JP7394923B2 - Carbon fiber sheet, gas diffusion electrode, membrane-electrode assembly, polymer electrolyte fuel cell, and method for producing carbon fiber sheet - Google Patents

Description

The present invention relates to carbon fiber sheets, gas diffusion electrodes, membrane-electrode assemblies, polymer electrolyte fuel cells, and methods for producing carbon fiber sheets.

Conventionally, carbon fiber sheets have been used as base materials for gas diffusion electrodes for fuel cells, as electrodes for electric double layer capacitors, and as electrodes for lithium ion secondary batteries by utilizing their conductivity and porosity. Its use is being considered.

As such a carbon fiber sheet, a fiber web is made by mixing carbon fibers and a binder for papermaking, and this fiber web is impregnated with a thermosetting resin such as a phenol resin, and after curing, the fiber web is heated at a temperature of 1000°C or more. Carbon fiber sheets manufactured by firing are known. Considering the mass production of carbon fiber sheets, it is desirable that carbon fiber sheets have the flexibility to be rolled into a roll. It was not possible to do so, had insufficient flexibility, and was difficult to handle.

In addition, the applicant proposed that ``a base material for a gas diffusion electrode made of a glass nonwoven fabric made of glass fibers with a binder containing an acrylic resin and/or a vinyl acetate resin attached, carbon black and a polytetrafluoroethylene resin or a polyfluoride resin. proposed a gas diffusion electrode in which a conductive paste containing vinylidene resin is deposited and fired (Patent Document 1), but glass fiber has high rigidity, and the conductive paste is deposited and fired. Furthermore, since the rigidity was increased, the flexibility was insufficient and the handleability was poor.

As a carbon fiber sheet that can solve such flexibility problems, the applicant of the present application has developed a carbon fiber sheet that includes a fibrous material having a first conductive material and a second conductive material connecting the first conductive material. 1. A conductive porous body, characterized in that the conductive porous body has a specific surface area of 100 m ² /g or more, a bending strength of 10 MPa or more, and a bending deflection of 1 mm or more. ” (Patent Document 2). Although this conductive porous material had a certain degree of flexibility, it had insufficient flexibility and was difficult to handle.

Japanese Patent Application Publication No. 2008-204945

JP 2017-59309 Publication

The present invention was made under these circumstances, and provides carbon fiber sheets with excellent flexibility and ease of handling, gas diffusion electrodes, membrane-electrode assemblies, polymer electrolyte fuel cells, and carbon fiber sheets. The purpose is to provide a manufacturing method.

The carbon fiber sheet of the present invention is a carbon fiber sheet in which particles or particle aggregates exist between carbon fibers, and the particles or particle aggregates have a diameter that is 1.5 times or more the average fiber diameter of the carbon fibers. Large particles or large particle aggregates are included between the carbon fibers in the thickness direction.

The carbon fiber sheet of the present invention is characterized in that large particles or large particle aggregates are present inside the carbon fiber sheet in the thickness direction, the large particles or large particle aggregates have a substantially spherical shape, or large particles or large particle aggregates are present inside the carbon fiber sheet in the thickness direction. Particle aggregates exist in point contact with carbon fibers, no film is formed between large particles or large particle aggregates and carbon fibers, and/or large particles or large particle aggregates exist. Preferably, the aggregate is electrically conductive.

Further, in the carbon fiber sheet of the present invention, the carbon fiber sheet in which large particles or large particle aggregates have conductivity can be suitably used as a base material for a gas diffusion electrode.

The present invention further includes a gas diffusion electrode in which a catalyst is supported on the carbon fiber sheet, a membrane-electrode assembly equipped with this gas diffusion electrode, and a polymer electrolyte fuel cell equipped with this gas diffusion electrode.

The method for producing a carbon fiber sheet of the present invention includes the steps of spinning precursor carbon fibers using a spinning solution containing a carbonizable resin, supplying particles or particle aggregates to the precursor carbon fibers, and forming a precursor carbon fiber sheet in which particles or particle aggregates exist between the precursor carbon fibers, carbonizing the precursor carbon fibers constituting the precursor carbon fiber sheet, and forming an average of the carbon fibers between the carbon fibers in the thickness direction; forming a carbon fiber sheet containing large particles or large particle aggregates having a diameter larger than the fiber diameter.

Note that the step of supplying particles or particle aggregates to the precursor carbon fibers is carried out by spraying a dispersion containing particles or particle aggregates onto the precursor carbon fibers. , is preferably carried out by fragmenting into electrically charged droplets and spraying them onto the precursor carbon fibers.

Further, it is preferable that the step of spinning the precursor carbon fiber is carried out by an electrostatic spinning method, and it is preferable that the large particles or large particle aggregates have electrical conductivity.

In the present invention, there are few contact points between carbon fibers due to the presence of large particles or large particle aggregates having a diameter larger than the average fiber diameter of the carbon fibers between the carbon fibers in the thickness direction. Due to the presence of large particles or large particle aggregates, the carbon fibers can be curved along the large particles or large particle aggregates, which is considered to have excellent flexibility and ease of handling. . Furthermore, since the large particles or large particle aggregates act like pillars in a house, they also have the effect of being less likely to be damaged by pressure in the thickness direction.

If large particles or large particle aggregates are present inside the carbon fiber sheet in the thickness direction, the flexibility will be better.

When the large particles or large particle aggregates have a substantially spherical shape, the contact area between the large particles or large particle aggregates and the carbon fibers is small, and the degree of freedom of the carbon fibers is not so restricted, making the carbon fiber sheet flexible. superior in nature.

When large particles or large particle aggregates exist in point contact with carbon fibers, the contact area between the large particles or large particle aggregates and the carbon fibers is small, and the degree of freedom of the carbon fibers is not very restricted. Carbon fiber sheets are more flexible because they are not

If a film is not formed between the large particles or large particle aggregates and the carbon fibers, the degree of freedom of the carbon fibers is not so restricted and the carbon fiber sheet has better flexibility. Moreover, the voids in the carbon fiber sheet can be effectively utilized. For example, it has excellent gas diffusivity and water permeability.

When the large particles or large particle aggregates have electrical conductivity, the carbon fiber sheet has excellent electrical conductivity and can be suitably used for applications requiring electrical conductivity.

A carbon fiber sheet in which large particles or aggregates of large particles have electrical conductivity has excellent electrical conductivity and is therefore suitable as a base material for a gas diffusion electrode.

Gas diffusion electrodes have large particles or aggregates of large particles that are electrically conductive, and a catalyst is supported on a carbon fiber sheet with excellent electrical conductivity, so they have low electrical resistance and are solid high-temperature electrodes that can exhibit sufficient power generation performance. Molecular fuel cells can be manufactured.

Since the membrane-electrode assembly includes the gas diffusion electrode, it is possible to manufacture a polymer electrolyte fuel cell that has low electrical resistance and can exhibit sufficient power generation performance.

Since the polymer electrolyte fuel cell is equipped with the gas diffusion electrode, it has low electrical resistance and can exhibit sufficient power generation performance.

Since the carbon fiber sheet manufacturing method of the present invention supplies particles or particle aggregates to the precursor carbon fibers, the carbon fiber sheet contains large particles or large particle aggregates between carbon fibers in the thickness direction. can be manufactured. In other words, a carbon fiber sheet with excellent flexibility and ease of handling can be manufactured.

Note that when a dispersion containing particles or particle aggregates is sprayed onto the precursor carbon fibers, it is easy to produce a carbon fiber sheet in which no film is formed between the large particles or large particle aggregates and the carbon fibers.

Furthermore, when a dispersion containing particles or particle aggregates is fragmented into charged droplets and sprayed onto the precursor carbon fiber, the large particles or large particle aggregates have a substantially spherical shape, and the large particles or large particle aggregates have a substantially spherical shape. It is easy to produce a carbon fiber sheet in which the aggregates exist in point contact with the carbon fibers and a film is not formed between the large particles or the large particle aggregates and the carbon fibers.

Furthermore, when the precursor carbon fiber is spun using an electrostatic spinning method, it is possible to spin the precursor carbon fiber which is thin, has a uniform fiber diameter, and is continuous. Therefore, it is possible to produce a carbon fiber sheet that has excellent conductivity, has a large surface area, and can effectively utilize the carbon fiber surface.

Furthermore, if the large particles or large particle aggregates have electrical conductivity, a carbon fiber sheet with excellent electrical conductivity can be produced.

Electron micrograph (2500x) of a cross section in the thickness direction of the carbon continuous fiber nonwoven fabric (carbon fiber sheet) in Example 1 Enlarged electron micrograph (20,000x) of carbon particle aggregates of carbon continuous fiber nonwoven fabric (carbon fiber sheet) in Example 1 Electron micrograph (5000x) of the surface of the carbon continuous fiber nonwoven fabric (carbon fiber sheet) in Example 1 Electron micrograph (2500x) of a cross section in the thickness direction of the carbon continuous fiber nonwoven fabric to which Ketjen black was applied in Comparative Example 3 Enlarged electron micrograph (20,000x) of carbon particle aggregates of carbon continuous fiber nonwoven fabric to which Ketjen black was applied in Comparative Example 3 Electron micrograph (5000x) of the surface of the carbon continuous fiber nonwoven fabric coated with Ketjenblack in Comparative Example 3

In the carbon fiber sheet of the present invention, large particles or large particle aggregates having a diameter larger than the average fiber diameter of the carbon fibers are present between the carbon fibers in the thickness direction, so that contact points between the carbon fibers are prevented. Due to the high degree of freedom of the carbon fibers and the presence of large particles or large particle aggregates, the carbon fibers can be curved along the large particles or large particle aggregates, and the carbon It is considered that the fiber sheet has excellent flexibility and is easy to handle because it has enough room to follow the deformation when the fiber sheet is bent. Furthermore, since the large particles or large particle aggregates act like pillars in a house, they also have the effect of being less likely to be damaged by pressure in the thickness direction. In addition, "thickness direction" means a direction parallel to a straight line perpendicular to both surfaces, and "both surfaces" refers to the widest surface of the carbon fiber sheet and the direction opposite to that surface. means face.

The carbon fiber of the present invention can be, for example, a PAN-based carbon fiber. The carbon fibers can also include electrically conductive materials. When the conductive material is contained in this way, the conductivity and mechanical strength are better. Such conductive materials include, for example, fullerenes, carbon nanotubes, carbon nanohorns, graphite, graphene, vapor grown carbon fibers, carbon black, metals (e.g. gold, platinum, titanium, nickel, aluminum, silver, zinc, iron). , copper, manganese, cobalt, stainless steel, etc.), and one or more types selected from the group of metal oxides of the above metals. Among these, carbon nanotubes are preferred because they have excellent electrical conductivity, are easily oriented in the length direction in carbon fibers, and can enhance electrical conductivity and mechanical strength.

It is preferable that the carbon fiber of the present invention has no voids inside the fiber because it has excellent mechanical strength and electrical conductivity. This "no voids inside the fibers" means that the cut plane in the thickness direction of the carbon fiber sheet has a continuous outline and a field of view that accommodates 1 to 3 carbon fibers with a clear cross-sectional shape. This means that the carbon fibers were observed at 10 locations and the number of carbon fibers with no voids observed was 70% or more. Note that the carbon fibers contain the above-mentioned conductive material, and when the carbon fiber sheet is cut in the thickness direction, voids that are clearly formed due to the conductive material falling off are not included in the above-mentioned voids. do not have. Furthermore, even if the conductive material itself is porous and contains voids, the conductive material is discontinuous and has little effect on the mechanical strength of carbon fibers. However, it is assumed that there are no voids inside the fiber.

The average fiber diameter of the carbon fibers of the present invention is not particularly limited, but in order to ensure that the carbon fibers themselves have excellent mechanical strength, there are some contact points between the carbon fibers, and the mechanical strength of the carbon fiber sheet is In order to have excellent conductivity, the thickness is preferably 10 nm to 20 μm, more preferably 50 nm to 10 μm, and even more preferably 100 nm to 5 μm.

The "average fiber diameter" in the present invention means the arithmetic average value of the fiber diameters at 40 points of carbon fiber, and the "fiber diameter" refers to the width perpendicular to the length direction when observing the carbon fiber in a micrograph. means.

Note that the carbon fibers are preferably continuous fibers so that they have excellent conductivity. In addition, when the carbon fiber sheet of the present invention is used as a base material for a gas diffusion electrode, there is also the effect that there are substantially no fiber ends of the carbon fibers and the solid polymer membrane is not damaged. Carbon fibers, which are such continuous fibers, can be produced by spinning continuous precursor carbon fibers (for example, by electrospinning method, spunbond method, etc.) using a spinning solution containing a carbonizable organic material, and then spinning the continuous precursor carbon fibers using a spinning solution containing a carbonizable organic material. It can be produced by carbonizing a carbonizable organic material (precursor carbon fiber). In addition, "continuous fiber" refers to an electron micrograph when a carbon fiber sheet is photographed using an electron microscope at a magnification such that one side of the photographed image is approximately 60 times the average fiber diameter of the carbon fibers. This means that the number of ends of carbon fiber per sheet is 0.3 or less. In other words, it means that the number of carbon fiber ends per electron micrograph is 0.3 or less, which is calculated by dividing the total number of carbon fiber ends in 20 electron micrographs by the number of images taken (20). Note that the photographs are taken at successively different locations in the center of the surface of the carbon fiber sheet that does not include the cut portion. For example, when confirming whether carbon fibers with an average fiber diameter of 1 μm are continuous fibers, photographs using an electron microscope are taken at continuously different points in the center of the surface of the carbon fiber sheet, not including the cut portion. , take 20 images (4 rows x 5 columns) at a magnification (2500 times) where one side of the image is about 60 μm, calculate the number of carbon fiber ends per electron micrograph, and calculate the number of carbon fiber edges per electron micrograph. For example, it can be thought of as a continuous fiber.

The carbon fiber sheet of the present invention does not need to be composed of one type of carbon fiber, but can be composed of two or more types of carbon fiber. For example, it can be composed of two or more types of carbon fibers that differ in at least one point selected from the presence or absence of a conductive material, the type of conductive material, the presence or absence of voids inside the carbon fiber, average fiber diameter, fiber length, etc. .

The carbon fibers constituting the carbon fiber sheet of the present invention are preferably bonded together so that they have excellent mechanical strength and conductivity. Note that the carbon fibers are preferably bonded to each other using a carbide of an organic material so that the carbon fibers have excellent adhesion, mechanical strength, and electrical conductivity. Such a carbon fiber sheet can be obtained, for example, by carbonizing a fiber sheet containing organic fibers containing a carbonizable organic material. The state in which carbon fibers are bonded to each other can be confirmed by an electron micrograph. For example, FIG. 2 is an enlarged electron micrograph of a carbon particle aggregate of a carbon fiber sheet, and it can be confirmed that the carbon fibers are bonded to each other.

The carbon fiber sheet of the present invention has particles or particle aggregates in addition to the carbon fibers as described above, and includes large particles or large particle aggregates having a diameter larger than the average fiber diameter of the carbon fibers. The presence of large particles or agglomerates of large particles may cause carbon fibers to Since it can be curved along large particles or large particle aggregates, it is believed that it has excellent flexibility and is easy to handle. In other words, there are few points of contact between the carbon fibers, the degree of freedom of the carbon fibers is relatively high, and if the carbon fibers are curved, there is room to follow the deformation when the carbon fiber sheet is bent. Carbon fiber is considered to have excellent flexibility.

As described above, if the large particles or large particle aggregates constituting the carbon fiber sheet of the present invention have a diameter larger than the average fiber diameter of the carbon fibers, bonding between the carbon fibers is hindered, resulting in excellent flexibility. In the carbon fiber sheet, the diameter of the large particles or large particle aggregates is preferably 1.5 times or more the average fiber diameter of the carbon fibers so that bonding between the carbon fibers can be reliably prevented. It is more preferably 2.0 times or more, and even more preferably 2.5 times or more. On the other hand, if the diameter of large particles or large particle aggregates is too large, there will be almost no bonding between carbon fibers, and the mechanical strength, electrical conductivity, etc. of the carbon fiber sheet will tend to deteriorate significantly. The diameter of the particle aggregate is preferably 60 times or less, more preferably 50 times or less, and even more preferably 40 times or less than the average fiber diameter of the carbon fibers. Note that the diameter of the large particles or large particle aggregates may be larger than the average fiber diameter of the carbon fibers, and is not particularly limited, but is preferably 0.1 μm to 1000 μm, and preferably 0.5 μm to 100 μm. It is more preferably 1 μm to 30 μm, even more preferably 1 μm to 10 μm.

The "diameter" of these large particles or large particle aggregates is determined by taking an electron micrograph of a cross section in the thickness direction of a carbon fiber sheet, and calculating the diameter of 50 large particles or large particle aggregates shown in the electron micrograph. Refers to the arithmetic mean value. In addition, if the shape of the large particle or large particle aggregate is non-circular in the photograph, the diameter of the circle having the same area as the area of the large particle or large particle aggregate on the photograph is determined as the diameter of the large particle or large particle aggregate. It is considered as the diameter of the particle aggregate.

The material constituting the large particles or large particle aggregates of the present invention may be conductive or non-conductive, but when utilizing the conductivity of carbon fibers, conductive large particles or Large particle aggregates are preferred. The former conductive material means a material with an electrical resistivity of 10 ³ Ω·cm or less, such as carbon (e.g., Ketjen black, acetylene black, carbon black, carbon nanotube, carbon nanofiber, fullerene, carbon nanohorn, graphite, graphene, etc.), metals (silver, aluminum, nickel, gold, palladium, platinum, cadmium, cobalt, copper, iron, zinc, etc.), conductive metal oxides (tin oxide doped indium oxide, tin oxide, zinc oxide, etc.) ) Carbon is suitable from the viewpoint of chemical resistance, electrical conductivity, etc., and it is particularly preferable to use electrically conductive carbon such as Ketjen black or acetylene black.

On the other hand, the latter non-conductive material refers to a material with an electrical resistivity exceeding 10 ³ Ωcm, such as non-conductive metal oxides (e.g. silicon oxide, aluminum oxide, iron oxide, manganese dioxide, copper oxide). , nickel oxide, cobalt oxide, zinc oxide, titanium-containing oxides, zeolites, catalyst-supported ceramics, etc.), organic resins (polyamide resins, polyester resins, polyolefin resins, fluorine resins, etc.), activated carbon powder (e.g. (steam-activated carbon, alkali-treated activated carbon, acid-treated activated carbon, etc.), ion exchange resin powder, plant seeds, and the like.

The shape of the large particles or large particle aggregates of the present invention is not particularly limited; , polyhedral shape, feather shape, or irregular shape. Among these, a substantially spherical shape is preferable because the contact area with the carbon fibers is small and the degree of freedom of the carbon fibers is less likely to be reduced.

In addition, the preferable approximately spherical shape means that the aspect ratio measured by the following method is 2.0 or less, preferably 1.5 or less, and 1.2 or less. More preferred.

(aspect ratio)
(1) Select large particles or large particle aggregates whose entire individual contours can be clearly observed in an electron micrograph of the surface of the carbon fiber sheet.
(2) Draw the longest line segment (longest line segment) connecting two points on the outer periphery of the selected large particle or large particle aggregate.
(3) Draw the longest line segment (orthogonal line segment) that is orthogonal to the longest line segment and connects two points on the outer periphery of the selected large particle or large particle aggregate.
(4) Find the ratio between the longest line segment (longer axis) and the orthogonal line segment (shorter axis). That is, the values of the longest line segment (longest axis)/orthogonal line segment (shortest axis) are determined.
(5) Perform the operations (1) to (4) above on 50 randomly selected particles, and calculate the arithmetic mean value of the longest line segment (longest axis)/orthogonal line segment (shortest axis) to determine the aspect ratio. shall be.

Note that the large particles or large particle aggregates may be hollow or solid.

In the carbon fiber sheet of the present invention, the large particles or large particle aggregates differ in at least one point selected from diameter, material, shape, conductivity or non-conductivity, and/or hollow or solid state. Large particles or aggregates of large particles may be mixed.

The carbon fiber sheet of the present invention contains large particles or large particle aggregates, and it is preferable that the large particles or large particle aggregates are present inside the carbon fiber sheet in the thickness direction. If the particles are present inside in this way, even if stress is applied due to bending, the large particles or large particle aggregates will relieve the stress, so it is thought that the flexibility will be superior. Note that large particles or large particle aggregates can exist only inside the carbon fiber sheet, but in order to have excellent flexibility, it is possible that the large particles or large particle aggregates exist not only inside the carbon fiber sheet but also near the surface. is preferable. That is, it is preferable that large particles or large particle aggregates exist throughout the thickness direction of the carbon fiber sheet. The "inside" of this carbon fiber sheet refers to the middle area when a cross-sectional photograph of the carbon fiber sheet is taken in the thickness direction, and the area between both surfaces is divided into thirds by a straight line parallel to the surface. The term "nearby" means a region including the surface when divided into thirds by the straight line.

In addition, when large particles or large particle aggregates are included inside the carbon fiber sheet in the thickness direction, it is not necessary that the large particles or large particle aggregates are uniformly present throughout the interior of the carbon fiber sheet, and the large particles Alternatively, layers of large particle aggregates and layers of carbon fibers may be present alternately.

Further, when the carbon fiber sheet contains large particles or large particle aggregates only inside, the vicinity of the surface can be composed only of carbon fibers, or only large particles or large particle aggregates.

In the carbon fiber sheet of the present invention, the large particles or large particle aggregates may be present in any manner, but the large particles or large particle aggregates may be present in point contact with the carbon fibers. is preferred. When the carbon fibers are in point contact in this way, the contact area between the large particles or large particle aggregates and the carbon fibers is small, and the degree of freedom of the carbon fibers is not restricted so much that the carbon fiber sheet is This is because it has better flexibility. Such point-contact state can be easily achieved when large particles or large particle aggregates having a substantially spherical shape are bonded to the carbon fibers without deforming their shape.

Further, in the carbon fiber sheet of the present invention, it is preferable that the large particles or large particle aggregates exist in a state where no film is formed between them and the carbon fibers. This is because if no film is formed in this way, the degree of freedom of the carbon fibers is not so restricted and the carbon fiber sheet has better flexibility. Note that, if no film is formed, there is also the effect that the voids in the carbon fiber sheet can be effectively utilized. In other words, if a film is formed between carbon fibers and large particles or large particle aggregates, the voids in the carbon fiber sheet in the vicinity of the film tend not to be used effectively, but if a film is formed, For example, the voids in the carbon fiber sheet can be effectively utilized. Such a state in which "no film is formed" is, for example, a state in which large particles or large particle aggregates are adhered to, attached to, welded to, or fixed to carbon fibers in spots or lines. Note that when large particles or large particle aggregates are adhered to carbon fibers using a liquid adhesive, a film is likely to be formed.

As described above, the carbon fiber sheet of the present invention contains large particles or large particle aggregates between the carbon fibers in the thickness direction, but the carbon fiber sheet is not limited to the thickness direction. It is preferable that large particles or large particle aggregates be included entirely or partially (staggered, square, random, etc.) in between. In particular, it is preferable that large particles or large particle aggregates be included between all the carbon fibers in the thickness direction and surface direction of the carbon fiber sheet. Even when large particles or large particle aggregates are included in the plane direction, the large particles or large particle aggregates have a substantially spherical shape and are in point contact with the carbon fibers, as in the case where they are present between carbon fibers in the thickness direction. It is preferable that the carbon fiber sheet is present so that no film is formed between the carbon fiber sheet and/or the carbon fiber sheet, and the flexibility of the carbon fiber sheet is not impaired.

In addition, the carbon fiber sheet of the present invention does not need to contain only large particles or large particle aggregates, but also small particles or small particle aggregates having a diameter equal to or smaller than the average fiber diameter of carbon fibers. It's okay to do so. In the carbon fiber sheet of the present invention, since it is thought that the large particles or large particle aggregates have flexibility, it is preferable that there are many large particles or large particle aggregates, and specifically, large particles or large particle aggregates are preferable. The number of aggregates in the thickness direction of the carbon fiber sheet is preferably one or more, more preferably 25 or more, even more preferably 50 or more. In addition, this number is 20 times the cross-sectional photograph in the thickness direction of the carbon fiber sheet at a magnification that allows both surfaces of the carbon fiber sheet to be accommodated and the cross section of the carbon fiber sheet occupies 80% or more of the photographed image area. It means the number of large particles or large particle aggregates per cross-sectional photograph when taken. In other words, it means the number of large particles or large particle aggregates per cross-sectional photograph, which is obtained by dividing the total number of large particles or large particle aggregates in 20 cross-sectional photographs by the number of photographs (20). Note that photographs are taken at continuously different locations in the center of the cross section of the carbon fiber sheet, not including the edges.

The carbon fiber sheet of the present invention contains carbon fibers and particles or particle aggregates, but the particles or particle aggregates account for 1 mass% or more of the entire carbon fiber sheet so that there are few contact points between the carbon fibers. is preferred, more preferably 5 mass% or more, and even more preferably 10 mass% or more. The large particles or large particle aggregates preferably account for 0.9 mass% or more of the entire carbon fiber sheet, more preferably 4.5 mass% or more, and preferably 9 mass% or more of the entire carbon fiber sheet. is even more preferable. On the other hand, if the amount of particles or particle aggregates is too large, the strength of the carbon fiber sheet will decrease, and from this point of view it tends to be difficult to handle. Therefore, particles or particle aggregates account for 99 mass% of the entire carbon fiber sheet. It is preferable that it occupies the following, it is more preferable that it occupies 95 mass% or less, and it is even more preferable that it occupies 90 mass% or less. It is preferable that the large particles or large particle aggregates of the present invention occupy a high proportion of the entire particles or particle aggregates, so it is preferable that the large particles or large particle aggregates account for 99 mass% or less of the entire carbon fiber sheet. Preferably, it accounts for 95 mass% or less, more preferably 90 mass% or less.

As mentioned above, the carbon fiber sheet of the present invention contains large particles or large particle aggregates between the carbon fibers in the thickness direction, and is in a porous state with few contact points between the carbon fibers. This makes it possible to effectively utilize the voids in the space. Specifically, it is preferably in a porous state with an apparent density of 0.40 g/cm ³ or less. The smaller the apparent density, the more effectively the internal voids can be utilized due to the porosity, so it is more preferably 0.20 g/cm ³ or less, and even more preferably 0.12 g/cm ³ or less. Note that if the apparent density is too small, the morphological stability tends to be poor, so it is preferably 0.01 g/cm ³ or more. This "apparent density" is a value calculated by dividing "fabric weight (unit: g/m ² )" by "thickness (unit: μm)". The mass of the sample obtained by cutting it into corners is measured and is the value converted to the mass of 1 m ^2. "Thickness" is the thickness gauge (manufactured by Mitutoyo Co., Ltd.: code No. 547-401). This refers to the value measured using a measuring force of 3.5 N or less.

Although the carbon sheet of the present invention contains carbon fibers and large particles or large particle aggregates, it can also contain other materials. For example, recycled fibers such as rayon, polynosic, and cupro; semi-synthetic fibers such as acetate fibers; nylon fibers, vinylon fibers, fluorine fibers, polyvinyl chloride fibers, polyester fibers, acrylic fibers, polyethylene fibers, polyolefin fibers, and polyurethane fibers. Synthetic fibers; inorganic fibers such as glass fibers and ceramic fibers; vegetable fibers such as cotton and hemp; animal fibers such as wool and silk; and the like can also be included.

The form of the carbon fiber sheet of the present invention is not particularly limited, but may be, for example, a nonwoven fabric in which carbon fibers are randomly oriented, or a woven or knitted fabric in which fibers are regularly oriented. In particular, a nonwoven fabric form is suitable because it has fine voids and a high porosity.

Although the basis weight of the carbon fiber sheet of the present invention is not particularly limited, it is preferably 0.5 to 500 g/m ² and 1 to 400 g/m ² so as to have excellent mechanical strength and conductivity. The amount is more preferably 3 to 300 g/m ² , even more preferably 5 to 200 g/m ² . The thickness is also not particularly limited, but is preferably from 1 to 2000 μm, more preferably from 3 to 1000 μm, even more preferably from 5 to 500 μm, even more preferably from 10 to 300 μm.

When the carbon fiber sheet of the present invention is used for applications requiring electrical conductivity, the electrical resistance is preferably 500 mΩ·cm ² or less, more preferably 100 mΩ·cm ² or less, and 20 mΩ·cm ² It is more preferable that it is as follows. This "electrical resistance" is a value obtained by the following procedure.
(1) A carbon fiber sheet is cut into 5 cm square pieces (area: 25 cm ² ) to prepare a sample.
(2) The sample is sandwiched between gold-plated metal plates from both sides, and the voltage (V) is measured while applying a current (I) of 1 A under pressure of 2 MPa in the stacking direction of the metal plates.
(3) Calculate resistance (R=V/I).
(4) Calculate the electrical resistance by multiplying the resistance (R) by the area (25 cm ² ) of the sample.

The carbon fiber sheet of the present invention is flexible and has excellent handling properties, and if the large particles or large particle aggregates have electrical conductivity, the carbon fiber sheet has even better electrical conductivity, so it can be suitably used as a base material for electrodes. be able to. For example, when used as an electrode for a lithium ion secondary battery or an electric double layer capacitor, a secondary battery or capacitor with a large capacity can be produced. Moreover, when used as a base material for a gas diffusion electrode of a polymer electrolyte fuel cell, a polymer electrolyte fuel cell that can exhibit excellent power generation performance can be produced.

When the large particles or large particle aggregates of the present invention have electrical conductivity and a carbon fiber sheet with excellent electrical conductivity is used as a base material for a gas diffusion electrode, the carbon fiber sheet of the present invention not only has excellent electrical conductivity. Since the sheet is porous, if the voids between the carbon fibers are not filled with anything, the gas diffusion electrode base material (carbon fiber sheet) has excellent drainage properties in the thickness direction and surface direction. Excellent diffusivity of supplied gas.

Note that when the fluororesin is contained in the gaps between the carbon fibers of the gas diffusion electrode base material (carbon fiber sheet), liquid water is easily pushed out, resulting in excellent drainage performance. Examples of the fluororesin include polytetrafluoroethylene (PTFE), polychlorotrifluoroethylene (PCTFE), polyvinylidene fluoride (PVDF), polyvinyl fluoride (PVF), perfluoroalkoxyfluororesin (PFA), and tetrafluorocarbon resin. ethylene/hexafluoropropylene copolymer (FEP), ethylene/tetrafluoroethylene copolymer (ETFE), ethylene/chlorotrifluoroethylene copolymer (ECTFE), vinylidene fluoride/tetrafluoroethylene/hexafluoro Examples include propylene copolymers and copolymers of various monomers constituting the resin.

In the gas diffusion electrode of the present invention, the large particles or aggregates of large particles have electrical conductivity, and the catalyst is supported on the highly conductive carbon fiber sheet, so the electrical resistance is low and sufficient power generation performance can be achieved. It is possible to manufacture polymer electrolyte fuel cells. Further, in the gas diffusion electrode of the present invention, a catalyst is supported on a carbon fiber sheet, for example, the catalyst is supported on the surface of carbon fibers and/or large particles or large particle aggregates, and electron conduction through contact between the catalysts is not sufficient. In addition, since electron conduction paths are also formed by carbon fibers and/or large particles or large particle aggregates, there are few catalysts isolated from electron conduction paths. Therefore, the catalyst can be used efficiently and the amount of catalyst can be reduced.

The gas diffusion electrode of the present invention has exactly the same structure as the conventional gas diffusion electrode except that the catalyst is supported on the carbon fiber sheet of the present invention as described above. For example, the catalyst may be platinum, platinum alloy, palladium, palladium alloy, titanium, manganese, magnesium, lanthanum, vanadium, zirconium, iridium, rhodium, ruthenium, gold, nickel-lanthanum alloy, titanium-iron alloy, etc. and one or more types of catalysts selected from these.

In addition to the catalyst, it is preferable to contain an electron conductor and a proton conductor, and the electron conductor is preferably made of a conductive material similar to large particles such as carbon black or large particle aggregates. The catalyst may be supported on this electron conductor. Further, as the proton conductor, an ion exchange resin is suitable.

Such a gas diffusion electrode can be produced, for example, by the following method. First, a catalyst (for example, carbon powder supporting a catalyst such as platinum) is added and mixed in a single or mixed solvent consisting of ethyl alcohol, propyl alcohol, butyl alcohol, ethylene glycol dimethyl ether, etc., and ion exchange resin is added to the mixture. Add the solution and mix uniformly using ultrasonic dispersion or the like to obtain a catalyst-dispersed suspension. Then, the carbon fiber sheet of the present invention can be coated or sprayed with the catalyst dispersion suspension and dried to produce a gas diffusion electrode.

Since the membrane-electrode assembly of the present invention is equipped with the gas diffusion electrode with excellent conductivity as described above, it is possible to manufacture a polymer electrolyte fuel cell that has low electrical resistance and can exhibit sufficient power generation performance. .

The membrane-electrode assembly of the present invention can be completely similar to a conventional membrane-electrode assembly, except that it includes a gas diffusion electrode as described above. For example, the solid polymer membrane may be a perfluorocarbon sulfonic acid resin membrane, a sulfonated aromatic hydrocarbon resin membrane, an alkylsulfonated aromatic hydrocarbon resin membrane, or the like.

Such a membrane-electrode assembly can be manufactured, for example, by sandwiching a solid polymer membrane between the respective catalyst-supporting surfaces of a pair of gas diffusion electrodes and joining them by hot pressing. Alternatively, after coating a catalyst dispersion suspension as described above on a support to form a catalyst layer, this catalyst layer is transferred to a solid polymer membrane, and then the carbon fiber sheet of the present invention is applied to the catalyst layer. It can also be manufactured by laminating them so that they are in contact with each other and hot pressing them.

Since the polymer electrolyte fuel cell of the present invention includes the gas diffusion electrode, it has low electrical resistance and can exhibit sufficient power generation performance.

The fuel cell of the present invention can be completely similar to a conventional fuel cell except that it includes the gas diffusion electrode as described above. For example, it has a structure in which a plurality of cell units in which a membrane-electrode assembly as described above is sandwiched between a pair of bipolar plates is stacked, and can be manufactured by, for example, stacking and fixing a plurality of cell units.

The bipolar plate is not particularly limited as long as it has high conductivity, does not allow gas to pass through, and has a flow path that can supply gas to the gas diffusion electrode, but examples include carbon molding materials, Carbon-resin composite materials, metal materials, etc. can be used.

The carbon fiber sheet of the present invention includes, for example, a step of spinning a precursor carbon fiber using a spinning solution containing a carbonizable resin, a step of supplying particles or particle aggregates to the precursor carbon fiber, and a step of spinning a precursor carbon fiber in the thickness direction. forming a precursor carbon fiber sheet in which particles or particle aggregates exist between carbon fibers, carbonizing the precursor carbon fibers constituting the precursor carbon fiber sheet, and forming an average fiber of the carbon fibers between the carbon fibers in the thickness direction; carbon fiber sheet containing large particles or large particle aggregates having a diameter larger than the carbon fiber sheet. In this way, by interposing large particles or large particle aggregates having a diameter larger than the average fiber diameter of the carbon fibers between the carbon fibers in the thickness direction, the number of points of contact between the carbon fibers in the thickness direction is reduced, Furthermore, since carbon fibers can be curved along large particles or large particle aggregates, it is thought that a carbon fiber sheet with excellent flexibility and ease of handling can be manufactured.

More specifically, first, a step of spinning a precursor carbon fiber using a spinning solution containing a carbonizable resin is performed. This carbonizable resin is not particularly limited, but examples include phenol resin, urea resin, melamine resin, unsaturated polyester resin, epoxy resin, xylene resin, urethane resin, silicone resin, thermosetting polyimide resin, and thermosetting resin. Thermosetting resins such as thermosetting polyimide resins and thermosetting polyamide resins; polystyrene resins, polyester resins, polyolefin resins, polyimide resins, polyamide resins, polyamideimide resins, polyvinyl acetate resins, vinyl chloride resins, fluororesins, polyacrylonitrile resins , thermoplastic resins such as acrylic resins, polyether resins, polyvinyl alcohol, polyvinylpyrrolidone, pitch, polyamino acid resins, and polybenzimidazole resins; cellulose (polysaccharides), tar; or copolymers containing monomers of these resins. Examples include acrylonitrile-butadiene-styrene copolymer, acrylonitrile-styrene copolymer, etc. Among these, it is preferable to use an acrylic resin, especially one containing 85% or more of polyacrylonitrile, because it has a high carbonization rate, has no voids inside the fiber, and can easily produce carbon fibers with high strength and high elastic modulus.

Note that a conductive material can also be mixed so that the conductivity and mechanical strength of the carbon fiber can be increased. This electrically conductive material can be any of the electrically conductive materials mentioned above, preferably carbon nanotubes.

In addition, instead of or in addition to the conductive material, silicone such as polydimethylsiloxane, metal alkoxide (methoxide, ethoxide, propoxide, butoxide, etc. of silicon, aluminum, titanium, zirconium, boron, tin, zinc, etc.) may be used. Polymers obtained by polymerizing known inorganic compounds, such as polymerized inorganic polymers, can also be mixed.

After preparing such materials, a spinning solution containing a carbonizable resin is prepared for spinning the precursor carbon fibers. Optionally, a spinning solution containing conductive materials, silicones, and/or inorganic polymers is prepared.

The solvent constituting the spinning solution is not particularly limited as long as it can dissolve the carbonizable resin, but examples include acetone, methanol, ethanol, propanol, isopropanol, tetrahydrofuran, dimethyl sulfoxide, 1,4- Dioxane, pyridine, N,N-dimethylformamide, N,N-dimethylacetamide, N-methyl-2-pyrrolidone, acetonitrile, formic acid, toluene, benzene, cyclohexane, cyclohexanone, carbon tetrachloride, methylene chloride, chloroform, trichloroethane, ethylene Examples include carbonate, diethyl carbonate, propylene carbonate, water, etc., and these solvents may be used alone or as a mixed solvent. Note that a poor solvent may be added as long as there is no problem with spinnability.

The solid content concentration of the carbonizable resin in the spinning solution is not particularly limited as long as it can form carbon fibers, but is preferably 1 to 50 mass%, more preferably 5 to 30 mass%. . This is because when it is less than 1 mass%, productivity is extremely reduced, and when it is more than 50 mass%, spinning tends to become unstable.

Next, the precursor carbon fiber is spun using a spinning solution as described above. Methods for forming this precursor carbon fiber include, for example, dry spinning method, electrostatic spinning method, and the method disclosed in Japanese Patent Application Laid-Open No. 2009-287138, in which a gas is applied to the spinning solution discharged from a liquid discharge section. One example is a method in which the spinning solution is discharged in parallel and a shearing force is applied to the spinning solution in a single straight line to form fibers. Among these, the electrostatic spinning method or the spinning method disclosed in JP-A No. 2009-287138 is preferable because it is easy to spin precursor carbon fibers having a small average fiber diameter of 3 μm or less. Further, dry spinning method or electrostatic spinning method is suitable because it is easy to spin precursor carbon fibers that are continuous fibers. In particular, the electrostatic spinning method makes it possible to spin precursor carbon fibers that are continuous fibers with small fiber diameters and uniform fiber diameters, have excellent conductivity, and have a wide surface area, making it possible to effectively utilize the carbon fiber surface. This is suitable because carbon fiber sheets can be manufactured.

In addition, the "precursor carbon fiber" in the present invention means a fiber in a state where the carbonizable resin is not carbonized, and since the carbonizable resin is carbonized to constitute carbon fiber, it becomes the base material of carbon fiber. In the sense of fiber, it is expressed as precursor carbon fiber.

A step of supplying particles or particle aggregates to the precursor carbon fibers is then performed. By supplying particles or particle aggregates, a precursor carbon fiber sheet containing large particles or large particle aggregates between carbon fibers in the thickness direction can be manufactured, resulting in excellent flexibility and ease of handling. Carbon fiber sheets with excellent properties can be manufactured.

The method for supplying the particles or particle aggregates is not particularly limited, but examples include a method in which particles or particle aggregates are jetted out by the action of compressed gas, a dispersion containing particles or particle aggregates is supplied by compressed gas or ultrasonic waves, etc. Examples of methods include a method in which a dispersion containing particles or particle aggregates is fragmented into electrically charged droplets by the action of static electricity and then sprayed. Among these supply methods, a method in which a dispersion containing particles or particle aggregates is atomized by the action of compressed gas or ultrasonic waves is preferable because it is easy to produce a precursor carbon fiber sheet without a film between precursor carbon fibers. It is. In particular, a method in which a dispersion containing particles or particle aggregates is fragmented into electrically charged droplets by the action of static electricity and sprayed is a method in which large particle aggregates have a substantially spherical shape, This is suitable because it is easy to produce a precursor carbon fiber sheet in which the particle aggregates are attached in point contact with the precursor carbon fibers and no film is formed between large particles or between the large particle aggregates and the precursor carbon fibers. It is. That is, since the droplets electrically repel each other and fragment, it is difficult to form a film containing large particles or large particle aggregates between the precursor carbon fibers. In particular, when a charged precursor carbon fiber is sprayed with droplets charged to the opposite polarity to that of the precursor carbon fiber, as in the case where the precursor carbon fiber is spun using an electrostatic spinning method, due to the action of the electric charge, Droplets containing large particles or large particle aggregates can be reliably attached to the precursor carbon fiber. For example, if droplets charged to the opposite polarity to the charge of the precursor carbon fibers are sprayed onto the flying precursor carbon fibers spun using the electrospinning method before they are accumulated on the collector, the charge will be reduced. By this action, droplets containing large particles or large particle aggregates can be reliably attached to the precursor carbon fiber. Note that when spraying charged droplets onto uncharged precursor carbon fibers, the polarity of the charged droplets is not particularly limited. For example, even when spinning by electrostatic spinning, when spraying charged droplets onto uncharged precursor carbon fibers that have accumulated on a grounded collector, polarity Freely charged droplets can be sprayed to deposit droplets containing large particles or large particle aggregates onto the precursor carbon fibers.

The supply of the particles or particle aggregates to the precursor carbon fibers may be performed, for example, for the precursor carbon fibers in a flying state after spinning, or for the precursor carbon fibers immediately after the precursor carbon fibers have been accumulated on the collecting body. Alternatively, it can be carried out on the precursor carbon fibers after the precursor carbon fibers have accumulated to some extent on the collector. In particular, if particles or particle aggregates are supplied to the precursor carbon fibers in a flying state after spinning, or to the precursor carbon fibers immediately after the precursor carbon fibers have been accumulated on a collector, the gaps between the precursor carbon fibers As a result of preventing contact and being able to curve the precursor carbon fibers along large particles or large particle aggregates, it is thought that it is easier to produce carbon fiber sheets with better flexibility and better handling properties. Preferably, particles or particle aggregates are provided at said stage.

As the particles or particle aggregates, conductive or non-conductive particles or particle aggregates as described above can be used. In particular, it is preferable that the particles or particle aggregates have electrical conductivity, since a carbon fiber sheet with excellent electrical conductivity can be produced. When increasing this conductivity, it is preferable to use conductive carbon such as Ketjen black or acetylene black as particles or particle aggregates.

In addition, when using a dispersion liquid of particles or particle aggregates, the solvent in the dispersion liquid is not particularly limited as long as it can disperse the particles or particle aggregates without dissolving them. The concentration of particles or particle aggregates in the dispersion is not particularly limited, but is preferably 1 to 80 mass%, more preferably 3 to 70 mass%, and preferably 5 to 60 mass%. More preferred.

Furthermore, when a dispersion of conductive particles or particle aggregates is used and the dispersion is fragmented into electrically charged droplets and then sprayed, fragmentation tends to be difficult, so a dispersion containing an organic resin is prepared. However, it is preferable that the liquid droplets be easily fragmented into charged droplets. Examples of such organic resins include nylon resin, vinylon resin, fluororesin, polyvinyl chloride resin, polyester resin, polyolefin resin, polyvinyl alcohol, and polyethylene glycol.

When preparing a dispersion containing an organic resin in this way, the solvent in the dispersion must be one that can disperse particles or particle aggregates without dissolving them, and that can also dissolve or disperse the organic resin. It is good if it exists and is not particularly limited. In addition, when the dispersion liquid contains an organic resin, the dispersion liquid may be one in which conductive particles or particle aggregates are dispersed in a melt of the organic resin.

Next, a step of forming a precursor carbon fiber sheet in which particles or particle aggregates are present between the precursor carbon fibers in the thickness direction is performed. For example, when the particles or particle aggregates are supplied to the precursor carbon fibers in a flying state after spinning, the precursor carbon fibers with the particles or particle aggregates attached are placed on the collector. can be aggregated to form a precursor carbon fiber sheet in which particles or particle aggregates are present between the precursor carbon fibers in the thickness direction. In addition, when particles or particle aggregates are supplied to the precursor carbon fibers immediately after the precursor carbon fibers have accumulated on the collection body, the precursor carbon fibers are accumulated on the precursor carbon fibers to which the particles or particle aggregates have adhered. Since the particles or particle aggregates are supplied to the precursor carbon fibers immediately after the accumulation, at the same time as the particles or particle aggregates are supplied to the precursor carbon fibers, the particles or particle aggregates are distributed between the precursor carbon fibers in the thickness direction. A precursor carbon fiber sheet can be formed in which the carbon fibers are present. Furthermore, when particles or particle aggregates are supplied to the sheet-shaped precursor carbon fibers after the precursor carbon fibers have accumulated to some extent on the collector, the sheet-shaped precursor carbon fibers to which the particles or particle aggregates have adhered are They can be laminated to form multilayer precursor carbon fiber sheets.

In the step of forming this precursor carbon fiber sheet, if particles or particle aggregates are supplied so that the particles or particle aggregates are present between the precursor carbon fibers in the thickness direction inside the precursor carbon fiber sheet, the thickness Carbon fiber sheets can be produced in which particles or particle aggregates are present in the direction. For example, when supplying particles or particle aggregates to precursor carbon fibers in a flying state after spinning, or when supplying particles or particle aggregates to precursor carbon fibers that are in a flying state after spinning, When supplying particles or particle aggregates to the precursor fibers constituting the interior of the precursor carbon fiber sheet, particles or particle aggregates can be supplied between the precursor carbon fibers in the thickness direction of the interior. A precursor carbon fiber sheet can be formed in which aggregates are present. In addition, when supplying particles or particle aggregates to sheet-shaped precursor carbon fibers that have accumulated to some extent on the collection body, the sheet-shaped precursor carbon fibers to which particles or particle aggregates have adhered constitute the interior. The carbon fiber sheets can be laminated in this manner to form a precursor carbon fiber sheet in which particles or particle aggregates are present between the precursor carbon fibers in the interior thickness direction.

Then, the precursor carbon fibers constituting this precursor carbon fiber sheet are carbonized, and the carbon fiber sheet of the present invention, that is, large particles or particles having a diameter larger than the average fiber diameter of the carbon fibers or Carbon fiber sheets containing large particle aggregates can be produced.

This carbonization step may be carried out as long as it can carbonize the carbonizable resin constituting the precursor carbon fiber, and is not particularly limited. It can be carried out by heating. The temperature increase rate is preferably 5 to 100,000°C/min, more preferably 5 to 1,000°C/min. Further, the holding time at the maximum temperature is preferably within 3 hours, and more preferably from 1 to 120 minutes.

The carbon fiber sheet of the present invention can be produced by the method described above, but when the carbonizable resin constituting the precursor carbon fiber is polyacrylonitrile resin, the fiber shape can be maintained even after carbonization treatment. Therefore, it is preferable to perform an infusibility step before the carbonization treatment. This infusibility can be carried out, for example, by heating in an oxidizing atmosphere at a temperature of 200 to 300° C. for 10 to 120 minutes. Note that the heating for infusibility can be carried out two or more times at different temperatures or times.

The above is an example of the method for manufacturing the carbon fiber sheet of the present invention, but physical properties suitable for each use can be imparted or improved by various post-processing. For example, when using the carbon fiber sheet of the present invention as a base material for a gas diffusion electrode in a polymer electrolyte fuel cell, polytetrafluorocarbon The carbon fiber sheet can be immersed in a fluororesin dispersion such as ethylene dispersion to apply the fluororesin, and then sintered at a temperature of 300 to 350°C. In addition, if the large particles or large particle aggregates are conductive, pressurizing the precursor carbon fiber sheet or carbon fiber sheet increases the adhesion between the precursor carbon fibers or between the carbon fibers and increases the conductivity. You can also do that. However, since the carbon fiber sheet of the present invention is preferably in a porous state with an apparent density of 0.40 g/cm ³ or less, it is preferable to adjust the pressing force as appropriate.

Note that a carbon fiber sheet made of carbon fibers with no internal voids can be easily produced by using a carbonizable resin with a high carbonization rate. For example, by using polyacrylonitrile, epoxy resin, phenol resin, etc., it is easy to manufacture a carbon fiber sheet made of carbon fibers with no internal voids.

Examples of the present invention will be described below, but the present invention is not limited to the following examples.

(Preparation of spinning solution)
Polyacrylonitrile (PAN) was added to N,N-dimethylformamide (DMF) and dissolved using a rocking mill to prepare a spinning solution with a solid content concentration of 15 mass%.

(Preparation of particle aggregate dispersion)
Ketjen black water dispersion [manufactured by Lion Corporation, Lion Paste W-311N, solid content: 16.5 mass%, primary particle diameter: 40 nm], polyethylene glycol [manufactured by Wako Pure Chemical Industries, Ltd., molecular weight: 500,000] and pure water to prepare a carbon dispersion (particle aggregate dispersion) with a Ketjen black concentration of 12 mass% and a polyethylene glycol concentration of 0.7 mass%.

(Example 1)
The precursor carbon continuous fibers obtained by spinning the above-mentioned spinning solution by the electrostatic spinning method under the following conditions are directly accumulated on a stainless steel drum serving as a counter electrode, and at the same time, the carbon dispersion liquid is electrostatically sprayed under the following conditions. The particles are fragmented into electrically charged droplets and sprayed from the top of the stainless steel drum toward the accumulation position of the precursor carbon continuous fibers. A precursor carbon continuous fiber nonwoven fabric (mass ratio of precursor carbon continuous fibers and Ketjen black = 70:30) in which chain black was mixed was produced.

Subsequently, the precursor carbon continuous fiber nonwoven fabric was subjected to oxidation treatment in air at a temperature of 220°C for 30 minutes and at a temperature of 260°C for 1 hour to infusible the PAN constituting the precursor carbon continuous fibers and It was made into a non-woven fabric with continuous fibers of fused precursor carbon.

Then, the infusible precursor carbon continuous fiber nonwoven fabric was subjected to carbonization firing treatment (heating rate: 10°C/min) for 1 hour at a maximum temperature of 800°C in a nitrogen gas atmosphere using a vacuum displacement electric furnace. A carbon continuous fiber nonwoven fabric (average fiber diameter: 350 nm, carbon continuous fibers have no voids inside the fibers) was produced.

When we took and observed a cross section in the thickness direction of this carbon continuous fiber nonwoven fabric, an enlarged view of the spherical large particle aggregates, and an electron micrograph of the surface (see Figures 1 to 3), we found that Ketjenblack was a spherical large particle aggregate. (diameter: 4 μm, aspect ratio: 1.2), and was contained between carbon fibers in the thickness direction (number of spherical large particle aggregates: 50). In addition, the large spherical particles of Ketjen black were in point contact with the carbon fibers and adhered to the carbon fibers without forming a film. Furthermore, while there were places where the carbon continuous fibers were joined to each other, there were also places where the carbon continuous fibers were joined to each other via Ketjenblack spherical large particle aggregates. Furthermore, some of the continuous carbon fibers were in a curved state due to Ketjenblack spherical large particle aggregates. Further, the carbon continuous fiber nonwoven fabric had Ketjenblack spherical large particle aggregates throughout the entire thickness direction including the interior, near the surface, and the entire surface direction.

Note <Electrospinning conditions>
Electrode: Metal nozzle (inner diameter 0.41mm) and grounded stainless steel drum Discharge rate: 3.0g/hour Distance between nozzle tip and stainless steel drum: 10cm
Applied voltage: +16kV
Temperature/Humidity: 25℃/30%RH

<Electrostatic spray conditions>
Electrode: Metal nozzle (inner diameter 0.41 mm) and grounded stainless steel drum Discharge amount: 2.5 g/hour Distance between nozzle tip and stainless steel drum: 10 cm
Applied voltage: +18kV
Temperature/Humidity: 25℃/30%RH

(Example 2)
A carbon continuous fiber nonwoven fabric was prepared in the same manner as in Example 1, except that the amount of carbon dispersion sprayed was increased and the mixed mass ratio of precursor carbon continuous fibers and Ketjen black in the precursor carbon continuous fiber nonwoven fabric was 50:50. (Average fiber diameter: 350 nm, carbon continuous fibers have no voids inside the fibers) were produced. Electron micrographs of the cross-section in the thickness direction, enlargement of spherical large particle aggregates, and surface of this carbon continuous fiber nonwoven fabric were taken and observed, and it was found that Ketjen Black was spherical large particle aggregates (diameter: 4 μm, aspect ratio: 1.2) was contained between carbon fibers in the thickness direction (number of spherical large particle aggregates: 80). In addition, the large spherical particles of Ketjen black were in point contact with the carbon fibers and adhered to the carbon fibers without forming a film. Furthermore, while there were places where the carbon continuous fibers were joined to each other, there were also places where the carbon continuous fibers were joined to each other via Ketjenblack spherical large particle aggregates. Furthermore, some of the continuous carbon fibers were in a curved state due to Ketjenblack spherical large particle aggregates. Further, the carbon continuous fiber nonwoven fabric had Ketjenblack spherical large particle aggregates throughout the entire thickness direction including the interior, near the surface, and the entire surface direction.

(Example 3)
A carbon continuous fiber nonwoven fabric was prepared in the same manner as in Example 1, except that the amount of carbon dispersion sprayed was increased and the mass ratio of precursor carbon continuous fibers and Ketjenblack in the precursor carbon continuous fiber nonwoven fabric was 20:80. (Average fiber diameter: 350 nm, carbon continuous fibers have no voids inside the fibers) were produced. Electron micrographs of the cross-section in the thickness direction, enlargement of spherical large particle aggregates, and surface of this carbon continuous fiber nonwoven fabric were taken and observed, and it was found that Ketjen Black was spherical large particle aggregates (diameter: 4 μm, aspect ratio: 1.2) was contained between carbon fibers in the thickness direction (number of spherical large particle aggregates: 150). In addition, the large spherical particles of Ketjen black were in point contact with the carbon fibers and adhered to the carbon fibers without forming a film. Furthermore, while there were places where the carbon continuous fibers were joined to each other, there were also places where the carbon continuous fibers were joined to each other via Ketjenblack spherical large particle aggregates. Furthermore, some of the continuous carbon fibers were in a curved state due to Ketjenblack spherical large particle aggregates. Further, the carbon continuous fiber nonwoven fabric had Ketjenblack spherical large particle aggregates throughout the entire thickness direction including the interior, near the surface, and the entire surface direction.

(Comparative example 1)
The procedure was the same as in Example 1, except that the carbon dispersion liquid was not sprayed by electrostatic spraying, and was made of only PAN-based carbon continuous fibers (average fiber diameter: 350 nm, carbon continuous fibers have no voids inside the fibers). A carbon continuous fiber nonwoven fabric was produced. When an electron micrograph was taken of the surface of this continuous carbon fiber nonwoven fabric and observed, it was found that the continuous carbon fibers were linear and the continuous carbon fibers were bonded to each other.

(Comparative example 2)
The precursor carbon continuous fibers obtained by spinning the spinning solution using the electrostatic spinning method under the same conditions as in Example 1 were directly accumulated on a stainless steel drum serving as a counter electrode to form a precursor carbon continuous fiber nonwoven fabric. The carbon dispersion was electrostatically sprayed under the same conditions as in Example 1, fragmented into charged droplets, and the precursor carbon continuous fibers, which had already been accumulated and formed into a nonwoven fabric, were sprayed from the top of the stainless steel drum. The mixture was sprayed onto a nonwoven fabric to produce a precursor carbon continuous fiber nonwoven fabric (mass ratio of precursor carbon continuous fibers and Ketjenblack = 70:30) to which Ketjenblack was attached.

Subsequently, in the same manner as in Example 1, the precursor carbon continuous fibers were infusible and carbonized by firing to produce a carbon continuous fiber nonwoven fabric (average fiber diameter: 350 nm, carbon continuous fibers had no voids inside the fibers). did.

Electron micrographs of the cross-section in the thickness direction, enlargement of spherical large particle aggregates, and surface of this carbon continuous fiber nonwoven fabric were taken and observed, and it was found that Ketjen Black was spherical large particle aggregates (diameter: 4 μm, aspect ratio: In the form of 1.2), it was contained between continuous carbon fibers in the plane direction in a bonded state without forming a film at points, but was not contained between carbon fibers in the thickness direction. . Furthermore, the continuous carbon fibers are linear, and there are places where the continuous carbon fibers are joined to each other, and there are no places where the continuous carbon fibers are joined to each other via Ketjen Black spherical large particle aggregates. There wasn't. In other words, there was no place where the bonding between the carbon continuous fibers was hindered by the Ketjenblack spherical large particle aggregates. Furthermore, the carbon continuous fiber nonwoven fabric had large spherical Ketjen black particles only on its surface, and the large spherical Ketjen black particle aggregates were likely to fall off.

(Comparative example 3)
A carbon continuous fiber nonwoven fabric prepared in the same manner as in Comparative Example 1 was placed in a Ketjen black water dispersion [manufactured by Lion Corporation, Lion Paste W-311N, solid content: 16.5 mass%, primary particle size: 40 nm]. After immersing it in water, it was dried at a temperature of 80°C for 1 hour to obtain a carbon continuous fiber nonwoven fabric with Ketjenblack (average fiber diameter: 350 nm, mass ratio of carbon continuous fibers and Ketjenblack = 45:55, carbon continuous fibers). produced a fiber with no voids inside.

When we photographed and observed a cross-section in the thickness direction of the carbon continuous fiber nonwoven fabric containing this Ketjen black, an enlarged view of the Ketjen black, and an electron micrograph of the surface (see Figures 4 to 6), we found that Ketjen black was a part of the film. In the form, the voids in the carbon fiber sheet were closed in the entire thickness direction and in the surface direction of the carbon continuous fiber nonwoven fabric. Note that no Ketjenblack spherical large particle aggregates were contained between the carbon fibers in the thickness direction. Note that the carbon continuous fibers are linear, and while there are places where the carbon continuous fibers are bonded to each other, there are also places where the carbon continuous fibers are bonded to each other with a film containing Ketjenblack.

(Comparative example 4)
Vinylidene fluoride/tetrafluoroethylene/hexafluoropropylene copolymer (THV) was added to N,N-dimethylformamide (DMF) and dissolved using a rocking mill to obtain a solution with a concentration of 10 mass%.

Next, carbon nanotubes (CNTs) synthesized by the CVD method [trade name: VGCF-H (manufactured by Showa Denko KK), fiber diameter: 150 nm, aspect ratio: 40, multi-walled carbon nanotubes] were mixed into the solution, After stirring, the mixture was further diluted by adding DMF to disperse the carbon nanotubes to obtain a dispersion solution.

Further, a carbonizable resin (EP) containing a cresol novolac epoxy resin as a main ingredient and a novolac type phenolic resin as a curing agent was added to the dispersion solution, so that the solid content mass ratio of CNT:THV:EP was 40:30:30. A first spinning solution having a solid content concentration of 16 mass% was prepared.

On the other hand, polyacrylonitrile (PAN) was added to N,N-dimethylformamide (DMF) and dissolved using a rocking mill to prepare a second spinning solution having a concentration of 20 mass%.

Next, the first precursor carbon continuous fibers are spun using the first spinning solution using the electrostatic spinning method under the following conditions and directly accumulated on a stainless steel drum serving as a counter electrode, and at the same time, the second spinning solution is electrostatically spun. According to the method, second precursor carbon continuous fibers are spun under the following conditions, and directly collided with and accumulated on the first precursor carbon continuous fiber aggregate to form the first precursor carbon continuous fibers and the second precursor carbon continuous fibers. A blended precursor carbon continuous fiber nonwoven fabric was prepared.

Description (electrospinning conditions)
(1) First spinning solution Electrode: Metal nozzle (inner diameter: 0.33 mm) and grounded stainless steel drum Discharge amount: 4 g/hour Distance between nozzle tip and stainless steel drum: 8 cm
Applied voltage: +10kV
Temperature/Humidity: 25℃/35%RH

(2) Second spinning solution Electrode: Metal nozzle (inner diameter: 0.41 mm) and grounded stainless steel drum Discharge amount: 2 g/hour Distance between nozzle tip and stainless steel drum: 12 cm
Applied voltage: +15kV
Temperature/Humidity: 25℃/35%RH

Next, after adjusting the thickness of the precursor continuous carbon fiber nonwoven fabric using a calendar roll (linear pressure 10 N/cm), heat treatment was performed for 1 hour in a hot air dryer set at a temperature of 150°C to obtain the first precursor. The epoxy resin constituting the carbon continuous fibers was cured to prepare a cured precursor carbon continuous fiber nonwoven fabric.

Subsequently, in the same manner as in Example 1, the cured precursor carbon continuous fibers were subjected to infusibility and carbonization firing treatment to obtain a carbon continuous fiber nonwoven fabric (average fiber diameter: 1.5 μm, carbon continuous fibers have voids inside). was manufactured.

When an electron micrograph of the surface of this carbon continuous fiber nonwoven fabric was taken and observed, it was found that the carbon continuous fibers were not linear but curved and joined to each other.

<Measurement of electrical resistance>
The electrical resistance of the carbon continuous fiber nonwoven fabrics of Examples 1 to 3 and Comparative Examples 1 to 4 was measured according to the following procedure. The results were as shown in Table 1.
(1) A carbon continuous fiber nonwoven fabric is cut into 5 cm square pieces (area: 25 cm ² ) to prepare a sample.
(2) The sample is sandwiched between gold-plated metal plates from both sides, and the voltage (V) is measured while applying a current (I) of 1 A under pressure of 2 MPa in the stacking direction of the metal plates.
(3) Calculate resistance (R=V/I).
(4) Calculate the electrical resistance by multiplying the resistance (R) by the area (25 cm ² ) of the sample.

<Evaluation of flexibility>
The carbon continuous fiber nonwoven fabrics of Examples 1 to 3 or Comparative Examples 1 to 4 were cut into 50 mm square pieces to prepare samples. Next, this sample was folded in half and sandwiched between metal plates to apply a load of 1.2 kPa for 5 seconds. Thereafter, the sample was taken out from between the metal plates, and the fracture state of the sample, that is, whether the sample cracked and broke into two pieces was checked. The results were as shown in Table 1.

From the results in Table 1, it was found that the carbon fiber nonwoven fabric had flexibility that did not break even when folded in half, by including large particles or large particle aggregates between the carbon fibers in the thickness direction. It was also found that it has excellent conductivity because it contains conductive large particles or large particle aggregates.

The carbon fiber sheet of the present invention is flexible and has excellent handling properties, and especially when the large particles or large particle aggregates have electrical conductivity, it also has excellent electrical conductivity, so it is suitable as a base material for electrodes. Can be used. For example, it can be suitably used as a base material for electrodes of lithium ion secondary batteries or electric double layer capacitors, and gas diffusion electrodes of polymer electrolyte fuel cells.

Claims (8)

This is a carbon fiber sheet in which approximately spherical particles or particle aggregates are present between carbon fibers in point contact with the carbon fibers , and the particles or particle aggregates have a diameter of 1.0 mm of the average fiber diameter of the carbon fibers. Large particles or large particle aggregates having a diameter of 5 times or more are included between carbon fibers in the thickness direction, and the carbon fibers are bonded to each other by a carbide of an organic material constituting the organic fiber containing a carbonizable organic material. A carbon fiber sheet that is characterized by : The carbon fiber sheet according to claim 1, wherein large particles or large particle aggregates are present inside the carbon fiber sheet in the thickness direction. The carbon fiber sheet according to claim 1 or 2 , characterized in that no film is formed between the large particles or large particle aggregates and the carbon fibers. The carbon fiber sheet according to any one of claims 1 to 3 , wherein the large particles or large particle aggregates have electrical conductivity. A carbon fiber sheet, characterized in that the carbon fiber sheet according to claim 4 is used as a base material for a gas diffusion electrode. A gas diffusion electrode, characterized in that a catalyst is supported on the carbon fiber sheet according to claim 4 . A membrane-electrode assembly comprising the gas diffusion electrode according to claim 6 . A polymer electrolyte fuel cell comprising the gas diffusion electrode according to claim 6 .

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