Oxygen reduction catalyst and preparation method thereof

Abstract

The invention provides an oxygen reduction catalyst and a preparation method thereof, and belongs to the technical field of oxygen reduction catalysts. The preparation method of the oxygen reduction catalyst comprises the following steps: S1, loading iron salt on biomass material powder to obtain a catalyst precursor; s2, mixing the catalyst precursor with a first pyrolyzable nitrogen-containing organic matter, and calcining in nitrogen-doped inert gas flow; 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 5000 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 oxygen reduction reaction (ORR) is a key factor in determining the energy conversion efficiency of proton exchange membrane fuel cells and zinc-air batteries. However, the reaction barrier of the oxygen reduction reaction is high and the kinetics are sluggish, so a catalyst with excellent performance is needed to accelerate this reaction. Precious metal Pt (platinum)-based materials are widely used as commercial catalysts for the oxygen reduction reaction due to their excellent catalytic activity. However, platinum is expensive and easily poisoned, which limits the large-scale commercial application of proton exchange membrane fuel cells and zinc-air batteries. Non-precious metal catalysts are expected to serve as alternatives to Pt-based catalysts.

[0003] Single-atom catalysts can expose more low-coordinated atoms to combine with reaction intermediates, showing excellent catalytic activity. Currently, single-atom catalysts are mainly prepared under the premise of sacrificing metal loading or using metastable support materials. These methods are not conducive to improving catalytic activity.

[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, there is provided a method for preparing an oxygen reduction catalyst, comprising the following steps:

[0007] Step S1, loading iron salt onto biomass material powder to obtain a catalyst precursor;

[0008] Step S2, mixing the catalyst precursor with the first pyrolyzable nitrogen-containing organic matter, and calcining in a nitrogen-doped inert gas flow; 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 5000 mg / L.

[0009] In an exemplary embodiment of the present disclosure, the average content of nitrogen element in the nitrogen-doped inert gas is 200 mg / L to 1500 mg / L; 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 to 30) mL / min: 1 g.

[0010] In an exemplary embodiment of the present disclosure, in step S1, the biomass material powder is a powder formed by washing, drying, grinding, and passing through a 300-mesh sieve of one or more of rush, cattail, corn stalks, and rice straw.

[0011] In an exemplary embodiment of the present disclosure, in step S1, after stirring the biomass material powder in an iron salt solution for 18 to 54 hours, the biomass material powder loaded with iron ions is separated and freeze-dried to obtain a catalyst precursor; wherein the ratio between the mass of the biomass material powder and the molar amount of the iron salt in the iron salt solution is 1g: (5 to 10) mM; and the concentration of the iron salt solution is 5 to 10 mmol / L.

[0012] In an exemplary 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; and the ratio of the mass of the first pyrolyzable nitrogen-containing organic matter to the mass of the catalyst precursor is between 1 and 5.

[0013] In an exemplary embodiment of the present disclosure, calcining the catalyst precursor in a nitrogen-doped inert gas atmosphere comprises:

[0014] Step S110, placing a mixture of a catalyst precursor and a first pyrolyzable nitrogen-containing organic compound in a first heating zone of a tube furnace; placing a second pyrolyzable nitrogen-containing organic compound in a second heating zone of the tube furnace; the first heating zone and the second heating zone can be heated independently;

[0015] 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;

[0016] 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 mixture of the catalyst precursor and the first pyrolyzable nitrogen-containing organic matter; 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;

[0017] Step S140, after the catalyst precursor is calcined, the first heating zone and the second heating zone are cooled down in sequence.

[0018] In an exemplary embodiment of the present disclosure, the second pyrolyzable nitrogen-containing organic matter is a mixture of one or more of melamine, urea, thiourea and dicyandiamide.

[0019] 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.

[0020] In an exemplary embodiment of the present disclosure, in step S130, the preset temperature is between 100°C and 355°C; the heating rate of the first heating zone is the same as the heating rate of the second heating zone.

[0021] According to another aspect of the present disclosure, an oxygen reduction catalyst is also provided, and the oxygen reduction catalyst is prepared by the above preparation method.

[0022] 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

[0023] 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.

[0024] Figure 1 This is a spherical aberration electron microscope image of the oxygen reduction catalyst prepared in Example 1.

[0025] Figure 2 This is a diagram showing the catalytic performance of the oxygen reduction catalyst prepared in Example 1. DETAILED DESCRIPTION

[0026] The present disclosure provides a method for preparing an oxygen reduction catalyst, which comprises the following steps:

[0027] Step S1, loading iron salt onto biomass material powder to obtain a catalyst precursor;

[0028] Step S2: After mixing the catalyst precursor with the first pyrolyzable nitrogen-containing organic matter, calcining in a nitrogen-doped inert gas flow, 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 5000 mg / L.

[0029] Biomass materials have natural tubular and reticular structures that are conducive to mass transfer; loading iron salts on biomass material powders can spatially isolate and fix the infiltrated iron elements with the help of the natural pipes and reticular structures of biomass materials, thereby improving the uniform distribution of iron elements in biomass materials and reducing the agglomeration of iron elements. In addition, biomass materials are pre-prepared into powders, which can facilitate the full penetration of iron elements in biomass materials, thereby increasing the iron element loading on biomass materials.

[0030] In step S2, the catalyst precursor is mixed with the first pyrolyzable nitrogen-containing organic matter and calcined. The biomass carrier will undergo partial graphitization during the calcination process, which can make the prepared oxygen reduction catalyst have high porosity and high conductivity. During the graphitization process of the biomass carrier, the formation of Fe (iron) -C (carbon) intermediates will induce the existing Fe (iron) -N (nitrogen) center to release HCN fragments. The first pyrolyzable nitrogen-containing organic matter can decompose during the calcination process to provide 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 massively supplementing 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, which will result 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 isolation and anchoring effect of nitrogen on iron atoms will be weakened, and the iron atoms will tend to migrate and aggregate during the calcination process.

[0031] In the embodiment of the present disclosure, the inert gas provided to the calcination area contains a nitrogen-containing component, and the CN free radicals provided by the nitrogen-containing component have strong reactivity with the iron atoms in the catalyst precursor, thereby continuously supplementing the nitrogen source for the catalyst precursor during the calcination process, maintaining or increasing the nitrogen content in the oxygen reduction catalyst, thereby inhibiting the migration and aggregation of iron atoms, so that the oxygen reduction catalyst has a high density of iron atom sites, and improving the activity of the oxygen reduction catalyst. In addition, the nitrogen-containing component can also consume the oxygen element in the biomass material, thereby causing more Fe-O bonds to be broken and iron atoms to be released, thereby increasing the proportion of iron elements involved in the formation of Fe-N bonds and the formation of iron nanoparticles, and further improving the catalytic ability of the oxygen reduction catalyst.

[0032] In step S2, the average content of nitrogen in the nitrogen-doped inert gas is 50 mg / L to 5000 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 difficulty in effectively increasing the nitrogen content in the oxygen reduction catalyst due to the low concentration of the nitrogen-containing component; on the other hand, this can increase the nitrogen content in the catalyst to the optimal content, thereby achieving a significant improvement in catalytic performance. In other words, during the calcination of the catalyst precursor, an inert gas doped with a suitable concentration of nitrogen-containing components is continuously provided around the catalyst precursor, and the nitrogen-containing components 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.

[0033] In the embodiment of the present disclosure, the iron element is spatially isolated by means of the special tubular structure of the biomass material powder, and the iron element is continuously anchored by doping the inert gas with nitrogen-containing components. Through the coordinated cooperation of the spatial isolation strategy and the continuous anchoring strategy, the density and proportion of iron atoms in the oxygen reduction catalyst can be effectively increased, thereby improving the activity of the oxygen reduction catalyst.

[0034] Optionally, the nitrogen-containing component is a carbon-nitrogen free radical or an ion, molecule or free radical that can provide a carbon-nitrogen free radical.

[0035] 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.

[0036] 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.

[0037] In one embodiment of the present disclosure, the biomass material powder is a powder formed by washing, drying, grinding and sieving one or more of rush, cattail, corn stalks and straw. These biomass materials are easily available and cheap, which is conducive to reducing the cost of oxygen reduction catalyst. It is understandable that other biomass materials can also be used when necessary.

[0038] In one example, the biomass material may be cleaned and then dried to remove the solvent used for cleaning, and then the cleaned and dried biomass material may be ground and sieved to obtain biomass material powder.

[0039] Optionally, the biomass material can be thoroughly soaked and cleaned with ultrapure water. Of course, a step-by-step cleaning method can also be used, such as first washing the biomass material with deionized water (e.g., soaking the biomass material with deionized water for 6 to 12 hours each time and filtering out the soaking liquid), and then washing the biomass material with ultrapure water (e.g., soaking the biomass material with ultrapure water for 6 to 12 hours each time and filtering out the soaking liquid). If necessary, the biomass material can be pre-cleaned with an organic solvent before washing the biomass material with water, such as using methanol, ethanol or acetone to pre-clean the biomass material.

[0040] Optionally, before washing the biomass material, the biomass material can be crushed to obtain biomass material coarse powder, and then the biomass material coarse powder is washed. Further, the biomass material coarse powder is sieved through a 300-mesh sieve to obtain fine powder. In this way, the washing efficiency and the washing effect can be improved, and the influence of metal ions, organic small molecules, etc. other than the carbon skeleton in the biomass material on the performance of the oxygen reduction catalyst can be reduced.

[0041] Optionally, the biomass material powder is obtained by filtering through a 300 mesh sieve. In this way, the iron element can fully penetrate into the biomass material powder, improve the distribution uniformity of the iron element in the biomass material powder, and thus help to improve the performance uniformity of the oxygen reduction catalyst. Not only that, the control of the particle size of the biomass material powder is conducive to controlling the porosity of the oxygen reduction catalyst formed after calcination and the proportion of voids with different pore sizes, which is conducive to regulating the material transfer process of the oxygen reduction catalyst in the catalytic process, and improving the performance of the oxygen reduction catalyst based on biomass materials.

[0042] In one embodiment of the present disclosure, the biomass material powder can be stirred in the iron salt solution for a long time, and then the biomass material powder loaded with iron ions can be separated and the separated biomass material powder can be freeze-dried to obtain a catalyst precursor. Optionally, after the biomass material powder is mixed with the iron salt solution, the mixture can be stirred at a low speed to improve the uniformity of the iron element in the iron salt solution, which is conducive to shortening the time of step S1. Optionally, the rotation speed during stirring is 200 to 500 rpm / min. Optionally, the iron salt solution is an iron salt aqueous solution.

[0043] In this embodiment, after the biomass material powder loaded with iron is separated from the iron salt solution, the biomass material powder is frozen and then freeze-dried. In this way, the migration of iron elements can be reduced during the drying process, and the iron elements that have penetrated into the biomass material powder are prevented from being enriched on the surface of the biomass material powder. In addition, the freeze-drying method can also maintain the biomass material powder in a relatively good fluffy state.

[0044] In one embodiment of the present disclosure, the concentration of the iron salt aqueous solution is between 5 and 10 mmol / L. If the iron salt concentration is too low, on the one hand, the iron element will penetrate into the biomass material too slowly, and the iron element loading of the biomass material infiltrated with iron element will be too low. If the iron salt concentration is too high, it may cause a significant difference in iron element concentration between the surface and the interior of the biomass material particles, reducing the uniformity of the oxygen reduction catalyst.

[0045] In one embodiment of the present disclosure, the ratio between the mass of the biomass material powder and the molar amount of the iron salt in the iron salt solution is 1g: (5-10) mM. In this way, the iron element in the catalyst precursor can have an appropriate content, which can avoid the iron element concentration in the biomass material powder being too low to reduce the density of the catalytic sites in the oxygen reduction catalyst, and avoid the iron element concentration in the biomass material powder being too high to cause the iron element to aggregate and the iron single atom distribution density to decrease.

[0046] Optionally, after the biomass material powder and the iron salt aqueous solution are mixed, the mixed state can be maintained and stirred for a long time, for example, the mixed state is maintained and stirred for 18 hours to 54 hours, so that the iron element can fully penetrate into the biomass material powder. Optionally, the mixture of the biomass material powder and the iron salt aqueous solution can be stirred at a low speed for 18 to 24 hours.

[0047] In the above-mentioned embodiment, after the biomass material powder is stirred in the iron salt solution, the biomass material powder loaded with iron ions is separated and freeze-dried to obtain a catalyst precursor. It is understandable that in some other embodiments of the present disclosure, the iron salt solution can be mixed with the biomass material powder in batches; for example, the biomass material powder can be mixed with the first iron salt aqueous solution, and the second iron salt aqueous solution can be added to the mixture after stirring for a period of time. In this way, the biomass material powder can be continuously in an environment of an iron salt aqueous solution of a suitable concentration, so as to realize the timely replenishment of the iron salt infiltrated into the biomass material powder. Furthermore, the concentration of the second iron salt aqueous solution can be greater than the concentration of the first iron salt aqueous solution, and the volume of the second iron salt aqueous solution is less than the volume of the first iron salt aqueous solution; for example, the concentration of the second iron salt aqueous solution is 3 to 5 times the concentration of the first iron salt aqueous solution, and the volume of the second iron salt aqueous solution is 0.3 to 0.5 times the volume of the first iron salt aqueous solution.

[0048] 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 oxygen reduction catalysts. It is understandable 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. For example, the iron salt in the iron salt solution may be FeCl3, which may also penetrate into the biomass material and provide anchoring for the iron element. During the calcination of the catalyst precursor, it will decompose and supplement the catalyst precursor with nitrogen to anchor the iron element during the decomposition process.

[0049] In the above examples, the iron salt solution is an aqueous solution of iron salt. It is understood that the solvent in the iron salt solution can also be adjusted as needed instead of using water. For example, the solvent in the iron salt solution can be alcohol, a mixed solvent of alcohol and water, etc. For example, the solvent in the iron salt solution can also be one of methanol, ethanol, a mixed solvent of methanol and water, a mixed solvent of ethanol and water, etc.

[0050] 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.

[0051] In one embodiment of the present disclosure, the ratio between the mass of the first pyrolyzable nitrogen-containing organic matter and the mass of the catalyst precursor is between 1 and 5, 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 usage amount. On the one hand, it can extend the duration of its pyrolysis process, and then provide a nitrogen source for the catalyst precursor for a longer period of 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.

[0052] Moreover, the first pyrolyzable nitrogen-containing organic matter is not extremely abundant, but can be basically completely pyrolyzed in the early stage of the calcination stage; on the one hand, this can avoid the mixture of the catalyst precursor and the first pyrolyzable nitrogen-containing organic matter from forming a pile that is too large, and reduce the amount of the first pyrolyzable nitrogen-containing organic matter to control the cost of the oxygen reduction catalyst. On the other hand, since the calcination temperature is much higher than the pyrolysis temperature of the first pyrolyzable nitrogen-containing organic matter, the pyrolysis rate of the first pyrolyzable nitrogen-containing organic matter at the calcination temperature is very fast, and the concentration of the nitrogen-containing component provided by the pyrolysis is very high; if the first pyrolyzable nitrogen-containing organic matter still exists in the later stage of the calcination stage, then the first pyrolyzable nitrogen-containing organic matter continues to provide a high concentration of nitrogen-containing components in the later stage of the calcination stage or even throughout the entire calcination process, which will cause the nitrogen content of the oxygen reduction catalyst to be too large, which may have an adverse effect on the conductivity of the oxygen reduction catalyst, and ultimately have an adverse effect on the performance of the oxygen reduction catalyst. In this embodiment, in the later stage of the calcination stage, the first pyrolyzable nitrogen-containing organic matter has been completely decomposed and can no longer continue to supplement the nitrogen source for the catalyst precursor. The additional nitrogen source for the catalyst precursor comes from the nitrogen-containing component continuously transmitted by the inert gas; the concentration of the nitrogen-containing component in the inert gas is limited to an appropriate range rather than a very high concentration, which can achieve the control of the nitrogen content in the oxygen reduction catalyst.

[0053] In one example, the average content of nitrogen in the nitrogen-doped inert gas is between 200 mg / L and 1500 mg / L, especially between 300 mg / L and 650 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.

[0054] 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.

[0055] In one embodiment of the present disclosure, the ratio between the flow rate of the nitrogen-doped inert gas and the mass of the catalyst precursor is between (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 the iron element cannot be anchored efficiently, which is not conducive to the improvement of the performance of the oxygen reduction catalyst. On the other hand, the inert gas flow is not large and the flow rate is low, which can prevent the airflow from blowing away or dispersing the catalyst precursor, thereby reducing the loss of the catalyst precursor. At the same time, since the gas flow rate is low and the catalyst precursor will not be blown away, the gas flow of the nitrogen-doped inert gas can be blown directly over the surface of the catalyst precursor, thereby accelerating the penetration of the nitrogen-containing component into the internal gap of the catalyst precursor, and improving the uniformity of the prepared oxygen reduction catalyst at different positions. In particular, when the amount of the prepared oxygen reduction catalyst is large, the diffusion of the nitrogen-containing component into the interior of the catalyst precursor stack can be effectively improved, thereby making the oxygen reduction catalyst inside and on the surface of the stack have more uniform performance. This can greatly improve the mass production of the oxygen reduction catalyst.

[0056] Not only that, the flow rate of the nitrogen-doped inert gas is small, and the nitrogen-doped inert gas flowing into the calcination area can also be quickly heated, avoiding the inflow of the nitrogen-doped inert gas causing the temperature of the calcination area to drop and affect the calcination effect. In particular, when the nitrogen-doped inert gas flows directly through the surface of the catalyst precursor, if the nitrogen-doped inert gas is not heated to a higher temperature in advance, the nitrogen-doped inert gas will cause the temperature of the catalyst precursor surface to drop significantly, thereby causing the actual calcination temperature difference of the catalyst precursor at different positions, which will cause the performance difference of the oxygen reduction catalyst at different positions, thereby reducing the mass production of the oxygen reduction catalyst. In the embodiment of the present disclosure, since the flow rate of the nitrogen-doped inert gas is small, it will be quickly heated and heated after flowing into the calcination area, which will not cause the surface temperature of the catalyst precursor to drop significantly, and thus will not affect the mass production of the oxygen reduction catalyst due to the temperature of the nitrogen-doped inert gas.

[0057] 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.

[0058] 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.

[0059] Step S110, placing a mixture of a catalyst precursor and a first pyrolyzable nitrogen-containing organic compound in a first heating zone of a tube furnace; placing a second pyrolyzable nitrogen-containing organic compound in a second heating zone of the tube furnace; the first heating zone and the second heating zone can be heated independently;

[0060] 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;

[0061] 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;

[0062] Step S140, after the catalyst precursor is calcined, the first heating zone and the second heating zone are cooled down in sequence.

[0063] 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 is beneficial to reducing the cost of the oxygen reduction catalyst. In addition, the second pyrolyzable nitrogen-containing organic matter and the stack of the catalyst precursor are 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.

[0064] 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.

[0065] 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 heating in the second heating zone to decompose the pyrolyzable nitrogen-containing organic matter, 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 inert gas doped with nitrogen, which reduces the equipment cost. In this way, the gas flowing into the tube furnace is an inert gas; after heating in the second heating zone to decompose the pyrolyzable nitrogen-containing organic matter, the gas flowing out of the second heating zone is an inert gas doped with nitrogen. 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 flow, it is calculated according to the volume of the inert gas at room temperature (eg, 25° C.).

[0066] 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.

[0067] 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.

[0068] In one example, the second heating zone may be heated first, and after the second heating zone is heated to a preset temperature, the first heating zone may be heated.

[0069] In another 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, the second heating zone reaches the preset temperature earlier than the first heating zone reaches the calcination temperature.

[0070] 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.

[0071] 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.

[0072] 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.

[0073] 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.

[0074] 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.

[0075] 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.

[0076] 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, which is greater than the heating time of the first heating zone.

[0077] 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.

[0078] 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 speed 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.

[0079] 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.

[0080] 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.

[0081] Optionally, the heating rate of the first heating zone is the same as the heating rate of the second heating zone.

[0082] 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.

[0083] 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 1.5 to 2.5 hours. 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. If the calcination time is too long, the migration of iron atoms will be aggravated during the calcination process, which may lead to a decrease in the distribution density of iron single atoms.

[0084] 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.

[0085] 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.

[0086] Embodiment 1:

[0087] The oxygen reduction catalyst was prepared by the following method:

[0088] Step S1, soaking and cleaning the rush thoroughly with ultrapure water, drying, grinding, and sieving through a 300-mesh sieve into powder for standby use, to obtain rush powder;

[0089] Add 1.25 g of rush powder to 75 mL of 7.5 mM FeCl3 aqueous solution and stir for 24 hours at a stirring speed of 300 rpm / min;

[0090] The FeCl3 aqueous solution was removed by filtration, and the separated rush powder was freeze-dried to obtain a catalyst precursor (rush powder loaded with iron salt).

[0091] Step S2, 1 gram of the catalyst precursor and 3 grams of urea (as the first pyrolyzable nitrogen-containing organic matter) are mixed evenly, transferred to a porcelain boat and placed in the first heating zone of the tube furnace; 7 grams of melamine (as the second pyrolyzable nitrogen-containing organic matter) are placed 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);

[0092] Inert gas is introduced into the tube 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;

[0093] The first heating zone and the second heating zone are heated simultaneously; the first heating zone is heated uniformly at a rate of 5°C / min to 950°C (calcination temperature), and the second heating zone is heated uniformly at a rate of 5°C / min to 355°C (preset temperature); after the first heating zone is heated to the calcination temperature, it is kept warm for 2 hours (calcination time);

[0094] After the first heating zone is cooled, the second heating zone is cooled again; the amount of solid material remaining in the second heating zone is very small; the catalyst precursor in the first heating zone is converted into an oxygen reduction catalyst (labeled as H-Fe@rush oxygen reduction catalyst 1).

[0095] Structural testing:

[0096] Figure 1 AC-HAADF–STEM (aberration-corrected high-angle annular dark-field scanning transmission electron microscopy) characterization image of H-Fe@rush oxygen reduction catalyst 1. Figure 1 , showing a uniform and high density of iron atoms in the entire field of view, and no aggregation of metal atoms occurs in areas such as the edge of the carbon matrix. The first pyrolyzable nitrogen-containing organic matter is physically mixed with the catalyst precursor in situ, which is equivalent to increasing the nitrogen content of the catalyst precursor itself, which in turn helps to increase the nitrogen doping amount in the oxygen reduction catalyst. Since the doped nitrogen itself has oxygen reduction activity, this is beneficial to improving the catalytic performance of the oxygen reduction catalyst. During the calcination stage, the second pyrolyzable nitrogen-containing organic matter is independently temperature-controlled and the pyrolysis rate is precisely controlled, which can continuously provide carbon-nitrogen free radicals to anchor metal iron atoms for the catalyst precursor throughout the calcination stage, reduce the aggregation of metal iron atoms at high temperatures, form high-density single iron atom active sites, and increase the density of single iron atoms.

[0097] Catalytic performance testing:

[0098] 2 mg of H-Fe@rush oxygen reduction catalyst 1 was dispersed in a 900 μL ethanol / 100 μL water mixed solvent containing 10 μL 5wt% Nafion, and ultrasonicated for 40 min to obtain a catalyst dispersion. 25 μL of the catalyst dispersion was drop-coated on the surface of a glassy carbon electrode (RRDE, surface area 0.2475 cm2) as a working electrode for ORR testing. All ORR tests were measured using a typical three-electrode device, with graphite as the counter electrode and a saturated calomel electrode (SCE) as the reference electrode. Linear sweep voltammetry (LSV) tests were performed in an O2-saturated 0.1 M HClO4 solution, and all potentials were converted to standard hydrogen potentials (RHE). Figure 2 The linear sweep voltammetry (LSV) curve of H-Fe@rush oxygen reduction catalyst 1. Its half-wave potential (E 1 / 2 ) is 0.82 V vs. RHE. Under the same loading conditions, the commercial Pt / C catalyst E 1 / 2 =0.78V, indicating that it has good catalytic performance.

[0099] Example 2

[0100] The same preparation method and test method as Example 1 were used, except that melamine was not added as the second pyrolyzable nitrogen-containing organic matter in the second heating zone when preparing the oxygen reduction catalyst. The obtained oxygen reduction catalyst was labeled as H-Fe@rush oxygen reduction catalyst 2.

[0101] The half-wave potential of H-Fe@rush oxygen reduction catalyst 2 is 0.75V, which is inferior to H-Fe@rush oxygen reduction catalyst 1. This is because the inert gas does not contain nitrogen-containing components and cannot continuously replenish nitrogen for the oxygen reduction catalyst. This results in that after the urea is completely thermally decomposed, the catalyst precursor cannot obtain nitrogen replenishment, the iron agglomeration increases, and the distribution density of iron atoms is reduced, which affects the catalytic performance.

[0102] Example 3

[0103] The same preparation method and test method as in Example 1 were used, except that the concentration of FeCl3 was changed to 5 mM when preparing the oxygen reduction catalyst. The obtained oxygen reduction catalyst was labeled as H-Fe@rush oxygen reduction catalyst 3.

[0104] The half-wave potential of H-Fe@Juncus rubrum oxygen reduction catalyst 3 is 0.80 V, which is close to that of H-Fe@Juncus rubrum oxygen reduction catalyst 1.

[0105] Example 4

[0106] The same preparation method and test method as in Example 1 were used, except that the concentration of FeCl3 was changed to 10 mM when preparing the oxygen reduction catalyst. The obtained oxygen reduction catalyst was labeled as H-Fe@Juncus junceus oxygen reduction catalyst 4.

[0107] The half-wave potential of H-Fe@Juncus rubrum oxygen reduction catalyst 4 is 0.81 V, which is close to that of H-Fe@Juncus rubrum oxygen reduction catalyst 1.

[0108] The catalytic performance of H-Fe@rush oxygen reduction catalyst 3, H-Fe@rush oxygen reduction catalyst 4 and H-Fe@rush oxygen reduction catalyst 1 are similar, which indicates that the internal structure of rush powder can effectively isolate iron ions and maintain a roughly consistent loading amount when the iron salt concentration fluctuates slightly. The performance of H-Fe@rush oxygen reduction catalyst 1 is slightly better than that of H-Fe@rush oxygen reduction catalyst 3 and H-Fe@rush oxygen reduction catalyst 4, which may be due to the fact that the concentration of iron salt can affect the loading amount and loading uniformity of iron salt. When the iron salt concentration is 7.5mM, a better balance can be achieved between slightly increasing the iron element loading amount and reducing the reduction of iron single atom density caused by reducing iron element enrichment.

[0109] Example 5

[0110] The same preparation method and test method as in Example 1 were used, except that, when preparing the oxygen reduction catalyst, the mass of melamine added as the second pyrolyzable nitrogen-containing organic matter in the second heating zone was 6 g. After the second heating zone was cooled, the solid material remaining in the second heating zone was close to a trace amount. The obtained oxygen reduction catalyst was labeled as H-Fe@rush oxygen reduction catalyst 5.

[0111] The half-wave potential of H-Fe@Juncus juncea oxygen reduction catalyst 5 is 0.76 V, which is lower than that of H-Fe@Juncus juncea oxygen reduction catalyst 1.

[0112] Example 6

[0113] The same preparation method and test method as in Example 1 were used, except that when preparing the oxygen reduction catalyst, the mass of melamine added as the second pyrolyzable nitrogen-containing organic matter in the second heating zone was 8 g. After the second heating zone was cooled, very little solid material remained in the second heating zone. The obtained oxygen reduction catalyst was labeled as H-Fe@rush oxygen reduction catalyst 6.

[0114] The half-wave potential of H-Fe@Juncus juncea oxygen reduction catalyst 6 is 0.77 V, which is lower than that of H-Fe@Juncus juncea oxygen reduction catalyst 1.

[0115] The performance of H-Fe@rush oxygen reduction catalyst 5 and H-Fe@rush oxygen reduction catalyst 6 is inferior to that of H-Fe@rush oxygen reduction catalyst 1. This may be because, compared with Example 1, the amount of melamine used in Example 5 is relatively low, resulting in a low concentration of nitrogen-containing components in the inert gas, especially in the later stage of the calcination stage, which makes it difficult to effectively ensure the nitrogen content in the catalyst. Compared with Example 1, the amount of melamine used in Example 6 is relatively high, resulting in a slightly higher concentration of nitrogen-containing components in the inert gas, which may cause a slightly higher nitrogen doping amount in the catalyst and may affect the conductivity of the catalyst, thereby affecting the catalytic performance.

[0116] Example 7

[0117] 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 2 g when preparing the oxygen reduction catalyst. The obtained oxygen reduction catalyst was labeled as H-Fe@rush oxygen reduction catalyst 7.

[0118] The half-wave potential of H-Fe@Juncus juncea oxygen reduction catalyst 7 is 0.74 V, which is lower than that of H-Fe@Juncus juncea oxygen reduction catalyst 1.

[0119] Example 8

[0120] 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 preparing the oxygen reduction catalyst. The obtained oxygen reduction catalyst was labeled as H-Fe@rush oxygen reduction catalyst 8.

[0121] The half-wave potential of H-Fe@Juncus juncea oxygen reduction catalyst 8 is 0.75 V, which is lower than that of H-Fe@Juncus juncea oxygen reduction catalyst 1.

[0122] The performance of H-Fe@rush oxygen reduction catalyst 7 and H-Fe@rush oxygen reduction catalyst 8 is not as good as that of H-Fe@rush oxygen reduction catalyst 1, which 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, its amount 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.

[0123] 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, loading iron salt onto biomass material powder to obtain a catalyst precursor; Step S2, mixing the catalyst precursor with the first pyrolyzable nitrogen-containing organic matter, and calcining in a nitrogen-doped inert gas flow; 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 5000 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 200 mg / L to 1500 mg / L; 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 to 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 biomass material powder is a powder formed by washing, drying, grinding and passing through a 300-mesh sieve one or more of rush, cattail, corn stalks and straw.

4. The method for preparing an oxygen reduction catalyst according to claim 1, characterized in that: In step S1, after stirring the biomass material powder in the iron salt solution for 18 to 54 hours, the biomass material powder loaded with iron ions is separated and freeze-dried to obtain a catalyst precursor; wherein the ratio between the mass of the biomass material powder and the molar amount of the iron salt in the iron salt solution is 1g: (5 to 10) mM; and the concentration of the iron salt solution is 5 to 10 mmol / L.

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

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 atmosphere comprises: Step S110, placing a mixture of a catalyst precursor and a first pyrolyzable nitrogen-containing organic compound in a first heating zone of a tube furnace; placing a second pyrolyzable nitrogen-containing organic compound in a second heating zone of the tube furnace; 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 mixture of the catalyst precursor and the first pyrolyzable nitrogen-containing organic matter; 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 a mixture of more than one 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.

9. The method for preparing an oxygen reduction catalyst according to claim 6, characterized in that: In step S130, the preset temperature is between 100°C and 355°C; the heating rate of the first heating zone is the same as the heating rate of the second heating zone.

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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