A method for preparing continuous framework porous carbon and its application

Abstract

This invention relates to a method for preparing continuous framework porous carbon and its applications. Using agricultural waste straw as a renewable carbon source, a polymer is introduced as a structural modifier into a KOH chemical activation system. Through a one-step carbonization / activation simultaneous strategy, the porous carbon structure is regulated and nitrogen atoms are in-situ doped, thereby constructing a continuous framework carbon material with highly interconnected efficient ion transport channels and charge storage units. A symmetrical supercapacitor assembled from the continuous framework porous carbon prepared according to this invention achieves a specific capacitance of 31.1 F / g at a high current density of 20 A / g and can provide an ultra-high power density of 10 kW / kg at an energy density of 4.32 Wh / kg. After 260,000 cycles in a 6M KOH aqueous electrolyte, the capacity retention rate remains 100%, demonstrating cycling stability far exceeding that of commercial activated carbon YP50F.

Description

Technical Field

[0001] This invention relates to a method for preparing porous carbon and its application. Background Technology

[0002] As the global energy structure transitions towards low-carbon and sustainable development, high-efficiency energy storage technology is gradually becoming a core driving force for the development of new energy systems. In commercial energy storage devices, supercapacitors, with their high power density, excellent rate performance, and ultra-long cycle life, are widely used in hybrid vehicles, power grids, and microelectronic products. Porous carbon materials, due to their tunable pore structure, good conductivity, and high specific surface area, are currently the most widely used electrode materials for supercapacitors. Generally, the fast charging / high rate performance of supercapacitors is mainly attributed to the advantages of porous carbon electrode materials, such as pore size distribution, porosity, and structural stability during ion transport.

[0003] In recent years, the preparation of porous carbon materials using low-cost, renewable biomass as a precursor has attracted widespread attention. These biomass raw materials are not only rich in cellulose and lignin, but their abundant surface pore structure and oxygen-containing active groups also make them ideal porous carbon precursors. However, the carbonization process of biomass often suffers from discontinuous skeletal structures, easily leading to insufficient electrochemical stability of the materials under ultrafast charge-discharge conditions (>40 A / g). Currently, while commonly used physical or chemical activation methods can increase the specific surface area of ​​biomass carbon, the degree of reaction with activators at high temperatures is difficult to precisely control, and they easily disrupt the original continuous skeletal structure of biomass (such as plant fibers), resulting in insufficient structural coherence of the obtained carbon materials, thus limiting their application in high-power supercapacitors. Summary of the Invention

[0004] To address the aforementioned technical problems, this invention provides a method for preparing continuous framework porous carbon and its application.

[0005] A method for preparing continuous framework porous carbon is specifically carried out according to the following steps:

[0006] I. Preparation of a polymer / straw / KOH mixture:

[0007] ① Dissolve KOH in deionized water to obtain a KOH solution; add straw to the KOH solution and stir continuously to obtain a straw / KOH solution;

[0008] ② Dry the straw / KOH solution, grind it into granules, and then mix it evenly with the polymer powder to obtain a polymer / straw / KOH mixture;

[0009] II. Calcination:

[0010] The polymer / straw / KOH mixture was transferred to an alumina pot, which was then placed in a tube furnace. Under an argon atmosphere, the temperature was first raised to 180°C and held, then raised to 800°C and held, then lowered to 300°C, and finally cooled to room temperature to obtain the reaction product. The reaction product was then washed with hydrochloric acid and deionized water in sequence and dried to obtain continuous porous carbon framework.

[0011] A continuous framework porous carbon is used as an electrode for a supercapacitor.

[0012] Advantages of this invention:

[0013] I. This invention proposes a method for preparing high-power biomass-based porous carbon materials with a continuous framework structure. Using agricultural waste straw as a renewable carbon source, this invention introduces high-molecular polymers, including polyvinyl alcohol (PVA), polyethylene glycol (PEG), polyacrylamide (PAM), and polyvinylpyrrolidone (PVP), as structural modifiers into a KOH chemical activation system. Through a one-step carbonization / activation simultaneous strategy, the porous carbon structure is regulated and nitrogen atoms are in-situ doped, thereby constructing a continuous framework carbon material with highly interconnected efficient ion transport channels and charge storage units. During calcination, a low-temperature holding period allows the molten polymer to carry KOH flow into weak points in the biomass structure. KOH etches PVP to generate gas, producing bubbles within the PVP. These bubbles contact the biomass structure, and KOH etches the biomass precursor on its surface. After the bubbles rupture, a three-dimensional interconnected pore structure is formed, achieving deeper, more uniform, and more thorough pore formation.

[0014] II. In existing technologies, straw is pre-carbonized by mixing it with alkali, and then polymer is added and calcined to prepare carbon material SK30. The specific surface area of ​​SK30, as determined by BET testing, is 1524.7178 m². 2 The microporous specific surface area is 1361.8985 m² / g, and the micropore ratio is 89.32%; while the continuous framework porous carbon prepared by this invention has a high specific surface area (1865.4 m² / g). 2 It has a high microporosity (99.07%) and a microporous specific surface area of ​​1847.99 m² / g. 2 g -1 This is beneficial for high volumetric energy density storage. The symmetrical supercapacitor assembled from continuous porous carbon with a continuous framework prepared based on this invention has a specific capacitance of 31.1 F / g at a high current density of 20 A / g and can provide an ultra-high power density of 10 kW / kg at an energy density of 4.32 Wh / kg. After 260,000 cycles in a 6M KOH aqueous electrolyte, the capacity retention rate is still 100%, which shows a cycle stability far exceeding that of commercial activated carbon YP50F. Attached Figure Description

[0015] Figure 1 The images are SEM images. In the figure, a is the SEM image of straw, b is the SEM image of straw / KOH solution after drying and grinding into particles in step one of Example 1, c is the SEM image of porous carbon (SK) prepared in Comparative Example 1, and d is the SEM image of continuous framework porous carbon (SK-30) prepared in Example 1.

[0016] Figure 2 Raman spectra of porous carbon (SK) prepared in Comparative Example 1 and continuous framework porous carbon (SK-30) prepared in Example 1;

[0017] Figure 3 (a) shows the isothermal adsorption / desorption curves of porous carbon (SK) prepared in Comparative Example 1 and continuous framework porous carbon (SK-30) prepared in Example 1, and (b) shows the pore size distribution of porous carbon (SK) prepared in Comparative Example 1 and continuous framework porous carbon (SK-30) prepared in Example 1.

[0018] Figure 4 High-resolution X-ray photoelectron absorption spectrum of continuous framework porous carbon (SK-30) prepared in Example 1;

[0019] Figure 5 (a) is the CV curve of the symmetrical supercapacitor assembled using the continuous framework porous carbon (SK-30) prepared in Example 1, and (b) is the CV curve of the symmetrical supercapacitor assembled using activated carbon (YP50F).

[0020] Figure 6 The rate performance of symmetrical supercapacitors assembled using porous carbon (SK) prepared in Comparative Example 1, continuous framework porous carbon (SK-30) prepared in Example 1, and activated carbon (YP50F) are shown in the figure.

[0021] Figure 7 Cyclic stability diagrams of symmetrical supercapacitors assembled using continuous framework porous carbon (SK-30) and activated carbon (YP50F) prepared in Example 1, respectively. Detailed Implementation

[0022] Specific Implementation Method 1: This implementation method is a method for preparing continuous framework porous carbon, specifically completed according to the following steps:

[0023] I. Preparation of a polymer / straw / KOH mixture:

[0024] ① Dissolve KOH in deionized water to obtain a KOH solution; add straw to the KOH solution and stir continuously to obtain a straw / KOH solution;

[0025] ② Dry the straw / KOH solution, grind it into granules, and then mix it evenly with the polymer powder to obtain a polymer / straw / KOH mixture;

[0026] II. Calcination:

[0027] The polymer / straw / KOH mixture was transferred to an alumina pot, which was then placed in a tube furnace. Under an argon atmosphere, the temperature was first raised to 180°C and held, then raised to 800°C and held, then lowered to 300°C, and finally cooled to room temperature to obtain the reaction product. The reaction product was then washed with hydrochloric acid and deionized water in sequence and dried to obtain continuous porous carbon framework.

[0028] Specific Implementation Method Two: This implementation method differs from Specific Implementation Method One in that: in step one ①, the mass ratio of KOH to deionized water in the KOH solution is (15g~25g):100mL; the straw mentioned in step one ① is corn straw with a mesh size of 5. Other steps are the same as in Specific Implementation Method One.

[0029] Specific Implementation Method Three: This implementation method differs from Specific Implementation Method One or Two in that: the mass ratio of straw to KOH in the straw / KOH solution mentioned in step one ① is 1:(1.5~2.5); the continuous stirring speed mentioned in step one ① is 600 r / min. -1 ~800r min -1 The time is 8h~12h. Other steps are the same as in specific implementation method one or two.

[0030] Specific Implementation Method Four: This implementation method differs from Specific Implementation Methods One to Three in that the polymer mentioned in step one ② is polyvinyl alcohol, polyethylene glycol, polyacrylamide, or polyvinylpyrrolidone. The other steps are the same as in Specific Implementation Methods One to Three.

[0031] Specific Implementation Method Five: This implementation method differs from Specific Implementation Methods One to Four in that: in step one ②, the mass ratio of polymer powder to straw in the polymer / straw / KOH mixture is (2~4):10; and the drying temperature in step one ② is 80℃. Other steps are the same as in Specific Implementation Methods One to Four.

[0032] Specific Implementation Method Six: This implementation method differs from Specific Implementation Methods One to Five in that the heating rate in step two is 3℃ / min to 5℃ / min; the cooling rate in step two is also 3℃ / min to 5℃ / min. The other steps are the same as in Specific Implementation Methods One to Five.

[0033] Specific Implementation Method Seven: This implementation method differs from Specific Implementation Methods One to Six in that the heat preservation time mentioned in step two is 2 to 3 hours. The other steps are the same as in Specific Implementation Methods One to Six.

[0034] Specific Implementation Method Eight: This implementation method differs from Specific Implementation Methods One to Seven in that: in step two, the reaction product is washed with hydrochloric acid 1 to 2 times to remove KOH and other impurities, and then washed with deionized water 2 to 4 times. The other steps are the same as in Specific Implementation Methods One to Seven.

[0035] Specific Implementation Method Nine: This implementation method differs from Specific Implementation Methods One to Eight in that: the mass fraction of hydrochloric acid in step two is 5% to 7%; the drying temperature in step two is 60℃ to 80℃, and the drying time is 10h to 12h. Other steps are the same as in Specific Implementation Methods One to Eight.

[0036] Specific Implementation Method 10: This implementation method uses continuous framework porous carbon as an electrode for a supercapacitor.

[0037] The beneficial effects of the present invention are verified using the following embodiments:

[0038] Example 1: A method for preparing continuous framework porous carbon, specifically carried out according to the following steps:

[0039] I. Preparation of a polymer / straw / KOH mixture:

[0040] ① Dissolve 20g of KOH in 100mL of deionized water to obtain a KOH solution; add 10g of corn stalks (5 mesh size) to the KOH solution and stir at 600 rpm. -1 The mixture was stirred continuously at a certain speed for 10 hours to obtain a straw / KOH solution.

[0041] ② Place the straw / KOH solution in an 80℃ forced-air drying oven to dry it into a solid, then grind it thoroughly into granules, and then mix it evenly with 3g of polymer powder to obtain a polymer / straw / KOH mixture;

[0042] The polymer mentioned in step 1② is polyvinylpyrrolidone (PVP);

[0043] II. Calcination:

[0044] The polymer / straw / KOH mixture was transferred to an alumina pot, which was then placed in a tube furnace. Under an argon atmosphere, the temperature was first increased from room temperature to 180°C at a rate of 3°C / min and held for 2 hours. Then, the temperature was increased to 800°C at a rate of 5°C / min and held for 2 hours. Finally, the temperature was decreased to 300°C at a rate of 5°C / min and cooled to room temperature to obtain the reaction product. The reaction product was washed once with 7% hydrochloric acid to remove KOH and other impurities, and then washed three times with deionized water. Finally, it was dried in an 80°C oven for 12 hours to obtain continuous porous carbon (SK-30).

[0045] Comparative Example 1: The preparation method of porous carbon (SK) is specifically carried out according to the following steps:

[0046] I. Preparation of straw / KOH mixture:

[0047] ① Dissolve 20g of KOH in 100mL of deionized water to obtain a KOH solution; add 10g of straw to the KOH solution and stir at 600 rpm. -1 The mixture was stirred continuously at a certain speed for 10 hours to obtain a straw / KOH solution.

[0048] ② Place the straw / KOH solution in an 80℃ forced-air drying oven to dry it into a solid, and then grind it thoroughly into granules to obtain a straw / KOH mixture;

[0049] II. Calcination:

[0050] The straw / KOH mixture was transferred to an alumina pot, which was then placed in a tube furnace. Under an argon atmosphere, the temperature was first increased from room temperature to 180°C at a rate of 3°C / min and held for 2 hours. Then, the temperature was increased to 800°C at a rate of 5°C / min and held for 2 hours. Finally, the temperature was decreased to 300°C at a rate of 5°C / min and cooled to room temperature to obtain the reaction product. The reaction product was washed once with 7% hydrochloric acid to remove KOH and other impurities, and then washed three times with deionized water. Finally, it was dried in an 80°C oven for 12 hours to obtain porous carbon (SK).

[0051] Figure 1 The images are SEM images. In the figure, a is the SEM image of straw, b is the SEM image of straw / KOH solution after drying and grinding into particles in step one of Example 1, c is the SEM image of porous carbon (SK) prepared in Comparative Example 1, and d is the SEM image of continuous framework porous carbon (SK-30) prepared in Example 1.

[0052] Figure 1(a) shows that the straw raw material has a natural layered fiber structure, which is relatively orderly arranged and has a relatively smooth surface, providing a structural basis for subsequent activation and carbonization; from Figure 1 (b) Significant changes in the material structure were observed, with localized erosion and roughening of the surface, indicating that KOH had begun to interact with the straw components; from Figure 1 (c) It was found that after chemical activation with KOH alone, the carbon skeleton was prone to local collapse or fracture during high-temperature treatment; Figure 1 (d) exhibits a highly interconnected continuous carbon framework microstructure, thereby improving the structural stability of the material and effectively reducing the tortuosity of ion transport.

[0053] Figure 2 Raman spectra of porous carbon (SK) prepared in Comparative Example 1 and continuous framework porous carbon (SK-30) prepared in Example 1;

[0054] Depend on Figure 2 It can be seen that the two materials are at approximately 1350cm -1 Approximately 1580cm -1 Distinct D-peaks and G-peaks appeared nearby. The D-peak originated from sp. 3 Hybridized carbon and structural defects; the G peak corresponds to sp in the graphite structure. 2 Hybridized carbon reflects the presence of graphitized ordered regions in the material. Furthermore, both materials exhibit characteristics around 1500 cm⁻¹. -1 A D3 peak is observed nearby, which is typically attributed to amorphous carbon components. Notably, this peak is only present at approximately 1200 cm⁻¹ in the spectrum of SK-30 material. -1 A peak I was observed nearby, which may be related to the stretching vibrations of C=C and C=C bonds in the carbon chain or the polyolefin-like structure. Calculations showed that the ID / IG values ​​of SK and SK-30 materials were 2.46 and 2.11, respectively. The relatively lower ID / IG value of SK-30 indicates that the introduction of PVP helps to increase the graphitization degree of the carbon material and improve its structural order, thereby enhancing the material's conductivity and facilitating higher capacity retention at high current densities.

[0055] Figure 3 (a) shows the isothermal adsorption / desorption curves of porous carbon (SK) prepared in Comparative Example 1 and continuous framework porous carbon (SK-30) prepared in Example 1, and (b) shows the pore size distribution of porous carbon (SK) prepared in Comparative Example 1 and continuous framework porous carbon (SK-30) prepared in Example 1.

[0056] from Figure 3(a) It can be seen that the adsorption capacity of both materials increases rapidly with increasing P / P0 in the relative pressure (P / P0) range of 0 to 1, exhibiting typical type I isotherm characteristics, indicating that the materials are rich in microporous structures; the specific surface area of ​​SK, as determined by BET testing, is 1828.68 m². 2 g -1 The specific surface area of ​​the micropores is 1760.65 m². 2 g -1 The micropore ratio is 96.28%. The surface area of ​​SK-30 is 1865.4 μm. 2 g -1 The specific surface area of ​​the micropores is 1847.99 m². 2 g -1 The micropore ratio is 99.07%.

[0057] from Figure 3 (b) It can be seen that the pore sizes of both SK and SK-30 are distributed in the range of 0.6-2 nm, further confirming their predominantly microporous pore structure, which is consistent with the isothermal analysis results. Compared with SK, SK-30 has a higher proportion of micropores and a higher specific surface area, indicating that the introduction of polymers (including PVA, PEG, PAM, PVP, etc.) optimizes the pore structure of the material, which is beneficial to improving its charge storage capacity as a supercapacitor electrode material.

[0058] Figure 4 High-resolution X-ray photoelectron absorption spectrum of continuous framework porous carbon (SK-30) prepared in Example 1;

[0059] Depend on Figure 4 It is known that the nitrogen component of SK-30 has a balanced and high content of electrochemically active nitrogen species (mainly pyrrole nitrogen N-5 and pyridine nitrogen N-6), which provides a significant contribution to the pseudocapacitance of the material and effectively improves its specific capacitance. At the same time, the material retains sufficient graphitized nitrogen (NQ), which helps to maintain good basic conductivity, thereby achieving efficient charge transport at high current densities.

[0060] Assembly of symmetrical supercapacitors:

[0061] Preparation of activated carbon electrode: The continuous framework porous carbon (SK-30), activated carbon (YP50F), and porous carbon (SK) prepared in Comparative Example 1 were used as active materials. The active materials, conductive agent (Super P), and binder (PVDF) were mixed in a ratio of 8:1:1 and ground in an agate mortar for more than 30 min to ensure that PVDF and Super P were uniformly dispersed in the activated carbon. Then NMP was added and stirred for 8 h to obtain activated carbon electrode slurry. Finally, it was coated on the surface of a 12 mm carbon cloth substrate with a glass rod and dried under vacuum overnight at 80 °C to obtain activated carbon negative electrode.

[0062] Preparation of KOH electrolyte: Add 0.67g KOH to 2mL of deionized water and stir for 6 hours to obtain KOH solution;

[0063] Symmetrical supercapacitor assembly: Based on the electrode sheets, KOH electrolyte, and glass fiber separator prepared above, the positive electrode shell, electrode material, separator, electrolyte, electrode material, gasket, spring, and negative electrode shell are sealed together in sequence using a hydraulic button cell sealing machine to obtain a dual-electrode symmetrical button supercapacitor.

[0064] Figure 5 (a) is the CV curve of the symmetrical supercapacitor assembled using the continuous framework porous carbon (SK-30) prepared in Example 1, and (b) is the CV curve of the symmetrical supercapacitor assembled using activated carbon (YP50F).

[0065] Depend on Figure 5 It can be seen that both materials exhibit approximately rectangular CV curves at different scan rates, indicating that both are dominated by double-layer capacitance behavior, possessing typical capacitance characteristics and good reversibility. Furthermore, at the same scan rate, the area enclosed by the CV curve of SK-30 is significantly larger than that of YP50F, indicating that SK-30 has a higher specific capacitance per unit mass. At the same time, at high scan rates, the CV curve of SK-30 still maintains a large response area, indicating that SK-30 has excellent capacitance retention capability, a large ion-accessible surface area, and low charge transport resistance, making it suitable for high-rate charge and discharge applications.

[0066] Figure 6 The rate performance of symmetrical supercapacitors assembled using porous carbon (SK) prepared in Comparative Example 1, continuous framework porous carbon (SK-30) prepared in Example 1, and activated carbon (YP50F) are shown in the figure.

[0067] Depend on Figure 6 It can be seen that SK-30 retains 60.32% of its capacity at 50 A / g, which is significantly higher than SK (41.3%) and YP50F (36.35%). This indicates that SK-30 material has excellent charge transport dynamics and structural stability, and can maintain a high capacitance under high rate conditions.

[0068] Figure 7 Cyclic stability diagrams of symmetrical supercapacitors assembled using continuous framework porous carbon (SK-30) and activated carbon (YP50F) prepared in Example 1, respectively.

[0069] from Figure 7The results show that the device assembled based on SK-30 maintained a capacity retention of 100.25% and a specific capacitance of 31.1 F / g after 260,000 cycles at a high current density of 20 A / g, demonstrating excellent electrochemical stability and cycle life. In contrast, the device assembled based on YP50F only maintained a capacity retention of 92.02% and a specific capacitance of 12.04 F / g after 50,000 cycles. This indicates that SK-30 material has significantly better cycle durability and capacitance retention than commercial activated carbon YP50F, making it suitable for high-power energy storage applications with stringent cycle life requirements.

Claims

1. A method for preparing continuous framework porous carbon, characterized in that... The preparation method is specifically carried out according to the following steps: I. Preparation of a polymer / straw / KOH mixture: ① Dissolve KOH in deionized water to obtain a KOH solution; add straw to the KOH solution and stir continuously to obtain a straw / KOH solution; ② Dry the straw / KOH solution, grind it into granules, and then mix it evenly with the polymer powder to obtain a polymer / straw / KOH mixture; II. Calcination: The polymer / straw / KOH mixture was transferred to an alumina pot, which was then placed in a tube furnace. Under an argon atmosphere, the temperature was first raised to 180°C and held, then raised to 800°C and held, then lowered to 300°C, and finally cooled to room temperature to obtain the reaction product. The reaction product was then washed with hydrochloric acid and deionized water in sequence and dried to obtain continuous porous carbon framework.

2. The method for preparing continuous framework porous carbon according to claim 1, characterized in that... In step 1①, the mass ratio of KOH to deionized water in the KOH solution is (15g~25g):100mL; the straw mentioned in step 1① is corn straw with a mesh size of 5 mesh.

3. The method for preparing continuous framework porous carbon according to claim 1, characterized in that... The mass ratio of straw to KOH in the straw / KOH solution in step 1 is 1:(1.5-2.5); the speed of the continuous stirring in step 1 is 600 r / min -1 800 r / min -1 for 8-12 h.

4. The method for preparing continuous framework porous carbon according to claim 1, characterized in that... The polymer mentioned in step 1② is polyvinyl alcohol, polyethylene glycol, polyacrylamide, or polyvinylpyrrolidone.

5. The method for preparing continuous framework porous carbon according to claim 1, characterized in that... In step 1②, the mass ratio of polymer powder to straw in the polymer / straw / KOH mixture is (2~4):10; the drying temperature in step 1② is 80℃.

6. The method for preparing continuous framework porous carbon according to claim 1, characterized in that... The heating rate in step two is 3℃ / min to 5℃ / min; the cooling rate in step two is 3℃ / min to 5℃ / min.

7. The method for preparing continuous framework porous carbon according to claim 1, characterized in that... The heat preservation time mentioned in step two is 2 to 3 hours.

8. The method for preparing continuous framework porous carbon according to claim 1, characterized in that... In step two, the reaction product is washed with hydrochloric acid 1 to 2 times to remove KOH and other impurities, and then washed with deionized water 2 to 4 times.

9. The method for preparing continuous framework porous carbon according to claim 1, characterized in that... The mass fraction of hydrochloric acid mentioned in step two is 5%~7%; the drying temperature mentioned in step two is 60℃~80℃, and the drying time is 10h~12h.

10. The application of the continuous framework porous carbon prepared by the preparation method according to any one of claims 1 to 9, characterized in that... The continuous framework porous carbon is used as an electrode for a supercapacitor.

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