Preparation method of two-step nitrogen-doped three-dimensional network phenolic resin-based composite carbon material and application thereof in supercapacitor electrode

Abstract

This invention discloses a two-step method for preparing nitrogen-doped three-dimensional network phenolic resin-based composite carbon materials and their applications. The method uses an alkaline environment provided by ammonia to induce addition condensation polymerization of phenol and formaldehyde to obtain a preliminary nitrogen-doped phenolic resin. This resin is then mixed with melamine resin to achieve a second step of nitrogen introduction, and after being composited with sodium alginate, it is cross-linked with CaCl2 to obtain a three-dimensional hydrogel precursor. After carbonization and activation treatment, a nitrogen-doped porous carbon material is obtained. This material possesses both micro- and mesoporous synergy and multiple types of nitrogen functional sites, exhibiting high specific surface area capacitance and excellent cycling stability in supercapacitors. Compared with existing schemes that only use NaOH without introducing dual-source nitrogen doping (such as CN119724955A), this invention achieves synergistic optimization of structure, chemical sites, and transport through ammonia pre-doping, secondary nitrogen doping with melamine resin, and a ternary interpenetrating network. The process is simple, uses inexpensive raw materials, and is suitable for large-scale production.

Description

Technical Field

[0001] This invention belongs to the field of energy materials and functional carbon material preparation technology, specifically relating to a two-step nitrogen-doped three-dimensional network phenolic resin-based composite carbon material preparation method and its application in supercapacitor electrodes. Background Technology

[0002] Supercapacitors, as a novel energy storage device, are widely used in new energy, smart grids, and electric transportation due to their high power density, long cycle life, and excellent safety. Their performance largely depends on the structural design and surface chemical properties of the electrode materials. Currently, the most mature electrode materials are carbon-based materials, such as activated carbon, carbon aerogels, graphene, and carbon nanotubes. Among numerous carbon precursors, phenolic resins are considered ideal matrices for preparing porous carbon materials due to their high carbon yield, controllable structure, and simple synthesis routes. However, traditional phenolic resin carbon materials still face technical bottlenecks. For example, the structure tends to be dense after carbonization, with a uniform pore size distribution and limited ion diffusion channels; nitrogen is difficult to introduce uniformly, resulting in insufficient surface active sites and limited pseudocapacitive contribution; and traditional alkaline catalytic systems (such as NaOH and KOH) often lead to pore structure collapse or residual alkali pollution, which is detrimental to environmental friendliness and electrochemical stability.

[0003] To improve pore structure and nitrogen doping uniformity, some studies have attempted to use natural polymers, such as alginate to introduce a sol-gel reaction to construct a composite structure; patent CN119724955A discloses a method using phenolic resin and sodium alginate as a matrix, through Ca... 2+ A method for preparing a semi-interpenetrating network structure by cross-linking. This technique achieves pore structure control through gel templates, which improves specific surface area and structural stability to some extent. However, this method still has the following shortcomings:

[0004] (1) When sodium hydroxide is used as a single alkaline catalyst, the system has a strong alkalinity, which can easily cause the resin polycondensation rate to run out of control and the pore walls to collapse locally.

[0005] (2) No effective dual-source nitrogen doping mechanism was introduced, the nitrogen content in the material was <1 wt%, and the contribution of pseudocapacitance on the electrode surface was limited;

[0006] (3) The network structure is only a binary system of phenolic resin and sodium alginate, lacking chemical cross-linking enhancement and conductivity improvement mechanisms.

[0007] (4) The capacitance performance can be improved by controlling the surface charge distribution of carbon materials through nitrogen doping. Normal nitrogen doping, especially in phenolic systems, results in insufficient reaction matching between the nitrogen source and the resin matrix, low nitrogen doping efficiency, and unstable chemical state of nitrogen. The two-step nitrogen doping method can provide certain active sites and solve the problems of limited doping depth and uneven distribution.

[0008] Therefore, there is an urgent need for a modification route with a reasonable structural design, high nitrogen doping efficiency, and green and simple process to realize the construction of a multi-level porous structure and efficient nitrogen doping of phenolic resin-based carbon materials, thereby significantly improving the specific capacitance, rate performance and cycle stability of supercapacitor electrodes without increasing the complexity of the process and cost.

[0009] To address the aforementioned issues, this invention proposes a "two-step nitrogen-doped ternary composite structure" strategy that combines pre-doped nitrogen with ammonia as the alkali source with composite cross-linking with melamine resin as the second nitrogen source. This strategy avoids strong alkali contamination while achieving efficient nitrogen introduction and synergistic regulation of pore structure, thereby significantly improving the electrochemical energy storage performance of carbon materials. Summary of the Invention

[0010] To overcome the problems of low nitrogen doping efficiency, simple pore structure, and contamination of alkaline catalysis system in existing phenolic resin-based carbon materials, this invention provides a two-step method for preparing nitrogen-doped three-dimensional network phenolic resin-based composite carbon materials and their application in supercapacitor electrodes.

[0011] This invention utilizes ammonia water to provide a mild alkaline environment for prepolymerization, achieving initial nitrogen introduction. Melamine resin is then used as a second nitrogen source for synergistic crosslinking with a phenolic system, and sodium alginate is used as a green polysaccharide support for Ca²⁺ crosslinking, constructing a stable three-dimensional interpenetrating network. After programmed heat treatment and chemical activation, a composite porous carbon material with high specific surface area, multiple active sites, and good electrical conductivity is obtained.

[0012] To achieve the above objectives, the present invention adopts the following technical solution:

[0013] A two-step method for preparing nitrogen-doped three-dimensional network phenolic resin-based composite carbon materials includes the following steps:

[0014] (1) First step of nitrogen doping: In the alkaline medium provided by ammonia water, phenol and formaldehyde are mixed in proportion and subjected to addition condensation reaction at 70-90°C for 90-180 min. The reaction is then terminated by cooling and weak acid titration to obtain a preliminary nitrogen-doped hot viscous phenolic resin.

[0015] (2) Second step of nitrogen doping and ternary composite reaction: The above-mentioned hot viscous phenolic resin and melamine resin are mixed and reacted at a mass ratio of 1:(0.5-1.0) so that the -NH2 group in the melamine molecule participates in the cross-linking reaction, realizing the second step of nitrogen introduction; then the mixed resin is dissolved in 1-2 wt% sodium alginate solution, and after thorough stirring, 0.3-0.5 MCaCl2 solution is added dropwise for ionic cross-linking to form insoluble three-dimensional hydrogel particles.

[0016] (3) Solvent replacement and heat treatment: After solvent replacement, the obtained gel is dried and pre-oxidized at 160-200°C for 4 h. Then, it is carbonized at 600-900°C in an inert atmosphere at a heating rate of 5-10°C / min to obtain primary carbon material. Then, it is mixed with KOH at a mass ratio of 1:(3-5), activated at 800°C for 1 h, and then acid washed and dried to obtain the target composite porous carbon material.

[0017] According to a preferred embodiment of the present invention, in step (1), the ammonia concentration is 25 wt% to 30 wt%. Furthermore, uniform alkalinity is maintained by constant temperature stirring during the initial stage of the reaction to avoid structural collapse caused by abrupt changes in the polycondensation rate.

[0018] According to a preferred embodiment of the present invention, in step (1), the addition polycondensation temperature in step (1) is 70-95°C and the time is 90-180 min.

[0019] According to a preferred embodiment of the present invention, in step (1), the reaction is terminated by cooling and neutralizing with 0.01-0.05 M acetic acid to pH=6.5-7.5, so as to stabilize the active sites and inhibit self-condensation.

[0020] According to a preferred embodiment of the present invention, in step (2), the amount of melamine resin is 50-100% of the mass of phenolic resin, and the mass ratio of the composite to sodium alginate is 1:(2-4).

[0021] According to a preferred embodiment of the present invention, in step (2), the reaction temperature of melamine resin and phenolic resin is 20-60°C and the reaction time is 1-3h.

[0022] According to a preferred embodiment of the present invention, in step (2), the reaction stirring rate of melamine resin and phenolic resin is 200-500 r / min.

[0023] According to a preferred embodiment of the present invention, in step (2), the crosslinking agent is a CaCl2 solution, the crosslinking time is ≥ 24 h, and the resulting gel is dried at room temperature or low temperature before entering the heat treatment process.

[0024] According to a preferred embodiment of the present invention, in step (3), after gelation, solvent replacement is performed, with ethanol as the solvent, 2-3 times, each time for 2-6 hours, and vacuum drying is carried out at -50-0 °C.

[0025] According to a preferred embodiment of the present invention, in step (3), the pre-oxidation temperature is 180°C and is maintained for 4 h; the carbonization temperature is increased to 600-900°C at a rate of 2-10°C / min and held for 0.5-2 h; during activation, the temperature is first increased to 350°C and held for 0.5 h, and then increased to 800°C and held for 1 h.

[0026] According to a preferred embodiment of the present invention, in step (3), the acid washing is performed using 2-5 M HCl followed by washing with deionized water until neutral.

[0027] According to a preferred embodiment of the present invention, in step (4), the nitrogen content of the composite carbon material is 2.0 to 3.0 wt%, the specific surface area is 500 to 1500 m² / g, the pore size distribution is concentrated in the mesoporous range of 2 to 10 nm, and the area specific capacitance under the three-electrode system is ≥0.4 F / m². 2 .

[0028] This invention provides a two-step method for preparing nitrogen-doped three-dimensional network phenolic resin-based composite carbon material and its application in supercapacitor electrodes. The material is prepared using the above-described method.

[0029] The technical features and beneficial effects of this invention are as follows:

[0030] (1) Dual-source nitrogen doping mechanism: Nitrogen is doped in two steps using ammonia and melamine resin, introducing nitrogen elements in the prepolymerization and composite stages respectively, forming multiple chemical states such as pyridine nitrogen, pyrrole nitrogen, and graphitic nitrogen. This design achieves hierarchical distribution of structural nitrogen and surface nitrogen, solves the problems of limited doping depth and uneven distribution, significantly enhances the pseudocapacitive contribution and electronic conductivity of the material, and enables the electrode to maintain a high specific capacitance at high rates.

[0031] (2) Ternary synergistic network structure: Through the preliminary nitrogen-doped resin, melamine resin and sodium alginate ternary interpenetrating network, the synergistic regulation of organic crosslinking and ionic crosslinking is achieved. This structure can be transformed into a stable three-dimensional porous framework during carbonization, effectively overcoming the problems of pore collapse and single pore size in traditional phenolic systems, and endowing the material with excellent structural stability and ion diffusion channels.

[0032] (3) Mild catalysis and green process: Ammonia water is used instead of traditional strong bases such as NaOH as the catalytic and nitrogen source medium, which not only provides an alkaline environment to promote addition and condensation polymerization, but also avoids inorganic salt pollution. The reaction conditions are mild (70-90°C), the system is environmentally friendly, and it is suitable for industrial promotion.

[0033] (4) Innovative reaction termination and activity control: The reaction is terminated by cooling and neutralizing with 0.01-0.05 M acetic acid to pH 6.5-7.5. This effectively neutralizes residual alkali, stabilizes the phenolic hydroxyl active sites, inhibits excessive polycondensation and self-crosslinking, ensures the activity of the resin molecular chains, and provides an ideal precursor for subsequent composite reactions. This measure significantly improves the uniformity of the resin and the controllability of crosslinking.

[0034] (5) Solvent replacement-assisted drying technology: After the sodium alginate gel is formed, ethanol is introduced to perform multiple solvent replacements, which can significantly reduce capillary stress and volume shrinkage during the drying process and avoid pore wall collapse. This step preserves the porous framework structure of the hydrogel, which increases the specific surface area of ​​the carbonized material by 15-25%, makes the pore size distribution more uniform, and makes the micro / mesoporous ratio more reasonable.

[0035] (6) Synergistic regulation of pore structure and chemical sites: The obtained material has a nitrogen content of 2.0-3.0 wt% and a specific surface area of ​​only 500-1500 m² / g. It has a specific capacitance of 240-260 F / g at 0.5 A / g under a symmetrical electrode system. Its rate performance and cycle stability are significantly better than existing technologies. The capacitance retention rate exceeds 98% after 5000 cycles. The process uses inexpensive raw materials and conventional equipment, and is feasible for scale-up. Attached Figure Description

[0036] Figure 1 SEM images of nitrogen-doped three-dimensional network phenolic resin-based composite carbon materials prepared in Example 1 and Comparative Examples 1-3 of the present invention; a) and b) are SEM images of sample Example 1; c) and d) are SEM images of sample Comparative Example 1; e) and f) are SEM images of sample Comparative Example 2; g) and h) are SEM images of sample Comparative Example 3.

[0037] Figure 2 The adsorption curves and pore size distribution curves of the nitrogen-doped three-dimensional network phenolic resin-based composite carbon materials prepared in Example 1 and Comparative Examples 1-3 of this invention are obtained by BET test.

[0038] Figure 3 The following are performance diagrams of the nitrogen-doped three-dimensional network phenolic resin-based composite carbon material prepared in Example 1 of the present invention: a) is the charge-discharge curve under different current densities in the three-electrode system; b) is the charge-discharge curve under different current densities in the symmetrical electrode system; and c) is the cycle stability curve at a current density of 10 A / g. Detailed Implementation

[0039] This invention proposes a two-step method for preparing nitrogen-doped three-dimensional network phenolic resin-based composite carbon materials and their application in supercapacitor electrodes. To make the advantages and technical solutions of this invention clearer and more explicit, the invention will be further described below with reference to specific embodiments.

[0040] Example 1

[0041] A two-step method for preparing nitrogen-doped three-dimensional network phenolic resin-based composite carbon materials includes the following steps:

[0042] (1) Add 2 ml of ethanol (C2H5OH), 5 ml of deionized water (H2O), 6 ml of ammonia (NH3·H2O), and 3 g of phenol to a three-necked flask equipped with a reflux condenser and a stirrer. When the reaction temperature reaches 75°C, add 8 ml of formaldehyde (37 wt%) using a separatory funnel, adjust the reaction temperature to 95°C, and allow the reaction time to be 150 min. Then, terminate the reaction by cooling and neutralizing with 0.03 M acetic acid to pH 6.5-7.5 to obtain a preliminary nitrogen-doped thermosetting phenolic resin.

[0043] (2) The obtained preliminary nitrogen-doped thermosetting resin and melamine resin were mixed at a mass ratio of 1:0.8 and reacted at 60°C for 2 hours. Then the mixed resin was poured into a pre-dissolved sodium alginate solution with a mass concentration of 1.5wt% at a mass ratio of 1:3 and stirred evenly.

[0044] (3) Then the solution mixture was dropped into the crosslinking agent solution (0.4 M CaCl2 solution). The soluble gel was rapidly converted into insoluble hydrogel particles. The solution was stirred for 24 hours, filtered, and the hydrogel particles were washed with ethanol 3 times for 3 hours each time. They were then placed in a vacuum freeze dryer and dried at low temperature for 12 hours to obtain nitrogen-doped phenolic resin-based composite material particles.

[0045] (4) Pre-oxidize in a forced-air drying oven at 180°C for 4 hours. Heat the resulting carbon material in a tube furnace to 600°C at a rate of 10°C / min and hold for 1 hour. Mix the resulting carbon material with saturated KOH at a mass ratio of 1:4 and dry under vacuum at 70°C. Heat to 350°C at a rate of 2°C / min and hold for 0.5 hours, then increase the temperature to 800°C at a rate of 10°C / min and hold for 1 hour. Finally, wash the sample with 5M HCl and deionized water.

[0046] Comparative Example 1

[0047] The preparation method is the same as that described in Example 1, except that:

[0048] In step (2), the melamine resin was replaced with p-phenylenediamine to obtain a nitrogen-doped resin. The remaining conditions were the same as in Example 1.

[0049] Comparative Example 2

[0050] The preparation method is the same as that described in Example 1, except that:

[0051] In step (2), the melamine resin was replaced with melamine to obtain a nitrogen-doped resin. The remaining conditions were the same as in Example 1.

[0052] Comparative Example 3

[0053] The preparation method is the same as that described in Example 1, except that:

[0054] In step (2), melamine resin was replaced with urea to obtain nitrogen-doped resin. The remaining conditions were the same as in Example 1.

[0055] Experimental Example 1

[0056] The microstructure of carbon materials obtained under different nitrogen source doping systems was observed using scanning electron microscopy (SEM), such as... Figure 1 As shown in the figure, Sample Example 1 exhibits a regular three-dimensional particle cluster morphology, with particles interconnected by carbon sheets to form a stable interpenetrating network. In contrast, Comparative Examples 1, 2, and 3 all show obvious pore collapse and pore wall aggregation. The results indicate that the two-step nitrogen doping strategy using ammonia water and melamine resin can significantly improve the dispersion uniformity and orderliness of the resin in the crosslinking system, resulting in a more uniform nitrogen doping distribution and the formation of a structurally ordered three-dimensional porous framework.

[0057] Experiment Example 2

[0058] The nitrogen adsorption-desorption isotherms of the sample are shown below. Figure 2 All curves exhibit typical Type IV characteristics, with a steep rise in the low-pressure region corresponding to microporous adsorption, and an H4-type hysteresis loop appearing in the medium- and high-pressure regions, indicating that the material possesses both microporous and mesoporous structures. The pore structure parameters calculated by the DFT method are shown in Table 1.

[0059] Table 1. Pore structure parameters calculated by DFT method

[0060]

[0061] The results show that the sample of Example 1 of this invention has the highest surface area specific capacitance, indicating that it possesses the optimal ordered structure and specific surface area utilization. Compared with the NaOH single-base system and sodium alginate binary structure sample in CN119724955A, this invention improves the activation efficiency and pore wall stability through dual-source nitrogen doping, and the pore size distribution is more concentrated in the 2-10 nm mesoporous range, thus balancing ion migration rate and charge storage capacity.

[0062] Experimental Example 3

[0063] The XPS test results are shown in Table 2. The N1s signal peak area ratio was the highest in the PF-MR-SA sample, with a nitrogen content of 2.6 wt%, which was significantly higher than that of the control sample (≤ 1.2 wt%). The nitrogen species mainly included pyridine nitrogen, pyrrole nitrogen, and graphitic nitrogen.

[0064] Table 2 Elemental Composition of Samples

[0065]

[0066] Electrochemical performance test results showed that, under a three-electrode system, the specific capacitance of sample Example 1 at 0.5 A / g was 430 F / g, and the areal capacitance was 0.43 F / m². 2 The specific capacitance of the symmetrical electrode system is 248 F / g, and the capacitance retention rate remains at 98% after 5000 cycles at 10 A / g, with a coulombic efficiency of 82%. This indicates that the dual-source nitrogen-doped structure significantly improves the active site density and electron transport capability, achieving high-rate and high-stability energy storage performance.

[0067] Example 2

[0068] Except for Example 1, all other conditions were the same, except that the melamine resin content was adjusted to 0.6 times the mass of the phenolic resin, and the sodium alginate solution concentration was 1.0 wt%. After drying and carbonization, the resulting gel particles had a specific surface area of ​​1320 m² / g and a nitrogen content of 1.8 wt%. The results show that although appropriately reducing the proportion of melamine resin maintains a good pore structure, the nitrogen doping amount and electrochemical performance decrease slightly.

[0069] Example 3

[0070] While maintaining a constant nitrogen doping ratio, the carbonization temperatures were adjusted to 700°C, 800°C, and 900°C, respectively. The results showed that the sample obtained at 800°C exhibited the optimal balance between structure and performance. Temperatures that were too low resulted in incomplete carbonization and insufficient conductivity; while temperatures that were too high caused the nitrogen-doped components to volatilize, reducing active sites. Therefore, the preferred carbonization temperature range of this invention is 750-850°C.

[0071] Example 4

[0072] The mass ratio of KOH to precursor was varied (1:2, 1:4, 1:6), while other conditions remained the same. Results showed that when the ratio was 1:4, the sample formed a uniform porous structure with the highest specific surface area (approximately 2280 m² / g) and the most reasonable pore volume distribution. Insufficient activator resulted in underdeveloped pore structure; excessive activator caused carbon framework collapse. Therefore, the preferred activator ratio in this invention is 1:(3-5).

[0073] Any parts not mentioned in this invention can be achieved by referring to existing technologies.

[0074] The above embodiments are merely illustrative of the technical concept and features of the present invention, and are intended to enable those skilled in the art to understand the content of the present invention and implement it accordingly. They should not be construed as limiting the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing a two-step nitrogen-doped three-dimensional network phenolic resin-based composite carbon material, characterized in that, Includes the following steps: (1) First step of nitrogen doping: In the alkaline medium provided by ammonia water, phenol and formaldehyde are mixed in proportion and subjected to addition condensation reaction at 70~95℃ for 90~180 min. The reaction is then terminated by cooling and weak acid titration to obtain the preliminary nitrogen-doped hot viscous phenolic resin. (2) Second step of nitrogen doping and ternary composite reaction: The above-mentioned hot viscous phenolic resin and melamine resin are mixed at a mass ratio of 1:(0.5~1.0) to achieve the second step of nitrogen introduction; then the mixed resin is dissolved in 1~2 wt% sodium alginate solution, and after thorough stirring, 0.3~0.5M CaCl2 solution is added dropwise to carry out ionic cross-linking to form insoluble three-dimensional hydrogel particles; (3) Solvent replacement and heat treatment: After solvent replacement, the obtained gel is dried and pre-oxidized at 160~200℃ for 4h. Then, it is carbonized at 600~900℃ in an inert atmosphere at a heating rate of 5~10℃ / min to obtain primary carbon material. Then, it is mixed with KOH at a mass ratio of 1:(3~5), activated at 800℃ for 1h, and then acid washed and dried to obtain the target composite porous carbon material. In step (1), the ammonia concentration is 25wt% to 30wt%, and uniform alkalinity is maintained by constant temperature stirring in the early stage of the reaction to avoid structural collapse caused by sudden changes in the polycondensation rate.

2. The method according to claim 1, characterized in that, In step (1), the reaction is terminated by cooling and neutralizing with 0.01~0.05M acetic acid to pH=6.5~7.5 in order to stabilize the active sites and inhibit self-condensation.

3. The method according to claim 1, characterized in that, In step (2), the reaction temperature of melamine resin and phenolic resin is 20~60℃ and the reaction time is 1~3 h.

4. The method according to claim 1, characterized in that, In step (2), the reaction stirring rate of melamine resin and phenolic resin is 200~500 r / min.

5. The method according to claim 1, characterized in that, In step (2), the crosslinking agent is CaCl2 solution, the crosslinking time is ≥24h, and the resulting gel is dried at room temperature or low temperature before entering the heat treatment process.

6. The method according to claim 1, characterized in that, In step (3), after gelation, solvent replacement is performed, with ethanol as the solvent, 2 to 3 times, each time for 2 to 6 hours, and vacuum drying is carried out at -50 to 0℃.

7. The method according to claim 1, characterized in that, In step (3), the pre-oxidation temperature is 180℃ and lasts for 4 hours; the carbonization temperature is increased to 600-900℃ at 2-10℃ / min and held for 0.5-2 hours; during activation, the temperature is first increased to 350℃ and held for 0.5 hours, and then increased to 800℃ and held for 1 hour.

8. The method according to claim 1, characterized in that, In step (3), the acid washing is performed with 2-5M HCl followed by deionized water until neutral.

9. The method according to claim 1, characterized in that, In step (3), the nitrogen content of the composite carbon material is 2.0 to 3.0 wt%, and the specific surface area is 500 to 1500 m². 2 / g, pore size distribution concentrated in the mesoporous range of 2 to 10 nm, and areal capacitance ≥0.4 F / m in a three-electrode system. 2 .

10. A two-step nitrogen-doped three-dimensional network phenolic resin-based composite carbon material prepared by any one of claims 1 to 9, characterized in that, The material has a three-dimensional interpenetrating porous framework structure, with nitrogen species including pyridine nitrogen, pyrrole nitrogen and graphitic nitrogen, forming multi-level charge conduction and pseudocapacitive active channels.

11. The application of the two-step nitrogen-doped three-dimensional network phenolic resin-based composite carbon material according to claim 10, characterized in that, When used as an electrode in a supercapacitor, this material exhibits high areal capacitance, excellent rate performance, and long-term cycling stability.

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