CN114335575A

CN114335575A - Preparation method of transition metal-heteroatom co-doped spiral carbonaceous nanotube for catalyzing oxygen reduction reaction

Info

Publication number: CN114335575A
Application number: CN202111549991.5A
Authority: CN
Inventors: 郑浩铨; 王彦智; 暴子佳; 曹睿
Original assignee: Shaanxi Normal University
Current assignee: Shaanxi Normal University
Priority date: 2021-12-17
Filing date: 2021-12-17
Publication date: 2022-04-12
Anticipated expiration: 2041-12-17
Also published as: CN114335575B

Abstract

The invention discloses a preparation method of a transition metal-heteroatom co-doped spiral carbon nanotube for catalyzing oxygen reduction reaction, which takes a chiral surfactant as a template, and pyrrole is directionally assembled into polypyrrole with a spiral structure through molecules of the chiral surfactant under the action of ammonium persulfate due to electrostatic interaction; then taking ferric triacetylacetonate, cobalt nitrate, nickel nitrate, manganese phthalocyanine (II), copper acetylacetonate and the like as transition metal sources, loading the transition metal into the spiral polypyrrole, refluxing the heteroatoms such as S, P and the like, enabling the heteroatoms to interact with the transition metal atoms and doping the heteroatoms into the spiral polypyrrole, and finally obtaining the transition metal-heteroatom co-doped spiral carbonaceous nanotube through pyrolysis. The raw materials are cheap and easy to obtain, the preparation method is simple, the method is suitable for industrial large-scale production, and the obtained material is used as a catalyst for oxygen reduction reaction and shows great activity and stability under alkaline and acidic conditions.

Description

Preparation method of transition metal-heteroatom co-doped spiral carbonaceous nanotube for catalyzing oxygen reduction reaction

Technical Field

The invention belongs to the technical field of electrocatalytic oxygen reduction, and particularly relates to a preparation method of a transition metal-heteroatom co-doped spiral carbon nanotube.

Background

The application of a large amount of primary energy mainly based on fossil fuel is an important cause of serious air pollution, and the development of sustainable green energy technology to replace traditional fossil energy is an important measure to improve the problem of environmental pollution. Metal-air batteries, fuel cells, lithium ion batteries, supercapacitors and other novel clean energy storage and conversion devices are continuously developed, and the worldwide problem of air pollution is expected to be improved in the future. The theoretical energy density of the metal-air battery can reach 2-10 times of that of the lithium ion battery. In addition, metal-air batteries utilize oxygen, which is abundant in air, as a cathode reactant, and are lighter in weight and lower in cost than fuel cells. And the metal-air battery has long average service life and zero carbon emission, and the research on improving the performance of the metal-air battery is very meaningful. Metal-air batteries have been developed to date, including Zn-air, Al-air, Fe-air, Li-O₂、Na-O₂、K-O₂Etc., of which Zn-air is the one of the most interest, with theoretical energy densities of up to 1350 Wh kg^-1The price is only $10 KW to $10 KW^-1 h^-1. Compared with other metal-air batteries, the characteristics of high performance, high safety and low cost of the Zn-air battery make the Zn-air battery become the most promising energy storage equipment.

For Zn-air batteries, the air cathode involves two major small molecule reactions during charging and discharging, namely, Oxygen Evolution Reaction (OER) and Oxygen Reduction Reaction (ORR). The kinetics of ORR and OER mainly determine the efficiency problem of Zn-air batteries, unfortunately, the kinetics of ORR and OER are particularly slow and are generally difficult to perform spontaneously, so that the reasonable design of ORR and OER catalysts is crucial to the development of energy storage and conversion equipment such as Zn-air batteries and the like. At present, precious metal-based electrocatalysts such as Pt, Pd and Ag are widely considered to be the ORR electrocatalysts with the highest activity. However, the large-scale practical application of the noble metal-based material is greatly limited by the defects of rare reserves, high cost, poor stability and the like of the noble metal-based material in the earth. Therefore, a non-noble metal catalyst which is low in price and easy to prepare in a large scale gradually becomes a research hotspot, but the catalytic activity of the catalyst is still not as good as that of a noble metal catalyst at present, and the catalytic performance still needs to be further improved by methods such as electronic structure regulation and control, material interface construction, electrode structure design and the like.

Nitrogen doped carbon materials (NC) have been considered as metal supports for promising non-noble metal catalysts for ORR. The common forms of NC-based materials include nanospheres, nanorods, nanotubes, etc. Helical structures are ubiquitous in nature, such as the alpha-helix of proteins in living systems and the double helix of deoxyribonucleic acid (DNA). The helical structure of biological macromolecules plays an important role in life functions. Inspired by the helix structure and function of biological macromolecules, people are interested in the research of the helix macromolecules. So far, NC materials of helical structure have rarely been used as electrocatalysts for ORR. More recently, polypyrrole, polyaniline, melamine, cyanamide, dicyandiamide, and covalent organic polymers have been prepared, all of which can serve as precursors for NC helical materials. The spiral structure has larger specific surface area and abundant surface grooves, which is more beneficial to the load of metal atoms and forms a single-atom catalytic site. The sulfur atom, phosphorus atom and nitrogen atom are p-block elements, but have different electronegativities and atomic radii. The relatively large atomic radii of the sulfur and phosphorus atoms may lead to carbon support defects, while the lower electronegativity is expected to change the electronic structure of the active center. Therefore, the doping of the hetero atom plays a very important role in improving the ORR activity.

Disclosure of Invention

The invention aims to provide a preparation method of a transition metal-heteroatom co-doped spiral carbon nanotube with higher catalytic activity and stability for catalytic oxygen reduction reaction, polypyrrole with a spiral structure is prepared through simple self-assembly, transition metal is loaded in abundant surface grooves of the polypyrrole, and hetero atoms such as S, P and the like are introduced, so that a high-efficiency and stable ORR catalyst is formed after high-temperature pyrolysis, and the requirements of application and development in related fields are met.

Aiming at the purposes, the technical scheme adopted by the invention comprises the following steps:

1. preparation of helical polyazoles

Dissolving a chiral surfactant in methanol at room temperature, adding pyrrole and deionized water, stirring for 10-20 minutes, adding a precooled 0-5 ℃ 2mol/L ammonium persulfate aqueous solution, continuously stirring for 20-40 minutes, filtering under reduced pressure, washing, and drying the obtained black solid to obtain spiral polypyrrole; wherein the hand surfactant is a linking C₁₂～C₁₈Chiral amino acids of carbon chains.

2. Preparation of transition metal-heteroatom-codoped spiral polypyrrole

Respectively dispersing the spiral polypyrrole and a transition metal source in methanol, then mixing and stirring the obtained dispersion liquid for 20-40 minutes, adding a heteroatom source, stirring and refluxing for 10-12 hours at the temperature of 60-70 ℃, naturally cooling to room temperature, carrying out suction filtration and washing on the reaction liquid, and drying the obtained black solid to obtain the transition metal-heteroatom co-doped spiral polypyrrole; wherein the transition metal source is one or two of ferric triacetylacetone, cobalt nitrate, nickel nitrate, manganese phthalocyanine (II) and copper acetylacetonate, and the heteroatom source is thiophene or phytic acid.

3. Preparation of transition metal-heteroatom co-doped helical carbonaceous nanotube

And (3) placing the transition metal-heteroatom co-doped spiral polypyrrole into a tubular furnace, pyrolyzing the transition metal-heteroatom co-doped spiral polypyrrole for 2-3 hours at 700-950 ℃ in an argon atmosphere, and cooling the pyrolysis product to room temperature to obtain the transition metal-heteroatom co-doped spiral carbonaceous nanotube.

In the step 1, the chiral amino acid is selected from any one of D-alanine, L-alanine, D-glutamic acid, L-glutamic acid, D-phenylalanine, L-phenylalanine, D-lysine, L-lysine, etc.

In the above step 1, the chiral surfactant is preferably used in combination withThe molar ratio of pyrrole to ammonium persulfate is 1: 20-50, the volume ratio of methanol to deionized water is 1: 4-8, and the concentration of the chiral surfactant in the methanol is 4.5 multiplied by 10^-3～7.5×10^-3mol/L。

In the step 2, the ratio of the spiral polypyrrole to the transition metal source and the heteroatom source is preferably 1mg to 0.01-0.02 mmol to 0.05-0.1 mmol.

In the step 3, the transition metal-heteroatom co-doped helical polypyrrole is preferably placed in a tube furnace, pyrolyzed at 900 ℃ for 2 hours in an argon atmosphere, and then cooled to room temperature.

In the step 3, the temperature rise rate of the pyrolysis is preferably 3-8 ℃/min, and the temperature is reduced to the room temperature at the temperature reduction rate of 3-8 ℃/min after the pyrolysis.

The invention has the following beneficial effects:

1. the method takes cheap and easily-obtained polypyrrole as a precursor, and the polypyrrole with a spiral structure is formed under the electrostatic action through the template induction action of a chiral surfactant, a large number of surface grooves brought by the spiral structure are beneficial to loading of transition metal, and doping of S, P and other heteroatoms can be combined with an active center of the transition metal, so that the electronic structure of the active center or the surrounding environment is changed, the catalytic performance is changed, and the stability is further improved. The transition metal-heteroatom co-doped spiral carbonaceous nanotube is obtained after pyrolysis, and the existence of high specific surface area and mesopores can improve the transfer of substances and charges in the catalysis process, so that the method has great benefit in the aspect of improving the electrocatalytic performance, effectively improves the mass transfer and charge transfer efficiency, and improves the electrochemical performance.

2. Compared with a commercial Pt/C catalyst, the transition metal-heteroatom co-doped helical carbon nanotube prepared by the invention is used as an oxygen reduction reaction catalyst, has excellent electrocatalytic activity and stability under alkaline conditions and acidic conditions, and is expected to be applied to solid Zn-air battery cathodes.

3. The preparation method is simple, rapid and economical, and is suitable for industrial large-scale production.

Drawings

FIG. 1 is a high power SEM image of CCNT @ Fe-S prepared in example 1.

FIG. 2 is a high power SEM image of CCNT @ Co-S prepared in example 2.

FIG. 3 is a high magnification SEM image of CCNT @ FeCo-S prepared in example 3.

FIG. 4 is a high power SEM image of CCNT @ Mn-S prepared in example 4.

FIG. 5 is a high power SEM image of CCNT @ Cu-S prepared in example 5.

FIG. 6 is a high power SEM image of CCNT @ Ni-S prepared in example 6.

FIG. 7 is a high power SEM image of CCNT @ Co-P prepared in example 7.

FIG. 8 is a plot of CCNT @ Fe-S prepared in example 1 and commercial Pt/C at O₂LSV profile in saturated 0.1mol/L KOH aqueous solution.

FIG. 9 is a comparison of CCNT @ Fe-S prepared in example 1 with commercial Pt/C at O₂Saturated 0.1mol/L HClO₄LSV curve in aqueous solution.

FIG. 10 is a LSV curve of CCNT @ Fe prepared in comparative example 1 versus CCNT @ Fe-S prepared in example 1 in 0.1mol/L KOH in water.

Detailed Description

The invention will be further described in detail with reference to the following figures and examples, to which, however, the scope of the invention is not limited.

Example 1

1. Preparation of helical polyazoles

0.0245g (0.08mmol) of N-stearoyl-L-glutamic acid (Angew. chem. int. Ed. 2018,57, 13187-.

2. Preparation of Fe-S co-doped spiral polypyrrole

Adding 30mg of L-PPy into 30mL of methanol, and ultrasonically dispersing for 30 minutes; adding 0.18g (0.5mmol) of ferric triacetylacetone into 5mL of methanol, and carrying out ultrasonic dispersion for 30 minutes; the two dispersions were then mixed and stirred for 30 minutes, followed by addition of 200mg (2.4mmol) of thiophene and stirring at reflux at 65 ℃ for 12 hours. And naturally cooling to room temperature, carrying out suction filtration on the reaction solution, washing with deionized water and ethanol, and finally drying the obtained black precipitate in an oven at 60 ℃ for 12 hours to obtain the Fe-S co-doped spiral polypyrrole (marked as L-PPy @ Fe-S).

3. Preparation of Fe-S co-doped spiral carbon nanotube

Putting L-PPy @ Fe-S into a porcelain boat, putting the porcelain boat into a tubular furnace, introducing argon for 30 minutes to remove air in the furnace, raising the temperature to 900 ℃ at a heating rate of 5 ℃/minute, pyrolyzing at the constant temperature for 2 hours, and then reducing the temperature to room temperature at a cooling rate of 5 ℃/minute to obtain black powder, namely the Fe-S co-doped spiral carbonaceous nanotube (marked as CCNT @ Fe-S). As shown in FIG. 1, the obtained Fe-S co-doped spiral carbonaceous nanotube has a spiral morphology.

Example 2

In step 2 of this example, 30mg of L-PPy was added to 30mL of methanol and ultrasonically dispersed for 30 minutes; 0.145g (0.5mmol) of Co (NO)₃)₂·6H₂Adding O into 5mL of methanol, and ultrasonically dispersing for 30 minutes; the two dispersions were then mixed and stirred for 30 minutes, followed by addition of 200mg (2.4mmol) of thiophene and stirring at reflux at 65 ℃ for 12 hours. And naturally cooling to room temperature, carrying out suction filtration on the reaction solution, washing with deionized water and ethanol, and finally drying the obtained black precipitate in an oven at 60 ℃ for 12 hours to obtain the Co-S Co-doped spiral polypyrrole (recorded as L-PPy @ Co-S). The other steps are the same as the example 1, and the Co-S Co-doped spiral carbon nano tube (marked as CCNT @ Co-S) is obtained. As shown in fig. 2, the obtained Co-doped helical carbonaceous nanotube has a left-handed helical morphology. Example 3

In step 2 of this example, 30mg of L-PPy was added to 30mL of methanol and ultrasonically dispersed for 30 minutes; 0.088g (0.25mmol) of ferric triacetylacetone and 0.073g (0.25mmol) of Co (NO)₃)₂·6H₂Adding O into 5mL of methanol, and ultrasonically dispersing for 30 minutes; the two dispersions are then mixed together and,stirring is carried out for 30 minutes, then 200mg (2.4mmol) of thiophene are added and stirring is carried out at reflux for 12 hours at 65 ℃. And naturally cooling to room temperature, carrying out suction filtration on the reaction solution, washing with deionized water and ethanol, and finally drying the obtained black precipitate in an oven at 60 ℃ for 12 hours to obtain FeCo-S co-doped spiral polypyrrole (marked as L-PPy @ FeCo-S). The other steps are the same as the example 1, and FeCo-S co-doped spiral carbon nano tube (marked as CCNT @ FeCo-S) is obtained. As shown in fig. 3, the obtained FeCo-S co-doped helical carbonaceous nanotube has a helical appearance.

Example 4

In step 2 of this example, 30mg of L-PPy was added to 30mL of methanol and ultrasonically dispersed for 30 minutes; adding 0.199g (0.5mmol) of manganese phthalocyanine (II) into 30mL of methanol, and ultrasonically dispersing for 30 minutes; the two dispersions were then mixed and stirred for 30 minutes, followed by addition of 200mg (2.4mmol) of thiophene and stirring at reflux at 65 ℃ for 12 hours. And naturally cooling to room temperature, carrying out suction filtration on the reaction solution, washing with deionized water and ethanol, and finally drying the obtained black precipitate in an oven at 60 ℃ for 12 hours to obtain the Mn-S co-doped spiral polypyrrole (recorded as L-PPy @ Mn-S). The other steps are the same as the example 1, and the Mn-S co-doped spiral carbon nano tube (marked as CCNT @ Mn-S) is obtained. As shown in fig. 4, the obtained Mn — S co-doped helical carbonaceous nanotube has a helical morphology.

Example 5

2. Preparation of L-PPy @ Cu-S

In step 2 of this example, 30mg of L-PPy was added to 30mL of methanol and ultrasonically dispersed for 30 minutes; adding 0.082g (0.5mmol) of copper acetylacetonate into 5mL of methanol, and carrying out ultrasonic dispersion for 30 minutes; the two dispersions were then mixed and stirred for 30 minutes, followed by addition of 200mg (2.4mmol) of thiophene and stirring at reflux at 65 ℃ for 12 hours. And naturally cooling to room temperature, carrying out suction filtration on the reaction solution, washing with deionized water and ethanol, and finally drying the obtained black precipitate in an oven at 60 ℃ for 12 hours to obtain the Cu-S co-doped spiral polypyrrole (recorded as L-PPy @ Cu-S). The other steps are the same as the example 1, and the Cu-S co-doped spiral carbon nanotube (marked as CCNT @ Cu-S) is obtained. As shown in fig. 5, the obtained Cu-S co-doped helical carbonaceous nanotube has a helical morphology.

Example 6

In step 2 of this example, 30mg of L-PPy was added to 30mL of methanol and ultrasonically dispersed for 30 minutes; 0.265mg (0.5mmol) of Ni (NO)₃)₂·6H₂Adding O into 5mL of methanol, and ultrasonically dispersing for 30 minutes; the two dispersions were then mixed and stirred for 30 minutes, followed by addition of 200mg (2.4mmol) of thiophene and stirring at reflux at 65 ℃ for 12 hours. And naturally cooling to room temperature, carrying out suction filtration on the reaction solution, washing with deionized water and ethanol, and finally drying the obtained black precipitate in an oven at 60 ℃ for 12 hours to obtain the Ni-S co-doped spiral polypyrrole (marked as L-PPy @ Ni-S). The other steps are the same as the example 1, and the Ni-S co-doped spiral carbon nano tube (marked as CCNT @ Ni-S) is obtained. As shown in fig. 6, the obtained Ni — S co-doped helical carbonaceous nanotube has a helical morphology.

Example 7

Step 1 of this example is the same as step 1 of example 1. In step 2 of this example, 30mg of L-PPy was added to 30mL of methanol and dispersed with ultrasound for 30 minutes; 0.145g (0.5mmol) of Co (NO)₃)₂·6H₂Adding O into 5mL of methanol, and ultrasonically dispersing for 30 minutes; the two dispersions were then mixed and stirred for 30 minutes, followed by the addition of 1.32g (2mmol) of phytic acid and stirring at reflux for 12 hours at 65 ℃. And naturally cooling to room temperature, carrying out suction filtration on the reaction solution, washing with deionized water and ethanol, and finally drying the obtained black precipitate in an oven at 60 ℃ for 12 hours to obtain the Co-P Co-doped spiral polypyrrole (recorded as L-PPy @ Co-P). In step 3 of this embodiment, L-PPy @ Co-doped helical carbonaceous nanotube (denoted as CCNT @ Co-P) is placed in a ceramic boat, the ceramic boat is placed in a tube furnace, argon is introduced for 30 minutes to remove air in the furnace, the temperature is raised to 800 ℃ at a heating rate of 5 ℃/minute, after pyrolysis is performed at a constant temperature for 2 hours, the temperature is lowered to room temperature at a cooling rate of 5 ℃/minute, and the obtained black powder is Co-doped helical carbonaceous nanotube (denoted as CCNT @ Co-P). As shown in fig. 7, the resulting Co-P Co-doped helical carbonaceous nanotube has a helical morphology.

Comparative example 1

In step 2 of example 1, 30mg of L-PPy was added to 30mL of methanol and ultrasonically dispersed for 30 minutes; adding 0.18g (0.5mmol) of ferric triacetylacetone into 5mL of methanol, and carrying out ultrasonic dispersion for 30 minutes; and then mixing the two dispersions, stirring for 30 minutes, carrying out suction filtration on the reaction solution, washing the reaction solution with deionized water and ethanol, and finally drying the obtained black precipitate in an oven at 60 ℃ for 12 hours to obtain the Fe-doped spiral polypyrrole (marked as L-PPy @ Fe). The other steps are the same as the example 1, and the Fe-doped spiral carbon nano tube (CCNT @ Fe) is obtained.

In order to prove the beneficial effects of the invention, the Fe-S co-doped spiral carbonaceous nanotube prepared in the embodiment 1 is used as a catalyst and is dripped on a glassy carbon electrode to be used as a working electrode, an Ag/AgCl electrode is used as a reference electrode, and a carbon rod is used as a counter electrode to construct a three-electrode system; for comparison, a three-electrode system was constructed by dropping a Pt/C catalyst onto a glassy carbon electrode as a working electrode. Then the three-electrode system is placed in O₂The LSV curve was tested in a saturated 0.1mol/L KOH aqueous solution and showed the initial potential (E) of CCNT @ Fe-S_onset) Can reach 1.02V and half-wave potential (E)_1/2) 0.91V, a further improvement compared to the Pt/C catalyst, and the results are shown in fig. 8.

Further employing the three-electrode system constructed as described above in O₂Saturated 1mol/L HClO₄The electrocatalytic oxygen reduction performance test of CCNT @ Fe-S was performed in aqueous solution, and the results are shown in FIG. 9. From FIG. 9, at O₂Saturated 1mol/L HClO₄Half-wave potential (E) of CCNT @ Fe-S in aqueous solution_1/20.78V) with half-wave potential (E) of Pt/C catalyst_1/20.75V) and exhibits high catalytic activity for oxygen reduction.

The oxygen reduction catalytic performance of CCNT @ Fe prepared in comparative example 1 was compared to CCNT @ Fe-S prepared in example 1, respectively, under basic conditions (0.1mol/L aqueous KOH) using the three-electrode system described above, and the results are shown in FIG. 10. As can be seen from FIG. 10, the half-wave potential of CCNT @ Fe is about 840mV, while the half-wave potential of CCNT @ Fe-S can reach 910mV, which indicates that the heteroatom interacts with the transition metal, and the electronic structure of the active center or the surrounding environment is changed, thereby further improving the activity of the electrocatalytic ORR.

Claims

1. A preparation method of a transition metal-heteroatom co-doped spiral carbonaceous nanotube for catalyzing oxygen reduction reaction is characterized by comprising the following steps:

(1) preparation of helical polyazoles

Dissolving a chiral surfactant in methanol at room temperature, adding pyrrole and deionized water, stirring for 10-20 minutes, adding a precooled 0-5 ℃ 2mol/L ammonium persulfate aqueous solution, continuously stirring for 20-40 minutes, filtering under reduced pressure, washing, and drying the obtained black solid to obtain spiral polypyrrole; wherein the chiral surfactant is a connecting C₁₂～C₁₈A chiral amino acid of a carbon chain;

(2) preparation of transition metal-heteroatom-codoped spiral polypyrrole

Respectively dispersing the spiral polypyrrole and a transition metal source in methanol, mixing and stirring the obtained dispersion liquid for 20-40 minutes, adding a heteroatom source, stirring and refluxing for 10-12 hours at 60-70 ℃, naturally cooling to room temperature, carrying out suction filtration and washing on the reaction liquid, and drying the obtained black solid to obtain the transition metal-heteroatom co-doped spiral polypyrrole; wherein the transition metal source is one or two of ferric triacetylacetone, cobalt nitrate, nickel nitrate, manganese phthalocyanine (II) and copper acetylacetonate, and the heteroatom source is thiophene or phytic acid;

(3) preparation of transition metal-heteroatom co-doped helical carbonaceous nanotube

And (3) placing the transition metal-heteroatom co-doped spiral polypyrrole into a tubular furnace, pyrolyzing the transition metal-heteroatom co-doped spiral polypyrrole for 2-3 hours at 700-950 ℃ in an argon atmosphere, and cooling the pyrolysis product to room temperature to obtain the transition metal-heteroatom co-doped spiral carbon nanotube.

2. The method for preparing a transition metal-heteroatom-codoped helical carbonaceous nanotube according to claim 1, wherein: in the step (1), the chiral amino acid is any one of D-alanine, L-alanine, D-glutamic acid, L-glutamic acid, D-phenylalanine, L-phenylalanine, D-lysine and L-lysine.

3. The method for preparing a transition metal-heteroatom-codoped helical carbonaceous nanotube according to claim 1, wherein: in the step (1), the molar ratio of the chiral surfactant to pyrrole to ammonium persulfate is 1: 20-50, the volume ratio of methanol to deionized water is 1: 4-8, and the concentration of the chiral surfactant in methanol is 4.5 x 10^-3～7.5×10^- ³mol/L。

4. The method for preparing a transition metal-heteroatom-codoped helical carbonaceous nanotube according to claim 1, wherein: in the step (2), the ratio of the spiral polypyrrole to the transition metal source and the heteroatom source is 1mg to 0.01-0.02 mmol to 0.05-0.1 mmol.

5. The method of preparing the transition metal-heteroatom-codoped helical carbonaceous nanotube of claim 1, wherein: in the step (3), the transition metal-heteroatom co-doped spiral polypyrrole is placed in a tube furnace, and is pyrolyzed for 2 hours at 900 ℃ in an argon atmosphere and then is cooled to room temperature.

6. The method of preparing the transition metal-heteroatom-codoped helical carbonaceous nanotube of claim 1 or 5, wherein: in the step (3), the heating rate of pyrolysis is 3-8 ℃/min, and the temperature is reduced to room temperature at the cooling rate of 3-8 ℃/min after pyrolysis.