Transition metal monatomic catalyst and preparation method and application thereof

Abstract

The invention relates to the technical field of electro-catalysis, in particular to a transition metal monatomic catalyst and a preparation method and application thereof. The transition metal monatomic catalyst takes transition metal monatomic as an active component and nitrogen-doped carbon nanofiber as a carrier; the transition metal monatomic catalyst has a hierarchical porous three-dimensional pore structure, the pore structure comprises micropores, mesopores and macropores, the pore volume of the micropores is 0.02-0.06 cm < 3 > / g, and the total pore volume of the mesopores and the macropores is 0.35-0.60 cm < 3 > / g; in the transition metal monatomic catalyst, the nitrogen content is 9 at.%-12 at.%. The transition metal monatomic catalyst prepared by the invention realizes electroreduction of CO2 into CO under industrial-grade current density, has high product selectivity and durability, solves the problem that the activity, selectivity and stability of the existing transition metal catalyst cannot be met at the same time, and has extremely high application value.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of electrocatalysis, in particular to a transition metal single-atom catalyst and a preparation method and application thereof. BACKGROUND

[0002] Among numerous ways of CO2 resource utilization, electroreduction of CO2 coupled with renewable electricity has attracted extensive attention due to its mild operating conditions and product diversity. Due to the extremely stable C=O double bond in CO2 molecule (bond energy up to 750 kJ / mol), the initial activation of CO2 molecule is often difficult. In addition, as the only competitive side reaction of CO2 electroreduction, the hydrogen evolution reaction is more favorable in solution. In recent years, researchers have developed a large number of electrocatalysts including metals and their oxides, single-atom catalysts, nano metals and molecular catalysts to solve the above problems, in order to achieve efficient CO2 electroreduction.

[0003] Among numerous electrocatalysts, single-atom electrocatalysts formed by anchoring transition metal atoms on nitrogen-doped carbon have shown excellent application prospects in the field of CO2 electroreduction due to their high atom utilization rate and simple synthesis steps. Although the CO selectivity of many transition metal single-atom catalysts can reach more than 90% now, the reduction activity is far from the industrial level (>300 mA cm -2 ). In addition, the long-term running stability of most transition metal single-atom electrocatalysts is not long, especially at a larger current density, which is often difficult to exceed several tens of hours.

[0004] In view of this, the present application is proposed. SUMMARY

[0005] In order to solve the above problems in the prior art, the present application provides a transition metal single-atom catalyst and a preparation method and application thereof. The transition metal single-atom catalyst has an industrial-level current density, high product selectivity and long-term running stability.

[0006] Based on this, the present application has the following technical solutions: In a first aspect, the present application provides a transition metal single-atom catalyst, which takes transition metal single atoms as active components and takes nitrogen-doped carbon nanofibers as carriers. The transition metal single-atom catalyst has a hierarchical porous three-dimensional pore structure, which includes micropores, mesopores and macropores. The micropore volume is 0.02-0.06 cm 3 / g, and the total pore volume of mesopores and macropores is 0.35-0.60 cm 3 / g. The nitrogen content in the transition metal single-atom catalyst is 9 at.%-12 at.%.

[0007] In the present application, the transition metal single-atom catalyst has a hierarchical porous three-dimensional pore structure, which is conducive to the exposure of active sites and the mass transfer of CO2 molecules; the large amount of nitrogen doping in the catalyst regulates the electronic structure of the central metal, which is conducive to the improvement of the intrinsic activity of the catalyst; the catalyst can realize the electrolysis of CO2 to CO at an industrial current density in an acidic system, and can be stably operated for at least 100 h.

[0008] Specifically, the transition metal single-atom catalyst provided by the present application is a transition metal single-atom as an active component and a hierarchical porous nitrogen-doped carbon nanofiber as a carrier. The micropores (<2 nm) in the hierarchical porous structure are conducive to improving the loading amount of the transition metal single-atom active component; the mesopores (2-50 nm) in the hierarchical porous structure are conducive to the exposure of the active component, thereby maximizing the utilization rate of the active component; the macropores (>50 nm) in the hierarchical porous structure are conducive to promoting the adsorption and transfer of the reactant CO2 molecules and reaction intermediates. The catalyst contains the above-mentioned content of electronegative nitrogen element doping, which regulates the electronic structure of the active component, optimizes the binding strength for the key intermediates, and is conducive to the improvement of the intrinsic activity of the catalyst.

[0009] According to the transition metal single-atom catalyst provided by the present application, the transition metal includes nickel, iron, cobalt, manganese or copper.

[0010] According to the transition metal single-atom catalyst provided by the present application, the diameter of the nitrogen-doped carbon nanofiber is between 500 and 600 nm.

[0011] According to the transition metal single-atom catalyst provided by the present application, the loading amount of the active component is 3wt.%~5wt.%.

[0012] In a second aspect, the present application provides a preparation method of the transition metal single-atom catalyst, comprising: S1: dissolving ZIF8 doped with a transition metal in a polyacrylonitrile solution to obtain a spinning solution; S2: electrospinning the spinning solution to obtain a nanofiber; S3: sequentially performing pre-oxidation treatment and carbonization treatment on the nanofiber.

[0013] The preparation method of the transition metal single atom provided by the application is characterized in that the transition metal-doped ZIF8 has a rhombohedron dodecahedron morphology and a controllable size, and will shrink in the subsequent high-temperature pyrolysis process, which is beneficial to the formation of macropores and mesopores; the transition metal-doped ZIF8 contains a large amount of low-boiling-point metal Zn, which is beneficial to the formation of micropores by evaporation at a high temperature (> 900 DEG C); the transition metal-doped ZIF8 is used as a spinning aid to form a hierarchical hierarchical pore morphology; in addition, the transition metal-doped ZIF8 is formed by doping transition metal while synthesizing ZIF8, transition metal ions replace part of Zn ions to coordinate with dimethyl imidazole, and transition metal preferentially occupies the surface of the ZIF8 crystal; in the subsequent high-temperature carbonization process, the transition metal is not easy to migrate and agglomerate under the anchoring of nitrogen elements in the ligand, thereby avoiding the formation of metal clusters and nanoparticles from the root, and the subsequent treatment to remove possible transition metal clusters and nanoparticles is not required in the preparation process, so that more transition metal single atoms can be doped and uniformly loaded; the particle size and the addition amount of the metal-doped ZIF8 can be controlled to realize the regulation of the pore structure of the catalyst, thereby maximizing the improvement of the catalytic activity, and the application has a good application prospect.

[0014] In step S1, the heating temperature during the dissolving of polyacrylonitrile is 40-80 DEG C, and the stirring time is 2-6h; the heating temperature during the dissolving of the transition metal-doped ZIF8 is 30-60 DEG C, and the stirring time is 10-24h.

[0015] According to the preparation method of the transition metal single atom catalyst provided by the application, the preparation method of the transition metal-doped ZIF8 comprises the following steps: 1) mixing a methanol solution of a metal salt and a methanol solution of dimethyl imidazole to obtain a mixed solution; wherein the metal salt comprises zinc nitrate hexahydrate and a transition metal salt; 2) placing the mixed solution at a constant temperature, and then sequentially performing centrifugation, washing and drying treatment.

[0016] In the application, in step 1), the metal salt can be dissolved in methanol by using ultrasonic or stirring, or the dimethyl imidazole can be dissolved in methanol, the stirring time is 10-20 min, and the ultrasonic time is 5-10 min.

[0017] In step 1), the stirring time during mixing is 20-30 min.

[0018] Further, in step 2) of the above technical solution, the temperature of the constant temperature is 30-60 DEG C; The standing time is 12-48h.

[0019] According to the application, a preparation method of a transition metal single-atom catalyst is provided, and a mass ratio of the polyacrylonitrile and the transition metal doped ZIF8 is 1: (0.5-2). Preferably, the polyacrylonitrile solution is a polyacrylonitrile dimethylformamide solution, and more preferably, a mass-volume ratio of the polyacrylonitrile and dimethylformamide is (0.08-0.125): 1 in g / mL.

[0020] According to the application, a preparation method of a transition metal single-atom catalyst is provided, and the transition metal salt includes nickel nitrate hexahydrate, nickel acetate tetrahydrate, nickel chloride hexahydrate, iron nitrate nonahydrate, cobalt nitrate hexahydrate, cobalt acetate tetrahydrate, manganese nitrate tetrahydrate, manganese acetate tetrahydrate, copper nitrate trihydrate, or copper acetate monohydrate.

[0021] Preferably, a molar ratio of the zinc nitrate hexahydrate and the transition metal salt is 1: (0.05-4).

[0022] Preferably, in the methanol solution of the metal salt, a ratio of the volume of methanol and the amount of substance of zinc nitrate hexahydrate is 300 ml: 0.01 mol-300 ml: 0.03 mol.

[0023] Preferably, a molar ratio of the dimethylimidazole and the zinc nitrate hexahydrate is (4-20): 1.

[0024] Preferably, in the methanol solution of the dimethylimidazole, a ratio of the volume of methanol and the amount of substance of dimethylimidazole is 100 ml: 0.6 mol-100 ml: 0.04 mol.

[0025] According to the application, a preparation method of a transition metal single-atom catalyst is provided, and the electrospinning condition includes that a voltage is 16-30 kV, and a feeding rate of the spinning solution is 0.5-0.8 ml / h. Preferably, a temperature of the pre-oxidation is 220-280℃. Preferably, a temperature of the carbonization is 900-1100℃.

[0026] Preferably, a pre-oxidation time is 60-120 min, and a carbonization time is 60-120 min.

[0027] In a third aspect, the application provides an application of the transition metal single-atom catalyst in preparation of carbon monoxide from carbon dioxide electro-reduction.

[0028] In a fourth aspect, the application provides a self-supporting electrode containing the transition metal single-atom catalyst, and preferably, the self-supporting electrode does not contain a binder.

[0029] The transition metal single-atom catalyst prepared by the method can be directly used as a self-supporting electrode, avoids the use of a binder, helps to improve the utilization rate of the catalyst, maximizes the three-phase interface, and avoids the stability problem caused by the falling of the binder.

[0030] Therefore, the technical scheme of the present application has the following beneficial effects: The transition metal single-atom catalyst provided by the present application is prepared through the processes of catalyst precursor preparation, electrostatic spinning precursor solution, pre-oxidized spinning fiber, and high-temperature carbonization of the pre-oxidized fiber in sequence. The prepared transition metal single-atom catalyst realizes the CO2 electro-reduction to CO under an industrial current density, and has high product selectivity and durability, solves the problem that the activity, selectivity, and stability of the existing transition metal catalyst cannot be satisfied simultaneously, and has extremely high application value. BRIEF DESCRIPTION OF DRAWINGS

[0031] In order to more clearly illustrate the technical solutions in the present application or the prior art, the following will briefly introduce the drawings needed to be used in the embodiments or the prior art description. Obviously, the drawings in the following description are some embodiments of the present application, and other drawings can be obtained by those skilled in the art without creative labor on the basis of these drawings.

[0032] Figure 1 The HR-TEM image of Example 1 provided by the present application.

[0033] Figure 2 The HR-TEM image of Comparative Example 1 provided by the present application.

[0034] Figure 3 The HAADF-STEM image of Example 1 provided by the present application.

[0035] Figure 4 The HAADF-STEM image of Comparative Example 1 provided by the present application.

[0036] Figure 5 The BET image of Example 1, Example 2, Example 3, and Comparative Example 1 provided by the present application.

[0037] Figure 6 The pore size distribution image of Example 1, Example 2, Example 3, and Comparative Example 1 provided by the present application.

[0038] Figure 7 The CO Faraday efficiency comparison image of Example 1, Example 2, Example 3, and Comparative Example 1 provided by the present application.

[0039] Figure 8Comparison of CO partial current density for Embodiment 1, Embodiment 2, Embodiment 3 and Comparative Example 1 provided by the present invention.

[0040] Figure 9 A comparison diagram of the full-cell polarization curves of Example 1 and Comparative Example 1 provided by the present invention.

[0041] Figure 10 A comparison chart of CO Faraday efficiency in full-cell tests of Example 1 and Comparative Example 1 provided by the present invention.

[0042] Figure 11 The diagram shows the full-cell stability test results for Embodiment 1 provided by this invention.

[0043] Figure 12 A comparison chart of CO Faraday efficiency between Example 4 and Comparative Example 2 provided by the present invention.

[0044] Figure 13 A comparison diagram of CO partial current density between Example 4 and Comparative Example 2 provided by the present invention. Detailed Implementation

[0045] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of this invention. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention.

[0046] Unless otherwise specified, all raw materials used in the examples and comparative examples are commercially available conventional raw materials, and the technical means used are conventional means well known to those skilled in the art.

[0047] Example 1 This embodiment provides a transition metal single-atom catalyst, the preparation method of which includes the following steps: First, add 2.98 g of zinc nitrate hexahydrate and 11.632 g of nickel nitrate hexahydrate to 300 ml of methanol, stir mechanically for 10 min, and then sonicate for 5 min to obtain mixed solution A. Then, add 16.42 g of dimethylimidazole to 100 ml of methanol solution, stir mechanically for 10 min, and then sonicate for 5 min to obtain mixed solution B.

[0048] Quickly pour solution B into solution A and mechanically stir for 20 minutes. Then place it in a forced-air drying oven and let it stand at 60°C for 24 hours.

[0049] After the reaction is completed, the precipitate is collected by centrifugation and the precipitate is washed with methanol for at least 3 times, and after the precipitate and the centrifugal supernatant are both free of green color, the solid product is placed in a vacuum oven at 60°C for drying for 12h to obtain a purple catalyst precursor Ni-ZIF8.

[0050] 0.8g of polyacrylonitrile is added to 8ml of dimethylformamide, and heated and stirred at 40°C for 4h to obtain a light yellow DMF solution of PAN, then 0.8g of Ni-ZIF8 powder is added thereto, and after stirring at 40°C for 12h, a spinning solution is obtained. The spinning solution is injected into a 5ml disposable syringe, and electrospinning is performed using an electrospinning machine (MECC NANON-01A), with a voltage of 20kV and a spinning solution feeding rate of 0.5ml / h, and an aluminum foil covered roller is used as a receiver. After the spinning is completed, the obtained nanofiber is placed in a muffle furnace and pre-oxidized at 230°C for 60min, and then the oxidized fiber is placed in a tube furnace, and heated to 900°C at a heating rate of 5°C / min under Ar atmosphere and kept for 60min, and after cooling to room temperature, a Ni-NCNF-Z catalyst is obtained.

[0051] The nitrogen content in the Ni-NCNF-Z catalyst obtained in this example is 9.84at.%; the pore structure of the Ni-NCNF-Z catalyst includes micropores, mesopores and macropores, wherein the micropore volume is 0.039cm 3 / g, and the total pore volume of the mesopores and macropores is 0.465cm 3 / g.

[0052] Example 2 This example provides a transition metal single-atom catalyst, and a preparation method thereof includes the following steps: First, 2.98 g of zinc nitrate hexahydrate and 9.954 g of nickel acetate tetrahydrate were added to 300 ml of methanol, and after mechanical stirring for 15 min, ultrasonic dispersion was performed for 10 min to obtain a mixed solution A. 16.42 g of dimethylimidazole was added to 100 ml of a methanol solution, and after mechanical stirring for 15 min, ultrasonic dispersion was performed for 10 min to obtain a mixed solution B. Solution B was quickly poured into solution A and mechanically stirred for 30 min, and then placed in a blast drying oven and left to stand at 50°C for 30 h. After the reaction was completed, the precipitate was collected by centrifugation and the precipitate was washed with methanol at least 3 times, and after the precipitate and the centrifugal supernatant were both free of green color, the solid product was placed in a vacuum oven and dried at 50°C for 24 h to obtain a purple catalyst precursor Ni-ZIF8. 0.8 g of polyacrylonitrile was added to 8 ml of dimethylformamide, and after heating and stirring at 50°C for 2 h, a DMF solution of PAN was obtained, and then 0.4 g of Ni-ZIF8 powder was added thereto, and after stirring at 50°C for 12 h, a spinning solution was obtained. The spinning solution was injected into a 5 ml disposable syringe, and an electrospinning machine (MECC NANON-01A) was used for electrospinning, with a voltage of 25 kV and a spinning solution feeding rate of 0.6 ml / h, and an aluminum foil-covered roller was used as a receiver. After the spinning was completed, the obtained nanofiber was placed in a muffle furnace and pre-oxidized at 240°C for 120 min to obtain an oxidized fiber, and then the oxidized fiber was placed in a tube furnace, and after being heated to 1000°C at a heating rate of 5°C / min under an Ar atmosphere and kept for 120 min, the Ni-NCNF-L catalyst was obtained after being cooled to room temperature.

[0053] The nitrogen content of the Ni-NCNF-L catalyst obtained in this example was 9.31 at.%; the pore structure of the Ni-NCNF-L catalyst included micropores, mesopores and macropores, wherein the micropore volume was 0.021 cm 3 / g, and the total pore volume of the mesopores and macropores was 0.3509 cm 3 / g.

[0054] Example 3 This example provides a transition metal single-atom catalyst, and a preparation method thereof includes the following steps: First, 2.98 g of zinc nitrate hexahydrate and 9.508 g of nickel chloride hexahydrate were added to 300 ml of methanol, and after mechanical stirring for 20 min, ultrasonic dispersion was performed for 8 min to obtain a mixed solution A. 16.42 g of dimethylimidazole was added to 100 ml of a methanol solution, and after mechanical stirring for 20 min, ultrasonic dispersion was performed for 8 min to obtain a mixed solution B. Solution B was quickly poured into solution A and mechanically stirred for 25 min, and then placed in a blast drying oven and left to stand at 60°C for 48 h. After the reaction was completed, the precipitate was collected by centrifugation and the precipitate was washed with methanol at least 3 times, and after the precipitate and the centrifugal supernatant were both free of green color, the solid product was placed in a vacuum oven and dried at 80°C for 20 h to obtain a purple catalyst precursor Ni-ZIF8. 0.8 g of polyacrylonitrile was added to 8 ml of dimethylformamide, and after heating and stirring at 80°C for 6 h, a DMF solution of PAN was obtained, and then 1.2 g of Ni-ZIF8 powder was added thereto, and after stirring at 50°C for 12 h, a spinning solution was obtained. The spinning solution was injected into a 5 ml disposable syringe, and an electrospinning machine (MECC NANON-01A) was used for electrospinning, with a voltage of 28 kV and a spinning solution feeding rate of 0.7 ml / h, and an aluminum foil-covered roller was used as a receiver. After the spinning was completed, the obtained nanofiber was placed in a muffle furnace and pre-oxidized at 280°C for 120 min to obtain an oxidized fiber, and then the oxidized fiber was placed in a tube furnace, and after being heated to 1000°C at a heating rate of 5°C / min under an Ar atmosphere and kept for 100 min, the Ni-NCNF-H catalyst was obtained after being cooled to room temperature.

[0055] The nitrogen content in the Ni-NCNF-H catalyst obtained in this example was 11.13 at.%; the pore structure of the Ni-NCNF-H catalyst included micropores, mesopores and macropores, wherein the micropore volume was 0.053 cm 3 / g, and the total pore volume of the mesopores and macropores was 0.5518 cm 3 / g.

[0056] Example 4 This example provides a transition metal single-atom catalyst, and a preparation method thereof includes the following steps: First, 2.98 g of zinc nitrate hexahydrate and 0.202 g of iron nitrate nonahydrate were added to 300 ml of methanol, and after mechanical stirring for 10 min, ultrasonic dispersion was performed for 10 min to obtain a mixed solution A. 3.448 g of dimethylimidazole was added to 100 ml of a methanol solution, and after mechanical stirring for 20 min, ultrasonic dispersion was performed for 10 min to obtain a mixed solution B. Solution B was quickly poured into solution A and mechanically stirred for 20 min, and then placed in a blast drying oven and left to stand at 60°C for 48 h. After the reaction was completed, the precipitate was collected by centrifugation and the precipitate was washed with methanol at least 3 times, and after the precipitate and the centrifugal supernatant were both free of yellow color, the solid product was placed in a vacuum oven and dried at 60°C for 12 h to obtain a nearly colorless catalyst precursor Fe-ZIF8. 0.8 g of polyacrylonitrile was added to 8 ml of dimethylformamide, and after heating and stirring at 60°C for 4 h, a PAN-dissolved DMF solution was obtained, and then 0.8 g of Fe-ZIF8 powder was added thereto, and after stirring at 50°C for 12 h, a spinning solution was obtained. The spinning solution was injected into a 5 ml disposable syringe, and an electrospinning machine (MECC NANON-01A) was used for electrospinning, with a voltage of 25 kV and a spinning solution feeding rate of 0.5 ml / h, and an aluminum foil-covered roller was used as a receiver. After the spinning was completed, the obtained nanofiber was placed in a muffle furnace and pre-oxidized at 230°C for 60 min to obtain an oxidized fiber, and then the oxidized fiber was placed in a tube furnace, and after being heated to 900°C at a heating rate of 5°C / min under an Ar atmosphere and kept for 120 min, the Fe-NCNF-Z catalyst was obtained after being cooled to room temperature.

[0057] The nitrogen content of the Fe-NCNF-Z catalyst obtained in this example was 9.55 at.%; the pore structure of the Fe-NCNF-Z catalyst included micropores, mesopores and macropores, wherein the micropore volume was 0.036 cm 3 / g, and the total pore volume of the mesopores and macropores was 0.507 cm 3 / g.

[0058] Comparative Example 1 This example provides a transition metal single-atom catalyst, and a preparation method thereof includes the following steps: A solution of PAN in DMF was prepared by dissolving 0.8 g of polyacrylonitrile in 8 ml of dimethylformamide at 40 °C under stirring for 4 h to obtain a light yellow solution. Then, 0.1 g of nickel nitrate hexahydrate was added into the solution, and the mixture was stirred at 40 °C for 12 h to obtain a spinning solution. The spinning solution was injected into a 5 ml disposable syringe, and electrospinning was performed using an electrospinning machine (MECC NANON-01A) at a voltage of 20 kV and a feeding rate of 0.5 ml / h. An aluminum foil covered drum was used as the receiver. After the electrospinning was completed, the obtained nanofibers were pre-oxidized in a muffle furnace at 230 °C for 60 min to obtain oxidized fibers. The oxidized fibers were then placed in a tube furnace, and the temperature was raised to 900 °C at a rate of 5 °C / min under an Ar atmosphere and maintained for 60 min. After the temperature was lowered to room temperature, a Ni-NCNF catalyst was obtained.

[0059] The nitrogen content of the Ni-NCNF catalyst obtained in the present example was 4.61 at.%. The pore structure of the Ni-NCNF catalyst included micropores, mesopores and macropores, wherein the pore volume of the micropores was 0 cm 3 / g, and the total pore volume of the mesopores and macropores was 0.1749 cm 3 / g.

[0060] Comparative Example 2 The present example provides a transition metal single-atom catalyst, and a preparation method thereof includes the following steps: A solution of PAN in DMF was prepared by dissolving 0.8 g of polyacrylonitrile in 8 ml of dimethylformamide at 40 °C under stirring for 4 h to obtain a light yellow solution. Then, 0.1 g of nickel nitrate hexahydrate was added into the solution, and the mixture was stirred at 40 °C for 12 h to obtain a spinning solution. The spinning solution was injected into a 5 ml disposable syringe, and electrospinning was performed using an electrospinning machine (MECC NANON-01A) at a voltage of 20 kV and a feeding rate of 0.5 ml / h. An aluminum foil covered drum was used as the receiver. After the electrospinning was completed, the obtained nanofibers were pre-oxidized in a muffle furnace at 230 °C for 60 min to obtain oxidized fibers. The oxidized fibers were then placed in a tube furnace, and the temperature was raised to 900 °C at a rate of 5 °C / min under an Ar atmosphere and maintained for 60 min. After the temperature was lowered to room temperature, a Ni-NCNF catalyst was obtained.

[0061] The HR-TEM image of Example 1 is shown in Figure 1 .

[0062] The HR-TEM image of Comparative Example 1 is shown in Figure 2 .

[0063] The HAADF-STEM image of Example 1 is shown in Figure 3 .

[0064] The HAADF-STEM image of Comparative Example 1 is shown in Figure 4 .

[0065] The BET image of Example 1 and Comparative Example 1 is shown in Figure 5 .

[0066] The pore size distribution image of Example 1 and Comparative Example 1 is shown in Figure 6 .

[0067] The CO Faraday efficiency comparison chart of Example 1, Example 2, and Example 3 is shown in Figure 7 .

[0068] The specific test method of CO Faraday efficiency is as follows: the CO Faraday efficiency of the catalyst under different applied potentials is tested in a conventional three-electrode system. The working electrode is the catalyst corresponding to the example or a self-supporting electrode, the reference electrode is an Ag / AgCl electrode with saturated KCl, the counter electrode is a Pt sheet electrode, the test electrolyte is 0.5M KHCO3 aqueous solution, and during the test, CO2 gas is continuously introduced into the electrolyte, the gas flow rate is 20sccm, and the electrolysis is operated at different potentials for 20min in a constant potential electrolysis mode. Before the electrolysis at each potential ends, the gas is introduced into the gas chromatograph online to check the concentration of CO in the product, so as to obtain the Faraday efficiency of CO.

[0069] The CO partial current density comparison chart of Example 1, Example 2, and Example 3 is shown in Figure 8 .

[0070] The test method of CO partial current density is as follows: the CO partial current density of the catalyst under different applied potentials is tested in a conventional three-electrode system. The working electrode is the catalyst corresponding to the example or a self-supporting electrode, the reference electrode is an Ag / AgCl electrode with saturated KCl, the counter electrode is a Pt sheet electrode, the test electrolyte is 0.5M KHCO3 aqueous solution, and during the test, CO2 gas is continuously introduced into the electrolyte, the gas flow rate is 20sccm, and the electrolysis is operated at different potentials for 20min in a constant potential electrolysis mode. Before the electrolysis at each potential ends, the gas is introduced into the gas chromatograph online to check the concentration of CO in the product, so as to obtain the Faraday efficiency of CO, and the current density at each potential multiplied by the CO Faraday efficiency at the corresponding potential can obtain the CO partial current density.

[0071] The full-cell polarization curve comparison chart of Example 1 and Comparative Example 1 is shown in Figure 9 .

[0072] The full cell polarization curve test method is as follows: the full cell is composed of two parts of cathode and anode, the cathode is a gas diffusion electrode coated with the catalyst of Example 1 or Comparative Example 1, the anode is an iridium-plated titanium felt, the cathode and the anode are separated by a Nafion 212 film, the cathode cavity is connected to dry CO2 gas, and the anode cavity is connected to 0.5M K2SO4 solution adjusted to pH = 1 with H2SO4. The polarization curve of the full cell is tested by using linear sweep curve.

[0073] The CO Faraday efficiency comparison chart of the full cell test of Example 1 and Comparative Example 1 is shown in Figure 10 .

[0074] The CO Faraday efficiency test method of the full cell test is as follows: the full cell is composed of two parts of cathode and anode, the cathode is a gas diffusion electrode coated with the catalyst of Example 1 or Comparative Example 1, the anode is an iridium-plated titanium felt, the cathode and the anode are separated by a Nafion 212 film, the cathode cavity is connected to dry CO2 gas, and the anode cavity is connected to 0.5M K2SO4 solution adjusted to pH = 1 with H2SO4. The method of constant current electrolysis is used to run at different current densities for 15min, and then the gas is tested online in the gas chromatograph before the end of electrolysis to test the concentration of CO, so as to obtain the CO Faraday efficiency.

[0075] The full cell stability test chart of Example 1 is shown in Figure 11 .

[0076] The full cell stability test method is as follows: the full cell is composed of two parts of cathode and anode, the cathode is a gas diffusion electrode coated with the catalyst prepared in Example 1, the anode is an iridium-plated titanium felt, the cathode and the anode are separated by a Nafion 212 film, the cathode cavity is connected to dry CO2 gas, and the anode cavity is connected to 0.5M K2SO4 solution adjusted to pH = 1 with H2SO4. The method of constant current electrolysis is used to run continuously at 400mA for 120h, and the gas is connected to the gas chromatograph online to detect the concentration of CO at intervals, so as to obtain the Faraday efficiency of CO.

[0077] The CO Faraday efficiency comparison chart of Example 4 and Comparative Example 2 is shown in Figure 12 .

[0078] The specific test method of CO Faraday efficiency is as follows: the CO Faraday efficiency of the catalyst at different applied potentials is tested in a conventional three-electrode system. The working electrode is the catalyst corresponding to Example 4 or Comparative Example 2 or a self-supporting electrode, the reference electrode is an Ag / AgCl electrode with saturated KCl, the counter electrode is a Pt sheet electrode, the test electrolyte is 0.5M KHCO3 aqueous solution, and during the test, CO2 gas is continuously introduced into the electrolyte at a flow rate of 20sccm. The electrolysis is carried out at different potentials for 20min, and before the electrolysis at each potential is completed, the gas is introduced on-line into a gas chromatograph to check the CO concentration in the product, so as to obtain the Faraday efficiency of CO.

[0079] The CO partial current density of Example 4 and Comparative Example 2 is shown in the following figure: Figure 13 .

[0080] The test method of CO partial current density is as follows: the CO partial current density of the catalyst at different applied potentials is tested in a conventional three-electrode system. The working electrode is the catalyst corresponding to Example 4 or Comparative Example 2 or a self-supporting electrode, the reference electrode is an Ag / AgCl electrode with saturated KCl, the counter electrode is a Pt sheet electrode, the test electrolyte is 0.5M KHCO3 aqueous solution, and during the test, CO2 gas is continuously introduced into the electrolyte at a flow rate of 20sccm. The electrolysis is carried out at different potentials for 20min, and before the electrolysis at each potential is completed, the gas is introduced on-line into a gas chromatograph to check the CO concentration in the product, so as to obtain the Faraday efficiency of CO, and the current density at each potential multiplied by the CO Faraday efficiency at the corresponding potential measured by the electrochemical workstation can obtain the CO partial current density.

[0081] Table 1

[0082] Table 2

[0083] Figure 1 It can be seen that the micro-morphology of the catalyst of Example 1 is nanofiber with a size of about 550nm, and there are obvious pore structures on the surface of the fiber, and no obvious metal nanoparticles appear.

[0084] Figure 2 It can be seen that the micro-morphology of the catalyst of Comparative Example 1 is nanofiber with a size of about 170nm, the surface of the fiber is smooth and almost no obvious pore structure can be observed, and no obvious metal nanoparticles appear.

[0085] Figure 3It can be seen that there are a large number of bright spots in the HAADF-STEM image of the catalyst of Example 1, which are uniformly dispersed and independent of each other, and can be attributed to the metal Ni with higher carbon contrast, and the uniform distribution of Ni monatomic on the surface of the photocatalyst.

[0086] Figure 4 It can be seen that there are also a large number of bright spots in the HAADF-STEM image of the catalyst of Comparative Example 1, which confirms the presence of Ni monatomic, and it can be obviously seen by comparison that Figure 3 the number of bright spots is more, indicating that the content of Ni monatomic in Example 1 is higher, which shows that the preparation method of the application can form more transition metal monatomic under the condition of the same amount of transition metal precursor.

[0087] Figure 5 It can be seen that the initial adsorption amount of Example 1, Example 2 and Example 3 in the low-pressure adsorption region (P / P0<0.1) is obviously improved, indicating that a large number of micropores exist in the three catalysts, and the initial adsorption amount of Comparative Example 1 is obviously smaller, indicating that the content of micropores is relatively small. In addition, Example 1, Example 2 and Example 3 appear obvious hysteresis loops in the medium-pressure adsorption region (0.5

[0088] Figure 6 The DFT pore size distribution of the catalysts can be seen that the pore structure of Example 1, Example 2 and Example 3 all contain micropores, mesopores and macropores, while Comparative Example 1 can only observe the existence of obvious mesopores and macropores, and the number of micropores, mesopores and macropores of the three examples is obviously more than that of Comparative Example 1. In addition, the micropore volume and (mesopore + macropore) pore volume of Example 1, Example 2 and Example 3 in Table 1 are obviously larger than those of Comparative Example 1.

[0089] Combined with the results of Figure 5 , Figure 6 and Table 1, the following conclusions can be drawn: after adding metal-doped ZIF8, the number of micropores, mesopores and macropores of the catalyst is obviously increased.

[0090] Figure 7 It can be seen that the CO faraday efficiency of Example 1, Example 2 and Example 3 in the whole potential range is better than that of Comparative Example 1, indicating that the CO selectivity of the catalyst after adding metal-doped ZIF8 is obviously increased.

[0091] Figure 8It can be seen that the CO partial current density of Examples 1, 2 and 3 is better than that of Comparative Example 1 in the entire potential range, indicating that the CO2 electroreduction activity of the catalyst is significantly increased after adding metal-doped ZIF8.

[0092] Figure 9 As can be seen, in the CO2 electrolysis test conducted in the acidic electrolytic device, the current density of Example 1 at the same potential is significantly better than that of Comparative Example 1. For example, at a full cell voltage of 3.5V, the current density of Example 1 reaches 1008 mAcm⁻¹. -2 Comparative Example 1 only had 365 mA cm⁻¹ -2 This indicates that the introduction of Ni-ZIF8 significantly enhances the CO2 electroreduction activity.

[0093] Figure 10 It can be seen that in the range of 100 to 600 mA cm -2 Product selectivity tests were conducted. The CO selectivity of Example 1 exceeded 80%, while Comparative Example 1 performed poorly, with the highest CO selectivity being less than 70%. This demonstrates that the introduction of Ni-ZIF8 significantly improves the CO Faraday efficiency under acidic CO2 electrolysis.

[0094] Figure 11 As can be seen, Example 1 can operate stably for at least 120 hours under constant current electrolysis test at 400mA, with a CO Faraday efficiency of over 80% and the full cell voltage showing virtually no significant change, demonstrating the excellent stability of the catalyst.

[0095] Figure 12 It can be seen that the CO Faraday efficiency of Example 4 is better than that of Comparative Example 2 across the entire voltage range, indicating the positive effect of the introduction of Fe-ZIF8 on CO selectivity and proving the universality of the improvement of CO selectivity by metal doping ZIF8.

[0096] Figure 13 It can be seen that the CO partial current density in Example 4 is better than that in Comparative Example 2 across the entire voltage range, indicating that the introduction of Fe-ZIF8 can improve the CO2 electroreduction activity of the catalyst, further demonstrating the universality of metal doping ZIF8 in improving CO2 reduction activity.

[0097] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A transition metal single-atom catalyst, characterized in that, Transition metal single atoms are used as active components, and nitrogen-doped carbon nanofibers are used as carriers; The transition metal single-atom catalyst has a hierarchical porous three-dimensional pore structure, which includes micropores, mesopores, and macropores, wherein the pore volume of the micropores is 0.02~0.06 cm³. 3 / g, the total pore volume of mesopores and macropores is 0.35~0.60cm³. 3 / g; The nitrogen content in the transition metal single-atom catalyst is 9 at.% to 12 at.%.

2. The transition metal single-atom catalyst according to claim 1, characterized in that, The transition metals include nickel, iron, cobalt, manganese, or copper; The nitrogen-doped carbon nanofibers have a diameter between 500 and 600 nm.

3. The transition metal single-atom catalyst according to claim 1 or 2, characterized in that, The loading of the active component is 3 wt.% to 5 wt.%.

4. The method for preparing the transition metal single-atom catalyst according to any one of claims 1 to 3, characterized in that, include: S1: Dissolve ZIF8 doped with transition metal in a polyacrylonitrile solution to obtain a spinning solution; S2: Electrospinning the spinning solution to obtain nanofibers; S3: The nanofibers are subjected to pre-oxidation and carbonization treatments in sequence.

5. The method for preparing a transition metal single-atom catalyst according to claim 4, characterized in that, The preparation method of the ZIF8 doped with transition metal includes the following steps: 1) A methanol solution of a metal salt is mixed with a methanol solution of dimethylimidazole to obtain a mixed solution; wherein the metal salt includes zinc nitrate hexahydrate and a transition metal salt; 2) The mixed solution was allowed to stand at a constant temperature, and then centrifuged, washed and dried in sequence.

6. The method for preparing a transition metal single-atom catalyst according to claim 4 or 5, characterized in that, The mass ratio of the polyacrylonitrile to the transition metal-doped ZIF8 is 1:(0.5~2). Preferably, the polyacrylonitrile solution is a polyacrylonitrile-dimethylformamide solution; more preferably, the mass-volume ratio of the polyacrylonitrile and dimethylformamide is (0.08~0.125):1, calculated in g / mL.

7. The method for preparing a transition metal single-atom catalyst according to any one of claims 4 to 6, characterized in that, The transition metal salts include nickel nitrate hexahydrate, nickel acetate tetrahydrate, nickel chloride hexahydrate, ferric nitrate nonahydrate, cobalt nitrate hexahydrate, cobalt acetate tetrahydrate, manganese nitrate tetrahydrate, manganese acetate tetrahydrate, copper nitrate trihydrate, or copper acetate monohydrate. Preferably, the molar ratio of zinc nitrate hexahydrate to the transition metal salt is 1:(0.05~4). Preferably, in the methanol solution of the metal salt, the ratio of the volume of methanol to the molar amount of zinc nitrate hexahydrate is 300 ml: 0.01 mol to 300 ml: 0.03 mol. Preferably, the molar ratio of dimethylimidazole to zinc nitrate hexahydrate is (4~20):1; Preferably, in the methanol solution of dimethylimidazole, the ratio of methanol volume to dimethylimidazole molar is 100ml:0.6mol to 100ml:0.04mol.

8. The method for preparing a transition metal single-atom catalyst according to any one of claims 4 to 7, characterized in that, The conditions for electrospinning include: a voltage of 16~30kV; and a spinning solution feed rate of 0.5~0.8ml / h. Preferably, the pre-oxidation temperature is 220~280℃; Preferably, the carbonization temperature is 900~1100℃.

9. The application of the transition metal single-atom catalyst according to claims 1-3 or the transition metal single-atom catalyst prepared by the preparation method according to claims 4-8 in the electroreduction of carbon dioxide to prepare carbon monoxide.

10. A self-supporting electrode, characterized in that, It contains the transition metal single-atom catalyst according to claims 1 to 3 or the transition metal single-atom catalyst prepared by the preparation method according to claims 4 to 8; preferably, it does not contain a binder.

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