Plant stem / lignin composite porous carbon and preparation thereof, and application of plant stem / lignin composite porous carbon in zinc ion supercapacitor

Abstract

The invention discloses plant stem / lignin composite porous carbon, preparation thereof and application of the plant stem / lignin composite porous carbon in a zinc ion supercapacitor. The preparation method comprises the following steps: respectively carrying out amination modification on plant stalks and lignin, and then adding glutaraldehyde for crosslinking; then adding a zinc acetate solution to generate a zinc carbonate / plant stem / lignin compound in situ; and finally, carrying out high-temperature carbonization to prepare the nitrogen-oxygen doped lignin composite porous carbon. The composite carbon takes plant stem porous carbon as a rigid framework, lignin porous carbon is filled in the rigid framework, collapse of the lignin porous carbon is inhibited, a similar reinforcing steel bar-concrete structure is formed, the structural stability is improved, and the specific surface area is increased; the micro-mesoporous ratio of the carbon material is adjusted through potassium carbonate and zinc carbonate dual activators, and storage and transmission of electrolyte ions by the carbon material are promoted; meanwhile, rich heteroatoms of the plant stems and lignin are utilized, efficient doping of nitrogen and oxygen elements is achieved, and the specific capacitance of the porous carbon is remarkably improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of biomass porous carbon, and particularly relates to a plant stalk / lignin composite porous carbon, a preparation method thereof, and an application thereof in a zinc-ion supercapacitor. Background Art

[0002] In order to address the issues of energy crisis and environmental pollution, the storage and conversion of sustainable energy by electrochemical energy storage devices have received extensive attention. Zinc-ion hybrid capacitors (ZIHCs) composed of a metal zinc negative electrode and a carbon-based positive electrode material combine the advantages of zinc-ion batteries and supercapacitors, featuring high energy density, high power density, and long cycle life. The positive electrode material is a key component determining the capacity of ZIHCs. Among them, porous carbon has advantages such as abundant pore channels and easy doping, and is a common positive electrode material that stores charge through reversible physical adsorption / desorption and pseudocapacitance introduced by heteroatom doping. However, due to problems such as the incompatibility between the pore structure of porous carbon and carriers, lack of active sites, and inaccessibility, the capacity of the porous carbon cathode is low, making it difficult to match the high-capacity zinc negative electrode.

[0003] According to the energy storage mechanism of porous carbon, researchers mainly use the following two methods to improve its specific capacity. One is to regulate the pore structure and pore size distribution of carbon materials, where micropores serve as active sites to adsorb electrolyte ions, and mesopores and macropores ensure the rapid diffusion of ions. The other is to regulate the surface properties of porous carbon through heteroatom doping to improve surface wettability and provide pseudocapacitance. Common dopings include elements such as nitrogen and oxygen. For example, Xiao et al. (Adv. Funct. Mater. 2024, 2405830) used phenol, melamine, and formaldehyde as raw materials, F127 as a template agent, and carried out high-temperature carbonization in different gas atmospheres after hydrothermal polymerization to prepare three nitrogen-doped carbon materials with different pore size distributions. Among them, the carbon material with a large number of micropores, mesopores, and an appropriate amount of macropores has the best performance, with an energy density as high as 126.6 Wh kg -1 . Jia et al. (Chem. Eng. J. 2024, 485:149820) selected a nitrogen-rich metal-organic framework (MOF), namely zinc-based triazole salt (Zn-MET), as a carbon precursor and nitrogen source, and prepared nitrogen-doped porous carbon through a simple direct pyrolysis strategy. Its nitrogen doping amount is as high as 16.2 at%, and it has a hierarchical porous structure with coexisting micropores / mesopores / macropores, achieving a high capacity of 164.2 mAh / g at 0.1 A / g. Although the above methods have successfully regulated the pore size and heteroatom doping, the raw materials used have too high costs and the preparation processes are complex, making it difficult to achieve industrial production.

[0004] Plant stalks in agricultural and forestry waste (such as tobacco stalks, tea stalks, corn straw, etc.) are an ideal carbon precursor due to their high carbon content, wide sources, abundant heteroatoms, and natural pore structure. The porous carbon with high specific surface area and heteroatom doping prepared by carbonization and activation is widely used in the field of electrochemical energy storage. Therefore, mixing plant stalks with an activator (such as KOH, ZnCl 2 etc.) and then carbonizing and activating can produce porous carbon with a high specific surface area. For example, Chen et al. (New Carbon Mater. 2017, 32(6): 592 - 599.) adopted a strategy of hydrothermal-assisted KOH activation of plant stalks (the mass ratio of plant stalks to KOH was 1:3) to prepare microporous-dominated porous carbon with a specific surface area of 1297.6 m 2 / g. However, due to the dense structure of plant stalks, the amount of corrosive activator used is too large and the activation effect is not good. Chinese patent application CN 111943200 A pre-carbonized plant stalks with a size of 2 - 5 mm at 400 - 700 °C in a nitrogen atmosphere, and then physically mixed the pre-carbonized sample with potassium oxalate and calcium carbonate (K 2 C 2 O 4 、CaCO 3 ) to carbonize and obtain tobacco stalk-based porous carbon. The prepared porous carbon has a specific surface area as high as 1842 m 2 / g. The structure of the pre-carbonized sample is loose, which is conducive to uniform mixing with the activator, improving the activation efficiency in the subsequent carbonization process, and thus reducing the amount of activator used. However, its pre-carbonization temperature is high, increasing the energy consumption. Tan et al. (Ind. Crops Prod. 2023, 191: 115981) used plant stalks as raw materials and K 2 C 2 O 4 、CaCO 3 as activators. After ball milling and mixing, they carbonized at 800 °C to prepare a plant stalk-based porous carbon with a high specific surface area (2749 m 2 / g) and a reasonable micro-mesoporous ratio. However, the pore size is concentrated at 1.2 nm and 2 - 4 nm, which does not match the size of hydrated zinc ions (0.86 nm) and is not suitable for use in zinc ion capacitors. In addition, many plant stalks contain abundant metal elements and heteroatom elements, which can be self-activated and self-doped. Chinese patent CN115497749B pre-oxidized plant stalks to achieve oxygen doping and strengthen the self-activation process, and after carbonization, 1435 m 2Microporous carbon with a specific surface area of / g and an oxygen-carbon ratio of 23.8% has a specific capacitance of 400 F / g at a current density of 0.5 A / g in a supercapacitor. However, the too-low specific surface area limits the further improvement of performance. Currently, there are the following problems in preparing porous carbon from plant stalks: (1) Plant stalks are insoluble in water or other solvents, and it is difficult to mix evenly with the activator physically, resulting in low activation efficiency, insufficient and uneven development of pore structures; (2) The structure of plant stalks is dense, and a large amount of corrosive activators (KOH, ZnCl 2 etc.) are often needed for activation to form pores. The large use of corrosive activators will increase production costs, corrode instrument equipment and cause pollution, which is not conducive to large-scale industrial production; (3) Although some plant stalks can prepare porous carbon with a relatively high specific surface area through self-activation, the pore size structure is single, resulting in poor capacitance performance.

[0005] In addition, industrial lignin comes from industries such as papermaking and biomass refining, with a high annual output, but there is also the problem of low high-value utilization rate. Lignin molecules are rich in benzene rings, with a carbon content as high as 60%, and have a three-dimensional network structure and abundant oxygen-containing functional groups, which are easy to adjust the structure and morphology and easy to dope. It is an ideal carbon precursor. For example, Klose et al. (ACS Sustainable Chem. Eng. 2017, 5, 4094 - 4102) used KOH to activate softwood lignin to prepare a porous carbon material with a specific surface area of 1886 m 2 / g and mainly micropores in pore size. There are fewer mesopores, and ion transport is blocked, which limits the performance in zinc-ion supercapacitors. Liu et al. (Carbon. 2019, 149: 105 - 116) prepared a porous carbon material by directly carbonizing kraft lignin. The prepared porous carbon material has a specific surface area of 1307 m 2 / g. Due to no additional activator added, lignin agglomerates and collapses during carbonization, resulting in insufficient specific surface area, which is not conducive to storing and transporting electrolyte ions. Chinese Patent CN201811375772.8 obtained a lignosulfonate / oxalate complex through self-assembly in an ethanol-water solvent, and activated lignin by means of oxalate pyrolysis to prepare porous carbon nanosheets. However, due to the weak assembly force, the activation effect of oxalate is not good, and the specific surface area of the prepared lignin porous carbon nanosheets is only 1069 m 2 / g, resulting in poor specific capacitance performance. Chinese Patent CN116332176B uses lignin as the raw material and metal oxalate as the activator and template agent. By evaporation-induced self-assembly in a urea-ethanol-water system and condensation modification of lignin, the structural stability and thermal stability of the lignin and metal oxalate complex are improved, and a porous carbon with a specific surface area of not less than 2500 m 2 / g, lignin-based mesoporous carbon with a mesopore rate of not less than 50%, but the pore sizes of the porous carbon are concentrated between 0.6 nm and 1.3 nm, which do not match the size of hydrated zinc ions (0.86 nm). Therefore, there are the following problems in preparing porous carbon from lignin: there are a large number of π-π interactions between benzene rings in lignin, which leads to easy aggregation, resulting in poor compatibility with the activator and limited activation effect; and compared with crude biomass, the structural stability of lignin is poor, it is prone to collapse during the carbonization process, the connectivity of the carbon skeleton is poor, and the pores are blocked, resulting in a large electrolyte transmission resistance and poor electrochemical performance.

[0006] Therefore, to solve the above problems, researchers combined the characteristics of different biomasses and compounded two biomasses to prepare porous carbon to achieve the purpose of complementing each other's advantages. Yu et al. (Electrochimica Acta. 2019, 327: 134999) based on the high electrical conductivity of coconut shell carbon and the hierarchical pore structure of bagasse carbon, combined the advantages of the two, and prepared porous carbon with a high specific surface area (3401 m 2 / g), high conductivity and hierarchical pores through a hydrothermal mixing strategy of bagasse and coconut shell, and achieved an energy density of 118 Wh / Kg and excellent cycle performance in zinc ion capacitors. Lin et al. (ACS Sustainable Chem. Eng. 2019, 7(12): 10393-10402) mixed rice husk and crab shell together through a hydrothermal-assisted mixing strategy and carried out activation carbonization with KOH. The porous carbon after composite carbonization combines the pore structure characteristics of rice husk and crab shell and has a higher specific surface area, showing a specific surface area of up to 3557 m 2 / g and a hierarchical pore structure, and the specific capacitance reaches 474 F / g at a current density of 0.5 A / g. Yang et al. (J. Energy Storage. 2024, 83: 110688) prepared porous carbon by hydrothermal mixing of bagasse and bovine bone meal with KOH activation. During the hydrothermal process, small molecule peptides in bovine bone react with lignocellulose in bagasse through Maillard and Mannich reactions, making hydroxyapatite in bovine bone uniformly adhere to the surface of bagasse. Under high-temperature carbonization, the combination of KOH activation and self-activation of hydroxyapatite prepared porous carbon with a high specific surface area (3103 m 2 / g) and a wide pore size distribution (0.5 - 10 nm). In 2 SO 4Excellent electrochemical performance has been demonstrated in supercapacitor systems with the solution as the electrolyte. In the above several research works, although two different crude biomass materials were effectively combined to regulate the structural morphology and surface properties of the carbon materials to achieve the purpose of synergistic enhancement, a single KOH was still used as the activator, which has high corrosiveness and high cost and is not conducive to large-scale production. Moreover, it is difficult to achieve uniform compounding of the two crude biomass materials through physical blending, resulting in uneven and insufficient development of the pore structure.

[0007] In summary, although researchers have adopted the strategy of effectively compounding two different biomass materials to optimize the pore structure characteristics and improve the electrochemical performance of biomass-based carbon. However, due to the insoluble characteristics of the crude biomass, it is difficult to mix evenly and has poor compatibility with the activator, resulting in low activation efficiency. Therefore, high-energy-consuming processes such as ball milling are used, and the production cost is too high. And a large amount of corrosive activator needs to be used, which is easy to corrode equipment and is not conducive to continuous large-scale industrial production. It also leads to low heteroatom content and difficult-to-control pore structure in the prepared biomass-based porous carbon, and it is difficult to improve the electrochemical performance as a capacitor electrode material. Summary of the Invention

[0008] To solve the disadvantages and deficiencies of the prior art, the primary object of the present invention is to provide a method for preparing a high specific surface area plant stalk / lignin composite porous carbon.

[0009] The method of the present invention uses plant stalks and lignin as raw materials, and through in-situ coprecipitation after glutaraldehyde cross-linking reaction, prepares a plant stalk / lignin composite porous carbon with a high specific surface area and high heteroatom doping. First, the plant stalks and lignin are respectively subjected to amination modification by ammonia hydrothermal method and Mannich reaction, and then glutaraldehyde is added to cross-link with the aminated plant stalk hydrochar and lignin. Then, a zinc acetate solution is added to in-situ generate a zinc carbonate / plant stalk / lignin composite. Finally, through high-temperature carbonization, a nitrogen and oxygen-doped lignin composite porous carbon is prepared, which uses the plant stalk porous carbon as a rigid framework, and the lignin porous carbon is filled therein to inhibit the collapse of the lignin porous carbon, forming a structure similar to "reinforced concrete", increasing the structural stability and improving the specific surface area. At the same time, the high heteroatoms in the plant stalks and lignin themselves are utilized to achieve efficient doping of nitrogen and oxygen elements.

[0010] During the preparation process of the method of the present invention, the plant stalks are pretreated by ammonia hydrothermal method, and at the same time, Mannich reaction is used to graft amino groups to the lignin, effectively increasing the amino group content in the plant stalk hydrochar and lignin, increasing the reaction sites for cross-linking reaction with glutaraldehyde. Further, glutaraldehyde is added for cross-linking reaction to construct a three-dimensional network structure, making the plant stalks and lignin evenly dispersed and improving the composite stability.

[0011] Another object of the present invention is to provide a plant stalk / lignin composite porous carbon prepared by the above method. The porous carbon has a hierarchical pore structure with abundant micropores and mesopores, a specific surface area of not less than 1700 m 2 / g, and has a mutually connected pore structure; at the same time, it contains abundant nitrogen and oxygen functional groups, which improve the surface characteristics of the porous carbon, can provide partial pseudocapacitance, and significantly improve the specific capacitance of the porous carbon.

[0012] Another object of the present invention is to provide the application of the above plant stalk / lignin composite porous carbon in a zinc ion supercapacitor.

[0013] The object of the present invention is achieved by the following technical solutions:

[0014] A preparation method of a plant stalk / lignin composite porous carbon, comprising the following steps:

[0015] (1) Dissolve lignin in a carbonate solution, then add a formaldehyde solution and an amination reagent to carry out an amino modification reaction to obtain an amino-modified lignin solution;

[0016] (2) Mix plant stalks and ammonia water in a hydrothermal autoclave, carry out a hydrothermal reaction at 150-220 °C for 0.5-4 h, filter, and dry the filter cake to obtain plant stalk hydrothermal carbon;

[0017] (3) Mix the plant stalk hydrothermal carbon obtained in step (2) with the amino-modified lignin solution obtained in step (1), add glutaraldehyde for a cross-linking reaction, then dropwise add a zinc acetate solution, stir, and evaporate to dryness to obtain a hydrothermal carbon / lignin / potassium carbonate / zinc carbonate composite;

[0018] (4) Carbonize the composite obtained in step (3) at high temperature in an inert gas atmosphere, wash, and dry to obtain a plant stalk / lignin composite porous carbon;

[0019] Based on parts by weight, the amounts of each reactant are as follows:

[0020] Lignin 10 parts

[0021] Carbonate 10-40 parts

[0022] Amination reagent 10-20 parts

[0023] 25-37 wt% formaldehyde solution 5-20 parts

[0024] Plant stalks 5-20 parts

[0025] 25-50 wt% aqueous glutaraldehyde solution 5-15 parts.

[0026] Preferably, based on parts by weight, the amounts of each reactant are as follows:

[0027] 10 parts of lignin

[0028] 20 - 30 parts of carbonate

[0029] 10 - 15 parts of amination reagent

[0030] 5 - 15 parts of 37 wt% formaldehyde solution

[0031] 5 - 15 parts of plant stalks

[0032] 5 - 10 parts of 50 wt% aqueous glutaraldehyde solution.

[0033] Preferably, the lignin in step (1) is at least one of enzymatic hydrolysis lignin extracted from biorefinery residues, alkali lignin extracted from kraft pulping black liquor, and lignosulfonate extracted from sulfite pulping red liquor.

[0034] Preferably, the carbonate in the carbonate solution in step (1) is at least one of potassium carbonate and sodium carbonate.

[0035] Preferably, the concentration of the carbonate solution in step (1) is 20 - 40 wt%; more preferably 20 wt%.

[0036] Preferably, the amination reagent in step (1) is at least one of ethylenediamine, diethylenetriamine, and triethylenetetramine.

[0037] Preferably, the concentration of the formaldehyde solution in step (1) is 25 - 37 wt%.

[0038] Preferably, the temperature of the amino modification reaction in step (1) is 55 - 85 °C, and the time is 1 - 5 h; more preferably, the temperature of the amino modification reaction is 55 - 65 °C, and the time is 1 - 3 h.

[0039] Preferably, the particle size of the plant stalks in step (2) is ≤ 200 mesh; obtained by mechanically crushing and screening the plant stalk raw materials.

[0040] Preferably, the plant stalks in step (2) are at least one of tobacco stalks, tea stalks, and corn straws; more preferably at least one of tobacco stalks and tea stalks.

[0041] Preferably, the concentration of the ammonia water in step (2) is 5 - 15 wt%; more preferably 5 - 10 wt%.

[0042] Preferably, the solid - liquid ratio of the hydrothermal reaction in step (2) is 1 (g of plant stalks): 5 - 10 (ml of ammonia water); more preferably 1 (g of plant stalks): 8 (ml of ammonia water).

[0043] Preferably, the temperature of the hydrothermal reaction in step (2) is 150 - 200 °C, and the time is 0.5 - 2 h.

[0044] Preferably, the glutaraldehyde described in step (3) is a 50 wt% aqueous glutaraldehyde solution.

[0045] Preferably, the cross-linking reaction of the glutaraldehyde, hydrothermal carbon, and lignin described in step (3) is carried out at 60-90 °C for 1-4 h; more preferably, the cross-linking reaction is carried out at 60-80 °C for 1-2 h.

[0046] Preferably, the concentration of the zinc acetate solution described in step (3) is 10-20 wt%; more preferably 10-15 wt%.

[0047] Preferably, the mass ratio of zinc acetate in the zinc acetate solution described in step (3) to the carbonate in step (1) is 2-20:10-40; more preferably 2-5:10-24.

[0048] Preferably, the stirring time described in step (3) is 1-6 h.

[0049] Preferably, the evaporation of water in step (3) is carried out using an oil bath at 70-105 °C.

[0050] Preferably, the inert gas described in step (4) refers to at least one of nitrogen, argon, and helium.

[0051] Preferably, the carbonization described in step (4) means holding at 600-900 °C for 1-4 h; the heating rate of carbonization is 5-10 °C / min; more preferably, the carbonization means holding at 600-750 °C for 1-2.5 h, and the heating rate of carbonization is 5-10 °C / min.

[0052] More preferably, before the carbonization described in step (4), it is necessary to hold at 105-250 °C for 0.5 h.

[0053] Preferably, the washing described in step (4) means washing the carbonized product in an acid solution with a concentration of 0.5-2 mol / L for 6-12 h, and then rinsing with deionized water.

[0054] More preferably, the 0.5-2 mol / L acid solution is at least one of hydrochloric acid, sulfuric acid, and nitric acid.

[0055] Preferably, the drying described in step (4) is at least one of freeze drying, forced air drying, vacuum drying, and infrared drying.

[0056] Preferably, the drying temperature described in step (4) is 50-90 °C, and the time is 4-8 h; more preferably, the drying temperature is 60-80 °C, and the time is 4-6 h.

[0057] A plant stalk / lignin composite porous carbon prepared by the above method.

[0058] The application of the above-mentioned plant stalk / lignin composite porous carbon in zinc ion supercapacitors.

[0059] The preparation method described in the present invention patent will be described in more detail below.

[0060] (1) Weigh 10 g of lignin and dissolve it in a carbonate solution. Stir until the lignin is completely dissolved. Slowly add 10 - 20 g of amination reagent and 5 - 20 g of 25 - 37 wt% formaldehyde solution, and react at 55 - 85 °C for 1 - 5 h to obtain an amino-modified lignin solution.

[0061] The purpose of adding potassium carbonate in this step is, on the one hand, to increase the alkalinity of the solution to dissolve lignin; on the other hand, it acts as an activator in the precursor to form a large number of micropores during the carbonization process, providing active sites for charge storage.

[0062] In this step, the Mannich reaction is carried out through lignin, amination reagent, and formaldehyde to achieve amination modification, increase amino groups, and provide a basis for subsequent cross-linking reactions.

[0063] (2) Use a high-speed crusher to crush and screen the plant stalk raw material to obtain plant stalk powder. Weigh 5 - 20 g of plant stalk powder and place it in a hydrothermal autoclave. Add 25 - 200 ml of 5 - 15 wt% ammonia water, stir well, and carry out hydrothermal reaction at 150 - 220 °C for 0.5 - 4 h. Filter and dry the filter cake to obtain amino-modified plant stalk hydrothermal carbon.

[0064] The purpose of crushing in this step is to enable the plant stalk powder to better contact the solution fully. Using ammonia water for hydrothermal reaction can strengthen the etching effect of hydrothermal treatment, better degrade cellulose and hemicellulose in biomass, remove ash and inorganic impurities, increase the specific surface area of the plant stalk and expand the original pores of the biomass, increase the contact area with the lignin solution, and at the same time introduce amino groups to provide a basis for subsequent cross-linking reactions.

[0065] (3) Add the dried plant stalk hydrothermal carbon obtained in step (2) to the solution in step (1), stir and mix evenly. Add an aqueous glutaraldehyde solution to the solution and react at 60 - 90 °C for 1 - 4 h to cross-link lignin and plant stalk hydrothermal carbon. Then slowly drop 20 - 50 ml of 10 - 20 wt% zinc acetate solution and stir for 1 - 6 h. Evaporate the water to obtain a hydrothermal carbon / lignin / potassium carbonate / zinc carbonate composite.

[0066] In this step, a cross-linking reaction is carried out between glutaraldehyde and amino groups to enhance the binding force between lignin and plant stalk. At the same time, the three-dimensional network structure formed by cross-linking can make the subsequent generated zinc carbonate precipitate evenly dispersed, enhancing the activation effect.

[0067] The purpose of adding zinc acetate in this step is to convert a part of potassium carbonate into zinc carbonate precipitation, forming a dual activator system of potassium carbonate-zinc carbonate. During the carbonization process, the carbon material generates micropores under the activation of potassium carbonate, and forms mesopores under the vapor phase exfoliation and template action of zinc carbonate. The micro-mesoporous ratio of the carbon material is adjusted by adjusting the ratio of the two activators to prepare a composite porous carbon with the best electrochemical performance.

[0068] (4) taking the plant stem hydrothermal carbon / lignin / potassium carbonate / zinc carbonate complex dried in step (3) and carbonizing it under an inert gas atmosphere, soaking and stirring the carbonized product in a dilute acid solution, washing it with a large amount of deionized water, and drying it in an oven to obtain a plant stem / lignin composite porous carbon material;

[0069] In this step, the carbonization atmosphere is an inert gas such as nitrogen and argon. The carbonization temperature is in the range of 600-900°C, the heating rate is 5-10°C / min, and the time is 1-4h. If the temperature is too low, the heating rate is too fast, and the time is too short, the carbonization will be incomplete, resulting in a low specific surface area of ​​the product; if the temperature is too high, the heating rate is too slow, and the time is too long, it will not only increase the production cost, but also have a negative impact on the yield and structural stability of porous carbon;

[0070] The dilute acid solution in this step is a solution of 0.5-2 mol / L hydrochloric acid or sulfuric acid, and the immersion time is controlled at 6-12 hours. If the acid concentration is too low or the immersion time is too short, metal oxide residues will remain and the specific surface area of ​​the product will be reduced; if the acid concentration is too high or the immersion time is too long, the pore structure of the porous carbon will be destroyed.

[0071] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0072] (1) The plant stem / lignin composite porous carbon prepared by the present invention is a hierarchical porous carbon with a reasonable micro-mesopore ratio and a small amount of macropores, and has an interconnected pore structure, a large specific surface area and a rich amount of N and O element doping. When used as a positive electrode material for zinc ion supercapacitors, it can provide sufficient active sites and provide channels for the rapid diffusion and transmission of electrolyte ions. In addition, the rich heteroatoms can enhance the wettability between the carbon material and the electrolyte, provide pseudocapacitance, show excellent specific capacitance, and have good potential application value.

[0073] (2) In the preparation process of the plant stalk / lignin composite porous carbon of the present invention, plant stalk powder and lignin are used as carbon sources, and weakly corrosive potassium carbonate and zinc carbonate are used as activators. By utilizing the strong carbon framework formed by the plant stalks during the carbonization process, the collapse of lignin during carbonization is prevented, and at the same time, lignin is stably cross-linked in the original pores of the plant stalks, filling the macroporous structure of the plant stalks during carbonization and increasing the degree of disorder of the carbon material, forming a structure similar to "reinforced concrete"; at the same time, potassium carbonate and zinc carbonate are used for activating pore formation to adjust the micro-mesopore ratio of the carbon material, promoting the storage and transmission of electrolyte ions during the electrochemical process. By effectively combining the advantages of the two biomasses, porous carbon with a rich pore structure and heteroatom content is prepared, which has potential application prospects in energy storage materials and realizes the high-value utilization of plant stalk waste and lignin. Description of the Drawings

[0074] Figure 1 It is a scanning electron microscope image of the plant stalk / lignin composite porous carbon prepared in Example 1 of the present invention.

[0075] Figure 2 It is the nitrogen adsorption-desorption curve and pore size distribution diagram of the plant stalk / lignin composite porous carbon prepared in Example 1 of the present invention.

[0076] Figure 3 It is the X-ray photoelectron spectroscopy image of the plant stalk / lignin composite porous carbon prepared in Example 1 of the present invention.

[0077] Figure 4 It is the cyclic voltammetry curve of the plant stalk / lignin composite porous carbon prepared in Example 1 of the present invention.

[0078] Figure 5 It is the galvanostatic charge-discharge curve of the plant stalk / lignin composite porous carbon prepared in Example 1 of the present invention. Detailed Embodiments

[0079] The present invention will be further described in detail below with reference to the embodiments and the drawings, but the embodiments of the present invention are not limited thereto.

[0080] For those not specified in the embodiments of the present invention, they are carried out according to the conventional conditions or the conditions recommended by the manufacturer. The raw materials, reagents, etc. without indicating the manufacturer can be obtained as conventional products through commercial purchase.

[0081] Example 1

[0082] Weigh 10 g of enzymatically hydrolyzed lignin and dissolve it in 50 ml of 20 wt% potassium carbonate solution. Then, sequentially add 10 g of diethylenetriamine and 5 g of 37 wt% formaldehyde solution, and react at 55 °C for 1 h to obtain an amino-modified lignin solution. Weigh 5 g of 200-mesh tobacco stem powder and put it into a hydrothermal reactor. Add 25 ml of 5 wt% ammonia water, stir well, and carry out hydrothermal reaction at 150 °C for 0.5 h. After cooling, filter, wash the filter cake, and dry it to obtain amino-modified hydrothermal carbon of tobacco stem. Add the dried hydrothermal carbon of tobacco stem to the amino-modified lignin solution, and add 5 g of 50 wt% glutaraldehyde aqueous solution. Continue to react at 60 °C for 1 h. Then, slowly drip 20 ml of 10 wt% zinc acetate solution into the mixture with a peristaltic pump at a pump speed of 3 rpm, continue to stir for 1 h, and then evaporate the water at 105 °C to obtain a tobacco stem hydrothermal carbon / lignin / potassium carbonate / zinc carbonate composite;

[0083] Put the composite obtained above into a ceramic boat and place it in a tube furnace. Carry out carbonization in a nitrogen atmosphere. The carbonization temperature is raised from room temperature to 250 °C and maintained for 0.5 h, then raised to 600 °C at a rate of 5 °C / min and maintained for 1 h, and then cooled to room temperature to obtain black powder;

[0084] Immerse the black powder obtained after carbonization in 150 mL of 0.5 mol / L hydrochloric acid solution, stir for 6 h, filter, wash with deionized water and absolute ethanol, and dry in an infrared oven at 50 °C for 4 h to obtain a tobacco stem / lignin composite porous carbon material.

[0085] Example 2

[0086] Weigh 10 g of alkali lignin and dissolve it in 200 ml of 20 wt% potassium carbonate solution. Then, sequentially add 20 g of diethylenetriamine and 20 g of 37 wt% formaldehyde solution, and react at 85 °C for 5 h to obtain an amino-modified lignin solution. Weigh 20 g of 200-mesh tobacco stem powder and put it into a hydrothermal reactor. Add 200 ml of 15 wt% ammonia water, stir well, and carry out hydrothermal reaction at 220 °C for 4 h. After cooling, filter, wash the filter cake, and dry it to obtain amino-modified hydrothermal carbon of tobacco stem. Add the dried hydrothermal carbon of tobacco stem to the amino-modified lignin solution, and add 15 g of 50 wt% glutaraldehyde aqueous solution. Continue to react at 90 °C for 4 h. Then, slowly drip 50 ml of 20 wt% zinc acetate solution into the mixture with a peristaltic pump at a pump speed of 3 rpm, continue to stir for 1 h, and then evaporate the water at 105 °C to obtain a tobacco stem hydrothermal carbon / lignin / potassium carbonate / zinc carbonate composite;

[0087] Put the obtained composite into a ceramic boat and place it in a tube furnace. Carbonize it in a nitrogen atmosphere. The carbonization temperature is raised from room temperature to 250 °C and maintained for 0.5 h, then raised to 900 °C at a rate of 10 °C / min and maintained for 4 h, and then cooled to room temperature to obtain black powder;

[0088] Immerse the black powder obtained after carbonization in 150 mL of 1.5 mol / L hydrochloric acid solution, stir for 12 h, filter, wash with deionized water and absolute ethanol, and dry in an infrared oven at 90 °C for 8 h to obtain tobacco stem / lignin composite porous carbon material.

[0089] Example 3

[0090] Weigh 10 g of enzymatically hydrolyzed lignin and dissolve it in 150 ml of 20 wt% potassium carbonate solution. Then add 15 g of diethylenetriamine and 15 g of 37 wt% formaldehyde solution in sequence, and react at 80 °C for 3 h to obtain an amino-modified lignin solution; weigh 20 g of 200-mesh tobacco stem powder and put it into a hydrothermal autoclave, add 200 ml of 10 wt% ammonia water, stir well, carry out hydrothermal reaction at 210 °C for 4 h, cool down, filter, wash the filter cake, and dry to obtain amino-modified tobacco stem hydrothermal carbon; add the dried tobacco stem hydrothermal carbon to the amino-modified lignin solution, add 10 g of 50 wt% glutaraldehyde aqueous solution, continue to react at 80 °C for 2 h, then slowly drip 100 ml of 20 wt% zinc acetate solution into the mixed solution with a peristaltic pump at a pump speed of 3 rpm, continue to stir for 1 h, and then evaporate the water at 105 °C to obtain a tobacco stem hydrothermal carbon / lignin / potassium carbonate / zinc carbonate composite;

[0091] Put the obtained composite into a ceramic boat and place it in a tube furnace. Carbonize it in a nitrogen atmosphere. The carbonization temperature is raised from room temperature to 250 °C and maintained for 0.5 h, then raised to 700 °C at a rate of 5 °C / min and maintained for 2 h, and then cooled to room temperature to obtain black powder;

[0092] Immerse the black powder obtained after carbonization in 150 mL of 1 mol / L hydrochloric acid solution, stir for 6 h, filter, wash with deionized water and absolute ethanol, and dry in an infrared oven at 50 °C for 12 h to obtain tobacco stem / lignin composite porous carbon material.

[0093] Example 4

[0094] Weigh 10 g of sodium lignosulfonate and dissolve it in 150 ml of 20 wt% potassium carbonate solution. Then, add 10 g of ethylenediamine and 15 g of 37 wt% formaldehyde solution in sequence, and react at 75 °C for 4 h to obtain an amino-modified lignin solution. Weigh 10 g of 200-mesh tobacco stem powder and put it into a hydrothermal reactor, add 70 ml of 5 wt% ammonia water, stir well, carry out hydrothermal reaction at 180 °C for 2 h, cool down, filter, wash the filter cake, and dry it to obtain amino-modified hydrothermal carbon of tobacco stem. Add the dried hydrothermal carbon of tobacco stem into the amino-modified lignin solution, and add 10 g of 50 wt% glutaraldehyde aqueous solution. Continue to react at 60 °C for 1 h, then slowly drip 40 ml of 15 wt% zinc acetate solution into the mixture with a peristaltic pump at a pump speed of 3 rpm, continue to stir for 1 h, and then evaporate the water at 105 °C to obtain a tobacco stem hydrothermal carbon / lignin / potassium carbonate / zinc carbonate composite;

[0095] Put the composite obtained above into a ceramic boat and place it in a tubular furnace. Carry out carbonization in a nitrogen atmosphere. The carbonization temperature is raised from room temperature to 250 °C and maintained for 0.5 h, then raised to 800 °C at a rate of 5 °C / min and maintained for 3 h, and then cooled to room temperature to obtain black powder;

[0096] Soak the black powder obtained after carbonization in 150 mL of 1.5 mol / L hydrochloric acid solution, stir for 6 h, filter, wash with deionized water and absolute ethanol, and dry in an infrared oven at 55 °C for 6 h to obtain a tobacco stem / lignin composite porous carbon material.

[0097] Example 5

[0098] Weigh 10 g of enzymatically hydrolyzed lignin and dissolve it in 100 ml of 20 wt% potassium carbonate solution. Then, add 15 g of triethylenetetramine and 5 g of 37 wt% formaldehyde solution in sequence, and react at 65 °C for 3 h to obtain an amino-modified lignin solution. Weigh 15 g of 200-mesh tea stem powder and put it into a hydrothermal reactor, add 120 ml of 10 wt% ammonia water, stir well, carry out hydrothermal reaction at 200 °C for 2 h, cool down, filter, wash the filter cake, and dry it to obtain amino-modified hydrothermal carbon of tea stem. Add the dried hydrothermal carbon of tea stem into the amino-modified lignin solution, and add 10 g of 50 wt% glutaraldehyde aqueous solution. Continue to react at 80 °C for 2 h, then slowly drip 20 ml of 15 wt% zinc acetate solution into the mixture with a peristaltic pump at a pump speed of 3 rpm, continue to stir for 1 h, and then evaporate the water at 105 °C to obtain a tea stem hydrothermal carbon / lignin / potassium carbonate / zinc carbonate composite;

[0099] Put the obtained composite into a ceramic boat and place it in a tube furnace. Carbonize it in a nitrogen atmosphere. The carbonization temperature is raised from room temperature to 250 °C and maintained for 0.5 h, then raised to 750 °C at a rate of 10 °C / min and maintained for 2.5 h, and then cooled to room temperature to obtain black powder;

[0100] Immerse the obtained black powder after carbonization in 150 mL of 1 mol / L hydrochloric acid solution, stir for 12 h, filter, wash with deionized water and absolute ethanol, and dry in an infrared oven at 80 °C for 6 h to obtain tea stalk / lignin composite porous carbon material.

[0101] Example 6

[0102] Weigh 10 g of enzymatically hydrolyzed lignin and dissolve it in 120 ml of 20 wt% potassium carbonate solution. Then add 15 g of ethylenediamine and 10 g of 37 wt% formaldehyde solution in sequence, and react at 65 °C for 3 h to obtain an amino-modified lignin solution; Weigh 15 g of 200-mesh tobacco stalk powder and put it into a hydrothermal reactor, add 75 ml of 10 wt% ammonia water, stir well, carry out hydrothermal reaction at 200 °C for 0.5 h, cool down, filter, wash the filter cake, and dry to obtain amino-modified tobacco stalk hydrothermal carbon; Add the dried tobacco stalk hydrothermal carbon to the amino-modified lignin solution, add 10 g of 50 wt% glutaraldehyde aqueous solution, continue to react at 90 °C for 4 h, then slowly drip 50 ml of 10 wt% zinc acetate solution into the mixture with a peristaltic pump at a pump speed of 3 rpm, continue to stir for 1 h, and then evaporate the water at 105 °C to obtain a tobacco stalk hydrothermal carbon / lignin / potassium carbonate / zinc carbonate composite;

[0103] Put the obtained composite into a ceramic boat and place it in a tube furnace. Carbonize it in a nitrogen atmosphere. The carbonization temperature is raised from room temperature to 250 °C and maintained for 0.5 h, then raised to 850 °C at a rate of 5 °C / min and maintained for 3 h, and then cooled to room temperature to obtain black powder;

[0104] Immerse the obtained black powder after carbonization in 150 mL of 0.5 mol / L hydrochloric acid solution, stir for 6 h, filter, wash with deionized water and absolute ethanol, and dry in an infrared oven at 75 °C for 5 h to obtain tobacco stalk / lignin composite porous carbon material.

[0105] Comparative Example 1 (compared with Example 1, only using plant stalks as raw materials)

[0106] Weigh 5 g of 200-mesh tobacco stem powder and put it into a hydrothermal reactor. Then add 25 ml of 5 wt% ammonia water, stir well, carry out hydrothermal reaction at 150 °C for 0.5 h, cool down, filter, wash the filter cake, and dry it to obtain amino-modified hydrothermal carbon of tobacco stem; add the dried hydrothermal carbon of tobacco stem into 50 ml of 20 wt% potassium carbonate solution, and slowly drip 20 ml of 10 wt% zinc acetate solution into the mixed solution at a pump speed of 3 rpm with a peristaltic pump, continue to stir for 1 h, and then evaporate the water at 105 °C to obtain a tobacco stem hydrothermal carbon / potassium carbonate / zinc carbonate composite;

[0107] Put the composite obtained above into a ceramic boat and place it in a tube furnace. Carry out carbonization in a nitrogen atmosphere. The carbonization temperature is raised from room temperature to 250 °C and maintained for 0.5 h, then raised to 600 °C at a rate of 5 °C / min and maintained for 1 h, and then cooled to room temperature to obtain black powder;

[0108] Soak the black powder obtained after carbonization in 150 mL of 0.5 mol / L hydrochloric acid solution, stir for 6 h, filter, wash with deionized water and absolute ethanol, and dry in an infrared oven at 50 °C for 4 h to obtain a tobacco stem / lignin composite porous carbon material.

[0109] Comparative Example 2 (compared with Example 1, only using lignin as the raw material)

[0110] Weigh 10 g of enzymatically hydrolyzed lignin and dissolve it in 50 ml of 20 wt% potassium carbonate solution. Then add 10 g of diethylenetriamine and 5 g of 37% formaldehyde solution in sequence, react at 55 °C for 1 h to obtain an amino-modified lignin solution; then slowly drip 20 ml of 10 wt% zinc acetate solution into the mixed solution at a pump speed of 3 rpm with a peristaltic pump, continue to stir for 1 h, and then evaporate the water at 105 °C to obtain a lignin / potassium carbonate / zinc carbonate composite;

[0111] Put the composite obtained above into a ceramic boat and place it in a tube furnace. Carry out carbonization in a nitrogen atmosphere. The carbonization temperature is raised from room temperature to 250 °C and maintained for 0.5 h, then raised to 600 °C at a rate of 5 °C / min and maintained for 1 h, and then cooled to room temperature to obtain black powder;

[0112] Soak the black powder obtained after carbonization in 150 mL of 0.5 mol / L hydrochloric acid solution, stir for 6 h, filter, wash with deionized water and absolute ethanol, and dry in an infrared oven at 50 °C for 4 h to obtain a tobacco stem / lignin composite porous carbon material.

[0113] Comparative Example 3 (compared with Example 1, the plant stem is not pretreated by hydrothermal ammonia)

[0114] Weigh 10 g of enzymatically hydrolyzed lignin and dissolve it in 50 ml of 20 wt% potassium carbonate solution. Then, add 10 g of diethylenetriamine and 5 g of 37% formaldehyde solution in sequence, and react at 55 °C for 1 h to obtain an amino-modified lignin solution. Weigh 5 g of 200-mesh tobacco stem powder and add it to the amino-modified lignin solution. Then, add 5 g of 50 wt% glutaraldehyde aqueous solution, and react at 60 °C for 1 h. Next, slowly drip 20 ml of 10 wt% zinc acetate solution into the mixture at a pump speed of 3 rpm using a peristaltic pump, continue stirring for 1 h, and then evaporate the water at 105 °C to obtain a tobacco stem hydrothermal carbon / lignin / potassium carbonate / zinc carbonate composite;

[0115] Put the composite obtained above into a ceramic boat and place it in a tubular furnace. Carbonize it in a nitrogen atmosphere. The carbonization temperature is raised from room temperature to 250 °C and maintained for 0.5 h, then raised to 600 °C at a rate of 5 °C / min and maintained for 1 h, and then cooled to room temperature to obtain a black powder;

[0116] Soak the black powder obtained after carbonization in 150 mL of 0.5 mol / L hydrochloric acid solution, stir for 6 h, then filter, wash with deionized water and absolute ethanol, and dry in an infrared oven at 50 °C for 4 h to obtain a tea stem / lignin composite porous carbon material.

[0117] Comparative Example 4 (compared with Example 1, lignin is not subjected to amino modification)

[0118] Weigh 10 g of enzymatically hydrolyzed lignin and dissolve it in 50 ml of 20 wt% potassium carbonate solution. Weigh 5 g of 200-mesh tobacco stem powder, put it into a hydrothermal autoclave, add 25 ml of 5 wt% ammonia water, stir well, carry out hydrothermal reaction at 150 °C for 0.5 h, cool down, filter, wash the filter cake, and dry to obtain amino-modified tobacco stem hydrothermal carbon. Add the dried tobacco stem hydrothermal carbon to the amino-modified lignin solution, add 5 g of 50 wt% glutaraldehyde aqueous solution, continue to react at 60 °C for 1 h, then slowly drip 20 ml of 10 wt% zinc acetate solution into the mixture at a pump speed of 3 rpm using a peristaltic pump, continue stirring for 1 h, and then evaporate the water at 105 °C to obtain a tobacco stem hydrothermal carbon / lignin / potassium carbonate / zinc carbonate composite;

[0119] Put the composite obtained above into a ceramic boat and place it in a tubular furnace. Carbonize it in a nitrogen atmosphere. The carbonization temperature is raised from room temperature to 250 °C and maintained for 0.5 h, then raised to 600 °C at a rate of 5 °C / min and maintained for 1 h, and then cooled to room temperature to obtain a black powder;

[0120] The black powder obtained after carbonization was immersed in 150 mL of 0.5 mol / L hydrochloric acid solution, stirred for 6 h, filtered, washed with deionized water and absolute ethanol, and dried in an infrared oven at 50 °C for 4 h to obtain the tobacco stem / lignin composite porous carbon material.

[0121] Comparative Example 5 (compared with Example 1, without using glutaraldehyde for cross-linking reaction)

[0122] Weigh 10 g of enzymatically hydrolyzed lignin and dissolve it in 50 ml of 20 wt% potassium carbonate solution. Then, add 10 g of diethylenetriamine and 5 g of 37% formaldehyde solution in sequence, and react at 55 °C for 1 h to obtain an amino-modified lignin solution; weigh 5 g of 200-mesh tobacco stem powder and put it into a hydrothermal reactor, add 25 ml of 5 wt% ammonia water, stir well, carry out hydrothermal reaction at 150 °C for 0.5 h, cool down, filter, wash the filter cake, and dry to obtain amino-modified tobacco stem hydrothermal carbon; add the dried tobacco stem hydrothermal carbon to the amino-modified lignin solution, stir at 60 °C for 1 h, then slowly drip 20 ml of 10 wt% zinc acetate solution into the mixture with a peristaltic pump at a pump speed of 3 rpm, continue to stir for 1 h, and then evaporate the water at 105 °C to obtain a tobacco stem hydrothermal carbon / lignin / potassium carbonate / zinc carbonate composite.

[0123] Put the above-obtained composite into a ceramic boat and place it in a tubular furnace. Carry out carbonization in a nitrogen atmosphere. The carbonization temperature is raised from room temperature to 250 °C and maintained for 0.5 h, then raised to 600 °C at a rate of 5 °C / min and maintained for 1 h, and then cooled to room temperature to obtain black powder.

[0124] The black powder obtained after carbonization was immersed in 150 mL of 0.5 mol / L hydrochloric acid solution, stirred for 6 h, filtered, washed with deionized water and absolute ethanol, and dried in an infrared oven at 50 °C for 4 h to obtain the tobacco stem / lignin composite porous carbon material.

[0125] Comparative Example 6 (compared with Example 1, using strong base potassium hydroxide as an activator)

[0126] Weigh 10 g of enzymatically hydrolyzed lignin and dissolve it in 50 ml of 20 wt% potassium hydroxide solution. Then, add 10 g of diethylenetriamine and 5 g of 37% formaldehyde solution in sequence, and react at 55 °C for 1 h to obtain an amino-modified lignin solution. Weigh 5 g of 200-mesh tobacco stem powder and put it into a hydrothermal reactor, add 25 ml of 5 wt% ammonia water, stir well, carry out hydrothermal reaction at 150 °C for 0.5 h, cool down, filter, wash the filter cake, and dry it to obtain amino-modified hydrothermal carbon of tobacco stems. Add the dried hydrothermal carbon of tobacco stems into the amino-modified lignin solution, add 5 g of 50 wt% glutaraldehyde aqueous solution, continue to react at 60 °C for 1 h, then slowly drop 20 ml of 10 wt% zinc acetate solution into the mixture with a peristaltic pump at a pump speed of 3 rpm, continue to stir for 1 h, and then evaporate the water at 105 °C to obtain a tobacco stem hydrothermal carbon / lignin / potassium hydroxide / zinc oxide composite;

[0127] Put the composite obtained above into a ceramic boat and place it in a tube furnace. Carry out carbonization in a nitrogen atmosphere. The carbonization temperature is raised from room temperature to 250 °C and maintained for 0.5 h, then raised to 600 °C at a rate of 5 °C / min and maintained for 1 h, and then cooled to room temperature to obtain black powder;

[0128] Soak the black powder obtained after carbonization in 150 mL of 0.5 mol / L hydrochloric acid solution, stir for 6 h, filter, wash with deionized water and absolute ethanol, and dry in an infrared oven at 50 °C for 4 h to obtain a tobacco stem / lignin composite porous carbon material.

[0129] Results and analysis:

[0130] Apply the plant stem / lignin composite porous carbon material prepared in Example 1 to the zinc ion supercapacitor electrode material and conduct material characterization and electrochemical tests. The results are shown in Table 1 and Figures 1 to 5 .

[0131] The microscopic morphology and structure of the samples are characterized by a scanning electron microscope (SEM, Hitachi S-550) and a high-resolution field emission transmission electron microscope (HRTEM, JEOL JEM-2100F, 200 kV). The specific surface area, pore structure, and X-ray photoelectron spectroscopy of the samples are tested using a fully automatic specific surface area and porosity analyzer (Micromeritics ASAP2020 instrument) and an X-ray photoelectron spectrometer (Thermo Scientific K-Alpha, ThermoFisher), respectively.

[0132] Electrochemical tests included cyclic voltammetry (CV) and galvanostatic charge-discharge (GCD) tests, both of which were carried out on an electrochemical workstation VMP-3E (Bio-Logic, France). The preparation process of the working electrode was as follows: The prepared plant stalk / lignin composite porous carbon powder, acetylene black, and polytetrafluoroethylene emulsion (solid content 60 wt%) were mixed evenly in absolute ethanol at a mass ratio of 8:1:1. After the ethanol evaporated, the mixed slurry was rolled into a film, cut into a square of 0.8×0.8 cm, and placed in a vacuum oven for drying for 12 h. The dried carbon film was pressed onto a stainless steel mesh through an infrared press to obtain the working electrode. The loading amount of the active material was about 1 - 1.5 mg / cm 2 . Subsequently, the carbon electrode was used as the anode, the zinc foil as the cathode, and the glass fiber as the separator. A 1 mol / L ZnSO 4 solution was used as the electrolyte to assemble a button cell. The CV test was completed at a scanning rate of 1 - 100 mv / s in a voltage window of 0.2 - 1.8 V. The GCD test was completed at a current density of 0.1 - 10.0 A / g in a voltage window of 0.2 - 1.8 V.

[0133] Table 1 Structural characteristics and electrochemical properties of the plant stalk / lignin composite porous carbon material and Comparative Examples 1 - 6

[0134]

[0135] Table 1 Explanation:

[0136] The specific surface area of the plant stalk / lignin composite porous carbon material prepared in Example 1 was 2154 m 2 / g, and the pore volume was 1.17 cm 3 / g; the O content was as high as 13.89%, and the N content was 2.31%. The specific capacitance at a current density of 0.1 A / g was 376 F / g. When the current density was 5 A / g, the specific capacitance of the carbon material was 194 F / g, and the specific capacitance retention rate, i.e., the rate performance, was 51.6%, showing obvious performance advantages among biomass porous carbon materials.

[0137] The specific surface areas of all the sample in the examples are relatively rich, with a reasonable ratio of micro and mesopores and a high content of nitrogen and oxygen functional groups, and they have a relatively high specific capacitance at a current density of 0.1 A / g. This is mainly due to the following two points: (1) The plant stalks and lignin are evenly mixed through a cross-linking reaction. The strong carbon framework formed by the plant stalks during the carbonization process prevents the collapse of lignin during carbonization. At the same time, lignin fills the macroporous structure of the plant stalks during carbonization to form abundant micro and mesopores, constituting a structure similar to "steel bars - concrete", effectively combining the advantages of the two biomasses, preparing porous carbon with a rich pore structure and heteroatom content, and realizing the high-value utilization of plant stalk waste and lignin; (2) By adding potassium carbonate and converting a part of it into zinc carbonate, a dual activator system is constructed, and the ratio of the two activators is adjusted, so that the prepared carbon material has a reasonable ratio of micro and mesopores, promoting the high-speed transmission of electrolyte ions and providing a large number of active sites.

[0138] Compared with Example 1, Comparative Example 1 uses a single plant stalk as the raw material. Due to the dense structure of the plant stalk and poor compatibility with the activator, the lack of lignin makes it impossible to uniformly compound with the generated zinc carbonate, and at the same time, its natural macroporous structure cannot be filled, resulting in too high a proportion of micropores (89.1%) in the obtained porous carbon and a slight decrease in specific surface area, causing a decline in electrochemical performance.

[0139] Compared with Example 1, Comparative Example 2 uses a single lignin as the raw material. The stability of the lignin / activator complex is poor, and the structure is prone to collapse during pyrolytic carbonization, resulting in a low specific surface area and non-connected structure of lignin carbon, causing difficulties in electrolyte transmission and low rate performance of the zinc ion capacitor.

[0140] Compared with Example 1, in Comparative Example 3, the plant stalks were not pretreated by ammonia hydrothermal treatment. The amino content in the plant stalks is too small to be stably cross-linked with lignin through glutaraldehyde, resulting in uneven mixing and poor stability during the pyrolysis process, and unbalanced development of the pore structure, which limits the electrochemical performance of the capacitor.

[0141] Compared with Example 1, in Comparative Example 4, lignin was not amino-modified. It cannot be stably cross-linked with the plant stalks through glutaraldehyde and is prone to agglomeration, resulting in uneven dispersion of lignin in the plant stalks, poor stability during the pyrolysis process, and unbalanced development of the pore structure, which limits the electrochemical performance of the capacitor.

[0142] Compared with Example 1, in Comparative Example 5, glutaraldehyde was not used for the cross-linking reaction. The plant stalks and lignin cannot be stably cross-linked, resulting in uneven dispersion of the plant stalks / lignin / zinc carbonate, poor stability during the pyrolysis process, and unbalanced development of the pore structure, which limits the electrochemical performance of the capacitor.

[0143] Compared with Example 1, in Comparative Example 6, strong base potassium hydroxide was used as the activator. Compared with carbonate activators such as potassium carbonate and zinc carbonate, potassium hydroxide has strong corrosiveness, resulting in a decrease in the heteroatom content; and potassium hydroxide is a microporous activator, and the prepared porous carbon is mainly composed of micropores, which is not conducive to the transport of electrolyte ions, resulting in low capacitance performance.

[0144] Figure 1 Figure 4 is the scanning electron microscope image of the plant stalk / lignin composite porous carbon prepared in Example 1 of the present invention. It can be seen from the figure that the prepared plant stalk / lignin composite porous carbon has a regular porous structure with interconnected pores.

[0145] Figure 2 Figure 5 is the nitrogen adsorption-desorption isotherm curve and pore size distribution map of the plant stalk / lignin composite porous carbon prepared in Example 1 of the present invention. It can be seen from the figure that the adsorption-desorption curve of the plant stalk / lignin composite porous carbon belongs to Type IV. In the region of relatively low relative pressure, the nitrogen adsorption amount increased rapidly, indicating that it has a microporous structure, while the hysteresis loop that appears in the region of relatively high relative pressure indicates that it has a mesoporous structure. The total BET specific surface area of the plant stalk / lignin composite porous carbon is 2154 m 2 / g, and the total pore volume is 1.17 cm 3 / g. Its reasonable pore structure and rich pore size distribution are beneficial to the improvement of electrochemical performance.

[0146] Figure 3 Figure 6 is the X-ray photoelectron spectroscopy image of the plant stalk / lignin composite porous carbon prepared in Example 1 of the present invention. It can be seen from the figure that the plant stalk-based porous carbon has a relatively high oxygen content, which is beneficial to improving the surface characteristics of the porous carbon, providing pseudocapacitance and thus enhancing the electrochemical performance.

[0147] Figure 4 Figure 7 is the cyclic voltammetry curve of the plant stalk / lignin composite porous carbon prepared in Example 1 of the present invention. It can be seen from the figure that the curves of the material at different scan rates are all quasi-rectangular shapes, indicating that the material has an ideal electric double layer capacitance. Even at the highest scan rate, the curve shape hardly changes, indicating that the material has excellent rate performance.

[0148] Figure 5 Figure 8 is the galvanostatic charge-discharge curve of the plant stalk / lignin composite porous carbon prepared in Example 1 of the present invention. It can be seen from the figure that the curve shapes of the prepared lignin cube carbon materials at different current densities are similar to isosceles triangles, indicating that the carbon materials have typical electric double layer capacitance characteristics.

[0149] The above embodiments are preferred embodiments of the present invention, but the embodiments of the present invention are not limited to the above embodiments. Any other changes, modifications, substitutions, combinations, and simplifications made without departing from the spirit and principle of the present invention shall be equivalent replacement methods and are all included in the protection scope of the present invention.

Claims

1. A method for preparing plant stem / lignin composite porous carbon, characterized in that: The following steps are involved: (1) dissolving lignin in a carbonate solution, then adding a formaldehyde solution and an aminating agent to carry out an amino modification reaction to obtain an amino-modified lignin solution; (2) mixing the plant stems and ammonia water in a hydrothermal kettle, performing a hydrothermal reaction at 150-220° C. for 0.5-4 h, filtering, and drying the filter cake to obtain the plant stem hydrothermal carbon; (3) mixing the plant stem hydrothermal carbon with the amino-modified lignin solution, adding glutaraldehyde for cross-linking reaction, then dropping zinc acetate solution, stirring, and evaporating the water to obtain a hydrothermal carbon / lignin / potassium carbonate / zinc carbonate complex; (4) Carbonizing the composite obtained in step (3) under an inert gas atmosphere, washing, and drying to obtain plant stem / lignin composite porous carbon.

2. The method for preparing a plant stem / lignin composite porous carbon according to claim 1, characterized in that: The amounts of each reactant are as follows in parts by weight: 10 parts of lignin; 10-40 parts of carbonate; 10-20 parts of amination reagent; 5 to 20 parts of 25 to 37 wt% formaldehyde solution; 5-20 parts of plant stems; 5 to 15 parts of a 25 to 50 wt% aqueous solution of glutaraldehyde; And / or, the mass ratio of zinc acetate in the zinc acetate solution of step (3) to the carbonate of step (1) is 2-20:10-40.

3. The method for preparing a plant stem / lignin composite porous carbon according to claim 1 or 2, characterized in that: The temperature of the amino modification reaction in step (1) is 55 to 85° C. and the time is 1 to 5 hours; And / or, the cross-linking reaction of glutaraldehyde with hydrothermal carbon and lignin in step (3) is carried out at 60-90° C. for 1-4 h; And / or, the carbonization in step (4) refers to keeping the temperature at 600-900°C for 1-4h; the heating rate of carbonization is 5-10°C / min; And / or, before the carbonization in step (4), the temperature is kept at 105-250° C. for 0.5 h.

4. The method for preparing a plant stem / lignin composite porous carbon according to claim 1 or 2, characterized in that: The lignin in step (1) is at least one of enzymatic lignin extracted from biorefining residues, alkali lignin extracted from black liquor of alkali pulping, and lignin sulfonate extracted from red liquor of sulfite pulping; And / or, the carbonate in the carbonate solution in step (1) is at least one of potassium carbonate and sodium carbonate; And / or, the amination reagent in step (1) is at least one of ethylenediamine, diethylenetriamine and triethylenetetramine; And / or, the concentration of the carbonate solution in step (1) is 20-40 wt%; And / or, the concentration of the formaldehyde solution in step (1) is 25-37 wt %.

5. The method for preparing a plant stem / lignin composite porous carbon according to claim 1 or 2, characterized in that: The particle size of the plant stems in step (2) is ≤200 mesh; And / or, the plant stems in step (2) are at least one of tobacco stems, tea stems and corn stalks; And / or, the concentration of the aqueous ammonia in step (2) is 5 to 15 wt %; And / or, the solid-to-liquid ratio of the hydrothermal reaction in step (2) is 1 gram of plant stem: 5 to 10 ml of ammonia water.

6. The method for preparing a plant stem / lignin composite porous carbon according to claim 1 or 2, characterized in that: The glutaraldehyde in step (3) is a 50wt% glutaraldehyde aqueous solution; And / or, the concentration of the zinc acetate solution in step (3) is 10-20wt%.

7. The method for preparing a plant stem / lignin composite porous carbon according to claim 1 or 2, characterized in that: The inert gas in step (4) is at least one of nitrogen, argon and helium; And / or, the washing in step (4) refers to washing the carbonized product in a 0.5-2 mol / L acid solution for 6-12 h; The 0.5-2 mol / L acid solution is at least one of hydrochloric acid, sulfuric acid and nitric acid.

8. The method for preparing a plant stem / lignin composite porous carbon according to claim 1 or 2, characterized in that: The amounts of each reactant are as follows in parts by weight: 10 parts of lignin, 20-30 parts of carbonate, 10-15 parts of amination reagent, 5 to 15 parts of 37 wt% formaldehyde solution, 5-15 pieces of plant stems, 5-10 parts of a 50wt% aqueous solution of glutaraldehyde; and / or, the mass ratio of zinc acetate in the zinc acetate solution of step (3) to the carbonate of step (1) is 2-5:10-24; And / or, the carbonization in step (4) refers to keeping the temperature at 600-750° C. for 1-2.5 hours, and the heating rate of carbonization is 5-10° C. / min; And / or, the concentration of the zinc acetate solution in step (3) is 10-15 wt%; And / or, the cross-linking reaction in step (3) is carried out at 60-80° C. for 1-2 h; And / or, the temperature of the hydrothermal reaction in step (2) is 150-200° C. and the time is 0.5-2 h; And / or, the concentration of the aqueous ammonia in step (2) is 5 to 10 wt %; And / or, the temperature of the amino modification reaction in step (1) is 55-65° C. and the time is 1-3 hours.

9. A plant stem / lignin composite porous carbon obtained by the preparation method according to any one of claims 1 to 8.

10. Use of the plant stem / lignin composite porous carbon according to claim 9 in zinc ion supercapacitors.

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