Bifunctional alloy electrocatalyst and preparation method and application thereof

Abstract

The invention discloses a bifunctional alloy electrocatalyst as well as a preparation method and application thereof, and belongs to the field of electrochemical energy devices. Firstly, a polymer nanofiber membrane is formed through electrostatic spinning, then MOFs and COFs are sequentially loaded on the fiber membrane, after high-temperature carbonization treatment, the fiber membrane is soaked in a mixed aqueous solution containing various metal ions, and finally the alloy electrocatalyst loaded by the porous carbon nanosheets is synthesized through high-temperature treatment. The prepared electrocatalyst has a stable hierarchical porous structure and abundant catalytic active sites, and shows excellent bifunctional electrocatalytic activity and remarkable chloride ion (Cl <->) corrosion resistance in an alkaline seawater medium aiming at oxygen reduction reaction (ORR) and oxygen evolution reaction (OER); a seawater electrolyte zinc-air battery assembled by taking the zinc-air composite material as a battery cathode catalyst has considerable power density and cycling stability; in addition, the assembled seawater-based flexible (solid-state) zinc-air battery can still keep stable circulation at different bending angles.

Description

Technical Field

[0001] The present invention relates to the technical field of electrochemical energy devices, and particularly to a bifunctional alloy electrocatalyst prepared by an electrospinning and porous organic framework composite strategy, and its application in seawater electrolyte zinc-air batteries (liquid / solid). Background Art

[0002] Zinc-air batteries (ZABs) exhibit significant advantages in the field of new energy storage devices due to their high theoretical energy density of up to 1086 Wh kg -1 and environmentally friendly characteristics. It is worth noting that existing research is mostly limited to using high-purity deionized water as the electrolyte. In contrast, zinc-air batteries constructed using natural seawater as the electrolyte (Seawater-based Zinc-Air Batteries, S-ZABs) can not only effectively relieve the pressure on fresh water resources, but also reduce the cost of the electrolyte, and have become a frontier research direction in the field of electrochemical energy storage.

[0003] In the S-ZABs system, the kinetic characteristics of the oxygen catalytic reaction (oxygen reduction reaction ORR / oxygen evolution reaction OER) at the air cathode directly determine the energy conversion efficiency and cycle life of the device. However, the inherent four-electron transfer lag process in ORR leads to slow kinetic response, while in the OER process, an overpotential as high as 370 mV (@10 mA cm-2) needs to be overcome. These two factors jointly restrict the energy output efficiency of the battery. More critically, the high concentration of chloride ions in the seawater electrolyte will cause competitive adsorption and chlorine evolution side reactions, resulting in electrode corrosion and catalyst deactivation. Therefore, the development of a bifunctional electrocatalyst with chloride ion tolerance, high intrinsic activity and structural stability has become the key to breaking through the technical bottleneck of seawater-based zinc-air batteries.

[0004] Electrospinning technology is a process that can prepare fibers with a scale in the micron or even nanometer level. By electrospinning, a polymer nanofiber membrane can be obtained, and then porous carbon nanofibers can be formed through high-temperature carbonization. Due to the high specific surface area and good electrical conductivity of the porous carbon nanofibers, it can provide a favorable path for the penetration of the electrolyte and the transport of ions and electrons, and at the same time expose more active sites. Thus, it has become an ideal substrate material for synthesizing electrocatalytic materials.

[0005] Porous organic frameworks (POFs), especially metal-organic frameworks (MOFs) and covalent organic frameworks (COFs), due to their ultra-high specific surface area, tunable pore structure, and abundant functional groups, can serve as ideal loading platforms for catalysts to achieve uniform dispersion of catalytic active centers and show potential application prospects in many fields such as electrocatalysis, photocatalysis, and water treatment. Further, combining MOFs and COFs with electrospun fibers can not only effectively improve the dispersibility of the catalyst, enhance its stability, but also promote the mass transfer process and reduce the diffusion resistance through the highly ordered structure of the fiber network. In addition, the high conductivity of carbon fibers helps to accelerate charge transfer, thereby improving the overall catalytic performance and providing a new design idea for the construction of an efficient electrocatalytic system. In the early stage of the inventors, first, a polymer nanofiber membrane was formed by electrospinning, then MOFs and COFs were successively loaded on the fiber membrane, and then it was immersed in a ruthenium ion or iridium ion solution and treated at high temperature to form a catalyst containing metal-nitrogen-carbon catalytic sites. Although it has highly efficient oxygen reduction active sites, its oxygen evolution ability is weak, and its activity and stability are low in seawater electrolytes. Therefore, the present invention proposes a preparation method for a bifunctional alloy electrocatalyst with stable structure and excellent electrochemical performance. Summary of the Invention

[0006] Aiming at the problems of low activity of the cathode catalyst, insufficient bifunctional cooperation, poor structural stability, and limited seawater corrosion resistance in the prior art, the present invention proposes a preparation method for a bifunctional alloy electrocatalyst with stable structure and excellent electrochemical performance.

[0007] To achieve the above object, the present invention adopts the following technical solutions:

[0008] A preparation method for a bifunctional alloy electrocatalyst, comprising the following steps:

[0009] (1) Preparation of an electrospun membrane: Dissolve 2-methylimidazole and a nitrogen-containing polymer hydrophilic polymer in an organic solvent to form a polymer solution, and form a polymer nanofiber membrane by electrospinning;

[0010] (2) Synthesis of a carbon nanosheet precursor: Dissolve 2-methylimidazole in water to form solution A; dissolve a zinc salt and at least one of a cobalt salt and an iron salt in water to form solution B, and add it to solution A under stirring to form a mixed solution; Immerse the fiber membrane obtained in step (1) in the above mixed solution and let it stand to obtain a fiber membrane with MOFs grown on the surface; Ultrasonically dissolve triformyl-m-truxene and 5,5'-diamino-2,2'-bipyridine in a solvent to form solution C, and immerse the fiber membrane with MOFs grown on the surface in solution C again and let it stand, and COFs continue to grow on the surface of the fiber membrane. After the reaction is completed, dry it to synthesize a carbon nanosheet precursor;

[0011] (3) Synthesis of hierarchically porous carbon nanosheets supported on carbon nanofibers: The carbon nanosheet precursor obtained in step (2) is subjected to high-temperature carbonization treatment in an inert gas to synthesize hierarchically porous carbon nanosheets supported on carbon nanofibers;

[0012] (4) Synthesis of the precursor: The hierarchically porous carbon nanosheets supported on carbon nanofibers obtained in step (3) are immersed in a mixed aqueous solution containing various metal ions, allowed to stand under heating conditions, and the porous carbon nanosheets adsorb the metal ions and are dried. The metal ions include any one of three transition metal ions, ruthenium ions or iridium ions, and platinum ions. The transition metal ions are preferably iron ions, cobalt ions, nickel ions, and copper ions. The molar ratio of the three transition metal ions, ruthenium ions or iridium ions, and platinum ions is 15 - 30:1:2;

[0013] (5) Synthesis of the alloy: Rapid Joule heat treatment is carried out in an inert gas or in vacuum to obtain the alloy electrocatalyst supported on the porous carbon nanosheets.

[0014] Preferably, the nitrogen-containing polymer hydrophilic polymer in step (1) is any one of polyacrylonitrile, polyimide, and polyethyleneimine; the organic solvents include but are not limited to any one or a mixture of dimethylformamide, dimethylacetamide, dimethyl sulfoxide, and acetone.

[0015] Preferably, in step (1), 2-methylimidazole and polyacrylonitrile are added to dimethylformamide and continuously stirred at room temperature for 12 h.

[0016] Preferably, the feeding speed of the spinning machine in step (1) is 0.5 - 1.5 mL h -1 , preferably 1.1 mL h -1 ; the positive voltage is set to 12 - 19 kV, preferably 15 kV; the negative voltage is 0 - 2 kV, preferably 2 kV; the spinning distance is 15 - 20 cm, preferably 19 cm.

[0017] Preferably, the zinc salt, cobalt salt, and iron salt in step (2) are water-soluble salts. The zinc salt can be a mixture of one or more of its nitrate, acetylacetonate, and chloride. The metal cobalt salt can be a mixture of one or more of its nitrate or chloride. The metal iron salt can be a mixture of one or more of its nitrate or chloride; the solvent is any one or a mixture of tetrahydrofuran, dioxane, mesitylene, and 1,2-dichlorobenzene. Preferably, in step (2), the fiber membrane stands and reacts in the mixed solution for 0.5 - 2 h, preferably 1 h, and the reaction condition is room temperature; it stands and reacts in solution C for 12 - 36 h, preferably 24 h, the reaction condition is room temperature, and the drying temperature is 30 - 100 °C, preferably 60 °C.

[0018] Preferably, the inert gas in step (3) is one or a mixture of more than one of nitrogen, helium, and argon.

[0019] Preferably, the high-temperature carbonization temperature in step (3) is 900-1000 °C, the carbonization time is 2-3 h, and the heating rate is 5-10 °C / min. -1 .

[0020] Preferably, the mixed aqueous solution in step (4) should contain metal platinum chloride, ruthenium or iridium chloride, and a mixture of one or more of nitrates, chlorides, and acetylacetonates of three or more elements among metal iron, cobalt, nickel, and copper.

[0021] Preferably, the molar ratio of the three transition metal ions is 1.

[0022] Preferably, the metal ions include iron ions, nickel ions, copper ions, iridium ions, and platinum ions, and the molar ratio of iron ions, nickel ions, copper ions, iridium ions, and platinum ions is 5:5:5:1:2, or the metal ions include iron ions, nickel ions, cobalt ions, ruthenium ions, and platinum ions, and the molar ratio of iron ions, nickel ions, cobalt ions, ruthenium ions, and platinum ions is 5:5:5:1:2.

[0023] Preferably, the heating time in step (4) is 20-30 h, preferably 24 h, the heating temperature is 70-90 °C in a water bath or oil bath, and the drying temperature is 30-100 °C, preferably 60 °C.

[0024] Preferably, the inert gas in step (5) is one or a mixture of more than one of nitrogen, helium, and argon.

[0025] Preferably, the set voltage of the rapid Joule heat treatment in step (5) is about 15-20 V, preferably 17 V; the current is about 15-25 A, preferably 20 A; the temperature is about 900-1100 °C, preferably 1000 °C; and the heat treatment time is 8-15 s, preferably 10 s.

[0026] The alloy supported on porous carbon prepared by the preparation method as described above has outstanding electrochemical performance and can be well applied to new energy storage devices. For example, the bifunctional alloy electrocatalyst is used as a cathode catalytic material, especially as a cathode catalyst for seawater electrolyte zinc-air batteries.

[0027] Therefore, the present invention has the following beneficial effects:

[0028] (1) The unique structure of the present invention can effectively prevent the structure from collapsing during the heat treatment process, increase the specific surface area, and expose more active sites; and the excellent stability and corrosion resistance of the alloy further improve the electrochemical performance.

[0029] (2) The alloy electrocatalyst supported on porous carbon exhibits ideal bifunctional catalytic performance for oxygen reduction and oxygen evolution in an alkaline seawater medium, with stable structural properties and easily achievable reaction conditions, and can be applied to the fields of electrocatalysis and new energy storage. In particular, it can be used as a cathode catalyst for seawater electrolyte zinc-air batteries, capable of providing a high power density and having good charge-discharge cycle stability; the assembled flexible (solid-state) zinc-air battery can maintain stable cycling even when bent at different angles. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 XRD pattern of the catalyst prepared in Example 1 of the present invention.

[0031] Figure 2 TEM image of the catalyst prepared in Example 2 of the present invention.

[0032] Figure 3 LSV polarization curves for oxygen reduction of the catalysts prepared in Example 1, Example 2 and Comparative Example 1 of the present invention in an O2-saturated 0.1 M KOH seawater solution (a), and LSV polarization curves for oxygen in a 1 M KOH seawater solution (b).

[0033] Figure 4 Accelerated durability test curves (3000 cycles) of Examples 1 and 2 of the present invention in a 0.1 M KOH seawater solution.

[0034] Figure 5 Discharge curves and power density diagrams (a) of the seawater electrolyte zinc-air batteries assembled from Example 1 and Comparative Example 1 of the present invention, and charge-discharge cycle diagram (b) of Example 1.

[0035] Figure 6 Charge-discharge cycle diagrams (a) of the flexible (solid-state) seawater electrolyte zinc-air battery assembled from Example 2 of the present invention at different bending angles, and practical application diagram (b). DETAILED DESCRIPTION OF THE INVENTION

[0036] It should be noted that the following detailed description is exemplary and is intended to provide further explanation of the present invention. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by those of ordinary skill in the technical field to which the present invention belongs.

[0037] Example 1

[0038] (1) Dissolve 0.2 g of 2-methylimidazole and 0.8 g of polyacrylonitrile in 4 g of dimethylformamide, and continuously stir at room temperature for 12 h to obtain a uniform and stable polymer solution. Form a polymer nanofiber membrane by electrospinning, and the feeding speed of the spinning machine is 1.1 mL h -1, the positive voltage is set to 15 kV, the negative voltage is 2 kV, and the spinning distance is 19 cm.

[0039] (2) Dissolve 1.97 g of 2-methylimidazole in 60 mL of deionized water to form solution A. Dissolve 0.84 g of zinc nitrate hexahydrate and 0.07 g of iron nitrate nonahydrate in 60 mL of deionized water to form solution B, and add it to solution A under stirring. Immerse 100 mg of the polymer nanofiber membrane in the mixed solution, let it stand at room temperature for 1 h, and after the reaction, rinse it with deionized water and absolute ethanol, and dry it in an oven at 60 °C to obtain a fiber membrane with MOFs grown on the surface.

[0040] (3) Weigh 20 mg of 5,5'-diamino-2,2'-bipyridine into a beaker, and weigh 15 mg of triformylphloroglucinol into another beaker. Add 10 mL of tetrahydrofuran liquid to each of the two beakers, then seal the beakers with plastic wrap and ultrasonicate for 20 min. Keep 5,5'-diamino-2,2'-bipyridine under stirring, add the triformylphloroglucinol solution to it, and continue stirring for 30 s. Immerse two samples obtained in step (2) in the solution and let it stand for 24 h. After the reaction, rinse it with tetrahydrofuran and dry it in an oven at 60 °C to obtain a fiber membrane with MOFs and COFs grown successively on the surface.

[0041] (4) Transfer the fiber membrane with MOFs and COFs grown successively on the surface to a tubular furnace, and perform high-temperature treatment (900 °C, 2 h) in an argon atmosphere, with a heating rate of 5 °C min -1 , and after carbonization is completed, naturally cool it to room temperature to synthesize hierarchically porous carbon nanosheets supported by carbon nanofibers;

[0042] (5) Weigh 0.5 mM of iron nitrate nonahydrate, nickel nitrate nonahydrate, and copper nitrate hexahydrate, 0.2 mM of chloroplatinic acid hexahydrate, and 0.1 mM of iridium chloride trihydrate, and dissolve them in 20 mL of deionized water to form a mixed solution. Weigh 20 mg of the hierarchically porous carbon nanosheets supported by carbon nanofibers obtained in step (4) and immerse them in the mixed solution. Keep it in an oil bath at 80 °C for 24 h with insulation and standing reaction. After the reaction, rinse it with deionized water and absolute ethanol, and dry it in an oven at 60 °C to synthesize the precursor;

[0043] (6) Transfer the precursor to a rapid Joule heating device, set the DC voltage to 16 V and the current to 20 A, and perform Joule heating in a vacuum for 10 s. After completion, naturally cool it to room temperature and collect it to obtain the product.

[0044] Example 2

[0045] This example is the same as Example 1 except for steps (2) and (5). Steps (2) and (5) are specifically as follows:

[0046] (2) Dissolve 1.97 g of 2-methylimidazole in 60 mL of deionized water to form solution A. Dissolve 0.84 g of zinc nitrate hexahydrate and 0.04 g of cobalt nitrate hexahydrate in 60 mL of deionized water to form solution B, and add it to the stirring solution A. Immerse 100 mg of the polymer nanofiber membrane in the mixed solution, let it stand at room temperature for reaction for 1 h. After the reaction, rinse it with deionized water and absolute ethanol, and dry it in an oven at 60 °C.

[0047] (5) Weigh 0.5 mM of iron(III) nitrate nonahydrate, nickel(II) nitrate nonahydrate, cobalt(II) nitrate hexahydrate, 0.2 mM of chloroplatinic acid hexahydrate, and 0.1 mM of ruthenium(III) chloride trihydrate, and dissolve them in 20 mL of deionized water to form a mixed solution. Weigh 20 mg of the hierarchically porous carbon nanosheets supported on carbon nanofibers obtained in step (4) and immerse them in the mixed solution. Keep it in an oil bath at 80 °C for static reaction for 24 h. After the reaction, rinse it with deionized water and absolute ethanol, and dry it in an oven at 60 °C.

[0048] Example 3

[0049] This example is the same as Example 1 except for step (5). Step (5) is specifically as follows:

[0050] (5) Weigh 1.0 mM of iron(III) nitrate nonahydrate, nickel(II) nitrate nonahydrate, copper(II) nitrate hexahydrate, 0.2 mM of chloroplatinic acid hexahydrate, and 0.1 mM of iridium(III) chloride trihydrate, and dissolve them in 20 mL of deionized water to form a mixed solution. Weigh 20 mg of the hierarchically porous carbon nanosheets supported on carbon nanofibers obtained in step (4) and immerse them in the mixed solution. Keep it in an oil bath at 80 °C for static reaction for 24 h. After the reaction, rinse it with deionized water and absolute ethanol, and dry it in an oven at 60 °C to synthesize the precursor.

[0051] Comparative Example 1

[0052] Take commercial ruthenium dioxide (RuO2) and platinum-carbon (Pt / C) directly as the oxygen evolution and oxygen reduction catalysts respectively. The preparation method of the catalyst solution is the same as that in the example.

[0053] Comparative Example 2

[0054] The difference between this comparative example and Example 1 lies in that steps (4) and (5) are different, and the others are the same.

[0055] (4) Weigh 0.5 mM of iron(III) nitrate nonahydrate, nickel(II) nitrate nonahydrate, copper(II) nitrate hexahydrate, 0.2 mM of chloroplatinic acid hexahydrate, and 0.1 mM of iridium(III) chloride trihydrate, and dissolve them in 20 mL of deionized water to form a mixed solution. Weigh 20 mg of the fiber membrane with MOFs and COFs grown on the surface in step (3) and immerse it in the mixed solution for adsorption. Keep it in an oil bath at 80 °C for static reaction for 24 h. After the reaction, rinse it with deionized water and absolute ethanol, and dry it in an oven at 60 °C;

[0056] (5) Transfer the fiber membrane adsorbed with mixed ions in step (4) to a tubular furnace, and perform high-temperature treatment (900 °C, 2 h) in an argon atmosphere, with a heating rate of 5 °C min -1 , and naturally cool to room temperature after carbonization is completed.

[0057] Figure 1 This is the XRD pattern of the catalyst prepared in this example, showing an fcc crystal structure conforming to the alloy characteristics.

[0058] Figure 2 This is the TEM image of the catalyst prepared in Example 2, showing uniform distribution of each element in the alloy nanoparticles.

[0059] Electrochemical testing method: Measured using a CHI 760E electrochemical workstation and a rotating disk electrode device at room temperature. A three-electrode system was adopted, with Hg / HgO as the reference electrode, a graphite electrode as the counter electrode, and a glassy carbon electrode (GC) as the working electrode. Grind the catalysts prepared in Examples 1-2 and Comparative Example 1 into powders. Take 5 mg of catalyst powder, 20 μL of Nafion solution, 780 μL of absolute ethanol, and 200 μL of deionized water, and ultrasonically form a catalyst solution. Pipette 10 μL of the catalyst solution and evenly drop it on the surface of the glassy carbon electrode and air-dry it naturally to form a film. Perform oxygen reduction testing in an O2-saturated 0.1 M potassium hydroxide seawater solution and oxygen evolution testing in a 1 M potassium hydroxide seawater solution. As Figure 3 shown, the catalyst prepared in Example 1 exhibits excellent oxygen reduction catalytic performance in an O2-saturated 0.1 M KOH seawater solution, and its half-wave potential and limiting current density are 0.882 V and 4.91 mA / cm 2 respectively. After the accelerated durability test, its half-wave potential only decays by 5 mV, demonstrating the good stability of the catalyst. In addition to having outstanding oxygen reduction catalytic activity, the prepared catalyst reaches an overpotential of 345 mV in a 1 M KOH seawater solution (@10 mA cm -2 ), showing bifunctional catalytic activity for oxygen reduction and oxygen evolution. The half-wave potential and limiting current density of the catalyst prepared in Example 2 are 0.871 V and 5.13 mA / cm 2 respectively; at the same time, it shows good stability, and the half-wave potential hardly decays after the accelerated durability test. The oxygen evolution overpotential corresponding to a current density of 10 mAcm -2 is 334 mV, showing bifunctional catalytic activity. The oxygen reduction catalytic performance of Comparative Examples 1 and 2 is lower than that of Examples 1 and 2; similarly, the overpotential (@10 mA cm -2 ) in the oxygen evolution reaction is higher than that of Examples 1 and 2, and their catalytic performance is far inferior to that of Examples 1 and 2.

[0060] Assembly method of liquid seawater electrolyte zinc-air battery: 200 μL of catalyst solution was dropped onto a hydrophobic carbon paper (catalyst loading was 1 mg / cm 2 ), which served as the cathode of the battery, a polished zinc plate served as the anode of the battery, and a seawater solution containing 6 M KOH served as the electrolyte. At room temperature, the performance of the seawater electrolyte zinc-air battery was tested using a BlueTEC test system. As Figure 5 shown, the power density of the seawater electrolyte zinc-air battery assembled with the cathode of Example 1 reached 160 mW / cm 2 , and it could stably cycle for more than 330 hours, while the power density of the liquid seawater electrolyte zinc-air battery assembled with Comparative Example 1 was relatively low.

[0061] Assembly method of flexible (solid-state) seawater electrolyte zinc-air battery: 200 μL of the catalyst solution prepared in Example 2 was dropped onto a 1×1 cm 2 carbon cloth, a polished zinc foil served as the anode of the battery, and a polyvinyl alcohol gel containing a seawater solution of 6 M KOH served as the electrolyte. They were assembled in the order of zinc foil, electrolyte, carbon cloth, and nickel foam, and encapsulated into a flexible zinc-air battery using an aluminum-plastic film. At room temperature, the charge-discharge cycle of the flexible zinc-air battery was tested using a BlueTEC test system. As Figure 6 shown, the flexible (solid-state) seawater electrolyte zinc-air battery assembled with this catalyst remained stable in cycling under bending, had the characteristics of being flexible and wearable, and was able to light up the LEDs of life jackets, lifebuoys, and buoy lights.

[0062] The alloy electrocatalysts prepared in Example 1 and Example 2 of the present invention both exhibited excellent bifunctional catalytic activities, and the assembled liquid (and solid-state) seawater electrolyte zinc-air batteries had considerable performances. The application of seawater electrolyte reduced the use of high-purity fresh water, lowered the cost of the battery, and had great application potential in energy storage and conversion devices as well as in alleviating the current fresh water shortage crisis; at the same time, the porous carbon-supported alloy catalyst prepared by the present invention also had broad development prospects in the field of flexible electronics.

[0063] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that they can still modify the technical solutions recorded in the foregoing embodiments, or perform equivalent replacements for some of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the spirit and scope of the technical solutions of the various embodiments of the present invention.

Claims

1. A preparation method of a bifunctional alloy electrocatalyst, characterized in that, It includes the following steps: (1) Prepare an electrospun membrane: Dissolve 2-methylimidazole and a nitrogen-containing polymer hydrophilic polymer in an organic solvent to form a polymer solution, and form a polymer nanofiber membrane by electrospinning; (2) Synthesize a carbon nanosheet precursor: Dissolve 2-methylimidazole in water to form solution A; dissolve a zinc salt and at least one of a cobalt salt and an iron salt in water to form solution B, and add it to solution A under stirring to form a mixed solution; Immerse the fiber membrane obtained in step (1) in the above mixed solution and let it stand to obtain a fiber membrane with MOFs grown on the surface; Ultrasonically dissolve triformylphloroglucinol and 5,5'-diamino-2,2'-bipyridine in a solvent to form solution C, and immerse the fiber membrane with MOFs grown on the surface in solution C again and let it stand. COFs continue to grow on the surface of the fiber membrane. After the reaction is completed, dry it to synthesize a carbon nanosheet precursor; (3) Synthesize hierarchically porous carbon nanosheets supported on carbon nanofibers: Perform high-temperature carbonization treatment on the carbon nanosheet precursor obtained in step (2) in an inert gas to synthesize hierarchically porous carbon nanosheets supported on carbon nanofibers; (4) Synthesize a precursor: Immerse the hierarchically porous carbon nanosheets supported on carbon nanofibers obtained in step (3) in a mixed aqueous solution containing various metal ions, let it stand under heating conditions, and the porous carbon nanosheets adsorb metal ions, and dry it. The metal ions include any one of three transition metal ions, ruthenium ions and iridium ions, and platinum ions. The transition metal ions are preferably iron ions, cobalt ions, nickel ions and copper ions. The molar ratio of the three transition metal ions, ruthenium ions or iridium ions and platinum ions is 15:1:2; (5) Synthesize an alloy: Perform rapid Joule heat treatment in an inert gas or vacuum to obtain the alloy electrocatalyst supported on the porous carbon nanosheets.

2. The preparation method of the bifunctional alloy electrocatalyst according to claim 1, characterized in that, The nitrogen-containing polymer hydrophilic polymer in step (1) is any one of polyacrylonitrile, polyimide, and polyethyleneimine; the organic solvent is any one or a mixture of dimethylformamide, dimethylacetamide, dimethyl sulfoxide, and acetone.

3. The preparation method of the bifunctional alloy electrocatalyst according to claim 1, characterized in that, In step (1), the feeding speed of the spinning machine is 0.5 - 1.5 mL / h -1 , the positive voltage is set to 12 - 19 kV, the negative voltage is 0 - 2 kV, and the spinning distance is 15 - 20 cm.

4. The preparation method of the bifunctional alloy electrocatalyst according to claim 1, characterized in that, The zinc salt, cobalt salt and iron salt in step (2) are water-soluble salts.

5. The preparation method of the bifunctional alloy electrocatalyst according to claim 1, wherein The high-temperature carbonization temperature described in step (3) is 900 - 1000 °C, the carbonization time is 2 - 3 h, and the heating rate is 5 - 10 °C / min -1 .

6. The preparation method of the bifunctional alloy electrocatalyst according to claim 1, wherein, The metal ions include iron ions, nickel ions, copper ions, iridium ions and platinum ions, and the molar ratio of iron ions, nickel ions, copper ions, iridium ions and platinum ions is 5:5:5:1:2, or the metal ions include iron ions, nickel ions, cobalt ions, ruthenium ions and platinum ions, and the molar ratio of iron ions, nickel ions, cobalt ions, ruthenium ions and platinum ions is 5:5:5:1:

2.

7. The preparation method of the bifunctional alloy electrocatalyst according to claim 1, wherein, In step (4), the heating time is 20-30 h, preferably 24 h, and the heating temperature is 70-90 °C in a water bath or oil bath.

8. The preparation method of the bifunctional alloy electrocatalyst according to claim 1, wherein In step (5), the set voltage of the rapid Joule heat treatment is about 15-20 V, the current is about 15-25 A, the temperature is about 900-1100 °C, and the heat treatment time is 8-15 s.

9. A bifunctional alloy electrocatalyst prepared by the method according to any one of claims 1-8.

10. Application of the bifunctional alloy electrocatalyst according to claim 9 as a cathode catalytic material, especially as a cathode catalyst for a seawater electrolyte zinc-air battery.

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