Nitrogen-doped yolk shell structure carbon sphere, electrode, supercapacitor and preparation method

Abstract

The invention relates to the technical field of electrochemical materials, and particularly discloses a nitrogen-doped yolk shell structure carbon sphere, an electrode, a supercapacitor and a preparation method. According to the invention, phenolic resin is taken as a core, catecholamine substances are taken as a shell precursor, and zinc salt is utilized to regulate and control the activity and fine structure of a carbon skeleton, so that an egg yolk shell structure with'core-shell-gap 'characteristics is prepared, and a mesopore-micropore interpenetrating multi-stage pore system is formed; the carbon sphere with the special structure has high specific surface area, good conductivity and high chemical stability, the maximum specific surface area is 567.04 m < 2 > / g, the specific capacitance of the supercapacitor prepared by taking the carbon sphere as an electrode is 225F / g when the current density is 0.5 A / g, the energy density is 16.7 Wh / kg, the cycle performance is excellent, the capacitance retention rate after 6000 cycles is 95.7%, and the specific capacitance retention rate is 95.7%. And the material shows extremely high application potential in the field of supercapacitors.

Description

Technical Field

[0001] The present invention relates to the technical field of electrochemical materials, and in particular to a nitrogen-doped yolk-shell structured carbon sphere, an electrode, a supercapacitor and a preparation method thereof. Background Art

[0002] Against the backdrop of rapidly growing global demand for clean energy and efficient energy storage technologies, supercapacitors, as an important electrochemical energy storage device, have shown great application potential in electric vehicles, smart grids, portable electronic devices, and other fields due to their high power density, rapid charge and discharge capabilities, long cycle life, and good environmental adaptability. As the core component of supercapacitors, the performance of electrode materials directly determines the overall energy storage performance of supercapacitors.

[0003] Carbon-based materials, with their advantages of widespread availability, excellent electrical conductivity, stable chemical properties, and flexible structural design, have become a key component of supercapacitor electrode materials. However, traditional carbon-based materials, such as activated carbon and carbon nanotubes, suffer from limited active sites, long ion diffusion pathways, and low specific surface area utilization, making their energy and power densities inadequate to meet the growing demand for high-performance energy storage.

[0004] Researchers are improving the performance of carbon materials by constructing specialized structures. The yolk-shell structure is a unique material structure, defined as a structure consisting of a smaller particle (the yolk) within a larger outer shell. This structure is achieved by controlling the material's growth and synthesis process to create a spatial separation between the inner particles and the outer shell. This structure offers numerous advantages in the materials field, such as providing additional ion / electron transport channels, buffering volume expansion, and enhancing structural stability, effectively improving the electrochemical performance of the material.

[0005] The currently commonly used methods for preparing yolk-shell carbon spheres have many shortcomings in terms of steps, raw materials and reaction conditions. First, the preparation process is often cumbersome, requiring multi-step reactions and complex operations, which not only increases production costs, but also reduces production efficiency, making it difficult to meet the needs of large-scale production. Secondly, the required raw materials are usually special or expensive, which further increases the preparation cost. In addition, the reaction conditions are relatively harsh, such as requiring high temperature, high pressure or a specific gas atmosphere, which places high demands on equipment and technology, and also limits its wide application to a certain extent. In terms of electrochemical performance, although the nitrogen-doped yolk-shell carbon spheres prepared by the existing methods show certain advantages in supercapacitors, there is still a gap with the expected goals in terms of specific capacity, rate performance and cycle stability. For example, the capacity retention rate at high current density needs to be improved, and the performance attenuation problem after long-term cycling still needs to be solved. These problems seriously restrict its application and development in actual supercapacitors. Summary of the Invention

[0006] Aiming at the problem that yolk-shell structured carbon spheres prepared by existing methods have poor specific capacity, rate performance and cycle stability, which seriously limits their application in supercapacitors, the present invention provides nitrogen-doped yolk-shell structured carbon spheres, electrodes, supercapacitors and preparation methods.

[0007] In order to solve the above technical problems, the technical solution provided by the present invention is:

[0008] A method for preparing nitrogen-doped yolk-shell structured carbon spheres comprises the following steps:

[0009] S1, adding phenolic substances and aldehydes to a mixed solution of ammonia water, anhydrous ethanol and water to carry out a polymerization reaction to obtain a phenolic resin reaction solution;

[0010] S2, adding catecholamine self-polymerizing monomers to the phenolic resin reaction solution, continuing the self-polymerization reaction, solid-liquid separation, and drying to obtain a precursor;

[0011] S3, dispersing the precursor in an alcohol solvent to obtain a precursor dispersion; adding zinc salt to the precursor dispersion, allowing it to stand, performing solid-liquid separation, washing, drying, and calcining under an inert atmosphere to obtain nitrogen-doped yolk-shell structured carbon spheres.

[0012] Compared with the prior art, the preparation method of nitrogen-doped yolk-shell structured carbon spheres provided by the present invention forms a uniform coating on the surface of the phenolic resin through the self-polymerization reaction of catecholamine substances. Catecholamine substances have rich nitrogen elements and self-polymerization properties. During the self-polymerization reaction, the nitrogen element can be evenly distributed in the polymer network. During the subsequent calcination process, the nitrogen element can be stably retained in the carbon sphere structure, forming a uniform nitrogen doping effect. After calcination, the organic nitrogen can be converted into active sites such as pyrrolic nitrogen, pyridinic nitrogen and graphitic nitrogen, introducing a large number of pseudocapacitive active sites on the carbon spheres, enhancing the adsorption and desorption ability of the carbon spheres for ions in the electrolyte, thereby improving the electrochemical activity and capacitive performance of the carbon spheres. At the same time, the catechol group in the molecular structure of the self-polymerizing monomer has strong redox activity and can contribute additional pseudocapacitance to a certain extent. It synergizes with the double-layer capacitance of the carbon material itself to significantly improve the specific capacitance and energy density of the carbon spheres. In addition, the coating layer formed by catecholamines can buffer the volume changes of carbon balls during charging and discharging to a certain extent, enhance the stability of the material structure, and improve its cycle life.

[0013] When zinc salt is added to the precursor dispersion, the zinc ions will coordinate with the active groups on the surface of the precursor (such as residual hydroxyl groups and amino groups of catecholamines) by virtue of their strong coordination ability, and be evenly adsorbed on the surface of the precursor or embedded in the interior of the precursor. During the subsequent calcination process under an inert atmosphere, the zinc salt gradually vaporizes and volatilizes or undergoes a decomposition reaction, generating a certain spatial driving force, which promotes the formation of gaps between the interior and surface of the precursor, thereby separating the interior of the precursor from the outer layer, and finally forming a yolk-shell structure.

[0014] During the static process of adding zinc salt to the precursor dispersion, zinc nitrate can etch a part of the dopamine shell and enrich the pore structure of the shell; in addition, the gas generated by the decomposition or volatilization of the zinc salt will form abundant pores inside the precursor and in the shell when it escapes. On the one hand, the rapid escape of the gas will produce a macroporous structure, providing a channel for the rapid penetration of the electrolyte; on the other hand, the zinc salt particles will leave tiny gaps locally during the decomposition and volatilization process. These gaps are interconnected to form mesoporous and microporous structures. The formation of multi-level pores greatly increases the specific surface area and pore volume of the carbon spheres, providing more space and paths for the storage and diffusion of ions, and effectively improving the electrochemical properties of the material.

[0015] Furthermore, in S1, the volume ratio of the ammonia water, anhydrous ethanol and water is (0.1-1):4:24.

[0016] Furthermore, in S1, the molar ratio of the phenolic substance to the aldehyde is (0.5-1):1.

[0017] Furthermore, in S1, the concentration of the phenolic substance in the mixed solution is 5 g / L to 10 g / L.

[0018] Furthermore, in S1, the polymerization reaction temperature is 20°C to 40°C, and the reaction time is 2h to 4h.

[0019] By controlling the molar ratio of phenolic substances to aldehydes, reaction temperature and time, the cross-linking degree and molecular weight of the phenolic resin can be precisely regulated to provide a stable carbon skeleton for the subsequent formation of the yolk-shell structure and ensure the integrity of the final carbon sphere structure.

[0020] Furthermore, in S1, the phenolic substance is at least one of resorcinol, aminophenol, phloroglucinol or p-aminophenol.

[0021] Furthermore, in S1, the aldehyde is at least one of formaldehyde, benzaldehyde or thiophenecarboxaldehyde.

[0022] Furthermore, in S2, the catecholamine self-polymerizing monomer is dopamine hydrochloride.

[0023] Furthermore, in S2, the molar ratio of the catecholamine self-polymerizing monomer to the phenolic substance is (0.4-0.8):1.

[0024] The optimal ratio of catecholamine self-polymerizing monomers creates a moderate nitrogen doping concentration within the carbon sphere structure and forms a moderately thick coating on the phenolic resin surface. This allows for the volatilization and decomposition of the zinc salt during subsequent calcination, resulting in a spatially well-spaced, porous yolk-shell structure. This structure ensures internal space for electrolyte infiltration and ion diffusion, while also providing the shell with sufficient mechanical strength to buffer volume changes during charge and discharge.

[0025] Furthermore, in S2, the temperature of the self-polymerization reaction is 20°C to 40°C, and the reaction time is 2h to 5h.

[0026] Furthermore, in S3, the concentration of the precursor dispersion is 5 g / L to 20 g / L.

[0027] Specifically, in S3, the alcohol solvent is anhydrous ethanol.

[0028] Furthermore, in S3, the mass ratio of the zinc salt to the precursor is (30-50):1.

[0029] It should be noted that in order to improve the uniformity of the zinc salt dispersion in the system, the zinc salt can be added in the form of an aqueous solution. Specifically, the concentration of the zinc salt aqueous solution is 1 mol / L to 3 mol / L.

[0030] Furthermore, in S3, the standing time is 2 hours to 6 hours.

[0031] The optimal standing time is conducive to promoting the uniform adsorption of zinc ions on the surface of the precursor or embedding into its internal structure, laying the foundation for the gasification, decomposition and structural formation of zinc salts at high temperatures, and ensuring that the final carbon spheres have an ideal yolk-shell morphology and multi-level pore structure.

[0032] Furthermore, in S3, the calcination temperature is 600°C to 1000°C, the heating rate is 1°C / min to 10°C / min, and the calcination time is 1h to 5h.

[0033] The optimal calcination temperature, time, and heating rate ensure sufficient carbonization of the precursor, forming a stable carbon skeleton structure. This facilitates the gradual decomposition or vaporization of the zinc salt, ensuring the stable formation of the yolk-shell structure and the stable presence of nitrogen within the carbon sphere structure. Furthermore, these calcination temperatures and heating rates facilitate fine-tuning of the carbon sphere's multi-level pore structure, improving its wettability and ion diffusion efficiency.

[0034] The present invention also provides a nitrogen-doped yolk-shell structured carbon sphere, which is prepared by any of the above-mentioned methods for preparing nitrogen-doped yolk-shell structured carbon spheres.

[0035] The present invention also provides an electrode comprising the nitrogen-doped yolk-shell structured carbon spheres.

[0036] The present invention also provides a supercapacitor comprising the above-mentioned electrode.

[0037] The nitrogen-doped yolk-shell structure carbon spheres prepared by the present invention use phenolic resin as the core and catecholamines as the shell precursor. Zinc salt is used to regulate the activity and fine structure of the carbon skeleton, thereby preparing a yolk-shell structure with "core-shell-void" characteristics and forming a multi-level pore system with interconnected mesopores and micropores. The carbon spheres with this special structure have high specific surface area, good electrical conductivity and high chemical stability, with a maximum specific surface area of ​​567.04 m 2 / g, and the supercapacitor prepared with it as an electrode has a specific capacitance of 225F / g at a current density of 0.5A / g and an energy density of 16.7Wh / kg. It also has excellent cycle performance, with a capacitance retention rate of 95.7% after 6000 cycles. It shows extremely high application potential in the field of supercapacitors and is expected to promote the innovation and development of energy storage technology. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Figure 1 The SEM images and TEM images of the carbon sphere materials prepared in Examples 1 to 3 of the present invention and Comparative Example 1, wherein: (a) SEM image of N-YCS in Comparative Example 1, (b) SEM image of N-YCS-1 in Comparative Example 2, (c) SEM image of N-YCS-2 in Example 2, (d) SEM image of N-YCS-4 in Comparative Example 3, (e) SEM image of uncarbonized N-YCS-2 prepared in Example 2, and (f) TEM image of N-YCS-2 in Example 2;

[0039] Figure 2 N2 adsorption-desorption isotherm and pore size distribution of nitrogen-doped yolk-shell carbon sphere materials prepared in Example 2 and Comparative Examples 1 to 3 of the present invention, (a) N2 adsorption-desorption isotherm, (b) pore size distribution;

[0040] Figure 3 IR spectra of RF@DA before and after immersion in aluminum nitrate in Example 2;

[0041] Figure 4 This is the XRD pattern of the nitrogen-doped yolk-shell carbon sphere material N-YCS-2 prepared in Example 2;

[0042] Figure 5This is the Raman spectrum of the nitrogen-doped yolk-shell carbon sphere material N-YCS-2 prepared in Example 2;

[0043] Figure 6 XPS graph of the nitrogen-doped yolk-shell carbon sphere material N-YCS-2 prepared in Example 2; wherein, (a) XPS full view, (b) C1s spectrum, (c) N1s spectrum, (d) O1s spectrum;

[0044] Figure 7 CV curves of the working electrodes prepared from the carbon spheres prepared in Example 2 and Comparative Examples 1 to 3 at 5 mV / s in a three-electrode system;

[0045] Figure 8 GCD curves of the working electrodes prepared from the carbon spheres prepared in Example 2 and Comparative Examples 1 to 3 at 1 A / g in a three-electrode system;

[0046] Figure 9 The relationship between current density and specific capacitance of the working electrodes prepared from the carbon spheres prepared in Example 2 and Comparative Examples 1 to 3 in a three-electrode system;

[0047] Figure 10 The Nyquist plots of the working electrodes prepared from the carbon spheres prepared in Example 2 and Comparative Examples 1 to 3 in a three-electrode system; wherein the inset is a partial enlarged view;

[0048] Figure 11 The Bode plots of the working electrodes prepared from the carbon spheres prepared in Example 2 and Comparative Examples 1 to 3 in a three-electrode system; wherein the inset is a partial enlarged view;

[0049] Figure 12 CV curves of the working electrode prepared from N-YCS-2 prepared in Example 2 in a three-electrode system at different scan rates in 6 M KOH electrolyte;

[0050] Figure 13 GCD curves of the working electrode prepared from N-YCS-2 prepared in Example 2 in a three-electrode system at different current densities in 6 M KOH electrolyte;

[0051] Figure 14 Comparison of capacitance contributions of working electrodes prepared from carbon spheres prepared in Example 2 and Comparative Examples 1 to 3 at a scan rate of 5 mV / s in a three-electrode system; (a) N-YCS, (b) N-YCS-1, (c) N-YCS-2, and (d) N-YCS-4;

[0052] Figure 15 The capacitance contribution of N-YCS-2 prepared in Example 2 at different scan rates in a three-electrode system;

[0053] Figure 16CV curves of N-YCS-2 prepared in Example 2 in a two-electrode system at different scan rates in 6 M KOH electrolyte;

[0054] Figure 17 GCD curves of N-YCS-2 prepared in Example 2 in a two-electrode system at different current densities in 6MKOH electrolyte;

[0055] Figure 18 This is a rate performance diagram of N-YCS-2 prepared in Example 2 at different current densities in a two-electrode system;

[0056] Figure 19 This is the Ragone diagram of N-YCS-2 prepared in Example 2 in a two-electrode system;

[0057] Figure 20 The capacitance contribution of N-YCS-2 prepared in Example 2 at different scan rates in a two-electrode system;

[0058] Figure 21 A comparison chart of capacitance contribution of N-YCS-2 prepared in Example 2 in a two-electrode system at a scan rate of 100 mV / s;

[0059] Figure 22 The capacity retention rate of N-YCS-2 prepared in Example 2 after different cycles in a two-electrode system is shown in the figure, where the inset is the GCD diagram before and after 6000 cycles;

[0060] Figure 23 This is a comparison of the EIS diagrams of N-YCS-2 prepared in Example 2 after different number of cycles in a two-electrode system, where the inset is a local magnified view. DETAILED DESCRIPTION

[0061] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention.

[0062] In order to better illustrate the present invention, further examples are given below.

[0063] Example 1

[0064] This embodiment provides a method for preparing nitrogen-doped yolk-shell structured carbon spheres, comprising the following steps:

[0065] S1: Ammonia water (0.8 mL, 25 wt%), anhydrous ethanol (4 mL), and deionized water (34 mL) were mixed and stirred for 1 h. Resorcinol (0.2 g) was added and stirred for 30 min. Then, formaldehyde solution (0.2 mL, 37 wt%) was added and stirred at 40 °C for 1 h. Then, 0.15 g of dopamine hydrochloride was added and stirred at 20 °C for 5 h. The mixture was washed by centrifugation and dried at room temperature to obtain the RF@DA precursor.

[0066] S2, the RF@DA precursor prepared above (0.1 g) was added to 10 mL of anhydrous ethanol, ultrasonically dispersed for 30 min, and then added with 20 mL of 2.0 M zinc nitrate aqueous solution. The mixture was soaked in a water bath at 25 °C for 4 h, centrifuged, washed, and dried at room temperature. Then, the mixture was heated to 1000 °C at a rate of 10 °C / min under a nitrogen atmosphere and pyrolyzed for 1 h to obtain nitrogen-doped yolk-shell structured carbon spheres.

[0067] Example 2

[0068] This embodiment provides a method for preparing nitrogen-doped yolk-shell structured carbon spheres, comprising the following steps:

[0069] S1: Ammonia water (0.1 mL, 25 wt%), anhydrous ethanol (4 mL), and deionized water (24 mL) were mixed and stirred for 1 h. Resorcinol (0.2 g) was added and stirred for 30 min. Then, formaldehyde solution (0.28 mL, 37 wt%) was added and stirred at 30 °C for 1 h. Then, 0.2 g of dopamine hydrochloride was added and stirred at 30 °C for 5 h. The mixture was washed by centrifugation and dried at room temperature to obtain the RF@DA precursor.

[0070] S2, the RF@DA precursor prepared above (0.2 g) was added to 20 mL of anhydrous ethanol, ultrasonically dispersed for 30 min, and then added with 25 mL of 2.0 M zinc nitrate aqueous solution. The mixture was soaked in a water bath at 25 °C for 4 h, centrifuged, washed, and dried at room temperature. Then, the mixture was heated to 800 °C at a rate of 5 °C / min under a nitrogen atmosphere and pyrolyzed for 2 h to obtain nitrogen-doped yolk-shell structured carbon spheres (N-YCS-2).

[0071] Example 3

[0072] This embodiment provides a method for preparing nitrogen-doped yolk-shell structured carbon spheres, comprising the following steps:

[0073] S1: Ammonia water (0.5 mL, 25 wt%), anhydrous ethanol (4 mL), and deionized water (16 mL) were mixed and stirred for 1 h. Resorcinol (0.2 g) was added and stirred for 30 min. Then, formaldehyde solution (0.35 mL, 37 wt%) was added and stirred at 20 °C for 4 h. Then, 0.25 g of dopamine hydrochloride was added and stirred at 40 °C for 2 h. The mixture was washed by centrifugation and dried at room temperature to obtain the RF@DA precursor.

[0074] S2, the RF@DA precursor prepared above (0.2 g) was added to 40 mL of anhydrous ethanol, ultrasonically dispersed for 30 min, and then 16 mL of 2.0 M zinc nitrate aqueous solution was added. The mixture was soaked in a water bath at 25 °C for 4 h, centrifuged, washed, and dried at room temperature. Then, the mixture was heated to 600 °C at a rate of 3 °C / min under a nitrogen atmosphere and pyrolyzed for 5 h to obtain nitrogen-doped yolk-shell structured carbon spheres.

[0075] Comparative Example 1

[0076] The only difference between this comparative example and Example 2 is that zinc nitrate is not added, and the specific steps are as follows:

[0077] S1: Ammonia water (0.1 mL, 25 wt%), anhydrous ethanol (4 mL), and deionized water (24 mL) were mixed and stirred for 1 h. Resorcinol (0.2 g) was added and stirred for 30 min. Then, formaldehyde solution (0.28 mL, 37 wt%) was added and stirred at 30 °C for 1 h. Then, 0.2 g of dopamine hydrochloride was added and stirred at 30 °C for 5 h. The mixture was washed by centrifugation and dried at room temperature to obtain the RF@DA precursor.

[0078] S2, the RF@DA precursor prepared above was pyrolyzed to 800 °C at a rate of 5 °C / min under a nitrogen atmosphere for 2 h to obtain N-YCS.

[0079] Comparative Example 2

[0080] The only difference between this comparative example and Example 2 is that the amount of dopamine hydrochloride in Example 2 is changed to 0.1 g. The comparative example specifically comprises the following steps:

[0081] S1: Ammonia water (0.8 mL, 25 wt%), anhydrous ethanol (4 mL), and deionized water (34 mL) were mixed and stirred for 1 h. Resorcinol (0.2 g) was added and stirred for 30 min. Then, formaldehyde solution (0.2 mL, 37 wt%) was added and stirred at 40 °C for 1 h. Then, 0.15 g of dopamine hydrochloride was added and stirred at 20 °C for 5 h. The mixture was washed by centrifugation and dried at room temperature to obtain the RF@DA precursor.

[0082] S2, the RF@DA precursor prepared above (0.1 g) was added to 10 mL of anhydrous ethanol, ultrasonically dispersed for 30 min, and then added with 20 mL of a 2.0 M zinc nitrate aqueous solution. The mixture was immersed in a water bath at 25 °C for 4 h, centrifuged, washed, dried at room temperature, and then heated to 1000 °C at a rate of 10 °C / min under a nitrogen atmosphere and pyrolyzed for 1 h to obtain N-YCS-1.

[0083] Comparative Example 3

[0084] The only difference between this comparative example and Example 2 is that the amount of dopamine hydrochloride in Example 2 is changed to 0.4 g. The specific steps include:

[0085] S1: Ammonia water (0.8 mL, 25 wt%), anhydrous ethanol (4 mL), and deionized water (34 mL) were mixed and stirred for 1 h. Resorcinol (0.2 g) was added and stirred for 30 min. Then, formaldehyde solution (0.2 mL, 37 wt%) was added and stirred at 40 °C for 1 h. Then, 0.4 g of dopamine hydrochloride was added and stirred at 20 °C for 5 h. The mixture was washed by centrifugation and dried at room temperature to obtain the RF@DA precursor.

[0086] S2, the RF@DA precursor prepared above (0.1 g) was added to 10 mL of anhydrous ethanol, ultrasonically dispersed for 30 min, and then added with 20 mL of 2.0 M zinc nitrate aqueous solution. The mixture was immersed in a water bath at 25 °C for 4 h, centrifuged, washed, dried at room temperature, and then heated to 1000 °C at a rate of 10 °C / min under a nitrogen atmosphere and pyrolyzed for 1 h to obtain N-YCS-4.

[0087] Material characterization

[0088] Figure 1 These are SEM images of nitrogen-doped yolk-shell structured carbon sphere materials prepared in Example 1 and Comparative Examples 1 to 3 of the present invention. Figure 1 (a) It can be seen that when zinc nitrate is not used as an activator, a core-shell structure is also formed. This may be because the phenolic resin undergoes condensation during carbonization, and the dopamine in the outer shell is a rigid material, so a core-shell structure is formed. Figure 1 (b) shows that a certain amount of zinc nitrate solution has etched away the dopamine shell, exposing the phenolic resin core. At this time, the material diameter is about 350nm. When the amount of dopamine is increased to 0.2g, the diameter of the phenolic resin core is about 350nm. Figure 1 (c) The complete core-shell structure can be seen. When the amount of dopamine used is increased to 0.4 g, Figure 1 (d) It can be seen that due to the excessive thickness of the shell, zinc nitrate only etches away a portion of the dopamine shell on the surface, which is now approximately 420 nm in diameter. Figure 1(e) It can be seen that N-YCS-2 without carbonization after impregnation with zinc nitrate exhibits a clear core-shell structure. This may be because the dopamine shell is partially dissolved, the zinc nitrate solution contacts the core resin, and the dibenzyl ether bond in the dopamine matrix is ​​oxidized to a carbonyl group by the zinc nitrate solution, while the nitrate ion is reduced to a nitro group.

[0089] TEM shows the internal structure of N-YCS-2, such as Figure 1 (f) As can be seen from the figure, N-YCS-2 as a whole has a core-shell separation morphology, with an outer wall thickness of about 32 nm. Its morphology is similar to the SEM image, in which the inner core diameter is about 310 nm and the overall diameter of the carbon sphere is about 400 nm.

[0090] Figure 2 (a) is the N2 adsorption-desorption isotherm curve of the nitrogen-doped yolk-shell structure carbon sphere material prepared in Examples 1 to 3 and Comparative Example 1, and Figure (2b) is the pore size distribution diagram. Figure 2 (a) shows that all samples exhibit type IV isotherms. The isotherms of N-YCS and N-YCS-2 exhibit a distinct hysteresis loop within the P / P0 range of 0.4 to 0.8, belonging to the H4 type hysteresis loop, indicating significant adsorption at the low end of the P / P0 range, which is related to micropore filling. N2 adsorption by N-YCS and N-YCS-2 increases significantly when P / P0 > 0.9, indicating the presence of a large number of cavities. However, due to the low amount of dopamine in the N-YCS-1 sample, zinc nitrate etches away the dopamine shell.

[0091] Figure 2 (b) shows the pore size distribution of each sample. It can be seen that N-YCS-2 has the largest BET surface area, reaching 567.04 m 2 / g, a large specific surface area will result in more active surface for charge storage. In supercapacitors, charge is stored in the double layer on the electrode surface, so a larger specific surface area will increase the specific capacitance, which means that the amount of charge that can be stored increases. More micropore volume also helps the electrolyte penetrate into the electrode material, which can promote the contact between the electrolyte and the electrode. At the same time, the micropores can also serve as channels for ion transmission, enhancing the conductivity of the electrode, which is crucial for improving the performance of double-layer supercapacitors. The structural parameters of each sample are shown in Table 1.

[0092] Table 1

[0093]

[0094] Figure 3 The infrared spectra of RF@DA before and after immersion in aluminum nitrate are shown in Figure 2. Compared with the IR spectra before immersion, the infrared spectra of RF@DA at 1530 and 1350 cm-1 after immersion are significantly higher than those before immersion. -1A new band appeared at 762 cm, which can be attributed to the vibration of the nitro group attached to the aromatic ring. -1 The nearby band is associated with the torsional vibration of benzene, 2983 cm -1 -CH 2 stretching vibration peak.

[0095] Figure 4 This is the XRD pattern of the nitrogen-doped yolk-shell carbon sphere material N-YCS-2 prepared in Example 2. N-YCS-2 displays three typical diffraction peaks near 2θ of 12°, 26°, and 42.5°. The high-intensity peaks in the low-angle region indicate the presence of a large number of micropores in the sample. The peaks at 26° and 42.5° correspond to the carbon (002) crystal plane and the amorphous structure (101) crystal plane, respectively, indicating that the carbon material is amorphous and has a low degree of graphitization.

[0096] Figure 5 This is the Raman spectrum of the nitrogen-doped yolk-shell carbon sphere material N-YCS-2 prepared in Example 2, 1347 cm -1 The peak at (D) indicates the degree of defects and disorder, while the peak at 158 ​​cm -1 The peak at (G) is related to the development of graphite structure. The intensity ratio (ID / IG) of N-YCS-2 is 1.13, which is higher than 0.8 of N-YCS, 0.81 of N-YCS-1 and 0.91 of N-YCS-4. D / I G The ratio increases with the increase of dopamine content. N-YCS-2 has the highest defect level, while N-YCS-4 has too much dopamine and too thick an outer shell, resulting in the absence of core-shell carbon spheres and a decrease in the defect level.

[0097] The elemental composition was analyzed by X-ray photoelectron spectroscopy (XPS). Figure 6 The XPS spectrum is shown in Figure 2 f, C1s main peak is located at 284.8eV, N1s main peak is located at 400.85eV, O1s main peak is located at 533.19eV. XPS elemental analysis shows that the contents of carbon, oxygen and nitrogen in N-YCS-2 are 89.88, 6.42 and 3.1wt%, respectively. These results confirm that nitrogen atoms have been successfully incorporated into the carbon framework. The specific form of each element can be determined by analyzing and fitting the high-resolution XPS spectrum of each individual element. Figure 6 As shown in (b), the C1s spectrum can be divided into two different peaks after deconvolution, corresponding to C=C at 284.8eV and CN at 285.8eV. Figure 6(c)) shows three different peaks at 398.3, 400.8 and 404.7 eV, which correspond to the contributions of pyridinic nitrogen (398.7 eV), pyrrolic nitrogen (400.8 eV) and pyridine N-oxide (404.7 eV). Pyridine nitrogen (398.3 eV) can provide active sites and help improve electrochemical performance; pyrrolic nitrogen can improve the conductivity of carbon materials, while pyridine N-oxide can change the electronic structure and surface properties of the material and promote the interaction between electrode materials and electrolytes. Since the electronegativity of N atoms is similar to that of C atoms, under the influence of the activator, N atoms are successfully doped into the vacancies therein to form zero-dimensional defects, which is consistent with the Raman spectrum. Deconvolution of O1s spectrum ( Figure 6 (d) Three distinct peaks were identified, corresponding to C=O at 531.53 eV, COC at 533.1 eV, and -OH at 536.4 eV. XPS results indicate that the addition of dopamine hydrochloride, doped with nitrogen, enhances the surface wettability of the material, improves the activity of the carbon material, and promotes the generation of pseudocapacitance.

[0098] Performance Testing

[0099] The viscous slurry (nitrogen-doped yolk-shell carbon spheres, carbon black, and polytetrafluoroethylene dissolved in anhydrous ethanol at a mass ratio of 8:1:1) was coated onto a nickel foam current collector of approximately 0.5 cm × 1 cm in size to prepare a working electrode. After drying at 100°C for 24 h, the mass of active material loaded on each working electrode was 4 to 5 mg.

[0100] Electrochemical measurements were performed using a three-electrode system with a 6 M KOH solution as the electrolyte on an electrochemical workstation (CHI 760E, Chenhua Instruments, China). Pt wire and Hg / HgO served as the counter and reference electrodes. Cyclic voltammetry (CV) was recorded over a potential range of -1 to 0 V at scan rates of 5 mV / s to 200 mV / s. Electrochemical impedance spectroscopy (EIS) measurements were performed with an AC perturbation of 5 mV over a frequency range of 100 kHz to 0.01 Hz.

[0101] Figure 7CV curves of the working electrodes prepared from the carbon spheres prepared in Example 2 and Comparative Examples 1 to 3 at 5 mV / s. At a scan rate of 5 mV / s, each electrode exhibits an approximately rectangular shape but with a slight deviation, indicating that it has a large specific capacitance value and good rate performance at a given voltage (-1 to 0 V), proving that double-layer capacitance behavior and pseudocapacitance behavior exist simultaneously on each working electrode, and that the pseudocapacitance behavior is related to the successful doping of nitrogen atoms. In addition, it can be seen from the figure that the CV integrated area size of each carbon sphere sample is N-YCS-2>N-YCS-4>N-YCS>N-YCS-1, indicating that the N-YCS-2 electrode has the largest capacitance value. Compared with N-YCS-2, the other samples may have lower activity due to the lack of a complete core-shell structure.

[0102] When the current density is 1A / g, the GCD curves of the working electrodes prepared from each carbon sphere are as follows: Figure 8 As shown. As can be seen from the figure, N-YCS-2 has a longer charge and discharge time, indicating that N-YCS-2 has the largest specific capacitance value. The electrodes of each carbon ball material present a symmetrical isosceles triangle, indicating typical double-layer behavior and high Coulomb efficiency, achieving efficient energy transfer. At a current density of 1A / g, the discharge time and corresponding capacitance of the working electrodes prepared from each carbon ball are: N-YCS-2>N-YCS-1>N-YCS-4>N-YCS. After calculation, the capacitances are 225F / g, 156F / g, 148.9F / g, and 142F / g, respectively. The high specific surface area of ​​N-YCS-2 allows a large amount of charge to accumulate at the electrode / electrolyte interface. The cavity structure provides a large additional pseudocapacitance, shortens the ion transport length, and makes it easier for ions to diffuse in the carbon nanospheres, thereby enhancing the capacitance. Calculated by the formula Figure 9 N-YCS-2 maintained high specific capacitances of 225F / g, 199F / g, 195F / g, 191F / g, 188F / g, 178F / g, and 164F / g at 1A / g, 2A / g, 3A / g, 4A / g, 5A / g, 10A / g, and 20A / g, which corresponded to capacitance retentions of 100%, 92%, 90%, 88%, 87%, 82%, and 76% at a current density of 1A / g. N-YCS-2 exhibited good rate performance.

[0103] EIS diagram can effectively obtain charge transfer information in electrochemical tests. The Nyquist diagram of N-YCSs electrode in the frequency range of 0.01 to 100 kHz is shown in Figure 2. Figure 10As shown. It can be seen that the approximate straight line in the low-frequency region corresponds to the capacitance behavior of the electrode material. Among them, the maximum slope of N-YCS-2 verifies its maximum specific capacitance. In the high-frequency region, the curve represents the charge transfer resistance of the material. The smaller the semicircle diameter, the faster the ion diffusion rate. Obviously, the R ct The value is smaller than that of other samples, indicating that the electron transfer rate between the N-YCS-2 electrode and the electrolyte is the fastest, which is consistent with the electric double layer capacitance (EDLC) behavior.

[0104] Figure 11 The Bode plots of the carbon sphere samples prepared in Example 2 and Comparative Examples 1 to 3 are shown, wherein the phase angles of N-YCS, N-YCS-1, N-YCS-2, and N-YCS-4 are 84°, 84.4°, 84.9°, and 82.4°, respectively, which are close to the ideal capacitance phase angle (-90°). The relaxation time (τ0) can be obtained according to the equation τ0=1 / f0, where f0 is the frequency at a phase angle of 45°. The relaxation time constants (τ0) of N-YCS, N-YCS-1, N-YCS-2, and N-YCS-4 at 45° are calculated to be 2.6s, 2.15s, 1.46s, and 2.6s, respectively, indicating that N-YCS-2 has excellent ion transport rate and diffusion capacity.

[0105] Figure 12 The CV curves of N-YCS-2 at scan rates of 5 to 100 mV / s are shown. As can be seen from the figure, even at a high scan rate of 100 mV / s, the material maintains its typical rectangular shape, primarily due to its core-shell cavity structure and the electron transport capability of its short-range channels.

[0106] Figure 13 The GC curves of N-YCS-2 at current densities of 0.5 to 20 A / g are shown. Even at a high current density of 20 A / g, the curve of N-YCS-2 is triangular and symmetrical, indicating that N-YCS-2 has good electrochemical reversibility and charge-discharge capabilities.

[0107] like Figure 14 As shown, the cyclic voltammetry curve was tested at a scan rate of 5 mv / s. After calculation, the capacitance contributions of N-YCS, N-YCS-1, N-YCS-2 and N-YCS-4 were 73.5%, 74.5%, 75% and 64.9%, respectively, among which N-YCS-2 had the largest capacitance contribution.

[0108] Figure 15It shows that when the scan rate increases from 5mV / s to 100mV / s, the capacitance contribution increases from 75% to 97%. As the scan rate increases, the double-layer capacitance gradually increases, while the diffusion capacitance gradually decreases, reflecting the characteristics of the sample as a double-layer supercapacitor.

[0109] To evaluate the applicability of N-YCS-2 in energy storage devices, the performance of N-YCS-2 in 6MKOH electrolyte was tested using a two-electrode system.

[0110] Working electrodes were prepared by coating a viscous slurry (N-YCS-2 sample, carbon black, and polytetrafluoroethylene dispersed in anhydrous ethanol at a mass ratio of 8:1:1) onto a nickel foam current collector approximately 0.5 × 1 cm in size. After drying at 100°C for 24 hours, the mass of active material loaded on each working electrode was 4–5 mg.

[0111] Electrochemical measurements were performed using a two-electrode system with 6 M KOH solution as the electrolyte on an electrochemical workstation (CHI 760E, Chenhua Instruments, China). Cyclic voltammetry (CV) was performed over a potential range of 0–1.4 V at scan rates of 5 mV / s to 100 mV / s. Electrochemical impedance spectroscopy (EIS) measurements were performed with an AC perturbation of 5 mV over a frequency range of 100 kHz to 0.01 Hz. GCD performance was evaluated at various current densities, ranging from 0.5 to 20 A / g.

[0112] Figure 16 The CV curves at a voltage of 1.4 V and a scan rate of 5 to 100 mV / s are shown. The approximately rectangular shape of the CV curves shows an ideal capacitive behavior.

[0113] Figure 17 The GCD curve of N-YCS-2 as a symmetric supercapacitor electrode at a current density of 0.5 to 20 A / g is shown. It can be seen that even at a current density of 20 A / g, the curve maintains a linear isosceles triangle, showing good rate performance. The specific capacitance of N-YCS-2 as a symmetric supercapacitor electrode is calculated from the GCD curve. When the current density is 0.5 A / g, it is 208.4 F / g. When the current density is 20 A / g, it is still 120 F / g. The current density increases to 20 A / g, which is equivalent to a capacitance retention rate of 53.2% ( Figure 18 ).

[0114] Ragone diagram of power and energy density levels based on two-electrode testing, such as Figure 19 When the power density is 720 W / kg, the energy density of N-YCS-2 can reach 16.7 Wh / kg, which is better than other reported carbon-based materials.

[0115] The capacitance contribution percentage of N-YCS-2 as a symmetric supercapacitor electrode material is as follows: Figure 20 and Figure 21 As shown in the figure, when the scan rate increases from 5mV / s to 100mV / s, the capacitance contribution increases from 70% to 97%. As the scan rate increases, the double-layer capacitance contribution gradually increases, while the diffusion capacitance gradually decreases, indicating that the capacitance storage at high rates in symmetric supercapacitors is mainly dominated by the surface adsorption process.

[0116] In order to test the stability of N-YCS-2 as a symmetric supercapacitor electrode material, 6000 GCD cycles were performed at a current density of 2 A / g. Figure 22 As can be seen from the figure, the capacitance retention rate is still as high as 95.7% after 6000 cycles. Figure 23 ), the Rs and Rct of the N-YCSs-2 electrode increased slightly after cycling.

[0117] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions or improvements made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A method for preparing nitrogen-doped yolk-shell carbon spheres, characterized in that: The steps include: S1, adding phenolic substances and aldehydes to a mixed solution of ammonia water, anhydrous ethanol and water to carry out a polymerization reaction to obtain a phenolic resin reaction solution; S2, adding catecholamine self-polymerizing monomers to the phenolic resin reaction solution, continuing the self-polymerization reaction, solid-liquid separation, and drying to obtain a precursor; S3, dispersing the precursor in an alcohol solvent to obtain a precursor dispersion; Zinc salt is added to the precursor dispersion, allowed to stand, solid-liquid separation is carried out, washing is carried out, drying is carried out, and calcination is carried out under an inert atmosphere to obtain nitrogen-doped egg yolk-shell structured carbon spheres.

2. The method for preparing nitrogen-doped yolk-shell structured carbon spheres according to claim 1, wherein: In S1, the volume ratio of ammonia water, anhydrous ethanol and water is (0.1-1):4:24; and / or In S1, the molar ratio of the phenolic substance to the aldehyde is (0.5-1):1; and / or In S1, the concentration of the phenolic substance in the mixed solution is 5 g / L to 10 g / L; and / or In S1, the polymerization reaction temperature is 20° C. to 40° C., and the reaction time is 2 h to 4 h.

3. The method for preparing nitrogen-doped yolk-shell structured carbon spheres according to claim 1, wherein: In S1, the phenolic substance is at least one of resorcinol, aminophenol, phloroglucinol or p-aminophenol; and / or In S1, the aldehyde is at least one of formaldehyde, benzaldehyde or thiophenecarboxaldehyde.

4. The method for preparing nitrogen-doped yolk-shell structured carbon spheres according to claim 1, wherein: In S2, the catecholamine self-polymerizing monomer is dopamine.

5. The method for preparing nitrogen-doped yolk-shell structured carbon spheres according to claim 1 or 4, wherein: In S2, the molar ratio of the catecholamine self-polymerizing monomer to the phenolic substance is (0.4-0.8):1; and / or In S2, the temperature of the self-polymerization reaction is 20°C to 40°C, and the reaction time is 2h to 5h.

6. The method for preparing nitrogen-doped yolk-shell structured carbon spheres according to claim 1, wherein: In S3, the concentration of the precursor dispersion is 5 g / L to 20 g / L; and / or In S3, the mass ratio of the zinc salt to the precursor is (30-50):1; and / or In S3, the standing time is 2 hours to 6 hours.

7. The method for preparing nitrogen-doped yolk-shell structured carbon spheres according to claim 1, wherein: In S3, the calcination temperature is 600°C to 1000°C, the heating rate is 1°C / min to 10°C / min, and the calcination time is 1h to 5h.

8. A nitrogen-doped yolk-shell carbon sphere, characterized in that: The carbon spheres are prepared by the method for preparing nitrogen-doped yolk-shell structured carbon spheres according to any one of claims 1 to 7.

9. An electrode, characterized in that Including the nitrogen-doped yolk-shell structured carbon spheres as described in claim 8.

10. A supercapacitor, characterized in that: Comprising the electrode according to claim 9.

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