Preparation method of high specific capacitance surface modified activated carbon and application thereof

Abstract

The application discloses a preparation method of high specific capacitance surface modified activated carbon and application thereof, and belongs to the technical field of biomass pyrolysis methods. Biomass such as pine sawdust is used as a raw material, and clean energy is fully utilized by using the biomass to prepare the activated carbon, so that renewable resources are fully utilized; and the preparation process comprises drying, crushing, pyrolysis, washing and surface modification. The activated carbon provided by the application is used to prepare a working electrode for specific capacitance test, the surface functional groups are regulated, and the specific capacitance is improved; hot hydrochloric acid is used for acid washing during washing, can effectively remove ash, HNO3 is a strong oxidant and has corrosivity, strongly corrodes internal pores of the biochar, changes pore size structure of the biochar, and forms a large number of oxygen-containing functional groups in the internal pores of the biochar. After HNO3 modification, the content of C=O in the biochar is increased, more pseudo capacitance can be provided for the biochar, the specific capacitance of the biochar is increased, and therefore, the electrochemical performance of the biochar is good.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of biomass pyrolysis method, and particularly relates to a preparation method of high specific capacitance surface modified activated carbon and application thereof. BACKGROUND

[0002] The activated carbon prepared by activating the biomass carbon obtained by biomass pyrolysis has high carbon content, less ash, developed pore structure, large specific surface area, can adsorb more negative charges to form an electromagnetic field; in addition, the biomass-based activated carbon contains more highly specific aromatic surface functional groups, and has stable physical and chemical properties, and is an ideal electrode material.

[0003] At present, the activated carbon is mainly prepared by selecting waste biomass, and the biomass is a renewable resource which is relatively rich in China and has not been widely utilized. The common treatment method is incineration treatment, which causes resource waste, so the biomass activated carbon can obtain ideal electrode materials and improve the resource utilization rate.

[0004] The present team finds that concentrated nitric acid can modify the activated carbon, the method of surface modification of activated carbon is adopted, the pyrolysis product generated by biomass pyrolysis is used as raw material, the surface is modified by concentrated nitric acid, and then the activated carbon with high specific capacitance is obtained. The activated carbon is used for electrochemical test. SUMMARY

[0005] In view of the deficiencies and problems in the prior art, the present application aims to provide a preparation method of high specific capacitance surface modified activated carbon and application thereof.

[0006] The present application provides a preparation method of high specific capacitance surface modified activated carbon, and the specific steps are as follows:

[0007] S1, after the selected biomass is crushed, zinc chloride is selected as an activator, impregnated for 4-12 hours, pyrolysis activation is carried out, after the reaction is completed, the temperature is reduced to room temperature, and then taken out and ground into a powder with a particle size of less than 50 nm, to obtain a mixed powder;

[0008] S2, the mixed powder is uniformly mixed with an HCl solution, heated to boiling, then washed with deionized water until neutral, and placed in a 50-105 DEG C air drying oven for drying, to obtain a dried product;

[0009] S3, concentrated nitric acid or hydrogen peroxide is added to the dried product, and activated carbon is added at the same time, and the shaking bed is used at room temperature, after the reaction is completed, the filter is extracted, and then washed with deionized water until neutral, and dried to obtain the surface modified activated carbon.

[0010] Further, in the step S1, the biomass and the activator are in a ratio of 1:3-1:5, the pyrolysis activation temperature is 500-900 DEG C, and the reaction time is 20-60 min.

[0011] Further, in the S2 step, the concentration of the HCl solution used is 0.1 mol / L-1 mol / L, the amount used is 800 ml-1000 ml, after boiling, the reaction is stopped after heating for 5 min-30 min, and the temperature of the deionized water is 50°C-100°C.

[0012] Further, in the S3 step, the amount of activated carbon added is 1 g, the amount of concentrated nitric acid or hydrogen peroxide added is 30-60 mL, and the shaker time is 6 h-8 h.

[0013] The application of the surface-modified activated carbon prepared by the above method, wherein the surface-modified activated carbon is used to prepare a working electrode, and the working electrode is used for cyclic voltammetry characteristic curve and constant current charge-discharge test, and the specific steps are as follows:

[0014] (1) uniformly mixing the surface-modified activated carbon, the binder and the conductive agent, and then ultrasonicating for 10 min-30 min to obtain a mixture;

[0015] (2) first cleaning the foamed nickel with acetone and anhydrous ethanol by ultrasonicating for 5 min-10 min, then cleaning the foamed nickel with hydrochloric acid by ultrasonicating for 5 min-10 min, cleaning the foamed nickel with deionized water, and drying the foamed nickel to obtain a treated foamed nickel, and then cutting the treated foamed nickel into a 1 cm*2 cm rectangle, weighing m1, and reserving the treated foamed nickel for use;

[0016] (3) uniformly dropping and coating the mixture in step (1) on the treated foamed nickel, drying, and compressing to obtain a to-be-soaked working electrode, and then weighing m2;

[0017] (4) soaking the to-be-soaked working electrode to obtain a working electrode;

[0018] (5) using an electrochemical workstation CHI760 to perform cyclic voltammetry, and selecting-0.1V—0.9V as the parameter and 0.1V / s, 0.05V / s, 0.025V / s, and 0.01V / s as the scanning speed to perform cyclic voltammetry characteristic curve test;

[0019] (6) using an electrochemical workstation CHI760 to perform chronopotentiometry, and selecting-0.1V—0.9V as the parameter and 1A / g as the current density to perform constant current charge-discharge test.

[0020] Further, the binder is polytetrafluoroethylene, and the anhydrous ethanol is ultrasonicated for 60 min-120 min at an ultrasonic frequency of 40 KHZ; and the conductive agent is acetylene black.

[0021] Further, the ratio of the surface-modified activated carbon, the binder and the conductive agent in step (1) is 8:1:1.

[0022] Further, the soaking condition in step (4) is soaking in 3 mol / L KOH solution for 6-12 hours.

[0023] Further, the specific capacitance value of the surface-modified activated carbon is calculated by formula (1), v is the scanning rate (V / s), which is the slope of the charge-discharge curve:

[0024]

[0025] Compared with the prior art, the present application has the following advantages: (1) The present application provides a preparation method of surface-modified activated carbon, which uses biomass to prepare activated carbon to make full use of clean energy and make full use of renewable resources. (2) The present application provides an activated carbon preparation method for working electrode specific capacitance test, which regulates the surface functional groups to improve the specific capacitance. Hot hydrochloric acid is used for washing, which can effectively remove ash. HNO3 is a strong oxidizing agent and has corrosion property, which can strongly corrode the internal pores of the biochar, change the pore size structure of the biochar, and form a large number of oxygen-containing functional groups in the internal pores of the biochar. After HNO3 modification, the content of C=O in the biochar increases, which can provide more pseudo-capacitance for the biochar and increase the specific capacitance of the biochar, so that the electrochemical performance of the biochar is good. (3) The present application has an effect on the microporous structure and mesoporous structure of the biochar, and the pore structure of the biochar is improved by etching the pore structure of the biochar. HNO3 can etch the biochar to generate new micropores, which can penetrate into the interior of the biochar and react with amorphous carbon to produce new defect sites, and further modify the existing micropores, thereby affecting the micropore distribution of the biochar. BRIEF DESCRIPTION OF DRAWINGS

[0026] Figure 1 The constant current charge-discharge curve of the modified biochar prepared in Example 1 is shown in Figure 1.

[0027] Figure 2 The CV graph of the modified biochar prepared in Example 1 is shown in Figure 2.

[0028] Figure 3 The AC impedance spectrum of the working electrode in the three-electrode test system in Example 1 is shown in Figure 3.

[0029] Figure 4 The frequency response characteristic graph of the modified biochar prepared in Example 1 is shown in Figure 4.

[0030] Figure 5 The frequency response characteristic graph of the modified biochar prepared in Example 1 is shown in Figure 4.

[0031] Figure 6 The constant current charge-discharge curve of the modified biochar prepared in Example 2 is shown in Figure 5.

[0032] Figure 7 The CV graph of the modified biochar prepared in Example 2 is shown in Figure 6.

[0033] Figure 8 Impedance spectra of the working electrode under the three-electrode test system in Example 2;

[0034] Figure 9 Frequency response characteristics of the modified biochar prepared in Example 2;

[0035] Figure 10 Frequency response characteristics of the modified biochar prepared in Example 2.

[0036] Figure 11 Constant current charge-discharge curves of the modified biochar prepared in Example 3;

[0037] Figure 12 CV curves of the modified biochar prepared in Example 3;

[0038] Figure 13 Impedance spectra of the working electrode under the three-electrode test system in Example 3;

[0039] Figure 14 Frequency response characteristics of the modified biochar prepared in Example 3;

[0040] Figure 15 Frequency response characteristics of the modified biochar prepared in Example 3. DETAILED DESCRIPTION

[0041] Example 1

[0042] A preparation scheme of modified activated carbon is shown in the following steps:

[0043] Step 1: Pine sawdust is crushed by a pulverizer and sieved to obtain pine sawdust powder. The pine sawdust powder is dried, and a certain amount of pine sawdust powder is mixed with zinc chloride at a ratio of 1:4 to obtain a mixture. The mixture is immersed at room temperature for 12 hours. After the immersion is completed, the mixture is placed in a tube furnace under N2 atmosphere and pyrolyzed and activated at 600°C for 30 minutes. After the reaction is completed, the temperature is reduced to room temperature, and the mixture is taken out and ground into powder.

[0044] Step 2: The ground powder is poured into a beaker, and a certain amount of deionized water is added. 0.1 moL / L HCl solution is added to the mixture until boiling. After boiling for 10 minutes, heating is stopped. Vacuum filtration is performed using a vacuum pump. The obtained activated carbon is repeatedly washed with deionized water at 80°C until it is neutral. The activated carbon is dried in a 105°C air drying machine to obtain the activated carbon.

[0045] Step 3: The obtained activated carbon is added to a selected solution in a ratio of 1 g to 45 mL of concentrated nitric acid, and shaken in a shaker at room temperature for 6 hours. After the reaction is completed, vacuum filtration is used, and deionized water at room temperature is used for repeated washing until neutral. The product is placed in a 105°C air drying machine for drying, and the surface-modified activated carbon is obtained.

[0046] The application of the surface-modified activated carbon prepared by the above method is used to prepare a working electrode, and the working electrode is used for cyclic voltammetry and constant current charge-discharge test. The specific steps are as follows:

[0047] (1) A certain amount of 60% PTFE emulsion is dropped into a suitable amount of anhydrous ethanol, and ultrasonic treatment is performed for 120 min to break and dilute the PTFE emulsion, and a binder is obtained;

[0048] (2) The mixture is uniformly mixed in a ratio of surface-modified activated carbon: binder: conductive agent = 8:1:1, and ultrasonic treatment is performed for 15 min to uniformly disperse the mixture;

[0049] (3) The foam nickel is ultrasonically cleaned with propanol and anhydrous ethanol for 5 min to remove surface oil stains, and then ultrasonically cleaned with 2 mol / L hydrochloric acid for 5 min to remove the surface oxide layer. The residual hydrochloric acid is washed with deionized water, and the foam nickel is cut into a 1 cm x 2 cm rectangle after drying and weighing m1 for standby;

[0050] (4) The mixture is uniformly dropped onto the foam nickel using a pipette, dried, and then pressed into a tablet on a tablet press at a pressure of 10 MPa for 1 min. Then, m2 is weighed for standby; (the amount of active substance = (m1-m2)*0.8)

[0051] (5) Before testing, the prepared working electrode is soaked in a 3 mol / L KOH solution for 6 hours to allow the electrolyte to fully contact the active substance, and the working electrode preparation is completed.

[0052] (6) The cyclic voltammetry method is used to test the cyclic voltammetry characteristic curve of the working electrode prepared in the above step, and the parameters are selected as -0.1V—0.9V, and the scanning speed is selected as 0.1V / s, 0.05V / s, 0.025V / s, and 0.01V / s.

[0053] (7) The chronopotentiometry method is used to test the constant current charge-discharge of the working electrode prepared in the above step, and the parameters are selected as -0.1V—0.9V, and the current density is selected as 1A / g. The specific capacitance value is calculated by the following formula: v is the scanning speed (V / s), which is the slope of the charge-discharge curve.

[0054] The working electrode of the supercapacitor prepared from the activated carbon obtained in the embodiment was tested by galvanostatic charge-discharge at different current densities in a three-electrode system (the working electrode of the embodiment, a counter electrode and a reference electrode) in a 3M potassium hydroxide electrolyte. The unmodified biochar and the concentrated nitric acid modified biochar were PC and NPC-45, respectively. As shown in Figure 1 the galvanostatic charge-discharge curves of the modified biochar all maintained an approximate "isosceles triangle" shape, indicating that the modified biochar had good charge-discharge reversibility and electrochemical performance. After HNO3 modification, the cyclic charge-discharge curve of the biochar under high current density conditions did not deform significantly, indicating that the biochar had good rate performance after modification. According to the galvanostatic charge-discharge curve, the specific capacitance and energy density equivalent series resistance of the biochar were calculated, and the results are shown in Table 1. The specific capacitance of the modified biochar was greatly improved compared with that before modification. The specific capacitance of sample NPC-45 was as high as 338.88 F / g at a current density of 1 A / g, which increased by 137.39%. The pore structure of sample NPC-45 did not change significantly, and the oxygen-containing functional groups on the surface of the sample provided a large amount of pseudo-capacitance.

[0055] Table 1 Specific capacitance and energy density equivalent series resistance

[0056]

[0057] The supercapacitor PC / / NPC (two groups of working electrodes of the embodiment) assembled from the working electrode of the embodiment was tested by cyclic voltammetry at a rate of 10 mV / s, 25 mV / s, 50 mV / s and 100 mV / s in a 3M potassium hydroxide electrolyte, and the results are shown in Figure 2 It can be seen that the CV curve shape of the modified biochar did not deform significantly, indicating that the modified biochar had good rate performance. The CV curve of the modified biochar showed a convex response current platform in the voltage range, indicating the appearance of a wide peak symbolizing pseudo-capacitance, indicating that the sample had redox reactions during the charge-discharge process, providing Faradic pseudo-capacitance. This also shows that the oxygen-containing functional groups on the surface of the biochar can participate in electrochemical reactions and provide additional capacitance.

[0058] The working electrode NPC in the three-electrode test system in the embodiment was prepared from Figure 3It can be found that the modified biochar has a semicircle in the low frequency region, indicating the capacitive characteristics of the electrode material. The medium frequency region indicates the transition of the electrode material from capacitive behavior to resistive behavior, and the high frequency region indicates resistance and capacitance. The semicircle in the low frequency region is believed to represent the contribution of the redox ability of the material to the capacitance. In the medium frequency region, the inclined line of the alternating current impedance curve shows a slope of about 45°, indicating the diffusion resistance of ions at the electrode / electrolyte interface, which is called "Warburg" impedance. The angle between the straight line in the high frequency region and the horizontal coordinate axis is called "phase angle". Generally, the closer the "phase angle" is to 90°, the better the electrochemical performance of the material. In the high frequency region, the "phase angle" of the modified biochar is greater than that of the unmodified biochar, that is, the electrochemical performance of the modified biochar has been significantly improved. Figure 4 and Figure 5 is the frequency response characteristics of the biochar electrode, including the real part capacitive capacity (C') and the imaginary part capacitive (C"). The real part capacitive of the modified biochar is higher than that of the unmodified biochar, which is due to the further improvement of the pore structure of the biochar after modification and the incorporation of oxygen-containing functional groups to provide additional pseudo-capacitance.

[0059] Example 2

[0060] A preparation scheme of modified activated carbon is as follows:

[0061] Step 1: Pine sawdust is crushed by a pulverizer and sieved to obtain pine sawdust powder. A certain amount of pine sawdust powder is dried and mixed with zinc chloride at a ratio of 1:4. After 12 hours of immersion at room temperature, it is placed in a tube furnace under N2 atmosphere and pyrolyzed and activated at 600°C for 30 minutes. After the reaction is completed, the temperature is reduced to room temperature, and the product is ground into powder.

[0062] Step 2: The ground powder is poured into a beaker and a certain amount of deionized water is added. Add 0.1 moL / L HCl solution to the solution until boiling. Stop heating after boiling for 10 minutes. Use a vacuum pump to filter. The obtained activated carbon is repeatedly washed with deionized water at 80°C until it is neutral, and then dried in a 105°C air drying machine to obtain the activated carbon.

[0063] Step 3: The obtained activated carbon is added to the selected solution at a ratio of 1g to 30mL concentrated nitric acid. Shake in a shaking bed at room temperature for 6 hours. After the reaction is completed, use a vacuum pump to filter, repeatedly wash with deionized water at room temperature until it is neutral, and dry in a 105°C air drying machine to obtain the product as surface modified activated carbon.

[0064] The application of the surface modified activated carbon prepared by the above method, the surface modified activated carbon is used for preparing a working electrode, and the working electrode is used for cyclic voltammetry characteristic curve and constant current charge and discharge test, and the specific steps are as follows:

[0065] (1) Take a certain amount of 60% PTFE emulsion, drop into a suitable amount of anhydrous ethanol, ultrasonic for 120 min, break and dilute the PTFE emulsion, and obtain the binder;

[0066] (2) According to the ratio of surface modified activated carbon: binder: conductive agent = 8:1:1, uniformly mix, and ultrasonic for 15 min to make the mixture uniformly dispersed;

[0067] (3) The foam nickel is ultrasonically cleaned with propanol and anhydrous ethanol for 5 min to remove surface oil stains, then ultrasonically cleaned with 2 mol / L hydrochloric acid for 5 min to remove the surface oxide layer, washed with deionized water to remove residual hydrochloric acid, and dried and cut into 1 cm x 2 cm rectangular strips with a weight of m1 for standby;

[0068] (4) The mixture is evenly dropped on the foam nickel using a pipette, dried, and then pressed on a tablet press at a pressure of 10 MPa for 1 min, and then weighed m2 for standby; (the amount of active substance = (m1-m2)*0.8)

[0069] (5) Before testing, the prepared working electrode is soaked in 3 mol / L KOH solution for 6 h to make the electrolyte fully contact with the active substance, and the working electrode preparation is completed;

[0070] (6) The electrochemical workstation CHI760 cyclic voltammetry method is adopted, the parameters are selected as -0.1V—0.9V, and the scan rate is selected as 0.1V / s, 0.05V / s, 0.025V / s and 0.01V / s for scanning, and the cyclic voltammetry characteristic curve test is carried out;

[0071] (7) The electrochemical workstation CHI760 chronopotentiometry method is adopted, the parameters are selected as -0.1V—0.9V, and the current density is selected as 1A / g, and the constant current charge and discharge test is carried out. The specific capacitance value is calculated by the following formula v is the scan rate (V / s), that is, the slope of the charge and discharge curve.

[0072] The working electrode of the supercapacitor prepared by the activated carbon obtained in this embodiment is tested by constant current charge and discharge test with different current densities in 3M potassium hydroxide electrolyte through three electrode system (working electrode, counter electrode and reference electrode of this embodiment). The unmodified biochar and concentrated nitric acid modified biochar are PC and NPC-30 respectively. From Figure 6As shown, the constant current charge-discharge curves of the modified biochar all maintain an approximate "isosceles triangle" shape, indicating that the modified biochar has good charge-discharge reversibility and electrochemical performance. After HNO3 modification, the micropores of the biochar become larger and the number thereof decreases, affecting the electrochemical performance of the sample. According to the constant current charge-discharge curves, the specific capacitance and energy density equivalent series resistance of the biochar are calculated, and the results are shown in Table 2. The specific capacitance of the modified biochar is greater than that of the unmodified biochar. Among them, the specific capacitance of the sample NPC-30 is as high as 274.88 F / g at a current density of 1 A / g. The pore size structure of the sample NPC-30 does not change significantly, and the oxygen-containing functional groups on the surface of the sample provide a large amount of pseudo-capacitance.

[0073] Table 2 Specific capacitance and energy density equivalent series resistance

[0074]

[0075] The supercapacitor PC / / NPC (two groups of working electrodes of the present embodiment) assembled by the working electrodes of the present embodiment is subjected to cyclic voltammetry test at a rate of 10 mV / s, 25 mV / s, 50 mV / s and 100 mV / s in a 3M potassium hydroxide electrolyte, and the results are shown in Figure 6. Figure 7 It can be seen that the CV curve shape of the modified biochar does not change significantly, indicating that the modified biochar has good rate performance.

[0076] In the three-electrode test system of the present embodiment, the working electrode NPC is prepared by Figure 8 It can be found that the modified biochar has a semicircle in the low frequency region, indicating the capacitive characteristics of the electrode material. The medium frequency region indicates the transition of the electrode material from capacitive behavior to resistive behavior, and the high frequency region indicates resistance and capacitance. The semicircle in the low frequency region is considered to represent the contribution of the redox ability of the material to the capacitance. In the medium frequency region, the inclined line of the alternating current impedance curve shows a slope of about 45°, indicating the diffusion resistance of ions at the electrode / electrolyte interface, which is called "Warburg" impedance. The angle between the straight line in the high frequency region and the horizontal coordinate axis is called "phase angle". Generally, the closer the "phase angle" is to 90°, the better the electrochemical performance of the material. In the high frequency region, the "phase angle" of the modified biochar is greater than that of the unmodified biochar, that is, the electrochemical performance of the modified biochar is obviously improved. Figure 9 and Figure 10 are the frequency response characteristics of the biochar electrode, including the real part capacitive capacity (C') and the imaginary part capacitive (C"). The real part capacitive of the modified biochar is higher than that of the unmodified biochar, which is due to the further improvement of the pore structure of the biochar after modification and the incorporation of oxygen-containing functional groups to provide additional pseudo-capacitance.

[0077] Example 3

[0078] A preparation scheme of modified activated carbon, as shown in the following steps:

[0079] Step 1: Pine sawdust is crushed by a crusher and sieved to obtain pine sawdust powder. The pine sawdust powder is dried, and a certain amount of pine sawdust powder is mixed with zinc chloride at a ratio of 1:4 and uniformly mixed at room temperature for 12 h. After the impregnation is completed, it is placed in a tube furnace under N2 atmosphere, and pyrolysis activation is carried out at 600℃ for 30 min. After the reaction is completed, the temperature is reduced to room temperature, and the product is ground into powder.

[0080] Step 2: The ground powder is poured into a beaker, and a certain amount of deionized water is added. 0.1 moL / L HCl solution is added to the solution until boiling. After boiling for 10 min, stop heating. Use a vacuum pump to filter, and then wash the obtained activated carbon with 80℃ deionized water until it is neutral. Dry it in a 105℃ air drying machine to obtain activated carbon.

[0081] Step 3: The obtained activated carbon is added to the selected solution at a ratio of 1 g to 60 mL of concentrated nitric acid. Shake in a shaker at room temperature for 6 h. After the reaction is completed, use a vacuum pump to filter, and then wash with room temperature deionized water until it is neutral. Dry it in a 105℃ air drying machine to obtain the product as surface modified activated carbon.

[0082] The application of the surface modified activated carbon prepared by the above method, the surface modified activated carbon is used to prepare a working electrode, and the working electrode is used for cyclic voltammetry characteristic curve and constant current charge and discharge test, and the specific steps are as follows:

[0083] (1) Take a certain amount of 60% PTFE emulsion, drop into a suitable amount of anhydrous ethanol, and ultrasonic for 120 min. Break and dilute the PTFE emulsion to obtain a binder;

[0084] (2) Mix uniformly according to the ratio of surface modified activated carbon: binder: conductive agent = 8:1:1, and ultrasonic for 15 min to make the mixture uniformly dispersed;

[0085] (3) Ultrasonic clean the foam nickel with propanol and anhydrous ethanol for 5 min to remove surface oil stains, then ultrasonic clean with 2 moL / L hydrochloric acid for 5 min to remove surface oxide layer, clean with deionized water to remove residual hydrochloric acid, dry and cut into 1 cm x 2 cm rectangular strips with a weight of m1 for standby;

[0086] (4) Use a pipette to evenly drop the mixture onto the foam nickel, dry, and then press the tablet on a tablet press at a pressure of 10 MPa for 1 min, and then weigh m2 for standby; (the amount of active substance = (m1-m2)*0.8)

[0087] (5) Before testing, immerse the prepared working electrode in a 3 mol / L KOH solution for 6 hours to ensure that the electrolyte and the active material are in full contact. The preparation of the working electrode is then complete.

[0088] (6) Cyclic voltammetry was performed using a CHI760 electrochemical workstation with parameters ranging from -0.1V to 0.9V and scan rates of 0.1V / s, 0.05V / s, 0.025V / s, and 0.01V / s, respectively, to test the cyclic voltammetric characteristic curves.

[0089] (7) A chronopotential method was performed using a CHI760 electrochemical workstation with parameters selected as -0.1V to 0.9V and a current density of 1A / g. Constant current charge-discharge tests were conducted. The specific capacitance value was calculated using the following formula. v is the scan rate (V / s), which is the slope of the charge-discharge curve.

[0090] The working electrode of the supercapacitor prepared from the activated carbon obtained in this embodiment was subjected to constant current charge-discharge tests at different current densities in 3M potassium hydroxide electrolyte using a three-electrode system (working electrode, counter electrode, and reference electrode in this embodiment). The unmodified biochar and the concentrated nitric acid-modified biochar were PC and NPC-60, respectively. Figure 11 As shown, the galvanostatic charge-discharge curves of the modified biochar all maintained an approximately "isosceles triangle" shape, indicating that the modified biochar has good charge-discharge reversibility and electrochemical performance. After HNO3 modification, some micropores of the biochar were transformed into mesopores, affecting the electrochemical performance of the sample. Based on the galvanostatic charge-discharge curves, the specific capacitance and energy density equivalent series resistance of the biochar were calculated, and the results are shown in Table 3. The specific capacitance of the modified biochar was significantly improved compared with that before modification, with the specific capacitance of sample NPC-60 reaching as high as 299.63 F / g at a current density of 1 A / g.

[0091] Table 3. Equivalent series resistance for specific capacitance and energy density

[0092]

[0093] The supercapacitor PC / / NPC (two sets of working electrodes in this embodiment) assembled with the working electrodes of this embodiment was subjected to cyclic voltammetry tests in 3M potassium hydroxide electrolyte at rates of 10mV / s, 25mV / s, 50mV / s, and 100mV / s. Figure 12 It can be seen that the CV curve of the modified biochar shows a bulge in the current plateau during the voltage range, with a broad peak indicating pseudocapacitance. This indicates that the sample underwent a redox reaction during charge and discharge, providing Faraday pseudocapacitance. This also demonstrates that the oxygen-containing functional groups on the surface of biochar can participate in electrochemical reactions and provide additional capacitance.

[0094] The working electrode NPC in the three-electrode test system in the embodiment is prepared by Figure 13 It can be found that the modified biochar has a semicircle in the low frequency region, indicating the capacitive characteristics of the electrode material. The medium frequency region indicates the transition of the electrode material from capacitive behavior to resistive behavior, and the high frequency region indicates resistance and capacitance. The semicircle in the low frequency region is considered to represent the contribution of the redox ability of the material to the capacitance. In the medium frequency region, the inclined line of the AC impedance curve shows a slope of about 45°, indicating the diffusion resistance of ions at the electrode / electrolyte interface, which is called "Warburg" impedance. The angle between the straight line in the high frequency region and the horizontal coordinate axis is called "phase angle". It is generally believed that the closer the "phase angle" is to 90°, the better the electrochemical performance of the material. In the high frequency region, the "phase angle" of the modified biochar is greater than that of the unmodified biochar, that is, the electrochemical performance of the modified biochar has been significantly improved. Figure 14 and Figure 15 are the frequency response characteristics of the biochar electrode, including the real part capacitive capacity (C') and the imaginary part capacitive (C"). The real part capacitive of the modified biochar is higher than that of the unmodified biochar, which is due to the further improvement of the pore structure of the biochar after modification and the incorporation of oxygen-containing functional groups to provide additional pseudo-capacitance.

[0095] The above only expresses the preferred embodiments of the present application, which are described in more detail and in more detail, but cannot be understood as limiting the scope of the patent of the present application. It should be noted that for ordinary skilled in the art, without departing from the concept of the present application, a number of modifications, improvements and substitutions can be made, which are within the scope of protection of the present application. Therefore, the scope of protection of the patent of the present application should be subject to the appended claims.

Claims

1. A method for preparing a high specific capacitance surface-modified activated carbon, characterized by, The specific steps are as follows: S1, after the selected biomass is crushed, zinc chloride is selected as the activator, impregnated for 4-12 hours, and then pyrolysis activation is performed, after the reaction is completed and the temperature is reduced to room temperature, the product is taken out and ground into a powder with a particle size of less than 50 nm, and a mixed powder is obtained; S2, after the mixed powder is uniformly mixed with an HCl solution and heated to boiling, it is washed with deionized water until it is neutral, and then dried in a 50-105°C air drying oven to obtain a dry product; S3, concentrated nitric acid or hydrogen peroxide is added to the dry product, and activated carbon is added at the same time, and the mixture is shaken at room temperature, then filtered, washed with deionized water until it is neutral, and dried to obtain a surface-modified activated carbon; In the S1 step, the ratio of biomass to activator is 1:4-1:5, the pyrolysis activation temperature is 500-900°C, and the reaction time is 20-60 minutes; The amount of activated carbon added is 1g, the amount of concentrated nitric acid or hydrogen peroxide added is 30-60mL, and the shaking time is 6-8 hours; In the S2 step, the concentration of the HCl solution used is 0.1-1mol / L, and the amount used is 800-1000ml. After boiling, the reaction is stopped after 5-30 minutes of heating. The temperature of the deionized water is 50-100°C.

2. Use of the surface-modified activated carbon produced according to claim 1, characterized in that The surface-modified activated carbon is used to prepare a working electrode, and the working electrode is used for cyclic voltammetry and constant current charge-discharge tests. The specific steps are as follows: (1) The surface-modified activated carbon, binder, and conductive agent are uniformly mixed and ultrasonicated for 10-30 minutes to obtain a mixture; (2) The foam nickel is first cleaned with acetone and anhydrous ethanol for 5-10 minutes, then cleaned with hydrochloric acid for 5-10 minutes, and then cleaned with deionized water and dried. The foam nickel is cut into 1cm*2cm rectangular strips, weighed m1, and used as a treated foam nickel; (3) The mixture obtained in step (1) is uniformly dropped onto the treated foam nickel, dried, and compressed using a tablet press to obtain a to-be-soaked working electrode, which is weighed m2; (4) The working electrode is prepared by soaking the to-be-soaked working electrode; (5) The cyclic voltammetry curve is tested by using an electrochemical workstation CHI760 with parameters of -0.1V—0.9V and scan speeds of 0.1V / s, 0.05V / s, 0.025V / s, and 0.01V / s; (6) The constant current charge-discharge test is performed by using an electrochemical workstation CHI760 with parameters of -0.1V—0.9V and a current density of 1A / g.

3. The use of the surface-modified activated carbon according to claim 2, characterized in that, The binder is polytetrafluoroethylene, which is ultrasonicated in anhydrous ethanol for 60-120 minutes at a frequency of 40KHZ. The conductive agent is acetylene black.

4. The use of the surface-modified activated carbon according to claim 2, characterized in that, The ratio of surface-modified activated carbon, binder, and conductive agent in step (1) is 8:1:

1.

5. The use of the surface-modified activated carbon according to claim 2, characterized in that, The soaking conditions in step (4) are soaking in a 3mol / L KOH solution for 6-12 hours.

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