Preparation method of boron-doped self-supporting three-dimensional non-metallic carbon-based catalytic electrode and application thereof in electrocatalytic nitrogen reduction

Abstract

This invention discloses a method for preparing a boron-doped self-supporting three-dimensional non-metallic carbon-based catalytic electrode and its application in electrocatalytic nitrogen reduction. Using starch, boric acid, and sodium carboxymethyl cellulose as raw materials, the boron-doped self-supporting three-dimensional non-metallic carbon-based catalytic electrode is obtained through freeze-drying and carbonization, or freeze-drying, pre-carbonization, and carbonization. This electrode can be used as a catalytic cathode for the electrochemical reduction of nitrogen. The product exhibits rich morphology and porosity. Boron doping intensifies the hybridization of the electronic structure and increases defects. This change effectively regulates nitrogen adsorption to promote the entire electrochemical reduction of nitrogen, resulting in an ammonia yield of 10 μg·h⁻¹. ‑1 ·mg ‑1 The invention eliminates the need for complex steps such as Nafion and carbon cloth treatment, making it convenient, efficient, and low-cost. The boron-doped self-supporting three-dimensional non-metallic carbon-based catalytic electrode produced is virtually pollution-free when discarded and easy to recycle.

Description

Technical Field

[0001] This invention relates to the technical field of catalyst materials, specifically to a method for preparing a boron-doped self-supporting three-dimensional non-metallic carbon-based catalytic electrode and its application in electrocatalytic nitrogen reduction. Background Technology

[0002] With increasing energy demand and worsening environmental pollution, finding clean energy sources to replace fossil fuels has become an urgent priority. Ammonia, with its advantages of zero carbon, high hydrogen content, and ease of liquefaction and storage, is considered a highly promising clean energy source and energy storage carrier. Meanwhile, as an important raw material for nitrogen fertilizers and chemical products, its global demand continues to rise. However, the current mainstream Haber-Bosch ammonia synthesis process suffers from high energy consumption, high emissions, and low conversion rates. Therefore, researchers are dedicated to developing novel synthetic routes, among which electrocatalytic nitrogen reduction (NRR) technology has become a research hotspot due to its ability to be driven by renewable energy under mild conditions. The key to this technology lies in developing efficient, stable, and low-cost catalysts.

[0003] Several studies have proposed different methods for catalyst preparation. For example, Chinese invention patent CN111632593A discloses a one-pot synthesis of ruthenium-rhodium alloy nanosphere catalysts using formaldehyde as a reducing agent, thus preparing a catalyst with electrocatalytic nitrogen reduction performance; Chinese invention patent CN111097452A discloses the preparation of a pre-reaction solution by adding an iron source reagent to an alkaline solution, heating it to obtain ferric oxide nanoparticles, and then performing a sulfidation reaction on a mixture of ferric oxide and graphene oxide to finally obtain graphene-supported ferrous sulfide with good catalytic effect; Chinese invention patent CN117512654A discloses the preparation of a unique fibrous nanocarbon structure by electrospinning and its application in electrocatalytic nitrogen reduction.

[0004] Although the above technologies have made some progress in improving the catalytic performance and stability of electrocatalytic nitrogen reduction, they still face the following challenges:

[0005] (1) Most studies on powder catalysis require Nafion or carbon cloth treatment, which makes it difficult to put into actual production; (2) Existing methods for preparing superior structures, such as extrusion and template methods (e.g., KOH activation), have problems such as high pollution, high consumption, and complex demolding, making them difficult to implement; (3) Most effective catalysts use metal raw materials, which cause serious pollution or are difficult to recycle when discarded, and are also costly.

[0006] Therefore, it is of great significance to develop a new method for preparing a catalyst with electrocatalytic nitrogen reduction performance. Summary of the Invention

[0007] To overcome the aforementioned defects and shortcomings in the prior art, this invention provides a method for preparing a boron-doped self-supporting three-dimensional non-metallic carbon-based catalytic electrode and its application in electrocatalytic nitrogen reduction.

[0008] The first objective of this invention is to provide a method for preparing a boron-doped self-supporting three-dimensional non-metallic carbon-based catalytic electrode.

[0009] The second objective of this invention is to provide a boron-doped self-supporting three-dimensional non-metallic carbon-based catalytic electrode prepared by the above-described preparation method.

[0010] A third objective of this invention is to provide the application of the aforementioned boron-doped self-supporting three-dimensional non-metallic carbon-based catalytic electrode in electrocatalytic nitrogen reduction.

[0011] The fourth objective of this invention is to provide a method for the electrocatalytic reduction of nitrogen to produce ammonia.

[0012] This invention claims protection for the following: A method for preparing a boron-doped self-supporting three-dimensional non-metallic carbon-based catalytic electrode includes the following steps: S1. Add sodium carboxymethyl cellulose to the starch paste containing boric acid. This step is to obtain a mixed gel solution of starch, boric acid and sodium carboxymethyl cellulose with a uniform three-dimensional cross-linked network. After thorough mixing and freeze-drying, a three-dimensional aerogel was obtained, which initially fixed the three-dimensional cross-linked network structure. The mass ratio of starch, boric acid, and sodium carboxymethyl cellulose is (38–42):(1–1.5):(0.18–0.22). S2. The three-dimensional aerogel is carbonized at 700–900 °C. After carbonization, a boron-doped self-supporting three-dimensional non-metallic carbon-based catalytic electrode is obtained.

[0013] This invention aims to simultaneously solve three interconnected problems: "macroscopic shaping and microstructure control of precursor gels", "maintaining and strengthening the three-dimensional structure during heat treatment", and "uniform introduction and efficient exposure of non-metallic active sites". Currently known starch carbonization, boron doping, or CMC gelation technologies cannot independently solve the above problems or serve the application purpose of this invention. This invention creatively designs a ternary synergistic gel system of soluble starch-boric acid-sodium carboxymethyl cellulose and combines it with a freeze-drying-gradient heat treatment process to solve the above problems.

[0014] Preferably, in step S1, the starch is soluble starch.

[0015] Preferably, in step S1, the mass ratio of starch, boric acid, and sodium carboxymethyl cellulose is 40:1:0.2.

[0016] Preferably, in step S1, the freeze-drying conditions are as follows: first, pre-freezing at -75℃ to -85℃ for 8 to 12 hours, followed by freeze-drying under vacuum conditions of -45℃ to -55℃ and 0.05 mbar to -0.1 mbar for a freeze-drying time of not less than 48 hours.

[0017] Preferably, in step S2, before carbonizing the three-dimensional aerogel at 700–900°C, it is pre-carbonized at 280–420°C to obtain a uniform three-dimensional structure with large pores. After pre-carbonization, the temperature is cooled to 23–27°C to fix the morphology before carbonization.

[0018] More preferably, the carbonization temperature is 800°C, and the pre-carbonization temperature is 300°C.

[0019] Preferably, in step S1, the method for preparing the boric acid-containing starch paste is as follows: mixing starch, boric acid and water, and heating at 85-95°C for 0.5-1.5 h.

[0020] More preferably, starch, boric acid and water are mixed and heated at 90°C for 1 h.

[0021] Preferably, in step S1, the temperature at which the mixture is fully mixed is 60–70°C.

[0022] More preferably, the temperature at which the mixture is fully mixed is 65°C.

[0023] Preferably, in step S2, the carbonization is carried out under an inert atmosphere, and the heating rate is 8-12°C / min.

[0024] More preferably, the carbonization is carried out in a nitrogen atmosphere at a heating rate of 10°C / min.

[0025] Preferably, the pre-carbonization is carried out under an inert atmosphere, with a heating rate of 3-7°C / min.

[0026] More preferably, the pre-carbonization is carried out under a nitrogen atmosphere at a heating rate of 5°C / min.

[0027] More preferably, the nitrogen gas flow rate is 150–250 mL / min.

[0028] More preferably, the nitrogen gas flow rate is 200 mL / min.

[0029] Preferably, the pre-carbonization and carbonization times are both 0.8 to 1.2 h.

[0030] More preferably, the pre-carbonization and carbonization times are both 1 h.

[0031] Preferably, in step S2, after carbonization, the product is washed with dilute hydrochloric acid, water, and ethanol.

[0032] The above preparation method yields a boron-doped self-supporting three-dimensional non-metallic carbon-based catalytic electrode.

[0033] The above-mentioned boron-doped self-supporting three-dimensional non-metallic carbon-based catalytic electrode is used in electrocatalytic nitrogen reduction.

[0034] A method for electrocatalytic reduction of nitrogen to ammonia, using the aforementioned boron-doped self-supporting three-dimensional non-metallic carbon-based catalytic electrode for electrocatalysis.

[0035] Compared with the prior art, the present invention has the following beneficial effects: This invention discloses a method for preparing a boron-doped self-supporting three-dimensional non-metallic carbon-based catalytic electrode and its application in electrocatalytic nitrogen reduction. The prepared boron-doped self-supporting three-dimensional non-metallic carbon-based catalytic electrode can be used as a catalytic cathode for the electrochemical reduction of nitrogen. The product of this invention has a rich morphology and porosity. The boron doping intensifies the hybridization of the electronic structure and increases defects. This change can effectively regulate nitrogen adsorption to promote the entire electrochemical reduction of nitrogen, resulting in an ammonia yield of 10 μg·h⁻¹. -1 ·mg -1 The invention eliminates the need for complex steps such as Nafion and carbon cloth treatment, making it convenient, efficient, and low-cost. The boron-doped self-supporting three-dimensional non-metallic carbon-based catalytic electrode produced is virtually pollution-free when discarded and easy to recycle. Attached Figure Description

[0036] Figure 1 The macroscopic structural diagrams of the boron-doped self-supporting carbon-based catalytic electrodes prepared in Examples 1, 5, and 6 are shown below: a: boron-doped self-supporting carbon-based catalytic electrode prepared in Comparative Example 5; b: boron-doped self-supporting carbon-based catalytic electrode prepared in Example 1; c: boron-doped self-supporting carbon-based catalytic electrode prepared in Comparative Example 6.

[0037] Figure 2 The following are macroscopic structural diagrams of the boron-doped self-supporting carbon-based catalytic electrodes prepared in Examples 1 and Comparative Examples 7-9: a: boron-doped self-supporting carbon-based catalytic electrode prepared in Comparative Example 7; b: boron-doped self-supporting carbon-based catalytic electrode prepared in Comparative Example 8; c: boron-doped self-supporting carbon-based catalytic electrode prepared in Example 1; d: boron-doped self-supporting carbon-based catalytic electrode prepared in Comparative Example 9.

[0038] Figure 3The following are SEM images of boron-doped self-supporting carbon-based catalytic electrodes: a: SEM image of Example 2 at 300 μm, b: SEM image of Comparative Example 1 at 300 μm, c: SEM image of Comparative Example 2 at 300 μm, d: SEM image of Example 1 at 300 μm, e: SEM image of Example 3 at 300 μm, f: SEM image of Comparative Example 3 at 300 μm, g: SEM image of Comparative Example 4 at 300 μm, h: SEM image of Example 1 at 300 μm, i: SEM image of Example 4 at 300 μm, j~m are EDX images of Example 1 at 200 μm.

[0039] Figure 4 LSV curves of boron-doped self-supporting carbon-based catalytic electrodes: a: LSV curves of boron-doped self-supporting carbon-based catalytic electrodes prepared in Examples 1-3 and Comparative Examples 1-2; b: LSV curves of boron-doped self-supporting carbon-based catalytic electrodes prepared in Examples 1, 4 and Comparative Examples 3-4.

[0040] Figure 5 Figures showing the ammonia yield and faradaic efficiency of boron-doped self-supporting carbon-based catalytic electrodes: a: Ammonia yield and faradaic efficiency of boron-doped self-supporting carbon-based catalytic electrodes prepared in Examples 1-3 and Comparative Examples 1-2; b: Ammonia yield and faradaic efficiency of boron-doped self-supporting carbon-based catalytic electrodes prepared in Examples 1, 4 and Comparative Examples 3-4.

[0041] Figure 6 The images show the Raman spectra of the boron-doped self-supporting carbon-based catalytic electrodes prepared in Examples 1, 4, and Comparative Examples 3-4. Detailed Implementation

[0042] The present invention will be further illustrated below with reference to specific embodiments, but the embodiments do not limit the present invention in any way. Unless otherwise specified, the reagents, methods, and equipment used in the present invention are conventional reagents, methods, and equipment in this technical field.

[0043] Unless otherwise specified, all reagents and materials used in the following examples are commercially available.

[0044] Example 1: A method for preparing a boron-doped self-supporting three-dimensional non-metallic carbon-based catalytic electrode This embodiment provides a method for preparing a boron-doped self-supporting three-dimensional non-metallic carbon-based catalytic electrode, including the following steps: S1. Dissolve 40 g of soluble starch and 1 g of boric acid in 100 mL of deionized water, and stir evenly at 600 rpm for 1 h in a 90℃ water bath to obtain a paste-like solution. S2. The paste-like solution was cooled to 65℃, and 0.2 g of sodium carboxymethyl cellulose was added as a gelling agent. The mixture was stirred until completely dissolved to obtain a mixed gel solution. The mixed gel solution was transferred to a 1.5×1.5×1.5 cm mold and pre-frozen in an ultra-low temperature freezer at -80℃ for 12 h. Then, it was freeze-dried in a freeze dryer at -50℃ and 0.1 mbar vacuum for 48 h to obtain boron-doped starch-based three-dimensional aerogel. S3. The boron-doped starch-based three-dimensional aerogel was placed in a tube furnace, and nitrogen gas was introduced at a flow rate of 200 mL / min. The temperature was increased to 300℃ at a heating rate of 5℃ / min for pre-carbonization for 1 h. Then, the morphology was fixed by natural cooling to obtain boron-doped self-supporting three-dimensional non-metallic carbon-based aerogel. S4. The boron-doped self-supporting three-dimensional non-metallic carbon-based aerogel was placed in a tube furnace for high-temperature carbonization. Nitrogen gas was introduced at a flow rate of 200 mL / min, and the temperature was increased to 800℃ at a heating rate of 10℃ / min for high-temperature calcination for 1 h. After natural cooling, the finished product was obtained. S5. Cut the finished product into 5×5×5 mm pieces, then clean it with 0.1 M dilute hydrochloric acid to remove excess boron oxide, then clean it with deionized water and ethanol and dry it to obtain a boron-doped self-supporting carbon-based catalytic electrode, named SCB-300-1.

[0045] Example 2: A method for preparing a boron-doped self-supporting three-dimensional non-metallic carbon-based catalytic electrode This embodiment provides a method for preparing a boron-doped self-supporting three-dimensional non-metallic carbon-based catalytic electrode according to Example 1, the difference being that: in step S3, pre-carbonization is not performed; the resulting boron-doped self-supporting carbon-based catalytic electrode is named SCB-0-1.

[0046] Example 3: A method for preparing a boron-doped self-supporting three-dimensional non-metallic carbon-based catalytic electrode This embodiment provides a method for preparing a boron-doped self-supporting three-dimensional non-metallic carbon-based catalytic electrode according to Example 1, the difference being that: in step S3, the pre-carbonization temperature is 400℃; the obtained boron-doped self-supporting carbon-based catalytic electrode is named SCB-400-1.

[0047] Example 4: A method for preparing a boron-doped self-supporting three-dimensional non-metallic carbon-based catalytic electrode This embodiment provides a method for preparing a boron-doped self-supporting three-dimensional non-metallic carbon-based catalytic electrode according to Example 1, the difference being that: in step S1, the amount of boric acid used is 1.5 g; the obtained boron-doped self-supporting carbon-based catalytic electrode is named SCB-300-1.5.

[0048] Comparative Example 1: A method for preparing a boron-doped self-supporting three-dimensional non-metallic carbon-based catalytic electrode This comparative example provides a method for preparing a boron-doped self-supporting three-dimensional non-metallic carbon-based catalytic electrode according to Example 1, the difference being that: in step S3, the pre-carbonization temperature is 100℃; the obtained boron-doped self-supporting carbon-based catalytic electrode is named SCB-100-1.

[0049] Comparative Example 2: A method for preparing a boron-doped self-supporting three-dimensional non-metallic carbon-based catalytic electrode This comparative example provides a method for preparing a boron-doped self-supporting three-dimensional non-metallic carbon-based catalytic electrode according to Example 1, the difference being that: in step S3, the pre-carbonization temperature is 200℃; the obtained boron-doped self-supporting carbon-based catalytic electrode is named SCB-200-1.

[0050] Comparative Example 3: A method for preparing a boron-doped self-supporting three-dimensional non-metallic carbon-based catalytic electrode This comparative example provides a method for preparing a boron-doped self-supporting three-dimensional non-metallic carbon-based catalytic electrode according to Example 1, the difference being that: in step S1, the amount of boric acid used is 0 g; the obtained boron-doped self-supporting carbon-based catalytic electrode is named SCB-300-0.

[0051] Comparative Example 4: A method for preparing a boron-doped self-supporting three-dimensional non-metallic carbon-based catalytic electrode This comparative example provides a method for preparing a boron-doped self-supporting three-dimensional non-metallic carbon-based catalytic electrode according to Example 1, the difference being that: in step S1, the amount of boric acid used is 0.5 g; the resulting boron-doped self-supporting carbon-based catalytic electrode is named SCB-300-0.5.

[0052] Comparative Example 5: A method for preparing a boron-doped self-supporting three-dimensional non-metallic carbon-based catalytic electrode This comparative example provides a method for preparing a boron-doped self-supporting three-dimensional non-metallic carbon-based catalytic electrode according to Example 1, the difference being that: in step S1, the amount of soluble starch used is 20 g; the obtained boron-doped self-supporting carbon-based catalytic electrode is named SCB-300-1-20.

[0053] Comparative Example 6: A method for preparing a boron-doped self-supporting three-dimensional non-metallic carbon-based catalytic electrode This comparative example provides a method for preparing a boron-doped self-supporting three-dimensional non-metallic carbon-based catalytic electrode according to Example 1, the difference being that: in step S1, the amount of soluble starch used is 60 g; the resulting boron-doped self-supporting carbon-based catalytic electrode is named SCB-300-1-60.

[0054] Comparative Example 7: A method for preparing a boron-doped self-supporting three-dimensional non-metallic carbon-based catalytic electrode This comparative example provides a method for preparing a boron-doped self-supporting three-dimensional non-metallic carbon-based catalytic electrode according to Example 1, the difference being that: in step S2, the amount of sodium carboxymethyl cellulose used is 0 g; the resulting boron-doped self-supporting carbon-based catalytic electrode is named SCB-300-1-0.

[0055] Comparative Example 8: A method for preparing a boron-doped self-supporting three-dimensional non-metallic carbon-based catalytic electrode This comparative example provides a method for preparing a boron-doped self-supporting three-dimensional non-metallic carbon-based catalytic electrode according to Example 1, the difference being that: in step S2, the amount of sodium carboxymethyl cellulose used is 0.1 g; the resulting boron-doped self-supporting carbon-based catalytic electrode is named SCB-300-1-0.1.

[0056] Comparative Example 9: A method for preparing a boron-doped self-supporting three-dimensional non-metallic carbon-based catalytic electrode This comparative example provides a method for preparing a boron-doped self-supporting three-dimensional non-metallic carbon-based catalytic electrode according to Example 1, the difference being that: in step S2, the amount of sodium carboxymethyl cellulose used is 0.3 g; the resulting boron-doped self-supporting carbon-based catalytic electrode is named SCB-300-1-0.3.

[0057] Test Example 1: Macroscopic Structure of Boron-Doped Self-Supported Three-Dimensional Non-metallic Carbon-Based Catalytic Electrode I. Experimental Methods The macroscopic structure of the boron-doped self-supporting carbon-based catalytic electrodes prepared in Example 1 and Comparative Examples 5-9 was observed with the naked eye.

[0058] II. Experimental Results The sample comparison diagrams of Example 1, Comparative Example 5, and Comparative Example 6 are shown below. Figure 1 As shown in the figure, it can be seen that the boron-doped self-supporting carbon-based catalytic electrode of Comparative Example 5 (using 20 g of soluble starch) has almost no macroscopic pores. Figure 1 (a) in the example may be due to the scarcity of raw materials, which provides ample space for carbon molecules to orient themselves, resulting in a slight collapse and compression of the structure. The boron-doped self-supporting carbon-based catalytic electrode of Example 1 (using 40 g of soluble starch) has an excellent pore structure. Figure 1 (b) If the amount of soluble starch is further increased, the boron-doped self-supporting carbon-based catalytic electrode of Comparative Example 6 (with 60 g of soluble starch) expands into an irregular shape instead. Figure 1 (c) This may be due to the rapid orientation of carbon molecules at high temperatures, while the already formed aerogel structure lacks sufficient space to complete structural changes, resulting in random orientation and expansion. Therefore, the amount of soluble starch was determined to be 40 g.

[0059] Comparison charts of samples from Example 1 and Comparative Examples 7-9 are shown below. Figure 2 As shown in the figure, it can be seen that the boron-doped self-supporting carbon-based catalytic electrode of Comparative Example 7 (with 0 g of sodium carboxymethyl cellulose) has almost completely random orientation. Figure 2 (a) The boron-doped self-supporting carbon-based catalytic electrode of Comparative Example 8 (using 0.1 g of sodium carboxymethyl cellulose) showed a certain orientation, which was more regular than that of the boron-doped self-supporting carbon-based catalytic electrode of Comparative Example 7. Figure 2 (b) The boron-doped self-supporting carbon-based catalytic electrode of Example 1 (using 0.2 g of sodium carboxymethyl cellulose) exhibits a more regular orientation and excellent structural porosity. Figure 2 (c) The boron-doped self-supporting carbon-based catalytic electrode of Comparative Example 9 (using 0.3 g of sodium carboxymethyl cellulose) showed a more regular orientation, but its pore structure disappeared. Figure 2 (d) This may be because sodium carboxymethyl cellulose acts as a gelling agent, forming a cross-linked network to increase the viscosity of the solution. However, excessive viscosity makes it more difficult for the carbon molecule structure to change during high-temperature calcination, thus maintaining a certain structure. But excessive addition results in carbon molecules having no room to move, and at high temperatures they can only be oriented in a certain regular pattern, which in turn causes macroscopic pore closure and a reduction in material volume. Therefore, the amount of sodium carboxymethyl cellulose was determined to be 0.2 g.

[0060] Test Example 2: Microstructure of a Boron-Doped Self-Supported Three-Dimensional Non-Metallic Carbon-Based Catalytic Electrode I. Experimental Methods The boron-doped self-supporting carbon-based catalytic electrodes prepared in Examples 1-4 and Comparative Examples 1-4 were observed by electron microscopy.

[0061] II. Experimental Results Scanning electron microscopy (SEM) images of the boron-doped self-supporting carbon-based catalytic electrodes prepared in Examples 1-4 and Comparative Examples 1-4 are shown below. Figure 3 As shown, it is evident that the material that has not undergone pre-carbonization ( Figure 3 (a) The structural voids are too large and unevenly distributed, and the voids are rough; after pre-carbonization at 100 and 200℃ ( Figure 3 b in Figure 3 (c) The morphology gradually transforms into a three-dimensional structure and a uniform high-density porous network structure; the temperature reaches 300℃ ( Figure 3 At point d), the pore size increases significantly, the pore distribution is moderate, and the overall structure of the material exhibits a honeycomb-like regular structure, showing a highly defective carbon skeleton. These are the basic characteristics of increasing the number of supported active centers; however, further increasing the calcination temperature to 400℃ ( Figure 3(e) In the case of the material, the structure collapses at the overheated temperature, and the pore size structure becomes disordered.

[0062] Figure 3 As can be seen from f in the figure, without boron doping, there is no honeycomb structure and the pore size is uneven; with the addition of boron, Figure 3 At g, a honeycomb-like porous structure with random orientation began to appear, but the pore size remained uneven; this continued until the amount of boric acid reached 1 g ( Figure 3 In the case of h), the pore distribution is uniform and the size is significantly increased; however, as the boron doping amount continues to increase, until 1.5 g ( Figure 3 In step i), it was found that the structural porosity actually decreased. Subsequent mapping scans (...) Figure 3 The presence of elements C, B, and O was verified by the (j~m) in the model.

[0063] Test Example 3: Effect of pre-carbonization temperature on the specific surface area, electrochemical performance, ammonia yield, and Faraday efficiency of a boron-doped self-supporting carbon-based catalytic electrode. I. Experimental Methods The boron-doped self-supporting carbon-based catalytic electrodes prepared in Examples 1-3 and Comparative Examples 1-2 were subjected to low-temperature nitrogen adsorption curve (BET) and linear sweep voltammetry (LSV) measurements, respectively. The specific steps of the linear sweep voltammetry are as follows: A boron-doped self-supporting carbon-based catalytic electrode was used as the working electrode. A linear voltage ranging from -2.4 V to 0 V was applied between the working electrode and the counter electrode (platinum electrode). The current changes through the electrodes within the potential window were recorded under argon and nitrogen atmospheres, respectively, at a potential scan rate of 5 mV·s. -1 .

[0064] Subsequently, the ammonia product was quantitatively tested using the Nessler's reagent method: 5 mL of solution was taken from each of the electrolytic cell and the absorption cell and added to a 50 mL colorimetric tube. Then, 1 mL of potassium sodium tartrate solution was added to each tube, and the mixture was stirred to adjust the pH to 11.8–12.4. 1.0 mL of Nessler's reagent was added and diluted with pure water to the 10 mL mark of the colorimetric tube. After mixing evenly, the tube was placed in the dark for 15 min, and then the ammonia production was measured at a wavelength of 425 nm using a UV-Vis spectrophotometer.

[0065] The formulas for calculating ammonia production efficiency and Faraday efficiency are as follows:

[0066] Y: Ammonia production efficiency; C: Concentration measured in electrolyte and absorbent (μg / mL); V: Volume (mL) in electrolyte and absorbent; T: Electrocatalytic ammonia production reaction test time (h); Mcat: Catalyst loading mass (mg); FE: Faraday efficiency (%); F: Faraday constant (96485 C / mol); Q: The amount of charge passing through the electrode (C).

[0067] II. Experimental Results The specific surface area, ammonia yield, and Faraday efficiency of the boron-doped self-supporting carbon-based catalytic electrodes prepared in Examples 1-3 and Comparative Examples 1-2 are shown in Table 1.

[0068] 1. Specific surface area The specific surface areas of SCB-0-1 (Example 2), SCB-100-1 (Comparative Example 1), SCB-200-1 (Comparative Example 2), SCB-300-1 (Example 1), and SCB-400-1 (Example 3) were 249.8, 46.2, 167.35, 438.6, and 290.8 m², respectively. 2 / g, of which SCB-300-1 prepared in Example 1 has the largest specific surface area and the best pore structure.

[0069] The specific surface area results show that the relationship is not entirely linear. When the pre-carbonization temperature is 0°C, the SCB-0-1 prepared in Example 2 has a larger specific surface area compared to the samples pre-carbonized at 100°C and 200°C. This may be because without pre-carbonization, the three-dimensional network lacks constraint, allowing internal gases to escape "unrestrainedly" during high-temperature carbonization, forming numerous but rough and disordered pores (249.8 m²). 2 / g); When the pre-carbonization temperature rises to 100–200℃, the carbon three-dimensional network crosslinks, but the network strength is uneven. Under the thermal stress of carbonization, this leads to premature structural collapse and overall densification, resulting in a low specific surface area (SCB-100-1 prepared in Comparative Example 1: 46.2 m). 2 / g, SCB-200-1 prepared in Comparative Example 2: 167.35 m 2 / g); When the pre-carbonization temperature is 300℃, the ternary network can be fully and uniformly cross-linked and solidified to form a strong and tough carbon skeleton, which can resist shrinkage and guide gas to form pores in an orderly manner, ultimately achieving ideal pores that are interconnected and highly active (SCB-300-1 prepared in Example 1: 438.6 m). 2 / g); However, a pre-carbonization temperature of 400℃ leads to over-carbonization, causing structural hardening, loss of plasticity, and pore fusion and closure, resulting in a decline in performance (SCB-400-1 prepared in Example 3: 290.8 m). 2 / g).

[0070] 2. Electrochemical performance The electrochemical performance of the boron-doped self-supporting carbon-based catalytic electrodes prepared in Examples 1-3 and Comparative Examples 1-2 is as follows: Figure 4 As shown in a, the effect trend is consistent with the specific surface area trend. The effect of the boron-doped self-supporting carbon-based catalytic electrode (SCB-300-1 prepared in Example 1) after pre-carbonization at 300℃ is twice that of the boron-doped self-supporting carbon-based catalytic electrode (SCB-0-1 prepared in Example 2) without pre-carbonization.

[0071] 3. Ammonia yield like Figure 5 As shown in a, the ammonia yield is 10.74 μg·h⁻¹ -1 ·mg -1 The cat (Example 2, pre-carbonization temperature 0°C) gradually decreased to 6.3 μg·h. -1 ·mg -1 cat (Comparative Example 1, pre-carbonization temperature 100℃), then increased to 8.16 μg·h -1 ·mg -1 cat (Comparative Example 2, pre-carbonization temperature of 200℃), reached a maximum of 21.96 μg·h at a pre-carbonization temperature of 300℃. -1 ·mg -1 cat (Example 1), when the pre-carbonization temperature is 400°C, drops to 11.86 μg·h -1 ·mg -1 cat (Example 3), trends and Figure 4 The LSV results for a in the model are consistent. The high specific surface area and excellent porosity at a pre-carbonization temperature of 300℃ provide more active sites for the reaction and promote mass transfer, thereby significantly improving the yield.

[0072] 4. Faraday efficiency like Figure 5 As shown in Figure a, the Faraday efficiency also reaches its highest point at a pre-carbonization temperature of 300°C (Example 1: 16.5%), and its lowest point at a pre-carbonization temperature of 400°C (Example 3: 9.9%). The Faraday efficiency of the boron-doped self-supporting carbon-based catalytic electrode with a pre-carbonization temperature of 100–200°C (Comparative Example 1: 10.9%, Comparative Example 2: 11.8%) is slightly higher than that of the boron-doped self-supporting carbon-based catalytic electrode with a pre-carbonization temperature of 0°C (Example 2: 10.5%). This may be related to the pore structure and selectivity, but overall it is still much lower than that of the boron-doped self-supporting carbon-based catalytic electrode with a pre-carbonization temperature of 300°C. This indicates that pre-carbonization at 300°C not only increases the number of active sites but also optimizes the electron transport path, thereby improving NRR selectivity.

[0073] Table 1. Specific surface area, ammonia yield, and Faraday efficiency of boron-doped self-supporting carbon-based catalytic electrodes

[0074] Test Example 4: Effect of Boric Acid Dosage on Specific Surface Area, Raman Spectroscopy, Electrochemical Performance, Ammonia Yield, and Faraday Efficiency of Boron-Doped Self-Supported Carbon-Based Catalytic Electrode I. Experimental Methods According to Test Example 3, the boron-doped self-supporting carbon-based catalytic electrodes prepared in Examples 1, 4 and Comparative Examples 3-4 were subjected to low-temperature nitrogen adsorption curve (BET) and linear sweep voltammetry, and the NH3 products were quantitatively tested using Nessler's reagent method.

[0075] Raman spectroscopy was performed on the boron-doped self-supporting carbon-based catalytic electrodes prepared in Examples 1, 4, and Comparative Examples 3-4 to compare the Raman I spectra of boron-doped self-supporting carbon-based catalytic electrodes prepared using different amounts of boric acid. D / I G value.

[0076] II. Experimental Results The specific surface area and Raman spectral density of the boron-doped self-supporting carbon-based catalytic electrodes prepared in Examples 1, 4, and Comparative Examples 3-4 are compared. D / I G The values, ammonia yield, and Faraday efficiency are shown in Table 2.

[0077] 1. Specific surface area and Raman spectroscopy analysis The specific surface areas of SCB-300-0 (Comparative Example 3), SCB-300-0.5 (Comparative Example 4), SCB-300-1 (Example 1), and SCB-300-1.5 (Example 2) were 384.6, 210.8, 438.6, and 179.6 m², respectively. 2 / g, among which SCB-300-1 (Example 1) has the largest specific surface area and the best pore structure.

[0078] Raman spectroscopy of SCB-300-0 (Comparative Example 3), SCB-300-0.5 (Comparative Example 4), SCB-300-1 (Example 1), and SCB-300-1.5 (Example 4) D / I G The values ​​were 1.015, 1.032, 1.048, and 1.036 respectively. Figure 6 The results showed that boron doping led to increased electronic hybridization and larger defects, which increased with increasing doping concentration, particularly in SCB-300-1 (Example 1). D / I G The highest value indicates that this amount of boron doping has the best effect on causing defects.

[0079] 2. Electrochemical performance Electrochemical performance such as Figure 4 As shown in b, the effect trend is consistent with the specific surface area trend. The effect of SCB-300-1 (Example 1) with 1 g of boric acid is 3 times that of SCB-0-1 (Example 2) without boric acid.

[0080] 3. Ammonia yield like Figure 5 As shown in b, the ammonia yield of SCB-300-0 prepared in Comparative Example 3 was extremely low (0.12 μg·h⁻¹). -1 ·mg - 1 The boric acid content was 0 g, which clearly proves that boron doping is the chemical basis for generating nitrogen reduction reaction (NRR) catalytic activity, while the simple porous carbon structure has almost no activity. The ammonia yield of SCB-300-0.5 prepared in Comparative Example 4 jumped to 8.95 μg·h⁻¹. -1 ·mg -1 The cat, with a boric acid content of 0.5 g, showed that despite its low specific surface area, the introduced boron active sites began to function. The ammonia yield of SCB-300-1 prepared in Example 1 reached a peak of 21.96 μg·h⁻¹. -1 ·mg -1 cat, with a boric acid dosage of 1 g. At this point, the maximum specific surface area (438.6 m²) is achieved. 2 The combination of / g) and suitable boron content produced a strong synergistic effect, providing abundant active sites while ensuring sufficient exposure of active sites and efficient mass transfer. The ammonia yield of SCB-300-1.5 prepared in Example 4 decreased to 11.05 μg·h⁻¹. -1 ·mg -1 The boric acid content in the cat is 1.5 g. Excessive boron doping leads to structural degradation, and active sites may be masked or aggregated, resulting in performance decline.

[0081] 4. Faraday efficiency like Figure 5 As shown in b, the SCB-300-0 prepared in Comparative Example 3 had an extremely low Faraday efficiency (0.26%), with a boric acid content of 0 g, further confirming that the NRR reaction can hardly proceed selectively without boron. The Faraday efficiencies of SCB-300-0.5 (0.5 g boric acid) prepared in Comparative Example 4 and SCB-300-1 (1 g boric acid) prepared in Example 1 were both high, at 16.2% and 16.5%, respectively. This indicates that the boron-doped self-supporting carbon-based catalytic electrodes prepared within this doping range have good selectivity for NRR. Among them, SCB-300-1 prepared in Example 1 achieved the highest yield while maintaining the highest efficiency, showing the best performance. The Faraday efficiency of SCB-300-1.5 (boric acid content of 1.5 g) prepared in Example 4 decreased to 9.06%, indicating that excessive boron doping may have changed the electronic structure of the material, exacerbated competing side reactions such as hydrogen evolution, and led to a significant decrease in reaction selectivity.

[0082] Table 2. Specific surface area, ammonia yield, and Faraday efficiency of boron-doped self-supporting carbon-based catalytic electrodes.

[0083] The above embodiments are preferred embodiments of the present invention, but the embodiments of the present invention are not limited to the above embodiments. Any changes, modifications, substitutions, combinations, or simplifications made without departing from the spirit and principle of the present invention shall be considered equivalent substitutions and shall be included within the protection scope of the present invention.

Claims

1. A method for preparing a boron-doped self-supporting three-dimensional non-metallic carbon-based catalytic electrode, characterized in that, Includes the following steps: S1. Add sodium carboxymethyl cellulose to a starch paste containing boric acid, mix thoroughly, and freeze-dry to obtain a three-dimensional aerogel; The mass ratio of starch, boric acid, and sodium carboxymethyl cellulose is (38–42):(1–1.5):(0.18–0.22). S2. The three-dimensional aerogel is carbonized at 700–900 °C. After carbonization, a boron-doped self-supporting three-dimensional non-metallic carbon-based catalytic electrode is obtained.

2. The production method according to claim 1, characterized by, In step S2, before carbonizing the three-dimensional aerogel at 700–900°C, it is first pre-carbonized at 280–420°C. After pre-carbonization, it is cooled to 23–27°C before carbonization.

3. The production method according to claim 1, characterized by, In step S1, the method for preparing the boric acid-containing starch paste is as follows: starch, boric acid and water are mixed and heated at 85-95°C for 0.5-1.5 h.

4. The method of claim 1, wherein, In step S1, the temperature at which the mixture is fully mixed is 60–70°C.

5. The preparation method according to claim 1, characterized in that, In step S2, the carbonization is carried out in an inert atmosphere, and the heating rate is 8-12℃ / min.

6. The preparation method according to claim 2, characterized in that, The pre-carbonization is carried out under an inert atmosphere with a heating rate of 3–7 °C / min.

7. The preparation method according to claim 2, characterized in that, The pre-carbonization and carbonization times are both 0.8 to 1.2 hours.

8. The boron-doped self-supporting three-dimensional non-metallic carbon-based catalytic electrode prepared by the preparation method according to any one of claims 1 to 7.

9. The application of the boron-doped self-supporting three-dimensional non-metallic carbon-based catalytic electrode according to claim 8 in electrocatalytic nitrogen reduction.

10. A method for electrocatalytic reduction of nitrogen to ammonia, characterized in that, Electrocatalysis was performed using the boron-doped self-supporting three-dimensional non-metallic carbon-based catalytic electrode as described in claim 8.

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