A biomass-based carbon nanotube material for supercapacitor electrodes and a method of preparing the same

Abstract

The application discloses a biomass-based carbon nanotube material for supercapacitor electrodes and a preparation method thereof. The biomass-based carbon nanotube material is prepared by using corn starch, pine wood, fungus and the like as raw materials, removing part of light components in the raw materials by a hydrothermal carbonization method to increase the carbon content, and then generating the carbon nanotube material by in-situ pyrolysis by using a one-step activation method. The carbon nanotube material prepared by the method has excellent conductivity and capacitance in supercapacitor tests. Moreover, the conductivity of the obtained carbon nanotube material can be adjusted by synthesis conditions, so that the electrochemical performance of the supercapacitor is improved. In addition, the preparation method is simple in process and has obvious cost advantages, and the obtained biomass-based carbon nanotube material is expected to be widely applied in the field of supercapacitor electrode materials.

Description

Technical Field

[0001] This invention belongs to the field of biomass resource utilization and new energy materials, specifically relating to a biomass-based carbon nanotube material for supercapacitor electrodes and its preparation method. Background Technology

[0002] Supercapacitors are a new type of energy storage device that falls between traditional capacitors and batteries. They feature high power density, long cycle life, and fast charge / discharge speeds, making them suitable for various environments and thus widely used and researched in recent years. Electrode materials are crucial to the performance of supercapacitors. Among various electrode materials, activated carbon has become the mainstream due to its advantages such as large specific surface area, tunable pore size, good conductivity, and low cost. A large specific surface area and a good pore structure can provide abundant active sites, thereby improving the specific capacitance of activated carbon materials; however, a well-developed pore structure can disrupt the graphitization structure of the material, leading to a decrease in conductivity. Excellent conductivity helps reduce internal resistance, thereby increasing power density, and a lower internal resistance can also reduce internal heat generation and extend the cycle life of the supercapacitor. To address this, carbon materials such as acetylene black, graphene, and carbon nanotubes are often added during electrode preparation to improve conductivity, but this undoubtedly increases the cost of the electrode and the preparation process. Therefore, developing an electrode material that balances excellent conductivity and capacitance is essential for the widespread application of supercapacitors.

[0003] Patent CN 109887760 A discloses "A Highly Conductive Activated Carbon and Its Preparation and Uses," which uses polymer pre-oxidized filaments as templates. After heating to 500-1200℃ in a carrier gas and maintaining this temperature for a period of time, water or CO2 is introduced for activation and pore formation. The disadvantages of this method are high raw material costs and demanding process conditions. CN 109850892 A discloses "An Industrialized Preparation of Highly Conductive Coconut Shell Activated Carbon for Supercapacitors through Two-Step Activation," which obtains porous activated carbon through physical activation, followed by multiple reactions in a CO2 atmosphere to remove surface functional groups, then acid washing and further functional group removal under an inert atmosphere. While this method reduces resistivity, the process is overly complex. In conclusion, finding a simple, low-cost, and practically applicable method for preparing supercapacitor electrode materials is of great significance. Summary of the Invention

[0004] This invention addresses the problem of poor conductivity of activated carbon used in supercapacitors, necessitating the use of conductive additives, by providing a method for preparing biomass-based carbon nanotubes for supercapacitor electrodes. The method of this invention generates carbon nanotubes in situ simultaneously with the activation and pore-forming process without the addition of external transition metal catalysts, ultimately yielding biomass-based carbon nanotube materials with excellent pore structure and conductivity. Furthermore, the hydrothermal carbonization technology employed in this invention also reduces energy consumption and environmental pollution.

[0005] The present invention adopts the following technical solution:

[0006] A method for preparing biomass-based carbon nanotube materials for supercapacitor electrodes includes the following steps:

[0007] (1) Grind the biomass raw material to 10-400 mesh, take the ground biomass and alkaline solution into a hydrothermal reactor according to the set mass ratio, mix evenly and then carry out hydrothermal carbonization treatment. After the reaction is completed, cool to room temperature and take out, dry and grind to obtain the precursor.

[0008] (2) Place the precursor and activator obtained in step (1) in a beaker according to the set mass ratio, add deionized water and stir thoroughly, dry, and then grind thoroughly.

[0009] (3) Place the mixture obtained in step (2) in a tube furnace, remove the air in the tube, and heat it at a certain heating rate to carry out the pyrolysis reaction. After the reaction is completed, cool it to room temperature to obtain the crude product containing carbon nanotubes.

[0010] (4) The crude product obtained in step (3) is purified by acid washing, filtered and washed with water until neutral, and dried to obtain the pure target product.

[0011] Furthermore, in step (1), the biomass raw material is wood ear fungus and also includes one or more of corn starch and pine wood, and the mass percentage of wood ear fungus in the raw material is at least 5%.

[0012] Further, in step (1), the alkaline solution is a sodium carbonate or sodium bicarbonate solution with pH = 7 to 11, and the mass ratio of biomass to alkaline solution is 1:1-10.

[0013] Furthermore, in step (1), the temperature of the hydrothermal carbonization treatment is 150-300℃, and the time of the hydrothermal carbonization treatment is 2-6h.

[0014] Further, in step (2), the activator is one or more of KOH, K2CO3, and C6H5K3O7.

[0015] Further, in step (2), the mass ratio of the precursor to the activator is 1:1-4.

[0016] Furthermore, in step (2), the stirring time is 1 to 18 hours.

[0017] Furthermore, in step (2), the drying temperature is 50-150℃ and the drying time is 12-24h.

[0018] Furthermore, in step (3), argon gas is introduced to remove the air from the tube.

[0019] Further, in step (3), the pyrolysis reaction is heated to 500-1000℃ at a heating rate of 5-20℃ / min and held for 1-3h.

[0020] The biomass-based carbon nanotube material obtained by the above preparation method has excellent conductivity and capacitance, and has broad application prospects in the field of supercapacitors.

[0021] The preparation of supercapacitor electrodes using the above-mentioned biomass-based carbon nanotube materials specifically includes the following steps:

[0022] The biomass-based carbon nanotube material and the binder polyvinylidene fluoride (PVDF) were mixed at a mass ratio of 90:10 and thoroughly ground in an agate mortar to obtain a mixture. An appropriate amount of solvent N-methyl-2-pyrrolidone (NMP) was added and mixed evenly to form a slurry. The slurry was dried in a vacuum drying oven at 100°C until anhydrous and then cooled to room temperature to obtain the electrode material.

[0023] The principle of this invention is as follows:

[0024] This invention uses inexpensive biomass such as wood ear mushrooms, corn starch, and pine wood as raw materials. First, during hydrothermal carbonization, the light components such as hemicellulose, cellulose, and lignin in the raw materials are decomposed into smaller fragments. Then, dehydration and decarboxylation reactions occur, increasing the C / H and C / O ratios and improving the carbon content of the raw materials. Alkaline substances promote the hydrothermal carbonization reaction. In the first-step activation process, KOH, K₂CO₃, and other potassium-containing substances are used... +Alkaline etching activates and pores the precursor, resulting in porous activated carbon materials. Simultaneously, due to the abundant iron content in wood ear mushrooms, free carbon species are converted into porous carbon nanotubes during high-temperature pyrolysis under the synergistic catalysis of Fe and K. The porous structure facilitates charge storage, while carbon nanotubes significantly improve the material's conductivity. In biomass-based activated carbon / carbon nanotube materials, activated carbon is the main component, and charge storage primarily relies on its large specific surface area and pore structure. However, as the pore structure develops, the degree of graphitization decreases, leading to reduced conductivity. This typically requires the subsequent addition of conductive additives such as acetylene black to improve conductivity. In-situ generated carbon nanotubes can effectively enhance the material's conductivity, achieving a balance between pore structure and conductivity, thus replacing or reducing the need for conductive additives.

[0025] Compared with existing technologies, the beneficial effects of this invention are as follows:

[0026] 1. This invention balances the pore structure and electrical conductivity of carbon materials, significantly improving the electrochemical performance of the material. The total pore volume of the material is approximately 0.6–0.9 cm³. 3 / g, with a specific surface area of ​​approximately 1300–2000 m². 2 / g, at a current density of 1A / g, the specific capacitance reaches 270~340F / g.

[0027] 2. This invention prepares biomass-based carbon nanotube supercapacitor electrode materials based on hydrothermal carbonization and one-step activation. The raw materials are inexpensive, the process equipment is simple, the environmental impact is small, energy consumption is saved, and it meets practical requirements and is suitable for industrial production.

[0028] 3. This invention prepares high aspect ratio multi-walled carbon nanotubes (diameter approximately 20–50 nm, length approximately 20–40 μm) in situ at extremely low cost, ensuring that the material has good conductivity. It can replace or reduce the addition of conductive agents such as acetylene black and graphite, reducing the cost of subsequent processes. Furthermore, the unique flexibility of the carbon nanotube structure helps to cope with volume changes during electrochemical reactions. Attached Figure Description

[0029] Figure 1 This is a flowchart illustrating the preparation process of the biomass-based carbon nanotube material of the present invention.

[0030] Figure 2 This is a scanning electron microscope (SEM) image of the biomass-based carbon nanotube material obtained in Example 1.

[0031] Figure 3 The image shows the powder X-ray diffraction (XRD) spectrum of the biomass-based carbon nanotube material obtained in Example 1.

[0032] Figure 4The constant current charge-discharge curves of the supercapacitor electrode assembled based on the biomass-based carbon nanotube material obtained in Example 1 are shown at different current densities.

[0033] Figure 5 The impedance Nyquist plots are for the supercapacitor electrodes assembled based on the materials obtained in Example 1, Comparative Example 1, and Comparative Example 2. Detailed Implementation

[0034] The present invention will be described below with reference to specific embodiments. Those skilled in the art will understand that these embodiments are for illustrative purposes only and do not limit the scope of the invention in any way.

[0035] Unless otherwise specified, the experimental methods used in the following examples are conventional methods. Unless otherwise specified, all reagents and materials used in the following examples are commercially available products.

[0036] Figure 1 The flowchart of the process for preparing biomass-based carbon nanotube supercapacitor electrode material in this embodiment of the invention is as follows: using biomass such as corn starch, pine wood and wood ear fungus as raw materials, a precursor is obtained after hydrothermal carbonization with alkaline solution; then the precursor is impregnated and stirred in an activator to activate and create pores, resulting in porous activated carbon material; then, porous carbon nanotube crude product is generated in situ through high-temperature pyrolysis, and finally, acid washing and purification are performed to obtain the pure target product, namely AC / CNTs.

[0037] Example 1

[0038] according to Figure 1 The process shown is for preparing a biomass-based carbon nanotube material and assembling a supercapacitor electrode. The specific operation steps are as follows:

[0039] Dry pine wood and wood ear mushroom raw materials were mixed, pulverized, and ground to 100 mesh in a mass ratio of 3:1. A mixture of the raw materials in a mass ratio of 1:4 and sodium bicarbonate solution (pH=11) was then subjected to hydrothermal carbonization in a hydrothermal reactor at 300℃ for 4 hours. After the reaction was complete, the solid product was cooled to room temperature, dried, and ground to obtain the precursor, with a solid yield of 67%. The precursor and KOH were placed in a beaker at a mass ratio of 1:2, and 60 mL of deionized water was added. The mixture was stirred thoroughly for 12 hours, then dried overnight in an oven at 100℃, followed by thorough grinding. The mixture was placed in a tube furnace, and after purging the air from the tubes, the temperature was increased to 850℃ at a rate of 5℃ / min and reacted for 1.5 hours. After the reaction was complete, the mixture was cooled to room temperature. The crude product was purified by acid washing, filtered, washed with water until neutral, and dried to obtain pure biomass-based carbon nanotube materials.

[0040] Figure 2The image shown is a scanning electron microscope (SEM) image of the biomass-based carbon nanotube material obtained in Example 1. It can be seen that the biomass-based carbon nanotubes prepared by the present invention have good morphology, with a diameter of about 20-50 nm and a length of about 20-40 μm. Figure 3 The powder X-ray diffraction (XRD) spectrum of the biomass-based carbon nanotube material obtained in Example 1 shows that the carbon (002) peak of this material is significantly sharper than that of Comparative Example 1, indicating that the formation of carbon nanotubes significantly improves the graphitization degree of the material and ensures that the material has excellent electrical conductivity.

[0041] The prepared biomass-based carbon nanotube material and the binder polyvinylidene fluoride (PVDF) were mixed at a mass ratio of 90:10 and thoroughly ground in an agate mortar to obtain a mixture. An appropriate amount of N-methyl-2-pyrrolidone (NMP) was added and mixed evenly to form a slurry. This slurry was dried in a vacuum drying oven at 100℃ until anhydrous, and then cooled to room temperature to obtain the electrode material. The obtained electrode material was uniformly coated onto the front end of a nickel foam as the working electrode. A three-electrode test was performed using platinum as the counter electrode, Hg / HgO as the reference electrode, and 6 mol / L KOH solution as the electrolyte.

[0042] Figure 4 The constant current charge-discharge curves of the electrode material at different current densities are shown. The results show that at a current density of 1 A / g, its specific capacity is as high as 337 F / g. Figure 5 The impedance Nyquist plots are shown for the supercapacitor electrodes assembled based on the materials obtained in Example 1, Comparative Example 1, and Comparative Example 2. The results show that the biomass-based carbon nanotube material prepared in this invention has lower internal resistance and higher conductivity when used as an electrode.

[0043] Example 2

[0044] according to Figure 1 The process shown is for preparing a biomass-based carbon nanotube material and assembling a supercapacitor electrode. The specific operation steps are as follows:

[0045] Dry corn starch and wood ear mushroom raw materials were mixed, pulverized, and ground to 100 mesh at a mass ratio of 3:1. A mixture of the raw materials at a mass ratio of 1:4 and sodium bicarbonate solution (pH=11) was then subjected to hydrothermal carbonization in a hydrothermal reactor at a temperature of 280℃ for 3 hours. After the reaction was complete, the solid product was cooled to room temperature, dried, and ground to obtain the precursor, with a solid yield of 61%. The precursor and KOH were placed in a beaker at a mass ratio of 1:1.5, and 60 mL of deionized water was added. The mixture was stirred thoroughly for 12 hours, then dried overnight in an oven at 110℃, followed by thorough grinding. The mixture was placed in a tube furnace, and after purging the air from the tubes, the temperature was increased to 900℃ at a rate of 10℃ / min and reacted for 2 hours. After the reaction was complete, the mixture was cooled to room temperature. The crude product was purified by acid washing, filtered, washed with water until neutral, and dried to obtain pure biomass-based carbon nanotube materials.

[0046] The prepared biomass-based carbon nanotube material and the binder polyvinylidene fluoride (PVDF) were mixed at a mass ratio of 90:10 and thoroughly ground in an agate mortar to obtain a mixture. An appropriate amount of N-methyl-2-pyrrolidone (NMP) was added and mixed evenly to form a slurry. This slurry was dried in a vacuum drying oven at 100℃ until anhydrous, and then cooled to room temperature to obtain the electrode material. The obtained electrode material was uniformly coated onto the front end of a nickel foam as the working electrode. A three-electrode test was conducted using platinum as the counter electrode, Hg / HgO as the reference electrode, and 6 mol / L KOH solution as the electrolyte. Charge-discharge test results showed that at a current density of 1 A / g, its specific capacity reached as high as 270 F / g.

[0047] Example 3

[0048] according to Figure 1 The process shown is for preparing a biomass-based carbon nanotube material and assembling a supercapacitor electrode. The specific operation steps are as follows:

[0049] Dry pine wood, corn starch, and wood ear mushroom raw materials in a mass ratio of 1.5:1.5:1 were mixed, pulverized, and ground to 100 mesh. A mass ratio of 1:5 of the raw materials and sodium bicarbonate solution (pH=11) was then subjected to hydrothermal carbonization in a hydrothermal reactor at 280℃ for 6 hours. After the reaction was complete, the solid product was cooled to room temperature, dried, and ground to obtain the precursor, with a solid yield of 58%. The precursor and KOH were placed in a beaker at a mass ratio of 1:3, and 60 mL of deionized water was added. The mixture was stirred thoroughly for 12 hours, then dried overnight in an oven at 100℃, followed by thorough grinding. The mixture was placed in a tube furnace, and after removing air from the tubes, the temperature was increased to 900℃ at a rate of 10℃ / min and reacted for 2 hours. After the reaction was complete, the mixture was cooled to room temperature. The crude product was purified by acid washing, then filtered, washed with water until neutral, and dried to obtain pure biomass-based carbon nanotube materials.

[0050] The prepared biomass-based carbon nanotube material and the binder polyvinylidene fluoride (PVDF) were mixed at a mass ratio of 90:10 and thoroughly ground in an agate mortar to obtain a mixture. An appropriate amount of N-methyl-2-pyrrolidone (NMP) was added and mixed evenly to form a slurry. This slurry was dried in a vacuum drying oven at 100℃ until anhydrous, and then cooled to room temperature to obtain the electrode material. The obtained electrode material was uniformly coated onto the front end of a nickel foam as the working electrode. A three-electrode test was conducted using platinum as the counter electrode, Hg / HgO as the reference electrode, and 6 mol / L KOH solution as the electrolyte. Charge-discharge test results showed that at a current density of 1 A / g, its specific capacity reached as high as 285 F / g.

[0051] Comparative Example 1

[0052] The specific steps for preparing biomass-based activated carbon materials using the hydrothermal method and assembling supercapacitor electrodes are as follows:

[0053] Dry pine wood and wood ear mushroom raw materials were mixed, pulverized, and ground to 100 mesh at a mass ratio of 4:0. A mixture of raw materials at a mass ratio of 1:4 and sodium bicarbonate solution (pH=11) was then subjected to hydrothermal carbonization in a hydrothermal reactor at a temperature of 300℃ for 4 hours. After the reaction was complete, the solid product was cooled to room temperature, dried, and ground to obtain the precursor, with a solid yield of 67%. The precursor and KOH were placed in a beaker at a mass ratio of 1:2, and 60 mL of deionized water was added. The mixture was stirred thoroughly for 12 hours, then dried overnight in an oven at 100℃, followed by thorough grinding. The mixture was placed in a tube furnace, and after purging the air from the tubes, the temperature was increased to 850℃ at a rate of 5℃ / min and reacted for 1.5 hours. After the reaction was complete, the mixture was cooled to room temperature. The crude product was purified by acid washing, filtered, washed with water until neutral, and dried to obtain pure biomass-based activated carbon material, denoted as Comparative Example 1-AC.

[0054] The prepared biomass-based activated carbon material, conductive carbon black, and binder polyvinylidene fluoride (PVDF) were mixed at a mass ratio of 80:10:10 and thoroughly ground in an agate mortar to obtain a mixture. An appropriate amount of N-methyl-2-pyrrolidone (NMP) was added and mixed evenly to form a slurry. This slurry was dried in a vacuum drying oven at 100°C until anhydrous, and then cooled to room temperature to obtain the electrode material. The obtained electrode material was uniformly coated onto the front end of a nickel foam as the working electrode. A three-electrode test was conducted using platinum as the counter electrode, Hg / HgO as the reference electrode, and 6 mol / L KOH solution as the electrolyte. Charge-discharge test results showed that at a current density of 1 A / g, its specific capacity was only 218 F / g, far lower than the specific capacity of 337 F / g in Example 1. This is because the introduction of wood ear fungus in Example 1 promoted the in-situ hydrothermal growth of carbon nanotubes from biomass, effectively enhancing their conductivity and achieving an effective balance between conductivity and pore structure; at the same time, the generated carbon nanotubes can replace or reduce the addition of conductive agents such as acetylene black and graphite, reducing the cost of subsequent processes.

[0055] Comparative Example 2

[0056] The biomass-based activated carbon material was prepared according to the conventional "carbonization-activation two-step method" and then assembled into a supercapacitor electrode. The specific operation steps are as follows:

[0057] Dry pine wood and wood ear mushroom raw materials in a mass ratio of 3:1 were mixed, crushed, and ground to 100 mesh. The mixture was then heated to 500℃ in a tube furnace at a rate of 5℃ / min and carbonized for 1.5 h to obtain carbonized material. After cooling, the carbonized material and KOH in a mass ratio of 1:2 were placed in a beaker, and 60 mL of deionized water was added. The mixture was stirred thoroughly for 12 h and then dried overnight in an oven at 100℃. Subsequently, the mixture was ground thoroughly. The mixture was placed in a tube furnace, and after the air inside the tube was purged, the temperature was increased to 850℃ at a rate of 5℃ / min and reacted for 1.5 h. After the reaction was completed, the mixture was cooled to room temperature. The crude product was purified by acid washing, then filtered and washed with water until neutral. After drying, pure biomass-based activated carbon material was obtained, denoted as Comparative Example 2-AC.

[0058] The prepared biomass-based activated carbon material, conductive carbon black, and binder polyvinylidene fluoride (PVDF) were mixed at a mass ratio of 80:10:10 and thoroughly ground in an agate mortar to obtain a mixture. An appropriate amount of N-methyl-2-pyrrolidone (NMP) was added and mixed evenly to form a slurry. This slurry was dried in a vacuum drying oven at 100°C until anhydrous, and then cooled to room temperature to obtain the electrode material. The obtained electrode material was uniformly coated onto the front end of a nickel foam as the working electrode. A three-electrode test was conducted using platinum as the counter electrode, Hg / HgO as the reference electrode, and 6 mol / L KOH solution as the electrolyte. Charge-discharge test results showed that at a current density of 1 A / g, its specific capacity was only 185 F / g, far lower than the specific capacity of 337 F / g in Example 1. This is because the hydrothermal carbonization method used in Example 1 decomposes the light components such as hemicellulose, cellulose, and lignin in the raw material during the hydrothermal carbonization process, generating smaller fragments. These fragments then undergo dehydration and decarboxylation reactions, increasing the C / H and C / O ratios and thus the carbon content of the raw material. The alkaline substances in this process promote the hydrothermal carbonization reaction. In the one-step activation process, the precursor is activated and porous by the etching effect of KOH, resulting in porous activated carbon material. This is beneficial for the subsequent high-temperature pyrolysis process, facilitates charge storage, and enhances the material's conductivity.

Claims

1. A method for preparing biomass-based carbon nanotube materials for supercapacitor electrodes, characterized in that, Includes the following steps: (1) The biomass raw material is crushed and ground. The ground biomass and alkaline solution are placed in a hydrothermal reactor according to the set mass ratio. After mixing evenly, hydrothermal carbonization is carried out. After the reaction is completed, the mixture is cooled to room temperature, dried, and ground to obtain the precursor. (2) Place the precursor and activator obtained in step (1) in a beaker according to the set mass ratio, add deionized water and stir thoroughly, dry, and then grind thoroughly; (3) Place the mixture obtained in step (2) in a tube furnace, remove the air from the tube, raise the temperature to carry out the pyrolysis reaction, and cool to room temperature after the reaction is completed to obtain the crude product containing carbon nanotubes. (4) The crude product obtained in step (3) is purified by acid washing, filtered and washed with water until neutral, and dried to obtain the pure target product; In step (1), the biomass raw material is wood ear fungus and also includes one or more of corn starch and pine wood, with wood ear fungus accounting for 25% of the raw material by mass; In step (1), the alkaline solution is a sodium carbonate or sodium bicarbonate solution with a pH of 7 to 11, and the mass ratio of biomass raw material to alkaline solution is 1:1-10.

2. The method for preparing biomass-based carbon nanotube material for supercapacitor electrodes according to claim 1, characterized in that, In step (1), the temperature of the hydrothermal carbonization treatment is 150-300℃, and the time of the hydrothermal carbonization treatment is 2-6 h.

3. The method for preparing biomass-based carbon nanotube material for supercapacitor electrodes according to claim 1, characterized in that, In step (2), the activator is one or more of KOH, K2CO3, and C6H5K3O7.

4. The method for preparing biomass-based carbon nanotube material for supercapacitor electrodes according to claim 1, characterized in that, In step (2), the mass ratio of the precursor to the activator is 1:1-4.

5. The method for preparing biomass-based carbon nanotube material for supercapacitor electrodes according to claim 1, characterized in that, In step (2), the drying temperature is 50-150℃ and the drying time is 12-24 h.

6. The method for preparing biomass-based carbon nanotube material for supercapacitor electrodes according to claim 1, characterized in that, In step (3), the pyrolysis reaction is heated to 500-1000℃ at a heating rate of 5-20℃ / min and held for 1-3 h.

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