JP5162817B2 - Network carbon material - Google Patents

Description

  The present invention relates to a network-like carbon material having a high specific surface area and thus exhibiting properties such as high substance adsorbability, substance permeability, electronic conductivity and / or chemical stability. Specifically, the present invention relates to a sheet-like carbon material in which carbon fibrils that are suitably used for an electrode material such as an electric double layer capacitor and a fuel cell are intertwined.

  Porous carbon materials are industrially used for electrodes and peripheral members of various energy devices, high-temperature filters, heat insulating materials, etc., but most of them are formed into a sheet by binding carbon powder or carbon fiber with a binder. Molded and used. Moreover, it is often used after carbonization after being bound with a binder. However, this method makes it difficult to precisely control the porous structure, and the degree of freedom in designing the carbon material is significantly hindered. As means for solving this problem, JP 2000-335909 A (Patent Document 1) describes that a porous carbonized film having fine continuous pores can be obtained by carbonizing a polyimide porous film. Yes.

On the other hand, it is known that woody cellulosic materials can be carbon precursors, but the structure has a rough fibril diameter on the order of several μm, so the specific surface area important for use as a functional material is several. In general, it is very small as m ² / g.

  As a cellulose-based fine fibril, Japanese Patent Application Laid-Open No. 2004-204380 (Patent Document 2) describes a method of obtaining a sheet-like material having a large specific surface area by drying bacterial cellulose produced by a microorganism. Yes. Bacterial cellulose, for example, produced by acetic acid bacteria, is known as a food material “Natadecoko”, has nano-level fibrillar properties, and in particular, cellulose in fibrils is highly oriented and crystallized.

However, production of a carbon material having a large specific surface area from bacterial cellulose has not been successful until now.
JP 2000-335909 A JP 2004-204380 A

  An object of the present invention is to provide a network-like carbon material having a large specific surface area and a sheet-like material thereof.

  1. Drying the gelatinous bacterial cellulose produced by the bacteria while maintaining its network structure;
  Contacting the dried bacterial cellulose with halogen vapor;
  Carbonizing bacterial cellulose contacted with halogen vapor at a temperature of 700 ° C. to 3200 ° C. in an inert gas atmosphere;
A method for producing a network-like carbon material, comprising:
  2. Drying the gelatinous bacterial cellulose produced by the bacteria while maintaining its network structure;
  Steaming and baking dried bacterial cellulose under pressure and sealing;
  Carbonizing the steamed bacterial cellulose at a temperature of 700 ° C. to 3200 ° C. in an inert gas atmosphere;
A method for producing a network-like carbon material, comprising:
  3. In the drying step, the drying is performed such that the apparent shrinkage remaining rate from the gel state before drying is 30% or more in the width and length directions and 10% or more in the thickness direction. The manufacturing method as described.
  4). 4. The method according to 3 above, wherein the drying is freeze-drying.
  5). The said carbonization temperature is 1500 degreeC-3200 degreeC, The manufacturing method in any one of said 1-4 characterized by the above-mentioned.
  6). 6. The production method according to any one of 1 to 5 above, comprising a step of compression-molding dried bacterial cellulose to form a sheet.
Further, the following matters of the present application are also disclosed.

1. A network-like carbon material produced without an activation treatment, in which carbon fibrils are entangled with each other to form a three-dimensional network structure, and a specific surface area is 30 m ² / g or more.

2. The carbon fibril has a branch,
2. The network-like carbon material as described in 1 above, wherein 50% by volume or more and 99.99% by volume or less of the constituent carbon fibrils are fibrils having a diameter of 1 nm to 300 nm.

  3. 3. The network-like carbon material as described in 1 or 2 above, which is produced from bacterial cellulose.

  4). 4. The network-like carbon material as described in any one of 1 to 3 above, wherein the graphitization rate is 30% or more.

5. Drying the gelatinous bacterial cellulose produced by the bacteria while maintaining its network structure;
And a step of carbonizing the dried bacterial cellulose at a temperature of 700 ° C. to 3200 ° C. in an inert gas atmosphere.

  6). 6. The production according to 5 above, wherein, in the drying step, drying is performed so that an apparent shrinkage remaining rate from a gel state before drying is 30% or more in the width and length directions and 10% or more in the thickness direction. Method.

  7). 7. The production method according to 6 above, wherein the drying is freeze-drying.

  8). The production method according to any one of 5 to 7 above, which comprises a step of bringing dried bacterial cellulose into contact with halogen vapor before the carbonization step.

  9. 8. The method according to any one of 5 to 7 above, further comprising a step of steaming and baking dried bacterial cellulose under pressure and sealing before the carbonization step.

  10. The said carbonization temperature is 1500 degreeC-3200 degreeC, The manufacturing method in any one of said 5-9 characterized by the above-mentioned.

  11. 11. The production method according to any one of 5 to 10 above, comprising a step of compression-molding dried bacterial cellulose to form a sheet.

  12 5. The network-like carbon material according to any one of 1 to 4, which is produced by the production method according to 11 above.

ADVANTAGE OF THE INVENTION According to this invention, it can manufacture without an activation process and can provide the network-like porous carbon material of 30 m < ² > / g or more with a very large specific surface area. In one embodiment of the present invention, it is possible to provide a network-like carbon material that is a fine fibril having a very small diameter and that is less likely to fall off and has excellent durability. In particular, since it has pores (mesh gaps) on the order of nm between networks, it can be used as various highly functional materials.

  Moreover, according to the manufacturing method of this invention, said network-like carbon material can be manufactured with a sufficient yield, maintaining the big surface area from bacterial cellulose as a raw material. Moreover, the graphitization rate can be changed according to a use by adjusting carbonization conditions, for example, a graphitization rate can be raised to 30% or more.

  In the present invention, the network shape means a state in which fibrils are entangled three-dimensionally, and it can be said that a network material formed into a sheet shape corresponds to a kind of nonwoven fabric.

In the present invention, carbon fibrils are intertwined with each other to form a three-dimensional network (three-dimensional network structure), and the specific surface area per weight is 30 m ² / g or more. In the conventional non-woven fabric of carbon fiber and the like, such a specific surface area could not be obtained because the fibril diameter was as thick as 10 to several μm. Conventional carbon materials have been subjected to a so-called activation treatment in which the surface is made porous by treatment with alkali or the like, and those having a specific surface area of 30 m ² / g or more are known, but the fiber diameter is basically Since it does not change, the property is completely different from the fine porous carbon material obtained from ultrafine fibrils of 1 μm or less as in the present invention.

The present invention is a carbon fibril obtained without an activation treatment, and has a specific surface area of 30 m ² / g or more. Such a configuration is completely new and unprecedented. More preferably, the specific surface area is 50 m ² / g or more. Moreover, since a thing with a very large specific surface area becomes difficult to manufacture gradually, it is generally 3000 m < ² > / g or less.

  The carbon fibril of the present invention preferably has a branch (branch). For example, a normal carbon fiber is a single straight fiber, but in a carbon fibril having a branch, one fibril is divided into two or more (many two) in the middle, and in many cases Further, it is further divided into a plurality (two are many). It can be said that three fibrils are bonded at a branch point divided into two. By having such a branch, the fibrils are further entangled with each other, and even when the carbon material of the present invention is repeatedly used, the fibrils are prevented from falling off, so that the durability is improved. In addition, it is advantageous from the viewpoint of electronic conductivity, thermal conductivity, and mechanical characteristics. Such characteristics are particularly suitable as an electrode for a high output capacitor having a wide use potential width.

  In the network-like carbon material, preferably, 50% by volume or more and 99.99% by volume or less of the constituent carbon fibrils have a diameter of 1 nm to 300 nm, more preferably 1 nm to 150 nm.

  The carbon material as described above is preferably produced using bacterial cellulose as described below as a starting material. Bacterial cellulose has a basic structure of thin cellulose fibrils on the order of nm, has a high specific surface area, material permeability, and porosity, and the orientation of cellulose molecules in the fibrils is also highly developed. It is extremely suitable as a precursor of the carbon material of the invention. Bacterial cellulose is also preferable in that cellulose fibrils have the above-mentioned branching based on bacterial growth.

  Next, the manufacturing method of the network-like carbon material of this invention is demonstrated.

  Bacterial cellulose used as a starting material in the present invention includes known ones such as Acetobacter xylinum subsp. Sucrofermentans, Acetobacter xylinum ATCC 23768. Acetobacter xylinum ATCC 23769, A. pasteurianus ATCC 10245, Acetobacter xylinum ATCC 14851, Acetobacter xylinum ATCC 11142 and Acetobacter xylinum ATCC 10821, etc. Rhizobium, Sarsina, Pseudomonas, Achromobacter, Alkaligenes, Aerobacter, Azotobacter Zooglea genus are produced by culturing various mutant strains created by mutation treatment by a known method using a Enterobacter or Kuryubera genus and NTG them (nitrosoguanidine), or the like. In the present invention, those that produce cellulose having a particularly long fiber length are preferred, and acetic acid bacteria and the like are preferred.

  Bacterial cellulose is a gel-like material containing water, as can be seen by looking at the properties of Nata de Coco, so it must be dried before carbonization. When this step is processed by ordinary hot air drying, the specific surface area is greatly reduced, and the specific surface area of the resulting carbon material is also reduced. Therefore, in the present invention, it is necessary to dry while maintaining the network structure of the bacterial cellulose in the gel state. Specifically, when shrinking after drying, from the state when the bacterial cellulose gel was first allowed to stand, the apparent shrinkage remaining rate (= length after shrinking / original length), width, 30% or more in the length direction and 10% or more in the thickness, preferably 50% or more in the length and length direction, and 20% or more in the thickness remain (in terms of shrinkage, width, length It is preferable to dry the film in a thickness direction of 50% or less and a thickness direction of 80% or less.

  Any method can be used as long as it can be dried in such a state. For example, freeze-drying is simple and preferable. As the lyophilization solvent, commonly used lyophilization solvents such as water and alcohol can be used, but those having a high vapor pressure in the frozen state are preferable, alcohol is particularly preferable, and t-butanol is particularly preferable. . When using a solvent other than water, the water contained in the gel may be replaced with a freeze-dried solvent and then freeze-dried.

  Bacterial cellulose after drying has a sponge-like property in which cellulose fibrils are intertwined. The carbonization treatment may be performed in this shape, but the shape of the carbon material finally produced can be defined by molding at this point. Usually, in the use used as a nonwoven fabric in a sheet form, it is preferable to form sponge-like bacterial cellulose after freeze-drying into a sheet form by uniaxial pressure compression.

  Carbonization of bacterial cellulose after drying is usually performed at a temperature of 700 ° C. to 3200 ° C. in an inert gas atmosphere. Preferably, it is 3000 degrees C or less. In order to promote carbon crystallization and increase the graphite quality, it is preferable to perform carbon treatment at a high temperature, and in order to obtain a network-like carbon material having a graphitization rate of 30% or more, the carbonization temperature is 1500. C. to 3200.degree. C. (preferably 3000.degree. C. or less), preferably 2000.degree. C. to 3000.degree. When the crystallite develops and the graphitization rate becomes 30% or more, the carbon hexagonal network can be easily observed with a transmission electron microscope. In order to increase the graphitization rate, the carbonization treatment may be performed in two stages. After the primary carbonization treatment of less than 1500 ° C, the secondary carbonization treatment (graphite at 1500 ° C or higher) Process). As the inert gas, nitrogen, argon or the like is preferable, and argon is particularly preferable in the treatment at a high temperature during graphitization. The graphitization rate is a value obtained by analyzing the wide-angle X-ray scattering intensity by the Gakushin method. Since the required physical properties differ depending on the application, it is preferable to change the graphitization rate in accordance with the application.

  In the carbonization step, it is preferable to treat the dried bacterial cellulose by sandwiching it between carbon plates. By such treatment, a carbon yield {= residual weight × 100 / dry cellulose weight} of about 10% by weight or more is obtained. On the other hand, for example, when the treatment is performed in a crucible, the carbon yield may be 7% or less, and in the worst case, 0%, and the specific surface area may be significantly lower than that of the precursor bacterial cellulose. The structure may become dense. This is presumably because bacterial cellulose fibrils are particularly sensitive and sensitive. Therefore, it is particularly preferable to perform the treatment by sandwiching between carbon plates when bacterial cellulose is used as a raw material.

  In one embodiment of the invention, it is preferred that the dried bacterial cellulose is contacted with halogen vapor prior to the carbonization step. As the halogen, bromine and iodine are preferable, and iodine is particularly preferable. This step may be performed by any method as long as the halogen vapor is brought into contact with the dried bacterial cellulose, and may be processed in a closed container or may be processed in a flow-through manner. The treatment atmosphere may be a space filled with pure halogen vapor alone, or air or an inert gas such as argon that does not adversely affect the reaction may coexist. The treatment conditions can be appropriately set in consideration of the vapor pressure. For example, the treatment conditions are about 20 to 120 ° C., preferably 50 to 100 ° C., about 10 minutes to 10 days, preferably about 6 hours to 3 days. Good.

  Bacterial cellulose thus treated with halogen is preferably carbonized by sandwiching it between carbon plates as described above, so that a carbon material having a high specific surface area can be obtained at a high yield of 15% by weight or more. can get.

  Furthermore, in one embodiment of the present invention, it is preferable to steam and dry the dried bacterial cellulose under pressure and sealing before the carbonization step. Steaming is carried out in an inert gas atmosphere by heating in a suitable closed container at a temperature that does not completely carbonize. Usually, the temperature is less than 700 ° C., preferably 500 ° C. or less and usually 200 ° C. or more. The treatment time is 10 minutes to 3 days, preferably 1 to 24 hours, more preferably 2 to 10 hours (for example, 5 hours). It is.

  Bacterial cellulose that has been steam-baked under pressure and hermetic sealing as described above is preferably carbonized by sandwiching it between carbon plates as described above, with a high yield of 15% by weight or more and a high ratio. A carbon material having a surface area is obtained.

  In the production method of the present invention, a halogen treatment and a steaming treatment may be used in combination. In that case, it is usually preferable to perform a steaming treatment after halogen treatment.

  The above treatment, that is, carbonization sandwiched between carbon plates, halogen treatment before the carbonization step and steaming treatment, can be applied to dry bacterial cellulose that has not been molded and to those that have been molded into a sheet. it can.

  The network-like carbon material obtained in this way, especially the sheet-like material, makes use of physical properties such as high substance adsorption, substance permeability, and electron conductivity due to its high specific surface area, so that it is an electrode material for an electric double layer capacitor. Can be suitably used. In particular, the crystallinity can be increased to improve the chemical stability of the electrode, and conversely, the crystallinity can be suppressed and the influence on the electrolyte solution can be suppressed. A good electrode material can be provided.

  Further, the carrier supporting the fuel cell catalyst requires a large surface area, but also requires a space and a gap through which hydrogen, oxygen, and water can easily pass. The product is also suitably used for the catalyst support of such a fuel cell. Furthermore, it can be suitably used for filters and other applications that require surface area and space.

  Next, the present invention will be described more specifically with reference to examples.

  The pore structure of the dried bacterial cellulose and network carbon material was measured by a nitrogen adsorption method. The specific surface area was calculated by the BET method. The pore size distribution was calculated by the Dollimore-Heal (DH) method using a nitrogen adsorption isotherm.

<Comparative Example 1>
A gel-like commercially available microbial-produced fine fibrous cellulose (bacterial cellulose) hydrated product was dried with hot air. The weight was 1% or less, and the appearance was reduced to 1/100 or less in the thickness direction and to 1/2 or less in the vertical and horizontal directions. As a result of observing the surface with a scanning electron microscope (SEM), it was found that the surface was densified. When the specific surface area was measured, it was 2 m ² / g or less.

<Example 1>
The bacterial cellulose hydrate used in Comparative Example 1 was immersed in t-butyl alcohol for 20 hours or more to perform solvent exchange between water and t-butyl alcohol. Thereafter, the sample was cooled and frozen, and freeze-dried. As a result, a white sponge-like dried product was obtained. From the gel material of the starting material, the shrinkage was 98% in the thickness direction and 95% in the vertical and horizontal directions.

As a result of SEM observation, it was found that both the surface (see FIG. 1) and the inside (see FIG. 2) have a network structure (porous structure) in which fine fibrils are connected by branching. The specific surface area of the obtained product was 50 m ² / g or more. Moreover, when pore diameter distribution was measured, it became clear that it has many pores with a diameter in the range of 2-10 nm (refer FIG. 3).

<Comparative example 2>
The dried bacterial cellulose obtained in Example 1 was allowed to stand in a carbon crucible and subjected to heat treatment at 1400 ° C. for 1 hour in a nitrogen atmosphere. Obtained. When the carbon yield was calculated, it was 10% or less.

<Example 2>
The bacterial cellulose dried product obtained in Example 1 was pressure-molded with a uniaxial compression molding machine so that the thickness was 1/10 of the original sample to obtain a sheet-like product. When this sample was measured for specific surface area and pore size distribution by nitrogen adsorption, it was confirmed that there was almost no change before and after compression.

<Example 3>
The sample obtained in Example 2 was sandwiched between two carbon plates, allowed to stand in a carbon crucible, and subjected to heat treatment at 1400 ° C. for 1 hour in a nitrogen atmosphere, whereby a sheet-like carbon film was obtained. The carbon yield was 14%. As a result of SEM observation, it was found that a porous structure in which carbon fibrils were connected on both the surface and the cross section was maintained. Further, the specific surface area was slightly increased to 70 m ² / g. The pore size distribution was shifted to the slightly larger side (see FIG. 3).

<Example 4> (steaming treatment)
The sheet-like material obtained in Example 2 was left standing in a small autoclave, the inside was replaced with nitrogen gas, and then sealed and heat-treated at 400 ° C. for 3 hours to obtain a brown sheet. The yield was 40%. When SEM observation was performed, it was confirmed that both the surface and the cross-section retained a porous structure in which carbon fibrils were connected (see FIGS. 4 and 5). Then, the carbonization process of 1400 degreeC similar to Example 3 was performed, and the black sheet thing was obtained. The yield was 45%, and the yield from the dried bacterial cellulose was 18%, confirming a great improvement. As a result of SEM observation, it was found that the surface and the cross section had a porous structure in which carbon fibrils were connected (see FIGS. 6 and 7). Further, the specific surface area was 70 m ² / g, which was slightly increased compared to the bacterial cellulose as a precursor. The pore size distribution was shifted to the slightly larger side. In the wide-angle X-ray scattering, broad scattering was confirmed in the vicinity of 2θ = 25 °. The degree of graphitization was 10% or less.

<Example 5> (Graphitization treatment)
The network-like porous carbon sheet produced in Example 4 was placed between a carbon plate in a graphitization furnace and allowed to stand, and heat treatment was performed at 3000 ° C. for 1.5 hours in an argon gas atmosphere. As a result, a carbon sheet having a dull gray luster was obtained. As a result of SEM observation, it was found that the surface had a porous structure in which carbon fibrils were connected, and the cross section had a structure in which fibrils were oriented in the sheet plane direction (see FIGS. 8 and 9). In addition, as a result of observation with a high-resolution transmission electron microscope (HRTEM), a large number of lattice stripes indicating the development of the carbon hexagonal network surface were observed (see FIG. 10). When wide-angle X-ray scattering was measured, 002 reflection indicating the growth of graphite crystals was clearly observed around 2θ = 26 °. The graphitization rate was estimated to be about 30%.

<Example 6> (iodine treatment)
0.3 g of the bacterial cellulose sheet obtained in Example 2 and 0.6 g of iodine were placed in a 500 cc glass container and sealed, followed by heat treatment at 80 ° C. for 24 hours. Thereafter, carbonization was performed at 1400 ° C. in the same manner as in Example 3 to obtain a black sheet. The carbon yield from bacterial cellulose was 22%, confirming a significant improvement. As a result of SEM observation, it was found that the surface had a porous structure in which carbon fibrils were connected. In the wide-angle X-ray scattering, broad scattering was confirmed in the vicinity of 2θ = 25 °. The degree of graphitization was 10% or less.

<Example 7> (graphitization treatment)
The network-like porous carbon sheet produced in Example 6 was placed between a carbon plate in a graphitization furnace and allowed to stand, and heat treatment was performed at 3000 ° C. for 1.5 hours in an argon gas atmosphere. As a result, a carbon sheet having a dull gray luster was obtained. As a result of SEM observation, it was found that the surface had a porous structure in which carbon fibrils were connected, and the cross section had a structure in which fibrils were oriented in the sheet plane direction (see FIGS. 11 and 12). In addition, as a result of observation with a high-resolution transmission electron microscope (HRTEM), a large number of lattice stripes indicating the development of the carbon hexagonal network surface were observed (see FIG. 13). Moreover, it turned out that the graphite crystal is orientating from the electron beam diffraction pattern of the same visual field (FIG. 14). When wide-angle X-ray scattering was measured, 002 reflection indicating the growth of graphite crystals was clearly observed in the vicinity of 2θ = 26 °, and the graphitization rate was about 35%.

2 is a SEM image of the surface of a dried bacterial cellulose produced in Example 1. 2 is an SEM image of the dried bacterial cellulose produced in Example 1. FIG. It is a graph which shows the pore size distribution by the DH method of the bacterial cellulose or carbon material of Examples 1 and 3. It is a SEM image of the bacterial cellulose surface after a steaming process in Example 4. It is a SEM image of the cross section of the bacterial cellulose after the steaming process in Example 4. 4 is a SEM image of the surface of a carbon material produced in Example 4. 6 is a SEM image of a cross section of the carbon material produced in Example 4. 6 is a SEM image of the surface of a carbon material produced in Example 5. 6 is a SEM image of a cross section of the carbon material produced in Example 5. 6 is an HRTEM image of a carbon material produced in Example 5. 7 is a SEM image of the surface of a carbon material produced in Example 7. 7 is a SEM image of a cross section of the carbon material produced in Example 7. 6 is an HRTEM image of the carbon material produced in Example 7. 14 is an electron diffraction pattern in the HRTEM image field of FIG. 13.

Claims (6)

Drying the gelatinous bacterial cellulose produced by the bacteria while maintaining its network structure;
Contacting the dried bacterial cellulose with halogen vapor;
And a step of carbonizing bacterial cellulose brought into contact with halogen vapor at a temperature of 700 ° C. to 3200 ° C. in an inert gas atmosphere. Drying the gelatinous bacterial cellulose produced by the bacteria while maintaining its network structure;
Steaming and baking dried bacterial cellulose under pressure and sealing;
And carbonizing the steamed and baked bacterial cellulose at a temperature of 700 ° C. to 3200 ° C. in an inert gas atmosphere. Wherein in the drying step, shrinkage residual ratio width apparent from the gel state prior to drying, 30% more than in the length direction, claim, characterized in that drying such that the thickness direction of 10% or more 1 or 2 The manufacturing method as described in. The manufacturing method according to claim 3 , wherein the drying is freeze-drying. The said carbonization temperature is 1500 degreeC-3200 degreeC, The manufacturing method in any one of Claims 1-4 characterized by the above-mentioned. The process according to claim 1, the dried bacterial cellulose by compression molding comprising a step of molding into a sheet.