Polyimide-derived graded porous carbon / carbon fiber composite electrode and preparation method thereof

Abstract

The invention relates to the technical field of new materials and electrochemical energy storage materials, and discloses a polyimide-derived graded porous carbon / carbon fiber composite electrode and a preparation method thereof.The preparation method comprises the steps that a carbon fiber felt is used as a substrate, and the polyimide-derived graded porous carbon / carbon fiber composite electrode is prepared through polyimide solution spraying, metal organic gel template deposition, gradient carbonization and eutectic activation processes; and a micropore-mesopore-macropore three-stage through structure is constructed. Through template collaboration and activation process optimization, the constructed composite electrode has a unique micropore-mesopore-macropore three-stage through structure, solves the problems of mass transfer limitation and insufficient active sites of a traditional carbon electrode, can be widely applied to all-vanadium, iron-chromium and organic system flow batteries, and has significant industrial application value.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of new materials and electrochemical energy storage materials, and particularly relates to a polyimide-derived hierarchical porous carbon / carbon fiber composite electrode and a preparation method thereof. BACKGROUND

[0002] Energy crisis and environmental pollution are the core challenges faced by global sustainable development at present. Promoting the construction of green, efficient and renewable energy system has become an international consensus. However, new clean energy represented by hydrogen energy and tidal energy is limited by uneven geographical distribution and supply volatility, and it is difficult to realize stable grid-connected output, so it is urgent to develop efficient energy storage technology. Under this background, the flow battery has become a strategic technical choice in the field of large-scale energy storage due to its modular design, capacity-power decoupling characteristics and ultra-long cycle life.

[0003] As the core component of the flow battery, the multi-scale mass transfer ability and electrochemical activity of the electrode material directly determine the energy efficiency and power density of the system. On the micro-nano scale, micropores (<2 nm) provide abundant redox reaction active sites through high specific surface area, mesopores (2-50 nm) act as ion diffusion channels to shorten the mass transfer path, and macroporous (>50 μm) networks significantly reduce the flow resistance of electrolyte, and the synergistic effect of the three can break through the kinetic bottleneck of traditional energy storage devices. However, the commercial carbon felt electrode is limited by single pore size distribution and low specific surface area (<5 m² / g), and its pore system presents a "mass transfer-reaction" contradiction: although the microporous modification of excessive pursuit of high specific surface area can improve the reaction site density, it leads to the increase of tortuosity factor and the decrease of ion transference number; while simply expanding the pore size can reduce the concentration polarization, but it leads to insufficient exposure of active sites. This imbalance between structure and function directly leads to the difficulty of flow battery to achieve simultaneous optimization of high energy efficiency and large power density in a wide operating range, which seriously restricts its commercial application in grid-level energy storage scenarios. SUMMARY

[0004] The present application aims at the deficiencies of the prior art, and provides a polyimide-derived hierarchical porous carbon / carbon fiber composite electrode and a preparation method thereof. Through polyimide solution spraying, metal organic gel (Fe-MOG) template deposition, gradient carbonization and eutectic activation process, a micropore-mesopore-macropore three-level through structure is constructed, solving the problems of mass transfer limitation and insufficient active sites of traditional carbon electrodes, and providing a new solution for the commercial deployment of megawatt-level flow battery power stations.

[0005] To achieve the above purpose, the present application provides the following technical solutions: A preparation method of a polyimide-derived hierarchical porous carbon / carbon fiber composite electrode, taking a carbon fiber felt as a substrate, through polyimide solution spraying, metal organic gel template deposition, gradient carbonization and eutectic activation process, a three-level through structure of micropore-mesopore-macropore is constructed, and a polyimide-derived hierarchical porous carbon / carbon fiber composite electrode is obtained, comprising the following steps: S1: uniformly spray a polyimide solution on the surface of the carbon fiber felt to form a polymer coating with a thickness of 10-200 µm, then perform heat curing at a temperature of 200-400 ℃ in an inert atmosphere, and keep the temperature for 1-3 hours to fully perform the cross-linking and curing reaction of the polyimide; S2: place the carbon fiber felt loaded with the coating in a mixed solution of FeCl3 and terephthalic acid PTA, make Fe³⁺ ions react with PTA ligands to form a metal organic framework MOF structure, and deposit a Fe-MOG gel on the surface of the polyimide coating; S3: perform heat treatment in an inert atmosphere, heat to 800-1200 ℃ at a rate of 5-15 ℃ / min through a pulse heating program, and keep the temperature for 2-6 hours to convert the polyimide into a carbon layer and form a chemical bond with the carbon fiber felt; S4: activation treatment: immerse the composite electrode in a KOH / urea eutectic mixture, and perform secondary activation at 700-900 ℃ for 1-3 hours to generate micropores with an average diameter of <2 nm and mesopores with a diameter of 2-50 nm; S5: post-treatment: remove residual metal oxides and byproducts through acid washing, and dry to obtain the polyimide-derived hierarchical porous carbon / carbon fiber composite electrode.

[0006] The polyimide in step S1 is one of PMDA-ODA type, PMDA-PABZ type, PDCA-PDA type and PABZ-ODA type; the polyimide solution is prepared by dissolving polyimide in N-methyl pyrrolidone NMP, and the solution concentration is 60-120 mg / ml.

[0007] In step S2, DMF is used as a solvent to prepare FeCl3 solution and PTA solution, respectively, and the molar ratio of PTA to FeCl3 is 1: (1-3), which is adjusted to adjust the deposition rate and quality of the Fe-MOG gel.

[0008] The formation of the Fe-MOG gel is completed through a mild mixing reaction process, which ensures no excessive deposition and maintains the uniformity of the electrode surface, and the reaction heating temperature is controlled at 60-120 ℃ and the reaction time is controlled at 6-12 hours.

[0009] In step S3, to avoid cracking of the carbon layer, the carbonization stage is kept at 30 minutes for every 100 ℃ increase in the 800-1200 ℃ interval.

[0010] In step S4, the mass ratio of the KOH / urea eutectic mixture is (1-5):1, and the impregnation time is 3-8 hours, so as to regulate the pore structure and activation effect.

[0011] In step S5, 1-3 mol / L HCl or HNO3 is used for soaking for 24 hours to remove residual K salt and pore-forming agent, and then deionized water is used for washing until neutral, and vacuum drying is carried out at 80-120 DEG C for 12 hours.

[0012] A polyimide-derived hierarchical porous carbon / carbon fiber composite electrode is prepared by the method, and the composite electrode has a three-level pore structure: Micropores: pore size 1-2 nm, specific surface area ≥100 m² / g; Mesopores: pore size 10-30 nm, volume ratio 20-40%; Macropores: pore size 100-500 nm, composed of primary pores of carbon felt and pores formed by the sacrificial template method.

[0013] The polyimide-derived hierarchical porous carbon / carbon fiber composite electrode is applied to the preparation of positive and negative electrodes of a flow battery.

[0014] Compared with the prior art, the present application has at least the following beneficial effects: (1) The present application uses carbon fiber felt as a substrate, and through polyimide solution spraying, metal organic gel template deposition, gradient carbonization and eutectic activation process, a three-level through structure of micropores-mesopores-macropores is constructed, and the preparation process is simple, the reaction conditions are easy to control, and industrialized preparation is easy.

[0015] (2) The polyimide-derived hierarchical porous carbon / carbon fiber composite electrode of the present application uses a multi-step activation and carbonization treatment process to successfully regulate the pore structure of the electrode. Compared with the prior art, the present application can form a hierarchical pore structure at multiple scales of micropores, mesopores and macropores, thereby increasing the specific surface area of the electrode and optimizing the permeability and ion transmission efficiency of the electrolyte.

[0016] (3) The present application forms a metal organic framework structure on the surface of the electrode by in-situ deposition of Fe-MOG gel, which significantly enhances the catalytic activity of the electrode and improves the charge and discharge efficiency of the battery. This modification method solves the problem of insufficient catalytic activity on the surface of the traditional carbon electrode, greatly improving the power density and energy conversion efficiency of the battery.

[0017] (4) The present application precisely controls the carbonization and activation process, and the prepared electrode material has high structural stability. Compared with traditional materials, the electrode prepared by the present application shows significant cycle stability during long-time charge and discharge cycle, reducing the problems of material aging and performance degradation.

[0018] (5) The present application combines the polyimide coating with the carbon fiber felt, which not only improves the conductivity of the electrode, but also enhances the mechanical strength and flexibility of the electrode. This makes the electrode more advantageous in the application of flow batteries, and can be used stably for a long time in complex working environments.

[0019] (6) The preparation method of the present application adopts simple and efficient steps, combines polyimide coating and Fe-MOG gel deposition with carbonization and activation treatment, avoids the cumbersome steps in traditional preparation methods, and reduces energy consumption and raw material consumption through a reasonable process flow, optimizing the cost structure; the method has strong operability, is suitable for large-scale production, and has good industrial application prospect. BRIEF DESCRIPTION OF DRAWINGS

[0020] The above and / or additional aspects and advantages of the present application will become apparent and more readily appreciated from the following description of the embodiments, taken in conjunction with the accompanying drawings, in which: Figure 1 is the energy efficiency diagram of the polyimide-derived hierarchical porous carbon / carbon fiber composite electrode prepared by each embodiment of the present application; Figure 2 is the capacity retention rate diagram of the polyimide-derived hierarchical porous carbon / carbon fiber composite electrode prepared by each embodiment of the present application; Figure 3 is the SEM diagram of the polyimide-derived hierarchical porous carbon / carbon fiber composite electrode prepared by embodiment 2 of the present application; Figure 4 is the SEM diagram of the polyimide-derived hierarchical porous carbon / carbon fiber composite electrode prepared by embodiment 4 of the present application. DETAILED DESCRIPTION

[0021] The present application will be further described in detail below in conjunction with the drawings and embodiments, so that those skilled in the art can implement it according to the description.

[0022] It should be understood that the terms such as "have", "contain" and "include" used herein do not exclude the presence or addition of one or more other elements or combinations thereof.

[0023] Unless otherwise specifically stated, the various raw materials, reagents, instruments and equipment used in the present application can be purchased from the market or can be prepared by existing methods.

[0024] Basic embodiment

[0025] A preparation method of a polyimide-derived hierarchical porous carbon / carbon fiber composite electrode, taking a carbon fiber felt as a substrate, constructing a three-level through structure of micropore-mesopore-macropore by polyimide solution spraying, metal organic gel template deposition, gradient carbonization and eutectic activation process, and obtaining a polyimide-derived hierarchical porous carbon / carbon fiber composite electrode, comprising the following steps: S1: uniformly spraying a polyimide solution on the surface of the carbon fiber felt to form a polymer coating with a thickness of 10-200 µm, and then performing heat curing at a temperature of 200-400 ℃ in an inert atmosphere for 1-3 hours to fully perform the crosslinking and curing reaction of the polyimide; The polyimide is one of PMDA-ODA type, PMDA-PABZ type, PDCA-PDA type and PABZ-ODA type; the polyimide solution is prepared by dissolving the polyimide in N-methyl pyrrolidone NMP, and the solution concentration is 60-120 mg / ml.

[0026] S2: placing the carbon fiber felt loaded with the coating in a mixed solution of FeCl3 and terephthalic acid PTA to make Fe³⁺ ions react with PTA ligands to form a metal organic framework MOF structure and deposit a Fe-MOG gel on the surface of the polyimide coating; Specifically, DMF is used as a solvent to prepare FeCl3 solution and PTA solution, wherein the molar ratio of PTA to FeCl3 is 1: (1-3), and the molar ratio is adjusted to adjust the deposition rate and quality of the Fe-MOG gel; the formation of the Fe-MOG gel is completed through a mild mixing reaction process to ensure no excessive deposition and maintain the uniformity of the electrode surface, and the reaction heating temperature is controlled at 60-120 ℃ and the reaction time is controlled at 6-12 hours; S3: heat treatment in an inert atmosphere, heating to 800-1200 ℃ at a rate of 5-15 ℃ / min through a pulse heating program, and holding 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, holding for 30 minutes at every 100 ℃ increase in the carbonization stage in the range of 800-1200 ℃; S4: activation treatment: impregnating the composite electrode with a KOH / urea eutectic mixture, and performing secondary activation at 700-900 ℃ for 1-3 hours to generate micropores with an average diameter of <2 nm and mesopores with a diameter of 2-50 nm; 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; S5: Post-treatment: acid washing to remove residual metal oxides and by-products, and drying to obtain a polyimide-derived hierarchical porous carbon / carbon fiber composite electrode.

[0027] Specifically, the sample was immersed in 1-3 mol / L HCl or HNO3 for 24 hours to remove residual K salt and pore-forming agent; then, the sample was washed with deionized water until neutral, and vacuum dried at 80-120°C for 12 hours.

[0028] A polyimide-derived hierarchical porous carbon / carbon fiber composite electrode is prepared by the method described above, and the composite electrode has a three-level pore structure: Micropores: pore diameter 1-2 nm, specific surface area ≥100 m² / g; Mesopores: pore diameter 10-30 nm, volume share 20-40%; Macropores: pore diameter 100-500 nm, composed of native pores of carbon felt and pores created by sacrificial template method.

[0029] The polyimide-derived hierarchical porous carbon / carbon fiber composite electrode is used in the preparation of positive and negative electrodes of a liquid flow battery.

[0030] The key points of the preparation method of this embodiment are: coating a polyimide solution on the surface of a carbon fiber felt and cross-linking and curing it; in situ depositing a 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 to the carbon fiber felt; activation by a KOH / urea mixture to generate microporous and mesoporous structures; and finally, acid washing to remove residual activator and drying to obtain a hierarchical porous carbon / carbon fiber composite electrode with excellent electrochemical properties.

[0031] The reaction principle of the embodiment of the present invention is as follows: The present invention uses polyimide as a carbon source and metal organic gel as a template to form a hierarchical porous carbon structure through a carbonization process and activation treatment. After the polyimide-metal organic framework is carbonized, the metal nodes are removed by pickling to generate a macroporous-mesoporous structure. In order to further optimize the pore structure of the electrode, a KOH / urea eutectic mixture is used for secondary activation treatment to generate a microporous-mesoporous structure, thereby improving ion transport and electrolyte permeability. Combining the special carbonization behavior of polyimide with the in-situ deposition of Fe-MOG gel, the pore structure and electrochemical performance of the electrode are optimized, and the power density and cycle stability of the battery are improved.

[0032] The following is combined with Figures 1-4 , described in detail with multiple specific embodiments.

[0033] Example 1

[0034] The polyimide-derived hierarchical porous carbon / carbon fiber composite electrode, the preparation method and the application thereof provided by the embodiment are specific applications on the basis of the foregoing basic embodiment, and the preparation method comprises the following steps: (1) Dissolve PMDA-ODA polyimide powder in N-methyl pyrrolidone (NMP), and magnetically stir at a speed of 500 rpm for 2 h until completely dissolved, and ultrasonically degas at a frequency of 40 kHz for 30 min to obtain a polyimide solution with a concentration of 80 mg / ml; (2) Adopt a high-voltage electrostatic spraying system, and spray on the surface of the carbon fiber felt under the conditions of a voltage of 20 kV, a nozzle diameter of 50 μm and a spraying distance of 15 cm, and repeat the spraying for 5 times with an interval of 10 min each time to form a uniform coating with a thickness of 10 μm; (3) Introduce nitrogen into a tube furnace at a flow rate of 50 mL / min, and heat at a rate of 5 ℃ / min to 150 ℃, keep for 30 min, and then heat at a rate of 8 ℃ / min to 200 ℃, keep for 30 min; (4) Take 0.3 M FeCl3·6H2O and dissolve in DMF, and ultrasonically disperse for 30 min to obtain solution A. Take 0.15 M terephthalic acid (PTA) and dissolve in DMF, and stir at 70 ℃ until transparent to obtain solution B; (5) Dip the coated carbon fiber felt into the mixed AB solution (Fe:PTA = 2:1), and magnetically stir at a speed of 300 rpm under constant temperature of 60 ℃ for 6 h, and then take out the carbon felt and dry for standby; (6) Under a nitrogen atmosphere, carbonize the composite carbon felt by a pulse heating program, heat to 500 ℃ at a rate of 5 ℃ / min and keep for 60 min, and then heat to 800 ℃ at a rate of 10 ℃ / min and keep for 60 min; (7) Mix KOH and urea in a mass ratio of 1:1, and dissolve in ethanol to obtain a transparent eutectic liquid; (8) Take the gradient carbonized composite carbon felt and immerse in the KOH / urea eutectic liquid, wherein the mass ratio of solid to liquid is 1:10, immerse for 3 h, take out and dry, and then activate at 700 ℃ under nitrogen for 1 h to obtain a hierarchical porous electrode; (9) Take the porous electrode and immerse in 1 mol / L HCl at room temperature for 24 h, centrifugally wash until neutral, and vacuum dry at 80 ℃ for 12 h to obtain a polyimide-derived hierarchical porous carbon / carbon fiber composite electrode.

[0035] The polyimide-derived hierarchical porous carbon / carbon fiber composite electrode prepared by the embodiment is used as a positive electrode and a negative electrode to assemble an organic flow battery, the electrolyte is 1 M MV / TEMPTMA (0.5 M NaCl), and the energy density of the battery is 100 Wh / kg at a current density of 200 mA cm -2The constant current charge and discharge test was carried out at a current density of 200 mA cm. The energy efficiency of the flow battery assembled with the electrodes prepared in Example 1 was 80.3%, and the capacity retention rate was 94% after 300 cycles. When the unmodified carbon fiber felt was used as the positive and negative electrodes, the capacity retention rate was 94% after 300 cycles. -2 At a current density of 1.5 GHz, the energy efficiency is only 65.1%, and the capacity retention rate is 76% after 300 cycles.

[0036] Example 2

[0037] The polyimide-derived hierarchical porous carbon / carbon fiber composite electrode, preparation method, and application thereof provided in this embodiment are specific applications based on the aforementioned basic embodiments. The preparation method includes the following steps: (1) PABZ-ODA polyimide powder was dissolved in NMP and magnetically stirred at 500 rpm for 4 h until completely dissolved. Ultrasonic degassing was performed at 40 kHz for 30 min to obtain a polyimide solution with a concentration of 100 mg / ml.

[0038] (2) Using a high-voltage electrostatic spraying system, the carbon fiber felt surface was sprayed five times with a voltage of 20 kV, a nozzle diameter of 50 μm, and a spray distance of 15 cm. The interval between each spraying was 10 min, forming a uniform coating with a thickness of 120 μm.

[0039] (3) Nitrogen was introduced into the tube furnace at a flow rate of 50 mL / min. The temperature was raised to 120°C at a rate of 5°C / min and held for 15 min. The temperature was then raised to 250°C at a rate of 10°C / min and held for 45 min.

[0040] (4) Dissolve 0.2 M FeCl3·6H2O in DMF and disperse by ultrasonication for 30 min to obtain solution A. Dissolve 0.2 M terephthalic acid (PTA) in DMF and stir at 70°C until transparent to obtain solution B.

[0041] (5) Immerse the coated carbon fiber felt in an equimolar mixed solution of AB (Fe:PTA = 1:1) and hydrothermally react at 120 °C for 6 h. Then take out the carbon felt and dry it for later use.

[0042] (6) In a nitrogen atmosphere, the composite carbon felt was carbonized using a pulse heating program: heating to 500°C at 10°C / min and holding for 20 min → heating to 800°C at 15°C / min and holding for 30 min → heating to 900°C at 10°C / min and holding for 60 min.

[0043] (7) KOH and urea were mixed and dissolved in ethanol at a mass ratio of 5:1 and stirred at 60°C until a transparent eutectic liquid was obtained.

[0044] (8) The composite carbon felt after gradient carbonization was immersed in a KOH / urea eutectic liquid with a solid-liquid mass ratio of 1:10. After immersion for 6 h, it was taken out and dried, and then activated at 700 °C for 1 h under nitrogen to obtain a graded porous electrode.

[0045] (9) The porous electrode was soaked in 3 mol / L HCl at room temperature for 24 h, washed by centrifugation until neutral, and dried in vacuum at 80 °C for 12 h to obtain a polyimide-derived hierarchical porous carbon / carbon fiber composite electrode.

[0046] See also Figure 3 The polyimide-derived hierarchical porous carbon / carbon fiber composite electrode prepared in this embodiment has a microstructure characterized by three levels of micropores, mesopores, and macropores. 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). At 200 mA cm -2 The constant current charge and discharge test was carried out at a current density of 200 mA cm. The energy efficiency of the flow battery assembled with the electrodes prepared in Example 2 was 89.6%, and the capacity retention rate was 98% after 300 cycles. When the unmodified carbon fiber felt was used as the positive and negative electrodes, the capacity retention rate was 98% after 300 cycles. -2 At a current density of 1.5 GHz, the energy efficiency is only 65.1%.

[0047] Example 3

[0048] The polyimide-derived hierarchical porous carbon / carbon fiber composite electrode, preparation method, and application thereof provided in this embodiment are specific applications based on the aforementioned basic embodiments. The preparation method includes the following steps: (1) PMDA-PABZ polyimide powder was dissolved in NMP and magnetically stirred at 500 rpm for 4 h until completely dissolved. Ultrasonic degassing was performed at 40 kHz for 30 min to obtain a polyimide solution with a concentration of 120 mg / ml.

[0049] (2) Using a high-voltage electrostatic spraying system, the carbon fiber felt surface was sprayed repeatedly six times at a voltage of 20 kV, a nozzle diameter of 50 μm, and a spray distance of 15 cm, with a single spraying interval of 10 min to form a uniform coating with a thickness of 200 μm.

[0050] (3) Nitrogen was introduced into the tube furnace at a flow rate of 50 mL / min. The temperature was raised to 150°C at a rate of 5°C / min and held for 30 min. The temperature was then raised to 250°C at a rate of 5°C / min and held for 60 min. The temperature was then raised to 400°C at a rate of 10°C / min and held for 90 min.

[0051] (4) Dissolve 0.3 M FeCl3·6H2O in DMF and disperse by ultrasonication for 30 min to obtain solution A. Dissolve 0.1 M terephthalic acid (PTA) in DMF and stir at 70°C until transparent to obtain solution B.

[0052] (5) Immerse the coated carbon fiber felt in an AB mixed solution (Fe:PTA = 3:1), stir at 120 °C for 12 h, and then take out the carbon felt and dry it for later use.

[0053] (6) In a nitrogen atmosphere, the composite carbon felt was carbonized using a pulse heating program: heating to 800 °C at 15 °C / min and keeping warm for 30 min → heating to 900 °C at 15 °C / min and keeping warm for 120 min → heating to 1000 °C at 10 °C / min and keeping warm for 60 min → heating to 1100 °C at 10 °C / min and keeping warm for 90 min → heating to 1200 °C at 5 °C / min and keeping warm for 60 min.

[0054] (7) KOH and urea were mixed and dissolved in ethanol at a mass ratio of 5:1 and stirred at 60°C until a transparent eutectic liquid was obtained.

[0055] (8) The composite carbon felt after gradient carbonization was immersed in a KOH / urea eutectic liquid with a solid-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.

[0056] (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°C for 12 h to obtain a polyimide-derived hierarchical porous carbon / carbon fiber composite electrode.

[0057] The polyimide-derived hierarchical porous carbon / carbon fiber composite electrode prepared in this example 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). -2 The constant current charge and discharge test was carried out at a current density of 200 mA cm. The energy efficiency of the flow battery assembled with the electrodes prepared in Example 3 was 83.7%, and the capacity retention rate was 95% after 300 cycles. When the unmodified carbon fiber felt was used as the positive and negative electrodes, the capacity retention rate was 95% after 300 cycles. -2 At a current density of 1.5 GHz, the energy efficiency is only 65.1%, and the capacity retention rate is 76% after 300 cycles.

[0058] Example 4

[0059] The polyimide-derived hierarchical porous carbon / carbon fiber composite electrode provided by the embodiment, a preparation method and applications thereof are specific applications based on the foregoing basic embodiment, and the preparation method comprises the following steps: (1) Dissolve PDCA-ODA polyimide powder in NMP, and magnetically stir at a speed of 500 rpm for 3 hours until completely dissolved, and ultrasonically degas at a frequency of 40 kHz for 30 minutes to obtain a polyimide solution with a concentration of 100 mg / ml.

[0060] (2) Adopt a high-voltage electrostatic spraying system, and spray repeatedly 4 times on the surface of the carbon fiber felt under the conditions of a voltage of 20 kV, a nozzle diameter of 50 μm, and a spraying distance of 15 cm, and the interval between single spraying is 10 minutes, so as to form a uniform coating with a thickness of 60 μm.

[0061] (3) Introduce nitrogen into a tube furnace at a flow rate of 50 mL / min, heat to 100 ℃ at a rate of 5 ℃ / min, keep for 30 min, heat to 250 ℃ at a rate of 5 ℃ / min, keep for 30 min, and then heat to 300 ℃ at a rate of 8 ℃ / min, keep for 30 min.

[0062] (4) Take 0.3 M FeCl3·6H2O and dissolve in DMF, and ultrasonically disperse for 30 min to obtain solution A. Take 0.3 M terephthalic acid (PTA) and dissolve in DMF, and stir at 70 ℃ until transparent to obtain solution B.

[0063] (5) Dip the coated carbon fiber felt into the mixed AB solution (Fe: PTA = 1:1), and stir at 80 ℃ for 10 h, and then take out the carbon felt for drying.

[0064] (6) Under a nitrogen atmosphere, carbonize the composite carbon felt by a pulse temperature rising program, heat to 800 ℃ at a rate of 10 ℃ / min and keep for 60 min, heat to 900 ℃ at a rate of 15 ℃ / min and keep for 60 min, and then heat to 1000 ℃ at a rate of 5 ℃ / min and keep for 60 min.

[0065] (7) Mix KOH and urea in a mass ratio of 4:1, and dissolve in ethanol to obtain a transparent eutectic liquid at 60 ℃.

[0066] (8) Dip the composite carbon felt after gradient carbonization into the KOH / urea eutectic liquid, wherein the mass ratio of solid to liquid is 1:10, dip for 6 h, take out for drying, and then activate at 800 ℃ for 2 h under a nitrogen atmosphere to obtain a hierarchical porous electrode.

[0067] (9) The porous electrode was immersed in 1 mol / L HNO3 for 24 h at room temperature, washed by centrifugation until neutral, and vacuum dried at 120℃ for 12 h to obtain the polyimide-derived hierarchically porous carbon / carbon fiber composite electrode.

[0068] Referring to Figure 4 The polyimide-derived hierarchically porous carbon / carbon fiber composite electrode prepared in this embodiment has a microstructure characterized by a three-level through micro-pore-mesopore-macropore. The polyimide-derived hierarchically 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 constant current charge-discharge test was carried out at a current density of 200 mA cm -2 The energy efficiency of the flow battery assembled with the electrode prepared in Example 4 was 87.1%, and the capacity retention rate was 96% after 300 cycles. When the unmodified carbon fiber felt was used as the positive and negative electrodes, the energy efficiency was only 65.1% at a current density of 200 mA cm -2

[0069] Example 5

[0070] The polyimide-derived hierarchically porous carbon / carbon fiber composite electrode, the preparation method and the application thereof provided in this embodiment are specific applications on the basis of the foregoing basic embodiments. The preparation method comprises the following steps: (1) The PDCA-ODA type polyimide powder was dissolved in NMP, and magnetically stirred at a speed of 500 rpm for 2 h until completely dissolved. The polyimide solution with a concentration of 90 mg / ml was obtained by ultrasonic degassing at a frequency of 40 kHz for 30 min.

[0071] (2) The high-voltage electrostatic spraying system was used to spray the surface of the carbon fiber felt under the conditions of a voltage of 20 kV, a nozzle diameter of 50 μm, and a spraying distance of 15 cm. The spraying was repeated 4 times with an interval of 10 min for each spraying to form a uniform coating with a thickness of 150 μm.

[0072] (3) Nitrogen was introduced into the tube furnace at a flow rate of 50 mL / min. The temperature was raised to 200℃ at a rate of 5℃ / min, and kept for 30 min. The temperature was raised to 250℃ at a rate of 5℃ / min, and kept for 60 min.

[0073] (4) 0.4 M FeCl3·6H2O was dissolved in DMF to obtain solution A, which was ultrasonically dispersed for 30 min. 0.2 M terephthalic acid (PTA) was dissolved in DMF to obtain solution B, which was stirred at 70℃ until transparent.

[0074] ​(5) The coated carbon fiber felt was immersed in the AB mixed solution (Fe: PTA = 2:1) and stirred at 80℃ for 10 h. The carbon felt was then taken out and dried for later use.

[0075] (6) The composite carbon felt was carbonized under a nitrogen atmosphere by a pulse temperature program, i.e. increased to 500℃ at 5 ℃ / min and maintained for 30 min, then increased to 800℃ at 10 ℃ / min and maintained for 60 min, then increased to 900℃ at 15 ℃ / min and maintained for 60 min, and then increased to 1000℃ at 10 ℃ / min and maintained for 30 min.

[0076] (7) KOH and urea were mixed in ethanol at a mass ratio of 2:1 and stirred at 60℃ until a transparent eutectic liquid was formed.

[0077] (8) The composite carbon felt after gradient carbonization was immersed in the KOH / urea eutectic liquid, with a solid-to-liquid mass ratio of 1:10. After 7 h of immersion, the carbon felt was taken out and dried, and then activated at 850℃ under nitrogen for 2 h to obtain a hierarchical porous electrode.

[0078] (9) The porous electrode was immersed in 2 mol / L HCl at room temperature for 24 h, washed by centrifugation until neutral, and vacuum dried at 100℃ for 12 h to obtain a polyimide-derived hierarchical porous carbon / carbon fiber composite electrode.

[0079] The polyimide-derived hierarchical porous carbon / carbon fiber composite electrode prepared in this example 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 constant current charge and discharge test was performed at a current density of 200 mA cm -2 The energy efficiency of the flow battery assembled with the electrode prepared in Example 5 was 86.8%, and the capacity retention rate was 97% after 300 cycles. When the unmodified carbon fiber felt was used as the positive and negative electrodes, the energy efficiency was only 65.1% at a current density of 200 mA cm -2

[0080] ​The preparation method of the composite electrode provided by each embodiment of the present application focuses on uniformly coating the polyimide solution on the surface of the carbon fiber, generating a Fe-MOG gel template in a FeCl3 / terephthalic acid system after pre-oxidation and solidification, realizing chemical bonding of the polyimide carbon layer and the fiber through gradient heating carbonization (800-1200 DEG C), and obtaining the composite porous electrode with high specific surface area and high conductivity after activating by using the eutectic mixture of KOH / urea, synchronously introducing micropores and mesopores, and acid washing. The polyimide-derived hierarchical porous carbon / carbon fiber composite electrode provided by each embodiment of the present application has high conductivity and catalytic activity, has high conductivity and high energy efficiency, and can be applied to various types of aqueous organic flow battery systems. The present application solves the problems of limited mass transfer and insufficient active sites of the traditional carbon electrode through template cooperation and activation process optimization, is suitable for all-vanadium, iron-chromium and organic system flow batteries, and has significant industrial application value.

[0081] It should be particularly pointed out that, within the range of the components, the ratio and the process parameters described in the present application, other technical solutions obtained by specific selection can achieve the technical effects of the present application, and therefore will not be listed one by one. At the same time, other technical solutions obtained by using other components similar to the components and solvents described in the present application are included in the protection scope of the present application.

[0082] In the description of the present application, the terms "one embodiment", "some embodiments", "specific embodiments" and the like are intended to mean that the specific features, structures, materials or characteristics described in connection with the embodiment or example are included in at least one embodiment or example of the present application. In the present application, the illustrative description of the above terms does not necessarily mean the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described can be combined in any one or more embodiments or examples in a suitable manner.

[0083] The above only describes the preferred embodiments of the present application and is not intended to limit the present application. For those skilled in the art, the present application can have various modifications and changes. Any modification, equivalent replacement, improvement, etc. made within the spirit and principles of the present application shall be included in the protection scope of the present application.

Claims

1. A method for preparing a polyimide-derived hierarchical porous carbon / carbon fiber composite electrode, characterized in that: Using carbon fiber felt as the substrate, a three-level interconnected structure of micropores, mesopores and macropores was constructed through polyimide solution spraying, metal organic gel template deposition, gradient carbonization and low-melting activation process to obtain 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 following steps are involved: S1: spraying a polyimide solution evenly onto the surface of the carbon fiber felt to form a polymer coating with a thickness of 10-200 µm, and then thermally curing the coating at a temperature range of 200-400°C under an inert atmosphere for 1-3 hours to allow the cross-linking and curing reaction of the polyimide to proceed fully; S2: The coated carbon fiber felt is placed in a mixed solution of FeCl3 and terephthalic acid (PTA), allowing Fe³⁺ ions to 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 in an inert atmosphere, heating the material to 800-1200°C at a rate of 5-15°C / min through a pulse heating program and keeping the temperature for 2-6 hours to convert the polyimide into a carbon layer and form a chemical bond with the carbon fiber felt; S4: Activation treatment: The composite electrode is impregnated with a KOH / urea eutectic mixture and secondary activated at 700-900°C 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 by-products, and drying to obtain a polyimide-derived hierarchical porous carbon / carbon fiber composite electrode.

3. 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 PMDA-ODA, PMDA-PABZ, PDCA-PDA and PABZ-ODA types; the polyimide solution is prepared by dissolving the polyimide in N-methylpyrrolidone (NMP) at a concentration of 60-120 mg / ml.

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

5. The method for preparing the polyimide-derived hierarchical porous carbon / carbon fiber composite electrode according to claim 4, characterized in that: The formation of the Fe-MOG gel is completed through a gentle 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.

6. The method for preparing the polyimide-derived hierarchical porous carbon / carbon fiber composite electrode according to claim 1, wherein: In step S3, in order to avoid cracking of the carbon layer, the carbonization stage is carried out in the range of 800-1200°C with each temperature increase of 100°C for 30 minutes.

7. The method for preparing the polyimide-derived hierarchical porous carbon / carbon fiber composite electrode according to claim 1, wherein: In step S4, the mass ratio of the KOH / urea eutectic mixture is (1-5):1, and the immersion time is 3-8 hours to control the pore structure and activation effect.

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

9. A polyimide-derived hierarchical porous carbon / carbon fiber composite electrode, characterized in that: It is prepared by the method according to any one of claims 1 to 8, and the composite electrode has a three-level pore structure: Micropores: pore diameter 1-2 nm, specific surface area ≥100 m² / g; Mesopores: pore diameter 10-30 nm, volume share 20-40%; Macropores: pore diameter 100-500 nm, composed of native pores of carbon felt and pores created by sacrificial template method.

10. Use of the polyimide-derived hierarchical porous carbon / carbon fiber composite electrode according to claim 9 in the preparation of positive and negative electrodes of a liquid flow battery.

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