CN115966710B - High-stability iron atom catalyst and preparation method and application thereof - Google Patents

Abstract

The invention provides a high-stability iron atom catalyst and a preparation method and application thereof, wherein the catalyst is a three-dimensional honeycomb net structure formed by nitrogen-doped carbon frameworks, the three-dimensional honeycomb net structure also comprises iron single atoms and iron nanoclusters, the iron nanoclusters are loaded in nanopores of the three-dimensional honeycomb net structure, the iron single atoms are combined on the nitrogen-doped carbon frameworks around the iron nanoclusters, gelatin hydrogel is used as a precursor and a template during preparation, and Fe nanoclusters (NCA/Fe _SA+NC) are loaded near Fe single atom sites in N-doped carbon aerogel by a simple two-step pyrolysis method. The high-stability iron atom catalyst prepared by the invention has better electrocatalytic activity and stability, and the flexible zinc-air battery assembled by taking NCA/Fe _SA+NC aerogel as a cathode catalyst has higher OCV and power density at room temperature and low temperature and excellent durability.

Description

A highly stable iron atom catalyst and its preparation method and application

Technical Field

The present invention belongs to the technical field of energy storage and conversion, and in particular relates to a highly stable iron atom catalyst and a preparation method and application thereof.

Background technique

The electrocatalytic oxygen reduction reaction (ORR) is the cornerstone of renewable energy conversion and storage devices such as metal-air batteries and fuel cells. Although platinum (Pt)-based catalysts have shown remarkable ORR performance, the development of non-precious metal catalysts remains imperative due to cost and sustainability reasons. Transition metal–nitrogen–carbon (M–N–C) composites have emerged as viable alternatives due to their high activity and low cost. Among them, (metal) single-atom catalysts (SACs) have received extensive attention. For SACs, their electrocatalytic activity can be optimized by regulating the coordination environment of atomically dispersed metal sites. In particular, Fe SACs, obtained by porous carbon embedded with FeN _x sites, show ORR activity and half-wave potential (E _1/2 ) comparable to commercial Pt/C catalysts in alkaline and acidic media. However, it is well known that carbon-based SACs show significant degradation during long-term electrode reactions due to demetallization, carbon oxidation, and carbon corrosion, resulting in poor stability, which has become a major obstacle for their practical application.

To alleviate these problems, research has mainly focused on two strategies, increasing the graphitization of carbon scaffolds and stabilizing metal active sites. For example, carbon corrosion can be alleviated by integrating metal sites into graphene or carbon nanotubes. In addition, a second metal atom can be introduced into Fe SACs to prevent harmful Fenton reactions. However, although these strategies do improve stability, the disadvantages are also obvious, such as the increase in graphitization leads to a decrease in carbon defects, thereby reducing activity; the structural engineering of metal sites requires cumbersome operations. Therefore, the development of simple and effective strategies to enhance the stability of SACs while maintaining high activity is of great significance for practical applications.

Summary of the invention

In order to overcome the technical problem that the activity and stability of iron single atom catalysts (Fe SACs) cannot be taken into account in the prior art, the present invention provides a highly stable iron atom catalyst, a preparation method and an application thereof, which achieves the regulation of electrons by introducing adjacent iron nanoclusters, which does not affect the electrocatalytic oxygen reduction reaction (ORR) activity of Fe SACs and can also improve its stability.

In order to solve the above technical problems, the technical solution proposed by the present invention is:

The first aspect of the present invention provides a highly stable iron atom catalyst, which is a three-dimensional honeycomb network structure composed of a nitrogen-doped carbon skeleton. The three-dimensional honeycomb network structure also contains iron single atoms and iron nanoclusters. The iron nanoclusters are loaded in the nanopores of the three-dimensional honeycomb network structure, and the iron single atoms are bound to the nitrogen-doped carbon skeleton around the iron nanoclusters.

The highly stable iron atom catalyst of the present invention has abundant pores, which helps to inhibit the excessive aggregation of metal atoms into large nanoparticles during high temperature processes.

As an optional embodiment, in the highly stable iron atom catalyst provided by the present invention, the diameter of the iron nanocluster is less than 10 nm, and the distance between the iron nanocluster and the iron single atom is less than 2 nm.

In the present invention, the diameter of the iron nanocluster is set to be less than 10nm, which meets the size standard of the nanocluster. The distance between the iron nanocluster and the iron single atom is less than 2nm. If the distance is too large, the effect of the nanocluster will be reduced. The distance between the iron nanocluster and the iron single atom is calculated as the distance between the outermost atom of the nanocluster and the iron single atom.

The second aspect of the present invention provides a method for preparing a highly stable iron atom catalyst, comprising the following steps:

S1. Mix the carbon source, porogen, iron salt, nitrogen source and water in a mass ratio, and self-assemble into a hydrogel at low temperature after being bathed in 55-65°C water;

S2, heating the hydrogel prepared in step S1 to 400-500° C. in an Ar or N ₂ atmosphere for pyrolysis, and etching to remove the porogen to obtain a semi-finished product;

S3. The semi-finished product prepared in step S2 is pyrolyzed at 900-950° C. in a mixed atmosphere of 97% Ar+3% H ₂ to prepare a highly stable iron atom catalyst.

In the present invention, the temperature in step S1 is set to 55-65°C, which is conducive to gel formation. If it exceeds this range, gel cannot be formed. Pyrolysis at 400-500°C in step S2 and 900-950°C in step S3 are both conducive to carbonization.

As an optional embodiment, in the preparation method provided by the present invention, the carbon source in step S1 is one of gelatin, chitosan, starch or agar.

As an optional embodiment, in the preparation method provided by the present invention, the nitrogen source is one of o-phenanthroline (PM) or melamine.

As an optional embodiment, in the preparation method provided by the present invention, the mass ratio of the carbon source, porogen, iron salt, nitrogen source and water in step S1 is (15-30): (5-12.5): (1-4): (2.97-11.9): (0.625-1.25).

As an optional embodiment, in the preparation method provided by the present invention, the porogen is one of silicon dioxide or aluminum oxide.

As an optional embodiment, in the preparation method provided by the present invention, the particle size of the silicon dioxide is 10 to 30 nm.

When the particle size of silica is set to be greater than 30 nm, the increase in particle size will reduce the specific surface area and the number of active sites will decrease.

As an optional embodiment, in the preparation method provided by the present invention, the aluminum oxide has a particle size of 10 to 50 nm.

As an optional embodiment, in the preparation method provided by the present invention, the iron salt is FeCl ₂ or Fe(NO ₃ ) ₂ .

As an optional embodiment, in the preparation method provided by the present invention, the heating rate in step S2 and step S3 is 5-10° C. per minute.

As an optional embodiment, in the preparation method provided by the present invention, the pyrolysis time in step S2 and step S3 is 2 to 3 hours.

As an optional embodiment, in the preparation method provided by the present invention, in step S2, the porogen is removed by etching with a 0.5-1.0 M strong alkaline solution at 75-85° C.

The third aspect of the present invention provides the use of the above-mentioned highly stable iron atom catalyst as an air cathode catalyst in a flexible zinc-air battery.

As an optional embodiment, in the application provided by the present invention, the highly stable iron atom catalyst is used at room temperature or at 0 to -40°C when used as an air cathode catalyst in a flexible zinc-air battery.

Fe-NC nanocomposites usually contain both durable and non-durable _FeNx sites. In the former, the metal center maintains the valence state of Fe(II) during the electrode reaction; while in the latter, the oxidation state of the metal center switches between Fe(III) and Fe(II) and is easily converted into iron oxide. This suggests that the stability of the _FeNx site can be enhanced by preventing the oxidation of Fe(II) to Fe(III). In theory, electron-rich metal nanoparticles in the ortho position can inhibit the oxidation of _FeNx sites through electron transfer. However, this electron transfer must be relatively weak so as not to impair the ORR activity. Therefore, the present invention can improve the stability of _FeNx by introducing smaller-sized metal nanoclusters in the ortho position. At the same time, the metal nanoclusters have good oxygen evolution reaction (OER) performance and can endow the nanocomposites with bifunctional oxygen catalytic activity, which is also a key feature of rechargeable metal-air batteries.

Compared with the prior art, the present invention has the following beneficial effects:

(1) In view of the shortcomings of the prior art that the activity and stability of iron single atom catalysts (Fe SACs) cannot be taken into account at the same time, the present invention provides a highly stable iron atom catalyst. By introducing iron nanoclusters near the _FeNx sites in N-doped carbon aerogels, a highly stable iron atom catalyst NCA/Fe _SA+NC is obtained. Electrochemical analysis shows that it has good ORR activity, a half-wave potential (E _1/2 ) of up to +0.92 V, and significantly improved stability compared with single iron atom catalysts without iron nanoclusters.

(2) The present invention can introduce iron nanoclusters near the _FeNx site in the N-doped carbon aerogel through a simple two-step pyrolysis method. First, the hydrogel is pyrolyzed at about 500°C for 2-3h to obtain the nitrogen-doped carbon aerogel, and then a secondary pyrolysis is performed at about 900°C for 2-3h. This can make some iron atoms that are weakly bound to the carbon skeleton aggregate into nanoclusters near the _FeNx site. The preparation method is simple and fast.

(3) In the present invention, a highly stable iron atom catalyst is used as the cathode catalyst, and the assembled flexible zinc-air battery exhibits high power density and remarkable durability at both room temperature and low temperature (as low as -40°C). The present invention provides a new idea for the electrocatalytic activity and stability of M-N-C composite materials, which is conducive to the further development of electrochemical energy technology.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

FIG1 is a schematic diagram of the preparation of NCA/Fe _SA+NC prepared in Example 1;

FIG2 is an SEM image of the freeze-dried G-Si/FePM hydrogel, NCA/Fe _SA+NC and NCA/Fe _SA prepared in Example 1;

FIG3 is a TEM image, a HAADF-STEM image and a HRTEM image of NCA/Fe _SA+NC prepared in Example 1;

FIG4 is a TEM and HAADF-STEM image of NCA/Fe _SA prepared in Comparative Example 1;

Figure 5 shows the N ₂ adsorption-desorption isotherms and pore size distribution, XRD and Raman spectra of NCA/Fe _SA and NCA/Fe _SA+NC ;

Figure 6 shows the XPS spectra of NCA/Fe _SA and NCA/Fe _SA+NC , the Fe 3p electron spectra (the arrow directions are Fe(0)3P _3/2 , Fe(Ⅱ)3P _3/2 , Fe(Ⅲ)3P _3/2 , satellite peaks, Fe(0)3P _1/2 , Fe(Ⅱ)3P _1/2 , Fe(Ⅲ)3P _1/2 ) and N1s electron spectra (the arrow directions are pyridinic N, metallic N, pyrrolic N, graphitic N and oxidized N);

Figure 7 shows the normalized Fe k-edge XANES spectra of NCA/Fe _SA+NC , NCA/Fe _SA , Fe foil and FePc (the inset is the magnified absorption edge), the Fourier transformed ^k3 -weighted Fe k-edge EXAFS spectra, the EXAFS fitting curves of NCA/Fe _SA+NC and NCA/FeSA, and the k-space curves and corresponding fitting curves of NCA/Fe _SA+NC and (f) NCA/Fe _SA ;

Figure 8 _shows ^{the 57} _Fe spectra and EPR spectra of NCA/Fe _SA+NC and NCA/Fe _SA ;

Figure 9 shows the optimized conformation diagrams of the simulated S1, S2 and S3 sites, as well as the free energy diagrams of the S1, S2, S3 and S0 sites at 0.90 V, the free energy diagrams at the limiting potential, and the magnetic moments of the S1, S2, S3 and S0 sites;

FIG10 is a schematic diagram of the reaction pathway of ORR at the S0 site in FIG9 ;

FIG11 is a schematic diagram of the reaction pathway of ORR at the S1 site in FIG9 ;

FIG12 is a schematic diagram of the reaction pathway of ORR at the S2 site in FIG9 ;

FIG13 is a schematic diagram of the reaction pathway of ORR at the S3 site in FIG9 ;

Fig. 14 shows the DOS of the S2 site and its Fe 3d (in FeN ₄ ) orbital in Fig. 9 , the DOS of Fe3d electrons at the S1, S2, and S3 sites, and the DOS of the five Fe 3d orbitals at the S2 site;

Figure 15 shows the DOS of the S1, S3 and S0 sites and their Fe 3d (in FeN ₄ ) orbitals in Figure 9; the DOS of the five Fe 3d orbitals at the S1 site, S2 site and S3 site;

Figure 16 shows the ORR polarization curves of NCA/Fe _SA+NC , NCA/Fe _SA and commercial Pt/C catalysts at 1600 rpm, a comparison of E _onset , E _1/2 and J _k , and the impedance spectra of NCA/Fe SA _+NC and NCA/Fe _SA. The inset is the corresponding equivalent circuit diagram and the Tafel curves of NCA/Fe _SA+NC , NCA/Fe _SA and Pt/C.

Figure 17 is a graph showing the LSV curves of Pt/C, (b) NCA/Fe _SA and (c) NCA/Fe _SA+NC at different rotation rates in O ₂ -saturated 0.1 M KOH;

FIG18 is a graph showing the LSV curves at different rotation rates in O ₂ saturated 0.1 M KOH and the calculated electron transfer number and H ₂ O ₂ yield results;

Figure 19 shows the TEM images of NCA/Fe _SA+NC after acid etching and the ORR polarization curves of NCA/Fe _SA+NC before and after acid etching;

FIG20 is a graph showing the durability test results of NCA/Fe _SA+NC and NCA/Fe _SA ;

Figure 21 shows the Fe 2p XPS spectra of NCA/Fe _SA and NCA/Fe _SA+NC before and after the durability test;

FIG22 is HRTEM images of NCA/Fe _SA+NC and NCA/Fe _SA after durability testing;

Figure 23 shows the OER polarization curves of NCA/Fe _SA+NC and NCA/Fe _SA in 1.0 M KOH. The inset shows the ΔE of the two aerogels, b is the Tafel curve, and c is the OER polarization curve of NCA/Fe _SA+NC before and after acid etching (1.0 M KOH);

FIG24 is a graph showing the OCV, power density and constant current discharge curves of Zn//NCA/Fe _SA+NC , Zn//NCA/Fe _SA and Zn//Pt/C-RuO ₂ quasi-solid batteries and the performance comparison results of flexible zinc-air batteries;

Figure 25 shows the constant current charge and discharge curves of Zn//NCA/Fe _SA+NC and Zn//NCA/Fe _SA batteries at a current density of 5 mA cm ^-2 , the compression results, repeated bending results and repeated compression test results of Zn//NCA/Fe _SA+NC batteries;

Figure 26 shows the OCV, power density, constant current discharge curve of the Zn//NCA/Fe _SA+NC quasi-solid-state battery at low temperature, the low temperature performance comparison of the flexible zinc-air battery, and the constant current charge and discharge curve result diagram when the current density is 5 mA cm ^-2 ;

Figure 27 shows photos of two Zn//NCA/Fe _SA+NC cells in series driving LEDs at different temperatures.

Detailed ways

In order to facilitate the understanding of the present invention, the present invention will be described more comprehensively and carefully below in conjunction with the accompanying drawings and preferred embodiments of the specification, but the protection scope of the present invention is not limited to the following specific embodiments.

Unless otherwise defined, all the professional terms used below have the same meanings as those generally understood by those skilled in the art. The professional terms used herein are only for the purpose of describing specific embodiments and are not intended to limit the scope of protection of the present invention.

Unless otherwise specified, various raw materials, reagents, instruments and equipment used in the present invention can be purchased from the market or prepared by existing methods.

1. Preparation of highly stable iron atom catalyst (NCA/Fe _SA+NC ).

Embodiment 1:

(1) Preparation of G-Si/FePM: 180.0 mg of gelatin, 90.0 mg of SiO ₂ , 23.9 mg of FeCl ₂ ·4H ₂ O and 71.4 mg of PM were added to 7.5 mL of ultrapure water and placed in a 60°C water bath for 20 min to form a blood-red dispersion. The resulting mixture was self-assembled into a hydrogel in a -4°C refrigerator, which was named G-Si/FePM.

(2) Preparation of NCA/Fe-500: The freeze-dried G-Si/FePM hydrogel was heated to 500°C at a rate of 5°C min ^-1 in an Ar mixed atmosphere and pyrolyzed for 2 h. It was etched with 0.5 M NaOH at 80°C to remove SiO ₂ , and was recorded as NCA/Fe-500.

(3) Preparation of NCA/Fe _SA+NC : NCA/Fe-500 was pyrolyzed in a mixed atmosphere of 97% Ar+3% H ₂ at a rate of 5°C min ^-1 to 900°C for 3 h to obtain NCA/Fe _SA+NC .

The preparation schematic diagram is shown in Figure 1.

Example 2

(1) 180.0 mg of chitosan, 90.0 mg of Al ₂ O ₃ , 23.9 mg of Fe(NO ₃ ) ₂ and 71.4 mg of melamine were added to 7.5 mL of ultrapure water and placed in a 55° C. water bath for 20 min to form a dispersion. The resulting mixture was self-assembled into a hydrogel in a -4° C. refrigerator.

(2) Preparation of NCA/Fe-500: The freeze-dried hydrogel was heated to 500°C at a rate of 5°C min ^-1 in an Ar mixed atmosphere and pyrolyzed for 3 h. It was etched with 0.5 M NaOH at 80°C to remove Al ₂ O ₃ and was recorded as NCA/Fe-500.

(3) Preparation of NCA/Fe _SA+NC : NCA/Fe-500 was pyrolyzed in a mixed atmosphere of 97% Ar+3% H ₂ at a rate of 5°C min ^-1 to 900°C for 2 h to obtain NCA/Fe _SA+NC .

Comparative Example 1

Preparation of iron single-atom catalysts without iron nanoclusters.

(1) Preparation of G-Si/FePM: 180.0 mg of gelatin, 90.0 mg of SiO ₂ , 23.9 mg of FeCl ₂ ·4H ₂ O and 71.4 mg of PM were added to 7.5 mL of ultrapure water and placed in a 60°C water bath for 20 min to form a blood-red dispersion. The resulting mixture was self-assembled into a hydrogel in a -4°C refrigerator, which was named G-Si/FePM.

(2) Preparation of NCA/Fe _SA : The freeze-dried G-Si/FePM hydrogel was heated to 900°C at a rate of 5°C min ^-1 in a mixed atmosphere of 97% Ar + 3% _H2 and pyrolyzed for 3 h. The _SiO2 template was then removed with 4% HF and dried in vacuum at 60°C for 1 h.

2. Assembly of Flexible Zinc-Air Battery

Synthesis of polyacrylic acid (PAA) hydrogel electrolyte: 4.2 mL of acrylic acid was dissolved in 9 mL of ultrapure water. Under strong magnetic stirring, 6.0 mg of methylenebisacrylamide and 60.0 mg of potassium persulfate were added. The solution was magnetically stirred for 20 min. The obtained solution was injected into a strip mold and placed in a 60°C oven overnight. Finally, the prepared PAA hydrogel was removed from the mold, dried naturally, and immersed in a solution containing 6 M KOH and 0.2 M Zn(AC) ₂ ·2H ₂ O for 72 h.

Synthesis of polyacrylamide (PAM) hydrogel electrolyte: 4 g acrylamide and 8 mL dimethyl sulfoxide were dissolved in 8 mL ultrapure water. Under strong magnetic stirring, 4.0 mg methylene bisacrylamide and 10.0 mg potassium persulfate were added and stirred for 20 min. The obtained solution was injected into a strip mold and placed in an oven at 60 ° C overnight. Finally, the prepared PAM hydrogel was immersed in a mixed solution of 6 M KOH and 0.2 M Zn(AC) ₂ ·2H ₂ O for 72 h.

The quasi-solid-state zinc-air battery adopts a typical sandwich structure. The prepared air electrode and zinc plate are placed on both sides of the PAA hydrogel electrolyte. The air electrode consists of a catalyst layer, a gas diffusion layer and a nickel foam layer. The catalyst, acetylene black and PTFE are fully mixed in a mass ratio of 6:1:3, moistened with ethanol, ground and rolled to prepare the catalyst layer. Then the catalyst layer, nickel foam and gas diffusion layer are stacked and pressed in sequence to obtain the air electrode, which is vacuum dried at 60°C for 3h and cut into 1.0cm×1.0cm slices for use.

For comparison, the cathode catalyst was prepared by the same method using commercial Pt/C- _RuO2 as the reference catalyst.

The assembly method of low-temperature zinc-air battery is the same as this one, except that PAM hydrogel electrolyte is used instead of PAA hydrogel electrolyte. Low-temperature measurement of zinc-air battery is carried out in a low-temperature test chamber (Haier, DW-60W151EU1).

3. Performance test of highly stable iron single atom catalyst

1. Morphology and structure of highly stable iron single-atom catalysts

The highly stable iron single atom catalyst (NCA/Fe _SA+NC ) prepared in Example 1 was taken as an example, and the N-doped carbon aerogel (NCA/Fe _SA ) prepared in Comparative Example 1, which only embedded Fe single atoms and did not contain iron nanoclusters, was used for comparison.

First, the morphology and structure of the carbon aerogel composite were studied by scanning electron microscopy (SEM) and transmission electron microscopy (TEM), and the results are shown in Figure 2. It can be seen from the SEM images of Figures 2b and 2c that the carbon aerogel retains a 3D structure with rich pores, which helps to inhibit the excessive aggregation of metal atoms into large nanoparticles during high temperature processes. The TEM image of Figure 3a shows that there are no larger metal nanoparticles in the NCA/Fe _SA+NC sample. In the high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) measurement, the results are shown in Figure 3b. It can be found that the NCA/Fe _SA+NC composite contains both metal nanoclusters (about 2nm in diameter) and metal single atoms, and many metal single atoms are located around the nanoclusters. As shown in Figure 3c, the lattice fringes of the nanoclusters are clearly visible, with a spacing of The interplanar spacing corresponds to the (100) crystal plane of metallic Fe (PDF#34-0529). The elemental distribution (EDS) diagram based on energy dispersive X-ray spectroscopy, as shown in Figure 3d, also confirms that the carbon aerogel contains both metallic Fe nanoclusters and single atoms. In contrast, the NCA/Fe _SA prepared by one-step pyrolysis contains only metal single atom sites without metal nanoclusters, as shown in Figure 4.

2. _N2 adsorption-desorption measurement experiment

In the N ₂ adsorption-desorption measurements, both NCA/Fe _SA+NC and NCA/Fe _SA composites exhibited type IV isotherms, indicating the formation of a complex porous network dominated by mesopores in the range of 5–15 nm. The results are shown in Figure 5 a. The specific surface area of NCA/Fe _SA is about 899 m ² g ^-1 , and the specific surface area of NCA/Fe _SA+NC is reduced to 579 m ² g ^-1 , which may be due to the blockage of some nanopores in NCA/Fe _SA by metal nanoclusters. In the XRD measurement, due to the (002) diffraction of graphitic carbon (PDF#65-6212), it can be seen that both NCA/Fe _SA+NC and NCA/Fe _SA show a broad diffraction peak at 2≈25°, indicating that the hydrogel precursor is effectively graphitized into carbon aerogel. The results are shown in Figure 5b. Compared with NCA/Fe _SA , two diffraction peaks located at 42.5° and 50.1° can be detected in NCA/Fe _SA+NC , which are attributed to the (100) and (101) planes of hexagonal Fe nanocrystals (PDF#34-0529), respectively, which is consistent with the results of HRTEM measurement mentioned above. In the Raman spectroscopy measurement, both NCA/Fe _SA+NC and NCA/Fe _SA showed a D band at 1348 cm ^-1 and a G band at 1590 cm ^-1 . The peak intensity ratio ( _ID / _IG ) of the former was slightly lower than that of the latter (0.89 vs. 0.92). The results are shown in Figure 5c, indicating that the former has a higher degree of graphitization, which can effectively promote electron transfer in the electrocatalytic reaction, thereby improving the catalytic activity.

3. Elemental composition and corresponding valence analysis of carbon aerogel

X-ray photoelectron spectroscopy (XPS) can be used to determine the elemental composition and corresponding valence state of carbon aerogels. In the XPS spectra of NCA/Fe _SA+NC and NCA/Fe _SA carbon aerogels, the results are shown in Figure 6a. The signal peaks at 284, 400, 530 and 710 eV correspond to C, N, O and Fe elements, respectively. The Fe content is 1.8 and 1.0 wt%, respectively, which is consistent with the results of inductively coupled plasma optical emission spectroscopy (ICP-OES). The results are shown in Table 1. From the HRXPS graph of Fe 2p electrons, it can be seen that NCA/Fe _SA+NC contains three pairs of peaks, as shown in Figure 6b, located at 708.0/721.2eV, 709.8/723.2eV and 713.9/727.3eV, respectively, belonging to Fe(0), Fe(Ⅱ) and Fe(Ⅲ); only two substances, Fe(Ⅱ) and Fe(Ⅲ), were detected in the NCA/Fe _SA sample, indicating that the former contains both Fe nanoclusters and Fe single atoms, while the latter contains Fe single atoms. It is worth noting that the binding energy of Fe(Ⅱ) and Fe(Ⅲ) in NCA/Fe _SA+ NC is about 0.3eV lower than that of NCA/Fe _SA , as shown in Table 2, indicating that there is an electron transfer effect from Fe nanoclusters to Fe single atom sites. In addition, the percentages of Fe(Ⅱ) and Fe(Ⅲ) in the NCA/Fe _SA+NC catalyst are significantly higher than those in NCA/Fe _SA (1.5vs.1.1), indicating that the antioxidant stability of NCA/Fe _SA+NC is relatively enhanced. In the O1s spectra of both carbon aerogel samples, C=O and CO/OH functional groups can be detected, located at 531.8eV and 532.0eV, respectively, as shown in Figures 6a and 6b; however, the corresponding metal-O (MO) peak at 530eV was not detected in either sample. The fitting results of the N 1s spectrum show that the samples contain 5 N species, namely pyridinic N (398eV), metal-N (MN) (399eV), pyrrolic N (400eV), graphitic N (401eV) and oxidized N (403eV), as shown in Figure 6c. The N 1s spectrum results show that the Fe single atoms in the carbon aerogel combine with N atoms to form FeN _x sites. It is worth noting that the binding energy of Fe-N species in NCA/Fe _SA+NC exhibits a negative shift of 0.4 eV compared with that in NCA/Fe _SA , as shown in Table 3, indicating that the electron cloud density of FeN ₄ sites in the former is larger, which may be due to the electron transfer from the neighboring Fe nanoclusters, which is consistent with the Fe 2p XPS results.

Table 1: Element contents (at%) determined by XPS and ICP-OES

Table 2: Iron content determined by XPS method

Table 3: N content determined by XPS method (%)

4. Analysis of coordination configuration of _FeNx sites

The coordination configuration of the _FeNx site was further studied by X-ray spectroscopy (XAS) measurements. Figure 7a and the inset depict the Fe k-edge X-ray absorption near-edge structure (XANES) spectra of the samples, where the order of white line intensity is Fe foil <NCA/Fe _SA+NC ≈FePc<NCA/Fe _SA , which is consistent with the result that NCA/Fe _SA+NC contains both Fe nanoclusters and Fe single atoms, while NCA/Fe _SA contains only Fe single atoms. As shown in Figure 7c, the corresponding change trend is also reflected in the absorption edge energy. The corresponding Fourier transform extended X-ray fine structure (EXAFS) curve is shown in Figure 7b. FePc, NCA/Fe _SA+NC and NCA/Fe _SA are all around A main peak appears at about 1000 nm, which is attributed to the Fe-N bond (from the above XPS measurement results, it can be seen that no Fe-O bond is formed); the second main peak of NCA/Fe _SA+NC appears at about 1000 nm. The peaks correspond to the Fe-Fe peaks in the Fe foil, which proves that the sample contains Fe nanoclusters. The coordination numbers (CN) and bond lengths of Fe-N and Fe-Fe are then determined by fitting the EXAFS data. The results are shown in Figures 7c to 7f. The specific values are shown in Table 4. It can be seen from Table 4 that the bond length corresponding to the Fe-Fe path of NCA/Fe _SA+NC is The bond length is consistent with that of Fe foil, but the CN is significantly lower (2 vs. 8), which is due to the smaller size of the metal clusters and the larger proportion of metal atoms with unsaturated coordination on the surface. For the Fe-N path, the CN of NCA/Fe _SA+NC is 3.6 and the bond length is Slightly smaller than NCA/Fe _SA (4.2 and ), which may be due to the presence of Fe clusters near the FeN ₄ site in NCA/Fe _SA+NC . These results indicate that the Fe in NCA/Fe _SA+NC is mainly composed of FeN ₄ sites and adjacent Fe clusters, while there is only FeN ₄ in NCA/Fe _SA .

Table 4: EXAFS fitting results

5. ^57Fe Spectroscopic and Electron Paramagnetic Resonance (EPR) Analysis

^57Fe Spectroscopy and _electron paramagnetic resonance (EPR) can be _used to explore ^the spin state of Fe. The spectrum contains at least two different pairs of peaks, D1 and D2, which are attributed to high-spin Fe(Ⅲ) and medium/low-spin Fe(Ⅱ), respectively. The two have similar isomeric shifts (δ) of 0.30-0.45 mm s ^-1 and different quadrupole splitting energies (ΔE _QS ) of 1.0 and 2.5 mm s ^-1 , respectively. The results are shown in Figures 8a and 8b. D1 and D2 can be detected in both NCA/Fe _SA+NC and NCA/Fe _SA samples. In addition, NCA/Fe _SA+NC has an additional peak, D3, δ = -0.06 mm s ^-1 , which can be attributed to Fe nanoclusters. Table 5 lists the percentages of D1 and D2 in the two carbon aerogels. It can be seen that D2 accounts for 50.7% in NCA/Fe _SA and 58.1% in NCA/Fe _SA+NC , which indicates that there is an effective chemical interaction between Fe single atoms and Fe nanoclusters, which is consistent with the XPS measurement results. According to literature reports, the D2 group exhibits better ORR stability than D1. It can be inferred that a higher D2 ratio in NCA/Fe _SA+NC is expected to lead to higher stability compared with NCA/ ^Fe _SA . The results of the spectrum are consistent with those of the samples of FeN4, as shown in Figure 8c. In the magnetic field strength range of 2000 to 5000 G, both NCA/Fe _SA+NC and NCA/Fe _SA show a signal peak centered at 3500 G, but the amplitude of the former is significantly weaker than that of the latter, further confirming that the adjacent Fe nanoclusters in NCA/Fe _SA+NC reduce the spin state of _FeN4 .

Table 5: ^57Fe The ratio of D1 to D2 in the test results

4. Theoretical Simulation

The present invention studies the interaction between Fe single atom sites and adjacent Fe nanoclusters through theoretical calculations. As shown in Figure 9a, we discuss four configurations: FeN ₄ (S ₀ site) on graphitic carbon, FeN ₄ and Fe nanoclusters separated by approximately (S1 site), (S2 site), (S3 site) of FeN ₄ -Fe ₁₃ configuration. The ORR reaction processes on the four configurations are shown in Figures 10 to 13. As shown in Figure 9b, from the free energy step diagram of ORR, it can be found that at a potential of +0.9V, the first and second electron transfer steps on S1, S2, S3 and S0 are exothermic reactions, while the third and fourth electron transfer steps are endothermic reactions, and the last step, the desorption of -OH is the rate-determining step (RDS). The desorption energy of -OH (ΔG _OH* ) at the S0 site is 0.36eV, and the desorption energy at the S1, S2, and S3 sites is slightly lower, which are 0.33eV, 0.30eV, and 0.34eV, respectively. This indicates that adjacent Fe nanoclusters can enhance the ORR activity of the FeN ₄ site. In this series, the S2 site has the lowest energy barrier (0.30V) and the highest limiting potential (0.60V), so it is thermodynamically and kinetically favorable for catalyzing ORR, as shown in Figure 9c.

Studies have found that the electronic spin state of the metal active site is closely related to the activity and/or stability of the _FeNx catalyst. Therefore, the present invention uses density functional theory (DFT) to calculate the magnetic moment of the Fe single atom at these sites to further explore the interaction between the Fe nanoclusters and the Fe single atom sites. The results are shown in Figure 9d. The S0 site is 1.94μB, and S1, S2 and S3 are reduced to 1.75, 1.68 and 1.65μB, respectively, which is consistent with the above ⁵⁷ Fe This is consistent with the measurement results. This shows that the adjacent Fe nanoclusters have a significant regulatory effect on the electrons of Fe single atoms, and it increases with the decrease in distance. The electron regulation effect can be further understood by analyzing the density of states (DOS). As can be seen from Figure 14a, for the S2 site, the Fe 3d electrons make a major contribution to the DOS near the Fermi level (marked with black arrows), indicating that the Fe atoms in FeN ₄ are the main active sites. Similar results can be obtained in the DOS diagrams of the S0, S1, and S3 sites, as shown in Figures 15a to 15c. Figure 14b compares the DOS of Fe3d electrons at the S1, S2, and S3 sites. As the nanoclusters get closer to FeN ₄ , the labeled state shifts negatively, indicating that the Fe center in FeN ₄ easily accepts electrons from neighboring Fe nanoclusters, and the labeled state of S2 is closest to the Fermi level, which is consistent with the results of the free energy step diagram.

Figures 14c, 15d and 15e show the DOS of the five Fe 3d orbitals at the S1, S2 and S3 sites, where the labeled states near the Fermi level are mainly associated with the _dxz orbital. However, the labeled states near the Fermi level at the S0 site are mainly associated with the _dxy orbital, as shown in Figure 15f. As the distance between the Fe nanocluster and the Fe single atom decreases (from S3 to S1), the electrons from the nanocluster form a fully filled _dxy orbital and a partially filled _dxz orbital in _FeN4 . This indicates that the electronic configuration and magnetic moment change significantly from S0 to S1, S2 and S3 due to the electron transfer from the Fe nanocluster to the Fe atom. This evolution of the electronic structure can lead to changes in catalytic activity, see the electrochemical measurements of ORR activity and stability below.

5. ORR catalytic performance of highly stable Fe single-atom catalysts

1. Evaluation of ORR activity of catalysts by rotating disk electrode (RDE)

The present invention uses a rotating disk electrode (RDE) at 1600rpm to evaluate the ORR activity of two carbon aerogels under 0.1MKOH conditions and compares them with commercial Pt/C. The results are as follows. As can be seen from Figures 16a and 16b, NCA/Fe _SA+NC exhibits good ORR performance, with an onset potential (E _onset ) of +1.05V and a half-wave potential (E _1/2 ) of +0.92V, which is better than NCA/Fe _SA (+1.01 and +0.90V) and commercial Pt/C catalysts (+0.98 and +0.86V). The electrochemical impedance spectroscopy (EIS) measurement results show that the charge transfer resistance (R _ct ) of NCA/Fe _SA+NC is 65.9Ω, which is significantly lower than NCA/Fe _SA (89.9Ω), as shown in Figure 16c. According to the polarization curves at different rotation speeds, as shown in Figure 17, the kinetic current density (J _k ) and Tafel slope at a certain potential can be calculated using the Koutecke-Levich equation. As shown in Figure 16b, the J _k of NCA/Fe _SA+NC at +0.85V is as high as 18.7mA cm ^-2 , which is about twice that of NCA/Fe _SA and Pt/C (10.8 vs. 9.5mA cm ^-2 ). The Tafel slope of NCA/Fe _SA+NC is 60mV dec ^-1 , which is close to the Tafel slope of NCA/Fe _SA (58mV dec ^-1 ), but lower than the Tafel slope of Pt/C (70mV dec ^-1 ), indicating a fast ORR reaction kinetic process, as shown in Figure 16d. The measurement results of the rotating ring disk electrode (RRDE) show that the ORR reaction on NCA/Fe _SA+NC and NCA/Fe _SA follows an efficient 4e ^- path, similar to Pt/C. In contrast, NCA/Fe _SA+NC exhibited the lowest average H ₂ O ₂ yield (2.65%) and the highest average electron transfer number (3.96) in the potential range of +0.2 V to +0.9 V, as shown in Figure 18. The remarkable ORR catalytic performance of NCA/Fe _SA+NC can be attributed to the electronic regulation of Fe single atomic sites by neighboring Fe nanoclusters.

2. Contribution of Fe nanoclusters and Fe single atoms to ORR activity

In order to distinguish the contribution of Fe clusters and single atoms to ORR electrocatalysis, the present invention treated NCA/Fe _SA+NC with acid etching. It can be seen from the TEM image that after etching in 0.5MH ₂ SO ₄ , the Fe clusters disappeared and obvious nanopores appeared, as shown in Figure 19a. The electrochemical test results showed that the removal of Fe clusters caused E _1/2 to shift negatively by 20mV to +0.90V, which is the same as the half-wave potential of NCA/Fe _SA , as shown in Figure 19b. The ORR activity of NCA/Fe _SA+NC comes from the Fe single atom sites, and the presence of Fe nanoclusters has a certain promoting effect on the catalytic activity. In addition, the electrocatalytic stability of NCA/Fe _SA+NC and NCA/Fe _SA was evaluated by cyclic voltammetry (CV) in the potential range of +0.2 to +1.2V. After 8000 continuous potential cycles, the ORR peak potential of the former shifted negatively by 5 mV, as shown in Figure 20a, which is less than one-tenth of the latter (54 mV, Figure 20b), confirming that the presence of Fe nanoclusters can significantly improve the stability of the catalyst. Even after 15,000 continuous cycles, the peak potential of NCA/Fe _SA+NC shifted negatively by only 14 mV, as shown in Figure 20a.

3. XPS measurement compares the structural changes of the two carbon aerogels before and after long-term cycle testing

The present invention uses XPS measurement to compare the structural changes of two carbon aerogels before and after long-term cycle testing. Comparing the Fe 2p spectra before and after 8000 CV cycles, it can be seen that the main peak of the NCA/Fe _SA+NC sample only has a positive shift of 0.1eV, which is significantly lower than NCA/Fe _SA (0.9eV), as shown in Figures 21a and 21b. It shows that the antioxidant capacity of the Fe species in the former is significantly enhanced. Note: The additives used in the electrode preparation process, such as Nafion and carbon black, make the 2p peak of Fe difficult to fit, so we compare the displacement of the main peak. HRXPS of the O1s spectrum further reveals the difference in the antioxidant properties of the two catalysts. As shown in Table 6, only 0.03% of metal-O species (MO) was detected in the NCA/Fe _SA+NC sample after the durability test, which is much lower than NCA/Fe _SA (0.19%), indicating that the Fe species in NCA/Fe _SA+NC have stronger antioxidant properties and stability. Table 7 lists the percentages of N species in carbon aerogels before and after the durability test. It can be seen that the difference between the two is mainly reflected in the change in the percentage of MN. The change in the percentage of MN in NCA/Fe _SA decreased from 0.45% to 0.11%, which is the main reason for the decrease in electrocatalytic activity. The percentage of MN in NCA/Fe _SA+NC only decreased slightly (0.55% to 0.45%), indicating that it can maintain a high catalytic activity.

As mentioned above, the attenuation of the ORR performance of _FeNx composites is largely due to the oxidation of Fe sites and aggregation into metal oxide nanoparticles. Therefore, we used TEM to investigate the microstructural changes of NCA/Fe _SA and NCA/Fe _SA+NC after stability testing. As can be seen from Figure 22 and the inset, although _{monoclinic Fe2O3} nanocrystals can be detected in both NCA/Fe _SA and NCA/Fe SA+ _NC , the number of nanoparticles in NCA/Fe _SA+NC is significantly lower than that in NCA/Fe _SA . This confirms that NCA/Fe _SA+NC has superior oxidation resistance, which is consistent with the XPS results.

Table 6: XPS determination of O content in carbon aerogel before and after stability test (%)

Table 7: XPS determination of N content in carbon aerogel before and after stability test (%)

6. Application of Highly Stable Fe Single Atom Catalysts in Low-Temperature Zn-Air Batteries

The above studies show that both _FeN4 sites and Fe nanoclusters may have catalytic activity for OER. Therefore, the present invention discusses the OER activity of these carbon aerogels and their application as bifunctional catalysts in rechargeable zinc-air batteries. Figures 23a and 24b show the OER polarization curves and Tafel curves of carbon aerogels under 1.0M KOH conditions. The corresponding overpotential (E _{OER, 10} ) of NCA/Fe _SA+NC is +1.57V when the OER current density is 10mA cm ^-2 , and the Tafel slope is 71.5mV dec ^-1 , both of which are lower than NCA/Fe _SA (+1.61V and 97.1mV dec ^-1 ). This shows that NCA/Fe _SA+NC has obvious advantages in OER electrocatalysis. Acid leaching experiments suggest that the OER activity of NCA/Fe _SA+NC comes from Fe single atoms, and Fe nanoclusters play an auxiliary enhancement role, as shown in Figure 23c. As shown in the inset of Figure 23a, the ORR and OER overpotential difference (ΔE＝ _{EOER, 10} - _E1/2 ) of NCA/FeSA _+NC is only 0.65 V, which is 60 mV lower than that of NCA/ _FeSA and is superior to most bifunctional oxygen electrocatalysts in the prior art.

1. Application in room temperature rechargeable zinc-air batteries

In order to explore the practical application of carbon aerogel in rechargeable zinc-air batteries, we assembled flexible zinc-air batteries with NCA/Fe _SA+NC or NCA/Fe _SA as air cathode catalysts, high-purity zinc plate as anode, and PAA hydrogel as flexible electrolyte. A comparative cell was assembled by using a mixture of commercial Pt/C and RuO ₂ catalysts. As shown in Figure 24a, the open circuit voltage (OCV) of the Zn//NCA/Fe _SA+NC battery was 1.50V, which was 30 and 120mV higher than that of the Zn//NCA/Fe _SA and Zn//Pt/C-RuO ₂ batteries, respectively. The maximum power density of the Zn//NCA/Fe _SA+NC battery was 236mW cm ^-2 , as shown in Figure 24b, which was much higher than that of the Zn//NCA/Fe _SA battery (170mW cm ^-2 ) and the Zn//Pt/C-RuO ₂ battery (119mW cm ^-2 ). In fact, the Zn//NCA/Fe _SA+NC battery shows a higher discharge voltage in the current density range of 5 to 50 mA cm ^-2 , as shown in Figure 24c. It is worth noting that the performance of this flexible Zn//NCA/Fe _SA+NC battery is better than most flexible zinc-air batteries and even better than the corresponding liquid batteries in terms of OCV and maximum power density, as shown in Figure 24d. Figure 25a shows the constant current charge and discharge curves at a current density of 5 mA cm ^-2 . After 770 consecutive charge and discharge cycles, the Zn//NCA/Fe _SA battery has a voltage gap of 0.90 V and a round-trip efficiency of 54.1%. In sharp contrast, the Zn//NCA/Fe _SA+NC battery has a narrower voltage gap of only 0.79 V, and even after 1800 cycles, the round-trip efficiency is still as high as 59.3%, indicating that its activity and durability are significantly improved. It is worth noting that when the PAA layer is compressed by 30% and 60%, the Zn//NCA/Fe _SA+NC battery still maintains 95% and 92% of the maximum power density, as shown in Figure 25b. When bent from 120° to 180°, the discharge-charge voltage remains almost unchanged, as shown in Figure 25c. After 1000 repeated compressions, the discharge voltage drop of the Zn//NCA/Fe _SA+NC battery is negligible, as shown in Figure 25d. All of these indicate that the Zn//NCA/Fe _SA+NC battery has good mechanical flexibility.

2. Application in zinc-air batteries at low temperatures

Low-temperature zinc-air batteries have great application potential in special environments such as polar expeditions and space exploration. However, the electrode reaction kinetics and ionic conductivity slow down significantly at low temperatures. Therefore, the operation of the device requires an effective electrode catalyst that can work even at such low temperatures. We assembled a low-temperature zinc-air battery using NCA/Fe _SA+NC as air cathode catalyst, a high-purity zinc plate as anode, and a PAM hydrogel containing dimethyl sulfoxide as an electrolyte. The results are shown in Figures 26a and 26b. The assembled Zn//NCA/Fe _SA+NC battery has a voltage of 1.49V and a maximum power density of 97.0mW cm ^-2 at -20°C; at -40°C, the voltage and maximum power density are 1.47V and 49.0mW cm ^-2 , respectively. The Zn//NCA/Fe _SA+NC battery also exhibits a stable discharge voltage in the current density range of 0.2 to 5.0 mA cm ^-2. At a current density of 1.0 mA cm ^-2 at -20°C and -40°C, the discharge voltage is 1.36 V and 1.31 V, respectively. The results are shown in Figure 26c. Figure 26d compares the performance of the recently reported related low-temperature zinc-air batteries. It can be seen that the performance of this Zn//NCA/Fe _SA+NC battery is significantly better than most batteries. As shown in Figure 26d, at -40°C and 1.0 mA cm ^-2 , after 2300 continuous constant current charge and discharge cycles, the Zn//NCA/Fe _SA+NC battery maintains a stable discharge platform with a round-trip efficiency of up to 81.4% and a voltage gap of only 0.32 V, proving that the synthesized carbon aerogel catalyst also has great practical application advantages at ultra-low temperatures. As shown in Figure 27, even at -40°C, only two Zn//NCA/Fe _SA+NC batteries connected in series can drive an LED with a rated voltage of 3.0 V. These results confirm that the NCA/Fe _SA+NC catalyst designed in the present invention has good application prospects in high-efficiency and freeze-resistant zinc-air batteries.

In summary, the present invention uses gelatin hydrogel as a precursor and template, and loads Fe nanoclusters (NCA/Fe _SA+NC ) near the Fe single atom sites in N-doped carbon aerogels through a simple two-step pyrolysis method. Spectral characterization, first-principles calculations and electrochemical test results all show that Fe nanoclusters have an electron-donating effect on the surrounding Fe single atoms, resulting in a decrease in magnetic moment, which can effectively promote their electrocatalytic activity and stability. The flexible zinc-air battery assembled with NCA/Fe _SA+NC aerogel as the cathode catalyst exhibits high OCV and power density at room temperature and low temperature (-40°C), as well as excellent durability. The results of this study provide a new paradigm for optimizing the stability and activity of M–N–C nanocomposite catalysts in electrochemical energy technology.

The above contents are further detailed descriptions of the present invention in combination with specific preferred embodiments, and it cannot be determined that the specific implementation of the present invention is limited to these descriptions. For ordinary technicians in the field to which the present invention belongs, several simple deductions or substitutions can be made without departing from the concept of the present invention, which should be regarded as falling within the scope of protection of the present invention.

Claims (8)

1. The preparation method of the high-stability iron atom catalyst is characterized by comprising the following steps of:

S1, uniformly mixing a carbon source, a pore-forming agent, ferric salt, a nitrogen source and water according to mass proportion, and performing low-temperature self-assembly to obtain hydrogel after water bath at 55-65 ℃; the carbon source is one of gelatin, chitosan, starch or agar, and the nitrogen source is one of phenanthroline or melamine;

S2, heating the hydrogel prepared in the step S1 to 400-500 ℃ in Ar or N ₂ atmosphere for pyrolysis, and etching with 0.5-1.0M strong alkali solution to remove the pore-forming agent to prepare a semi-finished product;

and S3, heating the semi-finished product prepared in the step S2 to 900-950 ℃ under the mixed atmosphere of 97% Ar+3% H ₂ for pyrolysis, and thus obtaining the high-stability iron atom catalyst.

2. The method for preparing a highly stable iron atom catalyst according to claim 1, wherein the pore-forming agent is one of silica or aluminum oxide, the particle size of the silica is 10-30 nm, and the particle size of the aluminum oxide is 10-50 nm.

3. The method for preparing a highly stable iron atom catalyst according to claim 2, wherein in step S1, the mass ratio of the carbon source, the pore-forming agent, the iron salt, the nitrogen source and water is (15-30): (5-12.5): (1-4): (2.97 to 11.9): (0.625 to 1.25).

4. The method for preparing a highly stable iron atom catalyst according to claim 1, wherein the heating rate in step S2 and step S3 is 5 to 10 ℃ per minute, and the pyrolysis time is 2 to 3 hours.

5. The method for preparing a highly stable iron atom catalyst according to claim 1, wherein the porogen is removed by etching at 75-85 ℃ in step S2.

6. The highly stable iron atom catalyst prepared by the preparation method according to any one of claims 1 to 5, wherein the catalyst is a three-dimensional honeycomb network structure formed by nitrogen-doped carbon frameworks, the three-dimensional honeycomb network structure further comprises iron monoatoms and iron nanoclusters, the iron nanoclusters are loaded in nanopores of the three-dimensional honeycomb network structure, the iron monoatoms are combined on the nitrogen-doped carbon frameworks around the iron nanoclusters, the diameter of the iron nanoclusters is smaller than 10nm, and the distance between the iron nanoclusters and the iron monoatoms is smaller than 2 nm.

7. Use of the highly stable iron atom catalyst of claim 6 as an air cathode catalyst in flexible zinc-air batteries.

8. The use according to claim 7, wherein the highly stable iron catalyst is used at room temperature or at 0 to-40 ℃ when used as an air cathode catalyst in flexible zinc-air batteries.