JP2013119690A - Method of manufacturing precursor sheet for porous electrode base material, method of manufacturing porous electrode base material, porous electrode base material, membrane-electrode assembly and polymer electrolyte fuel cell - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a precursor sheet for a porous electrode base material having a low manufacturing cost of precursor sheets and having reduced fiber bundles in which carbon fibers are bundled, and a method of manufacturing the same.SOLUTION: There is provided a method of manufacturing a precursor sheet for a porous electrode base material by papermaking fiber mixed slurry, in which (i) fibers in the fiber mixed slurry is a mixture of short carbon fibers (A) and precursor fibers (b), and a mixed ratio of the short carbon fibers (A) is 40% or more and 90% or less as a mass ratio to a whole amount of the fiber materials, (ii) a fiber concentration of the fiber mixture in the fiber mixed slurry is 0.8-1.8 g/L, and (iii) freeness of the fiber mixed slurry is 80-300 ml.

Description

  The present invention relates to a method for producing a precursor sheet for a porous electrode substrate used in a polymer electrolyte fuel cell, a method for producing a porous electrode substrate, a porous electrode substrate, a membrane-electrode assembly, and a solid The present invention relates to a molecular fuel cell.

  A polymer electrolyte fuel cell is characterized by using a proton-conducting polymer electrolyte membrane, and is an apparatus for obtaining an electromotive force by electrochemically reacting a fuel gas such as hydrogen and an oxidizing gas such as oxygen. is there. In such a polymer electrolyte fuel cell, two sets of gas diffusion electrodes each having a catalyst layer mainly composed of carbon powder supporting a noble metal catalyst and a gas diffusion electrode substrate are arranged on the inner side of each catalyst layer. Thus, the polymer electrolyte membrane has a structure bonded to both surfaces.

  The gas diffusion electrode substrate is generally composed of a carbonaceous material, and the precursor sheet of the gas diffusion electrode substrate is composed of a carbonaceous material and a carbonaceous precursor material. It is manufactured by a wet papermaking method using carbonaceous fibers. When a fiber bundle of carbonaceous fibers is present, unevenness occurs in gas diffusibility when used as a gas diffusion electrode substrate. Therefore, in the precursor sheet manufacturing method by the wet papermaking method, carbonaceous fibers are dispersed. The concentration of the slurry is adjusted to about 0.05 to 0.5 g / L to form a sheet. As a general gas diffusion electrode substrate precursor sheet and gas diffusion electrode substrate, for example, the following gas diffusion electrode substrate precursor sheet and gas diffusion electrode substrate are known.

  Patent Document 1: A carbon fiber nonwoven fabric comprising 70% by mass or more of polyacrylonitrile-based carbon short fibers, wherein the carbon fibers are hydroentangled with each other.

Patent Document 2: Under conditions of a thickness of 0.05 to 0.5 mm, a bulk density of 0.3 to 0.8 g / cm ³ , a strain rate of 10 mm / min, a fulcrum distance of 2 cm, and a specimen width of 1 cm. In the three-point bending test, a porous carbon electrode substrate for a fuel cell, having a bending strength of 10 MPa or more and a bending deflection of 1.5 mm or more.

  Patent Document 3: A mat comprising a plurality of carbon fibers; and a plurality of acrylic pulp fibers incorporated into the carbon fiber mat, the acrylic pulp fibers being cured and carbonized after being incorporated into the carbon fiber mat. Gas diffusion layer for a fuel cell.

Patent Document 4: In a method for producing a carbon fiber sheet by firing a polyacrylonitrile-based oxidized fiber sheet, the bulk density is obtained by compressing the polyacrylonitrile-based oxidized fiber sheet in the thickness direction under conditions of 150 to 300 ° C. and 10 to 100 MPa. Of carbon fiber sheet, characterized by obtaining an oxidized fiber sheet subjected to a compression treatment of 0.40 to 0.80 g / cm ³ and a compression ratio of 40 to 75%, and then firing the oxidized fiber sheet subjected to the compression treatment Method.

  Patent Document 5: Step (I) of feeding a reinforcing fiber bundle into a dispersion medium, step (II) of preparing a slurry in which reinforcing fibers constituting the reinforcing fiber bundle are dispersed in the dispersion medium, and step of transporting the slurry (III) and a step (IV) of obtaining a papermaking substrate containing reinforcing fibers by removing the dispersion medium from the slurry, and having a mass content of reinforcing fibers in the slurry prepared in the step (II) C1 and when the mass content of reinforcing fibers in the slurry at the start of the step (IV) is C2, C1 / C2 of the papermaking substrate containing reinforcing fibers in the range of 0.8 to 1.2 Production method. And the manufacturing method of the papermaking base material whose mass content rate of the reinforced fiber in a slurry is 0.01-1 mass%.

JP 2002-266217 A International Publication No. 2001/056103 Pamphlet JP 2007-273466 A International Publication No. 2002/042534 Pamphlet JP 2010-37666 A

  However, the carbon fiber nonwoven fabric disclosed in Patent Document 1 has a problem in handling due to low mechanical strength due to the entanglement of carbon fibers with high rigidity. The porous carbon electrode substrate disclosed in Patent Document 2 has high mechanical strength and surface smoothness and sufficient gas permeability and conductivity, but has a problem of high manufacturing cost. It was. The gas diffusion layer for fuel cells disclosed in Patent Document 3 can be manufactured at low cost, but there is little entanglement between carbon fiber and acrylic pulp when forming into a sheet, and handling is difficult was there. In addition, since acrylic pulp has almost no molecular orientation of the polymer compared to fibrous materials, the carbonization rate during carbonization is low, and it was necessary to add a large amount of acrylic pulp in order to improve handling properties. . The carbon fiber sheet and the carbon fiber non-woven fabric disclosed in Patent Document 4 can be manufactured at low cost, but the shrinkage during firing is large, the thickness unevenness of the obtained sheet or the like is large, and the sheet waviness ( There is a problem that the sheet cross-section is wavy or warped.

  In addition, when the precursor sheet of the gas diffusion electrode substrate and the gas diffusion electrode substrate are manufactured by a wet papermaking method, it is necessary to use a low-concentration slurry in order to reduce the fiber bundle in which the carbon fibers are concentrated. Since water is used and a large utility for dehydrating the water is required, the production cost of the precursor sheet is increased.

  Although the papermaking substrate manufacturing method disclosed in Patent Document 5 controls the dilution in each step of the papermaking slurry, it reduces the fiber bundle in which the carbon fibers are concentrated when the papermaking slurry concentration is high. For this reason, no technique has been presented, and the papermaking substrate used for the gas diffusion electrode substrate for fuel cells has a problem that many fiber bundles in which carbon fibers are concentrated are generated under the condition of a high concentration slurry.

  The present invention overcomes the problems as described above, has a low production cost when producing a precursor sheet, and has a small number of fiber bundles in which carbon fibers are converged, and production thereof And a porous electrode substrate having excellent handling properties and surface smoothness, improved sheet thickness unevenness and undulation, and sufficient gas permeability and conductivity. An object is to provide a manufacturing method.

  The above problems are solved by the following inventions [1] to [11].

[1] In a method for producing a porous electrode substrate precursor sheet by paper-making a fiber mixed slurry,
(I) The fiber in the fiber mixed slurry is a mixture containing carbon short fibers (A) and precursor fibers (b), and the mixing ratio of the carbon short fibers (A) is 40% or more by mass ratio with respect to the total amount of the fiber material. 90% or less,
(Ii) The fiber concentration in the fiber mixed slurry is 0.8 to 1.8 g / L,
And (iii) A method for producing a porous electrode substrate precursor sheet, wherein the drainage of the fiber-mixed slurry is 80 to 300 ml.

  [2] Step (1) of heating and pressurizing the porous electrode substrate precursor sheet obtained by the production method described in [1] above, and 1000 ° C of the precursor sheet heated and pressed in step (1) The manufacturing method of the porous electrode base material which has the process (2) which carbonizes at the above temperature.

  [3] The production method according to [2], further including a step (4) of oxidizing the heated and pressurized porous electrode substrate precursor sheet between the step (1) and the step (2).

  [4] Prior to step (1), the method according to [2] or [3] above, further comprising a step (3) of entanglement treatment of the porous electrode substrate precursor sheet obtained by the production method according to claim 1. Manufacturing method.

  [5] The step (5) of impregnating the porous electrode substrate precursor sheet obtained by the production method according to claim 1 with a carbonizable resin (c) before the step (1). [2] The production method according to [3].

  [6] The method according to [4], further including a step (5) of impregnating the porous electrode substrate precursor sheet with a carbonizable resin (c) between the step (3) and the step (1). Production method.

[7] The above-mentioned [1] to [1], wherein the porous electrode substrate precursor sheet is a porous electrode substrate precursor sheet in which the fiber bundle ratio of the short carbon fibers (A) is 2 or less. [6] The production method according to any one of [6].
<How to find the fiber bundle ratio>
(I) Light is applied from the back and top surfaces of the porous electrode substrate precursor sheet, and a 15 cm × 15 cm range is photographed using a digital camera having 2 million pixels or more.
(II) Based on the photograph taken in (I) above, the number of fiber bundles in a state where a plurality of short carbon fibers (A) are bundled in the range of 15 cm × 15 cm is measured.
(III) The above measurements (I) and (II) are performed at any three locations on the porous electrode substrate precursor sheet, and the average value is defined as the fiber bundle ratio.

[8] A porous material in which short carbon fibers (A) are bound by network carbon fibers (B) and / or resin carbide (C), and the fiber bundle ratio of the short carbon fibers (A) is 2 or less. Electrode substrate.
<How to find the fiber bundle ratio>
(I) Light is applied from the back and top surfaces of the porous electrode substrate, and a 15 cm × 15 cm range is photographed using a digital camera having 2 million pixels or more.
(II) Based on the photograph taken in (I) above, the number of fiber bundles in a state where a plurality of short carbon fibers (A) are bundled in the range of 15 cm × 15 cm is measured.
(III) The above measurements (I) and (II) are performed at any three locations on the porous electrode substrate, and the average value is defined as the fiber bundle ratio.

  [9] A porous electrode substrate obtained by the method according to any one of [2] to [7].

  [10] A membrane-electrode assembly using the porous electrode substrate according to [8] or [9].

  [11] A polymer electrolyte fuel cell using the membrane-electrode assembly according to [10].

  According to the present invention, the porous electrode base material precursor sheet, the manufacturing cost thereof, and the porous electrode base material are low in manufacturing cost when the precursor sheet is manufactured and the number of fiber bundles in which carbon fibers are bundled is small. It is possible to provide a porous electrode substrate having excellent handling properties and surface smoothness, improved sheet thickness unevenness and undulation, and sufficient gas permeability and conductivity, and a method for producing the same. .

  Hereinafter, the present invention will be described in detail.

<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)>
The reticulated carbon fiber (B) is a fiber that joins the short carbon fibers (A) to each other and exists in a state of being bent or curved at the joint, and each forms a network structure. The reticulated carbon fiber (B) is obtained by carbonizing the precursor fiber (b) by heating. The short carbon fiber (A) is bound by the reticulated carbon fiber (B) by mixing and firing the short carbon fiber (A) and the precursor fiber (b).

  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. A fibrillar carbon precursor fiber (b2) is preferred.

<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. Since the drainage of the slurry is changed by using the fibrillar carbon precursor fiber (b2), the freeness and mixing of the fibrillar carbon precursor fiber (b2) are adjusted so that the drainage of the slurry is 80 to 300 ml. It is preferable to adjust the ratio.

  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.

<Precursor sheet>
The precursor sheet is obtained by dispersing short carbon fibers (A) and precursor fibers (b), and may or may not form a three-dimensional entangled structure. The entanglement process for forming the three-dimensional entangled structure will be described later.

  The mass ratio of the short carbon fibers (A) constituting the precursor sheet in the sheet is 40% based on the total amount of the fiber material from the viewpoint of the sheet strength expression of the precursor sheet and the porous electrode substrate after heat treatment. More than 90% is preferable.

  In the present invention, the “total amount of fibrous material” refers to the amount of all fibrous materials present in the sheet. For example, when an organic polymer compound is used in the form of a fiber as a binder, the mass of the fibrous binder is added to the mass of the short carbon fiber (A) and the precursor fiber (b) as a fiber material, and the short carbon fiber The mass ratio of (A) to the total amount of the fiber material is determined.

  The precursor sheet can be produced by a wet method. The wet method is a method of making paper by dispersing short carbon fibers (A) and precursor fibers (b) in a liquid medium. Helps the short carbon fibers (A) to open into single fibers, prevents the opened single fibers from re-converging, and the short carbon fibers (A) and the precursor fibers (b) are intertwined into the sheet. In order to improve the strength and make it substantially binder-free, it is preferable to use a fibrillated carbon precursor fiber (b2) to produce by a wet method.

  Examples of the medium in which the short carbon fiber (A) and the precursor fiber (b) are dispersed include, for example, a medium in which the precursor fiber (b) is not dissolved, such as water and alcohol. preferable.

  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 presence of a bundle of short carbon fibers (A) in the precursor sheet causes local gas diffusivity and conductive spots when used as a porous electrode substrate, and is incorporated into a fuel cell. As a result, the power generation performance becomes uneven, which greatly affects battery durability. Therefore, it is preferable that the fiber bundle ratio, which is the ratio of the fiber bundles in which the short carbon fibers (A) in the precursor sheet are converged, is 2 or less.

<How to find the fiber bundle ratio>
(I) Light is applied from the back and top surfaces of the porous electrode substrate precursor sheet, and a 15 cm × 15 cm range is photographed using a digital camera having 2 million pixels or more.
(II) Based on the photograph taken in (I) above, the number of fiber bundles in a state where a plurality of short carbon fibers (A) are bundled in the range of 15 cm × 15 cm is measured.
(III) The above measurements (I) and (II) are performed at any three locations on the porous electrode substrate precursor, and the average value is defined as the fiber bundle ratio.

<Slurry>
When preparing a precursor sheet by a wet method, a slurry for sheet formation can be adjusted by dispersing a fiber mixture containing short carbon fibers (A) and precursor fibers (b) in a dispersion solvent.

  From the viewpoint of the sheet strength expression of the precursor sheet and the porous electrode substrate after the heat treatment, the mixing ratio of the short carbon fibers (A) in the fiber mixed slurry is 40% or more and 90% or less by mass ratio with respect to the total amount of the fiber substance. It is preferable to do.

  In the present invention, the “total amount of fibrous material” refers to the amount of all fibrous materials present in the slurry. For example, when an organic polymer compound is used in the form of a fiber as a binder, the mass of the fibrous binder is added to the mass of the short carbon fiber (A) and the precursor fiber (b) as a fiber material, and the short carbon fiber The mass ratio of (A) to the total amount of the fiber material is determined.

  In order to prevent generation of a bundle of short carbon fibers (A) in the precursor sheet, to reduce the amount of water used during slurry preparation, and to reduce utility during dehydration, the fiber concentration in the slurry Is preferably 0.8 to 1.8 g / L, and a slurry having a drainage of 80 to 300 ml described later is preferably used. If the fiber concentration is too high, a fiber bundle in which the short carbon fibers (A) are concentrated is generated in the precursor sheet. If the fiber concentration is too low, water and utilities increase, and the production cost of the precursor sheet increases. If the drainage is too high, a fiber bundle in which the short carbon fibers (A) dispersed in the slurry are converged again will be generated. If the drainage is too low, it will be difficult to dehydrate the water as the dispersion medium from the slurry. In addition to the increase in utility and the production cost of the precursor sheet, the sheet formation deteriorates because the dewatering position is not constant when continuously forming sheets.

  The “fiber concentration in the slurry” in the present invention is the concentration of all the fibers in the slurry. For example, when an organic polymer compound is used in the form of a fiber as a binder, the fibrous binder is also converted into a fiber in addition to the short carbon fiber (A) and the precursor fiber (b) to obtain the fiber concentration.

  For the evaluation of the freeness of the slurry, the freeness of the slurry is measured using a freeness meter used for measuring the freeness of fibers (JIS P8121 (pulp freeness test method: Canadian standard type)).

Slurry drainage (ml) = Paper slurry for papermaking was filled into a drainage meter, and the drainage was evaluated by a method based on JIS P8121 <Porous electrode substrate>
The porous electrode substrate of the present invention comprises a structure in which short carbon fibers (A) dispersed in a structure are bound by network carbon fibers (B), or a short carbon dispersed in a structure. The fibers (A) are bound together by the reticulated carbon fibers (B), and between the carbon short fibers (A) and between the carbon short fibers (A) and the reticulated carbon fibers (B) by the resin carbide (C). Is a structure in which carbon short 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, it is 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.

  If there is a bundle of short carbon fibers (A) bundled in the porous electrode substrate, local gas diffusibility and conductivity spots will cause spots in the power generation performance when assembled in a fuel cell. It has a great effect on battery durability. Therefore, it is preferable that the fiber bundle ratio, which is the ratio of the fiber bundles in which the short carbon fibers (A) in the porous electrode substrate are focused, is 2 or less.

  When a fiber bundle in which short carbon fibers (A) are concentrated is present in the porous electrode substrate of the present invention, local gas diffusibility and conductive spots are the cause, and power generation performance in a state where it is incorporated in a fuel cell. Spots appear on the battery, greatly affecting battery durability. Therefore, it is preferable that the fiber bundle ratio, which is the ratio of the fiber bundles in which the short carbon fibers (A) in the porous electrode substrate are focused, is 2 or less.

<How to find the fiber bundle ratio>
(I) Light is applied from the back and top surfaces of the porous electrode substrate, and a 15 cm × 15 cm range is photographed using a digital camera having 2 million pixels or more.
(II) Based on the photograph taken in (I) above, the number of fiber bundles in a state where a plurality of short carbon fibers (A) are bundled in the range of 15 cm × 15 cm is measured.
(III) The above measurements (I) and (II) are performed at any three locations on the porous electrode substrate, and the average value is defined as the fiber bundle ratio.

<Production method of the present invention>
In the production method of the present invention, the fibers in the fiber mixed slurry are a mixture containing carbon short fibers (A) and precursor fibers (b), and the mixing ratio of the carbon short fibers (A) is a mass ratio with respect to the total amount of the fiber material. A porous electrode substrate precursor sheet is produced by papermaking a fiber mixed slurry of 40% or more and 90% or less.

  As a method for producing the precursor sheet, a wet method in which the short carbon fibers (A) or the short carbon fibers (A) and the precursor fibers (b) are dispersed in a liquid medium to produce paper has a sheet uniformity. It is preferable from the viewpoint of high.

  When making paper by a wet method, the short carbon fibers (A) help to open into single fibers, prevent the opened single fibers from refocusing, and use water and utilities for making paper. In order to reduce the amount of the fiber mixture containing 40 to 90% by mass of the carbon short fiber (A) and the carbon fiber precursor short fiber (b1) and / or the fibrillar carbon precursor fiber (b2) in a mass ratio with respect to the total amount of the fiber material It is preferable to use a slurry having a fiber concentration of 0.8 to 1.8 g / L and a drainage of 50 to 300 ml. 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.

  The method for adjusting the drainage of the slurry to 80 to 300 ml is not particularly limited, but the fiber material of the precursor fiber (b) (for example, the fibrillar carbon precursor fiber (b2)) having a low fiber concentration or low freeness is not limited. A method for controlling the mass ratio relative to the total amount and a method for controlling the amount of paper strength enhancer such as polyacrylamide or carboxymethylcellulose are preferred. It is also preferable to use a combination of these methods. Increase the mass ratio of the fiber in the slurry and the low-freeness precursor fiber (b) (for example, fibrillar carbon precursor fiber (b2)) to the total amount of fiber material, paper such as polyacrylamide or carboxymethylcellulose By increasing the addition amount of the force enhancer, the drainage of the slurry can be reduced, and the fiber concentration in the slurry and the precursor fiber (b) having a low freeness (for example, the fibrillar carbon precursor fiber (b2) The drainage of the slurry can be increased by lowering the mass ratio of)) to the total amount of the fiber substance and by reducing the amount of paper strength enhancer such as polyacrylamide or carboxymethylcellulose. Specifically, the drainage may be determined based on the above-described method for evaluating the drainage of the slurry, and the drainage may be adjusted in accordance with the above-described correspondence.

  Examples of the method of mixing the short carbon fiber (A) and the precursor fiber (b) include a method of stirring and dispersing in water and a method of directly mixing these. From the viewpoint of uniform dispersion, stirring and dispersing in water. The method of making it preferable is. The strength of the precursor sheet is improved by mixing the short carbon 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.
In the precursor sheet, the carbon short fibers (A) are preferably 40% or more and 90% or less by mass ratio. By setting the mass ratio of the short carbon fibers (A) to 90% or less, the amount of the network carbon fibers (B) to be formed is appropriately increased. Therefore, the precursor sheet and the porous electrode substrate after the heat treatment are used. The strength of the sheet can be easily improved. By setting the mass ratio of the short carbon fibers (A) to 40% or more, the sheet can be easily contracted due to the short carbon fibers (A) that suppress the shrinkage of the precursor fibers (b) during carbonization. The strength of the precursor sheet and the porous electrode substrate sheet after heat treatment can be easily improved.

  The porous electrode substrate precursor sheet thus obtained is heated and pressurized (1), and the precursor sheet heated and pressurized in the process (1) is carbonized at a temperature of 1000 ° C. or higher. A porous electrode base material can be manufactured by passing through the process (2) to do.

  More preferably, a step (4) of oxidizing the heated and pressurized porous electrode substrate precursor sheet is provided between the step (1) and the step (2).

  Further, it is more preferable to provide a step (3) for entanglement the porous electrode substrate precursor sheet obtained by the above production method before the step (1).

  Furthermore, before the step (1) or between the step (3) and the step (1) when the step (3) is included, the carbonizable resin (c) is applied to the porous electrode substrate precursor sheet. It is preferable to provide an impregnation step (5).

  Hereinafter, steps (1) to (5) will be described.

<Step of heat and pressure treatment (1)>
A structure in which the short carbon fibers (A) in which the porous electrode base material is dispersed in the structure are sufficiently bound by the network carbon fibers (B), or the short carbon in which the porous electrode substrate is dispersed in the structure. The fibers (A) are sufficiently bound together by the reticulated carbon fibers (B), and between the carbon short fibers (A) and the carbon short fibers (A) and the reticulated carbon fibers (B) by the resin carbide (C). In order to obtain a structure in which the gap is sufficiently bound and to reduce unevenness in the thickness of the porous electrode substrate, it is preferable to heat and press mold the precursor 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 carbonization treatment (2)>
By the step (2), the precursor fiber (b) and / or the resin (c) is carbonized to form a reticulated 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 (3) which entangles a precursor sheet | seat can be included before a process (1). At this time, steps (1) and (2) 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 (3)>
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 (4) of oxidizing the heat-pressed precursor sheet | seat can be included between the heat-pressing process (1) and the carbonization-process process (2). 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.

<Oxidation process (4)>
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.

  Next, in this invention, the process (5) which impregnates a precursor sheet | seat with resin (c) can be included before a heating-pressing process (1). If there is a step (3), a step (5) is provided between the step (3) and the step (1). At this time, the steps (1), (2) and (3) are performed by the method described above. In this embodiment, since the carbon short fibers (A) and between the carbon short fibers (A) and the reticulated carbon fibers (B) can be bound with the resin carbide (C), the porous electrode base material 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 short carbon fibers described above is impregnated with a carbonizable resin or a mixture of a resin and a conductor, cured by heating and pressurization, and then carbonized to provide a porous fuel cell. An electrode substrate is used.

  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.

<Membrane-electrode assembly (MEA)>
The porous electrode substrate of the present invention can be suitably used for a membrane-electrode assembly. The membrane-electrode assembly is composed of a polymer electrolyte membrane, a catalyst layer, and a porous carbon electrode base material. A cathode side catalyst layer comprising a catalyst for oxidizing gas is provided on one surface of the polymer electrolyte membrane having proton conductivity. And an anode side catalyst layer made of a catalyst for fuel gas on the other surface, and a cathode side porous electrode substrate and an anode side porous electrode substrate are provided outside each catalyst layer. Yes.

<Solid polymer fuel cell>
The membrane-electrode assembly of the present invention can be suitably used for a polymer electrolyte fuel cell. The polymer electrolyte fuel cell includes a cathode side separator in which a cathode side gas flow path is formed and an anode side separator in which an anode side gas flow path is formed so as to sandwich a membrane-electrode assembly. Each separator is provided with an oxidizing gas inlet and an oxidizing gas outlet, and a fuel gas inlet and a fuel gas outlet.

  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”.

(1) Gas permeability According to JIS standard P-8117, the time taken for 200 mL of air to permeate was measured using a Gurley densometer, and the gas permeability (ml / Hr / cm ² / mmAq) was calculated.

(2) Thickness The thickness of the porous electrode base material 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.

(3) Through-direction resistance The electrical resistance (through-direction resistance) in the thickness direction of the porous electrode substrate is 10 mA / cm ² when the porous electrode substrate is sandwiched between gold-plated copper plates and pressed from above and below the copper plate at 1 MPa. The resistance value when a current was passed at a current density of was measured from the following equation.

(Equation 1)
Through-direction resistance (mΩ · cm ² ) = Measured resistance value (mΩ) × Sample area (cm ² )
(4) Waviness of the porous electrode base material The waviness of the porous electrode base material is calculated from the difference between the maximum value and the minimum value when the porous electrode base material having a length of 250 mm and a width of 250 mm is left on a flat plate. did.

(5) Power generation characteristics when incorporated in a fuel cell A catalyst layer (catalyst layer area: 25 cm ² , Pt adhesion amount: 0.3 mg / kg) composed of catalyst-supported carbon (catalyst: Pt, catalyst support amount: 50 mass%) on both sides cm ² ) perfluorosulfonic acid polymer electrolyte membrane (film thickness: 30 μm) with two sets of porous electrodes so that the three-dimensional structure side without a three-dimensional entangled structure is in contact with the polymer electrolyte membrane The MEA was obtained by sandwiching the substrates and joining them together. The MEA was sandwiched between two carbon separators having a bellows-like gas flow path to produce a polymer electrolyte fuel cell (single cell). And the power generation characteristic at the time of incorporating in a fuel cell was confirmed by measuring the voltage when hydrogen gas and air were supplied to the single cell made into 80 degreeC through the bubbler of 80 degreeC.

Example 1
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 fiber (b1), an acrylic short fiber (trade name: D122, manufactured by Mitsubishi Rayon Co., Ltd.) having an average fiber diameter of 4 μm and an average fiber length of 3 mm is prepared, and a fibrillated carbon precursor is prepared. As the fiber (b2), an easily splittable acrylic sea-island composite short fiber (b2-2) (manufactured by Mitsubishi Rayon Co., Ltd., product name) composed of an acrylic polymer fibrillated by beating and diacetate (cellulose acetate). Bonnell MVP-C651, average fiber length: 3 mm) was prepared.

  A precursor sheet and a porous electrode substrate were obtained by the following operations.

[Disaggregation of short carbon fiber (A)]
The carbon short fiber (A) was dispersed in water so that the fiber concentration was 5 g / L, and was disaggregated through a disc refiner (manufactured by Kumagai Riki Kogyo Co., Ltd.) to obtain a disaggregated slurry fiber (SA).

[Disaggregation of carbon fiber precursor short fiber (b1)]
The carbon fiber precursor short fiber (b1) is dispersed in water so that the fiber concentration becomes 5 / L, and is disaggregated through a disc refiner (manufactured by Kumagaya Rikyu Kogyo Co., Ltd.), and disaggregated slurry fiber (Sb1) It was.

[Disaggregation of fibrillar carbon precursor fiber (b2)]
The easily splittable acrylic sea-island composite short fibers are dispersed in water so that the fiber concentration is 5 g / L, and beaten and disaggregated through a disc refiner (manufactured by Kumagai Riki Kogyo Co., Ltd.). Sb2).

[Preparation of papermaking slurry]
The mass ratio of carbon short fiber (A), carbon fiber precursor short fiber (b1), and fibrillar carbon precursor fiber (b2) is 60:20:20, and the fiber concentration in the slurry is 1.4 g. The disaggregated slurry fiber (SA), the disaggregated slurry fiber (Sb1), the disaggregated slurry fiber (Sb2), and the dilution water were weighed so as to be / L and charged into the slurry supply tank. Furthermore, polyacrylamide (Daiichi Kogyo Seiyaku Co., Ltd., trade name: G9210) was added as a paper strength enhancer to prepare a papermaking slurry having a drainage of 132 ml.

[Manufacture of precursor sheets]
Using the adjusted papermaking slurry, paper can be manually made in accordance with JIS P-8209 method using a standard square sheet machine (manufactured by Kumagai Riki Kogyo Co., Ltd., trade name: No. 2555 standard square sheet machine). This was performed and dried to obtain a precursor sheet having a basis weight of 35 g / m ² . The produced precursor sheet had almost no fiber bundles in which short carbon fibers were bundled, and the fiber bundle ratio was 1.

[Heat and pressure molding]
Next, both sides of the precursor sheet were sandwiched between papers coated with a silicone-based release agent, and then heat-press molded for 3 minutes under conditions of 180 ° C. and 3 MPa in a batch press apparatus.

[Carbonization treatment]
Thereafter, the precursor sheet was carbonized in a batch carbonization furnace in a nitrogen gas atmosphere at 2000 ° C. for 1 hour to obtain a porous electrode substrate.

  The obtained porous electrode substrate had almost no fiber bundles in which short carbon fibers were bundled, and the fiber bundle ratio was 1. In addition, there was almost no in-plane shrinkage during carbonization treatment, the swell was as small as 2 mm or less, the surface smoothness was good, and the gas permeability, thickness, penetration direction resistance, thickness unevenness, and handling properties were also good. It was. The evaluation results are shown in Table 1.

(Examples 2 to 5)
A precursor sheet and a porous electrode base material were obtained in the same manner as in Example 1 except that the fiber concentration and drainage in the slurry were changed to the conditions shown in Table 1. The precursor sheet had almost no fiber bundles in which short carbon fibers were bundled, and the fiber bundle ratio was 1 or less. Furthermore, the obtained porous electrode substrate had almost no fiber bundles in which short carbon fibers were bundled, and the fiber bundle ratio was 1. In addition, there was almost no in-plane shrinkage during carbonization treatment, the swell was as small as 2 mm or less, the surface smoothness was good, and the gas permeability, thickness, penetration direction resistance, thickness unevenness, and handling properties were also good. It was. The evaluation results are shown in Table 1.

(Examples 6 to 8)
Example 1 except that the amount of polyacrylamide as a paper strength enhancer (Daiichi Kogyo Seiyaku Co., Ltd., trade name: G9210) added to the slurry was adjusted to the conditions shown in Table 1. In the same manner as above, a precursor sheet and a porous electrode substrate were obtained. The precursor sheet had almost no fiber bundles in which short carbon fibers were bundled, and the fiber bundle ratio was 1 or less. Furthermore, the obtained porous electrode substrate had almost no fiber bundles in which short carbon fibers were bundled, and the fiber bundle ratio was 1. In addition, there was almost no in-plane shrinkage during carbonization treatment, the swell was as small as 2 mm or less, the surface smoothness was good, and the gas permeability, thickness, penetration direction resistance, thickness unevenness, and handling properties were also good. It was. The evaluation results are shown in Table 1.

(Examples 9 to 13)
Except that the mass ratio of the carbon short fiber (A), the carbon fiber precursor short fiber (b1), and the fibrillar carbon precursor fiber (b2) and the drainage in the slurry were the conditions shown in Table 1, Examples In the same manner as in Example 1, a precursor sheet and a porous electrode substrate were obtained. The precursor sheet had almost no fiber bundles in which short carbon fibers were bundled, and the fiber bundle ratio was 1 or less. Furthermore, the obtained porous electrode substrate had almost no fiber bundles in which short carbon fibers were bundled, and the fiber bundle ratio was 1. In addition, there was almost no in-plane shrinkage during carbonization treatment, the swell was as small as 2 mm or less, the surface smoothness was good, and the gas permeability, thickness, penetration direction resistance, thickness unevenness, and handling properties were also good. It was. The evaluation results are shown in Table 1.

(Example 14)
As the fibrillar carbon precursor fiber (b2), a polyacrylonitrile-based pulp (b2-1) in which a large number of fibrils having a diameter of 3 μm or less were branched from the fibrous trunk and the drainage was set to the conditions shown in Table 1 were used. In the same manner as in Example 1, a precursor sheet and a porous electrode substrate were obtained. The precursor sheet had almost no fiber bundles in which short carbon fibers were bundled, and the fiber bundle ratio was 1. Furthermore, the obtained porous electrode substrate had almost no fiber bundles in which short carbon fibers were bundled, and the fiber bundle ratio was 1. In addition, there was almost no in-plane shrinkage during carbonization treatment, the swell was as small as 2 mm or less, the surface smoothness was good, and the gas permeability, thickness, penetration direction resistance, thickness unevenness, and handling properties were also good. It was. The evaluation results are shown in Table 1.

(Example 15)
A sheet-like material conveying device composed of a net that continuously rotates by connecting a net driving unit and a plain woven mesh made of plastic net having a width of 60 cm in a belt shape, a slurry supply unit width of 48 cm, and a supply slurry amount of 30 L / min. A papermaking slurry similar to that of Example 1 was supplied onto a plain weave mesh by a metering pump into a sheeting apparatus comprising a papermaking slurry supply apparatus and a vacuum dehydration apparatus disposed at the bottom of the net. The papermaking slurry was supplied after being widened to a predetermined size through a flow box for rectification into a uniform flow. Then, it left still, the part which carries out natural dehydration was passed, and it dehydrated with the pressure reduction dehydrator, and obtained the precursor sheet continuously. The target weight was 35 g / m ² . Further, a pressurized water jet treatment apparatus equipped with three water jet nozzles was disposed downstream of the sheet forming apparatus, and the obtained precursor sheet was loaded on the net of the pressurized water jet treatment apparatus. And the pressurized water flow injection pressure was set to 1 MPa, the precursor sheet was passed through the nozzle 1, the nozzle 2 and the nozzle 3 in this order, and the entanglement process was performed to obtain the entangled precursor sheet. This entangled precursor sheet had few fiber bundles in which short carbon fibers were converged, and the fiber bundle ratio was 1 or less. Furthermore, this precursor sheet subjected to the entanglement treatment was subjected to a heat and pressure molding treatment and a carbonization treatment in the same manner as in Example 1 to obtain a porous electrode substrate. The obtained porous electrode substrate had almost no fiber bundles in which short carbon fibers were bundled, and the fiber bundle ratio was 1. In addition, there is almost no in-plane shrinkage during carbonization treatment, swell is as small as 2 mm or less, surface smoothness is good, gas permeability, thickness, and penetration direction resistance are also good in thickness spots and handling properties. It was. The evaluation results are shown in Table 1.

(Example 16)
Porous in the same manner as in Example 15 except that the precursor sheet subjected to pressure and heat molding was oxidized in the atmosphere at 280 ° C. for 1 minute before the carbonization treatment step of the entangled precursor sheet in Example 15. A quality electrode substrate was obtained. The obtained porous electrode substrate had almost no fiber bundles in which short carbon fibers were bundled, and the fiber bundle ratio was 1. In addition, there was almost no in-plane shrinkage during carbonization treatment, the swell was as small as 2 mm or less, the surface smoothness was good, and the gas permeability, thickness, penetration direction resistance, thickness unevenness, and handling properties were also good. It was. The evaluation results are shown in Table 1.

(Example 17)
The entangled precursor sheet of Example 15 was impregnated with a water-dispersed phenol resin (trade name: PR-55464, manufactured by Sumitomo Bakelite Co., Ltd.), and sufficiently dried at room temperature. A phenol resin-impregnated sheet with a mass ratio of 60% was obtained. Further, the precursor sheet impregnated with this phenolic resin was subjected to heat and pressure molding treatment and carbonization treatment in the same manner as in Example 1 to obtain a porous electrode substrate. The obtained porous electrode substrate had almost no fiber bundles in which short carbon fibers were bundled, and the fiber bundle ratio was 1. In addition, there was almost no in-plane shrinkage during carbonization treatment, the swell was as small as 2 mm or less, the surface smoothness was good, and the gas permeability, thickness, penetration direction resistance, thickness unevenness, and handling properties were also good. It was. The evaluation results are shown in Table 1.

(Example 18)
[Manufacture of membrane-electrode assembly (MEA)]
Two sets of porous electrode substrates obtained in Example 1 were prepared as porous carbon electrode substrates for cathode and anode. Further, a catalyst layer (catalyst layer area: 25 cm 2, Pt adhesion) made of catalyst-supported carbon (catalyst: Pt, catalyst support amount: 50% by mass) on both sides of a perfluorosulfonic acid polymer electrolyte membrane (film thickness: 30 μm) An amount of 0.3 mg / cm ² ) was prepared. And this laminated body was pinched | interposed with the porous carbon electrode base material for cathodes and anodes, these were joined, and MEA was obtained.

[Evaluation of fuel cell characteristics of MEA]
The obtained MEA was sandwiched between two carbon separators having bellows-like gas flow paths to form a polymer electrolyte fuel cell (single cell).

The fuel cell characteristics were evaluated by measuring the current density-voltage characteristics of this single cell. Hydrogen gas was used as the fuel gas, and air was used as the oxidizing gas. The temperature of the single cell was 80 ° C., the fuel gas utilization rate was 60%, and the oxidizing gas utilization rate was 40%. Further, the humidification of the fuel gas and the oxidizing gas was performed by passing the fuel gas and the oxidizing gas through an 80 ° C. bubbler, respectively. As a result, when the current density was 0.8 A / cm ² , the cell voltage of the fuel cell was 0.68 V, the internal resistance of the cell was 2.4 mΩ, and good characteristics were exhibited.

(Comparative Example 1)
A precursor sheet and a porous electrode base material were obtained in the same manner as in Example 1 except that the fiber concentration and drainage in the slurry were changed to the conditions shown in Table 1. In the precursor sheet, fiber bundles in which short carbon fibers were bundled were observed, and the fiber bundle ratio was four. Furthermore, in the obtained porous electrode substrate, fiber bundles in which short carbon fibers were converged were observed, and the fiber bundle ratio was 4. There was almost no in-plane shrinkage at the time of carbonization treatment, waviness was as small as 2 mm or less, surface smoothness was good, thickness, penetration direction resistance, and handling properties were also good, but the part where the fiber bundle was observed It was observed that the gas permeability was lower than that of the other portions, and it was confirmed that the gas permeability spots were present. The evaluation results are shown in Table 1.

(Comparative Example 2)
A precursor sheet and a porous electrode base material were obtained in the same manner as in Example 1 except that the fiber concentration and drainage in the slurry were changed to the conditions shown in Table 1. The precursor sheet had almost no fiber bundles in which short carbon fibers were bundled, and the fiber bundle ratio was 1. However, when adjusting the slurry for papermaking, a large amount of dilution water was required to obtain a predetermined concentration. The manufacturing cost for the conversion has increased. Further, the obtained porous electrode substrate had almost no fiber bundles in which short carbon fibers were converged, and the fiber bundle ratio was 1. There was almost no in-plane shrinkage during the carbonization treatment, the swell was as small as 2 mm or less, the surface smoothness was good, and the gas permeability, thickness, penetration direction resistance, thickness unevenness and handling property were also good respectively. Since the production cost of the precursor sheet was high, the production cost of the obtained porous electrode substrate was also higher than that in Example 1. The evaluation results are shown in Table 1.

(Comparative Example 3)
Example 1 except that the amount of polyacrylamide as a paper strength enhancer (Daiichi Kogyo Seiyaku Co., Ltd., trade name: G9210) added to the slurry was adjusted to the conditions shown in Table 1. In the same manner as above, a precursor sheet and a porous electrode substrate were obtained. The precursor sheet had few fiber bundles in which short carbon fibers were bundled, and the fiber bundle ratio was 1. However, dehydration from the papermaking slurry was difficult during sheeting, increasing utility and increasing productivity. A decrease occurred, and the manufacturing cost for sheeting increased. Further, the obtained porous electrode substrate had almost no fiber bundles in which short carbon fibers were converged, and the fiber bundle ratio was 1. There was almost no in-plane shrinkage during the carbonization treatment, the swell was as small as 2 mm or less, the surface smoothness was good, and the gas permeability, thickness, penetration direction resistance, thickness unevenness and handling property were also good respectively. Since the production cost of the precursor sheet was high, the production cost of the obtained porous electrode substrate was also higher than that in Example 1. The evaluation results are shown in Table 1.

(Comparative Example 4)
Example 1 except that the amount of polyacrylamide as a paper strength enhancer (Daiichi Kogyo Seiyaku Co., Ltd., trade name: G9210) added to the slurry was adjusted to the conditions shown in Table 1. In the same manner as above, a precursor sheet and a porous electrode substrate were obtained. In the precursor sheet, fiber bundles in which short carbon fibers were bundled were observed, and the fiber bundle ratio was 3. Further, in the obtained porous electrode substrate, fiber bundles in which short carbon fibers were converged were observed, and the fiber bundle ratio was 3. There was almost no in-plane shrinkage at the time of carbonization treatment, waviness was as small as 2 mm or less, surface smoothness was good, thickness, penetration direction resistance, and handling properties were also good, but the part where the fiber bundle was observed It was observed that the gas permeability was lower than that of the other portions, and it was confirmed that the gas permeability spots were present. The evaluation results are shown in Table 1.

(Comparative Examples 5 and 6)
Except that the mass ratio of the carbon short fiber (A), the carbon fiber precursor short fiber (b1), and the fibrillar carbon precursor fiber (b2) and the drainage in the slurry were the conditions shown in Table 1, Examples In the same manner as in Example 1, a precursor sheet and a porous electrode substrate were obtained. The precursor sheet had almost no fiber bundles in which short carbon fibers were bundled, and the fiber bundle ratio was 1 or less. Furthermore, the obtained porous electrode substrate had almost no fiber bundles in which short carbon fibers were bundled, and the fiber bundle ratio was 1. Moreover, the in-plane shrinkage | contraction at the time of carbonization process and the wave | undulation compared with Example 1 also became large with 3 mm. The gas permeability, thickness, and penetration direction resistance were good, but the handling properties were worse than those of Example 1.

Claims (11)

In a method for producing a porous electrode substrate precursor sheet by paper-making a fiber mixed slurry,
(I) The fiber in the fiber mixed slurry is a mixture containing carbon short fibers (A) and precursor fibers (b), and the mixing ratio of the carbon short fibers (A) is 40% or more by mass ratio with respect to the total amount of the fiber material. 90% or less,
(Ii) The fiber concentration in the fiber mixed slurry is 0.8 to 1.8 g / L,
And (iii) A method for producing a porous electrode substrate precursor sheet, wherein the drainage of the fiber-mixed slurry is 80 to 300 ml.   The step (1) of heating and pressurizing the porous electrode substrate precursor sheet obtained by the production method according to claim 1 and the precursor sheet heated and pressed in the step (1) at a temperature of 1000 ° C. or more The manufacturing method of the porous electrode base material which has the process (2) which carbonizes.   The production method according to claim 32, further comprising a step (4) of oxidizing the porous electrode substrate precursor sheet subjected to the heat and pressure treatment between the step (1) and the step (2).   The manufacturing method according to claim 32 or 43, further comprising a step (3) of subjecting the porous electrode substrate precursor sheet obtained by the manufacturing method according to claim 1 to an entanglement treatment before the step (1).   The step (5) comprising a step (5) of impregnating the carbonizable resin (c) into the porous electrode substrate precursor sheet obtained by the production method according to claim 1 before the step (1). 3. The production method according to 3.   The manufacturing method according to claim 54, further comprising a step (5) of impregnating the porous electrode substrate precursor sheet with a carbonizable resin (c) between the step (3) and the step (1). The porous electrode substrate precursor sheet is a porous electrode substrate precursor sheet in which the fiber bundle ratio of the short carbon fibers (A) is 2 or less. The manufacturing method as described.
<How to find the fiber bundle ratio>
(I) Light is applied from the back and top surfaces of the porous electrode substrate precursor sheet, and a 15 cm × 15 cm range is photographed using a digital camera having 2 million pixels or more.
(II) Based on the photograph taken in (I) above, the number of fiber bundles in a state where a plurality of short carbon fibers (A) are bundled in the range of 15 cm × 15 cm is measured.
(III) The above measurements (I) and (II) are performed at any three locations on the porous electrode substrate precursor sheet, and the average value is defined as the fiber bundle ratio. A porous electrode base material in which short carbon fibers (A) are bound with reticulated carbon fibers (B) and / or resin carbide (C), and the fiber bundle ratio of short carbon fibers (A) is 2 or less. .
<How to find the fiber bundle ratio>
(I) Light is applied from the back and top surfaces of the porous electrode substrate, and a 15 cm × 15 cm range is photographed using a digital camera having 2 million pixels or more.
(II) Based on the photograph taken in (I) above, the number of fiber bundles in a state where a plurality of short carbon fibers (A) are bundled in the range of 15 cm × 15 cm is measured.
(III) The above measurements (I) and (II) are performed at any three locations on the porous electrode substrate, and the average value is defined as the fiber bundle ratio.   The porous electrode base material obtained by the method in any one of Claims 2-7.   A membrane-electrode assembly using the porous electrode substrate according to claim 8 or 9.   A polymer electrolyte fuel cell using the membrane-electrode assembly according to claim 10.