Fe-Sn-NC cathode oxygen reduction catalyst and preparation method and application

Abstract

The present invention provides a Fe-Sn-N-C cathode oxygen reduction catalyst, a preparation method, and an application, and belongs to the field of catalysts and their preparation technology. The preparation method includes: under a protective atmosphere, pyrolyzing ZIF-8 with a framework structure of a regular dodecahedron to obtain a carbonized nitrogen-carbon substrate, wherein the nitrogen-carbon substrate has a framework structure of a regular dodecahedron; mixing and heating a tin-based precursor with a nitrogen-carbon substrate to obtain a tin-intercalated nitrogen-carbon substrate; calcining the tin-intercalated nitrogen-carbon substrate to obtain a Sn-N-C composite solid; chemical vapor deposition of the Sn-N-C composite solid and an iron-based precursor to obtain a Fe-Sn-N-C cathode oxygen reduction catalyst. The present invention increases the number and density of active sites of the Fe-Sn-N-C cathode oxygen reduction catalyst, and utilizes the excellent graphite intercalation ability of tin salts to improve the stability of the Fe-Sn-N-C cathode oxygen reduction catalyst.

Description

Technical Field

[0001] The present invention relates to the technical field of catalysts and their preparation, and in particular to an Fe-Sn-NC cathode oxygen reduction catalyst and a preparation method and application thereof. Background Art

[0002] Proton exchange membrane fuel cells (PEMFCs) are a new type of energy conversion device that is relatively environmentally friendly. However, conventional fuel cells often use precious metal catalysts, particularly as cathode oxygen reduction catalysts. These catalysts have drawbacks such as high cost and limited resources, hindering the large-scale deployment of PEMFCs.

[0003] Non-precious metal single-atom catalysts offer low cost and high catalytic activity. However, in proton exchange membrane fuel cells, the dissolution of non-precious metal sites significantly reduces the loading of single-atom active centers, resulting in poor stability and low catalytic efficiency in practical applications. Summary of the Invention

[0004] In view of this, in order to at least partially solve at least one of the above-mentioned technical problems, the present invention provides a Fe-Sn-NC cathode oxygen reduction catalyst and a preparation method and application thereof.

[0005] According to an embodiment of one aspect of the present invention, a method for preparing an Fe-Sn-NC cathode oxygen reduction catalyst is provided, comprising: pyrolyzing ZIF-8 having a regular dodecahedral framework structure under a protective atmosphere to obtain a carbonized nitrogen-carbon substrate, wherein the nitrogen-carbon substrate has a regular dodecahedral framework structure; mixing and heating a tin-based precursor with the nitrogen-carbon substrate to obtain a tin-intercalated nitrogen-carbon substrate; calcining the tin-intercalated nitrogen-carbon substrate to obtain a Sn-NC composite solid; and chemically vapor depositing the Sn-NC composite solid and an iron-based precursor to obtain a Fe-Sn-NC cathode oxygen reduction catalyst.

[0006] According to an embodiment of the present invention, chemical vapor deposition of a Sn-NC composite solid and an iron-based precursor to obtain a Fe-Sn-NC cathode oxygen reduction catalyst includes: chemical vapor deposition of the Sn-NC composite solid and the iron-based precursor at 700-800°C under inert gas conditions for 2-4 hours to obtain a Fe-Sn-NC cathode oxygen reduction catalyst precursor; and pickling and drying the Fe-Sn-NC cathode oxygen reduction catalyst precursor to obtain a Fe-Sn-NC cathode oxygen reduction catalyst.

[0007] According to an embodiment of the present invention, mixing a tin-based precursor with a nitrogen-carbon substrate and heating the mixture includes: grinding the tin-based precursor and the nitrogen-carbon substrate at room temperature for 10 to 20 minutes to obtain a mixture; heating the mixture from room temperature to 300 to 400°C, maintaining the temperature at 300 to 400°C for 1 to 3 hours, and a heating rate of (4 to 6)°C / min.

[0008] According to an embodiment of the present invention, the molar ratio of the tin-based precursor to the nitrogen-carbon substrate is 1:(1-7.5), and the tin salt precursor includes at least one of tin tetrachloride, tin tetrachloride hydrate, and stannous chloride.

[0009] According to an embodiment of the present invention, calcining a tin-intercalated nitrogen-carbon substrate to obtain a Sn-NC composite solid includes: under a protective atmosphere, heating the tin-intercalated nitrogen-carbon substrate from (300-400)°C to 750°C at a heating rate of (4-6)°C / min, calcining at 750°C for 1.5-2.5h, then heating to 900°C at a heating rate of (4-6)°C / min, and continuing to calcine at 900°C for 0.5-1.5h to obtain a Sn-NC composite solid.

[0010] According to an embodiment of the present invention, the iron-based precursor includes at least one of ferrous chloride, ferrous chloride hydrate, and ferric chloride; and the molar ratio of the Sn-NC composite solid to the iron-based precursor is (1-1.5):1.

[0011] According to an embodiment of the present invention, the pH of the pickling is 0.5-1.5, and the drying temperature is 60°C.

[0012] According to an embodiment of the present invention, ZIF-8 is prepared by the following process: zinc nitrate or its hydrate and dimethylimidazole are dispersed in a methanol solution and mixed to obtain a mixed solution; the mixed solution is centrifuged and dried in sequence to obtain ZIF-8; the mass ratio of zinc nitrate or its hydrate and dimethylimidazole is (2~4): (5.5~7.5).

[0013] According to an embodiment of another aspect of the present invention, there is provided an Fe-Sn-NC cathode oxygen reduction catalyst prepared by the preparation method described above, comprising: a nitrogen-carbon substrate having a regular dodecahedral framework structure; and an active center comprising an Fe-Sn bimetallic single atom site supported within the nitrogen-carbon substrate; wherein the iron and tin are coordinated to form a bond, and the mass fraction of iron in the Fe-Sn-NC cathode oxygen reduction catalyst is 1-2%.

[0014] According to another embodiment of the present invention, there is provided an application of the above-mentioned Fe-Sn-NC cathode oxygen reduction catalyst in the field of cathode technology of proton exchange membrane fuel cells.

[0015] According to an embodiment of the present invention, a tin-based precursor is mixed and heated with a nitrogen-carbon substrate, and the graphite intercalation ability of the tin-based precursor is used to insert tin into the interior of the carbon-nitrogen substrate. After calcination, Sn-N4 active sites are formed between the tin and nitrogen and fixed to the interior of the carbon-nitrogen substrate. By vapor deposition of an iron-based precursor and a Sn-NC composite solid, iron replaces part of the tin in Sn-N4, forming an Fe-Sn bimetallic single-atom site supported on the interior of the nitrogen-carbon substrate, thereby producing an Fe-Sn-NC cathode oxygen reduction catalyst with a higher active site density and stronger stability.

[0016] The excellent graphite intercalation ability of the tin-based precursor allows it to be evenly distributed between the graphite layers of the nitrogen-carbon substrate. Interaction with nitrogen forms new active sites, increasing the number of catalytic sites available for the oxygen reduction reaction (ORR) on the NCN substrate, thereby improving the active site density and enhancing the catalytic activity of the Fe-Sn-NC cathode ORR catalyst. The addition of the tin-based precursor modifies the graphite interlayer spacing, increasing it during subsequent chemical vapor deposition (CVD). This facilitates the incorporation of iron into the NCN substrate, displacing tin and contributing to enhanced acidic ORR activity. The graphite intercalation ability of the tin-based precursor increases the graphitic degree of carbon within the NCN substrate, reducing defective carbon and improving the stability of the Fe-Sn-NC cathode ORR catalyst. When used in proton exchange membrane fuel cells, the immobilization of iron and tin within the NCN substrate reduces the dissolution of iron and tin active sites, thereby enhancing the cycling stability of the Fe-Sn-NC cathode ORR catalyst.

[0017] The preparation method of the invention is simple, easy to implement, environmentally friendly and suitable for large-scale production. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 A flow chart showing a method for preparing a Fe-Sn-NC cathode oxygen reduction catalyst according to an embodiment of the present invention is shown;

[0019] Figure 2a shows a scanning electron microscope image of a nitrogen-carbon substrate (NC) prepared in Example 1 of the present invention;

[0020] Figure 2b shows a scanning electron microscope image of the Sn-NC composite solid (SnNC) prepared in Example 1 of the present invention;

[0021] Figure 2c shows a scanning electron microscope image of the Fe-Sn-NC cathode oxygen reduction catalyst 1 (SnFeNC) prepared in Example 1 of the present invention;

[0022] Figure 3The figure shows a half-wave potential curve of the Fe-Sn-NC cathode oxygen reduction catalyst 1 prepared in Example 1 of the present invention in a 0.1M perchloric acid solution;

[0023] Figure 4 The figure shows the half-wave potential curves of the Fe-Sn-NC cathode oxygen reduction catalyst 1 prepared in Example 1 of the present invention in a 0.1M perchloric acid solution in the initial state, before and after 10,000 cycles, 20,000 cycles, 30,000 cycles, 50,000 cycles, and 70,000 cycles of accelerated aging test;

[0024] Figure 5 A graph showing the power density and voltage of the Fe-Sn-NC cathode oxygen reduction catalyst 1 prepared in Example 1 of the present invention as a function of current is shown;

[0025] Figure 6 The Raman shifts of the nitrogen-carbon substrate, Sn-NC composite solid, and Fe-Sn-NC cathode oxygen reduction catalyst 1 prepared in Example 1 of the present invention are shown;

[0026] Figure 7 The in situ Raman scattering patterns (SERS) of the Sn-NC composite solid prepared in Example 1 of the present invention, the Fe-Sn-NC cathode oxygen reduction catalyst 1, and the Fe-NC cathode oxygen reduction catalyst 1' (FeNC) prepared in Comparative Example 1 are shown, wherein a is an in situ Raman scattering pattern of FeNC; b is a bar graph showing the graphitization degree of FeNC; c is an in situ Raman scattering pattern of SnNC; d is a bar graph showing the graphitization degree of SnNC; e is an in situ Raman scattering pattern of SnFeNC; and f is a bar graph showing the graphitization degree of SnFeNC.

[0027] Figure 8 X-ray diffraction spectra of the nitrogen-carbon substrate, Sn-NC composite solid, Fe-Sn-NC cathode oxygen reduction catalyst 1 prepared in Example 1 of the present invention, and Fe-NC cathode oxygen reduction catalyst 1' prepared in Comparative Example 1 are shown. DETAILED DESCRIPTION

[0028] Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. However, it should be understood that these descriptions are exemplary only and are not intended to limit the scope of the present invention. In the following detailed description, for ease of explanation, many specific details are set forth to provide a comprehensive understanding of embodiments of the present invention. However, it is apparent that one or more embodiments may also be implemented without these specific details. In addition, in the following description, descriptions of known structures and technologies are omitted to avoid unnecessary confusion of the concept of the present invention.

[0029] The terms used herein are only for describing specific embodiments and are not intended to limit the present invention. The term "comprising" used herein indicates the existence of features, steps, operations, but does not exclude the existence or addition of one or more other features.

[0030] When expressions such as “at least one of A, B, and C, etc.” are used, they should generally be interpreted in accordance with the meaning commonly understood by those skilled in the art (for example, “a system having at least one of A, B, and C” should include but is not limited to systems having A alone, B alone, C alone, A and B, A and C, B and C, and / or A, B, and C, etc.). When expressions such as “at least one of A, B, or C, etc.” are used, they should generally be interpreted in accordance with the meaning commonly understood by those skilled in the art (for example, “a system having at least one of A, B, or C” should include but is not limited to systems having A alone, B alone, C alone, A and B, A and C, B and C, and / or A, B, and C, etc.).

[0031] In response to the phenomenon of single-atom site dissolution of non-precious metal single-atom catalysts in acidic oxygen reduction environments, related technologies mostly use methods such as introducing free radical scavengers to inhibit single-atom dissolution, designing highly stable local coordination structures of MNC catalysts, or increasing the density of single-atom active sites.

[0032] One approach in related technology is to introduce Ce 3+ / 4+ Redox couples act as free radical scavengers to protect single atom catalysts from free radical attacks, but since such free radical scavengers have no oxygen reduction activity, they have little effect on improving the initial power density of the battery.

[0033] Another approach in related technologies is to optimize the active site structure by constructing multi-center structures, introducing heteroatom groups, and locally modulating the MN coordination structure. However, it is difficult to effectively take into account the active site density, and the effect on improving the volume-to-power ratio is minimal.

[0034] In the process of realizing the concept of the present invention, it was found that the Fe-NC single-atom catalyst has good oxygen reduction activity under acidic conditions and can well catalyze the progress of oxygen reduction. However, when applied to proton exchange membrane fuel cells, due to the progress of the Fenton reaction under acidic conditions, the iron active sites are dissolved in the form of particles under operating conditions, which further reduces the catalytic ability and stability of the Fe-NC single-atom catalyst.

[0035] Furthermore, in order to solve the above problems, the present invention utilizes the good graphite intercalation ability of the tin-based precursor to insert tin into the interior of the nitrogen-carbon substrate, and then fixes the tin through calcination, and forms a Sn-N4 active site with the surrounding nitrogen, and then performs chemical vapor deposition with the iron-based precursor to replace part of the tin to form an Fe-Sn bimetallic single atom site. Based on the graphite intercalation ability of the tin-based precursor, the loading on the obtained single atom site is relatively high.

[0036] Specifically, according to an embodiment of one aspect of the present invention, a method for preparing a Fe-Sn-NC cathode oxygen reduction catalyst is provided. Figure 1 A flow chart of a method for preparing a Fe-Sn-NC cathode oxygen reduction catalyst according to an embodiment of the present invention is shown. Figure 1 As shown, the preparation method includes operations S101 to S104.

[0037] In operation S101 , ZIF-8 having a regular dodecahedron framework structure is pyrolyzed under a protective atmosphere to obtain a carbonized nitrogen-carbon substrate, wherein the nitrogen-carbon substrate has a regular dodecahedron framework structure.

[0038] According to an embodiment of the present invention, the use of ZIF-8, which has a regular dodecahedral framework structure, as a support can maintain a high specific surface area and a relatively rich pore structure after pyrolysis, which helps to increase the number of active sites for subsequent catalyst formation, thereby improving the catalytic effect. The protective atmosphere can be, for example, at least one of nitrogen and argon. At a relatively high pyrolysis temperature (for example, 900-1000°C), the organic ligands in ZIF-8 decompose into carbon at high temperatures, leaving behind a regular dodecahedral framework structure.

[0039] In operation S102 , a tin-based precursor is mixed with a carbon nitride substrate and heated to obtain a tin-intercalated carbon nitride substrate.

[0040] According to embodiments of the present invention, the tin-based precursor can be understood as a tin salt compound containing tin ions. Leveraging the unique graphite intercalation capability of the tin-based precursor, the tin salt precursor is able to penetrate deeply into the carbon nitride substrate. The intercalation of tin helps expand the interlayer spacing of the graphite nitride substrate, providing more space for subsequent iron deposition. This allows the iron to more easily enter the carbon nitride substrate, thereby increasing the number and density of active sites formed.

[0041] In operation S103 , the Sn-intercalated nitrogen-carbon substrate is calcined to obtain a Sn-NC composite solid.

[0042] According to an embodiment of the present invention, calcination helps to fix tin inside the nitrogen-carbon substrate, so that tin and nitrogen form a bond to form a Sn-N4 active center.

[0043] In operation S104 , a Sn-NC composite solid and an iron-based precursor are chemically vapor deposited to obtain a Fe-Sn-NC cathode oxygen reduction catalyst.

[0044] According to an embodiment of the present invention, iron is uniformly dispersed on the surface and inside of the nitrogen-carbon substrate by chemical vapor deposition, so that iron partially replaces tin and is fixed in the Fe-N4 and Sn-N4 inside the nitrogen-carbon substrate, where Fe and Sn are coordinated and bonded to form Fe-Sn bimetallic single atom sites, so that the single atom load in the nitrogen-carbon substrate is higher, and it has a higher half-wave potential and better cyclic stability in the acidic oxygen reduction reaction.

[0045] According to an embodiment of the present invention, the graphite interlayer spacing of the nitrogen-carbon substrate is modified by the intervention of a tin-based precursor. When an iron-based precursor is subsequently used for replacement, it is beneficial for iron to enter the interior of the nitrogen-carbon substrate, increasing the density of active sites, thereby improving the catalytic activity and stability of the Fe-Sn-NC cathode oxygen reduction catalyst. In addition, due to the insertion of the tin-based precursor, the degree of graphitization of the nitrogen-carbon substrate is improved. When subsequently used in an oxygen reduction process in an acidic environment, the iron undergoes less Fenton reaction and the decomposition of the nitrogen-carbon substrate is reduced, resulting in less dissolution of the active sites of iron and tin, further improving the catalytic activity and cyclic stability of the Fe-Sn-NC cathode oxygen reduction catalyst.

[0046] According to an embodiment of the present invention, ZIF-8 is prepared by the following process: zinc nitrate or its hydrate and dimethylimidazole are dispersed in a methanol solution and mixed to obtain a mixed solution; the mixed solution is centrifuged and dried in sequence to obtain ZIF-8.

[0047] In one embodiment, zinc nitrate or its hydrate is dissolved in a methanol solution to obtain a zinc nitrate solution. Dimethylimidazole is dissolved in a methanol solution to obtain a dimethylimidazole solution. The zinc nitrate solution is added to the dimethylimidazole solution, stirred and mixed to obtain a mixed solution. Zinc nitrate or its hydrate and dimethylimidazole have high solubility in methanol, and zinc ions and dimethylimidazole molecules are formed during the dissolution process. During the mixing process, the two interact through electrostatic attraction to form a complex. If necessary, the mixture can be allowed to stand for a period of time to promote the complex to assemble into the crystal structure of ZIF-8, and then the unreacted raw materials and by-products are removed by centrifugation, and the residual solvent is removed by drying.

[0048] According to an embodiment of the present invention, the mass ratio of zinc nitrate to dimethylimidazole is (2-4):(5.5-7.5), preferably 3:6.5. For example, 3 g of zinc nitrate hexahydrate can be dissolved in 40 mL of methanol solution to form a zinc nitrate solution, and 6.5 g of dimethylimidazole can be dissolved in 80 mL of methanol solution to form a dimethylimidazole solution.

[0049] Preferably, the mixture is stirred and mixed for 24 hours, centrifuged at a speed of 10,000 rpm for 3 minutes, and washed with methanol as needed after centrifugation, for example, three times. Drying can be performed, for example, by vacuum drying at a temperature of 55 to 60° C., preferably 60° C., for 12 hours.

[0050] In one embodiment, the pyrolysis conditions in operation S101 can be, for example, calcination at 950° C. for 1 hour, and the atmosphere used can be, for example, argon, and the argon flow rate can be 40 sccm. Most of the zinc ions in the nitrogen-carbon substrate are evaporated by calcination to form a graphitized structure of the nitrogen-carbon substrate.

[0051] According to an embodiment of the present invention, ZIF-8 having a regular dodecahedral framework structure is used as a carrier, which can maintain a high specific surface area and a relatively rich pore structure after pyrolysis, which helps to increase the number of active sites for subsequent catalyst formation, thereby improving the catalytic effect.

[0052] According to an embodiment of the present invention, operation S102 includes sub-operations S1021 and S1022.

[0053] In sub-operation S1021, the tin-based precursor and the nitrogen-carbon substrate are ground at room temperature for 10 to 20 minutes to obtain a mixture.

[0054] In sub-operation S1022, the mixture is heated from room temperature to (300-400)°C, for example, 300°C, 350°C, or 400°C, preferably 300°C. The temperature is maintained at (300-400)°C for 1-3 hours at a heating rate of (4-6)°C / min, preferably 5°C / min.

[0055] According to an embodiment of the present invention, the tin-based precursor and the nitrogen-carbon substrate are fully mixed and uniformly through grinding, which helps the two to interact after subsequent heating. By heating, the tin ions replace part of the zinc ions retained in the nitrogen-carbon substrate. Continuing to maintain the temperature at (300~400) ° C for a certain period of time helps the interaction between the tin ions and nitrogen to form Sn-N4 active sites, thereby fixing the tin inside the nitrogen-carbon substrate.

[0056] Preferably, the grinding time is 15 min.

[0057] More preferably, the molar ratio of the tin-based precursor to the nitrogen-carbon substrate is 1:(1-7.5), for example, 1:1, 1:2, 1:3, 1:4, 1:5, 1:5.5, 1:6, 1:6.5, 1:7, or 1:7.5, preferably 1:5. If too much tin-based precursor is added, tin particles are likely to form; if too little tin-based precursor is added, the single-atom loading of tin ions on the active sites is insufficient, resulting in poor catalytic activity. The tin salt precursor includes at least one of tin tetrachloride, tin tetrachloride hydrate, and stannous chloride, preferably tin tetrachloride or tin tetrachloride hydrate. The boiling points of these tin salt precursors are all below 700°C, lower than the boiling point of the iron-based precursor, which facilitates partial replacement by iron ions during the subsequent chemical vapor deposition process. When tin tetrachloride or tin tetrachloride hydrate is used as the tin-based precursor, due to its lower boiling point, it can be more easily fixed within the nitrogen-carbon substrate.

[0058] According to an embodiment of the present invention, operation S103 specifically includes: heating the Sn-intercalated carbon nitride substrate from (300-400)°C to 750°C at a heating rate of (4-6)°C / min, preferably 5°C / min, under a protective atmosphere; calcining at 750°C for 1.5-2.5 hours, preferably 2 hours; then heating to 900°C at a heating rate of (4-6)°C / min, preferably 5°C / min, and further calcining at 900°C for 0.5-1.5 hours, preferably 1 hour, to obtain a Sn-NC composite solid. Calcination at 750°C helps remove volatile components, and a second calcination at 900°C helps promote atomic rearrangement within the Sn-intercalated carbon nitride substrate, resulting in a more compact and structurally stable Sn-NC composite solid.

[0059] According to an embodiment of the present invention, operation S104 includes sub-operations S1041 - S1042 .

[0060] In sub-operation S1041, the Sn-NC composite solid and the iron-based precursor are chemically vapor deposited at 700-800° C. under inert gas conditions for 2-4 hours to obtain a Fe-Sn-NC cathode oxygen reduction catalyst precursor.

[0061] In sub-operation S1042 , the Fe—Sn—NC cathode oxygen reduction catalyst precursor is subjected to acid washing and drying to obtain the Fe—Sn—NC cathode oxygen reduction catalyst.

[0062] According to embodiments of the present invention, the iron-based precursor can transform into a gaseous state at the aforementioned high temperatures (700-800°C), enter the Sn-NC composite solid, and displace the remaining zinc ions and some tin ions. This allows the iron ions to form Fe-N4 active sites with the nitrogen, while also forming coordinated bonds between the iron ions and the tin ions. This reduces the dissolution of iron and tin ions during subsequent application in an acidic oxygen reduction environment, thereby improving the single-atom loading and cyclic stability of the Fe-Sn-NC cathode oxygen reduction catalyst. Acid washing helps remove impurities such as iron-based precursor particles from the surface of the Fe-Sn-NC cathode oxygen reduction catalyst precursor. Drying also helps remove moisture and other volatile substances from the surface of the Fe-Sn-NC cathode oxygen reduction catalyst precursor, ensuring the purity of the prepared Fe-Sn-NC cathode oxygen reduction catalyst.

[0063] For example, the chemical vapor deposition process can be understood as follows: an iron-based precursor is placed in the lower portion of a container, and a Sn-NC composite solid is placed in the upper portion. The upper and lower portions of the container are separated by a vapor membrane, such as a porcelain boat. Pyrolysis is then performed, transforming the iron-based precursor into a gaseous form. After passing through the vapor membrane, it is deposited onto the surface and interior of the Sn-NC composite solid.

[0064] Furthermore, the iron-based precursor includes at least one of ferrous chloride, ferrous chloride hydrate, and ferric chloride. Ferrous chloride easily decomposes at high temperatures (700-800°C), while ferric chloride has a boiling point of around 300°C. During the vapor deposition process, iron ions can easily enter the Sn-NC composite solid. The molar ratio of the Sn-NC composite solid to the iron-based precursor is (1-1.5):1, for example, 1:1, 1.1:1, 1.2:1, 1.3:1, 1.4:1, or 1.5:1, preferably 1:1. If too much iron-based precursor is added, iron particles are easily formed; if too little iron-based precursor is added, the single-atom loading of iron ions at the active sites is insufficient, resulting in poor catalytic activity.

[0065] Preferably, the pH of the pickling process is 0.5 to 1.5, more preferably, 1. The acidic solution used for pickling can be, for example, a 0.5 to 1 mol / L sulfuric acid solution. Pickling can be performed overnight to thoroughly remove impurities such as the iron-based precursor particles. The drying temperature is 60°C, and the drying method is vacuum drying. Drying can be performed overnight to fully remove moisture and other volatile substances.

[0066] In a specific embodiment, the preparation process of the Fe-Sn-NC cathode oxygen reduction catalyst is as follows:

[0067] Dissolve zinc nitrate hexahydrate in methanol solution to obtain a zinc nitrate solution.

[0068] Dissolve dimethylimidazole in methanol solution to obtain a dimethylimidazole solution.

[0069] The zinc nitrate solution was added to the dimethylimidazole solution, and the mixture was stirred to obtain a mixed solution.

[0070] The mixed solution was centrifuged, washed with methanol, and vacuum dried to obtain ZIF-8.

[0071] Under a protective gas atmosphere, ZIF-8 was pyrolyzed to obtain a carbonized nitrogen-carbon substrate.

[0072] A tin-based precursor is mixed with a nitrogen-carbon substrate and heated to obtain a tin-intercalated nitrogen-carbon substrate.

[0073] The Sn-intercalated nitrogen-carbon substrate is calcined to obtain a Sn-NC composite solid.

[0074] The Sn-NC composite solid and the iron-based precursor are chemically vapor deposited, and then acid-washed, filtered, and dried in sequence to obtain the Fe-Sn-NC cathode oxygen reduction catalyst.

[0075] According to an embodiment of another aspect of the present invention, there is provided an Fe-Sn-NC cathode oxygen reduction catalyst prepared by the preparation method described above, comprising: a nitrogen-carbon substrate having a regular dodecahedral framework structure; and an active center comprising an Fe-Sn bimetallic single atom site supported within the nitrogen-carbon substrate; wherein the iron and tin are coordinated to form a bond.

[0076] According to an embodiment of the present invention, the active center of iron is Fe-N4, and the active center of tin is Sn-N4. The iron and tin are coordinated to form a bond, forming a Fe-Sn bimetallic single-atom site supported on the surface and interior of the nitrogen-carbon substrate. The present invention is based on an intercalated graphite layer, and the single-atom loading of the obtained Fe-Sn-NC cathode oxygen reduction catalyst is high, and it has a high half-wave potential and cyclic stability when applied to acidic oxygen reduction reactions. By forming Fe-Sn bimetallic single-atom sites, the dissolution of iron and tin in the form of particles due to the Fenton phenomenon in the application to the cathode of a fuel cell is suppressed, and the single-atom stability of iron and tin is further improved.

[0077] Furthermore, the mass fraction of iron in the Fe-Sn-NC cathode oxygen reduction catalyst is 1-2%, for example, 1%, 1.5%, or 2%. This indicates that by fixing the above iron content on the nitrogen-carbon substrate, the electronic structure of the Fe-N4 active center can be optimized, thereby improving the catalytic efficiency of the Fe-Sn bimetallic single atom site.

[0078] According to another embodiment of the present invention, there is provided an application of the above-mentioned Fe-Sn-NC cathode oxygen reduction catalyst in the field of cathode technology of proton exchange membrane fuel cells.

[0079] According to an embodiment of the present invention, the iron and tin in the catalyst are monoatomic dispersions, which can form more active sites, making the catalytic activity of the Fe-Sn-NC cathode oxygen reduction catalyst per unit mass higher, and making the Fe-SNC cathode oxygen reduction catalyst have a higher half-wave potential, which can effectively catalyze the oxygen reduction reaction at a lower potential. Due to the coordination between iron and tin, and the graphitization of the nitrogen-carbon substrate, the occurrence of the Fenton reaction is reduced, thereby reducing the dissolution of metals on the active sites under operating conditions, improving the cyclic stability of the catalyst, and thus improving the voltage output and energy efficiency of the fuel cell. It also has high catalytic activity and stability, significantly improving the performance of the fuel cell and extending the service life of the battery.

[0080] In one embodiment, the half-wave potential of the Fe-Sn-NC cathode oxygen reduction catalyst can reach 0.85 V, which has relatively wide applicability in the field of proton exchange membrane fuel cell technology.

[0081] The present invention will be further described below by way of examples and related test experiments and results thereof. In the detailed description below, for ease of explanation, many specific details have been set forth to provide a comprehensive understanding of the embodiments of the present invention. However, it is apparent that one or more embodiments may be implemented without these specific details. Moreover, in the absence of conflict, the details in the following embodiments may be combined arbitrarily into other feasible embodiments.

[0082] It should be noted that the following specific examples are for illustration only and the scope of protection of the present invention is not limited thereto. The chemicals and raw materials used in the following examples were either commercially available or prepared in-house using recognized processing methods.

[0083] Example 1:

[0084] Add 3.0 g of zinc nitrate hexahydrate to 40 mL of methanol at room temperature and stir until completely dissolved to obtain a zinc nitrate solution. Add 6.5 g of dimethylimidazole to 80 mL of methanol and stir until completely dissolved to obtain a dimethylimidazole solution. Under stirring, slowly add 40 mL of zinc nitrate methanol solution to 80 mL of dimethylimidazole methanol solution, and then stir at room temperature for 24 hours to obtain a first suspension. The first suspension is centrifuged at 10,000 rpm for 3 minutes, washed three times with methanol solution, and then dried in a vacuum drying oven at 60°C for 12 hours to obtain ZIF-8.

[0085] ZIF-8 was placed in a tube furnace and calcined at 950 °C for 1 h under an argon atmosphere to obtain a nitrogen-carbon substrate after pyrolysis and carbonization.

[0086] Figure 2a FIG1 shows a scanning electron microscope image of the nitrogen-carbon substrate (NC) prepared in Example 1 of the present invention. Figure 2a As shown, it can be seen that the nitrogen-carbon substrate prepared in Example 1 has a regular dodecahedron structure.

[0087] A mixture of a nitrogen-carbon substrate and tin tetrachloride pentahydrate was ground for 15 minutes at a mass ratio of 1. The mixture was then heated from room temperature to 300°C at a rate of 5°C / min and held at 300°C for 2 hours to obtain a Sn-intercalated nitrogen-carbon substrate. This substrate was then placed in a tube furnace and heated from 300°C to 750°C at a rate of 5°C / min under pure nitrogen conditions. The substrate was calcined at 750°C for 2 hours, then heated to 900°C at a rate of 5°C / min and calcined for another hour to obtain a Sn-NC composite solid.

[0088] Figure 2b FIG1 shows a scanning electron microscope image of the Sn-NC composite solid (SnNC) prepared in Example 1 of the present invention. Figure 2b As shown, it can be seen that the Sn-NC composite solid still maintains a good regular dodecahedron structure.

[0089] The Sn-NC composite solid and ferrous chloride tetrahydrate were chemically vapor deposited at a molar ratio of 1 under pure nitrogen conditions at 750°C for 3 hours to obtain a Fe-Sn-NC cathode oxygen reduction catalyst precursor, which was then acid-washed with 60 mL of 0.5 M sulfuric acid for 8 hours, and then filtered and washed with 4 L of ultrapure water. The filter cake was placed in a vacuum drying oven and dried for 12 hours. Finally, it was ground to obtain a Fe-Sn-NC cathode oxygen reduction catalyst 1 with high oxygen reduction activity and stability.

[0090] Figure 2c FIG1 shows a scanning electron microscope image of the Fe-Sn-NC cathode oxygen reduction catalyst 1 (SnFeNC) prepared in Example 1 of the present invention. Figure 2c As shown, it can be seen that the structure of SnFeNC is still well maintained.

[0091] The performance of the Fe-Sn-NC cathode oxygen reduction catalyst 1 of Example 1 was tested:

[0092] Conduct three-electrode oxygen reduction activity test: Figure 3 The half-wave potential curve of Fe-Sn-NC cathode oxygen reduction catalyst 1 prepared in Example 1 of the present invention in 0.1M perchloric acid solution is shown. Figure 3As shown, the Fe-Sn-NC cathode oxygen reduction catalyst 1 exhibited a high oxygen reduction activity of 0.85V.

[0093] Conduct three-electrode stability test: Figure 4 The graphs show the half-wave potential curves of the Fe-Sn-NC cathode oxygen reduction catalyst 1 prepared in Example 1 of the present invention in 0.1M perchloric acid solution before and after accelerated aging tests at the initial state, 10,000 cycles, 20,000 cycles, 30,000 cycles, 50,000 cycles, and 70,000 cycles. Figure 4 As shown, under saturated oxygen conditions, the voltage range is 0.6-0.9V (vs RHE), the 7W cycle half-wave loss is 28mV, and the Fe-Sn-NC cathode oxygen reduction catalyst 1 exhibits relatively excellent acidic oxygen reduction stability.

[0094] Conducting tests on membrane electrode assemblies: Figure 5 The power density and voltage of the Fe-Sn-NC cathode oxygen reduction catalyst 1 prepared in Example 1 of the present invention are shown as a curve of the change of current. Figure 5 As shown in the figure, it can be clearly seen that the membrane electrode assembled with Fe-Sn-NC catalyst has a power of nearly 1.8W / cm under the condition of 2 back pressure hydrogen and oxygen. 2 Excellent performance.

[0095] Figure 6 The Raman shifts of the nitrogen-carbon substrate (NC), Sn-NC composite solid (SnNC), and Fe-Sn-NC cathode oxygen reduction catalyst 1 (SnFeNC) prepared in Example 1 of the present invention are shown. Figure 6 As shown, the poured A D / A G It represents the peak area ratio of the carbon peaks of the three materials. The specific values ​​are shown in Table 1 below. It can be seen that the ratio of SnFeNC is lower. The reason is that the degree of graphitization of the nitrogen-carbon substrate is higher, which reduces the carbon defects of the carbon layer.

[0096] Table 1 Comparison of carbon peak area ratios of three materials

[0097]

[0098] Inductively coupled plasma (ICP) testing was performed on the nitrogen-carbon substrate (NC), Sn-NC composite solid (SnNC), and Fe-Sn-NC cathode oxygen reduction catalyst 1 (SnFeNC), prepared in Example 1 of the present invention. The data obtained are shown in Table 2 below. It can be seen that the fixed Sn partially replaces the Zn. After subsequent chemical vapor deposition, Fe replaces part of the Sn and Zn. Even with a single-atom Fe loading of 1.37 wt%, the catalyst still maintains a half-wave potential of 0.85 V.

[0099] Table 2 Comparison of single atom loading on active sites of three materials

[0100]

[0101] Comparative Example 1:

[0102] The preparation process of Comparative Example 1 is substantially the same as that of Example 1, except that after obtaining the carbonized nitrogen-carbon substrate, ferrous chloride tetrahydrate is directly subjected to chemical vapor deposition to obtain the Fe-NC cathode oxygen reduction catalyst (FeNC).

[0103] Figure 7 The in-situ Raman scattering patterns (SERS) of the Sn-NC composite solid prepared in Example 1 of the present invention, the Fe-Sn-NC cathode oxygen reduction catalyst 1, and the Fe-NC cathode oxygen reduction catalyst 1' (FeNC) prepared in Comparative Example 1 are shown, wherein a is the in-situ Raman scattering pattern of FeNC; b is the columnar change pattern of the graphitization degree of FeNC; c is the in-situ Raman scattering pattern of SnNC; d is the columnar change pattern of the graphitization degree of SnNC; e is the in-situ Raman scattering pattern of SnFeNC; and f is the columnar change pattern of the graphitization degree of SnFeNC. Figure 7 As shown in a, the green box represents the change of Fenton radicals, and the corresponding D peak intensity / G peak intensity represents the degree of graphitization of carbon (the smaller the number, the fewer defects, the better the stability, and the smaller the increase, the better). Figure 7 As shown in Figures a to f, during the constant voltage polarization test, it can be seen that the degree of graphitization of SnFeNC is higher, resulting in less Fenton reaction, less decomposition of the nitrogen-carbon matrix, and less dissolution of the iron and tin active sites.

[0104] Figure 8 The X-ray diffraction spectra of the nitrogen-carbon substrate, Sn-NC composite solid, Fe-Sn-NC cathode oxygen reduction catalyst 1 prepared in Example 1 of the present invention and Fe-NC cathode oxygen reduction catalyst 1' prepared in Comparative Example 1 are shown. Figure 8 As shown in the figure, the carbon (002) peak corresponding to about 26° has an obvious shift to a lower angle in SnNC, indicating that tin ions with a larger radius are inserted into the graphite layer, resulting in a larger interlayer spacing and a shift of the diffraction angle to a lower angle. In the formed Fe-Sn-NC cathode oxygen reduction catalyst 1, this state of larger interlayer spacing and shifted diffraction angle is still maintained, verifying the modification of the graphite interlayer spacing by the intercalation of tin tetrachloride pentahydrate.

[0105] Example 2:

[0106] The preparation process of this Example 2 is substantially the same as that of Example 1, except that the nitrogen-carbon substrate and tin tetrachloride pentahydrate are ground in a molar ratio of 1:5.5 to prepare the Fe-Sn-NC cathode oxygen reduction catalyst 2.

[0107] The Fe—Sn—NC cathode oxygen reduction catalyst 2 prepared in Example 2 was subjected to a cyclic voltammetry test, and the half-wave potential of the Fe—Sn—NC cathode oxygen reduction catalyst 2 was found to be 0.846V.

[0108] Example 3:

[0109] The preparation process of this Example 3 is substantially the same as that of Example 1, except that the nitrogen-carbon substrate and tin tetrachloride pentahydrate are ground in a molar ratio of 1:6 to prepare Fe-Sn-NC cathode oxygen reduction catalyst 3.

[0110] The Fe—Sn—NC cathode oxygen reduction catalyst 3 prepared in Example 3 was subjected to a cyclic voltammetry test, and the half-wave potential of the Fe—Sn—NC cathode oxygen reduction catalyst 3 was found to be 0.844V.

[0111] Example 4:

[0112] The preparation process of this Example 4 is substantially the same as that of Example 1, except that the nitrogen-carbon substrate and tin tetrachloride pentahydrate are ground in a molar ratio of 1:6.5 to prepare Fe-Sn-NC cathode oxygen reduction catalyst 4.

[0113] The Fe—Sn—NC cathode oxygen reduction catalyst 4 prepared in Example 4 was subjected to a cyclic voltammetry test, and the half-wave potential of the Fe—Sn—NC cathode oxygen reduction catalyst 4 was found to be 0.843V.

[0114] Example 5:

[0115] The preparation process of this Example 5 is substantially the same as that of Example 1, except that the nitrogen-carbon substrate and tin tetrachloride pentahydrate are ground in a molar ratio of 1:7.5 to prepare Fe-Sn-NC cathode oxygen reduction catalyst 5.

[0116] The Fe—Sn—NC cathode oxygen reduction catalyst 5 prepared in Example 5 was subjected to a cyclic voltammetry test, and the half-wave potential of the Fe—Sn—NC cathode oxygen reduction catalyst 5 was found to be 0.84V.

[0117] Example 6:

[0118] The preparation process of this Example 6 is substantially the same as that of Example 1, except that the Sn-NC composite solid and ferrous chloride tetrahydrate are chemically vapor deposited at a molar ratio of 1:1 to prepare the Fe-Sn-NC cathode oxygen reduction catalyst 6.

[0119] The Fe—Sn—NC cathode oxygen reduction catalyst 6 prepared in Example 6 was subjected to a cyclic voltammetry test, and the half-wave potential of the Fe—Sn—NC cathode oxygen reduction catalyst 6 was found to be 0.85V.

[0120] Example 7:

[0121] The preparation process of this embodiment 7 is substantially the same as that of embodiment 1, except that the Sn-NC composite solid and ferrous chloride tetrahydrate are chemically vapor deposited at a molar ratio of 1:1.2 to prepare the Fe-Sn-NC cathode oxygen reduction catalyst 7.

[0122] The Fe—Sn—NC cathode oxygen reduction catalyst 7 prepared in Example 7 was subjected to a cyclic voltammetry test, and the half-wave potential of the Fe—Sn—NC cathode oxygen reduction catalyst 7 was found to be 0.852V.

[0123] Example 8:

[0124] The preparation process of this Example 8 is substantially the same as that of Example 1, except that the Sn-NC composite solid and ferrous chloride tetrahydrate are chemically vapor deposited at a molar ratio of 1:1.3 to prepare the Fe-Sn-NC cathode oxygen reduction catalyst 8.

[0125] The Fe—Sn—NC cathode oxygen reduction catalyst 8 prepared in Example 8 was subjected to a cyclic voltammetry test, and the half-wave potential of the Fe—Sn—NC cathode oxygen reduction catalyst 8 was found to be 0.853V.

[0126] Example 9:

[0127] The preparation process of this Example 9 is substantially the same as that of Example 1, except that the Sn-NC composite solid and ferrous chloride tetrahydrate are chemically vapor deposited at a molar ratio of 1:1.4 to prepare the Fe-Sn-NC cathode oxygen reduction catalyst 9.

[0128] The Fe—Sn—NC cathode oxygen reduction catalyst 9 prepared in Example 9 was subjected to a cyclic voltammetry test, and the half-wave potential of the Fe—Sn—NC cathode oxygen reduction catalyst 9 was found to be 0.854V.

[0129] Example 10:

[0130] The preparation process of this embodiment 10 is substantially the same as that of embodiment 1, except that the Sn-NC composite solid and ferrous chloride tetrahydrate are chemically vapor deposited at a molar ratio of 1:1.5 to prepare the Fe-Sn-NC cathode oxygen reduction catalyst 10.

[0131] The Fe—Sn—NC cathode oxygen reduction catalyst 10 prepared in Example 10 was subjected to a cyclic voltammetry test, and the half-wave potential of the Fe—Sn—NC cathode oxygen reduction catalyst 10 was found to be 0.856V.

[0132] Example 11:

[0133] The preparation process of this embodiment 11 is substantially the same as that of embodiment 1, except that the tin-intercalated nitrogen-carbon substrate is heated to 750° C. and calcined for 3 h to prepare the Fe-Sn-NC cathode oxygen reduction catalyst 11.

[0134] The Fe—Sn—NC cathode oxygen reduction catalyst 11 prepared in this Example 11 was subjected to a cyclic voltammetry test, and the half-wave potential of the Fe—Sn—NC cathode oxygen reduction catalyst 11 was found to be 0.852V.

[0135] Example 12:

[0136] The preparation process of this embodiment 12 is substantially the same as that of embodiment 1, except that the nitrogen-carbon substrate and tin tetrachloride pentahydrate are ground in a molar ratio of 1:7.5, and the Sn-NC composite solid and ferrous chloride tetrahydrate are chemically vapor deposited in a molar ratio of 1:1.5 to prepare the Fe-Sn-NC cathode oxygen reduction catalyst 12.

[0137] The Fe—Sn—NC cathode oxygen reduction catalyst 12 prepared in Example 12 was subjected to a cyclic voltammetry test, and the half-wave potential of the Fe—Sn—NC cathode oxygen reduction catalyst 12 was found to be 0.854V.

[0138] It can be seen from the preparation results of Examples 1 to 12 of the present invention that the Fe-Sn-NC cathode oxygen reduction catalyst prepared by the above steps can have high oxygen reduction catalytic activity while maintaining high cycle stability, and can be used in proton exchange membrane fuel cells.

[0139] The specific embodiments described above further illustrate the objectives, technical solutions and beneficial effects of the present invention in detail. It should be understood that the above are only specific embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A method for preparing a Fe-Sn-NC cathode oxygen reduction catalyst, comprising: Under a protective atmosphere, pyrolyzing ZIF-8 having a regular dodecahedron framework structure to obtain a carbonized nitrogen-carbon substrate, wherein the nitrogen-carbon substrate has a regular dodecahedron framework structure; Mixing a tin-based precursor with the nitrogen-carbon substrate and heating the mixture to obtain a tin-intercalated nitrogen-carbon substrate; calcining the tin-intercalated nitrogen-carbon substrate to obtain a Sn-NC composite solid; The Sn-NC composite solid was chemically vapor deposited with an iron-based precursor to obtain a Fe-Sn-NC cathode oxygen reduction catalyst; The heating conditions are as follows: heating the mixture from room temperature to 300-400°C, maintaining the temperature at 300-400°C for 1-3 hours, and heating at a rate of 4-6°C / min; The calcining of the tin-intercalated nitrogen-carbon substrate to obtain a Sn-NC composite solid comprises: Under a protective atmosphere, the Sn-intercalated nitrogen-carbon substrate is heated from 300-400°C to 750°C at a heating rate of 4-6°C / min, calcined at 750°C for 1.5-2.5 hours, then heated to 900°C at a heating rate of 4-6°C / min, and further calcined at 900°C for 0.5-1.5 hours to obtain the Sn-NC composite solid.

2. The preparation method according to claim 1, wherein The Sn-NC composite solid and the iron-based precursor are subjected to chemical vapor deposition to obtain the Fe-Sn-NC cathode oxygen reduction catalyst, which comprises: The Sn-NC composite solid and the iron-based precursor were chemically vapor deposited at 700-800°C under inert gas conditions for 2-4 hours to obtain the Fe-Sn-NC cathode oxygen reduction catalyst precursor. The Fe—Sn—NC cathode oxygen reduction catalyst precursor is subjected to acid washing and drying treatment to obtain the Fe—Sn—NC cathode oxygen reduction catalyst.

3. The preparation method according to claim 1, wherein The mixing of the tin-based precursor and the nitrogen-carbon substrate comprises: The tin-based precursor and the nitrogen-carbon substrate are ground at room temperature for 10 to 20 minutes to obtain a mixture.

4. The preparation method according to claim 3, wherein The molar ratio of the tin-based precursor to the nitrogen-carbon base is 1:1-7.5, and the tin-based precursor includes at least one of tin tetrachloride, tin tetrachloride hydrate, and stannous chloride.

5. The preparation method according to claim 1, wherein The iron-based precursor includes at least one of ferrous chloride, ferrous chloride hydrate, and ferric chloride; The molar ratio of the Sn-NC composite solid to the iron-based precursor is (1-1.5):

1.

6. The preparation method according to claim 2, wherein The pH of the pickling is 0.5-1.5, and the drying temperature is 60°C.

7. The preparation method according to claim 1, wherein ZIF-8 is prepared by the following process: Dispersing zinc nitrate or its hydrate and dimethylimidazole in methanol solution respectively and mixing them to obtain a mixed solution; The mixed solution is centrifuged and dried in sequence to obtain ZIF-8; The mass ratio of zinc nitrate or its hydrate and dimethylimidazole is (2~4): (5.5~7.5).

8. An Fe-Sn-NC cathode oxygen reduction catalyst prepared by the preparation method according to any one of claims 1 to 7, comprising: Nitrogen-carbon substrate with a regular dodecahedral framework structure; as well as Active centers, including Fe-Sn bimetallic single atom sites supported within the nitrogen-carbon substrate; Wherein, iron and tin are coordinated to form a bond, and the mass fraction of iron in the Fe-Sn-NC cathode oxygen reduction catalyst is 1-2%.

9. Use of the Fe-Sn-NC cathode oxygen reduction catalyst as claimed in claim 8 in the field of proton exchange membrane fuel cell cathode technology.

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