Gradient electrode, preparation method thereof and all-vanadium redox flow battery

Abstract

The invention relates to a gradient electrode, a preparation method thereof and an all-vanadium redox flow battery. The gradient electrode comprises a diffusion layer and a catalyst layer which are sequentially stacked, the diffusion layer comprises more than two diffusion sub-layers, the porosity of the diffusion sub-layers is sequentially increased in the direction away from the catalyst layer, the porosity difference value between the adjacent diffusion sub-layers is smaller than 3%, and the porosity of the catalyst layer is smaller than the porosity of the diffusion sub-layer in direct contact with the catalyst layer. After the gradient electrode disclosed by the invention is assembled, the porosity of each layer of the diffusion layer is consistent, and the porosity of the catalyst layer is smaller than that of the diffusion layer, so that the all-vanadium redox flow battery containing the gradient electrode disclosed by the invention has relatively high energy efficiency.

Description

Technical Field

[0001] This disclosure relates to the field of new energy, specifically to a gradient electrode and its preparation method, and an all-vanadium redox flow battery. Background Technology

[0002] In recent years, renewable energy has been vigorously developed to address issues such as energy shortages and environmental pollution, and to maintain a crucial path of sustainable development. However, wind and solar energy are characterized by intermittency, volatility, and time-varying nature, which can impact the stable operation of the power grid. Therefore, vanadium redox flow batteries (VRFBs), which can operate in conjunction with intermittent renewable energy sources, have attracted attention. VRFBs offer advantages such as independent capacity, high power output, long lifespan, and ease of operation, demonstrating broad development prospects in large-scale energy storage and creating significant demand in renewable energy power generation and smart grid construction.

[0003] For electrodes, higher porosity near the plate side is crucial for good mass transfer. Otherwise, reduced electrode utilization, especially in areas far from the plate side, inevitably increases battery polarization and degrades battery performance. While increasing porosity improves mass transfer to some extent, assembling flow batteries into a stack can lead to uneven electrode porosity, resulting in uneven electrolyte distribution and other problems. Furthermore, the electrode surface near the membrane side is the primary reaction region, making membrane-side electrochemical activity a major limiting factor.

[0004] Patent CN106558704A discloses a gradient electrode with a structure where the porosity decreases from the plate side to the film side along the thickness direction. However, this electrode structure cannot avoid the problem of uneven porosity after the battery is pressed together, and the improvement of activation polarization by adjusting the porosity alone is very limited. Patents CN117059828A and CN117080475A prepare integrated gradient electrodes by impregnating the electrode with resin and organic solvent, followed by freeze-drying, activation treatment, and other complex operations. The above methods have the disadvantages of cumbersome steps, toxic and harmful organic solvents, high cost, and difficulty in scale-up. Therefore, there is an urgent need to develop an integrated electrode material with a simple preparation method and low cost. Summary of the Invention

[0005] The purpose of this disclosure is to provide a gradient electrode, its preparation method, and an all-vanadium redox flow battery. After assembly, the gradient electrode of this disclosure has consistent porosity in each layer of the diffusion layer and the porosity of the catalyst layer is less than that of the diffusion layer. The electrolyte has good fluidity and uniform distribution in the electrode. The all-vanadium redox flow battery with the gradient electrode of this disclosure has high energy efficiency.

[0006] To achieve the above objectives, the first aspect of this disclosure provides a gradient electrode comprising a diffusion layer and a catalyst layer stacked sequentially; the diffusion layer comprises two or more diffusion sublayers, and the porosity of the diffusion sublayers increases sequentially along the direction away from the catalyst layer, the porosity difference between adjacent diffusion sublayers is less than 3%, and the porosity of the catalyst layer is less than the porosity of the diffusion sublayers in direct contact with the catalyst layer.

[0007] Optionally, the maximum porosity of the diffuser layer is 90-98%, preferably 92.5-95%; and the porosity of the catalyst layer is 50-92%, preferably 80-89%.

[0008] Optionally, the number of diffusion sublayers is 2-8, preferably 2-4, and the porosity difference between adjacent diffusion sublayers is 0.5-2%.

[0009] Optionally, the thickness of the gradient electrode is 2-6.5 mm, preferably 2.5-5.5 mm; the thickness of the diffusion layer is 30-90% of the thickness of the gradient electrode, preferably 45-75%.

[0010] Optionally, the surface of the diffusion layer contains oxygen-containing functional groups, which are carbonyl and / or carboxyl groups; the surface of the catalyst layer contains modified heteroatoms, which are selected from one or more of N, P and O.

[0011] Optionally, the materials of the diffuser layer and the catalyst layer are each independently one or more of carbon felt, carbon cloth, carbon paper and electrospun fiber felt.

[0012] The second aspect of this disclosure provides a method for preparing the gradient electrode provided in the first aspect of this disclosure. The method includes: stacking a catalytic layer and each diffuser layer in sequence along the thickness direction in order of increasing or decreasing porosity, and forming them into a single unit by longitudinal needle punching along the thickness direction of the stack.

[0013] Optionally, the method further includes: subjecting the catalyst layer precursor to plasma treatment, a first calcination treatment, or chemical treatment to obtain the catalyst layer; The plasma treatment conditions include: using a plasma source to perform plasma treatment on the catalyst layer precursor, wherein the plasma source is one or more of air, oxygen, argon, ammonia, nitrogen, nitric oxide, and nitrogen dioxide; The conditions for the first calcination treatment include: an atmosphere of ammonia and / or nitrogen, a temperature of 200-900℃, and a time of 0.5-8h; The chemical treatment is selected from one or more of the following: heat treatment, hydrothermal treatment, acid treatment, alkali treatment, and Fenton's reagent treatment.

[0014] Optionally, the method further includes: subjecting the diffuser layer precursor to a second calcination to obtain the diffuser layer; The conditions for the second roasting include: an initial temperature of 10-30℃, an final temperature of 200-700℃, a holding time of 0.5-10 hours, a heating rate of 1-10℃ / min, and an atmosphere of oxygen, ozone, water vapor, air, or carbon dioxide.

[0015] Optionally, the acupuncture conditions include: an acupuncture density of 3-55 needles / cm². 2 The needle insertion depth is 4-20mm.

[0016] A third aspect of this disclosure provides an all-vanadium redox flow battery, the all-vanadium redox flow battery including the gradient electrode provided in the first aspect of this disclosure.

[0017] The present disclosure has the following advantages through the above technical solution: (1) The gradient electrode of this disclosure has consistent porosity in each layer of the diffusion layer after assembly and the porosity of the catalyst layer is smaller than that of the diffusion layer. This makes the vanadium redox flow battery have sufficient electrode active sites, and also makes the electrolyte have good fluidity and uniform distribution in the electrode. The vanadium redox flow battery has high energy efficiency.

[0018] (2) The method for preparing gradient electrodes disclosed herein is simple, low-cost, and easy to scale up.

[0019] Other features and advantages of this disclosure will be described in detail in the following detailed description section. Attached Figure Description

[0020] The accompanying drawings are provided to further illustrate the present disclosure and form part of the specification. They are used together with the following detailed description to explain the present disclosure, but do not constitute a limitation thereof. In the drawings: Figure 1 This is a schematic diagram of a specific embodiment of the gradient electrode disclosed herein.

[0021] Explanation of reference numerals in the attached figures Detailed Implementation

[0022] The specific embodiments of this disclosure will be described in detail below with reference to the accompanying drawings. It should be understood that the specific embodiments described herein are for illustration and explanation only and are not intended to limit this disclosure.

[0023] The first aspect of this disclosure provides a gradient electrode, the gradient electrode comprising a diffusion layer and a catalyst layer stacked sequentially; the diffusion layer comprises two or more diffusion sublayers, the porosity of the diffusion sublayers increasing sequentially along the direction away from the catalyst layer, the porosity difference between adjacent diffusion sublayers being less than 3%, and the porosity of the catalyst layer being less than the porosity of the diffusion sublayers in direct contact with the catalyst layer.

[0024] In this disclosure, after assembling the gradient electrode into a battery, the porosity of each layer is consistent after compression, which effectively avoids the phenomenon of uneven distribution of electrolyte in the electrode; and the porosity of the catalyst layer is lower than that of the diffusion layer, giving the gradient electrode more active sites. This disclosure allows for Micro-CT testing of the gradient electrode, and the porosity of each layer can be obtained by modeling and analyzing the gradient electrode using computer software. The computer software used is well known to those skilled in the art and will not be described in detail here.

[0025] The gradient electrode disclosed herein comprises a diffusion layer consisting of multiple diffusion sublayers with different porosities. After assembly, the gradient electrode exhibits consistent porosity across all diffusion layers, with the catalytic layer having a lower porosity than the diffusion layer. This ensures that the vanadium redox flow battery possesses sufficient electrode active sites while also exhibiting good fluidity and uniform distribution of the electrolyte within the electrode. Consequently, the vanadium redox flow battery containing the gradient electrode of this disclosure exhibits high energy efficiency.

[0026] In one specific embodiment of this disclosure, the maximum porosity of the diffuser layer is 90-98%, preferably 92.5-95%; and the porosity of the catalyst layer is 50-92%, preferably 80-89%.

[0027] According to this disclosure, the number of diffuser layers can vary within a wide range. In one specific embodiment of this disclosure, the number of diffuser layers is 2-8, preferably 2-4, and the porosity difference between adjacent diffuser layers is 0.5-2%. Within the above preferred range, the gradient electrode of this disclosure has better electrode active sites and better electrolyte flowability and uniform distribution when used in vanadium redox flow batteries.

[0028] According to this disclosure, the thickness of the electrode is not specifically limited and can be selected according to actual needs. In one embodiment, the electrode thickness is 2-6.5 mm, preferably 2.5-5.5 mm. According to this disclosure, the thickness of the diffusion layer can vary within a wide range. In one specific embodiment, the thickness of the diffusion layer is 30-90% of the thickness of the gradient electrode, preferably 45-75%. This disclosure does not specifically limit the thickness of each diffusion sublayer and can also be selected according to actual needs, for example, it can be 0.5-3 mm.

[0029] According to this disclosure, the surface of the diffusion layer contains oxygen-containing functional groups, which are carbonyl and / or carboxyl groups; the surface of the catalyst layer contains modified heteroatoms, which are selected from one or more of N, P, and O. In the above embodiments, the diffusion layer of the gradient electrode has better hydrophilicity, and the catalyst layer has more active sites, thereby further improving the energy efficiency of the all-vanadium redox flow battery with the gradient electrode of this disclosure.

[0030] like Figure 1 As shown, in one specific embodiment of this disclosure, the gradient electrode includes a diffusion layer and a catalyst layer 3 stacked sequentially; the diffusion layer includes a first diffuser layer 1 and a second diffuser layer 2. The surfaces of the first diffuser layer 1 and the second diffuser layer 2 each independently contain oxygen-containing functional groups 4, and the surface of the catalyst layer contains modified heteroatoms 5.

[0031] In one specific embodiment of this disclosure, the materials of the diffuser layer and the catalyst layer are each independently one or more of carbon felt, carbon cloth, carbon paper and electrospun fiber felt.

[0032] The second aspect of this disclosure provides a method for preparing the gradient electrode provided in the first aspect of this disclosure. The method includes: stacking a catalytic layer and each diffuser layer in sequence along the thickness direction in order of increasing or decreasing porosity, and forming them into a single unit by longitudinal needle punching along the thickness direction of the stack.

[0033] The method disclosed herein can prepare a gradient electrode with uniform diffusion layer porosity and minimal catalyst layer porosity. The method is simple, low-cost, and easy to scale up. When the prepared gradient electrode is used in a vanadium redox flow battery, the battery exhibits high energy efficiency.

[0034] According to this disclosure, the thickness direction refers to the thickness direction of the stack formed by the superposition of the catalyst layer and each diffuser sublayer. In this disclosure, the needle tip of the needle is directed toward the main surface of the stack, and the needle tip can be inclined or vertical. In a specific embodiment of this disclosure, the process of forming a single unit by longitudinal needle punching along the thickness direction of the stack includes: forming a single unit by longitudinal needle punching in a direction perpendicular to the catalyst layer or diffuser sublayer.

[0035] In a preferred embodiment of this disclosure, the method includes: setting aside the diffuser layer with the largest porosity for later use; and during the needle punching process, sequentially stacking the remaining diffuser layers and the catalyst layer on the diffuser layer along the thickness direction in order of decreasing porosity until the needle punching is completed.

[0036] In another preferred embodiment of this disclosure, the method includes: setting aside the catalyst layer for later use, and during the needle-punching process, sequentially stacking diffuser layers on the catalyst layer along the thickness direction in order of increasing porosity until the needle-punching is completed. In the above embodiment, "diffusion process" means that each additional diffuser layer or catalyst layer is formed into a single unit through needle-punching.

[0037] To provide the catalyst layer with more active sites, in one specific embodiment of this disclosure, the method further includes: subjecting the catalyst layer precursor to plasma treatment, a first calcination treatment, or chemical treatment to obtain the catalyst layer, thereby introducing modified heteroatoms onto its surface. According to this disclosure, the plasma treatment method may include: performing plasma treatment on the surface of the graphite fiber felt using a plasma source, wherein the plasma source is one or more of air, oxygen, argon, ammonia, nitrogen, nitric oxide, nitrogen dioxide, and other plasma sources. The conditions for the first calcination treatment may include: an atmosphere of ammonia and / or nitrogen, a temperature of 200-900°C, and a time of 0.5-8 hours; the first calcination treatment can be performed in apparatus conventionally used by those skilled in the art, such as a muffle furnace. The chemical treatment is selected from one or more of heat treatment, hydrothermal treatment, acid treatment, alkali treatment, and Fenton's reagent treatment. Fenton's reagent treatment refers to treating the catalyst layer precursor with a highly oxidizing system composed of hydrogen peroxide and ferrous ions. The specific methods of the above treatments are well known to those skilled in the art and will not be described in detail here.

[0038] According to this disclosure, the diffuser layer precursor and the catalyst layer precursor are each independently selected from carbon felt, carbon cloth, carbon paper, and electrospun fiber felt, etc.

[0039] In one specific embodiment of this disclosure, the method further includes: subjecting the diffuser layer precursor to a second calcination to obtain the diffuser layer, thereby modifying the diffuser layer by introducing oxygen-containing functional groups onto its surface and improving the hydrophilicity of the diffuser layer. According to this disclosure, the conditions for the second calcination may include: an initial temperature of 10-30°C, preferably 15-25°C; an final temperature of 200-700°C, preferably 250-500°C; a holding time of 0.5-10 hours, preferably 3-6 hours; a heating rate of 1-10°C / min, preferably 3-7°C / min; and an atmosphere of oxygen, ozone, water vapor, air, or carbon dioxide.

[0040] In one specific embodiment of this disclosure, the acupuncture conditions include: an acupuncture density of 3-55 needles / cm². 2 The needle penetration depth is 4-20mm. The needle used in this disclosure can be a standard triangular needle, with barbs on both sides of the working section, and the upper and lower barbs are not on the same plane.

[0041] A third aspect of this disclosure provides an all-vanadium redox flow battery, the all-vanadium redox flow battery including the gradient electrode provided in the first aspect of this disclosure.

[0042] In one specific embodiment of this disclosure, the all-vanadium redox flow battery further includes a bipolar plate and a separator, wherein the diffusion layer of the gradient electrode is in contact with the bipolar plate and the catalytic layer is in contact with the separator.

[0043] The present invention will be further illustrated by the following examples, but the present invention is not limited thereto.

[0044] Unless otherwise specified, all raw materials used in the examples and comparative examples were commercially available.

[0045] Example 1 like Figure 1 As shown, in this embodiment, a gradient electrode is prepared using three layers of graphite fiber felt, wherein the thickness of the first graphite fiber felt is 1.7 mm and the porosity is 94.5%; the thickness of the second graphite fiber felt is 0.9 mm and the porosity is 93%; and the thickness of the third graphite fiber felt is 1.7 mm and the porosity is 89%.

[0046] The first and second graphite fiber felts serve as the first diffuser layer 1 and the second diffuser layer 2, respectively, and the third graphite fiber felt serves as the catalyst layer 3. The surfaces of the first and second graphite fiber felts contain oxygen-containing functional groups. The first and second graphite fiber felts are obtained by heat-treating the original graphite fiber felt in a muffle furnace for 5 hours at an initial temperature of 20°C, an final temperature of 500°C, and a heating rate of 3°C / min. The third graphite fiber felt contains modified nitrogen atoms on its surface and is prepared by immersing the original graphite fiber felt in a 15% by weight urea aqueous solution and holding it at 200°C for 4 hours via hydrothermal treatment.

[0047] First and second graphite fiber felts were stacked along the thickness direction and needled together longitudinally to form a single unit. Then, a third graphite fiber felt was stacked along the thickness direction and needled together to obtain a gradient electrode with a thickness of 4.3 mm. The needle-punching density was 10 needles / cm². 2 The needle depth was 11 mm. The needle used in the experiment was a standard triangular needle with barbs on both sides of the working section. The upper and lower barbs were not on the same plane.

[0048] The gradient electrode was subjected to Micro-CT testing and analysis. The gradient electrode was modeled and analyzed using computer software, which showed that the porosity of each fiber layer was basically the same under a pressure of 0.1 MPa.

[0049] Example 2 The gradient electrode was prepared using the same method as in Example 1, except that the first graphite fiber felt, the second graphite fiber felt and the third graphite fiber felt were not surface modified. That is, the surfaces of the first graphite fiber felt and the second graphite fiber felt did not contain oxygen-containing functional groups, while the surface of the third graphite fiber felt contained modified atoms.

[0050] Example 3 The gradient electrodes were prepared using the same method as in Example 1, except that the porosity of the first graphite fiber felt was 95% and the porosity of the second graphite fiber felt was 92.5%.

[0051] Example 4 The gradient electrode was prepared using the same method as in Example 1, except that the thickness of the first graphite fiber felt was 1.7 mm and the porosity was 94.5%; the thickness of the second graphite fiber felt was 0.9 mm and the porosity was 93%; and the thickness of the third graphite fiber felt was 1.7 mm and the porosity was 90%.

[0052] Example 5 The gradient electrode was prepared using the same method as in Example 1, except that in this example, four layers of graphite fiber felt were used to prepare the gradient electrode. The first graphite fiber felt had a thickness of 1.7 mm and a porosity of 96%; the second graphite fiber felt had a thickness of 0.9 mm and a porosity of 94%; the third graphite fiber felt had a thickness of 0.9 mm and a porosity of 93%; and the fourth graphite fiber felt had a thickness of 1.7 mm and a porosity of 89%.

[0053] The first, second, and third graphite fiber felts serve as the first, second, and third diffusing sublayers, respectively, while the fourth graphite fiber felt serves as the catalyst layer. The surfaces of the first, second, and third graphite fiber felts contain oxygen-containing functional groups, while the surface of the fourth graphite fiber felt contains modified atoms. The first, second, and third graphite fiber felts, containing oxygen-containing functional groups, were obtained by heat-treating the original graphite fiber felt in a muffle furnace for 5 hours at an initial temperature of 20°C, an final temperature of 500°C, and a heating rate of 3°C / min. The fourth graphite fiber felt, containing modified nitrogen atoms, was prepared by immersing the graphite fiber felt in a 15% by weight urea aqueous solution and holding it at 200°C for 4 hours via hydrothermal treatment.

[0054] Comparative Example 1 This comparative example uses a gradient electrode prepared with two layers of graphite fiber felt. The first graphite fiber felt has a thickness of 2.6 mm and a porosity of 94.5%, while the second graphite fiber felt has a thickness of 1.7 mm and a porosity of 89%. The first graphite fiber felt serves as the diffuser layer, and the second graphite fiber felt serves as the catalyst layer. The surface of the first graphite fiber felt contains oxygen-containing functional groups and was prepared using the same heat treatment method as in Example 1. The surface of the second graphite fiber felt contains modified atoms and was prepared using the same hydrothermal treatment method as in Example 1.

[0055] The first and second graphite fiber felts were stacked along their thickness direction and then longitudinally needled together to obtain a gradient electrode with a thickness of 4.3 mm. The needle-punching density was 10 needles / cm. 2 The needle depth was 11 mm. The needle used in the experiment was a standard triangular needle with barbs on both sides of the working section. The upper and lower barbs were not on the same plane.

[0056] Comparative Example 2 The gradient electrode was prepared using the same method as in Example 1, except that the thickness of the first graphite fiber felt was 1.7 mm and the porosity was 94.5%; the thickness of the second graphite fiber felt was 0.9 mm and the porosity was 91%; and the thickness of the third graphite fiber felt was 1.7 mm and the porosity was 89%.

[0057] Test case The gradient electrodes prepared in the examples and comparative examples were assembled into an all-vanadium redox flow battery, wherein the diffusion layer of the gradient electrode is close to the bipolar plate and the catalyst layer is close to the separator. The battery was assembled into an all-vanadium redox flow battery under the compression of end plates. The thickness of the three-layer gradient electrode after compression is approximately 3.2 mm, and the thickness of the four-layer gradient electrode after compression is approximately 4.0 mm, with an area of ​​28 cm². 2 The positive electrode electrolyte is 1.5M VO. 2+ 40 mL of 3M H₂SO₄ solution and 1.5 MV negative electrode electrolyte. 3+ 40 mL of 3M H2SO4 solution.

[0058] The obtained vanadium redox flow battery was subjected to constant current charge-discharge at a current density of 100 mA / cm². 2 Under the given conditions, a charge-discharge experiment was conducted, and the energy efficiency was recorded. The test results are shown in Table 1.

[0059] Table 1

[0060] As can be seen from the above, the gradient electrode prepared in the embodiments of this disclosure has consistent porosity in each layer of the diffusion layer after assembly, which makes the electrolyte have good fluidity and uniform distribution in the electrode. The all-vanadium redox flow battery containing the gradient electrode of this disclosure has high energy efficiency.

[0061] The preferred embodiments of this disclosure have been described in detail above with reference to the accompanying drawings. However, this disclosure is not limited to the specific details of the above embodiments. Within the scope of the technical concept of this disclosure, various simple modifications can be made to the technical solutions of this disclosure, and these simple modifications all fall within the protection scope of this disclosure.

[0062] It should also be noted that the various specific technical features described in the above embodiments can be combined in any suitable manner without contradiction. To avoid unnecessary repetition, this disclosure will not describe the various possible combinations separately.

[0063] Furthermore, various different embodiments of this disclosure can be combined in any way, as long as they do not violate the spirit of this disclosure, they should also be regarded as the content disclosed in this disclosure.

Claims

1. A gradient electrode, characterized in that, The gradient electrode includes a diffusion layer and a catalyst layer stacked sequentially; the diffusion layer includes two or more diffusion sublayers, and the porosity of the diffusion sublayers increases sequentially along the direction away from the catalyst layer, the porosity difference between adjacent diffusion sublayers is less than 3%, and the porosity of the catalyst layer is less than the porosity of the diffusion sublayers in direct contact with the catalyst layer.

2. The gradient electrode according to claim 1, wherein, The maximum porosity of the diffuser layer is 90-98%, preferably 92.5-95%; the porosity of the catalyst layer is 50-92%, preferably 80-89%.

3. The gradient electrode according to claim 1, wherein, The number of diffusion sublayers is 2-8, preferably 2-4, and the porosity difference between adjacent diffusion sublayers is 0.5-2%.

4. The gradient electrode according to claim 1, wherein, The thickness of the gradient electrode is 2-6.5 mm, preferably 2.5-5.5 mm; the thickness of the diffusion layer is 30-90% of the thickness of the gradient electrode, preferably 45-75%.

5. The gradient electrode according to claim 1, wherein, The surface of the diffusion layer contains oxygen-containing functional groups, which are carbonyl and / or carboxyl groups; the surface of the catalyst layer contains modified heteroatoms, which are selected from one or more of N, P and O.

6. The gradient electrode according to claim 1, wherein, The materials of the diffuser layer and the catalyst layer are each independently one or more of carbon felt, carbon cloth, carbon paper and electrospun fiber felt.

7. A method for preparing the gradient electrode according to any one of claims 1-6, characterized in that, The method includes: stacking the catalyst layer and each diffuser layer in sequence according to the order of increasing or decreasing porosity, and forming them into a single unit by longitudinal needle punching along the thickness direction of the stack.

8. The method according to claim 7, wherein, The method further includes: subjecting the catalyst layer precursor to plasma treatment, a first calcination treatment, or chemical treatment to obtain the catalyst layer; The plasma treatment method includes: performing plasma treatment on the catalyst layer precursor using a plasma source, wherein the plasma source is one or more of air, oxygen, argon, ammonia, nitrogen, nitric oxide, and nitrogen dioxide. The conditions for the first calcination treatment include: an atmosphere of ammonia and / or nitrogen, a temperature of 200-900℃, and a time of 0.5-8h; The chemical treatment is selected from one or more of the following: heat treatment, hydrothermal treatment, acid treatment, alkali treatment, and Fenton's reagent treatment.

9. The method according to claim 7, wherein, The method further includes: subjecting the diffuser sublayer precursor to a second calcination to obtain the diffuser sublayer; The conditions for the second roasting include: an initial temperature of 10-30℃, an final temperature of 200-700℃, a holding time of 0.5-10 hours, a heating rate of 1-10℃ / min, and an atmosphere of oxygen, ozone, water vapor, air, or carbon dioxide.

10. The method according to claim 7, wherein, The conditions for acupuncture include: a needle density of 3-55 needles / cm². 2 The needle insertion depth is 4-20mm.

11. A vanadium redox flow battery, characterized in that, The vanadium redox flow battery includes the gradient electrode as described in any one of claims 1-6.

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