Preparation method and application of nitrogen and oxygen dual-doped porous carbon-loaded graphite felt electrode

Abstract

The present invention belongs to the field of electrode material technology, and in particular relates to a preparation method and application of a nitrogen and oxygen dual-doped porous carbon-loaded graphite felt electrode. The present invention successfully synthesized a nitrogen and oxygen dual-doped MOF-derived porous electrode by combining an electrochemical deposition strategy with a pyrolysis method. This method can uniformly load ZIF-8-derived carbon on the graphite felt, thereby increasing the specific surface area of ​​the electrode, and significantly enhances the electrode's V by doping with nitrogen and oxygen functional groups. 3+ / V 2+ and VO 2+ / VO2 + Electrochemical activity of the VRFB assembled with ZIF‑8‑C‑GF as positive and negative electrodes at 100 mA cm ‑2 At a current density of 1.5 GHz, it can achieve a high energy efficiency of over 85.77% and has good long-term stability of 500 cycles.

Description

Technical Field

[0001] The present invention belongs to the technical field of electrode materials, and in particular relates to a preparation method and application of a nitrogen and oxygen dual-doped porous carbon-loaded graphite felt electrode. Background Art

[0002] In recent years, power generation from renewable energy sources such as solar and wind power has seen exponential growth. However, the mismatch between these fluctuating and intermittent renewable energy sources and grid demand has resulted in significant energy waste. To address this issue, large-scale energy storage technologies have become an attractive solution. Redox flow batteries (RFBs), due to their unique ability to decouple energy storage from power output, show great potential for development in various application scenarios. In particular, all-vanadium redox flow batteries (VRFBs), as a representative RFB technology, have become a popular choice due to their long lifespan, high safety, and design flexibility. However, their high initial investment cost has limited widespread adoption in the energy market. An effective way to reduce this cost is to increase the operating current density of VRFBs while maintaining high energy efficiency (EE), thereby reducing the system stack size and optimizing economics.

[0003] In VRFBs, redox reactions primarily occur at the interface between the electrode and the electrolyte. Therefore, the surface physical structure and electrochemical properties of the electrode fibers play a crucial role in battery performance. Specifically, the fiber surface physical structure influences the transport of electrons, ions, and species, while the electrochemical properties directly affect the rates of redox reactions. These factors, combined, influence activation loss, resistance loss, and concentration loss. Currently, the most commonly used electrode materials are carbon felt (CF) or graphite felt (GF) due to their high conductivity and good chemical stability. However, VRFBs using pristine CF or GF as electrodes typically operate only at low current densities, primarily due to their low electrochemical activity and limited specific surface area (SSA). Metal-organic frameworks (MOFs) are porous coordination polymers with high specific surface area, tunable pore structure, and broad application prospects. Carbon materials derived from MOFs have been widely used in energy storage devices and are ideal candidates for high-performance electrode materials. These carbon materials retain the properties of their parent MOFs, typically exhibiting large specific surface area (SSA) and high porosity. In addition, by regulating the carbonization temperature, the physical structure and surface chemical properties of carbon materials can be optimized, thereby improving charge transfer efficiency, electrical conductivity, and mass transport capacity, and accelerating the redox reaction process. Previous studies have shown that various MOF materials (such as Zn-MOF, Co-MOF, Ni-MOF, Sn-MOF, Zr-MOF, and W / Zr-MOF) can significantly improve the reaction kinetics of vanadium species, thereby endowing VRFBs with excellent performance. However, traditional in situ synthesis methods have problems such as complex operation and uneven growth of the MOF layer on the fiber surface, which limit the full performance of the battery. This challenge needs to be addressed urgently.

[0004] The present invention uses electrochemical deposition (ED) technology to quickly achieve uniform and dense MOF deposition on the electrode fiber surface. Subsequently, nitrogen and oxygen dual-doped porous carbon-supported graphite felt (ZIF-8-C-GF) was prepared by carbonization treatment. This dual-doping functionalization strategy significantly improves the hydrophilicity and electrochemical activity of the electrode surface, while optimizing the mass transfer performance of the electrode by utilizing the high porosity of MOF. VRFB based on ZIF-8-C-GF electrode at 100 mA cm -1 The electrode exhibited excellent performance at a current density of 100 mA cm-3, with an energy efficiency (EE) of 85.77%. -2 After 500 cycles under the same conditions, the energy efficiency still remains above 75.55%. At the same time, the VRFB can achieve 356 mW cm -2 The high peak power density provides new directions and possibilities for the development of high-performance and scalable VRFB electrodes. Summary of the Invention

[0005] The first object of the present invention is to provide a method for preparing a nitrogen and oxygen dual-doped porous carbon-supported graphite felt electrode, comprising the following steps:

[0006] S1. Preparation of ZIF-8 mother liquor

[0007] Dissolving zinc acetate and 2-methylimidazole in methanol separately, followed by stirring and ultrasonic treatment; then mixing the two solutions and stirring to form a homogeneous mother liquor;

[0008] S2. Preparation of ZIF-8@GF by electrochemical deposition

[0009] Graphite felt was cut into 2×3 mm sheets as the working electrode, and zinc foil was cut into the same size as the counter electrode. Both electrodes were simultaneously immersed in the mother liquor obtained in step S1 and subjected to electrodeposition treatment to prepare ZIF-8@GF. The prepared ZIF-8@GF was washed with methanol to remove unreacted precursors and then dried in an oven at 60 °C.

[0010] S3. Preparation of ZIF-8-C-GF

[0011] The ZIF-8@GF obtained in step 2 was carbonized by high-temperature calcination in an argon flow, and then the prepared material was immersed in a 10% H2SO4 solution at room temperature for 6 h to remove residual zinc; the sample was then washed with water and dried to obtain ZIF-8-C-GF.

[0012] Further, in step S1, 2-methylimidazole and Zn 2+ The molar ratio is 2:1.

[0013] Furthermore, in step S2 , the electrodeposition treatment is performed at room temperature with a current of 5 mA for 40 minutes.

[0014] Furthermore, in step S3, the high temperature calcination and carbonization process is as follows: the ZIF-8@GF synthesized in step S2 is heated to 100 mL min -1 Under argon flow at 5 °C min -1 The mixture was heated to 700-900 °C at a heating rate of 1000 ℃ and carbonized for 0.5 h.

[0015] The present invention also provides the use of the nitrogen and oxygen dual-doped porous carbon-supported graphite felt electrode in a redox flow battery.

[0016] Beneficial technical effects of the present invention: The present invention successfully synthesized a nitrogen and oxygen doped MOF-derived porous electrode by combining an electrochemical deposition strategy with a pyrolysis method. This method can evenly load ZIF-8-derived carbon on graphite felt, thereby increasing the specific surface area of ​​the electrode, and significantly enhances the electrode's V 3+ / V 2+ and VO 2+ / VO2 + The electrochemical activity of the VRFB assembled with ZIF-8-C-GF as positive and negative electrodes was 100 mA cm -2 At a current density of 1.5 GHz, a high energy efficiency exceeding 85.77% can be achieved, and good long-term stability for 500 cycles is demonstrated. This invention provides an efficient and reliable method for developing high-performance VRFB electrodes and opens up new directions for commercial electrode design. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 Schematic diagram of the synthesis route of the ZIF-8-C-GF electrode synthesized in Example 1.

[0018] Figure 2 SEM images of (a) pristine GF, (b) ZIF-8@GF, and (c) ZIF-8-C-GF.

[0019] Figure 3 EDS mapping images of ZIF-8-C-GF: (a) carbon, (b) nitrogen, and (c) oxygen elements.

[0020] Figure 4 XRD patterns of pristine GF, ZIF-8@GF, and ZIF-8-C-GF.

[0021] Figure 5 Raman spectra of original GF and ZIF-8-C-GF.

[0022] Figure 6 Nitrogen adsorption-desorption isotherms of pristine GF, ZIF-8@GF, and ZIF-8-C-GF.

[0023] Figure 7 Pore ​​size distribution curves of (a) ZIF-8-C-GF and (b) pristine GF.

[0024] Figure 8 The contact angles of the original GF (152.1°) (a) and ZIF-8-C-GF (105.5°) (b).

[0025] Figure 9 Schematic diagram of the wettability of the original GF (a) and the wettability of ZIF-8-C-GF (b).

[0026] Figure 10 (a) C1s XPS spectra of pristine GF and ZIF-8-C-GF, (b) N1s XPS spectra, and (c) O1s XPS spectra.

[0027] Figure 11 Cyclic voltammetry (CV) curves of pristine GF and ZIF-8-C-GF at a scan rate of 2 mV / s: (a) positive electrode reaction, (b) negative electrode reaction, and (c) bar graph of the Ipa / -Ipc ratio of the CV curves in the positive and negative reactions.

[0028] Figure 12 Nyquist plots of pristine GF and ZIF-8-C-GF at (a) the positive electrode and (b) the negative electrode.

[0029] Figure 13 For different samples at V 3+ / V 2+ and VO 2+ / VO2 + The charge transfer resistance (R CT ).

[0030] Figure 14 For different electrodes at 100 mA cm -2 VRFB charge and discharge curves at different current densities.

[0031] Figure 15 Coulombic efficiency (CE) (a), voltage efficiency (VE) (b), and energy efficiency (EE) (c) of pristine GF and ZIF-8-C-GF at different current densities.

[0032] Figure 16 CE, VE, and EE of pristine GF, ZIF-8-C-GF-700 °C, ZIF-8-C-GF-800 °C, and ZIF-8-C-GF-900 °C at different current densities.

[0033] Figure 17 VRFB polarization and power density curves of different electrodes.

[0034] Figure 18 VRFB of different electrodes at 100 mA cm -2 The cycle performance results graph below. DETAILED DESCRIPTION

[0035] Example 1

[0036] S1. Preparation of mother liquor for synthesis of ZIF-8@GF

[0037] 0.55 g of zinc acetate and 0.41 g of 2-methylimidazole (2-MIM) were dissolved in 20 mL of methanol, stirred, and ultrasonicated. The two solutions were then mixed and stirred to form a homogeneous mother liquor. In this mother liquor, 2-MIM and Zn 2+ The molar ratio is 2:1.

[0038] S2. Preparation of ZIF-8@GF by electrochemical deposition

[0039] Graphite felt was cut into 2 × 3 mm sheets as the working electrode, and zinc foil was cut into the same size as the counter electrode. Both electrodes were immersed in the mother solution and electrodeposited at 5 mA for 40 minutes at room temperature to produce ZIF-8@GF. The resulting ZIF-8@GF was washed with methanol to remove unreacted precursors and then dried in an oven at 60 °C.

[0040] S3. Preparation of ZIF-8-C-GF

[0041] The synthesized ZIF-8@GF was heated in an argon flow (50 mL min -1 ) at 5 °C min -1 The carbonization process was carried out by heating the sample to 800°C at a rate of 100°C and holding it for 0.5 hours. The resulting material was then immersed in a 10% H₂SO₄ solution at room temperature for 6 hours to remove residual zinc. The sample was then rinsed with water and dried to yield ZIF-8-C-GF.

[0042] Example 2

[0043] The difference between Example 2 and Example 1 is that the calcination and carbonization temperature in S3 is 700 ° C, and the other conditions are exactly the same.

[0044] Example 3

[0045] The difference between Example 3 and Example 1 is that the calcination and carbonization temperature in S3 is 900 °C, and the other conditions are exactly the same.

[0046] Comparative Example 1

[0047] Comparative Example 1 is original graphite felt. As a control, the original graphite felt is also subjected to the same heat treatment and acid treatment as in Example 1.

[0048] The samples prepared in Example 1 and Comparative Example 1 were characterized:

[0049] (1) Scanning electron microscopy characterization

[0050] The microstructure of the samples was observed by scanning electron microscopy (SEM SU3500) at a scanning voltage of 20 kV.

[0051] Figure 2 ac show the scanning electron microscopy (SEM) images of pristine graphite felt (GF), ZIF-8 coated graphite felt (ZIF-8@GF), and carbonized ZIF-8 derived porous carbon coated graphite felt (ZIF-8-C-GF), respectively. Figure 2 In a, the GF fiber has a smooth surface and a diameter of approximately 8 µm. Figure 2 b shows that the GF surface is evenly covered with a layer of ZIF-8 with a thickness of about 1.5 μm. Since ZIF-8 shrinks in volume during the carbonization process, Figure 2 The diameter of the ZIF-8-C-GF fiber in c is reduced to about 9 μm

[0052] (2) Energy dispersive X-ray spectroscopy (EDS) characterization

[0053] The types and distribution of elements in the samples were analyzed by energy dispersive X-ray spectroscopy (EDS).

[0054] Energy dispersive X-ray spectroscopy (EDS) analysis confirmed that nitrogen (N) and oxygen (O) elements were doped into the fiber surface ( Figure 3 ac), these elements will serve as reaction sites to promote the redox reaction of vanadium ions.

[0055] (3) X-ray diffraction (XRD) pattern characterization

[0056] X-ray diffraction (XRD) characterization was performed using a Rigaku Smartlab 3 X-ray powder diffractometer equipped with a copper sealed tube (λ = 1.54178 Å) at room temperature.

[0057] X-ray diffraction (XRD) pattern ( Figure 4 ) shows that ZIF-8@GF has the characteristic peaks of bulk ZIF-8. After calcination, ZIF-8 decomposes and transforms into porous carbon, with a diffraction peak appearing near the graphite (002) plane at 26.3°. Therefore, the characteristic peaks of ZIF-8 are not observed in the XRD pattern of ZIF-8-C-GF, indicating that ZIF-8 first nucleates and grows on the surface of the graphite felt fibers, and then decomposes to form porous carbon during calcination.

[0058] (4) Raman spectroscopy characterization

[0059] In order to further analyze the structure of carbon materials, Raman spectroscopy tests were carried out ( Figure 5 ), the laser wavelength was 532 nm. All samples were detected at 1345 cm -1 and 1590 cm -1Two prominent peaks appear at the center, corresponding to the D-band and G-band of typical carbon materials. The D-band to G-band intensity ratio (ID / IG) reflects the degree of disorder in the carbon material. The ID / IG ratio of ZIF-8-C-GF is 0.97, higher than the 0.84 of the pristine GF, indicating that ZIF-8-C-GF contains more defects and can provide more active sites.

[0060] (5) Nitrogen adsorption and desorption isotherm test

[0061] Nitrogen adsorption and desorption isotherms were measured using an AutosorbiQ automatic gas adsorption analyzer, and the specific surface area and pore size distribution were calculated by the Brunauer-Emmett-Teller (BET) method and Barret-Joyner-Halenda (BJH) analysis.

[0062] BET analysis at 77 K ( Figure 6 ) showed that the specific surface area of ​​the original GF increased from 1.923 m 2 / g increased to 11.971 m / g for ZIF-8-C-GF 2 / g. The higher specific surface area provides more active sites for the charge transfer of vanadium ions, promoting its charge transfer kinetics. In addition, the pore size distribution before and after modification was calculated by the BJH method, as shown in Figure 7 As shown, the results indicate that the significant increase in mesopores facilitates the diffusion of vanadium ions.

[0063] (6) Contact angle test

[0064] The contact angle was measured by a contact angle meter (Dataphysics-TP50). The hydrophilicity of the original GF and ZIF-8-C-GF was verified by testing the contact angle. Figure 8 Compared with the original GF, the contact angle of ZIF-8-C-GF is smaller, which is mainly attributed to the increase of N and O functional groups on its surface, which significantly improves the wettability of the material. In addition, the immersion test shows that ZIF-8-C-GF can sink to the bottom of the water due to its hydrophilicity, while the original GF floats on the water surface due to its hydrophobicity ( Figure 9 These results indicate that the excellent hydrophilicity of ZIF-8-C-GF can significantly increase its contact area with the electrolyte, thereby helping to improve its electrochemically active surface area.

[0065] (7) X-ray photoelectron spectroscopy (XPS) characterization

[0066] The elemental composition and chemical bonding of the original GF and ZIF-8-C-GF were characterized by X-ray photoelectron spectroscopy (XPS). Figure 10a) After deconvolution, two main peaks are shown, located at 284.8 eV and 285.4 eV, corresponding to C=C and C-C bonds, respectively. It is worth noting that an additional peak appears in the C1s spectrum of ZIF-8-C-GF, located at 287.9 ​​eV, which is caused by the formation of CO and CN bonds. This result indicates that nitrogen and oxygen atoms are introduced after the decomposition of ZIF-8, and they combine with the defective carbon to form nitrogen and oxygen functional groups. N1s spectra of pristine GF and ZIF-8-C-GF ( Figure 10 b) shows that no obvious nitrogen peaks were detected in the original GF, while four nitrogen species appeared in ZIF-8-C-GF: pyridinic nitrogen (398.7 eV), pyrrolic nitrogen (400.9 eV), graphitic nitrogen (401.9 eV) and oxidized nitrogen (402.3 eV). In addition, the O1s spectrum ( Figure 10 c) It can be deconvoluted into two components, corresponding to C-OH and -COOH. These two oxygen functional groups can effectively promote the redox reaction of vanadium ions on the electrode surface.

[0067] Example 4 Electrochemical Performance Test

[0068] Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were used to determine the VO 2+ / VO2 + and V 3+ / V 2+ The electrochemical performance of the redox couple on pristine GF and ZIF-8-C-GF was evaluated. 2+ / VO2 + In the CV curve of the reaction ( Figure 11 a) ZIF-8-C-GF exhibits significantly enhanced electrochemical activity, with an anodic peak current density (Ipa) reaching 65 mA cm -2 , the cathode peak current density (-Ipc) is 40mAcm -2 , which are significantly higher than the 51 mA cm of the original GF. -2 and 26 mA cm -2 This is mainly due to the significant increase in specific surface area (SSA) after the ZIF-8 derivatives modified the electrode surface.

[0069] In V 3+ / V 2+ In the CV curve of the reaction ( Figure 11 b) The difference between the two materials is more obvious. The original GF does not show V 3+ Restore to V 2+ In contrast, an obvious reduction peak appeared in the CV curve of ZIF-8-C-GF, indicating that ZIF-8 derivatives can effectively promote the 3+ / V 2+ The reduction reaction of the anode and cathode peak currents is closer to 1 ( Figure 11 c) ZIF-8-C-GF in VO 2+ / VO2 + and V 3+ / V 2+ It showed higher reversibility in the reaction, which was significantly better than the original GF.

[0070] The EIS curves of different electrodes are as follows Figure 12 The charge transfer resistance (R CT ). ZIF-8-C-GF as the positive and negative electrodes CT 1.06 Ω·cm 2 and 2.53 Ω·cm 2 , while the original GF acts as the positive and negative R CT 2.86 Ω·cm 2 and 159.3 Ω 2 ( Figure 13 Compared with the original GF, ZIF-8-C-GF showed significantly lower R CT , indicating faster charge transfer kinetics and excellent electrochemical activity. These results are consistent with the CV test results, further proving that the ZIF-8-derived porous carbon layer increases the specific surface area of ​​the electrode through the surface N and O functional groups, thereby effectively accelerating the conversion process of vanadium ions.

[0071] To evaluate the battery performance, VRFBs were assembled using ZIF-8-C-GF and pristine GF, respectively. Figure 14 Demonstrated at 100mAcm -2 The charge-discharge curves at different current densities. Compared with the VRFB using pristine GF, the VRFB using ZIF-8-C-GF exhibits a lower charge onset plateau and a higher discharge onset plateau, which is attributed to its lower cell polarization. -2 Their charge and discharge performances were evaluated at different current densities. The Coulombic efficiency (CE) of both VRFBs exceeded 95%, indicating that they have good sealing properties ( Figure 15 a). Polarization is a key factor affecting battery voltage efficiency (VE) and energy efficiency (EE) ( Figure 15 b, c). The experimental results show that at 100 mA cm -2At a current density of 200 mA cm-1, the VRFB using ZIF-8-C-GF achieved a voltage efficiency of 87.02% and an energy efficiency of 85.77%, while the VRFB using the original GF achieved 78.89% and 76.32%, respectively. As the current density increases, the VRFB using the original GF fails to continue to work, while the VRFB using ZIF-8-C-GF can still operate stably and maintain high energy efficiency at 200 mA cm-1. -2 The ZIF-8-derived N and O dual-doped carbon coating significantly improved the charge transfer rate and vanadium ion adsorption performance by increasing the specific surface area of ​​the electrode and increasing the number of active sites.

[0072] To further explore the effect of calcination temperature on electrode performance, experiments at different temperatures (700 °C, 800 °C, and 900 °C) were carried out. Figure 16 The battery performance data in Figures 16a and 16b show that the material calcined at 800 °C exhibits the best performance. Figure 17 The polarization curves at 50% state of charge are shown, and the results show that the VRFB using ZIF-8-C-GF has a smaller polarization than the VRFB using pristine GF. The peak power density of the VRFB using ZIF-8-C-GF reaches 356 mW cm -2 , which is significantly higher than the 210 mW cm of the original GF. -2 . Figure 18 The long-term cycling performance of VRFB was demonstrated. After 500 cycles, the VRFB using ZIF-8-C-GF still maintained a high energy efficiency (75.55%), indicating its good mechanical stability.

Claims

1. A method for preparing a nitrogen and oxygen dual-doped porous carbon-supported graphite felt electrode, comprising the following steps: S1. Preparation of ZIF-8 mother liquor Dissolving zinc acetate and 2-methylimidazole in methanol separately, followed by stirring and ultrasonic treatment; then mixing the two solutions and stirring to form a homogeneous mother liquor; S2. Preparation of ZIF-8@GF by electrochemical deposition Graphite felt was cut into 2×3 mm sheets as a working electrode, and zinc foil was cut into the same size as a counter electrode. Both electrodes were simultaneously immersed in the mother solution obtained in step S1 and subjected to electrodeposition at 5 mA for 40 minutes at room temperature to prepare ZIF-8@GF. The prepared ZIF-8@GF was washed with methanol to remove unreacted precursors and then dried in an oven at 60°C. S3. Preparation of ZIF-8-C-GF The ZIF-8@GF obtained in step S2 was carbonized by calcining at high temperature in an argon flow, and the carbonized material was immersed in a H2SO4 solution at room temperature to remove residual zinc; the sample was washed with water and dried to obtain ZIF-8-C-GF; In step S3, the high temperature calcination and carbonization process is as follows: ZIF-8@GF synthesized in step S2 is heated to 100 mL / min. -1 Under argon flow at 5 °C min -1 The samples were heated to 800°C at a heating rate of 1000 °C and carbonized for 0.5 h.

2. The method for preparing the nitrogen and oxygen dual-doped porous carbon-supported graphite felt electrode according to claim 1, characterized in that: In step S1, 2-methylimidazole and Zn 2+ The molar ratio is 2:

1.

3. Use of a nitrogen and oxygen dual-doped porous carbon-supported graphite felt electrode prepared by the preparation method according to any one of claims 1 to 2 in a redox flow battery.

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