Preparation method of nitrogen-doped activated carbon with hierarchical porous structure

Abstract

A preparation method of nitrogen-doped activated carbon with a hierarchical porous structure belongs to the field of activated carbon production, and comprises the following steps: step 1, adding a carbon source substance, a pore forming agent and a nitrogen source substance into deionized water, and uniformly mixing; 2, freezing the mixed material obtained in the step 1, and then freeze-drying the frozen mixed material; 3, carrying out primary calcination treatment on the material obtained after freeze drying treatment in the step 2; step 4, carrying out plasma treatment on the material obtained after the primary calcination treatment in the step 3; 5, mixing the material obtained by the plasma activation treatment in the step 4 with molten salt, and then carrying out secondary calcination treatment; and 6, removing molten salt from the material obtained by secondary calcination treatment in the step 5, and drying to obtain the nitrogen-doped activated carbon with the graded porous structure. According to the invention, precise construction of a hierarchical porous structure and efficient doping of nitrogen are realized.

Description

Technical Field

[0001] This invention belongs to the field of activated carbon production, specifically a method for preparing nitrogen-doped activated carbon with a hierarchical porous structure. Background Technology

[0002] Activated carbon, as an important porous carbon material, is widely used in many fields such as gas adsorption, water treatment, catalyst support, and energy storage due to its well-developed pore structure, large specific surface area, and excellent chemical stability.

[0003] With the continuous improvement of application demands, the single-pore structure of traditional activated carbon is no longer sufficient to meet the performance requirements of complex scenarios. Developing activated carbon materials with hierarchical porous structures (i.e., simultaneously containing micropores, mesopores, and macropores) has become a current research hotspot. Hierarchical porous structures can effectively meet the transport and storage needs of molecules of different sizes. Micropores provide high specific surface area to enhance adsorption capacity, while mesopores and macropores help shorten diffusion paths and improve mass transfer efficiency. Nitrogen doping is another effective means to improve the performance of activated carbon. By introducing nitrogen atoms into the carbon framework, the surface chemical properties of the material can be changed, increasing the surface polar sites and improving the affinity for polar molecules or ions. At the same time, nitrogen doping can also improve the electronic conductivity and electrochemical activity of the material, making it show broad application prospects in energy fields such as supercapacitors, fuel cells, and lithium-ion batteries. Activated carbon materials that combine hierarchical porous structures and nitrogen doping characteristics have important significance and practical value. Therefore, there is an urgent need for a high-performance activated carbon material that combines hierarchical porous structures and nitrogen doping characteristics. Summary of the Invention

[0004] This invention provides a method for preparing nitrogen-doped activated carbon with a hierarchical porous structure, thereby overcoming the deficiencies in the prior art.

[0005] This invention is achieved through the following technical solution: A method for preparing nitrogen-doped activated carbon with a hierarchical porous structure includes the following steps: Step 1: Add the carbon source, pore-forming agent, and nitrogen source to deionized water and mix thoroughly; Step 2: Freeze the mixture obtained in Step 1, and then freeze-dry it. Step 3: The material obtained after freeze-drying in Step 2 is subjected to a calcination process. Step 4: Perform plasma treatment on the material obtained after the first calcination treatment in Step 3; Step 5: Mix the material obtained from the plasma activation treatment in Step 4 with molten salt and then perform a second calcination treatment; Step Six: Remove the material obtained from the secondary calcination treatment in Step Five with molten salt and dry it to obtain nitrogen-doped activated carbon with a hierarchical porous structure.

[0006] The method for preparing nitrogen-doped activated carbon with a hierarchical porous structure as described above, wherein the carbon source in step one is any one or any two or more of lignin, cellulose, anthracite, chitosan and pitch mixed in any proportion; The pore-forming agent is any one or a mixture of any two or more of the following: nano-sized polystyrene microspheres, micron-sized polymethyl methacrylate microspheres, and nano-sized polyethylene glycol microspheres, in any proportion. The nitrogen source is any one or a mixture of any two or more of melamine, dicyandiamide, and urea in any proportion.

[0007] The preparation method of nitrogen-doped activated carbon with a hierarchical porous structure as described above, the specific operation of step one is as follows: the carbon source material and the nitrogen source material are crushed and passed through a 200-mesh sieve for later use; the crushed carbon source material, nitrogen source material and pore-forming agent are added to deionized water with a mass of 1.2 to 1.5 times their total mass and sent to a high-speed shear machine and stirred at a speed of 1800 to 1900 r / min for 20 to 30 min, and then taken out for later use. The mass ratio of the added carbon source material, nitrogen source material and pore-forming agent is 1:0.4 to 0.6:0.3 to 0.4.

[0008] The preparation method of nitrogen-doped activated carbon with hierarchical porous structure as described above, the specific operation of the freezing treatment in step two is as follows: the mixture obtained in step one is placed in a freeze dryer, and the temperature is reduced to -40 to -50°C at a cooling rate of 3 to 5°C / min, and frozen for 12 to 14 hours.

[0009] As described above, in the method for preparing nitrogen-doped activated carbon with a hierarchical porous structure, the freeze-drying process in step two is as follows: after the freezing operation is completed, the temperature is further reduced to -70 to -80°C at a cooling rate of 2 to 3°C / min, the vacuum degree is reduced to 30 to 50 Pa, and the freeze-drying process is carried out for 16 to 20 hours. Then, the temperature is raised to room temperature and restored to normal pressure before being taken out for use.

[0010] The preparation method of nitrogen-doped activated carbon with a hierarchical porous structure as described above, the specific operation of the first calcination treatment in step three is as follows: the material obtained after freeze-drying in step two is sent into a sealed calcination furnace and heated to 200-210°C at a heating rate of 10-15°C / min, and maintained at this temperature for 2-3 hours. Then, the temperature is raised to 380-400°C at a heating rate of 6-10°C / min and maintained at this temperature for 6-8 hours. Nitrogen gas is introduced for protection throughout the heating process. After the first calcination is completed, the material is cooled to room temperature and taken out for use.

[0011] The preparation method of nitrogen-doped activated carbon with a hierarchical porous structure as described above, the specific operation of the plasma treatment in step four is as follows: the material obtained after the first calcination treatment in step three is crushed and passed through a 200-mesh sieve and then fed into a plasma surface treatment machine. The radio frequency power of the plasma surface treatment machine is 1200-1500W, the volume of the plasma surface treatment machine is 100L, the weight of the material obtained after the first calcination treatment in step three is 20-30kg, the gas introduced into the plasma surface treatment machine is a mixed gas obtained by mixing ammonia and argon in a volume ratio of 1:1, the flow rate of the mixed gas is 30-40L / min, and the plasma treatment time is 30-40min.

[0012] The method for preparing nitrogen-doped activated carbon with a hierarchical porous structure as described above, wherein the molten salt is a mixture of potassium fluoride and anhydrous borax in a mass ratio of 1:1 to 2.

[0013] The preparation method of nitrogen-doped activated carbon with a hierarchical porous structure as described above, the specific operation of the secondary calcination treatment in step five is as follows: the material obtained from the plasma activation treatment in step four is mixed with molten salt particles at a mass ratio of 1:4 to 6. After mixing, the mixture is pulverized and passed through a 300-mesh sieve for later use. Then, it is added to a calcination furnace and heated to 280 to 290°C at a heating rate of 10 to 15°C / min. The temperature is maintained at this temperature for 20 to 30 minutes. Then, the temperature is raised to 520 to 560°C at a heating rate of 6 to 10°C / min. The temperature is maintained at this temperature for 3 to 4 hours. After cooling to room temperature, it is taken out for later use.

[0014] The preparation method of nitrogen-doped activated carbon with a hierarchical porous structure as described above, the specific operation of step six is ​​as follows: the material obtained from the secondary calcination treatment in step five is crushed and passed through a 200-mesh sieve, and then added to deionized water with a mass of 5 to 8 times the total mass of the material. The mixture is stirred at a temperature of 60 to 70°C and a rotation speed of 100 to 150 r / min for 15 to 20 minutes, and then filtered while hot. The above operation is repeated 2 to 4 times. Then it is sent to a drying oven and dried at a temperature of 60 to 80°C for 2 to 3 hours. After cooling to room temperature, the nitrogen-doped activated carbon with a hierarchical porous structure is obtained.

[0015] The advantages of this invention are as follows: This invention organically combines pre-freezing followed by freeze-drying, primary calcination, plasma treatment, and secondary calcination to achieve precise construction of a hierarchical porous structure and efficient nitrogen doping. Pre-freezing followed by freeze-drying allows for the initial formation of uniformly distributed ice crystals within the material, followed by sublimation through freeze-drying, leaving pores and laying the foundation for the subsequent formation of a hierarchical porous structure. Primary calcination, under nitrogen protection, completes the initial carbonization of the carbon source material, decomposing the pore-forming agent to form a preliminary pore structure, while simultaneously incorporating nitrogen from the nitrogen source material into the carbon skeleton. Plasma treatment uses a mixture of ammonia and argon. Argon maintains the stability of the plasma discharge and provides a high-energy particle bombardment effect, promoting etching and activation of the carbon material surface. Ammonia decomposes in the high-energy plasma environment to produce nitrogen atoms, amino radicals, and other particles. These particles can efficiently embed into defect sites and edge positions of the carbon skeleton, achieving deep nitrogen doping. Compared to the traditional high-temperature ammonia activation method, plasma treatment has advantages such as lower temperature and more efficient processing. The invention boasts advantages such as short processing time, high nitrogen doping efficiency, and uniform distribution, effectively avoiding thermal desorption of nitrogen atoms under high-temperature conditions and improving the stability and controllability of nitrogen doping. During the secondary calcination process, the molten salt system forms a eutectic mixture at high temperature, creating a liquid phase environment that promotes further graphitization and pore development of the carbon material. Potassium fluoride decomposes during calcination to produce gases such as hydrogen fluoride, which chemically etch the carbon matrix. This, combined with the melt penetration effect of anhydrous borax, precisely widens mesopores and connects micropores, forming a hierarchical interconnected multi-level pore network. At the same time, the high-temperature liquid phase environment of the molten salt helps to consolidate the nitrogen-doped structure introduced in the plasma treatment stage, making nitrogen atoms more stably embedded in the hybrid network of the carbon skeleton. Finally, through multiple hot water washing and drying processes, potassium fluoride and anhydrous borax are removed. The hot water washing process also further cleans the byproducts in the pores, ensuring the openness and connectivity of the pores. The preparation method of this invention has a compact process route, organically integrating the construction of hierarchical pore structures with the nitrogen doping process, and realizing the stable production of nitrogen-doped activated carbon with a hierarchical porous structure. Attached Figure Description

[0016] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0017] Figure 1 These are schematic diagrams of the specific surface area detection results of embodiments 1-3 of the present invention; Figure 2This is a schematic diagram of the total pore volume detection results of Embodiments 1-3 of the present invention; Figure 3 These are schematic diagrams of the micropore volume detection results in Examples 1-3 of the present invention; Figure 4 This is a schematic diagram of the mesoporous pore volume detection results of Embodiments 1-3 of the present invention; Figure 5 These are schematic diagrams of the macropore volume detection results in Embodiments 1-3 of the present invention; Figure 6 This is a schematic diagram of the nitrogen content detection results in Examples 1-3 of the present invention. Detailed Implementation

[0018] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention. Example 1

[0019] Step 1: Crush lignin and melamine and pass them through a 200-mesh sieve for later use; weigh the crushed lignin, melamine and nano-sized polystyrene microspheres at a mass ratio of 1:0.4:0.3; then add them to deionized water at a mass of 1.2 times their total mass and send them into a high-speed shear mill and stir at 1800 r / min for 30 min, then take them out for later use. Step 2: Place the mixture obtained in Step 1 into a freeze dryer and lower the temperature to -40°C at a cooling rate of 3°C / min. Freeze for 14 hours. After the freezing operation is completed, continue to lower the temperature to -70°C at a cooling rate of 2°C / min. Reduce the vacuum degree to 50Pa and freeze dry for 20 hours. Then, raise the temperature to room temperature and restore the atmospheric pressure before taking it out for use. Step 3: The material obtained after freeze-drying in Step 2 is fed into a sealed calcining furnace and heated to 200°C at a heating rate of 10°C / min. It is maintained at this temperature for 3 hours, and then heated to 380°C at a heating rate of 6°C / min. It is maintained at this temperature for 8 hours. Nitrogen gas is introduced for protection throughout the heating process. After one calcination is completed, the material is cooled to room temperature and taken out for use. Step 4: After crushing the material obtained from the first calcination treatment in Step 3 through a 200-mesh sieve, the material is fed into a plasma surface treatment machine. The RF power of the plasma surface treatment machine is 1200W, the volume of the plasma surface treatment machine is 100L, the weight of the material obtained from the first calcination treatment in Step 3 is 20kg, the gas introduced into the plasma surface treatment machine is a mixture of ammonia and argon in a volume ratio of 1:1, the gas flow rate is 30L / min, the plasma treatment time is 30min, and the material is taken out for use after the treatment is completed. Step 5: Mix the material obtained from the plasma activation treatment in Step 4 with molten salt particles (a mixture of potassium fluoride and anhydrous borax in a 1:1 mass ratio) in a mass ratio of 1:4. After mixing, pulverize the mixture through a 300-mesh sieve and set it aside. Then, add it to a calcining furnace and heat it to 280°C at a heating rate of 10°C / min. Maintain this temperature for 30 min, then heat it to 520°C at a heating rate of 6°C / min. Maintain this temperature for 4 h, then cool it to room temperature and set it aside. Step Six: Crush the material obtained from the secondary calcination treatment in Step Five through a 200-mesh sieve, then add five times the mass of the material to deionized water, stir at 60°C and 100 r / min for 20 min, and filter while hot. Repeat the above operation three times, then put it into a drying oven and dry at 60°C for 3 h. After cooling to room temperature, take it out to obtain nitrogen-doped activated carbon with a hierarchical porous structure. Example 2

[0020] Step 1: Crush lignin, cellulose, melamine, and urea and pass them through a 200-mesh sieve for later use; weigh the crushed lignin, cellulose, melamine, urea powder, nano-sized polystyrene microspheres, and micron-sized polymethyl methacrylate microspheres according to the mass ratio of 0.4:0.6:0.3:0.3:0.1:0.3; then add them to deionized water at a mass of 1.5 times their total mass and send them into a high-speed shear mill. Stir at 1900 r / min for 20 min and then remove for later use. Step 2: Place the mixture obtained in Step 1 into a freeze dryer and lower the temperature to -50°C at a cooling rate of 5°C / min. Freeze for 12 hours. After the freezing operation is completed, continue to lower the temperature to -80°C at a cooling rate of 3°C / min. Reduce the vacuum degree to 30Pa and freeze dry for 16 hours. Then, raise the temperature to room temperature and restore the atmospheric pressure before taking it out for use. Step 3: The material obtained after freeze-drying in Step 2 is fed into a sealed calcining furnace and heated to 210°C at a heating rate of 15°C / min. It is maintained at this temperature for 2 hours, and then heated to 400°C at a heating rate of 10°C / min. It is maintained at this temperature for 6 hours. Nitrogen gas is introduced for protection throughout the heating process. After one calcination is completed, the material is cooled to room temperature and taken out for use. Step 4: After crushing the material obtained from the first calcination treatment in Step 3 through a 200-mesh sieve, the material is fed into a plasma surface treatment machine. The RF power of the plasma surface treatment machine is 1500W, the volume of the plasma surface treatment machine is 100L, the weight of the material obtained from the first calcination treatment in Step 3 is 30kg, the gas introduced into the plasma surface treatment machine is a mixture of ammonia and argon in a volume ratio of 1:1, the gas flow rate is 40L / min, the plasma treatment time is 40min, and the material is taken out for use after the treatment is completed. Step 5: Mix the material obtained from the plasma activation treatment in Step 4 with molten salt particles (a mixture of potassium fluoride and anhydrous borax in a mass ratio of 1:2) in a mass ratio of 1:6. After mixing, pulverize the mixture through a 300-mesh sieve and set it aside. Then, add it to a calcining furnace and heat it to 290°C at a heating rate of 15°C / min. Maintain this temperature for 20 min, then heat it to 560°C at a heating rate of 10°C / min. Maintain this temperature for 3 h, then cool it to room temperature and set it aside. Step Six: Crush the material obtained from the secondary calcination treatment in Step Five through a 200-mesh sieve, then add it to deionized water at a mass of 8 times the total mass of the material. Stir at 70°C and 150 r / min for 15 min, then filter while hot. Repeat the above operation 4 times. Then put it into a drying oven and dry it at 80°C for 2 h. After cooling to room temperature, take it out to obtain nitrogen-doped activated carbon with a hierarchical porous structure. Example 3

[0021] Step 1: Crush lignin, cellulose, anthracite, chitosan, asphalt, melamine, dicyandiamide, and urea, and pass them through a 200-mesh sieve for later use. Weigh the crushed lignin, cellulose, anthracite, chitosan, asphalt, melamine, dicyandiamide, urea, nano-sized polystyrene microspheres, micron-sized polymethyl methacrylate microspheres, and nano-sized polyethylene glycol microspheres according to the following mass ratio: 0.3:0.2:0.2:0.1:0.2:0.2:0.1:0.2:0.1:0.1:0.2. Then add them to deionized water at a mass of 1.2 to 1.5 times their total mass and put them into a high-speed shear mill. Stir at a speed of 1800 to 1900 r / min for 20 to 30 minutes, and then remove them for later use. Step 2: Place the mixture obtained in Step 1 into a freeze dryer and lower the temperature to -40 to -50°C at a cooling rate of 3 to 5°C / min. Freeze for 12 to 14 hours. After the freezing operation is completed, continue to lower the temperature to -70 to -80°C at a cooling rate of 2 to 3°C / min. Reduce the vacuum degree to 30 to 50 Pa and freeze-dry for 16 to 20 hours. Then, heat the mixture to room temperature and restore it to normal pressure before taking it out for use. Step 3: The material obtained after freeze-drying in Step 2 is fed into a sealed calcining furnace and heated to 200-210°C at a heating rate of 10-15°C / min. It is maintained at this temperature for 2-3 hours, and then heated to 380-400°C at a heating rate of 6-10°C / min. It is maintained at this temperature for 6-8 hours. Nitrogen gas is introduced for protection throughout the heating process. After one calcination is completed, the material is cooled to room temperature and taken out for use. Step 4: After crushing the material obtained from the first calcination treatment in Step 3 through a 200-mesh sieve, feed it into a plasma surface treatment machine. The radio frequency power of the plasma surface treatment machine is 1200-1500W, the volume of the plasma surface treatment machine is 100L, the weight of the material obtained from the first calcination treatment in Step 3 is 20-30kg, the gas introduced into the plasma surface treatment machine is a mixture of ammonia and argon in a volume ratio of 1:1, the flow rate of the mixed gas is 30-40L / min, the plasma treatment time is 30-40min, and after the treatment is completed, it is taken out for use. Step 5: Mix the material obtained from the plasma activation treatment in Step 4 with molten salt particles (a mixture of potassium fluoride and anhydrous borax in a mass ratio of 1:1 to 2) in a mass ratio of 1:4 to 6. After mixing, pulverize the mixture through a 300-mesh sieve and set it aside. Then, add it to a calcining furnace and heat it to 280 to 290°C at a heating rate of 10 to 15°C / min. Maintain this temperature for 20 to 30 minutes. Then, heat it to 520 to 560°C at a heating rate of 6 to 10°C / min. Maintain this temperature for 3 to 4 hours. After cooling to room temperature, remove it for use. Step Six: Crush the material obtained from the secondary calcination treatment in Step Five through a 200-mesh sieve, then add it to 5-8 times the mass of the material in deionized water. Stir at 60-70℃ and 100-150 r / min for 15-20 min, then filter while hot. Repeat the above operation 2-4 times. Then put it into a drying oven and dry it at 60-80℃ for 2-3 h. After cooling to room temperature, take it out to obtain nitrogen-doped activated carbon with a hierarchical porous structure.

[0022] The specific surface area, pore size distribution, and nitrogen content of the nitrogen-doped activated carbon with hierarchical porous structure prepared in Examples 1-3 were tested. The specific surface area and pore size distribution were determined using the nitrogen adsorption-desorption isotherm method; the nitrogen content was determined using an elemental analyzer. The results are as follows: Figures 1-6 As shown.

[0023] Depend on Figures 1-6 It can be seen that the specific surface area of ​​the nitrogen-doped activated carbon with a hierarchical porous structure prepared in Example 1 is 2065 m². 2 / g, total pore volume is 1.26cm³ 3 / g, of which the micropore volume is 0.78cm³ 3 / g, mesoporous pore volume is 0.32cm³ 3 / g, macropore volume is 0.16cm³ 3 / g, exhibiting a hierarchical porous structure; nitrogen content is 8.9wt%; the specific surface area of ​​the nitrogen-doped activated carbon with a hierarchical porous structure prepared in Example 2 is 2150m². 2 / g, total pore volume is 1.28cm³ 3 / g, of which the micropore volume is 0.72cm³ 3 / g, mesoporous pore volume is 0.35cm³ 3 / g, macropore volume is 0.21cm³ 3 / g, exhibiting a hierarchical porous structure, with a nitrogen content of 9.1wt%; the nitrogen-doped activated carbon with a hierarchical porous structure prepared in Example 3 has a specific surface area of ​​2243 m². 2 / g, total pore volume is 1.34cm³ 3 / g, micropore volume is 0.77cm³ 3 / g, mesoporous pore volume is 0.37cm³ 3 / g, macropore volume is 0.20cm³ 3 / g, exhibiting hierarchical porous structure characteristics; nitrogen content is 9.5wt%.

[0024] The data above show that the nitrogen-doped activated carbon with a hierarchical porous structure prepared by this invention has a high specific surface area and a rich hierarchical pore structure. The ratio of micropores, mesopores and macropores is reasonable, which is beneficial to improving the adsorption performance and mass transfer efficiency of the material. At the same time, the high nitrogen content indicates that nitrogen atoms have been successfully embedded in the carbon framework and formed a stable doped structure. Therefore, the nitrogen-doped activated carbon with a hierarchical porous structure prepared by this invention has the excellent properties of both hierarchical porous structure and nitrogen doping characteristics, which is convenient for widespread promotion and application.

[0025] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for preparing nitrogen-doped activated carbon with a hierarchical porous structure, characterized in that: Includes the following steps: Step 1: Add the carbon source, pore-forming agent, and nitrogen source to deionized water and mix thoroughly; Step 2: Freeze the mixture obtained in Step 1, and then freeze-dry it. Step 3: The material obtained after freeze-drying in Step 2 is subjected to a calcination process. Step 4: Perform plasma treatment on the material obtained after the first calcination treatment in Step 3; Step 5: Mix the material obtained from the plasma activation treatment in Step 4 with molten salt and then perform a second calcination treatment; Step Six: Remove the material obtained from the secondary calcination treatment in Step Five with molten salt and dry it to obtain nitrogen-doped activated carbon with a hierarchical porous structure.

2. The method for preparing nitrogen-doped activated carbon with a hierarchical porous structure according to claim 1, characterized in that: In step one, the carbon source is any one or a mixture of any two or more of lignin, cellulose, anthracite, chitosan and pitch in any proportion. The pore-forming agent is any one or a mixture of any two or more of the following: nano-sized polystyrene microspheres, micron-sized polymethyl methacrylate microspheres, and nano-sized polyethylene glycol microspheres, in any proportion. The nitrogen source is any one or a mixture of any two or more of melamine, dicyandiamide, and urea in any proportion.

3. The method for preparing nitrogen-doped activated carbon with a hierarchical porous structure according to claim 1, characterized in that: The specific operation of step one is as follows: after crushing the carbon source material and nitrogen source material, pass them through a 200-mesh sieve for later use; add the crushed carbon source material, nitrogen source material and pore-forming agent to deionized water at a mass of 1.2 to 1.5 times their total mass and send them into a high-speed shearing machine and stir at a speed of 1800 to 1900 r / min for 20 to 30 minutes, then take them out for later use. The mass ratio of the added carbon source material, nitrogen source material and pore-forming agent is 1:0.4 to 0.6:0.3 to 0.

4.

4. The method for preparing nitrogen-doped activated carbon with a hierarchical porous structure according to claim 1, characterized in that: The specific operation of the freezing treatment in step two is as follows: the mixture obtained in step one is placed in a freeze dryer and the temperature is reduced to -40 to -50°C at a cooling rate of 3 to 5°C / min, and frozen for 12 to 14 hours.

5. The method for preparing nitrogen-doped activated carbon with a hierarchical porous structure according to claim 4, characterized in that: The freeze-drying process in step two is as follows: after the freezing operation is completed, continue to cool down to -70 to -80°C at a cooling rate of 2 to 3°C / min, reduce the vacuum degree to 30 to 50 Pa, freeze-dry for 16 to 20 hours, and then heat up to room temperature and restore normal pressure before taking it out for use.

6. The method for preparing nitrogen-doped activated carbon with a hierarchical porous structure according to claim 1, characterized in that: The specific operation of the first calcination treatment in step three is as follows: the material obtained after freeze-drying in step two is sent into a sealed calcination furnace and heated to 200-210°C at a heating rate of 10-15°C / min, and maintained at this temperature for 2-3 hours. Then, the temperature is raised to 380-400°C at a heating rate of 6-10°C / min and maintained at this temperature for 6-8 hours. Nitrogen gas is introduced for protection throughout the heating process. After the first calcination is completed, the material is cooled to room temperature and taken out for use.

7. The method for preparing nitrogen-doped activated carbon with a hierarchical porous structure according to claim 1, characterized in that: The specific operation of the plasma treatment in step four is as follows: the material obtained after the first calcination treatment in step three is crushed and passed through a 200-mesh sieve and then fed into a plasma surface treatment machine. The radio frequency power of the plasma surface treatment machine is 1200-1500W, the volume of the plasma surface treatment machine is 100L, the weight of the material obtained after the first calcination treatment in step three is 20-30kg, the gas introduced into the plasma surface treatment machine is a mixed gas obtained by mixing ammonia and argon in a volume ratio of 1:1, the flow rate of the mixed gas is 30-40L / min, and the plasma treatment time is 30-40min.

8. The method for preparing nitrogen-doped activated carbon with a hierarchical porous structure according to claim 1, characterized in that: The molten salt is a mixture of potassium fluoride and anhydrous borax in a mass ratio of 1:1 to 2.

9. The method for preparing nitrogen-doped activated carbon with a hierarchical porous structure according to claim 1, characterized in that: The specific operation of the secondary calcination treatment in step five is as follows: the material obtained from the plasma activation treatment in step four is mixed with molten salt particles at a mass ratio of 1:4 to 6. After mixing, the mixture is crushed and passed through a 300-mesh sieve for later use. Then, it is added to a calcination furnace and heated to 280 to 290°C at a heating rate of 10 to 15°C / min. The temperature is maintained at this temperature for 20 to 30 minutes. Then, the temperature is raised to 520 to 560°C at a heating rate of 6 to 10°C / min. The temperature is maintained at this temperature for 3 to 4 hours. After cooling to room temperature, it is taken out for later use.

10. The method for preparing nitrogen-doped activated carbon with a hierarchical porous structure according to claim 1, characterized in that: The specific operation of step six is ​​as follows: the material obtained from the secondary calcination treatment in step five is crushed and passed through a 200-mesh sieve, and then added to deionized water at a mass of 5 to 8 times the total mass of the material. The mixture is stirred at a temperature of 60 to 70°C and a speed of 100 to 150 r / min for 15 to 20 minutes, and then filtered while hot. The above operation is repeated 2 to 4 times. Then it is sent to a drying oven and dried at a temperature of 60 to 80°C for 2 to 3 hours. After cooling to room temperature, nitrogen-doped activated carbon with a hierarchical porous structure is obtained.

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