Preparation method and application of MOF-on-MOF heterojunction metal-organic framework material rich in oxygen defects

Abstract

The present invention discloses a preparation method and application of a MOF-on-MOF heterojunction metal-organic framework material rich in oxygen defects, belonging to the technical field of electrocatalytic oxygen evolution reaction. The present invention first grows Ni NDC nanosheets in situ on a carrier by a solvothermal method, and then grows Fe BDC nanosheets on the surface of the Ni NDC nanosheets by a solvothermal method to form Ni NDC@Fe BDC ultra-thin nanosheets. This method can synthesize a MOF-on-MOF type metal-organic framework under the premise of lattice mismatch. The synthesized Ni NDC@Fe BDC combines the advantages of multi-component material hybridization, ultra-thin nanosheets and defect induction, which can increase the number of active sites and promote the rapid transfer / redistribution of electrons in the catalytic process. The MOF-on-MOF heterojunction metal-organic framework has good electrocatalytic performance and excellent total water decomposition performance in alkaline oxygen evolution reaction.

Description

Technical Field

[0001] The present invention relates to the technical field of electrocatalytic oxygen evolution reaction, and in particular to a preparation method and application of an oxygen-defect-rich MOF-on-MOF heterojunction metal-organic framework material. Background Art

[0002] The process of water electrolysis includes two basic half reactions: oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). Generally speaking, OER is a more complex multi-electron transfer process compared with HER (OER, 4OH - →2H2O+O2+4e - ), involving the breaking of OH bonds and the formation of O=O bonds, which has a significant impact on the overall water splitting efficiency. Metal-organic frameworks (MOFs) are a class of porous coordination polymers, known for their unique structure, huge porosity and well-defined metal coordination environment, and are widely favored in the field of OER catalysts. Among various MOFs-based electrocatalysts, nickel-based MOFs have considerable research potential. This is because compared with precious metals, nickel resources are abundant and cost-effective, and the nickel metal sites have good redox activity. However, the catalytic performance of existing nickel-based MOFs in OER is still relatively low, which limits their further promotion and application in the field of OER. Further optimization based on nickel-based MOFs to develop catalysts with excellent OER catalytic performance remains the focus of research by those skilled in the art. Summary of the Invention

[0003] The purpose of the present invention is to provide a preparation method and application of an oxygen-defect-rich MOF-on-MOF heterojunction metal-organic framework material to solve the problems existing in the above-mentioned prior art.

[0004] To achieve the above object, the present invention provides the following solutions:

[0005] One of the technical solutions of the present invention is a method for preparing a MOF-on-MOF heterojunction metal-organic framework material rich in oxygen defects, which comprises sequentially growing NiNDC nanosheets (i.e., a nickel-based MOF with naphthalene dicarboxylic acid as a ligand) and Fe BDC nanosheets (i.e., an iron-based MOF with phthalic acid as a ligand) on a carrier surface through two solvothermal reactions (i.e., a solvothermal method) to obtain the MOF-on-MOF heterojunction metal-organic framework material rich in oxygen defects.

[0006] Furthermore, the sequential growth of NiNDC nanosheets and Fe BDC nanosheets on the support surface through two solvothermal reactions specifically includes the following steps:

[0007] The carrier is added to a reaction solution 1 composed of a nickel source, naphthalene dicarboxylic acid, and a solvent 1, and heated to perform a first solvothermal reaction to grow NiNDC nanosheets on the surface of the carrier. Then, the carrier (Ni NDC / CP) with the Ni NDC nanosheets grown on the surface is added to a reaction solution 2 composed of an iron source, phthalic acid, and a solvent 2, and heated to perform a second solvothermal reaction to grow Fe BDC nanosheets on the surface of the NiNDC nanosheets to form NiNDC@Fe BDC nanosheets, thereby obtaining the oxygen-deficient MOF-on-MOF heterojunction metal-organic framework material (abbreviated as NiNDC@Fe BDC or Ni NDC@Fe BDC / CP, ​​where CP represents the carrier).

[0008] The NiNDC nanosheets and Fe BDC nanosheets grown by the solvothermal method, as well as the Ni NDC@Fe BDC nanosheets composed of the NiNDC nanosheets and the Fe BDC nanosheets, are all in the form of ultrathin nanosheets. The thickness of the NiNDC@Fe BDC nanosheets is about 4.3 nm.

[0009] Furthermore, the nickel source is nickel chloride; the iron source is ferrous chloride; the naphthalene dicarboxylic acid is 2,6-naphthalene dicarboxylic acid (H2NDC); the phthalic acid is 1,4-terephthalic acid (H2BDC); the solvent 1 is a mixed solution of N,N-dimethylformamide (DMF) and anhydrous methanol (MeOH); the solvent 2 is a mixed solution of N,N-dimethylformamide, anhydrous ethanol (EtOH) and water; and the carrier is carbon fiber paper (CP).

[0010] Furthermore, the water is ultrapure water (DI).

[0011] The carbon fiber paper has good porosity and electrical conductivity, and is resistant to high temperatures and corrosion. Its skeleton will not be destroyed under extreme conditions, making it suitable as a carrier for the MOF-on-MOF heterojunction metal-organic framework material.

[0012] The two organic ligands play the following roles in the reaction process: In the first step of the solvothermal process, H2NDC and Ni 2+ Coordination, participating in the synthesis of NiNDC / CP; in the subsequent secondary solvothermal process, Ni NDC / CP is immersed in the reaction solution 2, during which two important processes occur: first, NiNDC / CP acts as a carrier for in-situ growth, facilitating the growth of the catalyst; second, because Ni and Fe have similar atomic radii, ion exchange occurs during the reaction, resulting in a small amount of Fe 2+ into the Ni NDC lattice, along with Fe 2+Fe BDC is generated by coordination with H2BDC and grows on the NiNDC surface, realizing the dynamic adaptive growth of MOFon MOF based on lattice mismatch.

[0013] In addition, H2NDC and H2BDC are both carboxylic acid ligands. Selecting them as ligands for nickel and iron, respectively, is more conducive to the morphology of long sheets in the subsequent synthesis of MOF onMOF.

[0014] Moreover, most MOFs are also prone to reconstruction during the OER process. Compared with other types of ligands, MOF-onMOF obtained by selecting H2NDC and H2BDC as ligands is easier to reconstruct during the OER process, and then quickly transformed into MOOH (M is metal) with high OER activity.

[0015] The reaction solvent plays the following roles in the reaction process: (1) stabilizing the structure of MOF-on-MOF by coordinating with metal ions to form coordination bonds; (2) regulating the polarity of the reaction solvent to control the nucleation and growth process of MOF.

[0016] In addition, the above solvents are selected as reaction solvents in the present invention because they have the following advantages: (1) DMF, as a polar solvent, can dissolve most organic substances and many inorganic substances, and can slowly release substances such as dimethylamine during the reaction process, which is beneficial to the synthesis of MOF; (2) By adding a small amount of anhydrous methanol, anhydrous ethanol and ultrapure water to DMF, the pH and polarity of the reaction solvent can be further adjusted, thereby affecting the nucleation and growth process of MOF during the reaction.

[0017] Furthermore, the nickel source is preferably nickel chloride hexahydrate (NiCl2·6H2O); the iron source is preferably ferrous chloride tetrahydrate (FeCl2·4H2O).

[0018] Furthermore, the molar ratio of the nickel source to naphthalene dicarboxylic acid in the reaction solution 1 is 4:3, and the usage ratio of naphthalene dicarboxylic acid to solvent 1 is 0.75 mmol:15 mL; the molar ratio of the iron source to phthalic acid in the reaction solution 2 is 8:15, and the usage ratio of phthalic acid to solvent 2 is 1.25 mmol:16 mL.

[0019] Furthermore, the volume ratio of N,N-dimethylformamide to anhydrous methanol in the solvent 1 is 1:1; and the volume ratio of N,N-dimethylformamide, anhydrous ethanol and water in the solvent 2 is 14:1:1.

[0020] Furthermore, the ratio of the nickel source to the carrier is 0.5 mmol: 0.5 cm 2 (carrier surface area).

[0021] Furthermore, the reaction solution 1 is prepared by mixing a nickel source, naphthalene dicarboxylic acid and solvent 1, and ultrasonically treating the mixture for 15 minutes; the reaction solution 2 is prepared by mixing a nickel source, phthalic acid and solvent 2, and ultrasonically treating the mixture for 15 minutes.

[0022] Furthermore, the temperature of the first solvent thermal reaction is 120° C., and the time is 9 hours; the temperature of the second solvent thermal reaction is 120° C., and the time is 2 hours.

[0023] Furthermore, before the carrier is added to the reaction solution 1, a pretreatment operation of the carrier is also included. The pretreatment operation specifically includes: heating the carrier in a nitric acid solution for reaction, and then washing and drying.

[0024] The hydrophobic carbon fiber paper was transformed into hydrophilic carbon fiber paper by heating in nitric acid solution, which was beneficial to the in situ growth of nanosheets.

[0025] Furthermore, the heating reaction temperature is 120° C. and the time is 3 hours.

[0026] Furthermore, the step of cutting the carrier (carbon fiber paper) into a size of 1 cm×2 cm is also included before heating the carrier in the nitric acid solution for reaction.

[0027] Furthermore, the washing is specifically washing with ultrapure water and anhydrous ethanol to neutrality (pH=7).

[0028] Furthermore, after the first solvent thermal reaction and the second solvent thermal reaction are completed, washing and drying operations are further included, specifically: washing the carbon paper after the reaction with anhydrous ethanol three times, and then vacuum drying at 60° C. for 6 hours.

[0029] The present invention synthesizes oxygen-deficient MOF-on-MOF heterojunction metal-organic framework materials through two solvothermal reactions. The preparation method has the following advantages: (1) simplified synthesis process: the two-solvothermal synthesis method is simple to operate and avoids the limitation of lattice matching strategy; (2) simple raw materials: only two organic ligands, two metal sources and organic solvents are required, and the sources are easily available;

[0030] (3) Improve product purity: The product grows on carbon fiber paper through solvent thermal reaction, which can reduce the generation of impurities.

[0031] The second technical solution of the present invention: an oxygen-deficient MOF-on-MOF heterojunction metal-organic framework material prepared by the above-mentioned method for preparing an oxygen-deficient MOF-on-MOF heterojunction metal-organic framework material.

[0032] The third technical solution of the present invention: an application of the above-mentioned oxygen-deficient MOF-on-MOF heterojunction metal-organic framework material in an electrocatalytic oxygen evolution reaction.

[0033] Furthermore, the oxygen-deficient MOF-on-MOF heterojunction metal-organic framework material is used as a catalyst for electrocatalytic oxygen evolution reaction.

[0034] Furthermore, the oxygen-deficient MOF-on-MOF heterojunction metal-organic framework material is used as an anode for electrocatalytic oxygen evolution reaction in a full water splitting device.

[0035] The present invention first solvothermally grows Ni NDC nanosheets in situ on a support, then solvothermally grows Fe BDC nanosheets on the surface of the Ni NDC nanosheets, forming ultrathin Ni NDC@Fe BDC nanosheets. This method allows the synthesis of a MOF-on-MOF metal-organic framework (MOF-on-MOF) under the premise of lattice mismatch. The resulting metal-organic framework not only possesses a MOF-on-MOF heterojunction structure but also possesses a large number of oxygen vacancies. Compared to a single metal MOF, the MOF-on-MOF heterojunction constructed in the present invention fully leverages the advantages of a two-component MOF. The introduced oxygen vacancies can adjust the catalyst surface electronic structure and optimize hydroxyl adsorption during the alkaline OER process. The synthesized Ni NDC@Fe BDC combines the advantages of multi-component material hybridization, ultrathin nanosheets, and defect induction, increasing the number of active sites and promoting rapid electron transfer / redistribution during the catalytic process. The synergistic effect of these factors gives the MOF-on-MOF heterojunction metal-organic framework good electrocatalytic performance in the alkaline oxygen evolution reaction and excellent overall water splitting performance.

[0036] In addition to being formed by the lattice mismatch method of the present invention, MOF-on-MOF materials can also be formed by lattice matching. However, most MOFs have different unit cell parameters and coordination modes. Therefore, the strategy of synthesizing MOF-on-MOF by lattice matching needs to consider that the two MOFs have similar lattice parameters and crystal plane matching, which has great limitations. The method of the present invention avoids the limitations of the lattice matching strategy.

[0037] The present invention discloses the following technical effects:

[0038] (1) The present invention grows oxygen-deficient MOF on MOF heterojunction metal-organic framework materials (Ni NDC@Fe BDC) on a support through a simple two-step solvothermal reaction. The designed Ni NDC@Fe BDC has the characteristics of ultrathin nanosheet morphology (about 4.3 nm), abundant oxygen defects and tight coupling at the heterojunction interface. These characteristics effectively promote interfacial charge transfer, optimize the d-band center and enhance its adsorption capacity for oxygen-containing intermediates during the reaction process. In 1.0MKOH alkaline medium, the Ni NDC@Fe BDC material exhibits excellent OER performance and long-term stability. In addition, the material also exhibits excellent catalytic performance and stability in the overall hydrolysis reaction. These experimental results fully demonstrate that the Ni NDC@Fe BDC material prepared by the preparation method of the present invention can be regarded as an efficient electrocatalytic oxygen evolution reaction catalyst with good application prospects.

[0039] (2) The NiNDC@FeBDC material preparation method provided by the present invention has the advantages of simplified process, easy access to raw materials, and significantly reduced preparation time and cost. These characteristics make the material easy to prepare on a large scale and facilitate commercial production, thus showing great commercial potential. More importantly, the prepared NiNDC@FeBDC material can be directly used as an anode material in a full hydrolysis system, greatly enhancing its practicality and convenience. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0041] Figure 1 X-ray powder diffraction patterns of Ni NDC@Fe BDC / CP prepared in Example 1 of the present invention and Ni NDC / CP and Fe BDC / CP prepared in Comparative Examples 1-2.

[0042] Figure 2 Raman spectra of Ni NDC@Fe BDC / CP prepared in Example 1 of the present invention and Ni NDC / CP and Fe BDC / CP prepared in Comparative Examples 1-2.

[0043] Figure 3 Electron paramagnetic resonance spectra of Ni NDC@Fe BDC / CP prepared in Example 1 of the present invention and Ni NDC / CP and Fe BDC / CP prepared in Comparative Examples 1-2.

[0044] Figure 4 These are electron micrographs of Ni NDC / CP and Fe BDC / CP prepared in Comparative Examples 1-2 of the present invention; wherein a is a scanning electron micrograph of Ni NDC / CP; and b is a scanning electron micrograph of Fe BDC / CP.

[0045] Figure 5 Various electron micrographs of Ni NDC@Fe BDC / CP prepared in Example 1 of the present invention; a is a scanning electron micrograph of Ni NDC@FeBDC / CP; bc are atomic force micrographs of Ni NDC@Fe BDC / CP; d is a transmission electron micrograph of Ni NDC@Fe BDC / CP; e is a high-resolution transmission electron micrograph of Ni NDC@Fe BDC / CP; and f is a distribution diagram of various elements.

[0046] Figure 6 X-ray photoelectron spectra of Ni NDC@Fe BDC / CP prepared in Example 1 of the present invention and Ni NDC / CP and Fe BDC / CP prepared in Comparative Examples 1-2; wherein a is the full XPS spectrum; b is the Ni 2p high-resolution XPS spectrum; c is the Fe2p high-resolution XPS spectrum; and de is the O1s high-resolution XPS spectrum.

[0047] Figure 7 Electrochemical performance test results of the Ni NDC@Fe BDC / CP material prepared in Example 1 of the present invention and the Ni NDC / CP, ​​Fe BDC / CP, ​​Ni NDC-BDC / CP, ​​Ni NDC@Fe / CP and RuO2 / CP prepared in Comparative Examples 1-4 and Comparative Example 6 in 1.0 M KOH; wherein a is the linear sweep voltammetry curve; b is the Tafel slope plot; c is 10 mA cm -2 The overpotential and Tafel slope of the catalysts at d are compared with those of previously reported catalysts; d is the double layer capacitance (C dl ) Figure; e and f are the ECSA normalized current density at a potential of 1.47 V vs. RHE and the current density of 1 mA cm -2 The overpotential at 100 mA cm is g, the OER mass activity of the catalyst is g, and h is 100 mA cm -2 Chronopotentiometry test of NiNDC@Fe BDC / CP.

[0048] Figure 8 The in-situ impedance diagrams of Ni NDC@Fe BDC / CP prepared in Example 1 of the present invention and Ni NDC / CP prepared in Comparative Example 1 in 1M KOH; a and b are Nyquist plots of Example 1 and Comparative Example 1 at different potentials in 1M KOH; c is R ctd and e are the Bode diagrams corresponding to the in situ EIS tests of Example 1 and Comparative Example 1 at different potentials in 1MKOH; f is the phase angle peak of Example 1 and Comparative Example 1 at different voltages; g is the molecular orbital diagram of Example 1.

[0049] Figure 9 In situ Raman images of the Ni NDC@Fe BDC / CP prepared in Example 1 of the present invention and the Ni NDC / CP prepared in Comparative Example 1; wherein a is an in situ Raman device image (WE, working electrode; CE, counter electrode; RE, reference electrode); b and c are the in situ Raman spectra of Example 1 and Comparative Example 1, respectively.

[0050] Figure 10 The whole water splitting process and results are shown; wherein, a is a schematic diagram of whole water splitting; b is the polarization curve of the two-electrode system composed of the Ni NDC@Fe BDC / CP material prepared in Example 1 of the present invention and the Pt / C / CP material prepared in Comparative Example 5, and the two-electrode system composed of the Pt / C / CP material prepared in Comparative Example 5 and the RuO2 / CP material prepared in Comparative Example 6 in 1.0 M KOH; c is a comparison of the voltage of the two-electrode system composed of the Ni NDC@Fe BDC / CP (This work) prepared in Example 1 of the present invention and the Pt / C / CP material prepared in Comparative Example 5 with the whole water splitting cell reported in the prior art; d is the stability test result of the two-electrode system composed of the Ni NDC@Fe BDC / CP prepared in Example 1 of the present invention and the Pt / C / CP material prepared in Comparative Example 5.

[0051] Figure 11 Characterization of the catalyst after the OER reaction; wherein, a is the HR-TEM of the Ni NDC@FeBDC / CP prepared in Example 1 of the present invention after OER; b is the EPR spectrum of the Ni NDC@Fe BDC / CP prepared in Example 1 of the present invention and the NiNDC / CP prepared in Comparative Example 1 after OER.

[0052] Figure 12 The density functional theory calculation diagram of the real active species participating in the OER reaction of Ni NDC@Fe BDC / CP prepared in Example 1 of the present invention and Ni NDC / CP prepared in Comparative Example 1; wherein a and b are the oxygen defects (O v ) of Ni(Fe)OOH; c is a schematic diagram of the bonding between the adsorbent (Ads.) and the electrocatalyst surface; d and e are the density of states and partial density of states diagrams; f is the Gibbs free energy diagram; g is a description of the four-electron transfer mechanism. DETAILED DESCRIPTION

[0053] Various exemplary embodiments of the present invention will now be described in detail. This detailed description should not be considered as limiting the present invention, but rather as a more detailed description of certain aspects, features, and embodiments of the present invention.

[0054] It should be understood that the terms described herein are intended only to describe particular embodiments and are not intended to limit the present invention. In addition, for numerical ranges herein, it should be understood that each intermediate value between the upper and lower limits of the range is also specifically disclosed. The intermediate value within any stated value or stated range, and each smaller range between any other stated value or intermediate value within the stated range, is also encompassed within the present invention. The upper and lower limits of these smaller ranges may be independently included or excluded within the scope.

[0055] Unless otherwise indicated, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the art. Although only preferred methods and materials are described herein, any methods and materials similar or equivalent to those described herein may also be used in the practice or testing of the present invention. All documents mentioned in this specification are incorporated by reference to disclose and describe the methods and / or materials associated with the documents. In the event of any conflict with any incorporated document, the contents of this specification shall prevail.

[0056] It will be apparent to those skilled in the art that various modifications and variations may be made to the specific embodiments described herein without departing from the scope or spirit of the invention. Other embodiments will be apparent to those skilled in the art from the description of the invention. The description and examples are intended to be exemplary only.

[0057] The words “include,” “including,” “have,” “contain,” etc. used in this document are open-ended terms, meaning including but not limited to.

[0058] Example 1

[0059] A MOF-on-MOF heterojunction metal-organic framework (NiNDC@Fe BDC / CP) rich in oxygen defects was prepared by two solvothermal reactions. The steps are as follows:

[0060] (1) Pretreatment of carrier: Cut carbon fiber paper into 1×2cm 2 The size was then heated in a nitric acid solution (concentration of 8 mol / L) at 120°C for 3 hours, followed by ultrasonic washing with ultrapure water and anhydrous ethanol until the pH of the washing solution was 7, and then naturally dried for use.

[0061] (2) Preparation of reaction solution 1: 1 mmol NiCl2·6H2O and 0.75 mmol H2NDC (2,6-naphthalene dicarboxylic acid) were dissolved in a mixed solution consisting of 7.5 mL DMF and 7.5 mL MeOH, and ultrasonicated for 15 minutes to obtain reaction solution 1.

[0062] (3) First solvent thermal reaction: The carbon fiber paper treated in step (1) was placed in the reaction solution 1 prepared in step (2), and the reaction was carried out at 120°C in an oven for 9 hours. After the reaction was completed, the carbon fiber paper was naturally cooled, and the carbon fiber paper after the reaction was rinsed three times with anhydrous ethanol, and then vacuum dried at 60°C for 6 hours to complete the growth of Ni NDC nanosheets on the surface of the carbon fiber paper.

[0063] (4) Preparation of reaction solution 2: 0.4 mmol FeCl2·4H2O and 1.25 mmol H2BDC (1,4-terephthalic acid) were dissolved in a mixed solution consisting of 14 mL DMF, 1 mL EtOH, and 1 mL DI, and ultrasonicated for 15 min to obtain reaction solution 2.

[0064] (5) Second solvent thermal reaction: The carbon fiber paper with Ni NDC nanosheets grown on the surface prepared in step (3) was placed in the reaction solution 2 prepared in step (4), and the reaction was carried out at 120°C in an oven for 2 hours. After the reaction was completed, the carbon fiber paper was naturally cooled, and the reacted carbon fiber paper was rinsed three times with anhydrous ethanol, and then vacuum dried at 60°C for 6 hours to complete the growth of Fe BDC nanosheets on the surface of NiNDC nanosheets to form NiNDC@Fe BDC ultra-thin nanosheets, thereby obtaining a MOF-on-MOF heterojunction metal-organic framework material rich in oxygen defects. The final product was recorded as Ni NDC@Fe BDC / CP.

[0065] Comparative Example 1

[0066] The NiNDC / CP material was prepared by a one-step solvothermal reaction as follows:

[0067] (1) Pretreatment of carrier: Cut carbon fiber paper into 1×2cm 2 The size was then heated in a nitric acid solution (concentration of 8 mol / L) at 120°C for 3 hours, followed by ultrasonic washing with ultrapure water and anhydrous ethanol until the pH of the washing solution was 7, and then naturally dried for use.

[0068] (2) Preparation of reaction solution 1: 1 mmol of NiCl2·6H2O and 0.75 mmol of H2NDC were dissolved in a mixed solution consisting of 7.5 mL of DMF and 7.5 mL of MeOH, and the mixture was sonicated for 15 minutes to obtain reaction solution 1.

[0069] (3) One-step solvent thermal reaction: The carbon fiber paper treated in step (1) is placed in the reaction solution 1 prepared in step (2), and the reaction is carried out in an oven at 120°C for 9 hours. After the reaction is completed, the carbon fiber paper is naturally cooled, and the carbon fiber paper after the reaction is rinsed three times with anhydrous ethanol, and then vacuum dried at 60°C for 6 hours to complete the growth of Ni NDC nanosheets on the surface of the carbon fiber paper to obtain Ni NDC / CP material.

[0070] Comparative Example 2

[0071] The Fe BDC / CP material was prepared by a one-step solvothermal reaction as follows:

[0072] (1) Pretreatment of carrier: Cut carbon fiber paper into 1×2cm 2 The size was then heated in a nitric acid solution (concentration of 8 mol / L) at 120°C for 3 hours, followed by ultrasonic washing with ultrapure water and anhydrous ethanol until the pH of the washing solution was 7, and then naturally dried for use.

[0073] (2) Preparation of reaction solution 2: 0.4 mmol FeCl2·4H2O and 1.25 mmol H2BDC were dissolved in a mixed solution consisting of 14 mL DMF, 1 mL EtOH, and 1 mL DI, and ultrasonicated for 15 minutes to obtain reaction solution 2.

[0074] (3) One-step solvent thermal reaction: The carbon fiber paper treated in step (1) is placed in the reaction solution 2 prepared in step (2), and the reaction is carried out in an oven at 120°C for 2 hours. After the reaction is completed, the carbon fiber paper is naturally cooled, and the carbon fiber paper after the reaction is rinsed three times with anhydrous ethanol, and then vacuum-dried at 60°C for 6 hours to complete the growth of Fe BDC nanosheets on the surface of the carbon fiber paper to obtain Fe BDC / CP material.

[0075] Comparative Example 3

[0076] The Ni NDC-BDC / CP material was prepared by two solvothermal reactions. The steps are as follows:

[0077] (1) Pretreatment of carrier: Cut carbon fiber paper into 1×2cm 2 The size was then heated in a nitric acid solution (concentration of 8 mol / L) at 120°C for 3 hours, followed by ultrasonic washing with ultrapure water and anhydrous ethanol until the pH of the washing solution was 7, and then naturally dried for use.

[0078] (2) Preparation of reaction solution 1: 1 mmol of NiCl2·6H2O and 0.75 mmol of H2NDC were dissolved in a mixed solution consisting of 7.5 mL of DMF and 7.5 mL of MeOH, and the mixture was sonicated for 15 minutes to obtain reaction solution 1.

[0079] (3) First solvent thermal reaction: The carbon fiber paper treated in step (1) was placed in the reaction solution 1 prepared in step (2), and the reaction was carried out at 120°C in an oven for 9 hours. After the reaction was completed, the carbon fiber paper was naturally cooled, and the carbon fiber paper after the reaction was rinsed three times with anhydrous ethanol, and then vacuum dried at 60°C for 6 hours to complete the growth of Ni NDC nanosheets on the surface of the carbon fiber paper.

[0080] (4) Preparation of reaction solution 2: 1.25 mmol of H2BDC was dissolved in a mixed solution consisting of 14 mL of DMF, 1 mL of EtOH, and 1 mL of DI, and the mixture was ultrasonically treated for 15 minutes to obtain reaction solution 2.

[0081] (5) Second solvent thermal reaction: The carbon fiber paper with Ni NDC nanosheets grown on the surface obtained in step (3) was placed in the reaction solution 2 prepared in step (4), and the reaction was carried out at 120°C in an oven for 2 hours. After the reaction was completed, the carbon fiber paper was naturally cooled, and the reacted carbon fiber paper was rinsed three times with anhydrous ethanol, and then vacuum dried at 60°C for 6 hours to obtain NiNDC-BDC / CP material.

[0082] Comparative Example 4

[0083] The NiNDC@Fe / CP material was prepared by two solvothermal reactions. The steps are as follows:

[0084] (1) Pretreatment of carrier: Cut carbon fiber paper into 1×2cm 2 The size was then heated in a nitric acid solution (concentration of 8 mol / L) at 120°C for 3 hours, followed by ultrasonic washing with ultrapure water and anhydrous ethanol until the pH of the washing solution was 7, and then naturally dried for use.

[0085] (2) Preparation of reaction solution 1: 1 mmol of NiCl2·6H2O and 0.75 mmol of H2NDC were dissolved in a mixed solution consisting of 7.5 mL of DMF and 7.5 mL of MeOH, and the mixture was sonicated for 15 minutes to obtain reaction solution 1.

[0086] (3) First solvent thermal reaction: The carbon fiber paper treated in step (1) was placed in the reaction solution 1 prepared in step (2), and the reaction was carried out at 120°C in an oven for 9 hours. After the reaction was completed, the carbon fiber paper was naturally cooled, and the carbon fiber paper after the reaction was rinsed three times with anhydrous ethanol, and then vacuum dried at 60°C for 6 hours to complete the growth of Ni NDC nanosheets on the surface of the carbon fiber paper.

[0087] (4) Preparation of reaction solution 2: 0.4 mmol FeCl2·4H2O was dissolved in a mixed solution consisting of 14 mL DMF, 1 mL EtOH, and 1 mL DI, and ultrasonicated for 15 min to obtain reaction solution 2.

[0088] (5) Second solvent thermal reaction: The carbon fiber paper with Ni NDC nanosheets grown on the surface obtained in step (3) was placed in the reaction solution 2 prepared in step (4), and the reaction was carried out at 120°C in an oven for 2 hours. After the reaction was completed, the carbon fiber paper was naturally cooled, and the reacted carbon fiber paper was rinsed three times with anhydrous ethanol, and then vacuum dried at 60°C for 6 hours to obtain NiNDC@Fe / CP material.

[0089] Comparative Example 5

[0090] Preparation of Pt / C / CP electrode materials

[0091] 2 mg of Pt / C (commercially available, Pt mass fraction 20%) was weighed and added to a mixed solution of 125 μL ultrapure water, 125 μL anhydrous ethanol and 25 μL Nafion solution. The mixture was dissolved by ultrasonication for 30 minutes. The Pt / C slurry homogenized by ultrasonication was then dropped on a 0.5 cm 2 Place on carbon fiber paper and dry at room temperature.

[0092] Comparative Example 6

[0093] Preparation of RuO2 / CP electrode materials

[0094] 2 mg of RuO2 (commercially available) was weighed and added to a mixed solution of 125 μL ultrapure water, 125 μL anhydrous ethanol, and 25 μL Nafion solution. The mixture was dissolved by ultrasonication for 30 minutes. The RuO2 slurry homogenized by ultrasonication was then dropped on a 0.5 cm 2 Place on carbon fiber paper and dry at room temperature.

[0095] Effect verification

[0096] 1. Phase structure and morphology characterization

[0097] (1) Figure 1 The X-ray powder diffraction patterns of Ni NDC@Fe BDC / CP prepared in Example 1 of the present invention and Ni NDC / CP and Fe BDC / CP prepared in Comparative Examples 1-2 are shown. Figure 1It can be seen that for Ni NDC@Fe BDC / CP, ​​the peaks near 7.4°, 14.8°, and 15.1° correspond to the (001), (012), and (011) crystal planes of Ni NDC, respectively. The peaks at approximately 8.8° and 9.4° are from the (048) and (139) crystal planes of Fe BDC, respectively. These results confirm the synthesis of MOF-on-MOF heterostructures. Figure 2 The Raman spectra of Ni NDC@Fe BDC / CP prepared in Example 1 of the present invention and Ni NDC / CP and Fe BDC / CP prepared in Comparative Examples 1-2 are shown. Figure 2 It can be seen that all materials are between 1100-1750cm -1 The characteristic peaks of organic ligands are shown in the range of 875 cm, which means the successful preparation of MOF. -1 The new peak at is derived from the CH vibration of the benzene ring in H2BDC, indicating the coexistence of two organic ligands, H2NDC and H2BDC. Meanwhile, compared with Ni NDC / CP, ​​the peak of Ni-O bond vibration in NiNDC@Fe BDC / CP is slightly blue-shifted ( Figure 2 Medium 520cm -1 This may be caused by the phonon confinement effect induced by surface oxygen vacancies, and also reveals the strengthening of Ni-O bonds in Ni NDC@Fe BDC / CP. Figure 3 Electron paramagnetic resonance spectra of Ni NDC@Fe BDC / CP prepared in Example 1 of the present invention and Ni NDC / CP and Fe BDC / CP prepared in Comparative Examples 1-2. Figure 3 It can be seen that compared with Ni NDC / CP and Fe BDC / CP, ​​Ni NDC@Fe BDC / CP shows abundant oxygen vacancies and produces a stronger EPR signal at g = 2.003, indicating that there are more oxygen defects at the lattice mismatch growth interface.

[0098] (2) Figure 4 The scanning electron microscope images of Ni NDC / CP and Fe BDC / CP prepared in Comparative Examples 1-2 of the present invention are shown in FIG. a is the Ni NDC / CP of Comparative Example 1; b is the Fe BDC / CP of Comparative Example 2. Figure 4 It can be seen that the Ni NDC / CP precursor presents a smooth ultrathin nanosheet morphology. If the Ni NDC / CP precursor is not used, the Fe BDC nanoparticles alone will show severe agglomeration when growing on carbon paper. Figure 5Various electron micrographs of Ni NDC@Fe BDC / CP prepared in Example 1 of the present invention; wherein, a is a scanning electron micrograph of Ni NDC@Fe BDC / CP, ​​from which it can be seen that the ultrathin nanosheet morphology is still maintained, but due to the introduction of Fe BDC, the surface of the nanosheet becomes slightly rough, which may be beneficial to the mass transfer process in electrocatalysis. d and e are high-resolution transmission electron micrographs (HR-TEM) of NiNDC@Fe BDC / CP, ​​where the lattice spacings at 0.99 and 0.93 nm correspond to the Fe BDC (048) and (139) crystal planes, and the lattice spacings at 1.18 and 0.58 nm correspond to the NiNDC (001) and (011) crystal planes, further indicating the successful synthesis of NiNDC@Fe BDC / CP. It is worth noting that in the HR-TEM image of Ni NDC@Fe BDC / CP, ​​a large number of dislocations and distortions were found in some lattice spacings (as shown in the dotted box in d), indicating that a large number of defects exist in the prepared catalyst. These lattice defects potentially expose more active sites, thereby improving the OER efficiency. (f) shows the elemental distribution (EDS), demonstrating the uniform distribution of Ni, Fe, and O elements in the Ni NDC@Fe BDC / CP. This suggests that the introduction of the H2BDC ligand does not induce heterogeneous coordination between the ligand functional groups and the metal ions. (b) and (c) show atomic force microscopy images and corresponding height profiles, confirming that the average thickness of the Ni NDC@Fe BDC / CP is approximately 4.3 nm.

[0099] (5) Figure 6 X-ray photoelectron spectra of Ni NDC@Fe BDC / CP prepared in Example 1 of the present invention and Ni NDC / CP and Fe BDC / CP prepared in Comparative Examples 1-2; wherein, a is the full XPS spectrum; b is the Ni 2p high-resolution XPS spectrum; c is the Fe2p high-resolution XPS spectrum; de is the O1s high-resolution XPS spectrum. The full spectrum of Ni NDC@Fe BDC / CP shows the coexistence of O, Ni and Fe, which is completely consistent with the previous EDS spectrum results. The Ni 2p spectrum supports the Ni 2+ The presence of oxidation states, the characteristic peaks at 856 and 873.1 eV are attributed to Ni 2+ 2p 3 / 2 and Ni 2+ 2p 1 / 2 Compared with pure Ni NDC / CP, ​​the Ni 2p peak of Ni NDC@Fe BDC / CP shifts to a lower binding energy, indicating that the local electronic environment of Ni has changed. As for the Fe 2p spectrum, the peaks at binding energies of 711.8 and 725.9 eV correspond to Fe 3+ , while the peaks at 710.4 and 723.4 eV belong to Fe 2+Obviously, the Fe in Ni NDC@Fe BDC / CP 3+ / Fe 2+ The ratio (1.92) is larger than that in Fe BDC / CP (1.48), which indicates that the Fe in MOF-on-MOF heterojunction is 3+ Similarly, in the O1s spectrum, the peaks near 529.8, 531.2, and 532.5 eV are defined as metal oxygen bonds (MO), oxygen vacancy defects (Ov), and absorbed water (H2O), respectively. ads It is worth noting that due to the absence of oxygen defects in single metal MOFs, the O 1s peak is fitted to CO / C=O, MO and H2O. ads In summary, these results indicate that the Ni-Fe synergistic effect and oxygen vacancies exist in NiNDC@Fe BDC / CP, ​​which provide the necessary conditions for its efficient OER catalytic performance.

[0100] 2. Electrochemical testing

[0101] Test method: All electrochemical measurements were performed using a multi-channel electrochemical workstation (Bio-logic VMP3, France) with a typical three-electrode system in 1.0 M KOH solution. The prepared catalyst material, carbon rod, and Hg / HgO electrode were used as the working electrode, counter electrode, and reference electrode, respectively. The voltage was set at 5 mV s in the range of 1.1–1.8 V (vs. RHE). -1 Linear sweep voltammetry (LSV) was performed at a scan rate of 1.5 Å, and all polarization curves were corrected for iR (95%). The measured potentials were converted to the RHE scale using the Nernst equation:

[0102] E RHE =E SCE +0.241+0.059pH-iR

[0103] Test results:

[0104] (1) Figure 7 Electrochemical performance test results of the Ni NDC@Fe BDC / CP material prepared in Example 1 of the present invention and the Ni NDC / CP, ​​Fe BDC / CP, ​​Ni NDC-BDC / CP, ​​Ni NDC@Fe / CP and RuO2 / CP prepared in Comparative Examples 1-4 and Comparative Example 6 in 1.0 M KOH; wherein a is the linear sweep voltammetry curve; b is the Tafel slope plot; c is 10 mA cm -2 The overpotential and Tafel slope of the catalysts at 200 nm are compared with those of previously reported catalysts (this work represents the NiNDC@Fe BDC / CP material prepared in Example 1 of the present invention); d is the double layer capacitance (Cdl ) Figure; e and f are the ECSA normalized current density at a potential of 1.47 V vs. RHE and the current density of 1 mA cm -2 g is the OER mass activity of the catalyst; h is 100 mA cm -2 Figure a shows that the optimized Ni NDC@Fe BDC / CP has a current density of 10 mA cm -2 The overpotential is 192 mV, which is lower than that of Ni NDC@Fe / CP (214 mV), Ni NDC@BDC / CP (280 mV), Ni NDC / CP (269 mV), RuO2 / CP (298 mV) and Fe BDC / CP (370 mV). -2 At a current density of 1.5 GHz, the overpotential of Ni NDC@FeBDC / CP is 239 mV, which is also competitive compared with other samples. The Tafel slope value of Ni NDC@Fe BDC / CP in Figure b is 38.8 mV dec. -1 , lower than Ni NDC@Fe / CP (46.9mV dec -1 )、Ni NDC@BDC / CP(74.1mV dec -1 )、Ni NDC / CP(105.5mV dec -1 )、RuO2 / CP(77.4mV dec -1 ) and Fe BDC / CP (64.6mV dec -1 ), indicating that Ni NDC@Fe BDC / CP has faster electrocatalytic kinetics. c is the overpotential and Tafel slope of Ni NDC@Fe NDC / CP (This work) compared with the catalysts reported in the prior art [S-NiFe / NFF(10.1016 / j.jelechem.2023.117365), NiFe-MOF-5(10.1039 / D2DT00127F), NiFe-MOF-74(10.1039 / C8CC03112F), CTGU-10c2(10.1002 / anie.201813634), Mn-MOF / CuO(10.1016 / j.fuel.2023.127638), Ni 0.75 Fe 0.25Se2@NF(6H)(10.1016 / j.apsusc.2019.05.220), Fe2O3@Ni-MOF-74(10.1021 / acs.inorgchem.9b01301), GO@Ce MOF(10.1002 / app.51571), MnSe@MWCNT(10.1039 / D1TA09864K), FeNiCo@NC / NF-600(10.1016 / j.jpowsour.2020.228866)]. Electrochemically active surface area (ECSA) is an important criterion that directly reflects the surface area activity: the larger the ECSA value, the more active sites. ECSA is related to double layer capacitance (C dl ) is linearly proportional, which can be calculated by cyclic voltammetry tests in the non-Faraday range at different scan rates. As shown in Figure d, Ni NDC@Fe BDC / CP shows a larger C dl (7.4mFcm -2 ), indicating that the catalyst has a high specific surface area. Next, the ECSA-normalized LSV curves were calculated to understand the intrinsic activity of different catalysts. As shown in Figure ef, the ECSA-normalized LSV curves indicate that the intrinsic activity of NiNDC@Fe BDC / CP has been substantially improved. At a potential of 1.47 V (vs. RHE), the ECSA-normalized current density of Ni NDC@Fe BDC / CP is estimated to be 0.56 mA cm -2 , which is higher than other catalysts. In addition, 1mA cm -2 The overpotential of Ni NDC@Fe BDC / CP is only 250mV, which is significantly lower than that of the original Ni NDC / CP (290mV). It is worth noting that the turnover frequency (TOF) is also an important parameter for comparing the intrinsic activity of the catalyst. The TOF value of Ni NDC@Fe BDC / CP is higher than that of other comparison samples, which strongly proves that NiNDC@Fe BDC / CP has more accessible active sites and higher intrinsic activity (Figure g). In addition to electrocatalytic activity, the stability of the catalyst is also crucial for practical applications. The long-term stability of NiNDC@Fe BDC / CP was verified by constant current testing in 1.0M KOH. At 100mAcm -2 At a current density of 1.5 GHz, the voltage of Ni NDC@Fe BDC / CP showed little change within 100 h (Figure h), which indicates its excellent stability.

[0105] Figure 8The in-situ impedance diagrams of Ni NDC@Fe BDC / CP prepared in Example 1 of the present invention and Ni NDC / CP prepared in Comparative Example 1 in 1M KOH; a and b are Nyquist plots of Example 1 and Comparative Example 1 at different potentials in 1M KOH; c is R ct d and e are the Bode plots corresponding to the in-situ EIS tests of Example 1 and Comparative Example 1 at different potentials in 1M KOH; f is the phase angle peak of Example 1 and Comparative Example 1 at different voltages; g is the molecular orbital diagram of Example 1. It can be clearly seen from the AC impedance spectra in a and b that NiNDC@Fe BDC / CP exhibits significantly lower charge transfer resistance within the applied potential range, indicating that the strong heterojunction coupling effect in NiNDC@Fe BDC / CP can significantly promote interfacial charge transfer and surface activation of the catalyst. ct It can be used to describe the adsorption resistance of oxygen-containing active intermediates, R ct The changing trend of R reveals the OH* change process on the catalyst surface. ct According to the fitting results of NiNDC@Fe BDC / CP under the applied potential, R ct The values ​​are lower than those of NiNDC / CP, ​​which indicates that it has faster adsorption kinetics for oxygen-containing intermediates during the OER process. From the df, it can be seen that the phase peak of Ni NDC@Fe BDC / CP in the low-frequency region shows a lower phase angle at each applied potential, which indicates that the deprotonation rate of the intermediates in the OER process is faster, and it is consistent with the C dl This is consistent with the results of ECSA normalized LSV. This enhanced intrinsic activity can be explained by crystal field theory. As shown in g, Ni in Ni NDC 2+ t 2g The orbital is fully occupied, resulting in Ni 2+ With bridge O 2- After the introduction of Fe BDC, there is a strong electron repulsion between Fe 3+ The coordinated H2BDC organic ligand is a strong field ligand whose d orbital splitting energy is higher than the pairing energy, resulting in Fe 3+ The valence electron configuration is t in a low spin state 2g 5 e g 0. Compared with Ni NDC, there are abundant oxygen defects on the heterojunction interface of NiNDC@Fe BDC, which not only promotes the delocalization of electrons on the Fe-O bond, but also causes part of the electrons on the Ni-O bond to migrate to the Ni active center, thereby optimizing the electronic structure and catalytic reaction activity of Ni NDC@FeBDC in this complex way. When electrons are transferred from Fe to Ni, the electrons in the Fe 3d orbital will rearrange to produce high-spin Fe species, which is beneficial to OER. In short, this electron transfer between NiFe optimizes the catalyst t 2g The electron distribution on the orbital is more conducive to the adsorption of active intermediates.

[0106] Figure 9 Figure 1 shows the in-situ Raman images of Ni NDC@Fe BDC / CP prepared in Example 1 of the present invention and Ni NDC / CP prepared in Comparative Example 1. Figure a shows the in-situ Raman setup (WE, working electrode; CE, counter electrode; RE, reference electrode). Figure b and c show the in-situ Raman spectra of Example 1 and Comparative Example 1, respectively. Clearly, as the applied potential increases, the characteristic peaks of the original MOF gradually disappear, replaced by Ni(Fe)OOH E g Bending vibration and A 1g Characteristic peaks of stretching vibrations. Compared with Ni NDC / CP, ​​Ni NDC@Fe BDC / CP can generate MOOH (M represents metal) phase at a lower potential, indicating that constructing defective heterojunctions on a single Ni MOF can enhance the intrinsic activity of Ni NDC and accelerate the conversion of the precatalyst into the actual active species Ni(Fe)OOH that participates in the reaction.

[0107] (2) Figure 10 The whole water decomposition process and results; wherein a is a schematic diagram of the whole water decomposition (wherein Pt / C represents Pt / C / CP prepared in Comparative Example 5, and Ni NDC@Fe BDC represents Ni NDC@Fe BDC / CP prepared in Example 1); b is the polarization curve of the two-electrode system composed of the Ni NDC@Fe BDC / CP material prepared in Example 1 of the present invention and the Pt / C / CP material prepared in Comparative Example 5, and the two-electrode system composed of the Pt / C / CP material prepared in Comparative Example 5 and the RuO2 / CP material prepared in Comparative Example 6 in 1.0MKOH. As can be seen from b, Ni NDC@Fe BDC / CP (+) ||Pt / C / CP (-) Only 1.43 and 1.54 V battery voltages are required to achieve 10 and 100 mA cm -2 The current density is better than RuO2 / CP (+) ||Pt / C / CP (-)The results show that the Ni NDC@Fe BDC / CP material prepared by the present invention has excellent full hydrolysis performance. c is a two-electrode system composed of NiNDC@Fe BDC / CP (This work) prepared in Example 1 of the present invention and Pt / C / CP materials prepared in Comparative Example 5 and a full water splitting battery reported in the prior art [NFN MOF / NF (10.1002 / aenm.201801065), CoNiRu / NT (10.1002 / adma.202107488), Co-NC@Ni2Fe-LDH (10.1002 / smll.202107739), Ni-250-2@NF (10.1039 / D2TA09369C), IrO2@Ir MOF (10.1039 / D0TA09168E), Ir@Ni NDC (10.1002 / ange.202302220), Ce@NiFe MOF(10.1021 / acs.inorgchem.2c04261), Ru-NiFe-P / Ni(10.1016 / j.apcatb.2019.118324), Mo-RuCoO x (10.1002 / adfm.202303073), FeNiMo (10.1021 / acsmaterialslett.4c00930), NiCo2O4 (10.1039 / D3GC01828H)] voltage comparison. d is the stability test result of the two-electrode system composed of Ni NDC@Fe BDC / CP prepared in Example 1 of the present invention and Pt / C / CP prepared in Comparative Example 5. It can be seen from d that NiNDC@Fe BDC / CP (+) ||Pt / C / CP (-) At 100mAcm -2 After continuous operation for 50 h at a constant current density, the potential showed no obvious decay, demonstrating impressive overall water splitting stability.

[0108] 3. Density functional theory calculations

[0109] Figure 11 : Characterization of the catalyst after OER reaction, where a is the HR-TEM of Ni NDC@FeBDC / CP prepared in Example 1 of the present invention after OER, and b is the EPR spectrum of Ni NDC@Fe BDC / CP prepared in Example 1 of the present invention and NiNDC / CP prepared in Comparative Example 1 after OER. It can be seen that Ni NDC / CP still has no oxygen defects after OER test. On the contrary, NiNDC@Fe BDC / CP has abundant oxygen defects, which illustrates the stability of oxygen defects in Ni NDC@Fe BDC / CP heterojunction. Combined with in situ Raman ( Figure 9 ) and EPR( Figure 11 From the test results in b), it can be inferred that Ni NDC@Fe BDC actually acts as a pre-catalyst in the OER reaction. It undergoes reconstruction during the OER process, and the generated Ni(Fe)OOH with rich oxygen defects is the real active species participating in the reaction; accordingly, Ni NDC is reconstructed into defect-free NiOOH in electrochemistry to participate in the OER.

[0110] Figure 12 The density functional theory calculation diagram of the real active species participating in the OER reaction of Ni NDC@Fe BDC / CP prepared in Example 1 of the present invention and Ni NDC / CP prepared in Comparative Example 1; wherein a and b are the oxygen defects (O v ) of Ni(Fe)OOH; c is a schematic diagram of the bonding between the adsorbent (Ads.) and the electrocatalyst surface; d and e are the state density and partial state density diagrams; f is the Gibbs free energy diagram; g is a diagram describing the four-electron transfer mechanism. b shows that there is a strong electronic interaction between Ni, Fe and O, and the O v The charge accumulation around the O atoms in Ni(Fe)OOH indicates that electrons are transferred from Ni / Fe to O atoms, which is consistent with the XPS results. The calculated density of states in d shows that the O v The state density of Ni(Fe)OOH near the Fermi level increases, which means that more charge carriers are directly involved in the catalytic reaction, thereby significantly improving the OER performance of the catalyst. v Compared with NiOOH (-1.25eV), O v The d-band center (εd) of Ni(Fe)OOH (-1.23 eV) is closer to the Fermi level, which is conducive to exposing active sites and adsorbing active intermediates ( Figure 12 According to the d-band center theory, the electronic interaction between the adsorbent and the electrocatalyst can be explained as the coupling between the valence state of the adsorbent and the d-band center of the transition metal, resulting in the formation of split bonding and anti-bonding states ( Figure 12 (c) in Figure 1). Bonding states below the Fermi level are usually filled, while the antibonding state of NiOOH increases after the introduction of iron doping and oxygen vacancies, resulting in an increase in the adsorption capacity of the reaction intermediates during the OER process. Based on the widely accepted AEM mechanism of the alkaline OER process and previous experimental results, (g) illustrates the best model for the Ni(Fe)OOH intermediate, in which the Ni site is considered as the adsorption site. As shown in (f), the conversion of *O to *OOH is O v Ni(Fe)OOH and O-free vNiOOH is the rate-determining step during the OER. v The △G3 (1.54eV) of Ni(Fe)OOH is lower than that of Ni(Fe)OOH without O v The △G3 of NiOOH (1.88 eV) further proves theoretically that the former has better OER kinetics.

[0111] The embodiments described above are merely descriptions of preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Without departing from the spirit of the present invention, various modifications and improvements made to the technical solutions of the present invention by persons skilled in the art should fall within the scope of protection defined by the claims of the present invention.

Claims

1. A method for preparing a MOF-on-MOF heterojunction metal-organic framework material rich in oxygen defects, characterized in that: Through two solvothermal reactions, NiNDC nanosheets and Fe BDC nanosheets are sequentially grown on the surface of the carrier to obtain the MOF-on-MOF heterojunction metal-organic framework material rich in oxygen defects.

2. The method for preparing the oxygen-deficient MOF-on-MOF heterojunction metal-organic framework material according to claim 1, wherein: The method of sequentially growing NiNDC nanosheets and Fe BDC nanosheets on the surface of the support through two solvothermal reactions specifically includes the following steps: The carrier is added to a reaction solution 1 composed of a nickel source, naphthalene dicarboxylic acid and a solvent 1, and heated to perform a first solvothermal reaction to grow NiNDC nanosheets on the surface of the carrier. Then, the carrier with the NiNDC nanosheets grown on the surface is added to a reaction solution 2 composed of an iron source, phthalic acid and a solvent 2, and heated to perform a second solvothermal reaction to grow Fe BDC nanosheets on the surface of the Ni NDC nanosheets to form NiNDC@Fe BDC nanosheets, thereby obtaining the oxygen-deficient MOF-onMOF heterojunction metal-organic framework material.

3. The method for preparing the oxygen-deficient MOF-on-MOF heterojunction metal-organic framework material according to claim 2, wherein: The nickel source is nickel chloride; the iron source is ferrous chloride; the naphthalene dicarboxylic acid is 2,6-naphthalene dicarboxylic acid; the phthalic acid is 1,4-terephthalic acid; the solvent 1 is a mixed solution of N,N-dimethylformamide and anhydrous methanol; the solvent 2 is a mixed solution of N,N-dimethylformamide, anhydrous ethanol and water; and the carrier is carbon fiber paper.

4. The method for preparing the oxygen-deficient MOF-on-MOF heterojunction metal-organic framework material according to claim 2, wherein: The molar ratio of the nickel source to naphthalene dicarboxylic acid in the reaction solution 1 is 4:3, and the usage ratio of naphthalene dicarboxylic acid to solvent 1 is 0.75 mmol:15 mL. The molar ratio of the iron source to phthalic acid in the reaction solution 2 is 8:25, and the usage ratio of phthalic acid to solvent 2 is 1.25 mmol:16 mL.

5. The method for preparing the oxygen-deficient MOF-on-MOF heterojunction metal-organic framework material according to claim 3, wherein: The volume ratio of N,N-dimethylformamide to anhydrous methanol in the solvent 1 is 1:1; the volume ratio of N,N-dimethylformamide, anhydrous ethanol and water in the solvent 2 is 14:1:

1.

6. The method for preparing the oxygen-deficient MOF-on-MOF heterojunction metal-organic framework material according to claim 2, wherein: The temperature of the first solvent thermal reaction was 120° C. and the time was 9 hours; the temperature of the second solvent thermal reaction was 120° C. and the time was 2 hours.

7. The method for preparing the oxygen-deficient MOF-on-MOF heterojunction metal-organic framework material according to claim 2, wherein: Before the carrier is added to the reaction solution 1, a pretreatment operation is also performed on the carrier. The pretreatment operation specifically includes: heating the carrier in a nitric acid solution for reaction, and then washing and drying.

8. The method for preparing the oxygen-deficient MOF-on-MOF heterojunction metal-organic framework material according to claim 7, wherein: The heating reaction temperature is 120° C. and the time is 3 hours.

9. An oxygen-deficient MOF-on-MOF heterojunction metal-organic framework material prepared by the method for preparing an oxygen-deficient MOF-on-MOF heterojunction metal-organic framework material according to any one of claims 1 to 8.

10. Use of the oxygen-deficient MOF-on-MOF heterojunction metal-organic framework material according to claim 9 in an electrocatalytic oxygen evolution reaction.

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