A surface-modified multi-Mott-Schottky heterostructure material and its application as a cathode catalyst for zinc-air batteries

Abstract

The present invention discloses a surface-modified multi-Mott-Schottky heterostructure bifunctional oxygen electrocatalytic material, a preparation method and its application, belonging to the technical field of electrocatalytic materials. The key points of the technical solution of the present invention are: during the growth of metal organic framework seed crystals, a cerium-containing polytungstate precursor (Ce-POT) is introduced into its shell in situ, and then a surface-modified N@C / Co / CoO / Ce@WO3 / N@C multi-heterostructure electrocatalytic material is derived under a reducing atmosphere. Its beneficial effect is that the entire material has electronic rectification characteristics under the action of multiple heterostructures, so that the outer shell is positively charged, which is beneficial to OH under alkaline conditions. – The adsorption of ions promotes the oxygen evolution reaction (OER); while the negative charge inside the material is conducive to the smooth progress of the four-electron oxygen reduction reaction (ORR), giving the material a double-sided catalytic effect.

Description

Technical Field

[0001] The present invention belongs to the technical field of electrocatalytic materials, and specifically relates to a material with a surface modified multiple Mott-Schottky heterostructure, a preparation method thereof, and its application as a cathode catalyst (bifunctional oxygen electrocatalyst) for zinc-air batteries. Background Art

[0002] Developing clean energy is the fundamental way to achieve the "dual carbon" goals and is also a distinctive feature of the world's technological and industrial changes today. Metal-air batteries, especially zinc-air batteries, are considered to be a highly promising green energy storage system due to their safety, low cost, and environmental protection. The continuous charging and discharging process of zinc-air batteries depends largely on the reversible oxygen reduction and oxygen evolution reaction (ORR / OER) at the air cathode. However, the slow kinetics and imbalance of ORR / OER have seriously restricted the rapid development of zinc-air batteries. Therefore, the design and synthesis of catalysts with bifunctional oxygen electrocatalytic activity has become the key to accelerating the application of metal-air batteries.

[0003] The strong adsorption ability of the catalyst for OOH* / OH* intermediates can promote the smooth progress of the OER reaction, but it has the opposite effect on the ORR reaction. Therefore, regulating the adsorption ability of the catalyst for intermediates is an important means to optimize the bifunctional catalytic activity of the catalyst. The electronic structure of the catalyst can significantly affect the adsorption ability of the material for intermediates. Surface engineering can affect the physical and chemical properties of the material from the size and surface properties of the material, and is an effective means to regulate the electronic structure of the material. Among them, interface engineering, as an important regulatory means in surface engineering, can not only use the significant coordination effect and synergistic effect of the interface to provide a strong regulatory ability for the local electron density of the catalyst, but also can form multiple active sites at the interface, which is an ideal solution for constructing excellent bifunctional catalysts.

[0004] The internal interfaces of the catalytic system can be roughly divided into two categories: isomorphous structures and heterostructures. By adjusting the growth and nucleation rates of different crystal faces of the same substance, grain boundaries composed of different crystal faces of the same substance can be formed. The atoms located at the grain boundaries are often disordered and the structure is relatively loose, resulting in more defects, vacancies and dangling bonds at the grain boundaries, which become potential high-activity catalytic active sites in the material. The heterostructure formed by different substances can not only form catalytic active sites at the interface, but also use the coupling effect and synergistic effect of the interface to regulate the local electronic structure of the material, thereby balancing the ORR / OER bifunctional catalytic activity of the material. In the heterostructure, the Mott-Schottky heterostructure that can form a Schottky contact will use the stronger embedded electric field in the material to transfer electrons from the side with a lower work function to the other side to maintain thermal balance in the structure, thereby forming a "double-sided" heterointerface with opposite charge density. The positively charged side is positive for OH– The strong adsorption capacity facilitates the smooth progress of the material's OER catalytic reaction. The negatively charged side can continuously provide electrons for the ORR reaction, significantly enhancing the material's ORR activity. This "double-sided" catalytic Mott-Schottky heterostructure offers unprecedented opportunities for balancing the material's ORR / OER dual functional properties.

[0005] However, in the process of implementing the technical solution of the present invention, the inventors of this application discovered that the above technology has at least the following technical problems:

[0006] Although a small number of Mott-Schottky heterojunctions have been synthesized and used in water splitting and zinc-air batteries, multiple Mott-Schottky heterostructures composed of multiple components have not yet been fully designed and synthesized. On the one hand, catalysts with multiple components can simultaneously contain components with different catalytic activities; on the other hand, the coupling and synergistic effects of multiple Mott-Schottky heterojunctions can fully adjust the electronic structure of the catalyst and provide more heterojunctions with "double-sided" catalytic properties, promoting the smooth progress of ORR and OER reactions. In addition, only heterojunctions on the surface of the material can fully contact the electrolyte and provide effective catalytic active sites. However, heterojunctions prepared by methods such as heteroepitaxial growth are often located inside the material and cannot fully exert their catalytic activity, which limits the state of the electrochemical reaction. Summary of the Invention

[0007] The technical problem to be solved by the present invention is to innovate the catalytic material synthesized by the existing technology and provide a catalytic material with a surface-modified multiple Mott-Schottky heterostructure, so that the catalytic material can provide an interface area with more catalytic active sites on the surface, and under the action of the multiple Mott-Schottky heterostructure, different interface catalytic active areas show two opposite catalytic effects, effectively improving the ORR / OER bifunctional catalytic activity and stability of the material in alkaline medium.

[0008] The present invention also provides a method for preparing the above-mentioned surface-modified multi-Mott-Schottky heterostructure catalytic material and its application as a cathode catalyst (ORR / OER bifunctional oxygen electrocatalyst) for zinc-air batteries.

[0009] To achieve the above object, the present invention adopts the following technical solutions:

[0010] A method for preparing a surface-modified multi-Mott-Schottky heterostructure material comprises the following steps:

[0011] Step S1: Prepare an aqueous solution containing Lindqvist-type cerium polytungstate anion clusters (Ce-POT) and 2-methylimidazole, and then add Co2+ The aqueous solution was stirred and reacted for 4-12 hours to obtain a blue-purple suspension. After solid-liquid separation, washing, and drying, a polyacid-embedded metal-organic framework precursor was obtained.

[0012] Step S2: Place the polyacid embedded metal organic framework precursor in a porcelain boat and heat it to 700-950°C under argon. o C, hold for 60–120 minutes, then introduce reducing ammonia for 10–30 minutes. Then, maintain under an argon atmosphere for another 60–120 minutes before cooling to room temperature to obtain a black material sample with a surface-modified multiple Mott-Schottky heterostructure. By adjusting the polyacid concentration, the thickness of the surface modification can be controlled.

[0013] In the above step S2, ammonia is used as a reducing gas and Ce-POT is used as an oxide source to prepare a multi-Mott-Schottky heterostructure, and the gas introduction time is 10-30 minutes.

[0014] The heterostructure material prepared above has a three-dimensional porous bougainvillea micromorphology, and the material particle size is between 0.4-1μm; the inner layer of the material is a single Mott-Schottky heterostructure of cobalt element coated with N@C, with a thickness between 0.3-0.8 μm; the outer layer of the material is a Co / CoO / Ce@WO3 multiple Mott-Schottky heterostructure coated with N@C, with a thickness between 0.1-0.2 μm.

[0015] Specifically, in step S1, 59-300 mg of Ce-POT and 709-1136 mg of 2-methylimidazole are dissolved in deionized water to obtain an aqueous solution containing Ce-POT and 2-methylimidazole. The molar concentration of 2-methylimidazole can be 93-758 times that of Ce-POT. The specific operation can be: first dissolve 59-300 mg of Ce-POT in 20 mL of deionized water, then add 709-1136 mg of 2-methylimidazole, and stir until all dissolved (stirring rate is 300 rpm min –1 – 800 rpm –1 Then, the syringe was used to increase the volume of the sample to 1 mL min –1 To the above stirring solution was added 20 mL of a solution containing 503–803 mg Co at a rate of 2+ The reaction is carried out for 4-12 hours.

[0016] Specifically, in step S1, Lindqvist-type cerium-containing polytungstates, Lindqvist-type rare earth-containing polytungstates, transition metal-containing polytungstates, etc. can be dissolved in an aqueous solution containing 2-methylimidazole to obtain an aqueous solution containing both polytungstates and organic ligands.

[0017] Specifically, in step S1, Co 2+ The added amount is 503 – 803 mg.

[0018] The present invention provides a surface-modified multi-Mott-Schottky heterostructure material prepared by the above-mentioned preparation method. The heterostructure material is composed of nitrogen-doped carbon (N@C) coated with multiple metal species; its core is a single Mott-Schottky heterostructure composed of N@C coated with metal cobalt; its outer shell is composed of N@C coated with Co / CoO / Ce@WO3 multi-Mott-Schottky heterostructure. In the material, electrons transfer from the outer N@C to Co through the Co-N bond, making the outer N@C layer positively charged, which is conducive to OH under alkaline conditions. – The adsorption of Ce@WO3 promotes the oxygen evolution reaction (OER). To maintain the material's thermal equilibrium, electrons further migrate toward CoO through the Co / CoO interface. Ce@WO3, forming a heterointerface with CoO, also transfers electrons to CoO through the interface, enabling CoO to continuously provide electrons for the ORR reaction and promoting the smooth progress of the four-electron ORR reaction. This catalytic material with "double-sided" catalytic properties has promising application prospects in metal-air batteries.

[0019] The present invention also provides the use of the surface modified multi-Mott-Schottky heterostructure material as a cathode catalyst for zinc-air batteries. Specifically, the present invention uses the surface modified multi-Mott-Schottky heterostructure material at a concentration of 1.2 mg cm –2 The loading amount is coated on an area of ​​0.8 cm –2 The carbon paper was used as the cathode of the zinc-air battery, a zinc sheet with a thickness of 1.5 mm was used as the positive electrode of the battery, and a mixed aqueous solution containing 0.2 M zinc acetate and 6.0 M KOH was used as the electrolyte. −2The cycling stability of zinc-air batteries was tested under charge-discharge conditions, with each charge / discharge cycle lasting 10 minutes. The material's primary catalytic active sites are the multiple Mott-Schottky heterojunctions on its surface. The embedded electric field generated by the interfaces redistributes the charge density at the material's interface, making the outer N@C substrate positively charged, facilitating the adsorption of hydroxyl species and increasing the battery's OER catalytic activity during charging. The CoO within the multiple heterojunctions can successfully receive electrons transferred from Co and Ce@WO3, promoting the smooth progress of the ORR reaction during discharge. This "Janus" heterojunction ORR / OER bifunctional oxygen electrocatalyst, achieved through "interface engineering," provides theoretical guidance and an experimental basis for the design and synthesis of efficient zinc-air battery cathode catalytic materials (bifunctional electrocatalysts).

[0020] The present invention introduces Lindqvist-type cerium-containing polytungstate (Ce-POT) to generate a zinc-air battery cathode catalyst with a surface-modified multiple Mott-Schottky heterostructure; the core of the cathode catalyst is a single Mott-Schottky heterostructure composed of a single metallic cobalt element coated with N@C; the outer shell is composed of a multiple Mott-Schottky heterostructure of Co / CoO / Ce@WO3 coated with N@C.

[0021] The surface-modified multiple Mott-Schottky heterostructure prepared by the method described in the present invention is used as a cathode catalyst for zinc-air batteries. Ce-POT is embedded in a metal-organic framework as a precursor and subjected to high-temperature pyrolysis. The multiple Mott-Schottky heterointerfaces generated on the material surface are used as the main catalytic active site region. The embedded electric field generated by the Mott-Schottky heterointerfaces redistributes the charge density at the material interface, resulting in multiple heterointerfaces on the material surface, exhibiting two opposite charge densities, positive and negative, and thus exhibiting opposite ORR / OER bifunctional catalytic properties, respectively contributing to the discharge and charge processes of the zinc-air battery. This "Janus" heterointerface ORR / OER bifunctional oxygen electrocatalyst, achieved through "interface engineering," provides theoretical guidance and experimental basis for the design and synthesis of efficient zinc-air battery cathode catalytic materials (bifunctional electrocatalysts).

[0022] Compared with the prior art, the present invention has at least the following technical effects or advantages.

[0023] 1) In the process of metal organic framework seed growth, the present invention utilizes the water-soluble characteristics of polyacid anion clusters and the interaction with 2-methylimidazole, Co 2+The interaction between cations and polyanion clusters encapsulates the outer layer of the metal-organic framework. Further, under a reducing atmosphere, the reducing atmosphere interacts with the metal oxygen clusters to derive an electrocatalyst with a surface-modified multi-Mott-Schottky heterostructure. The catalyst core consists of a single Mott-Schottky heterostructure composed of cobalt elemental metal encapsulated by N@C; the outer layer is composed of a multi-Mott-Schottky heterostructure composed of Co / CoO / Ce@WO3 encapsulated by N@C. The electrocatalyst exhibits a three-dimensional porous bougainvillea microstructure, with a particle size between 0.4 and 1 μm. The inner layer consists of a single Mott-Schottky heterostructure composed of cobalt elemental metal encapsulated by N@C, with a thickness between 0.3 and 0.8 μm; the outer layer consists of a multi-Mott-Schottky heterostructure composed of Co / CoO / Ce@WO3 encapsulated by N@C, with a thickness between 0.1 and 0.2 μm. This surface-modified multiple Mott-Schottky heterostructure can produce more effective, highly dispersed, and highly stable catalytic active sites on the catalyst surface, greatly improving the bifunctional electrocatalytic reaction efficiency of the material.

[0024] 2) This invention synthesizes a bifunctional ORR / OER catalyst containing multiple metal centers and abundant Mott-Schottky heterojunction interfaces by engineering the structure and composition of a polyacid-embedded metal-organic framework. The cerium-doped tungsten oxynitride (Ce@WO3) species derived from Ce-POT utilizes the reversible redox reaction of cerium ions to assist the catalyst in eliminating HO•, HOO•, and H• radicals formed during incomplete ORR catalysis, further enhancing the catalyst's ORR activity. Furthermore, the catalyst facilitates the reversible redox reaction of the multivalent cobalt species, enhancing the material's bifunctional ORR / OER activity.

[0025] 3) The present invention synthesizes ORR / OER bifunctional catalysts containing multiple Mott-Schottky heterostructures by designing the structure and composition of polyacid embedded metal organic framework materials. Among them, the WO3 species derived from Ce-POT has good stability and corrosion resistance in alkaline solution, and the W in Ce@WO3 species has good corrosion resistance. 6+ Cations can also give the surrounding non-metal atoms an extremely high charge density, thereby improving the charge transfer ability of the material and enhancing the synergistic effect between the components, providing a reference for the design and synthesis of ORR / OER bifunctional catalytic materials with new catalytic active sites.

[0026] 4) The present invention utilizes Ce-POT embedded in a metal-organic framework structure to derive multiple Mott-Schottky heterointerfaces, which can redistribute the charge density of the heterointerfaces by utilizing the inherent electric field, coupling, and synergistic effects of the heterointerfaces, so that the multiple interfaces distributed on the surface of the material exhibit opposite charge densities, thereby effectively improving the OER / ORR dual catalytic properties of the material. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 X-ray diffraction pattern (XRD) of the precursor prepared in step S1 of Example 1, Comparative Example 1, Comparative Example 2, and Comparative Example 3 (a); thermogravimetric graph of Example 1 and Comparative Example 1 (b); XRD pattern of the catalyst materials prepared in Example 1, Comparative Example 1, Comparative Example 2, and Comparative Example 3 (c);

[0028] Figure 2 Scanning electron microscopy (SEM) image (a) of the catalyst material prepared in Example 1; Transmission electron microscopy (TEM) images (b, c); High-resolution transmission electron microscopy (HRTEM) image (d); Elemental distribution map (e);

[0029] Figure 3 SEM images of the catalyst materials prepared in Comparative Example 1 (a), Comparative Example 2 (b), and Comparative Example 3 (c);

[0030] Figure 4 X-ray photoelectron spectroscopy (XPS) graphs of W 4f (a), N 1s (b), O 1s (c), C 1s (d), Ce 4f (e), and Co 2p (f) of the catalyst materials prepared in Example 1, Comparative Example 1, Comparative Example 2, and Comparative Example 3;

[0031] Figure 5 The Raman spectra of the catalyst materials prepared in Example 1 and Comparative Examples 1 to 3 are shown;

[0032] Figure 6ORR polarization curves of the catalyst materials prepared in Example 1 and Comparative Examples 1 to 3, and a commercial Pt / C catalyst in an oxygen-saturated 0.1 M KOH solution (a); ORR polarization curves of the catalyst material prepared in Example 1 at different rotation speeds (b); Electron transfer number of the catalyst material prepared in Example 1 at different potentials (c); ORR Tafel slopes of Example 1, Comparative Examples 1 to 3, and a commercial Pt / C catalyst (d); Comparison of ORR polarization curves of the catalyst material prepared in Example 1 before and after 55,000 seconds of operation (e); OER polarization curves of the catalyst materials prepared in Example 1 and Comparative Examples 1 to 3, and a commercial Ir / C catalyst in a 1.0 M KOH solution (f); OER Tafel slopes of the catalyst materials prepared in Example 1 and Comparative Examples 1 to 3, and a commercial Ir / C catalyst (g); OER polarization curves of the catalyst material prepared in Example 1 before and after 400 cycles of cyclic voltammetry (h); OER and ORR pressure differences (i) for Example 1, Comparative Examples 1 to 3, and commercial Pt / C||Ir / C catalysts;

[0033] Figure 7 The charge and discharge polarization curves of the zinc-air battery assembled with Example 1 and commercial Pt / C||Ir / C catalyst (a); power density diagram (b); and the charge and discharge polarization curves of the zinc-air battery assembled with Example 1 and commercial Pt / C||Ir / C catalyst at 5 mA cm −2 Zinc-air battery charge and discharge cycle stability test under charge and discharge conditions (c). DETAILED DESCRIPTION

[0034] The present invention introduces Ce-POT precursor into the shell of the metal organic framework seed crystal in situ during its growth process, and then derives the surface modified N@C / Co / CoOCe@WO3 / N@C multiple heterostructure electrocatalytic material under a reducing atmosphere. Its beneficial effect is that the entire material has electronic rectification characteristics under the action of multiple heterostructures. Electrons transfer from the outer N@C to Co through the Co-N bond, making the outer N@C layer positively charged, which is beneficial to the OH reaction under alkaline conditions. – The adsorption of CoO promotes the oxygen evolution reaction (OER). To maintain the thermal equilibrium of the material, electrons further migrate toward CoO through the Co / CoO interface. Ce@WO3, forming a heterointerface with CoO, also transfers electrons to CoO through the interface, allowing CoO to continuously provide electrons for the ORR reaction, promoting the smooth progress of the four-electron ORR reaction and giving the material a dual-sided catalytic effect.

[0035] In order to better understand the above technical solution, the above technical solution will be described in detail below in conjunction with the specification and specific implementation methods. However, this should not be understood as limiting the scope of the above subject matter of the present invention to the following embodiments. All technologies implemented based on the above content of the present invention fall within the scope of the present invention.

[0036] In the following examples, unless otherwise specified, all raw materials used are commercially available products in the art or prepared according to conventional methods in the art. For example, the Lindqvist-type cerium-containing polytungstate anion cluster (Ce-POT) can be found in the literature (John Iball, John N. Low, Timothy JR Weakley, Heteropolytungstate Complexes of the Lanthanoid Elements. Part III Crystal Structure of Sodium Decatungstocerate (IV) – Water (1 / 30), JCS Dalton ,1974, 2021 – 2024). Room temperature refers to 25 ± 5 o C.

[0037] Example 1

[0038] A method for preparing a surface-modified multi-Mott-Schottky heterostructure electrocatalyst material, the specific steps of which are:

[0039] Step S1: 118 mg of Ce-POT was dissolved in 20 mL of deionized water, and then 1136 mg of 2-methylimidazole was added and the mixture was stirred at 500 rpm min. –1 Stir at 1000 rpm until completely dissolved; then use a syringe to add 1 mL min –1 20 mL of Co-containing 2+ The counter cation aqueous solution (i.e., cobalt nitrate is dissolved in water to prepare, Co 2+ The amount of the mixture was 503 mg) and the mixture was stirred at a constant speed for 4 hours to obtain a blue-purple suspension. o C and vacuum dried overnight to obtain the Ce-POT embedded metal organic framework precursor;

[0040] Step S2: 40 mg of polyacid-embedded metal organic framework precursor was placed in a porcelain boat and heated to 700°C under argon. o C and maintain for 60 min, then at 20 mL min –1Reducing ammonia was introduced into the mixture at a rate of 20 min, and then calcined for 100 min under an argon atmosphere. o The rate of C / min was dropped to room temperature to obtain a black surface modified multi-Mott-Schottky heterostructure electrocatalyst material sample.

[0041] Comparative Example 1

[0042] Step S1: at 500 rpm min –1 1136 mg of 2-methylimidazole was dissolved in 20 mL of deionized water at a speed of 1 mL min using a syringe. –1 20 mL of Co-containing 2+ The counter cation aqueous solution (i.e., cobalt nitrate is dissolved in water to prepare, Co 2+ The amount added was 503 mg) and the mixture was stirred at a constant speed for 4 hours. After centrifugation and washing with ethanol several times, 60 o C and vacuum dried overnight to obtain a metal organic framework precursor without polyacid;

[0043] Step S2: 40 mg of the precursor was placed in a porcelain boat and heated to 700°C under argon. o C, maintain for 60 min and then increase to 20 mL min –1 Reducing ammonia was introduced into the mixture at a rate of 20 min, and then calcined under argon atmosphere for 100 min. o The reaction was cooled to room temperature at a rate of C / min to obtain a black sample containing only N@C / Co single heterostructure.

[0044] Comparative Example 2

[0045] Step S1: First, 59 mg of Ce-POT was dissolved in 20 mL of deionized water, and then 1136 mg of 2-methylimidazole was added and the mixture was stirred at 500 rpm min. –1 After stirring at a speed of 1 mL min until all the solution is dissolved, –1 20 mL of Co-containing 2+ The counter cation aqueous solution (i.e., cobalt nitrate is dissolved in water to prepare, Co 2+ The amount of addition was 503 mg), and the mixture was stirred at a constant speed for 4 hours to obtain a blue-purple suspension. Centrifuge, wash with ethanol several times, and 60 o C and vacuum dried overnight to obtain a low-concentration polyacid-intercalated metal-organic framework precursor;

[0046] Step S2: 40 mg of the precursor was placed in a porcelain boat and heated to 700°C under argon.o C, maintain for 60 min and then increase to 20 mL min –1 Reducing ammonia was introduced into the mixture at a rate of 20 min, and then calcined under argon atmosphere for 100 min. o The temperature was lowered to room temperature at a rate of C / min to obtain a black sample with low concentration of multiple heterostructures.

[0047] Comparative Example 3

[0048] Step S1: 300 mg of Ce-POT was dissolved in 20 mL of deionized water, and then 1136 mg of 2-methylimidazole was added and the mixture was stirred at 500 rpm min. –1 After stirring at a speed of 1 mL min until all the solution is dissolved, –1 20 mL of Co-containing 2+ The counter cation aqueous solution (i.e., cobalt nitrate is dissolved in water to prepare, Co 2+ The amount of addition was 503 mg), and the mixture was stirred at a constant speed for 4 hours to obtain a blue-purple suspension. Centrifuge, wash with ethanol several times, and 60 o C and vacuum dried overnight to obtain a high-concentration polyacid-intercalated metal-organic framework precursor;

[0049] Step S2: 40 mg of the precursor was placed in a porcelain boat and heated to 700°C under argon. o C, maintain for 60 min and then increase to 20 mL min –1 Reducing ammonia was introduced into the mixture at a rate of 20 min, and then calcined under argon atmosphere for 100 min. o The temperature was lowered to room temperature at a rate of C / min to obtain a black sample with high concentration of multiple heterogeneous structures.

[0050] The present invention uses X-ray diffraction (XRD) to conduct detailed determination of the components of the prepared examples and comparative examples before and after calcination. Figure 1 As can be seen in Figure a, both the embodiment and the comparative example showed the same diffraction peaks as the metal organic framework ZIF-L before calcination, and no characteristic diffraction peaks of Ce-POT appeared. This is due to the high dispersion and low content of polyacid in the material. To further prove that Ce-POT is compounded in the material, the present invention measured the material by thermal analysis technology. Figure 1As can be seen in Figure b, within the temperature range of 25-300 °C, the thermal weight loss of Comparative Example 1 without Ce-POT precursor is 5.2%, while the thermal weight loss of Example 1 is only 2.7%. This is because during the formation of Example 1, Ce-POT replaces part of the crystal water in the material, resulting in a decrease in the first step thermal weight loss. As the heating temperature rises to 800 o C, the thermal weight loss of Example 1 is only 48.9%, while the thermal weight loss of Comparative Example 1 without Ce-POT is 52.4%, which indirectly proves that Ce-POT is compounded in the pores of ZIF-L. After high temperature calcination, the comparative example 1 without Ce-POT only has 4 characteristic peaks, 44.21 o , 51.52 o , 75.85 o Corresponding to the (111), (200), and (220) crystal planes of elemental cobalt, 23 o The characteristic peak at 36.50 is the characteristic diffraction peak of N@C material formed by high temperature carbonization of organic 2-methylimidazole. In addition to the above characteristic diffraction peaks, Example 1, Comparative Example 2 and Comparative Example 3 also have the following characteristics: o and 42.40 o There are also two characteristic peaks at (111) and (200) planes of CoO. However, no diffraction peaks of Ce-POT-derived materials were found in XRD, which may be due to the amorphous state of Ce-POT-derived materials.

[0051] Since the cathode of zinc-air battery involves gas-liquid-solid three-phase reaction, the microstructure of the material has a very important influence on the cathode catalyst. The cathode catalyst with multi-level pore structure has a large specific surface area, which is conducive to the smooth adsorption and desorption of O2, and promotes the smooth progress of ORR and OER reactions involved in the discharge and charging process of zinc-air battery cathode. Based on this, the present invention uses scanning electron microscopy (SEM) to characterize the micromorphology of the material. Figure 2 As can be seen in Figure a, Example 1 presents a flower-like morphology similar to Bougainvillea, with a particle size of about 1 μm. In addition, the surface of the material is rough, which may be due to the formation of a large number of pores or nanoparticles in the material after pyrolysis. In order to further observe the surface morphology of Example 1, this experiment used a transmission electron microscope (TEM) to further test the material. The test showed that a large number of 10-50 nm particles were evenly distributed in the nano-flower-like morphology of the embodiment ( Figure 2 b, c), and high-resolution transmission electron microscopy (HRTEM) images show that the nanoparticles at the edge of the material contain two lattice fringes of 2.06 and 2.46 Å, corresponding to Co 0(111), CoO (111) crystal plane, and an amorphous substance without lattice fringes ( Figure 2 d), which is consistent with the XRD test results. In addition, CoO species are sandwiched between the amorphous material and the Co element, forming a special multiple heterogeneous structure. The element surface distribution can intuitively show the distribution state of the elements. Figure 2 As can be seen in Figure e, W species are closely combined with Co species, proving that this heterogeneous interface is formed by Ce-POT and ZIF-L during high-temperature pyrolysis. Figure 2 As can be seen in Figure e, the distribution states of the two elements Ce and W almost completely overlap, indicating that the Ce element does not form a doping or alloy state with Co or CoO.

[0052] For comparison, we also tested the micromorphology of Comparative Examples 1-3. The test showed that Comparative Example 1 without Ce-POT did not have the nano-flower-like morphology similar to Bougainvillea ( Figure 3 (a) shows that Ce-POT not only helps to form multiple heterogeneous interfaces on the material surface, but also acts as a morphology control agent, assisting in the formation of Bougainvillea-like nanoflower morphology with multi-level pores. As the Ce-POT concentration increases further, the material still maintains the Bougainvillea-like porous morphology, and only the concentration of the multiple heterogeneous structures in the material gradually increases with the increase of Ce-POT dosage ( Figure 3 (b, c).

[0053] To further determine the effect of Ce-POT concentration on material composition and electron cloud density distribution, we conducted X-ray photoelectron spectroscopy (XPS) tests on the examples. The experiments showed that W, N, O, Ce, and Co elements were all present in the examples. Figure 4 As shown in Figure a, the high-resolution XPS spectrum of W exhibits two peaks at 37.5 and 35.5 eV, both corresponding to W-O bonds; weaker peaks at 34.1 and 32.2 eV correspond to W-N bonds. This indicates the presence of WO3 in Example 1 and Comparative Examples 2 and 3, and that the WO3 species bind to the N@C via W-N bonds at the interface. With increasing Ce-POT content, the W-N bond content gradually increases, and the W 4f peak shifts negatively accordingly. This indicates that at the Mott-Schottky heterointerface formed between the metal element and the metal oxide in Examples 1 and Comparative Examples 2 and 3, the secondary W-N-C bonds are partially replaced by W-N-Co or W-O-Co bonds. Since Co has a stronger electron-donating ability than C, the electron cloud density around the W element increases.

[0054] Figure 4Figure b shows the N 1s spectra of Example 1 and Comparative Examples 1-3. As can be seen from the figure, the N 1s spectrum without Ce-POT has four distinct characteristic peaks: graphitic N (401.2 eV), pyrrolic N (40.1 eV), Co–N bond (399.2 eV), and pyridinic N (398.5 eV). As the Ce-POT content increases, the formation of Co–N bonds is affected by the formation of W–N bonds, causing the Co–N bond concentration in Example 1, Comparative Examples 2, and 3 to gradually decrease. At the same time, the pyrrolic N content in Example 1 and Comparative Example 2 is higher, which is because the addition of Ce-POT effectively blocks the connection between C and N. In previous studies, the relative content of N species directly affects the catalytic properties of the material, and which N species has the true catalytic activity has always been a controversial topic. Increasing the relative content of pyrrolic N by adding Ce-POT helps analyze the true catalytic activity of graphitic N, pyrrolic N, and pyridinic N.

[0055] Figure 4 In Figure c, the O 1s spectra of Example 1 and Comparative Examples 1-3 are shown. In Example 1 and Comparative Examples 2 and 3, there are four characteristic peaks at 532.8 eV, 531.6 eV, 530.8 eV, and 530.0 eV, corresponding to H–O, C=O, W–O, and Co–O bonds, respectively. However, there is no W–O bond signal in Comparative Example 1 without polyacid, which proves that the amorphous species contained in Examples 1, Comparative Examples 2 and 3 should be Ce@WO3 species derived from Ce-POT. At the same time, the C 1s spectra of the Examples and Comparative Examples contain five characteristic peaks, corresponding to C=C / C–C (284.5 eV), C–N (285.0 eV), C–O (285.8 eV), C=O (286.8 eV), O–C=O (288.5 eV) bonds ( Figure 4 d). The higher C–N bond content in Example 1 compared to Comparative Example 1 may be due to the higher W–N bond formation energy. The formation of graphitic N, pyridinic N, and Co–N bonds is affected by the polyacid content. In addition, the presence of Ce species pairs was detected in Example 1 and Comparative Examples 2 and 3, where v, v″, v‴, u, u″, and u‴ (882.5, 888.6, 898.7 eV, 901.4, 907.9, and 917.1 eV) all correspond to Ce. 4+ , v', u' (885.6, 904.6 eV) correspond to Ce 3+ ( Figure 4 e), indicating that Ce element is doped into amorphous Ce@WO3 in the form of ions. 4+ The presence of Ce ions indicates that Ce ions can act as reducing agents to assist Co 0When the dosage of polyacid is increased to 300 mg, that is, in Comparative Example 3, the original Ce 3+ The higher valence state causes the convolution peaks of u” and v” to disappear, and only the u' and v' peaks are more obvious.

[0056] from Figure 4 As can be seen in f, the two double peaks at 795.8 eV and 780.7 eV correspond to Co 2p 1 / 2 , Co 2p 3 / 2 , and the two satellite peaks at 786.6 and 803.1 eV correspond to Co 2+ . Co 2p 3 / 2 Three Co species can be further fitted, Co (778.6 eV), Co–O (780.2 eV), Co–N / C (781.7 eV). It is not difficult to find that when the content of Ce-POT is slightly increased, due to the reducing ability of Ce element, the Co in Example 1 and Comparative Example 2 is significantly different. 0 The content of Ce-POT gradually increased compared with that of Comparative Example 1. When the Ce-POT content reached 300 mg, the concentration of polyacid oxygen anion clusters was too high, which masked the reduction activity of Ce ions and made Co 0 At the same time, the Co 2p 1 / 2 and Co 2p 3 / 2 The binding energy shifts toward higher binding energy compared to Comparative Example 1, indicating that electrons in the material can flow from Co to WO3 species through the heterostructure.

[0057] Raman spectroscopy can further illustrate the composition of the material. –1 The characteristic peaks correspond to the F 2g 、E g 、F 2g 、F 1g Vibration peak. 670 cm –1 The strong peak at is mainly related to the CoO–N bond, indicating that CoO exists in Example 1, Comparative Examples 2 and 3, and interacts with N@C through the Co–N bond. The Co–N bond can effectively adjust the electron density of the material ( Figure 5 ). It is worth noting that at 881 cm –1 The peak at 37° intensifies with the increase of polyacid concentration, which is consistent with the W–O b 956 cm –1 The weak signal peak at corresponds to W–O t vibration, indicating that Ce@WO3 exists in Example 1, Comparative Examples 2 and 3. In addition, the Raman spectrum can also provide a deeper understanding of the degree of graphitization of the material. D band (1343 cm–1 ) and G belt (1587 cm –1 ) represent sp in the material 2 The disorder and crystallinity of carbon. The intensity of the D-band signal peak increases with the number of defects in the material. The shoulder peak next to the D-band is the vibration peak of the C–H bond, and its intensity gradually increases with the increase of polyacid content, indicating that the addition of polyacid has a certain influence on the degree of graphitization of the metal-organic framework D-band, which provides a good opportunity to study the true active sites of N@C.

[0058] Application test

[0059] Since the charge and discharge processes of the zinc-air battery correspond to OER and ORR reactions respectively, the present invention first uses a three-electrode system (saturated calomel electrode as reference electrode, carbon rod as counter electrode, and rotating disk electrode coated with catalyst as working electrode) to test the ORR / OER bifunctional catalytic activity of the catalyst. Among them, the catalysts are Example 1, Comparative Example 1, Comparative Example 2, and Comparative Example 3. The preparation method of the working electrode is as follows: 2 mg of target sample and 1 mg of carbon powder (Vulcan XC-72) are dispersed in 485 μL of isopropanol and 15 μL of 5wt% Nafion mixed solution, ultrasonicated for 30 minutes to form a uniform suspension, and then 25 μL of the suspension is dropped on the polished rotating disk electrode surface. The effective area of ​​the electrode is 0.196 cm –2 The electrolyte for ORR reaction is 0.1 M KOH aqueous solution; the electrolyte for OER reaction is 1.0 M KOH aqueous solution.

[0060] Figure 6 Figure a shows the ORR polarization curves of the catalyst materials prepared in Example 1, Comparative Examples 1 to 3, and commercial Pt / C in an oxygen-saturated 0.1 M KOH solution. It can be seen from the figure that the half-wave potential of Example 1 is the same as that of commercial Pt / C, both of which are 0.85 V, while the half-wave potentials (E 1 / 2) are only 0.79 V, 0.80 V and 0.66 V. This may be because the heterojunctions formed in Comparative Example 1 without Ce-POT and Comparative Example 2 with a lower Ce-POT content are relatively simple, and the local electronic structure of the material cannot be fully adjusted. When the Ce-POT content is too high, the oxide content formed in Comparative Example 3 is relatively high, resulting in a significant reduction in the catalytic activity and conductivity of the material, thereby reducing the ORR catalytic activity of Comparative Example 3. The multiple heterostructures containing multiple components formed in Example 1 can utilize the surface multiple Mott-Schottky heterojunctions as the main area of ​​catalytic active sites, and redistribute the interface charge density of the material through the embedded electric field generated by the interface, so that the CoO inside the multiple heterojunctions can smoothly receive the electrons transferred from Co and Ce@WO3, thereby promoting the smooth progress of the ORR reaction of the battery during discharge.

[0061] In addition to testing the ORR catalytic activity of Example 1, the present invention also tested the four-electron catalytic selectivity of Example 1 ( Figure 6 b, c). The Koutecký–Levich curves of Example 1 have a good linear relationship at different potentials, indicating that the ORR reaction of Example 1 follows the first-order reaction kinetics that varies with the oxygen concentration, and the number of electron transfers per oxygen molecule at different potentials is 4. At the same time, the present invention uses the Tafel slope to evaluate the reaction kinetics of Example 1 and Comparative Examples 1-3 ( Figure 6 Example 1 has the smallest Tafel slope (50.2 mV dec –1 ), indicating that the conversion of adsorbed O2 into OOH* during the catalytic process of Example 1 is the rate-determining step of the entire catalytic reaction. In addition, the time-current curve test shows that the example has good ORR catalytic activity and stability, and can still maintain the original catalytic activity after 55,000 seconds of continuous catalysis ( Figure 6 e). This shows that the surface-modified multi-Mott-Schottky heterogeneous interface electrocatalyst prepared by the present invention has good ORR catalytic activity and stability.

[0062] In order to verify the OER catalytic activity of the material, Example 1, Comparative Examples 1 to 3 and commercial Ir / C catalyst were tested for OER polarization curves in 1.0 M KOH solution ( Figure 6 f). The test results show that at 10 mA cm –2 At a current density of j=10 ) is 260 mV cm –2, which is significantly better than the overpotential of 302 mV of the commercial Ir / C catalyst and the overpotential of Comparative Examples 1-3 (Comparative Example 1: 377 mV, Comparative Example 2: 360 mV, Comparative Example 3: 405 mV, Commercial Ir / C: 309 mV). This is because Example 1 has multiple heterogeneous structures and a moderate concentration compared to Comparative Examples 1-3. It can adjust the electron cloud density through the thermal balance between the multiple heterogeneous structures, making the outer N@C layer of the material positively charged, which is beneficial to the OH reaction under alkaline conditions. – The adsorption of ions promotes the smooth progress of the oxygen evolution reaction (OER). Furthermore, the Tafel slopes of Example 1, Comparative Examples 1 to 3, and the commercial Ir / C catalyst were also tested to compare the OER catalytic performance of the materials ( Figure 6 The embodiment has the smallest Tafel slope (92.2 mV dec –1 ), while the Tafel slopes of commercial Ir / C and comparative examples 1 to 3 were 95.3 mV dec, –1 , 111.9 mV dec –1 , 99.7 mV dec –1 and 117.1 mV dec –1 , indicating that Example 1 has good OER catalytic activity. It is worth noting that after 500 cycles of cyclic voltammetry, the example still maintains good OER catalytic activity, indicating that Example 1 has good OER catalytic stability ( Figure 6 h).

[0063] In order to further observe the bifunctional catalytic performance of the material, the present invention conducted the pressure difference (ΔE = E j=10 – E 1 / 2 ) were compared ( Figure 6 (i) The voltage differences between Example 1 and commercial Pt / C||Ir / C are very close, at 679 mV and 680 mV, respectively, while those for Comparative Examples 1-3 are 817 mV, 792 mV, and 981 mV, respectively. The smaller voltage differences indicate that this example exhibits excellent bifunctional catalytic activity for both OER and ORR, making it a suitable alternative to commercial precious metal Pt / C||Ir / C in zinc-air battery assembly.

[0064] Based on this, the present invention used the catalytic material of Example 1 as the cathode catalyst for a rechargeable zinc-air battery to assemble a zinc-air battery. The performance of the zinc-air battery was compared with that of a commercial Pt / C||Ir / C catalyst. Specifically, the present invention used the catalytic material of Example 1 as the cathode catalyst for a rechargeable zinc-air battery, as well as a mixture of commercial Pt / C (20 wt%) and Ir / C (1:1 mass ratio) loaded on carbon paper as the battery cathode. A zinc sheet with a thickness of 1.5 mm was used as the positive electrode to assemble the zinc-air battery. The cathode catalyst loading was 1.2 mg cm –2 , the load area is 0.8 cm –2 ; A mixed aqueous solution of 0.2 M zinc acetate and 6.0 M KOH was used as the electrolyte at 5 mAcm −2 The zinc-air battery cycle stability test was carried out under charge and discharge conditions, with each charge and discharge cycle lasting 10 minutes.

[0065] Figure 7 The charge and discharge polarization curves (a) and power density diagram (b) of the zinc-air battery of Example 1 and commercial catalyst are given. The test results show that the battery assembled by Example 1 has a smaller charge and discharge voltage gap and a higher discharge current density than the commercial Pt / C||Ir / C battery, indicating that the zinc-air battery constructed by Example 1 has better rechargeability ( Figure 7 In addition, the zinc-air battery prepared with the cathode of Example 1 had a high current density at 183 mA cm –2 The power density is 146.9 mW cm –2 , which is superior to commercial Pt / C||Ir / C batteries at 183 mA cm –2 The power density at the current density is 127.9 mW cm –2 , indicating that Example 1 is an ideal catalyst to replace precious metals ( Figure 7 (b)

[0066] In addition, the present invention has a high –2 The charge and discharge cycle stability of the battery assembled from Example 1 and commercial Pt / C||Ir / C was tested under constant current charge and discharge conditions. Figure 7As shown in Figure c, Example 1 and the battery assembled with commercial Pt / C||Ir / C have similar charge and discharge potential windows. The initial charge and discharge potential gap (ζ) of the zinc-air battery constructed with Example 1 as the cathode catalyst is 1.35 V. After running for 930 cycles (charge / discharge time is 10 minutes), the charge and discharge potential gap of the zinc-air battery constructed in Example 1 can still be maintained at 1.36 V. In contrast, the zinc-air battery with commercial Pt / C||Ir / C as the cathode catalyst can only perform 500 charge and discharge cycles under the same conditions, further verifying the superiority of Example 1 as a cathode catalyst for zinc-air batteries.

[0067] Analysis of the experimental results above indicates that by controlling the Ce-POT content, the formation and concentration of multiple Mott-Schottky heterointerfaces on the catalyst surface can be regulated, increasing the number of effective catalytically active sites in the material. The multiple "double-sided" catalytic interfaces generated by these multiple Mott-Schottky heterointerfaces can simultaneously enhance the catalyst's bifunctional ORR and OER catalytic activity, enabling the material to fully exploit its dual catalytic effects during the charge and discharge processes of zinc-air batteries. This invention develops a strategy for localized electronic state manipulation through interface engineering. By leveraging the water-solubility of polyacids, polyacids are introduced into the surface pores of the synthesized material during the growth of metal-organic framework seeds. Subsequently, the polyacid's oxygen anion structure interacts with the reducing ammonia atmosphere to form N@C-coated Co / CoO / Ce@WO3 multiple Mott-Schottky heterointerfaces on the catalyst surface. These multiple heterointerfaces serve as the primary catalytically active regions, and the embedded electric fields generated by these heterointerfaces impart distinct charges and catalytic activity to the material interfaces, thereby achieving a comprehensive improvement in the dual ORR and OER catalytic activity and stability of the zinc-air battery cathode catalyst.

[0068] The above embodiments describe the basic principles, main features and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The above embodiments and descriptions are only for illustrating the principles of the present invention. Without departing from the scope of the principles of the present invention, the present invention may have various changes and improvements, and these changes and improvements all fall within the scope of protection of the present invention.

Claims

1. A method for preparing a surface-modified multi-Mott-Schottky heterostructure material, characterized in that: The steps include: Step S1: Prepare an aqueous solution containing Lindqvist-type cerium polytungstate anion cluster Ce-POT and 2-methylimidazole, and then add Co 2+ The aqueous solution is stirred for reaction for 4-12 hours, solid-liquid separation is performed, and the precursor of the polyacid embedded metal organic framework is obtained after washing and drying; Step S2: Heat the polyacid intercalation metal organic framework precursor to 700-950°C under argon atmosphere. o C, keep it for 60-120 min, then introduce reducing ammonia for 10-30 min, then keep it under argon atmosphere for another 60-120 min, and cool it to room temperature to obtain; In step S1, 59-118 mg of Ce-POT and 1136 mg of 2-methylimidazole are dissolved in deionized water to obtain an aqueous solution containing Ce-POT and 2-methylimidazole. The Ce element in the obtained Ce-POT solution has reducing ability, thereby obtaining Co in the heterostructure. 0 ; In step S1, Co 2+ The added amount is 503 – 803 mg.

2. The method for preparing a surface-modified multi-Mott-Schottky heterostructure material according to claim 1, wherein: In step S1, cobalt nitrate, cobalt sulfate or cobalt chloride is dissolved in water to obtain 2+ of aqueous solution.

3. A surface-modified multi-Mott-Schottky heterostructure material prepared by the preparation method according to any one of claims 1 to 2.

4. Use of the surface-modified multi-Mott-Schottky heterostructure material according to claim 3 as a cathode catalyst for zinc-air batteries.

5. The use of the surface modified multi-Mott-Schottky heterostructure material as a cathode catalyst for zinc-air batteries according to claim 4, characterized in that: The surface-modified multi-Mott-Schottky heterostructure material was coated on carbon paper as the battery cathode, a zinc sheet was used as the battery positive electrode, and a mixed aqueous solution of 0.2 M zinc acetate and 6.0 M KOH was used as the electrolyte to carry out the cycling stability test of the zinc-air battery.

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