A polyimide-derived hierarchically porous carbon / carbon fiber composite electrode and a preparation method thereof

Abstract

This invention relates to the field of new materials and electrochemical energy storage materials, and discloses a polyimide-derived hierarchical porous carbon / carbon fiber composite electrode and its preparation method. The preparation method involves using carbon fiber felt as a substrate, and constructing a three-level interconnected structure of micropores, mesopores, and macropores through polyimide solution spraying, metal-organic gel template deposition, gradient carbonization, and eutectic activation processes. This invention, through template synergy and optimized activation processes, enables the constructed composite electrode to possess a unique three-level interconnected structure of micropores, mesopores, and macropores, solving the problems of limited mass transfer and insufficient active sites in traditional carbon electrodes. It can be widely applied to all-vanadium, iron-chromium, and organic flow batteries, and has significant industrial application value.

Description

Technical Field

[0001] This invention relates to the field of new materials and electrochemical energy storage materials, specifically to a polyimide-derived hierarchical porous carbon / carbon fiber composite electrode and its preparation method. Background Technology

[0002] Energy crisis and environmental pollution are core challenges facing global sustainable development, and promoting the construction of a green, efficient, and renewable energy system has become an international consensus. However, new clean energy sources, such as hydrogen energy and tidal energy, are limited by uneven geographical distribution and supply fluctuations, making it difficult to achieve stable grid connection and output. Therefore, there is an urgent need to develop matching high-efficiency energy storage technologies. Against this backdrop, flow batteries, due to their modular design, capacity-power decoupling characteristics, and ultra-long cycle life, have become a strategic technology choice for large-scale energy storage.

[0003] As a core component of flow batteries, the multi-scale mass transfer capability and electrochemical activity of electrode materials directly determine the system's energy efficiency and power density. At the micro- and nano-scale, micropores (<2 nm) provide abundant redox reaction active sites through high specific surface area, mesopores (2-50 nm) shorten the mass transfer path as ion diffusion channels, and macro-scale macropores (>50 μm) significantly reduce electrolyte flow resistance. The synergistic effect of these three can overcome the kinetic bottlenecks of traditional energy storage devices. However, commercial carbon felt electrodes, limited by a single pore size distribution and low specific surface area (<5 m² / g), exhibit a deep contradiction in their pore system regarding mass transfer and reaction: excessive pursuit of high specific surface area through microporous modification can increase the density of reaction sites, but it leads to an increase in tortuosity factor and a decrease in ion transport number; while simply increasing the pore size can reduce concentration polarization, it results in insufficient exposure of active sites. This structure-function imbalance directly makes it difficult for flow batteries to achieve simultaneous optimization of high energy efficiency and high power density over a wide operating range, severely restricting their commercial application in grid-scale energy storage scenarios. Summary of the Invention

[0004] The purpose of this invention is to address the shortcomings of the prior art by proposing a polyimide-derived hierarchical porous carbon / carbon fiber composite electrode and its preparation method. Through polyimide solution spraying, metal-organic gel (Fe-MOG) template deposition, gradient carbonization, and eutectic activation processes, a three-level interconnected structure of micropores, mesopores, and macropores is constructed, solving the problems of limited mass transfer and insufficient active sites in traditional carbon electrodes, and providing a new solution for the commercial deployment of megawatt-level flow battery power plants.

[0005] To achieve the above objectives, the present invention provides the following technical solution:

[0006] A method for preparing a polyimide-derived hierarchical porous carbon / carbon fiber composite electrode, using carbon fiber felt as a substrate, involves a three-tiered interconnected structure of micropores, mesopores, and macropores through polyimide solution spraying, metal-organic gel template deposition, gradient carbonization, and eutectic activation processes to obtain the polyimide-derived hierarchical porous carbon / carbon fiber composite electrode. The method includes the following steps:

[0007] S1: Spray the polyimide solution evenly onto the surface of the carbon fiber felt to form a polymer coating with a thickness of 10-200 µm. Then, heat-cur it in an inert atmosphere at a temperature range of 200-400 °C for 1-3 hours to allow the cross-linking and curing reaction of the polyimide to proceed fully.

[0008] S2: The carbon fiber felt with the load coating is placed in a mixed solution of FeCl3 and PTA terephthalic acid, so that Fe³⁺ ions react with PTA ligands to form a metal-organic framework (MOF) structure, and Fe-MOG gel is deposited on the surface of the polyimide coating.

[0009] S3: Heat treatment is carried out in an inert atmosphere, with the temperature increased to 800-1200℃ at a pulse heating program of 5-15℃ / min and held for 2-6 hours to convert polyimide into a carbon layer and form a chemical bond with carbon fiber felt.

[0010] S4: Activation treatment: The composite electrode is impregnated with a KOH / urea eutectic mixture and activated twice at 700-900℃ for 1-3 hours to generate micropores with an average diameter of <2 nm and mesopores of 2-50 nm.

[0011] S5: Post-treatment: Acid washing removes residual metal oxides and byproducts, and drying yields a polyimide-derived hierarchical porous carbon / carbon fiber composite electrode.

[0012] The polyimide in step S1 is one of the following types: PMDA-ODA, PMDA-PABZ, PDCA-PDA, and PABZ-ODA. The polyimide solution is prepared by dissolving polyimide in N-methylpyrrolidone (NMP) with a concentration of 60-120 mg / ml.

[0013] In step S2, FeCl3 solution and PTA solution are prepared using DMF as solvent, wherein the molar ratio of PTA to FeCl3 is 1:(1~3). This molar ratio is adjusted to adjust the deposition rate and quality of Fe-MOG gel.

[0014] The Fe-MOG gel is formed through a mild mixing reaction process to ensure no excessive deposition and maintain the uniformity of the electrode surface. The reaction heating temperature is controlled at 60-120 °C, and the reaction time is controlled at 6-12 hours.

[0015] In step S3, to avoid cracking of the carbon layer, the carbonization stage is maintained for 30 minutes for every 100°C increase in temperature within the range of 800-1200°C.

[0016] In step S4, the mass ratio of the KOH / urea eutectic mixture is (1~5):1, and the impregnation time is 3-8 hours, in order to regulate the pore structure and activation effect.

[0017] In step S5, the sample is soaked in 1-3 mol / L HCl or HNO3 for 24 hours to remove residual K salt and pore-forming agent; then it is washed with deionized water until neutral and vacuum dried at 80-120 ℃ for 12 hours.

[0018] A polyimide-derived hierarchical porous carbon / carbon fiber composite electrode is prepared by the method described above, and the composite electrode has a tertiary pore structure.

[0019] Micropores: pore size 1-2 nm, specific surface area ≥100 m² / g;

[0020] Mesopores: pore size 10-30 nm, volume percentage 20-40%;

[0021] Macropores: pore size 100-500 nm, formed by the original pores of carbon felt and pores created by the sacrificial template method.

[0022] The application of the polyimide-derived hierarchical porous carbon / carbon fiber composite electrode in the preparation of positive and negative electrodes for flow batteries.

[0023] Compared with the prior art, the present invention has at least the following beneficial effects:

[0024] (1) The present invention uses carbon fiber felt as a substrate and constructs a three-level interconnected structure of micropore-mesopore-macropore through polyimide solution spraying, metal organogel template deposition, gradient carbonization and low eutectic activation process. The preparation process is simple, the reaction conditions are easy to control, and it is easy to prepare for industrialization.

[0025] (2) This invention utilizes a multi-step activation and carbonization process to successfully control the pore structure of a polyimide-derived hierarchical porous carbon / carbon fiber composite electrode. Compared with existing technologies, this invention can form a hierarchical pore structure at multiple scales, including micropores, mesopores, and macropores, thereby increasing the specific surface area of ​​the electrode and optimizing the permeability of the electrolyte and the ion transport efficiency.

[0026] (3) This invention forms a metal-organic framework structure on the electrode surface by in-situ deposition of Fe-MOG gel, which significantly enhances the catalytic activity of the electrode and improves the charge-discharge efficiency of the battery. This modification method solves the problem of insufficient catalytic activity on the surface of traditional carbon electrodes and greatly improves the power density and energy conversion efficiency of the battery.

[0027] (4) By precisely controlling the carbonization and activation processes, the electrode material prepared by this invention exhibits high structural stability. Compared with traditional materials, the electrode prepared by this invention shows significant cycle stability during long-term charge-discharge cycles, reducing the problems of material aging and performance degradation.

[0028] (5) By combining the polyimide coating with carbon fiber felt, this invention not only improves the conductivity of the electrode but also enhances its mechanical strength and flexibility. This makes the electrode more advantageous for use in flow batteries and enables it to be used stably for a long time in complex working environments.

[0029] (6) The preparation method of the present invention adopts simple and efficient steps. By combining polyimide coating and Fe-MOG gel deposition with carbonization and activation treatment, the cumbersome steps in the traditional preparation method are avoided. The energy consumption and raw material consumption are reduced through reasonable process flow, and the cost structure is optimized. The method has strong operability, is suitable for large-scale production, and has good prospects for industrial application. Attached Figure Description

[0030] The above and / or additional aspects and advantages of the present invention will become apparent and readily understood from the description of the embodiments taken in conjunction with the following drawings, in which:

[0031] Figure 1 This is an energy efficiency diagram of the polyimide-derived hierarchical porous carbon / carbon fiber composite electrode prepared in various embodiments of the present invention;

[0032] Figure 2 This is a capacity retention diagram of the polyimide-derived hierarchical porous carbon / carbon fiber composite electrodes prepared in various embodiments of the present invention.

[0033] Figure 3 This is a SEM image of the polyimide-derived hierarchical porous carbon / carbon fiber composite electrode prepared in Example 2 of this invention.

[0034] Figure 4 This is a SEM image of the polyimide-derived hierarchical porous carbon / carbon fiber composite electrode prepared in Example 4 of this invention. Detailed Implementation

[0035] The present invention will now be described in further detail with reference to the accompanying drawings and embodiments, so that those skilled in the art can implement it based on the description.

[0036] It should be understood that terms such as “having,” “comprising,” and “including” as used herein do not exclude the presence or addition of one or more other elements or combinations thereof.

[0037] Unless otherwise specified, all raw materials, reagents, instruments and equipment used in this invention can be purchased from the market or prepared by existing methods.

[0038] Basic Implementation

[0039] A method for preparing a polyimide-derived hierarchical porous carbon / carbon fiber composite electrode, using carbon fiber felt as a substrate, involves a three-tiered interconnected structure of micropores, mesopores, and macropores through polyimide solution spraying, metal-organic gel template deposition, gradient carbonization, and eutectic activation processes to obtain the polyimide-derived hierarchical porous carbon / carbon fiber composite electrode. The method includes the following steps:

[0040] S1: Spray the polyimide solution evenly onto the surface of the carbon fiber felt to form a polymer coating with a thickness of 10-200 µm. Then, heat-cur it in an inert atmosphere at a temperature range of 200-400 °C for 1-3 hours to allow the cross-linking and curing reaction of the polyimide to proceed fully.

[0041] The polyimide in question is one of the polyimides of the PMDA-ODA, PMDA-PABZ, PDCA-PDA, and PABZ-ODA types; the polyimide solution is prepared by dissolving polyimide in N-methylpyrrolidone (NMP) with a concentration of 60-120 mg / ml.

[0042] S2: The carbon fiber felt with the load coating is placed in a mixed solution of FeCl3 and PTA terephthalic acid, so that Fe³⁺ ions react with PTA ligands to form a metal-organic framework (MOF) structure, and Fe-MOG gel is deposited on the surface of the polyimide coating.

[0043] Specifically, FeCl3 solution and PTA solution are prepared using DMF as solvent, with a molar ratio of PTA to FeCl3 of 1:(1~3). This molar ratio is adjusted to regulate the deposition rate and quality of the Fe-MOG gel. The formation of the Fe-MOG gel is achieved through a mild mixing reaction process to ensure no over-deposition and maintain the uniformity of the electrode surface. The reaction heating temperature is controlled at 60-120 ℃, and the reaction time is controlled at 6-12 hours.

[0044] S3: Heat treatment is carried out in an inert atmosphere, with the temperature increased to 800-1200℃ at a pulse heating program of 5-15℃ / min, and held for 2-6 hours to convert the polyimide into a carbon layer and form a chemical bond with the carbon fiber felt; To avoid cracking of the carbon layer, during the carbonization stage, the temperature is increased by 30 minutes for every 100℃ increase in the 800-1200℃ range.

[0045] S4: Activation treatment: The composite electrode is impregnated with a KOH / urea eutectic mixture and activated twice at 700-900℃ for 1-3 hours to generate micropores with an average diameter of <2 nm and mesopores of 2-50 nm.

[0046] The mass ratio of the KOH / urea eutectic mixture is (1~5):1, and the impregnation time is 3-8 hours to regulate the pore structure and activation effect;

[0047] S5: Post-treatment: Acid washing removes residual metal oxides and byproducts, and drying yields a polyimide-derived hierarchical porous carbon / carbon fiber composite electrode.

[0048] Specifically, soak in 1-3 mol / L HCl or HNO3 for 24 hours to remove residual K salt and pore-forming agent; then wash with deionized water until neutral, and vacuum dry at 80-120 ℃ for 12 hours.

[0049] A polyimide-derived hierarchical porous carbon / carbon fiber composite electrode is prepared by the method described above, and the composite electrode has a tertiary pore structure.

[0050] Micropores: pore size 1-2 nm, specific surface area ≥100 m² / g;

[0051] Mesopores: pore size 10-30 nm, volume percentage 20-40%;

[0052] Macropores: pore size 100-500 nm, formed by the original pores of carbon felt and pores created by the sacrificial template method.

[0053] The application of the polyimide-derived hierarchical porous carbon / carbon fiber composite electrode in the preparation of positive and negative electrodes for flow batteries.

[0054] The key points of the preparation method in this embodiment are: coating a polyimide solution onto the surface of a carbon fiber felt and cross-linking and curing it; in-situ deposition of Fe-MOG gel by reacting FeCl3 and terephthalic acid; high-temperature heat treatment to convert the polyimide into a carbon layer and chemically bond it with the carbon fiber felt; activation with a KOH / urea mixture to generate microporous and mesoporous structures; and finally, removal of residual activator by acid washing and drying to obtain a hierarchical porous carbon / carbon fiber composite electrode with excellent electrochemical performance.

[0055] The reaction principle of this invention is as follows: This invention utilizes polyimide as a carbon source and metal-organic gel as a template to form a hierarchical porous carbon structure through carbonization and activation treatment. After carbonization of the polyimide-metal-organic framework, metal nodes are removed by acid washing to generate a macroporous-mesoporous structure. To further optimize the electrode pore structure, a secondary activation treatment using a KOH / urea eutectic mixture is performed to generate a microporous-mesoporous structure, thereby improving ion transport and electrolyte permeability. Combining the unique carbonization behavior of polyimide with the in-situ deposition of Fe-MOG gel optimizes the electrode pore structure and electrochemical performance, improving the battery's power density and cycle stability.

[0056] The following is in conjunction with the appendix Figure 1-4 The following is a detailed description using several specific embodiments.

[0057] Example 1

[0058] The polyimide-derived hierarchical porous carbon / carbon fiber composite electrode, its preparation method, and its application provided in this embodiment are specific applications based on the aforementioned basic embodiments. The preparation method includes the following steps:

[0059] (1) Dissolve PMDA-ODA polyimide powder in N-methylpyrrolidone (NMP), stir magnetically at 500 rpm for 2 h until completely dissolved, and degas ultrasonically at 40 kHz for 30 min to obtain a polyimide solution with a concentration of 80 mg / ml.

[0060] (2) A high-voltage electrostatic spraying system was used to spray the carbon fiber felt surface five times under the conditions of voltage 20 kV, nozzle diameter 50 μm and spraying distance 15 cm, with a single spraying interval of 10 min, to form a uniform coating with a thickness of 10 μm.

[0061] (3) Nitrogen gas is introduced into the tube furnace at a flow rate of 50 mL / min. The temperature is increased to 150 °C at a rate of 5 °C / min and held for 30 min. Then the temperature is increased to 200 °C at a rate of 8 °C / min and held for 30 min.

[0062] (4) Dissolve 0.3 M FeCl3·6H2O in DMF and sonicate for 30 min to obtain solution A. Dissolve 0.15 M terephthalic acid (PTA) in DMF and stir at 70℃ until transparent to obtain solution B;

[0063] (5) Immerse the coated carbon fiber felt in the AB mixed solution (Fe:PTA =2:1), stir magnetically at 300 rpm for 6 h at 60℃, and then take out the carbon felt and dry it for later use.

[0064] (6) Under a nitrogen atmosphere, the composite carbon felt was carbonized by a pulse heating program, heating at 5 ℃ / min to 500 ℃ and holding for 60 min → heating at 10 ℃ / min to 800 ℃ and holding for 60 min.

[0065] (7) Mix KOH and urea in a 1:1 mass ratio and dissolve them in ethanol. Stir at 60°C until a transparent eutectic liquid is obtained.

[0066] (8) The composite carbon felt after gradient carbonization was immersed in KOH / urea eutectic liquid, wherein the mass ratio of solid to liquid was 1:10. After immersion for 3 h, it was taken out and dried, and then activated at 700℃ for 1 h under nitrogen to obtain a graded porous electrode.

[0067] (9) The porous electrode was soaked in 1 mol / L HCl at room temperature for 24 h, centrifuged and washed until neutral, and vacuum dried at 80℃ for 12 h to obtain a polyimide-derived hierarchical porous carbon / carbon fiber composite electrode.

[0068] The polyimide-derived hierarchical porous carbon / carbon fiber composite electrode prepared in this embodiment was used as the positive and negative electrodes to assemble an organic flow battery. The electrolyte was 1 M MV / TEMPTMA (0.5 M NaCl), and the electrolyte temperature was 200 mA cm⁻¹. -2 Constant current charge-discharge tests were conducted at a current density of [insert current density here]. The flow battery assembled with the electrodes prepared in Example 1 had an energy efficiency of 80.3% and a capacity retention of 94% after 300 cycles. However, when unmodified carbon fiber felt was used as the positive and negative electrodes, [insert capacity density here] at 200 mA cm⁻¹ [insert current density here]. -2 At current densities, the energy efficiency is only 65.1%, and the capacity retention is 76% after 300 cycles.

[0069] Example 2

[0070] The polyimide-derived hierarchical porous carbon / carbon fiber composite electrode, its preparation method, and its application provided in this embodiment are specific applications based on the aforementioned basic embodiments. The preparation method includes the following steps:

[0071] (1) PABZ-ODA type polyimide powder was dissolved in NMP and magnetically stirred at 500 rpm for 4 h until completely dissolved. The solution was ultrasonically degassed at 40 kHz for 30 min to obtain a polyimide solution with a concentration of 100 mg / ml.

[0072] (2) A high-voltage electrostatic spraying system was used to spray the carbon fiber felt surface five times under the conditions of 20 kV voltage, 50 μm nozzle diameter and 15 cm spraying distance, with a 10 min interval between each spraying, to form a uniform coating with a thickness of 120 μm.

[0073] (3) Nitrogen gas is introduced into the tube furnace at a flow rate of 50 mL / min. The temperature is increased to 120℃ at 5℃ / min and held for 15 min. Then the temperature is increased to 250℃ at 10℃ / min and held for 45 min.

[0074] (4) Dissolve 0.2 M FeCl3·6H2O in DMF and sonicate for 30 min to obtain solution A. Dissolve 0.2 M terephthalic acid (PTA) in DMF and stir at 70℃ until transparent to obtain solution B.

[0075] (5) Immerse the coated carbon fiber felt in an equimolar mixture of AB (Fe:PTA = 1:1) and perform a hydrothermal reaction at 120℃ for 6 h. Then remove the carbon felt and dry it for later use.

[0076] (6) Under a nitrogen atmosphere, the composite carbon felt is carbonized by a pulse heating program: heating at 10℃ / min to 500℃ and holding for 20 min → heating at 15℃ / min to 800℃ and holding for 30 min → heating at 10℃ / min to 900℃ and holding for 60 min.

[0077] (7) Mix KOH and urea in ethanol at a mass ratio of 5:1 and stir at 60°C until a transparent eutectic liquid is obtained.

[0078] (8) The composite carbon felt after gradient carbonization was immersed in KOH / urea eutectic liquid with a solid-to-liquid mass ratio of 1:10. After immersion for 6 h, it was taken out and dried, and then activated at 700℃ for 1 h under nitrogen to obtain a graded porous electrode.

[0079] (9) The porous electrode was soaked in 3 mol / L HCl at room temperature for 24 h, centrifuged and washed until neutral, and vacuum dried at 80℃ for 12 h to obtain a polyimide-derived hierarchical porous carbon / carbon fiber composite electrode.

[0080] See Figure 3 The polyimide-derived hierarchical porous carbon / carbon fiber composite electrode prepared in this embodiment has a microstructure characterized by a three-tiered interconnection of micropores, mesopores, and macropores. Using this polyimide-derived hierarchical porous carbon / carbon fiber composite electrode as the positive and negative electrodes, an organic flow battery was assembled. The electrolyte was 1 M MV / TEMPTMA (0.5 M NaCl), and the electrolyte concentration was 200 mA cm⁻¹. -2 Constant current charge-discharge tests were conducted at a current density of [insert current density here]. The flow battery assembled with the electrodes prepared in Example 2 had an energy efficiency of 89.6% and a capacity retention of 98% after 300 cycles. However, when unmodified carbon fiber felt was used as the positive and negative electrodes, [insert capacity density here] at 200 mA cm⁻¹ [insert current density here]. -2 At current densities, the energy efficiency is only 65.1%.

[0081] Example 3

[0082] The polyimide-derived hierarchical porous carbon / carbon fiber composite electrode, its preparation method, and its application provided in this embodiment are specific applications based on the aforementioned basic embodiments. The preparation method includes the following steps:

[0083] (1) Dissolve PMDA-PABZ type polyimide powder in NMP, stir magnetically at 500 rpm for 4 h until completely dissolved, and degas ultrasonically at 40 kHz for 30 min to obtain a polyimide solution with a concentration of 120 mg / ml.

[0084] (2) A high-voltage electrostatic spraying system was used to spray the carbon fiber felt surface 6 times under the conditions of voltage 20 kV, nozzle diameter 50 μm and spray distance 15 cm, with a single spray interval of 10 min, to form a uniform coating with a thickness of 200 μm.

[0085] (3) Introduce nitrogen into the tube furnace at a flow rate of 50 mL / min. Increase the temperature to 150℃ at 5℃ / min and hold for 30 min. Increase the temperature to 250℃ at 5℃ / min and hold for 60 min. Then increase the temperature to 400℃ at 10℃ / min and hold for 90 min.

[0086] (4) Dissolve 0.3 M FeCl3·6H2O in DMF and sonicate for 30 min to obtain solution A. Dissolve 0.1 M terephthalic acid (PTA) in DMF and stir at 70℃ until transparent to obtain solution B.

[0087] (5) Immerse the coated carbon fiber felt in the AB mixed solution (Fe:PTA =3:1), stir at 120℃ for 12 h, and then take out the carbon felt and dry it for later use.

[0088] (6) Under a nitrogen atmosphere, the composite carbon felt was carbonized by a pulse heating program: heating at 15 ℃ / min to 800 ℃ and holding for 30 min → heating at 15 ℃ / min to 900 ℃ and holding for 120 min → heating at 10 ℃ / min to 1000 ℃ and holding for 60 min → heating at 10 ℃ / min to 1100 ℃ and holding for 90 min → heating at 5 ℃ / min to 1200 ℃ and holding for 60 min.

[0089] (7) Mix KOH and urea in ethanol at a mass ratio of 5:1 and stir at 60°C until a transparent eutectic liquid is obtained.

[0090] (8) The composite carbon felt after gradient carbonization was immersed in KOH / urea eutectic liquid with a solid-to-liquid mass ratio of 1:10. After immersion for 8 h, it was taken out and dried, and then activated at 900 °C for 3 h under nitrogen to obtain a graded porous electrode.

[0091] (9) The porous electrode was soaked in 3 mol / L HNO3 at room temperature for 24 h, centrifuged and washed until neutral, and vacuum dried at 120℃ for 12 h to obtain a polyimide-derived hierarchical porous carbon / carbon fiber composite electrode.

[0092] The polyimide-derived hierarchical porous carbon / carbon fiber composite electrode prepared in this embodiment was used as the positive and negative electrodes to assemble an organic flow battery. The electrolyte was 1 M MV / TEMPTMA (0.5 M NaCl), and the electrolyte concentration was 200 mA cm⁻¹. -2 Constant current charge-discharge tests were conducted at a current density of [insert current density here]. The flow battery assembled with the electrodes prepared in Example 3 had an energy efficiency of 83.7% and a capacity retention of 95% after 300 cycles. However, when unmodified carbon fiber felt was used as the positive and negative electrodes, [insert capacity density here] at 200 mA cm⁻¹ [insert current density here]. -2 At current densities, the energy efficiency is only 65.1%, and the capacity retention is 76% after 300 cycles.

[0093] Example 4

[0094] The polyimide-derived hierarchical porous carbon / carbon fiber composite electrode, its preparation method, and its application provided in this embodiment are specific applications based on the aforementioned basic embodiments. The preparation method includes the following steps:

[0095] (1) Dissolve PDCA-ODA type polyimide powder in NMP, stir magnetically at 500 rpm for 3 h until completely dissolved, and degas ultrasonically at 40 kHz for 30 min to obtain a polyimide solution with a concentration of 100 mg / ml.

[0096] (2) A high-voltage electrostatic spraying system was used to spray the carbon fiber felt surface four times under the conditions of 20 kV voltage, 50 μm nozzle diameter and 15 cm spraying distance, with a 10 min interval between each spraying, to form a uniform coating with a thickness of 60 μm.

[0097] (3) Nitrogen gas is introduced into the tube furnace at a flow rate of 50 mL / min. The temperature is increased to 100 ℃ at 5 ℃ / min and held for 30 min. The temperature is then increased to 250 ℃ at 5 ℃ / min and held for 30 min. Finally, the temperature is increased to 300 ℃ at 8 ℃ / min and held for 30 min.

[0098] (4) Dissolve 0.3 M FeCl3·6H2O in DMF and sonicate for 30 min to obtain solution A. Dissolve 0.3 M terephthalic acid (PTA) in DMF and stir at 70℃ until transparent to obtain solution B.

[0099] (5) Immerse the coated carbon fiber felt in the AB mixed solution (Fe:PTA = 1:1), stir at 80℃ for 10 h, and then take out the carbon felt and dry it for later use.

[0100] (6) Under a nitrogen atmosphere, the composite carbon felt is carbonized by a pulse heating program: heating at 10 ℃ / min to 800 ℃ and holding for 60 min → heating at 15 ℃ / min to 900 ℃ and holding for 60 min → heating at 5 ℃ / min to 1000 ℃ and holding for 60 min.

[0101] (7) Mix KOH and urea in ethanol at a mass ratio of 4:1 and stir at 60°C until a transparent eutectic liquid is obtained.

[0102] (8) The composite carbon felt after gradient carbonization was immersed in KOH / urea eutectic liquid with a solid-to-liquid mass ratio of 1:10. After immersion for 6 h, it was taken out and dried, and then activated at 800 °C for 2 h under nitrogen to obtain a graded porous electrode.

[0103] (9) The porous electrode was soaked in 1 mol / L HNO3 for 24 h at room temperature, centrifuged and washed until neutral, and vacuum dried at 120℃ for 12 h to obtain a polyimide-derived hierarchical porous carbon / carbon fiber composite electrode.

[0104] See Figure 4 The polyimide-derived hierarchical porous carbon / carbon fiber composite electrode prepared in this embodiment has a microstructure characterized by a three-tiered interconnection of micropores, mesopores, and macropores. Using this polyimide-derived hierarchical porous carbon / carbon fiber composite electrode as the positive and negative electrodes, an organic flow battery was assembled. The electrolyte was 1 M MV / TEMPTMA (0.5 M NaCl), and the electrolyte concentration was 200 mA cm⁻¹. -2 Constant current charge-discharge tests were conducted at a current density of [insert current density here]. The flow battery assembled with the electrodes prepared in Example 4 had an energy efficiency of 87.1% and a capacity retention of 96% after 300 cycles. However, when unmodified carbon fiber felt was used as the positive and negative electrodes, [insert capacity density here] at 200 mA cm⁻¹ [insert current density here]. -2 At current densities, the energy efficiency is only 65.1%, and the capacity retention is 76% after 300 cycles.

[0105] Example 5

[0106] The polyimide-derived hierarchical porous carbon / carbon fiber composite electrode, its preparation method, and its application provided in this embodiment are specific applications based on the aforementioned basic embodiments. The preparation method includes the following steps:

[0107] (1) Dissolve PDCA-ODA type polyimide powder in NMP, stir magnetically at 500 rpm for 2 h until completely dissolved, and degas ultrasonically at 40 kHz for 30 min to obtain a polyimide solution with a concentration of 90 mg / ml.

[0108] (2) A high-voltage electrostatic spraying system was used to spray the carbon fiber felt surface four times under the conditions of voltage 20 kV, nozzle diameter 50 μm and spray distance 15 cm, with a single spray interval of 10 min, to form a uniform coating with a thickness of 150 μm.

[0109] (3) Nitrogen gas is introduced into the tube furnace at a flow rate of 50 mL / min. The temperature is increased to 200 ℃ at 5 ℃ / min and held for 30 min. The temperature is then increased to 250 ℃ at 5 ℃ / min and held for 60 min.

[0110] (4) Dissolve 0.4 M FeCl3·6H2O in DMF and sonicate for 30 min to obtain solution A. Dissolve 0.2 M terephthalic acid (PTA) in DMF and stir at 70℃ until transparent to obtain solution B.

[0111] (5) Immerse the coated carbon fiber felt in the AB mixed solution (Fe:PTA =2:1), stir at 80℃ for 10 h, and then take out the carbon felt and dry it for later use.

[0112] (6) Under a nitrogen atmosphere, the composite carbon felt is carbonized by a pulse heating program: 5 ℃ / min to 500 ℃ and hold for 30 min → 10 ℃ / min to 800 ℃ and hold for 60 min → 15 ℃ / min to 900 ℃ and hold for 60 min → 10 ℃ / min to 1000 ℃ and hold for 30 min.

[0113] (7) Mix KOH and urea in ethanol at a mass ratio of 2:1 and stir at 60°C until a transparent eutectic liquid is obtained.

[0114] (8) The composite carbon felt after gradient carbonization was immersed in KOH / urea eutectic liquid with a solid-to-liquid mass ratio of 1:10. After immersion for 7 h, it was taken out and dried, and then activated at 850 °C for 2 h under nitrogen to obtain a graded porous electrode.

[0115] (9) The porous electrode was soaked in 2 mol / L HCl at room temperature for 24 h, centrifuged and washed until neutral, and then vacuum dried at 100℃ for 12 h to obtain a polyimide-derived hierarchical porous carbon / carbon fiber composite electrode.

[0116] The polyimide-derived hierarchical porous carbon / carbon fiber composite electrode prepared in this embodiment was used as the positive and negative electrodes to assemble an organic flow battery. The electrolyte was 1 M MV / TEMPTMA (0.5 M NaCl), and the electrolyte concentration was 200 mA cm⁻¹. -2 Constant current charge-discharge tests were conducted at a current density of [insert current density here]. The flow battery assembled with the electrodes prepared in Example 5 had an energy efficiency of 86.8% and a capacity retention of 97% after 300 cycles. However, when unmodified carbon fiber felt was used as the positive and negative electrodes, [insert capacity density here] at 200 mA cm⁻¹ [insert current density here]. -2 At current densities, the energy efficiency is only 65.1%, and the capacity retention is 76% after 300 cycles.

[0117] The composite electrode preparation method provided in the above embodiments of the present invention focuses on uniformly coating a polyimide solution onto the surface of carbon fibers, followed by pre-oxidation and curing to generate an Fe-MOG gel template in a FeCl3 / terephthalic acid system; achieving chemical bonding between the polyimide carbon layer and the fiber through gradient temperature carbonization (800-1200℃); and activating with a KOH / urea eutectic mixture to simultaneously introduce micropores and mesopores. After acid washing, a composite porous electrode with high specific surface area and high conductivity is obtained. The polyimide-derived hierarchical porous carbon / carbon fiber composite electrode provided in the above embodiments of the present invention exhibits high conductivity and catalytic activity, possessing both high conductivity and high energy efficiency, and can be applied to various types of aqueous organic flow battery systems. The present invention, through template synergy and optimized activation process, solves the problems of limited mass transfer and insufficient active sites in traditional carbon electrodes, making it suitable for all-vanadium, iron-chromium, and organic flow batteries, and has significant industrial application value.

[0118] It should be particularly noted that other technical solutions obtained by specifically selecting from the components, proportions, and process parameters described in this invention can all achieve the technical effects of this invention, and therefore will not be listed one by one. Furthermore, other technical solutions obtained by using other components and solvents similar to those described in this invention are included within the protection scope of this invention.

[0119] In the description of this invention, the terms "one embodiment," "some embodiments," "specific embodiment," etc., refer to a specific feature, structure, material, or characteristic described in connection with that embodiment or example, which is included in at least one embodiment or example of the invention. In this invention, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples.

[0120] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing a polyimide-derived hierarchical porous carbon / carbon fiber composite electrode, characterized in that, Includes the following steps: S1: Spray the polyimide solution evenly onto the surface of the carbon fiber felt to form a polymer coating with a thickness of 10-200 µm. Then, heat-cur it in an inert atmosphere at a temperature range of 200-400℃ for 1-3 hours to allow the cross-linking and curing reaction of the polyimide to proceed fully. S2: The loaded coated carbon fiber felt is placed in a mixed solution of FeCl3 and PTA terephthalate, so that Fe... 3+ Ions react with PTA ligands to form a metal-organic framework (MOF) structure, and Fe-MOG gel is deposited on the surface of the polyimide coating. S3: Heat treatment is carried out in an inert atmosphere, with the temperature increased to 800-1200℃ at a pulse heating program of 5-15℃ / min and held for 2-6 hours to convert polyimide into a carbon layer and form a chemical bond with carbon fiber felt. S4: Activation treatment: The composite electrode is impregnated with a KOH / urea eutectic mixture and activated twice at 700-900℃ for 1-3 hours to generate micropores with an average diameter of <2 nm and mesopores of 2-50 nm. S5: Post-treatment: Acid washing to remove residual metal oxides and byproducts, drying to obtain polyimide-derived hierarchical porous carbon / carbon fiber composite electrode; Using carbon fiber felt as a substrate, a three-level interconnected structure of micropores-mesopores-macropores was constructed through polyimide solution spraying, metal-organic gel template deposition, gradient carbonization and eutectic activation processes, resulting in a polyimide-derived hierarchical porous carbon / carbon fiber composite electrode.

2. The method for preparing the polyimide-derived hierarchical porous carbon / carbon fiber composite electrode according to claim 1, characterized in that, The polyimide in step S1 is one of the following types: PMDA-ODA, PMDA-PABZ, PDCA-PDA, and PABZ-ODA. The polyimide solution is prepared by dissolving polyimide in N-methylpyrrolidone (NMP) with a concentration of 60-120 mg / ml.

3. The method for preparing the polyimide-derived hierarchical porous carbon / carbon fiber composite electrode according to claim 1, characterized in that, In step S2, FeCl3 solution and PTA solution are prepared using DMF as solvent, wherein the molar ratio of PTA to FeCl3 is 1:(1~3). This molar ratio is adjusted to adjust the deposition rate and quality of Fe-MOG gel.

4. The method for preparing the polyimide-derived hierarchical porous carbon / carbon fiber composite electrode according to claim 3, characterized in that, The Fe-MOG gel is formed through a mild mixing reaction process to ensure no excessive deposition and maintain the uniformity of the electrode surface. The reaction heating temperature is controlled at 60-120 °C, and the reaction time is controlled at 6-12 hours.

5. The method for preparing the polyimide-derived hierarchical porous carbon / carbon fiber composite electrode according to claim 1, characterized in that, In step S3, to avoid cracking of the carbon layer, the carbonization stage is maintained for 30 minutes for every 100°C increase in temperature within the range of 800-1200°C.

6. The method for preparing the polyimide-derived hierarchical porous carbon / carbon fiber composite electrode according to claim 1, characterized in that, In step S4, the mass ratio of the KOH / urea eutectic mixture is (1~5):1, and the impregnation time is 3-8 hours, in order to regulate the pore structure and activation effect.

7. The method for preparing the polyimide-derived hierarchical porous carbon / carbon fiber composite electrode according to claim 1, characterized in that, In step S5, the sample is soaked in 1-3 mol / L HCl or HNO3 for 24 hours to remove residual K salt and pore-forming agent; then it is washed with deionized water until neutral and vacuum dried at 80-120 ℃ for 12 hours.

8. A polyimide-derived hierarchical porous carbon / carbon fiber composite electrode, characterized in that, It is prepared by the method described in any one of claims 1 to 7, and the composite electrode has a tertiary pore structure: Micropores: pore size 1-2 nm, specific surface area ≥100 m² 2 / g; Mesopores: pore size 10-30 nm, volume percentage 20-40%; Macropores: pore size 100-500 nm.

9. The application of the polyimide-derived hierarchical porous carbon / carbon fiber composite electrode according to claim 8 in the preparation of positive and negative electrodes for flow batteries.

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