A preparation method of onion carbon based on metal-organic framework and the onion carbon
The preparation of onion carbon by a two-step pyrolysis method based on metal organic frameworks has solved the problems of complex equipment, high energy consumption and high safety risks in the prior art, and achieved low cost, efficient production and excellent catalytic activity.
Patent Information
- Application Number
- CN202411410973.2
- Authority / Receiving Office
- CN · China
- Patent Type
- Patents(China)
- Current Assignee / Owner
- Filing Date
- 2024-10-10
- Publication Date
- 2025-07-08
- Estimated Expiration
- 2044-10-10
AI Technical Summary
The prior art has problems such as special equipment, high energy consumption, high cost, low production efficiency and high safety risks when preparing onion carbon, making it difficult to realize a simple, low cost and efficient preparation method.
Using a two-step pyrolysis method based on metal organic frames, the metal organic frame precursor is used to carbonize at high temperature under an inert atmosphere, and the gas released through melamine heat interpretation is etched to form uniform onion carbon particles. The iron element is anchored on the curved surface in the form of a single atom, reducing the reaction energy barrier and improving catalytic activity.
The simplicity, low cost and high efficiency of the preparation process are achieved, and the prepared onion carbon has uniform size and high purity, is suitable for large-scale production, and shows excellent catalytic activity in the catalytic redox process.
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Figure CN119430159B_ABST
Abstract
Description
Technical Field
[0001] The present invention relates to the technical field of inorganic non-metallic materials, and particularly relates to a preparation method of onion carbon based on metal-organic frameworks and the onion carbon. Background Art
[0002] Onion carbon is a relatively young member of the family of nanomaterials, first discovered by Iijima in 1980. It is a spherical carbon nanoparticle. However, different from fullerenes, they are composed of multiple graphene-like carbon shells stacked concentrically instead of one. Depending on the synthesis method and processing parameters, the size of onion carbon is approximately between 2 - 50 nm. Due to its unique physical and chemical properties, such as high conductivity, high specific surface area, extremely small nanoparticle size, and high curvature and surface energy, it can be used to enhance the electrochemical performance of certain electrode materials in electrocatalysis, fuel cells, supercapacitors, and lithium-ion batteries.
[0003] Functionalizing onion carbon as a catalyst has great potential in catalytic oxygen reduction. Some studies have shown (Angew.Chem.Int.Ed.2024,e202319370) that a tip-shaped Fe-N4 structure designed on a curved carbon surface can be used for efficient oxygen reduction reaction. The tip structure can induce a strong local electric field and a dense interfacial water layer, thereby enhancing the kinetics of the proton-coupled electron transfer step. Moreover, the electronic structure modulation of the Fe center at the tip also promotes the dissociation of oxygen-containing intermediates during the oxygen reduction process. Similarly, introducing the Fe-N4 structure onto the curved surface of onion carbon also has the potential to induce a strong local electric field and an interfacial water layer to accelerate the catalytic kinetics of oxygen reduction.
[0004] So far, onion carbon has been synthesized by various techniques, such as arc discharge, electron beam irradiation, heat treatment of nanodiamond, mechanical grinding, carbon ion beam implantation, and detonation reaction, etc. However, when using methods such as arc discharge, electron beam irradiation, and carbon ion beam implantation for preparation, special instrument equipment is required, the energy consumption is high, and the production efficiency is low. The heat treatment of nanodiamond often requires a high-temperature and high-pressure environment, increasing the manufacturing cost. The number of carbon shell layers of onion carbon prepared by the mechanical grinding method is difficult to control, and moreover, the production efficiency is low through mechanical grinding. Preparing onion carbon by detonation reaction has great safety risks during the preparation process and requires strict safety measures and operating procedures.
[0005] Therefore, there is an urgent need to develop a preparation method of onion carbon that is simple, low-cost, and efficient. Summary of the Invention
[0006] To address the deficiencies of the existing technology, the decomposition of carbon precursors has characteristics such as low energy loss and simple operation. Therefore, the present invention proposes a method for preparing onion carbon based on the pyrolysis of metal-organic frameworks. The synthesized onion carbon material has uniform particles with a diameter of 5 - 20 nm and a carbon layer spacing of 0.34 nm. In addition, the introduction of iron elements enables them to be anchored on the curved surface of onion carbon in the form of single atoms. The curved structure of onion carbon can lower the reaction energy barrier during the catalytic oxygen reduction process, and there is more electron transfer to the adsorbed oxygen-containing intermediates, thereby enhancing the oxygen reduction catalytic activity.
[0007] The technical solution of the present invention is as follows:
[0008] A method for preparing onion carbon based on metal-organic frameworks, comprising the following steps:
[0009] S1. Synthesis of metal-organic framework precursors: Zinc salts and iron salts are used as metal sources and added to a reaction tube, and a reaction solvent is added to obtain a mixture. The above mixture is dispersed evenly by ultrasonic waves, and then the organic ligand is added dropwise to the reaction tube under stirring. After reacting for 20 - 24 h, a suspension of the target metal-organic framework is obtained. After centrifugation, washing with an organic solution, and drying, a metal-organic framework material is obtained.
[0010] S2. Preliminary pyrolysis of metal-organic framework precursors: The metal-organic framework material obtained in S1 is spread flat at the bottom of a porcelain boat. Under an inert atmosphere and normal pressure, it is heated from room temperature to 800 - 1000 °C at a heating rate of 3 - 8 °C / min and held for 2 - 3 h. After cooling to room temperature, a preliminary carbon material is obtained.
[0011] S3. Etching and decomposition of the formed metal particles: The carbon material obtained in S2 is dispersed in an acid solution under the action of ultrasonic waves. After soaking, the metal nanoparticles on the carbon material are dissolved; then the carbon material is centrifuged, washed, and dried to obtain a carbon material powder without metal particles.
[0012] S4. Secondary pyrolysis of the preliminary pyrolysis carbon material: The carbon material obtained in S3 and melamine are placed in a tube furnace and heated to 900 - 1000 °C at a heating rate of 3 - 8 °C / min and held for 2 - 3 h. After cooling to room temperature, a target carbon material powder with a rich onion carbon structure is obtained.
[0013] Preferably, in S1, the molar ratio of zinc salt to iron salt is (50 - 500):1.
[0014] Preferably, in S1, the molar ratio of zinc salt to iron salt is 150:1.
[0015] Preferably, the zinc salt is ZnCl2, and the iron salts are FeCl2·4H2O respectively.
[0016] Preferably, in S1, the reaction solvent is a mixed solution of N,N-dimethylformamide, absolute ethanol, deionized water, and ammonium hydroxide.
[0017] Preferably, in S1, the organic ligand is 1-H-1,2,3-triazole.
[0018] Preferably, in S1, the washing solvent is absolute ethanol, and the number of washing times is 3 to 6 times.
[0019] Preferably, in S1, the washing solvent is absolute ethanol, and the number of washing times is 3 times.
[0020] Preferably, in S1, the drying is vacuum drying, the drying temperature is 60 °C, and the drying time is 12 h.
[0021] Preferably, in S2, the inert atmosphere is nitrogen, the initial pressure is -1 to 0 MPa, preferably -0.5 MPa, the initial atmosphere is nitrogen, and the system is kept sealed during the heat treatment.
[0022] Preferably, in S2, the heating rate is 5 °C / min, the pyrolysis temperature is 1000 °C, and the holding time is 2 h.
[0023] Preferably, in S3, the acid solution is a 3 mol / L hydrochloric acid solution, and the acid leaching time is 12 to 24 h.
[0024] Preferably, in S3, the acid solution is a 3 mol / L hydrochloric acid solution, and the acid leaching time is 24 h.
[0025] Preferably, in S3, the washing solution is deionized water, and the number of washing times is 3 to 6 times, preferably 3 times.
[0026] Preferably, in S4, the mass ratio of melamine to the carbon material is (1 to 3):1.
[0027] Preferably, in S4, the mass ratio of melamine to the carbon material is 2:1.
[0028] Preferably, in S4, in the tubular furnace, the initial heat treatment pressure is -0.5 MPa, the initial heat treatment atmosphere is N2, and the system is kept sealed during the heat treatment.
[0029] Preferably, in S4, the heating rate is 5 °C / min, the pyrolysis temperature is 1000 °C, and the holding time is 2 h.
[0030] An onion carbon is prepared by the above preparation method; the onion carbon has a diameter of 5 to 20 nm and a carbon layer spacing of 0.34 nm.
[0031] The beneficial effects achieved by the present invention are:
[0032] 1. The present invention uses metal-organic framework (MOF) as a precursor and adopts a two-step pyrolysis method. The precursor is carbonized by high temperature, and the carbon material is etched again by ammonia gas and other gases released by the thermal decomposition of melamine. Under this action, graphene rearranges in the form of concentric spherical layers to form onion carbon particles with uniform size. The temperature and reaction time in the preparation process can be precisely controlled, the preparation process is simple, and the prepared onion carbon powder has high purity and uniformity, so it has the effect of large-scale production.
[0033] 2. The introduction of iron element in the present invention enables it to be anchored on the curved surface of onion carbon in the form of single atoms. The curved structure of onion carbon can reduce the reaction energy barrier during the catalytic oxygen reduction process, and there is more electron transfer to the adsorbed oxygen-containing intermediates, thereby improving the catalytic activity of oxygen reduction.
[0034] 3. The preparation process of the present invention has low cost, simple operation, controllable conditions, and uniform product size, and has significant advantages among many methods for synthesizing onion carbon.
[0035] 4. The onion carbon prepared by the present invention has a definite carbon layer spacing and a uniform size distribution. Brief Description of the Drawings
[0036] Figure 1 is a flow chart of the preparation method of the present invention;
[0037] Figure 2 is the preparation process of onion carbon obtained in Example 1 of the present invention;
[0038] Figure 3 is the X-ray diffraction pattern of onion carbon obtained in Examples 1-4 and Comparative Example 1 of the present invention;
[0039] Figure 4 is the N2 adsorption isotherm and pore size distribution diagram of Example 1 of the present invention;
[0040] Figure 5 is the X-ray photoelectron spectroscopy diagram of Example 1 of the present invention;
[0041] Figure 6 is the scanning electron microscope image of onion carbon obtained in Example 1 of the present invention;
[0042] Figure 7 is the scanning electron microscope image of onion carbon obtained in Example 1 of the present invention;
[0043] Figure 8 is the oxygen reduction polarization curve diagram of Example 1 and Comparative Example 1 of the present invention. Detailed Embodiments
[0044] To facilitate the understanding of the present invention by those skilled in the art, the following describes the specific embodiments of the present invention with reference to the accompanying drawings.
[0045] It should be noted that: unless otherwise specified, the experimental methods described in the examples are all conventional methods; the reagents and materials, unless otherwise specified, are all obtained from commercial sources.
[0046] As Figure 1 shown, the present invention provides a method for preparing onion carbon based on metal-organic frameworks, which specifically includes the following steps:
[0047] S1. Synthesis of metal-organic framework precursors;
[0048] S2. Preliminary pyrolysis of metal-organic framework precursors;
[0049] S3. Etching and decomposition of the formed metal particles;
[0050] S4. Secondary pyrolysis of the preliminary pyrolyzed carbon material.
[0051] Example 1
[0052] S1. Add 4.97 g of ZnCl2 and 48 mg of FeCl2·4H2O into a reaction tube, and then add a mixed reaction solvent composed of 50 mL of N,N-dimethylformamide, 50 mL of absolute ethanol, 75 mL of deionized water and 20 mL of ammonium hydroxide to obtain a mixture. Disperse the above mixture evenly by ultrasonic wave, and then dropwise add 6.26 mL of 1H-1,2,3-triazole into the reaction tube under stirring. After reacting for 24 h, obtain a suspension of Fe@MET-6. After centrifugation, wash it 3 times with absolute ethanol and then place it in a vacuum drying oven at 60 °C overnight to obtain Fe@MET-6 powder material.
[0053] S2. Spread the Fe@MET-6 powder obtained in S1 on the bottom of a porcelain boat, place it in a tube furnace, and under N2 atmosphere and normal pressure, increase the temperature from room temperature to 900 °C at a heating rate of 5 °C / min and hold for 2 h. After cooling to room temperature, obtain a preliminary carbon material.
[0054] S3. Disperse the carbon material obtained in S2 in 3 mol / L HCl solution under ultrasonic wave, soak it for 24 h to dissolve the metal nanoparticles on the carbon material; then centrifuge, wash it 3 times with deionized water and place it in a vacuum drying oven at 60 °C overnight to obtain carbon material powder without metal particles.
[0055] S4. Place 60 mg of the carbon material obtained in S3 and 120 mg of melamine at both ends of a ceramic boat, and raise the temperature to 1000 °C at a heating rate of 5 °C / min in a tube furnace with programmable temperature control, and hold for 2 h. Set the initial pressure to -0.5 MPa. Then cool to room temperature with the furnace. Obtain the target carbon material with a rich onion carbon structure (p-Fe SAC(150-1)).
[0056] Example 2
[0057] S1. Add 4.90 g of ZnCl2 and 143 mg of FeCl2·4H2O to a reaction tube, and then add a mixed reaction solvent composed of 50 mL of N,N-dimethylformamide, 50 mL of absolute ethanol, 75 mL of deionized water, and 20 mL of ammonium hydroxide to obtain a mixture. Disperse the above mixture evenly with ultrasonic waves, and then dropwise add 6.26 mL of 1H-1,2,3-triazole to the reaction tube under stirring. After reacting for 24 h, obtain a suspension of Fe@MET-6. After centrifugation, wash it 3 times with absolute ethanol and place it in a vacuum drying oven at 60 °C overnight to obtain the Fe@MET-6 powder material.
[0058] S2. Spread the Fe@MET-6 powder obtained in S1 on the bottom of a porcelain boat, place it in a tube furnace, and under N2 atmosphere and normal pressure, raise the temperature from room temperature to 900 °C at a heating rate of 5 °C / min and hold for 2 h. After cooling to room temperature, obtain the preliminary carbon material.
[0059] S3. Disperse the carbon material obtained in S2 in a 3 mol / L HCl solution under ultrasonic waves, soak for 24 h to dissolve the metal nanoparticles on the carbon material; then centrifuge, wash it 3 times with deionized water and place it in a vacuum drying oven at 60 °C overnight to obtain a carbon material powder without metal particles.
[0060] S4. Place 60 mg of the carbon material obtained in S3 and 120 mg of melamine at both ends of a ceramic boat, and raise the temperature to 1000 °C at a heating rate of 5 °C / min in a tube furnace with programmable temperature control, and hold for 2 h. Set the initial pressure to -0.5 MPa. Then cool to room temperature with the furnace. Obtain the target carbon material with a rich onion carbon structure (p-Fe SAC(50-1)).
[0061] Example 3
[0062] S1. Add 4.99 g of ZnCl2 and 15 mg of FeCl2·4H2O into a reaction tube, and then add a mixed reaction solvent composed of 50 mL of N,N-dimethylformamide, 50 mL of absolute ethanol, 75 mL of deionized water, and 20 mL of ammonium hydroxide to obtain a mixture. Disperse the above mixture evenly by ultrasonic wave. Subsequently, dropwise add 6.26 mL of 1H-1,2,3-triazole into the reaction tube under stirring. After reacting for 24 h, a suspension of Fe@MET-6 is obtained. After centrifugation, wash it three times with absolute ethanol, and then place it in a vacuum drying oven at 60 °C overnight to obtain the Fe@MET-6 powder material.
[0063] S2. Spread the Fe@MET-6 powder obtained in S1 evenly on the bottom of a porcelain boat, place it in a tube furnace, and under N2 atmosphere and normal pressure, increase the temperature from room temperature to 900 °C at a heating rate of 5 °C / min and hold for 2 h. After cooling to room temperature, a preliminary carbon material is obtained.
[0064] S3. Disperse the carbon material obtained in S2 in a 3 mol / L HCl solution under ultrasonic wave, soak for 24 h to dissolve the metal nanoparticles on the carbon material; then centrifuge, wash it three times with deionized water, and place it in a vacuum drying oven at 60 °C overnight to obtain a carbon material powder without metal particles.
[0065] S4. Place 60 mg of the carbon material obtained in S3 and 120 mg of melamine at both ends of a ceramic boat respectively. In a tube furnace with programmable temperature control, increase the temperature to 1000 °C at a heating rate of 5 °C / min and hold for 2 h, and set the initial pressure to -0.5 MPa. Then cool it to room temperature with the furnace. The target carbon material (p-Fe SAC(500-1)) with a rich onion carbon structure is obtained.
[0066] Example 4
[0067] S1. Add 4.95 g of ZnCl2 and 72 mg of FeCl2·4H2O into a reaction tube, and then add a mixed reaction solvent composed of 50 mL of N,N-dimethylformamide, 50 mL of absolute ethanol, 75 mL of deionized water, and 20 mL of ammonium hydroxide to obtain a mixture. Disperse the above mixture evenly by ultrasonic wave. Subsequently, dropwise add 6.26 mL of 1H-1,2,3-triazole into the reaction tube under stirring. After reacting for 24 h, a suspension of Fe@MET-6 is obtained. After centrifugation, wash it three times with absolute ethanol, and then place it in a vacuum drying oven at 60 °C overnight to obtain the Fe@MET-6 powder material.
[0068] S2. Spread the Fe@MET-6 powder obtained in S1 evenly on the bottom of a porcelain boat, place it in a tube furnace, and under N2 atmosphere and normal pressure, heat it from room temperature to 900 °C at a heating rate of 5 °C / min and hold for 2 h. After cooling to room temperature, a preliminary carbon material is obtained.
[0069] S3. Disperse the carbon material obtained in S2 in 3 mol / L HCl solution under the action of ultrasonic waves, soak for 24 h to dissolve the metal nanoparticles on the carbon material; then centrifuge, wash with deionized water 3 times, and place in a 60 °C vacuum drying oven overnight to obtain a carbon material powder without metal particles.
[0070] S4. Place 60 mg of the carbon material obtained in S3 and 120 mg of melamine at both ends of a ceramic boat respectively, and in a tube furnace with programmed temperature control, heat it to 1000 °C at a heating rate of 5 °C / min and hold for 2 h, with the initial pressure set at -0.5 MPa. Then cool it to room temperature with the furnace. The target carbon material (p-Fe SAC(100-1)) with a rich onion carbon structure is obtained.
[0071] Comparative Example 1
[0072] Compared with Examples 1-4, the main difference is that FeCl2·4H2O is not added during the synthesis of MOF.
[0073] S1. Add 5 g of ZnCl2 to a reaction tube, then add a mixed reaction solvent composed of 50 mL of N,N-dimethylformamide, 50 mL of absolute ethanol, 75 mL of deionized water and 20 mL of ammonium hydroxide to obtain a mixture. Disperse the above mixture evenly with ultrasonic waves, and then dropwise add 6.26 mL of 1H-1,2,3-triazole to the reaction tube under stirring. After reacting for 24 h, a suspension of MET-6 is obtained. After centrifugation, wash with absolute ethanol 3 times and place in a 60 °C vacuum drying oven overnight to obtain MET-6 powder material.
[0074] S2. Spread the MET-6 powder obtained in S1 evenly on the bottom of a porcelain boat, place it in a tube furnace, and under N2 atmosphere and normal pressure, heat it from room temperature to 900 °C at a heating rate of 5 °C / min and hold for 2 h. After cooling to room temperature, a preliminary carbon material is obtained.
[0075] S3. To ensure the consistency of experimental operations, disperse the carbon material obtained in step S2 in 3 mol / L HCl solution under the action of ultrasonic waves, soak for 24 h, then centrifuge, wash with deionized water three times and place in a 60 °C vacuum drying oven overnight.
[0076] S4, 60 mg of carbon material and 120 mg of melamine obtained in S3 were placed at both ends of a ceramic boat, and the temperature was raised to 1000°C at a heating rate of 5°C / min in a tube furnace with program-controlled temperature and kept at that temperature for 2 hours, with the initial pressure set to -0.5 MPa. The mixture was then cooled to room temperature with the furnace, and the carbon material powder (NC) of Comparative Example 1 was obtained.
[0077] Figure 2 This is the preparation process of Example 1 of the present invention. It can be seen that after two pyrolysis steps, the initial metal organic framework material (MOF) is transformed into a graphene material with a concentric carbon layer structure.
[0078] Figure 3 The X-ray diffraction patterns of Examples 1 to 4 of the present invention and Comparative Example 1 show broad peaks at 25° and 44°, corresponding to the (002) and (101) crystal planes of graphene, respectively. When the amount of Fe added is large, a phase related to Fe appears, and when the amount of Fe added is small, no phase related to Fe appears.
[0079] Figure 4 The N2 adsorption isotherm and pore size distribution diagram of Example 1 of the present invention show a typical Type I adsorption-desorption isotherm, proving the existence of abundant micropores, which will also be beneficial to material transport and promote oxygen reduction to a certain extent.
[0080] Figure 5 This is the X-ray photoelectron spectrum of Example 1 of the present invention. It can be seen that the carbon material only contains C, N, and O elements. Due to the low content of Fe element, its existence was not detected. In addition, the material does not contain other elements.
[0081] Figure 6 This is a scanning electron microscope image of Example 1 of the present invention, showing a spherical morphology formed by accumulation of small particles.
[0082] Figure 7 This is a transmission electron microscope image of Example 1 of the present invention, in which the presence of onion carbon can be clearly seen.
[0083] Figure 8 It is the oxygen reduction polarization curve diagram of Examples 1 to 5 of the present invention and Comparative Example 1. Example 1 also has excellent oxygen reduction catalytic performance due to the presence of abundant onion carbon.
[0084] In summary, the carbon material synthesized in the present invention has a specific onion-shaped carbon layer structure, a certain carbon layer spacing and a uniform size distribution. In addition, iron is added during the synthesis, so it also shows a certain potential in catalyzing oxygen reduction.
[0085] The embodiments of the present invention described above do not constitute a limitation to the protection scope of the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the claims of the present invention.
Claims
1. A preparation method of onion carbon based on metal-organic frameworks, characterized in that, It includes the following steps: S1. Add 4.90 g of ZnCl2 and 143 mg of FeCl2·4H2O into a reaction tube, and then add a mixed reaction solvent composed of 50 mL of N,N-dimethylformamide, 50 mL of absolute ethanol, 75 mL of deionized water and 20 mL of ammonium hydroxide to obtain a mixture. Disperse the above mixture evenly by ultrasonic wave. Subsequently, dropwise add 6.26 mL of 1H-1,2,3-triazole into the reaction tube under stirring. After reacting for 24 h, obtain a suspension of Fe@MET-6; after centrifugation, wash it 3 times with absolute ethanol and then place it in a vacuum drying oven at 60 °C overnight to obtain a Fe@MET-6 powder material; S2. Spread the Fe@MET-6 powder obtained in S1 on the bottom of a porcelain boat, place it in a tube furnace, and under N2 atmosphere and normal pressure, increase the temperature from room temperature to 900 °C at a heating rate of 5 °C / min and hold for 2 h. After cooling to room temperature, obtain a preliminary carbon material; S3. Disperse the carbon material obtained in S2 in 3 mol / L HCl solution under ultrasonic wave, soak for 24 h to dissolve the metal nanoparticles on the carbon material; then centrifuge, wash it 3 times with deionized water and then place it in a vacuum drying oven at 60 °C overnight to obtain a carbon material powder without metal particles; S4. Place 60 mg of the carbon material obtained in S3 and 120 mg of melamine at both ends of a ceramic boat respectively, and increase the temperature to 1000 °C at a heating rate of 5 °C / min in a tube furnace with program-controlled temperature and hold for 2 h. Set the initial pressure to -0.5 MPa; then cool to room temperature with the furnace; obtain the target carbon material with a rich onion carbon structure; Or, S1. Add 4.95 g of ZnCl2 and 72 mg of FeCl2·4H2O into a reaction tube, and then add a mixed reaction solvent composed of 50 mL of N,N-dimethylformamide, 50 mL of absolute ethanol, 75 mL of deionized water and 20 mL of ammonium hydroxide to obtain a mixture. Disperse the above mixture evenly by ultrasonic wave. Subsequently, dropwise add 6.26 mL of 1H-1,2,3-triazole into the reaction tube under stirring. After reacting for 24 h, obtain a suspension of Fe@MET-6; after centrifugation, wash it 3 times with absolute ethanol and then place it in a vacuum drying oven at 60 °C overnight to obtain a Fe@MET-6 powder material; S2. Spread the Fe@MET-6 powder obtained in S1 on the bottom of a porcelain boat, place it in a tube furnace, and under N2 atmosphere and normal pressure, increase the temperature from room temperature to 900 °C at a heating rate of 5 °C / min and hold for 2 h. After cooling to room temperature, obtain a preliminary carbon material; S3. Disperse the carbon material obtained in S2 in 3 mol / L HCl solution under ultrasonic wave, soak for 24 h to dissolve the metal nanoparticles on the carbon material; then centrifuge, wash it 3 times with deionized water and then place it in a vacuum drying oven at 60 °C overnight to obtain a carbon material powder without metal particles S4. Place the 60 mg of carbon material obtained in S3 and 120 mg of melamine at both ends of a ceramic boat respectively, and raise the temperature to 1000 °C at a heating rate of 5 °C / min in a tube furnace with temperature-programmable control, and keep it at this temperature for 2 h. Set the initial pressure to -0.5 MPa; then cool it down to room temperature with the furnace; the target carbon material with a rich onion carbon structure is obtained.
2. An onion carbon, characterized in that: Prepared by the preparation method according to Claim 1; the onion carbon has a diameter of 5 - 20 nm and a carbon layer spacing of 0.34 nm.