Preparation method of nitrogen-phosphorus co-doped eucalyptus bark porous carbon supercapacitor electrode material

Abstract

The application discloses a preparation method of a nitrogen and phosphorus co-doped eucalyptus bark porous carbon supercapacitor electrode material, and the preparation method comprises the following steps: S1, washing, airing and crushing the eucalyptus bark, soaking in phosphoric acid and drying; S2, mixing the eucalyptus bark powder after the pretreatment of phosphoric acid with urea, completely covering with sufficient mixed chlorinated salt, and pre-carbonizing; washing and drying; S3, alkali impregnation; and S4, high-temperature activation. According to the application, the eucalyptus bark can be used to prepare the electrode material, and the electrode material has the advantages of high specific surface area, rich heteroatomic functional groups and high specific capacitance.

Description

Technical Field

[0001] This invention relates to the field of solid waste resource utilization, and in particular to a method for preparing porous carbon supercapacitor electrode materials derived from eucalyptus bark with molten salt-assisted nitrogen and phosphorus co-doping. Background Technology

[0002] In recent years, supercapacitors have played an increasingly important role in the energy storage field as electrochemical energy storage devices with considerable power density and long lifespan. Further development of supercapacitors depends on suitable, low-cost, and environmentally friendly materials as electrode active materials. Biomass-derived activated carbon possesses advantages such as excellent electrochemical performance, high specific surface area, high adsorption capacity, tunable surface chemistry, fast ion / electron transport, abundant functional groups, and low cost, making it a promising electrode material for supercapacitors and of great significance to the further development and application of electrochemical capacitors.

[0003] Eucalyptus trees are mostly native to the Australian continent, with smaller populations in neighboring New Guinea, Indonesia, and the Philippines. Introduced to various parts of the world in the 19th century, they are now mainly distributed in Oceania. In my country, they are also widely cultivated in provinces such as Yunnan, Guangdong, Guangxi, and Sichuan. With the implementation of the Western Development Strategy and the Yangtze and Pearl River Shelterbelt Projects, the progress of returning farmland to forest has accelerated, and the planting area of ​​eucalyptus has gradually expanded. Currently, eucalyptus is mainly used for making plywood, but the bark needs to be removed during processing, making the bark a waste material. To make full use of the inexpensive and readily available eucalyptus bark, many manufacturers process it into biomass pellets. However, biomass pellets have low added value and limited usage. If eucalyptus bark could be used as a raw material for the large-scale preparation of active materials for supercapacitors, it would further increase the added value of eucalyptus bark and expand the application areas of eucalyptus. Summary of the Invention

[0004] This invention discloses a method for preparing a nitrogen-phosphorus co-doped eucalyptus bark porous carbon supercapacitor electrode material. The electrode material can be prepared from eucalyptus bark and has the advantages of high specific surface area, rich heteroatom functional groups, and high specific capacitance.

[0005] To achieve the above objectives, the technical solution of the present invention is as follows:

[0006] A method for preparing a nitrogen-phosphorus co-doped eucalyptus bark porous carbon supercapacitor electrode material includes the following steps:

[0007] S1. Pretreatment: Wash, dry and crush the eucalyptus bark, then soak the eucalyptus bark powder in phosphoric acid solution for 2-10 hours, and then dry it to obtain eucalyptus bark powder pretreated with phosphoric acid.

[0008] S2. Molten Salt-Assisted Precarbonization: Phosphoric acid-pretreated eucalyptus bark powder and urea are mixed at a mass ratio of 1:1.8-2.5 and placed in a container. The mixture is then completely covered with sufficient mixed chloride salt and heated in a high-temperature furnace at a heating rate of 3-10℃ / min to 480-520℃. After precarbonization for 1.5-3 hours, the mixture is cooled to room temperature. The precarbonized mixture is washed with dilute acid solution and water, and after solid-liquid separation, a black solid powder and a mixed salt solution are obtained. The black solid powder is then dried to obtain biochar powder.

[0009] S3. Alkali impregnation: Mix biochar powder with alkaline solution evenly, soak for 2-10 hours, dry and grind into powder to obtain alkaline impregnated biochar powder.

[0010] S4. High-temperature activation: The alkali-impregnated biochar powder is placed in a tube furnace, nitrogen gas is introduced, and the temperature is heated to 580-620℃ at a heating rate of 3-10℃ / min for 1-5 hours. After cooling to room temperature, it is ground into powder, washed with dilute acid solution until neutral, and dried to obtain eucalyptus bark-derived porous carbon supercapacitor electrode material.

[0011] The mixed salt solution from step S2 can be recycled through recrystallization.

[0012] Furthermore, in step S1, the mass concentration of the phosphoric acid solution is 18-25 wt%, and the solid-liquid ratio of the eucalyptus bark powder to the phosphoric acid solution is 0.05-0.15 g / mL.

[0013] Furthermore, in step S2, the mixed chloride salt is a mixture of lithium chloride and potassium chloride.

[0014] Furthermore, in step S2, the molar ratio of lithium chloride to potassium chloride in the mixed chloride salt is 0.30–0.45:0.70–0.55.

[0015] Furthermore, in step S2, the high-temperature furnace is a muffle furnace.

[0016] Furthermore, in step S3, the alkaline solution is a 15-22 wt% potassium hydroxide solution; the mass ratio of biochar powder to alkaline solution is 1:8-15.

[0017] Furthermore, in step S4, the flow rate of nitrogen is 80–120 mL / min.

[0018] Furthermore, in step S2, the dilute acid solution used to wash the pre-carbonized mixture is a 2-6 wt% hydrochloric acid solution; in step S4, the dilute acid solution used for washing is a 2-6 wt% hydrochloric acid solution.

[0019] Furthermore, in step S2, the mass ratio of eucalyptus bark powder to urea is 1:2.

[0020] This invention also provides the application of the eucalyptus bark-derived porous carbon supercapacitor electrode material prepared by the above preparation method in the preparation of supercapacitor electrode materials.

[0021] The method for preparing nitrogen-phosphorus co-doped eucalyptus bark porous carbon supercapacitor electrode material described above involves using phosphoric acid as the phosphorus source and urea as the nitrogen source during the pre-carbonization process. Biochar is prepared in a molten salt medium. The molten salt medium enhances the reaction between phosphoric acid and urea and the biochar, resulting in a higher concentration of oxygen, nitrogen, and phosphorus heteroatom functional groups on the biochar surface. Simultaneously, the molten salt creates a limited oxygen environment, allowing atmospheric oxygen to etch the biochar during pre-carbonization, leading to more defects on the biochar surface. Alkali impregnation allows the heteroatom- and defect-rich biochar surface to adsorb an alkali activator. During subsequent high-temperature activation, the alkali activator reacts chemically with the biochar, further increasing its surface area. After washing and drying, nitrogen-phosphorus co-doped eucalyptus bark porous carbon is obtained.

[0022] The eucalyptus bark-derived porous carbon supercapacitor electrode material prepared by this invention has the following advantages:

[0023] (1) Because eucalyptus trees are widely distributed and grow quickly, eucalyptus bark is cheaper and easier to obtain than other biomass.

[0024] (2) Because the lignin content in eucalyptus bark can be as high as 19.69%, and the lignin structure is complex and has a wide decomposition temperature range, it is not easy to decompose at low temperatures, which makes eucalyptus bark have a high yield after carbonization.

[0025] (3) The present invention incorporates molten salt as a reaction medium, which enhances the reaction between phosphoric acid and urea and biochar during the carbonization process, resulting in more heteroatom functional groups on the surface of biochar. In addition, the limited oxygen environment it creates causes more defects on the surface of biochar, which can effectively increase the specific surface area of ​​the electrode material.

[0026] (4) The preparation method of the present invention is easy to operate and is conducive to industrial promotion.

[0027] (5) The eucalyptus bark-derived porous carbon supercapacitor electrode material prepared by the present invention has significant advantages such as high specific surface area, rich heteroatom functional groups, and high specific capacitance. Attached Figure Description

[0028] Figure 1 These are scanning electron microscope images and C / O / N / P elemental distribution energy spectrum diagrams of the material NP-BPC-1 obtained in Example 1;

[0029] Figure 2This is the X-ray diffraction pattern of the material NP-BPC-1 obtained in Example 1;

[0030] Figure 3 This is the nitrogen adsorption-desorption isotherm diagram of the material NP-BPC-1 obtained in Example 1;

[0031] Figure 4 This is a charge-discharge curve of the material NP-BPC-1 obtained in Example 1 under different current densities. Detailed Implementation

[0032] The present invention will be further described below with reference to specific embodiments, but the scope of protection of the present invention is not limited to the following embodiments.

[0033] In this embodiment of the invention, supercapacitor electrode materials are prepared and used for electrode fabrication. The prepared electrodes are then subjected to electrochemical performance testing. The following embodiments all employ the following method for electrode preparation: 0.08 g of supercapacitor electrode material, 0.01 g of acetylene black, and 0.01 g of polytetrafluoroethylene binder are weighed into 100 ml beakers, 20 ml of anhydrous ethanol is added, and the mixture is sonicated for 15 minutes. Afterward, it is dried in a drying oven until it reaches a clay-like consistency. The clay-like mixture is then evenly coated onto a 1 cm thick surface. 2 The square titanium mesh was pressed for 10 seconds under 10 MPa pressure using a tablet press and then dried in a vacuum drying oven. The active material (NP-BPC + acetylene black + polytetrafluoroethylene) loading on each electrode was 3 mg.

[0034] Example 1

[0035] S1. Pretreatment: After washing and drying the eucalyptus bark, pulverize it into powder using a pulverizer; mix the eucalyptus bark powder with 20wt% H3PO4 solution at a solid-liquid ratio of 0.1g / mL, soak for 8 hours, and then dry in an oven at 105℃ for 24 hours to obtain eucalyptus bark powder pretreated with phosphoric acid.

[0036] S2. Molten Salt-Assisted Precarbonization: Eucalyptus bark powder pretreated with phosphoric acid and urea are placed in a ceramic pot at a mass ratio of 1:2, and completely covered with sufficient mixed salt (lithium chloride and potassium chloride, molar ratio of 0.41:0.59). The mixture is heated to 500°C in a muffle furnace at a heating rate of 5°C / min and precarbonized for 2 hours, then cooled to room temperature. The precarbonized mixture is washed with 5% hydrochloric acid solution and ultrapure water, filtered, and black solid powder and mixed salt solution are obtained. The black solid powder is dried in an oven at 105°C for 24 hours to obtain biochar powder. The mixed salt solution can be recovered and reused through evaporation and crystallization.

[0037] S3. Alkali impregnation: Mix biochar powder with 20wt% potassium hydroxide solution, the mass ratio of biochar powder to potassium hydroxide solution is 1:12, stir, impregnate for 8 hours, place in a 105℃ forced-air drying oven to dry for 36 hours, grind into powder after drying to obtain alkali impregnated biochar powder.

[0038] S4. High-temperature activation: The alkali-impregnated biochar powder was placed in a tube furnace. Nitrogen flow rate was 100 mL / min, and the temperature was increased to 600°C at a rate of 5°C / min. Activation was carried out for 2 hours, followed by natural cooling to room temperature. The powder was then ground into a powder and washed with 5 wt% hydrochloric acid until neutral. The powder was then dried in a 105°C oven for 12 hours to obtain eucalyptus bark-derived porous carbon supercapacitor electrode material. The material obtained in this embodiment is designated NP-BPC-1.

[0039] Electrodes were prepared using NP-BPC-1, and their electrochemical performance was tested. In a three-electrode system, using 6 mol / L KOH solution as the electrolyte, a mercury oxide electrode as the reference electrode, a platinum sheet electrode as the counter electrode, and an electrode loaded with active material as the working electrode, the electrochemical performance was tested. The specific capacitances at different current densities are shown in Table 1 below.

[0040] Table 1

[0041] Current density (A / g) 0.5 1 2 5 10 20 Specific capacitance (F / g) 418.5 386.0 359.8 323.7 302.2 269.1

[0042] In a slow, oxygen-limited etching environment assisted by molten salt, NP-BPC-1 porous carbon material, prepared from eucalyptus bark as raw material, phosphoric acid as phosphorus source, and urea as nitrogen source, exhibits excellent capacitance performance after pre-carbonization and subsequent KOH activation.

[0043] Example 2

[0044] S1. Pretreatment: After washing and drying the eucalyptus bark, grind it into powder using a pulverizer; mix the eucalyptus bark powder with 25wt% H3PO4 solution at a solid-liquid ratio of 0.08g / mL, soak for 9h, and then dry in an oven at 105℃ for 24h to obtain eucalyptus bark powder pretreated with phosphoric acid.

[0045] S2. Molten Salt-Assisted Precarbonization: Phosphoric acid-pretreated eucalyptus bark powder and urea were placed in a ceramic pot at a mass ratio of 1:1.8 and completely covered with sufficient mixed salt (lithium chloride and potassium chloride, molar ratio of 0.30:0.70). The mixture was heated to 480°C in a muffle furnace at a heating rate of 4°C / min for 2 hours and then cooled to room temperature. The precarbonized mixture was washed with 5% hydrochloric acid solution and ultrapure water, filtered, and black solid powder and mixed salt solution were obtained. The black solid powder was dried in an oven at 105°C for 24 hours to obtain biochar powder. The mixed salt solution could be recovered and reused through evaporation and crystallization.

[0046] S3. Alkali impregnation: Mix biochar powder with 15wt% potassium hydroxide solution, the mass ratio of biochar powder to potassium hydroxide solution is 1:15, stir, impregnate for 8 hours, place in a 105℃ forced-air drying oven to dry for 36 hours, grind into powder after drying to obtain alkali impregnated biochar powder.

[0047] S4. High-temperature activation: The alkali-impregnated biochar powder was placed in a tube furnace. Nitrogen flow rate was 100 mL / min, and the temperature was increased to 620°C at a rate of 6°C / min for 2 hours. After natural cooling to room temperature, the powder was ground into a powder and washed with 5 wt% hydrochloric acid until neutral. The powder was then dried in a 105°C oven for 12 hours to obtain eucalyptus bark-derived porous carbon supercapacitor electrode material. The material obtained in this embodiment is designated NP-BPC-2.

[0048] Electrodes were prepared using NP-BPC-2, and their electrochemical performance was tested. In a three-electrode system, using 6 mol / L KOH solution as the electrolyte, a mercury oxide electrode as the reference electrode, a platinum sheet electrode as the counter electrode, and an electrode loaded with active material as the working electrode, the electrochemical performance was tested. The specific capacitances at different current densities are shown in Table 2 below.

[0049] Table 2

[0050] Current density (A / g) 0.5 1 2 5 10 20 Specific capacitance (F / g) 417.2 383.7 345.6 310.4 286.9 257.3

[0051] Example 3

[0052] S1. Pretreatment: After washing and drying the eucalyptus bark, grind it into powder using a pulverizer; mix the eucalyptus bark powder with 18wt% H3PO4 solution at a solid-liquid ratio of 0.15g / mL, soak for 8 hours, and then dry in an oven at 105℃ for 24 hours to obtain eucalyptus bark powder pretreated with phosphoric acid.

[0053] S2. Molten Salt-Assisted Precarbonization: Phosphoric acid-pretreated eucalyptus bark powder and urea were placed in a ceramic pot at a mass ratio of 1:2.5, and completely covered with sufficient mixed salt (lithium chloride and potassium chloride, molar ratio of 0.45:0.55). The mixture was heated to 520°C in a muffle furnace at a heating rate of 10°C / min for 3 hours of precarbonization, and then cooled to room temperature. The precarbonized mixture was washed with 5% hydrochloric acid solution and ultrapure water, filtered, and black solid powder and mixed salt solution were obtained. The black solid powder was dried in an oven at 105°C for 24 hours to obtain biochar powder. The mixed salt solution could be recovered and reused through evaporation and crystallization.

[0054] S3. Alkali impregnation: Mix biochar powder with 22wt% potassium hydroxide solution, the mass ratio of biochar powder to potassium hydroxide solution is 1:8, stir, impregnate for 8 hours, place in a 105℃ forced-air drying oven to dry for 36 hours, grind into powder after drying to obtain alkali impregnated biochar powder.

[0055] S4. High-temperature activation: The alkali-impregnated biochar powder was placed in a tube furnace. Nitrogen flow rate was 100 mL / min, and the temperature was increased to 580°C at a rate of 3°C / min for 2 hours. After natural cooling to room temperature, the powder was ground into a powder and washed with 5 wt% hydrochloric acid until neutral. The powder was then dried in a 105°C oven for 12 hours to obtain eucalyptus bark-derived porous carbon supercapacitor electrode material. The material obtained in this embodiment is designated NP-BPC-3.

[0056] Electrodes were prepared using NP-BPC-3, and their electrochemical performance was tested. In a three-electrode system, using 6 mol / L KOH solution as the electrolyte, a mercury oxide electrode as the reference electrode, a platinum sheet electrode as the counter electrode, and an electrode loaded with active material as the working electrode, the electrochemical performance was tested. The specific capacitances at different current densities are shown in Table 3 below.

[0057] Table 3

[0058] Current density (A / g) 0.5 1 2 5 10 20 Specific capacitance (F / g) 415.1 381.7 348.9 309.8 276.9 237.7

[0059] Comparative Example 1

[0060] S1. Pretreatment: After washing and drying the eucalyptus bark, pulverize it into powder using a pulverizer; mix the eucalyptus bark powder with 20wt% H3PO4 solution at a solid-liquid ratio of 0.1g / mL, soak for 8 hours, and then dry in an oven at 105℃ for 24 hours to obtain eucalyptus bark powder pretreated with phosphoric acid.

[0061] S2. Pre-carbonization: Phosphoric acid-pretreated eucalyptus bark powder and urea were placed in a ceramic pot at a mass ratio of 1:2. The mixture was heated to 500°C in a tube furnace under nitrogen protection at a nitrogen flow rate of 100 mL / min and a heating rate of 5°C / min. After pre-carbonization for 2 hours, the mixture was cooled to room temperature. The pre-carbonized mixture was washed with 5% hydrochloric acid solution and ultrapure water, filtered, and a black solid powder and a mixed salt solution were obtained. The black solid powder was dried in an oven at 105°C for 24 hours to obtain biochar powder.

[0062] S3. Alkali impregnation: Mix biochar powder with 20wt% potassium hydroxide solution, the mass ratio of biochar powder to potassium hydroxide solution is 1:12, stir, impregnate for 8 hours, place in a 105℃ forced-air drying oven to dry for 36 hours, grind into powder after drying to obtain alkali impregnated biochar powder.

[0063] S4. High-temperature activation: The alkali-impregnated biomass char powder was placed in a tube furnace. Nitrogen flow was introduced at a rate of 100 mL / min, and the temperature was increased to 600°C at a rate of 5°C / min. After activation for 2 hours, the powder was allowed to cool naturally to room temperature. After grinding, the powder was washed with 5 wt% hydrochloric acid until neutral and then dried in a 105°C oven for 12 hours to obtain the biomass porous carbon supercapacitor electrode material. The material obtained in this example is labeled N2-NP-BPC.

[0064] Electrodes were prepared using N2-NP-BPC, and their electrochemical performance was tested. In a three-electrode system, 6 mol / L KOH solution was used as the electrolyte, a mercury oxide electrode as the reference electrode, a platinum sheet electrode as the counter electrode, and an electrode loaded with active material as the working electrode. The specific capacitances at different current densities are shown in Table 4 below.

[0065] Table 4

[0066] Current density (A / g) 0.5 1 2 5 10 20 Specific capacitance (F / g) 259.0 246.4 236.6 225.5 215.4 201.8

[0067] The specific capacitance of the material pre-carbonized under a nitrogen atmosphere without molten salt is much lower than that pre-carbonized with molten salt, indicating that molten salt has a positive effect on improving the specific capacitance of the material.

[0068] Comparative Example 2

[0069] S1. Pretreatment: After washing and drying the eucalyptus bark, grind it into powder using a pulverizer, and then dry it in an oven at 105℃ for 24 hours to obtain pretreated eucalyptus bark powder.

[0070] S2. Molten Salt-Assisted Pre-Carbonization: Pretreated eucalyptus bark powder was placed in a ceramic pot and completely covered with a sufficient amount of mixed salt (lithium chloride and potassium chloride, molar ratio 0.41:0.59). The pot was heated to 500°C in a muffle furnace at a heating rate of 5°C / min for 2 hours, then cooled to room temperature. The pre-carbonized mixture was washed with 5% hydrochloric acid solution and ultrapure water, then filtered to obtain a black solid powder and a mixed salt solution. The black solid powder was dried in a 105°C oven for 24 hours to obtain biochar powder. The mixed salt solution could be recovered and reused through evaporation and crystallization.

[0071] S3. Alkali impregnation: Mix biochar powder with 20wt% potassium hydroxide solution, the mass ratio of biochar powder to potassium hydroxide solution is 1:12, stir, impregnate for 8 hours, and then place in a 105℃ forced-air drying oven to dry for 36 hours. After drying, grind into powder to obtain alkali-impregnated biochar powder.

[0072] S4. Activation: The alkali-impregnated biomass char powder was placed in a tube furnace. Nitrogen flow was introduced at a rate of 100 mL / min, and the temperature was increased to 600°C at a rate of 5°C / min. Activation was carried out for 2 hours, followed by natural cooling to room temperature. The powder was then ground into a powder and washed with 5 wt% hydrochloric acid until neutral. The powder was then dried in a 105°C oven for 12 hours to obtain the biomass porous carbon supercapacitor electrode material. The material obtained in this example is labeled BPC.

[0073] Electrodes were prepared using BPC, and their electrochemical performance was tested. In a three-electrode system, using 6 mol / L KOH solution as the electrolyte, a mercury oxide electrode as the reference electrode, a platinum sheet electrode as the counter electrode, and an electrode loaded with active material as the working electrode, the electrochemical performance was tested. The specific capacitances at different current densities are shown in Table 5 below.

[0074] Table 5

[0075] Current density (A / g) 0.5 1 2 5 10 20 Specific capacitance (F / g) 330.5 300.8 279.8 254.7 232.7 206.5

[0076] Comparative Example 3

[0077] S1. Pretreatment: After washing and drying the eucalyptus bark, pulverize it into powder using a pulverizer; mix the obtained eucalyptus bark powder with 20wt% H3PO4 solution at a solid-liquid ratio of 0.1g / mL, soak for 8h, and then dry in an oven at 105℃ for 24h to obtain eucalyptus bark powder pretreated with phosphoric acid.

[0078] S2. Molten Salt-Assisted Precarbonization: Eucalyptus bark powder pretreated with phosphoric acid is placed in a ceramic pot and completely covered with a sufficient amount of mixed salt (lithium chloride and potassium chloride, molar ratio of 0.41:0.59). The pot is heated to 500°C in a muffle furnace at a heating rate of 5°C / min and precarbonized for 2 hours, then cooled to room temperature. The precarbonized mixture is washed with 5% hydrochloric acid solution and ultrapure water, filtered, and black solid powder and mixed salt solution are obtained. The black solid powder is dried in an oven at 105°C for 24 hours to obtain biochar powder. The mixed salt solution can be recovered and reused through evaporation and crystallization.

[0079] S3. Alkali impregnation: Mix biochar powder with 20wt% potassium hydroxide solution, the mass ratio of biochar powder to potassium hydroxide solution is 1:12, stir, impregnate for 8 hours, place in a 105℃ forced-air drying oven to dry for 36 hours, grind into powder after drying to obtain alkali impregnated biochar powder.

[0080] S4. High-temperature activation: The alkali-impregnated biomass char powder was placed in a tube furnace. Nitrogen flow was introduced at a rate of 100 mL / min, and the temperature was increased to 600°C at a rate of 5°C / min. Activation was carried out for 2 hours, followed by natural cooling to room temperature. The powder was then ground into a powder and washed with 5 wt% hydrochloric acid until neutral. The powder was then dried in a 105°C oven for 12 hours to obtain the biomass porous carbon supercapacitor electrode material. The material obtained in this example is labeled P-BPC.

[0081] Electrodes were prepared using P-BPC, and their electrochemical performance was tested. In a three-electrode system, using 6 mol / L KOH solution as the electrolyte, a mercury oxide electrode as the reference electrode, a platinum sheet electrode as the counter electrode, and an electrode loaded with active material as the working electrode, the electrochemical performance was tested. The specific capacitances at different current densities are shown in Table 6 below.

[0082] Table 6

[0083]

[0084]

[0085] The lower specific capacitance of P-PBC compared to BPC in Comparative Example 2 is due to the overactivation effect caused by using phosphoric acid pretreatment alone without introducing urea, which reduces the microporosity of the material.

[0086] Comparative Example 4

[0087] S1. Pretreatment: After washing and drying the eucalyptus bark, grind it into powder using a pulverizer, and then dry it in an oven at 105℃ for 24 hours to obtain pretreated eucalyptus bark powder.

[0088] S2. Molten Salt-Assisted Pre-Carbonization: Pretreated eucalyptus bark powder and urea were placed in a ceramic pot at a mass ratio of 1:2, and completely covered with sufficient mixed salt (lithium chloride and potassium chloride, molar ratio of 0.41:0.59). The mixture was heated to 500°C in a muffle furnace at a heating rate of 5°C / min for 2 hours, and then cooled to room temperature. The pre-carbonized mixture was washed with 5% hydrochloric acid solution and ultrapure water, and then filtered to obtain a black solid powder and a mixed salt solution. The black solid powder was dried in an oven at 105°C for 24 hours to obtain biochar powder. The mixed salt solution could be recovered and reused through evaporation and crystallization.

[0089] S3. Alkali impregnation: Mix biochar powder with 20wt% potassium hydroxide solution, the mass ratio of biochar powder to potassium hydroxide solution is 1:12, impregnate for 8 hours, place in a 105℃ forced-air drying oven to dry for 36 hours, grind into powder after drying to obtain alkali impregnated biochar powder.

[0090] S4. High-temperature activation: The alkali-impregnated biomass char powder was placed in a tube furnace. Nitrogen flow was introduced at a rate of 100 mL / min, and the temperature was increased to 600°C at a rate of 5°C / min. After activation for 2 hours, the powder was allowed to cool naturally to room temperature. The activated product was then ground and washed with 5 wt% hydrochloric acid until neutral. It was then dried in a 105°C oven for 12 hours to obtain the biomass porous carbon supercapacitor electrode material. The material obtained in this example is labeled N-BPC.

[0091] Electrodes were prepared using N-BPC, and their electrochemical performance was tested. In a three-electrode system, using 6 mol / L KOH solution as the electrolyte, a mercury oxide electrode as the reference electrode, a platinum sheet electrode as the counter electrode, and an electrode loaded with active material as the working electrode, the electrochemical performance was tested. The specific capacitances at different current densities are shown in Table 7 below.

[0092] Table 7

[0093] Current density (A / g) 0.5 1 2 5 10 20 Specific capacitance (F / g) 384.8 356.7 337.0 312.7 291.9 264.5

[0094] Without phosphoric acid, the material has a low specific capacitance.

Claims

1. A method for preparing a nitrogen-phosphorus co-doped eucalyptus bark porous carbon supercapacitor electrode material, characterized in that... Includes the following steps: S1. Pretreatment: Wash, dry and crush the eucalyptus bark, then soak the eucalyptus bark powder in phosphoric acid solution for 2-10 hours, and then dry it to obtain eucalyptus bark powder pretreated with phosphoric acid. S2. Molten Salt-Assisted Precarbonization: Phosphoric acid-pretreated eucalyptus bark powder and urea are mixed at a mass ratio of 1:1.8-2.5 and placed in a container. The mixture is then completely covered with sufficient mixed chloride salt and heated in a high-temperature furnace at a heating rate of 3-10℃ / min to 480-520℃. After precarbonization for 1.5-3 hours, the mixture is cooled to room temperature. The precarbonized mixture is washed with dilute acid solution and water, and after solid-liquid separation, a black solid powder and a mixed salt solution are obtained. The black solid powder is then dried to obtain biochar powder. S3. Alkali impregnation: Mix biochar powder with alkaline solution evenly, soak for 2-10 hours, dry and grind into powder to obtain alkaline impregnated biochar powder. S4. High-temperature activation: The alkali-impregnated biomass char powder is placed in a tube furnace, nitrogen is introduced, and the temperature is heated to 580-620℃ at a heating rate of 3-10℃ / min for 1-5 hours. After cooling to room temperature, it is ground into powder and washed with dilute acid solution until neutral. After drying, eucalyptus bark-derived porous carbon supercapacitor electrode material is obtained. In step S1, the mass concentration of the phosphoric acid solution is 18-25 wt%, and the solid-liquid ratio of eucalyptus bark powder to phosphoric acid solution is 0.05-0.15 g / mL; In step S2, the molar ratio of lithium chloride to potassium chloride in the mixed chloride salt is 0.30–0.45:0.70–0.

55.

2. The preparation method according to claim 1, characterized in that: In step S2, the high-temperature furnace is a muffle furnace.

3. The preparation method according to claim 1, characterized in that: In step S3, the alkaline solution is a 15-22 wt% potassium hydroxide solution; the mass ratio of biochar powder to alkaline solution is 1:8-15.

4. The preparation method according to claim 1, characterized in that: In step S4, the flow rate of nitrogen is 80–120 mL / min.

5. The preparation method according to claim 1, characterized in that: In step S2, the dilute acid solution used to wash the pre-carbonized mixture is a 2-6 wt% hydrochloric acid solution. In step S4, the dilute acid solution used for washing is a 2-6 wt% hydrochloric acid solution.

6. The preparation method according to claim 1, characterized in that: In step S2, the mass ratio of eucalyptus bark powder to urea is 1:

2.

7. The application of the eucalyptus bark-derived porous carbon supercapacitor electrode material prepared according to any one of claims 1 to 6 in the preparation of supercapacitor electrode materials.

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