from being weakened by a stepwise change in the thickness of the core portion, and to easily prevent the strength from being lowered. Can do. In addition, when manufacturing a fiber (b2-1) by the said method, it may be difficult to keep the thickness of a core part constant by water vapor | steam scattering randomly, and the thickness of a core part may change. is there. However, since a gradual change in the thickness of the core portion tends to be seen when the water vapor to be sprayed cools into droplets, the core portion can be obtained by increasing the water vapor ejection pressure and temperature. Can be easily prevented from changing in steps.

<Fiber (b2-2)>
The fiber (b2-2) can be obtained by beating a long-fiber easily split sea-island composite fiber into an appropriate length and beating it with a refiner, a pulper, or the like. Long-fiber easy split sea-island composite fibers can be manufactured using two or more different types of polymers that are soluble in a common solvent and incompatible, and at least one type of polymer is carbonized. The residual mass in the treatment step is preferably 20% by mass or more. Among the polymers used for the easily splittable sea-island composite fibers, those having a residual mass in the carbonization treatment step of 20% by mass or more include acrylic polymers, cellulose polymers, and phenolic polymers. Among them, it is preferable to use an acrylic polymer containing 50% by mass or more of an acrylonitrile unit from the viewpoint of spinnability and the remaining mass in the carbonization treatment step.

  As the acrylic polymer, the same polymer as the carbon fiber precursor short fiber (b1) can be used.

  When one of the polymers used for the easily splittable sea-island composite fiber is a polymer having a residual mass of 20% by mass or more in the carbonization treatment step, the above-mentioned acrylic polymer is used as the other polymer. It is desirable that a spinning dope that dissolves in the same solvent as the acrylic polymer and dissolves both polymers stably exists. That is, it is desirable that the other polymer is incompatible with the acrylic polymer when dissolved in the same solvent as the acrylic polymer, and has a miscibility enough to form a sea-island structure during spinning. It is. This makes it possible to easily prevent inhomogeneity of the fiber that occurs when the degree of incompatibility of the two polymers is large when used as a spinning stock solution, and to easily prevent yarn breakage during spinning. Fiber shaping can be facilitated. In addition, it is desirable that other polymers are poorly soluble in water, so that when wet spinning, it is easy to prevent other polymers from dissolving in water and falling off in the coagulation tank and washing tank. Can do.

  Other polymers that satisfy these requirements include, for example, polyvinyl chloride, polyvinylidene chloride, polyvinylidene fluoride, polyvinyl pyrrolidone, cellulose acetate, acrylic resin, methacrylic resin, phenolic resin, etc. Resins and methacrylic resins are preferred from the viewpoint of the balance of demands described above. The other polymer may be one type or two or more types.

  The splittable sea-island composite fiber used for the fiber (b2-2) can be produced by a normal wet spinning method. First, a spinning stock solution is prepared by dissolving an acrylic polymer and another polymer in a solvent. Alternatively, a spinning stock solution obtained by dissolving an acrylic polymer in a solvent and a spinning stock solution obtained by dissolving another polymer in a solvent may be mixed with a static mixer or the like to obtain a spinning stock solution. As the solvent, dimethylamide, dimethylformamide, dimethyl sulfoxide and the like can be used. An easily split fiber sea-island composite fiber can be obtained by supplying these spinning stock solutions to a spinning machine, spinning from a nozzle, and performing wet heat drawing, washing, drying and dry heat drawing.

  The cross-sectional shape of the easily splittable sea-island composite fiber is not particularly limited. In order to suppress dispersibility and breakage due to shrinkage at the time of carbonization, the fineness of the easily split sea-island composite fiber is preferably 1 dtex or more and 10 dtex or less. The average fiber length of the splittable sea-island composite fiber is preferably 1 mm or more and 20 mm or less from the viewpoint of dispersibility after beating.

  The easily splittable sea-island composite fiber is beaten by peeling of the phase separation interface by a mechanical external force, and at least a part thereof is split and fibrillated. The beating method is not particularly limited, and for example, fibrillation can be performed by a refiner, a pulper, a beater, or a jet of pressurized water (water jet punching). When beating a splittable sea-island composite fiber by mechanical external force by peeling of the phase separation interface, the fibrillation state changes depending on the beating method and beating time. As a method for evaluating the degree of fibrillation, freeness evaluation (JIS P8121 (pulp freeness test method: Canadian standard type)) can be used. The freeness of the fiber (b2-2) is not particularly limited.

<Resin carbide (C)>
Resin carbide is a carbide that binds between short carbon fibers (A) and between short carbon fibers (A) and reticulated carbon fibers (B), and uses a carbon material obtained by carbonizing the resin by heating. Can do. As the resin (c) that can be carbonized by heating, it is known that carbon short fibers (A) and carbon short fibers (A) and reticulated carbon fibers (B) can be bound at the stage of carbonization. It can be used by appropriately selecting from these resins. From the viewpoint of easily remaining as a conductive substance after carbonization, the resin (c) is preferably a phenol resin, an epoxy resin, a furan resin, pitch, or the like, and particularly preferably a phenol resin having a high carbonization rate upon carbonization by heating. . As the phenol resin, a resol type phenol resin obtained by reaction of phenols and aldehydes in the presence of an alkali catalyst can be used. In addition, a novolac type phenolic resin showing solid heat-fusibility, which is produced by a reaction of phenols and aldehydes under an acidic catalyst by a known method, can be dissolved and mixed in a resol type flowable phenolic resin. In this case, a self-crosslinking type containing a curing agent such as hexamethylenediamine is preferred. As the phenol resin, a phenol resin solution dissolved in a solvent of alcohol or ketones, a phenol resin dispersion liquid dispersed in a dispersion medium such as water, or the like can be used.

  It is also preferable to mix a conductive substance in the resin carbide to further improve the conductivity. The conductive substance is preferably a carbonaceous material such as carbonaceous milled fiber, carbon black, acetylene black, or graphite powder from the viewpoint of conductivity and acid resistance. The mixing amount of the conductive substance in the resin (c) that can be carbonized by heating is preferably 1 to 10% by mass with respect to the resin. If the mixing amount is less than 1% by mass, it is disadvantageous in that the effect of improving the conductivity is small, and if it exceeds 10% by mass, the effect of improving the conductivity tends to saturate, and the cost increases. It is disadvantageous in terms.

  The content of the resin carbide (C) in the porous electrode substrate is preferably 10 to 90% by mass. In order to maintain sufficient conductivity and mechanical strength of the porous electrode substrate, the content of the resin carbide (C) is more preferably 15 to 80% by mass.

<Method for producing porous electrode substrate>
The present invention includes the following steps.
(1) A step of producing a precursor sheet in which carbon short fibers (A) or carbon short fibers (A) and precursor fibers (b) are dispersed in a two-dimensional plane.
(2) The process of heat-pressing the said precursor sheet | seat at the temperature of 100 to 300 degreeC.
(3) A step of carbonizing the heated and pressurized precursor sheet at a temperature of 1000 ° C. or higher.

Further, the following step (4) is performed between the steps (1) and (2), the following step (5) is performed before the step (2), and the following steps (2) and (3) are performed. Step (6) can be included.
(4) A step of entanglement the precursor sheet.
(5) Step of impregnating carbonizable resin (6) Step of oxidizing the heated and pressurized precursor sheet.

  Each step will be described in detail below.

<Process for producing precursor sheet (1)>
As a method for producing the precursor sheet, the short carbon fibers (A) or the short carbon fibers (A) and the precursor fibers (b) are dispersed in a liquid medium to make paper, and the short carbon fibers in the air. A paper making method such as a dry method in which the fiber (A) or the short carbon fiber (A) and the precursor fiber (b) are dispersed and piled up can be applied. However, it is preferable to use a wet method from the viewpoint of high uniformity of the sheet.

  It is preferable to use the precursor fiber (b) in order to help the short carbon fibers (A) to be opened into single fibers and to prevent the opened single fibers from refocusing. If necessary, wet papermaking can be performed using an organic polymer compound as a binder.

  This organic polymer compound plays a role as a binder (glue) that holds the components together in a precursor sheet containing carbon short fibers (A) or carbon short fibers (A) and precursor fibers (b). Have. As this organic polymer compound, polyvinyl alcohol (PVA), polyvinyl acetate, or the like can be used. In particular, polyvinyl alcohol is preferred because it has excellent binding power in the paper making process and the short carbon fibers are less likely to fall off. In the present invention, it is also possible to use this organic polymer compound in a fiber shape.

  In the case of using the precursor fiber (b), the carbon short fiber (A) and the precursor fiber (b) (for example, the fibrillar carbon precursor fiber (b2) can be formed without using the organic polymer compound as a binder. A precursor sheet can be obtained by moderate entanglement with)).

  Examples of the medium in which the carbon short fibers (A) or the carbon short fibers (A) and the precursor fibers (b) are dispersed include a medium in which the precursor fibers (b) such as water and alcohol are not dissolved. From the viewpoint of productivity, water is preferable.

  Examples of the method of mixing the carbon short fiber (A) or the carbon short fiber (A) and the precursor fiber (b) include a method of stirring and dispersing in water and a method of mixing these directly, but it is uniformly dispersed. From the viewpoint of achieving this, a method of diffusing and dispersing in water is preferable. The strength of the precursor sheet is improved by mixing the carbon short fiber (A) or the carbon short fiber (A) and the precursor fiber (b), and making a paper to produce the precursor sheet. Moreover, it can prevent that the carbon short fiber (A) peels from a precursor sheet | seat during the manufacture, and the orientation of a carbon short fiber (A) changes.

  The precursor sheet can be produced by either a continuous method or a batch method, but it is preferably produced by a continuous method from the viewpoint of the productivity and mechanical strength of the precursor sheet.

The basis weight of the precursor sheet may be 10 g / m ² or more and 200 g / m ² or less from the viewpoints of handling properties of the precursor sheet and gas permeability, conductivity, and handling properties when used as a porous electrode substrate. preferable.
The thickness of the precursor sheet is preferably 20 μm or more and 500 μm or less, more preferably 50 μm or more and 400 μm or less, from the viewpoint of gas permeability, conductivity, and handling properties.

  When the precursor fiber (b) is used, the network carbon fiber (B) in the porous electrode substrate finally obtained depending on the type, the mixing ratio with the short carbon fiber (A), and the presence or absence of oxidation treatment. The remaining proportion can be adjusted.

  The mixing ratio of the short carbon fiber (A) and the precursor fiber (b) is such that the precursor fiber (b) is 50 parts by mass or more and 300 parts by mass or less with respect to 100 parts by mass of the carbon short fiber (A). Is preferred. By setting the precursor fiber (b) to 50 parts by mass or more, the amount of the reticulated carbon fiber (B) formed is appropriately increased, so that the strength of the porous electrode substrate sheet can be easily improved. it can. By setting the precursor fiber (b) to 300 parts by mass or less, it is possible to easily suppress the shrinkage of the sheet due to the short carbon fibers (A) that suppress the shrinkage of the precursor fiber (b) during carbonization. The strength of the porous electrode substrate sheet can be easily improved.

<Step of heat and pressure molding (2)>
A structure in which the short carbon fibers (A) in which the porous electrode base material is dispersed in the structure are sufficiently joined by the network carbon fibers (B), or the short carbon fibers in the structure are dispersed. (A) are sufficiently joined together by reticulated carbon fibers (B), and between carbon short fibers (A) and between carbon short fibers (A) and reticulated carbon fibers (B) by resin carbide (C). In order to reduce the unevenness of the thickness of the porous electrode base material, and the structure in which the carbon short fibers (A) are sufficiently bound by the resin carbide (C) It is preferable to heat and press mold the sheet at a temperature of less than 300 ° C. Any technique can be applied to the heat and pressure molding as long as the technique can uniformly heat and mold the precursor sheet. For example, a method in which a smooth rigid plate is applied to both surfaces of the precursor sheet and heat-pressed, or a method using a continuous roll press device or a continuous belt press device can be mentioned.

  In the case of heating and press-molding a continuously produced precursor sheet, a method using a continuous roll press device or a continuous belt press device is preferable. Thereby, the carbonization process can be performed continuously. Examples of the pressing method in the continuous belt press apparatus include a method of applying pressure to the belt with a linear pressure by a roll press, a method of pressing with a surface pressure by a hydraulic head press, and the like. The latter is preferred in that a smoother porous electrode substrate can be obtained.

  The temperature at the time of heat and pressure molding is sufficient to bond the short carbon fibers (A), the precursor fibers (b), and the resin (c), and the surface of the porous electrode substrate precursor sheet is effectively used. In order to make it smooth, it is preferably less than 300 ° C, more preferably 120 to 290 ° C.

  The pressure at the time of heat and pressure molding is not particularly limited, but is preferably about 20 kPa to 10 MPa. If the press pressure is increased more than necessary at this time, there may be a problem that the short carbon fibers (A) are destroyed at the time of heat and pressure molding, a problem that the structure of the porous electrode substrate is too dense, or the like. is there.

  The time for heat and pressure molding can be, for example, 5 seconds to 10 minutes. When the precursor sheet is sandwiched between two rigid plates, or when heated and pressed by a continuous roll press device or a continuous belt press device, the precursor fiber (b) and / or resin (c ) Or the like is preferably applied in advance, or a release paper is preferably sandwiched between the precursor sheet and the rigid plate or roll or belt.

  Even if one precursor sheet is molded during the heat and pressure molding, a plurality of precursor sheets may be superimposed and integrated.

<Step of carbonizing (3)>
By the step (3), the precursor fiber (b) and / or the resin (c) are carbonized to form a network carbon fiber (B) and / or a resin carbide (C). Thereby, the mechanical strength and electroconductivity of the porous electrode base material obtained are improved.

  The carbonization treatment is preferably performed in an inert gas in order to increase the conductivity of the obtained porous electrode substrate. The carbonization treatment is usually performed at a temperature of 1000 ° C. or higher. 1000-3000 degreeC is preferable and, as for the temperature range which carbonizes, 1000-2200 degreeC is more preferable. The time for performing the carbonization treatment is, for example, about 10 minutes to 1 hour. Further, before the carbonization treatment, pretreatment by firing in an inert atmosphere of about 300 to 800 ° C. can be performed.

  When carbonizing the continuously manufactured precursor sheet, it is preferable to continuously perform the carbonizing process over the entire length of the precursor sheet from the viewpoint of reducing the manufacturing cost. If the porous electrode substrate is made long, the productivity of the porous electrode substrate can be further increased, and the subsequent MEA (Membrane Electrode Assembly) production can also be continuously performed. Manufacturing cost can be easily reduced. Moreover, it is preferable to wind up the manufactured porous electrode base material continuously from a viewpoint of productivity and reduction of manufacturing cost of a porous electrode base material and a fuel cell.

  Moreover, in this invention, as mentioned above, the process (4) which entangles a precursor sheet | seat can be included between a process (1) and a process (2). At this time, the steps (1), (2) and (3) are performed by the method described above. In this form, since the entanglement process step is included, the hand link property of the precursor sheet and the porous electrode substrate is improved. Furthermore, the electrical conductivity in the thickness direction of the porous electrode substrate is improved by the entanglement treatment. Hereinafter, the entanglement process step will be described in detail.

<Process of entanglement process (4)>
The entanglement process for entanglement of the short carbon fibers (A) or the short carbon fibers (A) and the precursor fibers (b) in the precursor sheet may be any method that forms an entangled structure, and is a known method. Can be implemented. For example, a mechanical entanglement method such as a needle punching method, a high pressure liquid injection method such as a water jet punching method, a high pressure gas injection method such as a steam jet punching method, or a combination thereof can be used. The high-pressure liquid injection method is preferable because breakage of the short carbon fibers (A) in the entanglement process can be easily suppressed and moderate entanglement can be easily obtained. Hereinafter, this method will be described in detail.

  The high-pressure liquid jet processing method is a method in which a precursor sheet is placed on a substantially smooth support member and, for example, a liquid columnar flow, a liquid fan-shaped flow, a liquid slit flow, or the like sprayed at a pressure of 1 MPa is applied. The carbon fiber (A) in the precursor sheet or the carbon fiber (A) and the precursor fiber (b) are entangled with each other. Here, the support member having a substantially smooth surface is selected as necessary from the one in which the ejected liquid is quickly removed without being formed in the entangled structure body where the pattern of the support member can be obtained. Can be used. Specific examples thereof include 30-200 mesh wire nets, plastic nets or rolls.

  It is preferable from the viewpoint of productivity that the precursor sheet is manufactured on a support member having a substantially smooth surface, and then the high-pressure liquid injection treatment can be continuously performed because the precursor sheet is entangled.

  The liquid used for the high-pressure liquid jet treatment may be anything other than a solvent that dissolves the fibers constituting the precursor sheet, but it is usually preferable to use water or warm water. The hole diameter of each injection nozzle in the high-pressure liquid injection nozzle is preferably 0.03 mm or more and 1.0 mm or less, and 0.05 mm or more and 0.3 mm or less from the viewpoint that a sufficient confounding effect is obtained in the case of a columnar flow. More preferred. The distance between the nozzle injection hole and the laminate is preferably 0.5 cm or more and 5 cm or less. The pressure of the liquid is preferably 0.5 MPa or more, and more preferably 1.0 MPa or more. The confounding process may be performed in a single row or in a plurality of rows. When performing in a plurality of rows, it is effective to increase the pressure of the high-pressure liquid jet processing in the second and subsequent rows rather than the first.

  The entanglement process by high-pressure liquid injection of the precursor sheet may be repeated a plurality of times. That is, after performing the high-pressure liquid injection process on the precursor sheet, the precursor sheet may be further laminated, and the high-pressure liquid injection process may be performed. Liquid ejection processing may be performed. These operations may be repeated.

  When continuously manufacturing the entangled structure precursor sheet, the high pressure liquid injection nozzle having one or more rows of nozzle holes is vibrated in the width direction of the sheet, resulting in the formation of a dense structure of the sheet in the sheet forming direction. It is possible to suppress the formation of a streak-like trajectory pattern. By suppressing the streak-like trajectory pattern in the sheet forming direction, the mechanical strength in the sheet width direction can be expressed. Further, when a plurality of high-pressure liquid jet nozzles having one or a plurality of rows of nozzle holes are used, the confounding structure precursor is controlled by controlling the frequency and the phase difference of the high-pressure liquid jet nozzles that vibrate in the width direction of the sheet. Periodic patterns appearing on the sheet can also be suppressed.

  Next, in this invention, the process (5) which impregnates a precursor sheet | seat with resin (c) can be included before a heating-pressing process (2). At this time, the steps (1), (2) and (3) are performed by the method described above. In this embodiment, the carbon short fibers (A) and the carbon short fibers (A) and the network carbon fibers (B) can be bound by the resin carbide (C). Hand linkability is improved. Furthermore, the conductivity in the thickness direction of the porous electrode substrate is improved. Hereinafter, the resin impregnation step will be described in detail.

<Resin impregnation step (5)>
In the present invention, the precursor sheet containing carbon short fibers described above is impregnated with carbonizable material, cured by heating and pressing, and then carbonized to obtain a porous electrode substrate for a fuel cell.

  As a method of impregnating the precursor sheet with resin or a mixture of a resin and a conductor, a method using a drawing device or a method of stacking a resin film on a carbon sheet is preferable. In the method using a squeezing device, a carbon sheet is impregnated in a resin solution or mixed solution, and the squeezing device applies the liquid to the entire carbon sheet uniformly, and the amount of liquid is adjusted by changing the roll interval of the squeezing device. It is a method to do. If the viscosity is relatively low, a spray method or the like can also be used.

  In the method using a resin film, first, a thermosetting resin is once coated on a release paper to obtain a thermosetting resin film. Thereafter, the film is laminated on a carbon sheet and subjected to heat and pressure treatment to transfer a thermosetting resin.

  Next, in this invention, the process (6) of oxidizing the heat-pressed precursor sheet | seat can be included between a heat-pressing process (2) and a carbonization process process (3). From the viewpoints of fusing the short carbon fibers (A) well with the precursor fibers (b) and improving the carbonization rate of the precursor fibers (b), the heated and pressurized precursor sheet is oxidized. It is preferable to do. Hereinafter, the oxidation treatment process will be described in detail.

<Step of oxidation treatment (6)>
The temperature of the oxidation treatment is preferably 200 ° C. or higher and lower than 300 ° C., and more preferably 240 ° C. or higher and 290 ° C. or lower from the viewpoint of improving the carbonization rate. The oxidation treatment time can be, for example, 1 minute to 2 hours. As oxidation treatment, continuous oxidation treatment by direct pressure heating using a heated perforated plate or continuous oxidation treatment by intermittent direct pressure heating using a heating roll or the like is low in cost and short carbon fiber (A) Is preferable in that it can be fused with the precursor fiber (b). When oxidizing the precursor sheet manufactured continuously, it is preferable to continuously oxidize the entire length of the precursor sheet. As a result, the carbonization treatment can be performed easily and continuously.

<Suppression of short-circuit current and reactive gas cross-leakage when used in fuel cells>
In order to suppress short-circuit current and reactive gas cross-leakage when used in a fuel cell, it is important to reduce damage to the electrolyte membrane by the porous electrode substrate. This damage to the electrolyte membrane is caused by the sticking out of the fallen carbon short fibers (A), reticulated carbon fibers (B), and resin carbide (C) in the porous electrode substrate. The porous electrode base material is used at a pressure of about 0.5 MPa to 3.5 MPa for the purpose of ensuring the sealing property of the reaction gas and reducing the electrical contact resistance between the respective materials when being incorporated in the fuel cell. It is concluded. When this pressure destroys the short carbon fibers (A), reticulated carbon fibers (B), and resin carbide (C) in the porous electrode base material and falls off the porous electrode base material, it pierces the electrolyte membrane and causes a short circuit current. As a result, cross-leakage of the reaction gas occurs and the durability of the fuel cell is greatly affected. Since the porous electrode substrate is a porous material made of a fiber, a large load is applied to the fiber intersection portion when pressure is applied. In order to suppress the destruction of the short carbon fiber (A), the net-like carbon fiber (B), and the resin carbide (C) due to this load, the area of the fiber intersection portion is increased and the number of intersection points per unit volume is increased. It is essential. In order to realize this, the bulk density of the porous electrode substrate is preferably 0.30 g / cm ³ or more. From the viewpoint of reducing the short-circuit current, the higher the bulk density of the porous electrode substrate, the better. However, considering gas diffusion of the porous electrode substrate, it is preferably 0.60 g / cm ³ or less.

In addition, the carbon short fiber (A), the network carbon fiber (B), and the resin carbide (C) in the porous electrode base material having a thickness of 30 μm or less from one surface in contact with the electrolyte membrane are separated from the electrolyte membrane. However, since the fallout within a thickness of 30 μm or more from one surface in contact with the electrolyte membrane is present in the porous electrode substrate, which is a porous substrate, the fuel Even the fastening pressure when incorporated in the battery does not cause damage to the electrolyte membrane.
Therefore, carbon short fibers and network carbon fibers that are insufficiently bonded on the surface of the porous electrode substrate and resin carbide are sufficiently reduced, and damage to the electrolyte membrane is reduced at the fastening pressure when incorporated in a fuel cell. For this purpose, the bulk density from at least one of the surfaces of the porous electrode substrate to a thickness of 30 μm is preferably 0.30 g / cm ³ or more. If the bulk density from at least one of the surfaces of the porous electrode substrate to a thickness of 30 μm is 0.30 g / cm ³ or more, the entire porous electrode substrate is almost uniformly bulk density 0.30 g / cm. ³ or more. If the bulk density from at least one of the surfaces of the porous electrode substrate to a thickness of 30 μm is 0.30 g / cm ³ or more, the bulk density of the surface from the surface is 30 μm or more. May be 0.30 g / cm ³ or less. Further, if the bulk density from the surface of at least one of the surfaces of the porous electrode substrate to a thickness of 30 μm is 0.30 g / cm ³ or more, the bulk density of the portion of the thickness of 30 μm or more from the surface is the thickness. It may be a bulk density of up to 30 μm.
The bulk density from at least one of the surfaces of the porous electrode base material to a thickness of 30 μm is better from the viewpoint of reducing the short-circuit current, but the higher the bulk density of the porous electrode base material, In consideration of gas diffusion of the porous electrode substrate, it is preferably 0.60 g / cm ³ or less.

The method of setting the bulk density from at least one of the surfaces of the porous electrode base material to a thickness of 30 μm to 0.30 g / cm ³ or more is not particularly limited. (B) It is preferable to increase the ratio, increase the resin (c) ratio, and increase the molding pressure in the heating and pressing step. These methods are preferably used alone or in combination.

When using only increasing the precursor fiber (b) ratio so that the bulk density from at least one of the surfaces of the porous electrode substrate to a thickness of 30 μm is 0.30 g / cm ³ or more. The ratio of the precursor fiber (b) to 50 mass% so that the content of the network carbon fiber (B) at least from one surface in the porous electrode substrate to a thickness of 30 μm is 30 to 60 mass%. It is preferable to control to% or more. When only increasing the resin (c) ratio is used, the content of the resin carbide (C) in at least a portion from one surface in the porous electrode substrate to a thickness of 30 μm is 30 to 60% by mass. It is preferable to control the resin (c) ratio to 70% by mass or more so that When only increasing the molding pressure in the heating and pressing step is used, the bulk density from at least one surface of the porous electrode substrate to a thickness of 30 μm is 0.30 g / cm ³ or more. It is preferable to control the molding pressure.

  In addition, when the bulk density is increased more than necessary, it is also preferable to change the bulk density in the thickness direction of the porous electrode base material when there is a possibility that a problem such as a decrease in gas diffusibility occurs. Although these methods are not particularly limited, such as stacking sheets having different fiber ratios or using a sheet in which the resin (c) is unevenly distributed so that the ratio of the precursor fibers (b) in the precursor sheet changes. Is preferred.

When laminating sheets with different fiber ratios, the bulk density from one surface in the porous electrode substrate to a thickness of 30 μm is 0.30 g / cm ³ or more, and one of the porous electrode substrates in the porous electrode substrate By making the bulk density of the portion having a thickness of 30 μm or more from the surface of the substrate become 0.30 g / cm ³ or less, it is possible to suppress a decrease in gas diffusibility of the porous electrode substrate. In order for the bulk density from one surface in the porous electrode substrate to a thickness of 30 μm to be 0.30 g / cm ³ or more, the portion from the one surface in the porous electrode substrate to the thickness of 30 μm It is preferable to control the precursor fiber (b) ratio to 50% by mass or more so that the content of the reticulated carbon fiber (B) is 30 to 60% by mass, from one surface in the porous electrode substrate. In order to make the bulk density of a part having a thickness of 30 μm or more 0.30 g / cm ³ or less, a mesh-like carbon fiber (B) having a part having a thickness of 30 μm or more from one surface in the porous electrode substrate is used. ) Content of precursor fiber (b) is preferably controlled to 40% by mass or less so that the content is less than 30% by mass.

  The method of laminating sheets with different fiber ratios is not particularly limited, but after preparing two or more sheets with different fiber ratios, a method of bonding them in the heating and pressurizing step or the entanglement process step, or two or more different fiber ratios A method of preparing and mixing the slurry can be used.

When using a sheet in which the resin (c) is unevenly distributed, the bulk density from one surface in the porous electrode substrate to a thickness of 30 μm is 0.30 g / cm ³ or more, By making the bulk density of the portion having a thickness of 30 μm or more from one surface of the material to be 0.30 g / cm ³ or less, it is possible to suppress a decrease in gas diffusibility of the porous electrode substrate. In order for the bulk density from one surface in the porous electrode substrate to a thickness of 30 μm to be 0.30 g / cm ³ or more, the portion from the one surface in the porous electrode substrate to the thickness of 30 μm The resin (c) ratio is preferably controlled to 70% by mass or more so that the content of the resin carbide (C) is 30 to 60% by mass, and the thickness is 30 μm or more from one surface in the porous electrode substrate. In order to make the bulk density of the portion of 0.30 g / cm ³ or less, the content of the resin carbide (C) in the portion of 30 μm or more in thickness from one surface in the porous electrode substrate is 30 It is preferable to control the resin (c) ratio to 60% by mass or less so as to be less than mass%.

  The method for producing the sheet in which the resin (c) is unevenly distributed is not particularly limited, but after producing two or more sheets with different resin ratios, a method of bonding in a heating and pressing process, or a sheet on one side such as a spray coat or a kiss coat From the viewpoint of impregnating the resin (c), a method in which the resin (c) is unevenly distributed can be used by using a sheet whose fiber composition changes in the thickness direction.

<Measurement method of bulk density of porous electrode substrate>
In the measurement of the bulk density of the porous electrode substrate in the present invention, the bulk density of the porous electrode substrate in which the bulk density is not changed in the thickness direction is determined from the weight per unit area and the thickness of the porous electrode substrate. What is necessary is just to calculate a bulk density. Since there is no change in the bulk density in the thickness direction, the bulk density thus determined is a bulk density from the surface to a thickness of 30 μm.

  The bulk density of the porous electrode base material in which the bulk density is changed in the thickness direction is determined by using a cross-section sample preparation device (trade name: Cross Section Polisher, manufactured by JEOL Ltd.). Cut the cross section, measured the density ratio and thickness ratio in the thickness direction with an electron microscope (trade name: scanning electron microscope, manufactured by JEOL Ltd.), and calculated the total bulk density from the weight and thickness per unit area. Can be used to calculate the bulk density from one surface to a thickness of 30 μm.

<Short-circuit current density>
The short-circuit current density is a current density that short-circuits both electrodes of the fuel cell. When this value is increased, not only the current density and energy that can be taken out of the fuel cell but also local heat is generated. It has a great influence on the durability. Although it is possible to increase the strength and thickness of the electrolyte membrane in order to suppress this, the proton conduction resistance increases with this, and the power generation efficiency decreases. Despite the thickness of the electrolyte membrane, the short-circuit current density 0.1 mA / cm ² or less, which will be described later, preferably the durability of the fuel cell by using a porous electrode base material may be 0.08 mA / cm ² or less , Can improve power generation performance.

<Measurement method of short circuit current density>
In the present invention, the short-circuit current density is measured under the following experimental conditions.

(1) A catalyst layer comprising catalyst-supported carbon (catalyst: Pt, catalyst support amount: 50% by mass) on both sides of a perfluorosulfonic acid polymer electrolyte membrane (Nafion NR211: DuPont Co., Ltd .: film thickness: 25 μm) A laminate formed with (catalyst layer area: 25 cm ² , Pt adhesion amount: 0.3 mg / cm ² ) is sandwiched between porous electrode base materials for cathode and anode, and these are joined to form a membrane for a fuel cell -Obtain an electrode assembly (MEA).

  (2) The MEA is sandwiched between two carbon separators having a bellows-like gas flow path attached to a JARI standard cell and fastened so that the surface pressure becomes 1.0 MPa. A solid polymer fuel cell (single cell) ).

  (3) Terminals are connected to both electrodes of the obtained single cell, and the potential difference between the electrodes is set to 1.0 V using a digital multimeter TR6487 (manufactured by Advantest), and the short-circuit current density is measured.

In the present invention, a porous electrode base material having a short-circuit current density of 0.1 mA / cm ² or less according to the measurement method is preferred. The lower the short circuit current density, the better. When the short-circuit current density is large, not only the electromotive force that can be taken out as the fuel cell is lowered, but also the durability is lowered.

  Hereinafter, the present invention will be described more specifically with reference to examples. Each physical property value in the examples was measured by the following method. “Part” means “part by mass”.

[Measurement method of short circuit current density]
Two sets of porous electrode base materials were prepared for the cathode and the anode. Further, a catalyst layer (catalyst comprising catalyst-supporting carbon (catalyst: Pt, catalyst loading: 50% by mass) on both surfaces of a perfluorosulfonic acid polymer electrolyte membrane (Nafion NR211: DuPont Co., Ltd .: film thickness: 25 μm) A laminate having a layer area of 25 cm ² and a Pt adhesion amount of 0.3 mg / cm ² was prepared. The laminate was sandwiched between cathode and anode porous electrode base materials and joined together to obtain a fuel cell membrane-electrode assembly (MEA).

  The MEA is sandwiched between two carbon separators having a bellows-like gas flow path attached to a JARI standard cell and fastened to a surface pressure of 1.0 MPa to form a polymer electrolyte fuel cell (single cell). did.

  Terminals were connected to both electrodes of the single cell, a digital multimeter TR6487 (manufactured by Advantest) was used, the potential difference between the electrodes was 1.0 V, and the short-circuit current density was measured.

[Bulk density within 30μm thickness from one surface]
The bulk density of the porous electrode substrate in which the bulk density did not change in the thickness direction was calculated from the weight per unit area and the thickness described later.

  The bulk density of the porous electrode base material in which the bulk density is changed in the thickness direction is determined by using a cross-section sample preparation device (trade name: Cross Section Polisher, manufactured by JEOL Ltd.). Cut the cross-section, measure the density ratio and thickness ratio in the thickness direction with an electron microscope (JEOL Ltd., trade name: scanning electron microscope), and calculate the total weight calculated from the weight per unit area and the thickness described later. Using the bulk density, the bulk density within a thickness of 30 μm from one surface was calculated.

[Gas permeability]
Based on JIS standard P-8117, the time required for 200 mL of air to permeate was measured using a Gurley densometer, and the gas permeability (ml / hr / cm ² / mmAq) was calculated.

[Thickness]
The thickness of the porous electrode substrate was measured using a thickness measuring device dial thickness gauge (manufactured by Mitutoyo Corporation, trade name: 7321). The size of the probe was 10 mm in diameter, and the measurement pressure was 1.5 kPa.

(Penetration direction resistance)
The electrical resistance (through-direction resistance) in the thickness direction of the porous electrode substrate is obtained by sandwiching the porous electrode substrate between gold-plated copper plates, pressurizing at 1.0 MPa from above and below the copper plate, and at a current density of 10 mA / cm ^2. The resistance value when a current was passed was measured and obtained from the following equation.

Through-direction resistance (mΩ · cm ² ) = Measured resistance value (mΩ) × Sample area (cm ² )
[Average diameter of reticulated carbon fiber (B)]
From the scanning electron micrograph of the surface of the porous electrode substrate, the diameters of the reticulated carbon fibers (B) at arbitrary 50 locations were measured and calculated from the average value.

[Content of reticulated carbon fiber (B)]
The content of the reticulated carbon fiber (B) was calculated from the following formula from the basis weight of the obtained porous electrode substrate and the basis weight of the carbon short fiber (A) used.

Content (% by mass) of reticulated carbon fiber (B) = [Porous electrode substrate basis weight (g / m ² ) −Carbon short fiber (A) basis weight (g / m ² )] ÷ Porous electrode substrate basis weight (G / m ² ) × 100
[Content of resin carbide (C)]
The content rate of the resin carbide (C) was calculated from the following formula or the carbonization rate of the used resin from the basis weight of the obtained porous electrode substrate and the basis weight of the carbon short fiber (A) used.

Content (% by mass) of resin carbide (C) = [Porous electrode substrate basis weight (g / m ² ) −Carbon short fiber (A) basis weight (g / m ² )] ÷ Porous electrode substrate basis weight (g / M ² ) × 100
[Example 1]
As carbon short fibers, 100 parts by mass of PAN-based carbon short fibers having an average diameter of 7 μm cut to a length of 3 mm and polyvinyl alcohol (PVA) fibers having a length of 3 mm (trade name: VBP105-1, manufactured by Kuraray Co., Ltd.) 11 After mass parts were dispersed in water and continuously formed on a wire mesh, they were dried to obtain carbon fiber paper.

  100 parts by mass of this carbon fiber paper is impregnated with a methanol solution of a phenol resin (trade name: Phenolite J-325, manufactured by Dainippon Ink and Chemicals), and the methanol is sufficiently dried at room temperature, so that the nonvolatile content of the phenol resin A phenol resin-impregnated carbon sheet with 80 parts by mass was obtained.

Two of the phenol resin-impregnated carbon sheets were stacked, and a roll press with a linear force of 10 × 10 ⁴ N / m was performed at a temperature of 250 ° C. to cure the phenol resin. Thereafter, the porous electrode base was continuously carbonized at 1900 ° C. in an inert gas (nitrogen) atmosphere, and carbon short fibers having a thickness of 170 μm and a bulk density of 0.31 g / cm ³ were bound with resin carbide. A material was obtained (the obtained porous electrode base material has no change in bulk density in the thickness direction, and the bulk density from the surface to a thickness of 30 μm is 0.31 g / cm ³ ). Moreover, the content rate of the resin carbide (C) of a porous electrode base material was 32%.

The short-circuit current density of the obtained porous electrode substrate was as low as 0.05 mA / cm ² , indicating good characteristics. The gas permeability and penetration direction resistance were also good. The evaluation results are shown in Table 1.

[Example 2]
A porous electrode in which carbon short fibers having a thickness of 170 μm and a bulk density of 0.35 g / cm ³ were bound with a resin carbide except that the nonvolatile content of the phenol resin was 110 parts by mass. A base material was obtained (the resulting porous electrode base material had no change in bulk density in the thickness direction, and the bulk density from the surface to a thickness of 30 μm was 0.35 g / cm ³ ). The resin carbide (C) content of the porous electrode substrate was 41%.

The short-circuit current density of the obtained porous electrode substrate was as low as 0.04 mA / cm ² , indicating good characteristics. The gas permeability and penetration direction resistance were also good. The evaluation results are shown in Table 1.

Example 3
A porous electrode in which carbon short fibers having a thickness of 170 μm and a bulk density of 0.44 g / cm ³ were bound with a resin carbide except that the nonvolatile content of the phenol resin was 180 parts by mass. A base material was obtained (the obtained porous electrode base material had no change in bulk density in the thickness direction, and the bulk density from the surface to a thickness of 30 μm was 0.44 g / cm ³ ). The resin carbide (C) content of the porous electrode substrate was 53%.

The short-circuit current density of the obtained porous electrode substrate was as low as 0.02 mA / cm ² , indicating good characteristics. The gas permeability and penetration direction resistance were also good. The evaluation results are shown in Table 1.

Example 4
As the carbon short fiber (A), a PAN-based carbon fiber having an average fiber diameter of 7 μm and an average fiber length of 3 mm was prepared. Further, as the carbon fiber precursor short fibers (b1), acrylic short fibers having an average fiber diameter of 4 μm and an average fiber length of 3 mm (manufactured by Mitsubishi Rayon Co., Ltd., trade name: D122), fibrillar carbon precursor fibers (b2) -2), an easily split fiber acrylic sea-island composite short fiber (Mitsubishi Rayon Co., Ltd., trade name: Bonnell MVP) consisting of an acrylic polymer fibrillated by beating and diacetate (cellulose acetate) . -C651, average fiber length: 3 mm). The carbon short fibers (A), the carbon fiber precursor short fibers (b1), and the fibrillar carbon precursor fibers (b2-2) are prepared in a mass ratio of 40:30:30 and continuously on the plain woven mesh. Made paper. The sheet thus prepared was entangled using a pressurized water jet treatment apparatus equipped with the following three water jet nozzles and then dried to obtain an entangled sheet.

  Nozzle 1: hole diameter φ0.15 mm, 501 holes, pitch between holes in the width direction 1 mm (1001 holes / width 1 m), one line arrangement, nozzle effective width 500 mm.

  Nozzle 2: hole diameter φ0.15 mm, 501 holes, pitch between holes in the width direction 1 mm (1001 holes / width 1 m), one row arrangement, nozzle effective width 500 mm.

  Nozzle 3: Hole diameter φ0.15 mm, 1002 holes, width-to-hole pitch 1.5 mm, three rows, row pitch 5 mm, nozzle effective width 500 mm.

  The respective pressurized water jet pressures were set to 2 MPa (nozzle 1), 3 MPa (nozzle 2), and 2 MPa (nozzle 3).

Next, both sides of this precursor sheet are sandwiched between papers coated with a silicone-based release agent, followed by roll pressing with a linear force of 10 × 10 ⁴ N / m at a temperature of 200 ° C. did.

Thereafter, the carbon was continuously carbonized at 1900 ° C. in an inert gas (nitrogen) atmosphere, and a porous carbon short fiber having a thickness of 150 μm and a bulk density of 0.31 g / cm ³ was bound with a network carbon fiber. An electrode substrate was obtained (the resulting porous electrode substrate had no change in bulk density in the thickness direction, and the bulk density from the surface to a thickness of 30 μm was 0.31 g / cm ³ ). The content rate of the mesh carbon fiber (B) of the porous electrode substrate was 31%.

The short-circuit current density of the obtained porous electrode substrate was as low as 0.07 mA / cm ² , indicating good characteristics. The gas permeability and penetration direction resistance were also good. The evaluation results are shown in Table 1.

Example 5
Except that the carbon short fiber (A), the carbon fiber precursor short fiber (b1), and the fibrillar carbon precursor fiber (b2-2) were prepared in a mass ratio of 35:35:30, the same as in Example 4. Thus, a porous electrode substrate in which carbon short fibers having a thickness of 130 μm and a bulk density of 0.34 g / cm ³ were bound with resin carbide was obtained (the obtained porous electrode substrate was bulk density in the thickness direction). The bulk density from the surface to a thickness of 30 μm is 0.34 g / cm ³ ). The resin carbide (C) content of the porous electrode substrate was 36%.

The short-circuit current density of the obtained porous electrode substrate was as low as 0.05 mA / cm ² , indicating good characteristics. The gas permeability and penetration direction resistance were also good. The evaluation results are shown in Table 1.

Example 6
The thickness is the same as in Example 4 except that the fibrillar carbon precursor fiber (b2-1) is a polyacrylonitrile pulp in which a large number of fibrils having a diameter of 3 μm or less are branched from a fibrous trunk produced by spray coagulation. Obtained was a porous electrode base material in which short carbon fibers having a bulk density of 0.31 g / cm ³ were bound with a network carbon fiber (the obtained porous electrode base material had a bulk density in the thickness direction). There is no change and the bulk density from the surface to a thickness of 30 μm is 0.31 g / cm ³ ). The content of reticulated carbon fiber (B) in the porous electrode substrate was 31%.

The short-circuit current density of the obtained porous electrode substrate was as low as 0.06 mA / cm ² and showed good characteristics. The gas permeability and penetration direction resistance were also good. The evaluation results are shown in Table 1.

Example 7
A porous material in which carbon short fibers having a thickness of 150 μm and a bulk density of 0.31 g / cm ³ are bound by a network carbon fiber in the same manner as in Example 6 except that the entanglement treatment by pressurized water flow injection was not performed. A porous electrode substrate was obtained (the resulting porous electrode substrate had no change in bulk density in the thickness direction, and the bulk density from the surface to a thickness of 30 μm was 0.31 g / cm ³ ). The content of reticulated carbon fiber (B) in the porous electrode substrate was 31%.
The short-circuit current density of the obtained porous electrode substrate was as low as 0.05 mA / cm ² , indicating good characteristics. The gas permeability and penetration direction resistance were also good. The evaluation results are shown in Table 1.

Example 8
A thickness of 150 μm and a bulk density of 0.31 g / min were the same as in Example 6 except that the pressure-heat-molded precursor sheet was oxidized at 280 ° C. for 1 minute in the air before the carbonization treatment step. A porous electrode base material in which short carbon fibers of cm ³ were bound with a network carbon fiber was obtained (the obtained porous electrode base material has no change in bulk density in the thickness direction and has a thickness of 30 μm from the surface. The bulk density is 0.31 g / cm ³ ). The content of reticulated carbon fiber (B) in the porous electrode substrate was 31%.

The short-circuit current density of the obtained porous electrode substrate was as low as 0.05 mA / cm ² , indicating good characteristics. The gas permeability and penetration direction resistance were also good. The evaluation results are shown in Table 1.

Example 9
Except that the short carbon fiber (A), the short carbon fiber precursor fiber (b1), and the fibrillar carbon precursor fiber (b2-2) were prepared in a mass ratio of 60:20:20, the same as in Example 4. The entangled sheet was obtained. 100 parts by mass of the entangled sheet was impregnated with a water-dispersed phenol resin (trade name: PR-55464, manufactured by Sumitomo Bakelite Co., Ltd.) and sufficiently dried at room temperature, to which 60 parts by mass of the nonvolatile content of the phenol resin was adhered. A phenol resin impregnated sheet was obtained.

This phenol resin impregnated sheet was roll-pressed at a temperature of 250 ° C. and a linear force of 10 × 10 ⁴ N / m to cure the phenol resin. Thereafter, the carbon is continuously carbonized at 1900 ° C. in an inert gas (nitrogen) atmosphere, and carbon short fibers having a thickness of 150 μm and a bulk density of 0.32 g / cm ³ are bound with reticulated carbon fibers and resin carbide. The obtained porous electrode base material was obtained (the obtained porous electrode base material has no change in bulk density in the thickness direction, and the bulk density from the surface to a thickness of 30 μm is 0.32 g / cm ³ ). The contents of the reticulated carbon fiber (B) and the resin carbide (C) of the porous electrode substrate were 13% and 25%, respectively.

The short-circuit current density of the obtained porous electrode substrate was as low as 0.03 mA / cm ² and showed good characteristics. The gas permeability and penetration direction resistance were also good. The evaluation results are shown in Table 1.

Example 10
A carbon short fiber having a thickness of 150 μm and a bulk density of 0.31 g / cm ³ is a reticulated carbon fiber and a resin carbide, except that 100 parts by mass of the nonvolatile content of the phenol resin is adhered. A bonded porous electrode base material was obtained (the obtained porous electrode base material has no change in bulk density in the thickness direction, and the bulk density from the surface to a thickness of 30 μm is 0.31 g / cm ³ ). . The contents of reticulated carbon fiber (B) and resin carbide (C) in the porous electrode substrate were 10% and 36%, respectively.

The short-circuit current density of the obtained porous electrode substrate was as low as 0.02 mA / cm ² , indicating good characteristics. The gas permeability and penetration direction resistance were also good. The evaluation results are shown in Table 1.

Example 11
A carbon short fiber having a thickness of 140 μm and a bulk density of 0.31 g / cm ³ is a reticulated carbon fiber and a resin carbide except that the linear force is set to 13 × 10 ⁴ N / m. A bonded porous electrode base material was obtained (the obtained porous electrode base material has no change in bulk density in the thickness direction, and the bulk density from the surface to a thickness of 30 μm is 0.31 g / cm ³ ). . The contents of the reticulated carbon fiber (B) and the resin carbide (C) of the porous electrode substrate were 13% and 25%, respectively.

The short-circuit current density of the obtained porous electrode substrate was as low as 0.03 mA / cm ² and showed good characteristics. The gas permeability and penetration direction resistance were also good. The evaluation results are shown in Table 1.

Example 12
The short carbon fiber (A), the short carbon fiber precursor fiber (b1), and the fibrillar carbon precursor fiber (b2-2) were prepared at a mass ratio of 60:20:20, and the basis weight was half that of Example 9. An entangled sheet was obtained in the same manner as in Example 9 except that two sheets obtained were obtained. 100 parts by mass of the entangled sheet is impregnated with a water-dispersed phenol resin (trade name: PR-55464, manufactured by Sumitomo Bakelite Co., Ltd.), and is thoroughly dried at room temperature. The nonvolatile content of the phenol resin is 40 and 80 parts by mass, respectively. Two sheets of phenol resin impregnated sheets adhered were obtained.

Two sheets of this phenol resin impregnated sheet were overlapped and a roll press with a linear force of 10 × 10 ⁴ N / m was performed at a temperature of 250 ° C. to cure the phenol resin. Thereafter, carbon is continuously carbonized at 1900 ° C. in an inert gas (nitrogen) atmosphere, and carbon short fibers having a thickness of 150 μm and a bulk density of the entire porous electrode substrate of 0.32 g / cm ³ are reticulated carbon. A porous electrode substrate bound with fibers and resin carbide was obtained. From cross-sectional observation, the bulk density from one surface to a thickness of 30 μm was 0.37 g / cm ³ . The contents of reticulated carbon fiber (B) and resin carbide (C) were 13% and 25%, respectively.

The short-circuit current density of the obtained porous electrode substrate was as low as 0.02 mA / cm ² , indicating good characteristics. The gas permeability and penetration direction resistance were also good. The evaluation results are shown in Table 1.

[Comparative Example 1]
A porous electrode in which carbon short fibers having a thickness of 170 μm and a bulk density of 0.28 g / cm ³ are bound with a resin carbide except that the nonvolatile content of the phenol resin is 60 parts by mass. A base material was obtained (the obtained porous electrode base material had no change in bulk density in the thickness direction, and the bulk density from the surface to a thickness of 30 μm was 0.28 g / cm ³ ). The resin carbide (C) content of the porous electrode substrate was 27%.

The obtained porous electrode substrate had good gas permeability and penetration direction resistance, but the short-circuit current density was as high as 0.11 mA / cm ² . The evaluation results are shown in Table 1.

[Comparative Example 2]
A porous electrode base material in which carbon short fibers having a thickness of 170 μm and a bulk density of 0.27 g / cm ³ are bound with reticulated carbon fibers in the same manner as in Example 9 except that no phenol resin was impregnated. (The obtained porous electrode base material has no change in bulk density in the thickness direction, and the bulk density from the surface to a thickness of 30 μm is 0.27 g / cm ³ ). The content of reticulated carbon fiber (B) in the porous electrode substrate was 17%.

Although the gas permeability and penetration direction resistance of the obtained porous electrode substrate were good, the short-circuit current density was as high as 0.15 mA / cm ² . The evaluation results are shown in Table 1.

Claims (2)

A porous electrode base material in which short carbon fibers are bound with reticulated carbon fibers and / or resin carbide, and the bulk density from the surface to a thickness of 30 μm is 0 on at least one surface of the porous electrode base material. A porous electrode base material of ³⁰ g / cm ³ or more. The porous electrode base material whose short circuit current density measured on the following experimental conditions is 0.1 mA / cm < ² > or less.
<Experimental conditions>
(1) A catalyst layer comprising catalyst-supported carbon (catalyst: Pt, catalyst support amount: 50% by mass) on both sides of a perfluorosulfonic acid polymer electrolyte membrane (Nafion NR211: DuPont Co., Ltd .: film thickness: 25 μm) A laminate formed with (catalyst layer area: 25 cm ² , Pt adhesion amount: 0.3 mg / cm ² ) is sandwiched between porous electrode base materials for cathode and anode, and these are joined to form a membrane for a fuel cell -Obtain an electrode assembly (MEA).
(2) The MEA is sandwiched between two carbon separators having a bellows-like gas flow path attached to a JARI standard cell and fastened so that the surface pressure becomes 1.0 MPa. A solid polymer fuel cell (single cell) ).
(3) Terminals are connected to both electrodes of the obtained single cell, and the potential difference between the electrodes is set to 1.0 V using a digital multimeter TR6487 (manufactured by Advantest), and the short-circuit current density is measured.