CN117154110A

CN117154110A - Method for synthesizing monoatomic catalyst through structure construction and monoatomic generation in parallel

Info

Publication number: CN117154110A
Application number: CN202210543995.0A
Authority: CN
Inventors: 王焕磊; 李平
Original assignee: Ocean University of China
Current assignee: Ocean University of China
Priority date: 2022-05-19
Filing date: 2022-05-19
Publication date: 2023-12-01

Abstract

The application belongs to the field of nano materials and electrocatalysis, and particularly relates to a single-atom electrocatalyst which is formed by carbonizing a precursor mixed by a nitrogen-containing organic matter and a transition metal salt in a mode of parallel formation of a structure and single atoms. The nitrogen-doped carbon nano-sheet prepared by the metal salt and having the characteristics of rich pore structure, large specific surface area, high graphitization degree and the like can capture monoatomic metal so as to form the monoatomic catalyst. In the preparation process, the transition metal salt is used as a template, a metal source and a pore-forming agent, and has a catalytic effect on the formed carbon material, so that the graphitization degree of the carbon material is improved. The monoatomic catalyst prepared by the method has high specific surface area and developed pore diameter structure, so that the exposure rate of active sites is improved, and carbon with high graphitization degree not only improves the conductivity, but also improves the corrosion resistance of the catalyst in alkaline environment, thereby improving the stability of the catalyst.

Description

Method for synthesizing monoatomic catalyst through structure construction and monoatomic generation in parallel

Technical Field

The application belongs to the technical field of catalyst preparation, and particularly relates to a preparation method of a single-atom catalyst.

Background

Clean, efficient and sustainable energy devices, zinc-air batteries, have received a great deal of attention as the most promising new energy devices to replace traditional fossil fuels. The zinc-air battery has the characteristics of high theoretical output capacity, low manufacturing cost, high safety, environmental friendliness and the like, and is considered to be one of the most promising future power sources.

However, the slow kinetics of the key reactive oxygen reduction (ORR) and Oxygen Evolution (OER) reactions in zinc-air cells severely hamper the large-scale application of these devices, requiring catalytic actuation of the catalyst. The current ORR and OER catalysts are mainly noble metal catalysts, but noble metals seriously obstruct the large-scale application of zinc-air batteries due to low reserves on the earth and high price. There is therefore a need to develop efficient, stable ORR and OER catalysts free of noble metals to facilitate large scale application of zinc-air batteries. In the course of research on zinc-air cell catalysts, monoatomic catalysts have received great attention due to their nearly 100% atomic utilization and high catalytic activity. The single-atom catalyst has uniform active sites, so that the single-atom catalyst has the common advantages of a homogeneous catalyst and a heterogeneous catalyst, and the defects of the two catalysts are overcome. The cost of the catalyst is greatly reduced by the monoatomic catalyst, and the catalyst can show the catalytic activity equivalent to or even better than that of the high-content metal catalyst only by the extremely low metal content, so that the utilization rate of the metal is improved by tens of times or even hundreds of times.

The preparation methods of the single-atom catalyst commonly used at present include a coprecipitation method, an impregnation method, a displacement reaction method, an atomic layer deposition method, an anti-Ostwald ripening method and the like. However, these methods have disadvantages of low yield, high cost, complicated equipment, etc., which prevent the large-scale application of the monoatomic catalyst. In view of the unprecedented superiority of the single-atom catalyst in the zinc-air battery field, it is particularly important to develop a general strategy for effectively preparing the single-atom catalyst by using multiple metals, wherein the general strategy has the advantages of simple synthesis method and low cost.

In the study of zinc-air battery catalysts, carbon materials are not indispensable. Pure carbon materials are not catalytically active and often require heteroatom doping and morphological construction of the carbon materials. Heteroatom doping tends to reduce the graphitization degree of the carbon material, while carbon with high graphitization degree has excellent conductivity and good corrosion resistance, while carbon with high graphitization degree lacks active sites for catalyzing ORR and OER. This presents a problem of balancing the relationship between the degree of graphitization of the carbon and the doping of the heteroatoms to simultaneously increase the catalytic activity and stability of the catalyst. The morphology of the carbon material is also important to improve the catalytic activity of the catalyst. The morphology of the carbon material is generally divided into a one-dimensional structure, a two-dimensional structure and a three-dimensional structure, wherein the two-dimensional structure can be effectively exposed in an electrolyte, and most active sites on the surface of the two-dimensional carbon material can participate in electrocatalytic reaction, so that the two-dimensional carbon material has high efficiency in a series of electrochemical reactions. However, in the above-mentioned techniques, the foregoing single-atom dispersion, balanced graphitization degree, heteroatom relationship, and two-dimensional structure construction can be carried out only individually, and each method has limited improvement in catalytic performance of the catalyst, so that a new method for preparing a single-atom catalyst capable of simultaneously realizing the foregoing features needs to be explored.

Disclosure of Invention

Aiming at the problems of complex preparation process, high cost and balance of heteroatom doping and graphitization of the carbon material in the prior art, the preparation process of the carbon material of the two-dimensional material is complex, the problem of subsequent treatment exists, and the research and application requirements in the field are met, one of the purposes of the application is to provide a method for preparing an ultra-stable single-atom catalyst which has simple flow, low cost and mass production, and the method is characterized in that the preparation is carried out by a method of parallel structure construction and single-atom generation.

According to the preparation method of the single-atom catalyst, a nitrogen-containing organic matter is used as a carbon precursor, metal salt is used as a template, a pore-forming agent, a metal source and a catalyst for catalyzing carbon, and the single-atom catalyst is finally obtained through mixing in a dissolving or ball milling mode, washing with hydrochloric acid and secondary carbonization. The metal salt is used as a template agent, a pore-forming agent, a doping agent and a catalyst for catalyzing carbon, so that the pore diameter adjustment, the morphology control and the graphitization degree control of the carbon material are realized while the single-atom preparation is realized. In the application, only the nitrogenous organic matters are used as the carbon precursor, and the metal salt is used as the template, the pore-forming agent, the metal source and the catalyst for catalyzing carbon, so that a new template or metal template or pore-forming agent or catalyst is not required to be additionally introduced, and compared with the existing method of the single-atom catalyst, the method has the advantages of simple flow, low cost and mass production.

The method organically combines the three methods of single atom preparation, a template method and an activation method to realize synchronous completion. And (3) selecting metal salt as a template agent, an activating agent and a metal source, and separating nitrogen-containing organic matters by the metal salt to form a two-dimensional structure. As the temperature increases, the metal salt is reduced by the carbon to elemental metal, and the carbon is consumed in this process to form a number of defects and pores in the carbon. The metal single substance has catalytic action on carbon at high temperature, so that graphitization degree of the carbon material is improved. Finally, the metal volatilizes at a high temperature, and the defective micropores on the carbon material have a capturing effect on the monoatomic metal to fix the metal, so that the metal monoatomic dispersion is formed. And in the secondary carbonization process, nitrogen doping is carried out on the carbon material through ammonia gas, so that more nitrogen elements are introduced.

The monoatomic catalyst prepared by the application is prepared by a mode of parallel structure construction and monoatomic generation. The transition metal salt has the functions of the template agent, the metal source, the pore-forming agent and the carbon catalyst in the preparation process, so that the preparation process of the single-atom catalyst is simplified. The obtained single-atom catalyst has the advantages of high stability, good conductivity, low price, high catalytic activity and the like. The monoatomic catalyst prepared by the application has the following characteristics: (1) The two-dimensional structure can effectively improve the contact between the electrolyte and the catalyst; (2) The large specific surface area improves the exposure rate of the active sites, so that more active sites participate in the reaction; (3) Abundant pore structures, which accelerate the transport of reactants and products to improve catalytic activity; (4) The high graphitization degree carbon has the corrosion resistance in alkaline environment while improving conductivity, so that the stability of the catalyst is improved; (5) Monoatomic dispersed metal active sites, monoatomic sites improve the activity of the catalyst.

The specific preparation method of the application comprises the following steps:

(a) Mixing: mixing a nitrogen-containing organic compound with a transition metal salt;

(b) Carbonizing: placing the uniformly mixed sample into a tube furnace, heating and maintaining under the protection of inert gas, so that the sample is carbonized and activated;

(c) Cleaning: pickling the carbonized sample in hydrochloric acid solution, then carrying out suction filtration to neutrality by using a large amount of deionized water, and putting the sample into a baking oven for baking;

(d) And (3) carrying out secondary carbonization on the dried sample under a mixed atmosphere of nitrogen and ammonia gas to obtain the single-atom catalyst.

In the preparation method of the present application, the nitrogen-containing organic matter used in the step (a) includes, but is not limited to: ammonium citrate, dopamine hydrochloride, urotropine, humic acid and the like; the transition metal salt is one or more than one of transition metals such as Fe salt, co salt, ni salt, mn salt and the like.

In the preparation method of the application, in the step (a), the mass ratio of the nitrogen-containing organic precursor to the transition metal salt is 1:3.

In the preparation method of the application, 10g of nitrogen-containing organic matters are dissolved in 30ml of water in the step (a), then 30g of transition metal salt is added and stirred for 1h to form a transparent solution, and finally the solution is frozen by liquid nitrogen fast and freeze-dried for 24h to form uniform solid powder.

In the preparation method of the application, in the step (b), the carbonization temperature is 700-1200 ℃, and the heating rate is 1-10 ℃ for min ^-1 The heat preservation time is 0-10 h.

In the preparation method, the prepared monoatomic catalyst comprises monoatomic transition metal and nitrogen-doped carbon nano-sheets with rich pore structures, large specific surface area, high graphitization degree and other characteristics.

In the preparation method of the application, in the step (c), the carbonized sample is pickled for 24 hours at 90 ℃ in a 2M hydrochloric acid solution, then is filtered to be neutral by a large amount of deionized water, and is put into an oven at 80 ℃ for drying.

The single-atom catalyst prepared by the preparation method is assembled into the zinc-air battery, and the charge-discharge cycle curve of the zinc-air battery can be known to be stable after 16000 cycles.

The mass of the transition metal salt is far higher than that of the nitrogenous organic precursor, and the transition metal salt plays roles of a template, a metal source and a pore-forming agent. The catalytic action of the transition metal on carbon gives carbon a rich defect that can trap the metal atoms that have evaporated to form a single atom catalyst.

The application discloses a preparation method of a single-atom catalyst, and the single-metal catalyst obtained by the method has good quality and high stability, so that the method can be applied to various fields applicable to the single-atom catalyst, and is particularly used in energy conversion devices, such as metal-air batteries.

Compared with the prior art, the application has the main beneficial effects and advantages that:

1) The structure construction and monoatomic generation parallel method adopted by the application simultaneously forms a two-dimensional lamellar structure and a monoatomic catalyst in the preparation process.

2) Compared with the existing method of the single-atom catalyst, the preparation process of the application has simple flow, low cost and large-scale production.

3) The transition metal salt used in the application plays roles of template, pore-forming, improving graphitization degree of carbon material and cobalt source.

4) The catalyst obtained by the preparation method has rich pore structure, large specific surface area, large interlayer spacing and high graphitization degree.

5) The catalyst obtained by the preparation method provided by the application has ultrahigh stability and is greatly superior to a monoatomic catalyst prepared by a traditional method.

Description of the drawings:

FIG. 1 scanning electron microscope image of the catalyst prepared in example 1

FIG. 2A scanning electron microscope image of the catalyst prepared in comparative example 1

FIG. 3A spherical aberration-induced electron microscope image of the catalyst prepared in example 1

FIG. 4 is a transmission electron microscope image of the catalyst prepared in example 1

FIG. 5 is an XRD pattern of the catalyst prepared in example 1

FIG. 6 is a graph showing the adsorption and desorption curves and pore diameter distribution of nitrogen for the catalyst prepared in example 1 and the catalyst prepared in comparative example 1

FIG. 7 is a linear sweep voltammogram of oxygen reduction performance for the example 1 catalyst and the comparative example 1 catalyst and commercial Pt/C

FIG. 8 is a schematic of example 1 catalyst and comparative example 1 catalyst and commercial IrO ₂ Linear sweep voltammogram of oxygen evolution performance of (2)

FIG. 9 is a schematic diagram of example 1 catalyst and commercial Pt/C+IrO ₂ Is a discharge curve and a power density map of (a)

FIG. 10 is a schematic illustration of example 1 catalyst and commercial Pt/C+IrO ₂ Is a charge-discharge cycle graph of (2)

FIG. 11 is a scanning electron microscope image of the catalyst obtained in example 2

FIG. 12 is an XRD pattern of the catalyst obtained in example 2

FIG. 13 is a graph showing the adsorption and desorption curves and pore diameter distribution of nitrogen for the catalyst prepared in example 2 and the catalyst prepared in comparative example 2

FIG. 14 is a linear sweep voltammogram of oxygen reduction performance for the example 2 catalyst and the comparative example 1 catalyst and commercial Pt/C

FIG. 15 is a schematic diagram of example 2 catalyst and commercial Pt/C+IrO ₂ Is a discharge curve and a power density map of (a)

FIG. 16 is a scanning electron microscope image of the catalyst obtained in example 3

FIG. 17 is a linear sweep voltammogram of oxygen reduction performance for the example 3 catalyst and the comparative example 2 catalyst and commercial Pt/C

FIG. 18 is a schematic of example 3 catalyst and comparative example 2 catalyst and commercial IrO ₂ Linear sweep voltammogram of oxygen evolution performance of (2)

FIG. 19 is a scanning electron microscope image of the catalyst obtained in example 4

FIG. 20 is a linear sweep voltammogram of oxygen reduction performance for the example 4 catalyst and the comparative example 3 catalyst and commercial Pt/C

The specific embodiment is as follows:

for a further understanding of the present application, reference is made to the following description of the application, taken in conjunction with the accompanying drawings and examples, which are not intended to limit the application in any way.

Example 1:

first 10g of ammonium citrate is dissolved in 30ml of water and then 30g of CoCl is added ₂ ·6H ₂ Stirring O for 1h to form a transparent solution, finally, rapidly freezing the solution with liquid nitrogen, and freeze-drying for 24h to form uniform solidAnd (3) powder. The solid powder formed was calcined in a tube furnace at 900 ℃ for 2 hours under nitrogen to give a black powder, which was then acid washed with 2m HCl at 90 ℃ for 24 hours. Calcining the acid-washed sample for 1h at 900 ℃ under a mixed atmosphere of nitrogen and ammonia gas to obtain the final catalyst.

Comparative example 1:

10g of ammonium citrate was dissolved in 30ml of water, and the solution was frozen with liquid nitrogen and lyophilized for 24 hours to form a powder. The resulting powder was calcined in a tube furnace at 900 ℃ for 2 hours under nitrogen to give a black powder, which was then acid washed with 2m HCl at 90 ℃ for 24 hours. Calcining the acid-washed sample for 1h at 900 ℃ under a mixed atmosphere of nitrogen and ammonia gas to obtain the final catalyst.

Fig. 1 and 2 are scanning electron microscope images of the catalysts obtained in example 1 and comparative example 1, respectively. FIG. 1 is a microscopic morphology of example 1, from which it can be seen that example 1 is a complete large sheet of two-dimensional material. Fig. 2 shows the microstructure of comparative example 1, with comparative example 1 being mainly a large block structure. This illustrates CoCl ₂ ·6H ₂ O is very important for forming a two-dimensional lamellar structure.

From the spherical aberration electron microscope image of example 1 of fig. 3, we can see that a large number of bright spots are on the carbon plate, respectively, which directly demonstrates the existence of monoatomic cobalt.

FIG. 4 is an electron projection microscope image of example 1, from which it can be analyzed that example 1 has a significant graphite lattice, which demonstrates that example 1 has a higher degree of graphitization and that the graphite interlayer spacing is much greater than the conventional graphite interlayer spacing, which is 0.401 nm.

Fig. 5 is an XRD pattern of the catalyst prepared using example 1. As shown, only two broad diffraction peaks appear in the XRD patterns at (002) and (100) planes corresponding to graphitic carbon at 26 ° and 43 °, and no characteristic peaks related to metallic cobalt appear.

Fig. 6 is a graph showing the adsorption and desorption curves and pore size distribution of nitrogen gas of example 1 and comparative example 1, from which it can be seen that example 1 has a large specific surface area and a developed pore structure. The specific surface area of example 1 was 1538.6m ² g ^-1 Pore volume of 1.311cm ³ g ^-1 This ratio ofThe specific surface area and pore volume of the catalyst obtained from comparative example 1 were much greater.

FIG. 7 shows a comparison of the ORR catalytic performance of example 1 catalyst with that of comparative example 1 catalyst and commercial Pt/C, and shows that example 1 has significantly improved ORR catalytic performance compared to comparative example 1 and even better than commercial Pt/C. The half-wave potential of example 1 was 0.87V and the limiting current was 5.2mA cm ^-2 。

Fig. 8 shows OER activity for example 1 catalyst, comparative example 1 catalyst and IrO 2. Example 1 at 10mA cm ^-2 The potential at the current density was 1.533V, which is higher than that of the catalyst of comparative example 1 and IrO ₂ The OER catalytic performance of the catalyst is good.

FIG. 9 is a discharge curve and power density curve of a catalyst-assembled zinc-air battery prepared in example 1 and commercial Pt/C+RuO ₂ A comparison was made. As can be seen from FIG. 9, the limiting power density of the catalyst material of the present application is about 255mW cm ^-2 170mW cm higher than commercial catalyst ^-2 . FIG. 10 shows the catalyst material prepared in example 1 and commercial Pt/C+RuO ₂ And the catalyst is assembled into a charge-discharge cycle curve of the zinc-air battery. The catalyst material of the application can still keep stable after 16000 cycles, and commercial Pt/C+RuO ₂ After 170 turns, a significant decay occurred.

Example 2:

first 10g of ammonium citrate is dissolved in 30ml of water and then 30g of MnCl is added ₂ ·4H ₂ O is stirred for 1h to form transparent solution, and finally the solution is quickly frozen by liquid nitrogen and freeze-dried for 24h to form uniform solid powder. The solid powder formed was calcined in a tube furnace at 900 ℃ for 2 hours under nitrogen to give a black powder, which was then acid washed with 2m HCl at 90 ℃ for 24 hours. Calcining the acid-washed sample for 1h at 900 ℃ under a mixed atmosphere of nitrogen and ammonia gas to obtain the final catalyst.

FIG. 11 is a scanning electron microscope image of the catalyst obtained in example 2. From fig. 11, it can be seen that the microstructure of the catalyst obtained in example 2 is a two-dimensional plate-like structure.

Fig. 12 is an XRD pattern of the catalyst prepared using example 2. As shown, only two broad diffraction peaks appear in the XRD patterns at (002) and (100) planes corresponding to graphite carbon at 21 ° and 43 °, and no characteristic peak related to manganese metal appears.

FIG. 13 is a graph showing the nitrogen adsorption and desorption curve and pore size distribution of the catalyst prepared in example 2, from which it can be seen that example 2 has a large specific surface area and a developed pore structure. Calculated we found that the specific surface area of the catalyst obtained in example 2 was 1967.2m ² g ^-1 The pore volume was 3.827cm ³ g ^-1 。

FIG. 14 is a comparison of the ORR catalytic performance of the catalyst of example 2 with the catalyst of comparative example 1 and commercial Pt/C, and shows that the ORR catalytic performance of example 2 is significantly improved or even better than commercial Pt/C when compared to comparative example 1. The half-wave potential of example 2 was 0.9V and the limiting current was 5.2mA cm ^-2 。

FIG. 15 is a discharge curve and power density curve of a catalyst-assembled zinc-air battery prepared in example 2 and commercial Pt/C+RuO ₂ A comparison was made. As can be seen from FIG. 15, the limiting power density of the catalyst material of the present application is about 190mW cm ^-2 170mW cm higher than commercial catalyst ^-2 。

Example 3:

10g of dopamine hydrochloride is dissolved in 30ml of water, and 15g of MnCl is then added ₂ ·4H ₂ O and 15g CoCl ₂ ·6H ₂ O is stirred for 1h to form transparent solution, and finally the solution is quickly frozen by liquid nitrogen and freeze-dried for 24h to form uniform solid powder. The solid powder formed was calcined in a tube furnace at 800℃for 2 hours under nitrogen to give a black powder, which was then acid-washed with 2M HCl90℃for 24 hours. Calcining the acid-washed sample for 1h at 800 ℃ under a mixed atmosphere of nitrogen and ammonia gas to obtain the final catalyst.

Comparative example 2:

10g of dopamine hydrochloride was dissolved in 30ml of water, and the solution was frozen with liquid nitrogen and lyophilized for 24 hours to form a powder. The resulting powder was calcined in a tube furnace at 800℃for 2 hours under nitrogen to give a black powder, which was then acid-washed with 2M HCl at 90℃for 24 hours. Calcining the acid-washed sample for 1h at 800 ℃ under a mixed atmosphere of nitrogen and ammonia gas to obtain the final catalyst.

FIG. 16 is a scanning electron microscope image of the catalyst obtained in example 3. From fig. 16, it can be seen that the microstructure of the catalyst obtained in example 3 is a two-dimensional plate-like structure.

FIG. 17 shows a comparison of the ORR catalytic performance of example 3 catalyst with the comparative example 2 catalyst and commercial Pt/C, and shows that example 3 has significantly improved ORR catalytic performance compared to comparative example 2 and even better than commercial Pt/C. Example 3 has a half-wave potential of 0.85V and a limiting current of 5.5mA cm ^-2 。

FIG. 18 shows the example 3 catalyst, comparative example 2 catalyst and IrO ₂ OER activity of (a). Example 3 at 10mA cm ^-2 The potential at the current density was 1.508V, which is higher than that of the catalyst of comparative example 1 and IrO ₂ The OER catalytic performance of the catalyst is good.

Example 4:

10g urotropine was dissolved in 30ml water and then 15g MnCl was added ₂ ·4H ₂ O and 15g CoCl ₂ ·6H ₂ O is stirred for 1h to form transparent solution, and finally the solution is quickly frozen by liquid nitrogen and freeze-dried for 24h to form uniform solid powder. The solid powder formed was calcined in a tube furnace at 800 ℃ for 2 hours under nitrogen to give a black powder, which was then acid washed with 2m HCl at 90 ℃ for 24 hours. Calcining the acid-washed sample for 1h at 800 ℃ under a mixed atmosphere of nitrogen and ammonia gas to obtain the final catalyst.

Comparative example 3:

10g of urotropin was dissolved in 30ml of water, and the solution was frozen with liquid nitrogen and lyophilized for 24 hours to form a powder. The resulting powder was calcined in a tube furnace at 800℃for 2 hours under nitrogen to give a black powder, which was then acid-washed with 2M HCl at 90℃for 24 hours. Calcining the acid-washed sample for 1h at 800 ℃ under a mixed atmosphere of nitrogen and ammonia gas to obtain the final catalyst.

FIG. 19 is a scanning electron microscope image of the catalyst obtained in example 4. From fig. 19, it can be seen that the microstructure of the catalyst obtained in example 3 is a two-dimensional plate-like structure.

FIG. 20 shows the catalyst of example 4 and the catalyst of comparative example 3 andcomparison of the ORR catalytic performance of commercial Pt/C it was found that example 4 had a significant improvement in the ORR catalytic performance of example 4 compared to comparative example 3, even comparable to commercial Pt/C. Example 4 has a half-wave potential of 0.84V and a limiting current of 5mA cm ^-2 。

Claims

1. A method for synthesizing a monoatomic catalyst by parallel structure construction and monoatomic generation is characterized in that: taking a nitrogenous organic matter as a carbon precursor, taking transition metal salt as a template agent, a pore-forming agent, a metal source and a catalyst for catalyzing carbon at the same time, mixing in a dissolving or ball milling mode, carbonizing, washing with hydrochloric acid, and carbonizing for the second time to finally obtain a single-atom catalyst; the transition metal salt is used as a template agent and a metal source, and can change the graphitization degree and the pore diameter structure of the carbon material; the method organically combines the three methods of single atom preparation, a template method and an activation method to realize synchronous completion.

2. The method of claim 1, wherein a transition metal salt is selected as a template agent, an activator, a metal source and a catalyst for catalyzing carbon, wherein the metal salt separates nitrogen-containing organic matters to form a two-dimensional structure, and carbon is reduced to a metal simple substance along with the increase of temperature, and carbon is consumed in the process to form a plurality of defects and holes; the metal single substance has catalytic action on carbon at high temperature, so that the graphitization degree of the carbon material is improved; finally, volatilizing the metal at a high temperature, wherein the defective micropores on the carbon material have a capturing effect on the monoatomic metal to fix the metal so as to form metal monoatomic dispersion; and in the secondary carbonization process, nitrogen doping is carried out on the carbon material through ammonia gas, so that more nitrogen elements are introduced.

3. The method according to any one of claims 1-2, characterized by the steps of:

4. A method according to claim 3, characterized in that: the nitrogen-containing organics used in step (a) include, but are not limited to: ammonium citrate, dopamine hydrochloride, urotropine, humic acid and the like; the transition metal salt is one or more than one of transition metals such as Fe salt, co salt, ni salt, mn salt and the like.

5. A method according to claim 3, characterized in that: in the step (a), the mass ratio of the nitrogen-containing organic precursor to the transition metal salt is 1:3.

6. A method according to claim 3, characterized in that: in step (a), 10g nitrogen-containing organic matter is dissolved in 30ml water, then 30g transition metal salt is added and stirred for 1h to form a transparent solution, and finally the solution is frozen by liquid nitrogen fast with nitrogen, and 24h is freeze-dried to form uniform solid powder.

7. A method according to claim 3, characterized in that: in the step (b), the carbonization temperature is 700-1200 ℃, and the heating rate is 1-10 ℃ for min ^-1 The heat preservation time is 0-10 h.

8. A method according to claim 3, characterized in that: the composition of the prepared monoatomic catalyst is monoatomic transition metal and nitrogen doped carbon nano-sheets with rich pore structures, large specific surface area, high graphitization degree and other characteristics.

9. A method according to claim 3, characterized in that: in step (c), the carbonized sample is pickled in 2M hydrochloric acid solution at 90 ℃ for 24h, then filtered to be neutral by a large amount of deionized water, and dried in an oven at 80 ℃.

10. A zinc-air battery, wherein the single-atom catalyst prepared by the method of any one of claims 1-9 is assembled into a zinc-air battery, and the charge-discharge cycle curve of the zinc-air battery can be known to be stable after 16000 cycles.