Membrane-packaged sulfur-mediated defect-rich NiFe layered hydroxide electrocatalyst, preparation method, electrode and application

Abstract

The invention discloses a membrane-packaged sulfur-mediated defect-rich NiFe layered hydroxide electrocatalyst, a preparation method, an electrode and application, and belongs to the technical field of electrocatalytic reduction of nitrate to prepare ammonia, the preparation method comprises the following steps: S1, placing foamy copper in a mixed solution of a strong oxidant and alkali to obtain Cu (OH) 2-loaded foamy copper, and pyrolyzing to obtain CuO NW; s2, putting the CuO NW into a mixed solution of nickel salt, ferric salt, a sulfur source, urea and water, and carrying out heating reaction to obtain S-NiFe-LDH coated CuO NW; s3, the S-NiFe-LDH (at) CuO NW is heated in inert gas, and S-D-NiFe-LDH (at) CuO NW is obtained; and S4, carrying out membrane packaging on the S-D-NiFe-LDH coated CuO NW by utilizing polyvinylidene fluoride, polyvinylpyrrolidone and N, N-dimethylformamide through phase inversion, so as to obtain the electrocatalyst. The electrocatalyst is used in the nitrate reduction process, and plays an important role in solving the problems of nitrate pollution and sustainable ammonia production.

Description

Technical Field

[0001] The present invention belongs to the technical field of electrocatalytic reduction of nitrate to ammonia, and relates to an electrocatalyst, in particular to a membrane-encapsulated sulfur-mediated defective NiFe layered double hydroxide electrocatalyst, a preparation method, an electrode and an application thereof. Background Art

[0002] Nitrate pollution is a global environmental problem. The electrocatalytic nitrate reduction reaction can convert nitrate into ammonia, which not only solves the nitrate pollution problem but also realizes sustainable ammonia production. The electrocatalytic reduction of nitrate to ammonia reaction (NO3-RR) plays an important role in solving nitrate pollution and sustainable ammonia production. However, under neutral conditions, due to insufficient proton supply and competitive hydrogen evolution reaction, the yield and Faraday efficiency of ammonia are limited. Therefore, regulating the generation and binding of active hydrogen is an important strategy to improve the activity of NO3-RR.

[0003] Two-dimensional transition metal layered double hydroxide (TM-LDH) nanosheets are considered to be efficient HER electrocatalysts due to their high catalytic activity, low cost and high stability. However, their inherent semiconductor properties and layered structure limit electron transfer, thereby restricting their electrocatalytic activity. Research shows that introducing defects can change the surface electron configuration of LDHs, generate mid-gap electron states, thus enhancing electron migration, promoting the cleavage of H-OH bonds and regulating the release of active hydrogen, breaking the bottleneck of limited active hydrogen supply under neutral conditions. In addition, the introduction of defects increases the number of dangling bonds and generates more unsaturated coordination sites, which is beneficial to the reconstruction and adsorption of intermediates in the NO3-RR process. However, these studies usually achieve high Faraday efficiency at high nitrate concentrations, but lead to serious hydrogen evolution reaction at low concentrations. Moreover, a large number of co-existing pollutants in sewage water bodies will inhibit the activity of reaction sites. For example, natural organic matters such as humic substances cover the active sites of the catalyst, and phosphates will bind to the metal active sites on the catalyst surface, resulting in catalyst poisoning, making it difficult to maintain the catalytic activity and seriously affecting the NO3-RR performance. Therefore, it is crucial to promote defect generation and regulate the inherent activity of defects to balance the generation and binding of active hydrogen, as well as improve the anti-interference performance of the catalyst, thereby enhancing the activity and stability of NO3-RR. Summary of the Invention

[0004] The present invention provides a membrane-encapsulated sulfur-mediated defective NiFe layered double hydroxide electrocatalyst, a preparation method, an electrode and an application thereof to overcome the defects of the prior art.

[0005] To achieve the above object, the present invention adopts the following technical solutions:

[0006] In a first aspect, the present invention provides a method for preparing a membrane-encapsulated sulfur-mediated defective NiFe layered hydroxide electrocatalyst, comprising the following steps:

[0007] S1. Preparation of copper foam loaded with CuO: Place copper foam in a mixed solution of a strong oxidant and an alkali to obtain copper foam loaded with Cu(OH)2, and pyrolyze it to obtain CuO NW, that is, copper foam loaded with CuO;

[0008] S2. Preparation of S-doped CuO NW loaded with NiFe-LDH: Place CuO NW in a mixed solution of a nickel salt, an iron salt, a sulfur source, urea and water, and heat and react to obtain S-NiFe-LDH@CuO NW, that is, S-doped NiFe-LDH@CuO NW;

[0009] S3. Preparation of S-doped defective NiFe-LDH@CuO NW: Heat S-NiFe-LDH@CuO NW in an inert gas to obtain S-D-NiFe-LDH@CuO NW, that is, S-doped defective NiFe-LDH@CuO NW;

[0010] S4. Preparation of membrane-encapsulated S-D-NiFe-LDH@CuO NW: Use polyvinylidene fluoride, polyvinylpyrrolidone and N,N-dimethylformamide to perform membrane encapsulation on S-D-NiFe-LDH@CuO NW by phase inversion to obtain an electrocatalyst.

[0011] Further, in S1, the strong oxidant is chromic acid or ammonium persulfate; the alkali is potassium hydroxide or sodium hydroxide; the molar ratio of the strong oxidant to the alkali is 1:3 to 50; the loading time of the copper foam is 0.5 to 2 h; the pyrolysis temperature is 250 to 350 °C, and the pyrolysis time is 2.5 to 3.5 h.

[0012] Further, the specific method of S2 is as follows: Add a nickel salt, an iron salt, a sulfur source and urea to water and stir for 30 to 90 min to obtain a mixed solution; put the CuO NW obtained in S1 into the mixed solution, and place it at 85 to 95 °C for 8 to 16 h; wash the reaction product with ethanol and water, and vacuum dry it at 50 to 70 °C for 8 to 10 h to obtain S-NiFe-LDH@CuO NW.

[0013] Further, in S2, the nickel salt is Ni(NO3)2·6H2O; the iron salt is Fe(NO3)3·9H2O; the sulfur source is a thiosulfate; the thiosulfate is potassium thiosulfate or sodium thiosulfate; the molar ratio of Ni(NO3)2·6H2O, Fe(NO3)3·9H2O, the thiosulfate and urea is 1 to 3:1 to 1.5:0.025 to 0.1:10 to 15, preferably 2:1.25:0.025 to 0.1:12.

[0014] Further, in S3, the inert gas is argon or helium; the heating temperature is 350 - 500 K, and the heating time is 1.5 - 2.5 h.

[0015] Further, in S4, the dosage ratio of polyvinylidene fluoride, polyvinylpyrrolidone, and N,N-dimethylformamide is 5 - 10 g : 0.03 - 0.2 g : 50 mL.

[0016] Further, the specific method of S4 is as follows: The mixed solution of polyvinylidene fluoride, polyvinylpyrrolidone, and N,N-dimethylformamide is evenly cast on the S-D-NiFe-LDH@CuO NW obtained in S3, and the S-D-NiFe-LDH@CuO NW is fixed on the perforated stainless steel plate with a casting knife; then the gap of the casting knife is adjusted, and then the casting solution is applied again; immediately afterwards, the stainless steel plate is immersed in water, and the membrane-encapsulated ME-St-D-NiFe-LDH@CuO NW is separated; it is soaked in water to remove the residual solvent, and the electrocatalyst is obtained.

[0017] In a second aspect, the present invention also provides a membrane-encapsulated sulfur-mediated defective NiFe layered hydroxide electrocatalyst prepared by the above preparation method.

[0018] In a third aspect, the present invention also provides an electrode comprising the above membrane-encapsulated sulfur-mediated defective NiFe layered hydroxide electrocatalyst.

[0019] Specifically, the electrocatalyst is mixed with acetylene black and polytetrafluoroethylene in absolute ethanol to obtain an electrode homogenate. The loading amount of the electrode is 3 - 5 mg / cm 2 .

[0020] In a fourth aspect, the present invention also provides the application of the above membrane-encapsulated sulfur-mediated defective NiFe layered hydroxide electrocatalyst in nitrate-to-ammonia production.

[0021] The beneficial effects of the present invention are as follows:

[0022] First, the present invention induces lattice distortion of NiFe-LDH through sulfur doping, promotes the generation of defects, optimizes the balance between active hydrogen generation and binding, and improves the NO3-RR activity.

[0023] Second, sulfur-mediated defects cause the d-band center displacement of Ni and Fe sites, effectively promoting the enrichment of active hydrogen and inhibiting the binding of active hydrogen. These defects construct a local environment enriched in NO3 - and H + , realizing the rapid hydrogenation of *NO and ensuring a high nitrate reduction reaction activity.

[0024] III. The present invention blocks the influence of background pollutants such as phosphates and humic substances through membrane encapsulation, improves the cyclic stability performance of the electrode in actual wastewater, and has practical application potential. Description of the Drawings

[0025] Figure 1 are the scanning electron microscope images of the preparation materials of Examples 1-5 and Comparative Examples 1-4;

[0026] Figure 2 are the XRD patterns of the preparation materials of Examples 1-4 and Comparative Example 3;

[0027] Figure 3 are the ESR spectra of the preparation materials of Examples 1-4 and Comparative Examples 1-4;

[0028] Figure 4 are the ammonia production rate and Faraday efficiency diagrams of the preparation materials of Examples 1-4 and Comparative Examples 1-4 during the adsorption electrocatalysis process;

[0029] Figure 5 are the density of states diagrams of the preparation materials of Examples 1-4 and Comparative Examples 1-4;

[0030] Figure 6 are the Faraday efficiency and ammonia production rate diagrams of the preparation materials of Examples 3 and 5 during 20 consecutive cycles at -0.4V vs. RHE in the actual industrial wastewater process. Detailed Description of the Invention

[0031] The present invention will be further described below in conjunction with specific embodiments.

[0032] Example 1

[0033] This example provides a sulfur-mediated defective NiFe layered hydroxide electrocatalyst for nitrate to ammonia production, and the preparation method includes the following steps:

[0034] S1. Preparation of copper foam loaded with CuO:

[0035] Clean 5 cm × 5 cm copper foam in a 5 mol / L hydrochloric acid solution in an ultrasonic bath for 60 min, and then wash it several times with ethanol and deionized water. Vertically place the copper foam in a mixture of 3 mol / L (NH4)2S2O8 solution and 10 mol / L KOH solution at room temperature for 60 min to form a Cu(OH)2 nanowire array loaded on the copper foam, i.e., Cu(OH)2 NW. Pyrolyze Cu(OH)2 NW in an argon atmosphere at 300 °C at a rate of 5 °C / min for 3 h to obtain CuO NW.

[0036] S2. Preparation of S-doped CuO NW loaded with NiFe-LDH:

[0037] 2 mmol of Ni(NO3)2·6H2O, 1.25 mmol of Fe(NO3)3·9H2O, 0.025 mmol of Na2S2O3 and 12 mmol of urea were added to 100 mL of deionized water and stirred for 40 min. The solution was transferred to a Teflon reactor and the CuO NW was placed therein for 15 h at 90 °C. The reaction product was repeatedly washed with deionized water and absolute ethanol and dried to obtain S1-NiFe-LDH@CuO NW.

[0038] S3. Preparation of S-doped defective NiFe-LDH@CuO NW:

[0039] S1-D-NiFe-LDH@CuO NW was obtained by heating S1-NiFe-LDH@CuO NW to 473 K at a rate of 1 °C / min in a H2 / Ar atmosphere, holding for 2 h, and then cooling to room temperature.

[0040] Example 2

[0041] This example provides a sulfur-mediated defective NiFe layered hydroxide electrocatalyst for ammonia production from nitrate, and the preparation method includes the following steps:

[0042] S1. Preparation of CuO-loaded copper foam:

[0043] The 5 cm×5 cm copper foam was cleaned with 5 mol / L hydrochloric acid in an ultrasonic bath for 60 min, and then washed several times with ethanol and deionized water. The copper foam was vertically placed in a mixture of 3 mol / L (NH4)2S2O8 solution and 10 mol / L KOH solution at room temperature for 60 min to form a Cu(OH)2 nanorod array loaded on the copper foam, namely Cu(OH)2NW. Cu(OH)2NW was pyrolyzed in an argon atmosphere at 300 °C at a rate of 5 °C / min for 3 h to obtain CuO NW.

[0044] S2. Preparation of S-doped NiFe-LDH-loaded CuO NW:

[0045] 2 mmol of Ni(NO3)2·6H2O, 1.25 mmol of Fe(NO3)3·9H2O, 0.050 mmol of Na2S2O3 and 12 mmol of urea were added to 100 mL of deionized water and stirred for 40 min. The solution was transferred to a Teflon reactor and the CuO NW was placed therein for 15 h at 90 °C. The reaction product was repeatedly washed with deionized water and absolute ethanol and dried to obtain S2-NiFe-LDH@CuO NW.

[0046] S3. Preparation of S-doped defective NiFe-LDH@CuO NW:

[0047] The S2-NiFe-LDH@CuO NW was heated to 473 K at a rate of 1 °C / min in an H2 / Ar atmosphere, held for 2 h, and then cooled to room temperature to obtain S2-D-NiFe-LDH@CuO NW.

[0048] Example 3

[0049] This example provides a sulfur-mediated defective NiFe layered hydroxide electrocatalyst for nitrate-to-ammonia production, and the preparation method includes the following steps:

[0050] S1. Preparation of copper foam loaded with CuO:

[0051] The 5 cm × 5 cm copper foam was cleaned with 5 mol / L hydrochloric acid in an ultrasonic bath for 60 min, and then washed several times with ethanol and deionized water. The copper foam was vertically placed in a mixture of 3 mol / L (NH4)2S2O8 solution and 10 mol / L KOH solution at room temperature for 60 min to form a Cu(OH)2 nanowire array loaded on the copper foam, namely Cu(OH)2 NW. The Cu(OH)2 NW was pyrolyzed in an argon atmosphere at 300 °C at a rate of 5 °C / min for 3 h to obtain CuO NW.

[0052] S2. Preparation of S-doped CuO NW loaded with NiFe-LDH:

[0053] 2 mmol Ni(NO3)2·6H2O, 1.25 mmol Fe(NO3)3·9H2O, 0.075 mmol Na2S2O3 and 12 mmol urea were added to 100 mL of deionized water and stirred for 40 min. The solution was transferred to a Teflon reactor, and the CuO NW was placed therein for 15 h at a temperature of 90 °C. The reaction product was repeatedly washed with deionized water and absolute ethanol and dried to obtain S3-NiFe-LDH@CuO NW.

[0054] S3. Preparation of S-doped and defect-induced NiFe-LDH@CuO NW:

[0055] The S3-NiFe-LDH@CuO NW was heated to 473 K at a rate of 1 °C / min in an H2 / Ar atmosphere, held for 2 h, and then cooled to room temperature to obtain S3-D-NiFe-LDH@CuO NW.

[0056] Example 4

[0057] This example provides a sulfur-mediated defective NiFe layered hydroxide electrocatalyst for nitrate-to-ammonia production, and the preparation method includes the following steps:

[0058] S1. Preparation of Copper Foam Loaded with CuO:

[0059] Clean a 5 cm × 5 cm copper foam in 5 mol / L hydrochloric acid under an ultrasonic bath for 60 min, and then wash it several times with ethanol and deionized water. Vertically place the copper foam in a mixture of 3 mol / L (NH4)2S2O8 solution and 10 mol / L KOH solution at room temperature for 60 min to form a Cu(OH)2 nanowire array loaded on the copper foam, namely Cu(OH)2 NW. Pyrolyze Cu(OH)2 NW in an argon atmosphere at 300 °C with a rate of 5 °C / min for 3 h to obtain CuO NW.

[0060] S2. Preparation of S-Doped NiFe-LDH@CuO NW Loaded on CuO NW:

[0061] Add 2 mmol Ni(NO3)2·6H2O, 1.25 mmol Fe(NO3)3·9H2O, 0.10 mmol Na2S2O3, and 12 mmol urea into 100 mL deionized water and stir for 40 min. Transfer the solution to a Teflon reactor and place CuO NW in it for 15 h at a temperature of 90 °C. Wash the reaction product repeatedly with deionized water and absolute ethanol and dry it to obtain S4-NiFe-LDH@CuO NW.

[0062] S3. Preparation of S-Doped Defect-Induced NiFe-LDH@CuO NW:

[0063] Heat S4-NiFe-LDH@CuO NW in a H2 / Ar atmosphere to 473 K at a rate of 1 °C / min, hold for 2 h, and then cool to room temperature to obtain S4-D-NiFe-LDH@CuO NW.

[0064] Example 5

[0065] This example provides a membrane-encapsulated sulfur-mediated defective NiFe layered hydroxide electrocatalyst for ammonia production from nitrate, and the preparation method includes the following steps:

[0066] S1. Preparation of Copper Foam Loaded with CuO:

[0067] Clean the 5 cm × 5 cm copper foam with 5 mol / L hydrochloric acid under an ultrasonic bath for 60 min, and then wash it several times with ethanol and deionized water. Vertically place the copper foam in a mixture of 3 mol / L (NH4)2S2O8 solution and 10 mol / L KOH solution at room temperature for 60 min to form a Cu(OH)2 nanowire array loaded on the copper foam, namely Cu(OH)2 NW. Pyrolyze Cu(OH)2 NW in an argon atmosphere at 300 °C with a rate of 5 °C / min for 3 h to obtain CuO NW.

[0068] Preparation of S2, S-doped NiFe-LDH@CuO NW:

[0069] Add 2 mmol Ni(NO3)2·6H2O, 1.25 mmol Fe(NO3)3·9H2O, 0.075 mmol Na2S2O3 and 12 mmol urea to 100 mL of deionized water and stir for 40 min. Transfer the solution to a Teflon reactor and place CuO NW in it for 15 h at a temperature of 90 °C. Wash the reaction product repeatedly with deionized water and absolute ethanol and dry it to obtain S-NiFe-LDH@CuO NW.

[0070] Preparation of S3, S-doped NiFe-LDH@CuO NW with induced defects:

[0071] Heat S-NiFe-LDH@CuO NW in a H2 / Ar atmosphere at a rate of 1 °C / min to 473 K, hold for 2 h, and then cool to room temperature to obtain S-D-NiFe-LDH@CuO NW.

[0072] Preparation of S4, membrane-encapsulated S-D-NiFe-LDH@CuO NW:

[0073] Mix 7 g of polyvinylidene fluoride, 0.05 g of polyvinylpyrrolidone and 50 mL of N,N-dimethylformamide, and evenly cast it on S-NiFe-LDH@CuO NW. Fix S-NiFe-LDH@CuO NW on a perforated stainless steel plate with a casting knife. Subsequently, adjust the gap of the casting knife and then apply the casting solution again. Then, immediately immerse the stainless steel plate in deionized water to obtain ME-S-D-NiFe-LDH@CuO NW. Soak the prepared material in deionized water for 48 h to remove the residual solvent and store it at 4 - 6 °C for later use.

[0074] Comparative Example 1

[0075] This comparative example provides a NiFe layered hydroxide electrocatalyst for ammonia production from nitrate. The preparation method includes the following steps:

[0076] S1. Preparation of copper foam supported with CuO:

[0077] Clean a 5 cm × 5 cm copper foam in a 5 mol / L hydrochloric acid solution under an ultrasonic bath for 60 min, and then wash it several times with ethanol and deionized water. Vertically place the copper foam in a mixture of 3 mol / L (NH4)2S2O8 solution and 10 mol / L KOH solution at room temperature for 60 min to form a Cu(OH)2 nanowire array supported on the copper foam, namely Cu(OH)2 NW. Pyrolyze Cu(OH)2 NW in an argon atmosphere at 300 °C with a rate of 5 °C / min for 3 h to obtain CuO NW.

[0078] S2. Preparation of NiFe-LDH@CuO NW:

[0079] Add 2 mmol Ni(NO3)2·6H2O, 1.25 mmol Fe(NO3)3·9H2O, and 12 mmol urea into 100 mL deionized water and stir for 40 min. Transfer the solution to a Teflon reactor and place CuO NW in it for 15 h at 90 °C. Wash the reaction product repeatedly with deionized water and absolute ethanol and dry it to obtain NiFe-LDH@CuO NW.

[0080] S3. Heat NiFe-LDH@CuO NW to 473 K at a rate of 1 °C / min in an H2 / Ar atmosphere, hold for 0.5 h, and then cool to room temperature to obtain D1-NiFe-LDH@CuO NW.

[0081] Comparative Example 2

[0082] This comparative example provides a NiFe layered hydroxide electrocatalyst for ammonia production from nitrate, and the preparation method includes the following steps:

[0083] S1. Preparation of copper foam supported with CuO:

[0084] Clean a 5 cm × 5 cm copper foam in a 5 mol / L hydrochloric acid solution under an ultrasonic bath for 60 min, and then wash it several times with ethanol and deionized water. Vertically place the copper foam in a mixture of 3 mol / L (NH4)2S2O8 solution and 10 mol / L KOH solution at room temperature for 60 min to form a Cu(OH)2 nanowire array supported on the copper foam, namely Cu(OH)2 NW. Pyrolyze Cu(OH)2 NW in an argon atmosphere at 300 °C with a rate of 5 °C / min for 3 h to obtain CuO NW.

[0085] S2. Preparation of NiFe-LDH@CuO NW:

[0086] 2 mmol of Ni(NO3)2·6H2O, 1.25 mmol of Fe(NO3)3·9H2O and 12 mmol of urea were added to 100 mL of deionized water and stirred for 40 min. The solution was transferred to a Teflon reactor and CuO NW was placed therein for 15 h at 90 °C. The reaction product was washed repeatedly with deionized water and absolute ethanol and dried to obtain NiFe-LDH@CuO NW.

[0087] S3. NiFe-LDH@CuO NW was heated to 473 K at a rate of 1 °C / min in an H2 / Ar atmosphere, held for 1 h, and then cooled to room temperature to obtain D2-NiFe-LDH@CuO NW.

[0088] Comparative Example 3

[0089] This comparative example provides a NiFe layered hydroxide electrocatalyst for ammonia production from nitrate, and the preparation method includes the following steps:

[0090] S1. Preparation of copper foam loaded with CuO:

[0091] The 5 cm × 5 cm copper foam was cleaned with 5 mol / L hydrochloric acid in an ultrasonic bath for 60 min, and then washed several times with ethanol and deionized water. The copper foam was vertically placed in a mixture of 3 mol / L (NH4)2S2O8 solution and 10 mol / L KOH solution at room temperature for 60 min to form a Cu(OH)2 nanowire array loaded on the copper foam, namely Cu(OH)2NW. Cu(OH)2NW was pyrolyzed in an argon atmosphere at 300 °C at a rate of 5 °C / min for 3 h to obtain CuO NW.

[0092] S2. Preparation of CuO NW loaded with NiFe-LDH:

[0093] 2 mmol of Ni(NO3)2·6H2O, 1.25 mmol of Fe(NO3)3·9H2O and 12 mmol of urea were added to 100 mL of deionized water and stirred for 40 min. The solution was transferred to a Teflon reactor and CuO NW was placed therein for 15 h at 90 °C. The reaction product was washed repeatedly with deionized water and absolute ethanol and dried to obtain NiFe-LDH@CuO NW.

[0094] S3. NiFe-LDH@CuO NW was heated to 473 K at a rate of 1 °C / min in an H2 / Ar atmosphere, held for 1.5 h, and then cooled to room temperature to obtain D3-NiFe-LDH@CuO NW.

[0095] Comparative Example 4

[0096] This comparative example provides a NiFe layered hydroxide electrocatalyst for ammonia production from nitrate, and the preparation method includes the following steps:

[0097] S1. Preparation of copper foam loaded with CuO:

[0098] Clean 5 cm×5 cm copper foam in a 5 mol / L hydrochloric acid solution in an ultrasonic bath for 60 min, and then wash it several times with ethanol and deionized water. Vertically place the copper foam in a mixture of 3 mol / L (NH4)2S2O8 solution and 10 mol / L KOH solution at room temperature for 60 min to form a Cu(OH)2 nanowire array loaded on the copper foam, namely Cu(OH)2 NW. Pyrolyze Cu(OH)2 NW in an argon atmosphere at 300 °C at a rate of 5 °C / min for 3 h to obtain CuO NW.

[0099] S2. Preparation of CuO NW loaded with NiFe-LDH:

[0100] Add 2 mmol Ni(NO3)2·6H2O, 1.25 mmol Fe(NO3)3·9H2O, and 12 mmol urea to 100 mL of deionized water and stir for 40 min. Transfer the solution to a Teflon reactor and place CuO NW in it for 15 h at 90 °C. Wash the reaction product repeatedly with deionized water and absolute ethanol and dry it to obtain NiFe-LDH@CuO NW.

[0101] S3. Heat NiFe-LDH@CuO NW to 473 K at a rate of 1 °C / min in a H2 / Ar atmosphere, hold for 2 h, and then cool to room temperature to obtain D4-NiFe-LDH@CuO NW.

[0102] The scanning electron microscope images of the materials prepared in Examples 1-5 and Comparative Examples 1-4 are as Figure 1 shown. The scanning electron microscope images show that when doped with sulfur, the nanosheet structure of NiFe layered hydroxide remains basically unchanged. However, after magnification, it is found that the surface of the sulfur-doped material becomes rough, and there are more defect sites and disordered regions, which are specifically reflected in the disordered and mismatched LDH lattice. By comparing the scanning electron microscope images before and after encapsulation, it can be found that when the nanosheets are encapsulated in the membrane, a closed microreactor is formed, which protects the material to a certain extent. Compared with the unencapsulated material, the anti-interference ability is significantly improved, the performance stability is optimized, and at the same time, the mass transfer of nitrate is enhanced, making the material have high catalytic and adsorption properties.

[0103] The XRD patterns of the materials prepared in Examples 1-4 and Comparative Example 3 are as Figure 2As shown. The XRD patterns show the diffraction peak positions and intensities of different samples, from which it can be judged whether the crystal structure of the material has changed. It is found in the figure that sulfur doping weakens the crystallinity of the NiFe-LDH phase, specifically manifested as the broadening of the full width at half maximum of the diffraction peak and the decrease in peak intensity, which is due to the lattice structure distortion caused by sulfur doping, thus leading to the formation of defects.

[0104] The ESR spectra of the materials prepared in Examples 1-4 and Comparative Examples 1-4 are as Figure 3 shown. The original NiFe-LDH@CuO NW shows a significant oxygen vacancy EPR signal at g = 2.003. After pyrolysis, the oxygen vacancy EPR signals of all samples of D x -NiFe-LDH@CuO NW do not change significantly, indicating that direct pyrolysis does not significantly promote the generation of defects in the LDH matrix. On the contrary, the oxygen vacancy signals of the series of samples of S x -D-NiFe-LDH@CuO NW increase significantly, which is due to the distortion of the LDH structure caused by sulfur doping, thus promoting the generation of defects and leading to an increase in oxygen vacancies.

[0105] The electrodes were prepared using the electrocatalysts prepared in Examples 1-4 and Comparative Examples 1-4 and used for electrocatalytic nitrate reduction to ammonia. The specific steps are as follows:

[0106] (1) The electrocatalyst was fully mixed with acetylene black and polytetrafluoroethylene at a mass ratio of 8:1:1, and a certain amount of absolute ethanol was added and ultrasonically dispersed for at least 1 h to form a uniform electrode slurry. The prepared electrode slurry was evenly coated on graphite paper with a film coater and air-dried at room temperature. The size of each electrode was about 5 cm × 5 cm and dried at room temperature to obtain the cathode.

[0107] (2) The NO3-RR test was carried out in a typical H-type electrolytic cell separated by a Nafion 117 membrane. The prepared electrode, Pt sheet electrode, and Ag / AgCl electrode were used as the working electrode, counter electrode, and reference electrode, respectively. Before the NO3-RR test, the electrode was activated by cyclic voltammetry (CV). Then, a chronopotentiometric test was carried out for 2 h in the potential range of -0.2 to -0.6 V vs. RHE.

[0108] (3) The prepared electrode was placed in a 0.5 M K2SO4 solution containing 100 mg-N L -1 KNO3. Electro-adsorption was carried out for 2 h at a voltage of 0.2 V and a rotation speed of 100 r / min. 1 mL of the solution was taken every 20 min, and the NO3 - concentration was quantified by UV-vis.

[0109] The ammonia production rates and Faraday efficiencies of Examples 1-4 and Comparative Examples 1-4 are asFigure 4 As shown. When sulfur is not doped, only D obtained by pyrolysis x -NiFe-LDH@CuO NW has a low ammonia production rate and Faraday efficiency, with the Faraday efficiency only about 75%. As found in the following figure, S x -D-NiFe-LDH@CuO NW has a high ammonia production rate and excellent Faraday efficiency, and the Faraday efficiency is maintained at about 95%. This shows that after doping with sulfur, not only the formation of defects is promoted, but also the hydrogen evolution reaction is inhibited, accelerating the combination of *NO and hydrogen, thus greatly promoting the process of electrocatalytic reduction of nitrate. The ammonia production rate and Faraday efficiency measured at -0.4V vs. RHE, where S3-D-NiFe-LDH@CuO NW shows the most excellent performance, and the ammonia production rate reaches 2800 μg h -1 cm -2 , and the Faraday efficiency is as high as 98%.

[0110] Using Examples 1 to 4 and Comparative Examples 1 to 4, the density of states diagrams of the Fe 3d orbitals of NiFe-LDH, D-NiFe-LDH, and S x -D-NiFe-LDH were calculated, and the results are as Figure 5 shown. To clarify the effects of S doping and O vacancies on the structure of layered hydroxides, DFT calculations were performed on the geometric and electronic structures of NiFe-LDH, D-NiFe-LDH, and S x -D-NiFe-LDH. The density of states results show that the density of states of D-NiFe-LDH and S x -D-NiFe-LDH gradually exceeds the Fermi level, enhancing the electron transfer ability. After doping with sulfur, the sulfur-mediated defects cause the d-band center of Ni and Fe sites to shift, getting closer to the Fermi level, thus effectively promoting the enrichment of active hydrogen and inhibiting the combination of active hydrogen. This kind of defect constructs a local environment enriched with NO3 - and H + , realizing the rapid hydrogenation of *NO and further promoting the process of electrocatalytic nitrate. The density of states diagram illustrates the important influence of sulfur-mediated induced crystal distortion on electrocatalytic nitrate.

[0111] The ammonia production rate and Faraday efficiency measured by cycling 20 times in the actual industrial wastewater process in Examples 3 and 5 are as Figure 6 shown. The material has a current density of ~20 mA cm -2It shows good stability during the reaction, and the Faraday efficiency is even as high as 90%. For the unencapsulated material, its Faraday efficiency drops to 60%, and the ammonia production rate significantly shows a downward trend. Through membrane encapsulation, not only the influence of background pollutants such as phosphates and humic substances is blocked, but also the cyclic stability performance of the electrode in real industrial wastewater is improved. Compared with the unencapsulated material, its anti-interference ability is enhanced, while the catalytic and adsorption properties are strengthened, and the stability and reusability are improved. Obviously, the electrocatalyst in the present invention realizes efficient NO3-RR performance in practical applications, achieving excellent Faraday efficiency and relatively high ammonia production rate.

[0112] In the present invention, unless otherwise specified, the scientific and technical terms used herein have the meanings commonly understood by those skilled in the art. Moreover, the reagents, materials and operation steps used herein are all widely used reagents, materials and conventional steps in the corresponding fields.

[0113] Finally, it should be noted that the above are only the preferred embodiments of the present invention and are not used to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, for those skilled in the art, they can still modify the technical solutions recorded in the foregoing embodiments, or perform equivalent replacements for some of the technical features. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A preparation method of a membrane-encapsulated sulfur-mediated defective NiFe layered hydroxide electrocatalyst, characterized in that: It includes the following steps: S1. Place copper foam in a mixed solution of a strong oxidant and an alkali to obtain copper foam loaded with Cu(OH)2, and pyrolyze to obtain CuO NW; S2. Place CuO NW in a mixed solution of a nickel salt, an iron salt, a sulfur source, urea and water, heat and react to obtain S-NiFe-LDH@CuO NW; S3. Heat S-NiFe-LDH@CuO NW in an inert gas to obtain S-D-NiFe-LDH@CuO NW; S4. Use polyvinylidene fluoride, polyvinylpyrrolidone and N,N-dimethylformamide to perform membrane encapsulation on S-D-NiFe-LDH@CuO NW to obtain an electrocatalyst.

2. The preparation method of the membrane-encapsulated sulfur-mediated defective NiFe layered hydroxide electrocatalyst according to claim 1, characterized in that: In S1, the strong oxidant is chromic acid or ammonium persulfate; The alkali is potassium hydroxide or sodium hydroxide; The molar ratio of the strong oxidant to the alkali is 1∶3-50; The loading time of copper foam is 0.5-2 h; The pyrolysis temperature is 250-350 °C, and the pyrolysis time is 2.5-3.5 h.

3. The preparation method of the membrane-encapsulated sulfur-mediated defective NiFe layered hydroxide electrocatalyst according to claim 1, characterized in that: The specific method of S2 is: add a nickel salt, an iron salt, a sulfur source and urea to water and stir for 30-90 min to obtain a mixed solution; put the CuO NW obtained in S1 into the mixed solution, and place it at 85-95 °C for 8-16 h; wash the reaction product with ethanol and water, and vacuum dry at 50-70 °C for 8-10 h to obtain S-NiFe-LDH@CuO NW.

4. The preparation method of the membrane-encapsulated sulfur-mediated defective NiFe layered hydroxide electrocatalyst according to claim 1, characterized in that: In S2, the nickel salt is Ni(NO3)2·6H2O; The iron salt is Fe(NO3)3·9H2O; The sulfur source is a thiosulfate; the thiosulfate is potassium thiosulfate or sodium thiosulfate; The molar ratio of Ni(NO3)2·6H2O, Fe(NO3)3·9H2O, thiosulfate and urea is 1-3∶1-1.5∶0.025-0.1∶10-15.

5. The preparation method of the membrane-encapsulated sulfur-mediated defective NiFe layered hydroxide electrocatalyst according to claim 1, characterized in that: In S3, the inert gas is argon or helium; The heating temperature is 350-500 K, and the heating time is 1.5-2.5 h.

6. The preparation method of the membrane-encapsulated sulfur-mediated defective NiFe layered hydroxide electrocatalyst according to claim 1, characterized in that: In S4, the dosage ratio of polyvinylidene fluoride, polyvinylpyrrolidone and N,N-dimethylformamide is 5-10 g∶0.03-0.2 g∶50 mL.

7. The preparation method of the film-encapsulated sulfur-mediated defective NiFe layered hydroxide electrocatalyst according to claim 1, wherein: The specific method of S4 is as follows: a mixed solution of polyvinylidene fluoride, polyvinylpyrrolidone and N,N-dimethylformamide is evenly cast on the S-D-NiFe-LDH@CuO NW obtained in S3, and the S-D-NiFe-LDH@CuO NW is fixed on a perforated stainless steel plate with a casting knife; then the gap of the casting knife is adjusted, and then the casting solution is applied again; immediately afterwards, the stainless steel plate is immersed in water, and the ME-S encapsulated with a membrane is separated t -D-NiFe-LDH@CuO NW; soak in water to remove the residual solvent to obtain the electrocatalyst.

8. The film-encapsulated sulfur-mediated defective NiFe layered hydroxide electrocatalyst prepared by the preparation method according to any one of claims 1 to 7.

9. An electrode, characterized in that: Comprising the film-encapsulated sulfur-mediated defective NiFe layered hydroxide electrocatalyst according to claim 8.

10. Application of the film-encapsulated sulfur-mediated defective NiFe layered hydroxide electrocatalyst according to claim 8 in ammonia production from nitrate.

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