Derivative plant-based porous carbon and preparation method and application thereof

Abstract

The application provides derived plant-based porous carbon and a preparation method and application thereof. The application uses bean sprout biomass as a precursor of activated carbon electrode material for supercapacitors, effectively improves the uniformity of the prepared activated carbon, and further explores a suitable green preparation method and an electrochemical performance optimization mechanism. Specifically, the application takes green bean sprouts as a representative derived plant as a precursor of activated carbon for supercapacitors, and on this basis, a series of related quality improvement modification work is carried out from two aspects of pore structure adjustment and nitrogen atom doping, provides a reference for high-value utilization of biomass and preparation of activated carbon for supercapacitors, and therefore has good practical application value.

Description

Technical Field

[0001] This invention belongs to the field of biochar material preparation technology, specifically relating to derivative plant-based porous carbon, its preparation method, and its application. Background Technology

[0002] The information disclosed in this background section is intended only to enhance understanding of the overall background of the invention and is not necessarily to be construed as an admission or in any way implying that such information constitutes prior art known to those skilled in the art.

[0003] With the increasing severity of the energy and environmental crisis, supercapacitors, as a novel energy storage device, have attracted widespread attention due to their high power density and long cycle life. Electrode materials are a key factor affecting the performance of supercapacitors. Common electrode materials for supercapacitors include carbon materials, conductive polymers, and metal oxides. Among these, carbon materials with p-conjugated structures, such as graphene, carbon nanotubes, ordered mesoporous carbon, and activated carbon, have been widely used. Plants, with their naturally ordered tissue and cellular structure, are excellent precursors for activated carbon used in supercapacitors.

[0004] Plants typically contain highly cross-linked structures such as parenchyma, transport tissues, and mechanical tissues. These morphologies can be well preserved after simple pyrolysis, resulting in sheet-like or hierarchically porous forms that facilitate electrolyte ion transport. However, due to the diversity of plant species and tissues, the resulting plant-based activated carbon exhibits poor homogeneity. The presence of non-structural impurities such as metals and alcohol extracts increases leakage current, impedance, and pore blockage, thus increasing energy consumption during preparation. Furthermore, while activated carbon's large specific surface area provides ample adsorption sites for electrolyte ions, its poor wettability and pore connectivity limit efficient utilization of the porous structure. Therefore, improving material homogeneity and quality by selecting superior biomass raw materials or optimizing process conditions while preserving the original good morphology of the plant material is essential for the large-scale commercial application of plant-based activated carbon. Summary of the Invention

[0005] To address the shortcomings of the existing technologies, the inventors, through long-term technical and practical exploration, have provided derivative plant-based porous carbon, its preparation method, and its applications. This invention utilizes bean sprout biomass as the activated carbon electrode material for supercapacitors, effectively improving the uniformity of the prepared activated carbon, and further exploring suitable green preparation methods and electrochemical performance optimization mechanisms. Based on the above research results, this invention has been completed.

[0006] To achieve the above technical objectives, the present invention adopts the following technical solution:

[0007] In a first aspect, the invention provides the application of derivative plants in the preparation of porous carbon electrode materials for supercapacitors.

[0008] The derived plants are bean sprouts, including but not limited to mung bean sprouts and soybean sprouts. Due to the clean growing environment, short growth cycle, and natural micron-sized thin-walled tissues, air cavities, and vessels of these bean sprouts, the biochar prepared from them exhibits better electrochemical performance than the bean plants themselves. Furthermore, the short growth cycle and aquatic nature of the plants prevent the absorption of excessive metal ions from the soil, resulting in lower impurity content and reduced energy consumption in subsequent impurity removal processes.

[0009] In a second aspect, the present invention provides a method for preparing a plant-based porous carbon material, the method comprising treating the plant-based material using a carbonization-KOH activation method.

[0010] The derived plant is bean sprout, also known as sprouted vegetables, which are edible "sprouts" cultivated from the seeds of various grains, beans, and trees, and are also called "living vegetables". The bean sprouts include, but are not limited to, mung bean sprouts and soybean sprouts, with mung bean sprouts being preferred.

[0011] Specifically, the preparation method includes:

[0012] S1. Place the bean sprouts in an inert atmosphere and pyrolyze them at high temperature;

[0013] S2. Add an activator to the product obtained in step S1 and perform a heating impregnation treatment;

[0014] S3. The product obtained in step S2 is placed in an inert gas for a second heating activation treatment;

[0015] S4. The product obtained in step S3 is obtained by acid washing, water washing and drying.

[0016] In step S1, the bean sprouts can be bean sprout powder obtained through simple pretreatment. The pretreatment method includes washing and drying the bean sprouts, crushing them, and then sieving them.

[0017] Specifically, the sieve mesh size is 80 mesh, which yields powder with a particle size of less than 178 μm for later use.

[0018] The inert atmosphere can be nitrogen, and the specific conditions for the high-temperature pyrolysis are: pyrolysis at 450-650℃ for 0.5-3 hours; the heating rate is controlled at 1-10℃·min. -1 Preferably 5℃·min -1 The nitrogen flow rate is 0.5-5 L / min. -1 Preferably 1 L·min -1 .

[0019] In step S2, the mass ratio of the activator to the product is 1-10:1, such as 1:1, 2:1, 5:1, 8:1 or 10:1, and the activator can be KOH;

[0020] The mass ratio of the activator to the product is called the impregnation ratio or the alkali-carbon ratio.

[0021] The specific conditions for the heat impregnation treatment are: impregnation at 70-90℃ for 1-3 hours (preferably impregnation at 80℃ for 2 hours);

[0022] In step S3, the specific conditions for the secondary heating activation treatment include: heating to 300-400℃ (preferably 350℃) at a heating rate of 1-10℃ (preferably 5℃), holding at this temperature for 10-50 min (preferably 30 min), then heating to 750-850℃ (preferably 800℃) at a heating rate of 1-10℃ (preferably 5℃) and holding at this temperature for 1-3 h (preferably 2 h), with nitrogen as the protective gas and a flow rate of 0.5-5 L·min. -1 Preferably 1 L·min -1 .

[0023] In step S4, the acid added in the pickling step can be hydrochloric acid, preferably 1.0M hydrochloric acid. The water washing step specifically involves repeatedly rinsing with water until the pH of the supernatant is neutral. The drying step specifically involves drying at 100-110℃ for 1-48 hours, preferably at 105℃ for 24 hours, and preferably by baking.

[0024] To further improve the uniformity of the final activated carbon, the inclusions in the bean sprouts are pretreated and removed. This pretreatment specifically includes the removal of non-structural components from the bean sprouts and acid washing of the bean sprout biomass for impurity removal. The removal of non-structural components from the bean sprouts can be achieved by reflux treatment of the biomass raw material with ethanol or ultrasound-assisted ethanol. The reflux conditions include: refluxing the bean sprouts with an ethanol solution at 70-90°C for 0.5-2 hours, cooling and removing the supernatant, repeating this process 2-3 times; preferably, refluxing at 80°C for 1 hour. The ethanol can be a 60-80% (preferably 70%) ethanol solution, and the mass-to-volume ratio of the bean sprouts to the ethanol solution is 1-5:10-80 (g / ml), preferably 2:50.

[0025] Furthermore, ultrasonic disruption is added during the ultrasonic-assisted ethanol reflux process, with the ultrasonic power controlled at 100-500W, preferably 300W.

[0026] Acid washing of bean sprout biomass is mainly used to deeply remove metal ions from bean sprouts. Specifically, the acid washing step includes placing the dried bean sprouts in a hydrochloric acid solution. The specific acid washing conditions are: treatment at 60-80℃ for 1-3 hours, preferably at 70℃ for 2 hours, and the concentration of the hydrochloric acid is 0.1-1 mol / L. -1 Preferably 0.5 mol L -1 The mass-to-volume ratio of bean sprouts to hydrochloric acid solution is 1-5:10-80 (g / ml), preferably 2:50. Furthermore, ultrasonic treatment can also be introduced during the pickling process. Introducing ultrasonic treatment promotes mass transfer, thereby improving the efficiency of pickling and impurity removal, shortening the processing time, and also improving the leaching effect of metal elements. Therefore, the processing time during ultrasonic pickling can be 0.5-2 hours, preferably 1 hour, and the ultrasonic power is 100-150W, preferably 120W.

[0027] The above-mentioned pretreatment to remove the contents in bean sprouts can be carried out before step S1, or the contents in bean sprout powder obtained after simple pretreatment of bean sprouts can be removed by pretreatment.

[0028] To further optimize the pore structure of bean sprouts and improve the electrochemical performance of activated carbon, a two-step activation method using H3PO4-KOH is preferred. Specifically, the preparation method includes phosphoric acid impregnation before step S1. The phosphoric acid impregnation method includes mixing bean sprouts with phosphoric acid, stirring, and drying before use.

[0029] The specific method for phosphoric acid impregnation includes: mixing mung beans and phosphoric acid at a mass ratio of 0.8-2:1, then adding water, controlling the phosphoric acid impregnation temperature at 80-140℃, preferably 120℃, and then drying (drying at 80-140℃ for 8-12 hours, preferably 10 hours) for later use. This invention has found that the product from the H3PO4-KOH activation method has a better hierarchical structure, and the early introduction of phosphoric acid activation effectively lowers the carbonization temperature, resulting in carbonized products with abundant mesopores and a certain amount of micropores, which is beneficial for further pore-forming reactions of KOH. Furthermore, KOH, based on phosphoric acid activation, can create well-developed channels and even more abundant micropores. The rational combination of phosphoric acid activation and KOH activation effectively reduces the amount of KOH used while imparting a higher specific capacitance to the product.

[0030] Furthermore, due to the diversity of material composition and structure, optimizing parameters such as morphology, pore structure, conductivity, and surface chemical composition of activated carbon can easily lead to unintended consequences and difficulty in achieving synergy. Based on obtaining suitable specific surface area and pore structure, nitrogen doping is beneficial for improving conductivity and wettability. It can also introduce pseudocapacitance through redox reactions, thereby increasing specific capacitance. Nitrogen-containing functional groups can promote the migration rate of electrons in the carbon framework, thus improving the conductivity of porous carbon materials.

[0031] This invention uses melamine with a high nitrogen content as a nitrogen dopant and KOH as an activator. The binding of carbon and nitrogen and the formation of pores occur synergistically at high temperature to achieve a reasonable pore size distribution in the target sample, with a variety of doped nitrogen atoms distributed within the carbon framework. Compared to conventional back-end doping, this synergistic process simplifies the production process and reduces costs while achieving good electrochemical performance of plant-based activated carbon, laying a solid foundation for large-scale application.

[0032] Therefore, the preparation method of the present invention further includes mixing the porous carbon material obtained in step S4 with melamine and calcining to obtain nitrogen-doped activated carbon material.

[0033] Specifically, the preparation method includes: mixing melamine with KOH and acid-washed bean sprout powder carbonized sample, heating to 300-400℃ (preferably 350℃) at a heating rate of 1-10℃ (preferably 5℃), holding at this temperature for 10-50 min (preferably 30 min), then heating to 750-850℃ (preferably 800℃) at a heating rate of 1-10℃ (preferably 5℃) and holding at this temperature for 1-3 h (preferably 2 h), using nitrogen as a protective gas at a flow rate of 0.1-1 L·min. -1 The preferred value is 0.6 L·min -1 Then, it is pickled, washed with water, and dried to obtain the final product.

[0034] The mass ratio of melamine to KOH and the carbonized bean sprout powder after acid washing is 1:5-50:1-10; more preferably 1:15-45:3-9.

[0035] A fourth aspect of the present invention provides a porous carbon electrode comprising the above-described derivative plant-based porous carbon material.

[0036] A fifth aspect of the present invention provides a supercapacitor comprising the aforementioned derivative plant-based porous carbon material or the aforementioned porous carbon electrode.

[0037] Compared with existing technical solutions, one or more of the above technical solutions have the following beneficial effects:

[0038] (1) Based on botanical theory, bean sprouts, a non-terrestrial plant with a hierarchical porous structure, were selected as precursors for activated carbon for supercapacitors. This can minimize the absorption of external metal elements. Bean sprout-based activated carbon synthesized from structures with a high proportion of thin-walled and transport tissues has a better pore structure.

[0039] (2) Pre-treatment and quality improvement of biomass from the perspective of removing the contents of bean sprout cells, optimize the initial structure and composition of biomass, and thus improve the uniformity of activated carbon preparation.

[0040] (3) Combining the characteristics of phosphoric acid activation method and KOH activation method improves the problem of the single activation method in the preparation of activated carbon with a single pore structure. It confirms the superiority of phosphoric acid-KOH secondary activation pore formation and reveals the contribution of different activators to secondary activation pore formation, increase internal surface area and the electrochemical performance of materials.

[0041] (4) Using bean sprouts as raw material and KOH and melamine as activator and nitrogen source material, respectively, a hierarchical porous carbon with excellent electrochemical performance was prepared by using nitrogen-doped structure. A method for synergistic regulation of porous carbon performance by high-temperature activation pore formation and structural nitrogen doping was obtained, revealing the transformation mechanism of nitrogen-containing functional groups at high temperature and the improvement mechanism of supercapacitor performance. The high proportion of NQ structure in nitrogen-doped porous carbon helps to improve the double-layer capacitance performance and cycle stability of the electrode, thus having good practical application value. Attached Figure Description

[0042] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an improper limitation of the invention.

[0043] Figure 1 This illustrates the effect of the phosphoric acid impregnation ratio on specific surface area and pore volume in Example 3 of the present invention.

[0044] Figure 2 This illustrates the effect of immersion temperature on surface area and pore volume in Example 3 of the present invention.

[0045] Figure 3 This illustrates the effect of carbonization temperature on pore volume and specific surface area in Example 3 of the present invention.

[0046] Figure 4 The effect of carbonization time on surface area and pore volume in Example 3 of the present invention.

[0047] Figure 5 This illustrates the effect of the KOH impregnation ratio on the surface area and pore volume in Example 3 of the present invention.

[0048] Figure 6The image shows the pore size distribution curves of HP and HPK samples in Example 3 of this invention.

[0049] Figure 7 The images are SEM images of (ac)HP and (df)HPK at different magnifications in Embodiment 3 of the present invention.

[0050] Figure 8 The XRD patterns of HPK and HP and the Raman spectrum are shown in Example 3 of this invention.

[0051] Figure 9 The following are examples of constant current charge-discharge curves of HPK at different current densities, constant current charge-discharge curves of HP at different current densities, and rate curves of HPK and HP in Embodiment 3 of the present invention.

[0052] Figure 10 The cyclic voltammetry curves of (a) HP and (b) HPK are shown in Embodiment 3 of the present invention.

[0053] Figure 11 The AC impedance curves of HP and HPK in Embodiment 3 of the present invention are shown.

[0054] Figure 12 The energy density and power density of HP and HPK in Embodiment 3 of the present invention are shown.

[0055] Figure 13 In embodiment 4 of the present invention, (a) MBSN 1:X (b) Nitrogen adsorption-desorption curve and pore size distribution curve.

[0056] Figure 14 In Embodiment 4 of the present invention, (ac)MBSN 1:3 and (df)MBSN 1:7 Magnified images at different magnifications.

[0057] Figure 15 MBSN in Embodiment 4 of the present invention 1:X (a) XRD and (b) Raman spectra.

[0058] Figure 16 MBSN in Embodiment 4 of the present invention 1:X (a) XPS total spectrum (b) XPS total spectrum of N1s.

[0059] Figure 17 MBSN in Embodiment 4 of the present invention 1:X (a) Constant current charge-discharge curve (b) Cyclic voltammetry curve (c) Rate curve (d) Impedance frequency scan curve.

[0060] Figure 18 MBS in Embodiment 4 of the present invention1:7 In 5A g -1 Cyclic performance at current density.

[0061] Figure 19 This is a diagram showing the power density and energy density in Embodiment 4 of the present invention. Detailed Implementation

[0062] It should be noted that the following detailed descriptions are illustrative and intended to provide further explanation of this application. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains.

[0063] It should be noted that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the exemplary embodiments according to this application. As used herein, the singular form is intended to include the plural form as well, unless the context clearly indicates otherwise. Furthermore, it should be understood that when the terms "comprising" and / or "including" are used in this specification, they indicate the presence of features, steps, operations, devices, components, and / or combinations thereof.

[0064] The present invention will be further illustrated below with specific examples. These examples are for illustrative purposes only and do not limit the scope of the invention. Experimental conditions not specifically specified in the examples are generally performed under conventional conditions or as recommended by the sales company; unless otherwise specified in the present invention, these conditions are commercially available.

[0065] Example 1: Preparation and electrochemical performance study of plant-derived porous carbon

[0066] Preparation of porous carbon

[0067] The fresh mung bean sprouts and mung beans used in the experiment were purchased from a local supermarket. The fresh mung bean sprouts, soybean sprouts, and mung beans were washed with clean water and then dried in a forced-air drying oven at 100℃ for 24 hours. After being pulverized in a grinder and passed through an 80-mesh sieve, powder with a particle size of less than 178μm was obtained for later use.

[0068] The preparation process of activated carbon mainly consists of carbonization, acid washing, impregnation, activation, acid washing again, and drying. First, the two types of biomass powders obtained are placed separately in a horizontal tube furnace and pyrolyzed at 600℃ in a nitrogen atmosphere for 2 hours, with a heating rate of 5℃·min. -1 The nitrogen flow rate is 1 L·min -1Impregnation process: Weigh the carbonized sample and activator according to the required impregnation ratio (mass ratio of activator to carbonized sample, including 2:1, 3:1, 4:1, 5:1, 6:1). Mix the carbonized sample and activator with deionized water in a nickel crucible and stir evenly. Then place it in a forced-air drying oven and impregnate at 80℃ for 2 hours. Activation process: Place the impregnated carbonized sample-activator mixture in an atmosphere muffle furnace and heat at 5℃·min. -1 The temperature was increased to 350℃ at a heating rate, held at that temperature for 30 minutes, and then increased at a rate of 5℃·min. -1 The temperature was raised to 800℃ at a heating rate and held for 2 hours, with nitrogen as the protective gas at a flow rate of 1 L / min. -1 Acid washing process: The activated sample was placed in a water bath and heated at a constant temperature of 80°C while being magnetically stirred. 1.0M hydrochloric acid was added dropwise until the pH reached 2. After magnetic stirring for 1 hour, the sample was repeatedly rinsed with deionized water until the pH of the supernatant reached 7. Finally, the acid-washed activated carbon was placed in a forced-air drying oven and dried at 105°C for 24 hours to obtain bean sprout / mung bean-based activated carbon.

[0069] Fabrication of supercapacitors

[0070] Conductive graphite, polytetrafluoroethylene solution, and activated carbon were weighed in a mass ratio of 1:1:8 and placed in a beaker. 35 mL of anhydrous ethanol was added, and the mixture was sonicated for 30 min. Then, it was dried in a 100℃ hot air drying oven until it reached a paste-like consistency. This paste was then evenly coated onto 1.5 cm diameter nickel foam current collectors, with approximately 10 mg of porous carbon coated on each current collector. The electrode sheets were then dried in a vacuum drying oven at 80℃ for 12 h. After removal, they were pressed for 1 min at 15 MPa using a hydraulic press. The cut aqueous diaphragm and electrode sheets were then assembled into a coin-type symmetrical supercapacitor in a 6 M KOH solution. After 12 h, its electrochemical performance was tested. All electrochemical performance characterizations were performed using a CS310H electrochemical workstation.

[0071] The conclusions drawn from this embodiment are as follows:

[0072] (1) Activated carbon derived from mung beans has a more reasonable hierarchical porous structure, and the properties of the activated carbon prepared are similar to those of mung bean-based activated carbon, but are superior to those of mung bean-based activated carbon.

[0073] (2) The mung bean sprout-based activated carbon achieved a maximum specific surface area of ​​3314.382 m² at an impregnation ratio of 5 (named MBS-5). 2 g -1 The specific pore volume is 2.289 cm³. 3 g -1 Its pore size is mainly concentrated between 0.5-0.7 nm, with a peak of 0.65 nm. This good pore size distribution is characteristic of the K+ in the KOH electrolyte. +and OH - The adsorption of ions provides an effective internal specific surface area.

[0074] (3) Mung bean sprout-based activated carbon is mainly composed of amorphous carbon with a small amount of graphitized crystals and does not have a two-dimensional graphene structure; at the same time, it contains a large number of hydrophilic groups such as C=O, O=CO, etc., which increases the wettability of the material.

[0075] (4) The mung bean sprout-based activated carbon electrode exhibits excellent electrochemical characteristics. MBS-5 achieved the highest specific capacitance, primarily due to double-layer capacitance, at 0.5 A g. -1 295 F g at current density -1 When the current density reaches 0.1 A g -1 Increase to 50Ag -1 At that time, the capacitance retention rate reached 78.8%. At 5.0 A g -1 After 5000 constant current charge-discharge cycles, the MBS-5 still maintains a capacitance retention rate of 93.7%.

[0076] Example 2: Quality Improvement and Modification Based on Bean Sprout Precursor

[0077] Although the non-soil growing environment of bean sprouts avoids the absorption of excessive metal ions from the outside world, they still contain inherent metal ions. Compared with plants with a higher degree of lignification, when bean sprouts germinate, the metal ions are transformed from the phytic acid-metal-protein chelate state in bean sprouts into free metal ions, which are therefore more easily absorbed by the human body. Moreover, bean sprouts have a lower structural strength, so metal leaching is easier to achieve during the acid washing process.

[0078] In this embodiment, non-productive components of mung bean sprouts were removed using an ethanol / ultrasound-assisted ethanol reflux treatment method on the biomass raw material. The reflux parameters were as follows: 20g of biomass was refluxed with 500ml of a 70% ethanol solution at 80°C for 1 hour, cooled, and the supernatant was removed; this process was repeated twice. The ultrasound-assisted ethanol reflux process involved refluxing 20g of biomass with 500ml of a 70% ethanol solution at 80°C for 1 hour, accompanied by ultrasonic disruption at a power of 300W; this process was repeated twice. The remaining activated carbon preparation process was the same as in Example 1. The activated carbon prepared from mung bean sprouts after ultrasound-assisted ethanol extraction was named MBSEU-5 (mung bean powder based activated carbon, EU represents ethanol and ultrasound pretreatments, 5 represents mass ratio). The activated carbon prepared from mung bean sprouts after ethanol extraction was named MBSE-5 (E represents ethanol pretreatment).

[0079] Meanwhile, this embodiment introduces an acid washing process to deeply remove metal ions from bean sprouts. Acid washing is a simple and effective way to remove alkali metals and alkaline earth metals from biomass, and acid pretreatment can also remove most of the hemicellulose components in the substrate, break down the compact structure of the substrate, change the surface morphology, and increase the porosity of the raw material.

[0080] To improve the leaching rate of metal ions and enhance pickling efficiency, an ultrasonic pretreatment step is introduced. During ultrasonic treatment, the bean sprouts undergo cell wall disruption, cell dissociation, and the dissolution of intracellular substances. The organic combination of these two processes shortens the reaction time, thereby increasing the leaching rate of metal ions from the biomass.

[0081] The pretreatment parameters are as follows: Place 20g of dried mung bean sprouts in 500ml of 0.5mol / L... -1 The acid washing process was carried out in hydrochloric acid solution at a temperature of 70℃ for 2 hours, followed by ultrasonic acid washing for 1 hour at a power of 120W. The remaining activated carbon preparation process was the same as above. The activated carbon prepared from bean sprouts after ultrasonic-assisted hydrochloric acid washing was named MBSAU-5 (mung bean powder-based activated carbon, AU represents acid and ultrasonic pretreatments, 5 represents mass ratio), and the activated carbon prepared from bean sprouts after hydrochloric acid washing was named MBSA-5 (Arepresents acid pretreatment).

[0082] The experimental conclusions of this embodiment are as follows:

[0083] (1) Acid washing promotes the leaching of metals from bean sprouts while also causing hydrolysis of cell walls, leading to the dissociation of the original structure of bean sprouts. MBSA-5 generates more micropores while also causing the collapse of mesopores. The higher alkali-to-carbon ratio further exacerbates the ablation of the pore structure. Ethanol extraction promotes the dissolution of non-structural components in bean sprouts, increases the number of mesopores in MBSA-5, and optimizes the pore structure, which in turn facilitates the rapid transport of electrolyte ions.

[0084] (4) The introduction of ultrasound promoted the mass transfer process, increased the depth of the pretreatment process, and saved pretreatment time. Ultrasound promoted the leaching of metal elements, and MBSAU-5 basically met the requirements of GB / T37386-2019 regarding metal element content. However, the unreasonable ultrasonic treatment time and power caused the effect of ultrasound to change from improving reaction efficiency to aggravating the damage to the original morphology of bean sprouts during the pretreatment process. Ultimately, this resulted in MBSAU-5 and MBSEU-5 producing more pores smaller than 0.55 nm, which are not easily accessible. The excessive number of micropores narrowed the mesopore distribution and reduced the number of mesopores.

[0085] (6) The four pretreatment methods not only improved the orderliness of the material but also increased the proportion of C=C and oxygen-containing functional groups, but their effect on the graphitization of the material was limited. The improvement of oxygen-containing functional groups was more effective in improving the wettability of the material and increasing the accessibility of the pore structure.

[0086] (7) MBSE-5 achieved the highest specific capacitance, primarily due to double-layer capacitance, at 0.5 A g. -1 367 F g at current density -1 When the current density reaches 0.5 A g -1 Increase to 50A g -1 At that time, the capacitance retention rate reached 82%.

[0087] (8) Bean sprouts have a low metal content, and the decomposition temperature does not increase significantly after acid washing. However, acid washing promotes the hydrolysis of hemicellulose and cellulose, and the biomass pores are opened and exposed, which is more conducive to heat transfer and the release of small molecule volatile substances, thus significantly promoting the maximum decomposition rate. The non-structural component of the ethanol extract improves the reactivity between the three major components and is conducive to the decomposition of structural substances. Therefore, the stability of the carbon skeleton of bean sprouts is improved after ethanol extraction.

[0088] Example 3: Preparation of porous carbon and its electrochemical properties using a two-step activation method with H3PO4-KOH

[0089] The experiment used mung bean sprouts as raw material and prepared activated carbon using a two-step H3PO4-KOH activation method. The specific steps included phosphoric acid impregnation, carbonization, KOH impregnation, and KOH activation. The phosphoric acid impregnation procedure was as follows: first, mung bean sprout powder and analytical grade phosphoric acid were mixed at a mass ratio of 0.8-2, then 100 ml of deionized water was added, and the mixture was stirred at room temperature for 1 hour. The phosphoric acid-biomass mixture was then dried in a forced-air drying oven at 80-140°C for 10 hours. Carbonization: the dried sample was placed in a tube furnace and carbonized at 450-600°C for 0.5-2 hours. The subsequent acid washing, KOH impregnation, KOH activation, and drying steps were the same as in Example 1, with an impregnation ratio of 1:5-1:2. Activated carbon prepared by the H3PO4-KOH two-step activation method was obtained. A comparison was made using only phosphoric acid activation without KOH activation.

[0090] To obtain the optimal preparation conditions for bean sprout-derived porous charcoal, an orthogonal experiment was conducted using five experimental factors: charcoalization temperature, charcoalization time, phosphoric acid impregnation temperature, KOH impregnation ratio, and charcoalization time. The orthogonal experimental factors are shown in Table 1. Based on the orthogonal experimental factors and levels in Table 1, an L(4)2 orthogonal model was designed. 5 There were a total of 16 experiments, and the experimental arrangements are shown in Table 2.

[0091] In this embodiment, the influencing factors are divided into the phosphoric acid impregnation process and the "phosphoric acid-biomass polymer" thermal process according to the mechanisms of the two activation methods. The phosphoric acid impregnation process mainly includes two influencing factors: phosphoric acid impregnation ratio and phosphoric acid impregnation temperature. The thermal process mainly includes three factors: carbonization temperature, KOH impregnation ratio, and carbonization time.

[0092] Table 1. Factors and levels of orthogonal experiment

[0093]

[0094] Table 2 Orthogonal Experimental Design

[0095]

[0096]

[0097] Table 3 lists the specific surface area, pore volume, and yield of activated carbon samples at various factor levels. The properties of porous carbon vary greatly under different preparation conditions.

[0098] The activated carbon samples prepared in experiments 1-16 were sequentially labeled No.1-No.16. Table 3 shows that the bean sprout activated carbon prepared under most experimental conditions possessed a large specific surface area (>3200 m²). 2 / g) and larger pore volume (>1.7cm) 3 g -1 This indicates that bean sprouts are an ideal biomass raw material for preparing activated carbon with high specific surface area. Activated carbon based on mung bean sprouts has great application potential in adsorption, gas storage, and as a carbon electrode material for supercapacitors.

[0099] Table 3 Specific surface area and pore volume of activated carbon under different preparation conditions

[0100]

[0101] Effect of phosphoric acid impregnation ratio

[0102] Figure 1 In the process, as the phosphate impregnation ratio increases, the specific surface area and pore volume both show a trend of first increasing and then decreasing. The peak values ​​of both specific surface area and total pore volume appear at a phosphate impregnation ratio of 1.2, which achieves a good hydrolysis effect when hydrolyzing substances such as cell walls without damaging their original structure.

[0103] Effect of phosphoric acid impregnation temperature

[0104] The effect of impregnation temperature on the impregnation process is mainly reflected in its promoting effect on the hydrolysis process. Figure 2Among the samples, the specific surface area and pore volume were maximized at an impregnation temperature of 120℃. Within a certain range, the higher the impregnation temperature, the more intense the hydrolysis. However, excessively high impregnation temperatures also lead to over-hydrolysis. Therefore, the trend of these values ​​is the same as that of the impregnation ratio, showing an initial increase followed by a decrease.

[0105] Effect of carbonization temperature

[0106] Figure 3 Within a certain range, as the carbonization temperature increases, the specific surface area and pore volume show a trend of first increasing and then decreasing, with the maximum specific surface area and pore volume at 500℃. At 500℃, the cross-linking effect is the best, and the phosphate ester bonds are not excessively damaged due to overheating, effectively preventing the shrinkage of the original plant structure while creating sufficient porous structure.

[0107] Effect of carbonization time

[0108] from Figure 4 It can be seen that extending the carbonization time has little effect on the specific surface area and pore volume of activated carbon. The specific surface area and pore volume of activated carbon are highest when the carbonization time is 2 hours. Extending the carbonization time is beneficial for the full reaction of phosphoric acid with bean sprout biomass; therefore, a carbonization time of 2 hours was chosen.

[0109] Effect of KOH impregnation ratio

[0110] Figure 5 In the process, the specific surface area and pore volume of activated carbon are maximized when the impregnation ratio is 4. KOH has a strong pore-forming ability, and the reaction is more vigorous and produces more micropores compared to phosphoric acid activation. When the impregnation ratio is low, the KOH content is relatively low, resulting in insufficient activation of the activated carbon. As the impregnation ratio increases, the raw materials are fully activated, thereby increasing the specific surface area of ​​the product. When the impregnation ratio is greater than 4, the carbon atoms that originally formed the pore structure are reacted, causing pore collapse and a decrease in specific surface area and pore volume.

[0111] Compared with simple alkali activation, the introduction of phosphoric acid pre-activation creates initial pores, which is conducive to full contact between KOH and activated carbon. It creates more accessible micropores on the basis of the original pores, avoiding the problem of low pore accessibility caused by KOH alone. Phosphoric acid pre-activation lowers the carbonization temperature, shortens the carbonization time, and makes the pore structure more reasonable.

[0112] Using the above-mentioned preferred conditions, the pore structure parameters of activated carbon prepared by the phosphoric acid method (HP) and the two-step activation method (HPK) are shown in Table 4 below:

[0113] Table 4. Pore structure parameters of activated carbon prepared by phosphoric acid method and two-step activation method

[0114]

[0115] HPK has a total pore volume of 2.295 cm³. 3 g -1 Its specific surface area also reached 3729.359 m². 2 g -1 The increase in specific surface area and pore volume is mainly attributed to the increase in the number of micropores, which is further confirmed by the increase in microporosity and the decrease in the average pore size. Figure 6 As can be seen, the micropores are mainly distributed between 0.5-0.8 nm, but the micropore portion of the HPK pore size distribution curve is generally above that of MBS-5, indicating a significant increase in micropore volume. Meanwhile, the HPK pore size distribution curve shows that it is mainly composed of micropores smaller than 0.7 nm, with very little mesopore content. This proves that under these conditions, phosphoric acid pre-activation primarily generated micropores. After phosphoric acid pre-activation, compared to the carbonized sample, the pore size distribution is more reasonable, allowing for more thorough contact between the alkali and activated carbon during subsequent alkali activation, thus reducing the amount of KOH used to some extent.

[0116] Micromorphology

[0117] Depend on Figure 7 (ac) shows that the HP surface is relatively rough, with pores mainly being circular, and the pore size distribution is uneven. Meanwhile, Figure 7 In (df), HP has an interconnected porous structure after secondary activation with KOH. Compared with the sample that is only activated by phosphoric acid, the surface of the material shows obvious and richer pores. Through the rich pores, the well-developed channels and hierarchical pore structure of the material can also be clearly seen.

[0118] microcrystalline structure

[0119] Figure 8 In (a), HPK does not have sharp diffraction peaks at the two diffraction angles of 23.5° and 44°, which proves that it is mainly composed of amorphous carbon with poor crystallinity. The poor diffraction peaks are because a large number of micropores are generated during the secondary activation process of KOH. The presence of these micropores hinders the parallel arrangement and bonding of microcrystals, thereby preventing the further growth of graphite microcrystals.

[0120] HP exhibits a distinct diffraction peak at 25.8°, which, compared to the standard 23.5° graphite diffraction peak, shows a significant rightward shift. According to Bragg's law: 2d·sinθ=λ, d… 002The decrease indicates that the interlayer spacing of carbon in activated carbon is smaller. Although it has a certain degree of graphitization, it is still poorly crystallized and has an amorphous structure. The graphite diffraction peak of HP at the (100) crystal plane at 2θ=44° is weak, and there are almost no carbon nanosheets. In addition to participating in the activation reaction of bean sprout biomass, phosphoric acid also reacts with some impurities in bean sprouts. After acid washing, the impurities are removed, which contributes to the uniformity of activated carbon.

[0121] Raman spectra of the two materials as follows Figure 8 As shown in (b), the G peaks of HPK and HP are at 1346.99 and 1351.74 cm⁻¹, respectively. -1 Nearby, peaks D are located at 1598.22 and 1584.38 cm⁻¹, respectively. -1 Nearby, I G / I D The values ​​are 1.187 and 1.172 respectively, with HPK having a higher I value than HP. G / I D It exhibits higher order and fewer defects. Compared to HPK, the smaller full width at half maximum (FWHM) at the 2D peak of HP demonstrates that HP has a more numerous few-layer graphite structure.

[0122] Surface functional groups

[0123] Table 5. Proportions of the four elements and carbon-oxygen atom ratio in HP and HPK.

[0124]

[0125] The carbon-to-oxygen ratios of HP and HPK are 8.3 and 15.96, respectively. The higher carbon-to-oxygen ratio of HPK can be attributed to the destruction of phosphorus- and oxygen-containing functional groups on the HP surface by KOH activation, which in turn increases the proportion of carbon atoms.

[0126] At the same time, the sp of both materials 2 C=C occupies the largest proportion in their respective C1s characteristic peaks, and the high content of C=C is beneficial to promoting charge transfer and improving the conductivity of the material. In addition, the presence of hydrophilic functional groups such as C=O and O=CO in the HP and HPK samples can effectively improve the wettability of the electrode material, increase the contact area of ​​ions, and further improve the specific capacitance.

[0127] Constant current charge-discharge and rate characteristics of activated carbon

[0128] Figure 9In (ab), the charge-discharge curves of HP and HPK are both approximately triangular in shape, indicating that the materials are mainly double-layer capacitors. However, the GCD curve of HP is slightly deformed. This may be because the low-temperature activation and the use of phosphoric acid make HP contain some nitrogen, oxygen and phosphorus functional groups. These functional groups can provide some pseudocapacitance for the material. Conversely, the GCD curve of HPK is more symmetrical, proving that HPK contains less pseudocapacitance and has a higher proportion of double-layer capacitors.

[0129] at the same time, Figure 9 In (a), the charge / discharge time of HPK is significantly longer than that of HP, indicating that HP has a lower specific capacitance at 0.5Ag. -1 At current density, the specific capacitance is only 83 F g -1 HPK, after secondary activation with KOH, at 0.5A g -1 The specific capacitance is 349F g. -1 It is worth noting that when the current density is 0.1 A g -1 Increase to 50A g -1 At that time, HPK's capacitance retention rate was 82.3%, while HP's was only 50A g. -1 At this point, its charging and discharging time was less than two seconds, the graph was severely distorted, and its charging and discharging voltage significantly exceeded the test voltage range. This indicates that the current density at this time had far exceeded its rated current density, and the capacitor was already in an unstable operating state. When the current density decreased from 0.1 A g... -1 Increased to 20A g -1 At that time, HP's capacitance retention rate was 28%, indicating severe degradation.

[0130] Cyclic voltammetric properties of activated carbon

[0131] like Figure 10 As shown, cyclic voltammetry tests were conducted on HP and HPK at different scan rates. It was found that the pseudocapacitive characteristics of HP were significant at all scan rates, with greater distortion in the cyclic voltammetry curves at higher scan rates. The pseudocapacitance can be attributed to the relatively low carbonization activation temperature of the phosphoric acid activation method and the protective effect of phosphoric acid on the carbon body at low temperatures. Biomass raw materials themselves contain certain nitrogen and oxygen elements, which form nitrogen- and oxygen-containing functional groups during the preparation process. Due to the relatively low temperature and the protective effect of phosphoric acid, these functional groups are not easily destroyed. Furthermore, the use of phosphoric acid introduces phosphorus-containing groups, which also contribute to the pseudocapacitance.

[0132] However, HPK can maintain a good rectangular shape at all scan rates, indicating that the specific capacitance of the KOH-activated porous carbon is also mainly provided by the double-layer capacitance. During the secondary activation process, the functional groups on the surface of HP are destroyed by the high temperature and KOH. The shedding of these nitrogen- and phosphorus-containing functional groups reduces the proportion of pseudocapacitance, resulting in good EDLC performance of the material.

[0133] Impedance characteristics of activated carbon

[0134] For an ideal electric double-layer capacitor, its Nyquist curve exhibits a slope close to 90° in the low-frequency region. In the figure, the slope of the Nyquist curve of HPK is significantly greater than that of the sample prepared using only phosphoric acid activation. The lower slope value of HP can be attributed to the pseudocapacitive effect of the large number of functional groups on its surface, while secondary activation effectively improves the electric double-layer capacitance characteristics of the material.

[0135] On the one hand, some functional groups are destroyed under the combined action of high temperature and activator, and the pseudocapacitive effect is weakened; on the other hand, the secondary activation of KOH effectively improves the pore connectivity of the material, and more channels are opened, thereby greatly increasing the effective area for forming the double layer.

[0136] like Figure 11 As shown, HPK has an ESR of 1.74Ω, which is higher than MBS-5, indicating a higher equivalent series resistance. HPK has a higher carbon-oxygen atom ratio than MBS-5. It is well known that oxygen-containing functional groups can effectively reduce the equivalent series resistance by improving the wettability of the material. HPK has a carbon-oxygen atom ratio of 15.96, which is greater than MBS-5. This means a lower oxygen content, resulting in poorer wettability and higher contact resistance.

[0137] Energy density and power density of activated carbon

[0138] The energy density and power density under different current densities were calculated, and the results are as follows: Figure 12 As shown, when the power density is 50W kg -1 At that time, the energy density was 13.13 Wh / kg. -1 Power density increased to 4658W kg -1 At that time, the energy density can still be maintained at 9.4 Wh / kg. -1 Therefore, the HPK electrode prepared in this paper exhibits excellent energy storage characteristics and is a reliable and highly promising electrode material. However, due to its lower specific capacitance, HPK has a much lower energy density than HPK, at 50 W / kg. -1 At that time, the energy density was only 3.82 Wh / kg at its highest. -1 .

[0139] In summary, this embodiment used mung bean sprouts as raw material and optimized the pore structure of the sprouts using a two-step activation method with H3PO4-KOH. Orthogonal experiments were conducted to study the effects of various influencing factors during the phosphoric acid impregnation and thermal processes on the pore structure, surface properties, graphitization degree, and electrochemical performance of activated carbon. Specific conclusions are as follows:

[0140] (1) The optimal preparation parameters were: KOH alkali-to-carbon ratio (4:1), phosphoric acid impregnation temperature (120℃), phosphoric acid impregnation ratio (1.2:1), carbonization temperature (500℃), and carbonization time (2h) (arranged in order of influence). The activated carbon from mung bean sprouts prepared under these conditions (named HPK) had a specific surface area as high as 3729.359 m². 2 g -1 The pore volume is 2.295 cm³. 3 g -1 The pore sizes are mainly concentrated in the 0.5-0.8 nm and 2-4 nm ranges, with HPK exhibiting a greater number of micropores. In a two-electrode system (6 M KOH electrolyte), the specific capacitance of HPK is 349 F g. -1 (Test current 0.5A g) -1 The EDLC performs well, with current ranging from 0.1A g. -1 Increase to 50A g -1 At that time, the capacitance retention rate was 82.3%.

[0141] (2) The phosphoric acid activation method has a simple preparation process and a relatively low activation temperature. The use of phosphoric acid has a certain protective effect on the carbon body during the carbonization process, allowing elements such as nitrogen and oxygen in the raw materials to be retained in the form of functional groups. The use of phosphoric acid also introduces some phosphorus-containing functional groups, so the activated products prepared by this method will have obvious pseudocapacitance.

[0142] (3) Compared with the phosphoric acid activation method and the carbonization-KOH activation method, the product of the H3PO4-KOH activation method has a better hierarchical structure. The early introduction of phosphoric acid activation effectively lowers the carbonization temperature, giving the carbonized product abundant mesopores and a certain amount of micropores, which is beneficial to the further pore-forming reaction of KOH. KOH, based on phosphoric acid activation, can create well-developed channels and more abundant micropores. The reasonable combination of phosphoric acid activation and KOH activation effectively reduces the amount of KOH used while giving the product a higher specific capacitance.

[0143] Example 4: Preparation of Bean Sprout-Based Porous Carbon via Synergistic Activation and Nitrogen Doping Process

[0144] In this experiment, melamine was used as the nitrogen source, acid-washed and dried mung bean sprout carbonized samples were used as raw materials, and KOH was used as the activator to prepare porous carbon materials. Each time, 5g of KOH and 1g of acid-washed mung bean sprout powder carbonized sample were weighed, then mixed with a certain amount of melamine, and placed in an atmosphere muffle furnace at 5℃ for 1 minute.-1 The heating rate was initially set to 350℃ and held for 0.5 h, then increased to 800℃ and held for 2 h, with high-purity nitrogen used as a protective gas at a flow rate of 0.6 L / min. -1 The obtained product was placed in a magnetically stirred water bath, and 0.1 mol L was added dropwise. -1 The solution was diluted with hydrochloric acid until it became neutral, then repeatedly rinsed with deionized water and finally dried. This group of products was labeled MBSN. 1：X The numbers under the subscripts represent the mass ratio of melamine to carbon. For example, MBSN 1：7 This indicates a sample prepared by synergistic nitrogen doping after mixing melamine and MBS-0 at a mass ratio of 1:7.

[0145] Effect of blending ratio on the pore structure and morphology of materials

[0146] Pore ​​structure characterization

[0147] Table 6 MBSN 1:X Hole structure parameters

[0148]

[0149] Table 6 shows the variation trend of pore structure with melamine blending ratio. In Table 6, the micropore volume and specific surface area show basically the same trend with blending ratio, proving that the specific surface area of ​​the material is mainly contributed by micropores. 1:x The specific surface area and micropore volume of the sample were both higher than those of MBS-5, demonstrating that the addition of melamine promoted the generation of more micropores during the alkali activation process. Among them, MBSN... 1:x Under lower melamine blending ratios, the specific surface area and total pore volume tend to increase with the blending ratio, indicating that melamine and KOH have a significant synergistic pore-forming effect at lower blending ratios. Specifically, MBSN... 1:5 and MBSN 1:7 It has the largest specific surface area and total pore volume, at 3558.184 m², respectively. 2 g -1 and 2.443cm 3 g -1 When the blending ratio is greater than 1:5, the pore volume and specific surface area of ​​the material tend to decrease. On the one hand, the pore-blocking effect of melamine gradually becomes obvious, and on the other hand, excessive melamine will intensify the reaction between KOH and activated carbon, leading to pore collapse.

[0150] Figure 13 In (a), the nitrogen adsorption-desorption curves of all nitrogen-doped samples show a significant increase in the low-pressure region. With further pressure increase, a hysteresis loop is observed, and the curve tails slightly rise. This demonstrates that MBSN... 1:XMicropores predominate, with a certain amount of mesopores present in all four samples. Meanwhile, MBSN... 1:7 The adsorption capacity and hysteresis loop area of ​​MBSN are both greater than those of other samples in the figure, proving that MBSN 1:7 It has a larger pore volume and a higher mesopore content. Figure 13 In (b), the micropores of all nitrogen-doped samples increased in the range of 0.5–0.8 nm, and the mesopores increased in the range of 2–4 nm. (MBSN) 1:7 The mesopore distribution range of MBSN is the same as that of MBS-5, but its pore size distribution curve is higher than that of MBS-5, proving that in the 2-4nm range, MBSN... 1:7 More mesoporous structures are formed, and a greater number of mesopores facilitates the transport of electrolyte ions. This is related to... Figure 13 In (a), MBSN 1:7 The conclusion is consistent with the larger hysteresis loop area.

[0151] In summary, in the synergistic activation and nitrogen doping process, at a low doping ratio, melamine mainly promotes the activation of KOH, while a high doping ratio intensifies the activation process of KOH and also blocks the pores.

[0152] Micromorphological characterization

[0153] Figure 14 In (a), MBSN can be seen at low magnification. 1:3 The surface contains numerous broken particles and flocculent material, with the accumulated layer being significantly thicker and unevenly distributed. MBSN 1:3 The flocculent material on the surface originates from the high-temperature condensation process of melamine, and a large amount of α-C3N4-like deposits cover the surface of activated carbon.

[0154] exist Figure 14 (d) At low magnification, MBSN 1:7 The tubular morphology is more distinct due to the lower melamine content in MBSN. 1:7 The surface flocculent material is no longer obvious, similar to MBSN. 1:3 Compared to a smoother surface, the addition of melamine intensifies the activation process, causing the thin-walled tissue on the vascular structure surface to be peeled off. The peeled-off thin-walled tissue fragments have abundant pores. Figure 14 (e) Within the frame, more tubular structures are exposed. These natural channels facilitate the entry of potassium vapor, melamine, and some pyrolysis products into the tube bundles, allowing them to fully contact the wall surface and react.

[0155] Figure 14 In (f), at a magnification of 50k, MBSN 1：7 Melamine flocculent deposits can still be observed on the surface, but compared with... Figure 14(c) In comparison, its effect on blocking the channel is not obvious.

[0156] Effect of nitrogen doping on the composition of porous carbon materials

[0157] Depend on Figure 15 (a) It can be seen that MBSN 1:X The broad peaks near 23.5° and 44° correspond to the crystal planes (002) and (100) of standard graphite, respectively. The weak diffraction intensities at these two locations indicate that the samples have poor crystallinity. This proves that the five samples in the figure are mainly composed of amorphous carbon with a low degree of graphitization and do not have a single-layer graphite structure.

[0158] Table 7 MBSN 1:X I G / I D value

[0159]

[0160] Table 7 shows the I values ​​for all nitrogen-doped samples. G / I D The values ​​have all improved compared to the original values, due to MBSN 1:9 The amount of melamine added was too small, therefore it affected I. G / I D The value increase is limited. (By...) Figure 6-3 (b) As shown in Table 6-2, with the increase of the blending ratio, I G / I D The value shows a trend of first increasing and then decreasing, indicating that when the blending amount exceeds the optimal value, excessive melamine will hinder graphitization. In addition, when the optimal value is exceeded, non-conductive melamine polymers will be generated in the material, which will also affect the improvement of the degree of graphitization.

[0161] This demonstrates that nitrogen doping can effectively improve the defect structure in carbon materials. The low peak intensity of the 2D peak indicates that few-layer graphite structures are rare, consistent with the XRD findings.

[0162] Effect of nitrogen doping on the composition of porous carbon materials

[0163] like Figure 16 As shown, MBSN 1:X It is mainly composed of three elements: C, N, and O. Furthermore, the diffraction peaks of N become more pronounced with increasing doping ratio, because MBSN... 1:9 If the nitrogen content is less than 1%, nitrogen-containing functional groups will not be analyzed. Figure 6-5In (b), the high-resolution N1s spectrum decomposes into four peaks: 399.0 eV (pyridine nitrogen N-6), 400.4–400.6 eV (pyrrole nitrogen N-5), 401.6 eV (graphite nitrogen NQ), and 402.3 eV (pyridine nitride NX). The N-6 structure refers to the nitrogen atom in the graphite sp... 2 The N-5 structure refers to a pyridine-like nitrogen atom bonded to two adjacent carbon atoms in a five-membered ring structure. The irregular pentagonal structure of N-5 is a defect structure and is unstable. The NQ structure refers to a nitrogen atom bonded to a carbon atom in a graphite sp... 2 In a network, nitrogen atoms are hybridized with three adjacent carbon atoms to form a graphitic nitrogen structure. The NX structure refers to nitrogen atoms that, in addition to hybridization with sp... 2 In addition to binding with carbon atoms, the network also binds with oxygen atoms and exhibits properties similar to those of oxygen-containing functional groups.

[0164] In Table 8, the N-5 content in all samples was less than that in MBSN. 1:7 This is because MBSN 1:7 The addition of melamine in the middle is relatively low, and the doping effect is not obvious. It is precisely this defect that leads to MBSN 1:7 Middle I G / I D The value is low, and the sample MBSN 1:7 Among them, the abundance of NQ was significantly higher than that of the other three samples. Compared with the structures of N-5 and N-6, NQ is located inside the carbon matrix, indicating that it is more stable. NQ can synergistically improve the conductivity and cycling performance of carbon materials. These results indicate that MBSN benefits from the synergistic effect of activation and nitrogen doping. 1:7 The distribution of nitrogen in the medium structure is relatively uniform. Nitrogen atoms are not only embedded in the edges of the carbon skeleton, but also firmly embedded in the carbon basal plane.

[0165] Table 8 MBSN 1:X Peak area ratio of nitrogen

[0166]

[0167] Electrochemical characterization

[0168] like Figure 17 As shown in (a), MBSN 1:7 The constant current charge-discharge curves exhibit an isosceles triangle with good symmetry, proving that the capacitance of the material is mainly composed of double-layer capacitance. Figure 17 In (b), as the scan rate increases, the cyclic voltammetric characteristic curve maintains a roughly rectangular shape. When the scan rate increases to 200 mV / s, the CV curve becomes spindle-shaped, proving that MBSN... 1:7 A small amount of pseudocapacitance exists, which is attributed to the small number of nitrogen-containing functional groups on the material surface.

[0169] Figure 17 In (c), when the current changes from 0.1A g -1 Increase to 50A g -1 At that time, MBSN 1：7 The specific capacitance is 411F g -1 Reduced to 306F g -1 The capacitance retention rate is 74.45%, and the high rate capability is due to the interconnected channels resulting from the increased internal surface area, the high degree of graphitization, and the nitrogen-containing structure incorporated into the carbon skeleton.

[0170] Figure 17 In (d), in the low-frequency region, the curves of the samples all show a straight line of approximately 90°, which indicates that the diffusion resistance is minimal and the double-layer performance is good.

[0171] Figure 18 MBSN in the middle 1：7 In 5A g -1 The capacitance retention rate after 5000 cycles at current density is determined by... Figure 18 As can be seen, the capacitance value increases slightly in the initial stage. This is because the electrolyte temperature rises with cycling, increasing the ion transport rate. This further promotes the Faraday reaction, resulting in a brief increase in specific capacitance. However, the pseudocapacitance provided by nitrogen-containing functional groups is unstable. After a certain number of cycles, the surface functional groups are destroyed, and the capacitance is provided solely by the electric double-layer capacitance. Therefore, the specific capacitance decreases again. The energy storage process of the electric double-layer capacitance involves only physical processes and is more stable than the pseudocapacitance; therefore, the capacitance retention rate of the material remains stable after 1000 cycles. After 5000 cycles, the capacitance retention rate of the material is still 96%, which is an improvement compared to MBS-5's 93.7%.

[0172] Figure 19 For MBSN 1:7 The curve showing the change in energy density as a function of power density. Its power density is 50 W / kg. -1 At that time, the energy density was 14.36 Wh / kg. -1 Power density increased to 12.47 kW kg -1 At that time, the energy density remained at 11.44 Wh / kg. -1 In summary, only through the synergistic effect of a suitable pore structure and a reasonable degree of graphitization can porous carbon electrodes possess excellent energy storage characteristics. Therefore, the MBSN prepared in this embodiment... 1:7 Electrodes possess excellent energy storage properties, making them a reliable and highly promising electrode material.

[0173] In MBSN 1:7In the preparation process, the activation of KOH and the doping of nitrogen atoms occur simultaneously. As the temperature increases, KOH continuously consumes carbon atoms, gradually forming interconnected macropores and mesopores. Simultaneously, when the temperature exceeds 650℃, melamine decomposes into intermediate products such as cyano groups, amino groups, and triazine rings. During continuous activation, nitrogen atoms are embedded into the carbon framework under thermal conditions. When the temperature reaches 760℃, liquid potassium metal transforms into potassium vapor. At this point, the pore-forming reaction gradually changes from a liquid-solid reaction to a gas-solid reaction.

[0174] Potassium vapor continuously enters and exits the three-dimensional porous structure, creating new micropores and further expanding the internal surface area, providing ample reaction space for melamine. Simultaneously, some nitrogen-containing small molecules also enter these newly formed micropores along with the potassium vapor, allowing nitrogen-containing groups to enter deeper reaction spaces.

[0175] Therefore, it can be considered that in the synthesis of MBSN 1:x During the process, potassium vapor not only contributes to the formation of more micropores but also affects the uniformity of nitrogen atoms in the carbon framework. This synergistic preparation strategy can control the distribution of doped nitrogen atoms in the carbon framework while simultaneously improving the wettability and conductivity of the material.

[0176] In summary, this embodiment used mung bean sprouts as the carbon source and melamine as the nitrogen source, employing a KOH activation combined with simultaneous doping process to prepare nitrogen-doped hierarchical porous carbon. The effects of the preparation process and the nitrogen source mixing ratio on the specific surface area, pore structure, electrochemical performance, structure, and composition of the material were investigated. Specific research results are as follows:

[0177] (1) The selection of precursors laid a good structural foundation for the preparation of hierarchical porous structures and ultra-high specific surface areas of nitrogen-doped porous carbon. Melamine played a certain synergistic role in KOH pore formation, which can effectively increase the specific surface area and micropore volume of the material. The potassium element generated by KOH during pore formation can effectively assist melamine to penetrate into the carbon skeleton for doping. Potassium vapor not only plays a role in forming more micropores, but also affects the uniformity of nitrogen atoms in the carbon skeleton.

[0178] (2) The porous carbon material prepared with a melamine blending ratio of 1:9 exhibits optimal electrochemical performance when used as a symmetrical capacitor. (0.1 Ag) -1 Its specific capacitance can reach 411 Fg at current density. -1 At 50A g -1 Its specific capacitance can still reach 306 F g at current density. -1 , in 5A g -1 After 5000 charge-discharge cycles at a given current density, its capacitance retention remains at 96%. Its highest energy density reaches 14.36 Wh / kg. -1The power density output at this time is 50W kg. -1 .

[0179] (3) The addition of melamine effectively improves the defective structure in carbon materials, but its effect on improving the degree of graphitization is not significant. MBSN 1:7 Middle I G / I D The value was the highest, and its NQ abundance was significantly higher than the other three samples. This is attributed to the synergistic effect of activation and nitrogen doping. 1:7 The distribution of nitrogen in the medium structure is relatively uniform. Nitrogen atoms are not only embedded in the edges of the carbon skeleton, but also firmly embedded in the carbon basal plane.

[0180] In summary, this invention addresses the problem of poor uniformity in plant-based activated carbon for supercapacitors. Based on botanical theory, it screened plant materials and ultimately selected mung bean sprouts as a representative derivative plant as a precursor for activated carbon in supercapacitors. Based on this, a series of related quality improvement and modification works were carried out from two aspects: pore structure adjustment and nitrogen atom doping. The main conclusions are as follows:

[0181] (1) By selecting the best plant species, we selected a "derived plant" (derived from other legumes) with good structure, fast growth and non-soil growth, such as mung bean sprouts, as a precursor. We used KOH as an activator and prepared activated carbon by carbonization-alkali activation method. We compared it with activated carbon prepared from mung beans to explore the potential of the material.

[0182] The results showed that, at an alkali-to-carbon ratio of 5, both materials achieved the largest specific surface area and pore volume, with MBS-5 exhibiting a specific surface area and pore volume of 3314.382 m². 2 g -1 and 2.2890cm 3 g -1 Its pore size is mainly concentrated between 0.5-0.7 nm, with a microporosity of 50.74%, making it a microporous activated carbon material. In a two-electrode system (6M KOH electrolyte), MBS-5 has a specific capacitance of 295 F / g. -1 (Test current 0.5A g) -1 After 5000 charge-discharge cycles, the capacitance retention rate reached 93.7%, with the current decreasing from 0.1A g. -1 Increase to 50A g -1 At that time, the capacitance retention rate was 78.8%.

[0183] Compared to mung beans, mung bean sprout-like "derived plants" (derived from legumes) are mainly composed of vascular tissue and parenchyma. The vascular bundles in the vascular tissue are accompanied by interconnected air cavities with a diameter of a few micrometers, and there are also gaps between the cells of the parenchyma, which lay a good foundation for the formation of a hierarchical porous structure. Due to the lack of a naturally occurring hierarchical porous structure, the mung bean-based activated carbon MBP-5 is inferior to MBS-5 in terms of the rationality of its pore structure and its electrochemical performance.

[0184] (2) Starting from the removal of metals and the removal of contents after alcohol extraction from plants, the bean sprout raw materials were subjected to acid washing pretreatment and ethanol solution extraction pretreatment, respectively, and ultrasound was introduced to enhance heat and mass transfer.

[0185] Thermogravimetric analysis revealed that acid washing increased the maximum biomass decomposition rate from 0.6% / ℃ to 0.8% / ℃. Ethanol extraction increased the maximum decomposition temperature from 310℃ to 340℃ and the maximum decomposition rate from 0.54% / ℃ to 0.6% / ℃. The introduction of ultrasound further enhanced the pretreatment effect. The presence of ethanol extraction promoted the reactivity among the three major components, facilitating the decomposition of structural substances. Acid washing promoted the removal of metal ions and the hydrolysis of cellulose and hemicellulose, thus opening and exposing the biomass pores, which is more conducive to heat transfer and the release of small molecule volatile substances, thereby significantly promoting the rate.

[0186] After acid washing, due to the hydrolysis of hemicellulose and other substances and the leaching of other small molecules, the number of micropores in MBSA-5 (0.55-0.7nm) increased significantly, and the microporosity increased from 50.74% in MBS-5 to 58.6%. However, the excessive number of micropores caused the mesopore distribution to narrow. At the same time, the introduction of ultrasound increased the number of less accessible micropores (smaller than 0.55nm) in MBSAU-5, which further exacerbated the reduction of mesopores.

[0187] After ethanol extraction, the dissolution of the extract reduced its blocking effect on the pores, resulting in an increase in both the number of micropores and mesopores in the prepared MBSE-5, with the pore volume increasing to 2.489 cm³. 3 g -1 Excessive ultrasonic power, while promoting mass transfer, can also damage the original morphology of bean sprouts. The average pore size of the MBSEU-5 micropores decreased from 0.6219 nm for MBS-5 to 0.5598 nm, and the excessive micropores caused the collapse of the mesopores. Specifically, MBSE-5, MBSA-5, MBSAU-5, and MBSEU-5 were prepared at 0.5 A g... -1 The current density reached 357,303,286 and 262 F g. -1 The metal ion content of MBSAU-5 prepared after ultrasonic acid washing conforms to GB / T 37386-2019.

[0188] (3) Using bean sprouts as raw material, the two-step activation method of H3PO4-KOH was used to make up for the shortcomings of the single phosphoric acid method and KOH, and the performance of bean sprout-based activated carbon was further optimized. The effects of impregnation process and thermal process on phosphoric acid-bean sprout polymer complex were studied through orthogonal experiments.

[0189] The optimal preparation process is as follows: KOH alkali-to-carbon ratio (4:1), phosphoric acid impregnation temperature (120℃), phosphoric acid impregnation ratio (1.2:1), carbonization temperature (500℃), and carbonization time (2h) (listed in order of influence). Activated carbon prepared under these conditions has a high specific surface area of ​​3729.359 m². 2 g -1 The pore volume is 2.295 cm³. 3 g -1 The pore sizes are mainly concentrated in the range of 0.5-0.8 nm and 2-4 nm, exhibiting a suitable hierarchical pore structure distribution. In the dual-electrode system (6 M KOH electrolyte), the specific capacitance of HPK is 349 F g. -1 (Test current 0.5A g) -1 The EDLC performs well, with current ranging from 0.1A g. -1 Increase to 50A g -1 At that time, the capacitance retention rate was 82.3%.

[0190] (4) Using melamine as the nitrogen source and bean sprouts as the carbon source, bean sprout activated carbon was synergistically modified by KOH activation and nitrogen doping. Melamine played a certain synergistic role in KOH pore formation, and MBSN prepared by KOH activation and nitrogen doping... 1:7 Its specific surface area is 3459.470 m². 2 g -1 The pore volume is 2.443 cm. 3 g -1 The potassium generated during KOH pore formation can effectively assist melamine in penetrating deeper into the carbon skeleton for doping. Potassium vapor not only helps to form more micropores, but also affects the uniformity of nitrogen atoms in the carbon skeleton.

[0191] The addition of melamine effectively improves the defective structure in carbon materials, but its effect on improving the degree of graphitization is not significant. (MBSN) 1:7 Middle I G / I D The value was the highest, and its NQ abundance was significantly higher than the other three samples. This is attributed to the synergistic effect of activation and nitrogen doping. 1:7 The distribution of nitrogen in the medium structure is relatively uniform. Nitrogen atoms are not only embedded in the edges of the carbon skeleton, but also firmly embedded in the carbon basal plane.

[0192] In a two-electrode system (6M KOH electrolyte), MBSN 1:7 The specific capacitance is 374 F g -1 (Test current 0.5A g) -1 The EDLC exhibits excellent performance, with a capacitance retention rate of up to 96% after 5000 charge-discharge cycles, demonstrating good cycle characteristics.

[0193] The above description is merely a preferred embodiment of this application and is not intended to limit this application. Various modifications and variations can be made to this application by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the protection scope of this application.

Claims

1. A method for preparing a derivative plant-based porous carbon electrode material, characterized in that, The preparation method includes treating the derivative plants using a carbonization-KOH activation method; Specifically, the preparation method includes: S1. Place the bean sprouts in an inert atmosphere and pyrolyze them at high temperature; S2. Add an activator to the product obtained in step S1 and perform a heating impregnation treatment; S3. The product obtained in step S2 is placed in an inert gas for a second heating activation treatment; S4. The product obtained in step S3 is obtained by acid washing, water washing, and drying. Pretreatment is performed to remove the contents of bean sprouts, specifically including the removal of non-structural components of bean sprouts and acid washing of bean sprout biomass to remove impurities; The above-mentioned pretreatment to remove the contents in bean sprouts is carried out before step S1, or the bean sprout powder obtained after simple pretreatment of bean sprouts is pretreated to remove the contents in bean sprout powder. The two-step activation method using H3PO4-KOH is adopted. Specifically, the preparation method includes phosphoric acid impregnation before step S1. The phosphoric acid impregnation method includes: mixing bean sprout powder with phosphoric acid, heating and stirring, and drying for later use. The preparation method further includes mixing the porous carbon material obtained in step S4 with melamine and calcining it to obtain nitrogen-doped activated carbon material.

2. The production method according to claim 1, wherein In step S1, the bean sprouts are mung bean sprout powder obtained through simple pretreatment. The pretreatment method includes: washing and drying the bean sprouts, crushing them, and sieving them. The sieve mesh size is 80 mesh, which yields powder with a particle size of less than 178 μm for later use. The inert atmosphere is nitrogen, and the high-temperature pyrolysis is performed at 450-650℃ for 0.5-3h, with a heating rate of 1-10℃·min -1 , and a nitrogen flow rate of 0.5-5L·min -1 .

3. The preparation method according to claim 1, characterized in that, In step S2, the mass ratio of the activator to the product is 1-10:1, and the activator is KOH; The specific conditions for heat impregnation treatment are: impregnation at 70-90℃ for 1-3 hours.

4. The preparation method according to claim 3, characterized in that, The specific conditions for the heat impregnation treatment are: impregnation at 80℃ for 2 hours.

5. The production method according to claim 1, wherein The specific conditions of the secondary temperature rising activation treatment in the step S3 include: rising the temperature to 300-400℃ at a temperature rising rate of 1-10℃, keeping the temperature for 10-50min, then rising the temperature to 750-850℃ at a temperature rising rate of 1-10℃ and keeping the temperature for 1-3h, with nitrogen as the protective gas and a flow rate of 0.5-5L·min -1 .

6. The preparation method according to claim 5, characterized in that, The specific conditions of the secondary temperature rising activation treatment in the step S3 include: rising the temperature to 350℃ at a rate of 5℃, keeping the temperature for 30 min, rising the temperature to 800℃ at a rate of 5℃ and keeping the temperature for 2 h, nitrogen as the protective gas, and the flow rate of 1 L·min -1 .

7. The preparation method according to claim 1, characterized in that, In step S4, the acid added in the pickling step is hydrochloric acid, the water washing step specifically involves repeatedly rinsing with water until the pH of the supernatant is neutral, and the drying step specifically involves drying at 100-110℃ for 1-48 hours.

8. The preparation method according to claim 7, characterized in that, In step S4, the acid added in the pickling step is 1.0M hydrochloric acid; the drying step is specifically carried out by drying at 105℃ for 24 hours using an oven-drying method.

9. The production method according to claim 1, wherein The method for removing non-structural components from bean sprouts using ethanol or ultrasound-assisted ethanol reflux treatment of biomass raw materials includes the following reflux conditions: refluxing bean sprouts with an ethanol solution at 70-90°C for 0.5-2 hours, cooling and removing the supernatant, repeating 2-3 times; wherein the ethanol is a 60-80% by mass solution, and the mass ratio of bean sprouts to the volume of the ethanol solution is 1-5:10-80; Ultrasonic disruption is added during the ultrasonic-assisted ethanol reflux treatment process, and the ultrasonic power is controlled to be 100-500W. Acid washing of bean sprout biomass is used to deeply remove metal ions from bean sprouts. Specifically, the acid washing step includes placing the dried bean sprouts in a hydrochloric acid solution. The acid washing conditions are: treatment at 60-80℃ for 1-3 hours, and the concentration of the hydrochloric acid is 0.1-1 mol·L⁻¹. -1 The ratio of the mass of the bean sprouts to the volume of the hydrochloric acid solution is 1-5:10-80; Ultrasonic treatment is introduced during the pickling process. The treatment time during ultrasonic pickling is 0.5-2 hours, and the ultrasonic power is 100-150W.

10. The production method according to claim 9, wherein The method for removing non-structural components from bean sprouts by reflux treatment of biomass raw materials with ethanol or ultrasound-assisted ethanol, wherein the reflux operation conditions include: refluxing bean sprouts with ethanol solution at 80°C for 1 hour, cooling and removing the supernatant, repeating 2-3 times; wherein the ethanol is a 70% ethanol solution by mass, and the mass ratio of bean sprouts to the volume of ethanol solution is 1-5:10-80. Ultrasonic disruption is added during the ultrasonic-assisted ethanol reflux treatment process, and the ultrasonic power is controlled at 300W. Acid washing of bean sprout biomass is used to deeply remove metal ions from bean sprouts; specifically, the acid washing step includes placing the dried bean sprouts in a hydrochloric acid solution, and the acid washing conditions are: treatment at 70℃ for 2 hours, and the concentration of the hydrochloric acid is 0.5 mol·L⁻¹. -1 The ratio of the mass of the bean sprouts to the volume of the hydrochloric acid solution is 2:

50. Ultrasonic treatment is introduced during the pickling process. The ultrasonic pickling process lasts for 1 hour and the ultrasonic power is 120W.

11. The production method according to claim 1, wherein The specific method for phosphoric acid impregnation includes: mixing bean sprout powder with phosphoric acid at a mass ratio of 0.8-2:1, then adding water, controlling the phosphoric acid impregnation temperature at 80-140℃, and then drying for later use. The drying conditions are drying at 80-140℃ for 8-12 hours. The specific preparation method of nitrogen-doped activated carbon material obtained by mixing and calcining the porous carbon material obtained in step S4 with melamine includes: mixing melamine with KOH and acid-washed bean sprout powder carbonized sample, heating to 300-400℃ at a heating rate of 1-10℃, holding at this temperature for 10-50 min, then heating to 750-850℃ at a heating rate of 1-10℃ and holding at this temperature for 1-3 h, using nitrogen as protective gas at a flow rate of 0.1-1 L·min. -1 Then, it is pickled, washed with water, and dried to obtain the product. The mass ratio of melamine to KOH and the carbonized bean sprout powder after acid washing is 1:5-50:1-10.

12. The production method according to claim 11, wherein The phosphoric acid impregnation temperature is controlled at 120℃, and then dried for later use. The drying conditions are drying at 80-140℃ for 10 hours. The specific preparation method of nitrogen-doped activated carbon material obtained by mixing and calcining the porous carbon material obtained in step S4 with melamine includes: mixing melamine with KOH and acid-washed bean sprout powder carbonized sample, heating to 350°C at a heating rate of 5°C, holding at this temperature for 30 min, then heating to 800°C at a heating rate of 5°C and holding at this temperature for 2 h, with nitrogen as the protective gas and a flow rate of 0.6 L·min. -1 Then, it is pickled, washed with water, and dried to obtain the product. The mass ratio of melamine to KOH and the carbonized bean sprout powder after acid washing was 1:15-45:3-9.

13. A porous carbon electrode, characterized in that, The porous carbon electrode comprises a derivative plant-based porous carbon electrode material obtained by the preparation method according to any one of claims 1-12.

14. A supercapacitor, characterized in that, The supercapacitor comprises a derivative plant-based porous carbon electrode material obtained by the preparation method of any one of claims 1-12 or the porous carbon electrode of claim 13.

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