Biomass-derived graded porous asymmetric coordination monatomic carbon-based electrocatalyst, and preparation method therefor and use thereof

Abstract

Disclosed in the present invention are a biomass-derived graded porous asymmetric coordination monatomic carbon-based electrocatalyst, and a preparation method therefor and the use thereof. The preparation method for the biomass-derived graded porous asymmetric coordination monatomic carbon-based electrocatalyst comprises the following steps: (1) stirring and mixing de-alkalized lignin with a transition metal salt, a nitrogen-containing compound and a template, i.e., magnesium oxide, in a solvent until uniform, so as to obtain a suspension, wherein the transition metal comprises iron and zinc; (2) heating, evaporating and drying off the suspension, so as to obtain a lignin / metal ion / nitrogen-containing compound / template mixed precursor; and (3) subjecting the mixed precursor to high-temperature pyrolysis under the protection of an inert gas, cooling same, and then sequentially washing and drying same, so as to obtain a monatomic carbon-based electrocatalyst. The monatomic carbon-based electrocatalyst provided in the present invention has graded pores of micropores, mesopores and macropores, a surface nanosheet and an asymmetric coordination structure of Fe-N3O at the same time, and can be applied to fuel cells and metal-air cells.

Description

A biomass-derived hierarchical porous asymmetric coordinated single-atom carbon-based electrocatalyst and its preparation method and application Technical field:

[0001] The present invention relates to the field of biochar preparation technology and electrocatalytic energy technology, and in particular to a biomass-derived hierarchical porous asymmetric coordinated single-atom carbon-based electrocatalyst, a preparation method thereof, and applications thereof. Background technology:

[0002] The oxygen reduction reaction (ORR) is an important component of new energy storage and conversion devices such as proton exchange membrane fuel cells and metal-air batteries. However, the slow kinetics of the oxygen reduction reaction lead to the reaction process being heavily dependent on the catalytic action of precious metals such as platinum, which has restricted the development of these new energy devices. Therefore, there is an urgent need to develop new, efficient, economical, and environmentally friendly ORR catalysts to replace the currently used precious metal catalysts. In recent years, single-atom catalysts have attracted extensive attention and research from researchers around the world due to their high atomic utilization and excellent catalytic activity. In particular, iron-nitrogen-carbon (Fe-NC) single-atom catalysts, formed by iron atoms anchored in a transition metal-nitrogen-carbon (TM-NC) structure, are considered to have the potential to replace precious metal catalysts such as platinum carbon in fuel cells and metal-air battery cathodes.

[0003] Currently, the synthesis of Fe-NC single-atom catalysts primarily uses the imidazolate framework molecular sieve material ZIF-8 as a template and precursor. Carbonization forms nitrogen-doped carbon with a rich microporous structure and large specific surface area, which further adsorbs and anchors Fe ions. Finally, secondary carbonization introduces Fe-N coordination structures onto the carbon support. This method effectively isolates metal ions and inhibits the aggregation of metal active sites during pyrolysis. However, this method suffers from disadvantages such as low yield, the toxicity of organic ligands, and a cumbersome synthesis process, making it difficult to synthesize on a large scale. Furthermore, direct carbonization of ZIF-8 inevitably leads to a shrinkage of the pore structure, resulting in a large number of active sites being buried in the carbon matrix and unable to be fully utilized, reducing the mass-specific activity of the catalyst. Furthermore, as a heterogeneous catalytic process, the oxygen reduction process places high demands on the formation of gas-liquid-solid three-phase boundaries. The predominantly microporous structure of ZIF-8, with its lack of mesopores and macropores, is not conducive to the transport of oxygen and product transfer across these three-phase boundaries during the ORR process.

[0004] To address the difficulties and low yields of Fe-NC single-atom catalysts, researchers are attempting to synthesize single-atom catalysts using cheaper, readily available biomass precursors instead of organic ligands. Lignin, the third most abundant renewable resource, is widely available (accounting for 20-30% of the total plant content). It contains three different phenylpropane monomer units, which are rich in oxygen-containing functional groups such as hydroxyl groups and can anchor metal ions through reactions such as ion exchange and coordination. Therefore, converting biomass waste lignin into single-atom ORR catalysts has significant economic and environmental benefits. At present, some research groups at home and abroad have developed methods to synthesize single-atom catalysts from lignin. For example, Liu Yun's research group at Beijing University of Chemical Technology developed a new method for synthesizing single-atom catalysts through independent assembly of lignin / metal supramolecular devices (ZL201810029041.1). The lignin-based cobalt single atoms developed by their method showed excellent activity in catalyzing the preparation of methyl benzoate from benzyl alcohol; Guo Haiwei and others from Hebei University of Technology developed a new Fe-NC single-atom catalyst using lignin as a carbon source, and successfully applied it in the field of lignin oxidative depolymerization (ZL202310266072.X).

[0005] However, despite reports on lignin-based transition metal-nitrogen-carbon single-atom catalysts, these catalysts still suffer from structural shortcomings similar to those of ZIF-8, namely, a predominance of micropores and a lack of mesoporous and macroporous structures. This still fails to meet the requirements of heterogeneous catalytic processes involving gas reactions, such as oxygen reduction. There are few reports on how to construct a hierarchical microporous-mesoporous-macroporous structure on lignin-based single-atom catalysts. Furthermore, the reported lignin-derived iron-nitrogen-carbon single-atom catalysts are still primarily based on an Fe-N4 coordination structure, with lignin primarily serving as a carbon support. Its rich oxygen-containing functional groups have not been fully utilized. Further research is needed to introduce the oxygen element from lignin into the coordination environment of the Fe single atom to form an asymmetric coordination structure. Summary of the invention:

[0006] The present invention solves the problems existing in the prior art and provides a biomass-derived hierarchical porous asymmetric coordinated single-atom carbon-based electrocatalyst and its preparation method and application. The biomass-derived hierarchical porous asymmetric coordinated single-atom carbon-based electrocatalyst obtained by the preparation method proposed in the present invention has a micropore-mesopore-macroporous hierarchical porous structure and a surface nanosheet structure, as well as an Fe-N3O ​​coordination structure, and can be applied to fuel cells and metal-air batteries.

[0007] The first object of the present invention is to provide a method for preparing a biomass-derived hierarchical porous asymmetric coordinated single-atom carbon-based electrocatalyst, comprising the following steps: (1) stirring and mixing dealkalized lignin with a transition metal salt, a nitrogen-containing compound, and a template agent, magnesium oxide, in a solvent to obtain a suspension, wherein the transition metal comprises iron and zinc; (2) heating the suspension obtained in step (1) in a water bath to evaporate and dry the suspension to obtain a lignin / metal ion / nitrogen-containing compound / template agent mixed precursor; (3) pyrolyzing the mixed precursor obtained in step (2) at a high temperature under the protection of an inert gas, cooling the mixture, and then washing and drying the mixture to obtain the biomass-derived hierarchical porous asymmetric coordinated single-atom carbon-based electrocatalyst.

[0008] The present invention utilizes coordination chemistry combined with a dual template strategy, using lignin as an organic ligand and carbon source, utilizing the abundant oxygen-containing functional groups on the surface of lignin to anchor metal ions, and utilizing the high-temperature volatility of zinc ions and the high-temperature stability of MgO to form a soft and hard dual template to construct a hierarchical porous structure. The introduction of metallic zinc ions during the carbonization process is beneficial to increasing the number of micropores in the carbon material. The reason is that as the temperature of the carbonization process increases, the zinc ions are gradually converted into ZnO and then reduced to metallic Zn by a carbon thermal reaction (ZnO+C→Zn+CO2). When the temperature rises to 900°C, metallic Zn volatilizes, causing more defects and microporous structures to appear on the surface of the carbon material. The magnesium oxide template undergoes a hydrogen oxidation reaction on the surface during the evaporation of the solvent, generating Mg(OH)2 that combines with lignin and covers the lignin surface to induce the formation of a carbon nanosheet structure. The carbonization process is beneficial to inhibiting the aggregation of surface metal ions. During the high-temperature carbonization process, nitrogen-containing compounds gradually decompose to produce ammonia that is doped onto the carbon, forming anchor sites such as pyridinic nitrogen and pyrrolic nitrogen, which can coordinate with Fe ions to form highly active Fe-N3O ​​sites.

[0009] Preferably, the solvent in step (1) is deionized water, and the mass volume ratio of lignin to deionized water is 1:100-1:150 g / mL.

[0010] Preferably, in step (1), the iron salt is one or more of ferric nitrate nonahydrate, ferric chloride and ferrous oxalate, and the zinc salt is one or more of zinc nitrate hexahydrate, zinc acetate dihydrate and zinc chloride.

[0011] Preferably, in step (1), the mass ratio of dealkalized lignin to iron salt is 5:1-20:1, and the molar ratio of iron salt to zinc salt is 1:10-1:30.

[0012] More preferably, in step (1), the mass ratio of dealkalized lignin to iron salt is 10:1, and the molar ratio of iron salt to zinc salt is 1:20.

[0013] Preferably, the nitrogen-containing compound in step (1) is selected from one or more of dicyandiamide, urea and melamine, the mass ratio of the nitrogen-containing compound to the dealkalized lignin is 1:2-2:1, and the mass ratio of the magnesium oxide to the dealkalized lignin is 1:3-2:1.

[0014] More preferably, the mass ratio of the nitrogen-containing compound to the dealkalized lignin in step (1) is 1:1, and the mass ratio of the magnesium oxide template to the dealkalized lignin is 1:1.3.

[0015] The specific steps of step (2) are as follows: heating and stirring the above suspension in a water bath at 75°C until the solvent evaporates, and further placing it in a vacuum drying oven at 60°C to dry it to obtain a lignin / metal ion / nitrogen-containing compound / template mixed precursor.

[0016] The inert gas described in step (3) is nitrogen or argon; the template agent magnesium oxide has a flaky structure.

[0017] Preferably, the high-temperature pyrolysis conditions in step (3) are: pyrolysis temperature of 900°C-1000°C, holding time of 0.5-3h, and heating rate of 3-20°C / min.

[0018] Further preferably, the high-temperature pyrolysis conditions in step (3) are: pyrolysis temperature is 910°C, holding time is 1h, and heating rate is 5°C / min.

[0019] Preferably, the specific steps of washing and drying (acid washing, water washing and drying) are: dispersing the product in a 1M hydrochloric acid solution and stirring overnight, filtering to remove the hydrochloric acid and washing with deionized water to pH = 6-7, then placing it in a 60°C vacuum drying oven, drying and then grinding to obtain a biomass-derived hierarchical porous asymmetric coordinated single-atom carbon-based electrocatalyst.

[0020] The second object of the present invention is to provide a biomass-derived hierarchical porous asymmetric coordinated single-atom carbon-based electrocatalyst obtained by the above-mentioned preparation method, wherein the single-atom carbon-based electrocatalyst simultaneously has a micropore-mesopore-macroporous hierarchical porous structure, a surface nanosheet structure and an asymmetric Fe-N3O ​​coordination environment.

[0021] The third object of the present invention is to provide the biomass-derived hierarchical porous asymmetric coordinated single-atom carbon-based electrocatalyst that can be applied to microbial fuel cells or metal-air battery cathodes.

[0022] Preferably, the single-atom carbon-based electrocatalyst is used as the cathode of a zinc-air battery or a microbial fuel cell.

[0023] Compared with the prior art, the present invention has the following advantages:

[0024] 1. Lignin contains a large number of oxygen-containing functional groups that can anchor metal ions and is inexpensive as an organic ligand and carbon source;

[0025] 2. The present invention realizes the preparation of hierarchical porous carbon-based single atoms through a simple process, which is easy to scale up production.

[0026] 3. The biomass-derived hierarchical porous asymmetric coordinated single-atom carbon-based electrocatalyst obtained by the preparation method proposed in the present invention has a hierarchical porous structure of micropores-mesopores-macroporosities.

[0027] 4. The biomass-derived hierarchical porous single-atom carbon-based electrocatalyst obtained by the preparation method proposed in the present invention has an Fe-N3O ​​asymmetric coordination structure. Description of the drawings:

[0028] FIG1 is a scanning electron micrograph of the biomass-derived hierarchical porous asymmetric coordinated single-atom carbon-based electrocatalyst obtained in Example 1;

[0029] FIG2 is a transmission electron micrograph of the biomass-derived hierarchical porous asymmetric coordinated single-atom carbon-based electrocatalyst obtained in Example 1;

[0030] FIG3 is a spherical aberration-corrected transmission electron microscopy image of the biomass-derived hierarchical porous asymmetric coordinated single-atom carbon-based electrocatalyst obtained in Example 1;

[0031] FIG4 is an X-ray extended edge fitting coordination structure of the biomass-derived hierarchical porous asymmetric coordinated single-atom carbon-based electrocatalyst obtained in Example 1;

[0032] FIG5 is a scanning electron micrograph of the biomass-derived microporous single-atom carbon-based electrocatalyst obtained in Example 2;

[0033] FIG6 is a transmission electron micrograph of the biomass-derived microporous single-atom carbon-based electrocatalyst obtained in Example 2;

[0034] FIG7 is a scanning electron micrograph of the biomass-derived mesoporous single-atom carbon-based electrocatalyst obtained in Example 3;

[0035] FIG8 is a nitrogen adsorption / desorption curve of the single-atom carbon-based electrocatalysts obtained in Example 1, Example 2, and Example 3;

[0036] FIG9 is a DFT pore size distribution analysis corresponding to the nitrogen adsorption / desorption curves of the single-atom carbon-based electrocatalysts obtained in Example 1, Example 2, and Example 3;

[0037] Figure 10 is a comparison curve of the ORR catalytic performance LSV test of the biomass-derived hierarchical porous asymmetric coordinated single-atom carbon-based electrocatalyst and the commercial platinum-carbon catalyst under alkaline conditions in Example 4;

[0038] FIG11 is a comparative curve of the ORR catalytic performance LSV test of the biomass-derived hierarchical porous asymmetric coordinated single-atom carbon-based electrocatalyst and the commercial platinum-carbon catalyst under neutral conditions in Example 5;

[0039] FIG12 is a charge and discharge test curve of the zinc-air battery assembled in Example 6;

[0040] FIG13 is a diagram of a zinc-air battery-driven electronic device assembled in Example 6;

[0041] FIG14 is a comparative curve of LSV test of ORR catalytic performance of samples with different pyrolysis temperatures under alkaline conditions in Example 7;

[0042] FIG15 is a comparative curve of the LSV test of the ORR catalytic performance of different zinc acetate dihydrate samples under alkaline conditions in Example 8. Specific implementation method:

[0043] The following examples are provided to further illustrate the present invention, but are not intended to limit the present invention.

[0044] Unless otherwise defined, all technical terms used hereinafter have the same meaning as those generally understood by those skilled in the art. The technical terms used herein are only for the purpose of describing specific embodiments and are not intended to limit the scope of protection of the present invention. Unless otherwise specified, the experimental materials and reagents herein are conventional commercial products in the art. Dealkalized lignin was purchased from Shanghai MacLean Biochemical Technology Co., Ltd., item number: L849279.

[0045] Example 1

[0046] The synthesis of biomass-derived hierarchical porous asymmetric coordinated single-atom carbon-based electrocatalysts includes the following steps:

[0047] 1) In a 100mL beaker, add 60mL of deionized water, add 0.438g of zinc acetate dihydrate (2mmol), and stir until dissolved. Using a pipette, pipette 200μL of a 200mg / mL Fe(NO3)3 solution (10mg Fe(NO3)3·9H2O dissolved in deionized water, acidify with 5.726mL of 36%–38% concentrated hydrochloric acid, and dilute to 50mL). Stir until the solution is uniform in color. Add 0.4g of lignin and 0.3g of MgO to the above solution, stir until the lignin is completely dispersed, and then add 0.4g of melamine.

[0048] 2) Place the beaker in a water bath and stir at 75°C until the solution is completely evaporated. Then, place the evaporated solid in a vacuum drying oven at 60°C overnight to further dry it to obtain a powdered precursor.

[0049] 3) placing the powdered precursor in a porcelain boat and pyrolyzing it in a vacuum tube furnace at 910°C under a nitrogen atmosphere at a heating rate of 5°C / min. After maintaining the temperature for 1 hour, the mixture was naturally cooled to room temperature to obtain powdered carbon;

[0050] 4) Grind the powdered carbon, disperse it in 50 mL of 1 M hydrochloric acid solution, stir it overnight, filter and wash it with deionized water until the pH is 6-7, and dry it in a vacuum oven at 60°C. After grinding evenly, a biomass-derived hierarchical porous single-atom carbon-based electrocatalyst can be obtained.

[0051] The resulting biomass-derived hierarchical porous asymmetric coordinated single-atom carbon-based electrocatalyst was tested, and its scanning electron microscope image is shown in Figure 1, and the transmission electron microscope image is shown in Figure 2. A large number of porous nanosheet structures were generated on the surface of the material. The spherical aberration-corrected transmission electron microscope image is shown in Figure 3, and the bright spots in the image are Fe single-atom sites. Figure 4 shows the coordination structure obtained by X-ray extended edge fitting, and the coordination structure of the Fe single atom is Fe-N3O. Figures 8 and 9 show that the pore structure of the biomass-derived hierarchical porous asymmetric coordinated single-atom carbon-based electrocatalyst contains a large number of micropores, mesopores, and macropores.

[0052] Example 2:

[0053] The method is the same as Example 1, except that: the MgO template is not added to the biomass-derived microporous single-atom carbon-based electrocatalyst during the raw material mixing stage to obtain the biomass-derived microporous single-atom carbon-based electrocatalyst.

[0054] The scanning electron microscope image is shown in Figure 5, and the transmission electron microscope image is shown in Figure 6. The material has a bulk structure. Figures 8 and 9 show that the pore structure of the biomass-derived microporous single-atom carbon-based electrocatalyst is mainly micropores.

[0055] Example 3:

[0056] The method is the same as Example 1, except that zinc acetate dihydrate is not added to the biomass-derived mesoporous single-atom carbon-based electrocatalyst during the raw material mixing stage to obtain the biomass-derived mesoporous single-atom carbon-based electrocatalyst.

[0057] The SEM image of the nanosheet structure is shown in Figure 7. Figures 8 and 9 show that the biomass-derived mesoporous single-atom carbon-based electrocatalyst has formed a clear mesoporous structure.

[0058] Example 4: Evaluation of Oxygen Reduction Activity of Electrocatalysts under Alkaline Conditions

[0059] 5 mg of the catalyst (biomass-derived hierarchical porous asymmetric coordination single-atom carbon-based electrocatalyst / commercial platinum-carbon obtained in Example 1) was dispersed in 1 mL of ethanol / water (ethanol:water volume ratio of 1:1). 10 μL of 5% Nafion solution was then added dropwise, and sonication was performed for 1 hour to uniformly disperse the catalyst ink. Using a pipette, 10 μL of the catalyst ink was dripped onto a platinum-carbon electrode. After drying at room temperature, the electrode was assembled on a rotating disk-changing electrode assembly, forming a three-electrode system with a graphite rod as the counter electrode, mercury / mercuric oxide as the reference electrode, and 0.1 M KOH as the electrolyte. Linear sweep voltammetry was performed over the voltage range of -0.6 to 0.3 V relative to the reference electrode at a scan rate of 10 mV / s and a rotation speed of 1600 rpm to determine the ORR performance of the catalyst. Figure 10 shows that the ORR half-wave potential of the biomass-derived hierarchical porous asymmetric coordination single-atom electrocatalyst is higher than that of the commercial platinum-carbon catalyst under alkaline conditions.

[0060] Example 5: Evaluation of oxygen reduction activity of electrocatalysts under neutral conditions

[0061] The same as Example 4, except that a 50 mM PBS solution was used as the electrolyte and a saturated silver / silver chloride reference electrode was used to form a three-electrode system. Figure 11 shows that under neutral conditions, the half-wave potential of the ORR process of the biomass-derived hierarchical porous asymmetric coordinated single-atom carbon-based electrocatalyst is higher than that of the commercial platinum-carbon catalyst.

[0062] Example 6: Application of biomass-derived hierarchical porous asymmetric coordinated single-atom carbon-based electrocatalysts in rechargeable zinc-air batteries

[0063] The biomass-derived hierarchical porous asymmetric coordinated single-atom carbon-based electrocatalyst synthesized in Example 1 and the commercial oxidant were mixed in a mass ratio of 1:1. The catalyst ink was prepared according to the method in Example 3, and the ink was dropped onto the hydrophobic carbon cloth. After drying, it was used as an air cathode (the area of ​​the catalyst loaded on the carbon cloth was 1 cm -2 , the catalyst loading was 3 mg cm -2 ). Assemble the air cathode and the polished zinc sheet on the zinc-air battery mold, and add 6 mol L -1 A KOH solution was used as the electrolyte for the zinc-air battery. The assembled zinc-air battery was subjected to charge and discharge tests using a battery testing system. Figure 12 shows that the zinc-air battery assembled with the biomass-derived hierarchical porous asymmetric coordinated single-atom carbon-based electrocatalyst can operate stably for 200 hours. Figure 13 shows that the assembled zinc-air battery can drive an electronic device timer.

[0064] Example 7:

[0065] The same as Example 1, except that the pyrolysis temperature is set to 850°C.

[0066] Its electrochemical performance is shown in Figure 14. The performance of the 850℃ sample under alkaline conditions is lower than that of the 910℃ sample. The reason is that the zinc ions cannot be completely volatilized at 850℃ and the degree of graphitization of the sample is low.

[0067] Example 8:

[0068] Same as Example 1, except that:

[0069] Two groups of samples with different addition amounts of zinc acetate dihydrate were added, 0.329 g (1.5 mmol) and 0.548 g (2.5 mmol), respectively, to form a control experiment with Example 1. The ORR performance of different addition amounts of zinc acetate dihydrate is shown in Figure 15, which shows that zinc acetate in Example 1 is the optimal addition amount.

[0070] Example 9:

[0071] Same as Example 1, except that:

[0072] The mass ratio of dealkalized lignin to iron salt is 5:1, and the molar ratio of iron salt to zinc salt is 1:10; the mass ratio of nitrogen-containing compound to dealkalized lignin is 1:2, and the mass ratio of magnesium oxide to dealkalized lignin is 1:3; the high-temperature pyrolysis conditions are: pyrolysis temperature is 900°C, holding time is 3h, and heating rate is 3°C / min.

[0073] Example 10:

[0074] Same as Example 1, except that:

[0075] The mass ratio of dealkalized lignin to iron salt is 20:1, and the molar ratio of iron salt to zinc salt is 1:30; the mass ratio of nitrogen-containing compound to dealkalized lignin is 2:1, and the mass ratio of magnesium oxide to dealkalized lignin is 2:1; the high-temperature pyrolysis conditions are: pyrolysis temperature is 1000°C, holding time is 0.5h, and heating rate is 20°C / min.

[0076] The description of the above embodiments is only used to help understand the technical solution and core ideas of the present invention. It should be pointed out that for those skilled in the art, several improvements and modifications can be made to the present invention without departing from the principles of the present invention. These improvements and modifications also fall within the scope of protection of the claims of the present invention.

Claims

1. A method for preparing a biomass-derived hierarchical porous asymmetric coordinated single-atom carbon-based electrocatalyst, characterized in that: The method comprises the following steps: (1) mixing dealkalized lignin with a transition metal salt, a nitrogen-containing compound, and a template agent magnesium oxide in a solvent and stirring the mixture to obtain a suspension, wherein the transition metal comprises iron and zinc; (2) heating and evaporating the suspension obtained in step (1), and drying the mixture to obtain a lignin / metal ion / nitrogen-containing compound / template agent mixed precursor; and (3) subjecting the mixed precursor obtained in step (2) to high-temperature pyrolysis under the protection of an inert gas, cooling the mixture, and washing and drying the mixture in sequence to obtain a biomass-derived hierarchical porous asymmetric coordinated single-atom carbon-based electrocatalyst.

2. The preparation method according to claim 1, characterized in that In step (1), the iron salt is one or more of ferric nitrate nonahydrate, ferric chloride and ferrous oxalate, and the zinc salt is one or more of zinc nitrate hexahydrate, zinc acetate dihydrate and zinc chloride.

3. The preparation method according to claim 1, characterized in that The solvent described in step (1) is deionized water, and the mass volume ratio of lignin to deionized water is 1:100-1:150 g / mL.

4. The preparation method according to any one of claims 1 to 3, characterized in that In step (1), the mass ratio of dealkalized lignin to iron salt is 5:1-20:1, and the molar ratio of iron salt to zinc salt is 1:10-1:

30.

5. The preparation method according to claim 1, characterized in that The nitrogen-containing compound in step (1) is selected from one or more of dicyandiamide, urea and melamine, the mass ratio of the nitrogen-containing compound to the dealkalized lignin is 1:2-2:1, and the mass ratio of the magnesium oxide to the dealkalized lignin is 1:3-2:

1.

6. The preparation method according to claim 1, characterized in that The high-temperature pyrolysis conditions described in step (3) are: pyrolysis temperature of 900°C-1000°C, holding time of 0.5-3h, and heating rate of 3-20°C / min.

7. The preparation method according to claim 1, characterized in that The template magnesium oxide described in step (1) has a flaky structure.

8. The biomass-derived hierarchical porous asymmetric coordinated single-atom carbon-based electrocatalyst obtained by the preparation method of claim 1, characterized in that: The single-atom carbon-based electrocatalyst has an asymmetric coordination structure of Fe-N3O, and also has a hierarchical porous structure of micropores-mesopores-macroporous structures and a surface nanosheet structure.

9. The biomass-derived hierarchical porous asymmetric coordinated single-atom carbon-based electrocatalyst according to claim 8 can be applied to the cathode of microbial fuel cells or metal-air batteries.

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