CN111146013A

CN111146013A - A kind of hollow microtube electrode material based on ramie and its synthesis method and application

Info

Publication number: CN111146013A
Application number: CN202010025099.6A
Authority: CN
Inventors: 孙东亚; 何丽雯; 赵怡灿; 陈瑞龙
Original assignee: Xiamen University of Technology
Current assignee: Xiamen University of Technology
Priority date: 2020-01-10
Filing date: 2020-01-10
Publication date: 2020-05-12

Abstract

The invention discloses a hollow microtube electrode material based on ramie and a synthesis method and application thereof. It uses ramie fiber as a carbon source, and through carbonization treatment, porous carbon microtubes are obtained. Then, using cobalt nitrate as a cobalt source and porous carbon microtubes as a carrier, _Co3O4 nanosheets are grown in situ _on the inner and outer walls of the porous carbon microtubes by a solvothermal method to obtain a composite. The composites were annealed to obtain ramie-based hollow microtube electrode materials. The synthesis method of the invention is simple, the obtained product has a uniform morphology and a large specific surface area, combines the advantages of biomass materials, and introduces heteroatoms to form pseudocapacitance, so that it has better performance when used as an electrode material of a super capacitor, Suitable for energy storage, conversion, catalyst and other related applications.

Description

Hollow micro-tube electrode material based on ramie, and synthesis method and application thereof

Technical Field

The invention belongs to the field of synthesis of nano electrode materials, and particularly relates to a ramie-based hollow microtube electrode material as well as a synthesis method and application thereof.

Background

Transition Metal Oxide (TMO) and its composites are considered ideal pseudocapacitive electrode materials due to their high theoretical specific capacitance. Because they can form nanostructures with large surface areas and rich oxidation states, efficient charge transfer by interfacial redox processes can be achieved. In recent years, researchers report several Co with special morphology through copying fine micro-nano structures of natural species such as cotton, sorghum stalks, wood and the like₃O₄A base electrode material. Most of them are dense blocks, lack efficient charge transport channels and are prone to self-aggregation, limiting their potentially high-performance large-scale practical applications. Therefore, there is a strong need to design some nanocomposite electrode materials with special structure and excellent performance to meet these challenges.

The ramie (nettle family) belongs to a sub-shrub or shrub plant, is a special crop which takes textile as a main purpose in China, is Chinese treasure, and the yield of the Chinese ramie accounts for more than 90 percent of the worldwide yield of the ramie, and is internationally called as 'Chinese grass'. The main component of ramie is cellulose, which is taken from the cortex of ramie stalks, and the shape of the ramie stalks is tubular in a Transmission Electron Microscope (TEM) picture after cleaning and drying. The natural ramie fiber has the characteristics of smoothness, softness and excellent mechanical property, and is mainly used as fiber modified reinforcing materials for textiles, artware and medicine extraction.

Disclosure of Invention

The invention aims to provide a ramie-based hollow microtube electrode material, a synthesis method and application thereof.

In order to achieve the purpose of the invention, the invention adopts the following technical scheme.

The invention provides a ramie-based synthesis method of a hollow microtube electrode material, which comprises the following steps: with ramie fiber as carbonCarbonizing the source to obtain a porous carbon microtube; then taking cobalt nitrate as a cobalt source, taking the porous carbon microtube as a carrier, and growing Co on the inner wall and the outer wall of the porous carbon microtube in situ by a solvothermal method₃O₄Nanosheets to obtain a composite; and annealing the composite to obtain the ramie-based hollow microtube electrode material.

Further, the carbonization treatment step includes: and (3) cutting the ramie fibers, removing water-soluble sugar and protein through ultrasonic water washing, drying, and carbonizing in Ar atmosphere to obtain the porous carbon microtubule.

Further: the carbonization temperature is 250-350 ℃, and the carbonization time is 0.3-1 h.

Further, the step of obtaining the compound by a solvothermal method comprises: immersing the porous carbon microtube into an ethanol solution containing the cobalt nitrate, and performing ultrasonic mixing to obtain a solid-liquid mixture; transferring the solid-liquid mixture into a high-pressure reactor, and carrying out solvothermal reaction to obtain a reaction product; and filtering, washing and drying the reaction product to obtain the compound.

Further, the concentration of the cobalt nitrate in the ethanol solution is 0.5-1.5 mol/L; the mass ratio of the porous carbon microtube to the ethanol solution containing the cobalt nitrate is 1: 30-1: 70, and the ultrasonic mixing time is 10-30 min.

Furthermore, the solvothermal reaction temperature is 150-250 ℃, and the solvothermal reaction time is 8-16 h.

Further, annealing the compound in an air atmosphere at 350-450 ℃ for 0.5-1.5 h.

The invention also provides a ramie-based hollow micro-tube electrode material, which is obtained according to the synthetic method of the ramie-based hollow micro-tube electrode material; the ramie-based hollow micro-tube electrode material comprises a tubular porous carbon micro-tube, wherein Co grows on the inner wall and the outer wall of the porous carbon micro-tube₃O₄The nanosheet is used for manufacturing an electrode of a supercapacitor.

The invention has the beneficial effects that:

1. the synthetic method is simple, the obtained product has uniform appearance and large specific surface area, combines the advantages of high specific surface area, large pore volume structure and low contact resistance of the biomass material, and introduces hetero atoms to form pseudo-capacitance, so that the pseudo-capacitance has larger capacitance, longer service life and good conductivity when being used as an electrode material of a super capacitor.

2. The composite material prepared by the invention has good electrochemical performance and electrochemical stability. The biologically derived carbon used as the matrix material improves the electronic conductivity of the electrode material and can increase the migration speed of electrons; the larger specific surface area improves the dispersibility of the active substance and provides more active surface interfaces for the Faraday reaction.

Drawings

FIG. 1(a) is an SEM image of Ramie porous micro carbon tube provided in test example 1;

fig. 1(b) is a scanning electron microscope image (high magnification image in upper right corner) of the CNM composite material provided in experimental example 1.

FIG. 2 shows CNM and pure Co provided in test example 2₃O₄An XRD spectrum (a) and a Raman spectrum (b).

FIG. 3 shows CNM and pure Co provided in test example 3₃O₄N of (A)₂Adsorption and desorption isotherms and pore size distributions thereof.

Fig. 4 shows XPS survey spectra (a), XPS local spectrum C1s (b), XPS local spectrum O1s (C), and XPS local spectrum Co 2p (d) of CNM provided in experimental example 4.

FIG. 5(a) is a CNM sample obtained in example 1 and pure Co obtained in comparative example 1, provided for test example 5₃O₄The sample concentration is 10 mV.s^-1CV curve at sweep rate;

FIG. 5(b) is a CV curve for CNM at different sweep rates;

FIG. 5(c) shows CNM, pure Co₃O₄And Ramie in 1A g for the sample obtained in comparative example 2^-1Discharge time at current density;

FIG. 5(d) is the discharge time of CNM at different current densities;

FIG. 5(e) shows CNM and pure Co₃O₄The Nyquist curve of (a);

FIG. 5(f) is the cycle life and Coulomb efficiency of CNM (TEM morphology of CNM after 15000 cycles in the middle inset), and pure Co₃O₄Cycle life diagram of (c).

Detailed Description

In order to make the objects, technical solutions and advantages of the present invention more apparent, the technical solutions in the embodiments of the present invention will be clearly and completely described below. The examples, in which specific conditions are not specified, were conducted under conventional conditions or conditions recommended by the manufacturer. The reagents or instruments used are not indicated by the manufacturer, and are all conventional products available commercially.

The hollow micro-tube electrode material based on ramie and the synthesis method thereof according to the embodiment of the present invention are specifically described below.

The invention provides a ramie-based synthesis method of a hollow microtube electrode material, which comprises the following steps:

and (3) taking ramie fibers as a carbon source, and carrying out carbonization treatment to obtain the porous carbon microtube.

Wherein the carbonization treatment step comprises: cutting ramie fibers, removing water-soluble sugar and protein through ultrasonic washing, drying, and carbonizing in Ar atmosphere to obtain the porous carbon microtubule.

Wherein the carbonization temperature is 250-350 ℃, and the carbonization time is 0.3-1 h.

Then taking cobalt nitrate as a cobalt source, taking the porous carbon microtube as a carrier, and growing Co on the inner wall and the outer wall of the porous carbon microtube in situ by a solvothermal method₃O₄Nanosheets to obtain the composite. The in-situ growth method can improve Co₃O₄The binding force of the nano sheet and the porous carbon micro tube is uniform in adhesion.

In one embodiment, the step of solvothermally obtaining the composite comprises: immersing the porous carbon microtube into an ethanol solution containing the cobalt nitrate, and performing ultrasonic mixing to obtain a solid-liquid mixture; transferring the solid-liquid mixture into a high-pressure reactor, and carrying out solvothermal reaction to obtain a reaction product; and filtering, washing and drying the reaction product to obtain the compound.

In the step, the concentration of cobalt nitrate in the ethanol solution is 0.5-1.5 mol/L; the mass ratio of the porous carbon microtube to the ethanol solution containing the cobalt nitrate is 1: 30-1: 70, and the ultrasonic mixing time is 10-30 min.

Wherein the solvothermal reaction temperature is 150-250 ℃, and the solvothermal reaction time is 8-16 h.

And after obtaining the compound, carrying out annealing treatment on the compound to obtain the ramie-based hollow microtube electrode material.

In one embodiment, the composite is annealed in an air atmosphere at 350-450 ℃ for 0.5-1.5 h. Annealing treatment can refine Co₃O₄Nanosheet, increasing Co₃O₄And the combination degree of the porous carbon micro-tubes reduces the contact resistance.

The synthetic method is simple, the obtained product has uniform appearance and large specific surface area, combines the advantages of high specific surface area, large pore volume structure and low contact resistance of the biomass material, and introduces hetero atoms to form pseudo-capacitance, so that the pseudo-capacitance has larger capacitance, longer service life and good conductivity when being used as an electrode material of a super capacitor.

The ramie-based hollow microtube electrode material has good electrochemical performance and electrochemical stability. The biologically derived carbon used as the matrix material improves the electronic conductivity of the electrode material and can increase the migration speed of electrons; the larger specific surface area improves the dispersibility of the active substance and provides more active surface interfaces for the Faraday reaction.

The embodiment of the present invention will be specifically described below by way of examples and comparative examples.

Example 1

The embodiment provides a ramie-based hollow microtube electrode material, which is synthesized by the following steps:

(1) the ramie fibers were chopped into small pieces of about 1mm in length by a household chopper. Removing water-soluble sugar and protein by ultrasonic water washing, baking the ramie fiber in the fragment section at the temperature of 100 ℃ for 10min, and carbonizing the dried fragment section at the temperature of 300 ℃ in Ar atmosphere for 0.5h to obtain a porous carbon microtubule;

(2) 1.5g of the prepared porous carbon microtube was immersed in 50ml of 1mol/L Co (NO)₃)₂·6H₂Carrying out ultrasonic treatment for 20 minutes in an O ethanol solution;

(3) after mutual infiltration, the mixed solution is transferred into a 100ml high-pressure reaction kettle for solvothermal reaction at 200 ℃ for 12 hours, and the temperature rise speed is 2 ℃ min^-1；

(4) The solvothermal reaction product was filtered, washed with pure water and dried, and then annealed in a tube furnace in an air atmosphere at a temperature of 400 ℃ for 1 hour. And grinding the sample after the annealing is finished. Obtaining the ramie-based hollow microtube electrode material which is marked as CNM.

Comparative example 1

This comparative example provides a pure Co₃O₄A crystal sample obtained by the following steps:

50ml of 1mol/L Co (NO)₃)₂·6H₂Placing the O ethanol solution into a 100ml high-pressure reaction kettle, heating at 200 deg.C for 12 hr at a temperature rise rate of 2 deg.C/min^-1(ii) a The solid-liquid mixture was filtered, and the obtained solid was washed with pure water and dried, followed by annealing at a temperature of 400 ℃ in a tube furnace in an air atmosphere for 1 hour. Grinding the sample after annealing to obtain pure Co₃O₄And (4) crystal samples.

Comparative example 2

This comparative example provides a porous carbon microtube obtained according to step (1) of example 1, and the resulting product is designated Ramie.

The samples obtained in example 1, comparative example 1 and comparative example 2 were characterized and analyzed as follows.

Experimental example 1 SEM microtube analysis

FIG. 1(a) is an SEM image (inset is a high magnification) of a Ramie sample of the carbonized Ramie fiber of comparative example 2, which shows a characteristic morphology of a porous carbon microtube with a diameter of 1-3 μm.

FIG. 1(b) shows the Co of the inner and outer tube walls of the CNM composite of example 1₃O₄Scanning electron microscopy of nanoplatelets (inset is high magnification). It can be seen that the tubular CNM has a typical "sandwich" structure (Co)₃O₄@C@Co₃O₄) In the middle is a porous carbon microtube, Co on the inner and outer walls₃O₄The layers being of ultrathin Co interconnected by a plurality of₃O₄And assembling the nano sheets.

Experimental example 2 XRD and Raman test analysis

As shown in FIG. 2, the zigzag line denoted "CNM" in FIG. 2(a) is the XRD pattern of the composite material CNM of example 1, denoted "pristine Co₃O₄"the jagged line of is pure Co of comparative example 1₃O₄XRD pattern of the crystal. All the characteristic diffraction peaks of CNM can be better matched with cubic spinel Co₃O₄The (111), (220), (311), (400), (511) and (440) crystal face indices of (JCPDS 43-1003) correspond, indicating the presence of Co in the composite CNM₃O₄. An unobvious broad peak of CNM around 24 degrees can be attributed to a diffraction peak of thin amorphous carbon, which indicates that carbon microtubules exist.

As shown in FIG. 2, the zigzag line denoted by "CNM" in FIG. 2(b) is the Raman spectrum of the composite CNM of example 1, denoted by "pristine Co₃O₄"the jagged line of is the single-phase Co of comparative example 1₃O₄Raman spectrum of the crystal. It can be seen that the positions are 470, 511, 606 and 674cm^-1Four peaks at are respectively assigned as cubic Co₃O₄Eg, F of_2g ¹、F_2g ²And A_g ¹And (4) acoustic mode. 1339 and 1587cm^-1The two peaks at (A) can be attributed to the D band of the graphitic carbon materialAnd a G band, which is similar to the Raman spectrum of typical carbon materials such as graphene. Wherein the D peak represents a defect of a lattice of C atoms, and the G peak represents a sp of C atoms²Hybrid in-plane stretching vibration. In contrast, pure Co₃O₄This pair of peaks is not present in the raman spectra of the phases, indicating the presence of carbon microtubes in the CNM material.

Test example 3N₂Adsorption-desorption isotherm

In FIG. 3, the two-dimensional Co of example 1 is contained₃O₄CNM N assembled by nanosheets and porous carbon microtubes₂Adsorption-desorption isotherms, typical of type IV isotherms, with hysteresis loops at p/p₀In the range of 0.45 to 0.95, which indicates that Co₃O₄The nano sheets are connected with each other to form a large number of mesoporous channels. In FIG. 3, pure Co of comparative example 1 was also included₃O₄Isotherms of powders having the characteristics of microporous materials. As shown in Table 1 below, the pore size distribution of CNM is in the range of 3-50 nm, and the average diameters are 4.0, 3.9 and 3.7nm, respectively. The BET specific surface areas of CNM were 127.6, 113.4 and 99.8m²·g^-1Much greater than pure Co₃O₄Powder (10.8 m)²·g^-1). The main sources of the CNM specific surface area increment of the composite material are the connection gaps between the nano sheets and the biological carbon skeleton. The CNM had pore volumes (pore volumes) of 0.39, 0.40 and 0.36cm, respectively³·g^-1And the raw material Co₃O₄Has a pore volume (pore volume) of only 0.11cm³·g^-1. The special mesoporous structure formed by the CNM can adapt to volume expansion, effective active interface reaction and electron transmission in the electrochemical reaction process, thereby avoiding the occurrence of failure situations such as self-discharge and the like caused by agglomeration of electrode materials.

TABLE 1 specific surface area, pore volume and mean pore diameter of the materials

Test example 4 XPS analysis

The X-ray photoelectron spectroscopy (XPS) graph of the composite CNM of example 1 is shown in fig. 4.

In fig. 4, fig. 4(a) is an XPS full spectrum of CNM, and the result further demonstrates that C, O and Co elements are contained in the CNM material.

In fig. 4, fig. 4(b) is an XPS local spectrum of CNM, and it is evident that characteristic peaks of C1s binding energy (b.e.) can be fitted to 4 peaks, respectively 282.7, 284.5, 286.6 and 288.5eV, which can be assigned to the presence of covalent bond of C-C, C-C, C-O and C-O of CNM, respectively. Porous carbon fiber and Co₃O₄The interaction between them can be demonstrated by the corresponding Co-O and C ═ O signals in XPS spectra.

In fig. 4, fig. 4(c) is an XPS local spectrum of CNM, showing that O1s signal is fitted to two peaks at 530.2 and 532.6eV at b.e., which can be attributed to Co₃O₄The lattice oxygen of the nano-sheet is connected with the fiber hydroxyl-OH.

In FIG. 4, FIG. 4(d) is an XPS local spectrum of CNM, and it can be seen that a pair of Co 2p_3/2And Co 2p_1/2(B.E. centered at 780.5 and 795.4eV) and literature (Kim J G, Pugmire D L, Battaglia D, et al₂O₄spinel surface with Auger and X-ray photoelectron spectroscopy[J]Co reported in Applied Surface science.2000,165: 70-84)₃O₄Co of (A)³⁺The corresponding b.e. positions are close. Different is that 2p_3/2And 2p_1/2Higher b.e. separation peaks at 782.0 and 796.5eV, corresponding to Co₃O₄Of (5) Co ²⁺2p of_3/2And 2p_1/2Peak(s). In addition, spinel Co was found at 790.1 and 804.9eV₃O₄Due to the classical CoO satellite peaks, two peaks that differ by 14.8eV (spin orbit splitting) can also confirm Co²⁺And Co³⁺The presence of ions.

Test example 5 electrochemical Performance test

Under this test item, the electrochemical performance test of the CNM material of example 1 was performed in 6mol/L KOH electrolyte, and the relevant electrochemical test characterization methods were: cyclic voltammetry, constant current charge and discharge, and electrochemical impedance spectroscopy.

FIG. 5(a) shows CNM and pure Co₃O₄At 10mV · s^-1CV curve at sweep rate; FIG. 5(b) is a CV curve of CNM at different sweep rates; FIG. 5(c) shows CNM, pure Co₃O₄And Ramie at 1A g^-1Discharge time at current density; FIG. 5(d) is the discharge time of CNM at different current densities; FIG. 5(e) shows CNM and pure Co₃O₄The Nyquist curve of (a); FIG. 5(f) is the cycle life and coulombic efficiency of the CNM (insert TEM morphology of CNM after 15000 cycles), and pure Co₃O₄Cycle life of (d).

The system for testing adopts a three-electrode testing system, the electrolyte is 6mol/L KOH, and the working voltage window is 0.2-0.6V (reference Hg/HgO).

As can be seen in FIG. 5(a), CNM and pure Co₃O₄There are a pair of redox peaks, indicating that the measured capacitance is mainly due to surface reversible faradaic reactions, as shown in equations (1) and (2) below.

Wherein, CNM and Co₃O₄The shift in redox peak position between is related to the presence of thin carbon layers in the "sandwich.

Fig. 5(b) clearly shows that the current density of the CNM electrode material increases with the increase of the scanning rate, which indicates that the CNM electrode material has good rate performance. The CV curves of the CNM composite electrode exhibited similar profiles at different scan rates, further indicating Co in CNM₃O₄High-rate response capability and reversible Faraday reaction characteristics between the nanosheets and the electrolyte. In FIG. 5(a), the integrated area of the CV curve of CNM is significantly larger than that of pure Co₃O₄This indicates that CNM has a purer Co₃O₄Better pseudocapacitance performance.

The charger shown in FIG. 5(c)The discharge curve (GCD) is consistent with the capacitance result calculated by the above equation (1). The discharge time of CNM is significantly longer than Ramie and pure Co₃O₄CNM specific capacitance 1280.6, pure Co₃O₄Specific capacitance of 40.0 F.g^-1It is shown that the magnitude of the specific capacitance is related to the specific surface area. The discharge times of CNM are Ramie and Co, respectively₃O₄24 times and 32 times, due to the introduction of carbon in CNM, relatively high specific surface area and special microstructure.

FIG. 5(d), Current densities of 1, 4, 8, 12 and 16 A.g., respectively^-1The specific capacitances of CNM calculated from the discharge time in GCD were 1280.6, 1122.8, 968.0, 892.8 and 865.5F g, respectively^-1. The decrease in capacitance with increasing current density may be due to incomplete faradaic redox reactions, failure of electrolyte molecules to enter the pores, and insufficient space charge storage between layers. It is well known that micropores and most mesopores below 5nm lose their activity. At low current densities, the electrolyte ions have sufficient time to penetrate deep into the CNM pores. However, at high current densities, the active species ions can only diffuse within the shallow interface range, with little probability of diffusing into the interior of the electrode, and therefore the utilization of the electroactive species in the electrode is significantly reduced.

Fig. 5(e) also shows that the ion transport studies of the different electrodes can be evaluated by Electrochemical Impedance Spectroscopy (EIS) with frequencies ranging from 0.01 to 100000 Hz. Generally, a lower Equivalent Series Resistance (ESR) value indicates a lower internal contact resistance and a faster charge and discharge rate. The results show that the ESR of CNM (less than or equal to 0.5 omega) is lower than that of pure Co₃O₄(0.55 Ω) and increased with the increase in carbon content, indicating that the porous carbon matrix can function as a conductive substrate, thereby effectively avoiding Co₃O₄Aggregation of nanocrystals.

Fig. 5(f) shows that after 15000 cycles of CNM, there is only a 3.11% capacitive decay with a high coulombic efficiency of 96%. The material retained almost the original sheet tube morphology over a long period of cycling (middle inset in fig. 5 (f)), indicating that the electrode had good cycling stability. In contrast, pure Co₃O₄The powder capacitance is lower, and the capacitance is obviously reduced in the first 1000 cycles (from 40 to 29.56 F.g)^-1) This indicates that its own dispersibility and conductivity are poor.

In conclusion, the ramie-based hollow microtube electrode material prepared by the method has excellent electrochemical performance, which is determined by the special nanostructure. Firstly, a hollow flaky pipe skeleton consisting of nanosheets and porous carbon microtubes provides space for electrolyte, and avoids Co in the charging and discharging process₃O₄To (3) is performed. Secondly, CNM consists of many nano-sheets, the microtube has a hollow structure (tube diameter is tens of nanometers), and electrolytes and active ions can be easily diffused from both sides of the tube and the tube wall. At the same time, ultra-thin Co₃O₄The nanosheets can help to increase the utilization of electroactive sites in the electrode. And thirdly, the porous ramie micro carbon tubes in the wall of the interlayer tube are used as a matrix material, so that the electronic conductivity of the electrode material is improved, a high-quality electronic transmission channel is provided for electrons, and the high-rate charge-discharge performance and the low capacitance attenuation are guaranteed.

The invention is made of Co₃O₄The composite material consisting of the bio-derived carbon matrix with special form and the porous ramie micro carbon tube provides a template for preparing valuable micro/nano functional materials by utilizing biomass through a simple and low-cost process, is suitable for energy storage, conversion, catalysts and other related application fields, and has a great prospect in the aspect of supercapacitors.

The above-described embodiments are merely some embodiments of the present invention and are not intended to be exhaustive or to limit the scope of the invention to the precise embodiments disclosed, and merely representative of selected embodiments of the invention. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.

Claims

1. a synthetic method based on ramie hollow microtube electrode material, its characteristic is, comprises the following steps:

Using ramie fiber as carbon source, through carbonization treatment, porous carbon microtubes are obtained;

Then, using cobalt nitrate as a cobalt source and the porous carbon microtubes as a carrier, by solvothermal method, _Co3O4 nanosheets are grown in situ _on the inner and outer walls of the porous carbon microtubes to obtain a composite;

The composite is annealed to obtain a ramie-based hollow microtube electrode material.

2. the synthetic method of the hollow microtube electrode material based on ramie according to claim 1, is characterized in that, the carbonization treatment step comprises:

The ramie fibers are chopped, washed with ultrasonic waves to remove water-soluble sugars and proteins, dried and then carbonized in an Ar atmosphere to obtain the porous carbon microtubes.

3 . The method for synthesizing the ramie-based hollow microtube electrode material according to claim 2 , wherein the carbonization temperature is 250-350° C., and the carbonization time is 0.3-1 h. 4 .

4. the synthetic method of the hollow microtube electrode material based on ramie according to claim 1, is characterized in that, the step that solvothermal method obtains compound comprises:

The porous carbon microtubes are immersed in an ethanol solution containing the cobalt nitrate, and a solid-liquid mixture is obtained after ultrasonic mixing;

The solid-liquid mixture is transferred to a high-pressure reactor, and a reaction product is obtained after a solvothermal reaction;

The complex is obtained after the reaction product is filtered, washed and dried.

5. The method for synthesizing ramie-based hollow microtube electrode material according to claim 4, wherein the concentration of cobalt nitrate in the ethanol solution is 0.5-1.5 mol/L; The mass ratio of the ethanol solution of the cobalt nitrate is 1:30～1:70, and the ultrasonic mixing time is 10～30min.

6 . The method for synthesizing the ramie-based hollow microtube electrode material according to claim 4 , wherein the solvothermal reaction temperature is 150-250° C., and the solvothermal reaction time is 8-16 h. 7 .

7 . The method for synthesizing the ramie-based hollow microtube electrode material according to claim 1 , wherein the composite is annealed in an air atmosphere, the annealing temperature is 350-450° C., and the annealing time is 0.5-1.5 h.

8. A ramie-based hollow microtube electrode material, characterized in that, obtained according to the method for synthesizing the ramie-based hollow microtube electrode material according to any one of claims 1 to 7, and the ramie-based hollow microtube electrode material is obtained. The electrode material includes tubular porous carbon microtubes, and Co ₃ O ₄ nanosheets are grown on both the inner wall and the outer wall of the porous carbon microtube.

9. The application of the ramie-based hollow microtube electrode material according to claim 8 in making electrodes for supercapacitors.