KR101173104B1 - Carbon nanotube as a hydrogen storage medium - Google Patents

Abstract

The present invention relates to a hydrogen storage medium consisting of carbon nanotubes prepared by ball milling at cryogenic temperatures.
According to the present invention, it is possible to manufacture carbon nanotubes with significantly improved hydrogen storage capability compared to conventional carbon nanotubes, and the carbon nanotubes can be used as a hydrogen storage medium such as a thermal battery, and a higher effect can be expected.

Description

Carbon nanotube as a hydrogen storage medium

The present invention relates to a carbon nanotube hydrogen storage medium, and more particularly to a hydrogen storage medium consisting of carbon nanotubes produced by ball milling at cryogenic temperatures.

Carbon nanotubes (CNTs) were first discovered by Ijima Sumio of Japan in 1991 in the analysis of carbon masses formed on the cathode of graphite using an electrical discharge method. It is called carbon nanotubes because it forms a phosphorus tube.

These carbon nanotubes have inherent high mechanical, chemical and electrical properties, and are used in various fields, and for this reason, many researches have been conducted over a decade.

Carbon nanotubes have attracted attention as energy storage media because of their unique structures, such as their cylindrical, multilayered and fluffy structures, and for this purpose, studies on the hydrogen storage potential of carbon nanotubes have been conducted.

Korean Patent Laid-Open No. 2001-0091479 discloses electrochemical and thermal methods for carbon nanotubes in which copper is injected by carbon nanotubes themselves or electroless plating, and carbon nanotubes doped with lithium and potassium by impregnation. A method of storing hydrogen is disclosed, and Korean Patent No. 10-0727237 discloses a thermal battery using hydrogen storage capacity of carbon nanotubes, that is, carbon nanotube storage for storing hydrogen or a hydrocarbon having 1 to 6 carbon atoms. And a heat supply unit for separating hydrogen or hydrocarbons from the carbon nanotubes by supplying heat to the carbon nanotube storage unit, a fuel cell unit for generating a voltage by redox-reacting hydrogen or hydrocarbons separated from the carbon nanotubes, and fuel. Thermo battery composed of final conductor terminals for applying the output voltage generated from the battery unit to the terminal of the layered material The methods for their use are disclosed.

As described above, many attempts have been made to utilize carbon nanotubes' hydrogen storage ability, such as fuel cells. It can be said to.

Accordingly, the present inventors have diligently researched to develop carbon nanotubes having improved hydrogen storage ability. When the ball milling of carbon nanotubes at an appropriate rotation speed under cryogenic conditions, the hydrogen storage capability is superior to that of conventional carbon nanotubes. It was confirmed that the improved carbon nanotubes can be prepared, and thus, the present invention was completed.

Therefore, the main object of the present invention is to provide a carbon nanotube with improved hydrogen storage capacity.

According to one aspect of the invention, the present invention provides a hydrogen storage medium consisting of carbon nanotubes prepared by ball milling at a temperature of -100 ℃ ~ -200 ℃.

In the hydrogen storage medium of the present invention, the speed of the ball milling is preferably set to 300rpm ~ 1000rpm.

In the hydrogen storage medium of the present invention, it is preferable to use multi-wall carbon nanotubes (MWCNTs) as the carbon nanotubes.

Hereinafter, the present invention will be described in more detail step by step.

The present invention relates to a hydrogen storage medium made of carbon nanotubes which have improved hydrogen storage capability by ball milling carbon nanotubes at cryogenic temperatures compared to conventional carbon nanotubes.

Carbon nanotubes (CNTs) are allotropic bodies of cylindrical carbon nanostructures and generally have a diameter of 1 to 100 nm and a length of several nm to several tens of μm. Carbon nanotubes have a honeycomb-shaped planar carbon structure in which one carbon atom is bonded to three other carbon atoms to form a tube. There are many types of carbon nanotubes, among which multi-walled carbon nanotubes (MWCNTs) or single-walled carbon nanotubes (SWCNTs), depending on the number of walls constituting the nanotubes. Can be divided into Among them, carbon nanotubes consisting of two or more walls are called multi-walled carbon nanotubes, and carbon nanotubes consisting of only one wall are called single-walled carbon nanotubes. Although both single-walled and multi-walled carbon nanotubes can be used as the hydrogen storage medium of the present invention, it is preferable to use multi-walled carbon nanotubes having a high possibility of storing hydrogen between several layers of walls.

As a method of manufacturing carbon nanotubes, various methods such as an electric discharge method, a laser deposition method, a thermochemical vapor deposition method, and a plasma chemical vapor deposition method are known. In the present invention, carbon nanotubes prepared by any of these methods may be used.

The ball milling method used in the present invention is a method of mechanically rubbing a ball of small size to crush the material therebetween, in the case of ball milling carbon nanotubes as in the present invention. It is effective to use hardened zirconia (ZrO ₂ ) or steel balls as grinding media.

In the case of the present invention, by using the ball milling in cryogenic conditions, rather than using a general ball milling method, the application of the cryogenic environment can maximize the ball milling effect by the strong embrittlement of the material. In addition, cryogenic ball milling has a great difference in that it can provide a high strain and low temperature environment, unlike a conventional mechanical alloying process that can improve dispersibility through grinding of metal powder. For this purpose, the ball milling temperature is preferably set to -100 ° C to -200 ° C. If the ball milling temperature is higher than -100 ℃, the satisfactory effect cannot be expected because the carbon nanotubes are not fully brittle, and if the ball milling temperature is lower than -200 ℃, fast brittleness can be expected. However, it is not preferable because it may cause unnecessary energy consumption and the carbon nanotubes may be structurally unstable.

Unlike in the present invention, carbon nanotubes may be manufactured by performing ball milling under conditions other than cryogenic conditions such as room temperature, but the hydrogen storage property of the carbon nanotubes thus prepared is much lower than that of the carbon nanotubes of the present invention. Can not do it.

In the production of carbon nanotubes of the present invention, not only the cryogenic temperature conditions as described above, but also the rotational speed during ball milling are important factors. As a result of comparing the characteristics of the manufactured carbon nanotubes with the change of revolutions per minute (rpm), the lower the rotational speed, the lower the characteristics of the manufactured carbon nanotubes, that is, dispersibility, surface area, hydrogen adsorption, The higher the rotational speed, the better the characteristics of the carbon nanotubes, but since it is not necessary to consume energy using an excessively high rotational speed, it is preferable to maintain the range of 300rpm to 1000rpm, more preferably to 700rpm or more. It is good.

The present inventors compared and analyzed various characteristics with the existing carbon nanotubes (carbon nanotubes without cryogenic ball milling) in order to confirm the effect of the carbon nanotubes prepared by cryogenic ball milling. In addition, the cryogenic ball milling rpm and the ball milling time were set differently to compare the effects.

First, untreated (not cryogenic ball milled) multiwalled carbon nanotubes and cryogenic ball milled multiwalled carbon nanotubes were dispersed in ethanol using ultrasonic dispersion, and their stability was observed after 24 hours. 2 is shown. Figure 2 (b) shows the dispersion state of the multi-wall carbon nanotubes cryogenic ball milling for 2 hours and 6 hours at 300rpm, it can be seen that the dispersion of the multi-walled carbon nanotubes well after 6 hours cryogenic ball milling. On the other hand, in the case of cryogenic ball milling for 2 hours, no change was observed compared to the untreated multi-walled carbon nanotubes (see FIG. 2A).

In the case of short ball milling time, sedimentation of multi-walled carbon nanotubes occurred as time passed according to their agglomeration phenomenon, and in case of cryogenic ball milling multi-walled carbon nanotubes at 700 rpm (see FIG. Since all multi-walled carbon nanotubes showed good suspension stability. The suspension stability is improved due to the high ball milling speed because the cryogenic ball milling process shortens the multi-walled carbon nanotubes and deagglomerates them, resulting in lighter weight and better dispersibility. As a result, the ball milling speed was more effective than the ball milling time for the dispersibility of multi-walled carbon nanotubes.

Scanning electron microscope (SEM) was used to confirm the dispersion and morphological characteristics of untreated multi-walled carbon nanotubes and cryogenic ball milled multi-walled carbon nanotubes. FIG. 3 shows the results before and after cryogenic ball milling (see a of FIG. 3) (see b and c of FIG. 3). The untreated multi-walled carbon nanotubes are dense and have a length of 200 nm. It was observed that the above-mentioned, cryogenic ball milled multi-walled carbon nanotubes are shorter as the ball milling speed increases, the dispersibility gradually increases. Some cryogenic ball milled multi-walled carbon nanotubes were observed to be closer to or smaller than 200 nm in size, and cryogenic ball milling at 700 rpm for 6 hours had no lumps. It indicates shortening and breakage according to the milling process. In other words, the cryogenic ball milling process is an effective way to reduce the particle size and agglomeration of multi-walled carbon nanotubes, and the high ball milling speed plays an important role in the size reduction and dispersibility of multi-walled carbon nanotubes. can do.

In order to verify the morphological and structural characteristics of multi-walled carbon nanotubes, transmission electron microscope (TEM), X-ray diffraction (XRD), selected area diffraction (SAD), Analysis using a Raman spectroscope was performed.

The inventors confirmed the open tip formation and the change of surface area of the multi-walled carbon nanotubes through the end tip and the surface contour image. 4 is a transmission electron microscope image of untreated and cryogenic ball milled multi-walled carbon nanotubes. As shown in FIG. 4a, the ends of the untreated multi-walled carbon nanotubes are mostly closed (black arrows and enlarged images). Was composed of multiple successive layers without defects or structural changes, and had the same cross section as the "Russian Doll" model. On the other hand, for 6 hours cryogenic ball milling at 300 rpm (Fig. 4b), the ends of the multi-walled carbon nanotubes were partially open (white arrow and enlarged image) and partially damaged (red arrow) wall shape, i.e. It had a torn, twisted or broken wall.

The surface profile (FIG. 4C) of the multiwall carbon nanotubes ball milled at 700 rpm for 6 hours was found to be significantly different from the open ends of the multiwall carbon nanotubes. The inner and outer walls had an extremely crushed structure that was twisted and twisted, the multi-layered carbon structure disappeared and was converted to amorphous carbon. This morphological change will improve adhesion properties and hydrogen storage without the need for other chemical improvements due to the improvement in surface area.

FIG. 5 shows a selective spatial diffraction pattern of the untreated and cryogenic ball milled multiwall carbon nanotubes shown in FIG. 4. The light emitting pattern and the clear ring pattern represent the polycrystalline structure of the carbon film and the multi-walled carbon nanotubes with many pores and amorphous carbon films. The multi-walled carbon nanotubes in which the diffraction ring patterns of (002), (101), (004), and (110) indicated by arrows in FIG. 5 are untreated (a in FIG. 5) and cryogenic ball milled (b, c in FIG. 5). ). The (002) and (101) patterns represent graphene layers and carbon atoms of multi-walled carbon nanotubes having an interlayer space of about 0.34 and 0.20 nm, respectively, and the patterns (004) and (110) are approximately The interlayer space of 0.17 and 0.12 nm is shown. The ring pattern of multi-walled carbon nanotubes untreated and cryogenically ball milled for 6 hours at 300 rpm means a polycrystalline structure, whereas the ring pattern for 6 hours cryogenic ball milling at 700 rpm is shown as the milling speed increases. It gradually darkens and partially disappears, indicating that the polycrystalline structure is becoming amorphous as the carbon atoms are disordered and the graphene layer breaks down.

In order to more clearly verify the structural change in the selective spatial diffraction pattern, X-ray diffraction analysis of untreated and cryogenic ball milled multiwall carbon nanotubes at 300 and 700 rpm was performed for 6 hours. 6 shows the change in the X-ray diffraction pattern, which clarifies the results of the earlier selective spatial diffraction analysis.

The untreated and cryoball milled multi-walled carbon nanotubes for 6 hours at 300 rpm showed no change in the polycrystalline properties of the graphene layer, resulting in near 26 °, 43.2 °, 53 °, and 79 ° (2θ) (002), The clear peaks of (101), (004) and (110) are shown. The strong, sharp peaks of the cryogenic ball milled multi-walled carbon nanotubes at this untreated and low milling rate show an interplanar spacing and high crystallinity of 0.34 nm, indicating that there is only a slight change in the graft. On the other hand, the peaks (002) and (101) decreased rapidly after 6 hours cryogenic ball milling at 700 rpm, and the peaks (004) and (110) gradually decreased or disappeared. This result indicates that the polycrystalline structure is amorphous by breaking the graphene layer of the inner outer wall of the multi-walled carbon nanotubes by the strong impact. The d-spaces of the (002) and (101) peaks were 3.45 and 2.101 ms, and the d-spaces of the (004) and (110) peaks were 1.70 and 1.22 ms.

The results of X-ray diffraction analysis indicate that the polycrystalline properties of multi-walled carbon nanotubes decrease with increasing milling speed. In particular, high milling speeds have a significant effect on the structure of multi-walled carbon nanotubes. do.

Raman analysis was performed to investigate the qualitative change of the multi-walled carbon nanotubes according to the cryogenic ball milling process. 7 is a low peak of the untreated and the cryogenic ball-milling a multiwall the Raman spectrum of the carbon nanotube, yes pie tick sp2 two main peaks that is, 1100 ~ 1500 ~ 1700㎝ 1400㎝ ^-1 high peak and a combination of ^-1 Are distinguished. The first peak (D-band) near 1300 cm ^-1 indicates the presence of carbon particles and the characteristics of an unsafe and disordered wall, and the second peak (G-band) near 1600 cm ^-1 indicates a continuous form of CC. This is a characteristic of the crystalline graphite layer of multi-walled carbon nanotubes. As shown in FIG. 7, the D-band and the G-band were found to gradually decrease with the milling speed. This phenomenon indicates that the milling process improved the production of residual detached graphite, fragmentation, and disordered carbon. In addition, the intensity ratio (R = ID / IG) of the D- and G-bands can be used to assess the degree of disorder or defect of multi-walled carbon nanotubes, and as the milling speed increases, the ratio of ID / IG increases initially. This indicates that cryogenic ball milling has converted the multi-walled carbon nanotubes into incomplete, disordered amorphous carbon, which is similar to Raman spectral results.

Hydrogen storage was analyzed using a volumetric method using a BET surface area and pore size analyzer. As shown in the transmission electron microscope results of FIG. 4, the surface profile of the cryogenic ball milled multi-walled carbon nanotubes tends to increase. This change in cross-sectional curve can improve the surface area and pore generation on the surface of multi-walled carbon nanotubes. Accordingly, the mass spectrometry of the untreated or cryogenic ball milled multiwall carbon nanotube surface was performed at 300 rpm and 700 rpm, and the BET specific surface area, pore volume and pore diameter were determined. Investigate. In the case of volume measurement, a sample dried at 200 ° C. for 6 hours was used.

The BET specific surface areas of the untreated samples and the cryoball milled samples for 6 hours at 300 rpm or 700 rpm were 201.3, 226.11 and 236.39, respectively (see FIG. 8). The surface area of the cryogenic ball milled sample at 700 rpm was about 17.4% higher than the untreated sample. Increased BET-specific surface area means that cryogenic ball milling is effective for the generation of surface area or voids of multi-walled carbon nanotubes, and the large surface area of such cryogenic ball milled multi-walled carbon nanotubes is found in small surface area. This is because more open ends and pore volume increased.

In addition, hydrogen storage of multi-walled carbon nanotubes is affected by pore volume and diameter. As shown in FIG. 9, the cryogenic ball milled multi-walled carbon nanotube at 700 rpm has a higher pore volume (34.9%) and a smaller pore size (74.8%) than the untreated multi-walled carbon nanotube. The pore diameter of untreated multi-walled carbon nanotubes is 103.54 nm, which is large porosity (macroporous, 50 nm ~), and the pore diameters of cryogenic ball milled multi-walled carbon nanotubes are 26.07 and 26.02 nm, which are mesoporous, 2-50. Nm) material. The decrease in pore diameter may be due to the increase in surface area and pore volume, resulting in increased hydrogen storage. Detailed surface area and pore data are shown in Table 1.

Sample S _BET (㎡ / g) V _p (cm 3 / g) Pore diameter peak (nm) Adsorption amount (mg / g) Untreated 201.30 0.7090 103.54 ^a 1.815 300 rpm / 6h 226.11 0.9038 28.07 ^b 2.131 700 rpm / 6h 236.39 0.9563 26.02 ^b 2.215

a: air hole (diameter: 50 nm ~)

b: medium pores (diameter: 2-50 nm)

The amount of hydrogen adsorption of untreated or cryogenic ball milled samples was measured from 77 K to 100 kPa (see FIG. 10). In FIG. 10, the adsorption amount of the untreated sample was 1.815 mg / g at 100 kPa, whereas the adsorption amount of the cryogenic ball milled sample at 700 rpm was increased by about 22% to 2.215 mg / g at 100 kPa. As the milling speed was increased, the surface area, pore volume, and hydrogen adsorption amount of the cryogenic ball milled multiwall carbon nanotubes increased compared to the untreated multiwall carbon nanotubes, and the pore diameter decreased. Therefore, cryogenic ball milling of multi-walled carbon nanotubes has a tremendous effect on surface area and surface porosity as the end is opened and the pore size is reduced by high energy ball milling. As a result, cryogenic ball milling increased the surface area and pore volume of multi-walled carbon nanotubes, and changed from macropore to mesopore to enhance hydrogen storage of the surface.

As described above, it was confirmed that the dispersion and hydrogen storage properties of the carbon nanotubes were improved by changing the milling speed and time of the carbon nanotube cryogenic ball milling. These results indicate that the cryogenic ball milling process, along with other additional refinements, is an effective way to improve the dispersibility of carbon nanotubes, and has great potential for applying carbon nanotubes to hydrogen storage media or new hybrid materials. It can be said that

As described above, according to the present invention, it is possible to manufacture carbon nanotubes having a significantly improved hydrogen storage capability compared to conventional carbon nanotubes, and expect higher effects by utilizing such carbon nanotubes as hydrogen storage media such as thermo batteries. Can be.

1 is an exemplary view showing a phenomenon occurring during cryogenic milling equipment (a) and milling that can be used in the present invention.
Figure 2 after sonication of the multi-walled carbon nanotubes prepared according to different milling speed [(a) untreated, (b) 300rpm, (c) 700rpm] and milling time (A: 2 hours, B: 6 hours) It is a photograph showing the degree of dispersion when 24 hours have elapsed.
3 is a scanning electron micrograph of multi-walled carbon nanotubes according to each treatment ((a) untreated, (b) 300 rpm 6 hours, (c) 700 rpm 6 hours).
4 is a transmission electron micrograph of the multi-walled carbon nanotubes according to each treatment ((a) untreated, (b) 300 rpm 6 hours, (c) 700 rpm 6 hours).
5 is an image showing a selective spatial diffraction pattern of multi-walled carbon nanotubes according to each treatment ((a) untreated, (b) 300 rpm 6 hours, (c) 700 rpm 6 hours).
6 is a graph showing an X-ray diffraction pattern of multi-walled carbon nanotubes according to each treatment.
7 is a graph showing Raman spectroscopy and intensity of multi-walled carbon nanotubes according to each treatment.
8 is a graph showing the BET specific surface area of multi-walled carbon nanotubes according to each treatment.
9 is a graph showing the volume distribution of pores having a diameter of the multi-walled carbon nanotubes of 200 nm or less according to each treatment.
10 is a graph showing hydrogen storage adsorption and desorption force of multi-walled carbon nanotubes according to each treatment.

Hereinafter, the present invention will be described in more detail with reference to Examples. These examples are only for illustrating the present invention, and the scope of the present invention is not to be construed as being limited by these examples.

Example 1.

In this example, multi-walled carbon nanotubes (MWCNTs) synthesized by catalytic chemical vapor deposition (CM-95, Hanwha Nanotech, Korea) were used. The diameter of the multi-walled carbon nanotubes used is 10 ~ 15nm, purity is more than 95%. Zirconia (ZrO ₂ ) balls (BNB Tech, Korea) having a diameter of 12.7 mm were used as the collision medium.

The multi-walled carbon nanotubes and the zirconia balls were placed in a cylindrical stainless steel chamber in an insulated box at a 1:10 weight ratio, and liquid nitrogen was continuously injected into the insulated box to maintain the temperature of the chamber at -180 ° C or lower. The milling speed applied to the stirrer was 300 rpm or 700 rpm, and the cryogenic ball milling was performed with the milling time of 2 hours or 6 hours.

After the milling process, the cryogenic milled multi-walled carbon nanotubes were dried in vacuo at 80 ° C. for 12 hours.

The apparatus and process used for cryogenic milling in this example are shown in FIG.

Experimental Example 1. Scanning electron microscopy

1 mg of cryogenic milled multi-walled carbon nanotubes or untreated multi-walled carbon nanotubes of Example 1 were each dispersed in 3 ml of ethanol, sonicated for 5 minutes, and then subjected to scanning electron microscope (SEM). ) Samples were prepared for observation. One drop of the prepared sample was placed on a silicon wafer (Si substrate) and observed with a scanning electron microscope, the results of which are shown in FIG. 3.

Experimental Example 2. Transmission Electron Microscopy

The same sample as Experimental Example 1 was placed on a 150 mesh copper grid (STEM150Cu, Okenshoji, Japan) coated with a carbon film, dried under vacuum at room temperature for 12 hours, and observed with a transmission electron microscope (TEM). The results are shown in FIG. 4.

Experimental Example 3 Selective Spatial Diffraction Analysis

Selective spatial diffraction analysis (SAD) was performed using the same sample and apparatus as Experimental Example 2, and the results are shown in FIG. 5.

Experimental Example 4 X-ray Diffraction Analysis

Samples were prepared by placing an appropriate amount of cryogenic milled multiwall carbon nanotubes or untreated multiwall carbon nanotubes of Example 1 between two slide glasses, and using an XRD analyzer (M18XHF-SRA, Mac Science, Japan). X-ray source: λ = 0.154 nm, 40 kV, 100mA was analyzed using the conditions, the results are shown in FIG.

Experimental Example 5. Raman spectroscopy

It was analyzed using a Raman spectrometer (NRS-3100, Jasco, Japan) under conditions of a laser exposure time of 10 seconds, power 10%, laser emission 250 mW, and laser wavelength 750 nm, and the results are shown in FIG. 7.

Experimental Example 6. BET Analysis

The cryogenic milled multi-walled carbon nanotubes or untreated multi-walled carbon nanotubes of Example 1 were charged to a BET analyzer (BELSORP-max, mini II, Bell-Japan), the pressure was raised to 100 kPa, and then gradually decreased. Thus, the amount of hydrogen molecules filled in the pores and the amount of hydrogen molecules released into the pores were measured. According to the results, specific surface area, pore volume, pore diameter, and hydrogen adsorption amount were analyzed and shown in FIGS. 8 to 10.

Claims (3)

Hydrogen storage medium consisting of carbon nanotubes produced by ball milling at a speed of 700rpm to 1000rpm at a temperature of -100 ℃ to -200 ℃. delete The hydrogen storage medium of claim 1, wherein the carbon nanotubes are multi-walled carbon nanotubes.

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