Iron and nitrogen co-doped carbon oxygen reduction catalyst with hierarchical porous structure as well as preparation method and application of iron and nitrogen co-doped carbon oxygen reduction catalyst

Abstract

The invention belongs to the technical field of fuel cell catalysts, and particularly discloses an iron and nitrogen co-doped carbon oxygen reduction catalyst with a hierarchical porous structure as well as a preparation method and application of the iron and nitrogen co-doped carbon oxygen reduction catalyst. The iron and nitrogen co-doped carbon oxygen reduction catalyst is prepared by taking ZnO / N / C-coated Fe-ZIF with a core-shell structure as a precursor, is of a rhombic dodecahedron structure and has a micropore-mesopore coexisting hierarchical porous structure, and the particle size of metal iron particles is 5-10 nm. The invention also discloses an application of the catalyst in a cathode oxygen reduction reaction of an alkaline fuel cell. The iron and nitrogen co-doped carbon oxygen reduction catalyst provided by the invention can avoid metal particle agglomeration, forms highly exposed and easily accessible active sites, and significantly improves the oxygen reduction catalytic activity.

Description

Technical Field

[0001] The present invention belongs to the technical field of fuel cell catalysts, and particularly relates to an iron and nitrogen co-doped carbon oxygen reduction catalyst with a hierarchical porous structure, a preparation method thereof, and an application thereof. Background Art

[0002] Hydrogen energy is an important guarantee for promoting energy interconnection and complementarity and accelerating the process of a low-carbon society. Fuel cells can efficiently convert hydrogen energy into electrical energy and have broad application prospects in fields such as electric vehicles, portable power sources, and stationary power plants. The cathode reaction of fuel cells has a high energy barrier, and noble metals are usually used to catalyze the oxygen reduction reaction. Noble metal-based catalysts such as composites of Pt, Ru, and Ir are widely used in the field of electrocatalysis due to their excellent catalytic performance and stability. Especially noble metal Pt exhibits extremely low onset potentials in both acidic and alkaline electrolytes, but its large-scale application is limited due to its scarcity, high cost, and poor durability.

[0003] Among the mainstream research directions of non-noble metal catalysts, compared with transition metal compounds, heteroatom-doped carbon catalysts, etc., transition metal and nitrogen co-doped carbon catalysts (TM-N-C, TM is Fe, Co, Ni, etc.) are considered to be the most promising and valuable catalysts due to their high catalytic activity and cyclic stability. ZIFs are a kind of zeolitic imidazolate metal framework materials with a periodic arrangement of metal-nitrogen, having advantages such as a large specific surface area, high porosity, adjustable components, and stable structure, and are usually used as precursors for the preparation of TM-N-C oxygen reduction catalysts by high-temperature pyrolysis. However, during the high-temperature pyrolysis process, the local structure of ZIFs is prone to shrinkage and collapse, resulting in active sites being buried in the carbon material and unable to contact the reactants, leading to insufficient utilization. In addition, the pyrolysis process will cause the precipitation and agglomeration of metal particles, making the catalyst face problems such as insufficient efficiency and decreased cyclic stability. Summary of the Invention

[0004] The purpose of the present invention is to provide an iron and nitrogen co-doped carbon oxygen reduction catalyst with a hierarchical porous structure, a preparation method thereof, and an application thereof, which can avoid the agglomeration of metal particles, finally form highly exposed and easily accessible active sites, and significantly improve the oxygen reduction catalytic activity.

[0005] To achieve the above purpose, the present invention is implemented by adopting the following technical solutions: According to one aspect of the present invention, there is provided an iron and nitrogen co-doped carbon oxygen reduction catalyst with a hierarchical porous structure, which is prepared by using a core-shell structured ZnO / N / C@Fe-ZIF as a precursor. The iron and nitrogen co-doped carbon oxygen reduction catalyst has a rhombic dodecahedron structure and has a hierarchical porous structure with coexisting micropores and mesopores, and the particle size of the metal iron particles is 5-10 nm.

[0006] Further, the iron and nitrogen co-doped carbon oxygen reduction catalyst with a hierarchical porous structure has a ZnO / N / C as the precursor core and a Fe-ZIF as the precursor shell.

[0007] Further, the precursor is in the shape of a rhombic dodecahedron with an average diameter of 450 - 550 nm.

[0008] With the above technical solution, the provided iron and nitrogen co-doped carbon oxygen reduction catalyst only contains four elements: Fe, C, N, and O. Its precursor ZnO / N / C@Fe-ZIF is a core-shell structure. Among them, ZnO / N / C is used as a seed to epitaxially grow Fe-ZIF to prepare the core-shell structure precursor. The catalyst prepared by high-temperature pyrolysis exhibits a hierarchical porous (0 - 20 nm) structure feature that is beneficial for mass transfer. The precursor core can play a role in pore formation, generating a hierarchical porous structure with coexisting micropores and mesopores. The active sites are highly exposed and have good dispersibility, accelerating the transport of reactants and products and improving the utilization rate of active sites. In addition, the precursor core, as a support, can promote the uniform pyrolysis of the precursor shell, avoiding the agglomeration of metal particles, that is, contributing to the dispersed distribution of metallic iron particles, and finally forming highly exposed and easily accessible active sites, significantly enhancing the oxygen reduction catalytic activity.

[0009] Compared with using Fe-ZIF or ZnO / N / C as the precursor, using the core-shell structure ZnO / N / C@Fe-ZIF as the precursor for preparing the iron and nitrogen co-doped carbon oxygen reduction catalyst can ensure that the obtained catalyst has a regular and complete morphology, improve the uniformity of the distribution of metallic iron particles, and contribute to enhancing the catalytic activity and stability of the obtained catalyst.

[0010] The iron and nitrogen co-doped carbon oxygen reduction catalyst obtained by the above technical solution retains the morphology of the ZnO / N / C@Fe-ZIF precursor, but the morphology is slightly shrunk, which may be due to the thermal decomposition of the precursor resulting in shrinkage. The uniform distribution of metallic iron particles and the formation of carbon nanotubes during the high-temperature pyrolysis process contribute to enhancing the electrical conductivity, accelerating the electron gain and loss process of the oxygen reduction reaction, and thus improving the catalytic performance.

[0011] According to one aspect of the present invention, a preparation method of an iron and nitrogen co-doped carbon oxygen reduction catalyst is provided, including the following steps: S1. Prepare the precursor core ZnO / N / C; S2. Epitaxially grow Fe-ZIF on the outside of the precursor core to obtain the precursor; S3. Calcinate the precursor in an inert atmosphere to obtain the iron and nitrogen co-doped carbon oxygen reduction catalyst.

[0012] Further, in step S1, the precursor core ZnO / N / C is obtained by calcining ZIF-8; The calcination temperature is 350 - 550 °C, and the heating rate during the calcination process is 1 - 5 °C / min -1 , and the heat preservation time is 0.5 - 2 h.

[0013] The precursor core of the catalyst is prepared by oxidizing ZIF-8 in air. This method can endow the catalyst with a hierarchical pore structure that is beneficial for mass transfer inside. Compared with the precursors and catalysts prepared with ZIF-8 as the core in the traditional way, the interior is often mainly composed of micropores.

[0014] Furthermore, the preparation method of ZIF-8 includes: dissolving zinc nitrate and 2-methylimidazole in organic solvents respectively, dispersing them evenly, fully mixing the two solutions under stirring conditions, then keeping them at 20 - 50 °C for 12 - 24 h, followed by suction filtration, washing and drying to obtain white powder ZIF-8; among them, the molar ratio of zinc nitrate to 2-methylimidazole is 1:2 - 1:16.

[0015] Preferably, the molar ratio of zinc nitrate to 2-methylimidazole is 1:2.35, the drying temperature is 60 - 100 °C, and the drying time is 8 - 12 h.

[0016] More preferably, the drying temperature is 60 - 100 °C, and the drying time is 6 - 10 h.

[0017] Furthermore, the step of epitaxially growing Fe-ZIF on the outside of the precursor core to obtain the precursor includes: Dissolve the precursor core, metal salt, and 2-methylimidazole in organic solvents respectively to obtain a precursor core dispersion, a metal salt solution, and a 2-methylimidazole solution. Then, pour the 2-methylimidazole solution and the metal salt solution into the precursor core dispersion in sequence, and react to obtain the precursor.

[0018] Furthermore, the reaction conditions for the 2-methylimidazole solution, the metal salt solution and the precursor core dispersion are to keep them at 100 - 150 °C for 5 - 10 h; after the reaction, perform suction filtration, washing and drying to obtain the precursor.

[0019] Preferably, the molar ratio of the metal salt to 2-methylimidazole is 1:10 - 1:70, the concentration of the metal salt in the metal salt solution is 1.2 - 3.6 mmol / L -1 , and the concentration of 2-methylimidazole in the 2-methylimidazole solution is 48 - 96 mmol / L -1 .

[0020] Preferably, the molar ratio of the metal salt to 2-methylimidazole is 1:20 - 1:50.

[0021] Preferably, the molar ratio of the metal salt to 2-methylimidazole is 1:30.

[0022] The ratio of metal salt to 2-methylimidazole is different, which will affect the morphology and structure of the obtained iron and nitrogen co-doped carbon oxygen reduction catalyst. When the amount of 2-methylimidazole used is relatively reduced, due to the small amount of 2-methylimidazole during the synthesis of the precursor, the coordinated metal salt is reduced, and the catalytic graphitization effect of the metal during the formation of the catalyst is weakened, resulting in a reduction in the carbon nanotubes on the surface of the iron and nitrogen co-doped carbon catalyst prepared by high-temperature pyrolysis. On the contrary, when the amount of 2-methylimidazole used is relatively increased, the number of metal particles and carbon tubes in the iron and nitrogen co-doped carbon catalyst prepared by high-temperature pyrolysis increases.

[0023] Preferably, the organic solvent is one or more of methanol, ethanol, propylene glycol, and isopropanol. More preferably, the organic solvent is methanol.

[0024] Preferably, the metal salt is a soluble salt of iron. More preferably, the metal salt is one or more of iron sulfate, iron nitrate, and iron chloride.

[0025] Further, in step S3, the calcination temperature is 900-1100 °C, and the heating rate during the calcination process is 1-10 °C min -1 , and the heat preservation time is 1-3 h.

[0026] Even further, in step S3, the temperature is raised to 950 °C at a heating rate of 5 °C min -1 , and heat preservation is carried out for 2 h.

[0027] According to one aspect of the present invention, there is provided the above-mentioned iron and nitrogen co-doped carbon oxygen reduction catalyst or the catalyst obtained by using any one of the above preparation methods in the cathodic oxygen reduction reaction of an alkaline fuel cell.

[0028] Compared with the prior art, the present invention has at least the following beneficial effects: 1. The iron and nitrogen co-doped carbon oxygen reduction catalyst provided by the present invention has a core-shell structure as the precursor of the catalyst. The core of the precursor can play a pore-forming role, generating a hierarchical porous structure with both micropores and mesopores, accelerating the transport of reactants and products, and improving the utilization rate of active sites. In addition, as a support, the core can promote the uniform pyrolysis of the shell layer, avoid the agglomeration of metal particles, improve the distribution uniformity of metal iron particles, and finally form highly exposed and easily accessible active sites, significantly enhancing the oxygen reduction catalytic activity.

[0029] 2. Different from adding iron salts into the mixed solution of zinc nitrate and 2-methylimidazole to prepare Fe-ZIF-8 and then calcining it to obtain the catalyst, in the present invention, ZIF-8 is first oxidized in air to obtain the precursor core, then Fe-ZIF is epitaxially grown to obtain the precursor, and finally the precursor is pyrolyzed in an inert atmosphere to prepare the catalyst. The oxidation process promotes the construction of the ZnO / N / C hierarchical porous structure. By this method, the obtained iron and nitrogen co-doped carbon oxygen reduction catalyst presents a hierarchical pore structure beneficial to mass transfer inside, while the traditional precursor and catalyst prepared with ZIF-8 as the core often mainly have micropores inside.

[0030] 3. The iron and nitrogen co-doped carbon oxygen reduction catalyst prepared by the present invention has highly exposed and uniformly dispersed active sites and a hierarchical porous structure beneficial to mass transfer, and its oxygen reduction catalytic activity and electrochemical stability are superior to those of commercial 20% Pt / C catalyst. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] The attached drawings forming a part of the present invention are used to provide a further understanding of the present invention. The schematic embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute an improper limitation to the present invention. In the drawings: Figure 1 It is the scanning electron microscope image of ZIF-8 in Example 1 of the present invention.

[0032] Figure 2 It is the scanning electron microscope image of ZnO / N / C in Example 1 of the present invention.

[0033] Figure 3 It is the scanning electron microscope image of ZnO / N / C@Fe-ZIF precursor in Example 1 of the present invention.

[0034] Figure 4 It is the transmission electron microscope image of the iron and nitrogen co-doped carbon oxygen reduction catalyst in Example 1 of the present invention.

[0035] Figure 5 It is the XPS spectrum of the iron and nitrogen co-doped carbon oxygen reduction catalyst in Example 1 of the present invention.

[0036] Figure 6 It is the transmission electron microscope image of the iron and nitrogen co-doped carbon oxygen reduction catalyst in Example 2 of the present invention.

[0037] Figure 7 It is the transmission electron microscope image of the iron and nitrogen co-doped carbon oxygen reduction catalyst in Example 3 of the present invention.

[0038] Figure 8 It is the transmission electron microscope image of the iron and nitrogen co-doped carbon oxygen reduction catalyst in Comparative Example 1 of the present invention.

[0039] Figure 9TEM image of the iron and nitrogen co-doped carbon oxygen reduction catalyst in Comparative Example 2 of the present invention.

[0040] Figure 10 Nitrogen adsorption - desorption curve and pore size distribution diagram of the iron and nitrogen co-doped carbon oxygen reduction catalyst in Example 1 of the present invention.

[0041] Figure 11 Polarization curve of the iron and nitrogen co-doped carbon oxygen reduction catalyst and commercial 20% Pt / C catalyst in Example 1 of the present invention.

[0042] Figure 12 Polarization curve of the iron and nitrogen co-doped carbon oxygen reduction catalyst and commercial 20% Pt / C catalyst in Example 2 of the present invention.

[0043] Figure 13 Polarization curve of the iron and nitrogen co-doped carbon oxygen reduction catalyst and commercial 20% Pt / C catalyst in Example 3 of the present invention.

[0044] Figure 14 Polarization curve of the iron and nitrogen co-doped carbon oxygen reduction catalyst and commercial 20% Pt / C catalyst in Comparative Example 1 of the present invention.

[0045] Figure 15 Polarization curve of the iron and nitrogen co-doped carbon oxygen reduction catalyst and commercial 20% Pt / C catalyst in Comparative Example 2 of the present invention.

[0046] Figure 16 Stability test curve of the nitrogen-doped carbon oxygen reduction catalyst and commercial 20% Pt / C catalyst in Example 1 of the present invention. Detailed Description of the Invention

[0047] The present invention will be described in detail below with reference to the accompanying drawings and in conjunction with embodiments. It should be noted that, without conflict, the embodiments of the present invention and the features in the embodiments can be combined with each other.

[0048] The following detailed descriptions are all exemplary descriptions, aiming to provide further detailed descriptions of the present invention. Unless otherwise specified, all technical terms used in the present invention have the same meaning as commonly understood by those of ordinary skill in the technical field to which the present invention belongs. The terms used in the present invention are only for describing specific embodiments and are not intended to limit the exemplary embodiments according to the present invention.

[0049] Example 1 Preparation of Carbon Oxygen Reduction Catalyst (1) Preparation of ZIF-8 Zn(NO3)2·6H2O and 2-methylimidazole were separately dissolved in methanol and ultrasonicated until evenly dispersed. The mass ratio of Zn(NO3)2·6H2O to methanol in the resulting methanol solution of zinc nitrate was 1:39, and the mass ratio of 2-methylimidazole to methanol in the resulting methanol solution of 2-methylimidazole was 1:60. Under stirring conditions, the above two solutions were fully mixed, and then kept at 30 °C for 12 h. After suction filtration, washing and drying, white powder ZIF-8 was obtained. The drying temperature was 80 °C and the drying time was 10 h. Among them, the molar ratio of Zn(NO3)2·6H2O to 2-methylimidazole was 1:2.35.

[0050] (2)Preparation of precursor core ZIF-8 was calcined in a muffle furnace at a calcination temperature of 450 °C, and the heating rate during the calcination process was 3 °C min -1 , and kept at 450 °C for 1 h. After grinding, the precursor core ZnO / N / C was obtained.

[0051] (3)Preparation of precursor ZnO / N / C@Fe-ZIF ZnO / N / C (40 mg) was dissolved in methanol (60 mL), ferric sulfate (28.7 mg, 0.072 mmol) and 2-methylimidazole (177.3 mg, 2.16 mmol) were separately dissolved in methanol (30 mL), and ultrasonicated for 30 min until evenly dispersed to obtain a precursor core dispersion, a metal salt solution and a 2-methylimidazole solution. Then, under the conditions of heating and stirring at 80 °C, the methanol solution of 2-methylimidazole was slowly added dropwise to the methanol solution of ZnO / N / C at a dropping rate of 0.5 mL s -1 . After reacting for 10 min, the methanol solution of ferric sulfate was added dropwise, and kept at 130 °C for 5 h; after suction filtration, washing and drying, yellow powder ZnO / N / C@Fe-ZIF was obtained; the drying temperature was 80 °C and the drying time was 10 h.

[0052] Among them, in the prepared precursor core dispersion, the mass-volume ratio of ZnO / N / C to methanol was 2 mg:3 mL; in the prepared metal salt solution, the concentration of ferric sulfate was 2.4 mmol L -1 ; in the prepared 2-methylimidazole solution, the concentration of 2-methylimidazole was 72 mmol L -1 ; the mass ratio of ZnO / N / C to ferric sulfate was 40:28.7; the molar ratio of ferric sulfate to 2-methylimidazole was 1:30.

[0053] (4)Preparation of iron and nitrogen co-doped carbon oxygen reduction catalyst Fe,N-HPC The as-prepared ZnO / N / C@Fe-ZIF was heated to 950 °C at a heating rate of 5 °C min -1 in an argon atmosphere, held for 2 h, and after cooling to room temperature, it was ground to obtain the iron and nitrogen co-doped carbon oxygen reduction catalyst Fe,N-HPC.

[0054] Preparation of the electrode (1)Preparation of the catalyst slurry Weigh the catalyst prepared above, and successively add absolute ethanol and 5 wt.% Nafion membrane solution to it with a pipette, and ultrasonicate until evenly dispersed to obtain the catalyst slurry.

[0055] Among them, the mass-volume ratio of the catalyst to absolute ethanol is 5 mg:1 mL; the mass-volume ratio of the catalyst to 5 wt.% Nafion membrane solution is 1 mg:2 μL. During the ultrasonication process, the water temperature is 25 °C.

[0056] (2)Preparation of the thin film electrode The working electrode of the electrochemical workstation (ALS-DY2323 Bi-Potentiostant, ALD Company) was polished successively with Al2O3 powder suspensions with particle sizes of 1.5 μm, 0.5 μm, and 50 nm, and the treatment time for each powder was 5 min, and then wiped clean with lens paper to ensure that the surface of the working electrode was smooth and bright.

[0057] Drop the prepared catalyst slurry onto the center of the working electrode disk, then adjust the rotation speed to 300 rpm, dry the catalyst slurry, and the amount of the catalyst slurry can cover the glassy carbon disk of the working electrode evenly.

[0058] Example 2 The difference between this example and Example 1 is that in the preparation of the iron and nitrogen co-doped carbon oxygen reduction catalyst, during the preparation of the precursor ZnO / N / C@Fe-ZIF, the amount of 2-methylimidazole used was reduced, and the molar ratio of iron sulfate to 2-methylimidazole was 1:10.

[0059] All other steps and conditions are the same.

[0060] Example 3 The difference between this example and Example 1 is that in the preparation of the iron and nitrogen co-doped carbon oxygen reduction catalyst, during the preparation of the precursor ZnO / N / C@Fe-ZIF, the amount of 2-methylimidazole used was increased, and the molar ratio of iron sulfate to 2-methylimidazole was 1:50.

[0061] All other steps and conditions are the same.

[0062] Comparative Example 1 The difference between this comparative example and Example 1 is as follows: In the preparation of the iron and nitrogen co-doped carbon oxygen reduction catalyst, Fe-ZIF was used as a precursor for high-temperature pyrolysis to prepare the iron and nitrogen co-doped carbon oxygen reduction catalyst Fe,N-HPC, which specifically included the following steps: (1) Preparation of the precursor Fe-ZIF Ferric sulfate (287 mg, 0.72 mmol) and 2-methylimidazole (1.78 g, 21.6 mmol) were respectively dissolved in 160 mL of methanol and sonicated until evenly dispersed. Then, under the conditions of heating and stirring at 80 °C, the methanol solution of ferric sulfate was added dropwise to the methanol solution of 2-methylimidazole. After stirring evenly, it was transferred to an oven and reacted at 130 °C for 5 h. After filtration and washing, it was dried to obtain a yellow powder Fe-ZIF. Among them, the molar ratio of ferric sulfate to 2-methylimidazole was 1:30, the concentration of ferric sulfate was 4.5 mmol / L -1 , and the concentration of 2-methylimidazole was 135 mmol / L -1 , the drying temperature was 80 °C, and the drying time was 10 h; (2) Preparation of the iron and nitrogen co-doped carbon oxygen reduction catalyst Fe,N-HPC Fe-ZIF was heated in an argon atmosphere at a heating rate of 5 °C / min -1 to 950 °C and held for 2 h. After cooling to room temperature, it was ground to obtain the Fe,N-HPC catalyst.

[0063] Comparative Example 2 The difference between this comparative example and Example 1 is as follows: In the preparation of the nitrogen-doped carbon oxygen reduction catalyst, ZnO / N / C was used as a precursor for high-temperature pyrolysis to prepare the nitrogen-doped carbon oxygen reduction catalyst N-HPC, including the following steps: (1) Preparation of ZIF-8 Zn(NO3)2·6H2O (5.45 mmol, 1.62 g) and 2-methylimidazole (12.8 mmol, 1.052 g) were respectively dissolved in a methanol solvent, sonicated and dissolved and dispersed evenly. Under stirring conditions, the above two solutions were fully mixed, and then held at 30 °C for 12 h. After filtration and washing, it was dried to obtain a white powder ZIF-8; among them, the molar ratio of Zn(NO3)2·6H2O to 2-methylimidazole was 1:2.35, the drying temperature was 80 °C, and the drying time was 10 h; (2) Preparation of the precursor ZnO / N / C ZIF-8 was calcined in a muffle furnace at a calcination temperature of 450 °C, and the heating rate during the calcination process was 3 °C / min -1, the heat preservation time was 1 h, and the precursor ZnO / N / C was obtained after grinding; (3)Preparation of nitrogen-doped carbon oxygen reduction catalyst N-HPC ZnO / N / C was heated to 950 °C at a heating rate of 5 °C min -1 in an argon atmosphere, held for 2 h, and after cooling to room temperature, it was ground to obtain the nitrogen-doped carbon oxygen reduction catalyst N-HPC.

[0064] Comparative Example 3 The difference between this comparative example and Example 1 is that: A commercial 20% Pt / C catalyst (Johnson Matthey, UK, batch number / specification: Hispec 3000) was used to prepare the electrode. The preparation of the electrode included the following steps: (1)Preparation of catalyst slurry Anhydrous ethanol and 5 wt.% Nafion membrane solution were successively added to the commercial 20% Pt / C catalyst and ultrasonicated until uniformly dispersed to obtain the Pt / C catalyst slurry.

[0065] Among them, the mass-volume ratio of the commercial 20% Pt / C catalyst to anhydrous ethanol was 5 mg: 3.91 mL; the mass-volume ratio of the commercial 20% Pt / C catalyst to 5 wt.% Nafion membrane solution was 1 mg: 10 μL. During the ultrasonic process, the water temperature was 25 °C.

[0066] (2)Preparation of thin film electrode The working electrode of the electrochemical workstation (ALS-DY2323 Bi-Potentiostant, ALD company) was polished successively with Al2O3 powder suspensions with particle sizes of 1.5 μm, 0.5 μm, and 50 nm, and the treatment time for each powder was 5 min, and then wiped clean with lens paper to ensure that the surface of the working electrode was smooth and bright.

[0067] The prepared catalyst slurry was dropped onto the center of the glassy carbon disk of the working electrode, and then the rotation speed was adjusted to 300 rpm, and the catalyst slurry was dried, and the amount of the catalyst slurry was sufficient to cover the glassy carbon disk of the working electrode evenly.

[0068] Performance test (1)Nitrogen adsorption and desorption test The nitrogen adsorption and desorption test used N2 to complete the physical adsorption and desorption process.

[0069] The Fe,N-HPC catalyst prepared in Example 1 was placed at 80 °C and dried for 2 h; then, the Fe,N-HPC catalyst was placed at 120 °C and degassed for 10 h; the specific surface area and porosity of the degassed Fe,N-HPC catalyst were tested by nitrogen adsorption and desorption on an instrument.

[0070] (2) Redox performance test The working electrodes coated with the catalyst slurry obtained in Examples 1-3 and Comparative Examples 1-3, together with a platinum wire and a saturated calomel electrode, were placed in an electrolyte under O2-saturated conditions to assemble a three-electrode system. Then, an electrochemical workstation was connected, and the room temperature was controlled at 25 °C to test the redox performance. The test was carried out in an O2-saturated electrolyte, the potential range was 0 to 1.2 V, the scanning direction was negative sweep, and the scanning rate was 5 mV s -1 , and a transient polarization curve (curve of voltage versus current density) was obtained.

[0071] Among them, the electrolyte was a KOH solution with a concentration of 0.1 mol / L.

[0072] (3) Catalyst stability test The working electrodes coated with the catalyst slurry obtained in Example 1 and Comparative Example 3 were respectively activated in an electrolyte under O2-saturated conditions, and then the redox performance test was carried out according to the above method to obtain the initial transient polarization curve test. After that, the working electrode, platinum wire, and saturated calomel electrode were placed in an electrolyte under N2-saturated conditions for 3000 cycles of cyclic voltammetry curve scanning, and then the redox performance test was carried out again in an O2-saturated electrolyte to obtain the test transient polarization curve after 3000 cycles, and the performance decay was observed by comparison. Among them, the cyclic voltammetry curve scanning was carried out in an N2-saturated electrolyte, the potential range was 0 to 1.2 V, the scanning rate was 0.1 V s -1 , and the scanning direction was negative sweep.

[0073] Among them, the electrolyte was a KOH solution with a concentration of 0.1 mol / L.

[0074] The scanning electron microscope images of the ZIF-8, ZnO / N / C, and ZnO / N / C@Fe-ZIF precursors obtained in Example 1 are respectively shown in Figures 1-3 , and the transmission electron microscope image of the Fe,N-HPC catalyst obtained in Example 1 is shown in Figure 4 . It can be seen that the ZIF-8, ZnO / N / C, and ZnO / N / C@Fe-ZIF precursors are all in the shape of a rhombic dodecahedron. The rhombic dodecahedron structure of the ZnO / N / C@Fe-ZIF precursor remains intact, the particle dispersion is good, and the diameter is 450-550 nm. See Figure 4, the Fe,N-HPC catalyst prepared by pyrolysis maintained the morphology of the ZnO / N / C@Fe-ZIF precursor, but the morphology was slightly shrunk, and the particle size range of metallic iron particles was 5-10 nm.

[0075] The XPS spectrum of the Fe,N-HPC catalyst prepared in Example 1 is shown in Figure 5 , as can be seen from the figure, high temperature caused the complete volatilization and disappearance of metallic Zn in the obtained catalyst, and the Fe,N-HPC catalyst only contained four elements: Fe, C, N, and O.

[0076] The transmission electron micrograph of the Fe,N-HPC catalyst prepared in Example 2 is shown in Figure 6 , compared with Figure 4 , in Example 2, the carbon nanotubes on the surface of the Fe,N-HPC catalyst prepared by high-temperature pyrolysis decreased. This may be due to the relatively small proportion of 2-methylimidazole in the synthesis of the precursor, resulting in a decrease in the coordinated metal salts and a weakened catalytic graphitization effect of the metal during the formation of the catalyst.

[0077] The transmission electron micrograph of the Fe,N-HPC catalyst prepared in Example 3 is shown in Figure 7 , compared with Figure 4 , in Example 3, the number of metal particles and carbon nanotubes on the surface of the Fe,N-HPC catalyst prepared by high-temperature pyrolysis increased.

[0078] The transmission electron micrograph of the Fe,N-HPC catalyst obtained in Comparative Example 1 is shown in Figure 8 , it can be found that the catalyst obtained in Comparative Example 1 presented an irregular morphology and a large number of metallic particles precipitated.

[0079] The transmission electron micrograph of the Fe,N-HPC catalyst obtained in Comparative Example 2 is shown in Figure 9 , it can be found that the N-HPC catalyst prepared in Comparative Example 2 presented a rhombic dodecahedron, the structure remained intact, and micropores and mesopores could be found on the particle surface.

[0080] The nitrogen adsorption-desorption isotherm of the Fe,N-HPC catalyst prepared in Example 1 is shown in Figure 10 , see Figure 10 (a). By the BET calculation method, it can be obtained that the specific surface area of the Fe,N-HPC catalyst reached 180.67 m 3 g -1 , see Figure 10 (b). It can be known that the Fe,N-HPC catalyst presented a hierarchical porous structure with coexisting micro-mesopores, and the pore size range was 0-20 nm.

[0081] The polarization curve comparison diagrams of the Fe,N-HPC catalysts prepared in Examples 1-3 and Comparative Example 1, the N-HPC catalyst prepared in Comparative Example 2, and the commercial 20% Pt / C catalyst in Comparative Example 3 are respectively shown in Figures 11-15 . The stability test curves of the Fe,N-HPC catalyst obtained in Example 1 and the commercial 20% Pt / C catalyst in Comparative Example 3 are shown in Figure 16 .

[0082] Combined with Figure 11 and Figure 16 , it can be seen that the Fe,N-HPC catalyst obtained in Example 1 has better catalytic activity than the commercial 20% Pt / C, the half-wave potential exceeds 46 mV, and the electrochemical stability ( Figure 8 ) is better than that of the commercial 20% Pt / C.

[0083] Combined with Figure 11 and Figure 12 , the Fe,N-HPC catalyst obtained in Example 2 has better electrocatalytic activity than the commercial 20% Pt / C, the half-wave potential exceeds 25 mV, and it decreases by 21 mV compared with Example 1.

[0084] Combined with Figure 11 and Figure 13 , the Fe,N-HPC catalyst obtained in Example 3 has better electrocatalytic activity than the commercial 20% Pt / C, the half-wave potential exceeds 34 mV, and it decreases by 12 mV compared with Example 1.

[0085] Combined with Figure 11 and Figure 14 , the Fe,N-HPC catalyst obtained in Comparative Example 1 has better electrocatalytic activity than the commercial 20% Pt / C, the half-wave potential exceeds 8 mV, and it decreases by 38 mV compared with Example 1.

[0086] Combined with Figure 11 and Figure 16 , the electrocatalytic activity of the N-HPC catalyst obtained in Comparative Example 2 is worse than that of the commercial 20% Pt / C, the half-wave potential decreases by about 12 mV, and it decreases by about 58 mV compared with Example 1.

[0087] Finally, it should be noted that the above examples 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 above examples, those of ordinary skill in the art should understand that: the specific implementation manners of the present invention can still be modified or equivalently replaced, and any modification or equivalent replacement without departing from the spirit and scope of the present invention should be covered within the protection scope of the claims of the present invention.

Claims

1. A Fe, N co-doped carbon oxygen reduction catalyst with a hierarchical porous structure, characterized in that it is prepared by using ZnO / N / C@Fe-ZIF with a core-shell structure as a precursor, and the Fe, N co-doped carbon oxygen reduction catalyst has a hierarchical porous structure with both micropores and mesopores, and the particle size of the metal iron particles is 5-10 nm.

2. The Fe, N co-doped carbon oxygen reduction catalyst according to claim 1, characterized in that the core of the precursor is ZnO / N / C, and the shell layer of the precursor is Fe-ZIF; the precursor is in the shape of a rhombic dodecahedron with an average diameter of 450-550 nm.

3. The preparation method of the iron and nitrogen co-doped carbon oxygen reduction catalyst according to claim 1 or 2, characterized in that, It includes the following steps: S1. Prepare the precursor core ZnO / N / C; S2. Epitaxially grow Fe-ZIF on the outside of the precursor core to obtain the precursor; S3. Calcinate the precursor in an inert atmosphere to obtain the Fe, N co-doped carbon oxygen reduction catalyst.

4. The preparation method according to claim 3, characterized in that in step S1, the precursor core ZnO / N / C is prepared by calcining ZIF-8; The calcination temperature is 350 - 550 °C, and the heating rate during the calcination process is 1 - 5 °C / min -1 , and the holding time is 0.5 - 2 h.

5. The preparation method according to claim 4, characterized in that the preparation method of ZIF-8 includes: dissolving zinc nitrate and 2-methylimidazole in organic solvents respectively, mixing the two solutions under stirring conditions, then keeping warm at 20-50 °C for 12-24 h, filtering, washing and drying to obtain white powder ZIF-8; wherein, the molar ratio of zinc nitrate to 2-methylimidazole is 1:2-1:

16.

6. The preparation method according to claim 3, characterized in that step S2 includes: dissolving the precursor core, metal salt and 2-methylimidazole in organic solvents respectively to obtain a precursor core dispersion, a metal salt solution and a 2-methylimidazole solution, and then pouring the 2-methylimidazole solution and the metal salt solution into the precursor core dispersion in sequence, and reacting to obtain the precursor.

7. The preparation method according to claim 6, characterized in that The molar ratio of the metal salt to 2-methylimidazole is 1:10 - 1:70, the concentration of the metal salt is 1.2 - 3.6 mmol / L -1 , and the concentration of 2-methylimidazole is 48 - 96 mmol / L -1 ; in step S2, the metal salt is a soluble salt of iron.

8. The preparation method according to claim 6, characterized in that the reaction conditions of the 2-methylimidazole solution, the metal salt solution and the precursor core dispersion are to keep warm at 100-150 °C for 5-10 h; after the reaction is completed, filter, wash and dry to obtain the precursor.

9. The preparation method according to claim 3, characterized in that In step S3, the calcination temperature is 900 - 1100 °C, and the heating rate during the calcination process is 1 - 10 °C / min -1 , and the heat preservation time is 1 - 3 h.

10. Application of the Fe, N co-doped carbon oxygen reduction catalyst according to claim 1 or 2 or the catalyst prepared by the preparation method according to any one of claims 4-9 in the cathode oxygen reduction reaction of an alkaline fuel cell.

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