CN113436900B

CN113436900B - Nitrogen-doped carbon-based electrode based on nickel-cobalt double hydroxide, preparation method and super capacitor

Info

Publication number: CN113436900B
Application number: CN202110716280.6A
Authority: CN
Inventors: 罗勇锋; 李祖伟; 周璀; 李贤军
Original assignee: Central South University of Forestry and Technology
Current assignee: Central South University of Forestry and Technology
Priority date: 2021-06-28
Filing date: 2021-06-28
Publication date: 2022-07-15
Anticipated expiration: 2041-06-28
Also published as: CN113436900A

Abstract

The nitrogen-doped carbon-based electrode based on the nickel-cobalt double hydroxide comprises a nitrogen-doped carbonized wood chip substrate, wherein a tube cell structure is arranged in the nitrogen-doped carbonized wood chip substrate, and the nickel-cobalt double hydroxide is electrodeposited on the inner wall of the tube cell structure in the nitrogen-doped carbonized wood chip substrate. In the invention, the concentration is 5mAcm^‑2The high specific capacity of NiCo-LDH @ NDC single electrode is 14.26mAhcm^‑2. The mixed type super capacitor with NiCo-LDH @ NDC as an anode and NDC electrode as a cathode is 5mAcm^‑2Has a width of 4.74Fcm^‑2The retention ratio of the capacitance after 8000 charge-discharge cycles of (2) was 93.15%. The maximum energy density is 1.48mWhcm^‑2And the maximum power density is 22.4mWcm^‑2。

Description

Nitrogen-doped carbon-based electrode based on nickel-cobalt double hydroxide, preparation method and super capacitor

Technical Field

The invention relates to an electrode material of a super capacitor, in particular to a nitrogen-doped carbon-based electrode based on nickel-cobalt double hydroxide and a preparation method thereof.

Background

The Super Capacitor (SCs) has the advantages of fast charge and discharge, long service life and high power density, and is an attractive energy storage device of a wind power generation system, military equipment, a distributed energy storage system and a hybrid electric vehicle. Generally, the size of the porous carbon material ranges from micro to nano-structured electrode material. Other additives must be introduced as binders to bind the particles together when manufacturing the electrode material.

Woody monolithic carbon materials having a tracheid arrangementWhen the material is an electrode material, a conductive agent or an adhesive is not needed, and the self-supporting electrode can be directly prepared. However, wood porous carbon has significant drawbacks as an electrode material. For example, an SC assembled based on a single carbon material has a low energy density and limited active sites for activating the surface of wood carbon, and due to the presence of various oxidation functional groups, the porous carbon material has a reduced energy storage capacity, and the cycle stability of the SCs is poor. In order to overcome the above-mentioned disadvantages, some measures, such as modifying the carbon material with a pseudocapacitive active material such as a metal oxide or a conductive polymer, are currently the most promising approaches to increase the energy density of the carbon material. In the pseudocapacitance material, transition metal compounds (such as oxide/hydroxide and Layered Double Hydroxide (LDH)) have reversible redox reaction on the surfaces of the active materials, and have wide application prospect in the field of SCs. For example, Co (OH)₂It has attracted a great deal of attention in terms of high theoretical capacitance, low cost and typical redox chemistry. Ni (OH)₂Has a uniform structure of layered interstitial nickel, which is a layered nickel hydroxide with high theoretical capacitance, and has attracted much attention due to its excellent electrochemical properties. The combination of the two materials described above can provide complementary advantages and has been widely studied. However, these active materials are not strongly adhered to the carbon substrate, resulting in poor stability of the electrode.

Disclosure of Invention

The invention aims to overcome the defects of the prior art and provides a nitrogen-doped carbon-based electrode based on nickel-cobalt double hydroxide and having high specific capacitance and good cycle stability and a preparation method thereof.

In order to solve the technical problems, the technical scheme provided by the invention is as follows: a nickel cobalt double hydroxide based nitrogen doped carbon based electrode comprising a nitrogen doped wood carbide sheet matrix having a cell structure therein with nickel cobalt double hydroxide (NiCo-LDH) electrodeposited on the interior walls of the cell structure within the nitrogen doped wood carbide sheet matrix.

In the above nitrogen-doped carbon-based electrode based on nickel-cobalt double hydroxide, preferably, the nickel-cobalt double hydroxide is attached to the inner wall of the tube cell structure in the nitrogen-doped wood carbide substrate in a mesh-like staggered structure.

The preparation method of the nitrogen-doped carbon-based electrode based on the nickel-cobalt double hydroxide comprises the following steps;

1) drying the wood, and cutting into wood chips with preset sizes;

2) carbonizing, namely placing the wood chips obtained in the step 1) in a hot air drying box for pre-carbonizing for 4-8 h, and carbonizing for 8-12 h at 800-1200 ℃ under the protection of Ar gas to obtain carbonized wood sheets (OWC);

3）CO₂and (3) activation: carbonizing wood flakes in CO₂Activating for 8-12 h in Ar mixed gas flow, and cutting or grinding to a preset size; the activation temperature is 650-850 ℃; ar gas flow rate of CO ₂3 times the flow rate of CO₂The flow rate is 80-120 sccm.

4) Removing impurities to obtain activated wood carbon sheets (OWC);

5) nitrogen doping: mixing the carbonized active wood electrode after impurity removal with potassium hydroxide and urea in deionized water, wherein the weight ratio of the carbonized active wood electrode to the potassium hydroxide to the urea is 1: 1: 1; stirring for more than 1 hour at room temperature by using a magnetic stirrer; after drying, keeping the temperature at 600-1000 ℃ for 1-3 hours; obtaining a nitrogen-doped carbonized active wood matrix (NDC);

6) co (NO) at 0.1mol/L₃)₂·6H₂O and 0.1mol/L of Ni (OH)₂·6H₂Electrodepositing nickel cobalt double hydroxide (NiCo-LDH) onto a nitrogen-doped carbonized activated wood substrate in a mixed solution of O under conditions of-1 vvs.sce; taking a nitrogen-doped carbonized active wood substrate as a working electrode, a platinum electrode as a counter electrode and a saturated calomel electrode as a reference electrode; the electrodeposition time is 1-6 hours; after drying, a nitrogen-doped carbon-based electrode of nickel cobalt double hydroxide (NiCo-LDH @ NDC) was obtained.

In the above method for preparing a nitrogen-doped carbon-based electrode based on nickel-cobalt double hydroxide, preferably, the removing impurities in step 4) includes the following steps;

respectively carrying out ultrasonic treatment on deionized water, hydrochloric acid with the weight concentration of 2% and absolute ethyl alcohol in an ultrasonic cleaning machine for 20 minutes to remove inorganic salts and amorphous carbon in the deionized water;

then sonicated with deionized water to bring its pH = 7;

the cleaned flakes were placed in a forced air drying oven and dried at 100 ℃ for 2 hours.

In the above method for preparing a nitrogen-doped carbon-based electrode based on nickel-cobalt double hydroxide, preferably, the nitrogen-doped carbonized active wood substrate obtained in step 6) is washed with 6% by weight hydrochloric acid and deionized water.

A super capacitor comprises the nitrogen-doped carbon-based electrode based on the nickel-cobalt double hydroxide.

Compared with the prior art, the invention has the advantages that: in the invention, after nitrogen doping, in the carbon-nitrogen layer, nitrogen can be tightly combined with metal ions, thereby effectively adjusting an electronic structure, forming a strong coupling effect, enhancing the interaction with nickel-cobalt double hydroxide (NiCo-LDH) and enabling the material to be more stable on a nitrogen-doped carbonized wood (NDC) material.

The invention adopts a wood monolithic carbon material as a conductive bracket. The carbonized wood is subjected to nitrogen doping treatment by adopting a green and simple method, and the novel electrode with excellent electrochemical performance is prepared. And (3) carrying out nitrogen doping treatment on the wood by using urea as a nitrogen source and KOH as an activating agent. The combination of nitrogen and carbon increases the hydrophilicity of the wood surface and increases the active sites of the wood. At 5mA cm⁻²The specific capacity of a carbon-based electrode (NiCo-LDH @ OWC) single electrode with nickel-cobalt double hydroxide is 4.72 mAh cm⁻². At 5mA cm⁻²The high specific capacity of a NiCo-LDH @ NDC single electrode is 14.26mAh cm⁻². By NiCo-The mixed super capacitor with LDH @ NDC as a cathode and NDC as an anode is at 5mA cm⁻²Has a width of 4.74F cm⁻²The retention ratio of the capacitance after 8000 charge-discharge cycles of (2) was 93.15%. The maximum energy density is 1.48mWh cm⁻²And the maximum power density is 22.4mW cm⁻²。

Drawings

FIG. 1 is a process of making NiCo-LDH @ NDC electrodes with high loading and stable structure.

FIG. 2 shows the angle of hydrophilicity tests of OWC and NDC slides.

FIG. 3 is an SEM image of a cross-section of a NiCo-LDH @ NDC slide.

Fig. 4 is a side view image showing a tunnel structure.

Fig. 5 is an enlarged image of fig. 4.

FIG. 6 is a transmission electron microscope image of NiCo-LDH sheets.

FIG. 7 is a high resolution TEM image of NiCo-LDH sheets.

FIG. 8 is a high resolution TEM image of NiCo-LDH sheets.

FIG. 9 is an electron diffraction pattern of selected regions of a NiCo-LDH sheet.

FIG. 10 is a TEM image and a C/O/N/Ni/Co element mapping image of NiCo-LDH @ NDC electrode slice.

FIG. 11 is an infrared spectrum of an OWC electrode, an NDC electrode, and a NiCo-LDH @ NDC electrode.

FIG. 12 is a Raman spectrum of an OWC electrode, an NDC electrode, and a NiCo-LDH @ NDC electrode.

FIG. 13 is an XPS measurement spectrum of a NiCo-LDH @ NDC electrode.

FIG. 14 is a high resolution C1s spectrum of a NiCo-LDH @ NDC electrode.

FIG. 15 is a high resolution O1s spectrum of a NiCo-LDH @ NDC electrode.

FIG. 16 is a high resolution N1s spectrum of a NiCo-LDH @ NDC electrode.

FIG. 17 is a high resolution Ni 1s spectrum of a NiCo-LDH @ NDC electrode.

FIG. 18 is a high resolution Co1s spectrum of a NiCo-LDH @ NDC electrode.

FIG. 19 is a graph showing the specific capacitance performance of NiCo-LDH @ NDC electrodes at different electrodeposition times.

FIG. 20 shows that NiCo-LDH @ NDC electrode material electrodeposited for 5h is 1-10mV s under a window of-0.2V to 0.5V^－1Cyclic voltammogram at sweep rate.

FIG. 21 is a schematic illustration of 5-30 mA cm of NiCo-LDH @ NDC electrode material electrodeposited for 5h, at a window of-0.2V to 0.4V^－2Charge and discharge curves at current density.

FIG. 22 is a graph of NiCo-LDH @ OWC and NiCo-LDH @ NDC electrodes at a current density of 100 mA cm^－2And capacity retention rate chart of 6000 times of charging and discharging.

FIG. 23 is an impedance profile of NiCo-LDH @ OWC and NiCo-LDH @ NDC electrodes.

Fig. 24 is a partial enlarged view of fig. 23.

FIG. 25 is a graph showing a scan rate of 5 mV s⁻¹CV curves for NiCo-LDH @ NDC electrode and NDC electrode.

FIG. 26 is a 5-30 mV s under the window of NiCo-LDH @ NDC// NDC HSC device (hybrid supercapacitor) -0.2V to 0.5V^-1CV curve at sweep speed.

FIG. 27 is a 5-30 mA cm mA/NDC HSC device with a-0.2V to 0.4V window for NiCo-LDH @ NDC// NDC HSC devices^-2GCD curve at current density.

FIG. 28 shows 5-30 mA cm^－2Area specific capacitance and mass specific capacitance of HSC devices at current density.

FIG. 29 is a NiCo-LDH @ NDC// NDC HSC device at 80 mA cm^－2Cyclability performance of HSC devices at current density.

FIG. 30 is an impedance profile of a NiCo-LDH @ NDC// NDC HSC device.

FIG. 31 is an illustration of a NiCo-LDH @ NDC// NDC HSC device and electronic ligation circuitry.

FIG. 32 is a schematic diagram of the 1.5V electronic watch being lit at different times

Detailed Description

In order to facilitate an understanding of the present invention, the present invention will be described more fully and in detail with reference to the preferred embodiments, but the scope of the present invention is not limited to the specific embodiments described below.

Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. The terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the scope of the present invention.

Example 1

The nitrogen-doped carbon-based electrode based on the nickel-cobalt double hydroxide comprises a nitrogen-doped carbonized wood chip substrate, wherein a tube cell structure is arranged in the nitrogen-doped carbonized wood chip substrate, and the nickel-cobalt double hydroxide is electrodeposited on the inner wall of the tube cell structure in the nitrogen-doped carbonized wood chip substrate. The nickel-cobalt double hydroxide is attached to the inner wall of the tracheid structure in the nitrogen-doped carbonized wood sheet substrate in a net-shaped staggered structure.

A preparation method of a nitrogen-doped carbon-based electrode based on nickel-cobalt double hydroxide comprises two major steps of:

firstly, preparing an NDC electrode:

the fir wood was dried and then crosscut to obtain a wood chip having a length of 6 cm, a width of 3 cm and a thickness of 2 mm, and dried for 6 hours for a pre-carbonization treatment. Then, the sheet was put into a 1000 ℃ tube furnace, carbonized at an argon flow rate of 400sccm, then placed into a 750 ℃ tube furnace, and activated for 10 hours by introducing argon of 300sccm and carbon dioxide gas of 100sccm, and then polished and ground into 0.8 mm thin pieces, which were then ultrasonically treated with deionized water, hydrochloric acid having a weight concentration of 2%, and absolute ethanol in an ultrasonic cleaner for 20 minutes, respectively, to remove inorganic salts and amorphous carbon therein, and then ultrasonically treated with deionized water to have a pH =7, and the cleaned thin pieces were put into a forced air drying furnace and dried at 100 ℃ for 2 hours to obtain activated wood carbon (OWC) pieces. The obtained OWC tablets were mixed with an activator (potassium hydroxide) and a nitrogen source (urea) in 100mL of deionized water, the weight ratio of the OWC tablets, potassium hydroxide and urea being 1: 1: 1. the flakes were stirred with a magnetic stirrer for 2 hours at room temperature. The precursor mixture was evaporated to dryness at 60 ℃ and then heated to 800 ℃ in a tube furnace for 2 hours. After cooling to room temperature, the sheet was washed with 6% by weight hydrochloric acid and deionized water to obtain an NDC sheet.

Secondly, preparing a NiCo-LDH @ NDC electrode:

at 0.1mol/L of Co (NO)₂·6H₂O and Ni (OH)₂·6H₂In a mixed solution of O, NiCo-LDH was electrodeposited onto NDC material under conditions of-1 Vvs. And carrying out electrochemical deposition in a three-electrode system by taking the NDC support as a working electrode, a platinum electrode as a counter electrode and a saturated calomel electrode as a reference electrode, wherein the deposition time is 5 h. NiCo-LDH @ NDC electrode material was then obtained by drying at 80 ℃.

The embodiment also provides an asymmetric hybrid supercapacitor, wherein the NDC electrode is a negative electrode, the NiCo-LDH @ NDC electrode is a positive electrode, the non-woven fabric is used as a diaphragm, and all materials are uniformly coated with the PVA-KOH electrolyte. The PVA-KOH electrolyte was prepared by immersing 1.2 g of PVA in 10 g of deionized water for 12 hours to allow it to absorb sufficient water for expansion, followed by stirring at 90 ℃ for 3 hours, cooling to room temperature, adding 0.6 g of KOH, and stirring sufficiently.

In this embodiment, fig. 1 illustrates a process for fabricating a ni-co double hydroxide based n-doped c-based electrode. The cedar wood is pre-carbonized and carbonized to obtain OWC slices. The pores of the OWC are regular and uniform, and the average size is about 20 mu m, so that enough space is provided for loading active substances on the inner wall of the OWC. The obtained OWC was then soaked in a liquid of an activator (KOH) and a nitrogen source (urea), and after sufficient soaking, fired at 800 ℃ using argon as a shielding gas to obtain NDC material. Nitrogen doping causes part of the carbon atoms on the OWC surface to be replaced by nitrogen atoms, and the wood surface has a very thin layer. The nitrogen carbon layer is a bridge formed by connecting the NDC material and metal ions, and is strongly coupled with NiCo-LDH, so that the active substance and the NDC material are combined more tightly.

FIG. 2 shows the test of the angle of hydrophilicity of an OWC sheet and an NDC sheet. Contact angle measurements were made on the OWC and NDC materials as a result of the contact angle decreasing from 75 to 36.2. The excellent hydrophilicity of the NDC material enables it to be more fully in ionic contact with the electrolyte. The excellent hydrophilicity of NDC materials allows them to be more fully in ionic contact with electrolytes. In addition, the electronic sites on the surface of the NDC material can be used as a transmission channel of electrons, so that the NDC material can be better transmitted, and the capability of storing electrons per se is improved.

FIG. 3 is an SEM image of a cross-section of a NiCo-LDH @ NDC slice. We can see that NDC provides sufficient space for loading of the active species NiCo-LDH with its excellent tube structure. FIG. 4 is a side view image showing a tunnel structure; fig. 5 is an enlarged image of fig. 4. It can be seen from fig. 4 and 5 that a large amount of NiCo-LDH nanosheets are uniformly supported on the inner wall of the NDC. The NiCo-LDH nano sheets are uniformly arranged and connected to form a staggered net structure, so that convenient conditions are provided for the contact of an electrode material and an electrolyte, and the ion transmission and electrochemical reaction are more sufficient.

FIG. 6 is a transmission electron microscope image of NiCo-LDH sheets. FIG. 6 Transmission Electron Microscopy (TEM) shows the ultrathin pleated structure of NiCo-LDH nanosheets intercalated with each other to form a layered network. Such layered nanostructures of NiCo-LDH are highly desirable for improving electrochemical performance, since these nanostructures not only provide a high specific surface area for sufficient electrochemical reactions, but also allow rapid diffusion of electrolyte ions within the structure.

The crystal properties of the samples were analyzed by HR-TEM images, with the lattice edges at about 0.15, 0.27 and 0.46 nm between the planes in fig. 7 and 8 corresponding to the (110), (100) and (001) planes of NiCo-LDH @ NDC, respectively. The polycrystalline nature of NiCo-LDH @ NDC is demonstrated by circular plaques in the SAED pattern in FIG. 9.

FIG. 10 is a TEM image and a C/O/N/Ni/Co element mapping image of NiCo-LDH @ NDC electrode slice. FIG. 10 shows the element mapping of EDX to NiCo-LDH @ NDC, from which it can be seen that the material contains 5 elements of C, N, O, Ni and Co, the N element is uniformly distributed on the surface of the carbon layer, and the O, Ni and Co are distributed on the carbon layer and the NiCo-LDH sheet, however, the mapping of Ni and Co on the NiCo-LDH sheet is more obvious.

In order to understand the chemical properties of the surface active substance of the electrode material deeply, the chemical properties of NiCo-LDH @ NDC are characterized by infrared spectroscopy, Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). Research on electrode material mesogens from infrared spectrumExistence of energy clusters. FIG. 11 is an infrared spectrum of an OWC electrode, an NDC electrode, and a NiCo-LDH @ NDC electrode. The broad peak in the infrared spectrogram 11 is 3462 cm^－1Due to tensile vibrations of the hydroxyl groups in the NiCo-LDH. At 1627 cm^－1Is due to bending vibrations of the absorbed water molecules bonded to the NiCo-LDH by hydrogen. At 1382 cm^－1The peak at (A) is derived from H-O deformation vibrations of the hydroxyl groups in NiCo-LDH. Furthermore, the peak value was 523 cm^－1And 633 cm^－1Caused by tensile and bending vibrations of the H-O bonds in the NiCo-LDH. By comparing infrared spectrograms of OWC and NDC, 1627 cm after nitrogen doping^－1The peak, the peak where the absorbed water molecules act on the material through hydrogen bonding, indicates that NDC material is more attractive to water.

FIG. 12 shows Raman spectra of OWC, NDC and NiCo-LDH @ NDC electrodes. At 1350 and 1580cm^－1The two peaks in (b) represent the D-band and G-band peaks, respectively. The degree of graphitization of carbon is generally described using the intensity ratio (ID/IG) of the D-band and G-band peaks. Degree of graphitization with I_D/I_GThe value decreases and increases. I of NDC_D/I_GBeing (1.15) higher than OWC (0.96) indicates more structural defects on NDC, which will favor the stable presence of NiCo-LDH on NDC material. I of NiCo-LDH @ NDC_D/I_GIs (0.86), lower than NDC (1.15) and OWC (0.96), which is caused by the catalytic reaction of Ni and Co to NiCo-LDH during electrodeposition.

In this example, FIG. 13 is an XPS measurement spectrum of a NiCo-LDH @ NDC electrode. FIG. 14 is a high resolution C1s spectrum of a NiCo-LDH @ NDC electrode. FIG. 15 is a high resolution O1s spectrum of a NiCo-LDH @ NDC electrode. FIG. 16 is a high resolution N1s spectrum of a NiCo-LDH @ NDC electrode. FIG. 17 is a high resolution Ni 1s spectrum of a NiCo-LDH @ NDC electrode. FIG. 18 is a high resolution Co1s spectrum for NiCo-LDH @ NDC electrodes. FIG. 13 measured spectra of NiCo-LDH @ NDC shows the presence of Ni, Co, N, O and C elements. A major peak was observed at 529.1 eV, which can be assigned as O1, indicating the presence of oxygen at the NiCo-LDH @ NDC surface. The C1s orbital in FIG. 14 is C-C, C-N/C-OH and C = O, respectively, according to the peak type and the binding energy position,the binding energies are in 284.8, 286.5 and 289.0 eV, respectively. The O1s peak in fig. 15 has four components, Co-O/Ni-O, Co-OH/Ni-OH, C = O and adsorbed water peaks, with binding energies at 530.2, 531.5, 532.6 and 534.0 eV, respectively. The peaks at the N1s orbitals, 399.3, 400.5, 401.6, 403.9 and 407.3 eV in FIG. 16 correspond to pyridinic N, pyrrole N, graphtic N, NO, respectively₂ ^－And NO₃ ^－。NO₃ ^－Is mainly due to partial NO during electrodeposition₃ ^－The intensity of the peaks remaining on NDC, pyridine N, pyrrole N, graphic N is weaker, indicating a lower amount of nitrogen doping.

Peaks 855.8 eV and 857.2 eV in the fine spectrum of Ni2p in FIG. 17 correspond to the orbital spin splitting peak (sum), the energy difference of the Ni2p orbital spin splitting peak is around 17.3 eV, the peaks 856.9 eV and 873.2 eV in the graph correspond to Ni-O, and the peaks 874.2 eV and 876.4 eV correspond to Ni-OH. Peaks 781.5 eV and 797.5 eV in the Co2p fine spectrum in FIG. 18 correspond to the orbital spin splitting peak (sum), respectively, the difference in energy between the orbital spin splitting peak (sum) in the Co2p fine spectrum is around 15.0 eV, peaks 780.9 eV and 796.2 eV correspond to Co-O, respectively, and peaks 782.2 eV and 797.8 eV correspond to Co-OH, respectively.

FIG. 19 is a graph of the specific capacitance performance of NiCo-LDH @ NDC electrodes at different electrodeposition times. FIG. 20 is a graph of electrodeposited 5h NiCo-LDH @ NDC electrode material at 1-10mV s under a window of-0.2V to 0.5V^－1Cyclic voltammogram at sweep rate. To further understand the electrochemical performance of the NiCo-LDH @ NDC electrode material, we performed relevant tests with the NiCo-LDH @ NDC electrode material as the working electrode, the platinum electrode as the counter electrode, and the saturated calomel electrode as the counter electrode material in the reference electrode (size (S) = 4 mm × 5 mm × 0.8 mm). Firstly, NiCo-LDH sheets are electrodeposited on NDC at different times under the same voltage, and the specific capacity of the electrode material obtained by electrodeposition at different times is shown in FIG. 19. The specific capacity of the electrode material is increased along with the increase of the electrodeposition time at the beginning, and the specific capacity is maximized when the electrodeposition time reaches 5 h. The electrochemical performance of the NiCo-LDH @ NDC electrode material is best when 5h of electrodeposition is carried out, and the loading capacity of the NiCo-LDH sheet is 0.34 g cm^－3. In that5 mA cm^－2The specific area capacity of the electrode under the current density is 14.26mAh cm^－2At 40 mA cm^－2The specific capacity of the electrode area is 9.31 mAh cm^－2. In FIG. 20, Cyclic Voltammetry (CV) was measured at a scan rate of 1 mV s^－1There is a distinct redox peak at all times, indicating that the electrochemical reaction occurring at the electrode is reversible. The chemical reaction that occurs on NiCo-LDH of NiCo-LDH @ NDC can be described by the following equation:

Ni(OH)₂+OH^－↔ NiOOH + H₂O + e^－ (1)

Co(OH)₂+OH^－↔ CoOOH + H₂O + e^－ (2)

CoOOH + OH^－↔ CoO₂+H₂O + e^－ (3)

with the increase of cyclic voltammetry sweep rate (1 mV s)^－1~10 mV s^－1) And the redox peak of the electrode material is polarized because of large loading capacity, and when the sweeping speed is increased, the redox reaction only stays on the surface of the electrode material, but the interior of the electrode material has no time to react. Constant current charge and discharge (GCD) tests were performed on the electrodes at different current densities. From FIG. 21, it can be seen that the GCD curve of NiCo-LDH @ NDC material has a distinct Faraday reaction platform, confirming the battery-type behavior of the electrode. From the charge-discharge curves of FIG. 21, the current densities of the electrodes were calculated to be 5, 10, 15, 20, and 30 mA cm^－2The area capacity at time of 14.26, 12.83, 11.90, 11.31 and 9.31 mAh cm^－2It has high rate capability of 65.29%. According to the amount of the loaded active substances, the corresponding specific mass capacities are 528.15, 475.19, 441.11, 418.89 and 344.81 mAh g^－1. FIG. 22 shows at 100 mA cm^－2After 6000 charge-discharge cycles, the capacity retention rates of the NiCo-LDH @ OWC and NiCo-LDH @ NDC electrode materials are 49.67% and 82.64% respectively. In order to better embody the electrochemical performance of the NiCo-LDH @ NDC electrode material, we performed an impedance test (EIS). FIGS. 23 and 24 are sections between the impedance curves and the solid axis in the high frequency partDistance represents electrode resistance and electrolyte resistance (R)_s) Using the diameter of the semicircle to measure the charge transfer resistance (R)_ct). The greater the slope of the low frequency portion of the curve, the smaller the diffusion resistance of the electrode. The high frequency region of Electrochemical Impedance Spectroscopy (EIS) shows a smaller equivalent series resistance R of the NiCo-LDH @ NDC electrode_s(5.62. OMEGA.) and a charge transfer resistance R_ct(0.44. omega.). Equivalent series resistance R of NiCo-LDH @ OWC_s(5.95. OMEGA.), charge transfer resistance R_ct(0.52 Ω). In a low-frequency region, the slope of a NiCo-LDH @ NDC electrode curve is obviously larger than that of a NiCo-LDH @ OWC electrode curve, which shows that the NiCo-LDH @ NDC electrode material has smaller diffusion resistance, larger conductivity and faster ion diffusion speed. By comparing the electrochemical performances of the NiCo-LDH @ OWC electrode and the NiCo-LDH @ NDC electrode, the fact that the capacitance of the electrode material is obviously improved by doping nitrogen can be found, and meanwhile, the cycle stability and the conductivity of the electrode material are also improved. Because the nitrogen doping improves the hydrophilicity of the wood, the contact between the electrode material and the electrolyte is promoted, and the excellent conductive network of the carbon-nitrogen layer provides convenience for the transmission of electrons and improves the conductivity of the carbon-nitrogen layer. Meanwhile, more connecting sites are exposed on the surface of the carbon layer due to the doping of nitrogen, and the connecting sites can enable NiCo-LDH to be better combined with an electrode material, so that the NiCo-LDH is larger in load capacity and less prone to falling off, and the capacitance and the cycling stability of the NiCo-LDH @ NDC electrode are improved.

A HSC device (hybrid supercapacitor) was assembled with NiCo-LDH @ NDC electrodes (size V = 4 mm. times.5 mm. times.0.8 mm) electrodeposited with 5h of NiCo-LDH as the anode, NDC electrodes (size V = 4 mm. times.5 mm. times.0.8 mm) as the cathode, nonwoven fabric as the separator, and PVA-KOH as the electrolyte. In order to balance the charge in the positive and negative electrodes and obtain maximum electrochemical performance, the mass of the electrode material should be calculated by the following equation:

(6)

wherein C is the specific capacitance (F cm)^－2) And V is the potential (V).

FIG. 25 shows NDC electrode and NiCo-LDH @ NDC electrode at 5 mV s^－1According to the CV curve at the same scanning speed, the voltage window of the HSC can reach 1.5V, and a NiCo-LDH @ NDC// NDC HSC device is obtained after positive and negative electrode charges are balanced. According to the balance equation, the optimum mass ratio of the positive and negative electrodes is 0.81. FIG. 26 shows HSC devices at 3 mV s^－1To 30 mV s^－1CV curve at scanning rate, potential window 1.5V. The CV curve is quasi-rectangular in shape, indicating that the HSC device has good capacitance performance. Fig. 27 shows the GCD curves for HSC devices with potential windows of 0V to 1.5V at different current densities. The area and mass specific capacitance of the HSC device calculated from the GCD curve integration is shown in fig. 28. At a current density of 5mA cm^－2When HSC devices have 4.74F cm^－2The specific capacitance of the total HSC device mass is 79.1F g^－1. When the current density reaches 30 mA cm^－2Then, the area specific capacitance still remains 2.46F cm^－2The mass specific capacitance is 41F g^－1This represents its excellent rate capability. Fig. 29 shows the excellent cycle stability of the supercapacitor, and the capacity retention rate is still 93.15% after 8000 cycles of charging and discharging. FIG. 30 shows the impedance spectrum of a NiCo-LDH @ NDC// NDC supercapacitor. To demonstrate the practical utility of our assembled HSC devices, we performed the use of assembled HSC devices to light an electronic watch with a voltage of 1.5V. Fig. 31 shows the connection between the HSC device and the electronic watch, from which we can see that the line is very simple. Fig. 32 shows a schematic diagram of different times for powering an electronic watch with an HSC device, the electronic watch still bright at 20 minutes and begins to dim at 30 minutes, with the HSC device actually powered for 35 minutes. We can see that a single HSC device can light the electronic watch for 35 minutes, and we can see that the assembled HSC device has great practical application value.

Claims

1. A nitrogen-doped carbon-based electrode based on nickel-cobalt double hydroxide is characterized in that: the wood chip comprises a nitrogen-doped carbonized wood chip substrate, wherein a tube cell structure is arranged in the nitrogen-doped carbonized wood chip substrate, and nickel-cobalt double hydroxide is electrodeposited on the inner wall of the tube cell structure in the nitrogen-doped carbonized wood chip substrate; the nickel-cobalt double hydroxide is attached to the inner wall of the tube cell structure in the nitrogen-doped carbonized wood sheet substrate in a net-shaped staggered structure.

2. The method of claim 1, wherein the nitrogen-doped carbon-based electrode is formed by: comprises the following steps;

1) drying the wood, and cutting into wood chips with preset sizes;

2) carbonizing, namely placing the wood chips obtained in the step 1) in a hot air drying box for pre-carbonizing for 4-8 h, and carbonizing for 8-12 h at 800-1200 ℃ under the protection of Ar gas to obtain carbonized wood sheets;

3）CO₂and (3) activation: carbonizing wood flakes in CO₂Activating for 8-12 h in Ar mixed gas flow, and cutting or grinding to a preset size; the activation temperature is 650-850 ℃; flow rate of Ar gas is CO₂3 times the flow rate of CO₂The flow rate is 80-120 sccm;

4) removing impurities to obtain activated charcoal pieces;

5) nitrogen doping: mixing the carbonized active wood electrode after impurity removal with potassium hydroxide and urea in 100mL of deionized water, wherein the weight ratio of the carbonized active wood electrode to the potassium hydroxide to the urea is 1: 1: 1; stirring for more than 1 hour at room temperature by using a magnetic stirrer; after drying, keeping the temperature at 600-1000 ℃ for 1-3 hours; obtaining a nitrogen-doped carbonized active wood matrix;

6) at 0.1mol/L of Co (NO)₂·6H₂O and 0.1mol/L of Ni (OH)₂·6H₂Electrodepositing nickel-cobalt double hydroxide on a nitrogen-doped carbonized active wood substrate in a mixed solution of O under the condition of-1 Vvs.SCE; taking a nitrogen-doped carbonized active wood substrate as a working electrode, a platinum electrode as a counter electrode and a saturated calomel electrode as a reference electrode; the electro-deposition time is 1-6 hours; and drying to obtain the nitrogen-doped carbon-based electrode of the nickel-cobalt double hydroxide.

3. The method of claim 2, wherein the nitrogen-doped carbon-based electrode is formed by a nickel-cobalt double hydroxide-based method, and the method comprises the following steps: the impurity removal in the step 4) comprises the following steps; respectively carrying out ultrasonic treatment on deionized water, hydrochloric acid with the weight concentration of 2% and absolute ethyl alcohol in an ultrasonic cleaning machine for 20 minutes to remove inorganic salts and amorphous carbon in the deionized water;

then sonicated with deionized water to bring its pH = 7;

the washed flakes were placed in a forced air drying oven and dried at 100 ℃ for 2 hours.

4. The method of claim 2, wherein the nitrogen-doped carbon-based electrode is formed by a nickel-cobalt double hydroxide-based method, and the method comprises the following steps: washing the nitrogen-doped carbonized activated wood substrate obtained in the step 6) with hydrochloric acid with a weight concentration of 6% and deionized water.

5. The method of claim 2, wherein the nitrogen-doped carbon-based electrode comprises: the wood is fir.

6. A supercapacitor, characterized by: comprising the nickel cobalt double hydroxide based nitrogen doped carbon based electrode of claim 1.