Multi-stage pore graphite felt for flow battery and preparation method of multi-stage pore graphite felt

Abstract

The invention discloses a multistage porous graphite felt for a flow battery and a preparation method of the multistage porous graphite felt, and belongs to the technical field of chemical energy storage. The invention aims to solve the problems that the existing graphite felt electrode is low in catalytic activity and the catalyst is easy to fall off. The preparation method comprises the following steps: uniformly mixing thermosetting resin, a pore-forming agent and a bismuth-containing compound to prepare a precursor mixed solution; then immersing the graphite felt in the mixed solution for solvent heat treatment, so that the precursor uniformly permeates and is firmly attached to the surface of the graphite felt fiber; and performing high-temperature pyrolysis in an inert atmosphere. In the pyrolysis process, the resin is converted into a conductive porous carbon layer in situ, the pore-forming agent is decomposed to form rich multi-stage pore channels, and the bismuth-containing compound is reduced into a highly dispersed metal bismuth nano-catalyst by carbon thermal reduction and is confined in a porous carbon skeleton. The graphite felt electrode prepared by the invention has high catalytic activity, excellent mass transfer channels and high stability, and is simple in preparation process, controllable in cost and suitable for large-scale production.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of liquid flow energy storage batteries in chemical energy storage technology, in particular to the electrode of all-vanadium redox flow battery. BACKGROUND

[0002] The liquid flow battery is a new type of energy storage technology with decoupling power and capacity, which has the characteristics of intrinsic safety, high efficiency, environmental protection, and high residual value. It is an important technology in the field of large-scale energy storage, and the performance of its electrode material directly determines the power density, energy efficiency and cycle life of the battery. The current liquid flow battery electrode mainly uses carbon-based materials such as carbon felt and graphite felt, which has the advantages of high electrical conductivity and chemical stability, but the fiber surface is smooth, resulting in fewer active sites, low reactivity and low mass transfer efficiency.

[0003] In order to overcome the above-mentioned defects, the prior art has carried out various modification explorations. Among them, the activity is improved by loading catalysts, for example, Chinese patent CN202410332105.0 discloses a method of anchoring bismuth nanometer carbon spheres on the surface of the electrode. Although this method can improve the electrode performance by introducing catalytically active substances, it often faces a core contradiction: the loaded nanometer catalyst particles are prone to agglomeration, or unevenly accumulated on the fiber surface under the scouring of electrolyte, which not only covers part of the active sites, but also may block the pores of the graphite felt, thus deteriorating the mass transfer performance and sacrificing part of the porosity to obtain catalytic activity. Therefore, how to find a balance point between efficient electrocatalytic performance and efficient mass transfer performance is a key problem that needs to be solved in the current electrode modification.

[0004] Patent CN202411319648.5 a high-activity graphite felt for vanadium battery and its preparation method, which mentions that a layer of polymer is electrodeposited on the graphite felt, then metal molybdenum is loaded by hydrothermal method, the metal molybdenum is chelated with phytic acid, and finally an organic framework-metal oxide-organic framework layered structure loaded graphite felt is obtained by high temperature pyrolysis. This method can also increase the specific surface area and active sites of the graphite felt, and can also protect the metal catalyst from falling off or being consumed under long-term catalytic reaction and electrolyte scouring. The electrodeposition method is difficult to achieve uniform polymer coating on the graphite felt with large area and complex three-dimensional structure, and the process flow is complex, which is not conducive to large-scale production and cost control.

[0005] In summary, the prior art has not yet well solved the contradiction between catalytic activity, mass transfer efficiency and preparation process in improving the performance of graphite felt electrode. Therefore, there is an urgent need in the art for a preparation method that is simple in process, controllable in cost, and can have high catalytic activity, excellent mass transfer channel and high stability composite functional layer on the graphite felt in one step. SUMMARY

[0006] In view of the deficiencies in the prior art, the purpose of the present application is to provide a high-performance active graphite felt and a preparation method thereof, by constructing a composite functional layer in which a metal bismuth catalyst is confined in a porous carbon framework on the surface of graphite felt fibers, thereby improving the catalytic activity, mass transfer efficiency and long-term running stability of the electrode.

[0007] To achieve the above object, the present application provides the following technical solutions. A preparation method of a multi-level porous graphite felt for a flow battery, comprising the following steps: Step 1: preparing a resin / pore-forming agent solution: dissolving or dispersing a thermosetting resin and a pore-forming agent in a solvent to obtain a resin / pore-forming agent solution.

[0008] In step 1, the thermosetting resin is selected from at least one of an epoxy resin, a phenolic resin, an unsaturated polyester resin and a furan resin.

[0009] The pore-forming agent is selected from at least one of polystyrene, polymethyl methacrylate, polyvinyl alcohol, polyphenylene sulfide and polyethylene glycol.

[0010] The mass ratio of the resin to the pore-forming agent is 20:1 to 1:10. The solvent is selected from at least one of ethanol, N,N-dimethylformamide (DMF) and dimethyl sulfoxide (DMSO).

[0011] Step 2: preparing a bismuth-containing precursor dispersion: dispersing a bismuth-containing compound in a solvent to obtain a bismuth-containing precursor dispersion.

[0012] The bismuth-containing compound is selected from bismuth nitrate. The solvent is selected from at least one of ethylene glycol, dimethyl sulfoxide (DMSO) and N,N-dimethylacetamide (DMAc). The concentration of the bismuth-containing precursor dispersion is 2-100 mg / mL.

[0013] Step 3: uniformly mixing the resin / pore-forming agent solution obtained in step 1 with the bismuth-containing precursor dispersion obtained in step 2 to obtain a final precursor mixture.

[0014] Further, in the precursor mixture, the total solid content of the resin and the pore-forming agent accounts for 1 / 100 to 1 / 10 of the total mass of the mixture.

[0015] Step 4: completely immersing a polyacrylonitrile-based or viscose-based graphite felt in the precursor mixture obtained in step 3, transferring it to a sealed reaction kettle, and performing solvothermal treatment at a temperature of 100-200°C for a holding time of 0.5-24 hours. After the reaction is completed, the graphite felt is taken out.

[0016] Step 5: The solvent-thermal treated and dried graphite felt is placed in a tube furnace, heated to 600-900℃ under inert atmosphere, and kept at this temperature for 0.5-6 hours for pyrolysis treatment. After the pyrolysis is completed, the graphite felt is naturally cooled to room temperature to obtain the graphite felt electrode material for high-performance flow batteries.

[0017] Further, the inert atmosphere is argon, nitrogen or a mixture thereof.

[0018] The solvent-thermal reaction in step 4 of the present application can reduce the viscosity of the solvent and enhance the penetration ability, so that all precursor components including the resin, pore-forming agent and bismuth source can be highly uniformly penetrated and deposited on the surface of each fiber in the complex three-dimensional network structure of the graphite felt, and uniform nanoparticles are formed in situ on the fiber surface. Compared with normal temperature impregnation or electrodeposition, the solvent-thermal treatment can achieve more excellent coating uniformity and stronger interfacial bonding force, which provides a basis for the subsequent formation of a high-quality composite functional layer.

[0019] In the high-temperature pyrolysis process in step 5 under inert atmosphere, the carbonized layer on the surface of the graphite felt is formed, i.e. the thermosetting resin is pyrolyzed and converted into a conductive porous carbon layer, which is coated on the surface of the original graphite fiber to improve its corrosion resistance. The pore-forming agent (such as PVA, PEG) is decomposed into gaseous small molecules and escapes, leaving a large number of micron or nanoscale pores in the space originally occupied by the pore-forming agent, thereby constructing a multi-level pore structure in the carbonized layer, greatly increasing the specific surface area of the electrode and constructing a high-efficiency mass transfer channel. The bismuth-containing compound is carbothermally reduced in situ by the carbon generated by the surrounding resin being carbonized at high temperature to form metallic bismuth, forming a stable composite structure of metallic bismuth-porous carbon.

[0020] The beneficial effects of the present application include: (1) The present application ensures the uniform penetration and distribution of all functional components in the three-dimensional network of the graphite felt through one-step solvent-thermal treatment of the resin, pore-forming agent and bismuth source. In the subsequent high-temperature pyrolysis process, the resin is converted into a porous carbon matrix in situ, and the pore-forming agent is decomposed to leave abundant pores. This forms a unique structure in which the catalytically active substance (metallic bismuth) is highly dispersed and confined in the porous carbon skeleton. This structure not only ensures extremely high reaction activity through highly dispersed catalysts and increased specific surface area, but also ensures rapid penetration of the electrolyte and efficient mass transfer of ions through the interconnected multi-level pore network, fundamentally solving the problem of catalyst blocking the pores in the traditional loading method.

[0021] (2) The carbonized layer formed by the application can effectively resist long-term erosion of electrolyte and volume change in the electrochemical cycle process, greatly inhibiting the migration, agglomeration and loss of catalyst particles. In addition, the carbonized layer also improves the corrosion resistance of the graphite fiber itself, which together ensures that the electrode can maintain stable performance in long-term cycling. BRIEF DESCRIPTION OF DRAWINGS

[0022] Figure 1 . Micro-morphology diagram of the electrode prepared in Example 1 by scanning electron microscope.

[0023] Figure 2 . Micro-morphology diagram of the electrode prepared in Example 2 by scanning electron microscope.

[0024] Figure 3 . Micro-morphology diagram of the electrode prepared in Example 3 by scanning electron microscope.

[0025] Figure 4 . Micro-morphology diagram of the electrode prepared in Example 4 by scanning electron microscope.

[0026] Figure 5 . Micro-morphology diagram of the electrode prepared in Comparative Example 1 by scanning electron microscope.

[0027] Figure 6 . Micro-morphology diagram of the electrode prepared in Comparative Example 2 by scanning electron microscope.

[0028] Figure 7 . Micro-morphology diagram of the electrode prepared in Comparative Example 3 by scanning electron microscope.

[0029] Figure 8 . Micro-morphology diagram of the electrode prepared in Comparative Example 4 by scanning electron microscope.

[0030] Figure 9 . Micro-morphology diagram of the electrode prepared in Comparative Example 5 by scanning electron microscope.

[0031] Figure 10 . Comparison chart of energy efficiency of electrodes prepared in different examples under different current densities.

[0032] Figure 11 Test data of the electrode in Comparative Example 1 in a single cell; Figure 12 Test data of the electrode in Comparative Example 2 in a single cell; Figure 13 Test data of the electrode in Comparative Example 3 in a single cell; Figure 14 Test data of the electrode in Comparative Example 4 in a single cell; Figure 15Test data of the electrode in Example 4 in a single cell at 200 mA cm-2current density and cycled for 300 times. Figure 16 Efficiency stability plot of Example 4 at 200 mA cm-2current density and cycled for 300 times. -2 Efficiency stability plot of Example 4 at 200 mA cm-2current density and cycled for 300 times.

[0033] Figure 17 Efficiency stability plot of Example 4 at 200 mA cm-2current density and cycled for 300 times. -2 Efficiency stability plot of Example 4 at 200 mA cm-2current density and cycled for 300 times. DETAILED DESCRIPTION

[0034] The following description of at least one exemplary embodiment is merely exemplary in nature and is in no way intended to limit the application or its application or uses. All other embodiments not explicitly described herein are within the scope of the application, which is defined by the claims. Example 1

[0035] Step S1: phenolic resin and polyvinyl alcohol (PVA 1788) were weighed according to a mass ratio of 1:1, and dissolved in DMSO to prepare a resin / pore-forming agent solution with a solid content (the total mass of phenolic resin and PVA accounting for the total mass of the solution) of 1 / 50.

[0036] Step S2: bismuth nitrate (Bi(NO3)3·5H2O) powder was dispersed in dimethyl sulfoxide (DMSO) to prepare a bismuth-containing precursor dispersion with a concentration of 10 mg / mL.

[0037] Step S3: the S1 and S2 solutions were mixed and stirred uniformly to obtain a precursor mixture. The precursor mixture was poured into a reaction kettle and polyacrylonitrile graphite felt was added for solvothermal reaction. The solvothermal reaction temperature was 120°C, and the solvothermal treatment time was 2h. After the graphite felt was taken out and dried, it was placed in a tube furnace, heated to 600°C in an argon atmosphere, and kept for 1h. After natural cooling, the modified graphite felt electrode material was obtained. Example 2

[0038] Step S1: furan resin and polymethyl methacrylate (PMMA) were weighed according to a mass ratio of 2:1, and dissolved in acetone to prepare a resin / pore-forming agent solution with a solid content of 1 / 50.

[0039] Step S2: bismuth nitrate powder was dispersed in ethylene glycol to prepare a bismuth-containing precursor dispersion with a concentration of 20 mg / mL.

[0040] Step S3: After mixing S1 and S2 solutions, the precursor mixed solution was obtained by stirring uniformly. The precursor mixed solution was poured into a reaction kettle to add polyacrylonitrile graphite felt for solvothermal reaction. The solvothermal reaction temperature was 100℃, and the time was 12h. The graphite felt was taken out and heated to 800℃ in an argon atmosphere, and kept for 1h. After natural cooling, the modified graphite felt electrode material was obtained. Example 3

[0041] Step S1: Phenolic resin and polyethylene glycol were dissolved in ethanol according to a mass ratio of 3:1 to prepare a resin / pore-forming agent solution with a solid content of 1 / 60.

[0042] Step S2: Bismuth nitrate powder was mixed with ethylene glycol to obtain a bismuth source dispersion solution with a concentration of 10mg / ml.

[0043] Step S3: After mixing S1 and S2 solutions, polyacrylonitrile raw material graphite felt was added, and the mixture was stirred uniformly and then poured into a reaction kettle for solvothermal reaction. The reaction temperature was 140℃, and the hydrothermal time was 4h. The graphite felt was taken out after heating to 700℃ in an argon atmosphere and keeping for 2h to obtain a modified graphite felt electrode material. Example 4

[0044] Step S1: Phenolic resin and polyethylene glycol were dissolved in ethanol according to a mass ratio of 1:1 to prepare a solution with a solid content of 1 / 40.

[0045] Step S2: Bismuth nitrate powder was mixed with N,N-dimethylacetamide to obtain a bismuth source dispersion solution with a concentration of 10mg / ml.

[0046] Step S3: After mixing S1 and S2 solutions, polyacrylonitrile raw material graphite felt was added, and the mixture was stirred uniformly and then poured into a reaction kettle for solvothermal reaction. The reaction temperature was 140℃, and the hydrothermal time was 4h. The graphite felt was taken out after heating to 700℃ in an argon atmosphere and keeping for 2h to obtain a modified graphite felt electrode material. Comparative Example 1

[0047] The unmodified original graphite felt, i.e. the original polyacrylonitrile-based graphite felt.

[0048] From the micro-morphology graph of Figure 5 It can be seen from the micro-morphology graph that the unmodified electrode has a smooth and uniform fiber surface, and no obvious pores can be observed. Comparative Example 2

[0049] Comparative Example 2 and Example 4 differ in that no pore-forming agent (polyethylene glycol) is added, and the rest of the operations are the same as Example 4.

[0050] From the micro-morphology graph of Figure 6The microstructure of the modified graphite felt electrode material can be observed under a scanning electron microscope. The fiber surface is uniformly coated with a carbon layer, but the surface is dense and has no obvious pores. Comparative Example 3:

[0051] Comparative Example 3 differs from Example 4 in that the mass ratio of phenolic resin and polyethylene glycol is adjusted to 100:1, and the rest of the operations are the same as in Example 4.

[0052] From Figure 7 The microstructure of the modified graphite felt electrode material can be observed under a scanning electron microscope. The fiber surface is uniformly coated with a carbon layer, but the surface is dense and has no obvious pores. Comparative Example 4:

[0053] Comparative Example 4 differs from Example 4 in that the mass ratio of phenolic resin and polyethylene glycol is adjusted to 1:20, and the rest of the operations are the same as in Example 4.

[0054] From Figure 8 The microstructure of the modified graphite felt electrode material can be observed under a scanning electron microscope. The fiber surface is uniformly coated with a carbon layer, but the surface is dense and has no obvious pores. Comparative Example 5:

[0055] Comparative Example 5 differs from Example 4 in that the solvent thermal treatment in step 3 is omitted.

[0056] S1-S2 operations are the same as in Example 4; Step S3: After mixing S1 and S2 solutions, add polyacrylonitrile raw material graphite felt and stir for 4 hours. Then heat to 700℃ in argon atmosphere and keep for 2h. Then take out the modified graphite felt electrode material.

[0057] Comparative Example 5 lacks the solvent thermal process, and the precursor is only attached to the surface layer of the graphite felt, which is uneven and not firm. The final electrode material, especially the performance and cycle stability under high current density, is lower than that of Example 4.

[0058] Electrochemical performance test:

[0059] The modified graphite felt prepared in Examples 1-4 and Comparative Examples is cut into an electrode sheet with an effective area of 4 cm² (2cm × 2cm). The cut electrode sheet is used as the positive and negative electrodes of the battery to assemble a full vanadium flow battery for testing. The battery is assembled with Kelun N-212 type as the separator, and the compression ratio of the battery is controlled at 25%. The electrolyte of the battery is 1.7M V+ 4.3M H2SO4.

[0060] The battery test system is used for constant current charge and discharge test.

[0061] Test procedure: rate capability test was performed at 100~300 mA / cm 2 , respectively, each at 5 cycles.

[0062] Charge-discharge condition: the charge cut-off voltage was set at 1.6 V, and the discharge cut-off voltage was set at 1.0 V.

[0063] Cycle stability test: the electrode of Example 4 (S4) with the best performance was selected and subjected to a cycle test at a large current density of 200 mA / cm2for 300 cycles.

[0064] Table 1. Comparison of energy efficiency of electrodes in different examples after being assembled into a full vanadium flow battery and tested at 100~300 mA / cm 2

[0065] The above merely describes preferred embodiments of the present application, but is not intended to limit the protection scope of the present application. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application shall be included in the protection scope of the present application.​

Claims

1. A method for preparing a multi-level porous graphite felt for a flow battery, characterized by, The method comprises the following steps: (1) dissolving or dispersing a thermosetting resin and a pore-forming agent in a solvent to obtain a resin / pore-forming agent solution; (2) dispersing a bismuth-containing compound in a solvent to obtain a bismuth-containing precursor dispersion; (3) mixing the resin / pore-forming agent solution and the bismuth-containing precursor dispersion to obtain a precursor mixture; (4) immersing a graphite felt in the precursor mixture of step (3) and performing a solvothermal treatment at a temperature of 100-200°C; (5) drying the graphite felt subjected to the solvothermal treatment, then heating it to 600-900°C in an inert atmosphere for pyrolysis, and naturally cooling to obtain a graphite felt with hierarchical pores.

2. The method for producing a multi-level porous graphite felt for a liquid flow battery according to claim 1, characterized by, The thermosetting resin is at least one selected from the group consisting of an epoxy resin, a phenolic resin, an unsaturated polyester resin, and a furan resin.

3. The method of claim 1, wherein the method further comprises: The pore-forming agent is at least one selected from the group consisting of polystyrene, polymethyl methacrylate, polyvinyl alcohol, polyphenylene sulfide, and polyethylene glycol.

4. The method of claim 1, wherein the multi-level porous graphite felt for a liquid flow battery is characterized by, The mass ratio of the resin to the pore-forming agent is 1:10-20:

1.

5. The method of claim 1, wherein the multi-level porous graphite felt for a liquid flow battery is characterized by, The bismuth-containing compound is bismuth nitrate.

6. The method of claim 1, wherein the multi-level porous graphite felt for liquid flow batteries is characterized by, In the precursor mixture of step (3), the total solid content of the resin and the pore-forming agent accounts for 1 / 100-1 / 10 of the total mass of the mixture.

7. The method of claim 1, wherein the multi-level porous graphite felt for liquid flow batteries is characterized by, The concentration of the bismuth-containing precursor dispersion is 2-100 mg / mL.

8. The method of claim 1, wherein the multi-level porous graphite felt for a flow battery is characterized by, The inert atmosphere is argon, nitrogen, or a mixture thereof.

9. A hierarchically porous graphite felt for a flow battery prepared by the method of any one of claims 1-8; comprising: The graphite felt substrate, the porous carbon layer coated on the surface of the graphite felt substrate, and the metal bismuth nanoparticles dispersed and confined in the porous carbon layer.

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