Nitrogen-doped hierarchical pore carbon material as well as preparation method and application thereof

Abstract

The invention provides a nitrogen-doped hierarchical pore carbon material as well as a preparation method and application thereof, and the preparation method comprises the following steps: sequentially dissolving urea, alkali metal hydroxide and / or a pore-forming agent in water to obtain a mixed solution, cooling the mixed solution, adding cellulose-based biomass, and dissolving the cellulose-based biomass to obtain transparent sol; freezing the transparent sol into a solid, and performing vacuum freeze drying to obtain a white loose solid; mixing and grinding the liquid polymeric monomer and the white loose solid, and performing heat treatment to obtain a solid product; and carbonizing the solid product in an inert atmosphere, and then pickling and washing the carbonized product. According to the preparation method, chain-shaped arrangement in cellulose-based biomass molecules is converted into three-dimensional net-shaped arrangement through dissolution, macropores are formed, a macropore framework is kept stable and does not collapse in the carbonization process through polymerization reaction, and the formed hierarchical pore carbon material has a micropore / mesopore / macropore coexisting three-dimensional structure and controllable specific surface area and pore size distribution; meanwhile, in-situ nitrogen doping can also be realized.

Description

Technical Field

[0001] This invention relates to a nitrogen-doped hierarchical porous carbon material, its preparation method and application, and belongs to the field of materials technology, particularly the field of porous carbon materials technology. Background Technology

[0002] Hierarchical porous carbon materials combine the advantages of different pore sizes, exhibiting excellent performance in various fields such as energy storage, catalysis, and adsorption. In particular, the nested hierarchical pore morphology of micropores / mesopores / macropores avoids the mass transfer obstruction caused by single micropores or the excessively low specific surface area caused by single mesopores / macropores. While ensuring a high specific surface area, it effectively shortens ion transport paths and improves mass transfer efficiency, better meeting the needs of various applications. Obtaining macropores usually requires the addition of template agents such as silica spheres or block copolymers, and the template needs to be removed during the preparation process using strong acids. To simplify the synthesis process and avoid the addition and removal of templates, template-free synthesis methods have been proposed and used to synthesize hierarchical porous carbon materials.

[0003] In template-free synthesis of hierarchical porous carbon, the selection of raw materials is particularly important. Biopolymers can reduce synthesis costs. Choosing biomass with abundant porous structures is crucial, ensuring that these structures do not deform or collapse during carbonization, and that the final carbon product possesses a similar porous structure. Cellulose-based biomass possesses excellent properties such as biodegradability, hydrophilicity, high aspect ratio, high surface area, and high porosity, and is widely available and abundant, making it suitable as a carbon source for hierarchical porous carbon materials. However, existing porous carbon synthesized from cellulose-based biomass or cellulose compounds, without special treatment, suffers from easily broken cross-linked polymer chains during carbonization, making it difficult to form a thermally stable porous framework. This results in carbon materials with low specific surface area or no hierarchical porous structure.

[0004] The following is a brief introduction to existing methods for preparing hierarchical porous carbon materials using cellulose or cellulose-based biomass as raw materials, based on relevant existing technologies.

[0005] CN114349002A discloses a method for preparing cellulose aerogel-MXene porous carbon electrode material, specifically: Step 1, drying cellulose; Step 2, adding NaOH and urea powder to deionized water, stirring evenly, then adding cellulose to obtain a mixture; Step 3, using HCl / LiF to etch Ti3AlC2 to prepare an MXene dispersion; Step 4, mixing the MXene dispersion and the mixture evenly, pre-cooling, freeze-drying, and then carbonizing; Step 5, mixing the sample obtained in Step 4 with KOH and performing activation treatment to obtain the cellulose aerogel-MXene porous carbon electrode material.

[0006] In this process, cellulose is also dissolved using a strong alkaline aqueous solution of urea. Two high-temperature carbonization processes occur in steps 4 and 5. A simple sodium hydroxide and urea system cannot achieve a high specific surface area for the carbon material in step 4; furthermore, the addition of potassium hydroxide in step 5 to increase the specific surface area of ​​the resulting porous carbon material may lead to a decrease in yield.

[0007] CN117163946A discloses a nitrogen-oxygen-doped porous carbon, its preparation method, and its application. It utilizes a "biomimetic mineralization coupled with seed-induced foaming method" to obtain a nitrogen-oxygen-doped porous carbon material with a hierarchical porous structure. The process involves mixing cellulose-based materials, magnesium salts, a strong alkali, a nitrogen-containing organic compound, and water. The resulting mixture is then subjected to freeze-drying and carbonization treatments sequentially to obtain a carbonization product. Finally, the carbonization product is subjected to acid treatment and separation sequentially to obtain the nitrogen-oxygen-doped porous carbon.

[0008] In this preparation process, magnesium hydroxide and urea are combined to dissolve cellulose and create pores in the later stages. However, this process does not protect the macroporous framework structure formed by cellulose, which is prone to collapse during subsequent carbonization.

[0009] CN109607509A discloses a method for preparing carbon aerogels from cellulose. In step (2), the preparation of the cellulose solution involves adding the dried cellulose from step (1) to a mixed solvent of lithium hydroxide, urea, and water at room temperature, stirring until a stable and transparent cellulose solution is obtained. This method is not applicable to cellulose-based biomass reactants; at room temperature, biomass such as cotton cannot dissolve in this system. In step (3), the preparation of the cellulose aerogel involves gelling the cellulose solution from step (2) at room temperature to form a cellulose hydrogel. This cellulose hydrogel is then immersed in water and washed until neutral to remove lithium hydroxide and urea, thus forming a neutral cellulose hydrogel. The neutral cellulose hydrogel is then immersed in an aqueous solution of tert-butanol, frozen to obtain a gel-state sample, and then fully sublimated and dried to room temperature to obtain the cellulose aerogel. In this step, lithium hydroxide and urea are removed.

[0010] Therefore, providing a novel nitrogen-doped hierarchical porous carbon material, its preparation method, and its application to transform various cellulose-based biomaterials into functional hierarchical porous carbon materials has become an urgent technical problem to be solved in this field. Summary of the Invention

[0011] To address the aforementioned shortcomings and deficiencies, the present invention aims to provide a nitrogen-doped hierarchical porous carbon material, its preparation method, and its applications. This invention avoids the reliance on external template agents for pore formation in existing technologies for preparing hierarchical porous carbon materials from cellulose-based biomass, and simultaneously solves the problem of the easy collapse of macroporous structures in cellulose-based biomass during carbonization.

[0012] To achieve the above objectives, on the one hand, the present invention provides a method for preparing nitrogen-doped hierarchical porous carbon materials, wherein the preparation method includes:

[0013] Step (1): Dissolve urea, alkali metal hydroxide and / or pore-forming agent in water in sequence to obtain a mixture, cool the mixture, add cellulose-based biomass and dissolve it to obtain a transparent sol;

[0014] Step (2): After freezing the transparent sol into a solid, it is then freeze-dried under vacuum to obtain a white, loose solid;

[0015] Step (3): The liquid polymerizable monomer is mixed and ground with a white loose solid and then heat-treated to obtain a solid product;

[0016] Step (4): Carbonize the solid product in an inert atmosphere, and then acid wash and water wash the carbonized product to obtain the nitrogen-doped hierarchical porous carbon material.

[0017] As a specific embodiment of the preparation method described above in this invention, in step (1), the mass ratio of water, alkali metal hydroxide, urea, pore-forming agent and cellulose-based biomass is 100:7-10:13-18:4-8:3-7.

[0018] As a specific embodiment of the preparation method described above in this invention, in step (1), the alkali metal hydroxide includes sodium hydroxide (NaOH) and / or potassium hydroxide (KOH), etc., preferably sodium hydroxide, which is more effective than potassium hydroxide in dissolving cellulose-based biomass.

[0019] As a specific embodiment of the preparation method described above in this invention, in step (1), the pore-forming agent includes one or more of potassium carbonate, potassium hydroxide, sodium hydroxide, sodium carbonate and potassium chloride, preferably potassium carbonate and / or potassium hydroxide.

[0020] In step (1) of the preparation method described above, when the pore-forming agent contains potassium hydroxide and / or sodium hydroxide, no additional alkali metal hydroxide is required. When the alkali metal hydroxide contains potassium hydroxide and / or sodium hydroxide, no additional pore-forming agent is required. In this case, potassium hydroxide and / or sodium hydroxide simultaneously play a dual role in dissolving cellulose-based biomass and in pore-forming during the carbonization stage.

[0021] As a specific embodiment of the preparation method described above in this invention, in step (1), the cellulose-based biomass includes one or more of cellulose, cotton, sugarcane bagasse, and bamboo fiber.

[0022] As a specific embodiment of the preparation method described above in this invention, in step (1), the cooling is to reduce the temperature of the mixture to -10 to -20°C.

[0023] In the preparation method (1) described above, the cellulose-based biomass is added in steps, and the specific number of steps is not required. It can be reasonably adjusted according to the actual operation needs, as long as the purpose of the present invention can be achieved.

[0024] In step (1) of the preparation method described above, the cellulose-based biomass is added and dissolved by continuous stirring; the cooling can be carried out in an ice-salt bath.

[0025] This invention does not impose specific requirements on the temperature and time of freezing and vacuum freeze-drying in step (2) of the preparation method described above. These requirements can be reasonably adjusted according to actual operational needs, as long as the purpose of this invention can be achieved. For example, in some embodiments of this invention, the temperature of the vacuum freeze-drying is -40°C.

[0026] As a specific embodiment of the preparation method described above in this invention, in step (3), for every 0.5g of white loose solid, a liquid polymeric monomer with a volume of 0.5-1.5mL, preferably 0.6-1.0mL, is added.

[0027] As a specific embodiment of the preparation method described above in this invention, in step (3), the liquid polymerizable monomer includes liquid organic polyaldehydes or aniline, etc.

[0028] As a specific embodiment of the preparation method described above in this invention, the liquid organic polyaldehyde includes one or more of glutaraldehyde, glyoxal, furfural, citral, salicylaldehyde, and cinnamaldehyde.

[0029] As a specific embodiment of the preparation method described above in this invention, in step (3), the temperature of the heat treatment is 150-200℃ and the time is 6-12h.

[0030] As a specific embodiment of the preparation method described above in this invention, in step (4), the carbonization is performed at 600-900℃ for 2-4 hours with a heating rate of 2-5℃ / min. Preferably, the carbonization is performed at 750-850℃ for 2-4 hours. In some embodiments of this invention, step (4) specifically includes: placing the solid product cooled to room temperature in a porcelain boat, and under argon protection, heating it from room temperature to a final temperature of 750℃ at a heating rate of 5℃ / min and maintaining it for 2 hours for carbonization treatment.

[0031] The present invention does not impose specific requirements on the inert atmosphere used in step (4) of the preparation method described above, and it can be reasonably selected and adjusted according to actual needs. For example, in some embodiments of the present invention, the inert atmosphere may be argon or the like.

[0032] On the other hand, the present invention also provides a nitrogen-doped hierarchical porous carbon material, wherein the nitrogen-doped hierarchical porous carbon material is prepared by the above-described method for preparing nitrogen-doped hierarchical porous carbon material, and has a three-dimensional morphology, including open micropores, mesopores and macropores.

[0033] As a specific embodiment of the nitrogen-doped hierarchical porous carbon material described above in this invention, the specific surface area of ​​the nitrogen-doped hierarchical porous carbon material is 1200-1900 m². 2 / g.

[0034] As a specific embodiment of the nitrogen-doped hierarchical porous carbon material described above in this invention, the nitrogen content is 0.5-5.0 wt% based on the total weight of the nitrogen-doped hierarchical porous carbon material as 100%.

[0035] The macropores in the nitrogen-doped hierarchical porous carbon material provided by this invention are macropores that have already formed in the white loose solid and are retained during the carbonization stage. This invention protects the macropores through polymerization reaction, while also introducing more carbon / nitrogen sources into the carbon material, thereby increasing the final porous carbon yield.

[0036] Furthermore, this invention also provides applications of the nitrogen-doped hierarchical porous carbon materials described above as adsorbents, electrode materials, or catalysts. Using the nitrogen-doped hierarchical porous carbon materials provided by this invention as adsorbents, electrode materials, or catalysts can be applied in fields such as energy and environmental protection.

[0037] Compared with the prior art, the beneficial technical effects achieved by the present invention include:

[0038] The method for preparing nitrogen-doped hierarchical porous carbon materials provided by this invention uses cellulose-based biomass as raw material to prepare nitrogen-doped hierarchical porous carbon materials. First, the chain morphology of cellulose-based biomass is transformed into a three-dimensional macroporous morphology by dissolving. Then, the cellulose-based biomass is dissolved and freeze-dried. The resulting macroporous solid, i.e., a white loose solid, has the characteristics of low density, large internal surface area, and open pores.

[0039] The preparation method provided by this invention can effectively improve the thermal stability of the three-dimensional macroporous framework by adding a polymerizable liquid reactant to coat the macroporous solid, thus preventing the pores from collapsing during high-temperature carbonization. It can also increase the carbonization yield of the material. Furthermore, the polymer layer coating the macroporous solid, i.e. the polymer layer formed by the polymerization of the liquid reactant, partially decomposes during the carbonization stage, which helps to form mesopores and micropores.

[0040] In this invention, the acquisition of a hierarchical porous structure does not depend on a template agent. The urea, alkaline hydroxide, and pore-forming agent added during the dissolution of cellulose-based biomass can continue to play a pore-forming role during the carbonization stage, decomposing to generate abundant mesopores and micropores, thereby obtaining a higher specific surface area and richer pore morphology. This can effectively improve the specific surface area and porosity of the hierarchical porous carbon material, while simultaneously achieving in-situ nitrogen doping.

[0041] The preparation method provided by this invention can convert biomass with high reserves into functional materials, avoiding the problems of high cost and great environmental harm when chemically synthesizing hierarchical porous carbon.

[0042] In summary, the method for preparing nitrogen-doped hierarchical porous carbon materials provided by this invention uses cellulose-based biomass as raw material. First, the chain-like arrangement of cellulose-based biomass molecules is transformed into a three-dimensional network arrangement through dissolution, achieving macropore formation. The macroporous framework is maintained stable and does not collapse during carbonization through polymerization. The resulting hierarchical porous carbon material has a three-dimensional structure with micropores, mesopores, and macropores coexisting. Furthermore, the material exhibits controllable specific surface area and pore size distribution, and in-situ nitrogen doping can be achieved. When the nitrogen-doped hierarchical porous carbon material provided by this invention is used as an electrode material for supercapacitors, it exhibits excellent conductivity and capacitance performance. Attached Figure Description

[0043] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0044] Figure 1 This is a process flow diagram of a method for preparing nitrogen-doped hierarchical porous carbon materials according to an embodiment of the present invention.

[0045] Figure 2a and Figure 2b This is a transmission electron microscope (TEM) image of the nitrogen-doped hierarchical porous carbon material provided in Embodiment 1 of the present invention.

[0046] Figure 2c Transmission electron microscope (TEM) image of carbon material provided for Comparative Example 2.

[0047] Figure 2d Transmission electron microscope image of carbon material provided for Comparative Example 1.

[0048] Figure 3 Thermogravimetric curves of the carbonized precursors provided in Embodiment 1 and Comparative Example 1 of the present invention are shown.

[0049] Figure 4 The AC impedance diagrams of the carbon materials provided in Examples 1-5 and Comparative Example 1 of this invention are shown.

[0050] Figure 5 The charge-discharge curves of the carbon materials provided in Examples 1-5 and Comparative Example 1 of this invention are shown. Detailed Implementation

[0051] It should be noted that the term "comprising" and any variations thereof in the specification, claims, and accompanying drawings of this invention are intended to cover non-exclusive inclusion. For example, a process, method, system, product, or device that includes a series of steps or units is not necessarily limited to those steps or units explicitly listed, but may include other steps or units not explicitly listed or inherent to such processes, methods, products, or devices.

[0052] The "range" disclosed in this invention is given in the form of a lower limit and an upper limit. It can be one or more lower limits and one or more upper limits, respectively. A given range is defined by selecting a lower limit and an upper limit. The selected lower and upper limits define the boundaries of the particular range. All ranges defined in this way are composable, meaning that any lower limit can be combined with any upper limit to form a range. For example, if ranges of 60-120 and 80-110 are listed for specific parameters, it is also expected that ranges of 60-110 and 80-120 are also expected. Furthermore, if the listed minimum range values ​​are 1 and 2, and the listed maximum range values ​​are 3, 4, and 5, then the following ranges are all expected: 1-3, 1-4, 1-5, 2-3, 2-4, and 2-5.

[0053] In this invention, unless otherwise specified, the numerical range "ab" represents a shortened representation of any combination of real numbers between a and b, where a and b are real numbers. For example, the numerical range "0-5" indicates that all real numbers between "0-5" have been listed in this invention, and "0-5" is simply a shortened representation of these numerical combinations.

[0054] In this invention, unless otherwise specified, all embodiments and preferred embodiments mentioned in this invention can be combined with each other to form new technical solutions.

[0055] In this invention, unless otherwise specified, all technical features and preferred features mentioned in this invention can be combined with each other to form new technical solutions.

[0056] In this invention, unless otherwise specified, all steps mentioned herein may be performed sequentially or randomly, but are preferably performed sequentially. For example, if the method includes steps (a) and (b), it means that the method may include steps (a) and (b) performed sequentially, or it may include steps (b) and (a) performed sequentially. For example, if the method may also include step (c), it means that step (c) may be added to the method in any order. For example, the method may include steps (a), (b), and (c), or it may include steps (a), (c), and (b), or it may include steps (c), (a), and (b), etc.

[0057] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying tables, drawings, and embodiments. The embodiments described below are some, but not all, embodiments of this invention, and are only used to illustrate the invention, and should not be considered as limiting the scope of the invention. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall be followed. Reagents or instruments whose manufacturers are not specified are all conventional products that can be purchased commercially.

[0058] Figure 1 This is a process flow diagram of a method for preparing nitrogen-doped hierarchical porous carbon materials according to an embodiment of the present invention, as shown below. Figure 1 As shown, the preparation method specifically includes the following steps:

[0059] S100: Urea, alkali metal hydroxide and / or pore-forming agent are dissolved in water in sequence to obtain a mixture. The mixture is then cooled, and cellulose-based biomass is added and dissolved to obtain a transparent sol.

[0060] It should be noted that S100 of this invention specifies the combination of raw materials and the order of addition. The order of addition is as follows: first, urea is added to water and dissolved; then, sodium hydroxide and a pore-forming agent are added. After all three reactants have dissolved, a certain amount of cellulose-based biomass is added as a carbon source. Cellulose-based biomass is generally insoluble in water, but in a low-temperature, strongly alkaline solution, the cellulose polymer chains swell. Urea molecules can enter and disrupt some of the cellulose molecular chains and the hydrogen bonds within them. Simultaneously, ions from sodium hydroxide and the pore-forming agent enter the spaces between the disrupted cellulose molecular chains, promoting the subsequent formation of macropores. This step ultimately yields a uniformly dissolved cellulose sol.

[0061] S200: After freezing the transparent sol into a solid, it is then freeze-dried under vacuum to obtain a white, loose solid.

[0062] It should be noted that during the vacuum freeze-drying process of the cellulose sol formed by S100, the dissolved cellulose forms a loose macroporous structure due to the rapid evaporation of water, and alkaline hydroxides such as urea and sodium hydroxide, as well as pore-forming agents, adhere to the inner and outer surfaces of the solid.

[0063] S300: Liquid polymerizable monomers are mixed and ground with white loose solids and then heat-treated to obtain a solid product.

[0064] S400: The solid product is carbonized in an inert atmosphere, and then the carbonized product is acid-washed and water-washed to obtain the nitrogen-doped hierarchical porous carbon material.

[0065] It should be noted that the study found that direct carbonization of the white porous solid resulted in a very low carbon product yield when the carbonization temperature was increased to 750℃, with severe collapse of the macroporous structure. Although carbonization of the white porous solid at lower temperatures could maintain its macroporous structure, the activating effect of the pore-forming agent could not be fully utilized, resulting in fewer micropores and mesopores and a lower surface area in the obtained product. Therefore, in S300 of this invention, the white porous solid is fully impregnated in liquid polymerizable monomers, such as liquid organic polyaldehydes or aniline, so that it is wrapped / attached to the inner and outer surfaces of the macroporous white porous solid. At this time, the pore-forming agent and alkaline hydroxides such as sodium hydroxide come into contact with the liquid polymerizable monomers. During the heat treatment process, the liquid polymerizable monomers undergo a polymerization reaction to form an outer shell consistent with the structure of the wrapped three-dimensional macroporous solid, i.e., a polymer layer. At this time, the white porous solid and the pore-forming agent are fixed together in the polymer layer, and the white porous solid is used as a template for forming a macroporous carbon skeleton. When carbonization reaches a certain stage, the cellulose skeleton begins to collapse and decompose into "sacrificial units," during which urea volatilizes and partially incorporates into the already formed macroporous shell, becoming nitrogen-doped active sites. Simultaneously, alkaline hydroxides such as sodium hydroxide and pore-forming agents work together at high temperatures to activate the macroporous carbon skeleton, introducing micropores and mesopores, significantly increasing the specific surface area and porosity of the product. Furthermore, the polymer layer also partially decomposes during the carbonization stage, contributing to the formation of mesopores and micropores.

[0066] The present invention will be further described below with reference to specific embodiments:

[0067] Example 1

[0068] This embodiment provides a nitrogen-doped hierarchical porous carbon material, which is prepared by a method including the following specific steps:

[0069] Step (1): Dissolve 14.0g urea, 8.0g sodium hydroxide and 4.0g potassium carbonate in 100mL of ultrapure water in sequence, place in an ice-salt bath, and wait for the solution to cool to -15℃. Add 3.0g of weighed cotton in portions and stir continuously until the cotton is completely dissolved and the whole solution becomes a transparent sol.

[0070] Step (2): After freezing the transparent sol, vacuum freeze-dry it at -40°C to obtain a white, loose solid.

[0071] Step (3): Weigh 0.5g of white loose solid, grind it, mix it with 0.8mL of glutaraldehyde, and then polymerize the solid-liquid mixture at 180℃ for 6h to obtain a solid product.

[0072] Step (4): The solid product cooled to room temperature is placed in a ceramic boat and carbonized under argon protection by raising the temperature from room temperature to a final temperature of 750°C and holding it for 2 hours. The heating rate is 5°C / min. The carbonized product is washed with 0.5M hydrochloric acid and then washed with ultrapure water until neutral. After drying, the nitrogen-doped hierarchical porous carbon material is obtained.

[0073] Example 2

[0074] This embodiment provides a nitrogen-doped hierarchical porous carbon material, which is prepared by a method including the following specific steps:

[0075] Step (1): Dissolve 18.0g urea, 10.0g sodium hydroxide and 6.0g potassium chloride in 100mL of ultrapure water in sequence, place in an ice-salt bath, and wait for the solution to cool to -15℃. Add 7.0g of weighed cellulose in portions and stir continuously until completely dissolved and the whole is in a transparent sol state, thus obtaining a transparent sol.

[0076] Step (2): Freeze the transparent sol and then freeze-dry it at -40°C to obtain a white, loose solid.

[0077] Step (3): Weigh 0.5g of white loose solid, grind it, mix it with 1.2mL of furfural, and then polymerize the solid-liquid mixture at 190℃ for 8h to obtain a solid product.

[0078] Step (4): The solid product cooled to room temperature is placed in a ceramic boat and carbonized under argon protection by raising the temperature from room temperature to a final temperature of 900℃ and holding it for 2 hours. The heating rate is 3℃ / min. The carbonized product is washed with 0.5M hydrochloric acid and then washed with ultrapure water until neutral. After drying, the nitrogen-doped hierarchical porous carbon material is obtained.

[0079] Example 3

[0080] This embodiment provides a nitrogen-doped hierarchical porous carbon material, which is prepared by a method including the following specific steps:

[0081] Step (1): Dissolve 13.0g of urea, 7g of sodium hydroxide and 4.0g of potassium hydroxide in 100mL of ultrapure water in sequence, place in an ice-salt bath, and wait for the solution to cool to -15℃. Add 5.0g of heat-treated sugarcane bagasse in portions and stir continuously until completely dissolved and the whole mixture becomes a transparent sol.

[0082] Step (2): After freezing the transparent sol, vacuum freeze-dry it at -40°C to obtain a white, loose solid.

[0083] Step (3): Weigh 0.5g of white loose solid, grind it, mix it with 0.5mL of cinnamaldehyde, and then polymerize the solid-liquid mixture at 160℃ for 12h to obtain a solid product.

[0084] Step (4): The solid product cooled to room temperature is placed in a ceramic boat and carbonized under argon protection by raising the temperature from room temperature to a final temperature of 700°C and holding it for 4 hours. The heating rate is 2°C / min. The carbonized product is washed with 0.5M hydrochloric acid and then washed with ultrapure water until neutral. After drying, the nitrogen-doped hierarchical porous carbon material is obtained.

[0085] Example 4

[0086] This embodiment provides a nitrogen-doped hierarchical porous carbon material, which differs from Embodiment 1 only in that:

[0087] In step (3), 0.5g of white loose solid was weighed, crushed, and then mixed and ground with 0.8mL of aniline.

[0088] Example 5

[0089] This embodiment provides a nitrogen-doped hierarchical porous carbon material, which differs from Embodiment 1 only in that:

[0090] In step (4), the solid product cooled to room temperature is placed in a porcelain boat and carbonized under argon protection by raising the temperature from room temperature to a final temperature of 600°C and holding it for 2 hours, with a heating rate of 5°C / min.

[0091] Comparative Example 1

[0092] This comparative example provides a carbon material whose preparation method differs from that of Example 1 only in that:

[0093] Step (3) is skipped, and the white loose solid is directly carbonized. The specific preparation steps are as follows: 14.0g of urea, 8.0g of sodium hydroxide and 4.0g of potassium carbonate are dissolved in 100mL of ultrapure water in sequence and placed in an ice-salt bath. After the solution is cooled to -15℃, 3.0g of weighed cotton is added in portions and stirred continuously until the cotton is completely dissolved and the whole is in a transparent sol state, and a transparent sol is obtained; the transparent sol is frozen and then freeze-dried at -40℃ to obtain a white loose solid; the white loose solid is placed in a porcelain boat and carbonized under argon protection, with the temperature rising from room temperature to a final temperature of 750℃ and held for 2h, wherein the heating rate is 5℃ / min. The carbonized product is washed with 0.5M hydrochloric acid and stirred, then washed with ultrapure water until neutral, and then dried to obtain the carbon material.

[0094] Comparative Example 2

[0095] This comparative example provides a carbon material whose preparation method differs from that of Example 1 only in that:

[0096] Step (3) is omitted, and the white loose solid is directly carbonized at different temperatures. The specific preparation steps are as follows: 14.0g of urea, 8.0g of sodium hydroxide and 4.0g of potassium carbonate are dissolved in 100mL of ultrapure water and placed in an ice-salt bath. After the solution is cooled to -15℃, 3.0g of weighed cotton is added in portions and stirred continuously until the cotton is completely dissolved and the whole is in a transparent sol state, and a transparent sol is obtained. The transparent sol is frozen and then freeze-dried in a vacuum at -40℃ to obtain a white loose solid. The white loose solid is placed in a porcelain boat and carbonized under argon protection, with the temperature rising from room temperature to a final temperature of 450℃ and held for 2h. The heating rate is 5℃ / min. The carbonized product is washed with 0.5M hydrochloric acid and stirred, then washed with ultrapure water until neutral, and then dried to obtain the carbon material.

[0097] Comparative Example 3

[0098] This comparative example provides a carbon material whose preparation method differs from that of Example 1 only in that:

[0099] No pore-forming agent is added in step (1). The specific preparation process is as follows: 14.0g of urea and 8.0g of sodium hydroxide are dissolved in 100mL of ultrapure water and placed in an ice-salt bath. After the solution is cooled to -15℃, 3.0g of weighed cotton is added in portions and stirred continuously until the cotton is completely dissolved and the whole is in a transparent sol state, thus obtaining a transparent sol. The transparent sol is frozen and then freeze-dried under vacuum at -40℃ to obtain a white loose solid. 0.5g of the white loose solid is weighed, crushed, and mixed with 0.8mL of glutaraldehyde. The solid-liquid mixture is then reacted at 180℃ for 6h to polymerize and obtain a solid product. The solid product is placed in a porcelain boat and carbonized under argon protection, with the temperature rising from room temperature to a final temperature of 750℃ and held for 2h. The heating rate is 5℃ / min. The carbonized product is washed with 0.5M hydrochloric acid and stirred, then washed with ultrapure water until neutral, and then dried to obtain the carbon material.

[0100] Test Example 1

[0101] This test example performs transmission electron microscopy analysis on the nitrogen-doped hierarchical porous carbon material provided in Example 1 of the present invention and the carbon materials provided in Comparative Examples 1 and 2, respectively. Figure 2a and Figure 2b Transmission electron microscopy (TEM) images of nitrogen-doped hierarchical porous carbon materials provided in Embodiment 1 of the present invention and Figure 2c and Figure 2d Transmission electron microscope (TEM) images of carbon materials provided in Comparative Example 2 and Comparative Example 1, respectively.

[0102] from Figure 2cAs can be seen, the carbon material provided in Comparative Example 2 exhibits a loose macroporous structure, with each layer of macropores overlapping and interlocking. This indicates that without the addition of liquid organic polyaldehydes or aniline, directly carbonizing the white, loose solid at 450°C can maintain the macroporous structure. However, when the carbonization temperature is increased to 750°C ( Figure 2d In Comparative Example 1), due to the instability of the macroporous framework of cellulose at high temperatures, the macroporous structure in the material almost completely collapses or twists, and the carbon sheets accumulate together, failing to form a hierarchical porous structure that maintains the macropores. Example 1 of this invention obtains a carbon material by adding glutaraldehyde to form a polymeric outer layer, followed by carbonization at 750°C. Figure 2a As can be seen, the carbon material exhibits an interlaced macroporous structure with an increased pore size. This is because the self-polymerizing liquid reactant, glutaraldehyde, can fully penetrate into the freeze-dried white porous solid, forming a polymer layer on the surface of the white porous solid during heat treatment. During carbonization, the cellulose skeleton encased in the polymer layer collapses under high temperature conditions, but the outer polymer layer has high thermal stability, maintaining the macroporous morphology and forming a stable macroporous carbon skeleton. Simultaneously, excess pore-forming agent and sodium hydroxide can undergo a synergistic activation process during carbonization, resulting in dense micropores and mesopores (such as...) on the macroporous carbon skeleton. Figure 2b As shown in the figure, this structure with progressively distributed pore sizes can effectively reduce mass transfer resistance, enabling the material to be rapidly transported into the pores of the carbon material in both gas-phase and liquid-phase reactions.

[0103] Test Example 2

[0104] In this test example, the specific surface area, mesopore area, and pore volume of the carbon materials provided in Examples 1-5 and Comparative Examples 1-3 of this invention were tested using the nitrogen adsorption-desorption test method. The experimental results are shown in Table 1.

[0105] Table 1

[0106]

[0107] The formation of pores in the material was further analyzed based on the nitrogen adsorption-desorption test results shown in Table 1. The carbon materials provided in Examples 1-5 of this invention all formed porous structures with large specific surface areas, and all contained micropores and mesopores (this method is not applicable to macropore testing). In Comparative Example 3, no pore-forming agent was added, and the resulting carbon material had a smaller specific surface area and pore volume, indicating that adding a pore-forming agent in step (1) is key to improving the specific surface area of ​​the hierarchical porous carbon material. The difference lies in the varying pore-forming effects of different pore-forming agents, as shown in Examples 1-5 in Table 1. The results of Comparative Examples 1 and 2 show that, without the addition of liquid organic polyaldehydes or aniline, the vacuum freeze-dried product of cellulose-based biomass, i.e., the white loose solid, although able to retain a macroporous framework during low-temperature carbonization, has a very low specific surface area and pore volume, only 36 m². 2 / g and 0.09cm 3 The / g indicates that this material contains only macropores, with almost no micropores or mesopores that can effectively increase the specific surface area, making further applications difficult. While increasing the carbonization temperature can produce a certain activation effect with NaOH at high temperatures, the porous framework is prone to collapse, leading to a decrease in the specific surface area of ​​the resulting carbon material. These results demonstrate that the self-polymerization of liquid organic polyaldehydes or aniline within the macropores of the cellulose-based freeze-dried product to form a shell, i.e., a polymer layer, is crucial for preventing the macroporous framework from collapsing during the carbonization stage.

[0108] Test Example 3

[0109] This test example analyzes the elemental composition of the carbon materials provided in Examples 1-5 and Comparative Examples 1-3 of the present invention, and the experimental results are shown in Table 2.

[0110] Table 2

[0111]

[0112] As shown in Table 2, the elemental composition analysis of the carbon materials in the various embodiments and comparative examples reveals that the carbon materials provided by Comparative Examples 2 and 3 have higher nitrogen content. This is because urea is added during the cellulose dissolution process. Urea molecules are embedded and fixed between the cellulose chains. During carbonization, the amino groups on the urea molecules react with the hydroxyl groups on the cellulose molecules to form nitrogen atoms doped on the carbon sheets, or decompose to produce NH3, which activates the material and performs nitrogen doping as it shuttles through the pores. The difference lies in the carbonization temperature used in Comparative Example 2, which allows the nitrogen-containing groups to be retained, thus increasing the nitrogen content of the carbon material. Comparative Example 3 did not add a pore-forming agent, resulting in a lower decomposition level of nitrogen-containing groups during high-temperature carbonization, which also increased the nitrogen content of the carbon material to some extent. The carbon materials provided in Examples 1-5 of this invention have lower nitrogen content, partly due to the addition of liquid organic polyaldehyde, which reduces the nitrogen element ratio, and partly because the pore-forming agent promotes the decomposition of nitrogen-containing groups during high-temperature carbonization. Compared to the carbon material provided in Example 1, the carbon material obtained in Example 4 using aniline as a polymerization raw material has a relatively higher nitrogen content. This is because a certain amount of nitrogen doping is also introduced during the aniline reaction. Since the carbonization temperature used in Example 5 is lower than that in Example 1, the degree of decomposition of nitrogen-containing groups is reduced, thereby increasing the nitrogen content in the resulting carbon material.

[0113] Test Example 4

[0114] In this test example, thermogravimetric analysis was performed on the carbonized precursors provided in Example 1 and Comparative Example 1, namely the solid product provided in step (3) of Example 1 and the white porous solid in Comparative Example 1. The obtained thermogravimetric curves are shown in the figure below. Figure 3 As shown. Comparison Figure 3 The two thermogravimetric curves show that in Comparative Example 1, when the white, loose solid was carbonized directly without polymer encapsulation, it rapidly lost nearly 50% of its weight in the temperature range of 80-200℃. This was due to the rapid decomposition of urea upon heating, with the degree of decomposition increasing again after 550℃. At this point, the macroporous carbon skeleton formed began to decompose and collapse, resulting in extremely low final yield. In Example 1, glutaraldehyde was added first to form a pre-encapsulated polymer shell before carbonization. The weight loss below 100℃ was caused by the volatilization of unreacted glutaraldehyde. Urea decomposed more slowly and in relatively low amounts below 400℃, which was conducive to the incorporation of residual N atoms into the carbon sheet interlayers. When higher temperatures were reached, the weight loss of the material was not significant, indicating that the skeleton was stable and no further decomposition occurred. Ultimately, the carbonization yield was increased while the macroporous skeleton was maintained.

[0115] Test Example 5

[0116] This test example evaluates the AC impedance performance of the nitrogen-doped hierarchical porous carbon materials provided in Examples 1-5 of this invention and the carbon material provided in Comparative Example 1. The electrochemical test conditions include: in a three-electrode system, using a platinum foil as the counter electrode, an Hg / HgO electrode as the reference electrode, and a nickel foam sheet carrying the aforementioned carbon material as the working electrode; the electrolyte is 6.0 mol / L. -1 For KOH solution, the AC impedance test was performed using the open-circuit voltage of the system, with a test frequency range of 0.01-10000Hz.

[0117] The AC impedance diagram obtained in this test example is as follows: Figure 4 As shown, from Figure 4 As can be seen, the Nyquist plots of the nitrogen-doped hierarchical porous carbon materials obtained in Examples 1-5 of this invention all exhibit typical quasi-vertical line characteristics at low frequencies, reflecting good capacitance characteristics. In the high-frequency region, the diameter of the semicircle reflects the electron transfer process, with the nitrogen-doped hierarchical porous carbon materials obtained in Examples 1 and 4 of this invention having the lowest charge transfer resistance. Compared to the nitrogen-doped hierarchical porous carbon material provided in Example 1, the carbon material provided in Comparative Example 1 has a larger semicircle arc in the high-frequency region, indicating that the carbon material has higher impedance and poor electron transport performance.

[0118] Test Example 6

[0119] This test example evaluates the impedance performance of the nitrogen-doped hierarchical porous carbon materials provided in Examples 1-5 of this invention and the carbon material provided in Comparative Example 1. The specific capacitance test voltage range is -1.0 to 0 V, and the galvanostatic charge-discharge curve (GCD) is within 0.2 A g. -1 Up to 10A g -1 The experiment was conducted at different current densities.

[0120] The charge-discharge curves obtained in this test example are as follows: Figure 5 As shown, from Figure 5 As can be seen from the results, the nitrogen-doped hierarchical porous carbon materials obtained in Examples 1-5 of this invention all exhibit superior capacitance performance. Among them, the nitrogen-doped hierarchical porous carbon material obtained in Example 1 has the highest specific capacitance value, at 0.2 A g. -1 At times, it can reach 340F g -1 This includes the double-layer capacitance provided by the porous structure and high specific surface area of ​​the material, as well as the pseudocapacitance provided by nitrogen doping. The nitrogen-doped hierarchical porous carbon material provided in this embodiment of the invention has significantly improved specific surface area and porosity, thereby enhancing the material's electronic conductivity and capacitance performance. Compared to the nitrogen-doped hierarchical porous carbon material provided in Example 1, the capacitance of the carbon material obtained in Comparative Example 1 is significantly reduced. This indicates that although the nitrogen content of the material is relatively high, its small specific surface area cannot provide sufficient surface for forming a double layer, and its uniform pore size distribution results in high mass transfer resistance.

[0121] The above description is merely a specific embodiment of the present invention and should not be construed as limiting the scope of the invention. Therefore, any substitution of equivalent components or equivalent changes and modifications made within the scope of protection of this patent should still fall within the scope of this patent. Furthermore, the technical features, technical features and technical inventions, and technical inventions in this invention can be freely combined and used.

Claims

1. A method for preparing a nitrogen-doped hierarchical porous carbon material, characterized in that, The preparation method includes: Step (1): Dissolve urea, alkali metal hydroxide and / or pore-forming agent in water in sequence to obtain a mixture, cool the mixture, add cellulose-based biomass and dissolve it to obtain a transparent sol; Step (2): After freezing the transparent sol into a solid, it is then freeze-dried under vacuum to obtain a white, loose solid; Step (3): The liquid polymerizable monomer is mixed and ground with a white loose solid and then heat-treated to obtain a solid product; Step (4): Carbonize the solid product in an inert atmosphere, and then acid wash and water wash the carbonized product to obtain the nitrogen-doped hierarchical porous carbon material.

2. The preparation method according to claim 1, characterized in that, In step (1), the mass ratio of water, alkali metal hydroxide, urea, pore-forming agent, and cellulose-based biomass is 100:7-10:13-18:4-8:3-7.

3. The preparation method according to claim 1 or 2, characterized in that, In step (1), the alkali metal hydroxide includes sodium hydroxide and / or potassium hydroxide.

4. The preparation method according to claim 1 or 2, characterized in that, In step (1), the pore-forming agent includes one or more of potassium carbonate, potassium hydroxide, sodium hydroxide, sodium carbonate, and potassium chloride.

5. The preparation method according to claim 1 or 2, characterized in that, In step (1), the cellulose-based biomass includes one or more of cellulose, cotton, bagasse and bamboo fiber.

6. The preparation method according to claim 1 or 2, characterized in that, In step (1), the cooling is to reduce the temperature of the mixture to -10 to -20°C.

7. The preparation method according to claim 1, characterized in that, In step (3), for every 0.5g of white loose solid, add 0.5-1.5mL of liquid polymer monomer.

8. The preparation method according to claim 1 or 7, characterized in that, In step (3), the liquid polymerizable monomer includes liquid organic polyaldehyde or aniline.

9. The preparation method according to claim 8, characterized in that, The liquid organic polyaldehydes include one or more of glutaraldehyde, glyoxal, furfural, citral, salicylaldehyde, and cinnamaldehyde.

10. The preparation method according to claim 1 or 7, characterized in that, In step (3), the heat treatment temperature is 150-200℃ and the time is 6-12h.

11. The preparation method according to claim 1, characterized in that, In step (4), the carbonization is carried out at 600-900℃ for 2-4 hours, and the heating rate is 2-5℃ / min.

12. A nitrogen-doped hierarchical porous carbon material, characterized in that, The nitrogen-doped hierarchical porous carbon material is prepared by the method for preparing nitrogen-doped hierarchical porous carbon material according to any one of claims 1-11, and has a three-dimensional morphology, including open micropores, mesopores and macropores.

13. The nitrogen-doped hierarchical porous carbon material according to claim 12, characterized in that, The nitrogen-doped hierarchical porous carbon material has a specific surface area of ​​1200-1900 m². 2 / g.

14. The nitrogen-doped hierarchical porous carbon material according to claim 12 or 13, characterized in that, The nitrogen content is 0.5-5.0 wt%, based on the total weight of the nitrogen-doped hierarchical porous carbon material as 100%.

15. The use of the nitrogen-doped hierarchical porous carbon material according to any one of claims 12-14 as an adsorbent, electrode, or catalytic material.

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