A copper sulfide-bismuth sulfide heterojunction nanosheet catalyst and its preparation and application methods

Abstract

The present invention relates to a copper sulfide-bismuth sulfide heterojunction nanosheet catalyst and its preparation and application methods, characterized in that the expression of the elemental composition is CuS-Bi2S3, and the components are CuS and Bi2S3 with a mass ratio of 1:4 to 1:1; the nanosheets contain the active components of the copper sulfide-bismuth sulfide heterojunction, the microcrystalline regions of copper sulfide and bismuth sulfide, and there is an interface between the two microcrystalline regions. The present invention uses bismuth nitrate hexahydrate as the bismuth source, soluble copper salt as the copper source, and L-cysteine as the sulfur source to prepare the copper sulfide-bismuth sulfide heterojunction nanosheet catalyst by a simple one-step solvothermal method. Due to its unique nanosheet structure, there are a large number of wrinkles on the surface of the catalyst, which can maximize the exposure of the active components and enable them to come into full contact with the reactants during the subsequent electrocatalytic process, thereby effectively improving the catalytic efficiency. The copper sulfide-bismuth sulfide heterojunction nanosheet catalyst has a novel and unique structure, a green and energy-saving preparation process, excellent catalytic performance and good stability, and has good application prospects.

Description

Technical Field

[0001] The present invention belongs to the field of preparation and application of nanomaterials, and relates to a copper sulfide-bismuth sulfide heterojunction nanosheet catalyst and a preparation and application method thereof. Specifically, it relates to a copper sulfide-bismuth sulfide heterojunction nanosheet catalyst for electrocatalytic reduction of carbon dioxide, and a preparation method and application thereof. Background Art

[0002] Carbon dioxide (CO2) widely exists in the earth's atmosphere and oceans, and its carbon content is 10 times that of coal, oil, and natural gas [International Journal of Hydrogen Energy. 2021, 46(9): 6180-6187]. Due to the rapid development of economy and industrialization, the global energy demand and fuel usage are constantly increasing, and a large amount of CO2 is still continuously emitted into the atmosphere. As of February 2021, the average global atmospheric CO2 concentration has reached 415.88 ppm. The continuous increase in CO2 concentration has caused great pressure on the environment, with the global temperature rising continuously and extreme weather occurring frequently. In order to alleviate the ecological environment problems caused by the increase in CO2 content, in addition to energy conservation, emission reduction, and the development of renewable energy, realizing the resource utilization of CO2 is one of the effective methods to solve the above problems. However, as the highest valence state of carbon element, CO2 has extremely stable chemical properties, and usually its conversion process requires the assistance of a catalyst.

[0003] Common chemical conversion methods of CO2 include thermal catalytic conversion, electrocatalytic conversion, and photocatalytic conversion. Compared with the other two catalytic conversion methods, the CO2 electrocatalytic reduction method (CO2RR) has the following advantages: (1) Using renewable energy as power to avoid additional carbon emissions; (2) The reaction process can be carried out at normal pressure and room temperature; (3) Hydrogen is not consumed additionally during the reaction process. Therefore, CO2RR has great research value and application prospects, and is currently a research hotspot in the field of CO2 resource utilization. However, in the CO2RR process, the formation potentials of various products (CO, formic acid, ethylene, etc.) are close, and coupled with the influence of hydrogen evolution from water electrolysis (HER), the product selectivity is poor. In addition, problems such as small current density, low catalyst stability, and sensitivity of products to voltage changes in the CO2RR process limit its industrial application.

[0004] Among numerous CO2RR products, formic acid has relatively high added value. Compared with the existing chemical methods for preparing formic acid, the electrocatalytic synthesis of formic acid from CO2 has obvious economic advantages. Therefore, the development of related catalysts has always been a hot topic of concern. Among numerous catalysts, metal Bi-based materials have become one of the important catalysts for preparing formic acid by CO2RR due to their low toxicity, environmental friendliness, and relatively low cost. Min et al. grew Bi nanodendrites on a highly conductive copper substrate by direct electrodeposition. This catalyst achieved a formate Faradaic efficiency of 89% at a potential of -0.74 V (ACS Catal., 2017, 7, 5071); Lee et al. deposited Bi nanosheets with more exposed edges on a copper sheet by pulsed electrodeposition. This catalyst could reach a formate Faradaic efficiency of 79.5% at an ultra-low potential of -0.4 V (Nano Energy, 2017, 39, 44); Jin et al. prepared ultrathin two-dimensional Bi nanosheets by liquid-phase exfoliation method. These ultrathin nanosheets have more active sites and can achieve a formate Faradaic efficiency of 86% and a current density of -16.5 mA cm -2 -2 (J. Am. Chem. Soc., 2001, 123, 9904). Although Bi-based materials have made certain progress in the preparation of formic acid by CO2RR, problems such as relatively low current density, poor long-term stability, and narrow potential window for generating formic acid with high Faradaic efficiency (greater than 90%) limit their industrial applications. Summary of the Invention

[0005] Technical Problems to be Solved

[0006] In order to avoid the deficiencies of the prior art, the present invention proposes a copper sulfide-bismuth sulfide heterojunction nanosheet catalyst and its preparation and application methods, aiming at the problems existing in the existing catalysts for electrocatalytic reduction of carbon dioxide based on bismuth. The present invention designs a copper sulfide-bismuth sulfide heterojunction nanosheet catalyst with excellent activity for electrocatalytic reduction of carbon dioxide to prepare formic acid. By means of copper doping, the current density of CO2RR is effectively increased, and at the same time, the stability of the bismuth-based catalyst is improved, laying a foundation for its industrial application.

[0007] Technical Solution

[0008] A copper sulfide-bismuth sulfide heterojunction nanosheet catalyst, characterized in that the expression of the element composition is CuS-Bi2S3, and the components are CuS and Bi2S3 with a mass ratio of 1:4 to 1:1; the nanosheet contains the active components of the copper sulfide-bismuth sulfide heterojunction, the microcrystalline regions of copper sulfide and bismuth sulfide, and there is an interface between the two microcrystalline regions.

[0009] The copper sulfide-bismuth sulfide heterojunction nanosheet catalyst is a flaky structure with a rough surface, and the size of the copper sulfide-bismuth sulfide heterojunction nanosheets is 200-300 nm and the thickness is 2-3 nm.

[0010] A preparation method of the copper sulfide-bismuth sulfide heterojunction nanosheet catalyst is characterized by the following steps:

[0011] Step 1: Take 25-40 mg of L-cysteine, 50-100 mg of bismuth nitrate, and 15-30 mg of copper salt, add them to 50-100 mL of solvent, and stir at room temperature for 30-60 minutes to obtain a mixed solution;

[0012] Step 2: Add 3-6 mL of ethanolamine and continue stirring for 20-40 minutes;

[0013] Step 3: Transfer the obtained solution to a hydrothermal reaction kettle, keep it at a temperature of 120-140 °C for 2-14 h; after cooling, obtain a crude sample of the copper sulfide-bismuth sulfide (i.e., CuS-Bi2S3) heterojunction nanosheet catalyst by filtration or centrifugation;

[0014] Step 4: Wash and centrifuge the CuS-Bi2S3 crude sample with a detergent to remove unreacted metal salts and ethanolamine;

[0015] Step 5: Then dry it under an inert gas atmosphere at 50-80 °C to obtain the copper sulfide-bismuth sulfide heterojunction nanosheet catalyst;

[0016] The dosages of the above reagents are enlarged or reduced in the same proportion.

[0017] The concentration of the L-cysteine solution is 1 mmol / L - 2 mmol / L.

[0018] The copper salt is one of copper nitrate, copper chloride, copper sulfate or a mixture of any proportion of them.

[0019] The solvent is deionized water or ethanol or a mixture of any proportion of both.

[0020] The detergent is deionized water, alcohols such as methanol and ethanol, and mixtures of any proportion of them. The optimal number of washing times is 2-3 times.

[0021] The inert gas is any one of helium, nitrogen, argon or a mixed gas of any proportion of them.

[0022] When the drying temperature in Step 5 is 50-80 °C, the drying time is 5-18 h.

[0023] An application method of the copper sulfide-bismuth sulfide heterojunction nanosheet catalyst is characterized by being used to catalyze the electroreduction of CO2 to prepare formic acid under alkaline conditions.

[0024] Beneficial Effects

[0025] The present invention proposes a copper sulfide-bismuth sulfide heterojunction nanosheet catalyst and a preparation and application method thereof, which relates to a copper sulfide-bismuth sulfide heterojunction nanosheet catalyst having excellent electrocatalytic reduction of carbon dioxide (CO2RR) to prepare formic acid activity. The copper sulfide-bismuth sulfide heterojunction nanosheet catalyst comprises a large number of active components; the active component is a copper sulfide-bismuth sulfide heterojunction; the copper sulfide-bismuth sulfide heterojunction nanosheet catalyst is a sheet structure; the nano copper sulfide-bismuth sulfide heterojunction nanosheet catalyst is directly prepared by a hydrothermal method. The present invention also provides a preparation method and application of the copper sulfide-bismuth sulfide heterojunction nanosheet catalyst. The copper sulfide-bismuth sulfide heterojunction nanosheet catalyst provided by the present invention exhibits excellent performance in the electrocatalytic reduction of CO2 to prepare formic acid. In a flow-type electrolytic cell, with it as the cathode and a 1M KOH aqueous solution as the electrolyte, the formic acid Faraday efficiency of more than 90% can be achieved within the voltage window of -0.35V to -1.40V (relative to the reversible hydrogen electrode). At a relatively low voltage of -1.06V (relative to the reversible hydrogen electrode), the current density can reach -400mA cm -2 The copper sulfide-bismuth sulfide heterojunction nanosheet catalyst has a novel and unique structure, a green and energy-saving preparation process, and excellent catalytic performance and good stability, and has good application prospects.

[0026] Compared with the prior art, the present invention has the following advantages:

[0027] (1) The copper sulfide-bismuth sulfide heterojunction nanosheet catalyst has a unique nanosheet structure with a large number of wrinkles on the surface, which allows the active components to be exposed to the maximum extent and fully contact with the reactants during the subsequent electrocatalytic process, thereby effectively improving the catalytic efficiency.

[0028] (2) With the help of copper doping, the current density and long-term stability of bismuth-based catalysts for CO2RR are effectively improved. The catalyst has a current density of up to -400 mA cm at -1.06 V (relative to the reversible hydrogen electrode) using 1 M KOH aqueous solution as the electrolyte. -2 When 5M KOH aqueous solution is used as the electrolyte, the current density can reach -710mA cm -2 The potential window for formic acid Faradaic efficiency above 90% exceeds 1050 mV; at -400 mA cm -2 It can work stably for more than 100 hours without deactivation under high current density, laying the foundation for industrial application.

[0029] (3) The preparation process of the copper sulfide-bismuth sulfide heterojunction nanosheet catalyst is simple and easy to scale up. Description of the Drawings

[0030] Figure 1 It is a schematic diagram of the preparation process of the CuS-Bi2S3 heterojunction nanosheet catalyst described in the present invention.

[0031] Figure 2 It is a scanning electron microscope photograph of the CuS-Bi2S3 heterojunction nanosheet catalyst prepared in Example 1. It can be observed from the figure that CuS-Bi2S3 has a curved nanosheet structure, and the nanosheets are stacked with each other to form a nanoflower-like assembly.

[0032] Figure 3 It is a high-resolution transmission electron microscope photograph of the CuS-Bi2S3 heterojunction nanosheet catalyst prepared in Example 1. It can be determined from the analysis of the crystal plane spacing in the figure that the CuS-Bi2S3 nanosheet is composed of a heterostructure of CuS and Bi2S3, and there is a clear heterojunction interface between the two.

[0033] Figure 4 It is the linear sweep voltammetry curves of the CuS-Bi2S3 / GDE electrode prepared by coating the CuS-Bi2S3 heterojunction nanosheet catalyst obtained in Example 1 on the surface of a gas diffusion electrode (GDE) in alkaline aqueous solutions saturated with Ar and CO2, respectively. It can be seen from the figure that CuS-Bi2S3 / GDE exhibits excellent CO2RR activity. In a 1M KOH solution, the voltage required for a current density of -100 mA cm -2 is only -0.59 V (versus the reversible hydrogen electrode).

[0034] Figure 5 It is the linear sweep voltammetry curves of the CuS-Bi2S3 / GDE and Bi2S3 / GDE electrodes prepared by coating the CuS-Bi2S3 heterojunction nanosheet catalyst obtained in Example 1 and the single Bi2S3 catalyst prepared in Example 6 on the surface of the GDE in a CO2-saturated alkaline aqueous solution. It can be seen from Figure 5 the figure that the current density of the CuS-Bi2S3 heterojunction nanosheet catalyst is significantly increased compared with the single Bi2S3 catalyst.

[0035] Figure 6 It is the formic acid Faraday efficiency ( Figure 6 a) and the partial current density diagram of formic acid ( Figure 6 b) of the CuS-Bi2S3 / GDE electrode prepared in Example 1 with a 1M KOH alkaline aqueous solution as the electrolyte at different overpotentials. It can be seen from Figure 6It can be seen that the CuS-Bi2S3 catalyst can maintain a formic acid Faraday efficiency of over 90% under an ultra-wide potential window of -0.35 to -1.4 V (versus the reversible hydrogen electrode). From Figure 6 It can be seen from b that when the overpotential is -1.06 V (versus the reversible hydrogen electrode), the partial current density for the formation of formic acid reaches -400 mA cm -2 , indicating the excellent activity of the CuS-Bi2S3 catalyst for formic acid production.

[0036] Figure 7 Figure is a performance comparison diagram of the CuS-Bi2S3 heterojunction nanosheet catalyst and some existing catalysts for electrocatalytic CO2 reduction to formic acid in a flow-through electrolytic cell. It can be seen from the figure that the CuS-Bi2S3 heterojunction nanosheet catalyst prepared in the present invention exhibits excellent CO2RR catalytic performance.

[0037] Figure 8 Figure is a long-term stability test of the CuS-Bi2S3 heterojunction nanosheet catalyst prepared in the present invention for electrocatalytic CO2 reduction to formic acid in a flow-through electrolytic cell ( Figure 8 a) and a comparison diagram of its long-term stability with existing catalysts for electrocatalytic CO2 reduction to formic acid ( Figure 8 b). It can be seen from the figure that the CuS-Bi2S3 heterojunction nanosheet catalyst prepared in the present invention exhibits excellent long-term stability (stable for up to 100 hours at -400 mA cm -2 ). Specific Embodiments

[0038] The present invention will be further described in conjunction with the embodiments and the accompanying drawings:

[0039] I. Preparation of Copper Sulfide-Bismuth Sulfide Heterojunction Nanosheet Catalyst

[0040] Example 1: CuS-Bi2S3 Heterojunction Nanosheet Catalyst

[0041] This embodiment provides a method for preparing a copper sulfide-bismuth sulfide heterojunction nanosheet catalyst, which includes the following steps: Take 30 mg of L-cysteine, 75 mg of bismuth nitrate hexahydrate, and 20 mg of copper nitrate hexahydrate, add them to 54 mL of deionized water, stir at room temperature for 30 minutes to dissolve, and obtain a mixed solution. Add 5 mL of ethanolamine to the above mixed solution, continue to stir at room temperature for 30 minutes, then transfer it to a 100 mL hydrothermal reaction kettle, keep it at 120 °C for 6 hours, and separate to obtain a crude sample of the copper sulfide-bismuth sulfide heterojunction nanosheet catalyst. Wash the obtained CuS-Bi2S3 crude sample with deionized water and ethanol respectively, and centrifuge twice to remove unreacted metal salts and ethanolamine, and then dry it in a nitrogen atmosphere at 65 °C for 12 hours to obtain a CuS-Bi2S3 heterojunction nanosheet catalyst.

[0042] Example 2: CuS-Bi2S3 heterojunction nanosheet catalyst

[0043] This embodiment provides a method for preparing a copper sulfide-bismuth sulfide heterojunction nanosheet catalyst, which is basically the same as that in Example 1, except that 54 mL of deionized water in the first step is replaced with 54 mL of ethanol to obtain a nanosheet-like CuS-Bi2S3-ethanol-120-75 catalyst.

[0044] Example 3: CuS-Bi2S3 heterojunction nanosheet catalyst

[0045] This embodiment provides a method for preparing a copper sulfide-bismuth sulfide heterojunction nanosheet catalyst, which is basically the same as that in Example 1, except that the reaction temperature of the hydrothermal kettle is changed from 120 °C to 100 °C to obtain a nanosheet-like CuS-Bi2S3-H2O-100-75 catalyst.

[0046] Example 4: CuS-Bi2S3 heterojunction nanosheet catalyst

[0047] This embodiment provides a method for preparing a copper sulfide-bismuth sulfide heterojunction nanosheet catalyst, which is basically the same as that in Example 1, except that the amount of bismuth nitrate hexahydrate is changed to 100 mg, and copper nitrate hexahydrate is changed to copper sulfate to obtain a nanosheet-like CuS-Bi2S3-H2O-100-100 catalyst.

[0048] Example 5: CuS-Bi2S3 heterojunction nanosheet catalyst

[0049] This embodiment provides a method for preparing a copper sulfide-bismuth sulfide heterojunction nanosheet catalyst, which is basically the same as that in Example 1, except that the amount of bismuth nitrate hexahydrate is changed to 50 mg to obtain a nanosheet-like CuS-Bi2S3-H2O-100-50 catalyst.

[0050] Example 6: Nano-sheet Bi2S3 Catalyst

[0051] This example provides a method for preparing a nano-sheet bismuth sulfide catalyst, which includes the following steps: Take 30 mg of L-cysteine and 75 mg of bismuth nitrate hexahydrate, add them to 54 mL of deionized water, stir at room temperature for 30 minutes to obtain a mixed solution. Add 5 mL of ethanolamine to the above mixed solution, continue to stir at room temperature for 30 minutes, then transfer it to a 100 mL hydrothermal reaction kettle, and keep it at 160 °C for 6 hours to obtain a crude bismuth sulfide catalyst product. Wash the obtained Bi2S3 crude product with deionized water and ethanol respectively, and centrifuge twice to remove unreacted metal salts and ethanolamine, and then dry it at 80 °C for 6 hours in an argon atmosphere to obtain a nano-sheet Bi2S3 catalyst.

[0052] II. Electrochemical Catalytic CO2 Reduction Performance Test of Copper Sulfide-Bismuth Sulfide Heterojunction Nanosheet Catalyst under Alkaline Conditions

[0053] Load the CuS-Bi2S3 catalyst prepared in Example 1 onto the surface of the GDE to prepare a CuS-Bi2S3 / GDE electrode for electrochemical catalytic CO2 reduction test. The specific process is as follows: (1) Mix 10 mg of the CuS-Bi2S3 catalyst with 1 mL of absolute ethanol and 50 μL of naphthol solution (5 wt%), ultrasonically disperse for 10 minutes, then take 100 μL of the above slurry and evenly coat it on the surface of the gas diffusion electrode, and let it dry naturally at room temperature to prepare a CuS-Bi2S3 / GDE working electrode; (2) Assemble the working electrode into a flow-type electrolytic cell. The electrolysis device includes a flow-type electrolytic cell, an Ag / AgCl reference electrode, a nickel foam counter electrode, and a CuS-Bi2S3 / GDE working electrode, and use 1 M KOH solution as the electrolyte.

[0054] The test results show that the CuS-Bi2S3 / GDE electrode can significantly inhibit HER and achieve electrochemical catalytic CO2 reduction at a lower overpotential. The results are as Figure 4 shown. When performing CO2RR, compared with the single Bi2S3 catalyst prepared in Example 6, the current density of the CuS-Bi2S3 heterojunction nanosheet catalyst is significantly increased. The results are as Figure 5 shown. In the ultra-wide potential window of -0.35 to -1.4 V (relative to the reversible hydrogen electrode), the Faraday efficiency of formic acid production can be maintained above 90%, as Figure 6 shown. At a high current of -400 mA cm -2 , the CuS-Bi2S3 heterojunction nanosheet catalyst can continuously and stably work for more than 100 hours. The results are as Figure 8As shown in a, it shows that the catalyst has excellent stability. The above analysis confirms that the prepared CuS-Bi2S3 heterojunction nanosheet catalyst of the present invention has excellent performance for preparing formic acid by CO2RR, and at the same time has a very wide potential window for efficient formic acid production and high long-term stability.

[0055] The present invention uses bismuth nitrate hexahydrate as the bismuth source, soluble copper salt as the copper source, and L-cysteine as the sulfur source to prepare a copper sulfide-bismuth sulfide heterojunction nanosheet catalyst by a simple one-step solvothermal method. Due to its unique nanosheet structure, there are a large number of wrinkles on the surface of the catalyst, which can maximize the exposure of the active components and enable full contact with the reactants during the subsequent electrocatalytic process, thereby effectively improving the catalytic efficiency. Thanks to the excellent electron interaction and antioxidant ability generated after copper doping, the copper sulfide-bismuth sulfide heterojunction nanosheet catalyst can effectively inhibit the occurrence of HER and the reaction of generating other reduction products in a flow-through electrolytic cell. When using 1M KOH aqueous solution as the electrolyte, the current density of this catalyst is as high as -400 mA cm at -1.06 V (versus reversible hydrogen electrode). -2 ; when using 5M KOH aqueous solution as the electrolyte, the current density can reach -710 mA cm. -2 The potential window with a formic acid Faraday efficiency higher than 90% exceeds 1050 mV, and it can stably work for more than 100 hours at a high current density of -400 mA cm without deactivation. In addition, the preparation method of the copper sulfide-bismuth sulfide heterojunction nanosheet catalyst described in the present invention has a simple process and is easy to scale up. -2 ​

Claims

1. A copper sulfide - bismuth sulfide heterojunction nanosheet catalyst, characterized in that The expression of the element composition is CuS-Bi2S3, and the components are CuS and Bi2S3 with a mass ratio of 1:4 to 1:1; the nano-sheets contain the active components of the copper sulfide-bismuth sulfide heterojunction, the microcrystalline regions of copper sulfide and bismuth sulfide, and there is an interface between the two microcrystalline regions.

2. The copper sulfide - bismuth sulfide heterojunction nanosheet catalyst according to claim 1, characterized in that: The copper sulfide-bismuth sulfide heterojunction nano-sheet catalyst is a sheet-like structure with a rough surface, and the size of the copper sulfide-bismuth sulfide heterojunction nano-sheets is 200-300 nm and the thickness is 2-3 nm.

3. A preparation method of the copper sulfide - bismuth sulfide heterojunction nanosheet catalyst according to claim 1 or 2, characterized in that The steps are as follows: Step 1: Take 25-40 mg of L-cysteine, 50-100 mg of bismuth nitrate, and 15-30 mg of copper salt, add them to 50-100 mL of solvent, and stir at room temperature for 30-60 minutes to obtain a mixed solution; Step 2: Add 3-6 mL of ethanolamine and continue to stir for 20-40 minutes; Step 3: Transfer the obtained solution to a hydrothermal reaction kettle, keep it at a temperature of 120-140 °C for 2-14 h; after cooling, obtain a crude sample of the copper sulfide-bismuth sulfide, i.e., CuS-Bi2S3, heterojunction nano-sheet catalyst by filtration or centrifugation; Step 4: Wash and centrifuge the CuS-Bi2S3 crude sample with a detergent to remove unreacted metal salts and ethanolamine; Step 5: Then dry it under an inert gas atmosphere at 50-80 °C to obtain the copper sulfide-bismuth sulfide heterojunction nano-sheet catalyst; The above reagent dosages are scaled up or down in the same proportion.

4. The method according to claim 3, characterized in that: The concentration of the L-cysteine solution is 1 mmol / L - 2 mmol / L.

5. The method according to claim 3, characterized in that: The copper salt is one of copper nitrate, copper chloride, copper sulfate or a mixture of any proportion of them.

6. The method according to claim 3, characterized in that: The solvent is deionized water or ethanol or a mixture of any proportion of the two.

7. The method according to claim 3, characterized in that: The detergent is deionized water, alcohols such as methanol and ethanol, and mixtures of any proportion of them. The optimal number of washing times is 2-3 times.

8. The method according to claim 3, characterized in that: The inert gas is any one of helium, nitrogen, argon or a mixed gas of any proportion of them.

9. The method according to claim 3, characterized in that: The drying time at a drying temperature of 50-80 °C in Step 5 is 5-18 h.

10. An application method of the copper sulfide - bismuth sulfide heterojunction nanosheet catalyst according to claim 1, characterized in that: It is used for catalytic electroreduction of CO2 to prepare formic acid under alkaline conditions.

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