High specific surface area nitrogen-doped mesoporous graphite carbon negative electrode supercapacitor material and preparation method thereof

Abstract

The application discloses a high specific surface area nitrogen-doped mesoporous graphite carbon negative electrode supercapacitor material, and is characterized in that 4,5-imidazole dicarboxylic acid sodium and zinc acetate dihydrate are used as raw materials, Zn-MOF is synthesized through a hydrothermal method, polyvinylpyrrolidone is used as an external carbon / nitrogen source, a direct carbonization method is adopted, PVP / Zn-MOF is used as a precursor mixed material, high-temperature carbonization is carried out under a nitrogen atmosphere, hydrochloric acid is used to remove Zn, a porous carbon material is obtained, the porous carbon material is mixed with KOH, activation is carried out under a nitrogen atmosphere, and the nitrogen-doped mesoporous graphite carbon negative electrode supercapacitor material is obtained. The mesoporous graphite carbon material has the advantages of a large specific surface area, uniform mesopore distribution and high pore volume. The application further discloses application of the nitrogen-doped mesoporous graphite carbon negative electrode supercapacitor material as a negative electrode material in preparation of a supercapacitor. The supercapacitor has the practical potential of high specific capacitance, excellent coulombic efficiency and good cycle performance.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of supercapacitor energy storage, and particularly relates to a high specific surface nitrogen-doped mesoporous graphite carbon negative electrode supercapacitor material and a preparation method thereof, and application of the high specific surface nitrogen-doped mesoporous graphite carbon negative electrode supercapacitor material in energy storage. BACKGROUND

[0002] With the development of science and technology, people's dependence on energy is increasing. In the face of the grim situation of the depletion of fossil energy, multidisciplinary intersection has become an inevitable trend, such as exploring and developing new energy materials through the mutual fusion of nanoscience and material chemistry, inorganic chemistry, analytical chemistry, etc. to meet the increasing demand for energy storage and conversion. Therefore, research on new energy materials with potential commercial development is the core and foundation of effective energy storage and conversion.

[0003] Supercapacitors, as an energy storage and conversion system, are a new type of energy storage device between traditional capacitors and rechargeable batteries. The device has the advantages of fast charging and discharging, high power density, and long cycle life, and is generally composed of a positive electrode material with high specific capacitance and good cycle stability, and a negative electrode material with excellent rate capability and conductivity. At present, commercial activated carbon is usually selected as the negative electrode material of supercapacitors to match the positive electrode material with large specific capacitance. However, the specific capacitance of commercial activated carbon is currently only 113 F·g -1 , which is extremely unbalanced with the positive electrode material, resulting in a small energy density, even only 1 / 20-1 / 10 of lithium ion battery materials.

[0004] Metal-organic framework (MOFs) derived porous carbon-based materials have become a new type of electrode material due to their unique three-dimensional spatial topology, ultra-large specific surface area (up to 3000 m 2 / g), and multiple active sites. Metal (oxide) / mesoporous carbon composite materials prepared by direct carbonization of MOFs as precursors inherit redox active sites and excellent conductivity, and are a kind of supercapacitor positive electrode material that can be used in multiple channels. The positive electrode material of supercapacitors can also be derived into nano-porous carbon materials through subsequent activation and other treatments, and the carbon materials have high specific surface area and ideal pore volume, which is beneficial to further improve the ion mobility and specific capacitance of supercapacitors, and is a very effective negative electrode material. SUMMARY

[0005] The application aims to provide a high specific surface area and high nitrogen-doped mesoporous graphite carbon material, which is a metal organic framework soft template (Zn-MOF) activated by acid washing and the like and is used as a negative electrode material of an energy storage device, can significantly improve the performance of the negative electrode material, provides a better choice for constructing a high energy density device, and has potential practical value.

[0006] The application aims to achieve the above-mentioned purpose by adopting the following technical scheme.

[0007] The high specific surface area and high nitrogen-doped mesoporous graphite carbon negative electrode supercapacitor material is prepared by using 4,5-imidazole dicarboxylic acid sodium and zinc acetate dihydrate as raw materials, synthesizing a metal organic framework soft template (Zn-MOF) through a hydrothermal method, using polyvinylpyrrolidone (PVP) as an external carbon / nitrogen source, using a direct carbonization method, using PVP / Zn-MOF as a precursor mixed material, high-temperature carbonization under a nitrogen atmosphere, removing Zn through hydrochloric acid, mixing the porous carbon material PNC with KOH, and activating under a nitrogen atmosphere to obtain the nitrogen-doped mesoporous graphite carbon negative electrode supercapacitor material.

[0008] Another purpose of the application is to provide a preparation method of the high specific surface area and high nitrogen-doped mesoporous graphite carbon negative electrode supercapacitor material.

[0009] Step (1), 4,5-imidazole dicarboxylic acid and sodium hydroxide are heated and reacted in pure water to obtain a 4,5-imidazole dicarboxylic acid sodium solution; zinc acetate dihydrate is added to a mixed solvent of anhydrous ethanol and water, stirred until uniform, 4,5-imidazole dicarboxylic acid sodium solution is added, stirred until a large amount of white precipitate is generated, ultrasonically treated for 15-30 min, and reacted at 100-140 DEG C for 12-48 h; after the reaction is completed, the mixture is cooled to room temperature, centrifuged to obtain a white solid, washed twice with pure water, then washed twice with anhydrous ethanol to remove excess impurities such as metal Zn ions and ligands (4,5-imidazole dicarboxylic acid), and freeze-dried to obtain the metal organic framework soft template Zn-MOF;

[0010] Step (2), Zn-MOF and polyvinylpyrrolidone (PVP) are dispersed in anhydrous ethanol, stirred and mixed until uniform, the solvent is evaporated, then the mixture is placed in a tube furnace, heated to 600-900 DEG C under a nitrogen atmosphere, and kept at a constant temperature for 1-2 h and then cooled to room temperature; concentrated hydrochloric acid with a concentration of 2-4 mol / L is added to the obtained black powder, stirred for 12-24 h, centrifuged, concentrated hydrochloric acid with a concentration of 2-4 mol / L is added again, stirred and centrifuged, and the above steps are repeated twice to remove excess Zn; the black powder is collected, washed with pure water until the filtrate is neutral, and freeze-dried to obtain the porous carbon material (PNC);

[0011] Step (3), the porous carbon material (PNC) and KOH are mixed uniformly at a mass ratio of 1:3, a small amount of pure water and anhydrous ethanol are added for soaking for 12 h, and the black viscous slurry is obtained by natural air drying or solvent evaporation drying in a ventilation kitchen, and then the black viscous slurry is placed in a tube furnace and activated at 700-750℃ under a nitrogen atmosphere for 2 h; the activated porous carbon PANC, i.e., a nitrogen-doped mesoporous graphite carbon negative electrode supercapacitor material, is obtained by neutralization with dilute hydrochloric acid, washing with pure water until neutral, centrifugal separation, and freeze-drying.

[0012] In step (1), the molar ratio of 4,5-imidazole dicarboxylic acid to sodium hydroxide is 1:2.

[0013] The reaction temperature of 4,5-imidazole dicarboxylic acid and sodium hydroxide is 70℃.

[0014] The concentration of the sodium 4,5-imidazole dicarboxylate solution is 0.2 mol / L.

[0015] The amount ratio of zinc acetate dihydrate to the mixed solvent of anhydrous ethanol and water is 5 mmol:30 mL.

[0016] The mixed solvent of anhydrous ethanol and water is prepared by mixing pure water and ethanol at a volume ratio of 1:2-2:1.

[0017] The molar ratio of sodium 4,5-imidazole dicarboxylate to zinc acetate dihydrate is 1:1.

[0018] The rotation speed of the centrifugation is 10000 r / min, and the centrifugation time is 5 min.

[0019] The freeze-drying time is 12-24 h.

[0020] In step (2), the mass ratio of Zn-MOF to PVP is 1:1-2:1.

[0021] 10-30 mL of anhydrous ethanol is used per 100 mg of Zn-MOF. Preferably, 15 mL of anhydrous ethanol is used per 100 mg of Zn-MOF.

[0022] Preferably, the Zn-MOF and polyvinylpyrrolidone are dispersed in anhydrous ethanol, stirred for 5 h to mix uniformly, and the solvent is evaporated at 75℃.

[0023] The heating rate is 3-5℃ / min, preferably 4℃ / min.

[0024] Preferably, the temperature is raised to 800-850℃ under a nitrogen atmosphere, and calcination is performed for 2 h.

[0025] Each 100 mg of Zn-MOF, 15-35 mL of 2-4 mol / L hydrochloric acid is used each time.

[0026] The centrifugal speed is 12000 r / min, and the centrifugal time is 5 min.

[0027] The freeze-drying time is 12-24 h.

[0028] In step (3), in order to control the proper spreading, dispersion and solvent evaporation of the PNC material, 0.5-1.5 mL of pure water and 1-3 mL of anhydrous ethanol are used for each 50 mg of PNC; preferably, 1 mL of pure water and 1 mL of anhydrous ethanol are used for each 50 mg of PNC.

[0029] The temperature for evaporating the solvent is 75 DEG C.

[0030] The temperature rising rate is 3-5 DEG C / min, preferably 4 DEG C / min.

[0031] The freeze-drying time is 12 h.

[0032] After the mesoporous graphite carbon material is characterized by XRD, BET, SEM, TEM, Raman spectrum and the like, it is determined that the mesoporous graphite carbon material has a large specific surface area (the specific surface area is greater than 850 m 2 / g), uniform mesopore distribution and high pore volume. In a three-electrode system, the electrochemical energy storage performance of the material as a supercapacitor negative material is tested, the application of the mesoporous graphite carbon material in electrochemical energy storage is clarified, and through data analysis, the supercapacitor shows practical potential such as high specific capacitance, excellent coulombic efficiency and good cycle performance. Another object of the present application is to provide the application of the high specific surface area nitrogen-doped mesoporous graphite carbon negative supercapacitor material as a negative electrode material in the preparation of a supercapacitor to meet the practical application of the supercapacitor.

[0033] A supercapacitor, which is a single electrode prepared by taking high specific surface area nitrogen-doped mesoporous graphite carbon negative supercapacitor material as a negative material, acetylene black and PVDF according to a mass ratio of 75:15:10, taking KOH as an electrolyte, taking a platinum wire electrode as a counter electrode, and taking a Hg / HgO electrode as a reference electrode to form a three-electrode system.

[0034] The present application has the following beneficial effects:

[0035] The traditional composite metal oxide / hydroxide method to improve the capacitance is easy to be damaged, low in heteroatom content and poor in functionality. The nitrogen atom-doped carbon-based material obtained by a soft template of a nitrogen-rich MOF is often high in nitrogen atom content and better in controllability. Research shows that (1) higher nitrogen atom doping indicates more material defects, improves electrode wettability and helps ion / electron migration; (2) the MOF-based soft template constructed by zinc ions and nitrogen-rich ligands at the molecular level is easy to remove the Zn core, easy to crystallize the carbon framework structure and conducive to anchoring nitrogen atoms, which is beneficial to the regulation and improvement of nitrogen content. The carbon material anchored by nitrogen atoms has high and controllable nitrogen content, good material wettability, rich surface-interface redox activity and special surface defect structure after multi-step activation of acid and alkali, thereby showing high electrochemical performance. Specifically, the electrochemical performance of the nitrogen-doped mesoporous graphite carbon material is more prominent than that of commercial activated carbon, and the nitrogen-doped mesoporous graphite carbon material has good commercial value.

[0036] (1) The Zn-MOF is used as the material in the present application, the Zn core is removed by hydrochloric acid, and the carbon skeleton is etched by KOH, so that the surface defects and redox sites of the nitrogen-doped mesoporous graphite carbon material are enriched, the electrochemical performance of the nitrogen-doped mesoporous graphite carbon material is more prominent than that of commercial activated carbon, and the nitrogen-doped mesoporous graphite carbon material has good commercial value.

[0037] (2) The nitrogen content of the nitrogen-doped mesoporous graphite carbon material is controllable, and the nitrogen content can be further regulated by adding a nitrogen-containing auxiliary agent PVP, and the carbon / nitrogen ratio and specific surface area of the nitrogen-doped carbon material can be changed by optimizing the carbonization temperature, so as to regulate the final electrochemical performance.

[0038] (3) The high-specific-surface-area nitrogen-doped mesoporous graphite carbon material can be used as a supercapacitor negative electrode material, and compared with other types of inorganic materials, the present application is more comprehensive, and the nitrogen-doped carbon material has higher chemical stability, and the ion migration rate and specific capacitance of the supercapacitor can be further improved due to the high specific surface area and ideal pore volume of the nitrogen-doped carbon material.

[0039] (4) The carbon purity of the nitrogen-doped mesoporous graphite carbon material is high, the metal Zn has been completely removed, the coulombic efficiency is high, the resistivity is low, the specific capacity is 2-3 times that of commercial activated carbon, and the cycle stability is good. BRIEF DESCRIPTION OF DRAWINGS

[0040] Figure 1 The infrared spectrum of Zn-MOF and ligand prepared in Example 1.

[0041] Figure 2 The thermogravimetric diagram of Zn-MOF prepared in Example 1.

[0042] Figure 3 The XRD diagram of mesoporous carbon PANC2 / 1-800 prepared in Example 1.

[0043] Figure 4The adsorption-desorption isotherm of PANC2 / 1-800 prepared in Example 1.

[0044] Figure 5 The pore size distribution diagram is shown for PANC2 / 1-800 prepared in Example 1.

[0045] Figure 6 The Raman spectrum of PANC2 / 1-800 prepared in Example 1.

[0046] Figure 7 SEM image of PANC2 / 1-800 prepared for Example 1.

[0047] Figure 8 TEM image of PANC2 / 1-800 prepared for Example 1.

[0048] Figure 9 The CP curves of PANC2 / 1-800 at different current densities prepared in Example 1.

[0049] Figure 10 The image shows the CV curves of the PANC2 / 1-800 prepared in Example 1 at different scan rates.

[0050] Figure 11 The electrochemical impedance spectroscopy of PANC2 / 1-800 prepared in Example 1.

[0051] Figure 12 A comparison of the specific capacitance of PANC2 / 1-800 prepared in Example 1 at different current densities, calculated from the CP curve.

[0052] Figure 13 PANC2 / 1-800 prepared for Example 1 was tested at 30 mV s. -1 Cyclic performance diagram of PANC2 / 1-800 in the lower three-electrode system.

[0053] Figure 14 The image shows the XRD pattern of the mesoporous carbon PANC2 / 1-850 prepared in Example 2.

[0054] Figure 15 The adsorption-desorption isotherm of PANC2 / 1-850 prepared in Example 2.

[0055] Figure 16 The pore size distribution of PANC2 / 1-850 prepared in Example 2 is shown.

[0056] Figure 17 The Raman spectrum of PANC2 / 1-850 prepared in Example 2.

[0057] Figure 18SEM image of PANC2 / 1-850 prepared for Example 2.

[0058] Figure 19 TEM image of PANC2 / 1-850 prepared for Example 2.

[0059] Figure 20 CP curve plot of PANC2 / 1-850 prepared for Example 2 at different current densities.

[0060] Figure 21 CV curve plot of PANC2 / 1-850 prepared for Example 2 at different scan rates.

[0061] Figure 22 Electrochemical impedance spectroscopy of PANC2 / 1-850 prepared for Example 2.

[0062] Figure 23 Specific capacitance vs. current density plot of PANC2 / 1-850 prepared for Example 2 calculated from CP curve.

[0063] Figure 24 XRD pattern of porous carbon PANC1 / 1-850 prepared for Example 3.

[0064] Figure 25 Adsorption-desorption isotherm plot of PANC1 / 1-850 prepared for Example 3.

[0065] Figure 26 Pore size distribution plot of PANC1 / 1-850 prepared for Example 3.

[0066] Figure 27 Raman spectroscopy plot of PANC1 / 1-850 prepared for Example 3.

[0067] Figure 28 SEM image of PANC1 / 1-850 prepared for Example 2.

[0068] Figure 29 TEM image of PANC1 / 1-850 prepared for Example 2.

[0069] Figure 30 CP curve plot of PANC1 / 1-850 prepared for Example 3 at different current densities.

[0070] Figure 31 CV curve plot of PANC1 / 1-850 prepared for Example 3 at different scan rates.

[0071] Figure 32 Electrochemical impedance spectroscopy of PANC1 / 1-850 prepared for Example 3.

[0072] Figure 33 The specific capacitance of PANC1 / 1-850 prepared in Example 3 at different current densities was plotted against the CP curve. DETAILED DESCRIPTION

[0073] Example 1

[0074] The high specific surface area nitrogen-doped mesoporous graphite carbon negative electrode supercapacitor material is prepared by the following preparation method, comprising the following steps:

[0075] Step (1), 100 mL of pure water was added to a beaker, then 3.120 g of 4,5-imidazole dicarboxylic acid and 1.600 g of sodium hydroxide (molar ratio of 4,5-imidazole dicarboxylic acid to sodium hydroxide was 1:2) were added, heated to 70°C, and stirred to dissolve, forming a 0.2 mol·L -1 4,5-imidazole dicarboxylic acid sodium solution; 1.098 g (5 mmol) of zinc acetate dihydrate was added to a mixed solvent containing 15 mL of anhydrous ethanol and 15 mL of water, stirred uniformly, and 25 mL of freshly prepared 0.2 mol·L -1 4,5-imidazole dicarboxylic acid sodium solution was added, and after sufficient stirring, a large amount of white precipitate was produced, which was ultrasonically dispersed for 30 min and transferred into a 100 mL polytetrafluoroethylene-lined reaction kettle, which was placed in a forced air drying oven and reacted at 120°C for 12 h; after the reaction was completed, it was cooled to room temperature and centrifuged at a speed of 10,000 r / min for 5 min to obtain a white solid, which was washed twice with pure water and then twice with anhydrous ethanol, and freeze-dried for 12 h to obtain a metal-organic framework soft template (Zn-MOF), the characterization of which and the ligand (4,5-imidazole dicarboxylic acid) are as follows: Figure 1 and 2 Both the functional group structure and the framework stability indicate that Zn is successfully coordinated with the ligand.

[0076] Step (2), 100 mg of Zn-MOF and polyvinylpyrrolidone (PVP K30) (mass ratio of Zn-MOF to polyvinylpyrrolidone was 2:1) were dispersed in 15 mL of anhydrous ethanol and stirred for 5 h to mix uniformly, then transferred into a porcelain boat and placed in an oven to evaporate the solvent at 75°C, then placed in a tube furnace and heated to 800°C at a heating rate of 4°C / min under a nitrogen atmosphere, and calcined for 2 h, then cooled to room temperature; 20 mL of 3 mol·L -1 HCl solution was added to the obtained black powder, stirred for 24 h, and centrifuged at a speed of 12,000 r / min for 5 min, then HCl solution was added again, and the stirring and centrifugation were repeated twice according to the foregoing operation to remove excess Zn, the black powder was collected, washed with pure water until the filtrate was neutral, and freeze-dried for 12 h to obtain a porous carbon material, which was recorded as PNC.

[0077] Step (3): Mix PNC (50 mg) and KOH at a mass ratio of 1:3, add 1 mL of pure water and 1 mL of anhydrous ethanol, soak for 12 h, transfer to a ceramic boat, place in a fume hood to air dry the solvent, and obtain a black viscous slurry spread evenly in the ceramic boat. Place in a tube furnace and heat to 700 °C at a rate of 4 °C / min under a nitrogen atmosphere for 2 h; 1 mol·L -1 Neutralize with dilute hydrochloric acid, wash repeatedly with pure water until the filtrate is neutral, centrifuge, freeze dry for 12 hours to obtain the activated mesoporous carbon sample, namely nitrogen-doped mesoporous graphitic carbon anode supercapacitor material, denoted as PANC2 / 1-800.

[0078] XRD pattern of PANC2 / 1-800 ( Figure 3 This indicates that Zn has been almost completely removed from the carbon material obtained after treatment. Figure 4 The nitrogen adsorption-desorption isotherm diagram of PANC2 / 1-800 is shown, from which the specific surface area of ​​PANC2 / 1-800 is obtained as 952.5 m². 2 / g. Pore size distribution diagram of PANC2 / 1-800 ( Figure 5 The average pore size of the material is 4.55 nm, mainly mesoporous, indicating that mesoporous carbon material was successfully obtained after heat treatment in step (3). The Raman spectrum of PANC2 / 1-800 (…) Figure 6 This confirmed that during pyrolysis, the original functional groups in the MOF were gradually removed, and the appearance of the D and G peaks indicated that the mesoporous carbon material was graphitized carbon, which is consistent with the XRD test results. From the SEM images ( Figure 7 It can be seen that PANC2 / 1-800 contains a large number of porous structures, and due to its special surface properties, it has a large specific surface area; TEM image ( Figure 8 The formation of sheet-like and graphene-like carbon materials is visible; this indicates that after the initial nitrogen atmosphere calcination, hydrochloric acid washing, KOH activation, and secondary nitrogen atmosphere calcination of Zn-MOF, there are no metal particles present in the material, and the material exhibits a sheet-like porous structure. Figure 7 and Figure 8 Furthermore, the presence of nitrogen was confirmed by energy dispersive spectroscopy (EDS) in SEM, indicating that nitrogen was successfully doped into the carbon lattice, which is consistent with the XRD results.

[0079] PANC2 / 1-800 was used as the negative electrode material, and a single electrode was prepared by mixing it with acetylene black and PVDF at a mass ratio of 75:15:10. A three-electrode system was formed using KOH as the electrolyte, a platinum wire electrode as the counter electrode, and a Hg / HgO electrode as the reference electrode. The CP curves at different current densities are shown below. Figure 9 The CV curves at different scan rates are as follows: Figure 10 . Figure 11 and Figure 12Electrochemical impedance spectroscopy and specific capacitance vs. current density plots calculated from CP curves, respectively.

[0080] By Figure 9 With Figure 10 It can be seen that, in the range of working voltage from -1.0 to 0V, the CP curves of PANC2 / 1-800 at different current densities are isosceles triangle, the charge-discharge reversibility is higher, and the chemical stability is good. The CV curves obtained at different scan rates show an approximately rectangular shape, and the current in the CV curve increases rapidly with the increase of the scan rate, indicating that the prepared mesoporous carbon material PANC2 / 1-800 has a fast response ability, showing a double-layer capacitance characteristic. Figure 11 The approximately vertical linear curve in the low frequency region indicates that the sample has excellent capacitance performance and low impedance. Figure 12 The results show that, when the current density is 1.0A·g -1 , the specific capacitance of PANC2 / 1-800 can reach 269.9F·g -1 , which is 2-3 times better than that of commercial activated carbon (1.0A·g -1 , ≈110F·g -1 ), and the device has very excellent cycle performance, as shown in Figure 13 Therefore, the nitrogen-doped mesoporous graphite carbon material prepared in this embodiment exhibits potential characteristics of a supercapacitor negative electrode material.

[0081] Example 2

[0082] The high specific surface area nitrogen-doped mesoporous graphite carbon negative electrode supercapacitor material is prepared by the following preparation method, comprising the following steps:

[0083] Step (1), add 100mL of pure water to a beaker, then add 3.120g of 4,5-imidazole dicarboxylic acid and 1.600g of sodium hydroxide, heat to 70℃, stir to dissolve, form a 0.2mol·L -1 4,5-imidazole dicarboxylic acid sodium solution; add 1.098g of zinc acetate dihydrate (5mmol) to a mixed solvent of 30mL of ethanol and 15mL of water, stir uniformly, add 25mL of freshly prepared 0.2mol·L -1 4,5-imidazole dicarboxylic acid sodium solution, after sufficient stirring, a large amount of white precipitate is generated, ultrasonic dispersion for 15min, transfer into a 100mL polytetrafluoroethylene lined reaction kettle, put into a forced air drying oven, react at 110℃ for 24h; after the reaction is completed, cool to room temperature, centrifuge at a speed of 10000r / min for 5min, obtain white solid, wash twice with pure water first, then wash twice with anhydrous ethanol, freeze-dry for 12h, obtain Zn-MOF.

[0084] Step (2): Dissolve 100 mg Zn-MOF and polyvinylpyrrolidone (PVP K30) (Zn-MOF to polyvinylpyrrolidone mass ratio 2:1) in 15 mL of anhydrous ethanol, stir for 5 h to mix evenly, transfer to a porcelain boat, place in an oven at 75 °C to evaporate the solvent, then place in a tube furnace, heat to 850 °C at a rate of 4 °C / min under nitrogen atmosphere, calcine for 2 h, and cool to room temperature; add 20 mL of 4 mol·L⁻¹ powder to the obtained black powder. -1 The HCl solution was stirred for 24 hours, centrifuged at 12000 r / min for 5 minutes, and then HCl solution was added again. The stirring and centrifugation were repeated twice to remove excess Zn. The black powder was collected, washed with pure water until the filtrate was neutral, and freeze-dried for 12 hours to obtain carbon material, denoted as PNC.

[0085] Step (3): Mix PNC (50 mg) and KOH at a mass ratio of 1:3, add 1 mL of pure water and 1 mL of anhydrous ethanol and soak for 12 h. Transfer to a porcelain boat and place in a forced-air drying oven at 75 °C. After evaporating the solvent, a black viscous slurry is obtained. Place in a tube furnace and heat to 750 °C at a rate of 4 °C / min under a nitrogen atmosphere for 2 h. 1 mol·L -1 Neutralize with dilute hydrochloric acid, wash repeatedly with pure water until the filtrate is neutral, centrifuge, freeze dry for 12 hours to obtain the activated mesoporous carbon sample, namely nitrogen-doped mesoporous graphitic carbon anode supercapacitor material, denoted as PANC2 / 1-850.

[0086] XRD pattern of PANC2 / 1-850 ( Figure 14 The presence of high purity carbon material obtained through multiple heat treatment steps, with no Zn or other impurity peaks, indicates that the metal elements have been completely removed. Figure 15 The nitrogen adsorption-desorption isotherm diagram for PANC2 / 1-850 is shown, from which the specific surface area of ​​PANC2 / 1-850 is obtained as 1006.1 m². 2 / g. Pore size distribution diagram of PANC2 / 1-850 ( Figure 16 The data shows that the material has an average pore size of 5.38 nm, mainly mesoporous, indicating that mesoporous carbon material was successfully obtained after heat treatment. The Raman spectrum of PANC2 / 1-850 is shown below. Figure 17 This confirms that during the pyrolysis process, the original functional groups in the MOF are gradually removed, and the appearance of the D and G peaks indicates that the mesoporous carbon material is graphitized carbon, which is consistent with the XRD test results.

[0087] From SEM image ( Figure 18 It can be seen from the TEM image that PANC2 / 1-850 contains a layered stacked structure with a rough surface, resulting in a large specific surface area. Figure 19) shows that the sheet layer is a porous thin layer structure. It is shown that after the primary nitrogen atmosphere calcination, hydrochloric acid pickling, KOH activation, and secondary nitrogen atmosphere calcination, there is no metal particle in the material, and the material presents a sheet-shaped porous structure. Similarly, through energy spectrum testing, it is also shown that after the N-containing precursor is carbonized at high temperature, the N element is uniformly distributed in the graphite carbon structure through in-situ doping, which is consistent with the XRD test result.

[0088] PANC2 / 1-850 was prepared as a negative electrode material, acetylene black, and PVDF according to a mass ratio of 75:15:10 to form a single electrode, KOH was used as an electrolyte, a platinum wire electrode was used as a counter electrode, and a Hg / HgO electrode was used as a reference electrode to form a three-electrode system. The CP curve diagram of PANC2 / 1-850 under different current densities is as follows Figure 20 , and the CV curve diagram of PANC2 / 1-850 under different scanning rates is as follows Figure 21 . Figure 22 and Figure 23 are respectively the electrochemical impedance diagram and the specific capacitance comparison diagram under different current densities calculated from the CP curve.

[0089] It can be seen from Figure 20 and Figure 21 that in the range of working voltage from-1.0 to 0V, the CP curve of PANC2 / 1-850 under different current densities is an isosceles triangle, and the CV curve obtained under different scanning rates presents an approximately rectangular shape, the charge-discharge reversibility is relatively high, and the chemical stability is good. And the current in the CV curve increases rapidly with the increase of the scanning rate, indicating that the prepared PANC2 / 1-850 also has a fast response capability. Compared with PANC2 / 1-800, the cyclic voltammetry area is slightly smaller. Figure 22 The approximately vertical linear curve in the low-frequency region indicates that the sample has excellent capacitance performance and low impedance. Figure 23 It is shown that when the current density is 1.0A·g -1 , the specific capacitance of PANC2 / 1-850 can reach 189.6F·g -1 , which is slightly lower than that of PANC2 / 1-800, but still better than that of the commercial activated carbon (1.0A·g -1 , ≈110F·g -1 ). Therefore, it is verified again that the nitrogen-doped carbon material prepared in the embodiment is a preferred material for the negative electrode of the supercapacitor.

[0090] Embodiment 3

[0091] The high-specific-surface-area nitrogen-doped mesoporous graphite carbon negative electrode supercapacitor material is prepared by the following preparation method, including the following steps:

[0092] Step (1), 100 mL of pure water was added to a beaker, then 3.120 g of 4,5-imidazole dicarboxylic acid and 1.600 g of sodium hydroxide were added, heated to 70℃, stirred and dissolved to form a solution with a concentration of 0.2 mol·L -1 4,5-Imidazole dicarboxylic acid sodium solution; 1.098 g of zinc acetate dihydrate (5 mmol) was added to a mixed solvent of 30 mL of ethanol and 15 mL of water, stirred uniformly, and 25 mL of freshly prepared 0.2 mol·L -1 4,5-Imidazole dicarboxylic acid sodium solution, after sufficient stirring, a large amount of white precipitate was produced, ultrasonic dispersion for 15 min, transferred into a 100 mL polytetrafluoroethylene lined reaction kettle, placed in a forced air drying oven, reacted at 100℃ for 48 h; after the reaction was completed, it was cooled to room temperature, centrifuged at a speed of 10000 r / min for 5 min, and a white solid was obtained, which was washed twice with pure water and then twice with anhydrous ethanol, and freeze-dried for 12 h to obtain Zn-MOF.

[0093] Step (2), 100 mg of Zn-MOF and polyvinylpyrrolidone (PVP K30) (mass ratio of Zn-MOF to polyvinylpyrrolidone 1:1) were dispersed in 15 mL of anhydrous ethanol and stirred for 5 h to mix uniformly, then transferred into a porcelain boat and placed in an oven to evaporate the solvent at 75℃, then placed in a tube furnace and heated to 850℃ at a heating rate of 4℃ / min under a nitrogen atmosphere, and calcined for 2 h, then cooled to room temperature; 20 mL of 2 mol·L -1 HCl solution was added to the obtained black powder, stirred for 24 h, centrifuged at a speed of 12000 r / min for 5 min, and the HCl solution was added again according to the previous operation, stirred and centrifuged, and the excess Zn was removed by repeating the operation twice, the black powder was collected, washed with pure water until the filtrate was neutral, and freeze-dried for 12 h to obtain a carbon material, which was denoted as PNC.

[0094] Step (3), PNC (50 mg) and KOH were mixed uniformly at a mass ratio of 1:3, soaked in 1 mL of pure water and 1 mL of anhydrous ethanol for 12 h, transferred into a porcelain boat, and placed in a forced air oven at 75℃ to evaporate the solvent, then obtained a black viscous slurry, placed in a tube furnace and heated to 750℃ at a heating rate of 4℃ / min under a nitrogen atmosphere, and activated for 2 h; 1 mol·L -1 HCl was neutralized, washed with pure water several times until the effluent was neutral, centrifuged, and freeze-dried for 12 h to obtain a porous carbon sample after activation, which was a nitrogen-doped mesoporous graphite carbon negative electrode supercapacitor material, PANC1 / 1-850.

[0095] The XRD pattern of PANC1 / 1-850 is shown in FIG. 1. Figure 24The results indicate that the carbon material obtained through multiple heat treatments has high purity, consistent with PANC2 / 1-850 and PANC2 / 1-800 samples, and has no Zn or other impurity peaks, indicating that the metal elements have been completely removed. Figure 25 The nitrogen adsorption-desorption isotherm diagram of PANC1 / 1-850 is shown, from which the specific surface area of ​​PANC1 / 1-850 is obtained as 855.5 m². 2 / g. Pore size distribution diagram of PANC1 / 1-850 ( Figure 26 The data shows that the average pore size of the material is 7.45 nm, still predominantly mesoporous, indicating that mesoporous carbon material was successfully obtained after heat treatment. The Raman spectrum of PANC1 / 1-850 is shown below. Figure 27 The presence of peaks D and G in the XRD results indicates that PANC1 / 1-850 has also been transformed into graphitized carbon, which is consistent with the XRD test results.

[0096] After initial nitrogen calcination, hydrochloric acid washing, KOH activation, and secondary calcination, no metal particles remain in the material, which exhibits a porous, sponge-like structure. (SEM) Figure 28 ) and TEM image ( Figure 29 This indicates that the sponge sheets are a layered porous structure composed of particles. Similarly, energy dispersive spectroscopy (EDS) tests also show that after the N-containing precursor is carbonized at high temperature, the N element is uniformly distributed in the graphite carbon structure through in-situ doping, which is consistent with the XRD test results.

[0097] PANC1 / 1-850 was used as the negative electrode material, and a single electrode was prepared by mixing it with acetylene black and PVDF at a mass ratio of 75:15:10. A three-electrode system was formed using KOH as the electrolyte, a platinum wire electrode as the counter electrode, and a Hg / HgO electrode as the reference electrode. The CP curves at different current densities are shown below. Figure 30 The CV curves at different scan rates are as follows: Figure 31 . Figure 32 and Figure 33 The figures show an electrochemical impedance spectroscopy diagram and a comparison of the specific capacitance at different current densities calculated from the CP curve.

[0098] Depend on Figure 30 and Figure 31 It can be seen that within the operating voltage range of -1.0 to 0V, similar to the PANC2 / 1-800 and PANC2 / 1-850 samples, the CP curves of PANC1 / 1-850 at different current densities also exhibit an isosceles triangle shape, while the CV curves obtained at different scan rates show an approximately rectangular shape. It has high charge-discharge reversibility and good chemical stability, but its specific capacity is lower than that of PANC2 / 1-800, and slightly better than that of PANC2 / 1-850. Figure 32 The near-vertical linear curve in the low-frequency region indicates that the sample has excellent capacitance performance and low impedance. Figure 33It is shown that the specific capacitance of PANC1 / 1-850 can reach 204.2 F·g -1 at a current density of 1.0 A·g -1 , which is still superior to that of commercial activated carbon (1.0 A·g -1 , ≈110 F·g -1 ). Therefore, it is further verified that the nitrogen-doped mesoporous graphite carbon material prepared in this embodiment is a potential candidate for supercapacitor negative electrode material.

Claims

1. A high specific surface area nitrogen-doped mesoporous graphitic carbon negative electrode supercapacitor material, characterized in that: It is to take 4,5-imidazole dicarboxylic acid sodium and zinc acetate dihydrate as raw materials, synthesize Zn-MOF by hydrothermal method, then take polyvinylpyrrolidone as external carbon / nitrogen source, adopt direct carbonization method, take PVP / Zn-MOF as precursor mixed material, carry out high temperature carbonization under nitrogen atmosphere, remove Zn by hydrochloric acid to obtain porous carbon material, mix the porous carbon material and KOH uniformly in mass ratio of 1:3, add pure water and anhydrous ethanol to soak for 12 h, dry naturally in ventilation kitchen or evaporate solvent to obtain black viscous slurry, place in tube furnace, heat to 700-750 DEG C under nitrogen atmosphere, constant temperature activation for 2 h to obtain nitrogen doped mesoporous graphite carbon negative electrode supercapacitor material; The molar ratio of 4,5-imidazole dicarboxylic acid sodium and zinc acetate dihydrate is 1:

1. The removing Zn by hydrochloric acid includes: using 15-35 mL of hydrochloric acid with a concentration of 2-4 mol / L for every 100 mg of Zn-MOF each time. 0.5-1.5 mL of pure water and 1-3 mL of anhydrous ethanol are used for every 50 mg of porous carbon material.

2. The method for preparing the high specific surface nitrogen-doped mesoporous graphite carbon negative electrode supercapacitor material of claim 1, characterized in that: The method comprises the following steps: Step (1), 4,5-imidazole dicarboxylic acid and sodium hydroxide are heated and reacted in pure water to obtain a 4,5-imidazole dicarboxylic acid sodium solution; zinc acetate dihydrate is added to a mixed solvent of anhydrous ethanol and water and stirred uniformly, the 4,5-imidazole dicarboxylic acid sodium solution is added, stirred until white precipitate is generated, ultrasonic treatment is performed for 15-30 min, and reaction is performed at 100-140 DEG C for 12-48 h; after the reaction is completed, the temperature is cooled to room temperature, centrifugation is performed to obtain white solid, the white solid is washed twice with pure water and then twice with anhydrous ethanol, and freeze-drying is performed to obtain Zn-MOF; Step (2), Zn-MOF and polyvinylpyrrolidone are dispersed in anhydrous ethanol, stirred and mixed uniformly, and the solvent is evaporated; the obtained black powder is placed in a tube furnace, heated to 600-900 DEG C under nitrogen atmosphere, and constant temperature calcination is performed for 1-2 h, and then the temperature is cooled to room temperature; concentrated hydrochloric acid with a concentration of 2-4 mol / L is added to the obtained black powder, stirred for 12-24 h, centrifuged, and then concentrated hydrochloric acid with a concentration of 2-4 mol / L is added again to perform stirring and centrifugation, and the operation is repeated twice; the black powder is collected, washed with pure water until the filtrate is neutral, and then freeze-drying is performed to obtain porous carbon material; Step (3), the porous carbon material and KOH are mixed uniformly in a mass ratio of 1:3, pure water and anhydrous ethanol are added to soak for 12 h, and then the solvent is dried naturally in a ventilation kitchen or evaporated to obtain black viscous slurry; the black viscous slurry is placed in a tube furnace, heated to 700-750 DEG C under nitrogen atmosphere, and constant temperature activation is performed for 2 h; neutralization is performed with dilute hydrochloric acid, washed with pure water until neutral, separated by centrifugation, and then freeze-drying is performed to obtain nitrogen doped mesoporous graphite carbon negative electrode supercapacitor material.

3. The method of claim 2, wherein the method further comprises: In step (1), the molar ratio of 4,5-imidazole dicarboxylic acid and sodium hydroxide is 1:2, and the concentration of the 4,5-imidazole dicarboxylic acid sodium solution is 0.2 mol / L.

4. The method of claim 2, wherein the method further comprises: In step (1), the amount ratio of zinc acetate dihydrate and the mixed solvent of anhydrous ethanol and water is 5 mmol:30 mL; the mixed solvent of anhydrous ethanol and water is prepared by mixing pure water and ethanol in a volume ratio of 1:2-2:

1.

5. The method of claim 2, wherein the method further comprises: In step (1), the molar ratio of sodium 4,5-imidazole dicarboxylate to zinc acetate dihydrate is 1:

1.

6. The method of claim 2, wherein the method further comprises: In step (2), the mass ratio of Zn-MOF to polyvinylpyrrolidone is 1:1-2:1; 10-30 mL of anhydrous ethanol is used per 100 mg of Zn-MOF; and 15-35 mL of 2-4 mol / L hydrochloric acid is used per 100 mg of Zn-MOF.

7. The method of claim 2, wherein the method further comprises: In step (3), 0.5-1.5 mL of pure water and 1-3 mL of anhydrous ethanol are used per 50 mg of porous carbon material.

8. The method of claim 2, wherein the method further comprises: In step (2), the temperature rising rate is 3-5℃ / min; and in step (3), the temperature rising rate is 3-5℃ / min.

9. The method of claim 8, wherein the method further comprises: In step (3), the temperature rising rate is 4℃ / min.

10. The use of the high specific surface area nitrogen-doped mesoporous graphite carbon negative electrode supercapacitor material of claim 1 as a negative electrode material in the preparation of a supercapacitor.

11. An ultracapacitor, characterized by: It is a single electrode prepared according to the mass ratio of 75:15:10 of the high specific surface area nitrogen-doped mesoporous graphite carbon negative electrode supercapacitor material of claim 1, acetylene black and PVDF, with KOH as an electrolyte, a platinum wire electrode as a counter electrode, and a Hg / HgO electrode as a reference electrode, to form a three-electrode system.

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