A preparation method and application of hollow / cavity nitrogen-doped carbon microspheres based on thermal expansion microspheres loaded with cobaltosic oxide

Abstract

This invention provides a method for preparing and applying hollow / cavity nitrogen-doped carbon microspheres based on thermally expandable microspheres loaded with cobalt tetroxide, belonging to the field of carbon materials technology. The method involves mixing and polymerizing carbon-carbon double-bonded monomers, a suspending agent, a high-boiling-point alkane, an initiator, and a first solution to obtain thermally expandable microspheres (TEMs). These TEMs are then hydrolyzed to obtain alumina-doped microspheres (ATEMs). These ATEMs are then mixed with 2-methylimidazole and a cobalt source to induce in-situ growth of ZIF-67, yielding ZIF-67@ATEMs. Finally, carbonization and calcination are performed to obtain hollow / cavity nitrogen-doped carbon microspheres loaded with cobalt tetroxide. This invention introduces electronegative groups through hydrolysis activation to induce in-situ growth of ZIF-67. During carbonization, the ZIF-67 layer prevents the microspheres from melting and agglomerating, resulting in an independently distributed structure. The obtained material, as an electrode material for supercapacitors, exhibits a specific capacitance of 287.33 F / g at a current density of 0.5 A / g, demonstrating excellent electrochemical performance.

Description

Technical Field

[0001] This invention belongs to the field of carbon materials technology, specifically relating to a method for preparing hollow / cavity nitrogen-doped carbon microspheres based on thermally expanded microspheres loaded with cobalt tetroxide and its application. Background Technology

[0002] The information disclosed in this background section is intended only to enhance understanding of the overall background of the invention and is not necessarily to be construed as an admission or in any way implying that such information constitutes prior art known to those skilled in the art.

[0003] Thermally expandable microspheres (also known as thermally expandable polymer microcapsules, thermally expandable polymer microspheres, thermally expandable microcapsules, etc.) consist of a thermoplastic polymer shell and a physical foaming agent core. When heated to a certain temperature, the wall material softens, and the core material undergoes a phase change, transforming from a liquid to a gaseous state. This generates sufficient internal pressure to cause the softened wall material to expand, achieving the purpose of foaming. The main synthetic monomer used in the shell layer of thermally expandable microspheres is nitrogen-containing acrylonitrile. After chemical treatment, the surface of the microspheres can exhibit electronegativity, thereby loading positively charged nanoparticles and giving the microspheres different functional properties in addition to thermal expansion.

[0004] Metal-organic frameworks (MOFs) are novel porous crystalline materials formed by the self-assembly of metal ions and organic molecules. They possess advantages such as large specific surface area, high porosity, and abundant active sites, showing broad application prospects in energy storage and conversion. Zeolite imidazole ester frameworks (ZIFs) are an important branch of MOFs, formed by crosslinking imidazole or imidazole derivatives with transition metals. Among them, Co-based ZIFs (ZIF-67) are characterized by structural stability and numerous active sites, and are often carbonized and calcined to obtain Co3O4 for use as electrode materials. However, to obtain a high specific surface area, the synthesized ZIF-67 particle size is generally in the nanometer range. The high surface energy of nanoparticles leads to particle agglomeration, and the polymer microspheres are prone to melting and adhesion during carbonization, thus losing the advantage of large specific surface area. Summary of the Invention

[0005] To address the shortcomings of the prior art, the present invention aims to provide a method for preparing hollow / cavity nitrogen-doped carbon microspheres based on thermally expanded microspheres loaded with cobalt tetroxide and its application. The carbon material provided by the present invention has excellent independent distribution and exhibits good electrochemical performance.

[0006] To achieve the above objectives, the present invention adopts the following technical solution: The first aspect of the present invention provides a method for preparing hollow / cavity nitrogen-doped carbon microspheres based on thermally expanded microspheres loaded with cobalt tetroxide, comprising the following steps: A monomer containing carbon-carbon double bonds, a suspending agent, a high-boiling-point alkane, an initiator, and a first solution are mixed and polymerized to obtain thermally expandable microsphere precursors TEMs. The TEMs are mixed with a second solution and subjected to a hydrolysis reaction to obtain ATEMs; ZIF-67@ATEMs were prepared by mixing ATEMs with 2-methylimidazole and a cobalt source. The ZIF-67@ATEMs were subjected to carbonization and calcination treatments in sequence to obtain hollow / cavity nitrogen-doped carbon microspheres based on thermally expanded microspheres loaded with cobalt tetroxide.

[0007] In a second aspect, the present invention provides hollow / cavity nitrogen-doped carbon microspheres based on thermally expandable microspheres loaded with cobalt tetroxide, prepared by the above preparation method.

[0008] A third aspect of the present invention provides the application of the above-mentioned hollow / cavity nitrogen-doped carbon microspheres based on thermally expandable microspheres loaded with cobalt tetroxide in supercapacitors.

[0009] Compared with the prior art, the technical solution of the present invention has the following beneficial effects: This invention is based on thermally expandable microspheres. Negatively charged groups are introduced onto the surface of the microspheres through a hydrolysis reaction, allowing ZIF-67 to grow in situ on the microsphere surface, resulting in a composite material with a uniform ZIF-67 layer. This in-situ growth method ensures that ZIF-67 and Co3O4 are firmly bonded to the substrate and uniformly distributed, effectively preventing cobalt tetroxide agglomeration caused by ZIF-67 aggregation, avoiding the fusion and aggregation of polymer microspheres, and maintaining a large electroactivity and specific surface area.

[0010] The method for preparing thermally expandable microspheres of the present invention is well compatible with existing mainstream industrial operation processes, does not involve additional equipment and devices, and is simple to operate, thus broadening the application scenarios of the original thermally expandable microspheres. Attached Figure Description

[0011] Figure 1 This is a scanning electron microscope image of ZIF-67@ATEMs prepared in Example 1 of this invention; Figure 2 This is a scanning electron microscope image of ZIF-67@ATEMs prepared in Comparative Example 1 of this invention; Figure 3 This is a scanning electron microscope (SEM) image of ZIF-67@ATEMs-C prepared in Example 1 of this invention. Figure 4 This is a scanning electron microscope image of TEMs prepared in Comparative Example 3 of this invention; Figure 5 This is a scanning electron microscope image of ZIF-67@ATEMs-CO-1 prepared in Example 1 of this invention; Figure 6 This is a scanning electron microscope image of the ZIF-67 / ATEMs composite material prepared in Comparative Example 2 of this invention. Detailed Implementation

[0012] It should be noted that the following detailed descriptions are exemplary and intended to provide further illustration of the invention. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.

[0013] It should be noted that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the scope of exemplary embodiments according to the invention. As used herein, the singular forms are intended to include the plural forms as well, unless the context clearly indicates otherwise. Furthermore, it should be understood that when the terms “comprising” and / or “including” are used in this specification, they indicate the presence of features, steps, operations, and / or combinations thereof.

[0014] As described in the background section, there are certain shortcomings in the existing technology. In order to solve the above-mentioned technical problems, the present invention provides a method for preparing hollow / cavity nitrogen-doped carbon microspheres based on thermally expandable microspheres loaded with cobalt tetroxide and its application.

[0015] In view of this, the present invention provides a method for preparing hollow / cavity nitrogen-doped carbon microspheres based on thermally expandable microspheres loaded with cobalt tetroxide, comprising the following steps: A monomer containing carbon-carbon double bonds, a suspending agent, a high-boiling-point alkane, an initiator, and a first solution are mixed and polymerized to obtain thermally expandable microsphere precursors TEMs. The TEMs are mixed with a second solution and subjected to a hydrolysis reaction to obtain ATEMs; ZIF-67@ATEMs were prepared by mixing ATEMs with 2-methylimidazole and a cobalt source. The ZIF-67@ATEMs were subjected to carbonization and calcination treatments in sequence to obtain hollow / cavity nitrogen-doped carbon microspheres based on thermally expanded microspheres loaded with cobalt tetroxide.

[0016] In some embodiments of the present invention, the carbon-carbon double bond monomer comprises a first component and a second component. Preferably, the first component comprises at least one of acrylonitrile, methacrylonitrile, acrylamide, and N,N-dimethylacrylamide, and the second component comprises one or more of methyl methacrylate, methyl acrylate, methacrylic acid, acrylic acid, and allyl methacrylate. Preferably, the first component of the carbon-carbon double bond monomer is acrylonitrile, and the mass percentage of acrylonitrile in the carbon-carbon double bond monomer is ≥80 wt%. The suspending agent includes one or more of the following: silica sol, silica powder, magnesium hydroxide, PVP-K30, PVP-K60, and sodium dodecyl sulfate. The high-boiling-point alkanes include one or more of isooctane, n-octane, and n-hexane; The initiator includes one or more of azobisisobutyronitrile, azobisisoheptanenitrile, azobisisobutyramidine hydrochloride, and benzoyl peroxide; The solvent of the first solution is water, and the solute is one or more of sodium chloride, potassium chloride, and magnesium chloride.

[0017] In some embodiments of the present invention, the mass ratio of the carbon-carbon double bond monomer, suspending agent, high-boiling-point alkane, initiator, and first solution is 10–20:1–2:2–6:0.005–0.2:40–100. After mixing, shear emulsification is performed to obtain an oil-in-water dispersion. The shear emulsification speed is 7000–10000 r / min to obtain an oil-in-water dispersion with uniform particle size.

[0018] In some embodiments of the present invention, the polymerization reaction temperature is 55–75°C, preferably 60–70°C; and the reaction time is 10–15 hours. Controlling the polymerization reaction temperature prevents excessively high temperatures from causing explosive polymerization or the escape of the foaming agent.

[0019] In some embodiments of the present invention, the second solution is an aqueous solution of an acid or a base, with a concentration of 0.5–1 mol / L; preferably, the acid is one of sulfuric acid and acetic acid; and the base is one of sodium hydroxide and potassium hydroxide. The present invention does not impose any special limitations on the solute in the second solution; adjustments can be made according to actual needs. For example, if silica sol is used as the suspending agent, sodium hydroxide can be used; if magnesium hydroxide is used as the suspending agent, hydrochloric acid can be used, etc.

[0020] In some embodiments of the present invention, the mass ratio of the TEM precursor to the second solution is 1–10:40–400; the hydrolysis reaction temperature is 20–80 °C and the time is 1–6 h; preferably, the hydrolysis reaction temperature is 30–60 °C and the time is preferably 2–4 h.

[0021] Microsphere polymerization uses a suspending agent. After the polymerization is completed, the suspending agent remains on the surface of the microspheres and is difficult to remove from the surface of the microspheres by simple washing and filtration. Hydrolysis can effectively remove the suspending agent from the surface of the microspheres, so that the surface of the microspheres is fully exposed.

[0022] In some embodiments of the present invention, after the hydrolysis reaction, the resulting product is filtered, washed until neutral, and then dried to obtain ATEMs.

[0023] In some embodiments of the present invention, the concentration of ATEMs is 80–170 g / L, preferably 85–160 g / L; the concentration of the 2-methylimidazole aqueous solution is 1–4 mol / L, preferably 2–4 mol / L; the concentration of the cobalt nitrate aqueous solution is 0.01–0.2 mol / L, preferably 0.05–0.15 mol / L; the impregnation time is 3–6 h, preferably 4–6 h; and after impregnation, the solution is filtered and dried to obtain ZIF-67@ATEMs.

[0024] Because TEMs have suspending agents attached to their surfaces, their surfaces are almost electrically neutral. However, after treatment, the electronegative functional groups (such as -CN and -OCOCH3) on the surface of ATEMs are fully exposed, and the electronegativity of the microsphere surface is significantly enhanced, enabling it to electrostatically adsorb Co from the solution. 2+ When 2-methylimidazole is added, ZIF-67 nucleates and grows in situ on the surface of ATEMs, forming a ZIF-67@ATEMs composite material. This in-situ growth method results in a strong and uniform bond between the ZIF-67 layer and the microsphere substrate, which is different from the method of simply physically mixing pre-synthesized ZIF-67 crystals with the substrate.

[0025] In some embodiments of the present invention, the carbonization is carried out in a nitrogen atmosphere, the carbonization temperature is 600-1100 °C, preferably 700-1000 °C, and the time is 1-3 h, preferably 2 h.

[0026] In some embodiments of the present invention, the calcination treatment is carried out in an air atmosphere, which initially decomposes the low-graphitization products and unstable structures, leaving a more stable structure, thereby improving the stability of the carbon material; at the same time, the cobalt metal produced by the decomposition of ZIF-67 is oxidized to cobalt tetroxide, thereby improving the electrochemical performance of the material. The calcination temperature is 200-400 °C, preferably 280-350 °C; the holding time is 1-3 h, preferably 2 h; the calcination is preferably carried out in a muffle furnace, and the heating rate to the calcination temperature is preferably 1-5 °C / min.

[0027] The present invention also provides a hollow / cavity nitrogen-doped carbon microsphere based on thermally expanded microspheres loaded with cobalt tetroxide, prepared by the above preparation method, wherein the microspheres have a hollow structure and are independently distributed among each other.

[0028] The present invention further provides the application of the above-mentioned hollow / cavity nitrogen-doped carbon microspheres based on thermally expanded microspheres loaded with cobalt tetroxide in supercapacitors, preferably as supercapacitor electrode materials.

[0029] To enable those skilled in the art to better understand the technical solution of the present invention, the technical solution of the present invention will be described in detail below with reference to specific embodiments.

[0030] Example 1 Acrylonitrile 12 g, methyl methacrylate 2 g, 30 wt% silica sol 5.8 g, isooctane 5.8 g, azobisisobutyronitrile 0.14 g, water 50 g, and sodium chloride 30 g. The mass ratio of the carbon-carbon double bond monomer, suspending agent, high-boiling-point alkane, initiator, and first solution is within the range of 10–20:1–2:2–6:0.005–0.2:40–100. The above formulation is mixed and sheared emulsified at 7000 r / min. The reaction is carried out in a reactor at 65°C for 15 h. After filtration, washing, drying, and collection, TEMs are obtained.

[0031] 0.5 g of TEMs was mixed with 40 mL of 1 mol / L sodium hydroxide solution. The hydrolysis reaction was carried out at 40 °C for 2 h. After the reaction was completed, the mixture was filtered, washed until neutral, and dried to obtain ATEMs.

[0032] After obtaining ATEMs, a 5.75 mL mixed solution was prepared by combining ATEMs, 2-methylimidazole, and cobalt nitrate hexahydrate, wherein the concentration of ATEMs was 87 g / L; the concentration of the 2-methylimidazole aqueous solution was 2.4 mol / L; the concentration of the cobalt nitrate aqueous solution was 0.067 mol / L; the impregnation time was 6 h; and after impregnation, the solution was filtered and dried to obtain ZIF-67@ATEMs.

[0033] ZIF-67@ATEMs were placed in a tube furnace and heated to 800°C at a rate of 5°C / min under a nitrogen atmosphere. After holding at this temperature for 2 hours, the temperature was cooled to room temperature to obtain ZIF-67@ATEMs-C. ZIF-67@ATEMs-C was then placed in a muffle furnace and heated to 350°C in air at a rate of 5°C / min. After holding at this temperature for 2 hours, the temperature was cooled to room temperature to obtain ZIF-67@ATEMs-CO-1.

[0034] Example 2 The preparation method of Example 1 was followed, except that 1.74 g of magnesium hydroxide was used instead of 5.8 g of silica sol as a suspending agent; and in step S2, 40 mL of 1 mol / L hydrochloric acid was used instead of sodium hydroxide solution for the hydrolysis reaction. The remaining steps were the same as in Example 1, yielding ZIF-67@ATEMs-CO-2.

[0035] Example 3 The preparation method of Example 1 was followed, except that: 12 g of acrylonitrile, 2 g of methyl acrylate, 5.8 g of 30 wt% silica sol, 5.8 g of isooctane, 0.14 g of azobisisobutyronitrile, 50 g of water, and 30 g of sodium chloride were used, and the remaining steps were the same as in Example 1 to obtain ZIF-67@ATEMs-CO-3.

[0036] Example 4 The preparation method of Example 1 was followed, except that: 12 g of N,N-dimethylacrylamide, 2 g of methyl methacrylate, 5.8 g of 30 wt% silica sol, 5.8 g of isooctane, 0.14 g of azobisisobutyronitrile, 50 g of water, and 30 g of sodium chloride were used, and the remaining steps were the same as in Example 1 to obtain ZIF-67@ATEMs-CO-4.

[0037] Example 5 The preparation method of Example 1 was followed, except that: 12 g of acrylonitrile, 2 g of methyl methacrylate, 5.8 g of 30wt% silica sol, 5.8 g of n-hexane, 0.14 g of azobisisobutyronitrile, 50 g of water, and 30 g of sodium chloride were used, and the remaining steps were the same as in Example 1 to obtain ZIF-67@ATEMs-CO-5.

[0038] Example 6 The preparation method of Example 1 was followed, except that: 12 g of acrylonitrile, 2 g of methyl methacrylate, 5.8 g of 30wt% silica sol, 5.8 g of isooctane, 0.14 g of azobisisobutyronitrile, 50 g of water, and 30 g of magnesium chloride were used, and the remaining steps were the same as in Example 1 to obtain ZIF-67@ATEMs-CO-6.

[0039] Example 7 The preparation method of Example 1 was followed, except that: 17 g of acrylonitrile, 1 g of methyl methacrylate, 5.8 g of 30wt% silica sol, 5.8 g of isooctane, 0.14 g of azobisisobutyronitrile, 50 g of water, and 30 g of sodium chloride were used, and the remaining steps were the same as in Example 1 to obtain ZIF-67@ATEMs-CO-7.

[0040] Example 8 The preparation method of Example 1 was followed, except that: 12 g of acrylonitrile, 2 g of methyl methacrylate, 5.8 g of 30wt% silica sol, 3 g of isooctane, 0.14 g of azobisisobutyronitrile, 50 g of water, and 30 g of sodium chloride were used, and the remaining steps were the same as in Example 1 to obtain ZIF-67@ATEMs-CO-8.

[0041] Example 9 The preparation method of Example 1 was followed, except that: 12 g of acrylonitrile, 2 g of methyl methacrylate, 5.8 g of 30wt% silica sol, 5.8 g of isooctane, 0.07 g of azobisisobutyronitrile, 50 g of water, and 30 g of sodium chloride were used, and the remaining steps were the same as in Example 1 to obtain ZIF-67@ATEMs-CO-9.

[0042] Example 10 The preparation method of Example 1 was followed, except that: 12 g of acrylonitrile, 2 g of methyl methacrylate, 5.8 g of 30wt% silica sol, 5.8 g of isooctane, 0.14 g of azobisisobutyronitrile, 50 g of water, and 60 g of sodium chloride were used, and the remaining steps were the same as in Example 1 to obtain ZIF-67@ATEMs-CO-10.

[0043] Example 11 The preparation method of Example 1 was followed, except that the polymerization temperature was 58°C and the time was 12h, while the remaining steps were the same as in Example 1, to obtain ZIF-67@ATEMs-CO-11.

[0044] Example 12 ZIF-67@ATEMs-CO-12 was prepared according to the method in Example 1, except that the concentration of ATEMs was 120 g / L.

[0045] Example 13 Prepared according to the method of Example 1, except that 0.5 g TEMs were mixed with 20 mL of 1 mol / L sodium hydroxide solution, the hydrolysis reaction was carried out at 25 °C for 4 h, and after the reaction was completed, the TEMs were filtered, washed until neutral, and dried to obtain ATEMs. The remaining steps were the same as in Example 1 to obtain ZIF-67@ATEMs-CO-13.

[0046] Example 14 ZIF-67@ATEMs was prepared according to the method in Example 1, except that ZIF-67@ATEMs was placed in a tube furnace and heated to 1000°C at a rate of 5°C / min under a nitrogen atmosphere. After holding at this temperature for 1.5 h, it was cooled to room temperature to obtain ZIF-67@ATEMs-C. ZIF-67@ATEMs-C was then placed in a muffle furnace and heated to 220°C in air at a rate of 5°C / min. After holding at this temperature for 3 h, it was cooled to room temperature to obtain ZIF-67@ATEMs-CO-14.

[0047] Comparative Example 1 Prepared according to the method of Example 1, except that the concentration of ATEMs was 174 g / L and no carbonization or calcination was performed.

[0048] Comparative Example 2 ZIF-67 was prepared according to the method in Example 1, except that it was synthesized separately: 2-methylimidazole was prepared into a 2.4 mol / L aqueous solution, and cobalt nitrate hexahydrate was prepared into a 0.067 mol / L aqueous solution. The two solutions were mixed in equal volumes (total volume 5.75 mL) and allowed to stand at room temperature for 6 h. After the reaction was completed, ZIF-67 powder was obtained by filtration and drying.

[0049] Weigh the pre-synthesized ZIF-67 powder as equivalent to the ZIF-67 grown on the surface of ATEMs in step S3 of Example 1. Mix the ZIF-67 powder and ATEMs in deionized water, and add deionized water to a total volume of 5.75 mL. Let stand at room temperature for 6 h to obtain a physically mixed ZIF-67 / ATEMs composite material. The sample after carbonization and calcination is named ZIF-67 / ATEMs-CO.

[0050] Comparative Example 3 Prepared according to step 1 in Example 1, except that the hydrolysis and in-situ nucleation growth of ZIF-67 on the surface of ATEMs are omitted. The TEMs are placed in a tube furnace and heated to 800°C at a heating rate of 5°C / min under nitrogen atmosphere protection. After holding at this temperature for 2 hours, the temperature is cooled to room temperature to obtain TEMs-C.

[0051] Characterization and Testing I. X-ray Diffraction (XRD) Analysis The crystal structure of the prepared material was characterized using an X-ray diffractometer.

[0052] The test conditions were as follows: Rigaku D / max-RC X-ray diffractometer manufactured by Rigaku Corporation of Japan was used, Cu Kα was used as the target material, the scanning rate was 10 ° / min, and the test voltage was 40 kV.

[0053] The results are shown in Table 1: Table 1. XRD data of the prepared materials

[0054] The XRD data of the TEMs obtained in Example 1 showed a polyacrylonitrile XRD crystallization peak at 2θ of 16.5°, indicating the successful synthesis of a thermally expandable microsphere precursor with polyacrylonitrile as the main component. The newly appearing diffraction peaks at 7.29°, 10.33°, 12.67°, and 17.9° in the XRD data of ZIF-67@ATEMs correspond to the positions of the characteristic diffraction peaks of ZIF-67, combined with... Figure 1The scanning electron microscope (SEM) image of ZIF-67@ATEMs in Example 1 shows that ZIF-67 was successfully grown on the surface of ATEMs, forming a ZIF-67@ATEMs composite material. The XRD data of ZIF-67@ATEMs-C from Example 1 shows diffraction peaks at 44.18°, 51.5°, and 75.66° consistent with Co metal; this indicates that during carbonization, the organic framework of ZIF-67 decomposes, cobalt ions are reduced to elemental cobalt metal, and the polymer shell is transformed into nitrogen-doped carbon. The XRD data of ZIF-67@ATEMs-CO-1 shows diffraction peaks at 18.9°, 31.26°, 36.83°, 44.8°, 59.3°, and 65.3° consistent with the characteristic diffraction peaks of cobalt tetroxide. The characteristic peaks of cobalt metal completely disappear, indicating that cobalt metal was successfully oxidized to cobalt tetroxide during calcination.

[0055] In Example 2 (treated with magnesium hydroxide / hydrochloric acid), ZIF-67@ATEMs-CO also showed complete Co3O4 characteristic peaks, indicating that Co3O4-loaded carbon microspheres can be successfully obtained regardless of whether silica sol / alkali treatment or magnesium hydroxide / acid treatment is used. Samples from Examples 3, 4, 5, 6, 7, 8, 9, 11, 12, 13, and 14 also showed Co3O4 characteristic peaks.

[0056] Example 10: Due to the large amount of sodium chloride used and the large amount of undissolved salt, the suspension system was uneven, some microspheres agglomerated, the Co3O4 loading was low, and the peak intensity was low. Although the characteristic peak of Co3O4 also appeared in the XRD spectrum of the physically mixed sample ZIF-67 / ATEMs-CO in Comparative Example 2, the peak intensity was significantly lower than that in Example 1. This indicates that some ZIF-67 failed to effectively adhere to the surface of the microspheres during the physical mixing process and was lost in subsequent processing, resulting in a lower final Co3O4 loading.

[0057] II. Scanning Electron Microscopy (SEM) Analysis The characterization results using scanning electron microscopy (SEM) are shown below: Depend on Figure 1 As can be seen, the surfaces of the ATEMs after hydrolysis activation are smooth and clean, and ZIF-67 crystals grow uniformly on the surface of the microspheres, forming a relatively complete coating layer. The ZIF-67 crystals exhibit a typical polyhedral morphology, are tightly attached to the surface of the microspheres, and show no obvious aggregation or self-nucleation phenomenon. This indicates that hydrolysis activation successfully introduced electronegative groups into the surface of the ATEMs, effectively inducing the in-situ nucleation and growth of ZIF-67.

[0058] Comparative Example 1 used ATEMs at a concentration of 174 g / L. (The rest of the text appears to be a fragment and requires further context for accurate translation.) Figure 2It is evident that ZIF-67 growth on the microsphere surface is uneven. In some areas, ZIF-67 overgrowth forms a thick coating layer, while in other areas it is clearly exposed. Simultaneously, a small number of self-nucleated ZIF-67 particles are present in the solution. This indicates that excessively high ATEM concentrations are detrimental to the uniform and dense coating of ZIF-67 on the TEM surface.

[0059] Depend on Figure 3 It is evident that after carbonization, the ZIF-67@ATEMs-C microspheres still maintain their complete spherical shape and independent distribution, with no melting or adhesion between the microspheres.

[0060] Depend on Figure 4 It is evident that after direct carbonization of TEMs without ZIF-67 loading, the TEMs-C microspheres underwent severe melting and agglomeration, with most microspheres adhering together to form an irregular blocky structure, completely losing their independently distributed microsphere morphology. Figure 3 In stark contrast, this fully demonstrates that the ZIF-67 layer plays a crucial role in physical isolation and protection during the carbonization process, and is key to obtaining independently distributed hollow carbon microspheres.

[0061] Depend on Figure 5 As can be seen, Co3O4 nanoparticles are uniformly distributed on the surface of ZIF-67@ATEMs-CO-1 microspheres, with no obvious agglomeration. This indicates that the ZIF-67 layer plays a protective role during the carbonization process, effectively preventing the microspheres from melting and sticking together, while successfully transforming into uniformly loaded Co3O4.

[0062] Depend on Figure 6 It is evident that ZIF-67 was almost not successfully loaded onto the surface of the ZIF-67 / ATEMs composite material, indicating that a uniform and robust loading of ZIF-67 cannot be achieved during simple physical mixing.

[0063] III. Electrochemical Performance Testing Three mg of active materials (ZIF-67@ATEMs-CO-1 from Example 1, the physically mixed ZIF-67 / ATEMs composite material from Comparative Example 2, and TEMs-C from Comparative Example 3) were weighed and pressed with nickel foam at a pressure of 8–10 MPa for 15 s to prepare three different working electrodes. Using a saturated calomel electrode as the reference electrode and a platinum sheet as the counter electrode in a 1 mol / L KOH solution, a three-electrode system was used for testing. The specific capacitance was calculated based on the constant current charge-discharge curve (GCD), and charge-discharge tests were performed. The results are shown in the table below. Table 2. GCD test data of electrode materials prepared by TEMs-C

[0064] Table 3. GCD test data of electrode materials prepared by ZIF-67@ATEMs-CO-1

[0065] Table 4. GCD test data of electrode materials prepared from ZIF-67 / ATEMs composite materials.

[0066] The electrode material prepared by TEMs-C has a specific capacitance of 131.17 F / g at a current density of 0.5 A / g; ZIF-67@ATEMs-CO-1 has a specific capacitance of 287.33 F / g at a current density of 0.5 A / g; while the ZIF-67 / ATEMs composite material has a specific capacitance of 152.2 F / g at a current density of 0.5 A / g, which is much worse than that of ZIF-67@ATEMs-CO-1.

[0067] Finally, it should be noted that the above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of them. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention. Although the specific embodiments of the present invention have been described above, they are not intended to limit the protection scope of the present invention. Those skilled in the art should understand that various modifications or variations that can be made by those skilled in the art without creative effort based on the technical solutions of the present invention are still within the protection scope of the present invention.

Claims

1. A method for preparing hollow / cavity nitrogen-doped carbon microspheres based on thermally expandable microspheres loaded with cobalt tetroxide, characterized in that, Includes the following steps: A monomer containing carbon-carbon double bonds, a suspending agent, a high-boiling-point alkane, a high-boiling-point alkane initiator, and a first solution are mixed and subjected to a polymerization reaction to obtain thermally expandable microsphere precursors TEMs. The TEMs are mixed with a second solution and subjected to a hydrolysis reaction to obtain ATEMs; ZIF-67@ATEMs were prepared by mixing ATEMs with 2-methylimidazole and a cobalt source. The ZIF-67@ATEMs were subjected to carbonization and calcination treatments in sequence to obtain hollow / cavity nitrogen-doped carbon microspheres based on thermally expanded microspheres loaded with cobalt tetroxide.

2. The preparation method according to claim 1, characterized in that, The carbon-carbon double bond monomer comprises a first component and a second component; preferably, the first component comprises at least one of acrylonitrile, methacrylonitrile, acrylamide, and N,N-dimethylacrylamide; the second component comprises one or more of methyl methacrylate, methyl acrylate, methacrylic acid, acrylic acid, and allyl methacrylate. The suspending agent includes one or more of the following: silica sol, silica powder, magnesium hydroxide, PVP-K30, PVP-K60, and sodium dodecyl sulfate. The high-boiling-point alkane includes one or more of isooctane, n-octane, and n-hexane; the initiator includes one or more of azobisisobutyronitrile, azobisisoheptanenitrile, azobisisobutyramidine hydrochloride, or benzoyl peroxide. The solvent of the first solution is water, and the solute is one or more of sodium chloride, potassium chloride, and magnesium chloride.

3. The preparation method according to claim 1 or 2, characterized in that, The mass ratio of the carbon-carbon double bond monomer, suspending agent, high-boiling-point alkane, initiator and first solution is 10-20:1-2:2-6:0.005-0.2:40-100.

4. The preparation method according to claim 1, characterized in that, The polymerization reaction is carried out at a temperature of 55–75°C for 10–15 hours.

5. The preparation method according to claim 1, characterized in that, The second solution is an aqueous solution of acid or base, with a concentration of 0.5–1 mol / L; Preferably, the acid is one of sulfuric acid, acetic acid, and hydrochloric acid; and the base is one of sodium hydroxide and potassium hydroxide.

6. The preparation method according to claim 1, characterized in that, The mass ratio of the TEM precursor to the second solution is 1–10:40–400; the hydrolysis reaction is carried out at a temperature of 20–80°C for 1–6 hours.

7. The preparation method according to claim 1, characterized in that, The concentration of ATEMs is 80–170 g / L, the concentration of 2-methylimidazole aqueous solution is 1–4 mol / L, the concentration of cobalt nitrate aqueous solution is 0.01–0.2 mol / L, and the immersion time is 3–6 h.

8. The preparation method according to claim 1, characterized in that, The carbonization is carried out in a nitrogen atmosphere at a temperature of 600–1100 °C for 1–3 h; the calcination is carried out in an air atmosphere at a temperature of 200–400 °C for 1–3 h.

9. A hollow / cavity nitrogen-doped carbon microsphere based on thermally expandable microspheres loaded with cobalt tetroxide, prepared by the preparation method according to any one of claims 1 to 8, characterized in that, The microspheres have a hollow structure and are independently distributed among each other.

10. The application of hollow / cavity nitrogen-doped carbon microspheres based on thermally expandable microspheres loaded with cobalt tetroxide in supercapacitors as described in claim 9.

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