JPH09157052A - Porous carbon sheet and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a porous carbon sheet having increased gas permeability in the thickness direction while keeping the mechanical strength relatively high and the specific resistance in the thickness direction relatively high. SOLUTION: Short carbon fibers dispersed in random directions in a practically two-dimensional plane are bonded to one another with carbon to obtain the objective porous carbon sheet having >=14.7MPa bending strength, >=0.49MPa compressive strength in the thickness direction, <=11.8MPa modulus of compressive elasticity in the thickness direction, <=0.01Ωm specific resistance in the thickness direction and >=3,000ml.mm/cm<2> /hr/mmAq gas permeability in the thickness direction in air.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a porous carbon plate suitable for constituting an electrode of, for example, a fuel cell, particularly a polymer electrolyte fuel cell, and a method for producing the same.

[0002]

2. Description of the Related Art Porous carbon plates used as electrode base materials for polymer electrolyte fuel cells have high conductivity, high mechanical strength, high gas permeability, and high corrosion resistance. Properties similar to those of the electrode base material for phosphoric acid fuel cells are required.

Among them, in the polymer electrolyte fuel cell, since the current density of the electrode is as high as 4 to 20 times that of the phosphoric acid type, hydrogen,
Since the amount of oxygen supplied and the amount of water produced by the reaction increased, and the operating temperature is low at around 100 ° C, water may block the gas supply channel and narrow the gas supply channel. In particular, high gas permeability is required.

Higher conductivity is also desirable, but resistance is increased by subjecting the porous carbon plate to a water repellent treatment with a fluorocarbon resin, and the contact resistance between the porous carbon plate and other materials is large. Therefore, it is desired to increase gas permeability even if the resistance of the porous plate increases to some extent.

It is desirable that the bending strength is also high, but since the electrode area is generally smaller than that of the phosphoric acid fuel cell, it is easy to handle, and some decrease in strength is allowed.

[0006] For the electrode base material of such a polymer electrolyte fuel cell, the material conventionally made for the phosphoric acid fuel cell was used as it is. For example, Japanese Patent Publication 53-18603
As described in the publication, a mixture of short carbon fibers and a paper-making medium containing an organic binder such as polyvinyl alcohol is made into a sheet to obtain a sheet-like intermediate substrate, and the intermediate substrate is heated to carbonize. A resin, for example, a self-curing type phenol resin is impregnated, and the intermediate substrate impregnated with the phenol resin is heated to carbonize the phenol resin, and carbon short fibers are bound to each other by a carbide of the phenol resin. Was used. However, the substrate produced by such a method is not sufficient in terms of conductivity and bending strength.

In order to solve this problem, Japanese Unexamined Patent Publication No.
JP-A-160867 discloses a method of using a mixed resin of a resol type phenol resin and a novolac type phenol resin instead of the self-curing type phenol resin impregnated in the intermediate substrate. However, the conditions shown therein do not give sufficient gas permeability for an electrode substrate of a polymer electrolyte fuel cell.

Further, Japanese Unexamined Patent Publication No. 7-105957 discloses an electrode using a woven fabric made of carbon fiber, paper or felt. In this case, there are problems that the gas permeability is sufficient but the specific resistance is high, the smoothness and the handling property are poor.

[0009]

SUMMARY OF THE INVENTION The object of the present invention is to solve the above-mentioned problems of the prior art and to maintain the gas permeability in the thickness direction while maintaining the mechanical strength and the specific resistance in the thickness direction relatively high. (EN) Provided are a porous carbon plate having a high height and a method for producing the same.

[0010]

In order to achieve the above object, the present invention has the following constitution.

(1) A porous carbon plate in which short carbon fibers dispersed in a random direction in a substantially two-dimensional plane are bound to each other by carbon and having a bending strength of 14.7 MPa or more. , Compressive strength in the thickness direction is 0.49
MPa or more, the compression modulus in the thickness direction is 11.8 MPa or less, the specific resistance in the thickness direction is 0.01 Ωm or less, and the gas permeability in the thickness direction by air is 3000 ml · mm / cm.
² / hr / mmAq or more, a porous carbon plate.

(2) The porous carbon plate according to the above (1), characterized in that the surface coverage of the binding carbon defined by the following formula and the carbon fiber in which the carbon is bound is 20% or less.

However, R _SC = S _SC / S _Total R _SC : Surface layer coverage S _SC : Boundary covering the surface layer Area of the part where carbon and carbon fiber are bound S _Total : Covered part Total area including uncovered portion (3) An electrode base material for a fuel cell comprising the porous carbon plate according to (1) or (2) above.

(4) An electrode base material for a polymer electrolyte fuel cell, comprising the porous carbon plate according to claim 1 or 2.

(5) Resole-type phenol resin R and novolac-type phenol resin N are added to short carbon fiber aggregates dispersed in a substantially two-dimensional random direction, R: N = 2: 1.
A step of impregnating a resin mixed at a ratio of 1 to 3 with 100 parts by weight of carbon fiber so that a phenol resin is 40 to 75 parts by weight to obtain an intermediate base material; A step of heating the intermediate base material to carbonize the mixed resin, and a method for producing a porous carbon plate.

(6) An intermediate substrate is obtained by impregnating a short carbon fiber aggregate dispersed in a substantially two-dimensional random direction with 70 to 220 parts by weight of melamine resin per 100 parts by weight of carbon fiber. A method for producing a porous carbon plate, comprising: a step of obtaining and a step of heating the intermediate base material impregnated with a melamine resin to carbonize the melamine resin.

[0017]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in more detail.

Short carbon fibers are polyacrylonitrile (P
AN) -based carbon fibers, pitch-based carbon fibers, rayon-based carbon fibers and the like can be used, but PAN-based carbon fibers or pitch-based carbon fibers are used to increase the bending strength of the base material. Is preferable, and it is more preferable to use PAN-based carbon fiber.

The fiber diameter of the carbon fiber is preferably about 4 to 20 μm, more preferably 4 to 13 μm, and especially 4 to 20 μm.
The thickness of 10 μm is more preferable in order to increase the strength of the base material. In the case of a carbon fiber having a flat cross section, the average of the major axis and the minor axis is the fiber diameter. The length of the short fibers is preferably about 3 to 20 mm, and more preferably about 5 to 15 mm in order to facilitate the production and increase the strength of the base material.

The present invention is a porous carbon plate in which carbon short fibers dispersed in random directions in a substantially two-dimensional plane are bonded to each other by carbon.

As the method for dispersing the short carbon fibers in a substantially two-dimensional random direction, a wet method in which the short carbon fibers are dispersed in a liquid medium to produce paper, or the short carbon fibers are dispersed in the air A dry method, etc., for accumulating can be applied.

The carbon for binding the short carbon fibers to each other can be obtained, for example, by heating a resin to carbonize it. The resin used is one that is carbonized by heating, such as phenol resin, epoxy resin, furan resin, melamine resin, pitch and the like.

As a method for producing a composite of short carbon fibers and a resin, there is a method of mixing and impregnating a short carbon fiber aggregate with a resin, or a method of making a paper together with a short carbon fiber and a resin.
It is preferable to impregnate the resin in a liquid state or to use a resin that melts in a later step in order to increase the strength of the substrate and reduce the specific resistance.

It is also a frequently used method to dissolve the resin in a solvent during the impregnation and remove the solvent after the impregnation. The mixture of the short carbon fibers and the resin is preferably molded by heating and pressurization before carbonization by heating.

By the molding, the improvement of the adhesiveness between the short carbon fibers and the resin and the improvement of the surface smoothness are achieved.

Although the temperature at the time of molding depends on the resin, the pressure is preferably about 0.0098 to 1.96 MPa,
It is more preferable that the pressure is 098 to 0.98 MPa. The substrate density can be controlled by the pressure during molding, but if the pressure is too low, the adhesion will deteriorate, and if the pressure is too high, an excessive flow will occur, and the material may collapse.

The temperature of carbonization by heating the mixture of short carbon fibers and resin is preferably 1300 ° C. or higher in order to increase bending strength, reduce specific resistance, reduce impurities and enhance corrosion resistance. And more preferably 2000 ° C. or higher.

According to the present invention, the above-mentioned porous carbon plate has a bending strength of 14.7 MPa or more and a compressive strength in the thickness direction of 0.4.
9 MPa or more, the compression elastic modulus in the thickness direction is 11.8 MPa
Below, the specific resistance in the thickness direction is 0.01 Ωm or less, and the gas permeability in the thickness direction by air is 3000 ml · mm / c.
It is characterized by having m ² / hr / mmAq or more.

Hereinafter, these characteristic values will be described.

First, the bending strength of the porous carbon plate of the present invention is 14.7 MPa or more, preferably 19.6 MPa.
Or more, More preferably, it is 24.5 MPa or more.

When the bending strength is less than 14.7 MPa,
It is not preferable because the specific resistance becomes high and the handleability is poor.

The bending strength is measured by a three-point bending test according to JIS K6911.

However, the width (W) of the test piece is 13 mm and the length (L) is 60 mm or more. The relationship between the distance between fulcrums (Lv) and the thickness (h) of the test piece is as follows.

Lv / h is Lv when the thickness is about 0.1 mm.
/ H is about 200, and Lv / h is reduced as the thickness increases. When the thickness is about 0.5 mm, Lv / h is about 100. Further, R of the fulcrum and the pressure wedge is 3 mm, and the load speed is 2 mm / min.

Next, the compressive strength in the thickness direction may be such that it can withstand the applied pressure when the electrode base material is processed into an electrode and when it is used as a fuel cell, and is 0.49 MPa or more, preferably. 0.98 MPa or more, more preferably 1.4
It is 7 MPa or more.

Further, the compressive elastic modulus in the thickness direction is low in order to reduce the contact resistance by absorbing the unevenness of the surface of the separator and the unevenness of the porous substrate itself which are stacked when the fuel cells are stacked and assembled. It is better, but if it is too low, the amount of deformation when pressure is applied becomes large and it becomes difficult to process, so it is set to 11.8 MPa or less, preferably 1.96 to 1
1.8 MPa, more preferably 3.92 to 9.8 MP
a.

With respect to the compressive strength and compressive elastic modulus in the thickness direction, as the load is increased, a displacement-load curve as shown in FIG. 1 is obtained. The point at which this changes linearly on the way and starts from the straight line is the breaking point. The thickness of the substrate is h, the area of the substrate is A,
The load at the breaking point is P, and the displacement up to the breaking point is Δh, which is calculated by the following equation.

Compressive strength = P / A Compressive elastic modulus = H / A × P / Δh When it is difficult to determine the breaking point, it is advisable to stack several substrates and measure. A commercially available micrometer is used to measure the thickness of the base material before the test, and when several sheets are stacked and measured, the sum of the thicknesses measured one by one is taken as the base material thickness.

Further, the specific resistance in the thickness direction is 0.01 Ωm.
It is preferably 0.005 Ωm or less.

The specific resistance in the thickness direction is calculated by the following equation from the voltage drop when a base material is sandwiched between mercury electrodes having a constant area and a constant current is applied between the electrodes.

Specific resistance = (voltage drop × electrode area) / (current × base material thickness) Gas permeability in the thickness direction by air is 3000 ml.mm/c
m ² / hr / mmAq or more, preferably 3600 ml ・ mm / cm ² / h
r / mmAq or more.

The gas permeability in the thickness direction by air is measured by the following method.

That is, 14 ml / cm ² / sec of air is applied to the base material.
(= 50400 ml / cm ² / hr) The pressure loss upon permeation is measured and calculated by the following formula.

Gas Permeability = (50400 × Substrate Thickness) / (Pressure Loss) Next, the surface coverage ratio of the binding carbon of the porous carbon plate and the carbon fiber to which the carbon is bound in the present invention is the gas permeability. From the relationship with sex, it is preferably 20% or less, more preferably 13% or less.

In order to obtain the surface layer coverage (R _SC ), the base material is magnified 200 times or more with an electron microscope or the like, and the binding carbon covering the surface layer is bound to the carbon of the carbon fibers. The area of the part, that is, the area of the covered part (S _SC ) is obtained. For example, in the case of FIG. 2, the shaded area in FIG. 3 is the covered portion.

Surface coverage (R _SC ) = S _SC / S _Total

However, S _Total is the total area of the covered and uncovered areas.

To eliminate the influence of variations due to location, S
_Total should be at least 0.5 mm ² or more. If it is difficult to determine the surface layer, divide the mesh into 0.1 mm × 0.1 mm meshes, and select the carbon bound to the 6th carbon fiber from the surface layer and the continuous carbon in each mesh.

Next, the method for producing the porous carbon plate of the present invention will be described.

In the method for producing a porous carbon plate of the present invention, the resole-type phenol resin R and the novolac-type phenol resin N are added to the short carbon fiber aggregates dispersed in a substantially two-dimensional random direction R: N = 2 :. Phenolic resin was added to 100 parts by weight of carbon fiber in a ratio of 1 to 1: 3.
0 to 75 parts by weight to obtain an intermediate base material by impregnation, and heating the intermediate base material impregnated with the mixed resin to carbonize the mixed resin. To do.

An organic binder may be attached to the short carbon fiber aggregate for the purpose of handling and maintaining the shape during resin impregnation. As the organic binder, polyvinyl alcohol, cellulose, polyester, epoxy resin, phenol resin, acrylic resin or the like can be used.
Further, the amount of adhesion may be about 1 to 30% by weight.

As for the mixing ratio of the resol type phenol resin R and the novolac type phenol resin N, if the amount of R is too large, the bending strength becomes low, the specific resistance in the thickness direction becomes high, and if the amount of N becomes too large. In the subsequent heating step, the mixed resin does not become sufficiently hard and becomes difficult to handle, and the carbon content remaining during the carbonization of the resin becomes small. Therefore, R: N = 2: 1 to 1: 3, and preferably R: N = 2: 1 to 1: 3. , R: N
= 3: 2 to 1: 2. Particularly when the amount of resin is smaller than that of short carbon fibers as in the present invention, it is important to suppress the amount of R for bending strength and specific resistance.

If the amount of the impregnated phenolic resin is too large, the density of the base material becomes high and the gas permeability becomes low, so that it becomes unsuitable as an electrode base material of a polymer electrolyte fuel cell. On the other hand, if the amount is too small, the density of the base material becomes too low, the bending strength, the compression strength and the elastic modulus are low, and the specific resistance becomes high. Therefore, the impregnation amount of the resin with respect to 100 parts by weight of the carbon fiber is 40 to 75. The amount is, by weight, preferably 55 to 75 parts by weight.

Another method for producing the porous carbon plate of the present invention is
The melamine resin was added to 100 parts by weight of the carbon fibers in the carbon short fiber aggregate dispersed in the substantially two-dimensional random direction.
It is characterized by including a step of obtaining an intermediate base material by impregnation so as to be 0 to 220 parts by weight, and a step of heating the intermediate base material impregnated with the melamine resin to carbonize the melamine resin.

The amount of the melamine resin impregnated in the above step is 70 to 220 parts by weight, preferably 100 to 200 parts by weight, more preferably 130 to 180 parts by weight, based on 100 parts by weight of the carbon fiber. Is preferable in terms of balance between gas permeability and specific resistance.

Further, a resin having a residual carbon rate of 30% or more, for example, a phenol resin, a furan resin, pitch or the like may be mixed with the melamine resin. However, the ratio of the melamine resin in the mixed resin is 60% or more, preferably 80% or more.
Further, the impregnated amount of the melamine mixed resin is the same as in the case of the above melamine alone.

[0057]

【Example】

Example 1 Toray Industries, Inc. polyacrylonitrile-based carbon fiber “Torayca” T300 (average short fiber diameter: 7 μm, single fiber number: 60)
(00 pieces) is cut into a length of 12 mm and well defibrated, then dispersed in water so that the content becomes 0.04% by weight, paper-making is performed on the wire mesh, and then 10 pieces of polyvinyl alcohol are added.
The sheet-shaped intermediate base material was obtained by immersing in a weight% aqueous solution, pulling it up and drying it, and about 30 weight% of polyvinyl alcohol as a binder adhered to 100 weight parts of carbon short fibers.

Next, the above-mentioned intermediate base material was immersed in a 6 wt% methanol solution of a mixed resin containing the same parts by weight of the novolac type phenol resin with respect to 100 parts by weight of the resol type phenol resin, and the carbon short fiber 100 was pulled up. Approximately 69 parts by weight of the mixed resin is attached to the parts by weight, and further at 90 ° C.
After heating and drying for 2 minutes, two sheets were stacked and a pressure of 0.69 MPa was applied for 15 minutes at a temperature of 145 ° C. to cure the resol-type phenol resin.

Next, the intermediate base material in which the mixed resin is hard is
The mixed resin was carbonized by heating at 2400 ° C. for 30 minutes in a nitrogen gas atmosphere to obtain a conductive base material. Examples 2 and 3 A conductive base material was obtained in the same manner as in Example 1 except that the mixed resin concentration was changed to 49,74 parts by weight of the mixed resin adhesion amount. Example 4 A conductive base material was obtained in the same manner as in Example 3 except that four sheet-shaped intermediate base materials impregnated with the mixed resin were stacked and the resin was cured by heating and pressing. Comparative Example 1 A conductive base material was obtained in the same manner as in Example 1 except that the concentration of the mixed resin was increased to 124 parts by weight of the mixed resin. Comparative Example 2 A conductive group was prepared in the same manner as in Example 1 except that four sheet-shaped intermediate base materials having a mixed resin concentration increased and a mixed resin adhesion amount of 150 parts by weight were stacked and the resin was cured by heating and pressing. I got the material. Example 5 Instead of the methanol solution of the mixed resin, 1 of the melamine resin was used.
A conductive base material was obtained in the same manner as in Example 1 except that a mixed solution of 0% by weight of water and methanol was used, the amount of resin adhered was 180 parts by weight, and drying was performed at room temperature for 24 hours. Comparative Example 3 The melamine resin concentration was increased to increase the melamine resin adhesion amount to 240.
A conductive base material was obtained in the same manner as in Example 5 except that the parts by weight were used.

The physical properties of the above base materials are summarized in Table 1. ★

[0061]

[Table 1] * As can be seen from Table 1, the porous carbon plate of the present invention slightly lowers bending strength and specific resistance as compared with the porous carbon plate of the comparative example, but realizes a great improvement in gas permeability.

[0062]

The porous carbon plate of the present invention has a bending strength of about 1
It is a base material with greatly improved gas permeability while maintaining a practical level of 4.7 MPa or more and a specific resistance in the thickness direction of 0.01 Ωm or less. It has a high current density and can supply and exhaust a large amount of reaction gas. It has a very excellent effect as a necessary electrode for a polymer electrolyte fuel cell.

[Brief description of the drawings]

FIG. 1 Displacement-load curve during compression test in the thickness direction

FIG. 2 is an electron micrograph showing an example of a porous carbon plate according to the present invention.

FIG. 3 is an electron micrograph showing a covered portion (hatched portion) for obtaining the surface layer coverage (R _SC ) in FIG.

Continuation of front page (51) Int.Cl. ⁶ Identification number Office reference number FI technical display location H01M 4/96 C04B 35/80 K

Claims (6)

[Claims] 1. A porous carbon plate obtained by binding carbon short fibers dispersed in random directions in a substantially two-dimensional plane to each other with a bending strength of 1
More than 4.7MPa, compressive strength in the thickness direction is 0.49MP
a or more and a compression elastic modulus in the thickness direction of 11.8 MPa or less,
The specific resistance in the thickness direction is 0.01Ωm or less, and the gas permeability in the thickness direction by air is 3000 ml · mm / cm ² / h.
Porous carbon plate characterized by r / mmAq or more. 2. The porous carbon plate according to claim 1, wherein the surface coverage of the binding carbon defined by the following formula and the carbon fiber in which the carbon is bound is 20% or less. However, R _SC = S _SC / S _Total R _SC : Surface layer coverage S _SC : Binder covering the surface layer Area of the part where carbon and carbon fiber are bound S _Total : Covered part and covered Total area including unfilled parts 3. An electrode base material for a fuel cell, comprising the porous carbon plate according to claim 1 or 2. 4. An electrode base material for a polymer electrolyte fuel cell, comprising the porous carbon plate according to claim 1 or 2. 5. A resol type phenol resin R and a novolac type phenol resin N are mixed with a carbon short fiber aggregate dispersed in a substantially two-dimensional random direction in a ratio of R: N = 2: 1 to 1: 3. A step of impregnating the resin so that the phenol resin is 40 to 75 parts by weight with respect to 100 parts by weight of carbon fiber to obtain an intermediate base material; and heating the intermediate base material impregnated with the mixed resin to perform the mixing. And a step of carbonizing the resin, the method for producing a porous carbon plate. 6. An intermediate base material is obtained by impregnating short carbon fiber aggregates dispersed in a substantially two-dimensional random direction with 70 to 220 parts by weight of melamine resin per 100 parts by weight of carbon fibers. A method for producing a porous carbon plate, comprising: a step of heating the intermediate base material impregnated with the melamine resin to carbonize the melamine resin.