A method for preparing a modified electrode for a vanadium redox flow battery and the vanadium redox flow battery itself.

Abstract

This invention belongs to the field of flow battery technology and discloses a method for preparing a modified electrode for an all-vanadium redox flow battery: a graphite felt electrode is immersed in an ammonium salt solution; then, the ammonium salt-immersed graphite felt electrode is placed in a tubular furnace quartz tube for high-temperature activation to obtain the modified electrode. This invention also discloses an all-vanadium redox flow battery incorporating this modified electrode. The modified electrode prepared by this invention has the characteristics of large electrochemical active area, abundant active sites, stable structure, and excellent performance, enabling the flow battery to operate at higher current densities and significantly improving its power density.

Description

Technical Field

[0001] This invention belongs to the field of flow battery technology, and particularly relates to a method for preparing a modified electrode for a vanadium redox flow battery and the vanadium redox flow battery itself. Background Technology

[0002] Vanadium redox flow batteries (VRBs) meet the requirements of long-term energy storage devices. VRB energy storage systems offer significant advantages such as intrinsic safety, reliable operation, and environmental friendliness throughout their entire lifecycle. Their output power and storage capacity are independent, allowing for flexible design and installation, making them suitable for large-scale, high-capacity, and long-term energy storage. They also boast high energy conversion efficiency, fast start-up, no phase change, and rapid response during charge / discharge state switching. VRB energy storage systems employ a modular design, facilitating system integration and scalability. However, the main factor hindering the development of VRBs is cost. Reducing the cost and improving the efficiency of key materials, including electrodes, ion exchange membranes, and electrolytes, is crucial for lowering the overall cost of vanadium batteries.

[0003] Among them, the electrode, as a key component of the all-vanadium redox flow battery, is the active material (VOC) in the electrolyte. 2+ / VO 2+ V 3+ / V 2+ The redox reaction of vanadium ions occurs at the surfaces of both electrodes, thus the energy efficiency of the battery largely depends on the electrocatalytic performance of the electrodes. Graphite felt is the preferred electrode material due to its large specific surface area, good stability, and good conductivity. However, raw graphite felt is hydrophobic, and its redox kinetics are poor due to the limited number of active sites, resulting in low battery efficiency. Therefore, it is necessary to modify graphite felt to improve the redox kinetics of vanadium ions on the graphite felt, reduce battery polarization, and thus improve energy efficiency, power density, and electrolyte utilization.

[0004] To address the aforementioned issues, current modification methods are mainly divided into intrinsic modification and catalyst-supported modification. Intrinsic modification is further divided into heat treatment, acid treatment, and electrochemical oxidation. Catalyst-supported modification primarily involves introducing materials such as metals, metal compounds, and carbon-based materials into the electrode through physical or chemical methods, which can enhance electrode reactivity or improve electrode selectivity. Common intrinsic modification strategies, such as acid and alkali treatment, generate additional waste acid and alkali, which are difficult to recycle and waste resources. After prolonged electrolyte rinsing, catalyst-supported electrodes may experience catalyst detachment, causing electrolyte contamination. Therefore, there is a need to research an environmentally friendly electrode modification method with stable electrode performance. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to overcome the deficiencies and defects mentioned in the background art above, and to provide a method for preparing a modified electrode for a vanadium redox flow battery and a vanadium redox flow battery.

[0006] To solve the above-mentioned technical problems, the technical solution proposed by this invention is as follows:

[0007] A method for preparing a modified electrode for an all-vanadium redox flow battery includes the following steps:

[0008] (1) Immerse the graphite felt electrode in an ammonium salt solution;

[0009] (2) The graphite felt electrode soaked in ammonium salt after step (1) is placed into the quartz tube of a tubular furnace for high-temperature activation to obtain the modified electrode.

[0010] In the preferred preparation method described above, in step (2), the graphite felt electrode soaked in ammonium salt is first placed in the non-insulated area on one side of the quartz tube of the tubular furnace. After the tubular furnace is heated to 500-1000℃, the graphite felt electrode soaked in ammonium salt is pushed into the insulation area to react for 5-60 minutes. This processing technology of the present invention causes the surface temperature of the graphite felt to rise sharply, thereby changing the microstructure of the carbon fiber surface. This rapid temperature change can create more defects (such as edge defects) and can more effectively achieve heteroatomic doping. If the temperature is directly raised in the temperature zone, it will not only cause the ammonium salt to volatilize, but it will also be discharged with the gas flow before doping and etching, resulting in the loss of ammonium salt, thereby reducing the doping source and etching source; moreover, the slow temperature rise is not conducive to the change of the carbon fiber surface and the formation of defects.

[0011] In the above preparation method, preferably, in step (2), the high-temperature activation is carried out in an air atmosphere.

[0012] In the above preparation method, preferably, in step (1), the ammonium salt solution includes ammonium salt, surfactant and water, wherein the concentration of ammonium salt is 0.1-1.5 mol / L and the mass ratio of surfactant to water is 1:500-1:2000.

[0013] In the above preparation method, preferably, the ammonium salt is a soluble ammonium salt, including at least one of ammonium sulfate, ammonium carbonate, ammonium chloride, ammonium nitrate, ammonium phosphate, ammonium fluoride, ammonium bromide, ferrous ammonium sulfate, ammonium molybdate, ammonium tungstate, dodecyltrimethylammonium chloride, tetraethylammonium bromide, and ammonium acetate.

[0014] In the above preparation method, preferably, the surfactant includes at least one of Tween 20, Tween 40, Span 20, Span 40, and sodium dodecyl sulfate.

[0015] In the above preparation method, preferably, in step (1), the graphite felt electrode is a polyacrylonitrile-based (PAN-based) graphite felt, pitch-based graphite felt, or adhesive-based graphite felt.

[0016] Based on a general inventive concept, the present invention also provides an all-vanadium redox flow battery, comprising a modified electrode prepared by the above-described preparation method.

[0017] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0018] (1) The modified electrode prepared by the present invention has the characteristics of large electrochemical active area, abundant active sites, stable structure and excellent performance, which can enable the flow battery to operate at a higher current density and greatly improve its power density.

[0019] (2) In the preparation process of the present invention, carbon defects are constructed by immersing the electrode material in an ammonium salt solution and activating it at high temperature, thereby improving the hydrophilicity of the electrode. High temperature etching increases the specific surface area of ​​the electrode. Heteroatom doping generates abundant reactive sites on the electrode surface, which improves the vanadium ion reaction rate and reduces energy loss during charging and discharging, so that the vanadium battery exhibits high energy efficiency in charging and discharging tests. Moreover, the modified electrode has stable mechanical and electrochemical properties, which allows the flow battery to operate stably for a long time at a higher current density.

[0020] (3) Compared with the hydrothermal ammoniation method, the preparation method of the present invention does not require the use of a special high-pressure reactor, has a weaker dependence on production equipment, and has lower operation difficulty and risk. Moreover, the heat treatment method and heating method adopted in the present invention can be used for continuous production in actual production.

[0021] (4) Compared with the molten salt method, the preparation method of the present invention does not require a large amount of chloride salt to provide a liquid phase environment, nor does it require additional oxygen and nitrogen sources. The present invention uses ammonium salt aqueous solution and requires fewer types and amounts of raw materials. The sample prepared after calcination can be used directly without cleaning. The preparation process is extremely simple and has low technical requirements.

[0022] (5) This invention can reduce electrode polarization, enabling the all-vanadium redox flow battery to operate at 200 mA cm⁻¹. -2 The energy efficiency reaches over 80% at current density.

[0023] (6) The present invention does not change the macroscopic structure of the electrode, does not affect the original assembly of the electrode, and does not affect the original battery structure.

[0024] (7) The raw material cost of the present invention is economical, the experimental process is scalable and easy to industrialize. Attached Figure Description

[0025] Figure 1Electron micrographs of the electrodes prepared in Comparative Examples 1, 2 and Example 4 of this invention.

[0026] Figure 2 The electrodes prepared using the electrodes of Example 4 and Comparative Example 1 of this invention were respectively assembled into all-vanadium redox flow batteries at 300 mA cm⁻¹. -2 Comparison of charge and discharge curves at current density.

[0027] Figure 3 The rate performance diagram shows the vanadium redox flow battery assembled using the electrodes prepared in Example 4 of this invention.

[0028] Figure 4 To assemble an all-vanadium redox flow battery using the electrodes prepared in Example 4 of this invention at 200 mA cm⁻¹ -2 Long-cycle performance at current density. Detailed Implementation

[0029] To facilitate understanding of the present invention, the present invention will be described more fully and in detail below with reference to the accompanying drawings and preferred embodiments, but the scope of protection of the present invention is not limited to the following specific embodiments.

[0030] Unless otherwise defined, all technical terms used herein have the same meaning as commonly understood by those skilled in the art. The technical terms used herein are for the purpose of describing particular embodiments only and are not intended to limit the scope of the invention.

[0031] 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.

[0032] Example 1:

[0033] A method for preparing a modified electrode for an all-vanadium redox flow battery according to the present invention includes the following steps:

[0034] (1) Immerse the PAN-based graphite felt electrode in an ammonium carbonate solution until the ammonium salt solution is completely absorbed by the graphite felt electrode. The solvent in the ammonium carbonate solution is distilled water, the surfactant is Tween 40, the concentration of ammonium carbonate is 1.27 mol / L, the total volume of the solvent is fixed at 10 mL, the mass ratio of distilled water to Tween 40 is 1200:1, the thickness of the PAN-based graphite felt electrode is 4.1 mm, and the length and width are 6 cm × 4 cm.

[0035] (2) Place the PAN-based graphite felt filled with ammonium carbonate solution in step (1) into a quartz ceramic boat and place it into one side of a quartz tube. When the tube furnace is heated to 650 °C, push the graphite felt into the heat preservation zone for high-temperature activation for 10 min. The atmosphere in the tube furnace is an air atmosphere. After high-temperature activation, a heteroatom-doped high-activity graphite felt electrode, i.e., a modified electrode, can be obtained.

[0036] The modified electrode was used as the negative electrode in a vanadium redox flow battery, and its charge-discharge performance was tested. The positive electrode used untreated virgin graphite felt, and the separator was Nafion 212. Both the positive and negative electrode electrolytes were 1.7 M V₂. 3+ +VO 2+ +4.3 M H₂SO₄, each with a volume of 45 mL. The charging cutoff voltage is 1.65 V, and the discharging cutoff voltage is 0.8 V. Test results show that at 200 mAcm⁻¹... -2 At the current density, the energy efficiency of the all-vanadium redox flow battery composed of the modified electrode of this embodiment is 77.47%.

[0037] Example 2:

[0038] The preparation method of the modified electrode in this embodiment is the same as that in Example 1, except that the solute in the solution for soaking the PAN-based graphite felt is ammonium sulfate, and the concentration of ammonium sulfate is 0.1 mol / L. Other processes and conditions are the same as in Example 1.

[0039] A full vanadium redox flow battery was assembled using the same method as in Example 1, and its electrochemical performance was tested in the same manner. The test results showed that at 200 mA cm⁻¹... -2 At the current density, the energy efficiency of the vanadium battery assembled using the electrode prepared in this embodiment as the negative electrode is 75.62%.

[0040] Example 3:

[0041] The preparation method of the modified electrode in this embodiment is the same as that in Example 1, except that the solute in the solution for soaking the PAN-based graphite felt is ammonium sulfate, and the concentration of ammonium sulfate is 0.5 mol / L. Other processes and conditions are the same as in Example 1.

[0042] A full vanadium redox flow battery was assembled using the same method as in Example 1, and its electrochemical performance was tested in the same manner. The test results showed that at 200 mA cm⁻¹... -2 At the current density, the energy efficiency of the vanadium battery assembled with the electrode prepared in this embodiment as the negative electrode is 78.53%.

[0043] Example 4:

[0044] The preparation method of the modified electrode in this embodiment is the same as that in Example 1, except that the solute in the solution for soaking PAN-based graphite felt is ammonium sulfate, and the concentration of ammonium sulfate is 1.27 mol / L. Other processes and conditions are the same as in Example 1.

[0045] A full vanadium redox flow battery was assembled using the same method as in Example 1, and its electrochemical performance was tested in the same manner. The test results showed that at 200 mA cm⁻¹... -2 At the current density, the energy efficiency of the vanadium battery assembled with the electrode prepared in this embodiment as the negative electrode is 80.17%.

[0046] Example 5:

[0047] The preparation method of the modified electrode in this embodiment is the same as that in Example 1, except that the solute in the solution for soaking PAN-based graphite felt is ammonium nitrate, and the concentration of ammonium nitrate is 1.27 mol / L. Other processes and conditions are the same as in Example 1.

[0048] A full vanadium redox flow battery was assembled using the same method as in Example 1, and its electrochemical performance was tested in the same manner. The test results showed that at 200 mA cm⁻¹... -2 At the current density, the energy efficiency of the vanadium battery assembled with the electrode prepared in this embodiment as the negative electrode is 76.80%.

[0049] Example 6:

[0050] The preparation method of the modified electrode in this embodiment is the same as that in Example 4, except that the activation temperature is 850°C and the activation time is 7 min. Other processes and conditions are the same as in Example 4.

[0051] A full vanadium redox flow battery was assembled using the same method as in Example 1, and its electrochemical performance was tested in the same manner. The test results showed that at 200 mA cm⁻¹... -2 At the current density, the energy efficiency of the vanadium battery assembled using the electrode prepared in this embodiment as the negative electrode is 78.4%.

[0052] Example 7:

[0053] The preparation method of the modified electrode in this embodiment is the same as that in Example 4, except that the activation temperature is 950°C and the activation time is 5 min. Other processes and conditions are the same as in Example 4.

[0054] A full vanadium redox flow battery was assembled using the same method as in Example 1, and its electrochemical performance was tested in the same manner. The test results showed that at 200 mA cm⁻¹... -2 At the current density, the energy efficiency of the vanadium battery assembled using the electrode prepared in this embodiment as the negative electrode is 78.14%.

[0055] Comparative Example 1:

[0056] This comparative example directly uses untreated PAN-based graphite felt electrodes as the positive and negative electrodes of the all-vanadium redox flow battery. The rest of the battery assembly and testing methods are the same as in Example 1. The test results show that at 200 mA cm⁻¹ -2 At the given current density, the energy efficiency of this vanadium redox flow battery is 67.35%.

[0057] Comparative Example 2:

[0058] The modified electrode in this comparative example was prepared by activating a PAN-based graphite felt electrode at 650 °C in air for 10 min to obtain the modified electrode.

[0059] The modified electrode was used as the negative electrode in a vanadium battery, while the positive electrode was an untreated graphite felt electrode. The rest of the battery assembly and testing methods were the same as in Example 1. Test results showed that the graphite felt electrode used in this comparative example served as the negative electrode, and the vanadium battery achieved a performance of 200 mA cm⁻¹. -2 The energy efficiency at current density is 70.9%.

[0060] Comparative Example 3:

[0061] The modified electrode of this comparative example was prepared as follows: PAN-based graphite felt electrode was placed in a 50 mL reaction vessel, 15 mL of 25% ammonia water was measured and fully moistened, and then placed in an oven at 180 °C for 15 h to keep it completely warm. After cooling, it was washed with distilled water until neutral and then dried in a vacuum chamber at 110 °C to obtain the modified electrode.

[0062] The modified electrode was used as the negative electrode in a vanadium battery, while the positive electrode was an untreated graphite felt electrode. The rest of the battery assembly and testing methods were the same as in Example 1. Test results showed that the vanadium battery prepared using the graphite felt electrode treated in this comparative example as the negative electrode performed well at 200 mA cm⁻¹. -2 The energy efficiency at current density is 69.68%.

[0063] Comparative Example 4:

[0064] The modified electrode of this comparative example was prepared as follows: 2.5 mmol Co(NO3)·6H2O and 20 mmol 2-methylimidazole were dissolved in 50 mL methanol, respectively. After vigorous stirring for 5 min, the two solutions were mixed and stirred for another 30 min to obtain a ZIF-67 solution. Graphite felt was then immersed in this solution and allowed to stand at room temperature for 18 h, followed by drying in a 60 °C oven for 24 h. Subsequently, the dried graphite felt was calcined at 800 °C for 4 h in an argon atmosphere. Finally, the calcined graphite felt was washed and dried with a mixed solution of hydrochloric acid and FeCl3 to obtain the modified electrode.

[0065] The modified electrode was used as the negative electrode of the vanadium battery, and the positive electrode was an untreated graphite felt electrode. The rest of the battery assembly and testing methods were the same as in Example 1.

[0066] Test results show that the graphite felt electrode used in this comparative example is the negative electrode, and the vanadium battery operates at 200 mA cm⁻¹. -2 The energy efficiency at current density is 74.35%.

[0067] Figure 1 The images shown are electron microscope images of the electrodes prepared in Comparative Examples 1, 2 and Example 4 of the present invention. The modified graphite felt electrodes in Comparative Examples 1 and 2 have smooth surfaces with almost no morphological changes; while the electrode in Example 4 has obvious porous structures on its surface. It is essentially a heteroatom-doped electrode based on ammonium salt modification prepared by the method of the present invention, which forms a large number of pores on the electrode surface, increasing the specific surface area of ​​the electrode and providing abundant active sites.

[0068] like Figure 2 As shown, the electrodes prepared in Example 4 and Comparative Example 1 of this invention are subjected to a temperature of 300 mA cm⁻¹. -2 The charge-discharge curves at current density show that the charge-discharge capacity in Example 1 reached 1400 mA h, while that in Comparative Example 1 was only about 750 mA h. The vanadium redox flow battery assembled with the electrode prepared in Example 4, compared to the electrode in Comparative Example 1, has a lower charging voltage and a higher discharging voltage, as well as a higher capacity, indicating more efficient utilization of the electrolyte. Compared to the graphite felt treated in Comparative Example 1, the modified electrode in Example 1 has an electrolyte utilization rate of 68.3%, which is 30.5% higher than that of the heat-treated graphite felt electrode in Comparative Example 1.

[0069] like Figure 3 As shown, the rate performance diagram of the all-vanadium redox flow battery assembled with electrodes prepared in Example 4 of this invention is shown. Figure 3 It can be seen that the vanadium redox flow battery assembled with the electrode prepared in Example 4 has excellent battery performance.

[0070] To test the long-cycle performance of this material at high current densities, the electrode material prepared in Example 4 was used as the negative electrode, and untreated raw graphite felt was used as the positive electrode. The cycle performance was tested at 200 mA cm⁻¹. -2 After 400 charge-discharge cycles at a current density, the results are as follows: Figure 4 As shown. By Figure 4 As can be seen, the battery's efficiency has not decreased significantly after long-term cycling, demonstrating excellent stability.

[0071] In summary, the preparation method of this invention is economical and effective, and the prepared highly active electrode material possesses excellent hydrophilicity, conductivity, stable mechanical properties, and catalytic performance. This invention achieves heteroatom-doped electrodes through rapid heat treatment, which not only enriches the number of active sites and increases the reaction surface area, but also results in a stable structure and excellent performance. It significantly reduces the polarization of the electrode surface, enabling the flow battery to operate stably at higher current densities, thus improving its energy efficiency and power density, and ultimately enhancing the performance of vanadium redox flow batteries in practical applications.

Claims

1. A method for preparing a modified electrode for an all-vanadium redox flow battery, characterized in that, Includes the following steps: (1) Immerse the graphite felt electrode in an ammonium salt solution; wherein the ammonium salt solution comprises an ammonium salt, a surfactant, and water, and the surfactant comprises at least one of Tween 20, Tween 40, Span 20, Span 40, and sodium dodecyl sulfate. (2) The graphite felt electrode soaked in ammonium salt in step (1) is placed into the quartz tube of a tubular furnace for high-temperature activation to obtain a modified electrode. Specifically, the high-temperature activation includes first placing the graphite felt electrode soaked in ammonium salt into the non-insulated area on one side of the quartz tube of the tubular furnace, and after heating the tubular furnace to 500-1000℃, pushing the graphite felt electrode soaked in ammonium salt into the insulation area for 5-60 minutes. The high-temperature activation is carried out in an air atmosphere.

2. The preparation method according to claim 1, characterized in that, In step (1), the concentration of ammonium salt is 0.1-1.5 mol / L, and the mass ratio of surfactant to water is 1:500-1:2000.

3. The preparation method according to claim 2, characterized in that, The ammonium salt is a soluble ammonium salt, including at least one of ammonium sulfate, ammonium carbonate, ammonium chloride, ammonium nitrate, ammonium phosphate, ammonium fluoride, ammonium bromide, ferrous ammonium sulfate, ammonium molybdate, ammonium tungstate, dodecyltrimethylammonium chloride, tetraethylammonium bromide, and ammonium acetate.

4. The preparation method according to claim 1, characterized in that, In step (1), the graphite felt electrode is a polyacrylonitrile-based graphite felt, pitch-based graphite felt, or adhesive-based graphite felt.

5. A vanadium redox flow battery, characterized in that, This includes the modified electrode prepared by the preparation method as described in any one of claims 1 to 4.

Images