Preparation methods and applications of nitrogen-doped carbon-supported Fe3C particles and Fe / Mn heteronuclear dual single-atom catalysts

Abstract

This invention discloses a method for preparing nitrogen-doped carbon-supported Fe3C particles and Fe / Mn heteronuclear dual-monoatom catalysts, and their applications. The disclosed preparation method includes the following steps: using MnFe2O4 as a template and HCl as an initiator, MnFe-PPy containing residual Fe oxides with incomplete template consumption is prepared using the in-situ polymerization principle of pyrrole monomers; the MnFe-PPy is then calcined in an inert atmosphere to obtain nitrogen-doped carbon-supported Fe3C particles and the Fe / Mn heteronuclear dual-monoatom catalyst Fe3C@FeMn-N-C. In the catalyst prepared by this method, the Fe3C particles and Fe / Mn heteronuclear dual-monoatoms are highly dispersed on the carbon support. The Fe3C particles reconstruct the electronic structure of the dual-monoatom sites, placing them in an electron-rich state, which is beneficial for catalytic reactions. Furthermore, the synergistic effect between the Fe / Mn heteronuclear dual-monoatoms can lower the reaction energy barrier of the catalyst during oxygen reduction, resulting in catalysts with extremely high electrocatalytic activity, stability, and methanol resistance.

Description

Technical Field

[0001] This invention relates to the field of non-precious metal catalysts; more specifically, it relates to a method for preparing nitrogen-doped carbon-supported Fe3C particles and Fe / Mn heteronuclear dual single-atom catalysts and their applications. Background Technology

[0002] Due to the scarcity of fossil fuels and the increasing environmental degradation, exploring next-generation energy storage and conversion technologies is of paramount importance to the world today. Zinc-air batteries are considered one of the most promising energy storage systems due to their low cost, excellent stability, and good safety. During operation, the oxygen reduction reaction (ORR) occurs at the cathode of a zinc-air battery. However, the sluggish kinetics of the cathode ORR limit the overall performance of the zinc-air battery. Therefore, improving the cathode ORR is crucial for enhancing the performance and large-scale commercialization of zinc-air batteries. Among currently known catalysts, Pt-based catalysts are considered to have the highest ORR catalytic activity; however, their high cost, low stability, and poor methanol resistance hinder their large-scale commercialization. Therefore, there is a need to vigorously develop cost-effective non-precious metal catalysts.

[0003] Transition metals M (M = Mn, Fe, Co, Ni, etc.) possess three-dimensional unoccupied orbitals that can accommodate foreign electrons, thereby reducing the bond strength between *OOH, *O, and *OH intermediates and giving them the potential to catalyze the reduction of O2. This is exemplified by metal-nitrogen-carbon (MN) groups. x -C) coordinated non-noble metal single-atom catalysts (SACs) have attracted considerable attention due to their high atom utilization, high intrinsic activity, and well-defined active sites and reaction mechanism models. However, despite increasing research showing that MN x While structure is the primary active site, single-atom MNC catalysts still suffer from drawbacks such as cumbersome synthesis and low metal loading. Furthermore, the symmetrical electronic structure distribution of a single single-atom site is unfavorable for the adsorption / desorption of oxygen intermediates, leading to poor catalytic activity. This is due to the linear scaling limit (SRL) at a single active site, meaning that the ORR pathway involves multiple steps, each with its own reaction energy barrier. Lowering the energy barrier of a step involving the active site typically leads to an increase in the energy barriers of other steps at the same site.

[0004] Therefore, constructing different types of diatomic central sites holds promise for overcoming the limitations of single-metal sites and is considered to possess more efficient ORR catalytic activity. Furthermore, introducing metal nanoparticles is considered another promising approach, involving the interaction of metal nanoparticles with MN... x The synergistic effect can accelerate the ORR dynamics process. Summary of the Invention

[0005] The main objective of this invention is to provide a method for preparing a catalyst containing Fe3C particles and Fe / Mn heteronuclear dual single atoms and its application. The Fe3C particles and Fe / Mn heteronuclear dual single atoms are dispersed on a nitrogen-doped carbon support with a hollow spherical morphology, exhibiting extremely high electrocatalytic activity, stability, and methanol resistance for the ORR reaction.

[0006] To achieve the aforementioned main objectives, the first aspect of the present invention provides a method for preparing nitrogen-doped carbon-supported Fe3C particles and Fe / Mn heteronuclear dual single-atom catalysts, comprising the following steps:

[0007] (1) The pyrrole monomer and MnFe2O4 template were evenly dispersed in an ethanol-water mixed solution. The initiator HCl was added to carry out the polymerization reaction until the MnFe2O4 template was not completely consumed and Fe oxide residues remained. Then the product was filtered, washed and dried to obtain MnFe-PPy containing Fe oxide residues.

[0008] (2) MnFe-PPy containing Fe oxide residues was calcined under an inert atmosphere to obtain nitrogen-doped carbon-supported Fe3C particles and Fe / Mn heteronuclear dual single-atom catalyst Fe3C@FeMn-NC, wherein the Fe3C particles and Fe / Mn heteronuclear dual single atoms are highly dispersed on the nitrogen-doped carbon support NC.

[0009] In the above technical solution, the Fe / Mn heteronuclear dual single-atom catalyst can synergistically catalyze the ORR reaction, lowering the reaction energy barrier. Simultaneously, MnFe-PPy containing template Fe oxide residues can form Fe3C particles during calcination under an inert atmosphere. Introducing Fe3C particles into the catalyst allows for electron accumulation at the active sites of the metal single atoms, further regulating the adsorption strength of the adsorbate. This results in a catalyst exhibiting extremely high electrocatalytic activity, stability, and methanol resistance for the ORR reaction.

[0010] Furthermore, a carbon atom coating layer is formed on the surface of the Fe3C particles. The presence of this carbon coating layer can prevent excessive etching of the internal active species by the electrolyte, thereby improving the catalytic stability of the catalyst.

[0011] Further, in step (1), the MnFe2O4 template is first dispersed evenly in water, then the pyrrole monomer is dissolved in ethanol and added to the MnFe2O4 template dispersion. After mixing evenly, HCl solution is added and polymerization reaction is carried out under ultrasonic conditions.

[0012] In the above technical solution, the polymerization reaction is carried out under ultrasonic conditions, and the reactants are uniformly dispersed during the reaction, thereby obtaining a polymer product with a relatively uniform morphology. Preferably, the reaction vessel is cooled and the temperature is controlled by condensate water during the ultrasonic process, with the temperature maintained at around room temperature (25°C) to avoid the influence of the solution temperature rise during the ultrasonic process on the polymerization reaction rate. This allows the polymerization reaction to proceed at a relatively stable reaction rate, thereby reliably controlling the reaction rate of the MnFe2O4 template and the residual Fe oxide.

[0013] Furthermore, in step (2), the calcination temperature is 750–850℃, and the calcination time is 1–2 hours; the inert atmosphere for calcination can be nitrogen. If the calcination temperature is too low, the carbon material in the catalyst will have a low degree of graphitization and poor conductivity, which is not conducive to ORR electrocatalysis; if the calcination temperature is too high, it may cause the collapse of the hollow spherical morphology of the catalyst, which is not conducive to sufficient contact between the catalyst and the electrolyte and reduces the catalytic performance.

[0014] Furthermore, the size of the MnFe2O4 template is 100nm to 300nm, and the polymerization reaction time is 1 to 2 hours.

[0015] Furthermore, the MnFe2O4 template has a hollow spherical structure, which enables the final catalyst to form a three-dimensional porous structure, which is beneficial to the migration of reactants and enhances the ORR reaction kinetics.

[0016] Furthermore, the MnFe2O4 template was obtained by the following method: manganese chloride tetrahydrate and ferric chloride hexahydrate were dispersed in ethylene glycol, ammonium acetate was added to adjust the pH, polyethylene glycol 6000 was added and stirred evenly, the mixed solution was subjected to a solvothermal reaction, and the resulting product was centrifuged, washed and dried to obtain a hollow MnFe2O4 template.

[0017] In one specific embodiment, the preparation of the MnFe2O4 template includes: first, dissolving 0.5 parts by weight of manganese chloride tetrahydrate and 1 part by weight of ferric chloride hexahydrate in 60 parts by volume of ethylene glycol, stirring to ensure complete dissolution, and then adding 5 parts by weight of ammonium acetate and 1.5 parts by weight of polyethylene glycol 6000 to the solution, wherein the relationship between parts by weight and parts by volume is g / mL.

[0018] Furthermore, the solvothermal reaction temperature for preparing the mixed solution of MnFe2O4 template was 200℃, and the holding time was 22h.

[0019] A second aspect of the present invention relates to the application of the Fe3C@FeMn-NC catalyst obtained by the aforementioned preparation method in the ORR reaction.

[0020] As detailed below, the technical solution of the present invention has the following beneficial effects:

[0021] 1. In the preparation method of this invention, Fe3C particles are formed by calcining MnFe-PPy containing template Fe oxide residues, and the Fe3C particles and Fe / Mn heteronuclear dual single atoms are highly dispersed on the nitrogen-doped carbon support NC. The Fe / Mn heteronuclear dual single atoms serve as reactive sites, and the synergistic effect between the two metal single-atom sites avoids the disadvantage of strong adsorption and difficult desorption of the ORR intermediate species OH* at Fe sites. The addition of Fe3C particles restructures the local coordination environment and electronic structure of the Fe / Mn heteronuclear dual single atoms, placing them in an electron-rich state, further reducing the reaction energy barrier of the catalyst in the oxygen reduction process, resulting in extremely high electrocatalytic activity, stability, and methanol resistance for the ORR reaction.

[0022] 2. The electrocatalytic ORR activity of MNC catalysts is closely related to their microstructure. Among various nanostructured carbons, two-dimensional and three-dimensional porous doped carbon structures can significantly enhance the catalytic activity of catalysts. Catalysts prepared using MnFe2O4 templates with hollow spherical structures exhibit porous hollow microsphere morphology, which is beneficial for the full exposure of active sites and the migration of reactants. In particular, the carbon support forms a porous hollow carbon nanosphere structure, in which the pore type mainly exists in the form of mesopores. Mesopores facilitate the migration of reactants, enhance ORR reaction kinetics, and thus improve catalyst performance.

[0023] To more clearly illustrate the purpose, technical solution, and advantages of the present invention, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. Attached Figure Description

[0024] Figure 1 In the image: a is the XRD pattern of the hollow MnFe2O4 template prepared in Example 1, and b is its SEM image;

[0025] Figure 2 In the middle: a is the XRD pattern of MnFe-PPy(2h) prepared in Comparative Example 1, and b is the XRD pattern of MnFe-PPy(4h) prepared in Comparative Example 2.

[0026] Figure 3 The images are SEM (a) and TEM (b) images of MnFe-PPy(2h) prepared in Comparative Example 1.

[0027] Figure 4 The image shows the SEM image of MnFe-PPy(4h) prepared in Comparative Example 2.

[0028] Figure 5 This is the XRD pattern of the Fe3C@FeMn-NC catalyst prepared in Example 1;

[0029] Figure 6In the image: a is a SEM image of the Fe3C@FeMn-NC catalyst prepared in Example 1, and b and c are its TEM images;

[0030] Figure 7 In the image: a is the SEM image of the FeMn-NC catalyst prepared in Comparative Example 3, and b is its TEM image.

[0031] Figure 8 In the image: a is a mapping diagram of the Fe3C@FeMn-NC catalyst prepared in Example 1, and b is a spherical aberration electron microscope image of it;

[0032] Figure 9 In the image: a and b are SEM images of the hollow Fe3O4 template prepared in Comparative Example 4, and c is its XRD pattern.

[0033] Figure 10 In the middle: a and b are SEM images of the H-Fe-NC catalyst prepared in Comparative Example 4, and c is its XRD pattern;

[0034] Figure 11 In the middle: a and b are SEM images of the Fe-NC catalyst prepared in Comparative Example 5, and c is its XRD pattern;

[0035] Figure 12 The nitrogen adsorption-desorption curves and pore size distribution diagrams of the Fe3C@FeMn-NC catalyst prepared in Example 1, the FeMn-NC catalyst prepared in Comparative Example 3, and the Fe-NC catalyst prepared in Comparative Example 5 are shown.

[0036] Figure 13 These are the cyclic voltammetry (CV) curves of the catalysts in Examples 1 and Comparative Examples 3-5 under O2 saturation and N2 saturation conditions;

[0037] Figure 14 This is a comparison of the linear sweep voltammetry (LSV) curves of Examples 1, Comparative Examples 3-5, and commercial Pt / C catalysts;

[0038] Figure 15 This is a comparison of the chronoamperometry (it) curves of the Fe3C@FeMn-NC catalyst prepared in Example 1 and the commercial Pt / C catalyst to resist methanol cross-effect.

[0039] Figure 16 This is a comparison of the chronoamperometry (it) curves of the Fe3C@FeMn-NC catalyst prepared in Example 1 and the commercial Pt / C catalyst;

[0040] Figure 17A and 17B These are electrochemical double-layer capacitance (Cdl) test graphs for Example 1, Comparative Example 3, Comparative Example 5, and a commercial Pt / C catalyst;

[0041] Figure 18 This is a TEM image of the MnFe-PPy powder after the polymerization reaction in Example 2;

[0042] Figure 19 These are XRD patterns of MnFe-PPy powders after polymerization in Examples 2-4 and Comparative Examples 6-8;

[0043] Figure 20 These are the XRD patterns of the catalysts prepared in Examples 2-4 and Comparative Examples 6 and 8;

[0044] Figure 21 This is a comparison graph of linear sweep voltammetry (LSV) curves for Example 2, Comparative Example 6, Comparative Example 8, and a commercial Pt / C catalyst. Detailed Implementation

[0045] This invention provides a method for preparing a Fe3C particle-coupled Fe / Mn heteronuclear dual single-atom catalyst. The catalyst obtained by this method has Fe3C particles and Fe / Mn heteronuclear dual single atoms highly dispersed on a nitrogen-doped carbon support (NC), exhibiting extremely high electrocatalytic activity, stability, and methanol resistance.

[0046] In some embodiments, the preparation method of Fe3C particle-coupled Fe / Mn heteronuclear dual single-atom catalyst includes the following steps:

[0047] The pyrrole monomer and MnFe2O4 template were evenly dispersed in an ethanol-water mixed solution. Then, the initiator HCl was added to carry out the polymerization reaction until the MnFe2O4 template was not completely consumed and Fe oxide residues remained. The product was then filtered, washed and dried to obtain MnFe-PPy containing Fe oxide residues.

[0048] MnFe-PPy containing residual Fe oxides is calcined under an inert atmosphere (e.g., nitrogen) to obtain nitrogen-doped carbon-supported Fe3C particles and Fe / Mn heteronuclear dual single-atom catalyst Fe3C@FeMn-NC. The preferred calcination temperature is 750–850℃ (e.g., 800℃), and the calcination time is 1–2 h (e.g., 1 h).

[0049] Furthermore, the size of the MnFe2O4 template is 100–300 nm, and the polymerization reaction time is 1–2 h.

[0050] Furthermore, the MnFe2O4 template preferably has a hollow spherical morphology, which can be obtained by the following preparation method: manganese chloride tetrahydrate and ferric chloride hexahydrate are dispersed in an appropriate amount of ethylene glycol, ammonium acetate is added to adjust the pH, and then polyethylene glycol 6000 is added and stirred evenly. After the mixed solution undergoes a solvothermal reaction, the resulting product is centrifuged, washed, and dried to obtain a hollow spherical MnFe2O4 template.

[0051] The present invention will now be described in more detail with reference to specific embodiments and comparative examples.

[0052] Example 1: Preparation of Fe3C@FeMn-NC catalyst

[0053] (1) First, dissolve 0.5g of manganese chloride tetrahydrate and 1g of ferric chloride hexahydrate in 60mL of ethylene glycol and stir until fully dissolved. Then, add 5g of ammonium acetate and 1.5g of polyethylene glycol 6000 to the solution and continue stirring for 1h. Next, subject the mixed solution to a solvothermal reaction at 200℃ for 22h. The product is then centrifuged, washed, and dried to obtain a hollow spherical MnFe2O4 template.

[0054] (2) First, disperse 100 mg of hollow spherical MnFe2O4 template in 60 mL of pure water and sonicate for 1 h to ensure uniform dispersion. Then, dissolve 1 mL of pyrrole monomer in 10 mL of ethanol and add it dropwise to the template solution. Mix thoroughly by stirring and sonicating for 30 min. Then, add 20 mL of 6 M HCl solution and perform polymerization for 2 h under sonication conditions while maintaining the reaction solution at room temperature. The resulting product is filtered, washed, and dried to obtain MnFe-PPy containing residual Fe oxide from the template.

[0055] (3) MnFe-PPy containing Fe oxide residue was placed in a tube furnace and calcined at 800℃ under N2 atmosphere for 1h to obtain catalyst Fe3C@FeMn-NC, in which Fe3C particles and Fe / Mn heteronuclear double single atoms are highly dispersed on nitrogen-doped carbon support NC.

[0056] Preparation of Comparative Example 1: FeMn-PPy(2h) Catalyst

[0057] The difference between this comparative example and Example 1 is that this comparative example did not undergo a final calcination treatment at 800°C under a N2 atmosphere for 1 hour.

[0058] (1) First, dissolve 0.5g of manganese chloride tetrahydrate and 1g of ferric chloride hexahydrate in 60mL of ethylene glycol and stir until fully dissolved. Then, add 5g of ammonium acetate and 1.5g of polyethylene glycol 6000 to the solution and continue stirring for 1h. Next, subject the mixed solution to a solvothermal reaction at 200℃ for 22h. The product is then centrifuged, washed, and dried to obtain a hollow spherical MnFe2O4 template.

[0059] (2) First, disperse 100 mg of hollow spherical MnFe2O4 template in 60 mL of pure water and sonicate for 1 h to ensure uniform dispersion. Then, dissolve 1 mL of pyrrole monomer in 10 mL of ethanol and add it dropwise to the template solution. Mix thoroughly by stirring and sonicating for 30 min. Then, add 20 mL of 6 M HCl solution and carry out the polymerization reaction for 2 h under sonication conditions while maintaining the reaction solution at room temperature. The obtained product is filtered, washed, and dried, and named MnFe-PPy(2h).

[0060] Preparation of Comparative 2FeMn-PPy(4h) Catalyst

[0061] The difference between this comparative example and Example 1 is that this comparative example did not undergo a final calcination treatment at 800°C under N2 atmosphere for 1 hour, and the polymerization time was 4 hours.

[0062] (1) First, dissolve 0.5g of manganese chloride tetrahydrate and 1g of ferric chloride hexahydrate in 60mL of ethylene glycol and stir until fully dissolved. Then, add 5g of ammonium acetate and 1.5g of polyethylene glycol 6000 to the solution and continue stirring for 1h. Next, allow the mixed solution to undergo a dendrothermal reaction at 200℃ for 22h. The product is then centrifuged, washed, and dried to obtain a hollow spherical MnFe2O4 template.

[0063] (2) First, disperse 100 mg of hollow spherical MnFe2O4 template in 60 mL of pure water and sonicate for 1 h to ensure uniform dispersion. Then, dissolve 1 mL of pyrrole monomer in 10 mL of ethanol and add it dropwise to the template solution. Mix thoroughly by stirring and sonicating for 30 min. Then, add 20 mL of 6 M HCl solution and carry out the polymerization reaction for 4 h under sonication conditions while maintaining the reaction solution at room temperature. The obtained product is filtered, washed, and dried, and named MnFe-PPy(4h).

[0064] Preparation of comparative 3FeMn-NC catalyst

[0065] The difference between this comparative example and Example 1 is that the polymerization reaction time in this comparative example is extended to 4 hours, so that the MnFe2O4 template is completely consumed and there is no Fe oxide residue.

[0066] (1) First, dissolve 0.5g of manganese chloride tetrahydrate and 1g of ferric chloride hexahydrate in 60mL of ethylene glycol and stir until fully dissolved. Then, add 5g of ammonium acetate and 1.5g of polyethylene glycol 6000 to the solution and continue stirring for 1h. Next, subject the mixed solution to a solvothermal reaction at 200℃ for 22h. The product is then centrifuged, washed, and dried to obtain a hollow spherical MnFe2O4 template.

[0067] (2) First, disperse 100 mg of hollow spherical MnFe2O4 template in 60 mL of pure water and sonicate for 1 h to ensure uniform dispersion. Then, dissolve 1 mL of pyrrole monomer in 10 mL of ethanol and add it dropwise to the template solution. Mix thoroughly by stirring and sonicating for 30 min. Next, add 20 mL of 6 M HCl solution and perform polymerization under sonication at room temperature for 4 h to completely consume the MnFe2O4 template. The resulting product is filtered, washed, and dried to obtain MnFe-PPy without Fe oxide residue.

[0068] (3) MnFe-PPy was placed in a tube furnace and calcined at 800℃ under N2 atmosphere for 1 hour to obtain the catalyst FeMn-NC, wherein FeMn-NC does not contain Fe3C particles.

[0069] Preparation of comparative 4H-Fe-NC catalyst

[0070] The difference between this comparative example and Example 1 is that the template used in this comparative example is a hollow Fe3O4 template.

[0071] (1) First, dissolve 1g of ferric chloride hexahydrate in 60mL of ethylene glycol and stir until fully dissolved. Then, add 5g of ammonium acetate and 1.5g of polyethylene glycol 6000 and continue stirring for 1h. Next, subject the mixed solution to a solvothermal reaction at 200℃ for 22h. The product is then centrifuged, washed, and dried to obtain a hollow Fe3O4 template.

[0072] (2) First, disperse 100 mg of hollow Fe3O4 template in 60 mL of pure water and sonicate for 1 h to ensure uniform dispersion. Then, dissolve 1 mL of pyrrole monomer in 10 mL of ethanol and add it dropwise to the template solution. Mix thoroughly by stirring and sonicating for 30 min. Then, add 20 mL of 6 M HCl solution and carry out the polymerization reaction for 4 h under sonication conditions while maintaining the reaction solution at room temperature. The resulting product is filtered, washed, and dried to obtain H-Fe-PPy.

[0073] (3) H-Fe-PPy was placed in a tube furnace and calcined at 800℃ under N2 atmosphere for 1 hour to obtain catalyst H-Fe-NC.

[0074] Preparation of Comparative 5Fe-NC Catalyst

[0075] The difference between this comparative example and Example 1 is that no template is used in the preparation process, but ferric chloride hexahydrate is used directly in the polymerization reaction.

[0076] (1) First, dissolve 2g of ferric chloride hexahydrate in 60mL of pure water and sonicate for 1h to ensure uniform dispersion. Then, dissolve 2mL of pyrrole monomer in 20mL of ethanol and add it dropwise to the template solution. Mix thoroughly by stirring and sonicating for 30min. Next, add 10mL of 6M HCl solution and carry out the polymerization reaction for 4h under sonication conditions while maintaining the reaction solution at room temperature. The resulting product is filtered, washed, and dried to obtain Fe-PPy.

[0077] (2) Fe-PPy was placed in a tube furnace and calcined at 800℃ under N2 atmosphere for 1 hour to obtain the catalyst Fe-NC.

[0078] Example 2: Preparation of P-Fe3C@FeMn-NC (1h) catalyst

[0079] (1) Disperse 2g of FeCl3·6H2O and 1g of MnCl2·4H2O in 60mL of pure water, and sonicate and stir to fully dissolve them to obtain solution I; then, dissolve 2.5g of NaOH in 10mL of pure water and add it dropwise to solution I to obtain a mixed solution; finally, transfer the mixed solution to a high-pressure reactor and hydrothermally react at 180℃ for 12h. The product is centrifuged, washed and vacuum dried to finally obtain P-MnFe2O4 template powder with a powder size in the range of 100nm to 300nm.

[0080] (2) First, disperse 100 mg of P-MnFe2O4 template powder in 60 mL of pure water and sonicate for 1 h to ensure uniform dispersion. Then, dissolve 1 mL of pyrrole monomer in 10 mL of ethanol and add it dropwise to the template solution. Mix thoroughly by stirring and sonicating for 30 min. Then, add 20 mL of 6 M HCl solution and polymerize for 1 h under sonication conditions while maintaining the reaction solution at room temperature. The resulting product is filtered, washed, and dried to obtain 1h-P-MnFe-PPy.

[0081] (3) Place 1h-P-MnFe-PPy in a tube furnace and calcine it at 800℃ under N2 atmosphere for 1h to obtain catalyst P-Fe3C@FeMn-NC(1h), wherein Fe3C particles and Fe / Mn single atoms are highly dispersed on nitrogen-doped carbon support NC.

[0082] Other embodiments and comparative examples

[0083] Examples 3-4 and Comparative Examples 6-8 are based on Example 2, with the polymerization reaction time adjusted while other conditions remain unchanged, as shown in Table 1 below.

[0084] Table 1. Polymerization reaction times of Examples 3-4 and Comparative Examples 6-8

[0085] Example 3 1.5h 1.5hP-MnFe-PPy <![CDATA[P-Fe3C@FeMn-N-C(1.5h)]]> Example 4 2h 2h-P-MnFe-PPy <![CDATA[P-Fe3C@FeMn-N-C(2h)]]> Comparative Example 6 0.5h 0.5hP-MnFe-PPy <![CDATA[P-Fe3C@FeMn-N-C(0.5h)]]> Comparative Example 7 3h 3h-P-MnFe-PPy <![CDATA[P-Fe3C@FeMn-N-C(3h)]]> Comparative Example 8 4h 4h-P-MnFe-PPy P-FeMn-NC(4h)

[0086] Morphology, dimensions and phase analysis of the examples and comparative examples

[0087] Figure 1 a is the X-ray diffraction (XRD) pattern of the hollow MnFe2O4 template in Example 1. Figure 1 b is its scanning electron microscope (SEM) image. (From...) Figure 1 As can be seen from a, the XRD pattern of the hollow MnFe2O4 template prepared by the hydrothermal method in Example 1 shows no other impurity peaks, indicating that it does not contain any other impurities, and the diffraction peak positions correspond to those of the standard MnFe2O4 PDF card. Figure 1 As can be seen from b, the MnFe2O4 template has a uniform spherical morphology and obvious hollow sphere characteristics, with a size in the range of 100nm to 300nm.

[0088] Figure 2 The XRD patterns of FeMn-PPy (2h) in Comparative Example 1 and FeMn-PPy (4h) in Comparative Example 2 are shown below. Figure 2 As shown in a, when the polymerization time is 2h, some of the remaining FeOOH and Fe3O4 phases in the MnFe2O4 template are not completely dissolved. However, when the polymerization time is extended to 4h (2b), all the diffraction peaks of the relevant phases of the template disappear, indicating that the template is completely dissolved and consumed and there is no Fe oxide residue.

[0089] Figure 3 ab are SEM and TEM images of the FeMn-PPy(2h) catalyst in Comparative Example 1. It can be seen that the prepared catalyst retains the morphological characteristics of the original hollow MnFe2O4 template. In addition, obvious metal species particles can be observed in the TEM image, with a lattice fringe spacing of 0.165 nm, corresponding to the (511) crystal plane of Fe3O4. This result is consistent with XRD.

[0090] Figure 4 The image shows the SEM image of the FeMn-PPy(4h) catalyst in Comparative Example 2. It can be seen that the morphology of the FeMn-PPy(4h) catalyst is similar to that of the FeMn-PPy(2h) catalyst, both being hollow microspheres with uniform size. The difference is that no metal oxide particles were observed on the FeMn-PPy(4h) catalyst, indicating that the MnFe2O4 template was completely consumed.

[0091] Figure 5The image shows the XRD pattern of the Fe3C@FeMn-NC catalyst in Example 1. As can be seen from the image, apart from the obvious diffraction peaks corresponding to the (002) crystal plane of the graphite phase, the other diffraction peaks of the Fe3C@FeMn-NC catalyst correspond well with the standard PDF card of Fe3C. This indicates that during the calcination process, the original residual Fe oxide phase was transformed into the Fe3C phase.

[0092] Figure 6 ac are SEM and TEM images of the Fe3C@FeMn-NC catalyst from Example 1. Figure 6 As can be seen from this, after calcination, the Fe3C@FeMn-NC catalyst still retains the morphological characteristics of the original template. The difference from the FeMn-PPy (2h) catalyst before calcination is that the catalyst surface is rougher after calcination. This may be due to the shrinkage of the carbon skeleton caused by further graphitization of carbon during the calcination process. Figure 6 As shown in bc, the Fe3C@FeMn-NC catalyst exhibits obvious hollow microsphere characteristics and contains Fe3C particles. Notably, there is a distinct carbon coating layer on the surface of the Fe3C particles, with a thickness of approximately 10-15 carbon atom layers. The presence of this carbon atom coating layer can prevent excessive etching of the internal active species by the electrolyte, thereby improving the catalytic stability of the catalyst.

[0093] Figure 7 ab are SEM and TEM images of the FeMn-NC catalyst in Comparative Example 3. As shown in the images, similar to the Fe3C@FeMn-NC catalyst, the FeMn-NC catalyst consists of uniformly sized hollow carbon microspheres with numerous mesoporous structures on its surface. The presence of these mesopores facilitates reactant migration. Furthermore, no metal carbide particles were observed in the TEM images, confirming that the FeMn-NC catalyst does not contain other metal-related phases, consistent with the XRD characterization results.

[0094] Figure 8 ab are the elemental mapping images and aberration-corrected high-angle annular dark-field scanning TEM (HAADS) images of the Fe3C@FeMn-NC catalyst prepared in Example 1. Figure 8 a indicates that N, Mn, and Fe elements are uniformly distributed on the carbon microspheres. Figure 8In section b, the yellow circles indicate the presence of a large number of atomically dispersed Fe / Mn atoms in the Fe3C@FeMn-NC catalyst, and the red boxes clearly show the presence of numerous adjacent Fe-Mn diatomic pairs. Furthermore, the presence of Fe3C particles can be observed near the single atoms. This indicates that in the prepared catalyst, a large number of Fe and Mn atoms are uniformly dispersed around the Fe3C particles in the form of double single-atom pairs, coupling with the Fe3C particles.

[0095] Figure 9 Figure ac shows the SEM and XRD patterns of the hollow Fe3O4 template prepared in Comparative Example 4. As can be seen from the figure, the Fe3O4 template is a hollow sphere with a diameter of about 300 nm. The corresponding diffraction peaks in the XRD pattern correspond to the Fe3O4 standard PDF card, indicating that the hollow spherical Fe3O4 template was successfully prepared.

[0096] Figure 10 Figure ac shows the SEM and XRD patterns of the H-Fe-NC catalyst prepared in Comparative Example 4. As can be seen from the figure, the H-Fe-NC catalyst retains the hollow microsphere morphology of the Fe3O4 template. In addition, the XRD results show that there is no metal oxide or carbide phase in the H-Fe-NC catalyst. In addition to the diffraction peak corresponding to the (002) crystal plane of graphite, a diffraction peak corresponding to the (101) crystal plane of graphite also appears, which indicates that the calcination treatment further increases the graphitization degree of the catalyst.

[0097] Figure 11 Figure ac shows the SEM and XRD patterns of the Fe-NC catalyst prepared in Comparative Example 5. As can be seen from the figure, the Fe-NC catalyst prepared by direct polymerization initiated by FeCl3 no longer has a hollow microsphere morphology, but is composed of disordered agglomeration of smaller particles. In addition, the XRD characterization results show that the Fe-NC catalyst mainly contains a carbon graphite phase, without the presence of other metal-related phases.

[0098] Figure 12 The nitrogen adsorption-desorption curves and pore size distribution diagrams of the Fe3C@FeMn-NC catalyst prepared in Example 1, the FeMn-NC catalyst prepared in Comparative Example 3, and the Fe-NC catalyst prepared in Comparative Example 5 are shown. The results indicate that the Fe3C@FeMn-NC catalyst and the FeMn-NC catalyst contain a large number of mesoporous structures, which is consistent with the SEM characterization results. Furthermore, the Fe3C@FeMn-NC and FeMn-NC catalysts, which exhibit hollow microsphere morphology, have a larger specific surface area (95.1013 m²). 2 g -1 ), is an Fe-NC catalyst (29.2020m 2 g -1This is approximately three times that of the electrolyte. This indicates that the catalyst prepared by the template method can have a larger contact area with the electrolyte, which is beneficial to the catalytic reaction.

[0099] Figure 18 The image shows a TEM image of the MnFe-PPy powder after the polymerization reaction in Example 2. As can be seen from the image, the prepared MnFe-PPy powder has a spherical particle morphology with a size of about 50 nm, and does not replicate the morphology and size of the MnFe2O4 template.

[0100] Figure 19 These are XRD patterns of MnFe-PPy powders after polymerization in Examples 2-4 and Comparative Examples 6-8. Figure 19 It can be seen that when the polymerization time is 0.5 h, the phase composition is still mainly MnFe2O4. This is because the polymerization time is too short, and most of the template oxide has not been dissolved. This also leads to the appearance of MnFe2O4 phase in addition to Fe3C phase after subsequent calcination / heat treatment. When the polymerization reaction time is 1 h, 1.5 h, 2 h, and 3 h, the XRD pattern of MnFe-PPy powder shows the phase of FeOOH (PDF#81-0464). As the polymerization time increases, the diffraction peak corresponding to MnFe2O4 phase gradually weakens. When the polymerization condition is 1.5 h, the MnFe2O4 phase itself basically disappears. As the polymerization time is extended to 4 h, all the Fe oxide in the template is dissolved, and only the diffraction peak corresponding to the (002) crystal plane of the graphite phase is observed in the XRD pattern.

[0101] Figure 20 These are the XRD patterns of the catalysts prepared in Examples 2-4 and Comparative Examples 6 and 8. Figure 20 As shown, the phase compositions of the 1h-Fe3C / MnFe-NC, 1.5h-Fe3C / MnFe-NC, and 2h-Fe3C / MnFe-NC catalysts are basically the same, with obvious diffraction peaks corresponding to the Fe3C phase in addition to the diffraction peaks corresponding to carbon. These results indicate that the MnFe2O4 template is gradually dissolved and transformed into FeOOH under the action of hydrochloric acid, and then the sample produces the Fe3C phase after heat treatment. As the polymerization time increases from 1h to 2h, the diffraction peaks corresponding to Fe3C in the final heat-treated product gradually weaken. This is because the remaining amount of oxides in the template gradually decreases with increasing polymerization time, leading to a gradual decrease in the Fe3C content in the final product.

[0102] Catalytic performance testing

[0103] Figure 13These are the cyclic voltammetry (CV) curves of the catalysts in Examples 1 and Comparative Examples 3-5 under O2 saturation and N2 saturation conditions. The CV curve corresponding to the Fe3C@FeMn-NC catalyst shows a clear reduction peak at 0.85 V. Although the FeMn-NC and Fe-NC catalysts also have clear reduction peaks, their peak positions show varying degrees of negative shift, indicating that the Fe3C@FeMn-NC catalyst has better ORR catalytic ability.

[0104] Figure 14 This is a comparison of the linear sweep voltammetry (LSV) curves of Examples 1, Comparative Examples 3-5, and commercial Pt / C catalysts. The Fe3C@FeMn-NC catalyst exhibits a higher limiting current density (j). L =4.95mA cm -2 ), initial potential (E) onset =1.032V) and half-wave potential (E 1 / 2 =0.870V), which is higher than that of FeMn-NC catalyst (j L =4.82mA cm -2 E onset =0.925V, E 1 / 2 =0.752V), Fe-NC catalyst (j L =3.27mAcm -2 E onset =0.918V, E 1 / 2 =0.713V) and commercial Pt / C catalysts (j L =4.36mAcm -2 E onset =0.986V, E 1 / 2 =0.845V). In particular, as can be seen from the test comparison between Example 1 and Comparative Example 3, the introduction of Fe3C particles further significantly improved the ORR catalytic activity of the Fe / Mn heteronuclear dual single-atom catalyst.

[0105] Figure 15 This is a comparison of the chronoamperometry (it) curves of the Fe3C@FeMn-NC catalyst prepared in Example 1 and the commercial Pt / C catalyst to withstand the methanol cross-effect. The results show that after adding methanol to the electrolyte, the it curve corresponding to the Fe3C@FeMn-NC catalyst only fluctuates slightly, while the commercial Pt / C catalyst shows a larger response. This indicates that the Fe3C@FeMn-NC catalyst has good methanol tolerance.

[0106] Figure 16This is a comparison of the chronoamperometry (it) curves of the Fe3C@FeMn-NC catalyst prepared in Example 1 and the commercial Pt / C catalyst. After a stability test of 50,000 s, the activity of the Fe3C@FeMn-NC catalyst decreased to 89.3% of its initial activity, while the current of the commercial Pt / C catalyst had already decreased to 83.3% of its initial current after 30,000 s. This indicates that the Fe3C@FeMn-NC catalyst has better stability.

[0107] Figure 17A and 17B These are electrochemical double-layer capacitance (Cdl) test graphs for Example 1, Comparative Example 3, Comparative Example 5, and the commercial Pt / C catalyst. The Fe3C@FeMn-NC catalyst exhibits the largest Cdl value. dl Value (30.16mF / cm) 2 This is far higher than that of the FeMn-NC catalyst (4.72 mF / cm). 2 Fe-NC catalyst (0.6 mF / cm) 2 ) and commercial Pt / C catalyst (8.74 mF / cm) 2 This indicates that the high ORR catalytic activity of the Fe3C@FeMn-NC catalyst is largely related to its large electrochemical active surface area, which exposes more active sites.

[0108] Figure 21 This is a comparison of the linear sweep voltammetry (LSV) curves of Example 2, Comparative Example 6, Comparative Example 8, and the commercial Pt / C catalyst. The test results are summarized in Table 2 below. Among them, the P-Fe3C@MnFe-NC (1h) catalyst exhibits the highest limiting current density (j L =6.04mA cm -2 ), initial potential (E) onset =0.93V), the ORR electrocatalytic performance is superior to that of the P-Fe3C@MnFe-NC (0.5h) catalyst (j L =4.50mA cm -2 E onset =0.922V), P-MnFe-NC(4h) catalyst (j L =4.09mA cm -2 E onset =0.920V) and commercial Pt / C catalyst (j L =4.36mA cm -2 E onset =0.986V).

[0109] Table 2. LSV test results of Example 2, Comparative Example 6, Comparative Example 8, and commercial Pt / C catalyst.

[0110]

[0111] Therefore, the introduction of Fe3C particles significantly improves the ORR catalytic activity of the catalyst. Appropriate polymerization time (the amount of residual Fe oxide in the template) is a key factor determining the ORR performance of the catalyst; a polymerization time of 1–2 hours is optimal. Excessive polymerization time results in no remaining Fe oxide in the template, leading to the final product containing no Fe3C particles. Conversely, insufficient polymerization time results in excessive residual template, reducing the carbon content, conductivity, and number of active sites in the final catalyst, thus degrading its performance.

[0112] The commercial Pt / C catalyst used as a comparative object in this invention was purchased from Johnson Matthey.

[0113] Although the present invention has been described above through specific embodiments, it should be understood that any equivalent improvements made by those skilled in the art in accordance with the present invention without departing from the scope of the present invention should be covered by the protection scope of the present invention.

Claims

1. A method for preparing nitrogen-doped carbon-supported Fe3C particles and Fe / Mn heteronuclear dual single-atom catalysts, characterized in that... Includes the following steps: (1) The pyrrole monomer and MnFe2O4 template were evenly dispersed in an ethanol-water mixed solution. The initiator HCl was added to carry out the polymerization reaction until the MnFe2O4 template was not completely consumed and Fe oxide residues remained. Then the product was filtered, washed and dried to obtain MnFe-PPy containing Fe oxide residues. (2) MnFe-PPy containing Fe oxide residue was calcined under an inert atmosphere to obtain nitrogen-doped carbon-supported Fe3C particles and Fe / Mn heteronuclear dual single-atom catalyst Fe3C@FeMn-NC, wherein the Fe3C particles and Fe / Mn heteronuclear dual single atoms are highly dispersed on the nitrogen-doped carbon support NC; The MnFe2O4 template has a hollow spherical structure, and the polymerization reaction time is 1~2h; the calcination temperature in step (2) is 750~850℃, and the calcination time is 1~2h.

2. The preparation method according to claim 1, characterized in that: The Fe3C particles have a carbon atom coating layer on their surface.

3. The preparation method according to claim 1, characterized in that: In step (1), the MnFe2O4 template is first dispersed evenly in water, then the pyrrole monomer is dissolved in ethanol and added to the MnFe2O4 template dispersion. After mixing evenly, HCl solution is added and polymerization reaction is carried out under ultrasonic conditions.

4. The preparation method according to claim 1, characterized in that: The inert atmosphere is nitrogen.

5. The preparation method according to claim 1, characterized in that: The size of the MnFe2O4 template is 100nm~300nm.

6. The preparation method according to claim 1, characterized in that, The MnFe2O4 template was obtained by the following method: Manganese chloride tetrahydrate and ferric chloride hexahydrate were dispersed in ethylene glycol, ammonium acetate was added to adjust the pH, polyethylene glycol 6000 was added and stirred until homogeneous, and the mixed solution was subjected to a solvothermal reaction. The resulting product was centrifuged, washed and dried to obtain a hollow MnFe2O4 template.

7. The preparation method according to claim 6, characterized in that, The preparation of the MnFe2O4 template includes: First, dissolve 0.5 parts by weight of manganese chloride tetrahydrate and 1 part by weight of ferric chloride hexahydrate in 60 parts by volume of ethylene glycol and stir until fully dissolved. Then, add 5 parts by weight of ammonium acetate and 1.5 parts by weight of polyethylene glycol 6000 to the solution. The relationship between the parts by weight and the parts by volume is g / mL.

8. The preparation method according to claim 6, characterized in that: The solvothermal reaction temperature of the mixed solution is 200℃, and the holding time is 22 h.

9. The application of the Fe3C@FeMn-NC catalyst obtained by the preparation method according to any one of claims 1-8 in the ORR reaction.

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