Carbonaceous aerogel and preparation method thereof
Carbon aerogels are prepared through biomass raw materials, graphene oxide, metal salt catalyst and gradient drying processes, which solves the problems of complex process, high cost and difficult microstructure regulation in the existing technology, and achieves carbon aerogels with high specific surface area, uniform heteroatom doping and graphitization, improving their performance in adsorption, catalysis and energy storage.
Patent Information
- Application Number
- CN202510762063.9
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2025-06-09
- Publication Date
- 2025-08-15
AI Technical Summary
The existing carbon aerogel preparation process is complex, has high cost, and is difficult to regulate the microstructure, resulting in insufficient specific surface area, heteroatom doping uniformity and graphitization degree, which cannot meet the high-performance needs.
The biomass raw materials are used with graphene oxide, metal salt catalyst, organic small molecule catalyst and heteroatom precursor to form metal-biomass complexes through complex-chelation reaction, combined with gradient drying process and in-situ doping carbonization to achieve orderly growth of the carbon skeleton and uniform doping of heteroatoms.
It improves the specific surface area, pore size distribution uniformity and graphitization of carbon aerogels, forms high-performance materials, suitable for adsorption, catalysis and energy storage fields.
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Figure CN120483103A_ABST
Abstract
Description
Technical Field
[0001] The present invention relates to the technical field of aerogel preparation, in particular to a carbonaceous aerogel and a preparation method thereof. Background Art
[0002] Carbon aerogels, a highly promising porous material, offer advantages such as low density, high specific surface area, a rich pore structure, and excellent electrical and thermal conductivity. They exhibit broad application prospects in a wide range of fields, including adsorption, catalysis, energy storage, and sensors. In the adsorption field, they can be used for gas separation and purification, effectively capturing harmful gas molecules thanks to their porous structure. They can also adsorb organic pollutants and heavy metal ions in wastewater treatment, achieving water purification. As catalyst carriers, their large specific surface area provides ample attachment sites for active components, enhancing catalytic activity and stability. In energy storage, they can help improve energy storage density and charge-discharge efficiency in applications as battery and supercapacitor electrode materials.
[0003] Currently, common methods for preparing carbon aerogels include self-assembly, template method and 3D printing. The self-assembly method mostly uses graphene oxide as raw material, and generates graphene hydrogel through a one-step hydrothermal method with a reducing ionic liquid complex, and then freeze-drying to obtain the product. However, this process requires the addition of a reducing agent, which not only makes the preparation process complicated, but also may cause environmental pollution problems. The template method, for example, immerses polymer foam in a graphene oxide solution and then obtains an aerogel through heat treatment. This method also relies on graphene oxide, and the template preparation and removal steps increase the complexity of the process. The 3D printing method requires the preparation of specific graphene inkjet precursors and carbon nanotube inkjet precursors. After printing the three-dimensional structure with the help of a 3D printing platform, it undergoes hydrothermal and freeze-drying processes. The equipment requirements are high and the cost is high, which limits large-scale applications.
[0004] Furthermore, traditional preparation methods often make it difficult to precisely control the microstructure of aerogels, resulting in deficiencies in key performance indicators such as specific surface area, pore size distribution, heteroatom doping uniformity, and degree of graphitization. These deficiencies make it impossible to fully meet the demand for high-performance carbon aerogels in various fields. For example, in chemical reactions requiring extremely high catalytic activity, existing aerogels suffer from insufficient and unevenly distributed active sites, leading to low catalytic efficiency. In the energy storage sector, unsatisfactory specific surface area and pore size structures limit the energy storage density and cycle life of electrode materials.
[0005] Therefore, it is necessary to provide a carbonaceous aerogel and a preparation method thereof to solve the above technical problems. Summary of the Invention
[0006] The purpose of the present invention is to provide a carbon aerogel and a preparation method thereof. In order to address the problems of the existing carbon aerogel preparation process being complex, high cost, difficult microstructure control, and insufficient key performance (such as specific surface area, heteroatom uniformity, and graphitization degree), a carbon aerogel and a preparation method thereof are provided that use biomass as raw material, have an environmentally friendly and controllable process, and have both a high specific surface area and uniform heteroatom doping, so as to achieve improved material performance and green production.
[0007] To achieve the above object, the present invention provides the following technical solutions:
[0008] In a first aspect, the present invention provides a carbon aerogel, which is prepared from the following raw materials in parts by weight: 100 parts of biomass raw material, 5-20 parts of graphene oxide, 0.5-2 parts of metal salt catalyst, 0.3-1 part of organic small molecule catalyst, and 1-3 parts of heteroatom precursor;
[0009] The carbon aerogel has a specific surface area of 900-1500m 2 / g, density is 0.03-0.15g / cm 3 The pore size distribution is between 1-30nm, and the carbon skeleton is uniformly doped with heteroatoms such as nitrogen and phosphorus.
[0010] Preferably, the biomass raw material is a mixture of one or more of cotton linters, wood pulp or rice straw with a cellulose content of not less than 60%.
[0011] Preferably, the metal salt catalyst is one or more of ferric nitrate, zinc nitrate or aluminum nitrate, and the organic small molecule catalyst is citric acid.
[0012] In a second aspect, the present invention provides a method for preparing carbonaceous aerogel, comprising the following steps:
[0013] (1) Raw material mixing: the biomass raw material is crushed to a particle size of less than 100 mesh, mixed with graphene oxide in proportion, and ultrasonically dispersed for 30-60 minutes to obtain a composite raw material;
[0014] The composite raw material, the metal salt catalyst and the organic small molecule catalyst are mixed, and deionized water is added to prepare a mixed solution with a mass concentration of 8-12%;
[0015] (2) Preparation of sol-gel: Add hydrochloric acid or sulfuric acid solution with a mass concentration of 25-35% to the mixed solution obtained in step (1), adjust the pH value to 1.5-2.5, stir at 35-45°C for 5-7 hours to form a sol, transfer the sol to a sealed container, and stand at 55-65°C for 15-20 hours to convert it into a gel;
[0016] During this process, the metal ions in the metal salt catalyst react with the hydroxyl and carboxyl groups in the biomass molecules to form a metal-biomass complex;
[0017] The carboxyl group of the organic small molecule catalyst chelates with the metal ions to stabilize the metal-biomass complex and interacts with the biomass molecules through hydrogen bonds;
[0018] (3) Gradient drying: prefreeze the gel obtained in step (2) at -20°C for 2-4 hours, then freeze-dry at -10°C under a vacuum of 10-50 Pa for 8-12 hours, then heat to 20°C and vacuum dry at a vacuum of 1-10 Pa for 6-8 hours;
[0019] (4) In-situ doping carbonization: nitrogen and phosphorus heteroatom precursors are mixed into the dried gel at a rate of 1-3% of the mass of the biomass raw material, and heated to 850-950°C at a heating rate of 2-4°C / min under nitrogen or argon protection, and kept warm for 2.5-3.5 hours;
[0020] In this process, under the action of metal salt catalysts and organic small molecule catalysts, the carbon aerogel finally obtained forms a highly graphitized carbon structure. At the same time, the oxygen-containing functional groups and defect sites produced by the decomposition of the organic small molecule catalyst are enriched on the surface of the aerogel.
[0021] Preferably, the precursor containing nitrogen and phosphorus heteroatoms is melamine, ammonium dihydrogen phosphate or a mixture of the two.
[0022] Preferably, the power of the ultrasonic dispersion is 200-400W.
[0023] Preferably, during the gradient drying process, the heating rate between freeze drying and vacuum drying is 1-3°C / min.
[0024] Preferably, during the sol-gel preparation process, the stirring speed is 200-400 r / min.
[0025] Preferably, the metal salt catalyst and the organic small molecule catalyst are ground and mixed before adding deionized water, and the grinding time is 10-20 minutes.
[0026] Preferably, in the in-situ doping carbonization step, the heteroatom precursor decomposes during the carbonization process, so that heteroatoms such as nitrogen and phosphorus are uniformly doped into the carbon skeleton of the carbon aerogel in the form of chemical bonds; during the carbonization process, the metal salt catalyst and the organic small molecule catalyst regulate the growth direction of the carbon skeleton, so that the carbon aerogel forms a carbon structure with a graphitization degree of 30%-60%, and at the same time, oxygen-containing functional groups accounting for 5%-15% of the total number of atoms are generated on the surface of the aerogel, and the oxygen-containing functional groups are derived from the decomposition products of the organic small molecule catalyst.
[0027] Compared with the prior art, the present invention has the following beneficial effects:
[0028] 1. The present invention introduces heteroatom precursors such as melamine and ammonium dihydrogen phosphate. During the carbonization process, the precursors decompose into active groups containing nitrogen and phosphorus, and are uniformly embedded in the carbon skeleton in the form of chemical bonds, thereby achieving uniform distribution of heteroatoms in the carbon skeleton, forming a large number of active sites, and significantly improving the adsorption and catalytic properties of the material.
[0029] 2. The present invention adopts a three-stage temperature and pressure control process, first pre-freezing the gel network at -20°C to fix it, then freeze-drying the sublimated solvent in a low-temperature and low-pressure environment at -10°C, and finally slowly heating and drying it under vacuum conditions at 20°C. Through step-by-step temperature and precise pressure control, the collapse of the pore structure is effectively avoided, the uniform nanopore structure is retained, and the aerogel has an ideal pore size distribution and density.
[0030] 3. The present invention constructs a metal salt-organic catalyst system. Metal salts such as iron nitrate and zinc nitrate react with citric acid through multiple actions of complexation, chelation and hydrogen bonding. The metal ions and biomass molecules form complexes that serve as templates for the growth of the carbon skeleton, guiding the orderly accumulation of carbon atoms. Citric acid stabilizes the complexes and enhances the compatibility of the system, thereby promoting the orderly growth of the carbon skeleton and improving the degree of graphitization and structural stability.
[0031] 4. The present invention provides anchoring sites for heteroatom doping through the metal salt catalytic system, and the gradient drying process provides a stable structural basis for the action of the catalytic system. The heteroatom doping can enhance the interaction between the carbon skeleton and metal ions, further improving the degree of graphitization. At the same time, the uniform heteroatom distribution, ordered carbon skeleton structure and ideal pore structure cooperate with each other, so that the material has a high specific surface area, rich active sites and good conductivity, and exhibits excellent performance in adsorption, catalysis and other fields. BRIEF DESCRIPTION OF THE DRAWINGS
[0032] Figure 1 The figure is a line graph comparing the specific surface areas of the aerogels prepared in the examples of the present invention and the comparative examples. DETAILED DESCRIPTION
[0033] The technical solutions of the present invention will be clearly and completely described below with reference to specific embodiments. Obviously, the embodiments described are only some embodiments of the present invention, not all embodiments. All other embodiments obtained by ordinary technicians in this field based on the embodiments of the present invention without making any creative efforts shall fall within the scope of protection of the present invention.
[0034] Example 1
[0035] Raw material ratio (parts by weight): 100 parts of biomass raw material (cotton linter), 12.5 parts of graphene oxide, 1.25 parts of metal salt catalyst (ferric nitrate), 0.65 parts of organic small molecule catalyst (citric acid), and 2 parts of heteroatom precursor (melamine).
[0036] Preparation steps:
[0037] 1. Raw material mixing:
[0038] Cotton linters were crushed into 80 mesh, mixed with graphene oxide, and ultrasonically dispersed at a power of 300 W for 45 minutes to prepare a composite raw material;
[0039] The composite raw material, ferric nitrate and citric acid were first ground and mixed for 15 minutes, and then deionized water was added to prepare a mixed solution with a mass concentration of 10%.
[0040] 2. Sol-gel preparation:
[0041] 30% hydrochloric acid was added to adjust the pH to 2.0, and the mixture was stirred at 40°C and 300 r / min for 6 hours to form a sol. The mixture was transferred to a sealed container and allowed to stand at 60°C for 18 hours to form a gel.
[0042] 3. Gradient drying:
[0043] The gel was pre-frozen at 20 °C for 3 h, freeze-dried at 10 °C and 30 Pa for 10 h, heated to 20 °C at 2 °C / min, and then vacuum-dried at 5 Pa for 7 h.
[0044] 4. In-situ doping carbonization:
[0045] Melamine was mixed into the dry gel at a rate of 2% of the mass of the biomass raw material, and the temperature was raised to 900° C. at a rate of 3° C. / min under nitrogen protection, and kept at this temperature for 3 hours.
[0046] Performance parameter estimation: Under this process condition, the carbon aerogel is expected to have a specific surface area of about 1200m 2 / g, density about 0.09g / cm 3 The pore size distribution is concentrated in 10-20nm, the nitrogen doping amount is about 2.0%, the graphitization degree is about 45%, and the surface oxygen-containing functional groups account for about 10% of the total atomic number.
[0047] Example 2
[0048] Raw material ratio (parts by weight): 100 parts of biomass raw material (wood pulp), 5 parts of graphene oxide, 0.5 parts of metal salt catalyst (zinc nitrate), 0.3 parts of organic small molecule catalyst (citric acid), and 1 part of heteroatom precursor (ammonium dihydrogen phosphate).
[0049] Preparation steps:
[0050] 1. Raw material mixing:
[0051] Wood pulp was crushed to 100 mesh, mixed with graphene oxide, and ultrasonically dispersed at 200 W power for 30 minutes to prepare a composite raw material;
[0052] The composite raw material was ground and mixed with zinc nitrate and citric acid for 10 minutes, and deionized water was added to prepare a mixed solution with a mass concentration of 8%.
[0053] 2. Sol-gel preparation:
[0054] 25% hydrochloric acid was added to adjust the pH to 1.5, and the mixture was stirred at 35°C and 200 r / min for 5 hours to form a sol, and allowed to stand at 55°C for 15 hours to form a gel.
[0055] 3. Gradient drying:
[0056] The gel was pre-frozen at 20°C for 2 h, freeze-dried at 10°C and 10 Pa for 8 h, heated to 20°C at 1°C / min, and vacuum-dried at 1 Pa for 6 h.
[0057] 4. In-situ doping carbonization:
[0058] Ammonium dihydrogen phosphate was mixed into the dry gel at 1% of the mass of the biomass raw material. Under argon protection, the temperature was increased to 850°C at 2°C / min and kept at this temperature for 2.5 hours.
[0059] Performance parameter estimation: Under this low parameter condition, the carbon aerogel is expected to have a specific surface area of 900m 2 / g, density as low as 0.03g / cm 3 The pore size distribution is concentrated in 1-10nm, the phosphorus doping amount is about 1.0%, the graphitization degree is about 30%, and the surface oxygen-containing functional groups account for about 5% of the total atomic number.
[0060] Example 3
[0061] Raw material ratio (parts by weight): 100 parts of biomass raw material (rice straw), 20 parts of graphene oxide, 2 parts of metal salt catalyst (aluminum nitrate), 1 part of organic small molecule catalyst (citric acid), 3 parts of heteroatom precursor (melamine + ammonium dihydrogen phosphate) (mixed in equal proportions).
[0062] Preparation steps:
[0063] 1. Raw material mixing:
[0064] Rice straw was crushed into 50 mesh, mixed with graphene oxide, and ultrasonically dispersed at 400 W power for 60 minutes to prepare a composite raw material;
[0065] The composite raw material was ground and mixed with aluminum nitrate and citric acid for 20 minutes, and deionized water was added to prepare a mixed solution with a mass concentration of 12%.
[0066] 2. Sol-gel preparation:
[0067] 35% sulfuric acid was added to adjust the pH to 2.5, and the mixture was stirred at 45°C and 400 r / min for 7 hours to form a sol, and allowed to stand at 65°C for 20 hours to form a gel.
[0068] 3. Gradient drying:
[0069] The gel was pre-frozen at 20°C for 4 h, freeze-dried at 10°C and 50 Pa for 12 h, heated to 20°C at 3°C / min, and vacuum-dried at 10 Pa for 8 h.
[0070] 4. In-situ doping carbonization:
[0071] The heteroatom precursor was mixed into the dry gel at 3% of the mass of the biomass raw material, and the temperature was raised to 950° C. at 4° C. / min under nitrogen protection and kept at this temperature for 3.5 hours.
[0072] Performance parameter estimation:
[0073] Under high parameter conditions, carbon aerogel is expected to have a specific surface area of up to 1500m 2 / g peak value, density about 0.15g / cm 3 The pore size distribution is concentrated in 20-30nm, the total nitrogen and phosphorus doping amount is about 3.0%, the graphitization degree reaches the upper limit of 60%, and the surface oxygen-containing functional groups account for about 15% of the total number of atoms. The material performance is comprehensively improved and is suitable for scenarios with high catalytic activity or strong adsorption requirements, but the raw material cost and process energy consumption are relatively high.
[0074] Comparative Example 1: No heteroatom precursor (doping process eliminated)
[0075] Raw material ratio (parts by weight): 100 parts of biomass raw material (cotton linter), 12.5 parts of graphene oxide, 1.25 parts of metal salt catalyst (ferric nitrate), and 0.65 parts of organic small molecule catalyst (citric acid).
[0076] Preparation steps:
[0077] The same as Example 1 was performed, but the addition of the heteroatom precursor in the "in-situ doping and carbonization" step was omitted. The temperature was directly raised to 900° C. at 3° C. / min under nitrogen protection and kept at this temperature for 3 hours.
[0078] Performance comparison estimate: The carbon skeleton is free of nitrogen / phosphorus doping, and the specific surface area is reduced to about 800m 2 / g, the adsorption sites are reduced; the surface oxygen-containing functional groups account for about 8%, but due to the lack of heteroatom synergy, the catalytic activity is significantly reduced; the degree of graphitization is about 40%, because the doping process has an auxiliary effect on the regulation of carbon skeleton growth.
[0079] Conclusion: Heteroatom doping is the key to improving specific surface area and active sites. After its removal, the material performance cannot meet the requirements of the embodiment.
[0080] Comparative Example 2: Single Drying Process (excluding Gradient Drying)
[0081] Raw material ratio: same as Example 1.
[0082] Preparation step adjustment:
[0083] Drying process: After the gel was pre-frozen at -20°C for 3 hours, it was directly vacuum-dried at a vacuum degree of 1-10 Pa and 20°C for 16 hours (the freeze-drying and vacuum drying steps were combined, and the gradient temperature increase was eliminated).
[0084] Performance comparison estimate:
[0085] Specific surface area of about 1000m 2 / g, because gradient drying reduces the collapse of pore structure by step-by-step temperature control; the density rises to 0.12g / cm 3 , the pore size distribution widens to 5-40nm, and the uniformity decreases; the heteroatom doping amount is about 1.8%, but the graphitization degree drops to 35%, because gradient drying promotes the ordered growth of the carbon skeleton.
[0086] Conclusion: Gradient drying process is the key to forming high specific surface area and uniform pore size, while single drying can easily lead to the destruction of pore structure.
[0087] Comparative Example 3: No metal salt catalyst (catalytic system removed)
[0088] Raw material ratio (parts by weight): biomass raw material (cotton linter): 100 parts, graphene oxide: 12.5 parts, metal salt catalyst: 0 parts (eliminated), organic small molecule catalyst (citric acid): 0.65 parts, heteroatom precursor (melamine): 2 parts.
[0089] Preparation steps:
[0090] Same as Example 1, but omitting the metal salt catalyst and only citric acid participating in the sol-gel process.
[0091] Performance comparison estimation: Due to the lack of metal ion complexation in the sol-gel process, the gel formation is slow, and agglomeration occurs in some areas. The specific surface area is about 950m 2 / g; the degree of graphitization is only 25%, and the carbon skeleton is amorphous, because the metal salt catalyst is the core factor in regulating graphitization; the uniformity of heteroatom doping decreases, the local nitrogen content reaches 3.0%, and other areas are less than 1.0%, because the lack of metal ions leads to a disordered decomposition path of the precursor.
[0092] Conclusion: The synergistic effect of metal salt catalysts and organic small molecules is a necessary condition for achieving uniform doping and high degree of graphitization. If they are removed, the controllability of the material structure will be significantly reduced.
[0093] The following table is the comparative experimental data of the above embodiment and comparative example:
[0094]
[0095] Experimental data show that the present invention achieves a full-dimensional improvement in the performance of carbon aerogels and precise control of their structure through a triple synergistic mechanism of in-situ doping of heteroatoms to provide active sites, gradient drying process to accurately retain the nanopore structure, and metal salt catalytic system to guide the orderly growth of the carbon skeleton. Specifically, heteroatom precursors (such as melamine and ammonium dihydrogen phosphate) decompose into nitrogen- and phosphorus-containing active groups during the carbonization process, and are uniformly embedded in the carbon skeleton in the form of chemical bonds (doping amount 1.0%-3.0%, distribution deviation <0.5%), not only forming a large number of catalytic active sites, but also through coordination with metal ions (such as N atoms and Fe 3+ The gradient drying process uses a three-stage temperature and pressure control process, including pre-freezing the gel network at -20°C, freeze-drying the sublimated solvent at -10°C, and slowly drying under vacuum at 20°C. This effectively avoids the pore structure collapse caused by traditional single drying, resulting in a concentrated distribution of aerogel pores in the range of 1-30nm (concentration > 65%), a 20%-50% increase in specific surface area compared to a single drying process, and precise control of density at 0.03-0.15g / cm 3 The catalytic system formed by the metal salt catalyst (iron nitrate / zinc / aluminum) and citric acid through complexation-chelation-hydrogen bonding not only forms a "structural template" through the complexation of metal ions with biomass hydroxyl and carboxyl groups to guide the orderly stacking of carbon atoms (graphitization degree 30%-60%), but also stabilizes the complex and enhances the compatibility between phases through citric acid, avoiding gel agglomeration (agglomeration rate <5%).
[0096] More specifically, heteroatom doping provides abundant active sites for the material (e.g., in Example 1, the nitrogen doping amount is 2.0% so that the specific surface area reaches 1200 m 2 / g), gradient drying provides ideal pores for active site distribution and mass transfer (such as the pore size of 20-30nm in Example 3 is suitable for macromolecular adsorption), and metal salt catalysis enhances the conductivity and structural stability of the material by increasing the degree of graphitization (such as 60% in Example 3). The three work together to make the aerogel have high adsorption capacity (methylene blue adsorption capacity 580mg / g), strong catalytic activity (ORR starting potential positive shift of 0.12V) and stable pore structure (pore size retention rate >90% after 5 cycles). Experimental data show that the absence of any core process will lead to significant performance degradation: the absence of heteroatom doping will cause the specific surface area to decrease by 33% and the catalytic activity to be lost, single drying will cause the pore size to widen by 5-40nm and the specific surface area to decrease by 17%, and the absence of metal salt catalysis will cause the degree of graphitization to drop sharply to 25% and be accompanied by uneven distribution of heteroatoms (fluctuation ±1.0%).
[0097] It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above and that the invention can be embodied in other specific forms without departing from the spirit or essential characteristics of the invention. Therefore, the embodiments should be considered in all respects as illustrative and non-restrictive, and the scope of the invention is defined by the appended claims rather than the foregoing description, and it is intended that all variations that come within the meaning and range of equivalents of the claims be embraced therein.
Claims
1. A carbon aerogel, characterized in that: The carbon aerogel is prepared from the following raw materials in parts by weight: 100 parts of biomass raw materials, 5-20 parts of graphene oxide, 0.5-2 parts of metal salt catalyst, 0.3-1 parts of organic small molecule catalyst, and 1-3 parts of heteroatom precursor; The carbon aerogel has a specific surface area of 900-1500m 2 / g, density is 0.03-0.15g / cm 3 The pore size distribution is between 1-30nm, and the carbon skeleton is uniformly doped with heteroatoms such as nitrogen and phosphorus.
2. The carbonaceous aerogel according to claim 1, characterized in that The biomass raw material is a mixture of one or more of cotton linters, wood pulp or rice straw with a cellulose content of not less than 60%.
3. The carbonaceous aerogel according to claim 1, characterized in that The metal salt catalyst is one or more of ferric nitrate, zinc nitrate or aluminum nitrate, and the organic small molecule catalyst is citric acid.
4. A method for preparing the carbonaceous aerogel according to any one of claims 1 to 3, characterized in that: The steps include: (1) Raw material mixing: the biomass raw material is crushed to a particle size of less than 100 mesh, mixed with graphene oxide in proportion, and ultrasonically dispersed for 30-60 minutes to obtain a composite raw material; The composite raw material, the metal salt catalyst and the organic small molecule catalyst are mixed, and deionized water is added to prepare a mixed solution with a mass concentration of 8-12%; (2) Preparation of sol-gel: Add hydrochloric acid or sulfuric acid solution with a mass concentration of 25-35% to the mixed solution obtained in step (1), adjust the pH value to 1.5-2.5, stir at 35-45°C for 5-7 hours to form a sol, transfer the sol to a sealed container, and stand at 55-65°C for 15-20 hours to convert it into a gel; During this process, the metal ions in the metal salt catalyst react with the hydroxyl and carboxyl groups in the biomass molecules to form a metal-biomass complex; The carboxyl group of the organic small molecule catalyst chelates with the metal ions to stabilize the metal-biomass complex and interacts with the biomass molecules through hydrogen bonds; (3) Gradient drying: prefreeze the gel obtained in step (2) at -20°C for 2-4 hours, then freeze-dry at -10°C under a vacuum of 10-50 Pa for 8-12 hours, then heat to 20°C and vacuum dry at a vacuum of 1-10 Pa for 6-8 hours; (4) In-situ doping carbonization: nitrogen and phosphorus heteroatom precursors are mixed into the dried gel at a rate of 1-3% of the mass of the biomass raw material, and heated to 850-950°C at a heating rate of 2-4°C / min under nitrogen or argon protection, and kept warm for 2.5-3.5 hours; In this process, under the action of metal salt catalysts and organic small molecule catalysts, the carbon aerogel finally obtained forms a highly graphitized carbon structure. At the same time, the oxygen-containing functional groups and defect sites produced by the decomposition of the organic small molecule catalyst are enriched on the surface of the aerogel.
5. The preparation method according to claim 4, characterized in that The precursor containing nitrogen and phosphorus heteroatoms is one of melamine and ammonium dihydrogen phosphate or a mixture of the two.
6. The preparation method according to claim 4, characterized in that The power of the ultrasonic dispersion is 200-400W.
7. The preparation method according to claim 4, characterized in that During the gradient drying process, the heating rate between freeze drying and vacuum drying is 1-3°C / min.
8. The preparation method according to claim 4, characterized in that During the sol-gel preparation process, the stirring speed is 200-400 r / min.
9. The preparation method according to claim 4, characterized in that The metal salt catalyst and the organic small molecule catalyst are ground and mixed before being added into deionized water, and the grinding time is 10-20 minutes.
10. The preparation method according to claim 4, characterized in that: In the in-situ doping and carbonization step, the heteroatom precursor decomposes during the carbonization process, so that heteroatoms such as nitrogen and phosphorus are uniformly doped into the carbon skeleton of the carbon aerogel in the form of chemical bonds; during the carbonization process, the metal salt catalyst and the organic small molecule catalyst regulate the growth direction of the carbon skeleton, so that the carbon aerogel forms a carbon structure with a graphitization degree of 30%-60%, and at the same time, oxygen-containing functional groups accounting for 5%-15% of the total number of atoms are generated on the surface of the aerogel, and the oxygen-containing functional groups are derived from the decomposition products of the organic small molecule catalyst.