Iron-nitrogen-carbon catalyst with high iron loading and preparation method and application thereof

Abstract

The application relates to the technical field of ORR catalyst preparation, in particular to an iron-nitrogen-carbon catalyst with high iron loading and a preparation method and application thereof. An efficient template-assisted strategy is provided for synthesizing the iron-nitrogen-carbon catalyst with high iron loading, so as to enhance the ORR performance. The iron-nitrogen-carbon catalyst with high iron loading prepared under a specific ratio of ferrous salt and template has atomically dispersed FeN4C active sites, and the iron loading is 5.45 wt%, which has been confirmed to have excellent ORR performance.

Description

Technical Field

[0001] This invention relates to the field of ORR catalyst preparation technology, specifically to a high iron-loaded iron-nitrogen-carbon catalyst, its preparation method, and its application. Background Technology

[0002] The oxygen reduction reaction (ORR) is a crucial electrochemical process in energy conversion devices such as fuel cells and zinc-air batteries. However, 4e... - The slow ORR kinetics during the transfer process usually require the use of electrocatalysts.

[0003] Atomically dispersed iron-nitrogen-carbon (Fe-NC) catalysts can accelerate the ORR process due to their high efficiency in catalyzing ORR, good resistance to poisoning, and low cost. However, the iron content in atomically dispersed Fe-NC catalysts is typically below 2 wt%; this is because iron atoms in Fe-NC catalysts tend to aggregate into clusters or nanoparticles during high-temperature preparation. Therefore, maintaining both atomic dispersion of iron and dense Fe-Nx sites in Fe-NC catalysts remains a challenging goal.

[0004] Zeolitic imidazolate frameworks (ZIFs) are considered excellent precursors for preparing atomically dispersed Fe-NC catalysts due to their high specific surface area and abundant pore structure. However, without template assistance, ZIF-derived Fe-NC catalysts typically exhibit a predominantly microporous structure, which is unfavorable for the transport of oxygen intermediates; furthermore, the Fe content in their atomically dispersed forms is mostly below 2 wt%. If the Fe loading in the ZIF precursor is high, Fe atom aggregation easily occurs during high-temperature pyrolysis, which is a key reason why it is difficult to prepare Fe-NC catalysts with high-density Fe-Nx active sites.

[0005] While the problems of excessive micropore ratio and Fe atom aggregation can be partially solved under template-assisted conditions, the template removal process often faces numerous challenges, such as the need to use environmentally harmful chemical reagents and the complexity and time-consuming nature of the process. Furthermore, obtaining a hierarchical porous Fe-NC catalyst while simultaneously ensuring sufficient iron loading to achieve a high density of Fe-Nx active sites presents certain difficulties.

[0006] Therefore, designing a green and efficient template strategy to prepare Fe-NC catalysts with high-density Fe-Nx active sites, hierarchical pore structure, and improved oxygen reduction reaction performance is of great significance. Summary of the Invention

[0007] The present invention aims to overcome the shortcomings of existing technologies that cannot simultaneously achieve a hierarchical porous structure and a high density of Fe-Nx active sites, and provides a high-iron-loaded iron-nitrogen-carbon catalyst, its preparation method, and its application to overcome the above-mentioned shortcomings.

[0008] To achieve the above objectives, the present invention provides the following technical solution:

[0009] This invention provides a method for preparing a high-iron-loaded iron-nitrogen-carbon catalyst, comprising:

[0010] S1. Zinc salt and ferrous salt are dispersed in methanol to obtain mixture A; 2-methylimidazolium and 1,2-phthalonitrile are dispersed in methanol and heated to obtain mixture B; the molar ratio between iron and 1,2-phthalonitrile in the ferrous salt is 3:(10~20).

[0011] S2. Mixture A and mixture B are stirred and mixed to obtain precursor powder. The precursor powder is calcined at 600~900℃ under an inert atmosphere to obtain a high iron-loaded iron-nitrogen-carbon catalyst.

[0012] This application proposes an efficient template (1,2-phthalonitrile)-assisted strategy for the synthesis of high-iron-loaded iron-nitrogen-carbon catalysts to enhance ORR performance. The high-iron-loaded iron-nitrogen-carbon catalysts prepared at specific ferrous salt and template ratios exhibit atomically dispersed FeN4C active sites with an iron loading of 5.45 wt%.

[0013] Extensive characterization results indicate that 1,2-phthalonitrile contains abundant nitrogen sources, which not only effectively improve the coordination environment of dense iron sites but also promote the formation of hierarchical porous structures in the catalyst. Furthermore, not all ratios of ferrous salt to template can yield catalysts simultaneously exhibiting hierarchical porous structures and single-atom Fe dispersion. This is because the ratio of ferrous salt to template plays a decisive role in the dispersion of single Fe atoms, while the hierarchical porous structure provides a nitrogen-doped carbon framework matrix with a large specific surface area, thus aiding in the dispersion of atoms. The combined effect of both results in the final dispersion of single Fe atoms. In other words, an appropriate amount of Fe must be introduced, along with a suitable amount of template agent, to control the dispersion of single Fe atoms.

[0014] On the one hand, iron atoms dispersed within the nitrogen-doped carbon framework can increase the iron loading while preventing agglomeration. On the other hand, iron atoms form a large number of FeN4C active sites with the C and N atoms of the nitrogen-doped carbon framework. Ultimately, the resulting catalyst not only possesses a hierarchical porous structure, enabling Fe atom dispersion, but also exhibits a high density of FeN4C active sites.

[0015] The catalyst prepared according to the method of this application exhibits a half-wave potential of 0.9 V (relative to the reversible hydrogen electrode) in 0.1 M KOH solution and 220 mW·cm⁻¹ in a zinc-air battery. -2 The maximum power density exceeds that of commercial Pt / C catalysts (half-wave potential 0.88 V vs RHE, maximum power density 194 mW·cm⁻¹). -2 ).

[0016] Preferably, the molar ratio between iron and 1,2-phthalonitrile in the ferrous salt is 3:(13~18).

[0017] Preferably, the molar ratio between iron and 1,2-phthalonitrile in the ferrous salt is 3:(14~16).

[0018] Preferably, in S1, the molar ratio between zinc in the zinc salt, iron in the ferrous salt, and 2-methylimidazole is 5:0.3:(40~42).

[0019] Preferably, in step S1, the heating conditions are: oil bath at 30~40℃.

[0020] Preferably, in step S2, the stirring and mixing time is 6-8 h, and after stirring and mixing, the precursor powder is obtained by vacuum drying at 60-80℃ for 12-24 h.

[0021] Preferably, in step S2, the inert atmosphere is nitrogen and / or argon.

[0022] Preferably, in step S2, the calcination temperature is 800~900℃; and / or the calcination time is 2~3 h.

[0023] Calcination at 800~900℃ can form more atomically dispersed Fe active sites.

[0024] Preferably, the calcination method is as follows: heating to 180~200℃ and holding for 2~3 h, then heating to 600~900℃ and holding for 2~3 h; and / or, the heating rate of the calcination is 2~10℃ / min.

[0025] Preferably, the temperature is increased to 180-200°C at a rate of 8-10°C / min, and then further increased to 600-900°C at a rate of 3-5°C / min.

[0026] The present invention provides a high iron-nitrogen-carbon catalyst, wherein the high iron-nitrogen-carbon catalyst comprises a pleated spherical nitrogen-doped carbon framework and iron atoms dispersed therein, wherein the iron atoms and the nitrogen-doped carbon framework form FeN4C active sites; wherein the iron loading in the high iron-nitrogen-carbon catalyst is ≥5.0 wt%.

[0027] This application utilizes ZIF8 as a framework to directly generate an iron-nitrogen-carbon catalyst. During the synthesis process, nitrogen and carbon mainly exist as nitrogen-doped carbon frameworks, with Fe atoms, the main active component, dispersed within the nitrogen-doped carbon framework. On one hand, the large specific surface area of ​​the nitrogen-doped carbon framework provides a more suitable matrix for Fe atom dispersion, avoiding Fe clusters or agglomeration. On the other hand, Fe atoms successfully achieve single-atom dispersion with the help of the nitrogen-doped carbon framework. After anchoring to the sites on the nitrogen-doped carbon framework, they can form FeN4C active sites with the C and N atoms of the nitrogen-doped carbon framework. The formation of a large number of active sites can reversely fix the position of Fe, allowing a large number of Fe atoms to be firmly loaded on the nitrogen-doped carbon framework, thereby increasing the iron loading.

[0028] Preferably, the high-iron-loaded iron-nitrogen-carbon catalyst has a hierarchical porous structure of micropores and mesopores, wherein the hierarchical porous structure is used to disperse iron atoms; and / or, the specific surface area of ​​the high-iron-loaded iron-nitrogen-carbon catalyst is 650~750 m². 2 / g.

[0029] This invention provides a zinc-air battery that uses a high-iron-loaded iron-nitrogen-carbon catalyst as the cathode catalyst material.

[0030] Therefore, the present invention has the following beneficial effects:

[0031] (1) The present invention uses 1,2-phthalonitrile as a template to control the acquisition of an iron-nitrogen-carbon catalyst with a hierarchical porous structure, and obtains an iron-nitrogen-carbon catalyst with a high iron loading that has both a hierarchical porous structure and a high density of FeN4C active sites by controlling the amount of ferrous salt and template.

[0032] (2) The iron-nitrogen-carbon catalyst of the present invention can reverse fix the position of Fe by forming a large number of active sites, so that a large number of Fe atoms can be firmly loaded in the hierarchical porous structure of nitrogen-doped carbon skeleton, thereby increasing the iron loading and obtaining an iron-nitrogen-carbon catalyst with high iron loading.

[0033] (3) The iron-nitrogen-carbon catalyst with high iron loading of the present invention has better performance than commercial Pt / C catalyst and has broad application prospects. Attached Figure Description

[0034] Figure 1 These are morphological and structural characterization diagrams of the catalytic materials obtained in Example 1 and Comparative Example 1, where a represents Fe-N / C from Example 1. DCB0.2 SEM images, b and c are Fe-N / C from Example 1. DCB0.2 TEM spectrum, d is Fe-N / C from Example 1 DCB0.2 HRTEM spectrum, e is Fe-N / C from Example 1 DCB0.2SAED spectrum, f is Fe-N / C of Example 1 DCB0.2 Aberration-corrected electron micrograph, g is Fe-N / C from Example 1. DCB0.2 Distribution diagram of Fe, N, and C elements.

[0035] Figure 2 The diagram shows the structure of Example 1, where a is the XRD pattern, b is the nitrogen adsorption-desorption isotherm curve, and c is the pore size distribution curve.

[0036] Figure 3 Comparative Example 1 (Fe-N / C) DCB0 The structural characterization diagrams are shown, where a is the XRD pattern, b is the nitrogen adsorption-desorption isotherm curve, and c is the pore size distribution curve.

[0037] Figure 4 The diagram shows the structural characterization of Comparative Example 4, where a is the XRD pattern, b is the pore size distribution curve, and c is the nitrogen adsorption-desorption isotherm curve.

[0038] Figure 5 The diagram shows the structural characterization of Comparative Example 5, where a is the XRD pattern, b is the nitrogen adsorption-desorption isotherm curve, and c is the pore size distribution curve.

[0039] Figure 6 The diagram shows the structural characterization of Comparative Example 6, where a is the XRD pattern, b is the nitrogen adsorption-desorption isotherm curve, and c is the pore size distribution curve.

[0040] Figure 7 The diagram shows the structural characterization of Comparative Example 7, where a is the XRD pattern, b is the nitrogen adsorption-desorption isotherm curve, and c is the pore size distribution curve.

[0041] Figure 8 The diagram shows the structural characterization of Comparative Example 8, where a is the XRD pattern, b is the nitrogen adsorption-desorption isotherm curve, and c is the pore size distribution curve.

[0042] Figure 9 The diagram shows the structural characterization of Comparative Example 9, where a is the XRD pattern, b is the nitrogen adsorption-desorption isotherm curve, and c is the pore size distribution curve.

[0043] Figure 10 The diagram shows the structural characterization of Comparative Example 10, where a is the XRD pattern, b is the nitrogen adsorption-desorption isotherm curve, and c is the pore size distribution curve.

[0044] Figure 11 The figures show the structural characterization of catalysts obtained at different calcination temperatures, where a~e correspond to SEM images at different calcination temperatures, f is the XRD pattern at different calcination temperatures, and g is a schematic diagram of structural evolution.

[0045] Figure 12 The diagram shows a comparison of the structural characterization of catalysts obtained at different calcination temperatures, where a is the nitrogen adsorption-desorption isotherm curve and b is the pore size distribution curve.

[0046] Figure 13 The images show the microstructure characterization of the catalytic materials obtained in Example 1 and Comparative Example 1, where a is the nitrogen adsorption-desorption isotherm, b is the pore size distribution curve, c is the XPS full spectrum, d and e are the high-resolution XPS N 1s spectrum and the content of different N dopants, respectively, and f is the Raman spectrum.

[0047] Figure 14 The microstructure characterization diagrams are shown for the catalytic materials obtained in Example 1 and Comparative Example 1, where a represents Fe-N / C. DCB0.2 And the Fe K-edge XANES spectra of reference materials (Fe foil, Fe2O3, FePc phthalocyanine), b is Fe-N / C DCB0.2 EXAFS spectra of Fe K-edge of reference materials (Fe foil, Fe2O3, FePc phthalocyanine), where c represents Fe-N / C. DCB0.2 Fe K-edge EXAFS wavelet transform spectra of reference materials (Fe foil, Fe2O3, FePc phthalocyanine).

[0048] Figure 15 Fe-N / C DCB0.2 High-resolution XPS Fe 2p spectrum.

[0049] Figure 16 The graphs show the Fe content (determined by ICP-MS) of Example 1 and Comparative Example 1.

[0050] Figure 17 Fe-N / C DCB0.2 Electrochemical performance graphs of Fe-N / C and commercial Pt / C catalysts are shown, where a is the CV curve, b is the ORR polarization curve, and c is the E curve. onset With E 1 / 2 Numerical comparison chart, d represents Fe-N / C DCB0.2 Compared with the E-C catalysts that have been reported to be excellent 1 / 2 And the comparison of Fe content (Table 4).

[0051] Figure 18 Fe-N / C DCB0.2 Electrochemical performance graphs of Fe-N / C and commercial Pt / C catalysts are shown, where a is the Tafel curve, b is the number of electrons transferred and H2O2 yield, c is the chronocurrent response stability test, and d is the chronocurrent response methanol resistance test (1 mL of methanol was added at approximately 200 seconds).

[0052] Figure 19Fe-N / C prepared for different amounts of 1,2-phthalonitrile DCB ORR polarization curves of a series of catalysts.

[0053] Figure 20 Fe-N / C DCB0.2 Electrochemical performance graph of the catalyst, where a represents Fe-N / C DCB0.2 ORR polarization curves of the catalyst at different rotational speeds; b represents Fe-N / C. DCB0.2 KL diagrams of catalysts at different potentials.

[0054] Figure 21 The diagram shows the performance of a zinc-air battery, where a is a schematic diagram of the zinc-air battery (ZAB) structure, and b is a Fe-N / C battery. DCB0.2 The galvanostatic discharge curves of Fe-N / C based zinc-air batteries at different current densities, where c represents Fe-N / C. DCB0.2 Polarization curves and power density curves of Fe-N / C based zinc-air batteries. DCB0.2 Open-circuit voltage curves of Fe-N / C based zinc-air batteries and Pt / C based zinc-air batteries, where e represents the open-circuit voltage curve of Fe-N / C based zinc-air batteries. DCB0.2 The specific capacity of a zinc-air battery assembled with Pt / C, normalized to the mass of zinc (Zn) consumed, f is the specific capacity of the battery using Fe-N / C. DCB0.2 A physical demonstration image of ZAB powering and charging a smartphone. Detailed Implementation

[0055] The present invention will be further described below with reference to specific embodiments. Those skilled in the art will be able to implement the present invention based on these descriptions. Furthermore, the embodiments of the present invention described below are generally only some, not all, of the embodiments of the present invention. Therefore, all other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort should fall within the scope of protection of the present invention.

[0056] In this section, FeSO4·7H2O, 2-methylimidazole (AR 98%), methanol (AR 99.5%), and 1,2-phthalonitrile (AR 98%) were purchased from Shanghai Aladdin Biochemical Technology Co., Ltd.; Zn(NO3)2·6H2O was purchased from Sinopharm Chemical Reagent Co., Ltd.; Pt / C catalyst (20%) was purchased from Suzhou Sinero Technology Co., Ltd.; and Nafion D520 (5%) was purchased from DuPont China Holdings Ltd. All reagents used in this section were not further purified.

[0057]

Example

[0058] Example 1

[0059] Si·Zn(NO3)2·6H2O (1.5024 g, 5 mmol) and FeSO4·7H2O (0.0828 g, 0.3 mmol) were dispersed in 60 mL of methanol to obtain mixture A. 2-Methylimidazole (3.3510 g, 40.8 mmol) and 1,2-phthalonitrile (0.20 g, 1.56 mmol) were dispersed in 60 mL of methanol, and then stirred and mixed in an oil bath at 30 °C to obtain mixture B.

[0060] S2. Add mixture A to mixture B and stir for 6 h. Centrifuge to collect the reddish-yellow precipitate, then wash three times with methanol to remove impurities. Dry in a vacuum drying oven at 80℃ overnight to obtain Fe-ZIF8 / DCB0.2 Precursor powder.

[0061] S3. Under nitrogen atmosphere, Fe-ZIF8 / DCB0.2 The precursor powder was pyrolyzed in a tube furnace. The temperature was first increased to 200℃ at a rate of 10℃ / min and held for 2 h, then increased further to 900℃ at a rate of 5℃ / min and held for 2 h. Calcination yielded Fe-N / C. DCB0.2 -900 (Fe-N / C) DCB0.2 )catalyst.

[0062] Example 2

[0063] This embodiment is basically the same as Embodiment 1, except that in S3, the temperature is increased to 800℃ at a rate of 5℃ / min and held for 2 hours. Calcination yields Fe-N / C. DCB0.2 -800 catalyst.

[0064] Example 3

[0065] This embodiment is basically the same as Example 1, except that in S3, the temperature is increased to 600℃ at a rate of 5℃ / min and held for 2 hours. Calcination yields Fe-N / C. DCB0.2 -600 catalyst.

[0066] Comparative Example 1

[0067] This comparative example is basically the same as Example 1, except that 1,2-phthalonitrile is not added in S1. A Fe-N / C catalyst is obtained.

[0068] Comparative Example 2

[0069] This comparative example is basically the same as Example 1, except that in S3, the temperature was increased to 400℃ at a rate of 5℃ / min and held for 2 hours. Calcination yielded Fe-N / C. DCB0.2 -400 catalyst.

[0070] Comparative Example 3

[0071] This comparative example is basically the same as Example 1, except that in S3, the temperature was increased to 1000℃ at a rate of 5℃ / min and held for 2 hours. Calcination yielded Fe-N / C. DCB0.2 -1000 catalyst.

[0072] Comparative Example 4 (CN119920918B Comparative Example 2)

[0073] Si,Zn(NO3)2·6H2O (1.5024 g, 5 mmol) and FeSO4·7H2O (0.0561 g, 0.2 mmol) were dispersed in 30 mL of methanol to obtain mixture A. 2-Methylimidazole (3.3510 g, 40.8 mmol) and 1,2-phthalonitrile (0.10 g, 0.78 mmol) were dispersed in 30 mL of methanol, and then stirred in an oil bath at 60 °C to obtain mixture B.

[0074] S2. Add mixture A to mixture B and stir for 6 h. Centrifuge to collect the reddish-yellow precipitate, then wash three times with methanol to remove impurities. Dry in a vacuum drying oven at 80℃ for 12 h to obtain Fe-ZIF8 / DCB0.1 Precursor powder.

[0075] S3. Under nitrogen atmosphere, Fe-ZIF8 / DCB0.1 The precursor powder was pyrolyzed in a tube furnace. The temperature was first increased to 200℃ at a rate of 10℃ / min and held for 2 h, then increased further to 900℃ at a rate of 5℃ / min and held for 2 h. Calcination yielded Fe-N / C. DCB0.1-60 catalyst.

[0076] Comparative Example 5

[0077] This comparative example is basically the same as Comparative Example 4, except that in S1, the oil bath temperature is 30℃. Calcination yields Fe-N / C. DCB0.1-30 catalyst.

[0078] Comparative Example 6

[0079] This comparative example is essentially the same as Example 1, except that in S1, the amount of 1,2-phthalonitrile used is 0.10 g (0.78 mmol). Calcination yields Fe-N / C DCB0.1 catalyst.

[0080] Comparative Example 7

[0081] This comparative example is essentially the same as Example 1, except that in S1, the amount of 1,2-phthalonitrile used is 0.30 g (2.34 mmol). Calcination yields Fe-N / C DCB0.3 catalyst.

[0082] Comparative Example 8

[0083] This comparative example is essentially the same as Example 1, except that in S1, the amount of 1,2-phthalonitrile used is 0.60 g (4.68 mmol). Calcination yields Fe-N / C DCB0.6 catalyst.

[0084] Comparative Example 9

[0085] This comparative example is basically the same as Example 1, except that in S1, the amount of FeSO4·7H2O used is 0.1095 g (0.4 mmol). Calcination yields 0.1 Fe-N / C DCB0.2 catalyst.

[0086] Comparative Example 10

[0087] This comparative example is essentially the same as Example 1, except that in S1, the amount of FeSO4·7H2O used is 0.1362 g (0.5 mmol), and the amount of 1,2-phthalonitrile used is 0.80 g (6.24 mmol). Calcination yields 0.13Fe-N / C DCB0.8 catalyst.

[0088] [Performance Testing]

[0089] 1. Morphology and structure

[0090] 1.1 The role of templates

[0091] Fe-N / C were analyzed by SEM and TEM. DCB0.2 (Example 1) The microstructure and structural composition of the catalyst were characterized. From... Figure 1 The SEM image of a shows that Fe-N / C DCB0.2 The catalyst exhibits a porous spherical structure with surface wrinkles, confirming the Fe-N / C... DCB0.2 The catalyst matrix is ​​a wrinkled, spherical nitrogen-doped carbon framework. This matrix structure facilitates the exposure of more active sites and accelerates electrolyte penetration, thereby improving ORR performance. Figure 1 b and Figure 1 In the image, 'c' represents the TEM image, Fe-N / C. DCB0.2 This also appears as a distinct spherical structure in the image. In contrast, the HRTEM image ( Figure 1 d) and SAED map ( Figure 1In e), no obvious nanoparticles (NPs) or clusters were observed, suggesting that Fe atoms may exist in an atomically dispersed form. Figure 1 The presence of g in the formula can prove that the three elements Fe, N, and C are in the Fe-N / C DCB0.2 It is evenly distributed in the middle.

[0092] Figure 2 Fe-N / C DCB0.2 (Example 1) Structural characterization diagram of the catalyst, Figure 2 The XRD results for a in Fe-N / C are as follows: the two significant diffraction peaks at 23.8° and 43.8° are identified as (002) and (100) diffraction peaks of low-crystallinity graphitic carbon. The presence of only carbon peaks in the XRD indicates that Fe exists in an atomically dispersed form in Fe-N / C. DCB0.2 In the catalyst. To verify the above hypothesis, Fe-N / C... DCB0.2 Aberration-corrected electron microscopy was performed. Figure 1 In the aberration-corrected electron microscope (EMS) of f, high-density and atomically isolated Fe atoms can be clearly observed, which is consistent with the XRD results, further proving that the Fe in Example 1 is dispersed in the form of single atoms on the nitrogen-doped carbon framework. Furthermore, Figure 2 b and Figure 2 In the figure, 'c' represents the pore size distribution curve and the nitrogen adsorption-desorption isotherm curve. Observation Figure 2 Fe-N / C can be found in b. DCB0.2 The catalyst exhibited a hierarchical porous structure with a specific surface area as high as 696.1 m². 2 ·g -1 ( Figure 2 (c) In theory, a high specific surface area and a hierarchical porous structure are beneficial for exposing more active sites and enhancing mass transfer efficiency, thereby promoting ORR performance improvement.

[0093] Figure 3 The diagram shows the structural characterization of the Fe-N / C catalyst (Comparative Example 1). Figure 3 In the XRD pattern of a, peaks at 35.4°, 53.5°, and 56.7° appeared, which are attributed to the (311), (422), and (511) diffraction peaks of Fe3O4 (PDF#03-0863). In contrast, in Fe-N / C DCB0.2 No diffraction peaks associated with iron species were detected. This result indicates that iron exists in the Fe-N / C catalyst as Fe3O4 clusters rather than as single Fe atoms. Furthermore, Figure 3 No hierarchical porous structure was observed in b. (Comparison) Figure 2 and Figure 3 The results clearly show that 1,2-phthalonitrile can act as a template agent to help the catalyst generate a hierarchical porous structure, thus giving the catalyst a larger specific surface area.

[0094] 1.2 Ratio of template to ferrous salt

[0095] XRD and BET tests were performed on the catalysts obtained in Comparative Examples 4-10, and the results were recorded in [the table / document / etc.]. Figures 4-10 Comparative Example 4 ( Figure 4 Comparative Example 5 Figure 5 Comparative Example 6 Figure 6 Comparative Example 7 Figure 7 Comparative Example 8 Figure 8 Comparative Example 9 Figure 9 Comparative Example 10 Figure 10 The XRD results of all samples showed diffraction peaks attributed to Fe3O4, indicating that the iron in the catalyst exists in the form of Fe3O4, and therefore is not dispersed as single Fe atoms. Furthermore, only the nitrogen adsorption-desorption isotherms of Comparative Examples 7-10 showed a hierarchical porous structure. Analysis shows that while a hierarchical structure increases the specific surface area, it does not necessarily lead to single Fe atom dispersion. This demonstrates that creating a hierarchical structure or a large specific surface area is not sufficient to achieve single Fe atom dispersion. It is speculated that the template and Fe... 2+ The ratio of Fe atoms plays a decisive role in the dispersion of Fe single atoms, while the hierarchical structure provides a matrix with a large specific surface area, thus aiding in the dispersion of atoms. The combined effect of both results in the final dispersion of Fe single atoms. In other words, Fe... 2+ The amount of Fe introduced must be appropriate, and the Fe single-atom dispersion can only be controlled with a suitable amount of template agent. Theoretically, Fe single-atom dispersion can help increase the iron loading, and subsequent XPS and ICP-MS verification showed that the iron loading of the catalyst obtained in Example 1 was much greater than that of Comparative Example 1, which did not form single-atom dispersion. Furthermore, Fe single-atom dispersion facilitates the formation of a large amount of FeN4C (from the C and N on the framework) by Fe. Figure 13 The XPS results confirmed the active sites, which significantly improved the ORR performance of the catalyst.

[0096] Table 1 Comparison of catalyst structures under different schemes

[0097]

[0098] 1.3 Calcination temperature

[0099] To investigate the effect of calcination temperature on the catalyst structure, the catalysts obtained in Example 1 (900℃), Example 2 (800℃), Example 3 (600℃), Comparative Example 2 (400℃), and Comparative Example 3 (1000℃) were characterized by SEM and XRD. The results are as follows: Figure 11 As shown. Observation Figure 11 It is evident that as the calcination temperature increases, the number of wrinkles on the catalyst surface gradually increases. This wrinkled structure is beneficial for increasing the specific surface area and constructing a hierarchical porous structure, ultimately exposing abundant Fe active sites for the ORR reaction. Furthermore, excessively high temperatures (1000℃) can easily trigger the structural collapse of the ZIF precursor-derived catalyst, leading to Fe-N / C... DCB0.2 Fe atom aggregation and Fe2O3 phase appear at -1000℃, thus reducing the exposed Fe active sites available for the ORR reaction. Previous studies have confirmed that temperatures above 800℃ are favorable for the formation of Fe-Nx active sites; and from Fe-N / C... DCB0.2 -800 and Fe-N / C DCB0.2 No Fe agglomeration was observed in the XRD pattern at -900°C, indicating that more atomically dispersed Fe active sites can be formed in the catalyst at 800°C and 900°C. However, the larger specific surface area and richer hierarchical porous structure formed at 900°C ( Figure 12 It is clearly Fe-N / C DCB0.2 The key to exposing more Fe active sites and resulting in better ORR performance is at pyrolysis temperatures below 600°C. Figure 11 (f) At 400°C, the crystal structure of the ZIF8 precursor remained unchanged, but as the temperature increased to 600°C, the ZnO phase appeared, indicating that the active Fe-Nx sites required for ORR are not easily formed below 600°C. Therefore, the above results suggest that a pyrolysis temperature of 900°C is more favorable for Fe-N / C DCB0.2 -900 achieves a larger specific surface area, a richer hierarchical porous structure, and densely exposed Fe-Nx sites, thereby enhancing its ORR performance.

[0100] 2. Microstructure characterization

[0101] XPS analysis was used to analyze the chemical composition and valence state of the catalyst, and the results are as follows: Figure 13 As shown. Figure 13 In c, Fe-N / C DCB0.2 Both Fe-N / C and Fe-N / C contain the four elements Fe, N, C, and O. DCB0.2 High-resolution N 1s spectrum of Fe-N / C ( Figure 13As shown in d), both catalysts contain five nitrogen species: pyridinic N (binding energy 398.5 eV), Fe-Nx (binding energy 399.2 eV), pyrrolic N (binding energy 400.3 eV), graphitic N (binding energy 401.0 eV), and oxidized N (binding energy 402.7 eV). Furthermore, compared to Fe-N / C, Fe-N / C... DCB0.2 The content of Fe-Nx and pyridine nitrogen is higher in the middle. Figure 13 (e). Existing studies have confirmed that Fe-Nx is a highly efficient active site for ORR. On the other hand, Fe-N / C DCB0.2 High-resolution Fe 2p XPS spectrum ( Figure 15 The result shows that it is at 710.6 eV (Fe). 2+ 2p3 / 2), 713.3 eV (Fe 3+ 2p3 / 2), 723.6 eV (Fe 2+ 2p1 / 2) and 727.4 eV (Fe 3+ A characteristic peak appears at 2p1 / 2); and Fe-N / C DCB0.2 The Fe content (5.53 wt.%) was higher than that of Fe-N / C (1.95 wt.%). These results further confirm that 1,2-phthalonitrile is effective against Fe-N / C. DCB0.2 High loading of Fe-Nx sites in the catalyst plays a positive role in its construction. Raman spectroscopy was used to investigate the Fe-N / C... DCB0.2 Analysis with Fe-N / C ( Figure 13 f in the middle). D band (1350 cm) -1 ) and G-band (1590 cm) -1 The strength of Fe-N / C corresponds to different degrees of graphitization in the material. DCB0.2 I D / I G The ratio (1.04) is higher than that of Fe-N / C (0.97), which indicates that the 1,2-phthalonitrile template can increase the defect density of the catalyst while reducing its crystallinity.

[0102] To investigate Fe-N / C DCB0.2 The coordination environment of Fe was characterized using X-ray absorption fine structure (XAFS) spectroscopy. Figure 14 The α,Fe K-edge X-ray absorption near-edge structure (XANES) spectrum shows that Fe-N / C DCB0.2 The absorption edge of the sample lies between the absorption edges of the reference samples (i.e., Fe foil, Fe2O3, and FePc phthalocyanine), and is very close to the absorption edge of Fe(II) in FePc. This indicates that Fe-N / CDCB0.2 It contains positively charged Fe atoms, whose oxidation states are between +2 and +3. Figure 14 b, Fe-N / C DCB0.2 Fourier transform (FT) k 3 Weighted extended X-ray absorption fine structure (EXAFS) spectra revealed two characteristic peaks at approximately 1.53 Å and 2.63 Å, corresponding to the first-shell scattering path of Fe-N and the axial coordination scattering path of Fe-C, respectively. This is similar to the coordination environment of Fe in FePc. Furthermore, no significant Fe-Fe coordination peak was detected at 2.2 Å, further confirming the Fe-N / C ratio. DCB0.2 The Fe in the middle layer is atomically dispersed, consistent with the observations of aberration-corrected electron microscopy. Figure 14 In the analysis of wavelet transform (WT) of Fe K-edge EXAFS, b shows that Fe-N / C DCB0.2 There are two WT maxima, located at approximately 4.8 Å. -1 and ≈6.2 Å -1 At this location, the signal originates from Fe-N coordination and is similar to the WT spectrum of FePc. It is noteworthy that, unlike the reference samples (Fe foil, Fe₂O₃), the Fe-N / C... DCB0.2 No maximum WT value corresponding to Fe-Fe coordination was observed, which further verifies the role of Fe in Fe-N / C DCB0.2 The atomic-level dispersion state in Fe-N / C. DCB0.2 The fitting curves in R-space agree well with the simulation model. According to the fitting results (Table 2), the isolated Fe center is coordinated with 4 N atoms in the first shell (i.e., Fe-N4 structure), with an average Fe-N bond length of 2.04 Å; at the same time, it is bonded to 1 C atom through axial coordination (i.e., Fe-C structure), with an average Fe-C bond length of 3.04 Å.

[0103] Table 2 Fitting results of the fine structure spectrum of Fe K-edge X-ray absorption

[0104]

[0105] a During the fitting process, S0 2 E0 is fixed at 0.75 and used as a global fitting parameter for optimization. b N is the coordination number. c R is the distance between the absorbing atom and the backscattering atom. Data range: 3.0 <k≤8.76Å -1 , 1.0≤R≤3.1Å. The total number of independent data points is 11.1.

[0106] also, Figure 16 ICP-MS compositional analysis results showed that Fe-N / CDCB0.2 The Fe content reached 5.45 wt% (close to the 5.53 wt% result of XPS in Table 3), which is not only higher than that of Fe-N / C catalysts (1.98 wt%, determined by ICP-MS), but also higher than most reported Fe-N / C catalysts (Table 4).

[0107] Table 3 Fe-N / C DCB0.2 Fe-N / C elemental content table (XPS determination)

[0108]

[0109] Table 4 Fe-N / C DCB0.2 Comparison table with previously reported Fe-NC catalysts in terms of half-wave potential (E1 / 2), H2O2 yield, and iron (Fe) loading.

[0110]

[0111] 3. Electrochemical performance

[0112] The ORR performance of the catalyst was characterized by cyclic voltammetry (CV) and linear sweep voltammetry (LSV) in an O2 / Ar saturated 0.1 mol / L KOH electrolyte. The test results are as follows: Figure 17 As shown.

[0113] like Figure 17 As shown in a, Fe-N / C DCB0.2 The ORR characteristic peak potential (0.90 V vs RHE) is more positive than that of Fe-N / C (0.84 V vs RHE). Figure 17 b in Figure 17 c in the figure shows that after introducing the 1,2-phthalonitrile template, the half-wave potential / initial potential (E) of the Fe-N / C catalyst is... 1 / 2 / E onset The value increased from 0.84 / 0.92 V vs RHE to 0.90 / 0.98 V vs RHE; while Figure 17 b and Figure 19 The display shows that Fe-N / C DCB0.2 The half-wave potential (0.90 V) and limiting current density (J_L, -5.53 mA·cm) -2 All are higher than Fe-N / C DCB0.1 (0.89 V, -5.32 mA·cm) -2 ), Fe-N / C DCB0.3 (0.86 V, -4.15 mA·cm) -2 ), Fe-N / C DCB0.6 (0.85 V, -4.50 mA·cm)-2 ) and commercial Pt / C catalyst (0.88 V, -4.89 mA·cm⁻¹) -2 ). Figure 17 In d, Fe-N / C DCB0.2 It exhibits faster ORR reaction kinetics, with a Tafel slope (76.1 mV·dec). -1 () is less than commercial Pt / C (78.3 mV·dec) -1 ) and Fe-N / C (80.4 mV·dec -1 The above results are consistent with the characterization results obtained by aberration-corrected electron microscopy, nitrogen adsorption-desorption, and SEM; compared with previously reported Fe-based ORR electrocatalysts ( Figure 18 (a in Table 4) Fe-N / C DCB0.2 Its superior ORR performance is attributed to its hierarchical porous structure and high-density exposed Fe single-atom active sites.

[0114] To further explore Fe-N / C DCB0.2 The ORR reaction kinetics were investigated, and the LSV curves of the catalyst were tested within the rotational speed range of 400–2025 rpm. Clearly, the Fe-N / C... DCB0.2 The limiting current density (J_L) increases with increasing rotational speed. Figure 20 In the figure 'a', the corresponding (KL) plots show good linearity and are parallel to each other, and the calculated electron transfer number (n) is close to 4.0. Figure 20 (b) indicates that Fe-N / C DCB0.2 It follows first-order reaction kinetics, and the ORR reaction pathway is 4e. - The electron transfer mechanism was investigated. On the other hand, the electron transfer number (n) and the hydrogen peroxide (H₂O₂) yield were measured using a rotating ring-disk electrode (RRDE). Figure 16 As can be seen from f, within the potential range of 0.3~0.8 V vs RHE, Fe-N / C DCB0.2 The electron transfer number (n) is high, with an average of 3.98, while the average yield of H2O2 is only 0.88%. Compared with commercial Pt / C and reported Fe-based ORR electrocatalysts (Table 4), its ORR performance is superior, further confirming that Fe-N / C has better ORR performance. DCB0.2 4e for ORR - The transfer path is highly selective. Furthermore, Figure 18 The c in the figure shows Fe-N / C DCB0.2 The current retention rate (90%) is higher than that of commercial Pt / C (87%), indicating that this catalyst has excellent ORR stability. More importantly, the current retention rate is higher than that of commercial Pt / C (87%). Figure 18 As can be seen from d, after methanol injection, Fe-N / C DCB0.2The current decay of Fe-N / C is negligible, while commercial Pt / C exhibits significant current decay, indicating that Fe-N / C... DCB0.2 It has a superior ability to resist methanol poisoning.

[0115] 4. Zinc-air battery

[0116] To further test Fe-N / C DCB0.2 To assess the performance of zinc-air batteries (ZABs) in practical applications, researchers assembled a Fe-N / C battery using a 6 mol / L KOH + 0.2 mol / L zinc acetate (Zn(CH3COO)2) aqueous solution as the electrolyte. DCB0.2 Homemade zinc-air battery with cathode catalyst ( Figure 21 (a) in the text. At 10, 15, 20, 25, and 30 mA·cm⁻¹ -2 At current densities, with Fe-N / C DCB0.2 The constant current discharge voltage of the zinc-air battery with Pt / C as the cathode is higher than that of the zinc-air battery with commercial Pt / C as the cathode; and both types of batteries can maintain a stable discharge plateau at different current densities. Figure 21 b), indicating Fe-N / C DCB0.2 The fabricated ZAB exhibits excellent rate performance and good voltage recovery capability. Furthermore, Fe-N / C DCB0.2 The open-circuit voltage (OCV) of the fabricated ZAB stabilized at 1.49 V. Figure 21 The c in the figure is higher than that of ZAB (1.45 V) made of commercial Pt / C. The maximum power density of this battery can reach 220 mW·cm. -2 ( Figure 21 The d in the middle not only exceeds the ZAB (197 mW·cm) produced by commercial Pt / C, but also exceeds the ZAB (197 mW·cm) produced by commercial Pt / C. -2 Furthermore, it outperforms most reported Fe-based catalysts used to prepare ZAB (Table 5). Meanwhile, at 10 mA·cm⁻¹... -2 At discharge current density, Fe-N / C DCB0.2 The specific capacity of the prepared ZAB was calculated to be 786 mAh·g. -1 ( Figure 21 The concentration of e in the Pt / C matrix is ​​higher than that of ZAB made with commercial Pt / C (736.9 mAh·g). -1 More importantly, Fe-N / C DCB0.2 The ZAB I made successfully charged a smartphone. Figure 21 f), which directly proves Fe-N / C DCB0.2 The above results demonstrate its potential for practical application in energy storage devices. DCB0.2 It exhibits excellent performance in terms of ORR catalytic activity and stability, and even has the potential to replace commercial Pt / C catalysts in practical applications.

[0117] Table 5 Fe-N / C DCB0.2 Performance comparison with zinc-air batteries using Fe-NC catalysts as cathodes

[0118]

Claims

1. A method for preparing a high-iron-loaded iron-nitrogen-carbon catalyst, characterized in that, include: S1. Zinc salt and ferrous salt are dispersed in methanol to obtain mixture A; 2-methylimidazole and 1,2-phthalonitrile are dispersed in methanol and mixed in an oil bath at 30~40℃ to obtain mixture B; the molar ratio between zinc in zinc salt, iron in ferrous salt, 1,2-phthalonitrile and 2-methylimidazole is 5:0.3:(1~2):(40~42); S2. Mixture A and mixture B are stirred to obtain precursor powder. The precursor powder is heated to 180~200℃ and held for 2~3 h in an inert atmosphere, and then heated to 600~900℃ and held for 2~3 h to obtain a high iron-loaded iron-nitrogen-carbon catalyst. The heating rate of the calcination is 2~10℃ / min.

2. The preparation method according to claim 1, characterized in that, In step S2, the stirring and mixing time is 6-8 h, and after stirring and mixing, the precursor powder is obtained by vacuum drying at 60-80℃ for 12-24 h.

3. The preparation method according to claim 1, characterized in that, In S2, the inert atmosphere is nitrogen and / or argon.

4. The high-iron-loaded iron-nitrogen-carbon catalyst prepared by the preparation method according to any one of claims 1 to 3, characterized in that, The high-iron-loaded iron-nitrogen-carbon catalyst comprises a pleated spherical nitrogen-doped carbon framework and iron atoms dispersed within it, wherein the iron atoms and the nitrogen-doped carbon framework form FeN4C active sites; and the iron loading in the high-iron-loaded iron-nitrogen-carbon catalyst is ≥5.0 wt%.

5. The high-iron-loading iron-nitrogen-carbon catalyst as described in claim 4, characterized in that, The iron-nitrogen-carbon catalyst with high iron loading has a hierarchical porous structure of micropores and mesopores, wherein the hierarchical porous structure is used to disperse iron atoms; and / or, the specific surface area of ​​the iron-nitrogen-carbon catalyst with high iron loading is 650~750 m². 2 / g.

6. A zinc-air battery, characterized in that, The high-iron-load iron-nitrogen-carbon catalyst as described in claim 4 or 5 is used as the catalyst material for the cathode.

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