Oxygen reduction catalytic material as well as preparation method and application thereof

Abstract

The invention relates to the field of catalysis, in particular to an oxygen reduction catalytic material and a preparation method and application thereof. The oxygen reduction catalytic material is of a core-shell structure with a substrate material as a core and a cobaltosic oxide nano array with the surface modified with a transition metal-covalent organic polymer as a shell. The catalytic material disclosed by the invention is of a core-shell structure, and the introduction of the Co3O4 nano array changes the electronic structure of a single transition metal atom, so that the activity and stability of the catalyst are remarkably improved.

Description

Technical Field

[0001] The present invention relates to the field of catalysis, and particularly to an oxygen reduction catalytic material, a preparation method thereof, and an application thereof. Background Art

[0002] Proton exchange membrane fuel cells (PEMFCs) play an important role in the process of achieving the green revolution due to their high energy conversion efficiency and zero emissions during operation. The development of efficient non-noble metal-based M-N-C catalysts for the acidic oxygen reduction reaction (ORR) cathode is crucial for accelerating the application of PEMFCs. In particular, atomically dispersed Co-N-C catalysts have been widely studied due to their efficient Co-N active sites and fewer Fenton reactions. However, Co-N-C catalysts are usually prepared by high-temperature pyrolysis of precursors containing metals and carbon sources, which inevitably leads to the internal collapse or even reconstruction of the prefabricated precursor structure, thus forming random and complex active sites. It is worth noting that the destruction of the internal structure buries many effective active sites, significantly reducing the catalytic activity.

[0003] Covalent organic polymers are macrocyclic compounds connected by covalent bonds. These molecules are locked in precise positions in the entire topological network, and specific parts of them can be used as reaction sites for catalytic conversion in the field of energy storage. Due to the atomic-level controllability of organic molecular components, as well as their strong structural possibilities and chemical properties, the direct use of COFs analogues as ORR catalysts has attracted extensive attention. For example, Yao et al. designed and synthesized stable metal-free thiophene-sulfur COFs as electrochemical catalysts for ORR. Chen et al. reported a graphene-based metal 1,4,8,11-tetraazacyclotetradecaene polymer with well-defined metal-N4 active sites, which exhibited excellent ORR kinetics and high turnover frequency. However, when directly used for acidic ORR electrocatalysis, the activity of the materials is still not satisfactory. Summary of the Invention

[0004] The purpose of the present invention is to overcome the problems of low utilization rate and poor activity of metal-N sites in catalytic materials existing in the prior art, and to provide an oxygen reduction catalytic material, a preparation method thereof, and an application thereof. The oxygen reduction catalytic material has a high utilization rate of metal-N sites, a well-defined active site structure, and high activity.

[0005] To achieve the above object, in the first aspect of the present invention, an oxygen reduction catalytic material is provided. The oxygen reduction catalytic material has a core-shell structure with a substrate material as the core and a cobalt tetroxide nanorod array modified with a transition metal-covalent organic polymer on the surface as the shell.

[0006] In the second aspect of the present invention, there is provided a method for preparing the oxygen reduction catalytic material described in the first aspect. The preparation method includes the following steps: (1) Synthesize a cobalt tetroxide nanoarray on the surface of a substrate material to obtain a precursor of the oxygen reduction catalytic material; (2) Perform solvothermal treatment on a mixture of the precursor of the oxygen reduction catalytic material and a cobalt-covalent organic polymer.

[0007] In the third aspect of the present invention, there is provided an application of the oxygen reduction catalytic material described in the first aspect in the preparation of the cathode of a proton exchange membrane fuel cell.

[0008] By the above technical solutions, the present invention has the following advantages: The catalytic material of the present invention has a core-shell structure. The introduction of the Co3O4 nanoarray changes the electronic structure of individual transition metal atoms, significantly improving the activity and stability of the catalyst.

[0009] The present invention innovatively constructs a catalytic material with good active sites and high hydrothermal stability through a non-pyrolysis strategy. Specifically, Co3O4 nanoarrays are in-situ grown on a substrate by a solvothermal method, and then a transition metal-covalent organic polymer with a well-defined transition metal-N4-C structure is anchored on the surface of the cobalt tetroxide nanoarray through an epitaxial growth strategy. The prepared catalytic material has excellent oxygen reduction activity and oxygen catalytic stability.

[0010] When the catalytic material of the present invention is used in the preparation of the cathode of a proton exchange membrane fuel cell, the material has an oxygenophilic three-phase interface, providing rich three-phase reaction sites and good transport channels for ORR, and providing good channels for continuous oxygen supply / gas supply. The material has a high oxygen concentration distribution and current density, accelerating the transfer efficiency of reaction gases and products and reducing mass transfer polarization loss. The material exhibits excellent ORR performance and is competitive compared with reported non-precious metal-based electrocatalysts. For example, when the transition metal is Co, in a H2-air atmosphere, the peak power density of the PEMFC can reach 0.413 W cm -2 , and after 100 hours of cyclic stability testing, the battery voltage has good stability, comparable to that of the currently highest-performance traditional pyrolytic Co-N-C. Generally speaking, the enhancement of activity and durability is attributed to the synergistic effect of cobalt tetroxide nanoarrays and cobalt-covalent organic polymers, resulting in a low ionization barrier and unobstructed mass transfer path for the electrode prepared from the catalytic material. This catalytic material solves the problem of limited mass transfer rates of reactants and protons at the membrane electrode level during the operation of PEMFCs and can timely discharge the generated water, which will effectively improve the performance of PEMFCs. Description of the Drawings

[0011] Figure 1Among them, (a), (b), and (c) are SEM images of the oxygen reduction catalytic material prepared in Example 1 at different magnifications; (d) and (e) are TEM images of the oxygen reduction catalytic material prepared in Example 1 at different magnifications; (f) and (g) are HAADF-STEM images of the oxygen reduction catalytic material prepared in Example 1; (h) is the elemental mapping analysis result image of the oxygen reduction catalytic material prepared in Example 1. Figure 2 Among them, (a) is the AFM image of the oxygen reduction catalytic material prepared in Example 1, and (b) is the array height distribution diagram of the oxygen reduction catalytic material prepared in Example 1. Figure 3 Among them, (a) is the contact angle schematic diagram of the catalytic material prepared in Comparative Example 2; (b) is the contact angle schematic diagram of the oxygen reduction catalytic material prepared in Example 1. Figure 4 Among them, (a) is the high-resolution C 1s spectrum of the x-ray photoelectron spectroscopy (XPS) of the oxygen reduction catalytic material prepared in Example 1; (b) is the spectral superposition diagram of the high-resolution spectrum (XPS) of Co 2p of the oxygen reduction catalytic materials prepared in Example 1 and Comparative Example 1. Figure 5 Among them, (a) is the spectral superposition diagram of the high-resolution spectrum (XPS) of Co 2p of the oxygen reduction catalytic material and the oxygen reduction catalytic material precursor prepared in Example 1; (b) is the Co K-edge x-ray absorption near-edge structure (XANES) spectrum of the oxygen reduction catalytic material prepared in Example 1; (c) is the k 3 Weighted Fourier transform (FT) extended x-ray absorption fine structure (EXAFS) spectrogram; (d) is the wavelet transform (WT) diagram of Co-foil; (e) is the wavelet transform (WT) diagram of Co3O4; (f) is the wavelet transform (WT) diagram of the oxygen reduction catalytic material prepared in Example 1. Figure 6 Among them, (a) is the ORR activity of the catalytic material and the catalytic material precursor prepared in Example 1, the catalytic material prepared in Comparative Example 1, 20 wt% Pt / C catalyst, and carbon paper by linear sweep voltammetry (LSV) at a scan rate of 5 mV s -1ORR polarization curves in 0.1 M HClO4 electrolyte saturated with O2; (b) Tafel plots corresponding to the catalytic material and its precursor prepared in Example 1, the catalytic material prepared in Comparative Example 1, 20 wt% Pt / C catalyst, and carbon paper; (c) Quasi-rectangular CV curves at different scan rates obtained from the voltage range of the non-Faradaic reaction region of the catalytic material and its precursor prepared in Example 1, and the catalytic material prepared in Comparative Example 1; (d) Trend chart of current retention rate of Co-COP@Co3O4 / CP and 20 wt% Pt / C tested within a 50,000 s time period; Figure 7 Among them, (a) Polarization curves of the catalytic material and its precursor prepared in Example 1, and the catalytic material prepared in Comparative Example 2 in an H2-O2 fuel cell; (b) Polarization curves of the catalytic material and its precursor prepared in Example 1, and the catalytic material prepared in Comparative Example 2 in an H2-Air cell; (c) Partial enlarged view of the polarization curves of the catalytic material prepared in Example 1 and the catalytic material prepared in Comparative Example 2 in different cells; (d) The catalytic material and its precursor prepared in Example 1, and the catalytic material prepared in Comparative Example 2 were used to test the mass transfer ability at a current density of 1 A cm -2 and the equivalent current diagram was obtained by simulation calculation using Gamry software; Figure 8 In an H2-Air cell, at a constant current density of 0.3 A cm -2 for 100 hours to evaluate the performance of each electrode (the catalytic material and its precursor prepared in Example 1, the catalytic material prepared in Comparative Example 2, and traditional phthalocyanine small molecules), fuel cell durability trend chart. Detailed Description of the Invention

[0012] In the ranges disclosed herein, the endpoints and any values are not limited to the exact ranges or values, and these ranges or values should be understood to include values close to these ranges or values. For numerical ranges, the endpoint values of each range, between the endpoint values of each range and individual point values, and between individual point values can be combined with each other to obtain one or more new numerical ranges, and these numerical ranges should be regarded as specifically disclosed herein.

[0013] The present invention provides an oxygen reduction catalytic material, which has a core-shell structure with a substrate material as the core and a cobalt tetroxide nanorod array surface-modified with a transition metal-covalent organic polymer as the shell.

[0014] The catalytic material of the present invention has a core-shell structure. The introduction of the Co3O4 nanorod array changes the electronic structure of individual transition metal atoms, significantly improving the activity and stability of the catalyst.

[0015] According to a preferred embodiment of the present invention, the contact angle of the oxygen reduction catalytic material is 110-150°, preferably 120-140°. The contact angle refers to the angle between the solid-liquid interface through the liquid interior to the gas-liquid interface at the three-phase junction of the solid, liquid, and gas of the material tested by a contact angle tester.

[0016] According to a preferred embodiment of the present invention, the cobalt tetroxide nanoarray is a rod-shaped array.

[0017] In the present invention, the transition metal can be a conventional choice in the art. The following is a demonstration, but it does not limit the scope of the present invention. According to a preferred embodiment of the present invention, the transition metal is selected from at least one of Fe, Ni, Cu, Mn, and Co, preferably Co.

[0018] According to a preferred embodiment of the present invention, the transition metal-covalent organic polymer is a compound with a porphyrin-like structure and a transition metal-N4 configuration, preferably a transition metal phthalocyanine polymer.

[0019] In the present invention, there is no particular requirement for the type of the substrate material, which can be a conventional choice in the art. The following is a demonstration, but it does not limit the scope of the present invention. According to a preferred embodiment of the present invention, the substrate material is a carbon material and / or a metal material, preferably a carbon material, and more preferably a carbon paper with a size of 2-10 cm 2 of carbon paper.

[0020] When the substrate material is carbon paper, pretreatment is required. The pretreatment method includes: first, ultrasonically clean with acetone and water respectively. Then immerse it in aqua regia (10 mL of 65% HNO3 and 30 mL of 37% HCl) and stir, then wash with water until the solution is neutral, and dry to obtain oxidized carbon paper.

[0021] According to a preferred embodiment of the present invention, the molar ratio of the transition metal element in the transition metal-covalent organic polymer to the cobalt element in the cobalt tetroxide is 0.05-0.1.

[0022] There is no particular requirement for the preparation method of the oxygen reduction catalytic material in the present invention, as long as the purpose of the present invention can be achieved. The following is a demonstration, but it does not limit the scope of the present invention. The preparation method of the oxygen reduction catalytic material includes the following steps: (1) Synthesize a cobalt tetroxide nanoarray on the surface of the substrate material to obtain a precursor of the oxygen reduction catalytic material; (2) Perform solvothermal treatment on the mixture of the precursor of the oxygen reduction catalytic material and the cobalt-covalent organic polymer.

[0023] The present invention innovatively constructs a catalytic material with good active sites and high hydrothermal stability through a non-pyrolysis strategy. Specifically, Co3O4 nanoarrays are in-situ grown on a substrate by a solvothermal method, and then a transition metal-covalent organic polymer with a well-defined transition metal-N4-C structure is anchored on the surface of the cobalt tetroxide nanoarrays through an epitaxial growth strategy. The prepared catalytic material has excellent oxygen reduction activity and oxygen catalytic stability.

[0024] According to a preferred embodiment of the present invention, step (1) includes: dissolving a substrate material, a first cobalt source, trimesic acid, NH4F, and urea in a first solvent, subjecting the obtained solution to hydrothermal treatment, washing, vacuum drying, and calcining in an inert atmosphere to obtain a precursor of the oxygen reduction catalytic material.

[0025] In the present invention, the optional range of the conditions for the hydrothermal treatment in step (1) is relatively wide. The following is a demonstration, but it does not limit the scope of the present invention. According to a preferred embodiment of the present invention, the conditions for the hydrothermal treatment include: the molar ratio of the first cobalt source in terms of cobalt element, trimesic acid, NH4F, and urea is 1∶2-4∶1-2∶1-2.

[0026] According to a preferred embodiment of the present invention, the conditions for the hydrothermal treatment include: the hydrothermal temperature is 120-160°C; the hydrothermal time varies with factors such as temperature, for example, it can be 6-10 h.

[0027] In the present invention, the inert atmosphere is an atmosphere composed of any gas that does not participate in the reaction, preferably a nitrogen atmosphere.

[0028] In the present invention, the conditions for calcining include: the heating rate is 2-10°C min -1 , heating up to 300-400°C; calcining for 1-4 h.

[0029] In the present invention, the first cobalt source can be a conventional choice in the art as long as the purpose of the present invention can be achieved. The following is a demonstration, but it does not limit the scope of the present invention. According to a preferred embodiment of the present invention, the first cobalt source is selected from at least one of cobalt nitrate, halide salts, and perhalate salts, preferably cobalt nitrate.

[0030] In the present invention, the first solvent can be a conventional choice in the art as long as the purpose of the present invention can be achieved. The following is a demonstration, but it does not limit the scope of the present invention. According to a preferred embodiment of the present invention, the first solvent is ethanol and / or water, preferably a mixed solvent with a volume ratio of ethanol to water of 0.5-2.

[0031] According to a preferred embodiment of the present invention, step (2) includes: mixing the organic ligand solution and the second cobalt source solution, adding a catalyst during mixing to obtain a cobalt-covalent organic polymer solution, and then adding a precursor solvent of the oxygen reduction catalytic material for solvothermal treatment.

[0032] In the present invention, the organic ligand can be a conventional choice in the art as long as it can achieve the purpose of the present invention. The following is a demonstration but does not limit the scope of the present invention. According to a preferred embodiment of the present invention, the organic ligand is 4-cyanobenzene.

[0033] In the present invention, the second cobalt source can be a conventional choice in the art as long as it can achieve the purpose of the present invention. The following is a demonstration but does not limit the scope of the present invention. According to a preferred embodiment of the present invention, the first cobalt source is selected from at least one of cobalt halide salts, carbonates, and perchlorates, preferably a cobalt halide salt.

[0034] In the present invention, the concentration of the organic ligand solution is not particularly limited. According to a preferred embodiment of the present invention, the concentration of the organic ligand solution is 0.03 - 0.07 mol / L. The present invention takes a concentration of 0.056 mol / L as an example to illustrate the advantages of the present invention.

[0035] In the present invention, the preparation of the organic ligand solution includes dissolving the organic ligand in a second solvent, and the second solvent is selected from at least one of DMF, ethylene glycol, and isopropanol, preferably DMF.

[0036] In the present invention, the concentration of the organic ligand solution is not particularly limited. According to a preferred embodiment of the present invention, the concentration of the organic ligand solution is 0.01 - 0.04 mol / L. The present invention takes a concentration of 0.028 mol / L as an example to illustrate the advantages of the present invention.

[0037] In the present invention, the preparation of the second cobalt source solution includes dissolving the organic ligand in a third solvent, and the third solvent is selected from ethylene glycol and / or isopropanol, preferably ethylene glycol.

[0038] According to a preferred embodiment of the present invention, in step (2), the mixing conditions include: the molar ratio of the organic ligand solution based on the organic ligand to the second cobalt source solution based on cobalt element is 2 - 4∶1.

[0039] According to a preferred embodiment of the present invention, in step (2), the mixing conditions include: the mixing temperature is 10 - 40°C; the mixing time is adjusted according to factors such as the mixing temperature, for example, the time is 10 - 30 min.

[0040] According to a preferred embodiment of the present invention, in the step (2), the conditions of the solvothermal treatment include: reacting at 160 - 200 °C for 12 - 36 h.

[0041] In the present invention, the catalyst can be a conventional selection in the art. The following is a demonstration but does not limit the scope of the present invention accordingly. According to a preferred embodiment of the present invention, the catalyst is selected from 1,8 - diazabicyclo(5,4,0)undec - 7 - ene.

[0042] The present invention provides an application of the described oxygen reduction catalytic material in the preparation of the cathode of a proton exchange membrane fuel cell.

[0043] The catalytic material of the present invention is used in the preparation of the cathode of a proton exchange membrane fuel cell. The material has an oxygen - philic three - phase interface, providing rich three - phase reaction sites and good transport channels for ORR, and good channels for continuous oxygen supply / gas supply. The material has a relatively high oxygen concentration distribution and current density, accelerating the transfer efficiency of reaction gases and products and reducing the mass transfer polarization loss. The material exhibits excellent ORR performance and is competitive compared with the reported non - noble metal - based electrocatalysts. For example, when the transition metal is Co, in a H2 - air atmosphere, the peak power density of the PEMFC can reach 0.413 W cm -2 , after 100 - hour cyclic stability test, the battery voltage has good stability and is comparable to that of the currently highest - performance traditional pyrolytic Co - N - C. Generally speaking, the enhancement of activity and durability is attributed to the synergistic effect of cobalt tetroxide nanoarrays and cobalt - covalent organic polymers, resulting in a low ionization barrier and unobstructed mass transfer path of the electrode prepared from the catalytic material. This catalytic material solves the problem of limited mass transfer rate of reactants and protons at the membrane - electrode level during the operation of PEMFCs and can timely discharge the generated water, which will effectively improve the performance of PEMFCs.

[0044] The present invention will be described in detail below through examples.

[0045] Example 1 Carbon paper pretreatment: Commercial carbon paper (CP) is first ultrasonically cleaned with acetone and water for 30 minutes respectively. Then it is immersed in aqua regia (10 mL 65% HNO3 and 30 mL 37% HCl) and stirred for 30 minutes, and then washed with excessive water until the solution is neutral, and oxidized CP is obtained after drying.

[0046] Preparation of the oxygen reduction catalytic material precursor: The treated 5 cm 2The CP was placed into a 30 mL mixed solution containing 2 mmol Co(NO3)2·6H2O, 6 mmol trimesic acid, 4 mmol NH4F and 4 mmol urea (ethanol: water = 1:2). Subsequently, it was sealed in a stainless-steel autoclave and placed in an oven at 140 °C for 8 hours. The CP was taken out, washed with deionized water and dried in vacuum at 60 °C. Subsequently, the dried precursor was placed in a tubular furnace, nitrogen was passed through, and the heating rate was set at 5 °C min -1 , heated to 350 °C and maintained for 2 hours to prepare the precursor of the oxygen reduction catalytic material - Co3O4 / CP.

[0047] Preparation of the oxygen reduction catalytic material: At room temperature of 20 °C, first dissolve 0.28 mmol 4-cyanobenzene (BTC) in 5 mL DMF, dissolve 0.14 mmol CoCl3 in 5 mL ethylene glycol, and then mix the two solutions and stir for 20 minutes. During the stirring process, 50 uL of 1,8-diazabicyclo(5,4,0)undec-7-ene (DBU) was gradually added to synthesize the cobalt phthalocyanine polymer Co-COP. The above solution was transferred to a stainless-steel high-pressure reactor, and the previously synthesized Co3O4 / CP was placed in the solution and reacted at 180 °C for 24 hours to obtain the oxygen reduction catalytic material - Co-COP@Co3O4 / CP.

[0048] The SEM images of the oxygen reduction catalytic material are as shown in Figure 1 (a), Figure 1 (b) and Figure 1 (c). It can be seen that the oxygen reduction catalytic material still maintains the original nanoarray structure, but compared with Co3O4 / CP, the surface becomes rougher, indicating that Co-COP has grown successfully on Co3O4; The TEM images of the oxygen reduction catalytic material are as shown in Figure 1 (d) and Figure 1 (e). It can be seen that the oxygen reduction catalytic material is a core-shell structure; The atomic resolution high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) of the oxygen reduction catalytic material is as shown in Figure 1 (f) and Figure 1 (g). The results show clear lattice fringes. 0.19 nm corresponds to the (4 0 0) crystal plane of Co3O4. At the same time, the presence of single-atom Co can be clearly seen and marked with a red circle; The elemental mapping analysis results of the oxygen reduction catalytic material are as shown in Figure 1 (h), indicating the presence and uniform distribution of C, N, O and Co elements in a single carbon fiber nanoarray; The atomic force microscope (AFM) image of the oxygen reduction catalytic material and its height distribution diagram are asFigure 2 (a), and Figure 2 as shown in (b), indicating that the material has a uniformly distributed nanorod array structure The contact angle of the oxygen reduction catalytic material Co-COP@Co3O4 / CP was measured using a contact angle tester as Figure 3 shown in (b), with a contact angle of 130°, which is significantly improved and more conducive to the timely discharge of the product water during the reaction The high-resolution C 1s spectrum of the x-ray photoelectron spectroscopy (XPS) of the oxygen reduction catalytic material Co-COP@Co3O4 / CP is as Figure 4 shown in (a), which can be divided into four peaks with binding energies of 284.6 eV, 285.2 eV, 286.3 eV, and 288.5 eV, corresponding to the functional groups of C=C, C=N, C-O, and C=O, respectively

[0049] The high-resolution spectrum (XPS) of Co 2p of the oxygen reduction catalytic material precursor - Co3O4 / CP is as Figure 5 shown in (a)

[0050] The high-resolution spectrum (XPS) of Co 2p of the oxygen reduction catalytic material Co-COP@Co3O4 / CP is as Figure 4 shown in (b) or Figure 5 shown in (a), indicating that compared with Co3O4 / CP, the binding energy of Co 2p in Co-COP@Co3O4 / CP has an obvious positive shift, and compared with Co-COP / CP, the binding energy has an obvious negative shift, indicating that there is an electronic interaction between Co3O4 and Co-COP The Co K-edge x-ray absorption near-edge structure (XANES) spectrum of the oxygen reduction catalytic material Co-COP@Co3O4 / CP is as Figure 5 shown in (b). The near-edge absorption threshold of Co-COP@Co3O4 / CP is between the thresholds of Co3O4 and Co foil, indicating that these Co atoms have partial positive charges. Compared with the Co3O4 reference phase, the spectrum of Co-COP@Co3O4 / CP has a smaller red shift, which may be due to coordination with atoms with smaller electronegativity such as N. In the pre-edge region of the Co-COP@Co3O4 / CP spectrum, due to the contribution of ligand-to-metal charge transfer and 1s→3d transitions, there is a weak peak at 7715 eV. The results show that the dominant coordination geometry around Co may be close to a square planar structure because this front-line peak is usually considered to be a square planar Co-N4 configuration with a porphyrin-like structure. In addition, both Co-COP@Co3O4 / CP and Co3O4 have a weak peak at 7709 eV, which is due to their octahedral structure C 3VIt consists of symmetry and two sets of 3 Co-O bonds.

[0051] For Co-COP@Co3O4 / CP k 3 Weighted Fourier transform (FT) extended X-ray absorption fine structure (EXAFS) spectrum Figure 5 (c) As shown, there is a prominent peak at 1.45 Å, which is attributed to the Co-N(O) scattering path. There is a weak broad peak near 2.5 - 3 Å, indicating the formation of Co-O-Co bonds. The results show that Co-N x and Co3O4 clusters may coexist.

[0052] The wavelet transforms (WT) of Co-foil, Co3O4, and Co-COP@Co3O4 / CP are respectively as Figure 5 (d), Figure 5 (e) and Figure 5 (f) shown, where the maximum intensity of Co-COP@Co3O4 / CP at 1.3 Å -1 can be attributed to Co-N / O bonds. Compared with the WT contour map of Co3O4, the peak intensity is significantly stronger, indicating the existence of obvious Co-N coordination bonds. 2.2 Å -1 corresponds to the Co-O-Co bond in Co3O4. At the same time, compared with Co-foil, no maximum intensity corresponding to Co-Co is observed, which further indicates that the amount of Co-Co is too small to be detected.

[0053] In summary, Co single-atom sites (Co-N4) and Co3O4 nanoclusters coexist in Co-COP@Co3O4 / CP, where Co atoms are mainly coordinated with N or O atoms.

[0054] Example 2 Same as Example 1, except that Prepare the precursor of the oxygen reduction catalytic material: Put the treated 5 cm 2 of CP into a 30 mL mixed solution containing 2 mmol Co(NO3)2·6H2O, 4 mmol trimesic acid, 2 mmol NH4F, and 2 mmol urea (ethanol:water = 1:2). Subsequently, seal it with a stainless steel autoclave and place it in an oven at 120 °C for 16 hours. Take out the CP, wash it with deionized water, and dry it in vacuum at 60 °C. Subsequently, place the dried precursor in a tubular furnace, introduce nitrogen, set the heating rate to 5 °C min -1 , heat it to 350 °C and hold for 2 hours to prepare the precursor of the oxygen reduction catalytic material - Co3O4 / CP-16; Prepare the oxygen reduction catalytic material: At room temperature, first dissolve 0.40 mmol of 4-cyanobenzene (BTC) in 5 mL of DMF, dissolve 0.1 mmol of CoCl2 in 5 mL of ethylene glycol, and then mix the two solutions and stir for 30 minutes. During the stirring process, gradually add 50 μL of 1,8-diazabicyclo(5,4,0)undec-7-ene (DBU). Transfer the above solution to a stainless-steel autoclave, put the previously synthesized Co3O4 / CP-16 into the solution, and react at 160 °C for 36 hours to obtain the oxygen reduction catalytic material - Co-COP@Co3O4 / CP-16.

[0055] Perform the same tests on the prepared oxygen reduction catalytic material as in Example 1, and the test results are basically similar to the spectra of the catalytic material prepared in Example 1.

[0056] Example 3 Same as Example 1, except that Prepare the precursor of the oxygen reduction catalytic material: Put the treated 5 cm 2 of CP into 30 mL of a mixed solution containing 2 mmol of Co(NO3)2·6H2O, 6 mmol of 1,3,5-benzenetricarboxylic acid, 4 mmol of NH4F, and 4 mmol of urea (ethanol:water = 1:2). Then seal it with a stainless-steel reaction kettle and place it in an oven at 160 °C for 24 hours. Take out the CP, wash it with deionized water, and dry it in vacuum at 60 °C. Subsequently, place the dried precursor in a tubular furnace, introduce nitrogen, set the heating rate to 5 °C min -1 , heat it to 350 °C and hold for 2 hours to prepare the precursor of the oxygen reduction catalytic material - Co3O4 / CP-24; Prepare the oxygen reduction catalytic material: At room temperature, first dissolve 0.6 mmol of 4-cyanobenzene (BTC) in 5 mL of DMF, dissolve 0.2 mmol of CoCl2 in 5 mL of ethylene glycol, and then mix the two solutions and stir for 20 minutes. During the stirring process, gradually add 50 μL of 1,8-diazabicyclo(5,4,0)undec-7-ene (DBU). Transfer the above solution to a stainless-steel autoclave, put the previously synthesized Co3O4 / CP-16 into the solution, and react at 200 °C for 12 hours to obtain the oxygen reduction catalytic material - Co-COP@Co3O4 / CP-24.

[0057] Perform the same tests on the prepared oxygen reduction catalytic material as in Example 1, and the test results are basically similar to the spectra of the catalytic material prepared in Example 1.

[0058] Example 4 Same as Example 1, except that the feeding amount is changed: During the preparation of the oxygen reduction catalytic material, at room temperature of 20 °C, first dissolve 0.14 mmol of 4-cyanobenzene (BTC) in 5 mL of DMF, dissolve 0.07 mmol of CoCl3 in 5 mL of ethylene glycol, and then mix the two solutions and stir for 20 minutes.

[0059] The prepared oxygen reduction catalytic material was tested in the same manner as in Example 1, and the test results were basically similar to the spectra of the catalytic material prepared in Example 1.

[0060] Example 5 Same as Example 1, except that CoCl2 is replaced by NiCl2.

[0061] The prepared oxygen reduction catalytic material was tested in the same manner as in Example 1, and the test results were basically similar to the spectra of the catalytic material prepared in Example 1.

[0062] Comparative Example 1 Same as Example 1, except that CP is used to replace the oxygen reduction catalytic material precursor to obtain the catalytic material - Co-COP / CP.

[0063] The high-resolution spectrum (XPS) of Co 2p of the oxygen reduction catalytic material Co-COP / CP is as Figure 4 (b) shown.

[0064] Comparative Example 2 The catalytic material - Co-COP / Co3O4 with the same composition as in Example 1 was prepared by the traditional spraying method.

[0065] The contact angle of Co-COP / Co3O4 is as Figure 3 (a) shown, and the contact angle is 105°.

[0066] Comparative Example 3 Same as Example 1, except that Co-ZIF is used to replace Co-COP to obtain the catalytic material - Co-ZIF@Co3O4 / CP.

[0067] Example 7 Three-electrode ORR performance test The catalytic material prepared in Example 1, the catalytic material precursor, the catalytic material prepared in Comparative Example 1, the 20 wt% Pt / C catalyst, and the carbon paper were directly used as the working electrode, and then a carbon rod and a Pt wire were used as the counter electrode and the reference electrode, respectively.

[0068] The ORR activity of each material was measured by linear sweep voltammetry (LSV) at a scan rate of 5 mV s -1Evaluated in 0.1 M HClO4 electrolyte saturated with O2, the ORR polarization curve is as Figure 6 (a) shown. It can be seen that compared with Co3O4 / CP ( E onset (onset potential) = 0.83 V, E 1 / 2 (half-wave potential) = 0.74 V) and Co-COP / CP ( E onset = 0.79 V, E 1 / 2 = 0.71V), Co-COP@Co3O4 / CP significantly enhances the ORR activity, E onset (0.94 V vs. RHE) and E 1 / 2 (0.846 V vs.RHE) are higher, comparable to 20% Pt / C ( E onset = 0.96 V; E 1 / 2 = 0.85 V vs. RHE).

[0069] In addition, the corresponding Tafel plots of each material are as Figure 6 (b) shown. It can be seen that the Tafel slope of Co-COP@Co3O4 / CP is 63 mV dec -1 , close to that of 20% Pt / C (57 mV dec -1 ), further proving its good ORR activity, indicating that Co-COP@Co3O4 / CP has more favorable ORR kinetics.

[0070] In addition, we obtained rectangular-like CV curves at different scan rates by selecting the voltage range of the non-Faradaic reaction region, as Figure 6 (c) shown, and calculated the double-layer capacitance (C dl ), and further deduced the electrochemically active surface area. The C dl value of Co-COP@Co3O4 / CP is 32 mF cm -2 , higher than that of Co3O4 / CP (18 mF cm -2 ) and Co-COP / CP (14 mFcm -2 ).

[0071] During the 50000 s period, the current retention rate change trends of Co-COP@Co3O4 / CP and 20 wt% Pt / C were tested, as Figure 6As shown in (d), Co-COP@Co3O4 / CP has a high current retention rate, while the decay rate of Pt / C is as high as 64% within the working time of 50000 s.

[0072] Therefore, the main reasons for the excellent electrocatalytic performance of Co-COP@Co3O4 / CP are as follows: on the one hand, the combination of Co single atoms and Co3O4 clusters further reduces the adsorption energy barrier of ORR intermediates; on the other hand, the existence of the nanoarray structure can provide a larger surface area to promote the transfer of reactants and protons.

[0073] The materials prepared in Example 4, Example 5 and Comparative Example 3 were tested by the same method, and it was found that the ORR activity was not as good as that of the material prepared in Example 1, and the electrochemical performances such as the current retention rate were also relatively poor. Moreover, Example 4 and Example 5 were significantly better than Comparative Example 3.

[0074] Example 8 The catalytic material prepared in Example 1, the catalytic material precursor, and the catalytic material prepared in Comparative Example 2 were used as the cathode catalysts of PEMFC. The test conditions were as follows: O2 and H2 were introduced into the anode and cathode respectively, and the gas flow rates were 0.8 L min -1 and 0.4 L min -1 , the working temperature was 80 °C, the relative humidity was 100%, and the back pressure was 1.5 bar; for the hydrogen-air test, air and H2 were introduced into the anode and cathode respectively, and the gas flow rates were both 1.0 L min -1 , the working temperature was 80 °C, the relative humidity was 100%, and the back pressure was 2.0 bar.

[0075] As Figure 7 shown in (a), in the H2-O2 fuel cell, the maximum power density of the Co-COP@Co3O4 / CP catalyst ( P max = 0.641 W cm -2 ) is greater than that of the traditional spray Co-COP@Co3O4 ( P max = 0.479 W cm -2 ).

[0076] As Figure 7 shown in (b), in the H2-Air atmosphere, the power density of Co-COP@Co3O4 / CP is also significantly higher than that of Co-COP@Co3O4 (0.413 vs. 0.306 W cm -2 ).

[0077] Polarization curve, enlarged view as Figure 7As shown in (c), under H2-O2 conditions, the voltages in the activation polarization region and ohmic polarization region of Co-COP@Co3O4 and Co-COP@Co3O4 / CP are basically the same, or even slightly higher. On the contrary, under H2-Air conditions, the voltage of Co-COP@Co3O4 / CP is significantly higher than that of Co-COP@Co3O4. This is because the insufficient oxygen supply at the Co-COP@Co3O4 electrode in air leads to a significant decrease in voltage, while the good mass transfer channel of the Co-COP@Co3O4 / CP array electrode ensures the timely supply of air. By in-situ growing the catalytic layer on the gas diffusion layer, the proton conduction path is greatly improved, enabling the water in the reaction products to be discharged in a timely manner and reducing the charge transfer resistance. In addition, this nanoarray structure can maximize the exposure of catalytic active sites and increase the accessible area of reactants.

[0078] Furthermore, the electrochemical impedance spectroscopy was further used to test the mass transfer ability at a current density of 1 A cm -2 , and the equivalent current diagram was obtained by simulation calculation using Gamry software, Figure 7 as shown in (d). It can be seen that compared with Co-COP@Co3O4 ( R ct = 0.15 Ω cm -2 , R ohm = 0.89 Ω cm -2 ) and Co3O4 / CP ( R ct = 0.18 Ω cm -2 , R ohm = 1.26 Ω cm -2 ), the Co-COP@Co3O4 / CP cathode exhibits a smaller charge transfer resistance ( R ct = 0.148 Ω cm -2 ) and total ohmic resistance ( R ohm = 0.81Ω cm -2 ).

[0079] It can be seen from the polarization curve that Co-COP@Co3O4 / CP has good ORR performance, which is consistent with the results of the half-cell.

[0080] In addition, the performance of each electrode of the membrane was also evaluated by maintaining a constant current density of 0.3 A cm -2 for 100 hours in an H2-Air battery, as Figure 8As shown. Compared with the traditional spray-coated electrode with a voltage drop to 58%, the voltage of the Co-COP@Co3O4 / CP battery still remains at 82% of the original, indicating that the membrane electrode based on Co-COP@Co3O4 / CP has excellent cycling stability. Maintaining a stable voltage under constant current means that there is no serious mass transfer loss or flooding in the membrane electrode, which is mainly attributed to the good oxygenophilic nanoarray structure and short mass transfer path. In addition, compared with the traditional phthalocyanine (Co-PC) small molecule, the Co-COP with a network structure in the present invention has high hydrothermal stability under acidic conditions due to the irreversible covalent bond reaction in kinetics.

[0081] The preferred embodiments of the present invention have been described in detail above. However, the present invention is not limited thereto. Within the scope of the technical concept of the present invention, various simple modifications can be made to the technical solutions of the present invention, including any other suitable combination of each technical feature. These simple modifications and combinations should also be regarded as the content disclosed by the present invention and fall within the protection scope of the present invention.

Claims

1. An oxygen reduction catalytic material, characterized in that: The oxygen reduction catalytic material presents a core-shell structure with a base material as a core and a cobalt tetroxide nanoarray with a transition metal-covalent organic polymer modified on the surface as a shell.

2. The oxygen reduction catalytic material according to claim 1, wherein The contact angle of the oxygen reduction catalytic material is 110-150°, preferably 120-140°.

3. The oxygen reduction catalytic material according to claim 1, wherein The cobalt tetroxide nanoarray is a rod-shaped array; and / or The transition metal is at least one selected from Fe, Ni, Cu, Mn and Co, preferably Co.

4. The oxygen reduction catalytic material according to claim 1 or 2, wherein: The transition metal-covalent organic polymer is a compound of transition metal-N4 configuration having a porphyrin-like structure, preferably a transition metal phthalocyanine polymer.

5. The oxygen reduction catalytic material according to any one of claims 1 to 4, wherein: The substrate material is a carbon material and / or a metal material, preferably a carbon material, and more preferably has a size of 2-10 cm. 2 of carbon paper.

6. The oxygen reduction catalytic material according to any one of claims 1 to 5, wherein: The molar ratio of the transition metal element in the transition metal-covalent organic polymer to the cobalt element in cobalt tetroxide is 0.05-0.

1.

7. The method for preparing the oxygen reduction catalytic material according to any one of claims 1 to 6, characterized in that: The preparation method comprises the following steps: (1) synthesizing cobalt tetroxide nanoarrays on the surface of a substrate material to obtain an oxygen reduction catalytic material precursor; (2) The mixture of the oxygen reduction catalyst precursor and the cobalt-covalent organic polymer is subjected to solvent thermal treatment.

8. The preparation method according to claim 7, wherein: The step (1) comprises: dissolving the substrate material, the first cobalt source, tris-benzoic acid, NH4F and urea in a first solvent, hydrothermally treating the obtained solution, washing, vacuum drying, and calcining under an inert atmosphere to obtain an oxygen reduction catalytic material precursor; preferably, the conditions for the hydrothermal treatment include: the molar ratio of the first cobalt source, tris-benzoic acid, NH4F and urea is 1:2-4:1-2:1-2 in terms of cobalt element; and / or the hydrothermal temperature is 120-160°C; and / or the hydrothermal time is 6-24h; and / or The step (2) comprises: mixing the organic ligand solution with the second cobalt source solution, adding a catalyst during the mixing to obtain a cobalt-covalent organic polymer solution, and then adding an oxygen reduction catalytic material precursor solvent for thermal treatment.

9. The preparation method according to claim 7 or 8, wherein: In the step (2), The mixing conditions include: the molar ratio of the organic ligand solution calculated as the organic ligand to the second cobalt source solution calculated as the cobalt element is 2-4:1; and / or the mixing temperature is 10-40° C.; and / or the mixing time is 10-30 min; and / or The conditions of the solvent thermal treatment include: 160-200° C. reaction for 12-36 hours.

10. Use of the oxygen reduction catalytic material according to any one of claims 1 to 6 in the preparation of a cathode of a proton exchange membrane fuel cell.

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