JP7372060B2 - carbon foam - Google Patents

Description

FIELD OF THE INVENTION This invention relates to carbon foam.

Carbon foam is a material obtained, for example, by heat-treating melamine resin foam (foam) in an inert gas atmosphere to carbonize it (see, for example, Patent Document 1), and its porosity, flexibility, and electrical It is used for various purposes depending on its characteristics.

Japanese Patent Application Publication No. 4-349178

Carbon foams are sometimes used as electrodes in redox flow batteries and the like. A redox flow battery cell including the electrode may have a structure in which a bipolar plate, a positive electrode, a diaphragm, a negative electrode, and a bipolar plate are laminated in this order (see Figure 1), in which the electrode is sandwiched between a bipolar plate and a diaphragm. Examples include cells that include a structure that Furthermore, in such a structure, from the viewpoint of improving the fluidity of the electrolyte within the cell, a channel through which the electrolyte flows may be provided in the bipolar plate (see FIG. 1).

If excessive pressure is applied when assembling cells, the electrode may fall into the flow path of the bipolar plate or the fibers in the electrode may break. There was a need for a carbon foam that would not easily fall into the flow path.
Therefore, an object of the present invention is to provide a carbon foam that is unlikely to fall into the flow path or break during cell assembly.

[1]
It has a linear part and a connecting part that connects the linear part,
The linear portion has an average fiber diameter of 0.1 to 5.0 μm,
Taber bending stiffness measured in accordance with JIS P8125 is 16gf・cm or more and 38gf・cm or less,
The thickness is 0.1 mm or more and 1.0 mm or less,
The porosity is 85% or more and 97% or less,
The carbon content is 90% by mass or more,
The density of the bonding portion is 15,000 pieces/mm ³ or more,
A carbon foam for fuel cells, redox flow batteries or water electrolysis, characterized by:
[2]
At least in part, the thickness direction of the carbon foam is the x direction, the direction perpendicular to the x direction is the y direction, and the direction perpendicular to the x direction and the y direction is the z direction;
The average value of the orientation angle with respect to the x direction of the linear portion included in a region of 300 μm x 300 μm x 300 μm is θavex,
The average value of the orientation angle with respect to the y direction is θavey,
The average value of the orientation angle with respect to the z direction is θavez,
When defined as
The carbon foam according to [1], wherein the difference θc between the maximum value and the minimum value among the θavex, θavey, and θavez is 3° or more.
[3]
The carbon foam according to [1] or [2], wherein the ratio of the number of linear parts to the number of bonding parts is 1.3 or more and 1.6 or less.
[4]
The carbon foam according to any one of [1] to [3], wherein the density of the bonding portion is 30,000 pieces/mm ³ or more.
[5]
The carbon foam according to any one of [1] to [4] , wherein the linear portion has an average fiber diameter of 0.5 to 4.0 μm.

Since the carbon foam of the present invention has the above configuration, it is possible to provide a carbon foam that is unlikely to fall into the flow path or break during cell assembly.

1 is a schematic diagram showing an example of a channel structure of a redox flow battery. 3 is a SEM image of the surface of carbon foam according to Comparative Example 1. 1 is an X-ray CT analysis image obtained from the carbon foam of Comparative Example 1. This is an image processed by performing line and node detection on the image of FIG. 3. 1 is a SEM image of the carbon foam surface according to Example 1. This is an enlarged SEM image of FIG. 5.

Hereinafter, a mode for carrying out the present invention (hereinafter referred to as "this embodiment") will be described in detail, but the present invention is not limited to the following description, and within the scope of the gist. It can be implemented with various modifications.

[Carbon foam]
The carbon foam of the present invention has a linear portion and a connecting portion that connects the linear portion, and has a Taber bending stiffness of 10 gf·cm or more as measured in accordance with JIS P8125.
Among these, Taber bending has a linear portion and a bonding portion that connects the linear portion, the average fiber diameter of the linear portion is 0.1 to 5.0 μm, and is measured in accordance with JIS P8125. Carbon foam having a stiffness of 10 gf·cm or more is preferred. The carbon foam also has a linear portion and a connecting portion that connects the linear portion, and at least in part, the thickness direction of the carbon foam is the x direction, the direction perpendicular to the x direction is the y direction, the x direction and the The direction perpendicular to the y direction is the z direction, and the average value of the orientation angles of the linear portions with respect to the x direction included in a region of 300 μm x 300 μm x 300 μm is θavex, and the average value of the orientation angles with respect to the y direction is θavey , when the average value of the orientation angles with respect to the z direction is defined as θavez, it is preferable that the difference θ _c between the maximum value and the minimum value among the θavex, θavey, and θavez is 3° or more.

The carbon foam of this embodiment is a carbon foam having a linear portion and a bonding portion that connects the linear portion, and a structure in which the linear portion is three-dimensionally continuous is preferable from the viewpoint of conductivity. .
The fact that at least 50% of the carbon foam is made up of linear parts and bonding parts that connect the linear parts provides excellent liquid permeability when a redox flow battery is formed, and provides a large electrode surface area and good performance. It is preferable from a certain point of view, more preferably 80% or more, still more preferably 90% or more.
Note that the ratio of the linear portion to the bonding portion may be the ratio of the total area occupied by the linear portion and the bonding portion to 100% of the total area occupied by the members constituting the carbon foam in any cross section of the carbon foam.
Examples of structures other than linear parts and joint parts include, for example, a structure in which the cross-sectional shape of the carbon fiber deviates significantly from a circle, and a wide structure in which the length of the long side of the cross-sectional shape is four times or more longer than the short side. It will be done. Since the permeability of the electrolyte decreases, the area of the above-mentioned structure other than the linear part and the joint part is preferably less than 20% of the total area of the parts constituting the carbon foam in any cross section of the carbon foam. , more preferably less than 10%, still more preferably less than 5%.

The carbon foam of this embodiment preferably has continuous voids from the viewpoint of liquid permeability when forming a redox flow battery. In addition, in a SEM image obtained by observing the surface or cross section of an arbitrary carbon foam at a magnification of 500 times, the area ratio of closed cells per unit area is 5% or less. The content is preferably 2% or less, more preferably 1% or less, and even more preferably 1% or less.

The carbon foam is preferably in the form of a sheet. The carbon foam may be a single-layer carbon foam consisting of one layer of carbon foam, or a laminated carbon foam consisting of two or more layers of single-layer carbon foam.

The carbon foam of this embodiment has a Taber bending stiffness of 10 gf/cm or more, which is advantageous in that it can further suppress falling into the flow path when pressurizing the cell set, and suppress pressure loss when the electrolyte is passed through. Therefore, it is preferably 15 to 200 gf·cm, more preferably 20 to 200 gf·cm, and still more preferably 25 to 150 gf·cm. Moreover, from the viewpoint of further reducing cell resistance, it may be 50 gf·cm or less, or may be 30 gf·cm or less.
Note that the Taber bending stiffness can be measured by the method described in Examples below.
The Taber bending stiffness is determined by, for example, the average fiber diameter of the carbon foam, the immersion conditions of the resin foam before carbonization (e.g., the compression conditions of the resin foam before immersion, the resin concentration in the solution to be immersed, and the excess adhering to the resin foam. It can be adjusted within the above range by adjusting the conditions for removing the immersion solution, etc.).

The surface area per gram of carbon foam of the present embodiment is preferably 3 m ² or more, more preferably 5 m ² or more, still more preferably 10 m ² or more, from the viewpoint of increasing the reaction area and reducing activation overvoltage. The upper limit of the surface area per 1 g of carbon foam is preferably 200 m ² or less, more preferably 180 m ² or less, even more preferably 150 m ² or less, even more preferably 120 m ² or less, and even more preferably is 100 m ² or less, more preferably 70 m ² or less, even more preferably 60 m ² or less, particularly preferably 40 m ² or less.
The surface area per gram of carbon foam can be determined by applying the BET analysis method to the nitrogen gas adsorption isotherm. Specifically, a pre-pulverized sample is dried under reduced pressure at 200° C., and then measured using an automatic nitrogen gas adsorption tester (manufactured by Quantachrome Japan, Autosorb 1). The measurement temperature is 77K, which is the liquid nitrogen temperature. The relative pressure range of 0.05-0.30 of the obtained adsorption isotherm is plotted by BET plotting, and the surface area can be determined from the slope.

The surface area per 1 cm ³ of the carbon foam of this embodiment is preferably 0.05 m ² or more, more preferably 0.1 m ² or more, and even more preferably 1 m ² from the viewpoint of increasing the reaction area and reducing activation overvoltage. That's all. The upper limit of the surface area per 1 cm ³ of the carbon foam is preferably 16 m ² or less, more preferably 10 m ² or less, and even more preferably 8 m ² or less, from the viewpoint of the pressure loss of the electrolyte.
The surface area per 1 cm ³ of carbon foam can be determined by multiplying the surface area per 1 g of carbon foam by the bulk density.

From the viewpoint of versatility, the carbon foam of this embodiment preferably has a surface with an area of 100 cm ² or more in plan view, preferably 225 cm ² or more, and more preferably 600 cm ² or more. Further, from the viewpoint of productivity, the planar area of the carbon foam is preferably 60,000 cm ² or less, more preferably 50,000 cm ² or less, still more preferably 30,000 cm ² or less, particularly preferably 25,000 cm ^{2 or} less.

From the viewpoint of resistance, the thickness of the carbon foam of this embodiment is preferably 1.0 mm or less, more preferably 0.8 mm or less, still more preferably 0.7 mm or less. The thickness is preferably 0.1 mm or more, more preferably 0.2 mm or more.

<Average fiber diameter of linear portion (carbon fiber)>
In the carbon foam of this embodiment, the average fiber diameter d of the linear portions (carbon fibers) constituting the carbon foam is preferably 0.1 μm or more and 5.0 μm or less. In this specification, the "diameter of carbon fiber" refers to the diameter of the linear portion that connects the bonding portions.
The average fiber diameter of the carbon fibers is preferably 0.1 μm or more, more preferably 0.2 μm or more, and even more preferably 0.1 μm or more, from the viewpoint of strength when used as an electrode, and from the viewpoint of ensuring physical strength and conductivity. .4 μm or more, more preferably 0.5 μm or more, even more preferably 0.6 μm or more, particularly preferably 0.8 μm or more. In addition, there are two points of view: preventing puncture into the diaphragm and suppressing short circuits due to membrane tearing, suppressing deterioration of the diaphragm surface, increasing the surface area by decreasing the average fiber diameter and reducing cell resistance, and during compression behavior. From the viewpoint of deformability and restorability, the thickness is preferably 5.0 μm or less, more preferably 4.5 μm or less, even more preferably 4.0 μm or less, particularly preferably 3.0 μm or less.
The average fiber diameter can be determined by analyzing an image obtained with a scanning electron microscope (SEM). Specifically, the fiber shape was observed at 10,000x magnification using a scanning electron microscope, and from the obtained observation image, the thickness of 20 randomly selected carbon fibers was measured. It can be calculated as the average thickness.
Note that the fiber diameter refers to the thickness of the linear part that connects the joint parts, and from the scanning electron microscope image obtained in the same manner as above, 20 points from one linear part (carbon fiber) were measured. The fiber diameter of each fiber can be measured by randomly selecting the fibers, measuring the thickness at each location, and determining the average thickness (average diameter) assuming that the cross-sectional shape is circular.

In the carbon foam of this embodiment, the ratio of the linear portions with a fiber diameter of more than 5 μm to the total linear portions should be less than 10% from the viewpoint of lowering the resistance to flowing electrolyte when passing the electrolyte. is preferable, more preferably less than 5%, still more preferably less than 5%.
Note that the proportion of linear parts having a fiber diameter of more than 5 μm may be determined from 100 linear parts randomly selected from the image obtained by the above-mentioned average fiber diameter method.

In the carbon foam of this embodiment, the average fiber length of the linear portions (carbon fibers) constituting the carbon foam is preferably 15 μm or more and 200 μm or less. As used herein, "carbon fiber length" refers to the length of the linear portion, specifically the length between any adjacent bonding portions.
The average fiber length of the carbon fibers is preferably 15 μm or more, more preferably 20 μm or more, and still more preferably 30 μm or more from the viewpoint of flexibility of the carbon foam. Further, the average fiber length of the carbon fibers is preferably 200 μm or less, more preferably 150 μm or less, and even more preferably 100 μm or less, from the viewpoint of the conductivity of the carbon foam.
Note that the average fiber length of the linear portions (carbon fibers) constituting the carbon foam can be determined by image analysis of a scanning electron microscope (SEM) image. Specifically, the carbon foam is observed using a scanning electron microscope at 10,000x magnification. From the obtained observation image, the lengths between 20 randomly selected adjacent joints are measured. This average length is defined as the above-mentioned average fiber length.

<Ratio R of the number N _l of linear parts to the number N _n of joint parts>
In the carbon foam of this embodiment, the ratio R of the number N _l of linear parts to the number N _n of bonding parts is preferably 1.3 or more and 1.6 or less. In other words, the ratio R is the average number of branches at the joint. The reason why R is set to be 1.3 or more is that when R is less than 1.3, the linear portions do not have a three-dimensional network structure connected at bonding portions, and the wires are not bonded like a nonwoven fabric. This is because a structure in which the shaped parts are in contact can be considered. Also, the reason why R is set to be 1.6 or less is because if R exceeds 1.6, the linear portion becomes band-like, for example, a porous structure covered with a honeycomb-like wall surface is considered. be.
The ratio R of the number N _l of linear parts to the number N _n of bonding parts is more preferably 1.35 or more, still more preferably 1.4 or more. Further, it is more preferably 1.55 or less, still more preferably 1.52 or less.

<Void ratio in plan view>
In the carbon foam of this embodiment, the area ratio of aggregates covering voids within a 0.5 mm x 0.5 mm area in plan view is preferably 50% or less, and preferably 30% or less. , more preferably 10% or less. It is preferable that the area ratio of the aggregates covering the voids be as small as possible. The lower limit of the area ratio of the aggregates covering the voids is not particularly limited, but may be 1% or more, 3% or more, or 5% or more.
In the carbon foam, the area ratio of voids within an area of 0.5 mm x 0.5 mm in plan view is preferably 50% or more, more preferably 70% or more, and still more preferably 90% or more.
Note that the above area can be measured using a SEM on the surface of the carbon foam.

In the carbon foam of this embodiment, the weight of the aggregates present on the surface of the carbon foam is 2% compared to the carbon foam 100, from the viewpoint of improving the Taber bending stiffness of the carbon foam and suppressing dropping into the flow path etc. The number is preferably 5 or more, more preferably 5 or more, and even more preferably 8 or more. In addition, from the viewpoint of increasing pressure loss due to aggregates blocking voids in the carbon foam, the weight of aggregates present on the surface of the carbon foam is preferably 400 or less, more preferably 200 or less, based on 100 carbon foam. Yes, and more preferably 100 or less.
In the carbon foam of this embodiment, the ratio of the area occupied by the aggregates present on the surface of the carbon foam to the surface area of the carbon foam is such that the fracture resistance of the linear portion is further improved, and the drop into the flow path etc. is further improved. From the viewpoint of increasing surface area and reducing cell resistance, it is preferably 1 to 70%, more preferably 3 to 50%, and still more preferably 1 to 30%.
The aggregate is preferably carbonized binder resin immersed in a resin foam.
Note that the ratio of the area occupied by aggregates to the surface area of the carbon foam can be determined by observing the carbon foam surface at a magnification of 10,000 times using a scanning electron microscope, and measuring any area (for example, It can be calculated by measuring the area occupied by carbon foam and the area occupied by aggregates (e.g., an area corresponding to 10 μm×10 μm). In addition, in the above calculation, the area occupied by the carbon foam and the aggregate may be expressed as the ratio of the carbon foam or the aggregate to the obtained image (image taken from one direction).
The ratio of the area occupied by aggregates to the surface area of the carbon foam is determined by, for example, the soaking conditions of the resin foam before carbonization (e.g., the compression conditions of the resin foam before soaking, the resin concentration in the dipping solution, the It can be adjusted by adjusting the conditions for removing excess immersion solution, etc.), the carbonization conditions, etc.

In the carbon foam of this embodiment, the ratio of the area occupied by the aggregates present on the surface of the linear part to the surface area of the linear part of the carbon foam is such that the fracture resistance of the linear part is further improved and the flow path From the viewpoint of further suppressing the drop in the cell resistance and increasing the surface area and reducing the cell resistance, the range is preferably 1 to 70%, more preferably 3 to 50%, and still more preferably 5 to 30%.
The ratio of the area occupied by the aggregates to the surface area of the linear portion of the carbon foam is the same as the ratio of the area occupied by the aggregates to the surface area of the carbon foam described above, except that the measurement target is changed to the linear portion. can be found.
The ratio of the area occupied by the aggregates to the surface area of the linear portion of the carbon foam depends on, for example, the immersion conditions of the resin foam before carbonization (e.g., the compression conditions of the resin foam before immersion, the resin concentration in the solution to be immersed, the resin It can be adjusted by adjusting the conditions such as the conditions for removing excess dipping solution adhering to the foam), the carbonization conditions, etc.

<Average diameter of aggregates>
In the carbon foam of this embodiment, the average diameter of the aggregates within the carbon foam is preferably 400 μm or less, more preferably 100 μm or less, and still more preferably 50 μm or less. The average diameter of the aggregates may be 1 nm or more, 5 nm or more, nmμm or more, or 20 nm or more.
In addition, the said average diameter can be measured using SEM of the carbon foam cross section.

<Orientation angle of linear part>
When a melamine resin foam is carbonized by heat treatment in a heat treatment furnace, the carbon foam has an isotropic structure in which the carbon fibers forming the skeleton of the carbon foam are spread evenly in all directions. In the case of such carbon foam, the average value of the orientation angle with respect to the x direction of the linear portion included in a region of 300 μm × 300 μm × 300 μm is θavex, the average value of the orientation angle with respect to the y direction is θavey, and the orientation angle with respect to the z direction is When the average value of is defined as θavez, the difference θc between the maximum value and the minimum value among θavex, θavey, and θavez is usually 1° or less.
Note that the above three directions include the thickness direction of the carbon foam as the x direction, the direction perpendicular to the x direction as the y direction, and the direction perpendicular to the x direction and the y direction as the z direction.

However, when compressive stress is applied to the resin foam that is the raw material of the carbon foam when heat-treating and carbonizing the melamine resin foam, a carbon foam with a skeletal structure having anisotropy in the spread of carbon fibers can be obtained. In the case of such a carbon foam, even when a compressive load is applied, it is possible to suppress breakage of carbon fibers (linear parts), reduce powder falling, and achieve high resilience. In order to obtain this effect, in the carbon foam of the present embodiment, the difference θc is preferably 3° or more, more preferably 5° or more, and even more preferably 8° or more.

It is preferable that at least a part of the carbon foam of the present embodiment includes an area portion of 300 μm in length x 300 μm in width x 300 μm in height that satisfies the above-mentioned specifications of θavex, θavey, and θavez, and 50% by volume or more satisfies the above-mentioned angle specifications. It is more preferable that the carbon foam satisfies the above density range, and it is even more preferable that the above density range is satisfied at 75% by volume or more, and it is particularly preferable that the above angle regulation is satisfied at any part of the carbon foam.

<Density of joint>
The density of the bonded portion of the carbon foam of this embodiment is preferably 8,000 pieces/mm ³ or more, more preferably 15,000 pieces/mm ³ or more, from the viewpoint of resilience when compressive load is applied. More preferably 30,000 pieces/mm ³ or more, particularly preferably 50,000 pieces/mm ³ or more. Further, from the viewpoint of flexibility of the carbon foam, the number is preferably 5,000,000 pieces/mm ³ or less, more preferably 4,000,000 pieces/mm ³ or less.

It is preferable that at least a part of the carbon foam of this embodiment has a part that satisfies the density of the bonded part, more preferably that the density is 50% by volume or more, and it is more preferable that the density is within the above density range by 75% by volume or more. It is more preferable if the above density range is satisfied, and it is particularly preferable that the above density range is satisfied at any part of the carbon foam.

In addition, in this specification, the number N _n of the bonded portions, the number N _l of the linear portions, the density of the bonded portions, and the orientation angle θ are determined by photographing the carbon foam using an X-ray CT (Computerized Tomography) device, From the obtained tomographic image data, after using the median filter as preprocessing, Otsu's binarization algorithm (Nobuyuki Otsu, "Automatic threshold selection method based on discriminant and least squares criterion", Electronic Information and Communication) D, Vol. J63-D, No. 4, pp. 346-356 (1980)) was used to divide the region into structure and space, and create a three-dimensional image of the structure including the inside of the carbon foam. This is the value determined using structural analysis software from the obtained three-dimensional image.

Specifically, the number of joints N _n and the number N _l of linear parts are determined by detecting the joints and linear parts included in the three-dimensional image obtained as described above and counting the number. Find it by From N _n and N _l obtained in this way, the ratio R of N _l to N _n can be determined.

Furthermore, the orientation angle θ of the linear part is the angle between each direction and the straight line connecting the joints at both ends of the linear part, and is determined for each of the three mutually orthogonal directions in the three-dimensional image, For each direction, the average value of the orientation angles of the linear portions is determined.

As a CT device used for structural analysis of the carbon foam, a CT device using low-energy, high-intensity X-rays, such as a high-resolution 3D X-ray microscope nano3DX manufactured by Rigaku Co., Ltd., can be used. Further, for image processing and structural analysis, for example, Centerline editor of the software Simpleware manufactured by JSOL Co., Ltd. can be used.
Note that the number N _n of bonded portions, the number N _l of linear portions, the density of bonded portions, and the orientation angle θ can be specifically measured by the measuring methods described in Examples.

<Carbon content>
From the viewpoint of conductivity, the carbon content of the carbon foam of this embodiment is preferably 51% by mass or more, 60% by mass or more, 65% by mass or more, 70% by mass or more, 75% by mass or more, 80% by mass or more. , 85% by mass or more, and 90% by mass or more. The upper limit is not particularly limited, but may be 100% by mass or less, 99% by mass or less, or 98% by mass or less.
The carbon content of the carbon foam can be determined by fluorescent X-ray measurement, and specifically by the method described in Examples below.

<Porosity>
From the viewpoint of flexibility, the porosity of the carbon foam of this embodiment is preferably 70% or more, more preferably 80% or more, still more preferably 85% or more, and particularly preferably 86% or more. The upper limit is not particularly limited, but is preferably 99.5% or less, more preferably 99% or less, even more preferably 95% or less.
Note that in this specification, porosity is a value determined from bulk density (described later) and true density (described later). Bulk density is the density based on the volume, including voids, contained in the carbon foam. True density, on the other hand, is the density based on the volume occupied by the material of the carbon foam.

[Measurement of bulk density]
First, the dimensions of the carbon foam are measured using a caliper or the like, and the bulk volume V _bulk of the carbon foam is determined from the obtained dimensions. Next, the mass M of the carbon foam is measured using a precision balance. From the obtained mass M and bulk volume V _bulk , the bulk density ρ _bulk of the carbon foam can be determined using the following equation (1).
ρ _bulk = M/V _bulk ...(1)
From the viewpoint of lowering the resistance when used as an electrode, the bulk density is preferably 3.0 kgm ^-3 or more, more preferably 3.5 kgm ^-3 or more, and even more preferably 4.0 kgm ^-3 or more. In addition, from the viewpoint of flexibility of the carbon foam, it is preferably 400 kgm ^-3 or less, more preferably 300 kgm ^-3 or less, still more preferably 200 kgm ^-3 or less.

[Measurement of true density]
The true density ρ _real of the carbon foam can be determined by the float-sink method using a liquid mixture consisting of n-heptane, carbon tetrachloride, and ethylene dibromide. Specifically, first, carbon foam of an appropriate size is placed in a stoppered test tube. Next, three types of solvents are appropriately mixed, added to a test tube, and placed in a constant temperature bath at 30°C. If the sample piece floats, add n-heptane, which has a lower density. On the other hand, if the test piece sinks, add ethylene dibromide, which has a high density. Repeat this operation until the test piece floats in the liquid. Finally, the density of the liquid is measured using a Gerussac pycnometer.

[Calculation of porosity]
From the bulk density ρ _bulk and true density ρ _real determined as described above, the porosity V _f,pore can be determined using the following equation (2).
V _f,pore = ((1/ρ _bulk )-(1/ρ _real ))/(1/ρ _bulk )×100 (%)...(2)

<Crystallite size>
The crystallite size Lc of the carbon foam of this embodiment is preferably 1.1 nm or more, and more preferably 1.5 nm or more from the viewpoint of conductivity. Further, from the viewpoint of physical fragility, the thickness is preferably 4.0 nm or less, more preferably 3.0 nm or less.

<Ratio of oxygen atoms>
The proportion of oxygen atoms in the carbon foam of this embodiment measured by surface analysis using fluorescent X-ray analysis is preferably 0.03% by mass or more, more preferably 0.05% by mass from the viewpoint of wettability to the electrolytic solution. It is at least 0.07% by mass, more preferably at least 0.07% by mass. Further, from the viewpoint of the resistance of the electrode, the content is preferably 10% by mass or less, more preferably 5% by mass or less, and still more preferably 3% by mass or less.

The carbon foam of this embodiment is preferably a carbon foam constructed from a single member without defects. In this embodiment, the term "defect" refers to a through hole H that passes through the carbon foam through the surface S with an area of 100 cm ² or more, and has an area of 2000 mm ² or more on the surface S. ing. That is, the carbon foam of this embodiment is preferably a carbon foam that does not include through holes H having an area of 2000 mm ² or more on the surface S. Note that the above-mentioned surface S means a surface composed of a single plane, and does not include, for example, a surface composed of a plurality of adjacent planes of a polyhedral surface.

In the carbon foam of this embodiment, the presence or absence of the through holes H is evaluated by visual inspection and inspection using an inspection device (for example, a pinhole inspection machine) equipped with a light source and a photodetector. Specifically, first, the surface S is visually observed to evaluate the presence or absence of the through hole H. If the presence of the through hole H cannot be visually confirmed, an inspection using an inspection device is performed. Specifically, a light source is placed on the surface S side of the carbon foam, and a photodetector is placed on the surface opposite to the surface S. Then, light is irradiated from the light source toward the surface S of the carbon foam. Then, if the carbon foam has through-holes H, the irradiated light passes through the through-holes H and reaches the photodetector. In this way, the through hole H can be detected. Note that the arrangement of the light source and the photodetector may be reversed. By using inspection equipment such as pinhole inspection machines currently on the market, it is possible to detect pinholes with a diameter of several μm, and if the through hole has an area of 2000 mm ² or more, the above visual inspection is possible. Even if it is missed, it can be reliably detected.

When the through hole H is detected by the above inspection, the area of the through hole H on the surface S is measured. This area can be measured using a microscope or an electron microscope. In this embodiment, in the case of a carbon foam in which no through hole H was detected by the inspection using the light source and photodetector, and in the case where a through hole H was detected but the area was less than 2000 mm ² , It is included in this embodiment as a carbon foam without defects. On the other hand, if the area of the through hole is 2000 mm ² or more, the carbon foam is considered to be defective and is not included in this embodiment.

Note that the shape of the through-hole H is not limited, and the through-hole H includes crack-like and linear shapes. In addition, holes that are made after the carbon foam is used, such as holes that are made by processing to incorporate the carbon foam into cells, are not defects and are therefore not included in the through-holes.
In addition, if there are multiple through holes H on the surface S, if the area of each of them is less than 2000 ^mm2 , they are included in this embodiment, and if the area of at least one of them is 2000 mm2 ^or more, this embodiment is included. Not included in the form.

From the viewpoint of the strength of the electrode, the carbon foam of this embodiment preferably has no through holes with an area of 2000 mm ² or more, more preferably has no through holes with an area of 1000 mm ² or more, and has an area of 500 mm ² or more. It is more preferable that there are no through holes, it is even more preferable that there are no through holes that have an area of 100 mm ² or more, and it is particularly preferable that there are no through holes that have an area of 10 mm ^{2 or} more.

In addition, from the viewpoint of strength and handling when used as an electrode, the carbon foam of this embodiment preferably has an area of 4000 mm ^{2 or} more without through holes with an area of 2000 mm 2 or more, and more preferably 6000 mm ² or more. It is preferably 8,000 mm ² or more, more preferably 10,000 mm ^{2 or more, and even more preferably 10,000 mm 2} or more.

Further, in the carbon foam of this embodiment, the area without through holes with an area of 1000 mm ² or more is preferably 2000 mm ² or more, more preferably 4000 mm ² or more, even more preferably 6000 mm ² or more, and even more preferably 8000 mm ² It is even more preferable that the area is at least 10,000 mm ² , particularly preferably at least 10,000 mm 2 .

Further, the carbon foam of this embodiment preferably has an area of 1000 mm ² or more without through holes with an area of 500 mm ² or more, more preferably 2000 mm ² or more, 4000 mm ² or more, 6000 mm ² or more, 8000 mm ^{2 or} more, or 10000 mm. ² or more.

Further, the carbon foam of this embodiment preferably has an area of 200 mm ² or more without through holes with an area of 100 mm ² or more, more preferably 500 mm ² or more, 1000 mm ² or more, 2000 mm ² or more, 4000 mm ^{2 or} more, or 6000 mm. ² or more, 8,000 mm ² or more, or 10,000 mm ² or more.

Further, the carbon foam of this embodiment preferably has an area of 20 mm ² or more without through holes with an area of 10 mm ² or more, more preferably 100 mm ² or more, 500 mm ² or more, 1000 mm ² or more, 2000 mm ^{2 or} more, or 4000 mm. ² or more, 6000 mm ² or more, 8000 mm ² or more, or 10000 mm ² or more.

The carbon foam of this embodiment is suitable for use by applying pressure and bringing it into close contact with a member having an uneven surface, such as a cavity or an uneven surface, because the linear portion is less prone to breakage or depression. Specifically, it can be used for fuel cells, redox flow batteries, water electrolysis, etc.
Note that the carbon foam of this embodiment is difficult to break in the linear portions and has little powder falling off, so it can also be used in applications in which it is brought into close contact with a member having a flat surface.

(Method for manufacturing carbon foam)
The method for manufacturing the carbon foam of this embodiment includes, for example, a pressing step of compressing the resin foam, a dipping step of immersing the compressed resin foam in a dipping solution containing a binder resin, and a carbonization step of carbonizing the soaked resin foam. , and the like. The above manufacturing method may further include an oxidation step of oxidizing the carbonized resin foam.

-Press process-
Examples of the resin foam used as a raw material for producing the carbon foam of this embodiment include any resin foam known as a raw material for carbon foam, such as melamine resin foam; urethane resin foam; phenol resin foam; acrylonitrile resin foam; Among these, melamine resin foam is preferred from the viewpoint of the size and uniformity of the diameter of the linear portion.

The above-mentioned melamine resin foam is produced after foaming an aqueous solution or dispersion containing, for example, a precondensate of melamines and formaldehyde, an emulsifier, a blowing agent, a hardening agent, and, if necessary, a well-known filler. It can be manufactured by performing a hardening treatment. The foaming treatment and the curing treatment may be performed by heating a solution consisting of the above-mentioned components to a temperature set depending on the type of foaming agent used (for example, a temperature equal to or higher than the boiling point of the foaming agent).

The melamines mentioned above include melamine, guanamine, N-butylmelamine, N-phenylmelamine, N,N-diphenylmelamine, N,N-diallylmelamine, N,N',N''-triphenylmelamine, N,N' , N''-trimethylolmelamine, benzoguanamine, 2,4-diamino-6-methyl-1,3,5-triazine, 2,4-diamino-6-butyl-1,3,5-triazine, 2,4- Diamino-6-benzyloxy-1,3,5-triazine, 2,4-diamino-6-butoxy-1,3,5-triazine, 2,4-diamino-6-cyclohexyl-1,3,5-triazine , 2,4-diamino-6-chloro-1,3,5-triazine, 2,4-diamino-6-mercapto-1,3,5-triazine, amerine (N,N,N',N'-tetra cyanoethylbenzoguanamine), etc. The above-mentioned melamines may be used alone or in combination of two or more.
Monomers other than melamines and formaldehyde may be used for the above-mentioned precondensate.
As the above-mentioned precondensate, for example, one having a melamine:formaldehyde ratio of 1:1.5 to 1:4 and an average molecular weight of 200 to 1000 can be used.
Conditions for condensation include, for example, pH 7 to 10 and temperature 70 to 100°C.

Examples of the emulsifier include sodium salts of alkylsulfonic acids and arylsulfonic acids. The emulsifier may be used in an amount of 0.5 to 5% by weight based on 100% by weight of the precondensate.

Examples of the blowing agent include pentane, hexane, trichlorofluoromethane, trichlorotrifluoroethane, hydroxyfluoroether, and the like. The blowing agent may be used in an amount of 1 to 50% by weight based on 100% by weight of the precondensate.

Examples of the curing agent include hydrochloric acid, sulfuric acid, formic acid, and the like. The curing agent may be used in an amount of 0.01 to 20% by weight based on 100% by weight of the precondensate.

As the melamine resin foam, for example, a melamine/formaldehyde condensation foam produced by the method disclosed in JP-A-4-349178 can be used.
Further, the urethane resin foam, the phenol resin foam, and the acrylonitrile resin foam can be appropriately produced by a known method.

The bulk density of the resin foam is preferably 0.001 to 0.1 g/cm ³ , more preferably 0.005 to 0.02 g/cm ³ .
Further, the porosity of the resin foam is preferably 60 to 99.9%, more preferably 80 to 99%.

The pressure applied to the resin foam in the above pressing step is preferably 0.1 to 50 MPa, more preferably 0.5 to 10 MPa, from the viewpoint of maintaining the porous structure and ensuring the permeability of the electrolyte.

The time for applying pressure to the resin foam in the above pressing step is preferably 1 to 300 minutes, more preferably 5 to 80 minutes, from the viewpoint of maintaining the porous structure and ensuring permeability of the electrolyte.

The above pressing step may be performed under heating.
The temperature at which pressure is applied to the resin foam in the above pressing step is preferably 200 to 600°C, more preferably 300 to 400°C, from the viewpoint of maintaining the shape after pressing.

In the above pressing step, after applying pressure, it is preferable to perform a dipping step after cooling (for example, after cooling to room temperature).

-Soaking process-
The binder resin contained in the above dipping solution is preferably a resin that carbonizes in the carbonization step, and includes organic resins, such as thermoplastic resins such as polyvinyl chloride latex, Saran latex, thermoplastic polyimide, and thermosetting resins. Thermosetting resins such as polyimide and phenol resin are preferred, and Saran latex is preferred from the viewpoint of binding properties and carbonization yield.

In the dipping solution, the binder resin is preferably diluted with a dispersion medium.
Examples of the dispersion medium include organic solvents such as γ-butyrolactone and N-methylpyrrolidone, and water. Among these, water is preferred from the viewpoint of drying speed and viscosity stability.

The solid content concentration of the binder resin in the above dipping solution is 0.1 to 50% by mass from the viewpoint of obtaining a carbon foam whose linear parts are more difficult to break and which can further suppress falling into the flow path during cell assembly. The amount is preferably 0.5 to 20% by weight, still more preferably 1 to 10% by weight, particularly preferably 2 to 5% by weight.

The method for dipping the resin foam in the dipping solution is not particularly limited, and any known method can be used. Specifically, the methods include dipping the resin foam in a dipping solution, passing the dipping solution through the resin foam, spraying the dipping solution from at least one direction of the resin foam, and applying the dipping solution from at least one direction of the resin foam. For example, a method of applying .

In the above dipping step, a step of removing excess dipping solution adhering to the resin foam may be added from the viewpoint of suppressing the formation of a film that blocks the voids in the carbon foam. Specific examples include centrifugation, compression, flow of air or inert gas, suction, etc. Among them, centrifugation is preferred from the viewpoint of suppressing aggregates in the carbon foam and causing less damage to the product.

In the above dipping step, the resin foam after dipping may be dried (eg, dried at a temperature of 50 to 200° C. for 0.5 to 5 hours). Steam drying may be vacuum drying.

The above-mentioned dipping step may be performed during the heating step of the carbonization step (for example, the step in which the resin foam is heated to about 300 to 600° C.).

-Carbonization process-
The above carbonization process involves applying a compressive load to the resin foam and carbonizing it by heat treatment in an inert gas stream such as nitrogen or in an inert atmosphere such as vacuum; the resin foam is placed in a heat treatment furnace. A step of introducing the raw material foam to be introduced, a temperature raising step of raising the temperature in the heat treatment furnace to the heat treatment temperature at a first temperature increase rate, and a step of holding the temperature at the heat treatment temperature for a predetermined time to carbonize the resin foam to produce carbon foam. A method including a carbonization step in which the carbon foam is heated, a temperature lowering step in which the temperature inside the heat treatment furnace is lowered to room temperature, and a carbon foam transport step in which the carbon foam is transported out of the heat treatment furnace; Here, the above-mentioned temperature raising step may be carried out at least in the first temperature range where the amount of decomposable desorption gas generated from the resin foam is large, while the inside of the heat treatment furnace is being evacuated under reduced pressure.

In the raw material foam introduction step, the heat treatment furnace for carbonizing the raw material resin foam is not limited as long as it is a furnace that can carbonize the resin foam to obtain carbon foam, for example, a reaction furnace that accommodates the raw material resin foam. A furnace, a heater that heats the inside of the reactor, a gas inlet that introduces inert gas into the reactor, a gas outlet that exhausts gas from the reactor, and a vacuum that reduces the pressure inside the reactor. A heat treatment furnace equipped with a vacuum pump can be used.

In the temperature raising step, it is preferable to carry out the heat treatment in the first temperature range where a large amount of decomposable gas is generated from the resin foam while the inside of the heat treatment furnace is being evacuated under reduced pressure.
When resin foam, which is the raw material for carbon foam, is heated, the active decomposition gas generated from the resin foam reacts with the carbon fibers that make up the carbon foam and decomposes, which can cause defects in the carbon foam. . The amount of the decomposable desorption gas generated depends on the temperature inside the furnace. In the temperature range (first temperature range) where a large amount of decomposable gas is generated from the resin foam in the temperature raising process, decompressing gas generated inside the resin foam can be removed by evacuating the inside of the heat treatment furnace. It is possible to promote the diffusion of outgassing out of the resin foam and prevent the formation of defects in the carbon foam.

The "temperature range (first temperature range) where a large amount of decomposable gas is generated from the resin foam" is determined by monitoring the weight of the raw material resin foam in the temperature raising process at intervals of 100 °C from 0 °C. , the temperature range in which the weight of the resin foam decreases by 5% or more of its initial weight per 100°C. For example, if the weight of the resin foam decreases by 5% or more of the initial weight per 100°C in all temperature ranges of 300°C or higher and lower than 400°C, 400°C or higher and lower than 500°C, and 500°C or higher and lower than 600°C, the The temperature range 1 is 300°C or more and less than 600°C.
When the resin foam is a melamine resin foam, the temperature range (first temperature range) in which a large amount of decomposable desorption gas is generated is a temperature range of 200°C or more and less than 800°C.

The above-mentioned evacuation can be performed using an evacuation means such as a vacuum pump. Preferably, evacuation is performed using a pump having an evacuation ability that can reduce the pressure inside the furnace to 1 Pa or less within 10 minutes.

For example, when the raw material resin foam is melamine resin foam, the rate of temperature increase to the heat treatment temperature (first temperature increase rate) is set to 10°C/min or less in order to suppress the amount of decomposable desorption gas generated. It is preferable to do so. Further, from the viewpoint of overall productivity, it is preferable that the first temperature increase rate is 1° C./min or more.

In addition, in the temperature range (first temperature range) where a large amount of decomposable desorption gas is generated from the resin foam, the temperature rise step is faster than the temperature rise rate (first temperature rise rate) up to the heat treatment temperature. It is also preferable to perform the heating at a lower temperature increase rate (second temperature increase rate). Thereby, the amount of decomposable desorption gas generated within the resin foam per unit time can be reduced, and the diffusion of the decomposable desorption gas outside the foam structure can be further promoted. When the temperature increase rate is reduced in the first temperature range (that is, when the temperature increase rate is changed to the second temperature increase rate), if the temperature inside the furnace exceeds the upper limit of the first temperature range, the temperature increase rate The temperature may be increased by returning the temperature to the first temperature increase rate.

Furthermore, the temperature raising step is performed in the second temperature region in a region (second temperature region) in which the rate of increase in the amount of decomposable desorption gas is high within the first temperature region in which the amount of desorption gas generated is large. It is preferable to carry out the heating at a lower heating rate (third heating rate) than the heating rate. Thereby, the amount of decomposable desorption gas generated within the resin foam per unit time can be further reduced, and the diffusion of the decomposable desorption gas outside the foam structure can be further promoted.

The "temperature range (second temperature range) in which the rate of increase in the amount of decomposable desorption gas generated from the resin foam is high" is defined as the weight of the resin foam as a raw material in the heating process at intervals of 100°C from 0°C. Monitor in advance and set the temperature range in which the weight of the resin foam decreases by 20% or more of the initial weight per 100°C. For example, if the weight of the resin foam decreases by 20% or more of the initial weight per 100°C in the temperature range of 300°C or higher and lower than 400°C and 400°C or higher and lower than 500°C, the second temperature range is 300°C or higher and lower than 500°C. Less than ℃.
When the raw material resin foam is a melamine resin foam, the temperature range (second temperature range) in which the rate of increase in the amount of desorbed gas generated from the resin foam is high is a temperature range of 300°C or more and less than 400°C. When the resin foam is a melamine resin foam, the heating rate is more preferably 5°C/min or less in the first temperature range, and particularly preferably 3°C/min or less in the second temperature range. .

Further, in the temperature raising step and the carbonization step described below, in order to prevent a decomposition reaction between oxygen and the carbon fibers constituting the carbon foam, the atmosphere in the furnace is preferably an inert gas atmosphere or a vacuum. Here, the term "vacuum" in the furnace means that the degree of vacuum in the furnace is less than 1 Pa. In addition, the inert gas atmosphere is preferably such that after the resin foam that is the raw material for the carbon foam is introduced into the heat treatment furnace (raw material foam introduction step), the inside of the furnace is depressurized and evacuated to remove air containing oxygen. After the inside of the furnace reaches a degree of vacuum of less than 1 Pa and air is sufficiently degassed, it is preferable to introduce nitrogen gas. In this way, the inside of the furnace can be made into a nitrogen gas atmosphere. After the inside of the furnace is made into an inert gas atmosphere or a vacuum in this way, temperature increase is started, and the inside of the furnace is depressurized and evacuated in the first temperature range.

Furthermore, in the region of 200° C. or higher and lower than 800° C. (first temperature region) where the amount of desorbed gas from the melamine resin foam is large, it is preferable to continue evacuation under reduced pressure while introducing an inert gas into the furnace. Thereby, a flow of an inert gas such as nitrogen gas or argon gas can be generated in the furnace, and the discharge of the decomposable desorption gas generated in the resin foam can be promoted.

When introducing the inert gas, the flow rate of the inert gas is preferably 1 L/min or more, more preferably 2 L/min or more. Further, the flow rate of the inert gas is preferably 40 L/min or less, more preferably 30 L/min or less, and particularly preferably 20 L/min or less.

In the carbonization step, the resin foam is carbonized into a carbon foam by holding the resin foam at the heated heat treatment temperature for a predetermined period of time. The heat treatment temperature is preferably a temperature equal to or higher than the softening point of the raw material resin foam. For example, when the resin foam is a melamine resin foam, the softening point of the melamine resin foam is 300° C. to 400° C., so the heat treatment temperature is set to a temperature equal to or higher than the softening point. The temperature is preferably 800°C or higher, more preferably 1000°C or higher. Further, from the viewpoint of physical fragility due to high crystallinity, the temperature is preferably 3000°C or lower, more preferably 2500°C or lower.

Further, the time for holding at the above heat treatment temperature (heat treatment time) is preferably a time for completely carbonizing the raw material resin foam. For example, when the raw material resin foam is a melamine resin foam, the holding time is preferably 0.5 hours or more, more preferably 1 hour or more, and still more preferably 2 hours or more. Further, from the viewpoint of productivity, the time is preferably 5 hours or less, more preferably 4 hours or less.

In the temperature lowering step, the temperature lowering rate during carbonization of the melamine resin foam is preferably 20° C./min or less from the viewpoint of mitigating damage to the heater and heat insulating material in the furnace due to rapid cooling. More preferably, it is 15°C/min or less. Further, from the viewpoint of overall productivity, the rate is preferably 5° C./min or more. More preferably, it is 10°C/min or more.

Note that by performing the temperature raising step and the carbonization step while applying a compressive load to the resin foam as a raw material, it is possible to obtain a carbon foam having a skeleton structure in which the spread of carbon fibers is anisotropic. Carbon foam having anisotropy can suppress breakage of carbon fibers, reduce powder falling, and realize high resilience even when a compressive load is applied.

The compressive load can be applied by placing a weight such as a graphite plate on the raw material resin foam. The compressive load to be applied is preferably 50 Pa or more, more preferably 200 Pa or more. Moreover, it is preferably 2000 Pa or less, more preferably 1500 Pa or less.

When applying a compressive load to the raw material resin foam, the diffusion of decomposable desorption gas is suppressed by a weight such as a graphite plate. Therefore, in the heating process, compared to the case where no compressive load is applied, the heating rate is reduced, and inert gas is supplied into the furnace while depressurizing exhaust is continued to promote the discharge of decomposable gases. is particularly preferred.

For example, when the raw material resin foam is melamine resin foam, in the temperature range of 200°C or more and less than 800°C (first temperature range), the temperature increase rate is preferably 5°C/min or less, and the desorption gas In the temperature range from 300°C to less than 400°C (second temperature range) where the rate of increase in the amount of generation is high, it is more preferable to set the temperature to 2°C/min or less. Further, in a temperature range of 200° C. or higher and lower than 800° C. (first temperature range), it is preferable to supply an inert gas such as nitrogen gas or argon gas into the heat treatment furnace.

Note that the compressive stress to the raw material resin foam may be applied not only from one direction but from two directions.

-Oxidation process-
It is preferable that the oxidation step is provided after the carbonization step.
Examples of the oxidation step include a method of heating in the presence of oxygen (for example, under a stream of air), a method of chemical oxidation, and the like.
The temperature in the oxidation step is 300 to 600°C, and the oxidation time is 0.5 to 5 hours.

The present invention will be explained in more detail below based on Examples, but the present invention is not limited to these Examples.

[evaluation]
The following measurements were performed on the carbon foams obtained in Examples and Comparative Examples.

<Porosity>
First, the dimensions of the carbon foam were measured using a caliper or the like, and the bulk volume V _bulk of the carbon foam was determined from the obtained dimensions. Next, the mass M of the carbon foam was measured using a precision balance. From the obtained mass M and bulk volume V _bulk , the bulk density ρ _bulk of the carbon foam was determined using the following equation (1).
ρ _bulk = M/V _bulk ...(1)
The true density ρ _real of the carbon foam was determined by the float-sink method using a liquid mixture consisting of n-heptane, carbon tetrachloride, and ethylene dibromide. Specifically, first, a carbon foam of an appropriate size was placed in a stoppered test tube, and then three types of solvents were appropriately mixed, added to the test tube, and immersed in a constant temperature bath at 30°C. If the sample pieces floated, low density n-heptane was added. On the other hand, if the test piece sank, high-density ethylene dibromide was added. This operation was repeated to allow the test piece to float in the liquid. Finally, the density of the liquid was measured using a Gerussac pycnometer.
From the bulk density ρ _bulk and true density ρ _real determined as described above, the porosity V _f,pore was determined using the following equation (2).
V _f,pore = ((1/ρ _bulk )-(1/ρ _real ))/(1/ρ _bulk )×100(%)...(2)

<Average fiber diameter>
The carbon foam was observed using SEM under 10000x magnification. The diameters of the carbon linear portions were measured at arbitrary 20 locations, and the average value was taken as the average fiber diameter (μm).

<Average fiber length>
The carbon foam was observed using SEM under 10000x magnification. Regarding the fiber length of the carbon linear portion, 20 fiber lengths between arbitrary adjacent joints were measured, and the average value was taken as the average fiber length (μm).

<Agglomerate ratio in plan view>
Using SEM, the surface of the carbon foam sheet was observed under 200 times magnification. The area of the membrane-like portion that closes the voids of the carbon foam within a 0.5 mm x 0.5 mm area was evaluated from the following viewpoints.
◎ (excellent): 10% or less ○ (good): 30% or less △ (fair): 50% or less × (poor): over 50%

<Diameter of aggregate>
The obtained carbon foam sheet was cut at an arbitrary point with a razor, and the cross section was magnified approximately 500 times using an SEM, and the cross section of the carbon foam sheet was observed. The diameters of 20 arbitrary aggregates within the carbon foam were measured, and the average value was evaluated from the following points of view.
◎ (Excellent): 50 μm or less ○ (Good): 100 μm or less △ (Normal): 400 μm or less × (Poor): Over 400 μm

<Carbon content>
The carbon content (mass %) of the carbon foam was determined from fluorescent X-ray measurements. For the fluorescent X-ray measurement, a fluorescent X-ray analyzer ZSX-100E (wavelength dispersion type, Rh tube) manufactured by Rigaku Co., Ltd. was used. A sample having a size of 20 mmφ or more was used.

<Ratio of oxygen atoms>
The oxygen content (% by mass) of the carbon foam was determined from X-ray fluorescence measurements. For the fluorescent X-ray measurement, a fluorescent X-ray analyzer ZSX-100E (wavelength dispersion type, Rh tube) manufactured by Rigaku Co., Ltd. was used. A sample having a size of 20 mmφ or more was used.

<Structural analysis by X-ray CT>
In order to make it easier to take X-ray images of the carbon foam produced in the examples, a test piece was taken after electroless copper plating and the structure was examined using a high-resolution 3D X-ray microscope nano3DX (manufactured by Rigaku Co., Ltd.). An analysis was performed. Specific electroless plating conditions and X-ray CT analysis conditions are as follows.
FIG. 3 shows an X-ray CT analysis image obtained from the carbon foam of Comparative Example 1, and FIG. 4 shows an example of the result after image processing in which lines and nodes were detected in the image of FIG.

[Electroless plating conditions]
The sample was immersed in OPC CondiClean MA (manufactured by Okuno Pharmaceutical Co., Ltd., diluted to 100 mL/L with distilled water) at 70°C for 5 minutes, and then washed with distilled water for 1 minute. Subsequently, it was immersed in OPC Predip 49L (manufactured by Okuno Pharmaceutical Co., Ltd., diluted with distilled water to 10 mL/L, added 98% sulfuric acid at 1.5 mL/L) for 2 minutes at 70°C, and then washed with distilled water for 1 minute. . Next, OPC Inducer 50AM (manufactured by Okuno Pharmaceutical Co., Ltd., diluted to 100 mL/L with distilled water) and OPC Inducer 50CM (manufactured by Okuno Pharmaceutical Co., Ltd., diluted with distilled water to 100 mL/L) were mixed in a 1:1 ratio. After being immersed in the solution at 45° C. for 5 minutes, it was washed with distilled water for 1 minute. Subsequently, it was immersed in OPC-150 Crysta MU (manufactured by Okuno Pharmaceutical Co., Ltd., diluted with distilled water to 150 mL/L) for 5 minutes at room temperature, and then washed with distilled water for 1 minute. Subsequently, it was immersed in OPC-BSM (manufactured by Okuno Pharmaceutical Co., Ltd., diluted with distilled water to 125 mL/L) at room temperature for 5 minutes. Next, a solution containing chemical copper 500A (manufactured by Okuno Pharmaceutical Industries, diluted to 250 mL/L with distilled water) and chemical copper 500B (manufactured by Okuno Pharmaceutical Industries, diluted with distilled water to 250 mL/L) at a ratio of 1:1. After soaking in water for 10 minutes at room temperature, the sample was washed with distilled water for 5 minutes. Thereafter, vacuum drying was performed at 90° C. for 12 hours to remove moisture.

[X-ray conditions]
X-ray target: Cu
X-ray tube voltage: 40kV
X-ray tube current: 30mA
[Shooting conditions]
Number of projections: 1500 images Rotation angle: 180°
Exposure time: 20 seconds/sheet Spatial resolution: 0.54μm/pixel

[X-ray CT analysis conditions]
The obtained three-dimensional image was processed using a median filter for 1 adjacent pixel, and was binarized using Otsu's algorithm.
Next, using Centerline editor (Ver. 7) of JSOL's simpleware software with default settings, lines of 2.16 μm or less were removed as noise, and then the coupling within the measurement field of 300 μm x 300 μm x 300 μm was performed. The number N _n of parts and the number N _l of linear parts were detected.

The above structural analysis revealed the number of joints included in the test piece N _n , the number of linear parts N _l , the density of joints (pieces/mm ³ ), and the orientation in three mutually orthogonal directions (x, y, z). The average value of the angle was determined. Note that the orientation angles in Table 1 are values obtained by determining the minimum value of θc with the compressive load application direction set as the x direction, and the y direction and the z direction set in the direction perpendicular to the compressive load application direction.

<Redox flow battery evaluation>
For redox flow battery evaluation, a cell consisting of a Viton rubber gasket, a Teflon (registered trademark) channel frame, a graphite separator, and a stainless steel end plate was used. A comb-shaped flow path was provided in a 33×30 mm portion of the graphite separator facing the electrode in parallel to the short side. The rib width, groove width, and groove depth of the channel were 1 mm, 1 mm, and 1.5 mm, respectively. Nafion 212 purchased from Aldrich was used as the electrolyte membrane. The thickness of the gasket was adjusted so that the compression ratio of the electrode was 80%.
The membrane cut into a size of 50 x 80 mm, the two sheets of carbon foam cut out into a size of 33 x 30 mm, and the cell constituent members were assembled in a predetermined order and fastened with stainless steel bolts at a predetermined torque. The assembled cell was connected to an electrolyte circulation system consisting of an electrolyte tank and a liquid pump. 30 ml of a vanadium sulfuric acid solution having a vanadium ion concentration of 1.5 M, a vanadium ion valence of 3.5, and a sulfate ion concentration of 4.5 M was added to the electrolyte tank, and the solution was circulated at a flow rate of 100 ml/min. The charge/discharge test was carried out by a constant current method using a potentiostat VSP manufactured by BioLogic. The voltage range was 1.00-1.55V, and the current density was 80mA/cm ² . Cell resistance was determined from the average voltages Vc and Vd during charging and discharging using the following formula.
Cell resistance (Ωcm ² )=(Vc-Vd)/(2×0.08)
Further, current efficiency was determined from the amount of discharge relative to the amount of charge at a current density of 80 mA/cm ² .

<Taber bending stiffness test>
The mechanical strength of carbon foam was evaluated by Taber bending stiffness. The test method was conducted in accordance with JIS P 8125. A carbon foam cut into a size of 70 mm x 38.1 mm was fixed to a Taber stiffness tester manufactured by Kumagai Riki Kogyo so that the load length was 50 mm. The bending moment when the test piece was bent left and right by 15 degrees was measured, and the average value thereof was taken as the Taber bending stiffness (gf·cm).

<Pressure loss>
The pressure drop at the supply port and discharge port when the electrolyte was flowing through the cell was measured using a manometer. A U-shaped glass tube was connected to a branch port provided at each of the supply port and the discharge port so that the straight pipe portion was oriented vertically to form a manometer. The pressure drop was determined from the difference in height of the electrolyte column that occurs when the electrolyte is flowing and the density of the electrolyte. Note that the electrolytic solution flow rate was 100 ml/min.

(Example 1)
Melamine foam (BASF Co. Ltd, BASOTECT G) cut into a size of 300 x 300 x 20 mm was pressed to a thickness of 0.6 mm with a metal spacer sandwiched between them using a vacuum press machine (KVHC-II) manufactured by Kitagawa Seiki Co., Ltd. In this press, the temperature was raised to 360° C. at a temperature increase rate of 5° C./min while reducing the pressure in a vacuum, and then pressed at a set pressure of 2.0 MPa, held for 10 minutes, and then cooled.
After taking out the sample, it was immersed in an aqueous solution of methylolmelamine (Nippon Carbide Co., Ltd., Nikaresin) with a concentration of 10 wt %, and then the excess solution contained inside was removed by centrifugal dehydration. Subsequently, the resin-modified melamine foam was dried under reduced pressure at 100° C. for 3 hours to obtain a coated melamine foam.
One graphite plate with dimensions of 300 x 300 x 4 mm was placed on the coated melamine foam, placed in a heat treatment furnace, and the inside of the furnace was depressurized and evacuated using a vacuum pump to reduce the degree of vacuum in the furnace to less than 1 Pa. While supplying nitrogen gas into the furnace at a flow rate of 2 L/min, the temperature inside the furnace was raised to 800°C at a heating rate of 5°C/min. When the temperature inside the furnace reached 800°C, the supply of nitrogen gas was stopped, and the temperature was raised to a heat treatment temperature of 1500°C at a heating rate of 5°C/min, and the coated melamine foam was heated for 1 hour. Carbonized.
Subsequently, the carbonized sample was heat-treated at 500° C. for 1 hour under a flow of dry air at 1 L/min to obtain a carbon foam with an oxidized surface. Details of the obtained carbon foam are shown in Table 1.
FIG. 5 shows a SEM image of the surface of the carbon foam according to Example 1, and FIG. 6 shows an enlarged SEM image of FIG.

(Example 2)
When carbonizing the coated melamine foam, the carbonization temperature was 1100°C and the surface oxidation treatment temperature was 300°C. Other than that, it was carried out under the same conditions as in Example 1.
Details of the obtained carbon foam are shown in Table 1.

(Example 3)
The thickness of the melamine foam used was 40 mm, and the thickness when pressed with a vacuum press was 1.0 mm. Furthermore, the carbonization temperature when carbonizing the coated melamine foam was set at 2000°C, and the surface oxidation treatment temperature was set at 500°C. Other than that, it was carried out under the same conditions as in Example 1. Details of the obtained carbon foam are shown in Table 1.

(Example 4)
A solution for impregnating melamine foam was prepared under the same conditions as in Example 1, except that polyvinyl chloride latex (manufactured by Asahi Kasei Corporation, solid content: 45% by mass) was used as the resin.

(Example 5)
When preparing a solution for impregnating melamine foam, the same conditions as in Example 1 were used except that the resin solution concentration was 15 wt%.

(Example 6)
When pressing the melamine foam, it was carried out under the same conditions as in Example 1 except that the thickness after pressing was 1 mm.

(Example 7)
Melamine foam pressed according to Example 1 was carbonized at 1500°C to produce carbon foam.
This carbon foam was impregnated with Saran latex (manufactured by Asahi Kasei, stock solution solids content: 45% by mass) prepared to have a solid content of 10% by mass. Excess solution contained inside was removed by centrifugal dehydration. Subsequently, carbon foam coated with Saran resin was obtained by drying under reduced pressure at 100° C. for 3 hours.
The coated carbon foam was carbonized again in the same manner as in Example 1, and then air oxidized.

(Example 8)
When preparing a solution for impregnating melamine foam, the same conditions as in Example 1 were used except that the resin solution concentration was 25 wt%.

(Comparative example 1)
A melamine foam cut into a size of 300 x 300 x 2 mm was placed in a heat treatment furnace, and the inside of the furnace was depressurized and evacuated using a vacuum pump to reduce the degree of vacuum in the furnace to less than 1 Pa. Nitrogen gas was introduced into the furnace while depressurizing and evacuating. While supplying at a flow rate of 2 L/min, the temperature inside the furnace was raised to 800° C. at a heating rate of 5° C./min. When the temperature in the furnace reached 800°C, the supply of nitrogen gas was stopped, and the temperature was raised to a heat treatment temperature of 1500°C at a heating rate of 5°C/min, and carbonized by holding for 1 hour.
FIG. 2 shows a SEM image of the carbon foam surface according to Comparative Example 1.

(Comparative example 2)
When preparing a solution for impregnating melamine foam, the same conditions as in Example 1 were used except that the resin solution concentration was 75 wt%.

(Comparative example 3)
When preparing a solution for impregnating melamine foam, the same conditions as in Example 1 were used except that the resin solution concentration was 2 wt%.

(Comparative example 4)
Redox flow battery evaluation was performed using carbon fiber paper (SIGRACET GFL39AA manufactured by SGL CARBON) as an electrode.

Claims (5)

It has a linear part and a connecting part that connects the linear part,
The linear portion has an average fiber diameter of 0.1 to 5.0 μm,
Taber bending stiffness measured in accordance with JIS P8125 is 16gf・cm or more and 38gf・cm or less,
The thickness is 0.1 mm or more and 1.0 mm or less,
The porosity is 85% or more and 97% or less,
The carbon content is 90% by mass or more,
The density of the bonding portion is 15,000 pieces/mm ³ or more,
A carbon foam for fuel cells, redox flow batteries or water electrolysis, characterized by: At least in part, the thickness direction of the carbon foam is the x direction, the direction perpendicular to the x direction is the y direction, and the direction perpendicular to the x direction and the y direction is the z direction;
The average value of the orientation angle with respect to the x direction of the linear portion included in a region of 300 μm x 300 μm x 300 μm is θavex,
The average value of the orientation angle with respect to the y direction is θavey,
The average value of the orientation angle with respect to the z direction is θavez,
When defined as
The carbon foam according to claim 1, wherein a difference θc between a maximum value and a minimum value among the θavex, θavey, and θavez is 3° or more. The carbon foam according to claim 1 or 2, wherein the ratio of the number of the linear parts to the number of the bonding parts is 1.3 or more and 1.6 or less. The carbon foam according to any one of claims 1 to 3, wherein the density of the bonded portions is 30,000 pieces/mm ³ or more. The carbon foam according to any one of claims 1 to 4 , wherein the linear portion has an average fiber diameter of 0.5 to 4.0 μm.

Images