A polyoxometalate-anchored cobalt monatomic catalyst, a preparation method and application thereof

Abstract

This invention belongs to the field of catalyst technology, specifically relating to a polyoxometalate-anchored cobalt single-atom catalyst, its preparation method, and its application. The invention uses a polyoxometalate as the anchoring substrate and electronic control unit. First, a cobalt-substituted polyoxometalate precursor is prepared via a coordination reaction, then composited with a ZIF-derived conductive carbon substrate, and finally reduced with hydrogen to obtain the target catalyst. This catalyst effectively inhibits cobalt single-atom aggregation, achieving a cobalt single-atom loading of 3.55 wt%. Under alkaline conditions, it exhibits excellent oxygen reduction catalytic activity, stability, and methanol resistance, with a half-wave potential of 0.826 V. When used in a zinc-air battery, it achieves an open-circuit voltage of 1.48 V and a flow rate of 239.6 mW / cm². 2 With maximum power density and excellent cycle stability, it can replace commercial platinum-carbon catalysts.

Description

Technical Field

[0001] This invention belongs to the field of catalyst technology, specifically relating to a polyoxometalate-anchored cobalt single-atom catalyst, its preparation method, and its application. Background Technology

[0002] Among various novel electrochemical energy storage systems, zinc-air batteries are considered one of the ideal power sources for next-generation portable electronic devices and electric vehicles due to their outstanding advantages such as a theoretical energy density as high as 1086 Wh / kg, low cost, and environmental friendliness, and they have broad prospects for commercial application. However, the practical performance of zinc-air batteries is limited by the oxygen reduction reaction (ORR) at its air cathode: this reaction involves a complex multi-electron transfer process, and the intrinsic reaction kinetics are slow. It is necessary to use a highly efficient cathode catalyst to lower the reaction energy barrier and accelerate the reaction process. The activity, stability, and tolerance of the catalyst directly determine the charge-discharge performance, energy efficiency, and cycle life of the zinc-air battery.

[0003] Currently, commercial platinum-carbon (Pt / C) catalysts are the most widely used ORR catalysts, but they suffer from insufficient stability and weak resistance to methanol interference, making it difficult to meet the requirements for long-term stable operation of zinc-air batteries. Cobalt-nitrogen-carbon (Co-NC) single-atom catalysts, with their near 100% atom utilization, unique electronic structure, and excellent ORR catalytic activity, have become core candidate materials to replace commercial platinum-carbon catalysts. However, existing Co-NC single-atom catalysts still face significant technical bottlenecks in preparation and application: on the one hand, during the high-temperature pyrolysis process of catalyst preparation, cobalt single atoms are prone to migration and aggregation, forming low-activity metal nanoparticles, resulting in the loss of active sites and a decrease in catalytic performance, while also making it difficult to achieve high loading of cobalt single atoms; on the other hand, the single Co-Nx coordination environment in traditional Co-NC catalysts cannot flexibly control the electronic structure of active sites, making it difficult to further optimize the adsorption energy of ORR reaction intermediates, thus limiting the potential for improving catalytic activity.

[0004] Polyoxometalates (POMs) are a class of polynuclear clusters formed by metal-oxygen polyhedra linked by oxygen atoms, possessing well-defined crystal structures, abundant metal active sites, and excellent electronic conductivity. Their abundant surface oxygen atoms can form stable coordination bonds with transition metal ions, providing natural anchoring sites for metal single atoms. Simultaneously, their polynuclear metal centers can modulate the electronic structure of single-atom active sites through electronic coupling effects, optimizing catalytic reaction pathways. Using polyoxometalates in the preparation of cobalt single-atom catalysts holds promise for solving the industry challenges of existing cobalt single-atom catalysts, such as easy aggregation, low loading, and insufficient catalytic activity and stability. Summary of the Invention

[0005] The present invention aims to provide a polyoxometalate-anchored cobalt single-atom catalyst, its preparation method and application, wherein the mass loading of cobalt single atoms is as high as 3.55wt%, which is significantly better than traditional Co-NC single-atom catalysts and greatly improves the active site density of the catalyst.

[0006] The objective of this invention is achieved through the following technical solution: This invention provides a polyoxometalate-anchored cobalt single-atom catalyst, comprising a conductive carbon substrate and a cobalt-substituted polyoxometalate supported on the conductive carbon substrate; the cobalt-substituted polyoxometalate uses the polyoxometalate as the anchoring substrate, cobalt is uniformly dispersed in the form of single atoms, and the cobalt single atoms are stably anchored by forming Co-O coordination bonds with oxygen atoms on the surface of the polyoxometalate cluster; the mass loading of the cobalt single atoms in the catalyst is up to 3.55 wt%.

[0007] Furthermore, the polyoxometalate is selected from any one of silicotungstic acid, phosphotungstic acid, and phosphomolybdic acid.

[0008] Furthermore, the conductive carbon substrate is a nitrogen-doped porous carbon material obtained by high-temperature pyrolysis of a Zn-ZIF precursor under an inert atmosphere. The Zn-ZIF precursor is prepared by reacting zinc nitrate with 2-methylimidazole in methanol.

[0009] Furthermore, the cobalt single atom exists in a 6-coordinate mode with Co-O bonds, and the average bond length of the Co-O bonds is 1.95. .

[0010] This invention also provides a method for preparing a polyoxometalate-anchored cobalt single-atom catalyst, comprising the following steps: (1) Preparation of cobalt-substituted polyoxometalate precursor: The polyoxometalate and soluble cobalt salt are dispersed in deionized water, heated and stirred, and cooled to crystallize, to obtain a cobalt-substituted polyoxometalate precursor in which cobalt atoms form stable coordination bonds with oxygen atoms of the polyoxometalate. (2) Preparation of conductive carbon substrate: Zinc nitrate and 2-methylimidazole were dispersed in methanol to react and obtain Zn-ZIF precursor. The Zn-ZIF precursor was placed in an inert atmosphere and pyrolyzed at high temperature to obtain conductive carbon substrate. (3) Composite and reduction: The cobalt-substituted polyoxometalate precursor is thoroughly mixed with a conductive carbon substrate and heat-treated in a hydrogen-containing reducing atmosphere to obtain a polyoxometalate-anchored cobalt single-atom catalyst.

[0011] Furthermore, in step (1), the molar ratio of the polyoxometalate to the soluble cobalt salt is 1:1 or the molar amount of cobalt ions is in excess; the soluble cobalt salt is cobalt sulfate; the heating and stirring temperature is 95°C, and the cooling and crystallization temperature is 5°C.

[0012] Furthermore, in step (2), the inert atmosphere is nitrogen or argon; the high-temperature pyrolysis process is as follows: the temperature is increased to 800-1000℃ at a heating rate of 5℃ / min, and held for 2 hours.

[0013] Furthermore, in step (3), the hydrogen-containing reducing atmosphere is a mixture of 5% H2 and 95% Ar by volume; the heat treatment process is as follows: heating to 160°C at a heating rate of 5°C / min and holding for 8-10 hours; before mixing, the conductive carbon substrate is immersed in a polyethyleneimine solution for surface modification, and the mass ratio of the cobalt-substituted polyoxometalate precursor to the conductive carbon substrate is 10:1.

[0014] The present invention also provides an application of a polyoxometalate-anchored cobalt single-atom catalyst, wherein the polyoxometalate-anchored cobalt single-atom catalyst according to any one of claims 1 to 4 is used as a cathode catalyst for the oxygen reduction reaction and applied in an electrochemical energy conversion device.

[0015] Furthermore, the electrochemical energy conversion device is a fuel cell or a metal-air battery; the metal-air battery is a zinc-air battery, and the electrolyte of the zinc-air battery is a 6M KOH aqueous solution.

[0016] The beneficial effects of this invention are as follows: This invention utilizes the abundant bridging oxygen and terminal oxygen sites on the surface of polyoxometalates to form stable Co-O coordination bonds with cobalt ions, providing strong anchoring sites for cobalt single atoms and effectively inhibiting the migration and aggregation of cobalt atoms during high-temperature heat treatment. Characterization verification shows that cobalt in the catalyst is uniformly dispersed in single-atom form with no obvious aggregation of metal nanoparticles. At the same time, the mass loading of cobalt single atoms is as high as 3.55 wt%, which is significantly better than traditional Co-NC single-atom catalysts, greatly improving the active site density of the catalyst.

[0017] The polyoxometalate used in this invention possesses a unique multinuclear metal cluster structure, where the metal center can form a strong electronic coupling effect with cobalt single atoms. This breaks the limitation of the single Co-Nx coordination environment in traditional Co-NC catalysts, allowing for flexible control of the electron cloud density of the cobalt active sites and optimizing the O2 and OH reactions during the oxygen reduction reaction. - The adsorption and desorption energy barriers of reaction intermediates are well defined. The prepared catalyst has an oxygen reduction reaction onset potential of 0.916V and a half-wave potential of 0.826V under alkaline conditions, which is superior to commercial Pt / C catalysts. The Tafel slope is as low as 23.6mV / dec, which is close to 22.3mV / dec of commercial Pt / C catalysts, and it has fast reaction kinetics comparable to commercial noble metal catalysts.

[0018] This invention achieves covalent anchoring of cobalt single atoms through Co-O coordination bonds, resulting in a robust anchoring structure that prevents the detachment and loss of active sites. Stability tests show that the catalyst retains up to 94.4% of the current density in the oxygen reduction reaction system and exhibits excellent resistance to methanol interference. This solves the problems of traditional commercial Pt / C catalysts, such as easy poisoning and rapid performance degradation during long-term operation, and significantly extends the catalyst's service life.

[0019] The zinc-air battery assembled using the catalyst prepared in this invention as the air cathode exhibits an open-circuit voltage as high as 1.48V, which is superior to the 1.42V of batteries assembled with commercial Pt / C catalysts; the maximum power density reaches 239.6mW / cm³. 2 This is significantly higher than the 200.6 mW / cm² of batteries assembled with commercial Pt / C catalysts. 2 At 10mA / cm 2 At the specified current density, the battery can cycle stably for more than 300 hours with no significant voltage decay during charging and discharging. This greatly improves the energy conversion efficiency and cycle life of the zinc-air battery, providing high-performance non-precious metal catalyst support for the commercial application of next-generation clean electrochemical energy storage devices. Attached Figure Description

[0020] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0021] Figure 1 CoSiW prepared in Example 1 of this invention 11 Scanning electron microscope (SEM) image of the @NC-900 single-atom catalyst; Figure 2 Transmission electron microscopy (TEM) image and lattice fringes of the polyoxometalate-anchored cobalt single-atom catalyst prepared in Example 1 of this invention; Figure 3 The image shows aberration-corrected high-angle annular dark-field scanning transmission electron microscope (HAADF-STEM) image of the catalyst prepared in Example 1 of this invention. Figure 4 The image shows the X-ray near-edge absorption structure (XAFS) of the catalyst prepared in Example 1 of this invention. Figure 5 The image shows the X-ray diffraction (XRD) pattern of the catalyst prepared in Example 1 of this invention. Figure 6The cyclic voltammetry (CV), linear sweep voltammetry (LSV), and corresponding Tafel slope curves of the catalysts in Examples 1-3 of this invention in O2-saturated 0.1 M KOH solution are shown. Figure 7 The cyclic voltammetry (CV) and linear sweep voltammetry (LSV) curves of Comparative Examples 1-2 of this invention in O2-saturated 0.1 M KOH solution are shown. Figure 8 The open-circuit voltage curve, power density curve, and cycle stability curve of the zinc-air battery assembled with the single-atom catalyst prepared in Example 1 of this invention as the positive electrode are shown. Detailed Implementation

[0022] Various exemplary embodiments of the present invention will now be described in detail. This detailed description should not be considered as a limitation of the present invention, but rather as a more detailed description of certain aspects, features, and embodiments of the present invention.

[0023] It should be understood that the terminology used in this invention is merely for describing particular embodiments and is not intended to limit the invention. Furthermore, with respect to numerical ranges in this invention, it should be understood that each intermediate value between the upper and lower limits of the range is also specifically disclosed. Every smaller range between any stated value or intermediate value within a stated range, and any other stated value or intermediate value within said range, is also included in this invention. The upper and lower limits of these smaller ranges may be independently included or excluded from the range.

[0024] Unless otherwise stated, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art. While only preferred methods and materials have been described herein, any methods and materials similar or equivalent to those described herein may be used in the implementation or testing of this invention. All references to this specification are incorporated by way of citation to disclose and describe methods and / or materials associated with those references. In the event of any conflict with any incorporated reference, the content of this specification shall prevail.

[0025] Various modifications and variations can be made to the specific embodiments described in this specification without departing from the scope or spirit of the invention, as will be apparent to those skilled in the art. Other embodiments derived from this specification will also be apparent to those skilled in the art. This specification and embodiments are merely exemplary.

[0026] The terms “include,” “including,” “have,” “contain,” etc., used in this article are all open-ended terms, meaning that they include but are not limited to.

[0027] The present invention will be further illustrated below through examples.

[0028] In the following examples and comparative examples of the present invention, all reagents used were commercially available analytical grade reagents, and the resistivity of the deionized water used was not less than 18.2 MΩ·cm; all electrochemical tests were performed on a CHI760e electrochemical workstation, and all zinc-air battery performance tests were performed on a Blue Electric CT3002A battery testing system.

[0029] Example 1: Polyoxometalate-anchored cobalt single-atom catalyst CoSiW 11 Preparation of @NC-900 In this embodiment, the catalyst uses silicotungstic acid as a polyoxometalate precursor, and the specific preparation steps are as follows: 1. Preparation of cobalt-substituted polyoxometalate precursors Weigh 28g of silicotungstic acid and dissolve it in 10mL of deionized water at 95℃, stirring until completely dissolved to obtain solution A; weigh 2.22g of cobalt sulfate and dissolve it in 100mL of deionized water at 60℃, stirring until completely dissolved to obtain solution B; weigh 31g of potassium acetate and dissolve it in 30mL of deionized water at 60℃, adjusting the pH of the solution to 7 with acetic acid to obtain solution C. Slowly add solutions B and C dropwise to solution A, stirring continuously for 10 minutes after the addition is complete; after rapid filtration, the mixture is cooled and crystallized at 5℃; the resulting crystals are redissolved in deionized water at 70℃, stirred thoroughly, and then recrystallized again at 5℃ for purification, finally obtaining {CoSiW}. 11 O39} crystal, i.e., cobalt-substituted polyoxometalate precursor.

[0030] 2. Preparation of conductive carbon substrate 3.67 g of zinc nitrate was weighed and dissolved in 123 mL of methanol, stirred until completely dissolved, to obtain solution A. 1.89 g of 2-methylimidazole was weighed and dissolved in another 123 mL of methanol, stirred until completely dissolved, to obtain solution B. Solution B was added to solution A, and the mixture was stirred continuously at 11000 rpm for 24 h to obtain a solid precipitate. The precipitate was collected by centrifugation, washed with deionized water, and dried in a vacuum oven at 60 °C to obtain the Zn-ZIF precursor. The Zn-ZIF precursor was placed in a magnetic boat, placed in a tube furnace, and purged with argon as a protective atmosphere. The temperature was increased to 900 °C at a rate of 5 °C / min and held for 2 h. The zinc element was volatilized and removed at the high temperature. After natural cooling to room temperature, a conductive carbon substrate, denoted as NC-900, was obtained.

[0031] 3. Surface modification of carbon substrate and catalyst composite The NC-900 obtained in step 2 was dispersed in 10 mL of ethanol to obtain dispersion A; an equal amount of polyethyleneimine (PEI) was weighed and dispersed in 10 mL of deionized water to obtain dispersion B; dispersion A and dispersion B were mixed and stirred continuously for 24 h; the product was collected by centrifugation and then dried in a vacuum oven to obtain PEI-modified conductive carbon substrate NC-900-PEI.

[0032] Disperse NC-900-PEI in 10 mL of ethanol to obtain dispersion C; weigh 10 times the mass of {CoSiW}. 11 O 39 Crystals were dispersed in 10 mL of deionized water to obtain dispersion D. Dispersion D was added to dispersion C, and the mixture was stirred continuously at room temperature for 24 h. After centrifugation and drying, the product was placed in a tube furnace and heated to 160 °C at a heating rate of 5 °C / min under a mixed atmosphere of 5% H₂ / 95% Ar, and held at this temperature for 8 h for reduction treatment. After natural cooling, a polyoxometalate-anchored cobalt single-atom catalyst, denoted as CoSiW, was obtained. 11 @NC-900.

[0033] Example 2: CoPW polyoxometalate-anchored cobalt single-atom catalyst 11 Preparation of @NC-900 The only difference between this embodiment and Example 1 is that in step 1, silicotungstic acid is replaced with 28g of phosphotungstic acid. All other preparation steps and process parameters are exactly the same as in Example 1, ultimately yielding the catalyst CoPW. 11 @NC-900.

[0034] Example 3: Polyoxometalate-anchored cobalt single-atom catalyst CoPMo 11 Preparation of @NC-900 The only difference between this embodiment and Example 1 is that in step 1, silicotungstic acid is replaced with 17g of phosphomolybdic acid. All other preparation steps and process parameters are exactly the same as in Example 1, ultimately yielding the catalyst CoPMo. 11 @NC-900.

[0035] Comparative Example 1: Catalyst CoSiW 11 Preparation of @NC-800 The only difference between this comparative example and Example 1 is that in step 2, the calcination temperature of the conductive carbon substrate is set to 800℃, and the holding time is still 2 hours. All other preparation steps and process parameters are exactly the same as in Example 1, ultimately yielding the catalyst CoSiW. 11 @NC-800.

[0036] Comparative Example 2: Catalyst CoSiW 11Preparation of @NC-1000 The only difference between this comparative example and Example 1 is that in step 2, the calcination temperature of the conductive carbon substrate is set to 1000℃, and the holding time remains 2 hours. All other preparation steps and process parameters are exactly the same as in Example 1, ultimately yielding the catalyst CoSiW. 11 @NC-1000.

[0037] Morphology and structural characterization of the catalyst in Experimental Example 1 1. Morphological characteristics CoSiW prepared in Example 1 11 The NC-900 catalyst was characterized by scanning electron microscopy (SEM), and the results are as follows: Figure 1 As shown, the results indicate that the catalyst exhibits a uniform dodecahedral morphology with a particle size of 1-2 μm. The surface of the dodecahedron is uniformly dispersed with polyacid cluster particles with a diameter of about 10 nm, and there is no obvious agglomeration.

[0038] Transmission electron microscopy (TEM) characterization results are as follows Figure 2 As shown, the results indicate that no lattice fringes corresponding to the metal element were observed in the catalyst, and no aggregation of metal nanoparticles was observed; the characterization results of aberration-corrected high-angle annular dark-field scanning electron microscopy (HAADF-STEM) are as follows. Figure 3 As shown, the results indicate that there are a large number of uniformly distributed bright spots on the carbon substrate surface, confirming that cobalt is uniformly dispersed on the substrate surface in the form of single atoms.

[0039] 2. Characterization of Crystal and Coordination Structure CoSiW prepared in Example 1 11 The NC-900 catalyst was characterized by X-ray diffraction (XRD), and the results are as follows: Figure 5 As shown in the results, only the characteristic diffraction peaks corresponding to the conductive carbon substrate appeared in the spectrum, and no characteristic diffraction peaks of metallic cobalt or its oxides appeared, further confirming that there was no agglomeration of metal nanoparticles in the catalyst and that cobalt existed in the form of single atoms.

[0040] The catalyst was characterized by X-ray near-edge absorption structure (XAFS), and the spectral results are as follows: Figure 4 As shown in Table 1, the extended X-ray absorption fine structure (EXAFS) of the CoK-edge was fitted and analyzed.

[0041] Table 1 CoSiW 11 EXAFS fitting results of @NC-900 catalyst

[0042] The fitting results show that each Co atom in the catalyst exists in a 6-coordinate mode of Co-O bonds, with an average Co-O bond length of 1.95 mm. This confirms that cobalt single atoms are stably anchored to the carbon substrate surface by forming Co-O coordination bonds with oxygen atoms on the surface of polyacid clusters; inductively coupled plasma optical emission spectroscopy (ICP-OES) tests show that the loading of cobalt single atoms in the catalyst prepared in this embodiment is as high as 3.55 wt%.

[0043] Experimental Example 2: Test of the oxygen reduction electrocatalytic performance of the catalyst 1. Preparation of catalyst dispersion Weigh 3 mg of the catalyst powder prepared in Examples 1-3 and Comparative Examples 1-2 respectively, add it to a mixture consisting of 0.70 mL of ultrapure water, 0.28 mL of anhydrous ethanol and 0.02 mL of 5% Nafion solution, and ultrasonically disperse until uniform to obtain the corresponding catalyst dispersion.

[0044] 2. Electrochemical testing methods Take 50 μL of catalyst dispersion and uniformly drop it onto the surface of a rotating disk glassy carbon electrode (RDE). Let it air dry at room temperature to serve as the working electrode. Construct a three-electrode test system with a platinum wire electrode as the counter electrode and an Hg / HgO electrode as the reference electrode. All electrochemical tests were performed in O2-saturated 0.1 M KOH aqueous solution at room temperature.

[0045] 3. Electrocatalytic performance test results The cyclic voltammetry (CV), linear sweep voltammetry (LSV), and corresponding Tafel slope curves of the catalysts in O2-saturated 0.1M KOH solutions in Examples 1-3 and Comparative Examples 1-2 are shown below. Figure 6-7 As shown.

[0046] Cyclic voltammetry (CV) results showed that the CoSiW prepared in Example 1... 11 Oxygen reduction characteristic potential E of NC-900 catalyst The value was 0.808V, which was significantly better than the catalysts of Comparative Example 1 and Comparative Example 2.

[0047] Linear sweep voltammetry (LSV) test results show that CoSiW 11 Oxygen reduction initiation potential E of @NC-900 catalyst oe Reaching 0.916V, half-wave potential E 1 / 2 The half-wave potentials reached 0.826 V, which is superior to the commercial 20wt% Pt / C catalyst. The catalysts prepared in Examples 2 and 3 also showed excellent ORR catalytic activity, with half-wave potentials of 0.812 V and 0.805 V, respectively, which are superior to the comparative catalysts.

[0048] According to the Tafel slope results obtained from LSV curve fitting, CoSiW 11 The Tafel slope of the @NC-900 catalyst was 23.6 mV / dec, which is close to the 22.3 mV / dec of the commercial 20 wt% Pt / C catalyst, indicating that the catalyst has fast ORR reaction kinetics; the Tafel slopes of the catalysts in Examples 2 and 3 were 45.2 mV / dec and 57.8 mV / dec, respectively, both lower than the comparative catalyst.

[0049] Stability test results show that, after constant potential testing, CoSiW 11 The @NC-900 catalyst exhibits a current density retention rate of up to 94.4% and demonstrates excellent resistance to methanol interference, significantly outperforming commercial Pt / C catalysts.

[0050] Experimental Example 3: Assembly and Performance Testing of Zinc-Air Batteries 1. Battery assembly method The catalysts prepared in Examples 1-3 and Comparative Examples 1-2 were used as active materials for air cathodes. The catalyst, acetylene black, and Nafion solution were mixed in a mass ratio of 7:2:1, and anhydrous ethanol was added and ultrasonically dispersed to form a uniform slurry. The slurry was uniformly coated on the surface of carbon paper and dried at room temperature to serve as the air cathode. A coin-type zinc-air battery was assembled using a polished zinc sheet as the anode and a 6M KOH aqueous solution as the electrolyte.

[0051] 2. Battery performance test results CoSiW prepared in Example 1 11 The open-circuit voltage curve, power density curve, and cycle stability curve of the zinc-air battery assembled with the @NC-900 catalyst are shown below. Figure 8 As shown, its open-circuit voltage reaches 1.48V, which is superior to that of a battery assembled with a commercial 20wt% Pt / C catalyst (1.42V); the battery's maximum power density reaches 239.6mW / cm³. 2 This is significantly higher than that of batteries assembled with commercial Pt / C catalysts (200.6 mW / cm²). 2 ).

[0052] Long-cycle stability test results show that at 10mA / cm 2 At current densities, with CoSiW 11 The @NC-900 is a zinc-air battery with a cathode. After 300 hours of cycle testing with each charge and discharge cycle lasting 30 minutes, the battery's charge and discharge voltage showed no significant decay, demonstrating excellent long-term cycle stability.

[0053] The zinc-air batteries assembled with the catalysts prepared in Examples 2 and 3 also exhibited superior charge-discharge performance and cycle stability compared to the comparative catalysts, demonstrating good potential for practical applications.

[0054] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A polyoxometalate-anchored cobalt single-atom catalyst, characterized in that, The catalyst includes a conductive carbon substrate and a cobalt-substituted polyoxometalate supported on the conductive carbon substrate. The cobalt-substituted polyoxometalate uses the polyoxometalate as an anchoring substrate, with cobalt uniformly dispersed in the form of single atoms. The cobalt single atoms are stably anchored by forming Co-O coordination bonds with oxygen atoms on the surface of the polyoxometalate cluster. The mass loading of the cobalt single atoms in the catalyst is up to 3.55 wt%.

2. The polyoxometalate-anchored cobalt single-atom catalyst according to claim 1, characterized in that, The polyoxometalate is selected from any one of silicotungstic acid, phosphotungstic acid, and phosphomolybdic acid.

3. The polyoxometalate-anchored cobalt single-atom catalyst according to claim 1, characterized in that, The conductive carbon substrate is a nitrogen-doped porous carbon material obtained by high-temperature pyrolysis of a Zn-ZIF precursor under an inert atmosphere. The Zn-ZIF precursor is prepared by reacting zinc nitrate with 2-methylimidazole in methanol.

4. The polyoxometalate-anchored cobalt single-atom catalyst according to claim 1, characterized in that, The cobalt single atom exists in a 6-coordinate mode with Co-O bonds, and the average bond length of the Co-O bonds is 1.

95. .

5. A method for preparing a polyoxometalate-anchored cobalt single-atom catalyst, characterized in that, Includes the following steps: (1) Preparation of cobalt-substituted polyoxometalate precursor: The polyoxometalate and soluble cobalt salt are dispersed in deionized water, heated and stirred, and cooled to crystallize, to obtain a cobalt-substituted polyoxometalate precursor in which cobalt atoms form stable coordination bonds with oxygen atoms of the polyoxometalate. (2) Preparation of conductive carbon substrate: Zinc nitrate and 2-methylimidazole were dispersed in methanol to react and obtain Zn-ZIF precursor. The Zn-ZIF precursor was placed in an inert atmosphere and pyrolyzed at high temperature to obtain conductive carbon substrate. (3) Composite and reduction: The cobalt-substituted polyoxometalate precursor is thoroughly mixed with a conductive carbon substrate and heat-treated in a hydrogen-containing reducing atmosphere to obtain a polyoxometalate-anchored cobalt single-atom catalyst.

6. The preparation method according to claim 5, characterized in that, In step (1), the molar ratio of the polyoxometalate to the soluble cobalt salt is 1:1 or the molar amount of cobalt ions is in excess; the soluble cobalt salt is cobalt sulfate; the heating and stirring temperature is 95°C, and the cooling and crystallization temperature is 5°C.

7. The preparation method according to claim 5, characterized in that, In step (2), the inert atmosphere is nitrogen or argon; the high-temperature pyrolysis process is as follows: the temperature is increased to 800-1000℃ at a heating rate of 5℃ / min and held for 2 hours.

8. The preparation method according to claim 5, characterized in that, In step (3), the hydrogen-containing reducing atmosphere is a mixture of 5% H2 and 95% Ar by volume; the heat treatment process is as follows: heating to 160°C at a heating rate of 5°C / min and holding for 8-10 hours; before mixing, the conductive carbon substrate is immersed in a polyethyleneimine solution for surface modification, and the mass ratio of the cobalt-substituted polyoxometalate precursor to the conductive carbon substrate is 10:

1.

9. An application of a polyoxometalate-anchored cobalt single-atom catalyst, characterized in that, The polyoxometalate-anchored cobalt single-atom catalyst according to any one of claims 1 to 4 is used as a cathode catalyst for the oxygen reduction reaction and applied in electrochemical energy conversion devices.

10. The application according to claim 9, characterized in that, The electrochemical energy conversion device is a fuel cell or a metal-air battery; the metal-air battery is a zinc-air battery, and the electrolyte of the zinc-air battery is a 6M KOH aqueous solution.

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