Ni < x > Co < 3-x > O < 4 > oxygen evolution catalyst based on Co-MOF derivative and preparation method thereof

Abstract

The invention discloses a Co-MOF derivative-based Ni < x > Co < 3-x > O < 4 > oxygen evolution catalyst and a preparation method thereof.The preparation method comprises the steps that 2-methylimidazole and cobalt salt are added into a methanol solution, stirring reaction is performed to obtain a nano rhombic precursor Co-MOF, then the Co-MOF is dispersed into a solution containing a nickel source, stirring reaction, centrifugal washing and drying are performed to obtain Co-Ni MOF, and finally the Co-Ni MOF is calcined in the air atmosphere to obtain the Co-MOF-based Ni < x > Co < 3-x > O < 4 > oxygen evolution catalyst. The temperature is raised to 300-375 DEG C for annealing, the Ni < x > Co < 3-x > O < 4 > oxygen evolution catalyst is obtained after cold annealing, and the Ni atom fraction in the Ni < x > Co < 3-x > O < 4 > oxygen evolution catalyst is controlled to be 4%-10%.

Description

Technical Field

[0001] The present invention belongs to the technical field of hydrogen production by alkaline electrolyzed water, and particularly relates to a Ni x Co 3-x CoO4 oxygen evolution catalyst based on Co-MOF derivatives and a preparation method thereof. Background Art

[0002] The large-scale utilization of fossil energy has triggered an energy crisis and ecological damage, seriously threatening sustainable development. Exploring an efficient renewable energy system has become a global consensus. Among them, hydrogen energy is regarded as one of the most potential clean energy solutions due to its ultra-high energy density and zero-emission characteristics. Among many hydrogen production technologies, hydrogen production by electrolyzed water has become a research focus in the energy field due to its green and sustainable potential. Compared with traditional alkaline water electrolyzers, proton / anion exchange membrane electrolyzers (PEM / AEM) have significant advantages. Currently, PEM water electrolysis catalysts mostly use noble metal catalysts such as platinum (Pt), ruthenium (Ru), and iridium (Ir), but their high prices severely limit the large-scale application of PEM water electrolysis technology. Therefore, for AEM water electrolysis that can use non-noble metals, it is particularly important to find transition metal-based catalysts with good performance and low cost to replace noble metal-based catalysts.

[0003] Metal-organic frameworks (MOFs) are multi-dimensional porous structures constructed by metal nodes (ions or clusters) and organic ligands, and have attracted much attention due to their adjustable pore structures, high specific surface areas, and abundant unsaturated sites. Based on the multivalent characteristics and conductive advantages of cobalt (Co), Co-MOF also exhibits good electrochemical activity, but its catalytic performance and durability still need to be further improved through doping, structure engineering, or defect engineering. Among many transition metals, the coupling of the 3d orbit of Ni and the 3d orbit of Co can adjust the position of the d-band center, weaken the adsorption energy of the *OOH intermediate, and significantly reduce the reaction energy barrier. Chinese patent application CN118904360A immersed a nickel foam substrate in a reaction solution to obtain a Co-MOF material (ZIF-67 / NF) supported by nickel foam, and then through calcination and hydrothermal treatment, a self-supporting cobalt-nickel bimetallic sulfide heterojunction electrocatalytic composite material Co3S4 / Ni3S2 / NF was obtained, which exhibited excellent catalytic performance and high stability due to its stable structure, large specific surface area, and abundant vacancies, and was significantly better than Ni3S2 / NF without loading Co-MOF. It shows that the composite and synergistic effects of metallic Co and Ni can have a favorable impact on the catalyst. However, self-supporting oxygen evolution catalysts using nickel foam substrates all have deficiencies in terms of corrosion resistance, loading process, and mass transfer efficiency. Therefore, the prior art mainly focuses on the research of powder catalysts, and powder catalysts also have problems of insufficient performance and poor stability.

[0004] Chinese Patent Document CN117587440A discloses a cobalt-copper-based oxygen evolution electrocatalyst and its preparation method. A cobalt-copper metal-organic framework compound is obtained through solution blending and ion exchange, and a non-self-supporting cobalt-copper MOF-derived oxide highly efficient oxygen evolution electrocatalyst with a low oxygen evolution potential and excellent stability is prepared by an air annealing process. However, its oxygen evolution catalytic activity is relatively low and further optimization is required. At the same time, copper is prone to migrate and agglomerate during high-temperature treatment or electrochemical cycling, the morphology of the catalyst is prone to collapse during the preparation process, and there is also a problem of poor uniformity in the size of the nanosheets. Summary of the Invention

[0005] In order to overcome the problems in the prior art, the present invention provides a Ni x Co 3-x O4 oxygen evolution catalyst based on Co-MOF derivatives and its preparation method, and a powder catalyst is prepared to improve the oxygen evolution reaction (OER) performance and stability of the catalyst.

[0006] In order to solve the above technical problems, the technical solution proposed by the present invention is as follows: The present invention provides a preparation method of a Ni x Co 3-x O4 oxygen evolution catalyst based on Co-MOF derivatives, comprising the following steps: S1. Add 2-methylimidazole and a cobalt salt to a solvent, stir and react, then let it stand, and obtain a nano-rhombic precursor Co-MOF after centrifugal washing and drying.

[0007] S2. Disperse the Co-MOF obtained in step S1 into a solution containing a nickel source, stir and react, and obtain Co-Ni MOF after centrifugal washing and drying.

[0008] S3. Slowly heat up the Co-Ni MOF obtained in step S2 in an air atmosphere to keep the morphology of the product the same as that of Co-MOF in step S1, then slowly heat up to 300-375 °C, and obtain Ni x Co 3-x O4 oxygen evolution catalyst after cooling; wherein, 0 < x < 3; the atomic fraction of Ni in the Ni x Co 3-x O4 oxygen evolution catalyst is 4%-10%.

[0009] In the present invention, a nano-rhombic precursor Co-MOF is prepared by reacting 2-methylimidazole and a cobalt salt. After ion exchange with Ni ions, a hetero-metal node (Co-Ni MOF) is formed, changing the local charge distribution. During this process, the coordination bonds in the Co-MOF structure will be cleaved by protons during the ion exchange process and Co 2+, at this time, the dodecahedral structure of Co-MOF will collapse to form nanosheets or nanorod morphologies. In the present invention, the morphology of the precursor Co-Ni MOF is regulated by controlling the content of doped Ni, and a Co-Ni MOF precursor with a hexagonal nanosheet morphology is obtained, effectively increasing the contact area between the catalyst and the electrolyte. After heat treatment, the metal node Co in the MOF is converted into a porous oxide (such as ), inheriting the nanosheet or spherical morphology of the original MOF, and 2-methylimidazole is carbonized into graphitized carbon, leaving a hollow carbon framework, achieving morphology replication and enhancing the oxygen evolution activity. In addition, the stability of the catalyst is greatly improved after heat treatment.

[0010] As an alternative embodiment, in the preparation method provided by the present invention, in step S3, the atomic fraction of Ni in the Ni x Co 3-x O4 oxygen evolution catalyst is 8%.

[0011] As an alternative embodiment, in the preparation method provided by the present invention, in step S3, the temperature is raised to 350 °C for reaction, and the reaction time > 5 h.

[0012] As an alternative embodiment, in the preparation method provided by the present invention, in step S1, 2-methylimidazole and cobalt salt are respectively added to a methanol solution, and the methanol solution containing the cobalt salt is uniformly dropped into the methanol solution containing 2-methylimidazole while stirring.

[0013] In the present invention, adding 2-methylimidazole and cobalt salt to the methanol solution respectively and then stirring to add the cobalt salt is beneficial to the contact between cobalt ions and organic ligands, improving the efficiency of the reaction.

[0014] As an alternative embodiment, in the preparation method provided by the present invention, in step S2, the nickel source is dissolved in a mixed solution of ethanol and water, and the volume ratio of ethanol to water is 4:1.

[0015] As an alternative embodiment, in the preparation method provided by the present invention, in step S2, the temperature of the stirring reaction is 40 - 60 °C, and the stirring time is 10 - 30 min.

[0016] In the present invention, controlling the stirring temperature is beneficial to promoting the hydrolysis reaction of Ni ions and Co ions and promoting NiCo ion exchange. Controlling within the above temperature range is beneficial to controlling the morphology and performance of the product.

[0017] As an alternative embodiment, in the preparation method provided by the present invention, the drying temperature in step S1 and step S2 is 50 - 60 °C.

[0018] As an alternative embodiment, in the preparation method provided by the present invention, in step S3, the calcination temperature is 150 - 300 °C and the time is 1.5 - 2.5 h.

[0019] As an alternative embodiment, in the preparation method provided by the present invention, in step S3, the heating rate is 5 °C·min -1 .

[0020] Based on the same technical concept, the present invention also provides the Ni x Co 3-x O4 oxygen evolution catalyst prepared by the preparation method of the Ni x Co 3-x O4 oxygen evolution catalyst.

[0021] Compared with the prior art, the beneficial effects of the present invention are as follows: (1) In the present invention, the rhombic dodecahedron structure of Co-MOF is directly synthesized by chemical potential driving at room temperature, avoiding the high temperature, high pressure and high energy consumption of the hydrothermal method and reducing the production cost.

[0022] (2) In the present invention, through ion exchange with Ni and composition optimization, a Co-Ni MOF precursor with a hexagonal nanosheet morphology is prepared by controlling the morphology, effectively increasing the contact area between the catalyst and the electrolyte, and then annealing treatment enables the Ni x Co 3-x O4 catalyst to maintain the morphology of the original MOF, enhancing the oxygen evolution activity, and in addition, the stability of the catalyst is greatly improved after heat treatment.

[0023] (3) Compared with the self-supported oxygen evolution catalysts with substrates such as nickel foam and carbon cloth, the powder catalyst prepared by the present invention has greater flexibility and strong designability. In an AEM electrolytic cell, the powder can be coated on an anion exchange membrane or a current collector (nickel felt, titanium felt, nickel foam) of any shape / material, adapting to the modular design of industrial electrolytic cells and having greater potential for large-scale application. Description of the Drawings

[0024] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for the description of the embodiments or the prior art. Obviously, the drawings in the following description are some embodiments of the present invention. For those of ordinary skill in the art, other drawings can be obtained based on these drawings without creative efforts.

[0025] Figure 1 FIG. is the SEM image of the precursor Co MOF at different magnifications, where (a) is the SEM image at 50,000 times and (b) is the SEM image at 30,000 times; Figure 2 The Co MOF prepared in Examples 1-4 was subjected to Ni 2+ SEM images of the Co-Ni MOF series precursors after ion exchange, where (a) is 4-Co-Ni MOF, (b) is 6-Co-Ni MOF, (c) is 8-Co-Ni MOF, and (d) is 10-Co-Ni MOF; Figure 3 8-Ni prepared in Example 3 x Co 3-x SEM images of the O4 catalyst at different magnifications, where (a) is the SEM image at 70,000 times magnification and (b) is the SEM image at 100,000 times magnification; Figure 4 4-Ni prepared in the example x Co 3-x O4, 6-Ni x Co 3-x O4, 10-Ni x Co 3-x O4 catalysts, where (a) is 4-Ni x Co 3-x O4, (b) is 6-Ni x Co 3-x O4, (c) is 10-Ni x Co 3-x O4; Figure 5 4-Ni prepared in the example x Co 3-x O4, 6-Ni x Co 3-x O4, 8-Ni x Co 3-x O4, 10-Ni x Co 3-x EDS spectra of O4, where (a) is 4-Ni x Co 3-x O4, (b) is 6-Ni x Co 3-x O4, (c) is 8-Ni x Co 3-x O4, (d) is 10-Ni x Co 3-x O4; Figure 6 The Co MOF, Co-Ni MOF, and Ni prepared in the example x Co 3-xXRD patterns of O4, where (a) is the XRD pattern of CoMOF and the Co MOF pattern calculated and simulated using VESTA, (b) is the XRD patterns of 4-Co-Ni MOF, 6-Co-Ni MOF, 8-Co-Ni MOF and 10-Co-Ni MOF and the comparison of Co-MOF and the calculated and simulated Ni MOF (CCDC 1494751) patterns, (c) is the partial enlarged view of (b), and (d) is 4-Ni x Co 3-x O4, 6-Ni x Co 3-x O4, 8-Ni x Co 3-x O4, 10-Ni x Co 3-x XRD patterns of O4; Figure 7 is 8-Ni x Co 3-x TEM detection results of O4, where (a) is the TEM image with a magnification of 50k and (b) is the TEM image with a magnification of 800k; Figure 8 is 8-Ni x Co 3-x XPS full spectrum of O4 and high-resolution spectra of Ni, Co, and O, where (a) is 8-Ni x Co 3-x XPS full spectrum of O4, and (b-d) are the high-resolution spectra of Ni, Co, and O respectively; Figure 9 is 4-Ni prepared in the example x Co 3-x O4, 6-Ni x Co 3-x O4, 10-Ni x Co 3-x Oxygen evolution catalytic activity results of the O4 catalyst, where (a) 4-Ni x Co 3-x O4, 6-Ni x Co 3-x O4, 10-Ni x Co 3-x Anodic polarization (LSV) curves of the O4 catalyst, and (b) is the Tafel slope diagram obtained by fitting the polarization curves; Figure 10 is the anodic polarization curve diagram of the catalyst products of 8-Co-Ni MOF at different annealing temperatures; Figure 11 is the anodic polarization curve diagram after loading the 8-Co-Ni MOF catalyst onto nickel foam and nickel felt; Figure 12 8-Ni prepared for Example 3 x Co 3-x Electrochemical detection results of CoO4, where (a) is for 8-Ni x Co 3-x Long-term electrolysis curve of CoO4 at a constant current density of 10 mA·cm -2 ; (b) is the comparative polarization curve before and after 5000 cycles of CV accelerated decay; Figure 13 For 4-Ni x Co 3-x Long-term electrolysis curves of CoO4, 6-Ni x Co 3-x Long-term electrolysis curves of CoO4, 10-Ni x Co 3-x Long-term electrolysis curves of CoO4 catalysts at a constant current density of 10 mA·cm -2 ; where (a) is for 4-Ni x Co 3-x CoO4, (b) is for 6-Ni x Co 3-x CoO4, (c) is for 10-Ni x Co 3-x CoO4. Detailed implementation manners

[0026] To facilitate the understanding of the present invention, the present invention will be described more comprehensively and in detail below in conjunction with the accompanying drawings of the specification and preferred embodiments, but the protection scope of the present invention is not limited to the following specific embodiments.

[0027] Unless otherwise defined, all professional terms used hereinafter have the same meaning as commonly understood by those skilled in the art. The professional terms used herein are only for the purpose of describing specific embodiments and are not intended to limit the protection scope of the present invention.

[0028] Unless otherwise specifically stated, various raw materials, reagents, instruments and equipment used in the present invention can be obtained through the market or can be prepared by existing methods.

[0029] The following reagents used in the present invention and the tests are all purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China), including cobalt nitrate hexahydrate (Co(NO3)2·6H2O), nickel chloride hexahydrate (NiCl2·6H2O), 2-methylimidazole (C5H8N2), methanol (CH4O), sodium hydroxide (NaOH), nitric acid (HNO3), sulfuric acid (H2SO4), potassium chloride (KCl) and 5 wt.% Nafion (DuPont D520). All chemical reagents are of analytical grade and can be directly used without further purification. The experimental water is deionized water (DI) purified by the Millipore system.

[0030] Example 1 Preparation method of a Ni x Co 3-x O4 oxygen evolution catalyst, comprising the following steps: (1) Preparation of Co-MOF First, 4.50 g of 2-methylimidazole and 2.00 g of cobalt(II) nitrate hexahydrate were respectively added to 160 mL and 100 mL of methanol, denoted as solution 1 and solution 2, and then magnetically stirred until completely dissolved. With solution 1 continuously magnetically stirred at room temperature, solution 2 was slowly added dropwise to solution 1 for mixing. After stirring for 30 min, it was sealed and allowed to stand for 12 h. The supernatant was poured off, and it was centrifugally washed three times with methanol and vacuum dried at 60 °C for 6 h to obtain Co MOF.

[0031] (2) Preparation of Co-Ni MOF 0.35 g of Co MOF was dispersed into a 75 mL ethanol / water mixed solution (ethanol-water volume ratio 4:1) containing 150 mg of nickel(II) chloride hexahydrate, and stirred at 50 °C for 20 min. The product was centrifugally washed three times with pure water and ethanol and vacuum dried at 60 °C for 6 h to obtain 8-Co-Ni MOF.

[0032] (3) Preparation of Ni x Co 3-x O4 Co-Ni MOF was heated to 200 °C at a heating rate of 5 °C·min -1 in an air atmosphere and held for 2 h, and then heated to 350 °C at a rate of 5 °C·min -1 and held for 2 h. After cooling with the furnace, Ni x Co 3-x O4 was obtained.

[0033] Examples 2-7 are different from Example 1 in the addition amount of nickel(II) chloride hexahydrate and the annealing temperature. For specific parameters, see Table 1.

[0034] Table 1: Addition amount of nickel(II) chloride hexahydrate and annealing temperature in Examples 1-7 By controlling the different addition amounts of nickel(II) chloride hexahydrate, Examples 1-4 obtained final products with Ni atomic fractions of 4%, 6%, 8% and 10%, named 4-Ni x Co 3-x O4, 6-Ni x Co 3-x O4, 8-Nix Co 3-x O4 and 10-Ni x Co 3-x O4. By controlling the final annealing temperature at 300 °C, 325 °C, and 375 °C, catalyst products of the precursor 8-Co-Ni MOF at different temperatures were obtained, named 8-Ni x Co 3-x O4-300, 8-Ni x Co 3-x O4-325 and 8-Ni x Co 3-x O4-375.

[0035] Performance detection The precursor Co MOF prepared in the example was detected, and the SEM images at different magnifications are as Figure 1 shown, where Figure 1 (a) is the SEM image at 50,000 times magnification, Figure 1 (b) is the SEM image at 30,000 times magnification. As shown in the figure, the shape of the precursor Co MOF is a nano rhombic dodecahedron, with uniform distribution, uniform size, relatively smooth surface, and no obvious agglomeration phenomenon.

[0036] The SEM images at 50,000 times magnification of the precursors 4-Co-Ni MOF, 6-Co-Ni MOF, 8-Co-Ni MOF, and 10-Co-Ni MOF after Co MOF is ion-exchanged with Ni 2+ are shown, where Figure 2 (a) is Figure 4 -Co-Ni MOF, Figure 2 (b) is 6-Co-Ni MOF, Figure 2 (c) is 8-Co-Ni MOF, Figure 2 (d) is 10-Co-Ni MOF. As shown in the figure, when the Ni 2+ content is relatively low, the nano rhombic dodecahedron structure of Co MOF begins to dissolve and gradually transforms into nano spherical particles. As the Ni content increases, the dodecahedron structure almost completely transforms into a nano spherical structure, and at the same time, it begins to transform into a hexagonal nano sheet structure. When the Ni content continues to increase, the hexagonal sheet structure begins to dissolve and forms a nano rod structure.

[0037] The 8-Ni x Co 3-x O4 catalyst prepared in Example 3 was detected, and the SEM images at different magnifications are as Figure 3 shown, where Figure 3 (a) is the SEM image at 70,000 times magnification, Figure 3(b) SEM image at 100,000 times magnification. Compared with Figure 2 (c), it can be found that the 8-Ni x Co 3-x O4 after annealing retains the hexagonal nanosheet structure of 8-Co-Ni MOF and obtains a rougher surface.

[0038] Figure 4 SEM images of 4-Ni x Co 3-x O4, 6-Ni x Co 3-x O4, and 10-Ni x Co 3-x O4 prepared in Examples 1-2 and 4 are shown, where Figure 4 (a) is 4-Co-Ni MOF, Figure 4 (b) is 6-Co-Ni MOF, Figure 4 (c) is 10-Co-Ni MOF. As shown in the figure, when the Ni content in the catalyst is low, the nano-spheres agglomerate to form a cellular structure. When the Ni content increases, the number of nanosheets increases and the sheet structures stack up. When the Ni content continues to increase, the formed nano-sheet structure begins to disintegrate, forming a rich number of ellipsoidal and rod-like structures.

[0039] EDS detection was carried out on 4-Ni x Co 3-x O4, 6-Ni x Co 3-x O4, 8-Ni x Co 3-x O4, and 10-Ni x Co 3-x O4 prepared in Examples 1-4. The spectra are as Figure 5 shown, confirming the elemental composition of the Ni x Co 3-x O4 catalyst. When performing EDS analysis, the catalyst was loaded on a silicon substrate, which led to a strong silicon peak in the energy spectrum. Among them, Figure 5 (a) is Figure 4 -Co-Ni MOF, Figure 5 (b) is 6-Co-NiMOF, Figure 5 (c) is 8-Co-Ni MOF, Figure 5 (d) is 10-Co-Ni MOF.

[0040] XRD detection was carried out on Co MOF, Co-Ni MOF, and Ni x Co 3-x O4 prepared in the examples. The results are as Figure 6 shown, Figure 6(a) The XRD pattern of Co MOF and the calculated and simulated Co MOF pattern (ZIF-67, CCDC-671073) using VESTA. As can be seen from the figure, the XRD pattern of Co MOF in the present invention matches well with the simulated pattern. The diffraction peaks at 7.37°, 10.43° and 12.78° correspond to the (011), (002) and (112) crystal planes of the simulated Co-MOF (CCDC 671073), respectively. The above results indicate the successful synthesis of Co MOF with a rhombic dodecahedron structure. Figure 6 (b) 2+ XRD patterns of the precursors 4-Co-Ni MOF, 6-Co-Ni MOF, 8-Co-Ni MOF and 10-Co-Ni MOF after Ni Figure 6 ion exchange, and the XRD patterns of the compared Co-MOF and the calculated and simulated Ni MOF (CCDC 1494751). As can be seen from the figure, first, when the Ni content is low, the XRD pattern of 4-Co-Ni MOF still basically maintains the peak shape of Co MOF, and the diffraction peak intensities at 7.37°, 10.43° and 12.78° all decrease to a certain extent. As the Ni content increases, the intensities of the three main peaks belonging to Co MOF gradually decrease until no obvious Co MOF peaks can be observed in the XRD pattern of 10-Co-Ni MOF. The diffraction peak at 11.84° on the simulated Ni MOF pattern corresponds to the (200) crystal plane, and this diffraction peak shifts to 11.06°, 11.10° and 11.20° respectively in the XRD patterns of 6-Co-Ni MOF, 8-Co-Ni MOF and 10-Co-Ni MOF ( Figure 6 (c)). This is because the ionic radius of Ni (0.69 Å) is slightly larger than that of Co (0.65 Å). According to Bragg's equation, as the proportion of Ni in the lattice increases, the lattice distorts and expands gradually, causing the diffraction peak to shift towards a smaller angle. At the same time, with the doping of Ni ions, the XRD pattern of Co-Ni MOF begins to show the characteristics of amorphous diffuse scattering peaks, indicating a certain degree of amorphization. Figure 6 (d) The XRD patterns of 4-Ni x Co 3-x O4, 6-Ni x Co 3-x O4, 8-Ni x Co 3-x O4, 10-Ni x Co 3-x O4. After analysis by the jade software, the PDF cards that correspond well to the 4 XRD patterns are Co3O4 (JCPDS 43-1003), Co2NiO4 (JCPDS 02-1074) and Co 1.29 Ni1.71 O4 (JCPDS 40 - 1191), the main peaks corresponding to the (311) crystal plane of the three PDF cards are located at 36.85°, 36.65° and 36.60° respectively. Correspondingly, since the 4 - Co - Ni MOF precursor still basically maintains the peak shape of Co MOF, 4 - Ni x Co 3-x the main peak in the O4 spectrum is closer to the main peak of the Co3O4 card, while 10 - Ni x Co 3-x the main peak in the O4 spectrum is closer to the main peak of the Co 1.29 Ni 1.71 O4 card. This change can be observed from the inset in Figure 6 (d), where the red line and the blue line correspond to the main peak values of Co3O4 and Co 1.29 Ni 1.71 O4 cards respectively. At the same time, the spectral shapes of the four samples are relatively close, and the degree of amorphization is relatively high. The reason is that Ni x Co 3-x O4 is formed by annealing Co - Ni MOF with a high degree of amorphization in air, and has a relatively low crystallinity. The low crystallinity is beneficial to exposing more catalytic active sites, accelerating the charge transfer between reaction intermediates and active sites to reduce the electrochemical impedance, and improving the activity of unit sites.

[0041] To analyze the microstructure and morphology of 8 - Ni x Co 3-x O4, TEM was used for detection, and the results are as Figure 7 shown. It can be seen from Figure 7 (a) and 7(b) that the nanosheet structure of 8 - Ni x Co 3-x O4 presents a regular hexagonal structure and is formed by the combination and splicing of nanocrystals with a width of 5 - 10 nm, and has good uniformity. This highly dispersed nanoparticle is beneficial to providing more catalytic active sites. The nanosheet structure of 8 - Ni x Co 3-x O4 presenting a regular hexagonal structure also confirms the view that the MOF - derived material in the present invention can retain the original MOF morphological structure, indicating that the morphology of the material does not collapse during the preparation process.

[0042] Figure 8 XPS full spectrum of 8 - Ni x Co 3-x O4 and high - resolution spectra of Ni, Co, and O. As shown in Figure 8 (a), 8 - Ni x Co 3- xThe full XPS spectrum of O4. By analyzing it, it can be clearly found that the main elements in the sample are Co, Ni, O, and C. Among them, the C signal may come from the calibration C. Figure 8 (b) is the high-resolution spectrum of Ni element. Two spin-orbit peaks can be fitted near 855.4 and 872.3 eV, corresponding to Ni 2p 3 / 2 and Ni 2p 1 / 2 respectively. Among them, the peaks with binding energies of 853.8 and 872.1 eV are attributed to Ni 2+ , while the peaks at 855.5 and 874.1 eV are attributed to Ni 3+ . The two peaks at 861.0 and 879.5 eV are the satellite peaks (labeled as Sat.) of Ni 2p 3 / 2 and Ni2p 1 / 2 respectively. Figure 8 (c) The Co 2p spectrum shown can also be fitted with two spin-orbit doublets. Among them, the peaks at 780.5 eV and 795.5 eV correspond to the 2p 2+ of Co 3 / 2 and 2p 1 / 2 respectively. The peaks at 781.7 eV and 797.2 eV correspond to the 2p 3+ of Co 3 / 2 and 2p 1 / 2 respectively. And the peak at 805.7 eV is the characteristic satellite peak of Co 2p 1 / 2 . Figure 8 (d) The high-resolution spectrum of O 1s shown. Among them, the peak at 529.3 eV corresponds to the typical Metal-O binding energy, the peak at 531.1 eV corresponds to the binding energy of oxygen vacancies, and the peak at 533.0 eV corresponds to the C-O bond binding energy. The valence electrons of high-valence Co 3+ and Ni 3+ are more abundant than those of low valence states. Their existence endows the prepared nanostructures with excellent catalytic activity in OER. At the same time, the above results show that the surface chemical valence states of the prepared 8-Ni x Co 3-x O4 are mainly Co 2+ , Co 3+ , Ni 2+ and Ni 3+ . The existence of Co / Ni nodes with multiple different metal valence states promotes the synergistic effect of the bimetals, thereby effectively improving the electrocatalytic performance of 8-Ni x Co 3- x O4.

[0043] To evaluate the OER performance of the catalyst, a standard three-electrode system was used to prepare a series of Ni in 1 M KOH solutionx Co 3-x Electrochemical tests were carried out on the Co

[0044] The electrochemical test method is as follows: All electrochemical tests were performed in a 1 mol·L -1 KOH solution at 25 °C under a standard three-electrode system in a CHI660B electrochemical workstation. A commercial glassy carbon electrode (GCE, with a bottom diameter of 6 mm and an effective central area diameter of 3 mm) was used as the modified electrode to load the catalyst as the working electrode, a saturated calomel electrode (SCE) and a graphite electrode were used as the reference electrode and the counter electrode, respectively.

[0045] The preparation process of the working electrode is as follows: 8 mg of catalyst powder was mixed with 80 μL of 5 wt.% Nafion, 460 μL of absolute ethanol, and 460 μL of deionized water. Then the mixture was ultrasonicated in a cold water bath for 1 h to obtain a uniform ink. Subsequently, the ink was evenly coated on the surface of the GCE, placed horizontally under an infrared lamp, and dried for 3 - 5 min before measurement. The glassy carbon electrode was used as the working electrode, and the surface catalyst loading was 1 mg·cm -2 .

[0046] With a overpotential of 10 mA cm -2 as the evaluation standard for the oxygen evolution catalytic activity, the results are as Figure 9 shown. As Figure 9 (a) The anodic polarization curve shows that the overpotential of 8-Ni x Co 3-x O4 is only 320 mV, slightly lower than that of 4-Ni x Co 3-x O4 (331 mV), 6-Ni x Co 3-x O4 (321 mV), 10-Ni x Co 3-x O4 (329 mV) at the same current density; and when the current density increases, 8-Ni x Co 3-x O4 only requires an overpotential of 391 mV to reach a current density of 50 mA cm -2 , significantly better than 4-Ni x Co 3-x O4 (430 mV), 6-Ni x Co 3-x O4 (408 mV) and 10-Ni x Co 3-xO4 (413 mV). Combining the above results, it can be speculated that when the Ni atomic ratio content is 8%, the synergistic effect strength between Ni atoms and Co atoms has the greatest increase in the OER activity of the catalyst, thereby improving the intrinsic catalytic activity of the catalyst. Combining the SEM results, when the Ni content is low, the surface of the catalyst is agglomerated with irregular small balls and flakes. When the Ni content is high, the formed nano-sheet structure begins to disintegrate. It can be speculated that the regular hexagonal nano-sheet morphology has more active sites, which is beneficial to the OER performance. Figure 9 (b) is the Tafel slope diagram obtained by fitting the polarization curve, 8-Ni x Co 3-x The Tafel slope of O4 is only 61.5 mV·dec -1 , which is lower than that of 4-Ni x Co 3-x O4 (74.4 mV·dec -1 ), 6-Ni x Co 3-x O4 (67.0 mV·dec -1 ), and 10-Ni x Co 3-x O4 (70.3 mV·dec -1 ), indicating that the Ni x Co 3-x O4 catalyst prepared by the present invention has excellent OER kinetic performance, and 8-Ni x Co 3-x O4 has the most excellent performance.

[0047] The catalyst products prepared from the same precursor 8-Co-Ni MOF prepared in Examples 5-7 at different annealing temperatures were detected, and their anodic polarization curves are as Figure 10 shown. At the same current density (10 mA cm -2 ), 8-Ni x Co 3-x O4-350 has the lowest overpotential (320 mV), which is better than that of 8-Ni x Co 3-x O4-300 (330 mV), 8-Ni x Co 3-x O4-325 (327 mV), 8-Ni x Co 3-x O4-375 (333 mV).

[0048] The 8-Ni x Co 3-x O4 catalyst prepared in Example 3 was loaded onto nickel foam and nickel felt to prepare a working electrode, and its oxygen evolution performance was tested.

[0049] The electrochemical test method is as follows: All electrochemical tests were carried out in a standard three-electrode system in a CHI660B electrochemical workstation at 25 °C in 1 mol·L -1 KOH solution. Foam nickel and nickel felt were used as the modified electrode to load the catalyst as the working electrode, saturated calomel electrode (SCE) and graphite electrode were used as the reference electrode and the counter electrode respectively.

[0050] The preparation process of the working electrode is as follows: 8 mg of catalyst powder was mixed with 80 μL of 5 wt.% Nafion, 460 μL of absolute ethanol and 460 μL of deionized water. Then the mixture was ultrasonicated in a cold water bath for 1 h to obtain a homogeneous ink. Subsequently, the ink was evenly coated on the foam nickel or nickel felt, placed horizontally under an infrared lamp, and dried for 3 - 5 min before measurement. Among them, the pretreatment steps of the foam nickel / nickel felt substrate were: cutting the substrate with a size of 1 cm * 1 cm and reserving a 0.5 cm * 1 cm tab, then successively ultrasonicating and washing with acetone, hydrochloric acid, and deionized water for 15 min, and then vacuum drying at 60 °C for storage. Referring to the loading amount of non-precious metals in the AEM electrolyzer, the catalyst loading amount on the surface of the foam nickel / nickel felt was 2 mg·cm -2 .

[0051] The test results are as Figure 11 shown. The overpotentials of 8-Ni x Co 3-x O4 / foam nickel and 8-Ni x Co 3-x O4 / nickel felt at a current density of 10 mA cm -2 were 231 and 243 mV respectively. Compared with the performance loaded on the glassy carbon electrode, there was an obvious optimization. The reason is that the specific surface area of the foam nickel and nickel felt substrates is larger, and the contact area between the catalyst powder loaded on them and the electrolyte also increases, thus exposing more active sites and improving the oxygen evolution efficiency. On the other hand, it is also related to the increase in the catalyst loading amount.

[0052] The 4-Ni x Co 3-x O4, 6-Ni x Co 3-x O4 and 10-Ni x Co 3-x O48-Ni x Co 3-x O4 prepared in the examples were detected. The long-term electrolysis curves at a constant current density of 10 mA·cm -2 and the comparative polarization curves before and after 5000 cycles of CV accelerated decay are as Figure 12 and13 as shown

[0053] As Figure 12 shown, the 8-Ni x Co 3-x O4 catalyst undergoes 32 h of constant current electrolysis, and its overpotential decay rate is only about 1.2%, and the electrolysis curves are relatively stable. After 5000 CV cycles of aging, the overpotential change of its polarization curve compared with the initial polarization curve at 10 mA·cm -2 is almost negligible, and the potential decay rate at 50 mA·cm -2 is only 0.8%. As Figure 13 shown, after 32 h of constant current electrolysis, Figure 13 (a) 4-Ni x Co 3-x O4 overpotential decay rate is about 2.7%; after 32 h of constant current electrolysis, Figure 13 (b) 6-Ni x Co 3-x O4 its overpotential decay rate is about 2.0%; after 32 h of constant current electrolysis, Figure 13 (c) 10-Ni x Co 3-x O4 overpotential decay rate is about 2.9%. The above results show that the electrochemically oxygen evolution stability effect of the catalysts prepared is better, and 8-Ni x Co 3-x O4 is the best.

[0054] The above content is a further detailed description of the present invention in combination with specific preferred embodiments. It cannot be determined that the specific implementation of the present invention is limited only to these descriptions. For those of ordinary skill in the art to which the present invention pertains, without departing from the concept of the present invention, several simple deductions or substitutions can still be made, which should all be regarded as belonging to the protection scope of the present invention.

Claims

1. A preparation method of a Ni x Co 3-x CoO4 oxygen evolution catalyst, characterized in that, It includes the following steps: S1. Add 2-methylimidazole and cobalt salt into a solvent, stir and react, then let it stand. After centrifuging, washing and drying, obtain a nano-rhombic precursor Co-MOF; S2. Disperse the Co-MOF obtained in step S1 into a solution containing a nickel source, stir and react, and obtain Co-Ni MOF after centrifuging, washing and drying; S3. Slowly heat up the Co-Ni MOF obtained in step S2 in an air atmosphere to maintain the morphology of the Co-MOF in step S1, and then slowly heat up to 300 - 375 °C. After cooling, Ni x Co 3-x O4 oxygen evolution catalyst; wherein, 0 < x < 3, the Ni x Co 3-x The Ni atomic fraction in the O4 oxygen evolution catalyst is 4% - 10%.

2. The preparation method of the Ni x Co 3-x CoO4 oxygen evolution catalyst based on Co-MOF derivative according to claim 1, characterized in that In step S3, Ni x Co 3-x The Ni atomic fraction in the O4 oxygen evolution catalyst is 8%.

3. The preparation method of the Ni x Co 3-x CoO4 oxygen evolution catalyst based on Co-MOF derivative according to claim 1, characterized in that In step S3, heat up to 350 °C for reaction, and the reaction time > 5 h.

4. The preparation method of the Ni x Co 3-x CoO4 oxygen evolution catalyst based on Co-MOF derivative according to claim 1, characterized in that, In step S1, add 2-methylimidazole and cobalt salt into a methanol solution respectively, and uniformly drip the methanol solution containing cobalt salt into the methanol solution containing 2-methylimidazole while stirring.

5. The preparation method of the Ni x Co 3-x CoO4 oxygen evolution catalyst based on Co-MOF derivative according to claim 1, characterized in that, In step S2, the nickel source is dissolved in a mixed solution of ethanol and water, and the volume ratio of ethanol to water is 4:

1.

6. The preparation method of the Ni x Co 3-x CoO4 oxygen evolution catalyst based on Co-MOF derivative according to claim 1, characterized in that, In step S2, the temperature of the stirring reaction is 40 - 60 °C, and the stirring time is 10 - 30 min.

7. The preparation method of the Ni x Co 3-x CoO4 oxygen evolution catalyst based on Co-MOF derivative according to claim 1, characterized in that The drying temperature in step S1 and step S2 is 50 - 60 °C.

8. The preparation method of the Ni x Co 3-x CoO4 oxygen evolution catalyst based on Co-MOF derivative according to claim 1, characterized in that, In step S3, the calcination temperature is 150 - 300 °C, and the time is 1.5 - 2.5 h.

9. The preparation method of the Ni x Co 3-x CoO4 oxygen evolution catalyst based on Co-MOF derivative according to claim 1, characterized in that In step S3, the heating rate is 5 °C·min -1 .

10. The Ni x Co 3-x O4 oxygen evolution catalyst prepared by the preparation method of the Co-MOF derivative-based Ni x Co 3-x O4 oxygen evolution catalyst.

Images