Oxygen reduction catalyst and preparation method thereof

Abstract

The invention relates to an oxygen reduction catalyst and a preparation method thereof, and relates to the technical field of oxygen reduction catalysts. The preparation method comprises the following steps: 1, obtaining melamine-formaldehyde resin microspheres, dispersing the melamine-formaldehyde resin microspheres in water, adding a pyrrole solution and an iron salt aqueous solution, carrying out a stirring reaction, and carrying out centrifugal washing and drying to obtain a catalyst precursor with a core-shell structure; 2, mixing the catalyst precursor with a first pyrolyzable nitrogen-containing organic matter, and calcining in nitrogen-doped inert gas flow to obtain an oxygen reduction catalyst; the nitrogen-doped inert gas is doped with a nitrogen-containing component, and the average content of the nitrogen element in the nitrogen-doped inert gas ranges from 50 mg / L to 1500 mg / L. The preparation method of the oxygen reduction catalyst can improve the performance of the oxygen reduction catalyst.

Description

Technical Field

[0001] The present disclosure relates to the technical field of oxygen reduction catalysts, and in particular, to an oxygen reduction catalyst and a preparation method thereof. Background Art

[0002] The development of highly active oxygen reduction catalysts is the key to catalyzing the kinetically retarded oxygen reduction reaction and promoting the development of advanced energy conversion devices such as fuel cells and metal-air batteries that use this reaction as the cathode reaction. The precious metal platinum has high activity in catalyzing the oxygen reduction reaction, but its reserves are low and its price is high, making it difficult to apply on a large scale. In recent years, researchers have developed a variety of non-precious metal catalysts as excellent substitutes for platinum. As an important member of them, single-atom catalysts have attracted great attention. In the current single-atom preparation process, the metal content is usually reduced to avoid the agglomeration of metal atoms, but this also reduces the density of active sites for the catalytic reaction. This in turn limits the further improvement of the catalyst activity.

[0003] In addition to single-atom active sites, nitrogen doping is also an important factor affecting the activity of carbon-based oxygen reduction catalysts. Nitrogen can act as a bridging atom to anchor metal atoms to the carbon skeleton, reduce the aggregation of metal atoms, and increase the content of active sites in the catalyst; different types of nitrogen have different electronic structures. The increase in nitrogen content and the increase in the content of specific types of nitrogen (such as pyridinic nitrogen, pyrrolic nitrogen, and graphitic nitrogen) significantly enhance the catalytic performance of carbon-based materials. In addition, there are some structures in the carbon matrix (such as sp 3 defects, etc.), and also have a beneficial effect on the catalytic performance of the material.

[0004] It should be noted that the information disclosed in the above background technology section is only used to enhance the understanding of the background of the present disclosure, and therefore may include information that does not constitute the prior art known to ordinary technicians in the field. Summary of the invention

[0005] The purpose of the present invention is to overcome the deficiencies of the above-mentioned prior art, provide an oxygen reduction catalyst and a preparation method thereof, and improve the performance of the oxygen reduction catalyst.

[0006] According to one aspect of the present disclosure, a method for preparing an oxygen reduction catalyst is provided, comprising the following steps:

[0007] Step S1, obtaining melamine-formaldehyde resin microspheres, dispersing the melamine-formaldehyde resin microspheres in water, adding pyrrole solution and iron salt aqueous solution, stirring for reaction, centrifugation washing and drying to obtain a catalyst precursor with a core-shell structure;

[0008] Step S2, mixing the catalyst precursor with a first pyrolyzable nitrogen-containing organic matter, and calcining the mixture in a nitrogen-doped inert gas flow to obtain an oxygen reduction catalyst;

[0009] The nitrogen-doped inert gas is doped with nitrogen-containing components, and the average content of nitrogen in the nitrogen-doped inert gas is between 50 mg / L and 1500 mg / L.

[0010] In an exemplary embodiment of the present disclosure, the average content of nitrogen in the nitrogen-doped inert gas is 50 to 800 mg / L;

[0011] The ratio between the flow rate of the nitrogen-doped inert gas flow and the mass of the catalyst precursor is (10-30) mL / min: 1 g.

[0012] In an exemplary embodiment of the present disclosure, in step S1, the method for preparing melamine-formaldehyde resin microspheres comprises:

[0013] The formaldehyde solution and melamine are reacted in an aqueous solution for 15 to 30 minutes, a formic acid solution is added and the reaction is continued for 30 to 60 minutes, and the melamine-formaldehyde resin microspheres are obtained by centrifugation, washing and drying.

[0014] The mass ratio of the formaldehyde solution to melamine is (2-5):1.

[0015] In an exemplary embodiment of the present disclosure, in step S1, the ratio of the volume of pyrrole in the pyrrole solution to the molar amount of iron in the iron salt aqueous solution is (30-70) μL:90 μmol; the concentration of the iron salt aqueous solution is between 5 mmol / L and 15 mmol / L.

[0016] In an exemplary embodiment of the present disclosure, in step S2, the first pyrolyzable nitrogen-containing organic matter is one or a mixture of more than one of urea, melamine, dicyandiamide, and thiourea;

[0017] The mass ratio of the catalyst precursor to the first pyrolyzable nitrogen-containing organic matter is 1:(2-4).

[0018] In an exemplary embodiment of the present disclosure, calcining the catalyst precursor in a nitrogen-doped inert gas flow comprises:

[0019] Step S110, placing the catalyst precursor and the first pyrolyzable nitrogen-containing organic matter in the first heating zone of the tube furnace, and placing the second pyrolyzable nitrogen-containing organic matter in the second heating zone of the tube furnace, wherein the first heating zone and the second heating zone can be heated independently;

[0020] Step S120, continuously introducing an inert gas into the tube furnace, wherein the second heating zone is located on a gas path between the first heating zone and the inert gas inlet;

[0021] Step S130, heating the second heating zone to a preset temperature, so that the second pyrolyzable nitrogen-containing organic matter is pyrolyzed at a preset rate and the nitrogen-containing components generated by the pyrolysis are released into the inert gas flowing through the second heating zone; heating the first heating zone to a calcination temperature to calcine the catalyst precursor; the start time of heating the second heating zone is no later than the start time of heating the first heating zone; the preset temperature is lower than the calcination temperature;

[0022] Step S140, after the catalyst precursor is calcined, the first heating zone and the second heating zone are cooled down in sequence.

[0023] In an exemplary embodiment of the present disclosure, the second pyrolyzable nitrogen-containing organic matter is a combination of one or more of melamine, urea, thiourea and dicyandiamide.

[0024] In an exemplary embodiment of the present disclosure, in step S130, the calcination temperature is between 600°C and 1100°C; after the first heating zone reaches the calcination temperature, the calcination temperature is maintained for 1 to 3 hours;

[0025] The preset temperature is between 100° C. and 355° C., and the heating rate of the first heating zone is the same as the heating rate of the second heating zone.

[0026] In an exemplary embodiment of the present disclosure, in step S130, the calcination temperature is between 900°C and 1000°C; after the first heating zone reaches the calcination temperature, the calcination temperature is maintained for 110 to 130 minutes.

[0027] According to another aspect of the present disclosure, an oxygen reduction catalyst is provided, wherein the oxygen reduction catalyst is prepared by the above preparation method.

[0028] It is to be understood that the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the present disclosure. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] The accompanying drawings herein are incorporated into the specification and constitute a part of the specification, illustrate embodiments consistent with the present disclosure, and together with the specification are used to explain the principles of the present disclosure. Obviously, the accompanying drawings described below are only some embodiments of the present disclosure, and for ordinary technicians in this field, other accompanying drawings can be obtained based on these accompanying drawings without creative work.

[0030] Figure 1 This is a spherical aberration electron microscope image of the oxygen reduction catalyst prepared in Example 1 of the present disclosure.

[0031] Figure 2This is a comparison chart of the catalytic performance of the oxygen reduction catalyst prepared in Example 1 of the present disclosure and commercial Pt / C. DETAILED DESCRIPTION

[0032] Example embodiments will now be described more fully with reference to the accompanying drawings. However, the example embodiments can be implemented in a variety of forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that the present disclosure will be comprehensive and complete and fully convey the concepts of the example embodiments to those skilled in the art. The same reference numerals in the figures represent the same or similar structures, and thus their detailed description will be omitted. In addition, the drawings are only schematic illustrations of the present disclosure and are not necessarily drawn to scale.

[0033] The terms "a", "an", and "the" are used to indicate the presence of one or more elements / components / etc.; the terms "including" and "having" are used to express an open-ended inclusion and mean that additional elements / components / etc. may exist in addition to the listed elements / components / etc.; the terms "first", "second", etc. are used merely as labels and are not intended to limit the quantity of their objects.

[0034] The present disclosure provides a method for preparing an oxygen reduction catalyst, which comprises the following steps:

[0035] Step S1, obtaining melamine-formaldehyde resin microspheres, dispersing the melamine-formaldehyde resin microspheres in water, adding pyrrole solution and iron salt aqueous solution, stirring for reaction, centrifuging, washing and drying to obtain a catalyst precursor with a core-shell structure.

[0036] Step S2: After mixing the catalyst precursor with the first pyrolyzable nitrogen-containing organic matter, calcining in a nitrogen-doped inert gas flow to obtain an oxygen reduction catalyst. The nitrogen-doped inert gas is doped with a nitrogen-containing component, and the average content of nitrogen in the nitrogen-doped inert gas is between 50 mg / L and 1500 mg / L.

[0037] In step S1, the catalyst precursor obtained has a core-shell structure, the melamine-formaldehyde resin microspheres form the core structure of the catalyst precursor, and the pyrrole solution and the iron salt aqueous solution form the shell structure of the catalyst precursor. That is to say, the resin microspheres prepared using melamine and formic acid are used as the "core" structure, and the pyrrole solution and the iron salt aqueous solution are complexed to provide the "core" of aggregation, so that the pyrrole solution and the iron salt aqueous solution are complexed to form a "shell" structure around the resin microspheres. When the pyrrole solution and the iron salt aqueous solution are shelled, the nitrogen atoms in the pyrrole are complexed with the iron atoms in the iron salt aqueous solution, and the iron atoms are anchored on the carbon skeleton formed by the polymerization of the pyrrole, so that the iron atoms are distributed more evenly, while avoiding the reunion of metal iron atoms, and realizing the spatial isolation and uniform dispersion of the iron element. Therefore, the iron salt aqueous solution is used as a catalyst for the polymerization reaction to form a "shell" structure, realizing the synthesis of the catalyst precursor of the core-shell structure, and at the same time, it is also used as a metal source to form a metal active site on the catalyst precursor after calcination.

[0038] In step S2, the catalyst precursor is mixed with the first pyrolyzable nitrogen-containing organic matter and calcined to obtain an oxygen reduction catalyst. During the pyrolysis process of the core-shell structure catalyst precursor, the entire catalyst precursor is similar to a micro-pressure "reactor". As the temperature rises during the pyrolysis process, the airflow in the core moves, forming more pores and holes on the core. As the pyrolysis temperature rises, the airflow in the core breaks through the "shell" structure, forming a large number of "holes" on the carbon matrix, thereby making the sp at the edge of the carbon matrix 3 The defective hybridized carbon content increases, which improves the catalytic performance of the prepared oxygen reduction catalyst; at the same time, the prepared oxygen reduction catalyst also has a small amount of iron nanoparticles. A large number of iron single atoms and a small amount of iron atom nanoclusters are also evenly distributed in the carbon matrix obtained after pyrolysis of the core-shell structure catalyst precursor. Therefore, the oxygen reduction catalyst prepared by the method provided in this embodiment has multiple metal active sites such as iron single atoms, iron atom nanoclusters, and iron nanoparticles.

[0039] The catalyst precursor and the first pyrolyzable nitrogen-containing organic matter are mixed and calcined. During the calcination process, the first pyrolyzable nitrogen-containing organic matter can be decomposed, thereby providing a nitrogen source for the catalyst precursor, thereby slowing down the decomposition of the Fe-N center or forming a new Fe-N center. Since the calcination temperature is generally much higher than the pyrolysis temperature of the first pyrolyzable nitrogen-containing organic matter, the first pyrolyzable nitrogen-containing organic matter can be rapidly pyrolyzed during the heating stage of the calcination treatment, quickly and in large quantities replenishing the nitrogen-containing component for the catalyst precursor, and playing an important role in inhibiting the migration and aggregation of iron elements in the early stage of calcination. Then, the first pyrolyzable nitrogen-containing organic matter will be quickly and completely pyrolyzed after being heated to the calcination temperature, resulting in the catalyst precursor being unable to continuously obtain the nitrogen source supplemented by pyrolysis from the first pyrolyzable nitrogen-containing organic matter during the calcination process. If the inert gas does not continuously provide additional nitrogen-containing components, the nitrogen doping amount of the catalyst precursor during the calcination process will be reduced, the anchoring effect of nitrogen on iron atoms will be weakened, and iron atoms will tend to migrate and aggregate during the calcination process.

[0040] In an embodiment of the present disclosure, the inert gas provided to the calcination zone contains a nitrogen-containing component. The CN free radicals provided by the nitrogen-containing component have a strong reaction activity with the iron atoms in the catalyst precursor, thereby continuously replenishing the nitrogen source for the catalyst precursor during the calcination process, maintaining or increasing the content of nitrogen in the oxygen reduction catalyst, thereby inhibiting the migration and aggregation of iron atoms, resulting in a high density of iron atom sites on the oxygen reduction catalyst, thereby improving the activity of the oxygen reduction catalyst.

[0041] In step S2, the average content of nitrogen in the nitrogen-doped inert gas is 50 mg / L to 1500 mg / L. It is understandable that the inert gas provided to the calcination area (the area where the catalyst precursor and the first pyrolyzable nitrogen-containing organic mixture are placed) continuously contains nitrogen-containing components, and the concentration of the nitrogen-containing components may fluctuate. During the calcination of the catalyst precursor, a suitable concentration of nitrogen-containing components can be maintained in the atmosphere around the catalyst precursor. On the one hand, this can avoid the concentration of nitrogen-containing components being too small, making it difficult to effectively increase or maintain the nitrogen content in the oxygen reduction catalyst. On the other hand, this can avoid the concentration of nitrogen-containing components in the atmosphere around the catalyst precursor being too high, avoiding the excessive nitrogen content in the oxygen reduction catalyst and causing possible negative effects on the conductivity of the oxygen reduction catalyst, thereby avoiding the decrease in the conductivity of the oxygen reduction catalyst and causing a decrease in the catalytic effect. In other words, during the calcination of the catalyst precursor, an inert gas doped with a nitrogen-containing component at an appropriate concentration is continuously provided around the catalyst precursor. The nitrogen-containing component provided around the catalyst precursor can continuously provide a nitrogen source for the catalyst precursor to anchor the iron element, thereby achieving a continuous anchoring effect.

[0042] In the embodiment of the present disclosure, based on the catalyst precursor of the core-shell structure, the carbon matrix obtained by the pyrolysis of the catalyst precursor has sp3 The defective hybridized carbon content increases, and the catalyst performance improves. At the same time, the carbon matrix obtained after pyrolysis has a variety of metal active sites such as iron single atoms, iron nanoclusters, and iron nanoparticles. The synergistic effect of the metal active sites of various structural scales can greatly improve the activity of the oxygen reduction catalyst.

[0043] In one embodiment of the present disclosure, the nitrogen-containing component is a carbon-nitrogen free radical or an ion, molecule or free radical capable of providing a carbon-nitrogen free radical.

[0044] In one example, the nitrogen-containing component may be a component in the pyrolysis product of a second pyrolyzable nitrogen-containing organic matter such as melamine, urea, thiourea or dicyandiamide. For example, melamine, urea, thiourea or dicyandiamide may be pyrolyzed by heating melamine, urea, thiourea or dicyandiamide, and the released pyrolysis components may be mixed into the inert gas as nitrogen-containing components, so that the inert gas is doped with the nitrogen-containing component.

[0045] In another example, the nitrogen-containing component can be a component in a plasma formed by plasma treatment of nitrogen-containing gas molecules (e.g., nitrogen-containing organic gases such as hydrocyanic acid, acetonitrile, methylamine, and ethylenediamine); for example, an inert gas containing nitrogen-containing organic gas molecules can be subjected to corona discharge treatment and then introduced into a calcination area to supplement the nitrogen source for the catalyst precursor. In this example, the nitrogen-containing organic gas molecules are gas molecules containing both nitrogen and carbon elements, such as nitrogen, ammonia, nitric oxide, and nitrogen dioxide, which are not the nitrogen-containing organic gas molecules used in this example.

[0046] In one embodiment of the present disclosure, the preparation method of melamine-formaldehyde resin microspheres is as follows: after adding water to formaldehyde solution and mixing and stirring, melamine is added to react for 15-30 minutes, and then formic acid solution is added to continue reacting for 30-60 minutes. The solution after the reaction is centrifuged, washed and dried to obtain melamine-formaldehyde resin microspheres. The mass ratio of formaldehyde solution to melamine is (2-5):1.

[0047] In one embodiment of the present disclosure, the ratio of the volume of pyrrole in the pyrrole solution to the molar amount of iron in the iron salt aqueous solution is (30-70) μL: 90 μmol. When the shell structure is formed, pyrrole and iron ions are complexed and adsorbed and deposited on the core structure, which enables the iron element to be anchored on the pyrrole. Not only that, some pyrroles will react with iron ions to polymerize to form polypyrroles of larger molecular weight and complex with iron ions. Polypyrroles complexed with iron ions can also be adsorbed and deposited on the core structure. Here, the iron salt aqueous solution is an oxidant for pyrrole polymerization and a source of metal active sites of the oxygen reduction catalyst prepared after calcination of the catalyst precursor, so it is necessary to select a suitable molar amount of pyrrole to molar amount ratio of iron element. On the one hand, if the molar amount of iron in the iron salt aqueous solution is too low, there are fewer iron atoms on the catalyst precursor, so that the metal active sites in the catalyst obtained after calcination are reduced, which reduces the catalytic activity of the catalyst. On the other hand, to avoid excessive molar amount of iron in the iron salt aqueous solution, which leads to aggregation of iron elements, the distribution density of iron single atoms is reduced.

[0048] In one example, the concentration of the iron salt aqueous solution is between 5mmol / L and 15mmol / L. If the concentration of the iron salt aqueous solution is too low, the polymerization reaction of pyrrole will be slow, the reaction time for forming the shell structure will be too long, and the iron loading on the pyrrole polymer will be reduced. If the concentration of the iron salt aqueous solution is too high, the iron element will aggregate and occupy the position, thereby reducing the number and distribution density of the iron single atom active sites in the oxygen reduction catalyst.

[0049] In one embodiment of the present disclosure, the iron salt may be selected from one or more of FeCl2, FeCl3, Fe2(SO4)3, etc. These iron salts are readily available and inexpensive, which helps to reduce the cost of the oxygen reduction catalyst. It is understood that other iron salts may also be selected as needed. For example, in some examples, iron salts containing organic ligands may also be used, especially iron salts containing nitrogen-containing organic ligands, such as the iron salt in the iron salt solution may be Fe(bpy)3Cl2.

[0050] In one embodiment of the present disclosure, melamine-formaldehyde resin microspheres, pyrrole solution and iron salt aqueous solution are mixed and stirred at a stirring speed of 200 to 500 rpm / min for 12 to 36 hours.

[0051] In one embodiment of the present disclosure, the average content of nitrogen in the nitrogen-doped inert gas is between 50 g / L and 800 mg / L, especially between 50 mg / L and 300 mg / L. In this way, the nitrogen-containing component in the inert gas has a more suitable concentration, which is more conducive to achieving continuous anchoring of iron atoms and affecting the nitrogen doping amount of the oxygen reduction catalyst.

[0052] In one embodiment of the present disclosure, during the calcination of the catalyst precursor, the maximum value of the content of nitrogen in the nitrogen-doped inert gas is 1.0 to 1.5 times the minimum value. The content of nitrogen in the nitrogen-doped inert gas may fluctuate, but generally remain stable; on the one hand, this is conducive to continuously replenishing nitrogen to the catalyst precursor and continuously inhibiting the decomposition of the Fe-N bond, and on the other hand, it can ensure that oxygen reduction catalysts prepared in different batches have substantially the same performance. In this way, the preparation method provided in this embodiment is used to prepare an oxygen reduction catalyst, which can achieve a balance between improving the performance of the oxygen reduction catalyst and maintaining the performance stability of the oxygen reduction catalyst between different batches, thereby improving the mass production of the oxygen reduction catalyst.

[0053] In one embodiment of the present disclosure, the ratio between the flow rate of the nitrogen-doped inert gas flow and the mass of the catalyst precursor is in the range of (10-30) mL / min: 1 g. On the one hand, this can ensure the amount of nitrogen-containing components flowing into the calcination area (the area where the catalyst precursor and the first pyrolyzable nitrogen-containing organic matter are placed), avoid a sharp drop in the concentration of the nitrogen-containing components after being consumed in the calcination area, and achieve timely replenishment of the nitrogen-containing components in the inert gas atmosphere of the calcination area. If the flow rate of the nitrogen-doped inert gas is too small, the nitrogen-containing components may not be replenished in time and inadequately and cannot be efficiently anchored to the iron element, which is not conducive to the improvement of the performance of the oxygen reduction catalyst. On the other hand, if the ratio of the flow rate of the inert gas flow to the mass of the catalyst precursor is too small, the formaldehyde remaining in the shell structure and the various by-products produced by the formaldehyde are difficult to be blown off in a timely and rapid manner, and the formaldehyde retained in the catalyst precursor may react with nitrogen or iron, which may lead to a reduction in nitrogen active sites and may lead to a reduction in the anchoring of nitrogen to iron.

[0054] In one embodiment of the present disclosure, the first pyrolyzable nitrogen-containing organic matter is a mixture of one or more of urea, melamine, dicyandiamide, and thiourea. Optionally, the first pyrolyzable nitrogen-containing organic matter and the catalyst precursor may be mixed evenly and then placed in a calcination area and calcined together at a high temperature.

[0055] In one embodiment of the present disclosure, the mass ratio of the catalyst precursor to the mass of the first pyrolyzable nitrogen-containing organic matter is 1: (2 to 4), for example, the mass of the first pyrolyzable nitrogen-containing organic matter is 3 times the mass of the catalyst precursor. In this way, the first pyrolyzable nitrogen-containing organic matter has a large amount, which can extend the duration of its pyrolysis process, and then provide a nitrogen source for the catalyst precursor for a long time, and provide a sufficient nitrogen source for the early stage of calcination of the catalyst precursor to achieve better anchoring. On the other hand, the partial voids remaining in the first pyrolyzable nitrogen-containing organic matter after pyrolysis can increase the porosity of the catalyst precursor stack, and then facilitate the nitrogen-doped inert gas to supplement the nitrogen source inside the catalyst precursor. Not only that, the first pyrolyzable nitrogen-containing organic matter is not extremely surplus, but can be basically completely pyrolyzed in the early stage of the calcination stage; this can avoid the stack formed by the mixture of the catalyst precursor and the first pyrolyzable nitrogen-containing organic matter being too large. Therefore, in this embodiment, in the later stage of the calcination stage, the first pyrolyzable nitrogen-containing organic matter has been completely decomposed and cannot continue to supplement the nitrogen source for the catalyst precursor. The additional nitrogen source of the catalyst precursor is supplemented by the nitrogen-containing component continuously transmitted by the inert gas; the concentration of the nitrogen-containing component in the inert gas can be limited to an appropriate range to achieve control of the nitrogen content in the oxygen reduction catalyst. Exemplarily, during the heating process of the calcination zone, the first pyrolyzable nitrogen-containing organic matter begins to pyrolyze. When the calcination zone is heated to the calcination temperature (the calcination temperature is much greater than the cracking temperature of the first pyrolyzable nitrogen-containing organic matter), the first pyrolyzable nitrogen-containing organic matter has almost been pyrolyzed. In the insulation stage after the calcination zone reaches the calcination temperature, it is only necessary to control the nitrogen content of the nitrogen-containing component in the inert gas to achieve control of nitrogen doping in the catalyst calcination stage, which simplifies the regulation of the nitrogen-containing component in the calcination zone.

[0056] In one embodiment of the present disclosure, in step S2, the catalyst precursor can be mixed with the first pyrolyzable nitrogen-containing organic matter and placed in the first heating zone of the tubular furnace, and the first heating zone is used as the calcination zone. Then, the nitrogen-doped inert gas is continuously introduced into the first heating zone of the tubular furnace. Then, while the nitrogen-doped inert gas is continuously introduced into the first heating zone of the tubular furnace, the first heating zone of the tubular furnace is heated and calcined. In this embodiment, the nitrogen-containing component can be doped into the inert gas carrier gas outside the tubular furnace, so that the concentration of the nitrogen-containing component can be accurately controlled.

[0057] In another embodiment of the present disclosure, the tube furnace has a first heating zone and a second heating zone, and the first heating zone and the second heating zone can be heated independently to achieve independent temperature control of each group. The tube furnace has a gas inlet and a gas outlet, and the second heating zone is located between the gas inlet and the first heating zone. In other words, the gas can enter the tube furnace from the gas inlet and flow through the second heating zone and the first heating zone in sequence, and flow out from the gas outlet. The method shown in the following steps S110 to S140 can be used to calcine the catalyst precursor.

[0058] Step S110, placing a mixture of a catalyst precursor and a first pyrolyzable nitrogen-containing organic compound in a first heating zone of a tubular furnace; placing a second pyrolyzable nitrogen-containing organic compound in a second heating zone of the tubular furnace; the first heating zone and the second heating zone can be heated independently.

[0059] Step S120, continuously introducing an inert gas into the tube furnace, wherein the second heating zone is located on a gas path between the first heating zone and an inert gas inlet.

[0060] Step S130, heating the second heating zone to a preset temperature, so that the second pyrolyzable nitrogen-containing organic matter is pyrolyzed at a preset rate and the nitrogen-containing components generated by the pyrolysis are released into the inert gas flowing through the second heating zone; heating the first heating zone to a calcination temperature to calcine the catalyst precursor; the start time of heating the second heating zone is no later than the start time of heating the first heating zone; the preset temperature is lower than the calcination temperature.

[0061] Step S140, after the catalyst precursor is calcined, the first heating zone and the second heating zone are cooled down in sequence.

[0062] Optionally, in step S110, the second pyrolyzable nitrogen-containing organic matter is a combination of one or more of melamine, urea, thiourea and dicyandiamide. The second pyrolyzable nitrogen-containing organic matter is readily available and inexpensive, which helps to reduce the cost of the oxygen reduction catalyst. In addition, the second pyrolyzable nitrogen-containing organic matter is mixed with the catalyst precursor and placed in the same tubular furnace, which can reduce the difficulty of providing nitrogen-doped inert gas. The second heating zone is separated from the first heating zone and heated independently, so the pyrolysis process of the second pyrolyzable nitrogen-containing organic matter can be controlled by independent temperature control of the second heating zone, so that the second pyrolyzable nitrogen-containing organic matter is continuously and pyrolyzed at a preset speed, so that the concentration of the nitrogen-containing component in the nitrogen-doped inert gas flowing into the first heating zone is within a set range.

[0063] In step S120, an inert gas is continuously introduced into the tube furnace, and the second heating zone is located on the gas path between the first heating zone and the inert gas inlet. The inert gas flows in from the gas inlet of the tube furnace and flows out from the gas outlet of the tube furnace; the inert gas forms an inert gas flow in the tube furnace; along the inert gas flow, the second heating zone is located upstream of the first heating zone. In one example, the inert gas is one of nitrogen and argon. When the inert gas is nitrogen, the nitrogen content in the nitrogen-doped inert gas flow is not considered when calculating the nitrogen content.

[0064] In step S130, the second heating zone is heated to a preset temperature so that the second pyrolyzable nitrogen-containing organic matter is pyrolyzed at a preset rate and the nitrogen-containing components generated by the pyrolysis are released into the inert gas flowing through the second heating zone. In this way, the gas flowing into the tube furnace is an inert gas; after the pyrolyzable nitrogen-containing organic matter is heated in the second heating zone to decompose, the gas flowing out of the second heating zone is an inert gas doped with nitrogen. This can effectively shorten the distance and time from the generation of the nitrogen-containing component to the flow into the first heating zone, and keep it in a high temperature environment throughout the process. On the one hand, it reduces the possible loss of the nitrogen-containing component on the transmission path and thus ensures the concentration of the nitrogen-containing component arriving at the first heating zone. On the other hand, it avoids setting up a special high-temperature pipeline to transmit the nitrogen-doped inert gas, which reduces the equipment cost. In this way, by controlling the flow rate of the inert gas, the flow rate of the nitrogen-doped inert gas gas flow can be controlled. In the embodiment of the present disclosure, when determining the flow rate of the inert gas gas flow, it is calculated according to the volume of the inert gas at room temperature (e.g., 25°C).

[0065] In this embodiment, the first heating zone is also heated to the calcination temperature to calcine the catalyst precursor. The start time of heating the second heating zone is no later than the start time of heating the first heating zone; the preset temperature is lower than the calcination temperature. In this way, before the first heating zone is heated to the calcination temperature, the second heating zone can reach the preset temperature in advance and the second pyrolyzable nitrogen-containing organic matter can be stably decomposed at a preset speed, which ensures that the catalyst precursor can be in a nitrogen-doped inert gas atmosphere before the temperature is raised to the calcination temperature.

[0066] In this embodiment, by independently controlling the temperature of the second heating zone, precise control of the decomposition process of the second pyrolyzable nitrogen-containing organic matter can be achieved, thereby providing the first heating zone with a suitable concentration of nitrogen-containing components, which is beneficial to controlling the nitrogen doping amount in the oxygen reduction catalyst and reducing the residual oxygen in the oxygen reduction catalyst.

[0067] In one example, the second heating zone and the first heating zone can be heated simultaneously; since the preset temperature for pyrolyzing the second pyrolyzable nitrogen-containing organic matter is lower than the calcination temperature, when the first heating zone is heated, the first pyrolyzable nitrogen-containing organic matter is pyrolyzed first to provide doped nitrogen for the catalyst precursor.

[0068] In step S130, the decomposition rate of the second pyrolyzable nitrogen-containing organic matter can be controlled by controlling the preset temperature and the initial mass of the second pyrolyzable nitrogen-containing organic matter (the mass of the second pyrolyzable nitrogen-containing organic matter before pyrolysis), thereby controlling the generation rate of the nitrogen-containing components generated by the decomposition of the second pyrolyzable nitrogen-containing organic matter. When the flow rate of the inert gas remains unchanged, the higher the preset temperature or the larger the initial mass of the second pyrolyzable nitrogen-containing organic matter, the faster the decomposition rate of the second pyrolyzable nitrogen-containing organic matter, the faster the generation rate of the nitrogen-containing components, the greater the concentration of the nitrogen-containing components in the nitrogen-doped inert gas flowing into the first heating zone, and the greater the content of nitrogen in the nitrogen-doped inert gas.

[0069] In one example of step S130, the second pyrolyzable nitrogen-containing organic matter can be prefabricated into a regular shape, for example, it can be prepared into regular shapes such as discs, strips, etc.; this is beneficial for controlling the decomposition rate of the second pyrolyzable nitrogen-containing organic matter and the release rate of the nitrogen-containing components, and thus is beneficial for the mass production of the oxygen reduction catalyst.

[0070] In an example of step S130, the generation rate of the nitrogen-containing component can be adjusted by adjusting the preset temperature. For example, the decomposition rate of the second pyrolyzable nitrogen-containing organic matter of the preset shape at different temperatures can be obtained through preliminary experiments. Thus, in step S130, the preset temperature used in step S130 can be determined according to the inert gas flow rate and the decomposition rate of the second pyrolyzable nitrogen-containing organic matter at different temperatures. When it is necessary to adjust the decomposition rate of the second pyrolyzable nitrogen-containing organic matter, the temperature adjustment method can also be determined according to the decomposition rate of the second pyrolyzable nitrogen-containing organic matter at different temperatures.

[0071] Optionally, a decomposition rate-temperature curve of the pyrolyzable nitrogen-containing organic matter of a preset shape can be fitted according to the decomposition rate of the second pyrolyzable nitrogen-containing organic matter of a preset shape at different temperatures. In step S130, the temperature of the second heating zone can be determined according to the decomposition rate-temperature curve and the required decomposition rate of the second pyrolyzable nitrogen-containing organic matter.

[0072] In another example of step S130, the generation rate of the nitrogen-containing component can be adjusted by adjusting the initial mass of the second pyrolyzable nitrogen-containing organic matter. When the preset temperature remains unchanged, the larger the initial mass of the second pyrolyzable nitrogen-containing organic matter, the greater the pyrolysis rate of the second pyrolyzable nitrogen-containing organic matter during the pyrolysis process, and the longer the pyrolysis time can be continued.

[0073] For example, the decomposition rate of the second pyrolyzable nitrogen-containing organic matter with different initial masses at a fixed temperature (as a preset temperature) can be obtained through preliminary experiments. Thus, in step S130, the initial mass of the second pyrolyzable nitrogen-containing organic matter used in step S130 can be determined according to the inert gas flow rate and the preset temperature. When the decomposition rate of the second pyrolyzable nitrogen-containing organic matter needs to be adjusted, the initial mass of the second pyrolyzable nitrogen-containing organic matter used can also be determined according to the decomposition rate of the second pyrolyzable nitrogen-containing organic matter with different initial masses at the preset temperature.

[0074] In some other examples of the present disclosure, the pyrolysis rate of the second pyrolyzable nitrogen-containing organic matter at different initial masses and different heating temperatures, and the time required for complete pyrolysis can be obtained in advance. In step S130, the pyrolysis rate of the second pyrolyzable nitrogen-containing organic matter can be determined according to the flow rate of the inert gas and the required concentration of the nitrogen-containing component; the required initial mass and heating temperature (i.e., preset temperature) of the second pyrolyzable nitrogen-containing organic matter can be determined according to the pyrolysis rate and heating time of the second pyrolyzable nitrogen-containing organic matter; wherein, the time required for complete pyrolysis of the second pyrolyzable nitrogen-containing organic matter needs to be greater than the heating time. The heating time is the time from the start of heating in the second heating zone to the start of cooling in the second heating zone.

[0075] It is understood that in the embodiment of the present disclosure, the preset temperature may fluctuate within a normal range. For example, when the preset temperature is 330°C, the temperature of the second heating zone within the range of 325°C to 335°C may be regarded as reaching the preset temperature.

[0076] In one example, the preset temperature is in the range of 100°C to 355°C, and in particular can be in the range of 300 to 355°C. Optionally, the preset temperature is not lower than the temperature at which the pyrolyzable nitrogen-containing organic matter begins to pyrolyze. Furthermore, the preset temperature is the temperature at which the pyrolyzable nitrogen-containing organic matter can be completely pyrolyzed. In this example, the pyrolyzable nitrogen-containing organic matter is gradually pyrolyzed during the heating process, and its heating surface can reach the temperature of complete pyrolysis and gradually completely pyrolyze. It can be understood that the preset temperature may be different for different pyrolyzable nitrogen-containing organic matter and different required decomposition rates of the pyrolyzable nitrogen-containing organic matter.

[0077] In one example, during the heating stage of the second heating zone, the temperature can be raised to a preset temperature at a heating rate of 5 to 10° C. / min.

[0078] In one example, during the heating stage in the first heating zone, the temperature may be raised to the calcining temperature at a heating rate of 5 to 10° C. / min.

[0079] Optionally, the heating rate of the first heating zone is the same as or different from the heating rate of the second heating zone. For example, the first heating zone and the second heating zone are heated at the same time, and the heating rate of the first heating zone is greater than the heating rate of the second heating zone, so that the first pyrolyzable nitrogen-containing organic matter disposed in the first heating zone is first pyrolyzed to provide doped nitrogen for the catalyst precursor, thereby avoiding the first pyrolyzable nitrogen-containing organic matter and the second pyrolyzable nitrogen-containing organic matter from being pyrolyzed at the same time to provide doped nitrogen, resulting in an excessively high nitrogen concentration.

[0080] In one example, in step S130, the calcination temperature is between 600°C and 1100°C. Further, the calcination temperature is between 900°C and 1000°C, such as 890°C to 910°C, 940°C to 960°C, or 990°C to 1100°C.

[0081] In one example, after the first heating zone reaches the calcination temperature, it is maintained at the calcination temperature for 1 to 3 hours, especially for 110 to 120 minutes. Insufficient calcination time will affect the degree of transformation of the catalyst precursor to the oxygen reduction catalyst, for example, it will cause problems such as low porosity and insufficient conductivity. Too long calcination time will lead to increased migration of iron atoms during the calcination process, which may lead to a decrease in the distribution density of iron single atoms.

[0082] In step S140, after the catalyst precursor is calcined, the first heating zone and the second heating zone are cooled in sequence. For example, the first heating zone may be cooled to a temperature lower than a preset temperature before the second heating zone is cooled; or the first heating zone may be cooled to a temperature substantially close to the ambient temperature before the second heating zone is cooled. In this way, the first heating zone is cooled before the second heating zone, which ensures that the oxygen reduction catalyst in the first heating zone is always in an atmosphere of nitrogen-doped inert gas during the high temperature stage.

[0083] As follows, the method for preparing the oxygen reduction catalyst provided by the present disclosure and the effect of the oxygen reduction catalyst prepared therefrom are verified through a number of specific examples.

[0084] Example 1

[0085] Step S1, 9.8g formaldehyde solution (37wt%) and 200mL water are mixed and stirred at 80°C, 2.5g melamine is added and mixed evenly, and the mixture is reacted for 20 minutes, and 0.3ml formic acid solution is added and the reaction is continued for 45 minutes. The solution after the reaction is centrifuged and washed, and the centrifugal washing is centrifuged at 10000rpm for 15 minutes; and dried to obtain melamine-formaldehyde resin microspheres, which are used as core structures.

[0086] 0.1 g of melamine-formaldehyde resin microspheres were ultrasonically dispersed in water, and then 50 μL of pyrrole solution and 10 mL of an aqueous solution of iron salt containing 0.0245 g of FeCl3·6H2O were added in sequence, and the mixture was stirred at low speed for 24 h at room temperature and then centrifuged for washing. The centrifugal washing was centrifuged at 10,000 rpm for 15 min, and dried to obtain a catalyst precursor powder having a core-shell structure. This step was repeated to obtain multiple batches of catalyst precursor powders, and the catalyst precursor powders obtained in multiple batches were mixed for subsequent calcination treatment.

[0087] Step S2, mix 1 g of the catalyst precursor obtained in step S1 with 3 g of urea (mass ratio 1:3, as the first pyrolyzable nitrogen-containing organic matter) evenly, transfer to a porcelain boat and place it in the first heating zone of the tubular furnace; place 1.5 g of melamine (as the second pyrolyzable nitrogen-containing organic matter) in the second heating zone (located upstream of the first heating zone, and the inert gas flows from the second heating zone to the first heating zone).

[0088] An inert gas is introduced into the tubular furnace, the flow rate of the inert gas is 30 mL / min, and the inert gas is argon; the inert gas flows through the second heating zone and the first heating zone in sequence.

[0089] The first heating zone and the second heating zone are heated simultaneously; the first heating zone is heated to 950°C (calcination temperature) at a uniform rate of 5°C / min, and the second heating zone is heated to 355°C (preset temperature) at a uniform rate of 5°C / min; after the first heating zone is heated to the calcination temperature, it is kept warm for 2 hours (calcination time).

[0090] After the first heating zone is cooled, the second heating zone is cooled again; almost no solid material remains in the second heating zone; the catalyst precursor in the first heating zone is transformed into an oxygen reduction catalyst (labeled as H-Fe@core-shell structure oxygen reduction catalyst 1).

[0091] Structural testing:

[0092] Figure 1 This is an AC-HAADF–STEM (aberration-corrected high-angle annular dark-field scanning transmission electron microscope) characterization image of the H-Fe@ core-shell structure oxygen reduction catalyst 1. The field of view shows uniform and high-density iron single atom bright spots, a small number of larger bright spots formed by iron atom nanoclusters, and nanoparticles larger than 5nm in size. In addition, during the pyrolysis process, the catalyst precursor microspheres of the entire core-shell structure are similar to a micro-pressure "reactor". As the pyrolysis temperature increases, surrounded by the shell, the air flow in the core moves, forming more pores and holes, and finally breaking through the shell structure, resulting in a large number of holes in the carbon matrix after pyrolysis, which also indicates that the sp 3The defective hybrid carbon content increases. Through the method provided in this embodiment, an oxygen reduction catalyst with coexistence of metal active sites of various types, structures and scales is successfully prepared, and under the synergistic effect of the above-mentioned various metal active sites, the catalytic performance of the oxygen reduction catalyst is greatly improved.

[0093] Catalytic performance testing:

[0094] 2 mg H-Fe@ core-shell structure oxygen reduction catalyst 1 was dispersed in 900 μL ethanol / 100 μL water mixed solvent containing 10 μL 5 wt% Nafion and ultrasonicated for 30 min to obtain a catalyst dispersion. 25 μL of the catalyst dispersion was dropwise applied to a glassy carbon electrode (RRDE, surface area 0.2475 cm 2 ) surface and used as the working electrode for ORR testing. The testing procedures of the commercial Pt / C catalyst used for comparison were the same as those of the H-Fe@core-shell structure oxygen reduction catalyst 1. All ORR tests were measured using a typical three-electrode setup with Pt wire as the counter electrode and Ag / AgCl as the reference electrode. Linear sweep voltammetry (LSV) tests were performed in an O2-saturated 0.1 M KOH solution, and all potentials were converted to standard hydrogen potential (RHE). Figure 2 The linear sweep voltammetry (LSV) curve of H-Fe@ core-shell structure oxygen reduction catalyst 1 is shown in Figure 1. Figure 2 As shown, the starting potential of the H-Fe@ core-shell structure oxygen reduction catalyst 1 is 1.03 V vs. RHE, and the half-wave potential is 0.91 V vs. RHE, which is better than the commercial Pt / C catalyst (starting potential of 0.98 V and half-wave potential of 0.85 V), indicating that the catalyst prepared in this example has good electrocatalytic performance.

[0095] Example 2

[0096] The same preparation method and test method as in Example 1 were used, except that, when preparing the oxygen reducing agent, the mass of urea (as the first pyrolyzable nitrogen-containing organic matter) mixed with the catalyst precursor was 2 g. The obtained oxygen reduction catalyst was labeled as H-Fe@core-shell structure oxygen reduction catalyst 2.

[0097] The onset potential of H-Fe@core-shell structure oxygen reduction catalyst 2 is 0.99 V and the half-wave potential is 0.87 V, which are comparable to the comparative Pt / C catalyst and inferior to H-Fe@core-shell structure oxygen reduction catalyst 1.

[0098] Example 3

[0099] The same preparation method and test method as in Example 1 were used, except that the mass of urea (as the first pyrolyzable nitrogen-containing organic matter) mixed with the catalyst precursor was 4 g when the oxygen reducing agent was prepared. The obtained oxygen reduction catalyst was labeled as H-Fe@core-shell structure oxygen reduction catalyst 3.

[0100] The onset potential of H-Fe@core-shell structure oxygen reduction catalyst 3 is 1.00 V and the half-wave potential is 0.90 V, which are close to the comparative Pt / C catalyst and inferior to H-Fe@core-shell structure oxygen reduction catalyst 1.

[0101] The performance of H-Fe@core-shell oxygen reduction catalyst 2 and H-Fe@core-shell oxygen reduction catalyst 3 is not as good as that of H-Fe@core-shell oxygen reduction catalyst 1. This shows that although the first pyrolyzable nitrogen-containing organic matter is rapidly and completely pyrolyzed in the early stage of calcination and cannot continuously replenish nitrogen for the catalyst precursor in the later period of calcination, its dosage may still affect the performance of the final catalyst. This may be because the amount of the first pyrolyzable nitrogen-containing organic matter may have a certain effect on the void distribution, nitrogen content, etc. of the oxygen reduction catalyst, which may in turn affect the performance of the oxygen reduction catalyst.

[0102] Example 4

[0103] The same preparation method and test method as Example 1 were used, except that the second pyrolyzable nitrogen-containing organic melamine was not added when preparing the oxygen reducing agent. The obtained oxygen reduction catalyst was labeled as H-Fe@core-shell structure oxygen reduction catalyst 4.

[0104] The onset potential of H-Fe@core-shell oxygen reduction catalyst 4 is 0.93 V, and the half-wave potential is 0.78 V, which is inferior to H-Fe@core-shell oxygen reduction catalyst 1, and the performance is also lower than that of commercial Pt / C. The reason may be that there is no nitrogen-containing component in the inert gas, which will lead to enhanced iron migration and agglomeration in the oxygen reduction catalyst and reduced density of single iron atoms.

[0105] Example 5

[0106] The same preparation method and test method as Example 1 were used, except that when preparing the oxygen reducing agent, the amount of the second pyrolyzable nitrogen-containing organic melamine added was 1 g, and the obtained oxygen reduction catalyst was labeled as H-Fe@core-shell structure oxygen reduction catalyst 5.

[0107] The onset potential of H-Fe@core-shell oxygen reduction catalyst 5 is 0.97 V, and the half-wave potential is 0.86 V, which is inferior to that of H-Fe@core-shell oxygen reduction catalyst 1. The reason may be that the reduction of nitrogen-containing components in the inert gas will lead to enhanced iron migration and agglomeration in the oxygen reduction catalyst, and the density of single iron atoms will decrease.

[0108] Example 6

[0109] The same preparation method and test method as Example 1 were adopted, with the only difference being that, when preparing the oxygen reducing agent, the amount of the second pyrolyzable nitrogen-containing organic matter melamine added was 2 g, and the obtained oxygen reduction catalyst was labeled as H-Fe@core-shell structure oxygen reduction catalyst 6.

[0110] The onset potential of H-Fe@core-shell structure oxygen reduction catalyst 6 is 0.99 V and the half-wave potential is 0.87 V, which are inferior to those of H-Fe@core-shell structure oxygen reduction catalyst 1.

[0111] Example 7

[0112] The same preparation method and test method as Example 1 were used, with the only difference being that, when preparing the oxygen reducing agent, the first heating zone was kept warm after being heated to a calcination temperature of 850°C. The obtained oxygen reduction catalyst was labeled as H-Fe@core-shell structure oxygen reduction catalyst 7.

[0113] The onset potential of H-Fe@core-shell structure oxygen reduction catalyst 7 is 0.99 V and the half-wave potential is 0.87 V, which are lower than those of H-Fe@core-shell structure oxygen reduction catalyst 1.

[0114] Example 8

[0115] The same preparation method and test method as Example 1 were used, with the only difference being that, when preparing the oxygen reducing agent, the first heating zone was kept warm after being heated to a calcination temperature of 1000°C. The obtained oxygen reduction catalyst was labeled as H-Fe@core-shell structure oxygen reduction catalyst 8.

[0116] The onset potential of H-Fe@core-shell structure oxygen reduction catalyst 8 is 0.99 V and the half-wave potential is 0.87 V, which are lower than those of H-Fe@core-shell structure oxygen reduction catalyst 1.

[0117] Example 9

[0118] The same preparation method and test method as Example 1 were used, with the only difference being that, when preparing the oxygen reducing agent, the first heating zone was heated to the calcination temperature and then kept warm for 1 hour. The obtained oxygen reduction catalyst was labeled as H-Fe@core-shell structure oxygen reduction catalyst 9.

[0119] The onset potential of H-Fe@core-shell structure oxygen reduction catalyst 9 is 1.00 V and the half-wave potential is 0.88 V, which are close to H-Fe@core-shell structure oxygen reduction catalyst 1.

[0120] Example 10

[0121] The same preparation method and test method as Example 1 were used, with the only difference being that, when preparing the oxygen reducing agent, the first heating zone was heated to the calcination temperature and then kept warm for 3 hours. The obtained oxygen reduction catalyst was labeled as H-Fe@core-shell structure oxygen reduction catalyst 10.

[0122] The starting potential of H-Fe@core-shell oxygen reduction catalyst 10 is 1.00 V, and the half-wave potential is 0.89 V, which is close to that of H-Fe@core-shell oxygen reduction catalyst 1. The reason may be that the calcination time is too long, which will lead to more Fe-N decomposition and more iron atoms migration and aggregation, thereby reducing the distribution density of iron single atoms, which is not conducive to the improvement of the performance of the oxygen reduction catalyst.

[0123] The performance of H-Fe@core-shell oxygen reduction catalyst 9 and H-Fe@core-shell oxygen reduction catalyst 10 is slightly lower than that of H-Fe@core-shell oxygen reduction catalyst 1. The reason may be that compared with Example 1, insufficient calcination time will affect the degree of transformation of the catalyst precursor to the oxygen reduction catalyst, for example, resulting in problems such as low porosity; too long calcination time will lead to increased migration of iron atoms during the calcination process, which may lead to a decrease in the distribution density of single iron atoms.

[0124] Those skilled in the art will readily appreciate other embodiments of the present disclosure after considering the specification and practicing the invention disclosed herein. This application is intended to cover any modification, use or adaptation of the present disclosure, which follows the general principles of the present disclosure and includes common knowledge or customary techniques in the art that are not disclosed in the present disclosure. The specification and examples are intended to be exemplary only, and the true scope and spirit of the present disclosure are indicated by the appended claims.

Claims

1. A method for preparing an oxygen reduction catalyst, characterized in that: The following steps are involved: Step S1, obtaining melamine-formaldehyde resin microspheres, dispersing the melamine-formaldehyde resin microspheres in water, adding pyrrole solution and iron salt aqueous solution, stirring for reaction, centrifugation washing and drying to obtain a catalyst precursor with a core-shell structure; Step S2, mixing the catalyst precursor with a first pyrolyzable nitrogen-containing organic matter, and calcining the mixture in a nitrogen-doped inert gas flow to obtain an oxygen reduction catalyst; The nitrogen-doped inert gas is doped with nitrogen-containing components, and the average content of nitrogen in the nitrogen-doped inert gas is between 50 mg / L and 1500 mg / L.

2. The method for preparing an oxygen reduction catalyst according to claim 1, characterized in that: The average content of nitrogen in the nitrogen-doped inert gas is 50 to 800 mg / L; The ratio between the flow rate of the nitrogen-doped inert gas flow and the mass of the catalyst precursor is (10-30) mL / min: 1 g.

3. The method for preparing an oxygen reduction catalyst according to claim 1, characterized in that: In step S1, the method for preparing melamine-formaldehyde resin microspheres comprises: The formaldehyde solution and melamine are reacted in an aqueous solution for 15 to 30 minutes, a formic acid solution is added and the reaction is continued for 30 to 60 minutes, and the melamine-formaldehyde resin microspheres are obtained by centrifugation, washing and drying. The mass ratio of the formaldehyde solution to melamine is (2-5):

1.

4. The method for preparing an oxygen reduction catalyst according to claim 1, characterized in that: In step S1, the ratio of the volume of pyrrole in the pyrrole solution to the molar amount of iron in the iron salt aqueous solution is (30-70) μL:90 μmol; the concentration of the iron salt aqueous solution is between 5 mmol / L and 15 mmol / L.

5. The method for preparing an oxygen reduction catalyst according to claim 1, characterized in that: In step S2, the first pyrolyzable nitrogen-containing organic matter is one or a mixture of more than one of urea, melamine, dicyandiamide, and thiourea; The mass ratio of the catalyst precursor to the first pyrolyzable nitrogen-containing organic matter is 1:(2-4).

6. The method for preparing an oxygen reduction catalyst according to any one of claims 1 to 5, characterized in that: Calcining the catalyst precursor in a nitrogen-doped inert gas flow comprises: Step S110, placing the catalyst precursor and the first pyrolyzable nitrogen-containing organic matter in the first heating zone of the tube furnace, and placing the second pyrolyzable nitrogen-containing organic matter in the second heating zone of the tube furnace, wherein the first heating zone and the second heating zone can be heated independently; Step S120, continuously introducing an inert gas into the tube furnace, wherein the second heating zone is located on a gas path between the first heating zone and the inert gas inlet; Step S130, heating the second heating zone to a preset temperature, so that the second pyrolyzable nitrogen-containing organic matter is pyrolyzed at a preset rate and the nitrogen-containing components generated by the pyrolysis are released into the inert gas flowing through the second heating zone; heating the first heating zone to a calcination temperature to calcine the catalyst precursor; the start time of heating the second heating zone is no later than the start time of heating the first heating zone; the preset temperature is lower than the calcination temperature; Step S140, after the catalyst precursor is calcined, the first heating zone and the second heating zone are cooled down in sequence.

7. The method for preparing an oxygen reduction catalyst according to claim 6, characterized in that: The second pyrolyzable nitrogen-containing organic matter is one or more of melamine, urea, thiourea and dicyandiamide.

8. The method for preparing an oxygen reduction catalyst according to claim 6, characterized in that: In step S130, the calcination temperature is between 600°C and 1100°C; after the first heating zone reaches the calcination temperature, the calcination temperature is maintained for 1 to 3 hours; The preset temperature is between 100° C. and 355° C., and the heating rate of the first heating zone is the same as the heating rate of the second heating zone.

9. The method for preparing an oxygen reduction catalyst according to claim 8, characterized in that: In step S130, the calcination temperature is between 900°C and 1000°C; after the first heating zone reaches the calcination temperature, the calcination temperature is maintained for 110 to 130 minutes.

10. An oxygen reduction catalyst, characterized in that The oxygen reduction catalyst is prepared by the preparation method according to any one of claims 1 to 9.

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