Flow battery composite electrode material and preparation method thereof

Abstract

The invention discloses a flow battery composite electrode material and a preparation method thereof.The method comprises the steps that a carbon material is placed in a solution containing a nitrogen precursor for a hydrothermal reaction, the reacted carbon material is calcined, and a carbon nitride carbon nanotube catalyst growing on the carbon material in situ is obtained; the carbon nitride-carbon composite electrode is obtained; meanwhile, a solution containing a nitrogen precursor is subjected to a hydrothermal reaction to obtain ordered carbon nitride nanotube array powder with hollow channels, and a diaphragm is coated with the ordered carbon nitride nanotube array powder to obtain a first catalyst layer and a second catalyst layer of the carbon nitride nanotube array powder with different concentration gradients to serve as a membrane electrode. According to the electrode material, the proton and electron conductivity is improved, and the battery polarization is reduced, so that the operation current density and the energy efficiency of the battery are improved; and the active site number and the active area of the reaction performance of the catalytic battery are increased, and the reaction performance of the battery is improved.

Description

Technical Field

[0001] The present invention relates to the technical field of membrane electrode preparation, and in particular to a composite electrode material for a liquid flow battery and a preparation method thereof. Background Art

[0002] Liquid flow energy storage battery is a large-scale electrochemical energy storage technology. Compared with other energy storage technologies, it has the advantages of high energy conversion efficiency, flexible system design, large storage capacity, free site selection, deep discharge, safety and environmental protection, and low maintenance costs. It can be widely used in wind power, solar energy and other renewable energy power generation and storage, emergency power supply systems, backup power stations and power system peak shaving and valley filling. At present, the more mature liquid flow battery system is the all-vanadium liquid flow battery, but the cost of liquid flow batteries is still relatively high, so it is necessary to further improve the battery performance, so it is necessary to further reduce the battery's ohmic internal resistance and polarization, and improve the battery's working current density and energy efficiency.

[0003] As one of the key components of liquid flow batteries, the conductivity, porosity, pore structure, and electrochemical catalytic activity of electrode materials affect the ohmic polarization, concentration polarization, and electrochemical polarization of the battery, thereby affecting the operating current density and energy efficiency of the battery. In other words, to increase the operating current density of the liquid flow battery, it is necessary to reduce the above polarization of the battery as much as possible and reduce the voltage loss.

[0004] Membrane electrodes can be used as electrode materials, but in traditional membrane electrodes, the catalyst particles and ionic polymers that serve as proton conductors are disorderly distributed, resulting in large mass transfer polarization, low catalyst utilization, and large catalyst loading.

[0005] Patent CN106558704A reports a gradient electrode for liquid flow batteries, which is composed of at least two layers of graphite fibers or carbon felts with different body densities stacked in order from low to high body density, and made into one piece by longitudinal acupuncture perpendicular to the electrode surface. This electrode can effectively reduce the resistance of the electrode body, reduce the flow resistance of the electrolyte and provide more reaction sites, and ultimately reduce the flow resistance of the electrolyte and provide more reaction sites, and ultimately reduce the ohmic polarization, electrochemical polarization and concentration polarization of the liquid flow battery. However, this patent uses graphite or carbon fiber as the electrode material, the fiber is relatively coarse, and the effective area is not high. Summary of the invention

[0006] In view of this, the main purpose of the present invention is to provide a composite electrode material for a liquid flow battery and a preparation method thereof, wherein the electrode material improves the conductivity of protons and electrons, reduces battery polarization, thereby improving the operating current density and energy efficiency of the battery; and increases the number of active sites and active area for catalytic battery reaction performance, thereby improving battery reaction performance.

[0007] To achieve the above-mentioned object of the invention, the first aspect of the present invention provides a method for preparing a composite electrode material for a flow battery, comprising:

[0008] 1) placing a carbonaceous material in a solution containing a nitrogen-containing precursor for hydrothermal reaction, calcining the carbonaceous material after the reaction, and obtaining a graphite-phase carbon nitride nanotube catalyst in situ grown on the carbonaceous material as a carbon nitride-carbon composite electrode;

[0009] 2) subjecting a solution of a nitrogen-containing precursor to a hydrothermal reaction to obtain a graphite-phase carbon nitride nanotube catalyst powder;

[0010] 3) preparing the graphite phase carbon nitride nanotube catalyst powder obtained in step 2) into a first catalyst layer slurry, coating the first catalyst layer slurry on one side or both sides of the diaphragm, and drying to obtain a membrane electrode having a first catalyst layer;

[0011] 4) preparing the graphite phase carbon nitride nanotube catalyst powder obtained in step 2) into a second catalyst layer slurry, coating the second catalyst layer slurry on the membrane electrode having the first catalyst layer in step 3), and drying to obtain a membrane electrode having different concentration gradients of the first catalyst layer and the second catalyst layer at the positive electrode and / or the negative electrode;

[0012] The mass content of the graphite phase carbon nitride nanotube catalyst powder in the first catalyst layer slurry is less than the mass content of the graphite phase carbon nitride nanotube catalyst powder in the second catalyst layer slurry;

[0013] 5) The membrane electrode and carbon nitride-carbon felt composite electrode obtained in step 4) are assembled in a liquid flow battery as electrode materials.

[0014] In a specific embodiment, when obtaining a membrane electrode having a first catalytic layer and a second catalytic layer with different concentration gradients at both the positive and negative electrodes, specifically: according to steps 3)-4), the first and second catalytic layer slurries are respectively coated on the surface of one side of the diaphragm to prepare a membrane electrode with a first catalytic layer of the positive electrode and a second catalytic layer of the positive electrode with different concentration gradients; then repeating steps 3)-4) to respectively coat the first and second catalytic layer slurries on the other side of the diaphragm to prepare a membrane electrode with a first catalytic layer of the negative electrode and a second catalytic layer of the negative electrode with different concentration gradients.

[0015] Furthermore, the nitrogen-containing precursor is one or more of urea, melamine or cyanamide, preferably a mixture of urea and melamine, wherein the molar ratio of urea to melamine is preferably 1 to 10:1.

[0016] Furthermore, the solvent in the solution of the nitrogen-containing precursor is a mixture of water and an alcohol reagent, the volume ratio of water:alcohol reagent is preferably 1-9:1, and the alcohol reagent is one or more of ethanol, ethylene glycol, and isopropanol.

[0017] Furthermore, the carbon material is carbon felt, carbon cloth, carbon paper or graphite felt.

[0018] Furthermore, the parameters of the hydrothermal reaction in step 1) include: conducting the hydrothermal reaction at 150-240° C. for 4-24 hours.

[0019] Furthermore, before calcining the reacted carbonaceous material in step 1), the reacted carbonaceous material is also washed and dried, preferably at 60-120° C. for 12-24 hours.

[0020] Furthermore, the calcination parameters in step 1) include: heating to 550-650° C. at a heating rate of 1-5° C. / min under inert atmosphere protection, and calcining for 4-8 hours.

[0021] Furthermore, the parameters of the hydrothermal reaction in step 2) include: conducting the hydrothermal reaction at 150-240° C. for 4-24 hours.

[0022] Furthermore, the first catalytic layer slurry in step 3) comprises: carbon nanotube nitride catalyst powder, binder, alcohol reagent and water, the solid content of the slurry is 0.2-2%; the mass ratio of alcohol reagent to water is 8-12:1, and the mass percentage between carbon nanotube nitride catalyst powder and binder is 50-60:50-40.

[0023] Furthermore, the temperature during coating in step 3) is 60-90°C, and the temperature during drying is 60-90°C.

[0024] Furthermore, the second catalytic layer slurry in step 4) comprises: carbon nanotube nitride catalyst powder, binder, alcohol reagent and water, the solid content of the slurry is 0.2-2%; the mass ratio of alcohol reagent to water is 8-12:1, and the mass percentage between carbon nanotube nitride catalyst powder and binder is 60-75:40-25.

[0025] Furthermore, the temperature during coating in step 4) is 60-90°C, and the temperature during drying is 60-90°C.

[0026] Furthermore, the diaphragm is a Nafion membrane.

[0027] Furthermore, the binder is a 5%-20% Nafion solution.

[0028] Furthermore, the total thickness of the first catalytic layer and the second catalytic layer is 500 nm-2 μm.

[0029] The second aspect of the present invention provides a composite electrode material for a liquid flow battery prepared by the above method, comprising a carbon nitride-carbon composite electrode and a membrane electrode.

[0030] Compared with the prior art, the present invention has the following advantages:

[0031] The present invention places a carbon material in a solution of a nitrogen-containing precursor for a hydrothermal reaction, and calcines the carbon material after the reaction to obtain a graphite-phase carbon nitride nanotube catalyst grown in situ on the carbon material (such as carbon felt), and obtains a carbon nitride-carbon composite electrode, wherein the carbon nitride nanotube structure has a higher nitrogen content than a single carbon felt electrode or a carbon felt-carbon nanotube composite structure, and the nitrogen doping is more uniform, and the catalyst conductivity is higher. At the same time, the present invention performs a hydrothermal reaction on the solution of a nitrogen-containing precursor to obtain an ordered carbon nitride nanotube array powder having a hollow channel, and obtains a positive and / or negative first catalytic layer and a second catalytic layer having different concentrations of carbon nitride nanotube array powder on the diaphragm as a membrane electrode;

[0032] The membrane electrode and carbon nitride-carbon composite electrode are applied to liquid flow batteries. From the membrane electrode catalyst layer to the composite electrode, the concentration of the ordered carbon nitride nanotube array is from low to high, which realizes the gradient transmission of protons and improves the conductivity of the electrode, thereby improving the conductivity of protons and electrons, reducing battery polarization, and thus improving the operating current density and energy efficiency of the battery. The ordered carbon nitride nanotube array has a hollow channel structure and high conductivity. As a catalyst layer, it further increases the number of active sites and active area of ​​the catalytic battery reaction performance, thereby improving battery performance.

[0033] Other features and advantages of the present invention will be described in detail through the following detailed description. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] The accompanying drawings are used to provide a further understanding of the present invention and constitute a part of the specification. Together with the following specific embodiments, they are used to explain the present invention but do not constitute a limitation of the present invention. In the accompanying drawings:

[0035] Figure 1 This is a schematic diagram of the combined structure of the ordered gradient membrane electrode and the composite electrode according to Example 1 of the present invention.

[0036] 1-carbon nitride-carbon felt composite electrode, 101-carbon felt, 102-carbon nitride nanotube catalyst, 2-membrane electrode, 201-Nafion membrane, 202-positive electrode first catalytic layer, 203-positive electrode second catalytic layer. DETAILED DESCRIPTION

[0037] The specific implementation of the present invention is described in detail below in conjunction with the accompanying drawings. It should be understood that the specific implementation described here is only used to illustrate and explain the present invention, and is not used to limit the present invention.

[0038] The endpoints and any values ​​of the ranges disclosed in this article are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of each range, the endpoint values ​​of each range and the individual point values, and the individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed in this article.

[0039] Unless otherwise specified, the materials used in the embodiments of the present invention can be obtained from commercial channels.

[0040] The present invention provides a method for preparing a flow battery electrode material, comprising:

[0041] 1) placing a carbonaceous material in a solution containing a nitrogen-containing precursor for hydrothermal reaction, calcining the carbonaceous material after the reaction, and obtaining a graphite-phase carbon nitride nanotube catalyst in situ grown on the carbonaceous material as a carbon nitride-carbon composite electrode;

[0042] 2) subjecting a solution of a nitrogen-containing precursor to a hydrothermal reaction to obtain a graphite-phase carbon nitride nanotube catalyst powder;

[0043] 3) preparing the graphite phase carbon nitride nanotube catalyst powder obtained in step 2) into a first catalyst layer slurry, coating the first catalyst layer slurry on one side or both sides of the diaphragm, and drying to obtain a membrane electrode having a first catalyst layer;

[0044] 4) preparing the graphite phase carbon nitride nanotube catalyst powder obtained in step 2) into a second catalyst layer slurry, coating the second catalyst layer slurry on the membrane electrode having the first catalyst layer in step 3), and drying to obtain a membrane electrode having different concentration gradients of the first catalyst layer and the second catalyst layer at the positive electrode and / or the negative electrode;

[0045] The mass content of the graphite phase carbon nitride nanotube catalyst powder in the first catalyst layer slurry is less than the mass content of the graphite phase carbon nitride nanotube catalyst powder in the second catalyst layer slurry;

[0046] 5) The membrane electrode and carbon nitride-carbon felt composite electrode obtained in step 4) are assembled in a liquid flow battery as electrode materials.

[0047] Figure 1 A schematic diagram of the combined structure of an ordered gradient membrane electrode and a composite electrode according to a specific embodiment of the present invention is illustrated.

[0048] In another specific embodiment, when obtaining a membrane electrode having a first catalytic layer and a second catalytic layer with different concentration gradients at both the positive and negative electrodes, specifically: according to steps 3)-4), the first and second catalytic layer slurries are respectively coated on one side of the diaphragm to prepare a membrane electrode having a first catalytic layer at the positive electrode and a second catalytic layer at the positive electrode with different concentration gradients; and then repeating steps 3)-4) to respectively coat the first and second catalytic layer slurries on the other side of the diaphragm to prepare a membrane electrode having a first catalytic layer at the negative electrode and a second catalytic layer at the negative electrode with different concentration gradients.

[0049] Furthermore, the nitrogen-containing precursor is one or more of urea, melamine or cyanamide, preferably a mixture of urea and melamine, wherein the molar ratio of urea to melamine is preferably 1 to 10:1, exemplarily 1:1, 2:1, 3:1, 4:1, 5:1, 6:1, 7:1, 8:1, 9:1, 10:1, etc.

[0050] Furthermore, the solvent of the solution of the nitrogen-containing precursor is a mixture of water and an alcohol reagent, and the volume ratio of water:alcohol reagent is preferably 1 to 9:1, exemplarily 1:1, 2:1, 3:1, 4:1, 5:1, 6:1, 7:1, 8:1, 9:1, etc., and the alcohol reagent is one or more of ethanol, ethylene glycol, and isopropanol.

[0051] Furthermore, the carbon material is carbon felt, carbon cloth, carbon paper or graphite felt.

[0052] Further, the parameters of the hydrothermal reaction in step 1) include: conducting the hydrothermal reaction at 150-240°C for 4-24 hours, and exemplary reaction temperatures are 150°C, 160°C, 170°C, 180°C, 190°C, 200°C, 210°C, 220°C, 230°C, 240°C, etc.

[0053] Furthermore, before calcining the reacted carbonaceous material in step 1), the reacted carbonaceous material is also washed and dried, preferably at 60-120° C. for 12-24 hours.

[0054] Furthermore, the calcination parameters in step 1) include: heating to 550-650° C. at a heating rate of 1-5° C. / min under inert atmosphere protection, and calcining for 4-8 hours.

[0055] Further, the parameters of the hydrothermal reaction in step 2) include: conducting the hydrothermal reaction at 150-240°C for 4-24 hours, and exemplary reaction temperatures are 150°C, 160°C, 170°C, 180°C, 190°C, 200°C, 210°C, 220°C, 230°C, 240°C, etc.

[0056] Furthermore, the first catalytic layer slurry in step 3) comprises: carbon nanotube nitride catalyst powder, binder, alcohol reagent and water, the solid content of the slurry is 0.2-2%; the mass ratio of alcohol reagent to water is 8-12:1, and the mass percentage between carbon nanotube nitride catalyst powder and binder is 50-60:40-50.

[0057] Furthermore, in step 3), the coating temperature is 60-90°C, preferably 70°C; and the drying temperature is 60-90°C, preferably 70°C.

[0058] Furthermore, the second catalytic layer slurry in step 4) comprises: carbon nanotube nitride catalyst powder, binder, alcohol reagent and water, the solid content of the slurry is 0.2-2%; the mass ratio of alcohol reagent to water is 8-12:1, and the mass percentage between carbon nanotube nitride catalyst powder and binder is 60-75:25-40.

[0059] Furthermore, in step 4), the coating temperature is 60-90°C, preferably 70°C; and the drying temperature is 60-90°C, preferably 70°C.

[0060] Furthermore, the diaphragm is a Nafion membrane.

[0061] Furthermore, the binder is a 5%-20% Nafion solution.

[0062] Furthermore, the total thickness of the first catalytic layer and the second catalytic layer is 500 nm-2 μm.

[0063] In the present invention, the membrane electrode and carbon nitride-carbon felt composite electrode are assembled in a flow battery as electrode materials, and the applicable electrolyte system is all-vanadium or zinc-iron electrolyte. In some examples, the flow rate of the zinc-iron electrolyte is 70 ml / min, and the charging current is 80-150 mA / cm 2 , discharge current is 80-150mA / cm 2 , the discharge cut-off voltage is 0.1V and the charging capacity is 20Ah / L.

[0064] The present invention is further described below with detailed exemplary embodiments, but these embodiments do not constitute any limitation to the present invention.

[0065] Example 1

[0066] 1) Grind 14.45g urea and 8g melamine into powder, dissolve in 100ml mixed solution of deionized water and ethanol (water-to-alcohol volume ratio is 1:1), ultrasonicate for 0.5h, stir for 2h, and mix well to obtain a precursor mixed solution. The mixed solution was transferred to a 100mL autoclave, 4mm commercial carbon felt was immersed in the above-mentioned precursor mixed solution, and hydrothermal reaction was carried out at 180℃ for 12 hours. After natural cooling, the reaction product was washed with water and ethanol for 3 times, filtered, and dried at 70℃ for 12h to obtain a precursor. The precursor was heated to 550℃ in nitrogen at a heating rate of 2℃ / min for calcination, the nitrogen flow rate was 60ml / min, and the calcination was 6 hours to obtain a hollow tubular carbon nitride nanotube catalyst grown in situ on the carbon felt, and a carbon nitride-carbon felt composite electrode was obtained.

[0067] 2) Grind 14.45g urea and 8g melamine into powder, dissolve in 100ml mixed solution of deionized water and ethanol (water-to-alcohol volume ratio is 1:1), ultrasonicate for 0.5h, stir for 2h, and mix well to obtain a precursor mixed solution. Transfer the mixed solution to a 100mL autoclave and perform hydrothermal reaction at 180℃ for 12 hours. After natural cooling, the reaction product is washed with water and ethanol 3 times each, filtered, and dried at 70℃ for 12h to obtain a precursor. The precursor is heated to 550℃ in nitrogen at a heating rate of 2℃ / min for calcination, with a nitrogen flow rate of 60ml / min, and calcined for 6 hours to obtain a nitrided carbon nanotube catalyst ground into powder.

[0068] 3) Mixing carbon nitride nanotube powder (C), 5% Nafion solution (I), isopropanol and water in a certain proportion to prepare two catalyst slurries with different I / C ratios, wherein the solid content of the slurry is 1%, the mass ratio of water to isopropanol is 9:1, the mass percentage of I / C of the first anode catalyst layer is 40:60, and the mass percentage of I / C of the second anode catalyst layer is 25:75; subjecting the above-mentioned mixed solution to ultrasonication at room temperature until it is completely dispersed to obtain a first catalyst layer slurry and a second catalyst layer slurry, which are uniformly coated on one side of a Nafion115 membrane at 70°C using a spray gun, and dried at 70°C to obtain a membrane electrode having a positive first catalyst layer and a second catalyst layer, and the catalyst loading is 0.5 mg / cm2;

[0069] 3) The prepared membrane electrode, carbon nitride-carbon felt composite electrode, and bipolar plate were assembled into a flow battery or battery stack for battery performance testing. The electrolyte system was zinc-iron electrolyte, the bipolar plate was a graphite plate, and the ion exchange membrane was a Nafion series membrane produced by DuPont with an effective area of ​​4 cm 2 .

[0070] The battery test parameters are as follows: the flow rate of zinc-iron electrolyte is 70ml / min, the charging current is 100mA / cm 2, discharge current is 100mA / cm 2 , the discharge cut-off voltage is 0.1V, the charging capacity is 20Ah / L, and the charge and discharge are repeated for 500 cycles. The battery performance is shown in Table 1 below.

[0071] Energy efficiency (%): The battery is tested using a charge and discharge tester to obtain current efficiency and voltage efficiency. Energy efficiency = current efficiency * voltage efficiency.

[0072] Example 2: The preparation method is similar to that of Example 1, except that in step 1), the calcination temperature in the preparation of the carbon nitride-carbon felt composite electrode is 650°C.

[0073] Example 3: The preparation method is similar to that of Example 1, except that in step 2), the mass percentage of I / C in the first catalytic layer of the positive electrode is 50:50, and the mass percentage of I / C in the second catalytic layer of the positive electrode is 40:60.

[0074] Example 4: The preparation method is similar to that of Example 1, except that in step 1), the amount of melamine is 6 g.

[0075] Example 5: The preparation method is the same as that of Example 1, except that: the first and second catalytic layers are coated on both sides of the diaphragm, that is, the membrane electrode has different concentration gradients of the first catalytic layer and the second catalytic layer at both the positive and negative electrodes. The assembled flow battery is tested in an all-vanadium electrolyte system, the flow rate of the all-vanadium electrolyte is 70 ml / min, and the charging current is 100 mA / cm 2 , discharge current is 100mA / cm 2 The charge cut-off voltage is 1.6V, the discharge cut-off voltage is 1.0V, and the charge and discharge are repeated for 500 cycles. The battery performance is shown in Table 1 below.

[0076] Comparative Example 1: The preparation method is the same as that of Example 1, but the membrane electrode has no catalytic layer, only a Nafion115 diaphragm, and there is no catalyst on the carbon felt.

[0077] Comparative Example 2: The preparation method is the same as that of Example 1, but the first and second catalytic layers have the same composition, and the mass percentage of I / C is 25:75. The battery performance is shown in Table 1 below.

[0078] Comparative Example 3: The preparation method of the composite electrode is the same as that of Example 1, but the catalysts used are all commercial carbon nanotube catalysts. According to step 2) of Example 1) the catalyst slurry preparation method is used, and they are coated on Nafion 115 diaphragm and 4 mm commercial carbon felt respectively, and dried at 70°C. The battery performance is shown in Table 1 below.

[0079] Comparative Example 4: A carbon nitride nanoparticle catalyst was prepared by conventional methods. 10 g of melamine was put into a crucible, placed in a muffle furnace, heated to 550°C at a heating rate of 2°C / min, and calcined for 6 hours. The obtained carbon nitride catalyst was then ground into powder and coated on a Nafion 115 diaphragm and a 4 mm commercial carbon felt according to step 2) of Example 1) for preparing the catalyst slurry. The powder was dried at 70°C. The battery performance is shown in Table 1 below.

[0080] Comparative Example 5: The preparation method is the same as that of Example 5, but the membrane electrode has no catalytic layer and is only a Nafion115 diaphragm.

[0081] Table 1

[0082]

[0083]

[0084] Obviously, the above embodiments of the present invention are merely examples for clearly explaining the present invention, and are not intended to limit the implementation methods of the present invention. For those skilled in the art, other different forms of changes or modifications can be made based on the above description. It is impossible to list all implementation methods here. All obvious changes or modifications derived from the technical solution of the present invention are within the spirit scope of the present invention.

Claims

1. A method for preparing a composite electrode material for a flow battery, characterized in that: include: 1) placing a carbonaceous material in a solution containing a nitrogen-containing precursor for hydrothermal reaction, calcining the carbonaceous material after the reaction, and obtaining a graphite-phase carbon nitride nanotube catalyst in situ grown on the carbonaceous material as a carbon nitride-carbon composite electrode; 2) subjecting a solution of a nitrogen-containing precursor to a hydrothermal reaction to obtain a graphite-phase carbon nitride nanotube catalyst powder; 3) preparing the graphite phase carbon nitride nanotube catalyst powder obtained in step 2) into a first catalyst layer slurry, coating the first catalyst layer slurry on one side or both sides of the diaphragm, and drying to obtain a membrane electrode having a first catalyst layer; 4) preparing the graphite phase carbon nitride nanotube catalyst powder obtained in step 2) into a second catalyst layer slurry, coating the second catalyst layer slurry on the membrane electrode having the first catalyst layer in step 3), and drying to obtain a membrane electrode having different concentration gradients of the first catalyst layer and the second catalyst layer at the positive electrode and / or the negative electrode; The mass content of the graphite phase carbon nitride nanotube catalyst powder in the first catalyst layer slurry is less than the mass content of the graphite phase carbon nitride nanotube catalyst powder in the second catalyst layer slurry; 5) The membrane electrode and carbon nitride-carbon felt composite electrode obtained in step 4) are assembled in a liquid flow battery as electrode materials.

2. The preparation method according to claim 1, characterized in that: When obtaining a membrane electrode having different concentration gradients of the first catalytic layer and the second catalytic layer at the positive electrode and the negative electrode, specifically: according to steps 3)-4), the first catalytic layer slurry and the second catalytic layer slurry are respectively coated on the surface of one side of the diaphragm to prepare a membrane electrode having different concentration gradients of the first catalytic layer of the positive electrode and the second catalytic layer of the positive electrode; and then repeating steps 3)-4) to respectively coat the first catalytic layer slurry and the second catalytic layer slurry on the other side of the diaphragm to prepare a membrane electrode having different concentration gradients of the first catalytic layer of the negative electrode and the second catalytic layer of the negative electrode.

3. The preparation method according to claim 1, characterized in that: The nitrogen-containing precursor is one or more of urea, melamine or cyanamide, preferably a mixture of urea and melamine, wherein the molar ratio of urea to melamine is preferably 1 to 10:1; and / or, The solvent in the solution of the nitrogen-containing precursor is a mixture of water and an alcohol reagent. Preferably, the volume ratio of water to the alcohol reagent is 1 to 9:1, and the alcohol reagent is one or more of ethanol, ethylene glycol, and isopropanol.

4. The preparation method according to any one of claims 1 to 3, characterized in that The carbon material is carbon felt, carbon cloth, carbon paper or graphite felt.

5. The preparation method according to any one of claims 1 to 4, characterized in that: The parameters of the hydrothermal reaction in step 1) include: performing the hydrothermal reaction at 150-240° C. for 4-24 hours; and / or, The calcination parameters in step 1) include: heating to 550-650° C. at a heating rate of 1-5° C. / min under inert atmosphere for calcination for 4-8 hours; and / or, Before calcining the reacted carbonaceous material in step 1), the reacted carbonaceous material is further washed and dried, preferably at 60-120° C. for 12-24 hours.

6. The preparation method according to any one of claims 1 to 5, characterized in that: The parameters of the hydrothermal reaction in step 2) include: performing the hydrothermal reaction at 150-240° C. for 4-24 hours.

7. The preparation method according to any one of claims 1 to 6, characterized in that: The first catalyst layer slurry in step 3) includes: carbon nanotube nitride catalyst powder, binder, alcohol reagent and water, the solid content of the slurry is 0.2-2%; the mass ratio of alcohol reagent to water is 8-12:1, and the mass percentage between carbon nanotube nitride catalyst powder and binder is 50-60:50-40.

8. The preparation method according to claim 1 or 7, characterized in that: The second catalyst slurry in step 4) includes: carbon nanotube nitride catalyst powder, binder, alcohol reagent and water, the solid content of the slurry is 0.2-2%; the mass ratio of alcohol reagent to water is 8-12:1, and the mass percentage between carbon nanotube nitride catalyst powder and binder is 60-75:40-25.

9. The preparation method according to any one of claims 1 to 8, characterized in that: The temperature during coating in step 3) is 60-90°C, and the temperature during drying is 60-90°C; the temperature during coating in step 4) is 60-90°C, and the temperature during drying is 60-90°C.

10. A composite electrode material for a liquid flow battery, comprising a carbon nitride-carbon composite electrode and a membrane electrode prepared by the method according to any one of claims 1 to 9.

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