Preparation and application of heteroatom-doped porous MXene nanosheets

Abstract

The embodiment of the present invention discloses a preparation method and application of heteroatom-doped porous metal carbon / nitride (MXene) nanosheets. The ternary metal carbon / nitrogen material MAX is a precursor of MXene, wherein M is an early transition metal element, A is aluminum or silicon, and X is carbon or nitrogen. The Al layer or Si layer of MAX is selectively etched away in an etching solution, and then MXene nanosheets are obtained by intercalation and ultrasound. The peeled MXene nanosheets are reacted in concentrated ammonia containing a heteroatom precursor at room temperature or under heating conditions for several hours to obtain heteroatom-doped porous MXene nanosheets. The material has a rich pore structure, a high specific surface area, exposed active edge sites, and heteroatom-doped active sites that are conducive to the transport, adsorption, and conversion of substances. Therefore, it can be used as an electrode component of metal batteries, metal ion batteries, and fuel cells, and exhibits good electrochemical properties.

Description

Technical Field

[0001] The present invention relates to the field of electrochemical energy, and in particular to a heteroatom-doped porous metal carbon / nitrogen compound nanosheet, a preparation method and application thereof. Background Art

[0002] Due to the non-renewable and environmentally polluting nature of traditional fossil fuels, the development of clean new energy sources and energy conversion devices is urgent. Fuel cells can directly and continuously convert the chemical energy in fuels into electrical energy, offering advantages such as high conversion efficiency and zero pollution, making them highly sought after by researchers. However, the slow kinetics and high overpotential of the oxygen reduction reaction at the fuel cell cathode hinder the improvement and development of fuel cell performance. The development of oxygen reduction cathode catalysts with high catalytic activity and stability is both desirable and highly desirable. Currently, the most commonly used cathode catalysts are Pt and Pt-based catalysts due to their high electrocatalytic activity. However, the scarcity, high cost, and susceptibility to poisoning of Pt have limited its development and application as a fuel cell electrode catalyst. Therefore, the development of non-precious metal catalysts with high catalytic activity and stability is of great significance for sustainable development.

[0003] MXenes are a novel class of transition metal carbon / nitrides with two-dimensional structures, such as Ti3C2 nanosheets. The Al layer in Ti3AlC2 can be selectively etched away using HF or HCl / LiF mixed solutions. The resulting Ti3C2 nanosheets have attracted widespread attention due to their hydrophilicity, high conductivity, flexible structure, and tunable surface chemistry. They are commonly used in lithium-ion batteries or water electrolysis reactions, but their catalytic activity as cathode oxygen reduction catalysts in fuel cells is poor. Although two-dimensional Ti3C2 nanosheets possess large surface area and abundant surface functional groups, they are prone to agglomeration and stacking, resulting in performance degradation. One effective strategy to address the aggregation problem is to create a porous structure on the surface of two-dimensional nanosheets. Furthermore, the introduction of heteroatoms such as nitrogen into Ti3C2 nanosheets can enhance their electrochemical performance, potentially improving their performance as cathode oxygen reduction catalysts in fuel cells. Summary of the Invention

[0004] The present invention discloses heteroatom-doped porous MXene nanosheets, their preparation methods, and applications, for use as cathode oxygen reduction electrocatalysts in fuel cells. The technical solution is as follows:

[0005] The present invention first provides a heteroatom-doped porous MXene nanosheet with rich pore structure, high specific surface area and a large number of exposed edge active sites, which can be used as a catalyst for the oxygen reduction reaction at the cathode of fuel cells.

[0006] In a preferred embodiment of the present invention, MXene is Ti3C2 nanosheets, the heteroatom is nitrogen, the pore diameter is 2 to 20 nm, and the specific surface area is 80 to 120 m 2 / g.

[0007] In a preferred embodiment of the present invention, the catalyst is tested using a three-electrode system, and the test voltage range is 0 to -1V.

[0008] The present invention also provides a method for preparing the nitrogen-doped porous Ti3C2 nanosheets, comprising:

[0009] First, the Al layer of the precursor Ti3AlC2 is selectively etched away in a mixed solution of HCl and LiF. Then, Ti3C2 nanosheets are obtained by intercalation and ultrasound. The Ti3C2 nanosheets are added to concentrated ammonia water, stirred at room temperature for 6 to 36 hours, centrifuged, washed with deionized water until neutral, and freeze-dried to obtain nitrogen-doped porous Ti3C2 nanosheets.

[0010] If other metal carbides or nitrides need to be prepared, the corresponding ternary metal carbon / nitrogen compound precursors need to be used;

[0011] If other heteroatom-doped MXene nanosheets are to be prepared, heteroatom precursors need to be added to concentrated ammonia solution for reaction;

[0012] By controlling the type and concentration of the alkali solution, the temperature and time of the reaction, the pore diameter and specific surface area of the nanosheets can be adjusted;

[0013] The above technical solution demonstrates that the present invention synthesizes porous nitrogen-doped TiC nanosheets by etching TiC nanosheets in an aqueous ammonia solution. This material possesses a large specific surface area and a rich pore structure, which facilitates the adsorption and transfer of O2. Furthermore, the etching process generates numerous active edge sites and nitrogen-doped active sites, enhancing the electrocatalytic oxygen reduction activity of the TiC nanosheets. Consequently, compared to pristine TiC nanosheets, the nitrogen-doped porous TiC nanosheets exhibit superior electrocatalytic oxygen reduction performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0015] Figure 1is the X-ray diffraction pattern of the nitrogen-doped porous Ti3C2 nanosheets prepared in Example 1;

[0016] Figure 2 TEM image of the nitrogen-doped porous Ti3C2 nanosheets prepared in Example 1;

[0017] Figure 3 This is the full XPS spectrum of the nitrogen-doped porous Ti3C2 nanosheets prepared in Example 1;

[0018] Figure 4 Graph showing the oxygen reduction polarization curves of the nitrogen-doped porous Ti3C2 nanosheets and the original Ti3C2 nanosheets prepared in Example 1. DETAILED DESCRIPTION

[0019] Ti3AlC2 is selectively etched from the Al layer in an HCl and LiF solution, and Ti3C2 nanosheets are obtained through intercalation and ultrasound. Subsequently, the Ti3C2 nanosheets are etched in concentrated ammonia at room temperature to obtain nitrogen-doped porous Ti3C2 nanosheets. This material has a rich pore structure and a high specific surface area. The present invention provides a fuel cell cathode oxygen reduction catalyst. It should be noted that the preparation and testing of the working electrode utilize existing technologies and are not limited to these methods in the present invention.

[0020] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. All other embodiments obtained by ordinary technicians in this field based on the embodiments of the present invention without creative work are within the scope of protection of the present invention.

[0021] First, the preparation method of Ti3C2 nanosheets is described. The specific process of Ti3C2 nanosheets used in the present invention includes: first, preparing 60mL of 9mol / L hydrochloric acid solution, slowly adding 3g of LiF to the solution, and stirring for 30 minutes to completely dissolve the LiF solid. 3g of Ti3AlC2 solid is added to the above solution and reacted in a 35°C oil bath for 24 hours. After the reaction is completed, the resulting product is centrifuged and washed until the pH value of the supernatant is 7. 100mL of deionized water is added to the resulting precipitate, and after ultrasonication in an Ar atmosphere for 2 hours, centrifugation for 30 minutes is performed. The upper layer is a colloidal solution of Ti3C2 nanosheets.

[0022] Example 1

[0023] 20 mL of 25-30% ammonia solution was added to 20 mL of 3 mg / mL Ti3C2 colloidal solution. After stirring at room temperature for 12 hours, the resulting product was centrifuged and washed with deionized water until the pH value was 7. Freeze-dried to obtain nitrogen-doped porous Ti3C2 nanosheets.

[0024] Example 2

[0025] 20 mL of 0.1 mol / L KOH solution was added to 20 mL of 3 mg / mL Ti3C2 colloidal solution, and 60 mg of urea was added to the above solution. After reacting in an 80°C oil bath for 24 hours, the resulting product was centrifuged and washed with deionized water until the pH value was 7. Freeze-dried to obtain nitrogen-doped porous Ti3C2 nanosheets.

[0026] Example 3

[0027] 20 mL of a 25-30% ammonia solution was added to 20 mL of a 3 mg / mL Ti3C2 colloidal solution. 60 mg of boric acid was then added to the solution. After reacting in an oil bath at 120°C for 24 hours, the resulting product was centrifuged and washed with deionized water until the pH reached 7. Freeze-drying yielded boron- and nitrogen-doped porous Ti3C2 nanosheets.

[0028] Example 4

[0029] 20 mL of a 25-30% ammonia solution was added to 20 mL of a 3 mg / mL Ti3C2 colloidal solution. 60 mg of phosphoric acid was then added to the solution. After reacting in an 80°C oil bath for 24 hours, the resulting product was centrifuged and washed with deionized water until the pH reached 7. Freeze-drying yielded phosphorus- and nitrogen-doped porous Ti3C2 nanosheets.

[0030] Sample characterization

[0031] 1. X-ray diffraction (XRD) analysis

[0032] The nitrogen-doped porous Ti3C2 nanosheets prepared in Example 1 of the present invention were analyzed by X-ray powder diffractometer (model: X Pert PRO MPD) produced by PANalytical of the Netherlands. The results are as follows: Figure 1 shown. Figure 1 The diffraction peaks appearing in the graph correspond to different crystal planes of Ti3C2 nanosheets, indicating the successful synthesis of nitrogen-doped porous Ti3C2 nanosheets.

[0033] 2. Transmission electron microscopy (TEM) analysis

[0034] The morphology and structure of the nitrogen-doped porous Ti3C2 nanosheets prepared in Example 1 of the present invention were analyzed using a transmission electron microscope (FEI Talos 200S). Figure 2 As shown. Figure 2 It can be observed that the nitrogen-doped porous Ti3C2 nanosheets maintain a relatively thin two-dimensional nanosheet structure, and many pore structures are formed on their surface. The diameter of most pores is 2 to 20 nm, indicating the successful synthesis of nitrogen-doped porous Ti3C2 nanosheets.

[0035] 3. X-ray photoelectron spectroscopy (XPS) analysis

[0036] The composition of the nitrogen-doped porous Ti3C2 nanosheets prepared in Example 1 of the present invention was analyzed by X-ray photoelectron spectroscopy (EscaLab 250Xi). Figure 3 shown. Figure 3 The characteristic peaks of C 1s, O 1s, N 1s and Ti 2p appeared, indicating the successful synthesis of nitrogen-doped porous Ti3C2 nanosheets.

[0037] synthesis.

[0038] Performance Testing

[0039] Working Electrode Preparation: A catalyst-coated glassy carbon electrode was used as the working electrode. Before adding the sample, the glassy carbon electrode required pretreatment. First, wipe the glassy carbon electrode with lens paper moistened with isopropyl alcohol to remove surface contaminants. Then, the glassy carbon electrode was polished in the shape of an "8" on chamois leather coated with 1.0µm, 0.3µm, and 0.05µm Al2O3 slurries, respectively, for 1 minute each. After each polishing, the Al2O3 slurry on the glassy carbon electrode surface was rinsed with deionized water. The polished electrode was ultrasonicated in anhydrous ethanol for 2 minutes. The test sample was prepared in a ratio of 2mg sample: 5% Nafion: ethanol: 25µL: 475µL. After ultrasonication for 30 minutes, the catalyst sample was drop-coated on the clean glassy carbon electrode surface, 1µL at a time, for a total of five times. Allow to air dry at room temperature before testing.

[0040] Comparative Example 1

[0041] Exfoliated Ti3C2 nanosheets obtained after Ti3AlC2 etching, intercalation, and sonication.

[0042] Ti3C2 nanosheets and nitrogen-doped porous Ti3C2 nanosheets were coated on glassy carbon electrodes as working electrodes, and the three-electrode system was used for testing on a CHI760E electrochemical workstation and RRDE-3A.

[0043] Figure 4The oxygen reduction polarization curves of Ti3C2 nanosheets and nitrogen-doped porous Ti3C2 nanosheets are shown in Figure 2. The results show that the half-wave potential of nitrogen-doped porous Ti3C2 nanosheets is 0.72V, which is higher than the half-wave potential of Ti3C2 nanosheets (0.65V); the onset potential of nitrogen-doped porous Ti3C2 nanosheets is 0.86V, which is higher than the onset potential of Ti3C2 nanosheets (0.80V); the limiting current density of oxygen reduction of nitrogen-doped porous Ti3C2 nanosheets is 4.48mA / cm 2 , which is higher than the oxygen reduction limiting current density of Ti3C2 nanosheets (3.01 mA / cm 2 The results show that the formation of nitrogen doping and porous structure can enhance the electrocatalytic oxygen reduction activity of Ti3C2 nanosheets.

[0044] The above is a detailed introduction to the preparation and application of the nitrogen-doped porous Ti3C2 nanosheets provided by the present invention. Specific examples are used herein to illustrate the principles and implementation methods of the present invention. The description of the above examples is only used to help understand the method of the present invention and its central idea. It should be pointed out that for ordinary technicians in this field, without departing from the principles of the present invention, the present invention can also be improved and modified in several ways, and these improvements and modifications also fall within the scope of protection of the claims of the present invention.

Claims

1. Application of heteroatom-doped porous metal carbon compound MXene nanosheets in cathode catalysts for fuel cells; The MXene nanosheet has an ultra-thin two-dimensional nanosheet structure, with abundant pore structures on the surface of the nanosheet and a heteroatom-doped surface structure. The preparation method of the heteroatom-doped porous metal carbon compound MXene nanosheets comprises the following steps: First, the Al layer of the precursor Ti3AlC2 is selectively etched away in a mixed solution of HCl and LiF, and then Ti3C2 nanosheets are obtained by intercalation and ultrasound. The Ti3C2 nanosheets are added to concentrated ammonia water, stirred at room temperature for 6 to 36 hours, centrifuged, washed with deionized water until neutral, and freeze-dried to obtain nitrogen-doped porous Ti3C2 nanosheets.

2. The use of heteroatom-doped porous metal carbon compound MXene nanosheets in a cathode catalyst of a fuel cell according to claim 1, characterized in that: The half-wave potential of the MXene nanosheet electrocatalytic oxygen reduction is 0.72-0.85 V, and the limiting current density is 4-6 mA / cm 2 , after 5000 cycles, the half-wave potential only dropped by 1 to 5 mV.

3. The use of heteroatom-doped porous metal carbon compound MXene nanosheets in a cathode catalyst of a fuel cell according to claim 1, characterized in that: The doping amount of the heteroatoms is 5 to 20 wt % and is evenly distributed on the surface of the nanosheets.

4. The use of heteroatom-doped porous metal carbon compound MXene nanosheets in a cathode catalyst of a fuel cell according to claim 1, characterized in that: The surface of the nanosheets has a rich pore structure with a pore diameter of 2 to 100 nm.

5. The use of heteroatom-doped porous metal carbon compound MXene nanosheets in a cathode catalyst of a fuel cell according to claim 1, characterized in that: The specific surface area of porous MXene nanosheets is between 50 and 200 m 2 / g.

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