Nitrogen-doped carbon nanotubes, methods of making and using the same, and methods of making hydrogen peroxide

Abstract

The present application relates to nitrogen-doped carbon nanotubes and a method for preparing and using the same and a method for preparing hydrogen peroxide. The nitrogen-doped carbon nanotubes comprise carbon nanotubes and pyridine groups grafted on the carbon nanotubes. The nitrogen-doped carbon nanotubes of the present application are modified carbon nanotubes with a clear functional structure under normal temperature and pressure and mild reaction conditions. The sp2 hybridized carbon nanotubes are modified by pyridine groups with a specific structure to generate pyridine-modified carbon nanotubes with a clear structure and electron transfer between the carbon plane and the substituent group, which can be used as electrode materials in the electrocatalytic preparation of hydrogen peroxide.

Description

Technical Field

[0001] This invention relates to the field of energy materials, specifically to a nitrogen-doped carbon nanotube, a method for preparing nitrogen-doped carbon nanotubes and the carbon nanotubes prepared therefrom, the application of nitrogen-doped carbon nanotubes in the preparation of hydrogen peroxide, and a method for preparing hydrogen peroxide. Background Technology

[0002] Hydrogen peroxide (H2O2) is an environmentally friendly oxidant and disinfectant, widely used in wastewater treatment, paper / pulp bleaching, and bacteria / virus elimination. Currently, about 99% of H2O2 is synthesized via the anthraquinone method, but this method faces significant energy consumption, environmental pollution, and safety concerns. In contrast, electrocatalytic preparation of H2O2, especially through the two-electron oxygen reduction reaction (2e-ORR), does not require expensive catalysts, does not require an environment where H2 and O2 coexist, and does not require large-scale reaction infrastructure. It enables in-situ electrochemical synthesis, offering advantages such as safety, portability, and environmental friendliness, and is receiving increasing attention. Oxygen reduction reactions typically involve two pathways: (1) the two-electron oxygen reduction process (2e-ORR) produces H2O2; (2) the four-electron oxygen reduction process (4e-ORR) produces H2O. It has been confirmed that certain catalyst materials, including noble metal-based (Pd-based, Pt-based), non-noble metal-based (Co-based and Ni-based), and carbon-based materials, possess excellent oxygen reduction activity and two-electron selectivity. Among them, carbon nanomaterials, with their excellent electronic conductivity, low cost, high structural stability, and easily tunable nanostructure, have become the most promising catalyst materials for the electrocatalytic synthesis of H2O2.

[0003] Specifically, carbon has very high reserves on Earth. Located in Group IV of the second period of the periodic table, carbon has four electrons in its outermost shell, making it difficult to gain or lose electrons. Therefore, carbon materials exhibit excellent stability and high designability, making them a widely studied material. The emergence of high-performance carbon materials has made significant contributions to fields such as energy, biology, and the environment. These materials, with their graphite or graphite-like structures, are... 2 Hybrid carbon materials, with their advantages of large specific surface area, strong corrosion resistance and low cost, have become the preferred carriers for electrocatalysts, providing charge conduction, dispersing active sites for reactions, and serving as reaction sites.

[0004] Due to the inertness of carbon material surfaces, people have conducted in-depth research and optimization on carbon supports, such as surface modification and activation modification, to improve the transfer of electrons, protons and ions in chemical and electrocatalytic reactions on the support surface; and molecular modification, which changes the charge density of surrounding carbon atoms, causing charge delocalization and improving the activity of carbon materials.

[0005] In the two-electron oxygen reduction strategy for preparing hydrogen peroxide using carbon-based nonmetallic electrocatalysts, N-doping can effectively modulate the electronic structure of carbon nanomaterials and is often used to improve the catalytic performance of carbon-based materials. Nitrogen doping in carbon materials typically exists in the form of pyridine nitrogen, graphitic nitrogen, and pyrrole nitrogen. Different doping structures, doping contents, and doping positions have different effects on ORR performance.

[0006] Hector et al. (Fernandez-Escamilla HN, Guerrero-Sanchez J, Contreras E, et al. Understanding the selectivity of the oxygen reduction reaction at the atomistic level on nitrogen-doped graphitic carbon materials[J]. Advanced Energy Materials, 2021, 11(3): 2002-459) explored the reaction pathways when the basic nitrogen defect type (pyridine-N or graphite-N) and different pyridine-N configurations (SV+1N, SV+2N, and SV+3N) act as oxygen reduction reaction centers using density functional theory (DFT) calculations. The results showed that under alkaline conditions, pyridine-N with graphite-N and SV+3N configurations tended to undergo the 2e-ORR pathway, while pyridine-N with SV+2N and SV+1N configurations tended to follow the 4e-ORR pathway. They further pyrolyzed a mixture of ferrocene and benzylamine at 600, 800, and 1000 °C to obtain various N-doped carbon nanotube materials with controllable graphene-N content and pyridine-N configuration. With increasing pyrolysis temperature, the pyridine-nitrogen content (ratio) gradually decreased, while the graphene-N content gradually increased. Correspondingly, the N-CNT-1000 material obtained by pyrolysis at 1000 °C showed the highest graphene-N content and the highest H2O2 selectivity (~55%). However, it is difficult to accurately prepare and characterize pyridine-N structures with different configurations (SV+1N, SV+2N, and SV+3N) in experiments, and whether the SV+3N type pyridine-N sites tend to follow the 2e-ORR pathway remains to be verified.

[0007] Li et al. (Li L, Tang C, Zheng Y, et al. Tailoring selectivity of electrochemical hydrogen peroxide generation by tunable pyrrolic-nitrogen-carbon[J]. Advanced Energy Materials, 2020, 10(21): 2000-789) hold different views, pointing out that pyrrolic-N is the main active site for the preparation of H2O2 via the 2e-ORR pathway. They prepared few-layer graphene nanosheets (NFLG) with tunable pyrrolic-N content by pyrolyzing a mixture of melamine and glycine at different temperatures, and used them for the electrocatalytic synthesis of H2O2. When the melamine / glycine ratio was low, the N-FLG-8 nanosheets obtained by pyrolysis had the highest pyrrolic-N content, and also exhibited the highest H2O2 yield and selectivity (>90%). Moreover, under a test voltage of 0.4V vs. RHE, the selectivity of this electrode material remained above 95% during an 8-hour cycle. That is, the content of pyrrole-N sites is positively correlated with the selectivity of H2O2 synthesis, rather than pyridine-N and graphite-N sites.

[0008] Currently, existing nitrogen-doped carbon materials have the following shortcomings:

[0009] (1) The preparation process usually requires heat treatment and / or calcination, which is complex, demanding, and costly.

[0010] (2) The structure of the product N obtained is unclear, usually a mixture of multiple types, which cannot be precisely controlled, resulting in insufficient electrocatalytic performance.

[0011] Therefore, there is a need to develop a more effective nitrogen-doped carbon material that can achieve low cost, mild preparation conditions, excellent product stability, and well-defined structure. Summary of the Invention

[0012] The purpose of this invention is to overcome the above-mentioned shortcomings of the prior art and provide a nitrogen-doped carbon nanotube, which comprises a carbon nanotube and a pyridine group of formula (1) grafted onto the carbon nanotube.

[0013]

[0014] R is selected from at least one of H, CH3, CHO and C5H4N.

[0015] The nitrogen-doped carbon nanotubes of this invention are modified carbon nanotubes with well-defined functional structures achieved under ambient temperature, ambient pressure, and mild reaction conditions. These sp2-hybridized carbon nanotubes are modified with pyridine groups of specific structures to generate pyridine-doped carbon nanotubes with well-defined structures and electron transfer between the carbon plane and the substituents. This material exhibits high conductivity, good stability, and nitrogen atom doping.

[0016] A second aspect of the present invention provides a method for preparing nitrogen-doped carbon nanotubes, the method comprising, in a three-electrode system, in the presence of an electrolyte and a solvent, electrochemically grafting carbon nanotubes with a pyridine compound of formula (2).

[0017]

[0018] R is selected from at least one of H, CH3, CHO and C5H4N.

[0019] A third aspect of the present invention provides nitrogen-doped carbon nanotubes prepared by the method of the present invention described above.

[0020] A fourth aspect of the invention provides the application of the nitrogen-doped carbon nanotubes of the present invention as a catalyst material for the two-electron oxygen reduction reaction.

[0021] A fifth aspect of the present invention provides a method for preparing hydrogen peroxide, the method comprising using nitrogen-doped carbon nanotubes provided by the present invention as catalyst materials to prepare hydrogen peroxide via an electrocatalytic redox reaction.

[0022] This invention focuses on two aspects. Firstly, it aims to develop inexpensive and stable carbon-based non-metallic electrode materials. Through gentle and environmentally friendly methods, the active sites of carbon nanotubes are modulated, and small molecules are used to functionalize the carbon nanotubes, thereby improving electrochemical catalytic performance, reducing costs, and enabling the use of metal catalysts. Secondly, the nitrogen-doped carbon nanotubes obtained through this electrochemical grafting can also serve as carbon supports. Modification of the carbon nanotubes with pyridine compounds results in pyridineized carbon nanotubes with well-defined structures and electron transfer between the carbon plane and the substituents.

[0023] Compared with existing technologies, this invention employs an electrochemical grafting method, which is simple, easy to implement, and avoids pyrolysis, enabling the preparation of carbon nanotube materials with well-defined functional structures at room temperature and pressure. By applying a constant potential in a three-electrode system, the pyridine compound molecule loses an electron, undergoing an oxidation reaction and successfully grafting onto the carbon plane of the carbon nanotube material. This method provides mild preparation conditions, does not significantly alter the structure of the carbon nanotubes, and yields pyridine-treated carbon nanotubes with a well-defined structure, making them suitable as electrode materials for the electrocatalytic production of hydrogen peroxide.

[0024] When the nitrogen-doped carbon nanotubes of the present invention are used as catalyst materials to prepare hydrogen peroxide via electrocatalytic redox reaction, they exhibit excellent two-electron oxygen reduction reaction activity, with a hydrogen peroxide yield reaching 90%. Moreover, after a long-term stability test, such as 10 hours, the hydrogen peroxide yield remains stable without any decay. Attached Figure Description

[0025] Figure 1 This is a schematic diagram of the experimental setup for the H-type electrolytic cell used in this invention.

[0026] Figure 2 This is a scanning electron microscope image of the nitrogen-doped carbon nanotubes prepared in Example 1 of the present invention.

[0027] Figure 3 This is the infrared spectrum of the nitrogen-doped carbon nanotubes prepared in Example 1 of this invention.

[0028] Figure 4 This is a Zeta potential diagram of the nitrogen-doped carbon nanotubes prepared in Example 1 of the present invention.

[0029] Figure 5 This is the XPS elemental analysis diagram of the nitrogen-doped carbon nanotubes prepared in Example 1 of this invention.

[0030] Figure 6 This is a rotating annular disk curve of the nitrogen-doped carbon nanotubes prepared in Example 1 of the present invention. Detailed Implementation

[0031] The present application will now be described in further detail with reference to the accompanying drawings and embodiments. Through these descriptions, the features and advantages of the present application will become clearer and more apparent.

[0032] The term “exemplary” as used herein means “serving as an example, embodiment, or illustration.” Any embodiment illustrated herein as “exemplary” is not necessarily to be construed as superior to or better than other embodiments. Although various aspects of embodiments are shown in the accompanying drawings, the drawings are not necessarily drawn to scale unless specifically indicated otherwise.

[0033] Furthermore, the technical features involved in the different embodiments of this application described below can be combined with each other as long as they do not conflict with each other.

[0034] A first aspect of the present invention provides a nitrogen-doped carbon nanotube comprising a carbon nanotube and a pyridine group of formula (1) grafted onto the carbon nanotube.

[0035]

[0036] R is selected from at least one of H, CH3, CHO and C5H4N.

[0037] According to a preferred embodiment of the present invention, R is H.

[0038] In this invention, the content of the pyridine group shown in formula (1) as a percentage of the nitrogen-doped carbon nanotube by weight (i.e., grafting rate) can generally reach 30-60%, which can be controlled by the feeding ratio and reaction conditions during the preparation process.

[0039] The term "carbon nanotube," also known as CNTs, refers to a carbon-containing cylinder with a diameter of 3-180 nm and a length many times, at least 10 times, that diameter. Other characteristics of these carbon nanotubes are ordered layers of carbon atoms, and they typically have a core with different morphologies. Synonyms for carbon nanotubes include, for example, "carbon fiber," "hollow carbon fiber," "carbon bamboo," or (in the case of wound structures), "nano scroll" or "nanoroll."

[0040] In this invention, the diameter of the carbon nanotubes can be 100-200 nm, and the length can be 5-15 μm; preferably, the diameter of the carbon nanotubes is 120-180 nm, and the length is 6-12 μm. According to a specific embodiment of this invention, the diameter of the carbon nanotubes is approximately 150 nm, and the length is approximately 10 μm.

[0041] In this invention, electron transfer occurs between the carbon plane of the carbon nanotube and the substituent pyridine group, i.e., a chemical bond is formed between them, resulting in a product with a well-defined structure and good stability.

[0042] A second aspect of the present invention provides a method for preparing nitrogen-doped carbon nanotubes, the method comprising, in a three-electrode system, in the presence of an electrolyte and a solvent, electrochemically grafting carbon nanotubes with a pyridine compound of formula (2).

[0043]

[0044] R is selected from at least one of H, CH3, CHO and C5H4N. The pyridine compound shown in formula (2) is selected from at least one of pyridine, 4-methylpyridine, 4-aldehyde-pyridine, and 4,4'-bipyridine.

[0045] According to a preferred embodiment of the present invention, R is H.

[0046] In this invention, the amount of pyridine compound shown in formula (2) relative to 1 part by weight of carbon nanotubes can be 5-20 parts by weight, preferably 8-12 parts by weight, and more preferably 10 parts by weight. By controlling the amounts of carbon nanotubes and pyridine compound shown in formula (2) within the above range, the degree of pyridine compound modification of carbon nanotubes can be better improved. If the amount of pyridine compound shown in formula (2) is less than 5 parts by weight relative to 1 part by weight of carbon nanotubes, the degree of pyridine modification of carbon nanotubes is weak; if the content of pyridine groups shown in formula (1) is higher than 20 parts by weight, there is a possibility of pyridine molecules self-polymerizing, which is detrimental to the modification of carbon nanotubes by pyridine molecules.

[0047] In this invention, the diameter of the carbon nanotubes can be 100-200 nm, and the length can be 5-15 μm; preferably, the diameter of the carbon nanotubes is 120-180 nm, and the length is 6-12 μm. According to a specific embodiment of this invention, the diameter of the carbon nanotubes is approximately 150 nm, and the length is approximately 10 μm.

[0048] In this invention, the three-electrode system includes a working electrode (WE), a counter electrode (CE), and a reference electrode (RE). The working electrode is the electrode on which the studied reaction takes place. The grafting reaction of carbon nanotubes with the pyridine compound shown in formula (2) in this invention occurs on the working electrode. The material of the working electrode can be a conventional choice in the art, such as a glassy carbon electrode, platinum (Pt), gold (Au), silver (Ag), etc. According to a specific embodiment of the invention, the working electrode can be a Pt sheet electrode. An auxiliary electrode forms a circuit with the working electrode and serves to conduct electricity; the auxiliary electrode used in this invention can be a Pt sheet electrode. The reference electrode is used as a reference for measuring or applying the working electrode potential; the reference electrode used in this invention can be a saturated calomel electrode (SCE).

[0049] According to a specific embodiment of the present invention, the apparatus for electrochemical grafting treatment can be an electrolytic cell, for example, as shown in the figure below. Figure 1 The H-type electrolytic cell shown is an H-type electrolytic cell. The H-type electrolytic cell includes cell A and cell B. Cell A contains a counter electrode, and cell B contains a working electrode and a reference electrode. Cells A and B are separated by a gasket and a diaphragm.

[0050] In this invention, the membrane used in the electrolytic cell can be a cation exchange membrane, used for the exchange of Li ions in the electrolyte solution between cells A and B of the H-type electrolytic cell during electrosynthesis. According to a preferred embodiment, the membrane of this invention can be a Nafion 117 membrane.

[0051] In this invention, both tank A and tank B of the electrolytic cell contain an electrolyte solution. The electrolyte solution contains both an electrolyte and a solvent. The electrolyte acts as a conductor in the solvent. The concentration of the electrolyte solution can vary within a wide range, for example, 0.3-0.8 M, preferably 0.4-0.6 M.

[0052] The electrolyte used in this invention can be selected from at least one of lithium perchlorate, sodium perchlorate, and tetrabutylammonium perchlorate. According to a preferred embodiment, the electrolyte is lithium perchlorate.

[0053] The solvent used in this invention may be selected from at least one of acetonitrile, DMF and N,N-dimethylformamide.

[0054] According to a preferred embodiment, the electrolyte solution is an anhydrous acetonitrile solution of lithium perchlorate.

[0055] In this invention, the conditions for electrochemical grafting treatment include at least the following: a potential of 1.2-1.4V and a time of 0.5-2 hours. The temperature is typically room temperature.

[0056] In this invention, the electrochemical grafting treatment is carried out in cell B of the electrolytic cell, therefore the electrolyte solution in cell B also contains carbon nanotubes and the pyridine compound shown in formula (2). Considering that carbon nanotubes are poorly soluble in water, this invention preferably uses ultrasound to assist in the dispersion of carbon nanotubes. Other substances can also be dissolved using ultrasound.

[0057] In this invention, considering the efficiency of the reaction and the yield of the product, the concentration of the pyridine compound shown in formula (2) can be 0.25-1M.

[0058] In this invention, the method for preparing nitrogen-doped carbon nanotubes may further include washing and drying steps following electrochemical grafting treatment. Washing and drying can be conventional operations in the art; for example, washing three times with anhydrous acetonitrile followed by drying in an oven.

[0059] According to a specific embodiment of the present invention, the method for preparing nitrogen-doped carbon nanotubes includes the following steps:

[0060] (1) Weigh a certain amount of lithium perchlorate and dissolve it in anhydrous acetonitrile by ultrasonication at room temperature to prepare mixed solution A;

[0061] (2) Weigh a certain amount of carbon nanotubes, lithium perchlorate, and pyridine compound shown in formula (2) and dissolve them in anhydrous acetonitrile at room temperature to prepare a mixed solution B;

[0062] (3) Pour the solutions from steps (1) and (2) into tanks A and B of the H-type electrolytic cell, and separate the two tanks with gaskets and diaphragms (Nafion117).

[0063] (4) The Pt sheet electrode is placed in cell B of an H-type electrolytic cell as the working electrode, and the counter electrode Pt sheet is placed in cell A. A saturated calomel electrode is selected as the reference electrode, and a Nafion 117 membrane is used as the diaphragm. A constant-potential anodic pyridinization process is performed in a three-electrode system. The experimental setup is as follows: Figure 1 As shown.

[0064] A third aspect of the present invention provides nitrogen-doped carbon nanotubes prepared by the method of the present invention described above.

[0065] A fourth aspect of the present invention provides the application of the above-mentioned nitrogen-doped carbon nanotubes as a catalyst material for the two-electron oxygen reduction reaction. The medium for the two-electron oxygen reduction reaction can be alkaline, neutral, or acidic.

[0066] A fifth aspect of the present invention provides a method for preparing hydrogen peroxide, comprising using the nitrogen-doped carbon nanotubes provided by the present invention as a catalyst material to prepare hydrogen peroxide via an electrocatalytic redox reaction. According to a specific embodiment, the method comprises coating nitrogen-doped carbon nanotubes onto an electrode, followed by performing an electrocatalytic redox reaction to prepare hydrogen peroxide. The medium for the electrocatalytic redox reaction can be alkaline, neutral, or acidic. Therefore, this application also relates to an electrode for preparing hydrogen peroxide via an electrocatalytic redox reaction, comprising an electrode substrate and nitrogen-doped carbon nanotubes of the present application loaded on the surface of the electrode substrate. Common electrode substrates may include glassy carbon electrodes, etc.

[0067] The present invention will be described in detail below through examples.

[0068] In the following examples, unless otherwise specified, all experimental instruments and raw materials involved are commercially available products.

[0069] Experimental apparatus:

[0070] Scanning electron microscope, JEOL JSM-6701F;

[0071] Fourier transform infrared spectroscopy, Thermo Fisher Scientific (USA), Nicolet 6700;

[0072] Raman spectroscopy, Horiba, France, RAMIS;

[0073] Zeta potential, Malvern instrument, NANO / ZS90;

[0074] X-ray photoelectron spectroscopy, Thermo Fisher Scientific (USA), ESCALAB250;

[0075] Rotating ring-disc electrode device, ALS, Japan, RRDE-3A;

[0076] Three-electrode system: such as Figure 1 The H-type electrolytic cell shown includes tank A containing solution A and tank B containing solution B, separated by a gasket and a diaphragm. A 1×2cm Pt sheet electrode is placed in tank B of the H-type electrolytic cell as the working electrode (WE), and a counter electrode Pt sheet (CE) is placed in tank A. A saturated calomel electrode is selected as the reference electrode (RE), and the diaphragm is a Nafion 117 membrane (DuPont, USA).

[0077] raw material:

[0078] Carbon nanotubes, Showa Denko Corporation, VGCF-H, with an average diameter of approximately 150 nm and an average length of approximately 10 μm;

[0079] Lithium perchlorate, Sinopharm Chemical Reagent, LiClO4, AR;

[0080] Pyridine, Sinopharm Chemical Reagent, C5H5N, AR;

[0081] 4-Aldehyde-pyridine, Macklin reagent, C6H5NO, 99%;

[0082] The remaining chemical reagents are standard commercially available products.

[0083] Example 1

[0084] This embodiment illustrates the nitrogen-doped carbon nanotubes and their preparation method according to the present invention.

[0085] 1.33 g of lithium perchlorate was dissolved in 25 mL of anhydrous acetonitrile at room temperature using ultrasonication to prepare mixed solution A. 100 mg of carbon nanotubes, 1.33 g of lithium perchlorate, and 1 mL of pyridine were dissolved in anhydrous acetonitrile using ultrasonication at room temperature to prepare mixed solution B. Solutions A and B were poured into tanks A and B of an H-type electrolytic cell, respectively. A constant-potential anodic pyridation process was carried out under a three-electrode system. A constant voltage of 1.4 V was selected, and the reaction was allowed to proceed for 1 hour. The mixture was then rinsed three times with the organic solvent anhydrous acetonitrile and dried in an oven.

[0086] Example 2

[0087] This embodiment illustrates the nitrogen-doped carbon nanotubes and their preparation method according to the present invention.

[0088] 1.33 g of lithium perchlorate was dissolved in 25 mL of anhydrous acetonitrile via ultrasonication at room temperature to prepare mixed solution A. 100 mg of carbon nanotubes, 1.33 g of lithium perchlorate, and 1 mL of pyridine were dissolved in anhydrous acetonitrile via ultrasonication at room temperature to prepare mixed solution B. Solutions A and B were poured into tanks A and B of an H-type electrolytic cell, respectively. A constant-potential anodic pyridation process was carried out under a three-electrode system. A constant voltage of 1.2 V was selected, and the reaction was allowed to proceed for 2 hours. The mixture was then rinsed three times with anhydrous acetonitrile (an organic solvent) and dried in an oven.

[0089] Example 3

[0090] This embodiment illustrates the nitrogen-doped carbon nanotubes and their preparation method according to the present invention.

[0091] 1.33 g of lithium perchlorate was dissolved in 25 mL of anhydrous acetonitrile at room temperature using ultrasonication to prepare mixed solution A. 100 mg of carbon nanotubes, 1.33 g of lithium perchlorate, and 1 mL of pyridine were dissolved in anhydrous acetonitrile using ultrasonication at room temperature to prepare mixed solution B. Solutions A and B were poured into tanks A and B of an H-type electrolytic cell, respectively. A constant-potential anodic pyridation process was carried out under a three-electrode system. A constant voltage of 1.3 V was selected, and the reaction was allowed to proceed for 0.5 h. The mixture was then washed three times with anhydrous acetonitrile and dried in an oven.

[0092] Example 4

[0093] This embodiment illustrates the nitrogen-doped carbon nanotubes and their preparation method according to the present invention.

[0094] 1.33 g of lithium perchlorate was dissolved in 25 mL of anhydrous acetonitrile via ultrasonication at room temperature to prepare mixed solution A. 100 mg of carbon nanotubes, 1.33 g of lithium perchlorate, and 2 mL of pyridine were dissolved in anhydrous acetonitrile via ultrasonication at room temperature to prepare mixed solution B. Solutions A and B were poured into tanks A and B of an H-type electrolytic cell, respectively. A constant-potential anodic pyridation process was carried out under a three-electrode system. A constant voltage of 1.4 V was selected, and the reaction was allowed to proceed for 1 hour. The mixture was then rinsed three times with anhydrous acetonitrile (an organic solvent) and dried in an oven.

[0095] Example 5

[0096] This embodiment illustrates the nitrogen-doped carbon nanotubes and their preparation method according to the present invention.

[0097] 1.33 g of lithium perchlorate was dissolved in 25 mL of anhydrous acetonitrile via ultrasonication at room temperature to prepare mixed solution A. 100 mg of carbon nanotubes, 1.33 g of lithium perchlorate, and 0.5 mL of pyridine were dissolved in anhydrous acetonitrile via ultrasonication at room temperature to prepare mixed solution B. Solutions A and B were poured into tanks A and B of an H-type electrolytic cell, respectively. A constant-potential anodic pyridation process was carried out under a three-electrode system. A constant voltage of 1.4 V was selected, and the reaction was allowed to proceed for 1 hour. The mixture was then rinsed three times with anhydrous acetonitrile and dried in an oven.

[0098] Example 6

[0099] This embodiment illustrates the nitrogen-doped carbon nanotubes and their preparation method according to the present invention.

[0100] 1.33 g of lithium perchlorate was dissolved in 25 mL of anhydrous acetonitrile via ultrasonication at room temperature to prepare mixed solution A. 100 mg of carbon nanotubes, 1.33 g of lithium perchlorate, and 1 mL of pyridine were dissolved in anhydrous acetonitrile via ultrasonication at room temperature to prepare mixed solution B. Solutions A and B were poured into tanks A and B of an H-type electrolytic cell, respectively. A constant-potential anodic pyridation process was carried out under a three-electrode system. A constant voltage of 1.4 V was selected, and the reaction was allowed to proceed for 0.5 h. The mixture was then washed three times with anhydrous acetonitrile and dried in an oven.

[0101] Comparative Example 1

[0102] A certain amount of carbon nanotubes was prepared into a slurry and used directly as an oxygen reduction catalyst.

[0103] Comparative Example 2

[0104] A nitrogen-doped carbon material, prepared via a one-step pyrolysis method using sugar as the carbon source and dicyandiamine as the nitrogen source, was used for the electrochemical oxygen reduction to produce hydrogen peroxide. (Qiao Minghua, Wang Dan. Nitrogen-doped carbon catalyst for electrocatalytic oxygen reduction to hydrogen peroxide and its preparation method [P]. Shanghai: CN112442708A, 2021-03-05.)

[0105] Test example:

[0106] The product prepared in Example 1 was analyzed and characterized.

[0107] (1) Scanning electron microscopy

[0108] The results are as follows Figure 2 As shown. From Figure 2 It can be seen that the morphology of carbon nanotubes remains consistent before and after pyridine grafting, and the presence of N element in the mapping distribution of the sample after the reaction indicates that N doping was successfully achieved.

[0109] (2) Infrared and Raman spectroscopy

[0110] The results are as follows Figure 3 As shown: Figure 3 a shows the infrared spectrum, where the upper line is the spectrum of Py-HCNT prepared in Example 1, and the lower line is the spectrum of HCNT, the raw material used in Example 1. Figure 3 b shows the Raman spectra, where the upper graph is the spectrum of Py-HCNT prepared in Example 1, and the lower graph is the spectrum of HCNT used as the raw material in Example 1. From Figure 3 It can be seen that the prepared nitrogen-doped carbon nanotubes exhibit a CN peak, which corresponds to the CN covalent bond between pyridine molecules and carbon nanotubes, proving the successful introduction of nitrogen. The Raman spectrum shows a shift of the G peak to higher wavenumbers, which is caused by charge transfer from the carbon plane to the pyridine compound molecules resulting from the grafting of pyridine molecules onto the carbon nanotubes. Simultaneously, the degree of disorder in the nitrogen-doped carbon nanotubes is also increased, manifested as I... D / I G The value increases.

[0111] (3) Zeta potential

[0112] Figure 4 It is the change of Zeta potential before and after the pyridine reaction. After the introduction of nitrogen doping, the Zeta potential becomes positive. This is because when pyridine is grafted onto carbon nanotubes, it causes charge transfer from the carbon nanotubes to the pyridine molecules, resulting in the carbon material surface exhibiting positive charge.

[0113] (4) XPS test

[0114] The results are as follows Figure 5 As shown: Figure 5 (a) In this graph, the upper line is the spectrum of Py-HCNT prepared in Example 1, and the lower line is the spectrum of HCNT, the raw material used in Example 1. From Figure 5 As can be seen, there is an N element. Figure 2-5 All of these indicate that pyridine molecules were successfully grafted onto carbon nanotubes.

[0115] (5) Electrocatalytic performance test

[0116] To investigate the electrocatalytic performance of the nitrogen-doped carbon nanotubes, the nitrogen-doped carbon nanotubes prepared in Example 1 were formulated into a slurry for electrocatalytic oxygen reduction to hydrogen peroxide production experiments. The specific implementation steps are as follows:

[0117] 5 mg of the prepared nitrogen-doped carbon nanotubes (pyridylated carbon nanotubes) Py-HCNT were dispersed in 1 mL of ethanol, and then 10 μL of 5 wt.% Nafion membrane solution was added. The mixture was sonicated until the slurry was uniformly dispersed and free of obvious particles. The resulting slurry was then added dropwise (10 μL of catalyst slurry in two separate drops, 5 μL each time, with the second drop added after drying) onto a glassy carbon electrode, followed by electrochemical testing.

[0118] The oxygen reduction performance test in this experiment was conducted at room temperature using 0.1M KOH as the alkaline electrolyte. The counter electrode was a C rod, and the reference electrode was an SCE electrode. The potentials in this experiment need to be converted to the standard hydrogen potential (RHE) using the following formula:

[0119] E RHE =E SCE +0.0591×pH+0.241………….………Equation (1-1)

[0120] Cyclic voltammetry (CV) curves. In this experiment, when testing the oxygen reduction performance, the catalyst was first activated at a scan rate of 100 mV / s. -1 After activation, perform CV testing, selecting a potential window of 0-1.2V and setting the scan rate to 50mV / s. -1 During the test, a stable flow of O2 or N2 is required to obtain the CV curve. By comparing the curves obtained from the saturated solutions under the two atmospheres, information such as the reduction peak potential and peak current density of the catalyst can be obtained.

[0121] Steady-state polarization profile (LSV). Potential window selected: 0-1.2V, with a 5mV / s interval. -1 The scan rate was tested in O2-saturated and N2-saturated KOH, while ensuring that the glassy carbon electrode had a certain rotation speed. The LSV curve of oxygen reduction obtained can be analyzed as follows.

[0122] Tafel analysis was performed on the LSV of the glassy carbon electrode at 1600 rpm, with potential E plotted on the ordinate, Log(J k Plotting the x-axis (J) k (This refers to the dynamic current density), from which the slope of the curve, i.e., the Tafel slope (b), is obtained. The relevant formulas are as follows: where J d Let J be the limiting diffusion current density, and J be the current density obtained from the LSV test.

[0123]

[0124] Rotating ring-disc electrode test at 1600 rpm, with a fixed potential of 1.5V for the platinum ring versus RHE, yielded the disk current (I). d ) and loop current (I rBased on the data, the hydrogen peroxide yield (%H2O2) and the number of transferred electrons (n) were calculated, where the collection coefficient N of the ring electrode was 0.37, as shown in the following formula:

[0125]

[0126]

[0127] The results are as follows Figure 6 (a)- Figure 6 As shown in (d). Wherein, Figure 6 (a) is the CV chromatogram, which shows that the Py-HCNT catalyst has a more positive reduction peak potential, indicating that its oxygen reduction performance is better. From Figure 6 (b) The LSV curve also shows that Py-HCNT has a higher onset potential and its loop current is greater than that of the control sample HCNT (carbon nanotubes). Figure 6 (c) It can be seen that Py-HCNT has a higher hydrogen peroxide yield, reaching about 90%, and the number of electrons transferred in its oxygen reduction process is closer to 2. Figure 6 (d) is the Tafel curve, which shows that Py-HCNT has a smaller Tafel slope and a faster oxygen reduction reaction kinetic rate. In general, nitrogen-doped carbon nanotubes have improved 2e-ORR performance.

[0128] (6) Electrocatalytic stability test

[0129] To investigate the electrocatalytic stability of the nitrogen-doped carbon nanotubes prepared in Example 1, the stability of the nitrogen-doped carbon nanotubes for electrocatalytic oxygen reduction to hydrogen peroxide production was tested. The specific implementation steps are as follows:

[0130] After the prepared slurry was coated onto the glassy carbon electrode, it was activated by CV and then subjected to a stability test under constant voltage. At this time, the disk voltage was set to 0.4V vs. RHE and the ring voltage was set to 1.5V vs. RHE.

[0131] The results are as follows Figure 6 (e) (The vertical axis of the upper curve represents the hydrogen peroxide yield, and the vertical axis of the lower curve represents the disk current) shows that nitrogen-doped carbon nanotubes have excellent two-electron oxygen reduction activity, and their hydrogen peroxide yield can reach 90%. Moreover, after a long-term stability test of 10 hours, the hydrogen peroxide yield remains stable without any decay.

[0132] For the remaining examples and comparative examples, the tests were conducted in the same manner as described above. The hydrogen peroxide yield of the nitrogen-doped carbon nanotubes prepared in Examples 2-6 was approximately 90%. The remaining results are listed in Table 1.

[0133] Table 1

[0134] Hydrogen peroxide yield (%) <![CDATA[Potential @ 1 mA cm -2 > Example 1 90％ 0.775 Comparative Example 1 54％ 0.755 Comparative Example 2 85％ 0.37

[0135] As can be seen from the results in the table above, the nitrogen-doped carbon nanotubes obtained have superior two-electron oxygen reduction performance, exhibiting higher onset potential and hydrogen peroxide yield. Furthermore, in the 10-hour stability test, the hydrogen peroxide yield remained stable at 90%, with no decay phenomenon.

[0136] The present application has been described above with reference to preferred embodiments; however, these embodiments are merely exemplary and illustrative. Various substitutions and modifications can be made to the present application based on these embodiments, all of which fall within the protection scope of the present application.

Claims

1. A nitrogen-doped carbon nanotube, characterized in that, The nitrogen-doped carbon nanotubes comprise carbon nanotubes and pyridine groups of formula (1) covalently grafted onto the carbon plane of the carbon nanotubes through electrochemical grafting. Equation (1) Wherein, R is selected from at least one of H, CH3 and CHO, and the content of the pyridine group shown in formula (1) accounts for 30-60% of the weight percentage of the nitrogen-doped carbon nanotubes; The nitrogen-doped carbon nanotubes are prepared by means of an electrochemical grafting process in a three-electrode system, in the presence of an electrolyte and a solvent, of carbon nanotubes with a pyridine compound of formula (2). Equation (2) Wherein, R is selected from at least one of H, CH3 and CHO; The conditions for the electrochemical grafting treatment include at least the following: a potential of 1.2-1.4V and a time of 0.5-2 hours.

2. The nitrogen-doped carbon nanotube according to claim 1, wherein, R is H.

3. The nitrogen-doped carbon nanotube according to claim 1, wherein, The carbon nanotubes have a diameter of 100-200 nm and a length of 5-15 μm.

4. A method for preparing nitrogen-doped carbon nanotubes, characterized in that, This method includes electrochemically grafting carbon nanotubes with a pyridine compound of formula (2) in a three-electrode system in the presence of an electrolyte and a solvent. Equation (2) R is selected from at least one of H, CH3 and CHO.

5. The method according to claim 4, wherein, R is H.

6. The method according to claim 4, wherein, The amount of the pyridine compound shown in formula (2) is 5-20 parts by weight relative to 1 part by weight of carbon nanotubes.

7. The method according to claim 4, wherein, The conditions for the electrochemical grafting treatment include at least the following: the concentration of the pyridine compound represented by formula (2) is 0.25-1M.

8. The method according to claim 4, wherein, The carbon nanotubes have a diameter of 100-200 nm and a length of 5-15 μm; The electrolyte is selected from at least one of lithium perchlorate, sodium perchlorate, and tetrabutylammonium perchlorate; The solvent is selected from at least one of acetonitrile and N,N-dimethylformamide.

9. Nitrogen-doped carbon nanotubes prepared by the method according to any one of claims 4-8.

10. The application of the nitrogen-doped carbon nanotubes according to any one of claims 1-3 and 9 as a catalyst material for the two-electron oxygen reduction reaction.

11. An electrode for preparing hydrogen peroxide by electrocatalytic redox reaction, comprising an electrode substrate and nitrogen-doped carbon nanotubes as described in any one of claims 1-3 and 9 supported on the electrode substrate.

12. A method for preparing hydrogen peroxide, characterized in that, The method includes preparing hydrogen peroxide via an electrocatalytic redox reaction using nitrogen-doped carbon nanotubes as described in any one of claims 1-3 and 9 as a catalyst material.

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