Preparation method of single-atom modified graphite felt electrode based on ZIF-L derivative and application thereof

Abstract

The application discloses a preparation method of a graphite felt electrode modified by single-atom based on ZIF-L and application thereof. The application takes two-dimensional ZIF-L as a precursor, grows an array structure on the surface of the graphite felt in situ, and performs pyrolysis under nitrogen or inert atmosphere to form a three-dimensional porous nitrogen-doped carbon material; then, a metal salt solution is impregnated in the obtained graphite felt electrode by utilizing the interaction between the nitrogen-doped carbon skeleton and metal ions, so that the metal precursor is fully adsorbed and uniformly distributed on the surface and in the pores of the electrode; subsequently, the solvent is removed through drying treatment; finally, secondary pyrolysis is performed under inert atmosphere to form a single-atom dispersed M-N X nitrogen-carbon material. The material is applied as an electrode in a vanadium flow battery system, the high dispersion structure of the single-atom sites and the synergistic effect of the nitrogen coordination environment effectively promote the electrode reaction kinetics, the material exhibits high electrochemical activity and energy efficiency in a wide current density range, and has good cycle stability.

Description

Technical Field

[0001] This invention belongs to the field of carbon materials and electrochemical energy storage, specifically relating to a method for preparing a single-atom nitrogen-doped carbon-modified graphite felt electrode based on an impregnation loading strategy and its application in vanadium redox flow batteries. Background Technology

[0002] With the large-scale development and utilization of renewable energy, electrochemical energy storage technology plays an increasingly prominent role in grid peak shaving and energy regulation. Vanadium redox flow batteries, due to their advantages such as independently adjustable capacity and power, high safety, and long cycle life, are considered an important technological route in the field of large-scale energy storage. However, their performance is still limited by slow electrode reaction kinetics and side reactions, restricting their application under high current density conditions. Currently, graphite felt, as a commonly used electrode material in vanadium redox flow batteries, has good conductivity and chemical stability, but its intrinsic electrocatalytic activity is low, making it difficult to effectively promote the positive and negative electrode reactions. In recent years, single-atom catalysts have shown promising application prospects in the field of electrocatalysis due to their high atomic utilization and tunable coordination structures. However, existing single-atom modified graphite felt electrodes still have limitations in structural construction, making it difficult to form an ordered structure on the carbon fiber substrate surface that combines high specific surface area, abundant exposed coordination sites, and continuous mass transfer channels. In particular, the lack of multi-level structural designs based on two-dimensional layered metal-organic frameworks leads to insufficient distribution of active sites and low utilization efficiency, thus affecting the overall electrochemical reaction kinetics of the electrode. Furthermore, conventional MOF-derived carbon materials are mostly based on three-dimensional precursors such as ZIF-8, and there is a lack of systematic research on the in-situ construction of ZIF-L, which has two-dimensional layered structure characteristics, on the surface of a three-dimensional conductive substrate and its derived single-atom electrode system. Therefore, developing a method to achieve the integrated construction of single-atom sites and graphite felt electrodes is of great significance for improving the stability of electrode structures and electrochemical performance.

[0003] Compared with the prior art, the present invention has the following beneficial effects: (1) Using ZIF-L with a two-dimensional layered structure as a precursor, an oriented nanosheet array structure is constructed in situ on the surface of a graphite felt substrate. Compared with traditional three-dimensional precursors such as ZIF-8, it is beneficial to form a continuous conductive network and open pore structure, thereby significantly improving the exposure of active sites and the mass transfer efficiency of reactants, thus realizing the integrated construction of structure and function on a conductive substrate. (2) By combining impregnation loading with secondary pyrolysis, the in-situ reconstruction and stable anchoring of metal species in nitrogen-doped carbon framework were realized, which effectively improved the dispersion of single atom sites and structural stability. (3) The constructed MN XActive sites can regulate the electronic structure of the electrode interface, reduce the energy barrier of electrochemical reactions and suppress side reactions, thereby improving the energy efficiency and cycle stability of vanadium redox flow batteries over a wider current density range. Summary of the Invention

[0004] The purpose of this invention is to provide a method for preparing a single-atom modified nitrogen-doped carbon graphite felt electrode and its application.

[0005] The single-atom modified nitrogen-doped carbon graphite felt electrode is a three-dimensional porous structure material with a nitrogen-doped carbon layer loaded on the surface of the graphite felt. The metal is anchored in the nitrogen-doped carbon framework in an atomically dispersed form and forms an MN with nitrogen atoms. X Coordination structure.

[0006] The method for preparing the single-atom modified nitrogen-doped carbon graphite felt electrode is as follows: Using graphite felt as a substrate, a ZIF-L precursor with a two-dimensional layered structure is grown in situ on its surface and pyrolyzed under nitrogen or an inert atmosphere to form nitrogen-doped carbon material. Then, the resulting electrode is immersed in a metal salt solution, adsorbed under stirring, and allowed to stand to allow metal ions to be uniformly distributed on the electrode surface and in the pore structure. After the reaction is complete, the electrode is removed, washed with deionized water, and dried. Subsequently, a second pyrolysis is performed under nitrogen or an inert atmosphere to allow metal species to be reconstructed in situ in the nitrogen-doped carbon framework and coordinated with nitrogen atoms, forming atomically dispersed MN atoms. X Active site.

[0007] The ZIF-L is a metal-organic framework material with a two-dimensional layered structure. It forms a sheet-like nanostructure array in situ on the surface of graphite felt. The structure is oriented along the surface of the graphite felt fibers and has abundant exposed coordination sites and interconnected pore structures.

[0008] The ZIF-L nanosheets have a thickness of 10-200 nm and form a continuous or semi-continuous covering structure on the surface of graphite felt fibers.

[0009] The metal in the metal salt is selected from one or more transition metals or main group metals, preferably one or more of Ni, Ag, and Bi.

[0010] The concentration of the metal salt solution is 0.1-5 wt% of the mass of the nitrogen-doped carbon material.

[0011] The stirring time during the impregnation process is 10-60 min, and the standing time is 6-24 h.

[0012] The pyrolysis temperature is 700-1000 ℃, the holding time is 1-4 h, and the heating rate is 2-10 ℃ min. -1 .

[0013] The flow rate of the nitrogen or inert gas is 10-200 mL / min. -1 .

[0014] The single-atom modified nitrogen-doped carbon graphite felt electrode prepared above can be used as an electrode material in vanadium redox flow battery systems.

[0015] This invention achieves uniform dispersion and stable anchoring of single-atom active sites on the electrode surface by in-situ constructing a ZIF-L-derived nitrogen-doped carbon layer on the surface of graphite felt and combining impregnation loading with a secondary pyrolysis strategy. The resulting electrode maintains good conductivity and structural stability while effectively improving electrode reaction kinetics and suppressing side reactions. It exhibits excellent electrochemical performance and cycling stability over a wide current density range, demonstrating promising application prospects. Attached Figure Description

[0016] Figure 1 The images shown are: transmission electron microscope (TEM) image (a) of the Ni single-atom modified graphite felt electrode prepared in Example 1, and corresponding scanning electron microscope (SEM) image and energy dispersive spectroscopy (EDS) elemental distribution map (b); X-ray diffraction (XRD) patterns (c) of the prepared Ni SAs / NC and samples with different Ni loadings (Ni SAs1 / NC, Ni SAs2 / NC, Ni SAs3 / NC); and corresponding Fourier transform extended X-ray absorption fine structure (FT-EXAFS) spectra (d), used for comparative analysis of the local single-atom coordination structure of Ni.

[0017] Figure 2 The electrochemical performance test results of the Ni single-atom modified graphite felt electrode prepared in Example 1 in the vanadium redox flow battery system include voltage efficiency (a), energy efficiency and coulombic efficiency (b) at different current densities, as well as efficiency change (c) and capacity retention (d) of the battery with NiSAs3 / NC electrode during long-term cycling.

[0018] Figure 3 Figure 1 shows scanning electron microscope (SEM) images, energy dispersive spectroscopy (EDS) elemental distribution maps (a, b), and X-ray diffraction (XRD) patterns (c) of the Ag / NC and Bi / NC electrodes prepared in Examples 2 and 3. Figure 2d shows a comparison of energy efficiency in the vanadium redox flow battery system. Detailed Implementation

[0019] Example 1 (1) Pretreatment of graphite felt: The graphite felt was cut into 3 cm × 3 cm × 0.3 cm pieces and placed in deionized water and anhydrous ethanol for ultrasonic treatment for 10 min each to remove surface impurities and organic contaminants. Then the graphite felt was transferred to a 68% nitric acid solution and stirred at 60 ℃ for 6 h. After treatment, the sample was taken out, rinsed repeatedly with a large amount of deionized water until the pH was neutral, and dried in a vacuum drying oven at 60 ℃ for later use.

[0020] (2) In-situ growth of ZIF-L precursor: 1.9 mmol Zn(NO3)2·6H2O was dissolved in 40 mL of deionized water to obtain solution A; 16 mmol 2-methylimidazole was dissolved in 40 mL of deionized water to obtain solution B. Under magnetic stirring at 800 rpm, solution B was quickly poured into solution A and stirred continuously for 5 min to obtain a homogeneous mixed solution C. Then, the graphite felt treated in step (1) was placed in the mixed solution C, and ultrasonically treated for 30 min. The graphite felt was then placed at an angle in the solution and allowed to stand at room temperature for 8 h to allow ZIF-L to grow in situ on the surface of the graphite felt fibers to form a nanosheet array structure. After the reaction was completed, the sample was taken out, rinsed repeatedly with deionized water, and dried in a vacuum drying oven at 60 ℃ for later use.

[0021] (3) Preparation of nitrogen-doped carbon-modified graphite felt (NC): The ZIF-L / graphite felt obtained in step (2) was placed in a ceramic boat and placed in the middle of a tube furnace. Under argon protection, a purging-evacuation cycle was performed three times to remove air, and then the furnace was heated at 5 °C for 1 minute. -1 The temperature was increased to 900 °C and held for 2 h. After pyrolysis, the material was naturally cooled to room temperature under an argon atmosphere to obtain a nitrogen-doped carbon-modified graphite felt electrode (NC).

[0022] (4) Preparation of Ni precursor solution: Weigh 20, 40 and 60 mg of Ni(NO3)2·6H2O and dissolve them in 100 mL of deionized water respectively. Stir until completely dissolved to obtain a homogeneous Ni precursor solution.

[0023] (5) Ni impregnation and loading: The NC electrodes obtained in step (3) were placed in the Ni precursor solution prepared in step (4) and stirred for 30 min under magnetic stirring at 200 rpm to allow metal ions to diffuse into the electrode surface and pore structure. They were then allowed to stand at room temperature for 12 h to enhance the Ni loading. 2+ Adsorption and coordination interactions between the metal and nitrogen-doped carbon framework. After the reaction, the electrode was removed, repeatedly rinsed with deionized water to remove unbound metal precursors, and dried in a vacuum drying oven for 12 h.

[0024] (6) Secondary pyrolysis to form single-atom sites: The sample obtained in step (5) is placed in a ceramic boat, put into a tube furnace, and purged under argon protection for 3 cycles, with the argon flow rate controlled at 50 mL / min. -1 Then at 5 ℃ min -1 The temperature was increased to 900 °C and held for 2 h at a high heating rate, causing the adsorbed Ni species to undergo in-situ reconstruction in the nitrogen-doped carbon framework and coordinate with nitrogen atoms to form Ni-N. X Structure. After the sample was naturally cooled to room temperature under an argon atmosphere, a single-atom Ni-modified nitrogen-doped carbon graphite felt electrode (Ni SAs / NC) was obtained.

[0025] Example 2 The difference from Example 1 is that: The metal precursor used in steps (4) and (5) was replaced with a silver salt solution by Ni(NO3)2·6H2O, while the other reaction conditions remained unchanged, to obtain the Ag / NC electrode.

[0026] Example 3 The difference from Example 1 is that: The metal precursor used in steps (4) and (5) was replaced with bismuth salt solution by Ni(NO3)2·6H2O, while the other reaction conditions remained unchanged, to obtain the Bi / NC electrode.

[0027] Test Results The Ni system was used as a representative model for systematic characterization, and the Ag and Bi systems showed similar structural and performance trends.

[0028] (1) Microstructure characterization: Transmission electron microscopy (TEM) results showed that the obtained Ni SAs / NC electrode retained the layered carbon structure derived from the ZIF-L precursor after pyrolysis treatment, and no obvious metal particles or agglomeration were observed (see Figure 1 a). Energy dispersive spectroscopy (EDS) elemental distribution results show that Ni is uniformly distributed in the Ni SAs / NC electrode (see... Figure 1 b); Ag and Bi elements are uniformly distributed in the corresponding samples (see Figure 3 (a, 3b) indicates that the metal species achieved good dispersion and anchoring on the carbon substrate.

[0029] (2) Phase structure analysis: X-ray diffraction (XRD) results showed that the Ni SAs / NC, Ag / NC, and Bi / NC electrodes exhibited characteristic diffraction peaks at approximately 26° and 44°, corresponding to the 002 and 101 crystal planes of graphitized carbon, respectively; no characteristic diffraction peaks of metallic Ni, Ag, Bi, or their oxides were detected, indicating that the metal species did not form detectable crystalline particles (see Figure 1 c and Figure 3c) (3) Synchrotron Radiation Structure Analysis: Synchrotron radiation X-ray absorption fine structure (EXAFS) analysis was performed on the Ni SAs / NC electrode. The R-space spectrum after Fourier transform showed a clear coordination peak at approximately 1.4–1.6 Å, corresponding to a Ni-N coordination structure. No Ni-Ni coordination peak was observed at approximately 2.2 Å, indicating that the metal species exist in an atomically dispersed form and have not formed detectable crystalline particles. Compared with the Ni foil reference sample, the coordination environment of Ni SAs / NC is significantly different, indicating that the Ni species exist in an atomically dispersed form and form a stable coordination structure with the nitrogen-doped carbon framework (see...). Figure 1 d) (4) Battery performance testing: Ni SAs / NC, Ag / NC, and Bi / NC electrodes were assembled into vanadium redox flow battery systems for performance testing. The performance was tested at 80-200 mA cm⁻¹. -2 Within a wide current density range, Ni SAs / NC, Ag / NC, and Bi / NC electrodes were assembled into vanadium redox flow battery systems for performance testing. The results show that, compared to the unmodified NC electrode, the energy efficiency of each modified electrode is improved by at least 5% at all current densities, and maintains a stable trend with changing current density, indicating that the modified electrodes exhibit excellent electrochemical performance enhancement over a wide current density range. (See...) Figure 2 and Figure 3 d).

Claims

1. A ZIF-L-derived single-atom modified graphite felt electrode, characterized in that: The electrode is a three-dimensional porous nitrogen-doped carbon layer constructed on the surface of a graphite felt substrate. This nitrogen-doped carbon layer is formed by the pyrolysis of a metal-organic framework precursor ZIF-L array and exhibits a three-dimensional continuous porous structure. Metal species are anchored in the nitrogen-doped carbon framework in an atomically dispersed manner, primarily in the form of MN. X The coordination structure exists; the nitrogen-doped carbon layer forms an array of oriented sheet structures along the surface of the graphite felt fiber, thereby constructing a continuous conductive network and a connected channel structure.

2. A method for preparing the electrode as described in claim 1, characterized in that... Includes the following steps: (1) Using graphite felt as a substrate, a ZIF-L precursor with a two-dimensional layered structure is grown in situ on its surface; (2) The precursor is pyrolyzed under an inert atmosphere to obtain a nitrogen-doped carbon-modified graphite felt electrode. (3) Immerse the obtained electrode in a metal salt solution to allow metal ions to be adsorbed and distributed in the nitrogen-doped carbon framework; (4) After drying, a second pyrolysis is carried out under an inert atmosphere to reconstruct the metal species in situ and coordinate them with nitrogen atoms to form atomically dispersed MN. X Active site.

3. The preparation method according to claim 2, characterized in that, The ZIF-L forms an oriented array of nanosheets on the surface of graphite felt fibers, with the nanosheets having a thickness of 5-200 nm.

4. The preparation method according to claim 2, characterized in that: The two-dimensional layered ZIF-L precursor is transformed into a continuous conductive carbon framework during pyrolysis while maintaining its layered structure, thereby increasing the exposure of active sites, reducing reactant diffusion resistance, and improving interfacial reaction kinetics.

5. The preparation method according to claim 2, characterized in that: The metal is selected from one or more of Ni, Cu, Co, Fe, Ag, and Bi.

6. The pyrolysis temperature is 700-1000 ℃, the holding time is 1-4 h, and the heating rate is 2-10 ℃ min. -1 .

7. The concentration of the metal salt solution is 0.1-5 wt%, the immersion time is 10-60 min, and the standing time is 6-24 h.

8. The electrode according to claim 1 exhibits higher energy efficiency, lower polarization and better cycle stability in vanadium redox flow batteries compared to unmodified graphite felt electrodes.

9. The application of the electrode according to any one of claims 1-8 in the electrode reaction of a vanadium redox flow battery.

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