Method for preparing rice hull-based hierarchical porous carbon based on alkali-assisted hydrothermal pretreatment as well as product and application of rice hull-based hierarchical porous carbon

Abstract

The invention belongs to the technical field of electrochemical energy storage and biomass resource utilization, and particularly relates to a method for preparing rice hull-based hierarchical porous carbon based on alkali-assisted hydrothermal pretreatment and a product and application of the rice hull-based hierarchical porous carbon. Alkali-assisted hydrothermal pretreatment: immersing the rice hull powder into an alkaline aqueous solution, mixing, transferring into a high-pressure reaction kettle, reacting, washing to be neutral, and drying to obtain a pretreated solid; pre-carbonization: carrying out heating and heat preservation on the pretreated solid in an inert atmosphere, and carrying out pre-carbonization to obtain a carbon precursor; chemical activation: uniformly mixing the carbon precursor with a chemical activator, heating and preserving heat for activation; and post-treatment: cooling the activated product, washing with a dilute acid solution to remove the residual activating agent and possibly residual ash, washing with deionized water to be neutral, and drying to obtain the graded porous carbon material. The method can be used for simply, conveniently and efficiently preparing the graded porous carbon material on a large scale.

Description

Technical Field

[0001] This invention belongs to the fields of electrochemical energy storage and biomass resource utilization technology, specifically relating to a method for preparing rice husk-based hierarchical porous carbon based on alkali-assisted hydrothermal pretreatment, its products, and applications. Background Technology

[0002] In the fields of new energy materials technology, electrochemical energy storage, and biomass resource utilization, hierarchical porous carbon preparation is crucial. However, existing technologies often suffer from limitations. Traditional activation methods result in single-pore structures: while direct activation of biomass using chemical activators such as KOH and ZnCl2 can generate high specific surface areas, the resulting pores are predominantly micropores, hindering rapid ion transport in the electrolyte and leading to poor rate performance in supercapacitors. Physical activation is also difficult to control: physical activation methods using CO2 and steam have limited ability to regulate the formation of mesoporous structures, making it difficult to obtain hierarchical porous carbon with an ideal micropore / mesopore ratio. Two-step processes are complex: existing technologies often require hard / soft template methods to obtain hierarchical porous carbon, which are cumbersome, costly, and may introduce pollution during template removal. Furthermore, biomass ash content plays a significant role: biomass raw materials such as rice husks contain a large amount of inorganic ash, primarily silica. During direct carbonization / activation, this ash can clog pores, reducing the conductivity and effective specific surface area of ​​the carbon material. Therefore, this invention provides a method for preparing rice husk-based hierarchical porous carbon based on alkali-assisted hydrothermal pretreatment, as well as its products and applications. Summary of the Invention

[0003] The purpose of this invention is to provide a method for preparing rice husk-based hierarchical porous carbon based on alkali-assisted hydrothermal pretreatment, along with its products and applications. This method enables the simple, efficient, and scalable preparation of hierarchical porous carbon materials. The method provided by this invention can effectively remove ash from biomass raw materials and simultaneously promote the formation of mesoporous structures. Through the synergy of alkali-assisted hydrothermal pretreatment and subsequent carbonization / activation processes, the specific surface area, pore volume, and micropore / mesopore ratio of biomass carbon materials can be optimized and controlled in a one-step process. A hierarchical porous carbon material with high specific surface area, large pore volume, suitable mesopore ratio, and good conductivity is obtained, which, when used as an electrode material for supercapacitors, exhibits high specific capacitance, excellent rate performance, and cycle stability.

[0004] The specific technical solution adopted by this invention is as follows: A method for preparing rice husk-based hierarchical porous carbon based on alkali-assisted hydrothermal pretreatment, characterized by the following steps: Step 1: Raw material preparation: Wash, dry, and crush the rice husks; Step 2: Alkali-assisted hydrothermal pretreatment: Immerse rice husk powder in an alkaline aqueous solution. Transfer the mixture to a high-pressure reactor and react at 140-220℃ for 4-12 hours. After the reaction, wash the product until neutral and dry to obtain a pretreated solid; the alkaline substance in Step 2 is selected from one or more of NaOH, KOH, and ammonia water, with a preferred concentration of 0.5-3 mol / L. Step 3: Pre-carbonization: The pretreated solid is heated to 400-600℃ in an inert atmosphere (N2 or Ar) at a heating rate of 2-10℃ / min and held for 0.5-3 hours to obtain the carbon precursor.

[0005] Step 4: Chemical activation: Mix the carbon precursor and chemical activator evenly according to the mass ratio. KOH is selected as the activator. The ratio of carbon precursor to activator is 1:2-5. Heat the mixture to 700-900℃ at a heating rate of 2-10℃ / min under an inert atmosphere and keep it at that temperature for 1-3 hours for activation.

[0006] Step 5: Post-processing: After the activated product is cooled, it is washed with dilute acid solution to remove residual activator and possible residual ash, then washed with deionized water until neutral, and dried to obtain hierarchical porous carbon material. The dilute acid solution used is HCl solution.

[0007] A method for preparing rice husk-based hierarchical porous carbon based on alkali-assisted hydrothermal pretreatment has been proposed. The prepared carbon exhibits a specific range of structural parameters, including a BET specific surface area > 2500 m². 2 / g, total pore volume >2.0cm³ 3 / g, the proportion of mesopore volume to total pore volume is 30%-55%.

[0008] The application of rice husk-based hierarchical porous carbon prepared by a method based on alkali-assisted hydrothermal pretreatment as an active material for supercapacitor electrodes.

[0009] The technical effects achieved by this invention are as follows: 1. High-efficiency ash removal and pore formation: Alkali-assisted hydrothermal pretreatment can remove more than 80% of the silica in rice husks and pre-construct a structure that is conducive to the formation of mesopores, resulting in extremely low ash content and well-developed pore structure in the final product.

[0010] 2. Excellent electrochemical performance: The prepared hierarchical porous carbon electrode material exhibits a specific capacitance exceeding 300 F / g at a current density of 1 A / g, significantly higher than the control sample (-190 F / g) that was directly activated without pretreatment. It also maintains high capacity even at a high current of 10 A / g, demonstrating excellent rate performance.

[0011] 3. The process is universal and environmentally friendly: the raw materials are cheap and readily available, the process is relatively simple, the use of complex templates is avoided, and the chemicals used, such as alkalis and acids, can be partially recovered or neutralized, resulting in a relatively small environmental impact.

[0012] 4. Stable product performance: After 5,000 charge-discharge cycles, the material retains a capacitance of over 88%, demonstrating excellent long-term cycle stability. Attached Figure Description

[0013] Figure 1 These are scanning electron microscope (SEM) images of the products obtained in Embodiment 1, Comparative Example 1, and Comparative Example 2 of the present invention. Figure a-1 is a 10 μm SEM image of Embodiment 1, Figure a-2 is a 100 μm SEM image of Embodiment 1, Figure b is a 10 μm SEM image of Comparative Example 1, and Figure c is a 10 μm SEM image of Comparative Example 2. Figure 2 This is a comparison diagram of the nitrogen adsorption-desorption isotherms and pore size distribution of the materials in Embodiment 1 of the present invention, Comparative Example 1, and Comparative Example 2. Detailed Implementation

[0014] To make the objectives and advantages of this invention clearer, the invention will be specifically described below with reference to embodiments. It should be understood that the following text is merely used to describe one or more specific embodiments of the invention and does not strictly limit the scope of protection specifically claimed by the invention.

[0015] Example 1: like Figure 1 As shown, a method for preparing rice husk-based hierarchical porous carbon based on alkali-assisted hydrothermal pretreatment is characterized by the following steps: Step 1: Raw material preparation: Wash, dry, and crush the rice husks; Step 2: Alkali-assisted hydrothermal pretreatment: Immerse rice husk powder in an alkaline aqueous solution. Transfer the mixture to a high-pressure reactor and react at 140-220℃ for 4-12 hours. After the reaction, wash the product until neutral and dry to obtain a pretreated solid; the alkaline substance in Step 2 is selected from NaOH, with a preferred concentration of 0.5-3 mol / L. Step 3: Pre-carbonization: The pretreated solid is heated to 400-600℃ in an inert atmosphere (N2 or Ar) at a heating rate of 2-10℃ / min and held for 0.5-3 hours to obtain the carbon precursor.

[0016] Step 4: Chemical activation: Mix the carbon precursor and chemical activator evenly according to the mass ratio. KOH is selected as the activator. The ratio of carbon precursor to activator is 1:2-5. Heat the mixture to 700-900℃ at a heating rate of 2-10℃ / min under an inert atmosphere and keep it at that temperature for 1-3 hours for activation.

[0017] Step 5: Post-processing: After the activated product is cooled, it is washed with dilute acid solution to remove residual activator and possible residual ash, then washed with deionized water until neutral, and dried to obtain hierarchical porous carbon material. The dilute acid solution used is HCl solution.

[0018] A method for preparing rice husk-based hierarchical porous carbon based on alkali-assisted hydrothermal pretreatment has been proposed. The prepared carbon exhibits a specific range of structural parameters, including a BET specific surface area > 2500 m². 2 / g, total pore volume >2.0cm³ 3 / g, the proportion of mesopore volume to total pore volume is 30%-55%.

[0019] The application of rice husk-based hierarchical porous carbon prepared by a method based on alkali-assisted hydrothermal pretreatment as an active material for supercapacitor electrodes.

[0020] Comparative Example 1: The difference from Example 1 is that in step 2, no NaOH alkali is added for activation during the hydrothermal process.

[0021] Comparative Example 2: The difference from Example 1 is that step 2 is omitted, and after pre-carbonization in step 3, step 4 is directly activated with KOH.

[0022] like Figure 1 as well as Figure 2 As shown, Figure 1 These are scanning electron microscope (SEM) images of the products obtained in Example 1, Comparative Example 1, and Comparative Example 2. Figure a-1 is a 10 μm SEM image of Example 1, Figure a-2 is a 100 μm SEM image of Example 1, Figure b is a 10 μm SEM image of Comparative Example 1, and Figure c is a 10 μm SEM image of Comparative Example 2. Figure 2 This is a comparison diagram of nitrogen adsorption-desorption isotherms and pore size distribution of the materials in Example 1, Comparative Example 1, and Comparative Example 2. from Figure 1 as well as Figure 2 As can be seen, the hierarchical porous carbon material prepared by the method disclosed in this invention has a specific range of structural parameters, for example, a BET specific surface area > 2500 m². 2 / g, total pore volume >2.0cm³ 3 / g, with mesopore volume accounting for 30%-55% of the total pore volume; efficient ash removal and pore formation: alkali-assisted hydrothermal pretreatment can remove more than 80% of silica from rice husks and pre-construct structures conducive to mesopore formation, resulting in extremely low ash content and well-developed pore structure in the final product. Excellent electrochemical performance: the prepared hierarchical porous carbon electrode material exhibits a specific capacitance exceeding 300 F / g at a current density of 1 A / g, significantly higher than the control sample directly activated without pretreatment (-190 F / g). It maintains high capacity even at a high current of 10 A / g, demonstrating excellent rate performance.

[0023] This invention features a technological innovation: the introduction of an "alkali-assisted hydrothermal pretreatment" step. Compared to simple physical or chemical activation, this step removes ash and modifies the precursor structure before activation, enabling the subsequent activation process to simultaneously and efficiently construct a hierarchical pore structure with both micropores and mesopores, rather than generating only carbon materials dominated by micropores. Compared to template methods, this method eliminates the need for introducing and removing additional templates, resulting in a simpler, lower-cost, and more environmentally friendly process.

[0024] The structural advantages of this invention are: the resulting carbon material has an exceptionally high specific surface area (up to 2500 m²). 2 (g or more) and total pore volume (up to 2.6 cm³) 3 The surface area is above 100 g / g, and the mesoporous ratio is significantly increased (up to 40%-50%). This unique combination of high specific surface area, large pore volume and high mesoporous ratio is difficult to achieve with traditional direct activation methods.

[0025] The invention enhances the following features: As a supercapacitor electrode material, due to its hierarchical pore structure (mesoporosis provides fast ion channels, micropores provide high energy storage area) and improved carbon skeleton conductivity due to pretreatment, this material exhibits extremely high specific capacitance (>300F / g@1A / g) in 6M KOH electrolyte, as well as excellent rate performance and cycle stability (>85% capacity retention after 5000 cycles).

[0026] Applications of this invention can be expanded beyond rice husks to include other high-ash biomass, such as straw and fruit shells, for the preparation of graded porous carbon. The resulting materials, in addition to their use in supercapacitors, also show potential applications in catalysis, adsorption, and lithium-sulfur batteries.

[0027] The above description is merely a preferred embodiment of the present invention. It should be noted that those skilled in the art can make various improvements and modifications without departing from the principles of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention. Structures, devices, and operating methods not specifically described or explained in this invention are implemented according to conventional methods in the art unless otherwise specified or limited.

Claims

1. A method for preparing rice husk-based hierarchical porous carbon based on alkali-assisted hydrothermal pretreatment, characterized in that: Includes the following steps: Step 1: Raw material preparation: Wash, dry, and crush the rice husks; Step 2: Alkali-assisted hydrothermal pretreatment: Immerse rice husk powder in an alkaline aqueous solution; transfer the mixture to a high-pressure reactor and react at 140-220℃ for 4-12 hours; after the reaction, wash the product until neutral and dry to obtain a pretreated solid; Step 3: Pre-carbonization: The pretreated solid is heated to 400-600℃ at a certain heating rate under an inert atmosphere and held for 0.5-3 hours to carry out pre-carbonization and obtain carbon precursor. Step 4: Chemical activation: Mix the carbon precursor and chemical activator evenly according to the mass ratio, wherein the carbon precursor: activator = 1:2-5; heat the mixture to 700-900℃ at a certain heating rate under an inert atmosphere and keep it at that temperature for 1-3 hours to activate it. Step 5: Post-processing: After the activated product is cooled, it is washed with dilute acid solution to remove residual activator and possible residual ash, then washed with deionized water until neutral, and dried to obtain hierarchical porous carbon material.

2. The method for preparing rice husk-based hierarchical porous carbon based on alkali-assisted hydrothermal pretreatment according to claim 1, characterized in that: In step 2, the alkaline substance is selected from one or more of NaOH, KOH, and ammonia water, and the concentration is preferably 0.5-3 mol / L.

3. The method for preparing rice husk-based hierarchical porous carbon based on alkali-assisted hydrothermal pretreatment according to claim 1, characterized in that: The inert atmosphere in step 3 is N2 or Ar, and the heating rate in step 3 is 2-10℃ / min.

4. The method for preparing rice husk-based hierarchical porous carbon based on alkali-assisted hydrothermal pretreatment according to claim 1, its products, and applications, characterized in that: In step 4, KOH is selected as the activator, and the heating rate in step 4 is 2-10℃ / min.

5. The method for preparing rice husk-based hierarchical porous carbon based on alkali-assisted hydrothermal pretreatment according to claim 1, characterized in that: In step 5, the dilute acid solution used is HCl solution.

6. The rice husk-based hierarchical porous carbon prepared by the method for preparing rice husk-based hierarchical porous carbon based on alkali-assisted hydrothermal pretreatment according to any one of claims 1-5, characterized in that: It has a specific range of structural parameters, in which: BET specific surface area > 2500 m² 2 / g, total pore volume >2.0cm³ 3 / g, the proportion of mesopore volume to total pore volume is 30%-55%.

7. The application of rice husk-based hierarchical porous carbon prepared by the method of preparing rice husk-based hierarchical porous carbon based on alkali-assisted hydrothermal pretreatment as described in any one of claims 1-5 as an active material for supercapacitor electrodes.

Images