Preparation method and application of iodine-doped cobalt-based electrocatalyst

Abstract

The application is a preparation method and application of an iodine-doped cobalt-based electrocatalyst. The method uses mild ammonium iodide to perform a doping and synchronous etching strategy, successfully anchoring cobalt in the form of single atoms and clusters on an iodine-doped carbon substrate in one step; the obtained iodine-doped cobalt-based electrocatalyst has tandem catalytic active sites composed of iodine-coordinated cobalt single atoms and cobalt clusters, which can simultaneously promote the deoxygenation and hydrogenation steps in the electrocatalytic reduction of nitrate. The electrocatalytic synthesis of ammonia obtained by the application exhibits an ammonia yield of more than 17000 μg h ‑1 mg cat. ‑1 and a Faraday efficiency of more than 95%, achieving the simultaneous coexistence of high ammonia yield and high Faraday efficiency, and providing a new idea for green, efficient and sustainable ammonia synthesis.

Description

Technical Field

[0001] This invention belongs to the field of electrocatalytic ammonia synthesis catalyst technology, specifically relating to an iodine-doped cobalt-based electrocatalyst and its preparation method. Background Technology

[0002] Ammonia is an extremely important compound, widely used in industry, agriculture, and energy. Currently, industry relies on the Haber-Bosch process for large-scale ammonia synthesis, but the harsh reaction conditions (350-500℃, 20-50 MPa) result in approximately 300 million tons of carbon dioxide emissions annually and consume about 2% of global energy. Therefore, it is necessary to explore more environmentally friendly and sustainable ammonia synthesis methods. Recently, electrocatalysis technology, using renewable energy electricity, nitrogen-containing species, and water as the driving force, nitrogen source, and proton source respectively, has shown great application potential in ammonia synthesis. As a pollutant widely present in wastewater, nitrates have high solubility in aqueous solution and more easily broken N=O bonds (204 kJ / mol). -1 This technology can effectively improve the reaction kinetics of electrocatalytic ammonia synthesis. Therefore, the electrocatalytic nitrate reduction reaction can convert nitrate pollutants into ammonia with higher added value under environmental conditions, which is beneficial for obtaining valuable chemicals while solving environmental pollution problems.

[0003] Because the electrocatalytic nitrate reduction reaction involves complex deoxygenation and hydrogenation processes, the design of the electrocatalyst determines its ammonia synthesis yield and Faradaic efficiency. Currently, technologies for preparing electrocatalysts for the electrocatalytic reduction of nitrate to ammonia have been reported. Patent CN118880375A discloses a method using Ti3C2T... x Ti3C2T prepared from copper nitrate x The / NENU-3 composite catalyst achieved a yield of 16370 μg / h for the reduction of nitrate to ammonia. -1 mg cat. -1 However, the Faraday efficiency was only 79.49%, indicating that serious competitive side reactions occurred during ammonia synthesis. Patent CN118407081A discloses a copper-molybdenum nanocrystalline alloy catalyst supported on nickel foam, which exhibited a Faraday efficiency of 98.9%, but the ammonia yield was only 479.2 μmol / h. -1 cm -2 (8150μg h -1 cm -2 This seriously affects the further practical application of the catalyst.

[0004] Therefore, designing and preparing an electrocatalyst that combines high ammonia yield and high Faradaic efficiency is of great significance for sustainable and efficient ammonia synthesis. Summary of the Invention

[0005] The purpose of this invention is to address the limitations of current technologies by providing a method for preparing and applying an iodine-doped cobalt-based electrocatalyst. This method utilizes a mild ammonium iodide-based simultaneous doping and etching strategy to successfully anchor cobalt in a single-atom and cluster coexistence form on an iodine-doped carbon substrate in one step. The resulting iodine-doped cobalt-based electrocatalyst possesses tandem catalytic active sites composed of iodine-coordinated cobalt single atoms and cobalt clusters, enabling simultaneous promotion of the deoxygenation and hydrogenation steps in the electrocatalytic nitrate reduction reaction. The electrocatalytic ammonia synthesis yielded by this invention exhibits a yield exceeding 17000 μg h⁻¹. -1 mg cat. -1 The ammonia yield and faradaic efficiency of over 95% achieve a combination of high ammonia yield and high faradaic efficiency, providing a new approach for green, efficient and sustainable ammonia synthesis.

[0006] The technical solution of this invention is as follows:

[0007] A method for preparing an iodine-doped cobalt-based electrocatalyst, the method comprising the following steps:

[0008] (1) Mix a methanol solution containing 2-methylimidazole with a methanol solution containing cobalt nitrate hexahydrate and stir for 30-45 minutes. Then let it stand for 2-5 hours to age. Then centrifuge, wash and vacuum dry to obtain the purple precursor, namely ZIF-67.

[0009] The mass ratio of cobalt nitrate hexahydrate to 2-methylimidazole is 1:1 to 1.5.

[0010] (2) The dried purple precursor was placed in a tube furnace filled with Ar atmosphere for pyrolysis at 700-800℃ and held for 3-5 hours to obtain black powder, i.e. Co nanoparticles anchored on carbon substrate (Co NP / C).

[0011] (3) Co NP / C and ammonium iodide were placed in the downstream and upstream of a tube furnace, respectively, and pyrolyzed in an argon gas atmosphere at 700-800°C for 1-3 hours to obtain an iodine-doped cobalt-based electrocatalyst.

[0012] The mass ratio is Co NP / C:ammonium iodide = 1:20-25. The heating rate is 1-5℃ / min.

[0013] In the methanol solution of cobalt nitrate hexahydrate, 400-500 mg of cobalt nitrate hexahydrate is added for every 40 mL of methanol.

[0014] In the methanol solution of 2-methylimidazole, 400-750 mg of 2-methylimidazole is added for every 20 mL of methanol.

[0015] The iodine-doped cobalt-based electrocatalyst prepared by the method is used for the electrocatalytic reduction of nitrate to synthesize ammonia.

[0016] Specifically, the steps include: in a three-electrode system, the electrolyte is electrolyzed for 1 to 2 hours using a constant voltage method to obtain ammonia;

[0017] In the three-electrode system, platinum sheet, Ag / AgCl and carbon paper supported on iodine-doped cobalt-based electrocatalyst are used as counter electrode, reference electrode and working electrode, respectively; the electrolytic cell type is H-type electrolytic cell, the diaphragm is Nafion 117 membrane; the electrolyte composition is 0.1-0.5M K2SO4 solution containing 0.1-0.5M KNO3.

[0018] In the working electrode, 0.5 to 1 mg of catalyst is loaded per 1 square centimeter of carbon paper;

[0019] The voltage range for electrolysis is -0.3V to -0.8V.

[0020] The essential features of this invention are:

[0021] This invention constructs a tandem catalytic active site consisting of iodine-coordinated cobalt single atoms and cobalt clusters, thereby simultaneously promoting the deoxygenation and hydrogenation steps of the electrocatalytic reduction of nitrate. It utilizes a mild ammonium iodide-based simultaneous doping and etching strategy, completing the doping and etching processes that would normally require two steps in a single step. During pyrolysis, ammonium iodide acts as both an etchant and a dopant. Specifically, the iodine vapor released from the decomposition of ammonium iodide not only achieves heteroatom doping by forming CI bonds with the carbon substrate, but also reacts with excess cobalt nanoparticles to form volatile cobalt iodide, which escapes from the catalyst with the gas flow. This results in an electrocatalyst (Co SAAC / IC) where cobalt single atoms and cobalt clusters coexist on iodine-doped carbon. This promotes the deoxygenation and hydrogenation processes in the electrocatalytic reduction of nitrate to ammonia, achieving both high ammonia yield and high Faradaic efficiency.

[0022] The present invention has the following beneficial effects:

[0023] 1. The electrocatalyst of the present invention can achieve doping of carbon substrate and etching of cobalt metal nanoparticles in one step through a mild ammonium iodide pyrolysis process.

[0024] 2. This invention cleverly constructs a series of catalytic active sites where iodine-doped cobalt single atoms and cobalt clusters coexist through a mild doping and simultaneous etching strategy. This can efficiently promote the deoxygenation and hydrogenation processes in the electrocatalytic reduction of nitrate, which not only improves the yield of electrocatalytic ammonia synthesis but also improves the Faraday efficiency.

[0025] 3. Due to the coordination of iodine atoms on cobalt single atoms, the electrocatalytic nitrate reduction reaction changes from a single adsorption mechanism to a tandem catalytic mechanism, thereby effectively improving the performance of electrocatalytic ammonia synthesis.

[0026] 4. An iodine-doped carbon structure with coexisting cobalt single atoms and cobalt clusters can be obtained through a simultaneous etching strategy using ammonium iodide doping. The cobalt single atoms coordinated by iodine change from favoring the adsorption of water and hydrogen to favoring the adsorption of nitrate, thus driving the electrocatalytic nitrate reduction reaction from a single adsorption mechanism to a tandem catalytic mechanism. Meanwhile, the adjacent cobalt clusters can provide abundant active hydrogen for the subsequent hydrogenation step by accelerating water dissociation, thereby forming a tandem catalytic active site with the iodine-coordinated cobalt single atoms to promote the electrocatalytic nitrate reduction reaction.

[0027] 5. Benefiting from the mechanism transformation caused by iodine doping and the construction of tandem catalytic active sites, the iodine-doped cobalt-based electrocatalyst simultaneously exhibited high ammonia yield (17790 μg / h) in the electrocatalytic nitrate reduction reaction. -1 mg cat. -1 The iodine-doped cobalt-based electrocatalyst prepared in this invention exhibits a high Faradaic efficiency (96.8%). The ammonia yield exceeds 17 mg / h. -1 mg cat. -1 Furthermore, the Faraday efficiency is greater than 95%, which simultaneously achieves the high level of previously reported ammonia synthesis electrocatalysts, providing new inspiration for the preparation of ammonia synthesis electrocatalysts with tandem catalytic active sites. Attached Figure Description

[0028] Figure 1 The image shows a scanning electron microscope image of the electrocatalyst prepared in Example 1.

[0029] Figure 2 The image shows a transmission electron microscope image of the electrocatalyst prepared in Example 1.

[0030] Figure 3 The image shows an aberration-corrected transmission electron microscope image of the electrocatalyst prepared in Example 1.

[0031] Figure 4 X-ray diffraction patterns of the electrocatalysts prepared in Examples 1, 2, and 3.

[0032] Figure 5 The image shows the near-edge X-ray absorption fine structure spectrum of the electrocatalyst prepared in Example 1.

[0033] Figure 6 The extended edge X-ray absorption fine structure spectrum of the electrocatalyst prepared in Example 1 is shown.

[0034] Figure 7Linear sweep voltammetry curves of the electrocatalysts prepared in Examples 1, 2, and 3 in the electrolyte.

[0035] Figure 8 The yields of ammonia synthesis from the electrocatalysts prepared in Examples 1, 2, and 3.

[0036] Figure 9 The Faraday efficiency of ammonia synthesis for the electrocatalysts prepared in Examples 1, 2, and 3. Detailed Implementation

[0037] The specific implementation steps and accompanying drawings of the present invention will be described in full and clearly below. It should be noted that the specific implementation of the present invention should not be considered limited to these descriptions. For those skilled in the art, other embodiments or inferences obtained without innovation should be considered to fall within the scope of protection of the present invention.

[0038] Example 1:

[0039] The specific preparation method of iodine-doped cobalt-based electrocatalysts is as follows:

[0040] (1) Dissolve 450 mg of cobalt nitrate hexahydrate (Co(NO3)2·6H2O) in 40 mL of methanol and stir for 30 minutes to obtain a methanol solution containing Co(NO3)2·6H2O.

[0041] (2) Dissolve 500 mg of 2-methylimidazole in 20 mL of methanol and sonicate for 30 minutes to obtain a methanol solution containing 2-methylimidazole.

[0042] (3) Mix all the methanol solutions containing Co(NO3)2·6H2O obtained above with the methanol solution containing 2-methylimidazole, stir for 30 minutes, let stand and age for 4 hours, then centrifuge, wash and vacuum dry to obtain ZIF-67 precursor.

[0043] (4) ZIF-67 was placed in a tube furnace filled with Ar atmosphere and heated to 800°C at a rate of 2°C / min and held for 3 hours to obtain cobalt nanoparticles (Co NP / C) loaded on a carbon substrate.

[0044] (5) Place 1g of Co NP / C and 25g of ammonium iodide at the downstream and upstream of a tube furnace, respectively. Heat to 800℃ at a rate of 10℃ / min under an Ar atmosphere with a gas flow rate of 40scm and hold for 1 hour, then allow to cool naturally to obtain an electrocatalyst (Co SAAC / IC) in which cobalt single atoms and cobalt clusters coexist on iodine-doped carbon.

[0045] The morphology of CoSAAC / IC was observed using scanning electron microscopy (SEM). Figure 1 As shown, Co SAAC / IC exhibits a slightly wrinkled rhombic dodecahedral structure, and no obvious cobalt nanoparticles were observed.

[0046] The morphology and structure of CoSAAC / IC were analyzed using transmission electron microscopy (TEM). For example... Figure 2 As shown, no aggregation of cobalt nanoparticles was observed at high resolution, indicating that the excess cobalt nanoparticles were successfully etched away by ammonium iodide.

[0047] The presence of cobalt in Co SAAC / IC was further observed using aberration-corrected transmission electron microscopy (AC-TEM). Figure 3 As shown, the coexistence of numerous cobalt single atoms and cobalt clusters can be clearly observed, with the bright spots in the dashed and solid circles representing cobalt single atoms and cobalt clusters, respectively. Notably, the cobalt clusters are almost entirely surrounded by cobalt single atoms, suggesting a possible synergistic effect between the two.

[0048] The phase composition of CoSAAC / IC was analyzed using X-ray diffraction (XRD). Figure 4 Only a broad diffraction peak at 26° was observed, representing the (002) plane of carbon. No diffraction peaks were observed for metallic cobalt, confirming the atomically dispersed form of the cobalt species.

[0049] The electronic structure and valence states of cobalt species in Co SAAC / IC were analyzed using near-edge X-ray absorption fine structure spectroscopy (XANES). Figure 5 As shown, the near-edge absorption energy of cobalt in Co SAAC / IC lies between that of cobalt foil and cobalt oxide, indicating that the valence state is between 0 and +2. Furthermore, Co SAAC / IC exhibits a nearly flat characteristic in the 7730-7780 eV range, corresponding to the combined contribution of cobalt single atoms and cobalt clusters.

[0050] The coordination environment of cobalt species in Co SAAC / IC was analyzed using extended-edge X-ray absorption fine structure spectroscopy (EXAFS). Figure 6 As shown, Co SAAC / IC in The peak at this location exhibits the same position as the first main peak of cobalt phthalocyanine, indicating the presence of a single-atom cobalt structure. Additionally, the peak located at... The characteristic peak at the same location as the main peak of the cobalt foil proves the existence of cobalt clusters.

[0051] The electrocatalytic performance of CoSAAC / IC was studied using linear sweep voltammetry. Figure 7 As shown, the Co SAAC / IC exhibited the lowest overpotential (135.1 mV) and the highest current density (89.53 mA cm⁻¹).-2 This indicates that Co SAAC / IC possesses excellent electrocatalytic activity, demonstrating that tandem catalytic active sites are crucial for promoting the electrocatalytic nitrate reduction reaction.

[0052] Specifically, the steps are as follows: (1) Electrolytic reduction of nitrate to ammonia synthesis was carried out on a CHI-760E electrochemical workstation of Shanghai Chenhua using a three-electrode system, wherein a platinum sheet, Ag / AgCl, and an area of ​​1 cm² were used. 2 The carbon paper-supported catalyst was used as the counter electrode, reference electrode, and working electrode, respectively. (2) Electrolysis test was carried out in an H-type electrolytic cell separated by a Nafion 117 membrane, containing an electrolyte of 0.1 M KNO3 and 0.5 M K2SO4. The Nafion 117 membrane was pretreated by first being treated in 5 wt% H2O2 solution at 80 °C for 1 hour and then soaked in deionized water for 30 minutes, then soaked in H2SO4 at 80 °C for 1 hour and then soaked in deionized water at 80 °C for 30 minutes. (3) The working electrode was prepared by dispersing 10 mg of catalyst in a mixed solution (50 μL Nafion solution + 0.45 mL deionized water + 0.45 mL anhydrous ethanol) and ultrasonically treating it for half an hour to form a uniform ink. 50 μL of catalyst ink was dropped into a 1 cm 2 (4) All electrolysis potentials involved in this work are reversible hydrogen electrode (RHE) potentials, and the conversion formula is E(RHE)=E(Ag / AgCl)+0.059×pH+0.197. (5) The electrocatalyst was subjected to chronoamperometry tests for 1 hour at different electrolysis potentials, and the electrolysis potential range was -0.3V~-0.8V. (6) After the chronoamperometry test was completed, 2mL of electrolyte was taken out and the ammonia synthesized by the electrocatalyst was qualitatively and quantitatively analyzed by ultraviolet spectrophotometer.

[0053] The amount of ammonia synthesized in the electrolyte after electrolysis of CoSAAC / IC using a UV spectrophotometer was determined. Figure 8 As shown, the ammonia yield of CoSAAC / IC reached 17790 μg / h under an applied voltage of -0.6V. - 1 mg cat. -1 This makes sustainable, large-scale ammonia synthesis possible. Figure 8 The Co SAAC / IC, Co NP / C, and CoSA / IC involved were all tested using a three-electrode system on a CHI-760E electrochemical workstation. The three-electrode system used platinum plates, Ag / AgCl electrodes, and electrodes with an area of ​​1 cm². 2Carbon paper-supported catalysts were used as the counter electrode, reference electrode, and working electrode, respectively. The electrolysis test method employed was chronoamperometry. The applied voltage range was -0.3V to -0.8V (constant voltage). The electrolyte composition was 0.5M K₂SO₄ containing 0.1M KNO₃. The ammonia yield after electrolysis was determined using a UV spectrophotometer.

[0054] The ammonia yield obtained from Co SAAC / IC at different voltages was used to calculate the corresponding Faraday efficiency. For example... Figure 9 As shown, the Co SAAC / IC maintains a Faradaic efficiency of over 80% across the electrolysis voltage range and reaches as high as 96.8% at an applied voltage of -0.6V, demonstrating its ability to improve the selectivity of the electrocatalytic reduction of nitrate to ammonia by suppressing competitive side reactions.

[0055] Example 2

[0056] The other steps are the same as in Example 1, except that step (5) of Example 1 is omitted, and the resulting sample is cobalt nanoparticles (Co NP / C) loaded on a carbon substrate;

[0057] The phase composition of Co NP / C was analyzed using X-ray diffraction patterns. For example... Figure 4 As shown, a typical broad peak is observed at 26°, which represents the (002) plane of carbon; three distinct sharp characteristic peaks are observed at 44.9°, 52.5° and 76.6°, which belong to the (111), (200) and (220) crystal planes of cobalt, respectively, indicating that cobalt nanoparticles are extensively aggregated on the carbon substrate.

[0058] The electrocatalytic performance of Co NP / C was analyzed using linear sweep voltammetry. Figure 7 As shown, the current density of Co NP / C at each potential is lower than that of Co SAAC / IC, indicating that the electrocatalytic activity of Co NP / C for nitrate reduction to ammonia synthesis is lower than that of Co SAAC / IC.

[0059] The ammonia synthesis in the electrolyte after electrolysis of Co NP / C under different constant voltages was detected using a UV spectrophotometer. Figure 8 As shown, the ammonia yield of Co NP / C at an applied voltage of -0.6V is 9535 μg / h. -1 mg cat. -1 .

[0060] The Faraday efficiency of the Co NP / C electrocatalyst was calculated using the ammonia yield at different voltages. Figure 9As shown, the Co NP / C electrocatalyst exhibits a Faraday efficiency of 72% at an applied voltage of -0.6V.

[0061] The ammonia yield and Faraday efficiency of Co NP / C at an applied voltage of -0.6V were 9535 μg h⁻¹. -1 mg cat. -1 Both were 72%, lower than Co SAAC / IC. Because the simultaneous etching step involving ammonium iodide doping was not performed, Co NP / C lacks iodine-doped single-atom structures, which is detrimental to the catalyst's deoxygenation process in the electrocatalytic nitrate reduction reaction, thus reducing the overall performance of electrocatalytic ammonia synthesis.

[0062] Example 3

[0063] The other steps are the same as in Example 1, except that step (6) is added after step (5) in Example 1. The Co SAAC / IC obtained in step (5) of Example 1 is dispersed in HCl solution and stirred for 24 hours. Then, the acid solution is washed with deionized water until neutral. Finally, it is freeze-dried to obtain a sample (Co SA / IC) with cobalt single atoms loaded on iodine-doped carbon.

[0064] X-ray diffraction patterns were used to analyze Co SA / IC. For example... Figure 4 As shown, only a broad characteristic diffraction peak at 26° belonging to carbon was observed, and no diffraction peaks related to the metallic state of cobalt were observed, which proves that the cobalt species exists in an atomically dispersed form.

[0065] The electrocatalytic performance of CoSA / IC was analyzed using linear sweep voltammetry. Figure 7 As shown, the current density of Co SA / IC at each potential is lower than that of Co SAAC / IC, indicating that the activity of Co SA / IC electrocatalyst in the electrocatalytic reduction of nitrate to ammonia is lower than that of Co SAAC / IC.

[0066] The ammonia synthesis in the electrolyte after electrolysis of CoSA / IC under different constant voltages was detected using a UV spectrophotometer. Figure 8 As shown, the CoSA / IC electrocatalyst achieved an ammonia yield of 6375 μg h⁻¹ at an applied voltage of -0.6 V. -1 mg cat. -1 .

[0067] The Faraday efficiency was calculated using the ammonia yield of the CoSA / IC electrocatalyst at different voltages. For example... Figure 9 As shown, the Co SA / IC has a Faraday efficiency of only 58% at an applied voltage of -0.6V.

[0068] The ammonia yield and Faraday efficiency of CoSA / IC at an applied voltage of -0.6V were 6375 μg / h, respectively. -1 mg cat. -1 The performance of CoSAAC / IC was 58%, far inferior to that of CoSAAC / IC. Immersion in HCl solution removed the cobalt clusters in CoSAAC / IC with acid washing, resulting in CoSA / IC containing only iodine-doped cobalt single atoms. Therefore, the performance degradation in Example 3 was due to the lack of cobalt clusters slowing down the hydrogenation step in the electrocatalytic nitrate reduction reaction, thus limiting the conversion of the reaction intermediate to the final product, ammonia.

[0069] Example 4

[0070] The other steps are the same as in Example 1, except that the mass of 2-methylimidazole in step (4) of Example 1 is replaced by 600 mg instead of 500 mg;

[0071] The resulting material properties are close;

[0072] Example 5

[0073] The other steps are the same as in Example 1, except that the annealing temperature in step (4) of Example 1 is replaced by 700°C instead of 800°C.

[0074] The resulting material properties are close;

[0075] Example 6

[0076] The other steps are the same as in Example 1, except that the mass of ammonium iodide in step (5) of Example 1 is replaced by 20g instead of 25g.

[0077] The resulting material properties are close;

[0078] Example 7

[0079] The other steps are the same as in Example 1, except that the pyrolysis time in step (5) of Example 1 is changed from 1 hour to 3 hours.

[0080] The resulting material properties are close;

[0081] In summary, the electrocatalyst prepared in this invention solves the problem that current electrocatalysts for ammonia synthesis cannot simultaneously achieve high yield and high Faradaic efficiency. As a highly efficient ammonia synthesis electrocatalyst, Co SAAC / IC possesses tandem catalytic active sites capable of simultaneously promoting the deoxygenation and hydrogenation processes in the electrocatalytic nitrate reduction reaction. Benefiting from the iodine-doped-induced mechanism transition and the tandem catalytic effect between cobalt single atoms and cobalt clusters, Co SAAC / IC simultaneously achieves high ammonia yield and high Faradaic efficiency in the electrocatalytic nitrate reduction reaction, providing a possibility for efficient and sustainable electrocatalytic ammonia synthesis.

[0082] The above content is merely an example and illustration of the concept of the present invention. It should be noted that those skilled in the art can make various improvements and additions without departing from the method of the present invention, and these improvements and additions should also be considered within the scope of protection of the present invention. Any modifications, alterations, and equivalent changes made by those skilled in the art based on the above-disclosed technical content without departing from the spirit and scope of the present invention are equivalent embodiments of the present invention. Furthermore, any modifications, alterations, and evolutions made to the above embodiments based on the essential technology of the present invention still fall within the scope of the technical solution of the present invention.

[0083] Matters not covered in this invention are common knowledge.

Claims

1. A method for preparing an iodine-doped cobalt-based electrocatalyst, characterized in that, The method includes the following steps: (1) Mix a methanol solution containing 2-methylimidazole with a methanol solution containing cobalt nitrate hexahydrate and stir for 30-45 minutes. Let stand for 2-5 hours, then centrifuge, wash and vacuum dry to obtain the purple precursor, namely ZIF-67. The mass ratio of cobalt nitrate hexahydrate to 2-methylimidazole is 1:1 to 1.

5. (2) The dried purple precursor was placed in a tube furnace filled with Ar atmosphere for pyrolysis at 700~800℃ and held for 3~5 hours to obtain black powder Co NP / C; (3) Co NP / C and ammonium iodide were placed in the downstream and upstream of a tube furnace, respectively, and pyrolyzed in an argon atmosphere at a temperature of 700-800°C for 1-3 hours to obtain an iodine-doped cobalt-based electrocatalyst. The mass ratio is Co NP / C:ammonium iodide = 1:20~25.

2. The method for preparing the iodine-doped cobalt-based electrocatalyst as described in claim 1, characterized in that, In the methanol solution of cobalt nitrate hexahydrate, 400-500 mg of cobalt nitrate hexahydrate is added for every 40 mL of methanol. In the methanol solution of 2-methylimidazole, 400-750 mg of 2-methylimidazole is added for every 20 mL of methanol.

3. The method for applying the iodine-doped cobalt-based electrocatalyst prepared by the method described in claim 1, characterized in that, It is used for the electrocatalytic reduction of nitrates to synthesize ammonia.

4. The application method as described in claim 3, characterized in that, The process includes the following steps: In a three-electrode system, the electrolyte is electrolyzed using a constant voltage method for 1-2 hours to obtain ammonia; In the three-electrode system, platinum sheets, Ag / AgCl, and carbon paper supported on iodine-doped cobalt-based electrocatalysts are used as the counter electrode, reference electrode, and working electrode, respectively; the electrolytic cell type is an H-type electrolytic cell, and the diaphragm is a Nafion 117 membrane; the electrolyte is a 0.1-0.5M K2SO4 solution containing 0.1-0.5M KNO3. In the working electrode, 0.5~1mg of catalyst is loaded per 1 square centimeter of carbon paper; The voltage range for electrolysis is -0.3V to -0.8V.

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