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, mixing a metal organic framework material with an iron salt solution, and separating the metal organic framework material loaded with iron ions as a catalyst precursor; and step S2, calcining the catalyst precursor in a 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 is between 50 mg / L and 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. In the prior art, iron salts can be mixed with precursor materials of metal-organic frameworks to generate metal-organic framework materials doped with iron ions in situ, and the metal-organic framework materials doped with iron ions are calcined to generate iron-based oxygen reduction catalysts. However, the performance of iron-based oxygen reduction catalysts prepared by this method needs to be further improved.

[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, mixing a metal organic framework material with an iron salt solution, and separating the metal organic framework material loaded with iron ions as a catalyst precursor;

[0008] Step S2, calcining the catalyst precursor in a nitrogen-doped inert gas flow, wherein 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 (20 to 40) mL / min: 1 g.

[0010] In an exemplary embodiment of the present disclosure, in step S1 , the metal organic framework material includes a mixture of one or more of MIL-100 (Fe) powder, ZIF-8 powder, and ZIF-67 powder.

[0011] In an exemplary embodiment of the present disclosure, in step S1, the ratio between the mass of the metal organic framework material and the molar amount of the iron salt is 1 g: (0.05-0.8) mmol; and the concentration of the iron salt solution is 0.1-1 mmol / L.

[0012] According to one aspect of the present disclosure, in step S1, the particle size of the metal organic framework material is in the range of 150 μm to 250 μm; after the metal organic framework material is mixed and stirred with an iron salt solution for 18 to 36 hours, the catalyst precursor is separated.

[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 the catalyst precursor and the pyrolyzable nitrogen-containing organic matter in the first heating zone and the second heating zone of the tube furnace respectively, wherein 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 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;

[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 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 the spherical aberration electron microscope image of Example 1.

[0025] Figure 2 It is a comparison chart of the catalytic performance of Example 1 and commercial Pt / C. DETAILED DESCRIPTION

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

[0027] Step S1, mixing a metal organic framework material with an iron salt solution, and separating the metal organic framework material loaded with iron ions as a catalyst precursor;

[0028] Step S2, calcining the catalyst precursor in a nitrogen-doped inert gas flow, wherein 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] The metal organic framework material has a nitrogen-containing skeleton, and the iron ions in the iron salt solution can penetrate into the metal organic framework material and be fixed in the grid or replace the metal ions of the metal organic framework material. In other words, the iron ions in the iron salt solution can be doped into the metal organic framework material and anchored and isolated with the help of the nitrogen-containing skeleton of the metal organic framework material.

[0030] In the embodiment of the present disclosure, iron salts are not added during the generation process of the metal organic framework material to achieve in-situ iron doping of the metal organic framework material, but an infiltration strategy is adopted to allow the iron salts to penetrate into the pre-generated metal organic framework material. On the one hand, this can ensure the uniformity of the metal organic framework material during the generation process, avoiding the addition of additional iron salts that may destroy the inherent structure of the metal organic framework material. On the other hand, this is conducive to controlling the doping amount of iron elements inside the metal organic framework material, reducing or avoiding the situation where iron elements agglomerate during the pyrolysis process of the metal organic framework material.

[0031] In step S2, when the catalyst precursor is calcined, the inert gas is doped with a nitrogen-containing component, which can continuously provide highly reactive carbon-nitrogen free radicals during the calcination of the catalyst precursor. The carbon-nitrogen free radicals can complex with the iron element in the catalyst precursor, thereby preventing or weakening the aggregation of iron elements on the catalyst precursor, so that the oxygen reduction catalyst has a high density of iron single atom sites, thereby improving the activity of the oxygen reduction catalyst.

[0032] In step S2, the metal organic framework material of the catalyst precursor will be partially graphitized 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. If the inert gas does not 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. In the embodiment of the present disclosure, the inert gas provided to the calcination area has a nitrogen-containing component, and the CN radicals provided by the nitrogen-containing component have a strong reactivity with the iron atoms in the catalyst precursor, and then continue to supplement the nitrogen source for the catalyst precursor during the calcination process, maintain or increase the content of nitrogen in the oxygen reduction catalyst, and then inhibit the migration and aggregation of iron atoms.

[0033] 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 is 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.

[0034] In the embodiment of the present disclosure, the iron element is spatially isolated by means of a metal organic framework material, and the iron element is continuously anchored by continuously doping the nitrogen-containing component in the inert gas. 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.

[0035] 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.

[0036] In one example, the nitrogen-containing component may be a component in the pyrolysis product of a pyrolyzable nitrogen-containing organic substance such as melamine, urea, thiourea or dicyandiamide. For example, a combination of one or more of melamine, urea, thiourea and dicyandiamide may be heated to pyrolyze melamine, urea, thiourea or dicyandiamide, and the released pyrolysis component may be mixed into the inert gas as a nitrogen-containing component, so that the inert gas is doped with the nitrogen-containing component.

[0037] 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.

[0038] In one embodiment of the present disclosure, the metal organic framework material includes a mixture of one or more of MIL-100 (Fe) powder, ZIF-8 powder, and ZIF-67 powder. Optionally, the particle size of the metal organic framework material is in the range of 150 μm to 250 μm to improve the penetration efficiency and penetration speed of the iron salt.

[0039] In one embodiment of the present disclosure, in step S1, the ratio between the mass of the metal organic framework material and the molar amount of the iron salt is 1g: (0.05-0.8)mmol; the concentration of the iron salt solution is 0.1-1mmol / L. In this way, by controlling the concentration of the iron salt solution, the amount and concentration of iron ions infiltrated into the metal organic framework material can be controlled, thereby controlling the loading amount of the iron element on the metal organic framework material. It can avoid the problem of iron element agglomeration and insufficient distribution density of iron single atoms due to excessive loading, and the problem of insufficient catalytic performance due to too low loading.

[0040] In this embodiment, the molar amount of iron salt is in a suitable range. If the molar amount of iron salt is too large, it will cause unnecessary waste of iron salt and the volume of iron salt solution is too large, which will cause waste of iron salt and increase in preparation cost;Not only that, it will also reduce the recovery rate of metal organic framework material, which is unfavorable for the yield of catalyst precursor. If the molar amount of iron salt is too small, as iron ions penetrate into metal organic framework material, the iron ion concentration in iron salt solution will appear a more obvious concentration decline;On the one hand, it is easy to cause the metal organic framework material to have iron ion distribution density difference inside and reduce the uniformity of oxygen reduction catalyst performance, on the other hand, it will cause the iron ion infiltration process to lengthen, and it will also cause the load to be less than expected;More importantly, it is difficult to effectively control the infiltration amount of iron ions between different batches, which is unfavorable for improving the performance uniformity of oxygen reduction catalysts in different batches.

[0041] Optionally, the solvent in the iron salt solution is an alcohol (e.g., methanol, ethanol, isopropanol) or an alcohol-water mixture. Preferably, the iron salt solution is a methanol solution of iron salt, so that the iron salt concentration in the iron salt solution can reach the target concentration and the solubility of the metal organic framework material in the iron salt solution is minimized, thereby improving the recovery rate of the metal organic framework material.

[0042] Optionally, the metal organic framework material can be stirred for a long time after being mixed with the iron salt solution, especially stirred at a low speed for a long time, to accelerate the penetration of iron ions into the metal organic framework material. In one example, the metal organic framework material is stirred for 18 to 36 hours after being mixed with the iron salt solution, and then separated to filter out the iron salt solution. If necessary, the metal organic framework material can also be washed to remove the iron salts remaining on the surface of the metal organic framework material and in the gaps between the stacks, so as to avoid these iron salt residues reducing the uniformity of the iron ion distribution in the catalyst precursor.

[0043] Optionally, after the mixture of the metal organic framework material and the iron salt solution is stirred, the metal organic framework material can be separated and dried. Further, the metal organic framework material can be separated by centrifugation, and the metal organic framework material can be washed multiple times with an alcohol solvent (such as anhydrous methanol) during the separation process; after vacuum drying, a dry metal organic framework material loaded with iron ions is obtained and used as a catalyst precursor.

[0044] Optionally, the mixture of the metal organic framework material and the iron salt solution can be

[0045] Stir at a speed of 200 to 500 rpm / min.

[0046] In the above-mentioned embodiment, after the metal organic framework material is stirred in the iron salt solution, the metal organic framework material loaded with iron ions is separated and 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 metal organic framework material in batches; for example, the metal organic framework material can be mixed with the first iron salt solution first, and the second iron salt solution can be added to the mixture after stirring for a period of time. In this way, the metal organic framework material can be continuously in an iron salt solution environment with a suitable concentration, so as to realize the timely replenishment of the iron salt infiltrated into the metal organic framework material. Furthermore, the concentration of the second iron salt solution can be greater than the concentration of the first iron salt solution, and the volume of the second iron salt solution is less than the volume of the first iron salt solution; for example, the concentration of the second iron salt solution is 3 to 5 times the concentration of the first iron salt solution, and the volume of the second iron salt solution is 0.3 to 0.5 times the volume of the first iron salt solution.

[0047] 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 is beneficial to reducing 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, such as iron salts in iron salt solutions may be Fe(bpy)3Cl2. In this example, bpy is 2,2'-bipyridine, which can partially penetrate into the metal organic framework material, and during the calcination of the catalyst precursor, it will decompose and supplement the catalyst precursor with nitrogen that anchors the iron element during the decomposition process.

[0048] In one embodiment of the present disclosure, 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. When the concentration of the nitrogen-containing component is too low, the catalyst precursor will have a low nitrogen content during the calcination process due to the difficulty in obtaining a sufficient concentration of nitrogen-containing components to supplement, which will lead to a weakened anchoring effect on iron atoms and lead to enhanced migration and agglomeration of iron atoms.

[0049] 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 iron-nitrogen centers, 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.

[0050] 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 (20 and 40) 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 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.

[0051] 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.

[0052] It is understandable that, although the flow rate of the inert gas is generally small, when the carrier in the catalyst precursor is different, the flow rate of the inert gas can be relatively different to match the changes in the calcination process of the carrier. For metal organic framework materials, the organic ligands themselves contain nitrogen, and the metals (such as zinc) in some organic framework materials need to be evaporated and removed, so the catalyst precursor with zinc-based metal organic framework materials as carriers needs to use a relatively large flow of inert gas. On the one hand, the concentration of nitrogen-containing components around the catalyst precursor can be mainly controlled by the concentration of nitrogen-containing components carried by the inert gas, reducing the influence of nitrogen-containing components released during the calcination of the metal organic framework material itself on the concentration of nitrogen-containing components in the surrounding gas atmosphere. Especially in the later stage of the calcination stage, the concentration of nitrogen-containing components around the catalyst precursor has a greater influence on the nitrogen content in the oxygen reduction catalyst, the distribution density of iron single atoms, etc.; the appropriate high inert gas flow rate can improve the control accuracy of the concentration of nitrogen-containing components in the inert gas atmosphere around the catalyst precursor. On the other hand, a proper increase in the inert gas flow rate can ensure the removal efficiency of the evaporated species during the decomposition of the metal organic framework material, especially the removal efficiency of the metal atoms (such as zinc atoms) in the evaporated species.

[0053] In one embodiment of the present disclosure, in step S2, the catalyst precursor can be 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.

[0054] 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.

[0055] Step S110, placing a catalyst precursor in a first heating zone of a tubular furnace; placing a pyrolyzable nitrogen-containing organic matter in a second heating zone of the tubular furnace; the first heating zone and the second heating zone can be heated independently;

[0056] 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;

[0057] Step S130, heating the second heating zone to a preset temperature, so that the 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;

[0058] Step S140, after the catalyst precursor is calcined, the first heating zone and the second heating zone are cooled down in sequence.

[0059] Optionally, in step S110, the pyrolyzable nitrogen-containing organic matter is a combination of one or more of melamine, urea, thiourea and dicyandiamide. The 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 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 pyrolyzable nitrogen-containing organic matter can be controlled by independent temperature control of the second heating zone, so that the 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. In particular, this can avoid the situation where the pyrolyzable nitrogen-containing organic matter has been completely decomposed before the calcination process is completed.

[0060] 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.

[0061] In step S130, the second heating zone is heated to a preset temperature, so that the 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.).

[0062] Optionally, the flow rate of the inert gas can be kept consistent, for example, the ratio between the flow rate of the inert gas and the mass of the catalyst precursor is within the range of (20-40) mL / min:1 g, for example, the ratio between the flow rate of the inert gas and the mass of the catalyst precursor is 30 mL / min:1 g. In this way, the efficiency of blowing off the evaporable species generated during the calcination of the catalyst precursor in the first heating zone can be ensured.

[0063] 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 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.

[0064] In this embodiment, by independently controlling the temperature of the second heating zone, precise control of the decomposition process of the 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.

[0065] 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.

[0066] In another example, the second heating zone and the first heating zone can be heated simultaneously; since the preset temperature for pyrolyzing the 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.

[0067] In step S130, the decomposition rate of the pyrolyzable nitrogen-containing organic matter can be controlled by controlling the preset temperature and the initial mass of the pyrolyzable nitrogen-containing organic matter (the mass of the pyrolyzable nitrogen-containing organic matter before pyrolysis), thereby controlling the generation rate of the nitrogen-containing components generated by the decomposition of the 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 pyrolyzable nitrogen-containing organic matter, the faster the decomposition rate of the 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 the nitrogen element in the nitrogen-doped inert gas.

[0068] In one example of step S130, the pyrolyzable nitrogen-containing organic matter can be prefabricated into regular shapes, for example, it can be prepared into regular shapes such as discs, strips, etc.; this is conducive to controlling the decomposition rate of the pyrolyzable nitrogen-containing organic matter and the release rate of the nitrogen-containing components, and thus is conducive to the mass production of the oxygen reduction catalyst.

[0069] 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 pyrolyzable nitrogen-containing organic matter of a 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 pyrolyzable nitrogen-containing organic matter at different temperatures. When it is necessary to adjust the decomposition rate of the pyrolyzable nitrogen-containing organic matter, the temperature adjustment method can also be determined according to the decomposition rate of the pyrolyzable nitrogen-containing organic matter at different temperatures.

[0070] 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 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 pyrolyzable nitrogen-containing organic matter.

[0071] In another example of step S130, the generation rate of the nitrogen-containing component can be adjusted by adjusting the initial mass of the pyrolyzable nitrogen-containing organic matter. When the preset temperature remains unchanged, the larger the initial mass of the pyrolyzable nitrogen-containing organic matter, the greater the pyrolysis rate of the pyrolyzable nitrogen-containing organic matter during the pyrolysis process, and the longer the pyrolysis time can be continued.

[0072] For example, the decomposition rate of 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 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 pyrolyzable nitrogen-containing organic matter needs to be adjusted, the initial mass of the pyrolyzable nitrogen-containing organic matter used can also be determined according to the decomposition rate of the pyrolyzable nitrogen-containing organic matter with different initial masses at the preset temperature.

[0073] In some other examples of the present disclosure, the pyrolysis rate of the pyrolyzable nitrogen-containing organic matter at different initial masses and different heating temperatures, and the time required for the complete pyrolysis can be obtained in advance. In step S130, the pyrolysis rate of the 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 pyrolyzable nitrogen-containing organic matter can be determined according to the pyrolysis rate and heating time of the pyrolyzable nitrogen-containing organic matter; wherein, the time required for the complete pyrolysis of the 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.

[0074] 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.

[0075] 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.

[0076] In one example, during the heating stage of the second heating zone, the temperature may be increased by 5 to 10°C.

[0077] The temperature is raised to the preset temperature at a heating rate of / min.

[0078] In one example, during the heating stage of the first heating zone, the temperature may be increased by 5 to 10°C.

[0079] The temperature was raised to the calcination temperature at a heating rate of / min.

[0080] Optionally, the heating rate of the first heating zone is the same as the heating rate of the second heating zone.

[0081] 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.

[0082] 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.

[0083] 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.

[0084] 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.

[0085] Embodiment 1:

[0086] The oxygen reduction catalyst was prepared by the following method:

[0087] Step S1, dissolving 1.17 g zinc nitrate hexahydrate in 8 mL deionized water, and dissolving 22.70 g 2-methylimidazole in 80 mL water; adding the zinc nitrate hexahydrate aqueous solution dropwise to the 2-methylimidazole aqueous solution under stirring, and continuously stirring; washing with deionized water by centrifugation and drying in a vacuum oven to obtain ZIF-8 powder;

[0088] 300 mg of ZIF-8 powder was slowly added to a FeCl3 anhydrous methanol solution with a concentration of 0.5 mmol / L and a volume of 100 mL, and stirred for 24 hours at a stirring speed of 200 rpm / min. The ZIF-8 powder was centrifuged and washed three times with anhydrous methanol and vacuum dried to obtain an iron-loaded ZIF-8 powder as a catalyst precursor. This step was repeated to obtain multiple batches of catalyst precursors, and the catalyst precursors obtained in multiple batches were mixed for subsequent calcination treatment.

[0089] Step S2, transferring the catalyst precursor (1 g) into a porcelain boat and placing it in the first heating zone of the tube furnace; placing 7.5 g of melamine (as a 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);

[0090] 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;

[0091] 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);

[0092] 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@ZIF oxygen reduction catalyst 1).

[0093] Structural testing:

[0094] Figure 1 AC-HAADF–STEM (aberration-corrected high-angle annular dark-field scanning transmission electron microscopy) characterization image of H-Fe@ZIF oxygen reduction catalyst 1. Figure 1 , no obvious metallic iron atom aggregation is found in the entire field of view, and no selective nucleation occurs in the nucleation-prone region (such as the edge region). The entire sample shows high-density and uniformly distributed iron atoms. The stable coordination of transition metal atoms relies heavily on nitrogen atoms. The number of nitrogen atoms present in ZIF is not enough to stabilize it, and iron-carbon species may induce the release of HCN species, further leading to a reduction in the number of active centers. In this embodiment 1, by accurately controlling the pyrolysis of pyrolyzable nitrogen-containing organic matter, it is possible to accurately and continuously provide carbon-nitrogen free radicals to anchor transition metal iron atoms under high temperature conditions during the graphitization of the precursor and the self-assembly of the active sites, forming a high-density of stably coordinated FeN4 species.

[0095] Catalytic performance testing:

[0096] 3 mg H-Fe@ZIF oxygen reduction catalyst 1 was dispersed in 225 μL ethanol / 250 μL water mixed solvent containing 25 μL 5 wt% Nafion and ultrasonicated for 40 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 cm2 ) surface and used as the working electrode for ORR testing. The preparation of the commercial Pt / C catalyst dispersion for comparison followed exactly the same procedure as the sample. All ORR tests were measured using a typical three-electrode setup with graphite as the counter electrode and a saturated calomel electrode (SCE) as the reference electrode. Cyclic voltammetry (CV) tests were carried out in an O2 / N2-saturated 0.1M KOH solution, and linear sweep voltammetry (LSV) tests were carried out in an O2-saturated 0.1M KOH solution, and all potentials were converted to standard hydrogen potential (RHE). Figure 2 The linear sweep voltammetry (LSV) curve of H-Fe@ZIF oxygen reduction catalyst 1. Its onset potential (Eonset) is 1.13 V vs. RHE, and its half-wave potential (E 1 / 2 ) is 0.95V vs.RHE, and the diffusion limiting current density (J L ) is 6.2mA / cm 2 , which is superior to commercial Pt / C catalysts (E onset =1.01V, E 1 / 2 =0.86V and J L =5.2 mA cm -2 ), indicating that it has good catalytic performance.

[0097] Example 2

[0098] The same preparation method and test method as Example 1 were used, except that when preparing the oxygen reduction catalyst, the concentration of FeCl3 was 0.75 mmol / L, and melamine as a pyrolyzable nitrogen-containing organic matter was not added in the second heating zone. The obtained oxygen reduction catalyst was labeled as H-Fe@ZIF oxygen reduction catalyst 2.

[0099] The performance parameters of H-Fe@ZIF oxygen reduction catalyst 2 are: onset =1.05V; E 1 / 2 =0.88V; J L =5.7mAcm -2 .

[0100] Example 3

[0101] The same preparation method and test method as Example 1 were used, except that melamine as a pyrolyzable nitrogen-containing organic matter was not added to the second heating zone when preparing the oxygen reduction catalyst. The obtained oxygen reduction catalyst was labeled as H-Fe@ZIF oxygen reduction catalyst 3.

[0102] The performance parameters of H-Fe@ZIF oxygen reduction catalyst 3 are: onset =1.06V; E 1 / 2 =0.90V; J L=5.2mAcm -2 .

[0103] Example 4

[0104] The same preparation method and test method as in Example 1 were used, except that when preparing the oxygen reduction catalyst, the concentration of FeCl3 was 0.25 mmol / L, and melamine as a pyrolyzable nitrogen-containing organic matter was not added in the second heating zone. The obtained oxygen reduction catalyst was labeled as H-Fe@ZIF oxygen reduction catalyst 4.

[0105] The performance parameters of H-Fe@ZIF oxygen reduction catalyst 4 are: onset =1.05V; E 1 / 2 =0.89V; J L =5.3mAcm -2 .

[0106] In Examples 2 to 4, different concentrations of iron salt solutions were mixed with metal organic framework materials to prepare metal organic framework materials loaded with iron ions, and the performance of the H-Fe@ZIF oxygen reduction catalyst finally obtained was comparable to or better than that of the commercial Pt / C catalyst. Compared with the H-Fe@ZIF oxygen reduction catalyst 1 prepared in Example 1, the performance has declined. 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 reduce the density of iron single atoms. If nitrogen-containing organic matter is added in the second heating zone to achieve further anchoring of the iron element during the calcination process, the performance of the obtained oxygen reduction catalyst will be further improved.

[0107] Example 5

[0108] The same preparation method and test method as Example 1 were used, except that: when preparing the oxygen reduction catalyst, melamine as a pyrolyzable nitrogen-containing organic matter was not added in the second heating zone; and the calcination temperature was 900° C. The obtained oxygen reduction catalyst was labeled as H-Fe@ZIF oxygen reduction catalyst 5.

[0109] The performance parameters of H-Fe@ZIF oxygen reduction catalyst 5 are: onset =1.02V; E 1 / 2 =0.88V; J L =5.2mAcm -2 , which is comparable to the performance of commercial Pt / C catalysts. Compared with the H-Fe@ZIF oxygen reduction catalyst 1 prepared in Example 1, the performance is reduced.

[0110] Example 6

[0111] The same preparation method and test method as Example 1 were used, except that: when preparing the oxygen reduction catalyst, melamine as a pyrolyzable nitrogen-containing organic matter was not added in the second heating zone; and the calcination temperature was 1000° C. The obtained oxygen reduction catalyst was labeled as H-Fe@ZIF oxygen reduction catalyst 6.

[0112] The performance parameters of H-Fe@ZIF oxygen reduction catalyst 6 are: onset =1.09V; E 1 / 2 =0.90V; J L =5.2mAcm -2 , which is better than the performance of commercial Pt / C catalyst. Compared with the H-Fe@ZIF oxygen reduction catalyst 1 prepared in Example 1, the performance is reduced. 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 iron single atoms.

[0113] Example 7

[0114] 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 to the second heating zone was 10 g. The obtained oxygen reduction catalyst was labeled as H-Fe@ZIF oxygen reduction catalyst 7.

[0115] The performance parameters of H-Fe@ZIF oxygen reduction catalyst 7 are: onset =1.07V, E 1 / 2 =0.91V and J L =6.0mAcm -2 , which is better than the performance of commercial Pt / C catalyst. Compared with the H-Fe@ZIF oxygen reduction catalyst 1 prepared in Example 1, the performance is decreased.

[0116] Example 8

[0117] The same preparation method and test method as in Example 1 were used, except that when preparing the oxygen reduction catalyst, 5 g of melamine was added to the second heating zone. The obtained oxygen reduction catalyst was labeled as H-Fe@ZIF oxygen reduction catalyst 8.

[0118] The performance parameters of H-Fe@ZIF oxygen reduction catalyst 8 are: onset =1.09V, E 1 / 2 =

[0119] 0.92V and J L =5.6 mA cm -2, which is better than the performance of commercial Pt / C catalyst. Compared with the H-Fe@ZIF oxygen reduction catalyst 1 prepared in Example 1, the performance is reduced. The reason is that the concentration of nitrogen-containing components in the inert gas is too low, which may lead to enhanced iron migration and agglomeration in the oxygen reduction catalyst and reduce the density of iron single atoms.

[0120] 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, mixing a metal organic framework material with an iron salt solution, and separating the metal organic framework material loaded with iron ions as a catalyst precursor; Step S2, calcining the catalyst precursor in a nitrogen-doped inert gas flow, wherein 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 (20 to 40) mL / min: 1 g.

3. The method for preparing an oxygen reduction catalyst according to claim 1, characterized in that: In step S1 , the metal organic framework material includes one or a mixture of multiple powders selected from the group consisting of MIL-100 (Fe) powder, ZIF-8 powder, and ZIF-67 powder.

4. The method for preparing an oxygen reduction catalyst according to claim 1, characterized in that: In step S1, the ratio between the mass of the metal organic framework material and the molar amount of the iron salt is 1g: (0.05-0.8)mmol; the concentration of the iron salt solution is 0.1-1mmol / L.

5. The method for preparing an oxygen reduction catalyst according to claim 1, characterized in that: In step S1, the particle size of the metal organic framework material is in the range of 150 μm to 250 μm; after the metal organic framework material is mixed and stirred with an iron salt solution for 18 to 36 hours, the catalyst precursor is separated.

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 the catalyst precursor and the pyrolyzable nitrogen-containing organic matter in the first heating zone and the second heating zone of the tube furnace respectively, 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 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 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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