Preparation and Application of a Cathode Material for Bromine Redox Flow Batteries

Abstract

The present invention relates to a positive electrode material for a bromine-based flow battery. The positive electrode uses a conductive current collector as a substrate, and a layered porous carbon nitride material with bromine fixation function is supported on the substrate. The substrate is carbon felt, graphite felt or carbon cloth. The electrode material of the present invention has beneficial characteristics such as simple preparation method, low cost, strong bromine fixation ability, and good chemical stability. The prepared electrode material inhibits the migration and diffusion of bromine, reduces the self-discharge reaction of the battery, effectively improves the capacity retention rate and cycle stability of the bromine-based flow battery, and greatly improves the battery life.

Description

Technical Field

[0001] The present invention relates to the field of positive electrode materials for flow batteries, especially to the field of preparation and application of positive electrode materials for bromine-based flow batteries. Background Art

[0002] With the gradual depletion of fossil energy and the continuous progress of new energy technologies, new energy power generation technologies such as wind energy and solar energy have achieved large-scale development and account for an increasingly large proportion in the overall power generation. However, due to their inherent instability and discontinuity, they will impact the power grid in practical applications and also have an adverse effect on the power consumption of off-grid users. Therefore, the introduction of an energy storage system is one of the keys to realizing the large-scale application of renewable energy. Flow battery technology has received attention in the field of large-scale energy storage technologies due to its advantages such as independently designed output power and energy storage capacity, high energy efficiency, safety and reliability. At the same time, bromine-based flow batteries have the advantages of rich sources, low cost, and high energy density, and have broad application prospects in the field of user-side energy storage.

[0003] Although zinc bromine flow batteries have been in the commercial demonstration stage, the further commercialization and industrialization of bromine-based flow batteries still face many challenges. Among them, Br2 has strong volatility and diffusivity. On the one hand, it volatilizes into the environment and causes pollution, which may trigger safety problems; on the other hand, it leads to a reduction in the active substances of the battery, reducing the capacity, efficiency, and service life of the battery. And Br2 diffuses to the negative electrode and often reacts chemically with the negative electrode active substances when migrating to the negative electrode, resulting in self-discharge of the battery and reducing the capacity, efficiency, and service life of the battery. The currently commonly used solutions are as follows: 1) adding complexing agents to the electrolyte to reduce the concentration of free bromine; 2) modifying the diaphragm material to reduce the penetration of bromine through the pore size screening effect. However, the polybromides formed by the additives and bromine are insoluble in the aqueous phase, reducing the conductivity and mass transfer rate of the electrolyte; the diaphragm materials suitable for bromine-based flow batteries are also limited. Among them, ion exchange membranes are expensive and have high resistance, and porous membranes cannot completely prevent the migration of bromine, and the membrane resistance increases after modification, which is not conducive to the reaction of Br2 / Br - -. Summary of the Invention

[0004] To solve the above technical problems, the present invention aims to provide a preparation method of a positive electrode material with a bromine-fixing effect and its application in a bromine-based flow battery. By pyrolyzing a cyano compound in air for the first time and then in ammonia for the second time to prepare a layered porous carbon nitride material, which has a certain adsorption effect on bromine, and at the same time captures bromine and polybromides in the layered porous structure by means of the size effect. When used as the positive electrode material of a bromine-based flow battery, it can significantly inhibit the diffusion of bromine, slow down the capacity decay, and thus improve the battery efficiency.

[0005] To achieve the above object, the technical solution adopted by the present invention is as follows:

[0006] On the one hand, the present invention provides a positive electrode material for a bromine-based flow battery. The positive electrode material uses a conductive current collector as a substrate, and a layered porous carbon nitride material with a bromine fixation effect is supported on the substrate. The pore diameter of the carbon nitride material is 0.5 nm to 2 nm, and the interlayer spacing is

[0007] Based on the above scheme, preferably, the pore diameter of the carbon nitride material is 0.6 nm to 1.3 nm, and the interlayer spacing is

[0008] Based on the above scheme, preferably, the loading amount of the carbon nitride material on the substrate is 1-15 mg / cm 2 , and more preferably 5-12 mg / cm 2 .

[0009] Based on the above scheme, preferably, the substrate is carbon felt, graphite felt or carbon cloth.

[0010] On the other hand, the present invention provides a preparation method of the above positive electrode material, including the following steps:

[0011] 1), subjecting an organic compound containing C, N, O, and H elements to a first pyrolysis in air in a tube furnace; the pyrolysis temperature is 400-700 °C, the pyrolysis time is 6-10 h, and after cooling to room temperature, a first pyrolysis product is obtained.

[0012] 2), subjecting the first pyrolysis product to a second pyrolysis in ammonia, the pyrolysis temperature is 500-600 °C, the pyrolysis time is 1-3 h, to obtain a layered porous carbon nitride material.

[0013] 3), loading the layered porous carbon nitride material on a conductive current collector substrate to obtain the positive electrode material.

[0014] Based on the above scheme, preferably, the organic compound containing C, N, O, and H elements in step 1) is urea or a cyanide compound; the cyanide compound includes monocyanamide, dicyandiamide, melamine, etc., and preferably melamine.

[0015] Based on the above scheme, preferably, the pyrolysis temperature in step 1) is 500-600 °C; the pyrolysis time is 7-8 h.

[0016] Based on the above scheme, preferably, the heating rate in step 1) is 1-10 °C / min, preferably 2-5 °C / min.

[0017] Based on the above scheme, preferably, the pyrolysis temperature in step 2) is 520-550 °C; the pyrolysis time is 1-2 h.

[0018] Based on the above scheme, preferably, the heating rate in step 2) is 1-10 °C / min, preferably 2-5 °C / min.

[0019] Based on the above scheme, preferably, in step 3), the loading method of the layered porous carbon nitride material is as follows: adding the carbon nitride material into isopropanol, adding a binder at the same time, and spraying it on the conductive current collector matrix after ultrasonic dispersion.

[0020] Based on the above scheme, preferably, the binder in step 3) is Nation, PVDF or PTFE, preferably Nation, and the mass ratio of the binder to the porous carbon nitride is 2-8, preferably 4.

[0021] Based on the above scheme, preferably, the mass ratio of isopropanol to the porous carbon nitride in step 3) is 25-100, preferably 60.

[0022] Advantages of the present invention:

[0023] (1) For the electrode material prepared by the present invention, the layered porous carbon nitride material can promote the adsorption of bromine, and at the same time, with the help of the size effect, bromine and polybromides can be confined in the layered porous structure. As a positive electrode material applied to a bromine-based flow battery, it can significantly inhibit the diffusion of bromine, slow down the capacity decay, and thus improve the battery efficiency;

[0024] (2) The electrode material prepared by the present invention has good bromine fixation ability, providing a novel method to solve the problem of bromine diffusion from the perspective of electrode materials;

[0025] (3) The process of the present invention is simple, easy to operate, low in cost, and safe and environmentally friendly. Description of the drawings

[0026] Figure 1 SEM diagrams of Comparative Example 3, Example 1, and Example 2.

[0027] Figure 2 XRD patterns of Comparative Example 3, Example 1, and Example 2.

[0028] Figure 3 Battery performance diagrams of Comparative Example 1, Example 1, Example 2, and Example 3.

[0029] Figure 4 Self-discharge performance diagrams of Comparative Example 1, Example 1, Example 2, and Example 3.

[0030] Figure 5 UV adsorption diagrams of Comparative Example 1, Example 1, Example 2, and Example 3.

[0031] Figure 6Bromine fixation capacity diagrams for Comparative Example 2, Comparative Example 4, Example 4, Example 5, and Example 6. Detailed implementation manners

[0032] The following examples further illustrate the present invention rather than limiting the scope of the present invention.

[0033] Comparative Example 1:

[0034] Commercial raw carbon felt with dimensions of 3 cm * 3 cm * 5 mm.

[0035] The current density is 80 mA cm- 2 Under this condition, battery performance tests are carried out: assemble a zinc-bromine flow battery. Both the positive and negative electrodes use the raw carbon felt electrodes, the separator is a commercial Daramic porous membrane, and the electrolyte is 2M ZnBr2 + 3M KCl + 0.4M MEP.

[0036] Self-discharge performance test: Assemble a zinc-bromine flow battery. Both the positive and negative electrodes use untreated raw carbon felt electrodes, the separator is a commercial Daramic porous membrane, and the electrolyte is 2M ZnBr2 + 3M KCl + 0.4M MEP. Under the condition of a current density of 80 mA cm -2 Perform 15 charge-discharge cycles, and then leave it for 24 h and test the Coulomb efficiency of the 16th cycle.

[0037] Bromine adsorption capacity test: Cut out a circular raw carbon felt with a diameter of 6 mm and a thickness of 2 mm, place it in 3 ml of bromine water with a concentration of 20 mmol / L, soak it for five minutes, and then use a UV-visible spectrophotometer to test the concentration of bromine water. The wavelength range is set to 240 - 320 nm during the test.

[0038] Comparative Example 2:

[0039] Polish a rotating ring-disk electrode with a diameter of 6 mm (the ring is Pt and the disk is glassy carbon) with 30 - 50 nm of Al2O3 polishing powder until the surface is smooth and traceless, rinse it repeatedly with deionized water, and dry it for standby.

[0040] Bromine fixation capacity test: Use the rotating ring-disk electrode technique to evaluate the bromine fixation performance of the material. An electrochemical workstation (Gamry Multichannel System installation, Reference 3000 and Reference 1000) is used to control the voltage and current of the test system. The test conditions are as follows: the working electrode is the above-mentioned rotating ring-disk electrode, the reference electrode is an Ag / AgCl electrode, the counter electrode is a graphite plate, and the rotation speed of the ring-disk electrode is 2000 rpm. The oxidation current on the glassy carbon disk electrode is maintained at 1 mA for 60 s, and the reduction potential on the Pt ring is maintained at 0.5 V vs. Ag / AgCl.

[0041] Comparative Example 3:

[0042] Weigh 5.0 g of melamine and spread it flat in a corundum boat, place it in a tubular furnace, and pyrolyze it in air at a pyrolysis temperature of 520°C, a pyrolysis time of 8 h, and a heating rate of 2°C / min. Then, pyrolyze it in ammonia at a pyrolysis temperature of 520°C, a pyrolysis time of 4 h, and a heating rate of 2°C / min.

[0043] Comparative Example 4:

[0044] The pretreatment process of the rotating ring disk electrode with a diameter of 6 mm is the same as that of Comparative Example 2. Take 2.5 mg of the material prepared in Comparative Example 3, mix it with 0.4 g of isopropanol and 10 μL of 0.05 wt.% Nation solution by ultrasonication to form a uniform slurry. Use a microinjector to transfer 10 μL of the above slurry to the glassy carbon electrode. After the solvent evaporates completely, a thin film electrode with a smooth surface is obtained as the test object.

[0045] The bromine fixation capacity test device and conditions are the same as those in Comparative Example 2, except that the ring disk electrode treated as above is used as the working electrode.

[0046] Embodiment 1:

[0047] Weigh 5.0g of melamine and spread it in a corundum boat, place it in a tube furnace, and perform pyrolysis in air at a temperature of 520℃, a time of 8h, and a heating rate of 2℃ / min, then perform pyrolysis in ammonia at a temperature of 520℃, a time of 1h, and a heating rate of 2℃ / min. After fully grinding, weigh 0.05g of the prepared material, mix it evenly with 0.2g of 0.05wt% Nation solution and 3g of isopropanol, and spray it on the original carbon felt with a size of 3cm*3cm*5mm under ultrasonication for several hours.

[0048] The current density is 80mA em -2 The battery performance test was carried out under the following conditions: a zinc-bromine flow battery was assembled, an untreated original carbon felt electrode was used for the negative electrode, and the treated carbon felt was used for the positive electrode, and other conditions were the same as those in the above comparative example 1.

[0049] Self-discharge performance test: zinc-bromine liquid flow batteries were assembled, and untreated original carbon felt electrodes were used for both positive and negative electrodes, and the treated carbon felt was used for the positive electrode. Other conditions were the same as those in the comparative example 1 above.

[0050] Bromine adsorption capacity test: The treated carbon felt was cut into a circular shape with a diameter of 6 mm and a thickness of 2 mm. Other conditions were the same as those of Comparative Example 1.

[0051] Embodiment 2:

[0052] Weigh 5.0 g of melamine and spread it flat in a corundum boat. Place it in a tubular furnace and pyrolyze it in air. The pyrolysis temperature is 520 °C, the pyrolysis time is 8 h, and the heating rate is 2 °C / min. Then pyrolyze it in ammonia gas. The pyrolysis temperature is 520 °C, the pyrolysis time is 2 h, and the heating rate is 2 °C / min. After sufficient grinding, weigh 0.05 g of the prepared material and mix it evenly with 0.2 g of 0.05 wt% Nation solution and 3 g of isopropanol, and spray it evenly on the original carbon felt with dimensions of 3 cm * 3 cm * 5 mm by ultrasonic treatment for several hours.

[0053] The current density is 80 mA em -2 Under this condition, the battery performance test is carried out: Assemble a zinc-bromine flow battery. The negative electrode uses the untreated original carbon felt electrode, and the positive electrode uses the carbon felt after the above treatment. Other conditions are the same as those in Comparative Example 1 above.

[0054] Self-discharge performance test: Assemble a zinc-bromine flow battery. Both the positive and negative electrodes use the untreated original carbon felt electrodes, and the positive electrode uses the carbon felt after the above treatment. Other conditions are the same as those in Comparative Example 1 above.

[0055] Bromine adsorption capacity test: Cut out a circular piece with a diameter of 6 mm and a thickness of 2 mm from the carbon felt after the above treatment. Other conditions are the same as those in Comparative Example 1 above.

[0056] Example 3:

[0057] Weigh 5.0 g of melamine and spread it flat in a corundum boat. Place it in a tubular furnace and pyrolyze it in air. The pyrolysis temperature is 520 °C, the pyrolysis time is 8 h, and the heating rate is 2 °C / min. Then pyrolyze it in ammonia gas. The pyrolysis temperature is 520 °C, the pyrolysis time is 2 h, and the heating rate is 2 °C / min. After sufficient grinding, weigh 0.1 g of the prepared material and mix it evenly with 0.4 g of 0.05 wt% Nation solution and 6 g of isopropanol, and spray it evenly on the original carbon felt with dimensions of 3 cm * 3 cm * 5 mm by ultrasonic treatment for several hours.

[0058] The current density is 80 mA em -2 Under this condition, the battery performance test is carried out: Assemble a zinc-bromine flow battery. The negative electrode uses the untreated original carbon felt electrode, and the positive electrode uses the carbon felt after the above treatment. Other conditions are the same as those in Comparative Example 1 above.

[0059] Self-discharge performance test: Assemble a zinc-bromine flow battery. Both the positive and negative electrodes use the untreated original carbon felt electrodes, and the positive electrode uses the carbon felt after the above treatment. Other conditions are the same as those in Comparative Example 1 above.

[0060] Bromine adsorption capacity test: Cut out a circular piece with a diameter of 6 mm and a thickness of 2 mm from the carbon felt after the above treatment. Other conditions are the same as those in Comparative Example 1 above.

[0061] Example 4:

[0062] The pretreatment process of the rotating ring-disk electrode with a diameter of 6 mm is the same as that of Comparative Example 2. Take 2.5 mg of the layered porous carbon nitride material prepared in Example 1, and ultrasonically mix it with 0.4 g of isopropanol and 10 μL of 0.05 wt.% Nafion solution to form a uniform slurry. Use a microsyringe to aspirate 10 μL of the above slurry and transfer it onto the glassy carbon electrode. After the solvent has completely evaporated, a thin-film electrode with a flat surface is obtained for testing.

[0063] The bromine fixation capacity test device and conditions are the same as those of Comparative Example 2, except that the above-treated ring-disk electrode is used as the working electrode.

[0064] Example 5:

[0065] The pretreatment process of the rotating ring-disk electrode with a diameter of 6 mm is the same as that of Comparative Example 2. Take 2.5 mg of the layered porous carbon nitride material prepared in Example 2, and ultrasonically mix it with 0.4 g of isopropanol and 10 μL of 0.05 wt.% Nafion solution to form a uniform slurry. Use a microsyringe to aspirate 10 μL of the above slurry and transfer it onto the glassy carbon electrode. After the solvent has completely evaporated, a thin-film electrode with a flat surface is obtained for testing.

[0066] The bromine fixation capacity test device and conditions are the same as those of Comparative Example 2, except that the above-treated ring-disk electrode is used as the working electrode.

[0067] Example 6:

[0068] The pretreatment process of the rotating ring-disk electrode with a diameter of 6 mm is the same as that of Comparative Example 2. Take 2.5 mg of the layered porous carbon nitride material prepared in Example 2, and ultrasonically mix it with 0.4 g of isopropanol and 10 μL of 0.05 wt.% Nafion solution to form a uniform slurry. Use a microsyringe to aspirate 20 μL of the above slurry and transfer it onto the glassy carbon electrode. After the solvent has completely evaporated, a thin-film electrode with a flat surface is obtained for testing.

[0069] The bromine fixation capacity test device and conditions are the same as those of Comparative Example 2, except that the above-treated ring-disk electrode is used as the working electrode.

[0070] Figure 1 SEM images of Comparative Example 3, Example 1, and Example 2. It can be seen from the figure that when the second pyrolysis time is 1 h (Example 1), the material prepared by the present invention exhibits a layered porous structure, with relatively tight stacking between layers and an interlayer spacing of When the second pyrolysis time is 2 h (Example 2), the material prepared by the present invention is still a layered porous structure, but the space between its layers becomes loose, and the interlayer spacing significantly increases to When the pyrolysis time is 4 h (Comparative Example 3), the layered structure disappears, and the lamellae curl into hollow rod-like shapes.

[0071] Figure 2 In the XRD pattern, the characteristic peaks of Example 2 shifted significantly to the left compared with those of Example 1, indicating that with the increase of the secondary pyrolysis time, the interlayer spacing of the prepared electrode material increased. The characteristic peaks in Comparative Example 3 weakened or even disappeared, corresponding to the disappearance of the lamellar structure.

[0072] Figure 3 When the current density is 80 mA cm -2 The Coulomb efficiency diagrams of the zinc-bromine flow batteries assembled with Comparative Example 1, Example 1, Example 2, and Example 3 under this condition. It can be seen from the figure that the Coulomb efficiency of the original carbon felt (Comparative Example 1) fluctuates around 96%, while the Coulomb efficiency of the batteries assembled with the materials prepared in the present invention (Example 1, Example 2, Example 3) increases significantly. The larger interlayer spacing and the higher loading amount correspond to a higher Coulomb efficiency. The Coulomb efficiency of Example 3 is as high as 98.5% and remains stable for 100 cycles without obvious attenuation or mutation. The increase in Coulomb efficiency means an improvement in capacity retention, indicating a reduction in the migration and diffusion of bromine, that is, the positive electrode material prepared in the present invention is beneficial to inhibiting the volatilization and reduction of active substances.

[0073] Figure 4 When the current density is 80 mA cm -2 The self-discharge performance diagrams of the zinc-bromine flow batteries assembled with Comparative Example 1, Example 1, Example 2, and Example 3 under this condition. It can be seen from the figure that the Coulomb efficiency of the original carbon felt (Comparative Example 1) can only be maintained at 39% after being placed for 24 h, while the Coulomb efficiencies of the batteries assembled with the materials prepared in the present invention (Example 1, Example 2, Example 3) can reach 50%, 56%, and 63%. The self-discharge performance is significantly improved, indicating that the structure of the electrode material in the present invention is beneficial to inhibiting the migration and diffusion of bromine and slowing down the capacity loss of the battery.

[0074] Figure 5 This is a test diagram of bromine adsorption capacity. It can be seen from the figure that the concentration of bromine water after soaking in Comparative Example 1 decreased slightly because the carbon felt itself has a certain adsorption effect on bromine, while the concentration of bromine water in Example 1, Example 2, and Example 3 decreased significantly. The larger interlayer spacing corresponds to a larger specific surface area and more adsorption sites, and the adsorption capacity for bromine is stronger.

[0075] Figure 6This is a diagram for testing the bromine fixation ability using a rotating ring-disk electrode. According to Faraday's law, the quantitatively generated Br2 on the disk electrode is divided into two parts: one part is captured in the material of the disk electrode through pore size sieving or adsorption, and the other part of the un-captured Br2 diffuses to the ring electrode and is reduced by the reduction potential on the ring electrode. That is, the current value on the ring can represent the amount of un-captured bromine, and the smaller the current value, the stronger the bromine fixation ability of the material. It can be seen from the figure that compared with the original ring-disk electrode (Comparative Example 2) and the non-layered carbon nitride material (Comparative Example 4), after modifying the electrode material of the present invention (Examples 4, 5, and 6), the limiting current value is significantly reduced, indicating that the electrode material of the present invention has excellent bromine fixation ability, can inhibit the migration and diffusion of bromine, and slow down the self-discharge of the battery.

Claims

1. A positive electrode material for a bromine-based flow battery, characterized in that: The positive electrode material uses a conductive current collector as a substrate, and a layered porous carbon nitride material is supported on the substrate. The pore diameter of the carbon nitride material is 0.5 nm to 2 nm, and the interlayer spacing is The preparation method of the positive electrode material includes: 1), subjecting an organic compound containing C, N, O, and H elements simultaneously to a first pyrolysis in air, with a pyrolysis temperature of 400 - 700 °C, a pyrolysis time of 6 - 10 h, and cooling to room temperature to obtain a first pyrolysis product; The organic compound is one or more of monocyanamide, dicyandiamide, and melamine; 2), subjecting the first pyrolysis product to a second pyrolysis in ammonia gas, with a pyrolysis temperature of 500 - 600 °C and a pyrolysis time of 1 - 3 h to obtain a layered porous carbon nitride material; 3), loading the layered porous carbon nitride material onto a conductive current collector matrix to obtain the positive electrode material.

2. The cathode material according to claim 1, characterized in that: The pore size of the carbon nitride material is 0.6 nm to 1.3 nm, and the interlayer spacing is 3. The cathode material according to claim 1, characterized in that: The loading amount of the carbon nitride material on the substrate is 1-15 mg / cm 2 , and the substrate is carbon felt, graphite felt or carbon cloth.

4. The cathode material according to claim 3, characterized in that: The loading amount of the carbon nitride material on the substrate is 5-12 mg / cm 2 .

5. The cathode material according to claim 1, characterized in that: In step 1), the pyrolysis temperature is 500 - 600 °C, the pyrolysis time is 8 h, and the heating rate is 1 - 10 °C / min; in step 2), the pyrolysis temperature is 520 - 550 °C, the pyrolysis time is 1 - 2 h, and the heating rate is 1 - 10 °C / min.

6. The cathode material according to claim 5, characterized in that: In step 1), the organic compound is melamine, and the heating rate is 2 - 5 °C / min; in step 2), the heating rate is 2 - 5 °C / min.

7. The cathode material according to claim 1, characterized in that: In step 3), the loading method of the layered porous carbon nitride material is: adding the carbon nitride material into isopropanol, adding a binder simultaneously, and after ultrasonic dispersion, spraying it on the conductive current collector matrix.

8. The cathode material according to claim 7, characterized in that: The binder is one of Nafion, PVDF, or PTFE, and the mass ratio of the binder to the porous carbon nitride is 2 - 8; the mass ratio of the isopropanol to the porous carbon nitride is 25 - 100.

9. Application of the positive electrode material according to any one of claims 1 - 8 in a bromine-based flow battery.

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