Method for preparing lignin-based supercapacitor carbon through combination of supercritical alcohol-alkali pretreatment and activation

Abstract

The invention aims to provide a method for preparing lignin-based supercapacitor carbon through combination of supercritical alcohol-alkali pretreatment and activation, and belongs to the technical field of porous carbon material preparation, the method comprises the following steps: firstly, mixing lignin, an alkali catalyst and an alcohol solvent, and carrying out pretreatment in a reaction kettle under a supercritical condition to obtain a precursor; then mixing the precursor with alkali again, and performing high-temperature activation in an inert atmosphere; and finally, carrying out acid pickling, water washing and drying on the activated product to obtain the final supercapacitor carbon material. According to the invention, through supercritical alcohol-alkali pretreatment, the stubborn structure of lignin is effectively destroyed, and activation sites are introduced, so that the lignin is easier to form a developed hierarchical pore structure in the subsequent activation process. The supercapacitor carbon prepared by the method has high specific surface area and optimized pore size distribution, and shows high specific capacitance and excellent rate capability when being used as a supercapacitor electrode material.

Description

Technical Field

[0001] This invention belongs to the field of porous carbon material preparation technology, specifically relating to a method for preparing lignin-based supercapacitor carbon by supercritical alcohol-alkali pretreatment combined with activation. Background Technology

[0002] Supercapacitors have attracted much attention in the field of energy storage due to their advantages such as high power density, long cycle life, and fast charge and discharge speed. Their performance depends primarily on the electrode material, and porous carbon is the most commonly used electrode material due to its high specific surface area, good conductivity, and chemical stability.

[0003] In the preparation of existing lignin-based supercapacitor electrode materials, the one-step KOH activation method has significant drawbacks: under high temperature (1000℃) conditions, lignin is prone to excessive carbonization, the carbon material skeleton collapses, the pore structure becomes disordered and mesopores are scarce, affecting ion transport and charge storage; the strong corrosiveness of KOH triggers non-selective reactions, destroys the graphitized structure, reduces conductivity, and the residual alkali metal ions are difficult to remove, easily causing secondary corrosion of the material; at the same time, it has strict requirements for the purity of raw materials, and the alkaline waste generated requires additional environmental protection treatment, which is not in line with the trend of green and low-carbon development.

[0004] Lignin is the second most abundant natural polymer in the world. It is a byproduct of the paper industry and biomass refining, and is mostly incinerated, resulting in low value. Converting it into high-value-added supercapacitor char has significant economic and environmental benefits. However, the traditional KOH activation method for treating lignin suffers from the aforementioned common problems.

[0005] Supercritical fluid technology has shown unique advantages in biomass conversion, with its powerful dissolution and mass transfer capabilities effectively depolymerizing biomass macromolecules. However, using supercritical technology alone to prepare lignin-based supercapacitor electrode materials has significant limitations: while it can utilize the high diffusivity of supercritical fluids to improve the mass transfer efficiency of lignin, it lacks a targeted activation mechanism, making it difficult to effectively control the pore structure of carbon materials (leading to problems such as excessively high micropore ratio), resulting in limited specific surface area and charge storage sites; at the same time, it cannot directionally modify the chemical structure of lignin, resulting in low graphitization and poor conductivity of carbon materials, and its effect on impurity removal and cross-linking reconstruction of lignin is limited when used alone, ultimately restricting the electrochemical performance of the electrode materials and failing to meet the core requirements of supercapacitors for high specific capacitance and high cycle stability. Summary of the Invention

[0006] This invention addresses the shortcomings of existing lignin-based supercapacitor electrode material preparation technologies, such as difficulty in controlling pore structure and poor conductivity, which exist in one-step activation with single KOH, supercritical technology, and high-temperature activation. By synergistically combining supercritical pretreatment with high-temperature activation and leveraging the directional modification effect of the alcohol-alkali system, this invention provides a preparation method that is process-controllable, energy-efficient, and environmentally friendly. Ultimately, it yields lignin-based supercapacitor electrode materials with high specific surface area, reasonable hierarchical pore distribution, good conductivity, and excellent electrochemical performance, meeting the core performance requirements of electrode materials for the industrial application of supercapacitors.

[0007] The present invention adopts the following technical solution: A method for preparing lignin-based supercapacitor carbon by supercritical alcohol-alkali pretreatment combined with activation includes the following steps: S1. Supercritical pretreatment: Lignin, alkaline catalyst and alcohol solvent are mixed in proportion, placed in a reaction vessel, and reacted under supercritical conditions. After the reaction is completed, the precursor is obtained by washing with water and drying. S2, High-temperature activation: The precursor obtained in S1 is mixed with the base in a certain proportion and activated at high temperature under a nitrogen atmosphere; S3. Post-processing: The product activated by S2 is sequentially acid-washed and water-washed until neutral, and then dried to obtain lignin-based supercapacitor carbon material.

[0008] Furthermore, the alkaline catalyst described in S1 includes one or two of potassium hydroxide, potassium carbonate, potassium bicarbonate, sodium carbonate, and sodium bicarbonate. The alcohol solvents include one or more of methanol, ethanol, and isopropanol.

[0009] Furthermore, the ratio of lignin, alkali catalyst, and alcohol solvent in S1 is 10g:2.5g:25mL.

[0010] Furthermore, the supercritical reaction conditions described in S1 are a reaction temperature of 240-330℃ and a reaction time of 0.5-4h.

[0011] Further, the mass ratio of the precursor to the base in S2 is 1:0.5-3; the base includes one or two of potassium hydroxide, potassium carbonate, potassium bicarbonate, sodium carbonate, and sodium bicarbonate.

[0012] Furthermore, the high-temperature activation temperature described in S2 is 600-800℃, and the activation time is 0.5-4h.

[0013] Furthermore, the pickling described in S3 involves soaking and washing with a hydrochloric acid solution with a concentration of 1-3 mol / L for 12-36 hours.

[0014] In this invention, lignin, an alkaline solvent, and an alcohol solvent are mixed in a specific ratio and placed in a high-pressure reactor. The system is heated and pressurized to reach a supercritical state in the alcohol solvent, where the reaction proceeds. During this process, the alkali acts as a catalyst, selectively breaking unstable bonds such as hydrogen bonds between lignin molecules, making lignin more easily permeable by the supercritical fluid. Simultaneously, through synergy with the supercritical alcohol, the initial degradation degree of lignin can be controlled, preventing disordered carbonization, preserving carbon skeleton precursors conducive to subsequent activation, and providing more reaction sites.

[0015] During high-temperature activation, alkali acts as an activator, reacting with the carbon skeleton of lignin after carbonization to produce gases such as CO and CO2. The gas escapes to achieve "etching", forming a microporous-mesoporous hierarchical pore structure.

[0016] The combination of supercritical pretreatment and high-temperature activation in the preparation of lignin-based supercapacitor electrode materials can exert significant synergistic advantages: supercritical fluids can efficiently penetrate the interior of lignin, breaking intermolecular forces, enhancing reactivity, and initially regulating the structure, laying a uniform microscopic foundation for subsequent activation; high-temperature activation can directionally etch mesoporous-microporous hierarchical channels under low energy consumption and mild conditions, while optimizing the graphitization degree of carbon materials, ultimately obtaining electrode materials with high specific surface area, good conductivity, and ion transport efficiency, effectively making up for the shortcomings of single methods in pore structure regulation, energy consumption control, and performance improvement.

[0017] The beneficial effects of this invention are as follows: 1. Synergistic effect: The core function of supercritical pretreatment is to break down the dense structure of lignin by utilizing high temperature, high pressure, and an alcohol-alkali environment, and to introduce pores and functional groups into it. This process aims to create a large number of readily reactive active sites for subsequent activation, rather than simply dissolving it.

[0018] 2. Optimize the final product structure: This "pre-activated" precursor has a more uniform and efficient etching reaction during high-temperature activation, which is conducive to the formation of a more developed hierarchical porous structure (micropores with high specific surface area and mesopores that facilitate ion transport), thereby improving the electrochemical performance of the material.

[0019] 3. Process innovation: The combination of supercritical technology as a pretreatment step with chemical activation is a novel approach that fully utilizes the advantages of both technologies and overcomes the limitations of using a single technology. Attached Figure Description

[0020] Figure 1 This is a process flow diagram of the present invention. Detailed Implementation

[0021] To make the objectives, technical solutions, and advantages of this invention clearer, specific embodiments of the invention are described in detail, but are not limited thereto. Unless otherwise specified, the raw materials used in the embodiments are all commercially available products; and unless otherwise specified, the methods used are methods commonly used in the art.

[0022] Example 1 A method for preparing lignin-based supercapacitor carbon by supercritical alcohol-alkali pretreatment combined with activation includes the following steps: S1. Supercritical Pretreatment: Weigh 10g lignin, 2.5g KOH, and 25mL methanol into a beaker and mix thoroughly. Transfer the mixture to a 100mL high-pressure reactor, seal it, and heat it to 240℃. React at this temperature for 2 hours. After the reaction is complete, cool, remove the solid product, wash it with deionized water until neutral, and dry it for 12 hours to obtain the pretreated precursor. S2. High-temperature activation: The dried precursor and KOH are mixed and ground at a mass ratio of 1:1. The mixture is placed in a ceramic boat and then placed in a tube furnace. Under a nitrogen atmosphere, the temperature is increased to 700°C at a rate of 5°C / min and held at that temperature for 1 hour for activation. S3. Post-treatment: After the activated product is allowed to cool naturally, it is soaked in 2 mol / L HCl solution for 24 hours, and then washed with a large amount of deionized water until the filtrate is neutral. Finally, the solid product is dried for 12 hours to obtain the supercapacitor carbon sample.

[0023] Characterization and Testing: The sample was characterized and subjected to electrochemical tests. Its BET specific surface area was 784.57 m². 2 / g; in KOH electrolyte, at a current density of 1 A / g, its specific capacitance is 224.1 F / g; the solid yield is 19.87%, and the ash content is 7.74%.

[0024] Example 2 A method for preparing lignin-based supercapacitor carbon by supercritical alcohol-alkali pretreatment combined with activation includes the following steps: S1. Supercritical Pretreatment: Weigh 10g lignin, 2.5g KOH, and 25mL methanol into a beaker and mix thoroughly. Transfer the mixture to a 100mL high-pressure reactor, seal it, and heat it to 300℃. React at this temperature for 2 hours. After the reaction is complete, cool, remove the solid product, wash it with deionized water until neutral, and dry it for 12 hours to obtain the pretreated precursor. S2. High-temperature activation: The dried precursor and KOH are mixed and ground at a mass ratio of 1:1. The mixture is placed in a ceramic boat, placed in a tube furnace, and activated by heating to 800°C at 5°C / min under a nitrogen atmosphere and holding at that temperature for 1 hour. S3. Post-treatment: After the activated product is allowed to cool naturally, it is soaked in 2 mol / L HCl solution for 24 hours, and then washed with a large amount of deionized water until the filtrate is neutral. Finally, the solid product is dried for 12 hours to obtain the supercapacitor carbon sample.

[0025] Characterization and Testing: The sample was characterized and subjected to electrochemical tests. Its BET specific surface area was 1018.66 m². 2 / g. In KOH electrolyte, at a current density of 1 A / g, its specific capacitance is 280.6 F / g. The microporosity is 75.47%, the ash content is 4.60%, and the average pore size is 2.44 nm.

[0026] Example 3 A method for preparing lignin-based supercapacitor carbon by supercritical alcohol-alkali pretreatment combined with activation includes the following steps: S1. Supercritical Pretreatment: Weigh 10g lignin, 2.5g KOH, and 25mL methanol into a beaker and mix thoroughly. Transfer the mixture to a 100mL high-pressure reactor, seal it, and heat it to 300℃. React at this temperature for 2 hours. After the reaction is complete, cool, remove the solid product, wash it with deionized water until neutral, and dry it for 12 hours to obtain the pretreated precursor. S2. High-temperature activation: The dried precursor and KOH are mixed and ground at a mass ratio of 1:1. The mixture is placed in a ceramic boat and then placed in a tube furnace. Under a nitrogen atmosphere, the temperature is increased to 600°C at 5°C / min and held at that temperature for 1 hour for activation. S3. Post-treatment: After the activated product is allowed to cool naturally, it is soaked in 2 mol / L HCl solution for 24 hours, and then washed with a large amount of deionized water until the filtrate is neutral. Finally, the solid product is dried for 12 hours to obtain the supercapacitor carbon sample.

[0027] Characterization and Testing: The sample was characterized and subjected to electrochemical tests. Its BET specific surface area was 487.2 m². 2 / g. In KOH electrolyte, at a current density of 1 A / g, its specific capacitance is 287.9 ​​F / g. The microporosity is 74.17%, the ash content is 2.58%, and the average pore size is 2.57 nm.

[0028] Example 4 A method for preparing lignin-based supercapacitor carbon by supercritical alcohol-alkali pretreatment combined with activation includes the following steps: S1. Supercritical Pretreatment: Weigh 10g lignin, 2.5g KOH, and 25mL methanol into a beaker and mix thoroughly. Transfer the mixture to a 100mL high-pressure reactor, seal it, and heat it to 330℃. React at this temperature for 2 hours. After the reaction is complete, cool, remove the solid product, wash it with deionized water until neutral, and dry it for 12 hours to obtain the pretreated precursor. S2. High-temperature activation: The dried precursor and KOH are mixed and ground at a mass ratio of 1:1. The mixture is placed in a ceramic boat and then placed in a tube furnace. Under a nitrogen atmosphere, the temperature is increased to 700°C at a rate of 5°C / min and held at that temperature for 1 hour for activation. S3. Post-treatment: After the activated product is allowed to cool naturally, it is soaked in 2 mol / L HCl solution for 24 hours, and then washed with a large amount of deionized water until the filtrate is neutral. Finally, the solid product is dried for 12 hours to obtain the supercapacitor carbon sample.

[0029] Characterization and Testing: The sample was characterized and electrochemically tested. Its BET specific surface area was 816.63 m². 2 / g. In KOH electrolyte, at a current density of 1 A / g, its specific capacitance is 264.9 F / g. The microporosity is 77.87%, the ash content is 3.07%, and the average pore size is 2.46 nm.

[0030] Example 5 S1. Supercritical Pretreatment: Weigh 10g lignin, 2.5g KOH, and 25mL methanol into a beaker and mix thoroughly. Transfer the mixture to a 100mL high-pressure reactor, seal it, and heat it to 270℃. React at this temperature for 2 hours. After the reaction is complete, cool, remove the solid product, wash it with deionized water until neutral, and dry it for 12 hours to obtain the pretreated precursor. S2. High-temperature activation: The dried precursor and KOH are mixed and ground at a mass ratio of 1:1. The mixture is placed in a ceramic boat and then placed in a tube furnace. Under a nitrogen atmosphere, the temperature is increased to 700°C at a rate of 5°C / min and held at that temperature for 1 hour for activation. S3. Post-treatment: After the activated product is allowed to cool naturally, it is soaked in 2 mol / L HCl solution for 24 hours, and then washed with a large amount of deionized water until the filtrate is neutral. Finally, the solid product is dried for 12 hours to obtain the supercapacitor carbon sample.

[0031] Characterization and Testing: The sample was characterized and subjected to electrochemical tests. Its BET specific surface area was 739.38 m². 2 / g. In KOH electrolyte, at a current density of 1 A / g, its specific capacitance is 255.9 F / g. The microporosity is 73.97%, the ash content is 6.83%, and the average pore size is 2.67 nm.

[0032] Example 6 A method for preparing lignin-based supercapacitor carbon by supercritical alcohol-alkali pretreatment combined with activation includes the following steps: S1. Supercritical Pretreatment: Weigh 10g lignin, 2.5g KOH, and 25mL methanol into a beaker and mix thoroughly. Transfer the mixture to a 100mL high-pressure reactor, seal it, and heat it to 300℃. React at this temperature for 2 hours. After the reaction is complete, cool, remove the solid product, wash it with deionized water until neutral, and dry it for 12 hours to obtain the pretreated precursor. S2. High-temperature activation: The dried precursor and KOH are mixed and ground at a mass ratio of 1:0.5. The mixture is placed in a ceramic boat and then placed in a tube furnace. Under a nitrogen atmosphere, the temperature is increased to 700°C at 5°C / min and held at that temperature for 1 hour for activation. S3. Post-treatment: After the activated product is allowed to cool naturally, it is soaked in 2 mol / L HCl solution for 24 hours, and then washed with a large amount of deionized water until the filtrate is neutral. Finally, the solid product is dried for 12 hours to obtain the supercapacitor carbon sample.

[0033] Example 7 A method for preparing lignin-based supercapacitor carbon by supercritical alcohol-alkali pretreatment combined with activation includes the following steps: S1. Supercritical Pretreatment: Weigh 10g lignin, 2.5g KOH, and 25mL methanol into a beaker and mix thoroughly. Transfer the mixture to a 100mL high-pressure reactor, seal it, and heat it to 300℃. React at this temperature for 2 hours. After the reaction is complete, cool, remove the solid product, wash it with deionized water until neutral, and dry it for 12 hours to obtain the pretreated precursor. S2. High-temperature activation: The dried precursor and KOH are mixed and ground at a mass ratio of 1:3. The mixture is placed in a ceramic boat and then placed in a tube furnace. Under a nitrogen atmosphere, the temperature is increased to 600°C at 5°C / min and held for 1 hour for activation. S3. Post-treatment: After the activated product is allowed to cool naturally, it is soaked in 2 mol / L HCl solution for 24 hours, and then washed with a large amount of deionized water until the filtrate is neutral. Finally, the solid product is dried for 12 hours to obtain the supercapacitor carbon sample.

[0034] Comparative example (traditional direct activation method) Without supercritical pretreatment, the original lignin and KOH are directly mixed in a 1:1 mass ratio.

[0035] The subsequent activation, acid washing, water washing, and drying steps were exactly the same as in Example 1, resulting in a comparative sample.

[0036] Characterization and Testing: The sample was characterized and subjected to electrochemical tests. Its BET specific surface area was 2.56 m² / g. In KOH electrolyte, at a current density of 1 A / g, its microporosity was 2.05%, its ash content was 7.51%, and its average pore size was 42.48 nm.

[0037] From the above, we can see that: The lignin-based supercapacitor carbon prepared by supercritical alcohol-alkali pretreatment and activation process exhibits excellent structural and electrochemical properties, which are significantly affected by process parameters. In terms of specific surface area, the BET values ​​of different embodiments range from 487.2 to 1018.66 m². 2 / g, with the highest activation temperature of 800℃ (Example 2), and the measured result was 1018.66 m. 2 / g, while the lowest was at 600℃ (Example 3), at 487.2 m. 2 / g indicates that activation temperature is a key factor in the regulation of specific surface area.

[0038] Regarding the microporous structure, all samples exhibited high microporosity, ranging from 73.97% to 77.87%, with the highest microporosity observed at a supercritical temperature of 330℃ (Example 4), at 77.87%. The average pore size ranged from 2.44 to 2.67 nm, reflecting that the supercritical temperature has a certain regulatory effect on the pore size.

[0039] In terms of electrochemical performance, Example 3 (activated at 600℃) exhibited the highest specific capacitance at 287.9 ​​F / g and the lowest ash content (2.58%), while Example 1 (supercritical at 240℃) had the highest ash content (7.74%) and a specific capacitance of 224.1 F / g. Compared to Example 1, the optimal sample showed an increase in specific capacitance of over 28% and a reduction in ash content of over 66%. Furthermore, increasing the KOH ratio (as in Example 7) further optimized the pore structure and electrochemical performance, demonstrating that the synergistic control of process parameters is crucial for material performance.

[0040] Compared with the comparative example (traditional direct activation method), the supercapacitor carbon prepared by supercritical alcohol-alkali pretreatment combined with activation has significant advantages in all performance indicators. The BET specific surface area of ​​the comparative example is only 2.56 m² / g, and the microporosity is 2.05%, while the lowest specific surface area in the examples reaches 487.2 m² / g, and the lowest microporosity is 73.97%. This fully demonstrates that the supercritical alcohol-alkali pretreatment step can significantly improve the structure of lignin, increase the porosity and specific surface area of ​​the supercapacitor carbon, thereby enhancing its electrochemical performance.

[0041] The technical features of the above embodiments can be combined arbitrarily. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as the combination of these technical features does not contradict each other, it should be considered within the scope of this specification. The above embodiments only illustrate several implementation methods of this application, and their descriptions are relatively specific and detailed, but they should not be construed as limiting the scope of the invention patent. It should be noted that for those skilled in the art, several modifications and improvements can be made without departing from the concept of this application, and these all fall within the protection scope of this application. Therefore, the protection scope of this patent application should be determined by the appended claims.

Claims

1. A method for the preparation of a lignin-based supercapacitor carbon by supercritical alcohol alkali pretreatment in combination with activation, characterized by: The method comprises the following steps: S1, supercritical pretreatment: mixing lignin, alkali catalyst and alcohol solvent in proportion, placing in a reaction kettle, and reacting under supercritical conditions; after the reaction is completed, washing with water and drying to obtain a precursor; S2, high-temperature activation: mixing the precursor obtained in S1 with alkali in proportion, and performing high-temperature activation under a nitrogen atmosphere; S3, post-treatment: sequentially performing acid washing, water washing to neutral, and then drying on the product after the activation in S2 to obtain a lignin-based supercapacitor carbon material.

2. A method of preparing a lignin-based supercapacitor carbon by supercritical alcohol alkali pretreatment combined with activation according to claim 1, characterized by: The alkali catalyst in S1 comprises one or two of potassium hydroxide, potassium carbonate, potassium bicarbonate, sodium carbonate and sodium bicarbonate. The alcohol solvent comprises one or several of methanol, ethanol and isopropanol.

3. A method of preparing a lignin-based supercapacitor carbon by supercritical alcohol alkali pretreatment combined with activation according to claim 1, characterized by: The use amount ratio of the lignin, alkali catalyst and alcohol solvent in S1 is 10g:2.5g:25mL.

4. A method of preparing a lignin-based supercapacitor carbon by supercritical alcohol alkali pretreatment combined with activation according to claim 1, characterized by: The reaction temperature of the supercritical condition in S1 is 240-330℃, and the reaction time is 0.5-4h.

5. A method of preparing a lignin-based supercapacitor carbon by supercritical alcohol alkali pretreatment combined with activation as claimed in claim 1, characterized by: The mass ratio of the precursor to alkali in S2 is 1:0.5-3; the alkali comprises one or two of potassium hydroxide, potassium carbonate, potassium bicarbonate, sodium carbonate and sodium bicarbonate.

6. A method of preparing a lignin-based supercapacitor carbon by supercritical alcohol alkali pretreatment combined with activation as claimed in claim 1, characterized by: The high-temperature activation temperature in S2 is 600-800℃, and the activation time is 0.5-4h.

7. A method of preparing a lignin-based supercapacitor carbon by supercritical alcohol alkali pretreatment combined with activation as claimed in claim 1, wherein: The acid washing in S3 uses a hydrochloric acid solution with a concentration of 1-3mol / L to soak and wash for 12-36 hours.

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