A Co that Enriches Lewis Acid Sites 3 O 4 / Ni Array Electrode and Its Preparation Method and Application

Abstract

The present invention provides a Co3O4 / Ni array electrode enriched with Lewis acid sites, a preparation method thereof and an application thereof, relating to the technical field of electrocatalysts. The present invention prepares a Co3O4 / Ni array electrode enriched with Lewis acid sites by a two-step hydrothermal-pyrolysis method, and this electrode can significantly improve the performance of electroreduction of nitrate to ammonia. The electrode of the present invention exhibits an ammonia synthesis rate of 128.70 mg h ‑1 cm ‑2 and a Faraday efficiency (FE) of 95.31%, as well as an effective current density for ammonia synthesis of 1623.17 mA cm ‑2 in a sodium hydroxide electrolyte. The electrode of the present invention has the advantages of controllable preparation process, simple operation, etc. The introduction of Lewis acid sites and the self-supporting array configuration significantly improve the nitrate adsorption and activation ability, mass transfer and electron transfer ability, and long-term stability during the catalytic process, providing guarantee for the research and development of hydrogen storage materials and the development of low-carbon energy, and having important significance for the industrial application of ammonia synthesis.

Description

Technical Field

[0001] The present invention relates to the technical field of electrocatalysts, and particularly to a Co 3 O 4 / Ni array electrode and its preparation method and application. Background Art

[0002] Ammonia, as an energy storage intermediate and a carbon-free energy carrier, is of great significance for alleviating the global energy crisis and facilitating the achievement of the dual-carbon goal. The Haber-Bosch process, which uses nitrogen and hydrogen as raw materials, has been the main means of industrial ammonia synthesis for nearly a century. However, the harsh synthesis conditions such as high temperature and high pressure, as well as the large amount of carbon dioxide emissions, have restricted the sustainable development of the Haber-Bosch process. Therefore, people are urgently seeking a mild, green, and sustainable alternative process for ammonia synthesis. Given the relatively low dissociation energy (204 kJ mol -1 ) of the N=O bond in nitrate and the controllable solid-liquid reaction interface, electrocatalytic nitrate reduction reaction, using water as the proton source and driven by renewable energy, is a promising alternative process for ammonia synthesis.

[0003] The process of nitrate reduction for ammonia synthesis involves eight-electron and nine-proton transfer. The complex reaction path makes it extremely difficult to explore the reaction mechanism and develop catalysts. Among them, the weak nitrate adsorption energy on the electrode surface in the aqueous solution and the slow electron transfer driving force during the catalytic process are the main factors limiting the selectivity and yield of ammonia synthesis. At the same time, traditional metal-based catalytic materials can only maintain good ammonia synthesis performance at low potentials and small currents. The severe hydrogen evolution reaction during high-current operation seriously restricts the yield and selectivity of ammonia synthesis. In addition, the use of binders in the preparation process of nitrate reduction electrodes reduces the conductivity and catalytic stability of the electrodes. Therefore, how to overcome the problems of low ammonia synthesis yield, serious hydrogen evolution phenomenon, and poor stability caused by metal-based catalysts in electrocatalytic nitrate synthesis for ammonia, and obtain a green, efficient, and sustainable ammonia synthesis method is of great significance for realizing industrial ammonia synthesis and alleviating the global energy crisis. Summary of the Invention

[0004] To solve the above problems, the present invention provides a preparation method for a Co 3 O 4 / Ni array electrode enriched with Lewis acid sites. The present invention uses a two-step hydrothermal-pyrolysis method to prepare a Co 3 O 4 / Ni array electrode enriched with Lewis acid sites, and the performance of electro-reducing nitrate to ammonia is greatly improved after optimization. The electrode of the present invention has the advantages of controllable preparation process, simple operation, etc. The introduction of Lewis acid sites and the self-supporting array configuration significantly improve the nitrate adsorption and activation ability, mass transfer and electron transfer ability, and long-term stability during the catalytic process.

[0005] The preparation method of the Co 3 O 4 / Ni array electrode enriched with Lewis acid sites includes the following steps:

[0006] (1) Add cobalt source, ammonium fluoride, urea and sodium citrate into deionized water in sequence, and stir until completely dissolved to obtain a mixed solution;

[0007] (2) Put the foam Ni into acetone for ultrasonic cleaning for 20 - 40 min, then put it into 3 mol / L hydrochloric acid solution for ultrasonic cleaning for 20 - 40 min, then put it into absolute ethanol for ultrasonic cleaning for 20 - 40 min, then put it into deionized water for ultrasonic cleaning for 20 - 40 min, and finally dry it with high-purity argon to obtain the cleaned foam Ni;

[0008] (3) Transfer the mixed solution into a reaction kettle, and then put the cleaned foam Ni into the reaction kettle;

[0009] (4) Transfer the reaction kettle to an electrothermal blast drying oven for reaction. After the reaction is completed, cool the reaction kettle to room temperature, take out the foam board of Co(CO 3 ) 0.5 (OH)·0.11H 2 O / Ni, rinse it 2 - 3 times with deionized water and absolute ethanol respectively, and then put it into a vacuum drying oven at 60 - 80 °C for drying for 10 - 14 h to obtain the Co(CO 3 ) 0.5 (OH)·0.11H 2 O / Ni foam board;

[0010] (5) Heat the Co(CO 3 ) 0.5 (OH)·0.11H 2 O / Ni foam board in an argon atmosphere, take out the sample after cooling to room temperature, rinse it 2 - 3 times with deionized water and absolute ethanol respectively, and finally dry it with high-purity argon to obtain the Co 3 O 4 / Ni array electrode enriched with Lewis acid sites.

[0011] Furthermore, the cobalt source in the step (1) is cobalt chloride, cobalt acetate, cobalt sulfate or cobalt nitrate.

[0012] Furthermore, the molar ratio of cobalt salt, ammonium fluoride and urea in the step (1) is 1:5:5, the weight molar ratio of sodium citrate to cobalt salt is 0.0025 g:1 mmol, and the addition amount of deionized water is 20 - 30 mL.

[0013] Further, in the step (2), the specification of the Ni foam is 4.0 cm × 2.0 cm.

[0014] Further, in the step (3), the addition amount of the mixed solution accounts for 40% - 60% of the volume of the reaction kettle.

[0015] Further, in the step (3), the weight ratio of the mixed solution to the area of the Ni foam is 25 mL : 8 cm 2 .

[0016] Further, in the step (4), the reaction temperature is 100°C - 140°C, and the reaction time is 4 - 8 h.

[0017] Further, in the step (5), the heating temperature is 350°C - 550°C, and the heating time is 1 - 3 h.

[0018] The present invention also provides a Co 3 O 4 / Ni array electrode enriched with Lewis acid sites prepared according to the above method.

[0019] Another object of the present invention is to provide the application of the Co 3 O 4 / Ni array electrode enriched with Lewis acid sites in electrocatalytic nitrate reduction to ammonia, which specifically includes the following steps:

[0020] Using the Co 3 O 4 / Ni array electrode as the working electrode, a Pt mesh as the counter electrode, and Ag / AgCl as the reference electrode, using nitrate as the reactant and sodium hydroxide solution as the electrolyte to perform electrocatalytic nitrate reduction to ammonia at normal temperature and pressure.

[0021] Further, the specification of the Co 3 O 4 / Ni array electrode is 1.0 cm × 1.0 cm.

[0022] Further, the nitrate is sodium nitrate or potassium nitrate.

[0023] Further, the concentration of the nitrate is 0.01 - 0.5 mol / L, and the concentration of the sodium hydroxide solution is 0.1 - 5 mol / L.

[0024] Further, the electrolysis voltage is -0.4 - -0.8 V vs. RHE.

[0025] In the process of electrocatalytic nitrate reduction to ammonia at normal temperature and pressure of the present invention, a three-electrode system is adopted, and the relevant electrochemical properties of nitrate reduction to ammonia are tested by a VMP3 electrochemical workstation produced by Bio-Logic of France. The ammonia generated in the cathode electrolyte is detected by the indophenol blue colorimetric method after being diluted 50-200 times.

[0026] The indophenol blue colorimetric method is as follows: Take 2 mL of the diluted cathode electrolyte, and successively add 2 mL of a 1 mol / L sodium hydroxide solution containing 5 wt% salicylic acid and 5 wt% sodium citrate, 1 mL of a 0.05 mol / L sodium hypochlorite solution, and 0.2 mL of a 1 wt% sodium nitroprusside solution. After shaking the above mixture well, place it in the dark and let it stand for 1 h. Detect the absorbance at the maximum absorption wavelength by a UV-visible spectrophotometer, and finally calculate the concentration of the product ammonia through a standard ammonium chloride solution.

[0027] Co 3 O 4 / Ni rich in Lewis acid sites prepared by the present invention can greatly improve the adsorption and activation ability of nitrate on the electrode surface. At the same time, the integrated array configuration accelerates the mass transfer and electron transfer abilities of the nitrate reduction process and improves the catalytic stability of the electrode. Compared with the Co 3 O 4 / Ni array electrode calcined in air atmosphere and the non-in-situ grown Co 3 O 4 nanoparticles, the Co 3 O 4 / Ni array electrode rich in Lewis acid sites exhibits excellent ammonia synthesis performance.

[0028] Compared with the prior art, the beneficial technical effects of the present invention are as follows:

[0029] The electrode of the present invention has the advantages of controllable preparation process and simple operation. The introduction of Lewis acid sites and the self-supporting array configuration significantly improve the nitrate adsorption and activation ability, mass transfer and electron transfer abilities, and long-term stability during the catalytic process. The Co 3 O 4 / Ni array electrode rich in Lewis acid sites prepared by the present invention exhibits an ammonia synthesis yield of 128.70 mgh -1 cm -2 and a Faraday efficiency (FE) of 95.31% in a sodium hydroxide electrolyte, as well as an effective current density for ammonia synthesis of 1623.17 mAcm -2 . The Co 3 O 4 / Ni array electrode pyrolyzed in air atmosphere at high temperature (relative to the test conditions) and the non-in-situ grown Co 3 O4 Nanoparticles have further promoted the industrialization process of ammonia synthesis, provided guarantees for the research and development of hydrogen storage materials and the development of low-carbon energy, and are of great significance for the industrial application of ammonia synthesis. Brief Description of the Drawings

[0030] The present invention will be further described below in conjunction with the description of the drawings.

[0031] Figure 1 It is a characterization diagram of an X-ray diffractometer for the electrocatalysts prepared in Example 1, Comparative Example 1, and Comparative Example 2 of the present invention;

[0032] Figure 2 (a) is a scanning electron microscope photograph of the electrocatalyst in Comparative Example 1 of the present invention, Figure 2 (b) is a scanning electron microscope photograph of the electrocatalyst in Example 1 of the present invention, Figure 2 (c) is a transmission electron microscope photograph of the electrocatalyst in Comparative Example 2 of the present invention; Figure 2 (d) is a high-resolution transmission electron microscope image of the electrocatalyst in Comparative Example 1 of the present invention, Figure 2 (e) is a high-resolution transmission electron microscope image of the electrocatalyst in Example 1 of the present invention, Figure 2 (f) is a high-resolution transmission electron microscope image of the electrocatalyst in Comparative Example 2 of the present invention;

[0033] Figure 3 (a) is the Co2p XPS spectrum of Comparative Example 1 of the present invention; Figure 3 (b) is the Co2p XPS spectrum of Example 1 of the present invention; Figure 3 (c) is the O1s XPS spectrum of Comparative Example 1 of the present invention; Figure 3 (d) is the O1s XPS spectrum of Example 1 of the present invention;

[0034] Figure 4 (a) shows the change in ammonia synthesis yield with the applied potential for Example 1, Comparative Example 1, and Comparative Example 2 of the present invention; Figure 4 (b) shows the change in the Faraday efficiency of ammonia synthesis with the applied potential for Example 1, Comparative Example 1, and Comparative Example 2 of the present invention; Figure 4 (c) shows the change in the effective current density of ammonia synthesis with the applied potential for Example 1, Comparative Example 1, and Comparative Example 2 of the present invention; Figure 4 (d) shows the change in ammonia synthesis yield and Faraday efficiency with electrolysis time when Example 1 of the present invention conducts a continuous cycling experiment at -0.7V vs. RHE; Figure 4 (e) shows the change in the output current density with electrolysis time when Example 1 of the present invention conducts continuous electrolysis at -0.7V vs. RHE;

[0035] Figure 5Linear sweep voltammograms of Example 1 of the present invention under different test conditions;

[0036] Figure 6 (a) Cyclic voltammograms of Example 1 of the present invention at different scan rates; Figure 6 (b) Cyclic voltammograms of Comparative Example 1 of the present invention at different scan rates; Figure 6 (c) Cyclic voltammograms of Comparative Example 2 of the present invention at different scan rates; Figure 6 (d) Variation of capacitive current density of Example 1, Comparative Example 1 and Comparative Example 2 of the present invention with scan rate. Detailed implementation manners

[0037] The technical solutions provided by the present invention are further described below in conjunction with embodiments.

[0038] The foam Ni in the embodiments of the present invention is commercial foam Ni, purchased from Kunshan Guangjiayuan New Materials Co., Ltd.

[0039] Example 1

[0040] (1) 4 mmol of cobalt chloride, 20 mmol of ammonium fluoride, 20 mmol of urea and 0.01 g of sodium citrate were successively added to a beaker containing 25 mL of deionized water and stirred until completely dissolved;

[0041] (2) The commercially available foam Ni cut into a size of 4.0 cm × 2.0 cm was ultrasonically cleaned in acetone for 30 min, then ultrasonically cleaned in 3 mol / L hydrochloric acid solution for 30 min, then ultrasonically cleaned in absolute ethanol for 30 min, then ultrasonically cleaned in deionized water for 30 min, and finally the cleaned foam Ni was dried with high-purity argon;

[0042] (3) The 25 mL of the mixed solution in step (1) was transferred into a 50 mL reaction kettle, and then the foam Ni cleaned according to step (2) was put into the above reaction kettle;

[0043] (4) The reaction kettle in step (3) was transferred to an electrothermal blast drying oven, heated to 120 °C, and reacted for 6 h. After the reaction was completed and the reaction kettle was cooled to room temperature, the grown Co(CO 3 ) 0.5 (OH)·0.11H 2 O / Ni foam board was taken out, rinsed 2 - 3 times with deionized water and absolute ethanol, and then placed in a vacuum drying oven at 70 °C for drying for 12 h;

[0044] (5) The Co(CO 3 ) 0.5 (OH)·0.11H 2The O / Ni foam board was heated at 450 °C for 2 h in an argon atmosphere. After the tubular furnace was cooled to room temperature, the sample was taken out, rinsed 2 - 3 times with deionized water and absolute ethanol respectively, and finally dried with high-purity argon, thus obtaining the Co 3 O 4 / Ni array electrode enriched with Lewis acid sites of the present invention, labeled as Ar-Co 3 O 4 / Ni.

[0045] Using the Ar-Co 3 O 4 / Ni array prepared by the above method as the working electrode, applied to the reaction of electrocatalytic nitrate reduction to synthesize ammonia at normal temperature and pressure. Using 0.1 mol / L sodium nitrate as the reactant and 1 mol / L sodium hydroxide solution as the electrolyte, after electrolysis at an electrolysis voltage of -0.8 V vs. RHE, the cathode electrolyte was diluted 100 times, and finally the ammonia production was detected by the indophenol blue colorimetric method. After electrochemical testing, the ammonia synthesis yield and effective current density of Ar-Co3O4 / Ni were 128.70 mg h - 1 cm -2 and 1623.17 mA cm -2 ; the Faraday efficiency of ammonia synthesis was 95.31% at an electrolysis voltage of -0.4 V vs. RHE.

[0046] Example 2:

[0047] (1) 2 mmol of cobalt chloride, 10 mmol of ammonium fluoride, 10 mmol of urea and 0.005 g of sodium citrate were successively added to a beaker containing 20 mL of deionized water and stirred until completely dissolved;

[0048] (2) The commercially available foam Ni with a size of 4.0 cm × 2.0 cm cut was ultrasonically cleaned in acetone for 30 min, then ultrasonically cleaned in 3 mol / L hydrochloric acid solution for 30 min, then ultrasonically cleaned in absolute ethanol for 30 min, then ultrasonically cleaned in deionized water for 30 min, and finally the cleaned foam Ni was dried with high-purity argon;

[0049] (3) The 20 mL of the mixed solution in step (1) was transferred into a 50 mL reaction kettle, and then the foam Ni cleaned according to step (2) was put into the above reaction kettle;

[0050] (4) The reaction kettle in step (3) was transferred to an electrothermal blast drying oven, heated to 100 °C, and reacted for 4 h. After the reaction ended and the reaction kettle was cooled to room temperature, the grown Co(CO 3 ) 0.5 (OH)·0.11H 2Take out the O / Ni foam board, rinse it with deionized water and absolute ethanol 2-3 times, and then put it into a vacuum drying oven at 60 °C for drying for 10 h;

[0051] (5) Heat the Co(CO 3 ) 0.5 (OH)·0.11H 2 O / Ni foam board in an argon atmosphere at 450 °C for 2 h. After the tubular furnace cools to room temperature, take out the sample, rinse it with deionized water and absolute ethanol 2-3 times respectively, and finally dry it with high-purity argon, that is, the Co 3 O 4 / Ni array electrode enriched with Lewis acid sites of the present invention is prepared, marked as Ar-Co 3 O 4 / Ni.

[0052] Using the Air-Co 3 O 4 / Ni array prepared by the above method as the working electrode, using 0.1 mol / L sodium nitrate as the reactant and 1 mol / L sodium hydroxide solution as the electrolyte. After electrochemical testing, the ammonia synthesis yield and effective current density of Air-Co 3 O 4 / Ni are 125.29 mg h -1 cm -2 and 1580.30 mA cm -2 respectively when the electrolysis voltage is -0.8 V vs. RHE; the Faraday efficiency of ammonia synthesis is 97.50% when the electrolysis voltage is -0.4 V vs. RHE.

[0053] Example 3:

[0054] (1) Add 6 mmol cobalt chloride, 30 mmol ammonium fluoride, 30 mmol urea and 0.015 g sodium citrate to a beaker containing 30 mL of deionized water in sequence, and stir until completely dissolved;

[0055] (2) Put the commercially available foam Ni with a cut size of 4.0 cm × 2.0 cm into acetone for ultrasonic cleaning for 30 min, then put it into a 3 mol / L hydrochloric acid solution for ultrasonic cleaning for 30 min, then put it into absolute ethanol for ultrasonic cleaning for 30 min, then put it into deionized water for ultrasonic cleaning for 30 min, and finally dry the cleaned foam Ni with high-purity argon;

[0056] (3) Transfer the 30 mL mixed solution in step (1) into a 50 mL reaction kettle, and then put the foam Ni cleaned according to step (2) into the above reaction kettle;

[0057] (4) Transfer the reactor in step (3) to an electrothermal blast drying oven, heat it up to 140 °C, react for 8 h, and after the reaction is completed and the reactor is cooled to room temperature, take out the grown Co(CO 3 ) 0.5 (OH)·0.11H 2 O / Ni foam board, rinse it with deionized water and absolute ethanol 2 - 3 times, and then put it into a vacuum drying oven at 80 °C and dry it for 14 h;

[0058] (5) Heat the Co(CO 3 ) 0.5 (OH)·0.11H 2 O / Ni foam board prepared in step (4) at 450 °C for 2 h under an argon atmosphere. After the tubular furnace is cooled to room temperature, take out the sample, rinse it with deionized water and absolute ethanol 2 - 3 times respectively, and finally dry it with high-purity argon, thus preparing the Co 3 O 4 / Ni array electrode enriched with Lewis acid sites of the present invention, marked as Ar-Co 3 O 4 / Ni.

[0059] Using the Air-Co 3 O 4 / Ni array prepared by the above method as the working electrode, using 0.1 mol / L sodium nitrate as the reactant and 1 mol / L sodium hydroxide solution as the electrolyte. After electrochemical testing, for Air-Co 3 O 4 / Ni, when the electrolysis voltage is -0.8 V vs. RHE, the ammonia synthesis yield and the effective current density are 123.35 mg h -1 cm -2 and 1555.79 mA cm -2 respectively; when the electrolysis voltage is -0.4 V vs. RHE, the Faraday efficiency of ammonia synthesis is 95.49%.

[0060] Comparative Example 1:

[0061] (1) Steps (1) to (4) are the same as in Example 1;

[0062] (5) Heat the Co(CO 3 ) 0.5 (OH)·0.11H 2 O / Ni foam board prepared in step (4) at 450 °C for 2 h in an air atmosphere. After the muffle furnace is cooled to room temperature, take out the sample, rinse it with deionized water and absolute ethanol 2 - 3 times respectively, and finally dry it with high-purity argon, thus preparing the Co 3 O 4 / Ni array electrode of the present invention, marked as Air-Co3 O 4 / Ni.

[0063] Using the Air-Co 3 O 4 / Ni array prepared by the above method as the working electrode, 0.1 mol / L sodium nitrate as the reactant, and 1 mol / L sodium hydroxide solution as the electrolyte. After electrochemical testing, when the electrolysis voltage of Air-Co 3 O 4 / Ni is -0.8 V vs. RHE, the ammonia synthesis yield and effective current density are 115.00 mg h -1 cm -2 and 1425.20 mA cm -2 respectively; when the electrolysis voltage is -0.4 V vs. RHE, the Faraday efficiency of ammonia synthesis is 92.48%.

[0064] Comparative Example 2:

[0065] (1) Step (1) is the same as in Example 1;

[0066] (2) Transfer 25 mL of the mixed solution in step (1) into a 50 mL reaction kettle;

[0067] (3) Transfer the reaction kettle in step (2) to an electrothermal blast drying oven, heat it to 120 °C, react for 6 h. After the reaction is completed and the reaction kettle is cooled to room temperature, take out the generated suspension, centrifuge it, and wash it repeatedly with deionized water and absolute ethanol. The centrifugation condition is that the centrifuge runs at 10,000 rpm for 5 min to complete the centrifugation process. The water washing and alcohol washing conditions are that the centrifuge runs at 10,000 rpm for 5 min as one cycle, and each is repeated 3 times. Then put it into a vacuum drying oven at 70 °C and dry it for 12 h to obtain Co(CO 3 ) 0.5 (OH)·0.11H 2 O / Ni powder;

[0068] (4) Heat the Co(CO 3 ) 0.5 (OH)·0.11H 2 O / Ni powder prepared in step (3) at 450 °C for 2 h in an argon atmosphere. After the tubular furnace is cooled to room temperature, take out the sample, wash it repeatedly with deionized water and absolute ethanol. The water washing and alcohol washing conditions are that the centrifuge runs at 10,000 rpm for 5 min as one cycle, and each is repeated 3 times. Finally, put it into a vacuum drying oven at 70 °C and dry it for 12 h to prepare non-in-situ grown Co 3 O 4 nanoparticles, labeled as Co 3 O 4 NPs;

[0069] (5) Disperse 5 mg of Co 3 O 4 NPs prepared by the above method in 450 μL of absolute ethanol, add 50 μL of Nafion solution, mix well, disperse ultrasonically for 20 min, then coat the evenly dispersed catalyst ink on the surface of pretreated foam Ni (1.0 cm × 1.0 cm), and finally place it in a vacuum drying oven at 40 °C for 4 h;

[0070] (6) Heat the foam board obtained in step (5) at 450 °C for 2 h under an argon atmosphere. After the tubular furnace cools to room temperature, take out the sample, rinse it 2 - 3 times with deionized water and absolute ethanol respectively, and finally dry it with high-purity argon, and label it as Co 3 O 4 NPs.

[0071] Using Co 3 O 4 NPs prepared by the above method as the working electrode, using 0.1 mol / L sodium nitrate as the reactant and 1 mol / L sodium hydroxide solution as the electrolyte. After electrochemical testing, when the electrolysis voltage of Co 3 O 4 NPs is 0.8 V vs. RHE, the ammonia synthesis yield and the effective current density are 59.17 mg h -1 cm -2 and 753.12 mA cm -2 ; when the electrolysis voltage is -0.4 V vs. RHE, the Faraday efficiency of ammonia synthesis is 76.81%.

[0072] Test Example 1

[0073] The following is a comparative test on the electrode materials prepared in Example 1 and Comparative Examples 1 - 2 and their electrocatalytic nitrate reduction to synthesize ammonia, and the following results are obtained.

[0074] It can be seen from Figure 1 that the XRD diffraction patterns of Example 1 and Comparative Example 1 of the present invention have the same peak positions, proving that Co 3 O 4 has successfully grown on the surface of foam Ni to form a self-supporting material. While the XRD diffraction peaks of Comparative Example 2 all originate from spinel-type Co 3 O 4 , proving the successful synthesis of the Co 3 O 4 phase.

[0075] It can be seen from Figure 2 (a) and Figure 2 (b) that in-situ grown Co on the surface of foam Ni prepared by hydrothermal-hydrolysis method 3O 4 exhibits a uniform nanowire array. However, compared with Comparative Example 1, the Co 3 O 4 nanowires prepared in Example 1 have a rougher surface, which is due to the stronger etching effect produced by calcination in an argon atmosphere. Figure 2 (c) is the transmission electron microscopy photograph corresponding to Comparative Example 2. It can be seen that when there is no foam Ni substrate as a support, the prepared Co 3 O 4 is composed of nanograins sized 15 - 20 nm piled up, and there is obvious agglomeration between the grains. Figure 2 (d), Figure 2 (e) and Figure 2 (f) respectively correspond to the high - resolution transmission electron microscopy pictures of Comparative Example 1, Example 1 and Comparative Example 2 of the present invention. The corresponding lattice fringes in the figures respectively correspond to the (311), (311) and (220) crystal planes of Co 3 O 4 , and this result is consistent with the conclusion of XRD analysis.

[0076] Figure 3 Corresponding are the X - ray photoelectron spectroscopy (XPS) diagrams of Example 1 and Comparative Example 1 of the present invention. Among them Figure 3 (a) is the Co2p XPS spectrum of Comparative Example 1, Figure 3 (b) is the Co2p XPS spectrum of Example 1. The fitting peaks at 796.2 / 781.0 eV and 794.8 / 779.8 eV in the figure respectively come from Co 2+ and Co 3+ . From the comparison between Figure 3 (a) and Figure 3 (b), it can be seen that compared with Comparative Example 1, the ratio of Co 2+ / Co 3+ in Example 1 increases significantly, and this phenomenon is beneficial to the formation of oxygen vacancies in Co 3 O 4 . Figure 3 (c) is the O1s XPS spectrum of Comparative Example 1, Figure 3 (d) is the O1s XPS spectrum of Example 1. The characteristic peaks after fitting of the O1s spectrum are respectively located at 529.9 eV, 531.1 eV, 532.1 eV and 533.0 eV, and respectively correspond to four different types of oxygen: lattice oxygen (OL), vacancy oxygen (OV), hydroxyl oxygen (OOH) and adsorbed oxygen (OA). After calculation, the OV content in Example 1 is as high as 38.67%, which is significantly higher than that in Comparative Example 1 (the OV content is 17.84%), proving that Co 3 O 4More oxygen vacancies are generated on the surface, which is consistent with the XPS results of Co2p. As Lewis acid sites, oxygen vacancies exhibit stronger nitrate adsorption ability during the electrocatalytic process. Therefore, the electrocatalyst of Example 1 has stronger ammonia synthesis potential.

[0077] Figure 4 (a) shows the variation of ammonia synthesis yield with the applied potential for Example 1, Comparative Example 1, and Comparative Example 2 of the present invention. Figure 4 (b) shows the variation of ammonia synthesis Faraday efficiency with the applied potential for Example 1, Comparative Example 1, and Comparative Example 2 of the present invention. Figure 4 (c) shows the variation of ammonia synthesis effective current density with the applied potential for Example 1, Comparative Example 1, and Comparative Example 2 of the present invention. By Figure 4 (a), Figure 4 (b), and Figure 4 (c), it can be seen that the integrated array configuration and abundant Lewis acid sites prepared in Example 1 of the present invention increase the number of active sites on the electrode surface, significantly improving the electrocatalytic nitrate reduction to ammonia performance of Example 1. Figure 4 (d) and Figure 4 (e) respectively correspond to the variation of ammonia synthesis yield, Faraday efficiency, and output current density with the electrolysis time when Example 1 conducts continuous cycling experiments at -0.7 V vs. RHE. It can be seen from the above two figures that Example 1 maintains excellent cycling stability within 30 cycles of continuous cycling.

[0078] Figure 5 This is the linear sweep voltammetry curve of Example 1 of the present invention under different test conditions. To clarify the role of Lewis acid sites (i.e., oxygen vacancies) in the electroreduction of nitrate, the present invention conducts a poisoning experiment on Example 1 using potassium thiocyanate (KSCN) as a poison. As a strong Lewis base, KSCN can firmly bind to Lewis acid and inactivate it. As Figure 5 shown, the addition of KSCN significantly inhibits the current density of the nitrate reduction reaction, while the current density of the hydrogen evolution reaction remains basically unchanged. By Figure 5 it can be seen that oxygen vacancies as Lewis acid sites are more conducive to nitrate adsorption, which is consistent with the excellent ammonia synthesis yield and favorable catalytic selectivity of Example 1.

[0079] Figure 6 (a), Figure 6 (b), and Figure 6 (c) are the cyclic voltammetry curves (CV) of Example 1, Comparative Example 1, and Comparative Example 2 of the present invention at different scan rates respectively. The double-layer capacitance value is measured according to the CV curve, and the electrochemically active surface area of the above three samples is estimated therefrom. From Figure 6It can be seen from (d) that the catalyst prepared in Example 1 of the present invention has the largest active surface area, which means that more catalytic sites can be exposed during the nitrate reduction process, thereby improving the performance of the electrocatalyst for ammonia synthesis in Example 1.

[0080] In summary, the binder-free Co 3 O 4 nanowire array electrode prepared by the in-situ growth method with foam Ni as the conductive substrate can significantly improve the yield, selectivity, effective current density, and cycling stability of electrocatalytic nitrate reduction to ammonia;

[0081] (2) During the preparation of the Co 3 O 4 nanowire array in the present invention, a large number of oxygen vacancies can be introduced into Co 3 O 4 by heating under the protection of an inert gas (argon) during pyrolysis. The oxygen vacancies, as Lewis acid sites, can significantly improve the adsorption and activation ability of the catalyst surface for nitrates, thereby showing more excellent ammonia synthesis performance.

[0082] Specific examples are used in this article to elaborate on the principle and implementation mode of the present invention. The description of the above examples is only used to help understand the method and its core idea of the present invention; at the same time, for those of ordinary skill in the art, according to the idea of the present invention, there will be changes in the specific implementation mode and application scope. In summary, the content of this specification should not be construed as a limitation to the present invention.

Claims

1. Application of Co 3 O 4 / Ni array electrode in electrocatalytic nitrate reduction to ammonia It is characterized in that it includes the following steps: Using a Co 3 O 4 / Ni array electrode as the working electrode, a Pt mesh as the counter electrode, and Ag / AgCl as the reference electrode, nitrate as the reactant, and a sodium hydroxide solution as the electrolyte, electrocatalytic reduction of nitrate to ammonia is carried out at normal temperature and pressure; The Co that enriches Lewis acid sites 3 O 4 / Ni array electrode preparation method, comprising the following steps: (1) Add a cobalt source, ammonium fluoride, urea, and sodium citrate to deionized water in sequence, and stir until completely dissolved to obtain a mixed solution; (2) Ultrasonically clean the foam Ni successively with acetone, hydrochloric acid solution, absolute ethanol, and deionized water, and finally dry it to obtain the cleaned foam Ni; (3) Transfer the mixed solution into a reaction kettle, and then put the cleaned foam Ni into the reaction kettle; (4) Transfer the above reaction kettle to an electrothermal blast drying oven for reaction. After the reaction is completed, cool the reaction kettle to room temperature, take out the foam board of Co(CO 3 ) 0.5 (OH)·0.11H 2 O / Ni, rinse it 2 - 3 times with deionized water and absolute ethanol respectively, and then put it into a vacuum drying oven at 60 - 80 °C for drying for 10 - 14 h to obtain the Co(CO 3 ) 0.5 (OH)·0.11H 2 O / Ni foam board; (5) Heat Co(CO 3 ) 0.5 (OH)·0.11H 2 O / Ni foam board in an argon atmosphere, take it out after cooling to room temperature, rinse it 2 - 3 times with deionized water and absolute ethanol respectively, and finally dry it to obtain a Co 3 O 4 / Ni array electrode; In the step (1), the molar ratio of the cobalt salt, ammonium fluoride, and urea is 1:5:5, the weight molar ratio of sodium citrate to the cobalt salt is 0.0025 g:1 mmol, and the addition amount of deionized water is 20 - 30 mL; In the step (4), the reaction temperature is 100°C - 140°C, and the reaction time is 4 - 8 h.

2. The application according to claim 1, it is characterized in that In the step (3), the volume ratio of the mixed solution to the area of the Ni foam is 25 mL: 8 cm 2 .

3. The application according to claim 1, it is characterized in that In the step (5), the heating temperature is 350°C - 550°C, and the heating time is 1 - 3 h.

4. The application according to claim 1, it is characterized in that the nitrate is sodium nitrate or potassium nitrate.

5. The application according to claim 1, it is characterized in that the concentration of the nitrate is 0.01 - 0.5 mol / L, and the concentration of the sodium hydroxide solution is 0.1 - 5 mol / L.

6. The application according to claim 1, it is characterized in that the electrolysis voltage is -0.4 - 0.8 V vs. RHE.

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