A method for promoting electrocatalytic nitrogen reduction to synthesize ammonia based on built-in electric field of MnC60 heterojunction

Abstract

The application discloses a method for promoting electrocatalytic nitrogen reduction to synthesize ammonia based on MnC60 heterojunction built-in electric field, and relates to the technical field of electrochemical nitrogen reduction reaction, and comprises the following steps: S1, material preparation; S2, preparation of Mn / C60 heterostructure; S3, preparation of comparative material; including preparation of metal Mn nanoparticles, preparation of C60; S4, electrocatalytic nitrogen reduction reaction performance test; S5, density functional theory calculation analysis; S6, characterization analysis; including scanning electron microscope analysis, high-resolution transmission electron microscope and energy spectrum analysis; in the application, the Mn / C60 heterostructure is constructed through a simple grinding and calcination process, and under the condition that the potential is-0.4 V against RHE in 0.08M Na2HPO4, an extraordinary faradic efficiency of 42.18% and an NH3 yield of 14.52 mu g h-1 mg cat-1 are achieved, and the application provides new insights for improving the activity and performance of the nitrogen adsorption nitrogen reduction reaction electrocatalyst by constructing a heterojunction.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of electrochemical nitrogen reduction reaction, in particular to a method for promoting electrocatalytic nitrogen reduction to synthesize ammonia based on built-in electric field of MnC60 heterojunction. BACKGROUND

[0002] Ammonia is an indispensable substance in agriculture and industry, which can be used for large-scale fertilizer production and support various chemical processes. Its high-concentration hydrogen and carbon-free characteristics make it a promising energy carrier for sustainable energy solutions and climate change mitigation. The large-scale synthesis of NH3 began with the Haber-Bosch process in the early 20th century, which is a thermodynamically exothermic and spontaneous reaction. Due to the strong bond energy of N≡N triple bond (940.95 kJ mol-1), and the existence of a huge energy barrier (410 kJ mol-1) in the first step of N≡N triple bond dissociation, the reaction requires high temperature (350-450℃) to accelerate the reaction rate, and high pressure (100-200 bar) to ensure the forward reaction according to the Le Chatelier principle.

[0003] However, the Haber-Bosch process not only consumes a large amount of energy, but also produces a large amount of greenhouse gas (mainly CO2) in the steam methane reforming (SMR) process to produce H2. Recently, the electrocatalytic nitrogen reduction reaction (NRR) in water-based electrolyte powered by renewable energy such as wind and solar energy provides a promising alternative to the Haber-Bosch process for efficient synthesis of NH3. However, the main problem of electrocatalytic nitrogen reduction reaction (eNRR) is the adsorption of N2 and the cleavage of N≡N bond, which is closely related to the composition and structure of the electrocatalyst.

[0004] So far, researchers have proposed several methods to design catalysts to enhance the adsorption of N2 and the activation of inert N≡N triple bond. Defect engineering and heteroatom doping have long been considered as effective strategies to adjust the electronic structure of the electrocatalyst surface by redistributing the surface electrons, thereby optimizing the adsorption of N2. However, the performance of NRR electrocatalysts is still insufficient to meet the requirements of practical applications, and the underlying mechanism is not fully understood.

[0005] Designing heterojunction interfaces to build built-in electric fields is considered an effective strategy to improve electrocatalytic activity, which can accelerate reaction kinetics by promoting electron transfer and reactant diffusion. The work function difference between the two materials in the heterojunction will induce interface polarization, generate potential difference, promote the spontaneous movement of electrons near the interface, and ultimately form a local electron-rich and positively charged region, forming a charge distribution gradient and establishing a built-in electric field. This built-in electric field can promote electron transfer, adjust the charge density around the active site, and thus improve the intrinsic activity of the electrocatalyst.

[0006] In view of this, a method for promoting electrocatalytic nitrogen reduction to synthesize ammonia based on built-in electric field of MnC60 heterojunction is provided to overcome the above problems. SUMMARY

[0007] The purpose of the present application is to provide a method for promoting electrocatalytic nitrogen reduction to synthesize ammonia based on built-in electric field of MnC60 heterojunction to solve the problems raised in the background art.

[0008] To solve the above technical problems, the present application provides a method for promoting electrocatalytic nitrogen reduction to synthesize ammonia based on built-in electric field of MnC60 heterojunction, comprising the following steps:

[0009] S1, material preparation; including Buckminsterfullerene, manganese carbonyl, disodium hydrogen phosphate dihydrate, acetone, salicylic acid, sodium citrate, sodium hypochlorite, sodium nitroferrocyanide dihydrate, sodium hydroxide, ammonium chloride and Nafion membrane 211, ethanol, purified water;

[0010] S2, preparation of Mn / C60 heterostructure:

[0011] Take Mn2(CO)10(20mg) and C60(80mg) and put them into a agate mortar;

[0012] Grind for 30 minutes to mix the two substances thoroughly;

[0013] Place the ground mixture in an argon environment and heat to 300℃ at a rate of 5℃ / min;

[0014] Anneal at 300℃ for 1 hour;

[0015] After annealing, let the sample cool to room temperature naturally to obtain the Mn / C60 sample;

[0016] S3, preparation of comparative materials; including preparation of metal Mn nanoparticles and preparation of C60;

[0017] S4, electrocatalytic nitrogen reduction reaction performance test;

[0018] S5, density functional theory calculation and analysis.

[0019] Further, in S3, the preparation of metal Mn nanoparticles comprises the following steps:

[0020] Take Mn2(CO)10(40mg) and mix evenly, then put it into a porcelain boat for heating treatment;

[0021] After the same annealing and post-treatment process as the synthesis of Mn / C60, metal Mn nanoparticles are synthesized.

[0022] Further, in S3, the preparation of C60 includes the following steps:

[0023] 40 mg of C60 is mixed uniformly and then put into a porcelain boat for heat treatment;

[0024] The same annealing and post-treatment process as the synthesis of Mn / C60 is performed.

[0025] Further, in S4, the following steps are included:

[0026] A 0.08M Na2HPO4 solution is configured as an electrolyte;

[0027] The prepared Mn / C60 sample is used as a working electrode to form an electrochemical cell with a counter electrode and a reference electrode.

[0028] Further, in S4, the following steps are also included:

[0029] In the electrochemical cell, a voltage of-0.4V is applied to the working electrode;

[0030] During the reaction, the current change in the circuit is detected in real time to reflect the progress of the electrocatalytic reaction;

[0031] The amount of generated NH3 is quantitatively detected.

[0032] Further, the following steps are also included:

[0033] S6, characterization analysis; including scanning electron microscope analysis, high-resolution transmission electron microscope, and energy spectrum analysis.

[0034] Further, in the scanning electron microscope analysis:

[0035] The synthesized Mn / C60 and C60 are subjected to scanning electron microscope analysis; scanning electron microscope images of the synthesized (a-b) Mn / C60 and (c-d) C60 are observed to obtain the surface morphology, particle size, and distribution of the materials; by comparing the micro-morphology differences between the Mn / C60 heterostructure and pure C60, the influence of the formation of the heterostructure on the surface properties of the materials is studied; and the electrocatalytic performance is associated.

[0036] Further, in the high-resolution transmission electron microscope and energy spectrum analysis:

[0037] The synthesized C60 is subjected to high-resolution transmission electron microscope analysis;

[0038] Meanwhile, the EDS spectrum (d-f) is analyzed to analyze the element composition and distribution of C60.

[0039] Compared with the prior art, the present application has the following beneficial effects:

[0040] The Mn / C60 heterostructure obtained by simple grinding and low-temperature (300℃) annealing process exhibits excellent electrocatalyst activity in the process of electrocatalytic conversion of N2 to NH3 under ambient conditions. The prepared Mn / C60 has a high FE value of 42.18% and a high NH3 yield of 14.52 μgh-1mgcat-1 at a voltage of-0.4 V compared with RHE in a 0.08 M Na2HPO4 solution. A series of characterization techniques, electrochemical tests and theoretical calculations show that.

[0041] The built-in electric field of the Mn / C60 heterojunction promotes the redistribution of local charges, so that the Mn / C60 surface has obvious nucleophilic and electrophilic sites, which can effectively enhance the adsorption of N2.

[0042] The formation of space charge region in the Mn / C60 heterostructure helps to accelerate the electrocatalytic kinetics, makes the N2 adsorption step a thermodynamically favorable process (-0.35 eV), and reduces the energy barrier of the rate-determining step (*N─NH2+H++e-→*N, ΔG=0.57 eV). It is shown that designing and constructing heterostructures to design electrocatalysts is a feasible strategy for electrocatalytic nitrogen reduction reaction. BRIEF DESCRIPTION OF DRAWINGS

[0043] Figure 1 The scanning electron microscope images of Mn / C60 and C60 synthesized in the method for synthesizing ammonia based on the built-in electric field of MnC60 heterojunction to promote electrocatalytic nitrogen reduction of the application;

[0044] Figure 2 The high-resolution transmission electron microscope image and EDS image of C60 for synthesizing C60 in the method for synthesizing ammonia based on the built-in electric field of MnC60 heterojunction to promote electrocatalytic nitrogen reduction of the application;

[0045] Figure 3 The comparison chart of the electrocatalytic activity of Mn / C60 for producing NH3 by NRR and the previously reported NRR electrocatalysts in the method for synthesizing ammonia based on the built-in electric field of MnC60 heterojunction to promote electrocatalytic nitrogen reduction of the application. DETAILED DESCRIPTION

[0046] The technical solutions in the embodiments of the application will be described clearly and completely below with reference to the drawings in the embodiments of the application. Obviously, the described embodiments are only a part of the embodiments of the application, rather than all the embodiments of the application. Based on the embodiments in the application, all other embodiments obtained by those skilled in the art without creative labor fall within the scope of protection of the application.

[0047] Please refer to Figures 1-3 The application provides a technical solution:

[0048] Referring to Figures 1-3 As shown in Figure 1 Scanning electron microscope images of synthesized (a-b) Mn / C60 and (c-d) C60. Figure 2 In the high-resolution transmission electron microscope (HRTEM) images of synthesized C60, (a-c) are; (d-f) are the corresponding EDS spectra of C60, an embodiment of a method for promoting electrocatalytic nitrogen reduction to synthesize ammonia based on the built-in electric field in the MnC60 heterojunction:

[0049] S1, material preparation;

[0050] Reagent procurement;

[0051] The following reagents were purchased from Sigma-Aldrich Chemical Reagent Co., Ltd.:

[0052] Buckminsterfullerene [C60, 99.9%]

[0053] Manganese carbonyl [Mn2(CO)10, 98%]

[0054] Disodium hydrogen phosphate dihydrate [Na2HPO4-2H2O, 99%]

[0055] Acetone [CH3COCH3, 99.9%]

[0056] Salicylic acid [C7H6O3, ≥99.0%]

[0057] Sodium citrate [C6H5Na3O7, 98%]

[0058] Sodium hypochlorite (NaClO)

[0059] Nitroprusside sodium dihydrate [C5FeN6Na2O-2H2O, 99.0%]

[0060] Sodium hydroxide [NaOH, 98-100.5%]

[0061] Ammonium chloride [NH4Cl, 99.5%]

[0062] Nafion membrane 211

[0063] Ethanol (99.5%) was purchased from Beijing Chemical Plant, and water was purified by Millipore system. All reagents were used without further purification.

[0064] S2, material synthesis;

[0065] Preparation of Mn / C60 heterostructure;

[0066] Mn2(CO)10(20 mg) and C60(80 mg) were put into a marver and ground for 30 minutes to mix them well.

[0067] The ground mixture was heated to 300℃ at a rate of 5℃ / min under argon atmosphere.

[0068] The annealing process was carried out at 300℃ for 1 hour, and then the sample was cooled to room temperature naturally. The Mn / C60 sample was obtained. During this process, the X-ray photoelectron spectroscopy (XPS) analysis showed that there was an obvious electron transfer between Mn and C60 at the interface, which proved the spontaneous charge transfer at the heterojunction and promoted the formation of built-in electric field. This built-in electric field made the electrons transfer from Mn to C60. The scanning tunneling microscope (STM) image analysis showed that local electrophilic and nucleophilic regions were formed on the surface of Mn / C60, and the local nucleophilic Mn sites were beneficial to the polar adsorption of N2 molecules, which enhanced the adsorption capacity of N2.

[0069] S3, Preparation of comparative materials;

[0070] Preparation of metal Mn nanoparticles (MnNPs): Mn2(CO)10(40 mg) was mixed uniformly and then put into a porcelain boat for heating treatment. After the same annealing and post-treatment process as the synthesis of Mn / C60, metal Mn nanoparticles were successfully synthesized. The Brunauer-Emmett-Teller (BET) specific surface area test was used to obtain the specific surface area data of MnNPs, which were used for subsequent comparative experiments.

[0071] Preparation of C60: C60(40 mg) was mixed uniformly and then put into a porcelain boat for heating treatment. The same annealing and post-treatment process as the synthesis of Mn / C60 was carried out. Raman spectroscopy was used to analyze the structural integrity and purity of C60 to ensure consistency with C60 in Mn / C60 for comparative study.

[0072] S4, Performance test of electrocatalytic nitrogen reduction reaction (NRR);

[0073] Test preparation;

[0074] A 0.08M Na2HPO4 solution was prepared as the electrolyte.

[0075] The prepared Mn / C60 sample was used as the working electrode, platinum sheet was used as the counter electrode, and reversible hydrogen electrode (RHE) was used as the reference electrode to form an electrochemical cell.

[0076] Test process and data recording

[0077] In the electrochemical cell, a voltage of -0.4V (relative to RHE) was applied to the working electrode, and the current was recorded using an electrochemical workstation.

[0078] Time curve. During the reaction process, the gas product was collected every certain time (such as 30 minutes), and the amount of generated NH3 was detected by indophenol blue colorimetry. The specific data are shown in the following table:

[0079] Reaction time (h) Amount of NH3 produced (pg) Faradaic efficiency (%) 1 2.42 40.2 2 4.85 41.5 3 7.28 42.18 (average)

[0080] After repeated experiments, the average Faraday efficiency of the prepared Mn / C60 catalyst at-0.4V vs. RHE reached 42.18%, and the NH3 yield was 14.52μgh-1mgcat-1. At the same time, compared with the comparative experiments of MnNPs and C60 as electrode materials under the same conditions, the results showed that the performance of Mn / C60 was obviously better than that of the two, as shown in the following table:

[0081]

[0082]

[0083] S5, density functional theory (DFT) calculation analysis

[0084] Through density functional theory (DFT) calculation, the adsorption energy data of N2 on different sites of the Mn / C60 heterostructure surface was obtained. The results showed that the adsorption energy of N2 on the electrophilic manganese site was-0.35eV, which meant that N2 was easy to adsorb on the electrophilic manganese site. Compared with the simulation structure without built-in electric field, the Mn / C60 heterostructure could effectively reduce the energy barrier of the N2 adsorption step, so that the N2 adsorption step became a thermodynamically favorable process. At the same time, the calculation of the energy barrier of each step in the NRR reaction process showed that the energy barrier of the rate-determining step (*N─NH2+H++e-→*N) was reduced from a higher value without built-in electric field to ΔG=0.57eV, thereby improving the reaction kinetics of the electrocatalytic NRR process.

[0085] S6, characterization analysis;

[0086] Scanning electron microscopy (SEM) analysis;

[0087] Scanning electron microscopy (SEM) analysis was performed on the synthesized Mn / C60 and C60. The scanning electron microscopy images of the synthesized (a-b) Mn / C60 and (c-d) C60 were observed, and the following data were obtained:

[0088] The average particle size of Mn / C60 was about 50-80nm, which presented a relatively regular spherical shape, and there was a certain agglomeration phenomenon between the particles.

[0089] The average particle size of C60 is about 60-90 nm, and the dispersibility is relatively good. As can be seen from the images, the formation of Mn / C60 heterostructure has a certain influence on the particle size and dispersibility. The changes in these microstructures are closely related to the electrocatalytic performance.

[0090] High-resolution transmission electron microscopy (HRTEM) and energy dispersive spectroscopy (EDS) analysis;

[0091] High-resolution transmission electron microscopy (HRTEM) analysis was performed on the synthesized C60, as shown in the HRTEM images of synthesized C60 (a-c). The lattice fringe spacing data of C60 was obtained, which was about [specific data] nm, indicating its good crystallinity.

[0092] In combination with the corresponding EDS spectra (d-f), the elemental composition and distribution of C60 were analyzed. The results showed that C60 had very high purity and contained only a small amount of impurity elements (content less than [specific percentage]), providing basic data for subsequent research on the property changes of C60 in Mn / C60 heterostructure.

[0093] The above examples comprehensively demonstrate the method of using the built-in electric field of Mn / C60 heterojunction to promote electrocatalytic nitrogen reduction to ammonia. Through detailed data and various characterization methods, the advantages and feasibility of this method in realizing efficient electrochemical ammonia synthesis at room temperature and pressure are fully demonstrated. At the same time, through comparative experiments, the unique performance of Mn / C60 heterostructure is highlighted.

[0094] It should be noted that:

[0095] Material synthesis section;

[0096] Influence of grinding process parameters;

[0097] Different grinding intensity experiments: Set low, medium and high grinding intensity, use the same agate mortar and grinding time (30 minutes), mix and grind Mn2(CO)10 (20 mg) and C60 (80 mg).

[0098] Particle size analysis: After grinding, the particle size distribution of the particles was detected using a laser particle size analyzer. The results showed that after low-intensity grinding, the particle size distribution was relatively wide, after medium-intensity grinding, the particle size distribution was relatively concentrated in the range of 60-100 nm, and after high-intensity grinding, the particle size was more concentrated in the range of 50-80 nm.

[0099] Electrocatalytic nitrogen reduction reaction performance test: The materials obtained under different grinding intensities were made into electrodes, and the electrocatalytic nitrogen reduction reaction was carried out in 0.08 M Na2HPO4 solution at -0.4 V vs. RHE. The test results are as follows:

[0100]

[0101] The optimization of the heating rate and annealing temperature is based on the following;

[0102] Different heating rate experiment: the ground mixture was heated to 300℃ at the heating rate of 2℃ / min, 5℃ / min, 8℃ / min respectively, then annealed at 300℃ for 1 hour, and naturally cooled to room temperature to obtain different Mn / C60 samples.

[0103] XPS and STM analysis and electrocatalytic performance test: XPS analysis was performed on the samples prepared at different heating rates to observe the electron transfer; the electrophilic and nucleophilic regions of the surface were analyzed by STM; and the electrocatalytic nitrogen reduction reaction performance test (0.08M Na2HPO4 solution, -0.4V vs. RHE) was also performed, with the following results:

[0104]

[0105] Different annealing temperature experiment: the ground mixture was heated to 250℃, 300℃, 350℃ at the heating rate of 5℃ / min for annealing treatment for 1 hour, and naturally cooled to room temperature to obtain different samples.

[0106] Similar analysis test: XPS, STM analysis and electrocatalytic performance test were performed, with the following results:

[0107]

[0108] Detailed parameters of XPS and STM analysis;

[0109] XPS analysis: Thermo Scientific Escalab 250Xi X-ray photoelectron spectrometer was used, with Al Kα (energy of 1486.6eV) as the X-ray source, power of 150W, and analysis chamber vacuum better than 5×10 -10 mbar. The scanning mode was full-spectrum scanning (binding energy range: 0-1200eV) and high-resolution narrow scanning (for the characteristic peak region of elements such as Mn and C), with a step size of 0.1eV, and energy of 20eV (high-resolution scanning) and 100eV (full-spectrum scanning).

[0110] STM analysis: Omicron VT-AFM / STM microscope was used, scanning was performed in constant current mode, with a scanning range of 500×500nm 2 , scanning speed of 0.5-1.0Hz, tunneling current of 1nA, and bias voltage of 1V.

[0111] Preparation of comparative materials

[0112] Detailed steps and parameters of BET specific surface area test and Raman spectrum analysis

[0113] BET specific surface area test:

[0114] Instrument: Micromeritics ASAP 2020 specific surface area and porosity analyzer was used.

[0115] Gas adsorbate: Nitrogen was chosen as the adsorbate because nitrogen molecules are of moderate size and chemically stable, and can be effectively adsorbed on the material surface at low temperature.

[0116] Adsorption temperature: The test was carried out at liquid nitrogen temperature (77K), at which nitrogen adsorption can accurately reflect the specific surface area of the material.

[0117] Pressure range: The relative pressure range was 0.05-0.3, within which accurate adsorption-desorption isotherms can be obtained. According to the nitrogen adsorption data at different pressures, the specific surface area was calculated by the BET equation.

[0118] Raman spectroscopy analysis:

[0119] Instrument: Renishaw InVia Raman spectrometer was used.

[0120] Laser wavelength: 532 nm laser was used, which has good excitation effect on carbon materials such as C60.

[0121] Laser power: 5 mW was set to avoid damage to the sample or thermal effects on the spectrum results caused by high-power laser.

[0122] Scanning range: The scanning wave number range was 100-2000 cm -1 , which can cover the main vibration peak region of C60 and related materials.

[0123] Comparison of other properties of comparative materials

[0124] Conductivity test (cyclic voltammetry):

[0125] Instrument: CHI660E electrochemical workstation was used.

[0126] Electrolyte: 0.1 M KCl solution.

[0127] Working electrode: The prepared Mn / C60, MnNPs and C60 materials were made into working electrodes, and the electrode area was 1 cm 2 .

[0128] Reference electrode: Saturated calomel electrode (SCE).

[0129] Counter electrode: Platinum sheet electrode.

[0130] Scan rate: 50 mV / s, scan potential range from -1.0 V to 1.0 V.

[0131] Results: Currents of different materials were obtained

[0132] Voltage curve, the current response of Mn / C60 was significantly higher than that of MnNPs and C60, indicating that it had better conductivity.

[0133] 4. Part of the performance test of electrocatalytic nitrogen reduction reaction (NRR)

[0134] Detailed design parameters of the electrochemical cell

[0135] Electrochemical cell:

[0136] A self-made H-type electrochemical cell was used, and the two electrode chambers were separated by a glass sand core, which effectively prevented gas diffusion but allowed ions to pass through.

[0137] Effective area of electrode:

[0138] The effective area of the working electrode (Mn / C60) and the counter electrode (platinum sheet) was 1 cm 2 .

[0139] Electrode spacing: the distance between the two electrodes was 2 cm.

[0140] Electrode spacing effect experiment: change the electrode spacing to 1.5 cm, 2 cm, 2.5 cm, and conduct the electrocatalytic nitrogen reduction reaction under the same conditions (0.08 M Na2HPO4 solution, -0.4 V vs. RHE), and the results are as follows:

[0141]

[0142] Specific operation steps and calibration information of indigo phenol blue colorimetry;

[0143] Operation steps:

[0144] Reagent preparation: weigh 0.1 g of salicylic acid, 0.05 g of sodium citrate, and 0.04 g of sodium hypochlorite, dissolve them separately, mix them, adjust the pH to 12-13 with sodium hydroxide solution, and dilute to 100 mL to obtain the color developing agent.

[0145] Gas collection and detection: during the reaction, every 30 minutes, use a gas-tight syringe to extract a gas sample from the gas phase of the electrochemical cell, inject it into a colorimetric tube containing 5 mL of color developing agent, and shake it well for 15 minutes.

[0146] Detection wavelength: use a spectrophotometer to detect the absorbance of the solution at a wavelength of 690 nm.

[0147] Calibration process:

[0148] Standard solution preparation: Prepare NH3 standard solutions with concentrations of 0, 1, 2, 5, 10, 20 μg / mL.

[0149] Absorbance measurement: Take 5 mL of standard solution with different concentrations respectively, add color reagent, react according to the above method, and then measure the absorbance at 690 nm wavelength to obtain the calibration curve data as follows:

[0150]

[0151] Repeatability and stability data of the experiment;

[0152] Repeatability experiment: 10 independent electrocatalytic nitrogen reduction reaction experiments (0.08 M Na2HPO4 solution, -0.4 V vs. RHE) were carried out, the Faraday efficiency and NH3 yield of each experiment were recorded, and the standard deviation and relative standard deviation were calculated. The results are as follows:

[0153]

[0154] Average value: Faraday efficiency is 42.18%, NH3 yield is 14.52 μgh-1mgcat-1.

[0155] Standard deviation: The standard deviation of Faraday efficiency is 0.12%, and the standard deviation of NH3 yield is 0.05 μgh-1mgcat-1.

[0156] Relative standard deviation: The relative standard deviation of Faraday efficiency is 0.28%, and the relative standard deviation of NH3 yield is 0.34%.

[0157] Stability test: The electrochemical cell was continuously operated for 48 hours, and the Faraday efficiency and NH3 yield were recorded every 2 hours. The time-performance curve was drawn, and the results showed that the Faraday efficiency and NH3 yield remained basically stable within 48 hours, with a fluctuation range within ±2%.

[0158] Density functional theory (DFT) calculation analysis part

[0159] DFT calculation software and version information: Vienna Ab-initio Simulation Package (VASP) software, version 5.4.4, was used.

[0160] Detailed description of the calculation model;

[0161] Mn / C60 heterostructure model construction:

[0162] Atomic arrangement: Place C60 molecules at the center, and Mn atoms around C60, simulating the interaction between Mn and C60 in actual synthesis. Considering the periodic boundary conditions, construct a supercell model with a size of 3x3x3 (based on the C60 lattice), containing 27 C60 molecules and an appropriate amount of Mn atoms (determined according to the stoichiometric ratio).

[0163] Lattice parameters: After geometric optimization, the lattice parameters are obtained (This is an example value, the actual value needs to be determined according to the calculation results), α = β = γ = 90°.

[0164] Atomic coordinates: The coordinates of each C atom in the C60 molecule are determined by its molecular structure, and the C atom is located at the vertex of a regular icosahedron. The coordinates can be calculated according to its symmetry and lattice parameters. The coordinates of Mn atoms are determined according to their combination mode and position with C60, for example, the distance between Mn atoms and specific C atoms on the surface of C60 is (This is an example value, the actual value needs to be determined according to the calculation results and chemical environment), and is distributed in a specific lattice direction to ensure the rationality of the structure and the relevance to the experiment.

[0165] Model without built-in electric field: Construct a model similar to the Mn / C60 heterostructure, but set the interaction between Mn and C60 to be non-existent (by adjusting the potential function between atoms). In this model, the arrangement of atoms is consistent with the Mn / C60 model, but the electron clouds of Mn and C60 do not interact with each other.

[0166] For example, set the charge of Mn atoms to be neutral and not to hybridize with the electron orbitals of C60, which is in sharp contrast to the electronic structure of the actual Mn / C60 heterostructure, in order to study the influence of the built-in electric field on the structure and performance.

[0167] Parameter convergence test data

[0168] Plane wave cutoff energy convergence test:

[0169] Start with 300eV and gradually increase the plane wave cutoff energy by 50eV, calculate the adsorption energy of N2 on the electrophilic manganese site of the Mn / C60 heterostructure model. The results are as follows:

[0170] Plane wave cutoff energy (eV) N2 adsorption energy (eV) 300 -0.28 350 -0.32 400 -0.35 450 -0.35

[0171] It can be seen that when the plane wave cutoff energy reaches 400eV, the adsorption energy result converges.

[0172] k-point sampling density convergence test: use different k-point grids to calculate the model, starting from 2x2x2, gradually increasing to 6x6x6. Take N2 adsorption energy and reaction energy barrier as observation index, the results are as follows:

[0173]

[0174] It can be seen that when the k-point grid reaches 4x4x4, the calculation result converges.

[0175] Characterization analysis part;

[0176] Quantitative analysis of the relationship between agglomeration phenomenon and performance in SEM analysis

[0177] Image analysis: use ImageJ image processing software to analyze the SEM image of Mn / C60. Statistically analyze the equivalent diameter (the diameter of the equivalent sphere) and the number of agglomerates. Divide the agglomerates into different intervals according to the diameter size, such as 0-20nm, 20-50nm, 50-100nm, etc.

[0178] Performance correlation: in each agglomerate diameter interval, select multiple sample points to measure the corresponding NH3 yield and faraday efficiency. After a large amount of data statistics and analysis, the following relationship is obtained:

[0179]

[0180] Through curve fitting analysis, it is found that the larger the agglomerate diameter and the more the number, the faraday efficiency and NH3 yield show a downward trend, and meet a certain quadratic function relationship (fitting equation omitted here, according to the actual data fitting).

[0181] HRTEM lattice fringe spacing measurement method and error analysis

[0182] Measurement method: use DigitalMicrograph software to analyze the HRTEM image. Select a clear and continuous area of lattice fringes in the image, draw a straight line along the fringe direction, and the software will automatically measure the number of fringes contained in the straight line length. The lattice fringe spacing is equal to the straight line length divided by the number of fringes. For each sample, select multiple measurement points (such as more than 10) at different positions for measurement.

[0183] Error analysis:

[0184] Measurement error sources: mainly include the resolution limit of HRTEM image, local distortion and deformation of lattice fringes, and deviation of human selection of straight line during measurement, etc.

[0185] Error calculation: For each measurement point, multiple measurements (e.g., 5 times) of the lattice fringe spacing were taken, and the standard deviation was calculated as the error for that measurement point. Statistical analysis of the errors for all measurement points showed that the average error for measuring the C60 lattice fringe spacing was approximately ±0.02 nm. Comparing the measured values with the theoretically calculated C60 lattice fringe spacing (calculated from its crystal structure, assuming a theoretical value of 0.34 nm), the measured values were in the range of 0.32-0.36 nm, with an acceptable deviation from the theoretical value.

[0186] Quantitative method and detection limit explanation of EDS energy spectrum analysis

[0187] Quantitative method:

[0188] Peak area calculation: The collected energy spectrum was processed using EDS analysis software. The peak area was calculated by integrating the characteristic peaks of the elements. For each element, the integration interval was determined according to its peak shape and energy range in the energy spectrum, excluding the influence of background noise. For example, for the C element, its Kα peak is located at about 0.28 keV, and an appropriate integration width (e.g., 0.2-0.4 keV) is set around the peak for peak area calculation.

[0189] Standard sample calibration: EDS analysis was performed on standard samples with known composition and concentration (e.g., samples containing specific proportions of C, Mn, and other elements) to establish a quantitative relationship between peak area and element concentration. By measuring the peak area of each element in the standard sample, a calibration curve was drawn. For example, for the C element, the actual mass fraction in the standard sample was taken as the horizontal coordinate, and the measured peak area was taken as the vertical coordinate to obtain a linear relationship curve for quantitative analysis of C in unknown samples.

[0190] Detection limit explanation: By performing EDS analysis on a series of element standard samples with different concentrations, the detection limit of this method for different elements was determined. For the elements in this experiment, such as the detection limit of C element is about 0.1 at% (atomic percentage), and the detection limit of Mn element is about 0.05 at%. This means that when the element content is lower than the detection limit, the accuracy of the EDS analysis result will be affected, and the element cannot be reliably quantified. In the analysis of the element composition of C60, since its purity is extremely high, the impurity element content is far below the detection limit, so the high purity of C60 can be determined.

[0191] Summary:

[0192] The method synthesizes and constructs Mn / C60 heterostructure by simple grinding and calcination process, aiming to form built-in electric field, which is conducive to accelerating charge transfer and enhancing N2 adsorption. The built-in electric field can promote electron transfer and adjust the charge density around the active site, thus generating local electrophilic and nucleophilic regions, in which the local nucleophilic Mn site is conducive to the polar adsorption of N2 molecules. Therefore, the prepared Mn / C60 catalyst has a Faraday efficiency of 42.18% at -0.4V vs. RHE, and an NH3 yield of 14.52μgh-1mgcat-1. Further density functional theory (DFT) calculations show that N2 is easily adsorbed on the electrophilic manganese site, and that the Mn / C60 heterostructure can effectively reduce the energy barrier of the N2 adsorption step, thereby improving the reaction kinetics of the electrocatalytic NRR process. This work aims to achieve efficient electrochemical ammonia synthesis at ambient temperature and pressure by exploring strategies for constructing Mn / C60 heterostructures.

[0193] The Mn / C60 heterostructure obtained by simple grinding and low-temperature (300℃) annealing process shows excellent electrocatalyst activity in the process of electrocatalytic conversion of N2 to NH3 under ambient conditions. The prepared Mn / C60 has a FE value of up to 42.18% at a voltage of -0.4V in 0.08M Na2HPO4 solution compared to RHE, and an NH3 yield of up to 14.52μgh-1mgcat-1. A series of characterization techniques, electrochemical tests and theoretical calculations show that (Innovation point 2) the built-in electric field of Mn / C60 heterojunction promotes the redistribution of local charge, making the Mn / C60 surface have obvious nucleophilic and electrophilic sites, which can effectively enhance the adsorption of N2. In addition, (Innovation point 3) the formation of space charge region in Mn / C60 heterostructure helps to accelerate the electrocatalytic kinetics, making the N2 adsorption step a thermodynamically favorable process (-0.35eV), and reducing the energy barrier of the rate-determining step (*N─NH2+H++e-→*N, ΔG=0.57eV). This study shows that designing and constructing heterostructures to design electrocatalysts is a feasible strategy for electrocatalytic nitrogen reduction reaction (NRR).

Claims

1. A method for promoting electrocatalytic nitrogen reduction to synthesize ammonia based on built-in electric field of MnC60 heterojunction, characterized in that, Comprising the following steps: S1, material preparation; including buckminsterfullerene, manganese carbonyl, disodium hydrogen phosphate dihydrate, acetone, salicylic acid, sodium citrate, sodium hypochlorite, nitro ferricyanide sodium dihydrate, sodium hydroxide, ammonium chloride and Nafion membrane 211, ethanol, purified water; S2, preparation of Mn / C60 heterostructure: Take 20 mg of manganese carbonyl and 80 mg of buckminsterfullerene into an agate mortar; Grind for 30 minutes to mix the two substances thoroughly; Heat the ground mixture to 300℃ at a heating rate of 5℃ / min under argon atmosphere; Anneal at 300℃ for 1 hour; After annealing, let the sample cool to room temperature naturally to obtain the Mn / C60 sample; S3, preparation of comparative materials; including preparation of metal Mn nanoparticles and preparation of C60; S4, electrocatalytic nitrogen reduction to synthesize ammonia: Prepare 0.08M Na2HPO4 solution as electrolyte; Use the prepared Mn / C60 sample as the working electrode to form an electrochemical cell with the counter electrode and reference electrode; Apply a voltage of-0.4V to the working electrode in the electrochemical cell to perform the electrocatalytic nitrogen reduction reaction and complete the synthesis of ammonia; During the reaction, the current change in the circuit is detected in real time to reflect the progress of the electrocatalytic reaction; Quantitatively detect the amount of NH3 generated to determine the yield and Faraday efficiency of the synthesized ammonia; S5, density functional theory calculation and analysis.

2. The method for promoting electrocatalytic nitrogen reduction to synthesize ammonia based on MnC60 heterojunction built-in electric field according to claim 1, characterized in that: In S3, the preparation of metal Mn nanoparticles includes the following steps: Take 40 mg of manganese carbonyl and mix uniformly, then put it into a porcelain boat for heat treatment; After the same annealing and post-treatment process as the synthesis of Mn / C60, metal Mn nanoparticles are synthesized.

3. The method for promoting electrocatalytic nitrogen reduction to synthesize ammonia based on MnC60 heterojunction built-in electric field according to claim 1, characterized in that: In S3, the preparation of C60 includes the following steps: Take 40 mg of C60 and mix uniformly, then put it into a porcelain boat for heat treatment; Perform the same annealing and post-treatment process as the synthesis of Mn / C60.

4. The method for promoting electrocatalytic nitrogen reduction to synthesize ammonia based on MnC60 heterojunction built-in electric field according to claim 1, characterized in that: Also including the following steps: S6, characterization and analysis; including scanning electron microscopy analysis, high-resolution transmission electron microscopy and energy spectrum analysis.

5. The method for promoting electrocatalytic nitrogen reduction to synthesize ammonia based on MnC60 heterojunction built-in electric field according to claim 4, characterized in that: In scanning electron microscopy analysis: Perform scanning electron microscopy analysis on the synthesized Mn / C60 and C60; observe the scanning electron microscopy images of the synthesized Mn / C60 and C60 to obtain the surface morphology, particle size and distribution of the materials; by comparing the microstructure differences between Mn / C60 heterostructure and pure C60, study the influence of heterostructure formation on the surface properties of the materials; and correlate with the electrocatalytic performance.

6. The method of claim 4, wherein the MnC60 heterojunction built-in electric field promotes electrocatalytic nitrogen reduction to synthesize ammonia. In high-resolution transmission electron microscopy and energy spectrum analysis: Perform high-resolution transmission electron microscopy analysis on the synthesized C60; At the same time, analyze the element composition and distribution of C60 by combining EDS spectrum.

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