Production of single-walled carbon nanotubes by a hydrogen arc discharge method

[0002] The present invention relates to a large-scale hydrogen arc discharge method for the synthesis of single-walled carbon nanotubes (“SWNTs”). Under an atmosphere of H₂, a large-size anode composed of graphite powders, catalyst metals and growth promoter is consumed with the growth of SWNTs. The angle between the anode and cathode can be adjusted in the range of 30°˜80°. The catalysts used can be two or more kinds of metals selected from Fe, Co, Ni and Y, which have proven to be essential for the growth of SWNTs. The proportion of catalysts is modified in the range of 2.5˜5.0 at %. By these processes, large-scale SWNTs with a large diameter can be synthesized with the advantages of semi-continuous synthesis, low cost and high quality, and the SWNTs obtained can then be employed as a kind of promising hydrogen storage material.

[0003] To eliminate the gasoline shortage and environmental pollution produced by automobiles, various technologies are being developed to replace the gasoline-powered internal combustion engine. Among the alternative fuel technologies, the hydrogen fuel cell is believed to be ideal because of its advantages of renewable and non-pollution performance. For the purpose of identifying the practicability of the fuel cell powered vehicle, the United States Department of Energy (“D.O.E.”) has issued energy density goals for vehicular hydrogen storage: 6.5 wt % H₂or 62 kg H₂/m³, which indicates that the identified fuel cell can provide a 350 mile range driving in a vehicle.

[0004] Compared to all the other hydrogen storage systems, such as liquid hydrogen systems, compressed hydrogen systems, metal hydride systems and super active carbon systems, carbon nanotube systems, especially single-walled carbon nanotubes (SWNTs) are expected to be utmost close to the D.O.E. energy density goals and can be made into an ideal hydrogen storage system, which needs to be light, compact, relatively inexpensive, safe, easy to use and reusable without the need for regeneration.

[0005] The most widely used method to produce SWNTs is the electric arc discharge method, which was first employed by S. Iijima and Bethune in 1993. However, the purity of the products they obtained was very poor, so that the characterization and utilization of SWNTs were limited to a great extent. In 1997, C. Journet et al obtained SWNTs with higher yield and purity through the optimization of synthesis parameters. The main characteristics of their method include: use of He at 660 torr as a buffer gas, use of two graphite rods as electrodes, and the size of the electrodes were: anode—Φ 16×40 mm, cathode—Φ 16×100 mm, and a Φ 3.5×40 mm pore was drilled along with the axis of the anode to fill with a mixture of graphite powders and catalysts (Y, Ni). The anode and the cathode were perpendicular with each other. The morphologies of the products can be classified as follows: (1) Rubbery soot formed on the internal wall of the reactor; (2) Web-like substance between the cathode and reactor wall; (3) Cylinder deposit on the tip of the cathode; and (4) Porous, light, hoop-like substance formed around the cylinder deposit. From then on, no remarkable progress happened with respect to the electric arc discharge method. Although the obtained SWNTs have nearly defect-free microstructure, the traditional electric arc discharge method is difficult to apply to commercial or large-scale synthesis because of its shortcomings: i.e., the yield is limited by the size of electrodes and the reactor; the producing process is un-continuous; and the content of impurities such as amorphous carbon is high.

[0006] Since SWNTs have aroused a great deal of interest from the fundamental viewpoint as well as for potential applications, enormous high-quality SWNTs are demanded for both research and commercial purposes. Consequently, there is an urgent need for a new method which can produce SWNTs in a semi-continuous or continuous synthesis in large-scale, high quality and low cost.

[0007] In accordance with the present invention, there is a hydrogen arc discharge method provided for the production of SWNTs with high hydrogen storage capacity. The main characteristics or parameters of the method include:

[0008] (a) Large-scale SWNTs are produced with: (i) graphite powders, (ii) catalyst metals selected from Fe, Co, Ni, Y, (iii) sulfur or solid sulfide as a growth promoter, and (iv) H₂at a pressure of 50˜400 torr as a buffer gas. After being evenly dispersed, the reactants are filled into holes provided on the upper surface of a large anode or mold-pressed into targets as the anode, and the anode will be consumed with the growth of SWNTs under the atmosphere of H₂; and

[0009] (b) The obtained SWNTs are (i) soaked in acids or oxidative reactants, (ii) heated in vacuum, or (iii) through a combined pre-treatment of (i) and (ii), to prepare high-capacity hydrogen storage materials.

[0010] In preferred embodiments of the present invention, the diameter of the anode is about 10˜20 times larger than that of the cathode, so that the synthesis time is enlonged to produce large-scale SWNTs.

[0011] In other preferred embodiments of the present invention, the angle between the anode and the cathode can be adjusted from 30° to 80°. By changing the angle, the quality and morphology of the products can be changed.

[0012] In still other preferred embodiments of the present invention, argon (no more than 20 vol %) can be mixed into the H₂as a buffer gas.

[0013] In yet other preferred embodiments of the present invention, the pre-treatment techniques for the SWNTs may include: (a) soaking in (i) HNO₃acid (20˜65%), (ii) HCl acid (10˜37%) or (iii) oxidative reactants, (b) vacuum heating at 400˜900° C. or (c) the combination of (a) and (b).

[0014] FIG. 1 is a schematic diagram of the apparatus used for synthesizing SWNTs in large scale by a semi-continuous hydrogen arc discharge method (in this apparatus, several holes are drilled on the upper surface of the big cylinder anode to fill in with reactants);

[0015] FIG. 2 is a schematic diagram of another apparatus used for sythesizing SWNTs in large scale by a semi-continuous hydrogen arc discharge method (in this apparatus, the reactants are pressed into cylinder target as part of the anode directly);

[0016] FIG. 3 is an SEM image of the SWNTs prepared by the hydrogen arc discharge method;

[0017] FIG. 4 is a TEM image of the SWNTs prepared by the hydrogen arc discharge method;

[0018] FIG. 5 is a Raman spectrum of the SWNTs prepared by the hydrogen arc discharge method; and

[0019] FIG. 6 is a schematic diagram of the apparatus used for testing the hydrogen storage capacity of SWNTs.

[0020] In a stainless shell, copper-lined large-size reactor, an electric arc discharge is generated between the electrodes under an atmosphere of H₂or a mixture of H₂and Ar. The anode is a graphite cylinder (Φ400×35 mm) on whose upper surface several holes (Φ10×15 mm) are drilled to fill in with the reactants, or a reactants cylinder target (Φ400×35 mm) is prepared by model-pressing. The anode is then placed onto a rotatable disc. The cathode is a graphite rod (Φ10 mm) that can be moved toward the anode such that a distance of about 2 mm remains between them. Typically, the anode diameter is larger than the cathode diameter, wherein the ratio of the anode diameter to the cathode diameter is 10˜20.

[0021] The reactants are weighed individually and then evenly dispersed in an alcohol solution. After having been dried, the reactants are filled into the holes on the upper surface of the anode, or the reactants are model-pressed into a cylinder target to be used directly as the anode (e.g., the reactants may be model-pressed under a pressure of ˜10 MPa).

[0022] Under an atmosphere of H₂, a current (100˜150 A, DC) is passed through the electrodes to create a plasma. By the effect of high temperature, the reactants are co-evaporated to synthesize SWNTs. With the co-evaporation of the reactants, the cathode is continuously translated to keep a constant distance between the electrodes. After the reactants in one hole (or one part of the target) are consumed, the synthesis process is stopped, and the disc under the anode is rotated properly to sublimate the reactants in the next hole (or the next part of the target). At the same time, taking into account the consumption of H₂, the buffer gas is pumped out and re-filled in the interval. The typical reaction time is 3 minutes.

[0023] Pure hydrogen gas (H₂) may be used in this process. In some processes, however, Ar (no more than 20 vol %) is incorporated with the H₂to be employed as the buffer gas.

[0024] In order to improve the yield and quality of the products, as compared to the traditional electric arc discharge method, some innovation to the apparatus and optimization to the reactants are employed in the present invention.

[0025] Compared with traditional buffer gas—He, the price of H₂is much cheaper, so that the synthesis cost can be lowered by the usage of H₂. At the same time, some intrinsic characteristics of hydrogen are beneficial to the growth of SWNTs: high conductivity is helpful to cool down the plasma rapidly and create a high temperature gradient to improve the growth of SWNTs; hydrogen can selectively corrode amorphous carbon impurities and thereby improve the purity of the products; hydrogen can promote the vaporization of metal catalysts and prevent the catalysts from being poisoned.

[0026] In order to prepare SWNTs at high yield, the synthesis apparatus is also innovated. The reactor is cylindrical shaped (at least Φ600×400 mm), and the electrodes are not vertically opposite but make an oblique angle (30°˜80°), which are beneficial to preparing SWNTs in large scale. In our comparative experiments, no evident differences in the yield of products were observed when the angle was adjusted (as an instance, see examples 1 and 2), but a suitable angle can accelerate the flowing of the plasma and then is considered to be helpful to synthesize SWNTs in large scale. In most examples of the present invention, the angle between the anode and cathode is selected to be 50°.

[0027] Moreover, the reactants were mixed evenly and filled into the holes on the upper surface of the anode cylinder to co-evaporate graphite powders and catalysts more homogeneously. The rotatable anode makes this synthesis process a semi-continuous or continuous one. The system is more convenient to operate when the reactants are compressed into the shape of a cylinder and then used as a raw material target.

[0028] The application of a sulfur-containing promoter is essential to the present invention (e.g., in an amount of 0.5 to 1 atom percent). Without the sulfur-containing prompter, SWNTs are barely obtained. When a proper sulfur-containing promoter is employed, this invention is capable of producing large-scale, high quality SWNTs.

[0029] There are mainly three kinds of products obtained: (a) web-like substances; (b) aligned SWNT ropes that hang between the cathode and the upper chamber wall; and (c) thin films formed on the upper chamber wall, which can be peeled off in large slices (˜200 cm²), and are self-fluttering, self-adhering to hands and vessels. The existing change in the form of the products may be attributed to the different opposite orientations of the electrodes, which lead to different ways that the plasma flows.

[0030] To obtain a high-capacity hydrogen storage material, the as-prepared SWNTs are soaked in acids such as HNO₃(20˜65%) or HCl (10˜37%) and then heat-treated in vacuum for 2˜4 hours. The soaking may last for 24 to 48 hours, and the heat treatment may be at a temperature from 400° C. to 1200° C. The results of hydrogen absorption experiments based on the pretreated SWNTs indicate that the hydrogen storage capacity of the SWNTs is higher than 3% by weight, and typically about 3˜5 wt % at moderate high pressure and room temperature, and the hydrogen storage capacity of the SWNTs can be further improved if the quality is improved and the pre-treatment parameters are optimized.

[0031] In brief, in this invention, hydrogen, rather than helium, was selected as the buffer gas, so that high-quality SWNTs can be synthesized at low cost. The synthesis apparatus is innovated to realize a semi-continuous synthesis process, and the yield can be high. Because the diameter of the SWNTs obtained is large (mean value: 1.85 nm), they can uptake a large amount of hydrogen at room temperature. The hydrogen storage capacity of SWNTs is further improved by acid soaking and heat treatment, and the hydrogen storage capacity is measured to be 3˜5 wt %. Through this hydrogen arc discharge method, SWNTs can be prepared with the advantages of semi continuity, low cost, large scale and high quality. The processes of acid soaking and heat treatment give high hydrogen storage capacity of SWNTs, which indicates that SWNTs are a promising candidate for hydrogen storage.

[0032] The present invention will be illustrated in more detail with reference to the following examples, which should not be construed to be limiting in scope of the present invention.

[0033] The apparatus used is shown in FIG. 1.

[0034] 2.0 g of reactants containing 2.6 at % Ni, 0.7 at % Fe, 0.7 at % Co, 0.75 at % FeS and 95.25 at % graphite powder were filled into a hole on the upper surface of the anode cylinder. The angle between the anode and the cathode was modified to 40°. The electric arc was operated under an atmosphere of H₂using 150 A, dc. The distance between the electrodes remained ˜2 mm, and the reaction time was typically 3 minutes. The product obtained was a rope-like, film-like and web-like substance with a total weight of 185 mg.

[0035] The apparatus used is shown in FIG. 1.

[0036] 2.0 g of reactants containing 2.6 at % Ni, 0.7 at % Fe, 0.7 at % Co, 0.75 at % FeS and 95.25 at % graphite powder were filled into a hole on the upper surface of the anode cylinder. The angle between the anode and the cathode was modified to 70°. The electric arc was operated under an atmosphere of H₂using 150 A, dc. The distance between the electrodes remained ˜2 mm, and the reaction time was typically 3 minutes. The product obtained was a rope-like, film-like and web-like substance with a total weight of 180 mg.

[0037] The apparatus used is as shown in FIG. 1.

[0038] 2.0 g of reactants containing 2.6 at % Ni, 1.0 at % Co, 0.75 at % FeS and 95.65 at % graphite powder were filled into a hole on the upper surface of the anode cylinder. The angle between the anode and the cathode was modified to 50°. The electric arc was operated under an atmosphere of H₂using 150 A, dc. The distance between the electrodes remained ˜2 mm, and the reaction time was typically 3 minutes. The product obtained was a rope-like, film-like and web-like substance with a total weight of 165 mg.

[0039] The apparatus used is shown in FIG. 1.

[0040] 2.0 g of reactants containing 2.6 at % Ni, 1.0 at % Y, 0.75 at % FeS and 95.65 at % graphite powder were filled into a hole on the upper surface of the anode cylinder. The angle between the anode and the cathode was modified to 50°. The electric arc was operated under an atmosphere of H₂using 150 A, dc. The distance between the electrodes remained ˜2 mm, and the reaction time was typically 3 minutes. The product obtained was a rope-like, film-like and web-like substance with a total weight of 175 mg.

[0041] The apparatus used is shown in FIG. 1.

[0042] 2.0 g of reactants containing 2.6 at % Ni, 1.0 at % Fe, 0.75 at % FeS and 95.65 at % graphite powder were filled into a hole on the upper surface of the anode cylinder. The angle between the anode and the cathode was modified to 50°. The electric arc was operated under an atmosphere of H₂using 150 A, dc. The distance between the electrodes remained ˜2 mm, and the reaction time was typically 3 minutes. The product obtained was a rope-like, film-like and web-like substance with a total weight of 160 mg.

[0043] The apparatus used is shown in FIG. 1.

[0044] 2.0 g of reactants containing 2.6 at % Ni, 0.7 at % Fe, 0.7 at % Co, 1.0 at % S and 95 at % graphite powder were filled into a hole on the upper surface of the anode cylinder. The angle between the anode and the cathode was modified to 50°. The electric arc was operated under an atmosphere of H₂using 150 A, dc. The distance between the electrodes remained ˜2 mm, and the reaction time was typically 3 minutes. The product obtained was a rope-like, film-like and web-like substance with a total weight of 170 mg.

[0045] The apparatus used is shown in FIG. 1.

[0046] 2.0 g of reactants containing 2.6 at % Ni, 0.7 at % Fe, 0.7 at % Co, 0.75 at % FeS and 94.95 at % graphite powder were filled into a hole on the upper surface of the anode cylinder. The angle between the anode and the cathode was modified to 50°. The electric arc was operated under a mixed atmosphere of H₂(200 torr) and Ar (50 torr) using 150 A, dc. The distance between the electrodes remained ˜2 mm, and the reaction time was typically 3 minutes. The product obtained was a rope-like, film-like and web-like substance with a total weight of 170 mg.

[0047] The apparatus used is shown in FIG. 1.

[0048] 2.0 g of reactants containing 2.6 at % Ni, 0.7 at % Fe, 0.7 at % Co, 0.75 at % FeS and 95.25 at % graphite powder were filled into a hole on the upper surface of the anode cylinder. The angle between the anode and the cathode was modified to 50°. The electric arc was operated under an atmosphere of H₂using 150 A, dc. The distance between the electrodes remained ˜2 mm, and the reaction time was typically 3 minutes. The product obtained was a rope-like, film-like and web-like substance with a total weight of 175 mg.

[0049] The apparatus used is shown in FIG. 1.

[0050] 2.0 g of reactants containing 2.6 at % Ni, 0.7 at % Fe, 0.7 at % Co, 0.75 at % FeS and 95.25 at % graphite powder were filled into a hole on the upper surface of the anode cylinder. The angle between the anode and the cathode was modified to 50°. The electric arc was operated under a mixed atmosphere of H₂(200 torr) and Ar (50 torr) using 150 A, dc. The distance between the electrodes remained ˜2 mm, and the reaction time was typically 3 minutes. The product obtained was a rope-like, film-like and web-like substance with a total weight of 167 mg.

[0051] The apparatus used is shown in FIG. 1.

[0052] Totally, 10 g of reactants were filled in five holes on the upper surface of the anode cylinder, such that 2.0 g of reactants were filled into each hole with the proportion of 2.6 at % Ni, 0.7 at % Fe, 0.7 at % Co, 0.75 at % FeS and 95.25 at % graphite powder. The angle between the anode and the cathode was modified to 50°. The electric arc was operated under an atmosphere of H₂using 150 A, dc. The distance between the electrodes remained ˜2 mm. The reaction time was typically 3 minutes for each hole, and then the buffer gas was changed to discharge the reactant in the next hole. The total reaction time was 30 minutes. The product obtained was a rope-like, film-like and web-like substance with a total weight of 940 mg.

[0053] The apparatus used is shown in FIG. 2.

[0054] A homogeneous mixture of 2.6 at % Ni, 0.7 at % Fe, 0.7 at % Co, 0.75 at % FeS and 95.25 at % graphite powder was model-pressed into a cylinder shape to act as the anode. The angle between the anode and the cathode was modified to 50°. The electric arc was operated under an atmosphere of H₂using 150 A, dc. The distance between electrodes remained ˜2 mm. The reaction time was typically 3 minutes and then the buffer gas was changed and the anode was rotated to discharge the reactant in another part of the anode cylinder. The total reaction time was 30 minutes. The product obtained was a rope-like, film-like and web-like substance with a total weight of 980 mg.

[0055] The apparatus used is shown schematically in FIG. 6.

[0056] The as-prepared SWNTs of ˜600 mg were soaked in 37% HCl acid for 48 hours, rinsed by de-ionized water and then dried in an oven. After being weighed accurately, the sample was put into the sample cell as shown in FIG. 6 to undergo vacuum for 5 hours under the condition of 150° C. Then the hydrogen storage capacity of SWNTs was measured at moderate high pressure (100˜120 atm) and room temperature. The hydrogen storage capacity of the SWNTs tested was determined to be 4.02 wt %.

[0057] The apparatus used is shown schematically in FIG. 6.

[0058] The as-prepared SWNTs of ˜600 mg were soaked in 37% HCl acid for 48 hours, rinsed by de-ionized water and then dried in an oven. The sample obtained was further vacuum heat-treated at 500° C. for 2 hours. After being weighed accurately, the sample was put into the sample cell as shown in FIG. 6 to undergo vacuum for 5 hours under the condition of 150° C. Then the hydrogen storage capacity of the SWNTs was measured at moderate high pressure (100˜120 atm) and room temperature. The hydrogen storage capacity of the SWNTs tested was determined to be 4.58 wt %.

[0059] The apparatus used is shown schematically in FIG. 6.

[0060] The as-prepared SWNTs of ˜600 mg were vacuum heat-treated at 400° C. for 2 hours. After being weighed accurately, the sample was put into the sample cell as shown in FIG. 6 to undergo vacuum for 5 hours under the condition of 150° C. Then the hydrogen storage capacity of the SWNTs was measured at moderate high pressure (100˜120 atm) and room temperature. The hydrogen storage capacity of the SWNTs tested was determined to be 4.24 wt %.