JP2008006399A - Gas storage material - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a gas storage material of an enhanced gas storage capacity which secures a sufficient quantity of voids by releasing monolayer carbon nanotubes from their close adhesion by decoupling and dispersing bundles of the carbon nanotubes to reduce the bundle size. <P>SOLUTION: The gas storage material comprised of monolayer carbon nanotubes forming a bundle structure is characterized in that the number of the monolayer carbon nanotubes constituting individual bundles is not more than 10 in average. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a gas storage material in which single-walled carbon nanotubes form a bundle structure and a method for producing the same.

  Carbon nanotubes (CNT) are expected as an excellent gas storage material that can obtain a large gas storage capacity by adsorbing gas in the internal space of the tube and the space between the tubes. For example, it is extremely useful for the construction of a storage system for fuel gas (hydrogen, methane, natural gas, etc.) mounted on automobiles.

  What should be considered about the adsorption in the carbon nanotube inner space is not only that the gas molecules are directly adsorbed on the inner wall of the tube, but also more gas molecules are adsorbed on the adsorbed gas molecules within the range of the adsorption action from the inner wall of the tube In addition, it is repeated that gas molecules are adsorbed on it. Such multiple adsorption increases the amount of adsorption of the carbon nanotubes into the internal space, and high gas storage efficiency is obtained. Therefore, it is necessary to have a sufficiently large tube diameter that can secure a large internal space in which multiple gas molecules to be occluded can be adsorbed. However, if the tube diameter is too large, the central region of the inner space of the tube becomes a useless space because the adsorption action from the tube wall does not reach, so the occlusion efficiency decreases. Therefore, in order to obtain high occlusion efficiency, it is necessary to set the tube diameter to an appropriate value according to the size and adsorption characteristics of gas molecules to be occluded.

  From the above viewpoint, for example, the tube diameter suitable for occlusion of hydrogen (molecular diameter 0.28 nm) having the smallest molecular size in the fuel gas is about 1.5 to 2 nm. On the other hand, single-walled carbon nanotubes currently on the market have an average tube diameter of about 1 nm at the maximum, and high occlusion efficiency cannot be obtained as it is. Therefore, in order to increase the diameter of the carbon nanotube, a method of fusing the carbon nanotubes by heating to about 1500 to 200 ° C. in a vacuum or in an inert gas atmosphere is known.

  On the other hand, what should be considered about the adsorption | suction in the space | gap between tubes of a carbon nanotube is that several carbon nanotubes aggregate by van der Waals force, and form a bundle structure. By securing an appropriate space between the carbon nanotubes constituting the bundle and increasing the amount of gas molecules adsorbed as a whole bundle, the gas storage efficiency of the gas storage material as the bundle aggregate can be increased.

  However, the countless carbon nanotubes that make up a huge bundle are densely packed and in close contact with each other. In order to release this crowding and release the tubes from close contact to form an appropriate gap between the tubes, the bundle itself must be dispersed to reduce the bundle size.

  In Patent Document 1, in order to form a super bundle, a suspension of a single-walled carbon nanotube substrate is prepared using a dilute acid or an acidic solution as a solvent, and this is stirred with ultrasonic energy. The use of 98% or more single-walled carbon nanotubes is disclosed. However, there is no suggestion about distributing super bundles.

  Patent Document 2 discloses that a graphite intercalation compound (GIC) is doped with an alkali metal and nano-structured by milling to provide an appropriate gap between the GICs to allow gas adsorption. Yes.

  Patent Document 3 discloses that introduction of defects into carbon nanotubes promotes coalescence of carbon nanotubes and facilitates an increase in diameter.

  Patent Document 4 discloses that an opening is formed in a carbon nanotube, and the D / G ratio in the laser Raman spectroscopy of the opening is 0.02 to 0.10, thereby increasing the hydrogen storage capacity. Yes.

  Patent Document 5 discloses that the specific surface area is increased by irradiating the single-walled carbon nanotube with an electron beam to increase the hydrogen storage amount.

  Patent Documents 2 to 5 have no suggestion about dispersing the bundle.

Special table 2003-520178 gazette JP 2004-181383 A JP 2005-324971 A Japanese Patent Laid-Open No. 2004-290793 JP 2003-26413 A

  An object of the present invention is to provide a gas storage material in which a bundle is dispersed to reduce a bundle size, thereby ensuring a space between single-walled carbon nanotubes and improving a gas storage capacity.

  In order to achieve the above object, the gas storage material of the present invention has an average of 10 single-walled carbon nanotubes constituting each bundle in the gas storage material in which single-walled carbon nanotubes form a bundle structure. It is characterized by the following.

  In the present invention, the number of single-walled carbon nanotubes constituting the bundle is limited to an average of 10 or less and the bundle size is reduced, so that an appropriate gap is secured between the tubes in the bundle, and the specific surface area is increased. A large gas storage material can be obtained.

  As already described, generally, when the bundle size is large, the carbon nanotubes in the bundle are in a dense state and are in close contact with each other. In order to release this crowding and release the tubes from close contact to form an appropriate gap between the tubes, the bundle itself must be dispersed to reduce the bundle size.

  As a specific means, in the present invention, the number of single-walled carbon nanotubes constituting the bundle is limited to an average of 10 or less. By setting the bundle size within this range, the specific surface area of the gas storage material which is an aggregate of a large number of bundles is remarkably increased.

  In a desirable embodiment of the present invention, the single-walled carbon nanotube has an average diameter of 1.5 to 2 nm, and the S / L ratio is S when the cross-sectional area at an arbitrary portion of the tube is S and the perimeter is L. 1.0 to 1.2. When the average tube diameter is 1.5 to 2 nm, it is suitable for occlusion of hydrogen gas and an equivalent relatively small molecular size, and when the tube S / L ratio is 1.0 to 1.2, the tube cross-sectional shape is It is indeterminate from the perfect circle, which is advantageous for securing the space between tubes. Furthermore, when the S / L ratio is 1.1 to 1.2, it is advantageous for securing a pore volume effective for gas adsorption.

[Example 1]
Using the commercially available single-walled carbon nanotube as a starting material, the following two-stage heat treatment was performed, and various characteristics were examined.

Single-walled carbon nanotubes manufactured by CNI were used as starting materials. In an as-received state, 20 to 30 wt% of Fe, which is a constituent component of the catalyst used for carbon nanotube production, remains as impurities. Therefore, the residual Fe was dissolved and removed by treatment in an HCl solution at 60 ° C. (HCl: H ₂ O = 1: 2) for 3 hours. Then, pure water washing was performed, and then vacuum drying was performed by heating to 200 ° C. in a vacuum of about 10 Pa.

  Next, as pre-stage heat treatment, it was held at various temperatures in the range of 1100 to 1600 ° C. for 12 hours in a high vacuum of 10 to 3 Pa. The heating rate was 20 ° C./min.

  Further, as a subsequent heat treatment, it was heated to 1800 ° C. at the same rate of temperature rise and held for 2 hours. For comparison, only the post-stage heat treatment was performed on the sample without the pre-stage heat treatment. This latter heat treatment is a heat treatment for expanding the tube diameter by fusing the single-walled carbon nanotubes. In general, commercially available single-walled carbon nanotubes have a tube diameter of about 1 nm, but the diameter is expanded to nearly 2 nm by this treatment.

  The obtained sample was subjected to the following measurement and analysis.

(1) Measurement of specific surface area The specific surface area was measured by the BET method (temperature 77K, nitrogen adsorption) using a fully automatic gas adsorption amount measuring device (Autosorb manufactured by Cantalome).

(2) Measurement of purity Purity was measured by the TGA method (Thermogravimetric Analysis).

(3) Analysis of tube cross section The tube cross section was observed for several tens of fields with a transmission electron microscope, and the bundle size (the number of bundle constituent tubes) was measured. When there was no large variation in the number of constituent tubes for each bundle, five representative visual fields were selected, and the following bundle analyzes (I) and (II) were performed. When the bundles are dispersed, the tubes become single or the number of constituent tubes is extremely reduced. Therefore, the focal point is moved to prevent the bundles from being overlooked in each field of view.

(I) Measurement of the number of constituent tubes for each bundle in each field of view (II) Evaluation of tube cross-sectional shape: The cross-sectional area S and the perimeter L were measured at an arbitrary part of each tube, and the S / L ratio was obtained. The S / L ratio is about 1.4 in a perfect circle, and decreases as it deviates from the perfect circle.

FIG. 1 shows the average number of tubes (N) and specific surface area (A) per bundle for various pre-treatment heat treatment temperatures. The case where only the post-stage heat treatment is performed without the pre-stage heat treatment is also shown at the left end of the figure. When the pre-stage heat treatment is performed at 1100 to 1400 ° C., the average number of tubes (N) per bundle is 10 or less, and the specific surface area (A) increases correspondingly to 1400 to 1450 m ² / It is about g. In particular, when the pre-stage heat treatment temperature is 1100 to 1200 ° C., the average number of tubes (N) per bundle is extremely small, 2 to 4.

When the temperature of the pre-stage heat treatment is outside the above range, that is, 1000 ° C., 1500 to 1600 ° C. and without the pre-stage heat treatment, the average number of tubes (N) per bundle is about 15 to 20 and 25, respectively. The specific surface area (A) is about 1100 m ² / g.

  FIG. 2 shows the average tube diameter (D) and cross-sectional shape (S / L ratio) for various pre-treatment heat treatment temperatures. The case where only the post-stage heat treatment is performed without performing the pre-stage heat treatment is also shown at the left end of the figure.

  Here, the “average tube diameter” is an average value of two or more arbitrary positions when close to a perfect circle, and an average value of the longest part and the shortest part when deviating from the perfect circle. As shown in FIG. 2, the average tube diameter did not change greatly depending on the temperature of the previous heat treatment, and was 1.5 to 2 nm.

  The cross-sectional shape (S / L ratio) is 1.1 to 1.2 when the pre-stage heat treatment temperature is 1100 to 1400 ° C., and 1.3 to 1.4 when there is no other temperature and no pre-stage heat treatment. Met. Since the cross-sectional shape becomes a perfect circle at an S / L ratio (“R” in FIG. 2) slightly larger than 1.4, the pre-stage heat treatment temperature of 1100 to 1400 ° C. where the S / L ratio is 1.1 to 1.2 In the case of, it can be seen that it is far from the perfect circle. However, if the S / L ratio is less than 1.0, the S / L ratio is desirably 1.0 or more because it is too far from the perfect circle and it is difficult to obtain an effective pore volume. Therefore, a desirable S / L ratio range is 1.0 to 1.2. An S / L ratio of 1.1 to 1.2 is more desirable because an effective pore volume can be easily obtained with certainty.

[Example 2]
The cap of the carbon nanotube tip was opened for the sample of the pre-stage heat treatment 1100 ° C. and the post-stage heat treatment 1800 ° C. in which the specific surface area was remarkably increased in Example 1 above. The cap was opened by heating to 420 ° C. in the atmosphere to cause partial oxidation. About the sample after cap opening, it measured by BET method on the same conditions as the above, and obtained 1920 m < ² > / g as a specific surface area.

  At present, there is a tendency for the purity to decrease with opening of the cap. According to the result of TGA measurement, about 30 wt% of impurities due to graphite were contained. In general, it is known that such graphite is produced by a tube diameter expansion treatment (usually heat treatment at 1800 ° C.). However, in the case of the sample of this example, the amount of graphite was 20 wt% when confirmed by TGA measurement before the cap was opened. Therefore, it is considered that the ratio of graphite further increased due to the tube burning during the partial oxidation treatment when the cap was opened. It is a future problem to suppress such impurities.

If the purity is assumed to be 100%, the specific surface area should be approximately 2770 m ² / g, which is the theoretical specific surface area of graphene. In this sense, it can be said that the present invention has a very remarkable effect. In this case, it is considered that N ₂ adsorption occurs on the outer surface of the tube.

  When the hydrogen occlusion amount at a pressure of 30 MPa and room temperature was measured for the sample with the cap opened, 1.7 mass% was obtained.

  The obtained sample is typically then formed into a high density and can be used as a storage material for hydrogen and natural gas, mainly as a high-pressure storage tank for automobiles.

  According to the present invention, there is provided a gas storage material in which a bundle is dispersed to reduce the bundle size, thereby ensuring a space between single-walled carbon nanotubes and improving gas storage capacity.

FIG. 1 is a graph showing the average number of tubes per bundle (N) and specific surface area (A) with respect to the temperature of various pre-stage heat treatments for samples subjected to two-stage heat treatment. However, each data plotted at the left end of the graph shows the result when only the post-stage heat treatment is performed without performing the pre-stage heat treatment. FIG. 2 is a graph showing the average tube diameter (D) and cross-sectional shape (S / L ratio) with respect to the temperature of various pre-stage heat treatments for the samples subjected to the two-stage heat treatment. However, each data plotted at the left end of the graph shows the result when only the post-stage heat treatment is performed without performing the pre-stage heat treatment.

Claims (3)

In the gas storage material in which single-walled carbon nanotubes form a bundle structure,
A gas storage material characterized in that the average number of single-walled carbon nanotubes constituting each bundle is 10 or less.   2. The single-walled carbon nanotube according to claim 1, wherein the single-walled carbon nanotube has an average diameter of 1.5 to 2 nm, a cross-sectional area at an arbitrary portion of the tube is S, and an S / L ratio is 1. Gas storage material characterized by being 0-1.2.   3. The gas storage material according to claim 2, wherein the S / L ratio is 1.1 to 1.2.

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