阅读说明：本技术 一种紫根水葫芦基分级多孔炭材料及其制备方法与在超级电容器中的应用 (Purple-root water hyacinth-based graded porous carbon material, preparation method thereof and application thereof in super capacitor ) 是由 杨东杰 王鹏 李致贤 邱学青 林绪亮 秦延林 谭海城 于 2020-11-25 设计创作，主要内容包括：本发明公开了一种紫根水葫芦基分级多孔炭材料及其制备方法与在超级电容器中的应用。本发明方法首先将钾盐水溶液浸渍的紫根水葫芦进行密闭微波反应处理并干燥,然后加入钙盐球磨后高温炭化制备得到分级多孔炭。本发明制备的紫根水葫芦基分级多孔炭与传统化学活化法相比具有较大的比表面积、合理的孔道结构、丰富的孔径分布；将其应用于超级电容器电极材料,可为电化学过程中电解质离子的快速扩散与传输提供良好的通道,表现出更高的比电容和优异的倍率性能,具有良好的潜在应用价值。(The invention discloses a purple-root water hyacinth-based graded porous carbon material, a preparation method thereof and application thereof in a super capacitor. The method comprises the steps of carrying out closed microwave reaction treatment on the purple-root water hyacinth dipped by the sylvite aqueous solution, drying, adding calcium salt, carrying out ball milling, and carrying out high-temperature carbonization to prepare the graded porous carbon. Compared with the traditional chemical activation method, the purple-root water hyacinth-based graded porous carbon prepared by the invention has larger specific surface area, reasonable pore structure and abundant pore size distribution; the high-specific-capacitance and high-rate-of-charge capacity super-capacitor electrode material is applied to super-capacitor electrode materials, can provide a good channel for rapid diffusion and transmission of electrolyte ions in an electrochemical process, shows higher specific capacitance and excellent rate capability, and has good potential application value.)

1. A preparation method of a purple-root water hyacinth-based graded porous carbon material is characterized by comprising the following steps:

(1) mechanically crushing the purple-root water hyacinth to obtain purple-root water hyacinth powder, adding a certain mass of purple-root water hyacinth powder into a potassium salt aqueous solution with a certain concentration, placing the mixture in a closed microwave reactor to react for a certain time, and drying to obtain potassium salt-purple-root water hyacinth composite powder;

(2) mixing the calcium salt with the potassium salt-purple root water hyacinth composite powder obtained in the step (1), and then carrying out ball milling treatment to obtain potassium salt-calcium salt-purple root water hyacinth composite powder;

(3) carbonizing the potassium salt-calcium salt-purple root water hyacinth composite powder in the step (2) in the atmosphere of nitrogen or inert gas, then soaking in an acid solution, washing and drying to obtain the purple root water hyacinth-based graded porous carbon material.

2. The preparation method of the water hyacinth-based graded porous carbon material according to claim 1, wherein the mass ratio of the water hyacinth powder, the potassium salt and the water in the step (1) is 1-5: 1: 10-50, wherein the potassium salt is at least one of potassium carbonate and potassium oxalate.

3. The method for preparing the water hyacinth-based graded porous carbon material according to claim 1, wherein the mass ratio of the calcium salt in the step (2) to the water hyacinth powder in the step (1) is 1: 1-6; the calcium salt is at least one of calcium carbonate and calcium oxalate.

4. The method for preparing the purple root water hyacinth based graded porous carbon material according to claim 1, wherein the power of the closed microwave reactor in the step (1) is 100-500W, the temperature is 80-180 ℃, and the time is 1-10 min.

5. The method for preparing the purple root water hyacinth based graded porous carbon material according to claim 1, wherein the rotation speed of the ball milling treatment in the step (2) is 100-500 rpm, and the time is 2-8 h.

6. The method for preparing the purple-root water hyacinth-based graded porous carbon material as claimed in claim 1, wherein the temperature of the carbonization in the step (3) is 550-850 ℃ and the time is 2-4 h.

7. The method for preparing the water hyacinth-based graded porous carbon material according to claim 1, wherein the particle size of the water hyacinth powder in the step (1) is 100-200 meshes; the concentration of the acid solution in the step (3) is 0.05-2 mol/L; the acid solution is at least one of hydrochloric acid, sulfuric acid and nitric acid; the soaking time is 1-24 h.

8. The method for preparing the water hyacinth-based graded porous carbon material according to claim 1, wherein the ball mill used in the ball milling treatment of the step (2) is at least one of a tubular ball mill, a horizontal ball mill and a planetary ball mill; and (3) drying is at least one of forced air drying, vacuum drying, infrared drying and spray drying.

9. A water hyacinth-based graded porous carbon material prepared by the method of any one of claims 1 to 8.

10. The use of the water hyacinth-based graded porous carbon material as in claim 9 in an electrode material of a supercapacitor.

Technical Field

The invention belongs to the technical field of porous carbon materials, and particularly relates to a purple-root water hyacinth-based graded porous carbon material, a preparation method thereof and application thereof in a super capacitor.

Background

The purple-root water hyacinth serving as biomass for gene improvement of the water hyacinth has a root system accounting for more than 80% of the plant volume, and a root crown which is nearly 20 times of that of the common water hyacinth, has a higher lignification degree compared with other biomass mainly comprising stems, leaves, barks and the like, and is an ideal biomass carbon source for preparing the porous carbon material. The porous carbon material has the advantages of good thermal stability, high mechanical stability, excellent conductivity, developed pore structure, large specific surface area and the like, and is widely applied to the fields of supercapacitors, lithium ion batteries and the like. The biomass-based hierarchical porous carbon has special pore size distribution of micropores, mesopores and macropores and developed porosity, can promote the rapid transfer of ions in pore channels, has high desalting capacity and rapid desalting rate, can greatly improve the mass transfer of reactants/products, further promotes the electrochemical reaction process, and can be potentially applied to electrode materials of supercapacitors.

At present, the following problems exist in the preparation of porous carbon by using biomass as a raw material: (1) because the biomass mainly comprising leaves, stems, barks and the like of the plant is insoluble in water, the contact and compounding of the biomass and the activating agent are difficult, particularly, the biomass represented by the purple-root water hyacinth is mostly taken as roots, the structure is more compact, and the porous carbon material with high specific surface area is difficult to obtain; (2) most of porous carbon prepared by research has single pore channel structure and mainly has a microporous structure; (3) the existing porous carbon preparation process is complex, a large amount of solvent and activating agent are needed, and the preparation process is not green.

In order to solve the above problems, the current research mainly adopts impregnation, hydrothermal method and the like to realize the compounding of the biomass and the activating agent. Toudaiyong et al (CN 108117073A method for preparing porous carbon material from water hyacinth) by adding water hyacinth into hydrothermal reaction kettle, performing hydrothermal pretreatment, and adding a certain amount of metal salt (Na)₂CO₃-K₂CO₃Mixed salt) is put into a tube furnace for high-temperature carbonization to obtain a porous carbon material with the specific surface of 1501m²The specific capacitance value is 346F/g at a current density of 0.2A/g. Sun Jie et al (CN109850896A A preparation method and application of a native Eichhornia crassipes Living mass carbon porous electrode material) firstly dries and then crushes water hyacinth, then uniformly stirs and mixes the water hyacinth with zinc chloride powder and deionized water, pours the mixture into an open container, dries the mixture by distillation, then puts the dried mixture into a tube furnace, and then injects nitrogen to calcine the dried mixture to obtain a porous carbon material with a specific surface area of 800m²(g) total pore volume of 1.108cm³(ii) in terms of/g. Luqiufeng et al (CN106966391A biomass porous carbon material based on watermelon peel and preparation method and application thereof) mix watermelon peel powder with KOH solution to obtain precursor, and prepare the obtained porous carbon material through one-step high-temperature anaerobic carbonization process, and the discharge specific capacitance of the porous carbon material under the current density of 1A/g can reach 208F/g. However, the whole process uses a large amount of solvent and activating agent, and is not environment-friendly. Ma G et al (Bioresource technology,2015,197:137-142) use potato residues as carbon source, zinc chloride (ZnCl)₂) As an activating agent, melamine is used as a nitrogen doping agent to prepare nitrogen-doped porous carbon with the surface area of 1052m²The specific capacitance obtained in 2M KOH electrolyte is up to 255F/g. However, the ZnCl used in this process₂It is easy to volatilize in a large amount at high temperature, and can generate environmental pollution while corroding production equipment, and the yield of the material is low. In order to reduce the use of an activating agent, a biological porous activated carbon (CN109516459A, a biomass porous activated carbon, a preparation method thereof and application thereof as an electrode material of an electric double layer capacitor) is prepared by crushing a biomass raw material by a high-energy crusher, and heating to an activation temperature under argon in a protective atmosphereAnd introducing carbon dioxide gas into the furnace for activation treatment, and then cooling to normal temperature under a protective atmosphere to obtain the biomass porous activated carbon material with the discharge specific capacitance of 138F/g under the current density of 1A/g. Yin H et al (Environmental science)&technology,2014,48(14): 8101-. However, the method has long process carbonization time and high energy consumption, and the molten salt treatment process has high requirements on equipment and high risk, and is not beneficial to industrial large-scale production.

In conclusion, researchers have improved the complexing effect of biomass and activating agent to some extent by methods such as impregnation and hydrothermal treatment. However, for biomass mainly containing roots (such as purple-root water hyacinth and the like), due to the difficulty in compounding with an activator, the specific surface area of the prepared porous carbon is still not ideal, and the pore channels are mainly distributed in micropores, so that the rapid transfer of ions in the pore channels is not facilitated, and the electrochemical performance of the prepared porous carbon applied to the supercapacitor is influenced to a certain extent. In addition, the existing activation method has the problems of complex process, serious corrosion to equipment, high production cost and the like.

Disclosure of Invention

In order to overcome the defects and shortcomings in the prior art, the invention mainly aims to provide a preparation method of a purple root water hyacinth based graded porous carbon material.

The invention adopts two steps of microwave-ball milling to realize the uniform composition of the purple-root water hyacinth powder and the activating agent, and prepares the graded porous carbon material by a sylvite and calcium salt double activation method. Firstly, soaking the water hyacinth powder with the purple root in a potassium salt solution, placing the solution in a microwave reactor for microwave treatment, drying the obtained mixture to obtain a potassium salt-water hyacinth compound, then mixing the potassium salt-water hyacinth compound with calcium salt, carrying out ball milling, and finally carbonizing the mixture at high temperature to prepare the graded porous carbon.

In the preparation process of the method, after the purple-root water hyacinth dipped in the sylvite aqueous solution is subjected to closed microwave treatment, the tissue structure is loose, the number of interstitial pores is increased, and then the physical and chemical actions among the purple-root water hyacinth powder, the activating agent and the grinding aid are enhanced under the action of the grinding aid and the mechanical force, so that the nano-crystallization of the mixed powder can be realized, and the effective and uniform compounding of the purple-root water hyacinth powder and the activating agent is facilitated.

Compared with the traditional activating agent (KOH, ZnCl)₂) The carbon layer is etched and pore-formed mainly by an activating agent, carbonate and oxalate in a calcium salt/sylvite activating agent are decomposed in the heating process to generate a large amount of carbon dioxide and carbon monoxide gas, the material can be subjected to blasting treatment, a product after high-temperature decomposition can be used as a template agent to participate in the subsequent pore-forming stage, and the preparation of the graded porous carbon can be realized by utilizing the intercalation mechanism of cations and the synergistic action of gas phase stripping under the heat treatment.

The invention also aims to provide the purple-root water hyacinth-based graded porous carbon material prepared by the method.

The invention further aims to provide application of the purple root water hyacinth-based graded porous carbon material in a supercapacitor electrode material.

The purpose of the invention is realized by the following technical scheme:

a preparation method of a purple-root water hyacinth-based graded porous carbon material comprises the following steps:

(1) mechanically crushing the purple-root water hyacinth to obtain purple-root water hyacinth powder, adding a certain mass of purple-root water hyacinth powder into a potassium salt aqueous solution with a certain concentration, placing the mixture in a closed microwave reactor to react for a certain time, and drying to obtain potassium salt-purple-root water hyacinth composite powder;

(2) mixing the calcium salt with the potassium salt-purple root water hyacinth composite powder obtained in the step (1), and then carrying out ball milling treatment to obtain potassium salt-calcium salt-purple root water hyacinth composite powder;

(3) carbonizing the potassium salt-calcium salt-purple root water hyacinth composite powder in the step (2) in the atmosphere of nitrogen or inert gas, then soaking in an acid solution, washing and drying to obtain the purple root water hyacinth-based graded porous carbon material.

Preferably, the processing power of the closed microwave reactor in the step (1) is 100-500W, the temperature is 80-180 ℃, and the time is 1-10 min.

Preferably, the particle size of the water hyacinth powder with the purple root in the step (1) is 100-200 meshes.

Preferably, the mass ratio of the purple root water hyacinth powder, the potassium salt and the water in the step (1) is 1-5: 1: 10-50, wherein the potassium salt is at least one of potassium carbonate and potassium oxalate.

Preferably, the mass ratio of the calcium salt in the step (2) to the water hyacinth powder of the purple root in the step (1) is 1: 1-6; the calcium salt is at least one of calcium carbonate and calcium oxalate.

Preferably, the rotation speed of the ball milling treatment in the step (2) is 100-500 rpm, and the time is 2-8 h.

Preferably, the ball mill used for the ball milling treatment in step (2) is at least one of a tubular ball mill, a horizontal ball mill and a planetary ball mill.

Preferably, the carbonization temperature in the step (3) is 550-850 ℃, and the carbonization time is 2-4 h.

Preferably, the concentration of the acid solution in the step (3) is 0.05-2 mol/L; the acid solution is at least one of hydrochloric acid, sulfuric acid and nitric acid.

Preferably, the soaking time in the step (3) is 1-24 h.

Preferably, the washing in step (3) is water washing.

Preferably, the drying of step (3) is at least one of forced air drying, vacuum drying, infrared drying and spray drying.

The purple-root water hyacinth-based graded porous carbon material prepared by the method.

The specific surface area of the purple-root water hyacinth-based graded porous carbon material ranges from 1900 m to 3056m²The specific surface area of the micropores ranges from 100 m to 2000m²The mesoporous specific surface area ranges from 100 m to 2600m²The ratio of mesopores is 70.5%, the pore size is 0.5-100 nm, and the pore volume is 0.5-1.6 cm³/g。

The application of the purple-root water hyacinth-based graded porous carbon material in the electrode material of the super capacitor is provided.

Compared with the prior art, the invention has the following advantages and beneficial effects:

(1) compared with the traditional chemical activation method, the purple-root water hyacinth-based graded porous carbon prepared by the invention has larger specific surface area, reasonable pore structure and abundant pore size distribution; the high-specific-capacitance and high-rate-of-charge capacity super-capacitor electrode material is applied to super-capacitor electrode materials, can provide a good channel for rapid diffusion and transmission of electrolyte ions in an electrochemical process, shows higher specific capacitance and excellent rate capability, and has good potential application value.

(2) As more than 80 percent of the purple-root water hyacinth is root, the purple-root water hyacinth has compact structure, and the traditional method is difficult to realize the full compounding of the purple-root water hyacinth and the activating agent. The microwave-ball milling treatment method has obvious loosening effect on the water hyacinth with the radix violae, and promotes the full compounding of soluble potassium salt, insoluble calcium salt and the water hyacinth with the radix violae; the method has the advantages that weakly corrosive calcium salt/potassium salt is used as an activating agent, a template agent and a grinding aid, the raw materials are wide in source, cheap and easily available, the preparation process is simple and environment-friendly, the final material can be obtained through microwave-ball milling-carbonization, the method has the characteristics of simple process and green process, the corrosion degree on equipment is low, large-scale preparation is easily realized, high added value utilization of the purple-root water hyacinth is realized, the environmental protection is facilitated, and the method is suitable for industrial production.

Drawings

FIG. 1 is a scanning electron microscope image of a purple-root water hyacinth-based graded porous carbon prepared in example 1 of the present invention.

FIG. 2 is a transmission electron microscope image of a purple-root water hyacinth-based graded porous carbon prepared in example 1 of the present invention.

Fig. 3 is a nitrogen adsorption/desorption graph and a pore size distribution graph of the purple root water hyacinth-based graded porous carbon prepared in example 1 of the present invention.

Fig. 4 is a constant dc charge-discharge curve diagram of the purple root water hyacinth based graded porous carbon prepared in example 1 of the present invention.

Detailed Description

The present invention will be described in further detail with reference to examples and drawings, but the embodiments of the present invention are not limited thereto.

Those who do not specify specific conditions in the examples of the present invention follow conventional conditions or conditions recommended by the manufacturer. The raw materials, reagents and the like which are not indicated for manufacturers are all conventional products which can be obtained by commercial purchase.

Example 1

Mechanically crushing the purple-root water hyacinth to obtain 150-mesh purple-root water hyacinth powder, mixing and stirring 3g of the purple-root water hyacinth powder, 1.5g of potassium carbonate and 30ml of deionized water uniformly, placing the mixture in a closed microwave reactor, performing microwave treatment at 160 ℃ and 500W for 10min, and drying the obtained mixture to obtain the potassium salt-purple-root water hyacinth composite powder. According to the mass ratio of the purple root water hyacinth powder to the calcium salt of 2: 1, uniformly stirring the potassium salt-purple root water hyacinth composite powder and 1.5g of calcium oxalate, and sealing the mixture into a ball milling tank, wherein the ball milling time is 4 hours, and the ball milling rotating speed is set to be 400 rpm. And then placing the mixture obtained by ball milling into a porcelain ark and placing into a high-temperature tube furnace, calcining for 2 hours at 800 ℃ by taking nitrogen as protective gas, placing into 1mol/L hydrochloric acid after cooling, stirring for 24 hours, and washing away redundant impurities and templates. Washing with water, filtering, and vacuum drying to obtain the purple root water hyacinth base graded porous carbon material.

Example 2

Mechanically crushing the purple-root water hyacinth to obtain 150-mesh purple-root water hyacinth powder, mixing and stirring 3g of the purple-root water hyacinth powder, 1.5g of potassium carbonate and 30ml of deionized water uniformly, placing the mixture in a microwave reactor, performing microwave treatment at 120 ℃ and 400W for 10min, and drying the obtained mixture to obtain the potassium salt-purple-root water hyacinth composite powder. According to the mass ratio of the purple root water hyacinth powder to the calcium salt of 2: 1, uniformly stirring the potassium salt-purple root water hyacinth composite powder and 1.5g of calcium oxalate, and sealing the mixture into a ball milling tank, wherein the ball milling time is 4 hours, and the ball milling rotating speed is set to be 400 rpm. And then placing the mixture obtained by ball milling into a porcelain ark and placing into a high-temperature tube furnace, calcining for 2 hours at 800 ℃ by taking nitrogen as protective gas, placing into 1mol/L hydrochloric acid after cooling, stirring for 24 hours, and washing away redundant impurities and templates. Washing with water, filtering, and vacuum drying to obtain the purple root water hyacinth base graded porous carbon material.

Example 3

Mechanically crushing the purple-root water hyacinth to obtain 150-mesh purple-root water hyacinth powder, mixing and stirring 3g of the purple-root water hyacinth powder, 2g of potassium carbonate and 30ml of deionized water uniformly, placing the mixture in a microwave reactor, performing microwave treatment at 160 ℃ and 500W for 10min, and drying the obtained mixture to obtain the potassium salt-purple-root water hyacinth composite powder. According to the mass ratio of the purple root water hyacinth powder to the calcium salt of 3: 1, uniformly stirring the potassium salt-purple root water hyacinth composite powder and 1g of calcium oxalate, and sealing the mixture into a ball milling tank, wherein the ball milling time is 4 hours, and the ball milling rotating speed is set to be 400 rpm. And then placing the mixture obtained by ball milling into a porcelain ark and placing into a high-temperature tube furnace, calcining for 2 hours at 800 ℃ by taking nitrogen as protective gas, placing into 1mol/L hydrochloric acid after cooling, stirring for 24 hours, and washing away redundant impurities and templates. Washing with water, filtering, and vacuum drying to obtain the purple root water hyacinth base graded porous carbon material.

Example 4

Mechanically crushing the purple-root water hyacinth to obtain 150-mesh purple-root water hyacinth powder, mixing and stirring 3g of the purple-root water hyacinth powder, 1.5g of potassium carbonate and 30ml of deionized water uniformly, placing the mixture in a microwave reactor, performing microwave treatment at 160 ℃ and 500W for 10min, and drying the obtained mixture to obtain the potassium salt-purple-root water hyacinth composite powder. According to the mass ratio of the purple root water hyacinth powder to the calcium salt of 2: 1, uniformly stirring the potassium salt-purple root water hyacinth composite powder and 1.5g of calcium oxalate, and sealing the mixture into a ball milling tank, wherein the ball milling time is 2 hours, and the ball milling rotating speed is set to be 200 rpm. And then placing the mixture obtained by ball milling into a porcelain ark and placing into a high-temperature tube furnace, calcining for 2 hours at 800 ℃ by taking nitrogen as protective gas, placing into 1mol/L hydrochloric acid after cooling, stirring for 24 hours, and washing away redundant impurities and templates. And washing, filtering and vacuum drying to obtain the purple root water hyacinth base graded porous carbon material.

Example 5

Mechanically crushing the purple-root water hyacinth to obtain 150-mesh purple-root water hyacinth powder, mixing and stirring 3g of the purple-root water hyacinth powder, 1.5g of potassium carbonate and 30ml of deionized water uniformly, placing the mixture in a microwave reactor, performing microwave treatment at 160 ℃ and 500W for 10min, and drying the obtained mixture to obtain the potassium salt-purple-root water hyacinth composite powder. According to the mass ratio of the purple root water hyacinth powder to the calcium salt of 2: 1, uniformly stirring the potassium salt-purple root water hyacinth composite powder and 1.5g of calcium oxalate, and sealing the mixture into a ball milling tank, wherein the ball milling time is 2 hours, and the ball milling rotating speed is set to be 400 rpm. And then placing the mixture obtained by ball milling into a porcelain ark and placing into a high-temperature tube furnace, calcining for 2 hours at 700 ℃ by taking nitrogen as protective gas, placing into 1mol/L hydrochloric acid after cooling, stirring for 24 hours, and washing away redundant impurities and templates. Washing with water, filtering, and vacuum drying to obtain the purple root water hyacinth base graded porous carbon material.

Comparative example 1

Mechanically crushing the purple-root water hyacinth to obtain 150-mesh purple-root water hyacinth powder, mixing and stirring 3g of the purple-root water hyacinth powder and 30ml of deionized water uniformly, placing the mixture in a microwave reactor, performing microwave treatment at 160 ℃ and 500W for 10min, drying the obtained mixture, and then mixing the dried mixture according to the mass ratio of the purple-root water hyacinth powder to the calcium salt of 2: 1, uniformly stirring the purple-root water hyacinth powder and 1.5g of calcium oxalate, and sealing the mixture into a ball milling tank, wherein the ball milling time is 4 hours, and the ball milling rotating speed is set to be 400 rpm. And then placing the mixture obtained by ball milling into a porcelain ark and placing into a high-temperature tube furnace, calcining for 2 hours at 800 ℃ by taking nitrogen as protective gas, placing into 1mol/L hydrochloric acid after cooling, stirring for 24 hours, and washing away redundant impurities and templates. Washing with water, filtering, and vacuum drying to obtain the purple root water hyacinth base graded porous carbon material.

Comparative example 2

Mechanically crushing the purple-root water hyacinth to obtain 150-mesh purple-root water hyacinth powder, mixing and stirring 3g of the purple-root water hyacinth powder, 1.5g of potassium carbonate and 30ml of deionized water uniformly, placing the mixture in a microwave reactor, performing microwave treatment at 160 ℃ and 500W for 10min, and drying the obtained mixture to obtain the potassium salt-purple-root water hyacinth composite powder. And (3) sealing the potassium salt-purple root water hyacinth composite powder into a ball milling tank, wherein the ball milling time is 4 hours, and the ball milling rotating speed is set to be 400 rpm. And then placing the mixture obtained by ball milling into a porcelain ark and placing into a high-temperature tube furnace, calcining for 2 hours at 800 ℃ by taking nitrogen as protective gas, placing into 1mol/L hydrochloric acid after cooling, stirring for 24 hours, and washing away redundant impurities and templates. Washing with water, filtering, and vacuum drying to obtain the purple root water hyacinth base graded porous carbon material.

Comparative example 3

Drying the water hyacinth with the purple root until the water content is 18%, placing 10g of the water hyacinth with the purple root and the water content of 18% in a microwave oven, carrying out microwave treatment for 2min under the power of 800 watts, mechanically crushing the expanded water hyacinth powder with 150 meshes, and selecting particles according to the mass ratio of the water hyacinth powder to salt (calcium salt and potassium salt) of 1:1, uniformly mixing 3g of purple-root water hyacinth powder, 1.5g of calcium oxalate and 1.5g of potassium carbonate, and sealing the mixture in a ball milling tank, wherein the ball milling time is 2 hours, and the ball milling rotating speed is set to be 400 rpm. And then placing the mixture obtained by ball milling into a porcelain ark and placing into a high-temperature tube furnace, calcining for 2 hours at 800 ℃ by taking nitrogen as protective gas, placing into 1mol/L hydrochloric acid after cooling, stirring for 24 hours, and washing away redundant impurities and templates. Washing with water, filtering, and vacuum drying to obtain the purple root water hyacinth base graded porous carbon material.

Comparative example 4

After mechanically crushing the expanded purple root water hyacinth, selecting 150-mesh particles according to the mass ratio of the purple root water hyacinth powder to KOH of 1:1 dissolving 3g of purple root water hyacinth powder and 3g of potassium hydroxide in 100 g of deionized water, and heating and stirring at 80 ℃ until the water is evaporated to dryness. And carbonizing the obtained solid at 800 ℃ for 2h in an inert atmosphere, soaking the product in 1mol/L hydrochloric acid for washing for 24h, and then washing, filtering and vacuum-drying to obtain the purple-root water hyacinth-based porous carbon material.

The prepared purple root water hyacinth-based graded porous carbon is applied to a supercapacitor electrode material and subjected to material characterization and electrochemical test, and the results are shown in tables 1-2 and figures 1-4.

The microscopic morphology and structure of the samples were characterized by scanning electron microscopy (SEM, Hitachi S-550) and high-resolution field emission transmission electron microscopy (HRTEM, JEOL JEM-2100F,200 kV). The specific surface area and channel structure of the samples were tested using a fully automated specific surface and porosity analyzer (Micromeritics ASAP 2020 instrument).

Electrochemical tests were performed on an electrochemical workstation (CHI660E, shanghai chenghua), all tests using a three-electrode system. The working electrode preparation process comprises the following steps: dispersing the prepared lignin porous carbon nanosheet, acetylene black and polytetrafluoroethylene emulsion (solid content is 60 wt%) in absolute ethyl alcohol according to the mass ratio of 8:1:1, fully grinding, coating the ethanol on foamed nickel with the thickness of 1cm multiplied by 1cm after the ethanol is completely volatilized, and tabletting to obtain the working electrode. The counter electrode adopts a platinum sheet electrode, and the reference electrode adopts a saturated calomel electrode. The constant direct current charging and discharging curve test is completed in a voltage window of-1 to 0V at a current density of 1.0 to 20.0A/g.

Table 1 shows the pore structure of the purple root water hyacinth-based hierarchical porous carbon prepared in examples 1 to 5 and the porous carbon prepared in comparative examples 1 to 4.

Table 2 shows the electrochemical properties of the multi-layer carbon prepared in examples 1 to 5 as compared with those of the multi-layer carbon prepared in comparative examples 1 to 4.

TABLE 1 pore canal structure of purple-root water hyacinth-based graded porous carbon material

TABLE 2 electrochemical Properties of purple-rooted water hyacinth-based graded porous carbon Material

Tables 1 to 2 illustrate:

the specific surface area of the water hyacinth-based graded porous carbon prepared in example 1 was 3056m²The proportion of mesoporous volume is 70.5%, which is obviously superior to other embodiments under other process conditions. The specific capacitance of the purple-root water hyacinth graded porous carbon prepared in the embodiment 1 is 255F/g and 136F/g under the current density of 1.0A/g and 20.0A/g, the specific capacitance retention rate is 53.3%, the specific capacitance has good rate performance, and the biomass porous carbon material has obvious performance advantages.

The specific surface area, the mesoporous rate and the electrochemical performance of the porous carbon prepared by the embodiment are obviously improved compared with the comparative ratio. The main reason is that gas released in the thermal decomposition process of carbonate and oxalate in calcium salt and potassium salt has the stripping and activating effects on the hyacinth powder, and metal oxide nanoparticles generated by high-temperature decomposition have the etching and hard template effects on the hyacinth-based porous carbon, and hierarchical porous carbon with rich pore channel structures can be formed after the template is washed away, so that the rapid transmission and diffusion of electrolyte ions in the electrochemical process are promoted, and the electrochemical performance is improved. Comparative examples 1-2 are samples activated with potassium and calcium salts alone, respectively, lacking the potassium + calcium salt synergy; the comparative example 3 is a sample ball-milled by a potassium salt and calcium salt one-pot method, and the composite effect of the purple root water hyacinth and the activating agent is poorer than that of the microwave-ball milling in the embodiment; comparative example 4 is a conventional impregnation activation method, and the pore size distribution of the sample is narrow, mainly consisting of micropores, increasing the resistance to electrolyte ion diffusion in the electrochemical process, resulting in a severe decrease in specific capacitance.

FIG. 1 is a scanning electron micrograph of a graded porous carbon material prepared in example 1 of the present invention. It can be seen from the figure that the prepared hierarchical porous carbon material has a significant pore channel structure.

FIG. 2 is a transmission electron microscope image of a water hyacinth-based graded porous carbon material prepared in example 1 of the present invention. As can be seen from the figure, the hierarchical porous carbon has graphene-like wrinkle lines and has very rich pore channel structures.

FIG. 3 is a drawing showing nitrogen desorption of a purple-root water hyacinth-based hierarchical porous carbon material prepared in example 1 of the present invention. The hierarchical porous carbon material belongs to IV-type desorption, the nitrogen adsorption capacity is rapidly increased in a region with lower relative pressure, the hierarchical porous carbon material is shown to have a microporous structure, a hysteresis loop in a region with higher relative pressure is shown to have a mesoporous structure, the pore channel of the prepared hierarchical porous carbon material has an obvious hierarchical structure, and the reasonable pore channel structure and rich pore diameter distribution are beneficial to improvement of electrochemical performance.

FIG. 4 is a constant DC charging and discharging graph of the purple root water hyacinth based graded porous carbon prepared in example 1 of the present invention. The charging and discharging curves of the purple-root water hyacinth-based graded porous carbon are similar to isosceles triangles under different current densities, which shows that the purple-root water hyacinth-based graded porous carbon has typical electric double layer capacitance characteristics, and the charging and discharging time of the curves is approximately equal, which shows that the purple-root water hyacinth-based graded porous carbon has higher coulombic efficiency.

The above embodiments are preferred embodiments of the present invention, but the present invention is not limited to the above embodiments, and any other changes, modifications, substitutions, combinations, and simplifications which do not depart from the spirit and principle of the present invention should be construed as equivalents thereof, and all such changes, modifications, substitutions, combinations, and simplifications are intended to be included in the scope of the present invention.