COF-coated Bi-based catalyst as well as preparation method and application thereof

Abstract

The invention relates to the field of material preparation and electro-catalysis application, in particular to a COF-coated Bi-based catalyst and a preparation method and application thereof. The COF-coated Bi-based catalyst disclosed by the invention comprises a flower-ball-shaped Bi2O2CO3 material and a COFs (Covalent Organic Frameworks) material coated on the surface of the flower-ball-shaped Bi2O2CO3 material; the flower-ball-shaped Bi2O2CO3 material is prepared by assembling nano-sheet-shaped Bi2O2CO3, and the diameter of the Bi2O2CO3 is 10-20 microns The COFs material is of a coralline structure. According to the catalyst disclosed by the invention, the COFs are coated on the surface of the flower-ball-shaped Bi2O2CO3 material, so that the CO2 adsorption capacity of the catalyst is improved, the electron transmission rate is increased, the hydrophobicity is enhanced, the hydrogen evolution reaction (HER) is inhibited, the oxygen vacancy is increased, and the efficiency of electrocatalytic CO2 reduction is further improved. The catalyst keeps high activity in a wide potential window, and formate selectivity exceeding 95% can be obtained at most in a flow type electrolytic tank.

Description

Technical Field

[0001] The present invention relates to the fields of material preparation and electrocatalytic applications, and particularly to a COF-coated Bi-based catalyst, a preparation method thereof, and an application thereof. Background Art

[0002] With the acceleration of the industrialization process, the combustion of a large amount of fossil fuels has led to a significant increase in the concentration of carbon dioxide (CO2) in the atmosphere, triggering a series of serious problems such as global warming and environmental pollution. How to effectively reduce carbon dioxide emissions and convert it into high-value chemicals or fuels has become a hot spot and focus of global scientific research. As a green and sustainable solution, the electrochemical carbon dioxide reduction reaction (CO2RR) can use electrical energy to convert CO2 into valuable chemicals (such as formic acid, carbon monoxide, methanol, etc.) or fuels, which not only helps to alleviate the energy crisis but also reduces the dependence on traditional fossil fuels, and has important environmental and economic significance.

[0003] In the field of electrocatalytic CO2 reduction, Bi-based catalysts have shown good electrocatalytic performance due to their unique structural characteristics and have become one of the research hot spots. Bi-based catalysts have high activity and selectivity in the CO2 reduction reaction, can effectively reduce the reaction overpotential, and improve the reaction efficiency. However, existing Bi-based catalysts still have some limitations in practical applications. First, Bi-based catalysts may catalyze multiple reactions simultaneously during the CO2 reduction process, resulting in poor product selectivity and difficulty in achieving efficient single-product formation. Second, the catalytic efficiency of Bi-based catalysts may be strictly limited by reaction conditions (such as temperature, pressure, catalyst concentration, etc.), especially when performing CO2 conversion under mild conditions, the efficiency is low. In addition, the electron-hole pairs in Bi-based catalysts tend to recombine rather than effectively catalyze the reaction, reducing the catalytic efficiency. Moreover, the number of active sites in Bi-based catalysts is limited and is easily oxidized, further restricting the improvement of their performance.

[0004] To overcome the above problems, researchers have tried to improve the performance of Bi-based catalysts through various modification strategies. For example, by compounding with other materials, adjusting the material structure, etc., to increase the active sites of the catalyst, improve the electron transfer efficiency, and increase the specific surface area, thereby enhancing the selectivity and catalytic performance of CO2 reduction. Bismuth oxycarbonate (Bi2O2CO3, abbreviated as BOC) has a unique layered structure and rich Bi-O bonds, which can provide suitable reaction sites for electrocatalytic carbon dioxide reduction and exhibits good electrocatalytic performance. In related technologies, Chinese invention patent CN119287438A discloses a preparation method of a Cu-doped Bi2O2CO3 nanosheet catalyst, which significantly improves the selectivity of carbon dioxide reduction to formic acid. Chinese invention patent CN117385403A discloses a CeO with rich oxygen vacancies x-Bi2O2CO3 catalyst electrochemical catalytic CO2 reduction material preparation method, by introducing more economical rare earth element cerium (Ce) to construct CeO with abundant oxygen vacancies x -Bi2O2CO3 catalyst, making the catalyst show better performance in the electrocatalytic CO2RR conversion to formate. Although the above catalysts have improved the selectivity and performance of CO2 reduction to formate to a certain extent, there are still problems such as insufficient product selectivity, insufficient stability, catalytic efficiency needs to be improved, complex preparation methods or high costs, difficulty in regulating oxygen vacancies, poor reusability and strong dependence on reaction conditions. These problems limit the widespread promotion and long-term stability of bismuth-based catalysts in practical applications. Therefore, further optimizing the design and preparation methods of catalysts, improving their performance and stability, and reducing costs are important directions for future research. Summary of the invention

[0005] The present invention provides a COF-coated Bi-based catalyst and a preparation method and application thereof, so as to solve the problems of existing Bi-based catalysts such as insufficient selectivity, low catalytic efficiency, insufficient stability, and complex preparation methods.

[0006] According to the first aspect of the present invention, the present invention provides a COF-coated Bi-based catalyst, comprising a flower-ball-shaped Bi2O2CO3 material and a COFs material coated on the surface of the flower-ball-shaped Bi2O2CO3 material; the flower-ball-shaped Bi2O2CO3 material is assembled from nano-sheet Bi2O2CO3; the COFs material has a coral rod-like structure.

[0007] The COF-coated Bi-based catalyst of the present invention comprises a flower-like Bi2O2CO3 material and a COFs material coated on the surface of the flower-like Bi2O2CO3 material. The flower-like Bi2O2CO3 material is assembled from nano-sheet Bi2O2CO3, has a large specific surface area, can provide more active sites, and is beneficial to the adsorption of CO2 and the progress of the catalytic reaction. As a new type of porous material, covalent organic frameworks (COFs) have the characteristics of high porosity, specific surface area, stability and easy combination with other nano-materials. The COFs material has a coral rod-like structure and is coated on the surface of the flower-like Bi2O2CO3 material. This structural design enables a synergistic effect to be formed between the COFs and Bi2O2CO3. The high specific surface area and abundant active sites of the COFs can further enhance the CO2 adsorption capacity. At the same time, the COFs will accelerate the electron transfer, effectively overcome the high dissociation energy of the C=O bond and the energy barrier of proton-coupled electron-transfer (PCET), accelerate the catalytic reaction rate, inhibit the occurrence of side reaction processes, and have excellent electrocatalytic CO2 reduction performance. A formate selectivity of more than 95% is obtained in a gas diffusion electrolytic cell, and the current density is superior to that of most current bismuth-based catalysts. In addition, the coating of the COFs can play a certain protective role on the Bi2O2CO3 material, preventing its structural change or oxidation during the reaction process, thereby improving the stability of the catalyst.

[0008] Further, the diameter of the flower-like Bi2O2CO3 material is 4-5 μm, preferably 4.5 μm.

[0009] Further, the diameter of the COFs material is 10-20 nm, preferably 14 nm.

[0010] By defining the diameter of the flower-like Bi2O2CO3 material and the thickness of the COFs material, the microstructure of the catalyst can be more precisely controlled, thereby optimizing its performance. Appropriate size and thickness can ensure that the catalyst has the best activity and selectivity during the reaction process. Reasonable size and thickness also help to increase the specific surface area of the catalyst and the exposure degree of active sites, further enhancing the CO2 adsorption and catalytic conversion efficiency.

[0011] Further, the COFs material comprises a covalent organic framework formed by the condensation reaction of phloroglucinol trialdehyde and 4,4',4''-(1,3,5-triazine-2,4,6-triyl)tri-aniline; this specific material combination has excellent chemical stability and good electron transport performance, and can effectively improve the performance of the catalyst.

[0012] Preferably, the molar ratio of the trihydroxybenzene tricarboxaldehyde to the 4,4',4''-(1,3,5-triazine-2,4,6-triyl)triphenylamine is (1 - 2):1. The above molar ratio range helps to achieve the optimal COFs structure and performance during the synthesis process, further improving the activity and stability of the catalyst.

[0013] According to the second aspect of the present invention, the present invention also provides a method for preparing the above-mentioned COF-coated Bi-based catalyst, comprising the following steps: Step (1) Preparation of the flower-like Bi2O2CO3 material: Dissolve bismuth nitrate pentahydrate and polyvinylpyrrolidone in ethylene glycol respectively, and stir to obtain solution A and solution B; dissolve potassium hydroxide in water to obtain solution C; while stirring, drop the solution B and the solution C into the solution A in sequence to obtain a mixed solution D; perform a hydrothermal reaction on the mixed solution D; after the reaction is completed, collect the product by centrifugation, wash it, and then dry it to obtain the flower-like Bi2O2CO3 material; Step (2) Preparation of the COF-coated Bi-based catalyst: Put the flower-like Bi2O2CO3 material, trihydroxybenzene tricarboxaldehyde and 4,4',4''-(1,3,5-triazine-2,4,6-triyl)triphenylamine obtained in step (1) into a mixed solvent, ultrasonically stir, then add acetic acid solution and continue to ultrasonically mix evenly, and then perform a hydrothermal reaction; after the reaction is completed, centrifuge and wash the obtained product, and dry it in the air.

[0014] The method for preparing the COF-coated Bi-based catalyst of the present invention includes the preparation of the flower-like Bi2O2CO3 material and the COF coating process, making the preparation of the catalyst operable and repeatable, facilitating large-scale production and application. Through steps such as hydrothermal reaction, the preparation of the catalyst can be achieved under mild conditions, the preparation method is simple, avoiding harsh conditions such as high temperature and high pressure, reducing the preparation cost and energy consumption, and at the same time ensuring the quality and performance of the catalyst.

[0015] Furthermore, in the step (2), the weight ratio of the flower-like Bi2O2CO3 material, the trihydroxybenzene tricarboxaldehyde and the 4,4',4''-(1,3,5-triazine-2,4,6-triyl)triphenylamine is (1 - 2):1:(1 - 2.5). By limiting the weight ratio of the flower-like Bi2O2CO3 material, trihydroxybenzene tricarboxaldehyde and 4,4',4''-(1,3,5-triazine-2,4,6-triyl)triphenylamine within a reasonable range value, it can ensure that the components can fully react with each other during the preparation process, forming a uniform and stable COF coating layer, thereby improving the performance of the catalyst.

[0016] Furthermore, the mixed solvent includes 1,4-dioxane and mesitylene; preferably, the volume ratio of the 1,4-dioxane to the mesitylene is (1-5): 1. 1,4-dioxane and mesitylene are selected as mixed solvents, and their preferred volume ratio range is given. This solvent combination can provide good solubility and reaction environment, which is beneficial to the formation and coating process of COFs.

[0017] Furthermore, in the step (2), the flower ball-shaped Bi2O2CO3 material, trialdehyde phloroglucinol and 4,4',4''-(1,3,5-triazine-2,4,6-triyl)triphenylamine obtained in the step (1) are placed in a mixed solvent and ultrasonically stirred for 25-35 minutes, and then an acetic acid solution is added and ultrasonicated for 1-3 minutes. By limiting the specific time of ultrasonic stirring to a reasonable range, it is possible to ensure that the raw materials are fully mixed, thereby forming a uniform COF coating layer, and ensuring that the performance of the catalyst is optimal.

[0018] Furthermore, the temperature of the hydrothermal reaction in step (2) is 100-150°C, the time is 48-72h, and CO2 gas is continuously passed during the reaction. By limiting the time and temperature of the hydrothermal reaction to a reasonable range, it is possible to ensure that the raw materials react fully to form a uniform COF coating layer, while avoiding overreaction or incomplete reaction, ensuring that the performance of the catalyst is optimal. Continuously passing CO2 gas during the hydrothermal reaction can provide a suitable atmosphere for the reaction, help form a stable COFs structure and improve the activity of the catalyst.

[0019] Furthermore, in step (1), the molar ratio of the bismuth nitrate pentahydrate to the potassium hydroxide is 1:(30-50). By limiting the molar ratio of bismuth nitrate pentahydrate to potassium hydroxide within a reasonable range, it is possible to ensure that in the process of preparing the flower-shaped Bi2O2CO3 material, the raw materials can fully react with each other to form a uniform and stable nanostructure, thereby improving the performance of the catalyst.

[0020] Furthermore, the mass amount of the polyvinyl pyrrolidone is 25-35% of the mass amount of the bismuth nitrate pentahydrate. By limiting the mass amount of the polyvinyl pyrrolidone to a reasonable range, it can be ensured that in the process of preparing the flower ball-shaped Bi2O2CO3 material, the raw materials can fully react with each other to form a uniform and stable nanostructure, thereby improving the performance of the catalyst.

[0021] Further, in the solution A, the concentration of bismuth nitrate pentahydrate is 0.05 - 0.2 mol / L; in the solution B, the concentration of polyvinylpyrrolidone is 0.01 - 0.05 g / mL; in the solution C, the concentration of potassium hydroxide is 0.4 - 0.5 g / mL. By precisely controlling the concentrations of each solution, the uniform dispersion and full reaction of each component during the reaction can be ensured, further improving the quality and performance of the flower-shaped Bi2O2CO3 material.

[0022] Further, in the hydrothermal reaction in step (1), the temperature is 100 - 150 °C and the time is 1 - 3 h. By optimizing the temperature and time of the hydrothermal reaction, the preparation process of the flower-shaped Bi2O2CO3 material can be ensured to proceed smoothly, and at the same time, its good crystallinity and purity can be ensured, providing a high-quality precursor for the subsequent COF coating process.

[0023] Further, in step (1), deionized water and ethanol are used for washing; the drying is vacuum drying, the drying temperature is 55 - 65 °C, and the drying time is 4 - 6 h. By optimizing the washing and drying conditions, the preparation efficiency can be improved, the preparation cost can be reduced, and at the same time, the quality and performance of the catalyst can be ensured.

[0024] According to the third aspect of the present invention, the present invention also provides the application of the above-mentioned COF-coated Bi-based catalyst or the COF-coated Bi-based catalyst prepared by the above-mentioned preparation method in the electrocatalytic reduction of CO2 to formate.

[0025] The COF-coated Bi-based catalyst of the present invention has excellent structures and properties, such as high specific surface area, abundant active sites, good electron transport performance and stability, etc. Therefore, it can exhibit higher activity, selectivity and stability in the reaction of electrocatalytic reduction of CO2 to formate, having significant application advantages.

[0026] Further, the application method includes: mixing the COF-coated Bi-based catalyst with ethanol and Nafion reagent, and performing ultrasonic treatment to prepare a catalyst solution; then coating the catalyst solution on a conductive substrate to prepare a working electrode.

[0027] Preferably, in the catalyst solution, the concentration of the COF-coated Bi-based catalyst is 0.001 - 0.003 g / mL, and the volume ratio of ethanol to Nafion reagent is (60 - 70):1. By optimizing the concentration of the COF-coated Bi-based catalyst in the catalyst solution and the volume ratio of ethanol to Nafion reagent, a uniform and stable coating of the catalyst on the working electrode can be formed, improving the activity and stability of the catalyst, and further enhancing the performance of electrocatalytic reduction of CO2 to formate.

[0028] Furthermore, a flow-through electrolytic cell was adopted, and an electrochemical workstation was used to conduct tests in a three-electrode system. The prepared working electrode was used as the working electrode, a mercury oxide electrode (Hg / HgO) was used as the reference electrode and installed at the cathode, and a nickel foam electrode was used as the counter electrode and installed at the anode. Both LSV and CV tests were carried out in a potential window of -0.5 to -2.3 V. The liquid products were collected by electrolysis at a voltage of -0.6 to -1.3 V vs. RHE for 1 h, and 1H NMR of the water suppression peak was used to quantify formic acid.

[0029] Advantages of the present invention: The present invention provides a COF-coated Bi-based catalyst, which includes a flower-like Bi2O2CO3 material and a COFs material coated on the surface of the flower-like Bi2O2CO3 material. The flower-like Bi2O2CO3 material is assembled from nano-sheet Bi2O2CO3; the COFs material has a coral rod-like structure. The introduction of the COFs material provides the catalyst with abundant active sites and loading sites. The high specific surface area characteristic improves the CO2 adsorption capacity. The organic framework of the COFs can regulate the electronic state of the metal active sites through π-π interaction or coordination bonds, optimize the adsorption energy of intermediates (such as *COOH, *CO), and enhance the reaction kinetics. The conjugated structure of the COFs can promote charge transfer, while Bi2O2CO3 provides high conductivity, and the two work together to reduce the reaction overpotential. The coating of the COFs can also improve the hydrophobicity of the catalyst, thereby inhibiting the HER reaction. The present invention is of great significance for the study of electrocatalytic CO2 reduction. Description of the drawings

[0030] In order to more clearly illustrate the technical solutions in the present invention or the prior art, the following will briefly introduce the drawings required for the description of the embodiments or the prior art. Obviously, the drawings in the following description are some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0031] Figure 1 Synthesis schematic diagram of the COF / BOC catalyst provided in Example 1 of the present invention.

[0032] Figure 2 X-ray diffraction patterns of the COF / BOC catalyst and the flower-like BOC material provided in Example 1 of the present invention.

[0033] Figure 3 Transmission electron micrograph of the COF / BOC catalyst provided in Example 1 of the present invention.

[0034] Figure 4 Element distribution map of the COF / BOC catalyst provided in Example 1 of the present invention.

[0035] Figure 5 The EPR diagram of the COF / BOC catalyst provided in Example 1 of the present invention.

[0036] Figure 6 The water contact angle diagram of the COF / BOC catalyst and the flower-like BOC material provided in Example 1 of the present invention.

[0037] Figure 7 For the COF / BOC materials, COF 2d / BOC materials and flower-like BOC materials as catalysts in Example 2, Example 4 and Comparative Example 1 of the present invention, the comparative diagram of the Faraday efficiency of the products generated by electrocatalytic reduction of CO2. Detailed implementation manners

[0038] To make the objectives, technical solutions and advantages of the present invention clearer, the technical solutions in the present invention will be clearly and completely described below with reference to the accompanying drawings in the present invention. Obviously, the described embodiments are some but not all of the embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art based on the embodiments in the present invention without making creative efforts shall fall within the protection scope of the present invention.

[0039] Example 1 This example provides a preparation method of a COF-coated Bi-based catalyst, and its preparation process is referred to Figure 1 as shown, and the preparation method includes the following steps: Step (1) Preparation of flower-like Bi2O2CO3 (BOC) material Dissolve 2 mmol of bismuth nitrate pentahydrate in 20 mL of ethylene glycol, stir at room temperature for 10 min to obtain solution A. Dissolve 0.3 g of polyvinylpyrrolidone (PVP) in 10 mL of ethylene glycol, stir at room temperature for 10 min to obtain solution B. Dissolve 4.48 g of potassium hydroxide in 10 mL of deionized water to obtain solution C. While stirring, sequentially drop solution B and solution C into solution A, and continue to stir for 10 min to obtain a mixed solution D. Transfer the mixed solution D into a 50 mL mechanical stirring autoclave lined with Teflon, continuously introduce CO2 gas, carry out a hydrothermal reaction at 120 °C for 2 h, and naturally cool to room temperature after the reaction is completed. Collect the product by centrifugal washing, and after washing alternately with ethanol and deionized water 3 times, dry it in a vacuum drying oven at 60 °C for 6 h to obtain flower-like BOC material powder.

[0040] Step (2) Preparation of COF-coated Bi-based catalyst Put 20 mg of the flower-like BOC material obtained in (1), 0.12 mmol (16.10 mg) of phloroglucinol trialdehyde, and 0.08 mmol (28.35 mg) of 4,4',4''-(1,3,5-triazine-2,4,6-triyl)triphenylamine into a 10 mL glass vial. After adding 0.8 mL of 1,4-dioxane and 0.2 mL of mesitylene solvent, ultrasonically stir for 30 min, and then add 0.2 mL of 3M acetic acid solution and continue ultrasonically stirring for 2 min. Subsequently, open the lid of the glass vial and transfer it to a 100 mL hydrothermal reaction kettle, and place it in an oven for hydrothermal reaction at 120 o C for 72 h. After the reaction is completed, centrifuge and wash 3 times with tetrahydrofuran solution, and air dry naturally in the fume hood to obtain a COF-coated Bi-based catalyst for electrochemical catalytic CO2 reduction (named COF / BOC catalyst).

[0041] Carry out a series of experimental characterizations on the obtained COF / BOC catalyst: Figure 2 is the X-ray diffraction pattern of the COF / BOC catalyst and the flower-like BOC material. The diffraction peaks at 12.9°, 23.9°, 30.2°, and 46.9° in the XRD pattern correspond well to the standard card of Bi2O2CO3 (JCPDS No. 41-1488). At the same time, a weak diffraction peak appears at 5.5°, corresponding to the (100) crystal plane of the COFs material.

[0042] Figure 3 is the transmission electron microscopy image of the COF / BOC catalyst. It can be seen that the COF / BOC catalyst includes the flower-like BOC material and the COFs material coated on the surface of the flower-like BOC material; the flower-like BOC material is assembled by nano-sheet BOC materials with a diameter of 4.5 μm; the COFs material has a coral rod-like structure with a coral rod diameter of 14 nm, which provides a larger specific surface area and exposes more active sites beneficial to the CO2RR reaction.

[0043] Figure 4 is the elemental distribution map of the COF / BOC catalyst and the flower-like BOC material. The uniform distribution of Bi, C, O, and N indicates the uniform distribution of the material elements.

[0044] Figure 5 is the EPR spectrum of the COF / BOC catalyst and the flower-like BOC material. The symmetric peak at g value of 2.004 represents the electrons captured by oxygen vacancies. Compared with pure BOC, the enhanced peak of COF / BOC indicates that the coating of COF introduces more oxygen vacancies and can achieve better electron transport.

[0045] Figure 6It is the contact angle experimental test diagram of the COF / BOC catalyst and the flower-shaped BOC material. The contact angle formed by COF / BOC and water is 145°, which is significantly larger than the water contact angle of BOC (114°), indicating that the coating of the COF material significantly improves the hydrophobicity of the catalyst, and thus can effectively inhibit the hydrogen evolution reaction (HER).

[0046] Example 2 The COF / BOC catalyst of Example 1 was used for the test of electrochemically reducing CO2 to produce HCOOH: Step (1): 2 mg of the COF / BOC catalyst was mixed with 985 μL of ethanol and 15 μL of Nafion reagent, and ultrasonically treated for 15 min to prepare a catalyst solution. 500 μL of the catalyst solution was evenly dropped on a 1×1 cm conductive carbon paper and dried naturally to prepare a working electrode.

[0047] In a flow-through electrolytic cell, a three-electrode system with the COF / BOC material as the working electrode, nickel foam as the counter electrode, and a mercury oxide electrode (Hg / HgO) as the reference electrode and 1 M KOH electrolyte was constructed. I-t tests were carried out for 15 min under an externally applied bias voltage of different voltages (-0.6 V, -0.7 V, -0.8 V, -0.9 V, -1.0 V, -1.1 V, -1.2 V, -1.3 V vs. RHE) applied to the working electrode.

[0048] Step (2): Under the conditions of step (1), the concentration of HCOOH in the electrolyte was detected by a nuclear magnetic resonance spectrometer, and the Faraday efficiency of electrochemically reducing CO2 to produce HCOOH at different voltages was calculated; at the same time, a gas chromatograph was used to sample and detect and analyze the gas amounts of H2 and CO generated at regular intervals, and the Faraday efficiencies of H2 and CO generated at different voltages were calculated.

[0049] Comparative Example 1 The flower-shaped BOC material in Example 1 was used for the test of electrochemically reducing CO2 to produce HCOOH: The difference from Example 2 is only that the flower-shaped BOC material was used to replace the COF / BOC catalyst, and other implementation conditions were the same as those in Example 2.

[0050] Example 3 This example provides a preparation method of a COF-coated Bi-based catalyst. The difference from Example 1 is that: the hydrothermal reaction time described in step (2) of Example 1 was changed to 48 h, and other steps were the same as those in Example 1, and a COF-coated Bi-based catalyst (named COF 2d / BOC catalyst) for electrochemically catalyzing CO2 reduction was also obtained.

[0051] Example 4 Using the COF in Example 32d Electrochemical reduction of CO2 to HCOOH was tested using the COF / BOC catalyst: The difference from Example 2 is only that the COF 2d / BOC catalyst was used to replace the COF / BOC catalyst, and other implementation conditions were the same as those in Example 2.

[0052] As Figure 7 shown, from the electrocatalytic reduction of CO2 test results of COF / BOC, COF 2d / BOC and pure-phase BOC, it can be seen that the catalyst material coated with COF has significantly improved current density and selectivity compared to BOC. For the pure-phase BOC material, within a wide voltage window (-0.6 to -1.1 V vs. RHE), the Faraday efficiency of HCOOH remains below 90%. For the COF / BOC catalyst synthesized for 3 days, both the current density and selectivity are better than those of BOC and COF 2d / BOC, which can efficiently electrocatalytically reduce CO2 to HCOOH. Within a wide voltage window (-0.6 to -1.1 V vs. RHE), the Faraday efficiency of HCOOH remains above 90%, up to 95.2% at most, and only a small amount of CO and H2 are produced.

[0053] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that they can still modify the technical solutions recorded in the foregoing embodiments, or perform equivalent replacements for some of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A COF-coated Bi-based catalyst, characterized in that, It includes flower-like Bi2O2CO3 materials and COFs materials coated on the surface of the flower-like Bi2O2CO3 materials; the flower-like Bi2O2CO3 materials are assembled by nano-sheet Bi2O2CO3; the COFs materials have a coral rod-like structure.

2. The COF-coated Bi-based catalyst according to claim 1, wherein The diameter of the flower-like Bi2O2CO3 materials is 4 - 5 μm; and / or, the diameter of the COFs materials is 10 - 20 nm.

3. The COF-coated Bi-based catalyst according to claim 1 or 2, characterized in that, The COFs materials include a covalent organic framework formed by the condensation reaction of phloroglucinol trialdehyde and 4,4',4''-(1,3,5-triazine-2,4,6-triyl)triphenylamine; Preferably, the molar ratio of phloroglucinol trialdehyde to 4,4',4''-(1,3,5-triazine-2,4,6-triyl)triphenylamine is (1 - 2):

1.

4. The preparation method of the COF-coated Bi-based catalyst according to any one of claims 1-3, characterized in that, It includes the following steps: Step (1) Preparation of flower-like Bi2O2CO3 materials: Dissolve bismuth nitrate pentahydrate and polyvinylpyrrolidone in ethylene glycol respectively, stir to obtain solution A and solution B; dissolve potassium hydroxide in water to obtain solution C; While stirring, sequentially drop solution B and solution C into solution A to obtain a mixed solution D; Perform hydrothermal reaction on the mixed solution D; after the reaction is completed, collect the product by centrifugation, wash it, and then dry it to obtain flower-like Bi2O2CO3 materials; Step (2) Preparation of COF-coated Bi-based catalyst: Put the flower-like Bi2O2CO3 materials, phloroglucinol trialdehyde and 4,4',4''-(1,3,5-triazine-2,4,6-triyl)triphenylamine obtained in step (1) into a mixed solvent, ultrasonically stir, then add acetic acid solution and continue to ultrasonically mix evenly, and then perform hydrothermal reaction; after the reaction is completed, centrifuge and wash the obtained product, and air-dry it.

5. The preparation method according to claim 4, characterized in that, In step (2), the weight ratio of the flower-like Bi2O2CO3 materials, phloroglucinol trialdehyde and 4,4',4''-(1,3,5-triazine-2,4,6-triyl)triphenylamine is (1 - 2):1:(1 - 2.5); and / or, the mixed solvent includes 1,4-dioxane and mesitylene; preferably, the volume ratio of 1,4-dioxane to mesitylene is (1 - 5):

1.

6. The preparation method according to claim 4 or 5, characterized in that, In step (2), put the flower-like Bi2O2CO3 materials, phloroglucinol trialdehyde and 4,4',4''-(1,3,5-triazine-2,4,6-triyl)triphenylamine obtained in step (1) into a mixed solvent and ultrasonically stir for 25 - 35 min, then add acetic acid solution and continue to ultrasonically stir for 1 - 3 min; and / or, the temperature of the hydrothermal reaction in step (2) is 100 - 150 °C, the time is 48 - 72 h, and CO2 gas is continuously introduced during the reaction process.

7. The preparation method according to claim 4, wherein In step (1), the molar ratio of bismuth nitrate pentahydrate to potassium hydroxide is 1:(30 - 50); and / or, the mass dosage of polyvinylpyrrolidone is 25 - 35% of the mass dosage of bismuth nitrate pentahydrate; And / or, in the solution A, the concentration of bismuth nitrate pentahydrate is 0.05 - 0.2 mol / L; in the solution B, the concentration of polyvinylpyrrolidone is 0.01 - 0.05 g / mL; in the solution C, the concentration of potassium hydroxide is 0.4 - 0.5 g / mL.

8. The preparation method according to claim 4 or 7, characterized in that The temperature of the hydrothermal reaction in the step (1) is 100 - 150 °C, and the time is 1 - 3 h; And / or, in the step (1), deionized water and ethanol are selected for the washing; the drying is vacuum drying, the drying temperature is 55 - 65 °C, and the drying time is 4 - 6 h.

9. Application of the COF-coated Bi-based catalyst according to any one of claims 1 - 3 or the COF-coated Bi-based catalyst prepared by the preparation method according to any one of claims 4 - 8 in electrocatalytic reduction of CO2 to formate.

10. The application according to claim 9, characterized in that, The application method includes: mixing the COF-coated Bi-based catalyst with ethanol and Nafion reagent, and ultrasonicating to prepare a catalyst solution; then coating the catalyst solution on a conductive substrate to prepare a working electrode; Preferably, in the catalyst solution, the concentration of the COF-coated Bi-based catalyst is 0.001 - 0.003 g / mL, and the volume ratio of ethanol to the Nafion reagent is (60 - 70):1.

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