A nitrogen-doped porous carbon felt material and its application in bromine-based flow batteries

Abstract

The present invention relates to a positive electrode material for a bromine-based flow battery, wherein the positive electrode material is nitrogen-doped porous carbon felt, and the doping amount of nitrogen in the carbon fiber of the porous carbon felt is 2wt%-8wt%; the carbon fiber constituting the carbon felt has a porous structure, a pore size of 0.1nm-30nm, and a pore volume of 0.001-0.008cm3 / g. The porous structure increases the pore volume, specific surface area, and hydrophilicity of the carbon felt, can enhance the electrolyte wettability and adsorption capacity for bromine, provide more reactive sites, thereby improving the reaction kinetics of the bromine electrode pair and inhibiting bromine diffusion. Ultimately, the purpose of reducing battery polarization and improving battery efficiency, capacity, and life is achieved. This type of material has the advantages of simple preparation method, good conductivity, small electrochemical polarization, good chemical stability, etc., and the bromine-based flow battery assembled with this material as the positive electrode of the battery has high voltage efficiency, energy efficiency, and cycle stability.

Description

Technical Field

[0001] The present invention relates to the field of liquid flow battery electrode materials, and in particular to application of a nitrogen-doped porous carbon felt material in bromine-based batteries. Technical Background

[0002] To meet people's dual demands for energy and the environment, vigorously developing and utilizing renewable energy sources such as wind and solar energy has become a very effective way to meet people's needs. However, these energy sources are discontinuous and unstable, making them difficult to connect directly to the grid, resulting in high rates of wind and solar power curtailment and waste of resources. Therefore, they need to be combined with efficient energy storage technologies to ensure smooth power output. Energy storage technology improves the utilization rate and stability of renewable energy through the storage and release of energy, and is a key technology for the development and utilization of new energy. Liquid flow batteries are an electrochemical energy storage technology suitable for large-scale energy storage. Their characteristic is that energy is stored in an electrolyte, achieving the independence of power and energy.

[0003] Bromine redox couples are often used as positive active couples in batteries due to their advantages of high electrode potential, high solubility, abundant sources and low price. A battery can be formed by selecting a suitable negative electrode couple to pair with them. This type of battery with bromine couples as positive active couples is called bromine-based flow battery, such as zinc-bromine flow battery, sodium polysulfide / bromine flow battery, hydrogen-bromine flow battery and vanadium-bromine flow battery. Recently, a variety of new bromine-based flow batteries have been proposed, such as quinone / bromine battery, lithium / bromine battery and magnesium / bromine battery. However, bromine-based flow batteries also face some common problems, which can be divided into the following three aspects: (1) The strong corrosiveness and oxidizing properties of Br2 place high demands on the chemical stability of the materials of various battery components; (2) The strong volatility and diffusivity of Br2 will reduce the safety and life of the battery. Reducing the volatilization of Br2 into the environment will pollute the environment and may cause safety problems; on the other hand, it will lead to a reduction in battery active materials, reducing the capacity, efficiency and service life of the battery. When Br2 diffuses to the negative electrode, it often reacts chemically with the negative electrode active material, causing the battery to self-discharge and reduce battery efficiency. (3) Br2 / Br - The reactivity of the electrode pair is relatively low, resulting in large electrochemical polarization of the battery, low battery power density, and increased battery cost. Summary of the Invention

[0004] In view of the above problems, the present invention improves the Br2 / Br -The reactivity of the electrode pair increases the adsorption of bromine, inhibiting the diffusion of bromine, thereby reducing battery polarization and self-discharge. The method is simple to operate, and the prepared material has the characteristics of good conductivity, strong electrolyte permeability, and good chemical stability. The bromine-based flow battery assembled with the prepared material has high voltage efficiency, energy efficiency, cycle stability and power density.

[0005] To achieve the above purpose, the specific technical solutions of the present invention are as follows:

[0006] On the one hand, the present invention provides a positive electrode material for a bromine-based flow battery, wherein the positive electrode material is a nitrogen-doped porous carbon felt, wherein the nitrogen doping amount in the carbon fibers of the porous carbon felt is 2wt%-8wt%; and the carbon fibers constituting the carbon felt have a porous structure with a pore diameter of 0.1nm-30nm and a pore volume of 0.001-0.008cm 3 / g.

[0007] The “porous” in the nitrogen-doped porous carbon felt material does not refer to a three-dimensional pore structure formed by interweaving carbon felt fibers, but refers to a porous structure formed by pores on the carbon fibers of the carbon felt.

[0008] Based on the above solution, preferably, the nitrogen doping amount in the carbon fibers of the porous carbon felt is 4 wt % to 6 wt %.

[0009] Based on the above scheme, preferably, the pore volume of the carbon felt is 0.001-0.01cm 3 / g.

[0010] On the other hand, the present invention provides a method for preparing the nitrogen-doped porous carbon felt material, the preparation process of which is as follows:

[0011] (1) dissolving dopamine in water and stirring to obtain a uniform dopamine aqueous solution, wherein the concentration of the dopamine aqueous solution is 100-500 mg / 100 mL of water;

[0012] (2) soaking the original carbon felt material in the dopamine aqueous solution prepared in step (1), stirring, so that the dopamine aqueous solution fully permeates the original carbon felt material;

[0013] (3) adding a tris(hydroxymethyl)aminomethane (Tris) solution to the dopamine aqueous solution containing carbon felt prepared in step (2) and stirring to ensure that dopamine is completely self-polymerized on the carbon fibers of the carbon felt; wherein the molar concentration of the Tris solution is 5-20 mM; and the volume ratio of the tris(hydroxymethyl)aminomethane (Tris) solution to the dopamine solution is 1×10 -4 -9×10 -4 ;

[0014] (4) The carbon felt treated in step (3) is taken out and dried, and then calcined at 800-1200° C. in a N 2 atmosphere for 2-5 h to obtain the positive electrode material.

[0015] Based on the above scheme, preferably, in step (1), the stirring time is greater than 2 hours; in step (2), the stirring time is greater than 0.5 hours; in step (3), the stirring time is greater than 24 hours.

[0016] Based on the above scheme, preferably, in step (4), the drying temperature is 40-80°C, the drying time is more than 12 hours, the calcination temperature is 900-1000°C, and the calcination time is 2-4 hours.

[0017] Based on the above solution, preferably, the original carbon felt material is a commercial carbon felt material with a thickness of 3-6 mm and a porosity of 90-98%.

[0018] The nitrogen-doped porous electrode material prepared by the present invention is used in bromine-based flow batteries, including but not limited to hydrogen-bromine flow batteries, vanadium-bromine flow batteries, sodium polysulfide / / bromine flow batteries, zinc-bromine flow batteries, quinone-bromine flow batteries, lithium-bromine flow batteries or magnesium / bromine flow batteries, etc.

[0019] Beneficial effects of the present invention:

[0020] (1) Constructing a porous structure on carbon felt fibers can increase the pore volume and roughness of the original carbon felt material, thereby increasing the specific surface area and hydrophilicity of the electrode, and further improving the electrolyte wettability and bromine adsorption capacity of the electrode. Bromine adsorption is Br2 / Br - The rate-controlling step of the redox reaction is the electrochemical reaction, and the improvement of bromine adsorption capacity can effectively promote the Br2 / Br - The improvement of electrolyte wettability can promote the reduction of active material transmission resistance and accelerate the Br2 / Br - Reaction kinetics of the electrode pair.

[0021] (2) Nitrogen doping can make the carbon felt fiber have nitrogen-containing groups, which have high electronegativity, stronger bromine adsorption capacity and hydrophilicity. Therefore, nitrogen doping can further increase the Br2 / Br - Reactivity and reaction kinetics of electrode pairs.

[0022] (3) The improvement of the electrode's adsorption capacity for bromine can inhibit the diffusion of bromine, reduce the occurrence of battery self-discharge reaction, and improve the battery's capacity retention rate.

[0023] (4) The type of nitrogen-containing groups is mainly graphitic nitrogen, which can improve the electronic conductivity of the material.

[0024] (5) This type of material has the characteristics of simple preparation method, good conductivity, good electrolyte wettability and good chemical stability. Therefore, the bromine-based flow battery assembled with this material has high voltage efficiency, energy efficiency and cycle stability. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 This is the surface morphology of electrode A1 in Example 1;

[0026] Figure 2 : is the cycle performance diagram of electrode A1 in Example 1;

[0027] Figure 3 This is the surface morphology of electrode a1 in comparative example 1. DETAILED DESCRIPTION

[0028] The following examples are provided to further illustrate the present invention and are not intended to limit the scope of the present invention. The original carbon felt used in the following examples and comparative examples has a thickness of 5 mm and a porosity of 95%.

[0029] Example 1

[0030] (1) Dissolve 0.2 g of dopamine in 100 mL of water and stir for 3 h to prepare a uniform dopamine aqueous solution;

[0031] (2) soaking the original carbon felt material in the dopamine aqueous solution prepared in step (1) and stirring for 1 hour to allow the dopamine solution to fully penetrate the original carbon felt material;

[0032] (3) Add 75 μL of Tris solution to the dopamine solution containing carbon felt prepared in step (2), wherein the molar concentration of the Tris solution is 10 mM;

[0033] (4) The solution obtained in step (3) was stirred for 30 h to ensure that dopamine was completely self-polymerized onto the carbon felt fibers. The carbon felt was then removed and dried at 60° C. for 24 h. It was then calcined at 900° C. in a nitrogen atmosphere for 3 h. A nitrogen-doped porous carbon felt material (defined as electrode A1) was prepared.

[0034] The electrode A1 was characterized and its morphology was as follows Figure 1 As shown in the figure, there are a large number of pores distributed on the carbon felt fibers, with a pore diameter of 0.1-30 nm, a nitrogen mass content of 2.5%, and a pore volume of 0.0015 cm 3 / g.

[0035] Electrode A1 was used as the positive electrode and the original carbon felt material was used as the positive electrode to assemble the zinc-bromine flow battery, wherein the catalyst layer was activated carbon felt, the bipolar plate was a graphite plate, and the membrane effective area was 9 cm 2 , the current density is 80mA cm- 2The electrolyte composition is: 2 mol / L zinc bromide + 3 mol / L potassium chloride + 0.4 mol / L N,N-methylethylpyrrolidine bromide (complexing agent). The charging process adopts the constant capacity charging mode with a charging time of 0.5 hours; the discharge process adopts the voltage cutoff mode with a cutoff voltage of 0.8V. The coulombic efficiency of the zinc-bromine flow battery assembled with electrode A1 as the positive electrode is 99.73%, which is higher than the coulombic efficiency of the zinc-bromine flow battery assembled with the original carbon felt material as the positive electrode (97.62%). This is because the porous structure and highly electronegative nitrogen-containing groups of the electrode enhance the adsorption capacity of bromine, effectively inhibit the diffusion of bromine, and reduce the self-discharge reaction of the battery. The voltage efficiency reached 88.46%, significantly higher than the voltage efficiency of a zinc-bromine flow battery assembled with pristine carbon felt as the positive electrode (78.63%). This is because the porous structure of the carbon fibers in the nitrogen-doped porous carbon felt increases the pore volume and specific surface area of ​​the pristine carbon felt, providing more active sites for the redox reaction of the Br2 / Br- pair. Furthermore, the porous structure and nitrogen doping enhance the electrode's hydrophilicity, promoting electrolyte penetration into the carbon felt and thus enhancing the reaction kinetics of the Br2 / Br- pair. Furthermore, the enhanced bromine adsorption capacity of the electrode effectively promotes the reaction kinetics of the Br2 / Br- pair. These advantages of nitrogen-doped porous carbon felt can reduce battery polarization and improve the voltage efficiency of the zinc-bromine flow battery. Therefore, the energy efficiency of the zinc-bromine flow battery assembled with electrode A1 as the positive electrode reached 88.22%, significantly higher than the energy efficiency of the zinc-bromine flow battery assembled with pristine carbon felt as the positive electrode (76.76%) (Table 1). This demonstrates that the performance of electrode A1 as a positive electrode is superior to that of the pristine carbon felt. The zinc-bromine flow battery assembled with electrode A1 was tested for self-discharge (the battery was first -2 The charge and discharge cycles were carried out at a current density of 150 mA cm -2 The capacity retention rate after charging for 0.5h at a current density of , then discharging after standing for 24h with a cut-off voltage of 0.8V is 70%, which is significantly higher than the capacity retention rate (30%) of the zinc-bromine flow battery assembled with the original carbon felt after the self-discharge test, indicating that electrode A1 can indeed effectively inhibit the diffusion of bromine, reduce the self-discharge reaction of a single cell, and improve the capacity retention rate. In addition, the zinc-bromine flow battery assembled with electrode A1 can operate continuously and stably for more than 300 cycles without obvious performance degradation, showing excellent stability, and the capacity retention rate after 300 cycles is 98%, which is higher than the capacity retention rate (70%) of the zinc-bromine flow battery assembled with the original carbon felt as the positive electrode after 300 cycles, showing that electrode A1 has a better ability to inhibit bromine diffusion ( Figure 2 ).

[0036] Example 2

[0037] A nitrogen-doped porous carbon felt electrode material (defined as electrode A2) was prepared according to the method (process and conditions) described in Example 1, except that the calcination temperature was 1100°C.

[0038] Electrode A2 was characterized. The pore size of the carbon felt fiber was 0.1-30 nm, the N mass content was 2.5%, and the pore volume was 0.0021 cm 3 / g.

[0039] A zinc-bromine flow battery was assembled using electrode A2 as the positive electrode and the original carbon felt material as the positive electrode, wherein the catalyst layer was activated carbon felt, the bipolar plate was a graphite plate, and the membrane effective area was 9 cm 2 , the current density is 80 mA cm -2 , the electrolyte composition is: 2mol / L zinc bromide + 3mol / L potassium chloride + 0.4mol / L N,N-methylethylpyrrolidine bromide (complexing agent). The charging process adopts constant capacity charging mode, and the charging time is 0.5h; the discharge process adopts voltage cutoff mode, and the cutoff voltage is 0.8V. The coulombic efficiency of the zinc-bromine flow battery assembled with electrode A2 as the positive electrode is 99.81%, which is higher than the coulombic efficiency of the zinc-bromine flow battery assembled with the original carbon felt material as the positive electrode; the voltage efficiency is 88.35%, which is significantly higher than the voltage efficiency of the zinc-bromine flow battery assembled with the original carbon felt material as the positive electrode; the energy efficiency is 88.00%, which is higher than the energy efficiency of the zinc-bromine flow battery assembled with the original carbon felt material as the positive electrode (Table 1). This shows that the performance of electrode A2 as a positive electrode is better than that of the original carbon felt as a positive electrode. And the zinc-bromine flow battery assembled with electrode A2 has a good self-discharge test (the battery is first discharged at 80mA cm -2 The charge and discharge cycles were carried out at a current density of 15 mA cm -2 The capacity retention rate after charging for 0.5h at a current density of 1000 nm and then discharging after standing for 24h with a cut-off voltage of 0.8V is 69%, which is significantly higher than the capacity retention rate of the zinc-bromine flow battery assembled with the original carbon felt after the self-discharge test, indicating that electrode A2 can indeed effectively inhibit the diffusion of bromine, reduce the self-discharge reaction of the single cell, and improve the capacity retention rate. In addition, the zinc-bromine flow battery assembled with electrode A2 can operate continuously and stably for more than 250 cycles without obvious performance degradation, showing excellent stability, and the capacity retention rate after 250 cycles is 96%, which is higher than the capacity retention rate (72%) of the zinc-bromine flow battery assembled with the original carbon felt as the positive electrode after 250 cycles. The reason why its cycle performance and capacity retention rate are weaker than the cycle performance of the zinc-bromine flow battery assembled with electrode A1 as the positive electrode is that the higher calcination temperature increases the electrode pore volume and specific surface area and has a certain impact on the stability of the carbon felt itself, but it can still show excellent performance (Table 2).

[0040] Example 3

[0041] A nitrogen-doped porous carbon felt electrode material (defined as electrode A3) was prepared according to the method (process and conditions) described in Example 1, except that the calcination time was 4 h.

[0042] Electrode A3 was characterized. The pore size of the carbon felt fiber was 0.1-30 nm, the N mass content was 2.5%, and the pore volume was 0.0015 cm 3 / g.

[0043] The zinc-bromine flow battery was assembled using electrode A3 as the positive electrode and the original carbon felt material as the positive electrode, wherein the catalyst layer was activated carbon felt, the bipolar plate was a graphite plate, and the membrane effective area was 9cm. 2 , the current density is 80 mA cm -2 , the electrolyte composition is: 2mol / L zinc bromide + 3mol / L potassium chloride + 0.4mol / L N,N-methylethylpyrrolidine bromide (complexing agent). The charging process adopts constant capacity charging mode, and the charging time is 0.5h; the discharge process adopts voltage cutoff mode, and the cutoff voltage is 0.8V. The coulombic efficiency of the zinc-bromine flow battery assembled with electrode A3 as the positive electrode is 99.54%, which is higher than the coulombic efficiency of the zinc-bromine flow battery assembled with the original carbon felt material as the positive electrode; the voltage efficiency is 88.21%, which is significantly higher than the voltage efficiency of the zinc-bromine flow battery assembled with the original carbon felt material as the positive electrode; the energy efficiency is 87.80%, which is higher than the energy efficiency of the zinc-bromine flow battery assembled with the original carbon felt material as the positive electrode (Table 1). This shows that the performance of electrode A3 as a positive electrode is better than that of the original carbon felt as a positive electrode. And the zinc-bromine flow battery assembled with electrode A3 has a good self-discharge test (the battery is first discharged at 80mA cm -2 The charge and discharge cycles were carried out at a current density of 15 mA cm -2 After charging at a current density of 100 nm for 0.5 h and then discharging at a cutoff voltage of 0.8 V for 24 h, the capacity retention rate was 69%, significantly higher than the 30% capacity retention rate of the zinc-bromine flow battery assembled with pristine carbon felt after self-discharge testing. This indicates that electrode A3 effectively inhibits bromine diffusion, reduces the self-discharge reaction within a single cell, and improves capacity retention. Furthermore, the zinc-bromine flow battery assembled with electrode A3 maintained stable operation for over 250 cycles without significant performance degradation, demonstrating excellent stability. The capacity retention rate after 250 cycles was 96%, exceeding the 72% capacity retention rate of the zinc-bromine flow battery assembled with pristine carbon felt as the positive electrode. The reason for this weaker cycling performance and capacity retention than that of the zinc-bromine flow battery assembled with electrode A1 is that prolonged high-temperature calcination does not increase the nitrogen content, pore volume, or specific surface area of ​​the electrode, but does affect the stability of the carbon felt itself. Despite this, the battery still exhibited excellent performance (Table 2).

[0044] Comparative Example 1

[0045] A nitrogen-doped porous carbon felt electrode material (defined as electrode a1) was prepared according to the method (process and conditions) described in Example 1, except that the calcination temperature was 700°C.

[0046] The electrode a1 was characterized and its morphology was as follows Figure 3 As shown, there are no holes in the carbon felt fibers, and the N content thereof is 2.5% by mass.

[0047] A zinc-bromine flow battery was assembled using electrode a1 as the positive electrode and the original carbon felt material as the positive electrode, wherein the catalyst layer was activated carbon felt, the bipolar plate was a graphite plate, and the membrane effective area was 9 cm 2 , the current density is 80 mA cm -2 The electrolyte composition is: 2 mol / L zinc bromide + 3 mol / L potassium chloride + 0.4 mol / L N,N-methylethylpyrrolidine bromide (complexing agent). The charging process adopts the constant capacity charging mode with a charging time of 0.5h; the discharge process adopts the voltage cutoff mode with a cutoff voltage of 0.8V. The coulombic efficiency of the zinc-bromine flow battery assembled with electrode a1 as the positive electrode is 98.15%, which is higher than the coulombic efficiency of the zinc-bromine flow battery assembled with electrode A1 as the positive electrode. This is because electrode a1 does not have a porous structure, has a low specific surface area, has a low adsorption capacity for bromine, and has a weaker ability to inhibit bromine diffusion than electrode A1. However, the presence of highly electronegative nitrogen-containing groups improves the adsorption capacity for bromine, making its ability to inhibit bromine diffusion greater than that of the original carbon felt. The voltage efficiency was 79.27%, higher than that of a zinc-bromine flow battery assembled with pristine carbon felt as the positive electrode, but lower than that of a zinc-bromine flow battery assembled with electrode A1 as the positive electrode. This is because, although electrode a1 lacks a porous structure, it still contains nitrogen-containing groups with high electronegativity and excellent catalytic ability, which can promote the reaction kinetics of the bromine electrode pair and enhance its reactivity. Furthermore, the generation of graphitic nitrogen increases the electronic conductivity of the carbon felt. However, compared with the generation of a porous structure on carbon fibers, the effect of nitrogen doping is weaker (Table 2). The energy efficiency of the zinc-bromine flow battery assembled with electrode a1 as the positive electrode was 77.80%, higher than that of a zinc-bromine flow battery assembled with pristine carbon felt as the positive electrode, but lower than that of a zinc-bromine flow battery assembled with electrode A1 as the positive electrode (Table 1). This indicates that the performance of electrode a1 as the positive electrode is better than that of the pristine carbon felt as the positive electrode, but worse than that of electrode A1. Furthermore, the zinc-bromine flow battery assembled with electrode a1 exhibited a high self-discharge rate (the battery was first charged at 80 mA cm -2 The charge and discharge cycles were carried out at a current density of 15 mA cm -2The capacity retention rate after charging for 0.5h at a current density of 1000 nm and then discharging after standing for 24h with a cut-off voltage of 0.8V is 32%, which is slightly higher than the capacity retention rate (30%) of the zinc-bromine flow battery assembled with the original carbon felt after the self-discharge test, indicating that electrode a1 can also effectively inhibit the diffusion of bromine, but the effect is very weak. In addition, the capacity retention rate of the zinc-bromine flow battery assembled with electrode a1 as the positive electrode after 300 cycles is only 72%, which is similar to the capacity retention rate of the zinc-bromine flow battery assembled with the original carbon felt as the positive electrode, but lower than the capacity retention rate of the zinc-bromine flow battery assembled with electrode A1 as the positive electrode. This is because there is no porous structure generated, the electrode's adsorption capacity for bromine is reduced, the self-discharge reaction of the single cell is serious, and the capacity retention rate is low (Table 2).

[0048] Comparative Example 2

[0049] A nitrogen-doped porous carbon felt electrode material (defined as electrode a2) was prepared according to the method (process and conditions) described in Example 1, except that the calcination temperature was 1300°C.

[0050] Electrode a2 was characterized. The pore size of the carbon felt fiber was 0.1-50 nm, the N mass content was 2.5%, and the pore volume was 0.012 cm 3 / g.

[0051] A zinc-bromine flow battery was assembled using electrode a2 as the positive electrode and the original carbon felt material as the positive electrode, wherein the catalyst layer was activated carbon felt, the bipolar plate was a graphite plate, and the membrane effective area was 9 cm 2 , the current density is 80 mA cm -2The electrolyte composition is: 2mol / L zinc bromide + 3mol / L potassium chloride + 0.4mol / L N,N-methylethylpyrrolidine bromide (complexing agent). The charging process adopts the constant capacity charging mode with a charging time of 0.5h; the discharge process adopts the voltage cutoff mode with a cutoff voltage of 0.8V. The coulombic efficiency of the zinc-bromine flow battery assembled with electrode a2 as the positive electrode was 94.32%, lower than that of the zinc-bromine flow battery assembled with the pristine carbon felt material and electrode A1 as the positive electrode. This is because the calcination temperature was too high, and the intensified pore formation process resulted in excessively large pores, which caused the carbon fiber filaments to break and reduced electrode stability (Table 2). The voltage efficiency was 88.61%, higher than that of the zinc-bromine flow battery assembled with the pristine carbon felt material and electrode A1 as the positive electrode. This is because the larger pore volume and specific surface area of ​​electrode a2 provide more reactive sites, reducing battery polarization, and high temperature favors the formation of graphitic nitrogen, which improves the electronic conductivity of the electrode (Table 2). The energy efficiency was 83.58%, higher than that of the zinc-bromine flow battery assembled with the pristine carbon felt material as the positive electrode, but lower than that of the zinc-bromine flow battery assembled with electrode A1 as the positive electrode (Table 1). This shows that electrode a2 is superior to the pristine carbon felt as the positive electrode but inferior to electrode A1 as the positive electrode. The zinc-bromine flow battery assembled with electrode a2 was tested for self-discharge (the battery was first -2 The charge and discharge cycles were carried out at a current density of 150 mA cm -2 The capacity retention rate after charging at a current density of 0.5 h, then discharging after 24 h at a cutoff voltage of 0.8 V was 25%, lower than the capacity retention rate (30%) of the zinc-bromine flow battery assembled with the original carbon felt after self-discharge testing. In addition, the zinc-bromine flow battery assembled with a2 as the positive electrode can only operate continuously and stably for more than 50 cycles, indicating poor cycling performance.

[0052] Comparative Example 3

[0053] A nitrogen-doped porous carbon felt electrode material (defined as electrode a3) was prepared according to the method (process and conditions) described in Example 1, except that the calcination time was 1 h.

[0054] Electrode a3 was characterized. The pore size of the carbon felt fiber was 0.01-20 nm, the N mass content was 2.5%, and the pore volume was 0.0001 cm 3 / g.

[0055] A zinc-bromine flow battery was assembled using electrode a3 as the positive electrode and the original carbon felt material as the positive electrode, wherein the catalyst layer was activated carbon felt, the bipolar plate was a graphite plate, and the membrane effective area was 9 cm 2 , the current density is 80 mA cm -2The electrolyte composition is: 2mol / L zinc bromide + 3mol / L potassium chloride + 0.4mol / L N,N-methylethylpyrrolidine bromide (complexing agent). The charging process adopts the constant capacity charging mode with a charging time of 0.5h; the discharge process adopts the voltage cutoff mode with a cutoff voltage of 0.8V. The coulombic efficiency of the zinc-bromine flow battery assembled with electrode a3 as the positive electrode is 99.17%, which is higher than the coulombic efficiency of the zinc-bromine flow battery assembled with electrode A1 as the positive electrode; the voltage efficiency is 82.84%, which is higher than the original carbon felt material but lower than the voltage efficiency of the zinc-bromine flow battery assembled with electrode A1 as the positive electrode. This is because the calcination time is too short and too few pores are generated on the carbon felt fibers, resulting in a small pore volume and specific surface area of ​​electrode a3, weak adsorption capacity for bromine, and fewer reaction active sites, which reduces the electrode's ability to inhibit bromine diffusion and reduce battery polarization (Table 2); the energy efficiency of the zinc-bromine flow battery assembled with electrode a3 as the positive electrode is 82.15%, which is higher than the energy efficiency of the zinc-bromine flow battery assembled with the original carbon felt material as the positive electrode, but lower than the energy efficiency of the zinc-bromine flow battery assembled with electrode A1 as the positive electrode (Table 1), indicating that the performance of electrode a3 as the positive electrode is better than that of the original carbon felt as the positive electrode but worse than that of electrode A1 as the positive electrode.

[0056] Comparative Example 4

[0057] A nitrogen-doped porous carbon felt electrode material (defined as electrode a4) was prepared according to the method (process and conditions) described in Example 1, except that the calcination time was 5 h.

[0058] Electrode a4 was characterized. The pore size of the carbon felt fiber was 0.1-60 nm, the N mass content was 2.5%, and the pore volume was 0.0015 cm 3 / g.

[0059] A zinc-bromine flow battery was assembled using electrode a4 as the positive electrode and the original carbon felt material as the positive electrode, wherein the catalyst layer was activated carbon felt, the bipolar plate was a graphite plate, and the membrane effective area was 9 cm 2 , the current density is 80 mA cm -2The electrolyte composition is: 2mol / L zinc bromide + 3mol / L potassium chloride + 0.4mol / L N,N-methylethylpyrrolidine bromide (complexing agent). The charging process adopts the constant capacity charging mode with a charging time of 0.5h; the discharge process adopts the voltage cutoff mode with a cutoff voltage of 0.8V. The coulombic efficiency of the zinc-bromine flow battery assembled with electrode a4 as the positive electrode is 93.33%, which is lower than the coulombic efficiency of the zinc-bromine flow battery assembled with the original carbon felt material and electrode A1 as the positive electrode. Since the calcination time is too long, the crystallite size and interlayer spacing of the carbon fiber will change, causing the carbon fiber to break easily and reducing the stability of the electrode (Table 2); the voltage efficiency is 88.47%, which is higher than the voltage efficiency of the zinc-bromine flow battery assembled with the original carbon felt material as the positive electrode, and similar to the voltage efficiency of the zinc-bromine flow battery assembled with electrode A1 as the positive electrode. This is because a longer calcination time does not increase the pore volume of the electrode (Table 2); the energy efficiency is 82.60%, which is higher than the energy efficiency of the zinc-bromine flow battery assembled with the original carbon felt material as the positive electrode, but lower than the energy efficiency of the zinc-bromine flow battery assembled with electrode A1 as the positive electrode (Table 1), indicating that the performance of electrode a4 as the positive electrode is better than that of the original carbon felt as the positive electrode but worse than that of electrode A1 as the positive electrode. Moreover, the zinc-bromine flow battery assembled with a4 as the positive electrode can only operate continuously and stably for more than 50 cycles, and the cycle performance is poor.

[0060] Comparative Example 5

[0061] A nitrogen-doped porous carbon felt electrode material (defined as electrode a5) was prepared according to the method (process and conditions) described in Example 1, except that the mass fraction of dopamine was 0.

[0062] Electrode a5 was characterized. The pore size of the carbon felt fiber was 0.1-30 nm, the N mass content was 0, and the pore volume was 0.0015 cm 3 / g.

[0063] A zinc-bromine flow battery was assembled using electrode a5 as the positive electrode and the original carbon felt material as the positive electrode, wherein the catalyst layer was activated carbon felt, the bipolar plate was a graphite plate, and the membrane effective area was 9 cm 2 , the current density is 80 mA cm -2The electrolyte composition is: 2mol / L zinc bromide + 3mol / L potassium chloride + 0.4mol / L N,N-methylethylpyrrolidine bromide (complexing agent). The charging process adopts the constant capacity charging mode with a charging time of 0.5h; the discharge process adopts the voltage cutoff mode with a cutoff voltage of 0.8V. The coulombic efficiency of the zinc-bromine flow battery assembled with electrode a5 as the positive electrode is 98.23%, which is higher than that of the original carbon felt material but lower than that of the zinc-bromine flow battery assembled with electrode A1 as the positive electrode. This is because the lack of nitrogen-containing groups reduces the electrode's adsorption capacity for bromine and reduces the effect of inhibiting bromine diffusion; the voltage efficiency is 77.96%, which is lower than the voltage efficiency of the zinc-bromine flow battery assembled with the original carbon felt material and electrode A1 as the positive electrode. This is because the porous structure of the carbon fiber on electrode a5 reduces its electronic conductivity and increases the battery polarization (Table 2); the energy efficiency is 76.58%, which is lower than the energy efficiency of the zinc-bromine flow battery assembled with the original carbon felt material and electrode A1 as the positive electrode (Table 1), indicating that the performance of electrode a5 as the positive electrode is better than that of the original carbon felt as the positive electrode but worse than that of electrode A1 as the positive electrode.

[0064] Comparative Example 6

[0065] A nitrogen-doped porous carbon felt electrode material was prepared according to the method (process and conditions) described in Example 1, except that the carbon felt dried at 60° C. for 24 h was directly used as the positive electrode (defined as electrode a6).

[0066] Electrode a6 was characterized and found to have no pores on the carbon felt fibers and a nitrogen content of 2.5% by mass.

[0067] A zinc-bromine flow battery was assembled using electrode a6 as the positive electrode and the original carbon felt material as the positive electrode, wherein the catalyst layer was activated carbon felt, the bipolar plate was a graphite plate, and the membrane effective area was 9 cm 2 , the current density is 80 mA cm -2The electrolyte composition is: 2 mol / L zinc bromide + 3 mol / L potassium chloride + 0.4 mol / L N,N-methylethylpyrrolidine bromide (complexing agent). The charging process uses a constant-capacity charging mode with a charging time of 0.5 h; the discharge process uses a voltage-cutoff mode with a cutoff voltage of 0.8 V. The coulombic efficiency of the zinc-bromine flow battery assembled with electrode a6 as the positive electrode is 98.26%, which is higher than the coulombic efficiency of the zinc-bromine flow battery assembled with the original carbon felt material but lower than the coulombic efficiency of the zinc-bromine flow battery assembled with electrode A1 as the positive electrode. This is because electrode a6 has not undergone a calcination process, and the carbon fibers lack a porous structure, which reduces the adsorption capacity of bromine. However, the nitrogen-containing groups can still adsorb bromine, increasing the coulombic efficiency of the battery. The voltage efficiency is 78.96%, which is higher than that of the zinc-bromine flow battery assembled with the original carbon felt material as the positive electrode, but lower than that of the zinc-bromine flow battery assembled with electrode A1 as the positive electrode. This is because the strong catalytic activity of the nitrogen-containing groups enhances the reactivity of the bromine electrode couple and reduces battery polarization. The energy efficiency is 77.59%, which is higher than the energy efficiency of the zinc-bromine flow battery assembled with the original carbon felt material as the positive electrode, but lower than the energy efficiency of the zinc-bromine flow battery assembled with electrode A1 as the positive electrode (Table 1). This shows that the performance of electrode a1 as the positive electrode is better than that of the original carbon felt as the positive electrode but worse than that of electrode A1 as the positive electrode. This is because electrode a6 has not undergone a calcination process and there is no porous structure on the carbon fiber (Table 2).

[0068] Table 1 Performance of zinc-bromine flow battery with different electrode materials as positive electrode

[0069]

[0070] Table 2 Summary of the conclusions of the examples and comparative examples

[0071]

[0072]

Claims

1. Application of a cathode material in a bromine-based flow battery, characterized in that: The positive electrode material is nitrogen-doped porous carbon felt, and the nitrogen doping amount in the carbon fibers of the porous carbon felt is 2wt%-8wt%; the carbon fibers constituting the carbon felt have a porous structure with a pore diameter of 0.1nm-30nm and a pore volume of 0.001-0.008cm 3 / g; The preparation method of the positive electrode material comprises the following steps: (1) dissolving dopamine in water and stirring to obtain a uniform dopamine aqueous solution, wherein the concentration of the dopamine aqueous solution is 100-500 mg / 100 mL of water; (2) soaking the original carbon felt material in the dopamine aqueous solution prepared in step (1), stirring, so that the dopamine aqueous solution fully permeates the original carbon felt material; The original carbon felt material is a commercial carbon felt material with a thickness of 3-6 mm and a porosity of 90-98%; (3) adding tris(hydroxymethyl)aminomethane solution to the dopamine aqueous solution containing carbon felt prepared in step (2) and stirring to ensure that dopamine is completely self-polymerized on the carbon fibers of the carbon felt; wherein the molar concentration of the tris(hydroxymethyl)aminomethane solution is 5-20 mM; and the volume ratio of the tris(hydroxymethyl)aminomethane solution to the dopamine solution is 1×10 -4 -9×10 -4 ; (4) The carbon felt treated in step (3) is taken out and dried at a temperature of 40-80° C. for more than 12 hours; and then calcined at 1100-1200° C. in a N 2 atmosphere for 3-4 hours to obtain the positive electrode material.

2. The use according to claim 1, characterized in that: The nitrogen doping amount in the carbon fibers of the porous carbon felt is 4wt%-6wt%.

3. The use according to claim 1, characterized in that: In step (1), the stirring time is greater than 2 hours; in step (2), the stirring time is greater than 0.5 hours; in step (3), the stirring time is greater than 24 hours.

4. The use according to claim 1, characterized in that: The bromine-based flow battery includes a hydrogen-bromine flow battery, a vanadium-bromine flow battery, a sodium polysulfide / bromine flow battery, a zinc-bromine flow battery, a quinone-bromine flow battery, a lithium-bromine flow battery or a magnesium-bromine flow battery.

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