Microporous carbon and preparation method thereof, zinc ion capacitor positive electrode and zinc ion capacitor

Abstract

The invention provides microporous carbon and a preparation method thereof, a zinc ion capacitor positive electrode and a zinc ion capacitor, and belongs to the technical field of capacitors. The preparation method of the microporous carbon comprises the following steps: mixing a straw precursor with a template agent, carrying out a hydrothermal reaction, mixing the dried product after the hydrothermal reaction with a pore-forming agent, carrying out high-temperature pyrolysis, and finally carrying out acid treatment and drying on the product after the high-temperature pyrolysis to obtain the microporous carbon. The straw precursor is straw powder sequentially pretreated by strong acid and strong alkali; the template agent comprises Mg2 (OH) 2CO3, and the pore forming agent comprises KOH. The pore diameter structure of the microporous carbon prepared by the preparation method of the microporous carbon can be matched with the ion size in the electrolyte of the zinc ion capacitor, so that the dynamic balance of a carbon positive electrode and a battery type negative electrode prepared from the microporous carbon can be balanced to the greatest extent, and the capacity, the rate capability and the cycling stability of the zinc ion capacitor are further improved.

Description

Technical Field

[0001] The present application relates to the technical field of capacitors, and in particular to a microporous carbon and a preparation method thereof, a zinc ion capacitor positive electrode and a zinc ion capacitor. Background Art

[0002] With the development and application of electrochemical energy storage, it is particularly urgent to develop energy storage devices with excellent electrochemical performance. At present, batteries and supercapacitors each have their own advantages and disadvantages: batteries have high energy density, but poor fast charging performance and limited cycle life. Supercapacitors have high power density, long cycle life and fast charging and discharging capabilities, but low capacity. In order to integrate the advantages of the two, hybrid ion capacitors came into being and became a research hotspot, such as lithium ion capacitors, sodium ion capacitors and zinc ion capacitors. Among them, zinc ion capacitors (ZICs) stand out for their unique advantages. On the one hand, zinc metal reserves are abundant, low cost, non-toxic and stable. On the other hand, the zinc metal negative electrode also has a capacity of up to 820mAh g -1 It has a high specific capacity and a low redox potential (-0.76V vs standard hydrogen electrode), and has good compatibility with water.

[0003] Currently, carbon materials are widely used as positive electrodes for zinc-ion capacitors (Zn-ion capacitors). However, their low surface area and disordered pore structure restrict the adsorption and desorption of ions from the electrolyte, leading to a mismatch in reaction kinetics and capacity between the carbon cathode and the battery-type anode. Despite this, carbon electrode materials that combine high conductivity with good stability continue to attract researchers. Designing and manipulating the structure of carbon materials is an effective approach to improving carbon electrode materials, leading to the emergence of porous carbon materials. As a key characteristic of porous carbon materials, pore size is crucial for improving this type of electrode. Research has found that a pore size that matches the size of the ions in the electrolyte is crucial for efficient ion adsorption. Excessively large pore sizes can lead to Coulombic ordering, resulting in overscreening and double-layer loss during charge storage, thereby increasing the impedance of Zn-ion capacitor devices. Conversely, excessively small pore sizes hinder the adsorption of solvated ions, limiting charge density and, consequently, the power and energy density of Zn-ion capacitor devices. Summary of the Invention

[0004] The present application provides a microporous carbon and a preparation method thereof, a zinc ion capacitor positive electrode and a zinc ion capacitor, which can be used to assemble into a carbon positive electrode for use in a zinc ion capacitor, thereby improving the capacity, rate performance and cycle stability of the zinc ion capacitor.

[0005] The embodiment of the present application is implemented as follows:

[0006] In the first aspect, the present application example provides a method for preparing microporous carbon, which includes: first mixing a straw precursor with a template and performing a hydrothermal reaction, then mixing the dried product after the hydrothermal reaction with a pore-forming agent and performing high-temperature pyrolysis, and finally treating the product after the high-temperature pyrolysis with acid and drying it to obtain microporous carbon; the straw precursor is a straw powder that has been pretreated with a strong acid and a strong base in sequence; the template includes Mg2(OH)2CO3, and the pore-forming agent includes KOH; the temperature of the hydrothermal reaction is 120℃~140℃, and the temperature of the high-temperature pyrolysis is 700℃~900℃.

[0007] In the above technical solution, the preparation method of microporous carbon of the present application uses straw as raw material, and provides rich oxygen-containing functional groups for microporous carbon without additional heteroatom additives. After the microporous carbon is made into the positive electrode of zinc ion capacitor, the oxygen-containing functional groups it has can be used in Zn 2+ After being adsorbed into the carbon cathode material, a redox reaction occurs with it to form zinc-containing functional groups (CO x -Zn, etc.) and provide capacitance contributions to the electrode material; then, a dual molten salt (Mg2(OH)2CO3 and KOH) activation strategy is adopted. The hydrothermal reaction of the straw precursor with the template agent Mg2(OH)2CO3 can deeply disintegrate, converting macromolecules such as lignin, cellulose, and hemicellulose into small molecules. At the same time, it is conducive to the deep burial of Mg2(OH)2CO3 within the product as a reaction template, providing favorable conditions for subsequent sufficient pyrolysis and carbonization. The dried product after the hydrothermal reaction is mixed with the pore-forming agent KOH for pyrolysis, and the reaction temperature is controlled to ensure the pyrolysis efficiency. At high temperature, Mg2(OH)2CO3 will be transformed into a liquid molten salt carrying KOH into the material for high-temperature pyrolysis and activation. Thanks to the small size of KOH and the penetration of Mg2(OH)2CO3, the pore structure of the prepared carbon material can match the ion size in the electrolyte of the zinc ion capacitor, thereby maximizing the kinetic equilibrium between the carbon positive electrode made of microporous carbon and the battery-type negative electrode, thereby improving the capacity, rate performance and cycle stability of the zinc ion capacitor.

[0008] In some possible implementation schemes, the hydrothermal reaction time is 12 hours to 16 hours, and / or the mass ratio of the straw precursor to the template is 1:0.5-2.

[0009] In some possible embodiments, the high-temperature pyrolysis time is 1.5 h to 3 h, and / or the mass ratio of the dried hydrothermal reaction product to the pore-forming agent is 0.5 to 2:1, and / or the high-temperature pyrolysis is carried out under inert gas protection.

[0010] In some possible embodiments, after the hydrothermal reaction is completed, the product of the hydrothermal reaction is dried to obtain a dry hydrothermal reaction product, and then the dry hydrothermal reaction product is ground with a pore-forming agent to obtain a mixed powder, and then the mixed powder is subjected to high-temperature pyrolysis.

[0011] In some possible embodiments, the straw precursor is prepared by the following method:

[0012] First, the crushed straw powder is placed in a strong alkaline solution for soaking, and a first precursor is obtained after the soaking in the strong alkaline solution is completed. Then, the first precursor is placed in a strong acid solution, and after the soaking in the strong alkaline solution is completed, it is filtered, washed to neutrality and dried in sequence to obtain a straw precursor; wherein, the concentration of the strong alkaline solution is 8 mol / L to 12 mol / L, and / or; the strong alkaline solution includes sodium hydroxide solution and / or potassium hydroxide solution, and / or; the soaking time of the strong alkaline solution is 24h to 36h, and / or; the volume ratio of acid to water in the strong acid solution is 1:3 to 1:1, and / or; the strong acid solution includes at least one of hydrochloric acid solution, sulfuric acid solution and nitric acid solution; and the soaking time of the strong acid solution is 12h to 16h.

[0013] In the above technical scheme, this application uses waste straw powder as raw material, and removes a large amount of ash, silicate and other impurities in the biomass through washing, standing, filtering and drying with strong alkali and strong acid solutions, and can promote the hydrolysis of straw cell walls into macromolecular substances such as lignin, cellulose, hemicellulose, etc., which is beneficial to subsequent reactions.

[0014] In a second aspect, the present application provides a microporous carbon, which is prepared according to the preparation method of the microporous carbon in the above embodiment.

[0015] In the above technical solution, the microporous carbon of the present application uses waste straw as raw material, and provides rich oxygen-containing functional groups for the microporous carbon without additional heteroatom additives. After the microporous carbon is made into the positive electrode of the zinc ion capacitor, the oxygen-containing functional groups it has can be used in the Zn 2+ After being adsorbed into the carbon cathode material, a redox reaction occurs with it to form zinc-containing functional groups (CO x -Zn, etc.) and provide capacitance contribution to the electrode material.

[0016] In some possible embodiments, the specific surface area of the microporous carbon is ≥ 2000 m 2 / g, and / or; the microporous carbon has micropores, in which the micropores with a pore diameter of 0.8nm to 2nm account for ≥50%, and the micropores with a pore diameter of 1.1nm to 1.3nm account for ≥40%.

[0017] In the above technical solution, the high specific surface area of microporous carbon provides a good environment for the distribution of active sites and defects, which can efficiently attract Zn 2+ The intercalation layer is the key to improving the capacity of zinc ion capacitors. The pore size of the carbon material of this application is at least 50% distributed in the range of 0.8nm to 2nm, and at least 40% distributed in the range of 1.1nm to 1.3nm. A large distribution of pore sizes of 0.8nm to 2nm can effectively improve the specific surface area of the carbon material and provide more active sites. A large distribution of pore sizes of 1.1nm to 1.3nm facilitates the rapid transfer and storage of zinc ions. Specifically, for zinc ion capacitor electrolytes, the solvated ions in most zinc salt electrolytes are [Zn(H2O)6] 2+ , its size is approximately (0.86nm). In the micropore range of 0.8nm to 2nm, the larger pore size of carbon materials will make [Zn(H2O)6] 2+ The double electric layer shielding effect occurs during the transmission process, which limits the capacity, and the small pore size of the carbon material will make [Zn(H2O)6] 2+ During the transmission process, it interacts with the carbon wall, hindering rapid transmission. Therefore, the pore structure of the carbon electrode at 1.1nm to 1.3nm can maximize the kinetic balance between the carbon positive electrode made of microporous carbon and the battery-type negative electrode, thereby improving the capacity, rate performance and cycle stability of the zinc ion capacitor.

[0018] In a third aspect, the present application provides an example of a zinc ion capacitor positive electrode, which includes microporous carbon prepared by the preparation method of microporous carbon in the above embodiment or microporous carbon according to the above embodiment.

[0019] In the above technical solution, the positive electrode of the zinc ion capacitor of the present application is made of the microporous carbon material in the above embodiment, which has ultra-fast ion transport capability, ultra-high capacity and excellent cycle stability.

[0020] In a fourth aspect, the present application provides an example of a zinc ion capacitor, which includes the zinc ion capacitor positive electrode in the above embodiment.

[0021] In the above technical solution, the zinc ion capacitor of the present application adopts the zinc ion capacitor positive electrode in the above embodiment, which is beneficial to improving the electrochemical performance of the zinc ion capacitor. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the following is a brief introduction to the drawings required for use in the embodiments. It should be understood that the following drawings only show certain embodiments of the present application and therefore should not be regarded as limiting the scope. For ordinary technicians in this field, other relevant drawings can be obtained based on these drawings without creative work.

[0023] Figure 1 This is an SEM image of the carbon material prepared in Example 1 of the present application;

[0024] Figure 2 This is a pore size distribution diagram of the microporous carbon prepared in Example 1 of the present application;

[0025] Figure 3 This is a pore size ratio diagram of the microporous carbon prepared in Example 1 of the present application;

[0026] Figure 4 The microporous carbon species prepared in Example 1 of this application have different pore sizes for Zn(H2O)6 2+ Schematic diagram of adsorption energy;

[0027] Figure 5 This is the XRD pattern of the microporous carbon prepared in Example 1 of the present application;

[0028] Figure 6 This is the Raman image of the microporous carbon prepared in Example 1 of the present application;

[0029] Figure 7 This is the XPS graph of the microporous carbon prepared in Example 1 of the present application;

[0030] Figure 8 This is an SEM image of the carbon material prepared in Example 2 of the present application;

[0031] Figure 9 This is an SEM image of the carbon material prepared in Example 3 of the present application;

[0032] Figure 10 This is an SEM image of the carbon material prepared in Example 4 of the present application;

[0033] Figure 11 This is an SEM image of the carbon material prepared in Example 5 of the present application;

[0034] Figure 12 This is an SEM image of the carbon material prepared in Example 6 of the present application;

[0035] Figure 13 This is an SEM image of the carbon material prepared in Example 7 of the present application;

[0036] Figure 14 This is an SEM image of the carbon material prepared in Example 8 of the present application;

[0037] Figure 15 This is an SEM image of the carbon material prepared in Comparative Example 1 of this application;

[0038] Figure 16 This is an SEM image of the carbon material prepared in Comparative Example 2 of this application;

[0039] Figure 17 This is an SEM image of the carbon material prepared in Comparative Example 3 of this application;

[0040] Figure 18 This is an SEM image of the carbon material prepared in Comparative Example 4 of this application;

[0041] Figure 19 This is an SEM image of the carbon material prepared in Comparative Example 5 of this application;

[0042] Figure 20 This is the SEM image of the carbon material prepared in Comparative Example 6 of this application. DETAILED DESCRIPTION

[0043] The embodiments of the present application will be described in detail below with reference to the examples, but it will be understood by those skilled in the art that the following examples are merely illustrative of the present application and should not be considered as limiting the scope of the present application. In the examples, if specific conditions are not specified, the conditions are carried out according to conventional conditions or manufacturer recommendations. The reagents or instruments used are not specified by the manufacturer and are conventional products that can be purchased commercially.

[0044] The following is a detailed description of a microporous carbon and its preparation method, a zinc ion capacitor positive electrode, and a zinc ion capacitor according to the embodiments of the present application:

[0045] The present application provides a method for preparing microporous carbon, which comprises the following steps:

[0046] S1. Preparation of straw precursor

[0047] First, the waste straw is ground or ball-milled into powder, and then the straw powder is placed in a strong alkaline solution for soaking. After the soaking in the strong alkaline solution, a first precursor is obtained. Then, the first precursor is placed in a strong acid solution. After the soaking in the strong alkaline solution, it is filtered, washed to neutrality and dried to obtain a straw precursor.

[0048] The present application does not limit the source of the waste straw. For example, the waste straw may be waste corn straw.

[0049] Wherein, the strong alkaline solution includes sodium hydroxide solution and / or potassium hydroxide solution.

[0050] As an example, the strong base solution may include only sodium hydroxide solution or potassium hydroxide solution, or may be a mixture of sodium hydroxide solution and potassium hydroxide solution.

[0051] The concentration of the strong alkaline solution is 8 mol / L to 12 mol / L.

[0052] As an example, the concentration of the strong base solution may be 8 mol / L, 9 mol / L, 10 mol / L, 11 mol / L or 12 mol / L.

[0053] The soaking time in strong alkaline solution is 24h to 36h.

[0054] As an example, the soaking time of the strong alkaline solution can be 24 hours, 26 hours, 28 hours, 30 hours, 32 hours, 34 hours or 36 hours.

[0055] The strong acid solution includes at least one of a hydrochloric acid solution, a sulfuric acid solution and a nitric acid solution.

[0056] As an example, the strong acid solution may include only hydrochloric acid solution, sulfuric acid solution or nitric acid solution, or may be a mixture of hydrochloric acid solution and sulfuric acid solution, or may be a mixture of hydrochloric acid solution and nitric acid solution, or may be a mixture of sulfuric acid solution and nitric acid solution, or may be a mixture of hydrochloric acid solution, sulfuric acid solution and nitric acid solution.

[0057] The volume ratio of acid to water in the strong acid solution is 1:3 to 1:1.

[0058] As an example, the volume ratio of acid to water in the strong acid solution may be 1:3, 1:2.5, 1:2, 1:1.5 or 1:1.

[0059] The soaking time in strong acid solution is 12h to 16h.

[0060] As an example, the soaking time of the strong acid solution may be 12 hours, 13 hours, 14 hours, 15 hours or 16 hours.

[0061] It should be noted that excessively high concentrations of strong bases and strong acids, or excessively long rest times, can waste resources and potentially damage the material structure. Conversely, excessively low concentrations of strong bases and strong acids, or excessively short rest times, cannot effectively remove impurities from the precursor.

[0062] This application uses waste straw powder as raw material, and removes a large amount of ash, silicate and other impurities in the biomass through washing, standing, filtering and drying with strong alkali and strong acid solutions, and can promote the hydrolysis of straw cell walls into macromolecular substances such as lignin, cellulose, hemicellulose, etc., which is beneficial to subsequent reactions.

[0063] S2. Hydrothermal reaction

[0064] The straw precursor and the template are mixed and dissolved in deionized water, and then transferred to a reactor for a hydrothermal reaction. The template includes Mg2(OH)2CO3.

[0065] The temperature of the hydrothermal reaction is 120° C. to 140° C., and the time of the hydrothermal reaction is 12 h to 16 h.

[0066] As an example, the temperature of the hydrothermal reaction may be 120° C., 125° C., 130° C., 135° C., or 140° C., and the time of the hydrothermal reaction may be 12 h, 13 h, 14 h, 15 h, or 16 h.

[0067] The mass ratio of the straw precursor to the template is 1:0.5-2.

[0068] As an example, the mass ratio of the straw precursor to the template can be 1:0.5, 1:0.8, 1:1, 1:1.5 or 1:2.

[0069] It should be noted that the purpose of the hydrothermal reaction is to hydrolyze macromolecular substances such as lignin and cellulose in the precursor into small molecular substances. The reaction time and reaction temperature can effectively promote the hydrolysis of macromolecular substances within the ranges provided above.

[0070] S3, high temperature pyrolysis

[0071] The product after the hydrothermal reaction is dried and mixed with a pore-forming agent, and then ground or ball-milled to obtain a mixed powder. The mixed powder is then subjected to high-temperature pyrolysis under the protection of an inert gas, and the pore-forming agent includes KOH.

[0072] The inert gas includes at least one of nitrogen, helium and argon, and the flow rate of the inert gas is 50 mL / min to 70 mL / min.

[0073] The temperature of the high-temperature pyrolysis is 700° C. to 900° C., and the time of the high-temperature pyrolysis is 1.5 h to 3 h.

[0074] As an example, the temperature of the high-temperature pyrolysis may be 700° C., 750° C., 800° C., 850° C., or 900° C., and the time of the high-temperature pyrolysis may be 1.5 h, 2 h, 2.5 h, or 3 h.

[0075] The mass ratio of the dried hydrothermal reaction product to the pore-forming agent is 0.5 to 2:1.

[0076] As an example, the mass ratio of the dried hydrothermal reaction product to the pore-forming agent may be 0.5:1, 0.7:1, 1:1, 1.2:1, 1.5:1, 1.8:1 or 2:1.

[0077] By controlling the ratio of the pore-forming agent and the hydrothermal product, the activation and etching degree of the precursor can be effectively regulated, the honeycomb-shaped high specific surface area porous carbon can be more fully constructed, and the distribution of the pore structure can be controlled, thereby adjusting the electrochemical properties, such as capacity and cycle stability.

[0078] S4. Preparation of microporous carbon

[0079] The product after high-temperature pyrolysis is sequentially immersed in an acid solution, allowed to stand, filtered, and dried to remove unremoved ash and inadequately reacted pore-forming agent, thereby obtaining microporous carbon.

[0080] The acid solution includes a hydrochloric acid solution, the volume ratio of hydrogen chloride to water in the hydrochloric acid solution is 1:3 to 1:1, and the standing time is 8 hours to 18 hours.

[0081] The preparation method of microporous carbon of the present application uses straw as raw material, and provides rich oxygen-containing functional groups for microporous carbon without additional heteroatom additives. After the microporous carbon is made into the positive electrode of zinc ion capacitor, the oxygen-containing functional groups can be used in Zn 2+ After being adsorbed into the carbon cathode material, a redox reaction occurs with it to form zinc-containing functional groups (CO x -Zn, etc.) and provide capacitance contributions to the electrode material; then, a dual molten salt (Mg2(OH)2CO3 and KOH) activation strategy is adopted. The hydrothermal reaction of the straw precursor with the template agent Mg2(OH)2CO3 can deeply disintegrate, converting macromolecules such as lignin, cellulose, and hemicellulose into small molecules. At the same time, it is conducive to the deep burial of Mg2(OH)2CO3 within the product as a reaction template, providing favorable conditions for subsequent sufficient pyrolysis and carbonization. The dried product after the hydrothermal reaction is mixed with the pore-forming agent KOH for pyrolysis, and the reaction temperature is controlled to ensure the pyrolysis efficiency. At high temperature, Mg2(OH)2CO3 will be transformed into a liquid molten salt carrying KOH into the material for high-temperature pyrolysis and activation. Thanks to the small size of KOH and the penetration of Mg2(OH)2CO3, the pore structure of the prepared carbon material can match the ion size in the electrolyte of the zinc ion capacitor, thereby maximizing the kinetic equilibrium between the carbon positive electrode made of microporous carbon and the battery-type negative electrode, thereby improving the capacity, rate performance and cycle stability of the zinc ion capacitor.

[0082] The present application also provides a microporous carbon, which is prepared according to the preparation method of the microporous carbon in the above embodiment.

[0083] Optionally, the specific surface area of the microporous carbon is ≥ 2000 m 2 / g.

[0084] Optionally, the microporous carbon has micropores, and in the microporous carbon, the micropores with a pore diameter of 0.8 nm to 2 nm account for ≥50%, and the micropores with a pore diameter of 1.1 nm to 1.3 nm account for ≥40%.

[0085] The high specific surface area of microporous carbon provides a good environment for the distribution of active sites and defects, which can efficiently attract Zn 2+The intercalation layer is the key to improving the capacity of zinc ion capacitors. The pore size of the carbon material of this application is at least 50% distributed in the range of 0.8nm to 2nm, and at least 40% distributed in the range of 1.1nm to 1.3nm. A large distribution of pore sizes of 0.8nm to 2nm can effectively improve the specific surface area of the carbon material and provide more active sites. A large distribution of pore sizes of 1.1nm to 1.3nm facilitates the rapid transfer and storage of zinc ions. Specifically, for zinc ion capacitor electrolytes, the solvated ions in most zinc salt electrolytes are [Zn(H2O)6] 2+ , its size is approximately (0.86nm). In the micropore range of 0.8nm to 2nm, the larger pore size of carbon materials will make [Zn(H2O)6] 2+ The double electric layer shielding effect occurs during the transmission process, which limits the capacity, and the small pore size of the carbon material will make [Zn(H2O)6] 2+ During the transmission process, it interacts with the carbon wall, hindering rapid transmission. Therefore, the pore structure of the carbon electrode at 1.1nm to 1.3nm can maximize the kinetic balance between the carbon positive electrode made of microporous carbon and the battery-type negative electrode, thereby improving the capacity, rate performance and cycle stability of the zinc ion capacitor.

[0086] Optionally, the microporous carbon has a honeycomb shape.

[0087] The honeycomb structure not only creates a fast electron conduction network system, but also provides a high-speed channel for the adsorption and desorption of ions. The honeycomb structure is crucial for electrodes loaded with high-quality active materials. As the loading quality of the active material increases, the utilization rate of the active material decreases, and pore blockage is a major factor. The honeycomb carbon material structure allows the active material to maintain multiple ion transmission channels even under high loads, effectively improving the disadvantage of electrodes with high active material loading and slow ion transmission.

[0088] The microporous carbon of the present invention uses waste straw as raw material, and provides rich oxygen-containing functional groups for the microporous carbon without additional heteroatom additives. After the microporous carbon is made into the positive electrode of the zinc ion capacitor, the oxygen-containing functional groups it possesses can be used in Zn 2+ After being adsorbed into the carbon cathode material, a redox reaction occurs with it to form zinc-containing functional groups (CO x -Zn, etc.) and provide capacitance contribution to the electrode material.

[0089] The present application also provides a zinc ion capacitor positive electrode, which includes microporous carbon prepared by the preparation method of microporous carbon in the above embodiment or microporous carbon according to the above embodiment.

[0090] The positive electrode of the zinc ion capacitor of the present application is made of the microporous carbon material in the above embodiment, which has ultra-fast ion transport capability, ultra-high capacity and excellent cycle stability.

[0091] The present application also provides a zinc ion capacitor, which includes the zinc ion capacitor positive electrode in the above embodiment.

[0092] The zinc ion capacitor of the present application adopts the zinc ion capacitor positive electrode in the above embodiment, which is beneficial to improving the electrochemical performance of the zinc ion capacitor.

[0093] The microporous carbon and its preparation method, the positive electrode of the zinc ion capacitor and the zinc ion capacitor of the present application are further described in detail below with reference to the embodiments.

[0094] Example 1

[0095] The present invention provides a method for preparing microporous carbon, which comprises the following steps:

[0096] S1. Preparation of straw precursor

[0097] First, the discarded corn straw is ground into powder, and then the straw powder is placed in a sodium hydroxide solution with a concentration of 10 mol / L and soaked for 12 hours. After soaking in a strong alkaline solution, a first precursor is obtained. The first precursor is then placed in a hydrochloric acid solution with a volume ratio of hydrogen chloride to water of 1:3 and soaked for 12 hours. After soaking in the hydrochloric acid solution, it is filtered, washed to neutrality and dried to obtain a straw precursor.

[0098] S2. Hydrothermal reaction

[0099] The straw precursor and the template Mg2(OH)2CO3 were mixed and dissolved in 200 mL of deionized water, with the mass ratio of the straw precursor and the template Mg2(OH)2CO3 being 1:1, and transferred to a reactor for a hydrothermal reaction at a temperature of 120°C and a time of 12 h.

[0100] S3, high temperature pyrolysis

[0101] The product after the hydrothermal reaction is dried and mixed with the pore-forming agent KOH, and ground to obtain a mixed powder. The mass ratio of the dried hydrothermal reaction product and the pore-forming agent KOH is 1:1. The mixed powder is then placed under the protection of an inert gas for high-temperature pyrolysis. The high-temperature pyrolysis temperature is 800°C, the high-temperature pyrolysis time is 2 hours, the inert gas is nitrogen, and the nitrogen flow rate is 50 mL / min.

[0102] S4. Preparation of microporous carbon

[0103] The product after high-temperature pyrolysis is sequentially immersed in a hydrochloric acid solution with a volume ratio of hydrogen chloride to water of 1:3 for 12 hours, allowed to stand, filtered, and dried to remove the remaining ash and the incompletely reacted pore-forming agent to obtain microporous carbon.

[0104] Example 2

[0105] The embodiment of the present application provides a method for preparing microporous carbon. Compared with Example 1, the mass ratio of the product after the hydrothermal reaction of drying in S3 and high-temperature pyrolysis to the pore-forming agent KOH is changed to 0.5:1, while other parameters remain unchanged.

[0106] Example 3

[0107] The embodiment of the present application provides a method for preparing microporous carbon. Compared with Example 1, the mass ratio of the product after the hydrothermal reaction of S3 and the drying in the high-temperature pyrolysis to the pore-forming agent KOH is changed to 2:1, while other parameters remain unchanged.

[0108] Example 4

[0109] The embodiment of the present application provides a method for preparing microporous carbon. Compared with Example 1, the mass ratio of S2, the straw precursor and the template Mg2(OH)2CO3 in the hydrothermal reaction is changed to 1:2, while other aspects remain unchanged.

[0110] Example 5

[0111] The embodiment of the present application provides a method for preparing microporous carbon. Compared with Example 1, the mass ratio of S2, the straw precursor and the template Mg2(OH)2CO3 in the hydrothermal reaction is changed to 1:0.5, while other parameters remain unchanged.

[0112] Example 6

[0113] The embodiment of the present application provides a method for preparing microporous carbon. Compared with Example 1, the method changes S2 and the temperature of the hydrothermal reaction to 140° C., while keeping the other parameters unchanged.

[0114] Example 7

[0115] The embodiment of the present application provides a method for preparing microporous carbon. Compared with Example 1, the temperature of S3 and high-temperature pyrolysis is changed to 700° C., while other parameters remain unchanged.

[0116] Example 8

[0117] The embodiment of the present application provides a method for preparing microporous carbon. Compared with Example 1, the temperature of S3 and high-temperature pyrolysis is changed to 900° C., while other parameters remain unchanged.

[0118] Comparative Example 1

[0119] The comparative example of the present application provides a method for preparing microporous carbon, which comprises the following steps:

[0120] S1. Preparation of straw precursor

[0121] First, the discarded corn straw is ground into powder, and then the straw powder is placed in a sodium hydroxide solution with a concentration of 10 mol / L and soaked for 12 hours. After soaking in a strong alkaline solution, a first precursor is obtained. The first precursor is then placed in a hydrochloric acid solution with a volume ratio of hydrogen chloride to water of 1:3 and soaked for 12 hours. After soaking in the hydrochloric acid solution, it is filtered, washed to neutrality and dried to obtain a straw precursor.

[0122] S2. Hydrothermal reaction

[0123] The straw precursor and the template Mg2(OH)2CO3 were mixed and dissolved in 200 mL of deionized water, with the mass ratio of the straw precursor and the template Mg2(OH)2CO3 being 1:1, and transferred to a reactor for a hydrothermal reaction at a temperature of 120°C and a time of 12 h.

[0124] S3, high temperature pyrolysis

[0125] The product after the hydrothermal reaction was ground to obtain a powder, and then the powder was subjected to high-temperature pyrolysis under the protection of an inert gas. The high-temperature pyrolysis temperature was 800°C, the high-temperature pyrolysis time was 2 h, the inert gas was nitrogen, and the nitrogen flow rate was 50 mL / min.

[0126] S4. Preparation of microporous carbon

[0127] The product after high-temperature pyrolysis is sequentially immersed in a hydrochloric acid solution with a volume ratio of hydrogen chloride to water of 1:3 for 12 hours, allowed to stand, filtered, and dried to remove the remaining ash and the incompletely reacted pore-forming agent to obtain microporous carbon.

[0128] Comparative Example 2

[0129] The present invention provides a method for preparing microporous carbon, which comprises the following steps:

[0130] S1. Preparation of straw precursor

[0131] First, the discarded corn straw is ground into powder, and then the straw powder is placed in a sodium hydroxide solution with a concentration of 10 mol / L and soaked for 12 hours. After soaking in a strong alkaline solution, a first precursor is obtained. The first precursor is then placed in a hydrochloric acid solution with a volume ratio of hydrogen chloride to water of 1:3 and soaked for 12 hours. After soaking in the hydrochloric acid solution, it is filtered, washed to neutrality and dried to obtain a straw precursor.

[0132] S2. High temperature pyrolysis

[0133] The straw precursor and the pore-forming agent KOH were mixed and ground to obtain a mixed powder. The mass ratio of the straw precursor and the pore-forming agent KOH was 1:1. The mixed powder was then subjected to high-temperature pyrolysis under inert gas protection. The high-temperature pyrolysis temperature was 800°C, the high-temperature pyrolysis time was 2 hours, the inert gas was nitrogen, and the nitrogen flow rate was 50 mL / min.

[0134] S3. Preparation of microporous carbon

[0135] The product after high-temperature pyrolysis is sequentially immersed in a hydrochloric acid solution with a volume ratio of hydrogen chloride to water of 1:3 for 12 hours, allowed to stand, filtered, and dried to remove the remaining ash and the incompletely reacted pore-forming agent to obtain microporous carbon.

[0136] Comparative Example 3

[0137] The comparative example of the present application provides a method for preparing microporous carbon. Compared with Example 1, S2 and the template agent in the hydrothermal reaction are changed to MgCO3, while other aspects remain unchanged.

[0138] Comparative Example 4

[0139] The comparative example of the present application provides a method for preparing microporous carbon. Compared with Example 1, S2 and the template agent in the hydrothermal reaction are changed to Mg(OH)2, while other aspects remain unchanged.

[0140] Comparative Example 5

[0141] The comparative example of the present application provides a method for preparing microporous carbon. Compared with Example 1, the pore-forming agent in S3 and high-temperature pyrolysis is changed to NH4Cl, while other aspects remain unchanged.

[0142] Comparative Example 6

[0143] The comparative example of the present application provides a method for preparing microporous carbon. Compared with Example 1, the pore-forming agent in S3 and high-temperature pyrolysis is changed to ZnCO3·2Zn(OH)2·H2O, while other parameters remain unchanged.

[0144] Test Example 1

[0145] The specific surface area, pore size distribution and morphology of the microporous carbon prepared in Examples 1 to 8 and Comparative Examples 1 to 6 were measured and are shown in Table 1. Figure 1 is the SEM image of the microporous carbon obtained in Example 1, Figure 2 is the pore size distribution diagram of the microporous carbon obtained in Example 1, Figure 3 This is a pore size ratio diagram of the microporous carbon prepared in Example 1 of the present application. Figure 4 The microporous carbon species prepared in Example 1 of this application have different pore sizes for Zn(H2O)6 2+ Schematic diagram of adsorption energy, Figure 5 is the XRD pattern of the microporous carbon prepared in Example 1, Figure 6 is the Raman image of the microporous carbon obtained in Example 1, Figure 7 This is the XPS graph of the microporous carbon obtained in Example 1. Figures 8 to 20 They are SEM images of Examples 2 to 8 and Comparative Examples 1 to 6 respectively.

[0146] The test method is as follows:

[0147] 1. Specific surface area and pore size

[0148] The BET physical adsorption instrument uses gas molecules (N2) as a "measuring tool" to measure the specific surface area and pore structure of the material, and uses the adsorption characteristics of the solid material to measure the surface area and pore size distribution and adsorption-desorption curve of the material.

[0149] 2. Appearance

[0150] Scanning electron microscopy uses secondary electron signals to accurately image the surface microstructure of a sample. In this experiment, an S-4800 scanning electron microscope (accelerating voltage 5kV, current 10μA, working distance 6mm) was used to characterize the microstructure of the material.

[0151] Table 1 Parameters of microporous carbon obtained in Examples 1 to 8 and Comparative Examples 1 to 6

[0152]

[0153]

[0154] from Figure 1 It can be seen that the prepared waste straw-based honeycomb-shaped high specific surface area rich microporous carbon material presents a honeycomb porous structure, and the pore structures are connected by thin sheets of carbon material, which builds a stable conductive network for the carbon electrode material. The pore size distribution diagram, XRD diagram, XPS diagram and Raman diagram of Experimental Example 1 are further supplemented. Figure 2 Pore size distribution and Figure 3 In the pore size ratio chart, the results show that more than 50% of the pore size of the carbon electrode material is concentrated in the range of 0.8nm to 2nm, and more than 40% of the pore size is concentrated in the range of 1.1nm to 1.3nm. The large concentration of pore size in the range of 0.8nm to 2nm can effectively increase the specific surface area of the material. As shown in Table 1, Example 1 shows the highest surface area of 2559m 2 g -1 The concentration of pores in the range of 1.1 nm to 1.3 nm can effectively improve the adsorption capacity of the carbon cathode to electrolyte ions. This is because the pore size is between 1.1 nm and 1.3 nm, which is just in the range of [Zn(H2O)6] 2+ The optimal range of conduction and the matching pore structure reduce the kinetic differences between the carbon positive electrode and the battery-type negative electrode. Figure 4, the DRT theory was used to calculate the Zn(H2O)6 2+ The adsorption energy results of different pore sizes indicate that a pore structure that is too large will cause over-shielding response and limit capacity, while a pore structure that is too small will interact with the carbon wall. Therefore, the pore size of 1.1-1.3 nm is considered to be the best pore size for zinc storage. In addition, XRD ( Figure 5 ) and Raman ( Figure 6 ) The results show that the prepared waste straw-based honeycomb-shaped high specific surface area microporous carbon material is an amorphous carbon with abundant defects. The abundant defects and amorphous structure provide sufficient sites for the intercalation of zinc ions, effectively improving the capacity of zinc ion capacitors. In addition, the XPS graph ( Figure 7 ) showed that the microporous carbon contained abundant CO functional groups, which could promote the adsorption of zinc ions by carbon materials and undergo redox reactions with zinc ions to form new functional groups, such as CO x The formation of these new functional groups can provide additional capacitance contribution to zinc ion capacitors, improving the energy storage capacity of the device to a certain extent.

[0155] Figure 8 、 9 , 10, and 11 represent the activation conditions of changing the ratio of the pore-forming agent KOH or the template agent Mg2(OH)2CO3. Combined with the analysis of the specific surface area and pore size ratio in Table 1, it can be seen that changing the ratio of KOH or Mg2(OH)2CO3 within a relatively small dosage range will not cause qualitative changes in the parameters such as the morphology and pore size of the carbon material. Similarly, Examples 6, 7, and 8 change the hydrothermal or carbonization temperature within a given range, and the corresponding SEM images ( Figure 12 、 13 , 14) and the pore size parameters in Table 1 indicate that changing the hydrothermal temperature or carbonization temperature within the above given range will not cause qualitative changes in the parameters such as the morphology and pore size of the carbon materials.

[0156] In contrast, Figure 15 Comparative Example 1 uses only Mg2(OH)2CO3 for activation and pyrolysis to prepare carbon materials without adding KOH, and the corresponding morphology is mainly flaky. This shows that basic magnesium carbonate plays a major role as a template during the activation process. It is converted into a molten state at high temperature and immersed in the straw powder material, and is distributed inside the material to act as a template. As the temperature rises, basic magnesium carbonate is converted into magnesium oxide and gas, thereby exploiting the block structure of the straw powder and constructing a flaky structure, while MgO remains in the flaky structure and is washed away in the subsequent acid washing, leaving mesopores. Figure 16Comparative Example 2 shows the SEM image of the direct pyrolysis of straw powder with KOH. The carbon material in the figure presents a block structure, which indicates that without the infiltration of basic magnesium carbonate, KOH can only react outside the material and cannot further activate it. Comparative Examples 3 and 4 are carbon materials prepared by replacing the template in Example 1 with MgCO3 and Mg(OH)2, respectively. The corresponding SEM images ( Figure 17 and Figure 18 ) and pore size parameters (Table 1) show that the activation effect of the two is not good, and they cannot play a good template role during activation, resulting in poor specific surface area and pore size distribution of the materials. Comparative Examples 5 and 6 are carbon materials prepared by replacing the pore-forming agent in Example 1 with NH4Cl and ZnCO3·2Zn(OH)2·H2O, respectively. The corresponding SEM images ( Figure 19 and Figure 20 ) and pore size parameters (Table 1) indicate that these two pore-forming agents cannot effectively etch carbon materials and cannot make the carbon materials exhibit high specific surface area and corresponding honeycomb-like microporous morphology.

[0157] Test Example 2

[0158] Zinc ion capacitors were prepared using the microporous carbons of Examples 1 to 8 and Comparative Examples 1 to 6. Performance tests of the zinc ion capacitors were then performed, and the results are shown in Tables 2 to 3.

[0159] The test steps are as follows: charge and discharge capacity test, rate performance test, and cycle stability test. The zinc ion capacitor preparation process is as follows: the prepared waste straw-based honeycomb-shaped high-specific surface area and rich microporous carbon material is used as the active material, uniformly mixed with Super-P and polytetrafluoroethylene in a mass ratio of 8:1:1 and dispersed in N-methylpyrrolidone. After magnetic stirring for 12 hours, the material is evenly applied to carbon paper and then transferred to a drying oven. It is first dried at 55°C at normal pressure for 4 hours to remove the macromolecular solvent, and then dried at 80°C in a vacuum for 12 hours. After completion, it is removed and cut into 12mm diameter discs.

[0160] The zinc ion capacitor was assembled at room temperature, wherein the electrode shell used a 2032 button half-cell, the pole piece was the above-mentioned disc, the diaphragm used a glass fiber diaphragm, the negative electrode was a zinc foil (size: 16 mm in diameter, 1 mm in thickness), the electrolyte was 150 μL of 2M / L zinc sulfate, and the supporting and conductive materials used were a stainless steel gasket and spring with a diameter of 16 mm and a thickness of 1 mm and 1.2 mm, respectively.

[0161] The electrochemical performance tests were completed on a Xinwei battery testing system with a test voltage range of 0.2 to 1.8 V and a current density of 0.3 to 30 A / g.

[0162] Table 2 Capacities of zinc ion capacitors made from microporous carbon of Examples 1 to 8 and Comparative Examples 1 to 6 at different current densities

[0163]

[0164]

[0165] Table 3 Rate performance and cycle performance of zinc ion capacitors made from microporous carbon of Examples 1 to 8 and Comparative Examples 1 to 6

[0166] project <![CDATA[Rate performance (0.1 - 30Ag -1 )]]> <![CDATA[10Ag -1 Capacity retention after 100,000 cycles]]> Example 1 53.3％ 98.7％ Example 2 48.4％ 96.0％ Example 3 47.1％ 95.4％ Example 4 50.1％ 97.6％ Example 5 47.8％ 95.8％ Example 6 48.5％ 96.1％ Example 7 48.9％ 95.3％ Example 8 49.8％ 97.2％ Comparative Example 1 21.9％ 71.1％ Comparative Example 2 28.5％ 82.3％ Comparative Example 3 29.4％ 85.8％ Comparative Example 4 29.1％ 83.4％ Comparative Example 5 31.6％ 86.3％ Comparative Example 6 30.3％ 86.1％

[0167] From Table 2, it can be seen that the waste straw-based honeycomb-shaped high specific surface area and rich microporous carbon material provided by the present application has excellent zinc storage performance. Among them, the composite materials in Examples 1 to 8 have an excellent zinc storage performance at 0.3Ag -1 The specific capacity at the current density of 186.8 mAh g -1 、170.1mAh g -1 、165.3mAh g -1 , 180.1mAh g -1 , 168.3mAh g -1 , 173.6mAh g -1 , 175.4mAh g -1 , 178.3mAh g -1 The results show that the dual molten salt activation strategy of basic magnesium carbonate and KOH can effectively improve the capacity and corresponding energy density of carbon materials at low current density, even at 30 A g -1 At a high current density of 99.6 mAh g -1 、82.3mAh g -1 , 77.9mAh g -1 , 90.2mAh g -1 、80.4mAh g -1 , 84.2mAh g -1 , 85.8mAh g -1 , 88.8mAh g -1 The specific capacity of the activated porous structure is 53.3%, 48.4%, 47.1%, 50.1%, 47.8%, 48.5%, 48.9% and 49.8%, respectively (Table 3), which reveals the good reversible performance and rate performance of the porous structure constructed by this activation strategy. In particular, Table 3 also shows that at 10A g -1 At high current density, the cycling stability of Examples 1-8 is proportional to the pore size content of 1.1-1.3 nm and the specific surface area. This indicates that a large specific surface area and appropriate pore size effectively improve the zinc ion storage capacity and corresponding electrochemical performance.

[0168] In contrast, in Comparative Example 1, straw powder was mixed with basic magnesium carbonate for a hydrothermal reaction without being mixed with KOH for pyrolysis, resulting in a flaky, ultra-fine microporous structure. The results in Tables 2 and 3 show that the rate performance and cycling stability of the carbon cathode material with this structure are far inferior to those of the honeycomb-shaped, high-specific-surface-area, microporous carbon material in Example 1. This suggests that a pore structure that is mismatched with the electrolyte ions cannot provide a carbon electrode with both high reversible capacity and good cycling stability.

[0169] In Comparative Example 2, only KOH was used to activate the straw powder. The prepared carbon material only showed a bulk mesoporous structure, and the corresponding cycle stability and rate performance were the worst among the above materials. This is mainly because without the infiltration of basic magnesium carbonate, KOH can only react on the outside of the material and cannot further activate it. The surface pore structure cannot provide a suitable environment for the carbon positive electrode material to store zinc ions, thus limiting the performance of the zinc ion capacitor.

[0170] Comparative Examples 3 to 4 use templates other than Mg2(OH)2CO3, which makes the prepared carbon materials less than ideal in specific surface area and pore size distribution, thereby limiting the performance of the zinc ion capacitor.

[0171] Comparative Examples 5 to 6 use pore-forming agents other than KOH, resulting in unsatisfactory specific surface area and pore size distribution of the prepared carbon materials, thereby limiting the performance of the zinc ion capacitor.

[0172] The foregoing description is merely a specific embodiment of the present application and is not intended to limit the present application. Persons skilled in the art will readily appreciate that various modifications and variations are possible. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of the present application shall be included within the scope of protection of the present application.

Claims

1. A method for preparing microporous carbon, characterized in that: The preparation method of the microporous carbon comprises: firstly mixing a straw precursor with a template agent and performing a hydrothermal reaction, then mixing the dried product after the hydrothermal reaction with a pore-forming agent and performing high-temperature pyrolysis, and finally treating the product after the high-temperature pyrolysis with acid and drying it to obtain the microporous carbon; The straw precursor is straw powder that has been pretreated with strong acid and strong alkali in sequence; The template includes Mg2(OH)2CO3, and the pore-forming agent includes KOH; The temperature of the hydrothermal reaction is 120°C to 140°C, and the temperature of the high-temperature pyrolysis is 700°C to 900°C.

2. The method for preparing microporous carbon according to claim 1, wherein: The hydrothermal reaction time is 12h to 16h, and / or; The mass ratio of the straw precursor to the template is 1:0.5-2.

3. The method for preparing microporous carbon according to claim 1, wherein: The high temperature pyrolysis time is 1.5h to 3h, and / or; The mass ratio of the dried product after the hydrothermal reaction to the pore-forming agent is 0.5 to 2:1, and / or; The high-temperature pyrolysis is carried out under the protection of inert gas.

4. The method for preparing microporous carbon according to any one of claims 1 to 3, characterized in that: After the hydrothermal reaction is completed, the product after the hydrothermal reaction is dried to obtain a dried product after the hydrothermal reaction, and then the dried product after the hydrothermal reaction is ground with the pore-forming agent to obtain a mixed powder, and then the mixed powder is subjected to the high-temperature pyrolysis.

5. The method for preparing microporous carbon according to any one of claims 1 to 3, characterized in that: The straw precursor is prepared by the following method: First, the crushed straw powder is placed in a strong alkaline solution and soaked to obtain a first precursor; then the first precursor is placed in a strong acid solution and soaked in the strong alkaline solution, and then filtered, washed to neutrality, and dried to obtain a straw precursor; Wherein, the concentration of the strong alkaline solution is 8 mol / L to 12 mol / L, and / or; The strong alkaline solution includes sodium hydroxide solution and / or potassium hydroxide solution, and / or; The soaking time of the strong alkaline solution is 24h to 36h, and / or; The volume ratio of acid to water in the strong acid solution is 1:3 to 1:1, and / or; The strong acid solution includes at least one of a hydrochloric acid solution, a sulfuric acid solution and a nitric acid solution; The soaking time of the strong acid solution is 12 hours to 16 hours.

6. A microporous carbon, characterized in that The microporous carbon is prepared according to the method for preparing microporous carbon according to any one of claims 1 to 5.

7. The microporous carbon according to claim 6, characterized in that The specific surface area of the microporous carbon is ≥2000m 2 / g, and / or; The microporous carbon has micropores. In the microporous carbon, the micropores with a pore diameter of 0.8 nm to 2 nm account for ≥50%, and the micropores with a pore diameter of 1.1 nm to 1.3 nm account for ≥40%.

8. A zinc ion capacitor positive electrode, characterized in that The positive electrode of the zinc ion capacitor comprises microporous carbon prepared by the method for preparing microporous carbon according to any one of claims 1 to 5 or the microporous carbon according to any one of claims 6 to 7.

9. A zinc ion capacitor, characterized in that The zinc ion capacitor comprises the zinc ion capacitor positive electrode according to claim 8.

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