A composite electrode for a vanadium redox flow battery and a method of making the same

Abstract

The application belongs to the technical field of batteries, and discloses a composite electrode for a full vanadium redox flow battery and a preparation method thereof. The application performs surface modification on an acid-treated carbon felt substrate, sequentially constructs a porous high-entropy oxide (HEO) core and a Bi monatomic shell base (Bi-N4@NC), and modifies a hydrogen evolution inhibition base (Cr2O3 nano-island modification). Through multi-element catalysis of the porous high-entropy oxide core, electronic regulation of the bismuth monatomic shell, and local passivation of the Cr2O3 nano-island site shielding base, precise division of elements and spatial function partition are realized, technical difficulties such as high-density operation of the composite electrode, radical treatment of side reactions, and low cost and long service life are solved, contradictions between hydrogen evolution inhibition and reaction activity are balanced, and three functions of charging catalysis, discharging catalysis and hydrogen evolution inhibition are simultaneously realized on a single electrode, thereby providing a core solution for electrode design of a high-power flow battery.

Description

Technical Field

[0001] This invention belongs to the field of battery-related technology, specifically relating to a composite electrode for vanadium redox flow batteries and its preparation method. Background Technology

[0002] The surge in global energy demand and increasing environmental pressures are driving the transition of power systems to renewable energy. However, the inherent intermittency and volatility of renewable energy sources such as wind and solar power pose challenges to grid stability, necessitating efficient energy storage technologies to improve energy utilization efficiency. Vanadium redox flow batteries (VRFBs) are considered a promising energy storage solution due to their high safety, long cycle life, good environmental compatibility, and significant cost-effectiveness over their entire lifecycle. Electrode materials, as core components affecting the electrochemical performance of VRFBs, are crucial in their properties. Currently, widely used carbon-based electrode materials (such as graphite, carbon paper, carbon felt, and carbon cloth) are often limited by issues such as limited effective electrochemical active area, insufficient intrinsic catalytic activity, structural aging and degradation during long cycles, and hydrogen evolution reaction (HER). Therefore, performance optimization of carbon-based electrode materials is urgently needed.

[0003] Currently, common modification strategies for carbon-based electrode materials include catalyst loading, surface etching, heteroatom doping, and composite material construction. For VRFB systems, an ideal modified electrode needs to synergistically enhance the catalytic activity of vanadium ion redox reactions (charge / discharge) and effectively suppress hydrogen evolution side reactions. However, existing technologies lack a composite optimization scheme that can simultaneously achieve these three functions. For example, Chinese invention patent CN104319409 A discloses a highly active asymmetric electrode based on bismuth-based catalyst-deposited carbon nanofibers. While this electrode can suppress HER and improve capacity retention to some extent, its lack of optimization of the ion desolvation process results in low charging efficiency and poor cycle stability. Chinese invention patent CN118472289 A discloses a high-entropy alloy composite electrode for all-vanadium redox flow batteries. This electrode mainly relies on the conductivity of the high-entropy alloy, lacks targeted optimization of key electrode reaction kinetics, and does not integrate a hydrogen evolution suppression mechanism. Summary of the Invention

[0004] The present invention aims to solve at least one of the technical problems existing in the prior art. To this end, the present invention proposes a composite electrode for vanadium redox flow batteries and a method for preparing the same. This composite electrode can not only simultaneously improve the charging / discharging catalytic activity of vanadium redox flow batteries, but also effectively suppress hydrogen evolution side reactions, thereby achieving complementary functions and lifespan assurance.

[0005] To address the aforementioned technical problems, a first aspect of the present invention provides a method for preparing a composite electrode, comprising the following steps:

[0006] (1) The carbon felt substrate is acid-treated to obtain an acid-treated carbon felt substrate.

[0007] (2) The acid-treated carbon felt substrate is immersed in a high-entropy oxide precursor solution and subjected to a hydrothermal reaction; then a pore-forming agent is added to the reaction product and calcined under a reducing atmosphere to obtain a porous high-entropy oxide core and carbon felt composite, denoted as HEO / carbon felt composite.

[0008] The high-entropy oxide precursor solution contains a metal compound and a surfactant, wherein the metal compound includes equimolar amounts of Bi(NO3)3 and (NH4)6Mo7O. 24 (NH4) 10 W 12 O 41 and Ce(NO3)3;

[0009] (3) The HEO / carbon felt composite is immersed in a metal-organic framework precursor solution, which includes bismuth salt and organic ligand; it is allowed to stand to form a ZIP-8 type MOF group; then the ZIP-8 type MOF group is pyrolyzed to form a nitrogen-carbon coated Bi single-atom shell group on the surface of the porous high-entropy oxide core, denoted as HEO / Bi-N4@NC, to obtain the HEO / Bi-N4@NC / carbon felt composite.

[0010] (4) The HEO / Bi-N4@NC / carbon felt composite is pulse-deposited in an electrolyte containing chromium salt and ammonium salt; after annealing, Cr2O3 crystals are generated to obtain the Cr2O3 nano-island modified HEO / Bi-N4@NC / carbon felt composite, which is the composite electrode.

[0011] In the fabrication of the composite electrode of this invention, the carbon felt substrate is first acid-treated to remove the surface inert layer, forming a micron-scale trench structure, increasing the specific surface area, and introducing oxygen-containing functional groups. Then, a high-entropy oxide (HEO) core is constructed, and a pore-forming agent is added to form porous HEO nanoparticles, improving reaction kinetics, exposing highly active crystal faces, and enhancing structural stability, thus forming an HEO / carbon felt composite. Next, a Bi single-atom shell (Bi-N4@NC) is constructed, utilizing the MOF confinement effect to anchor Bi single atoms, achieving atomic-level catalytic redox and enhancing interfacial bonding. Finally, hydrogen evolution inhibitory groups (Cr2O3 nanoislands) are added for precise site shielding to suppress hydrogen evolution activity.

[0012] Specifically, this invention employs porous BiMoWCeO2 high-entropy oxide, which, compared to traditional porous high-entropy alloys (such as ZrSnMnBiCo), has at least the following superior technical effects: 1) Corrosion resistance and structural stability. The Ce-O bond energy in BiMoWCeO2 is as high as 795 kJ / mol, which is beneficial for forming a protective oxide layer, significantly reducing the corrosion rate of the electrode surface, and Ce... 3 + / Ce 4+ Redox pairs drive the formation and filling of oxygen vacancies through reversible valence state switching, achieving a closed loop of "release-replenishment" of lattice oxygen, alleviating structural degradation, and endowing materials with self-healing capabilities. However, porous high-entropy alloys accelerate electrolyte penetration during cycling, dissolving active metals and easily causing electrode pulverization. 2) Electrochemical activity and reaction kinetics. BiMoWCeO2 contains a high concentration of oxygen vacancies, forming localized electron-rich regions, enhancing electronic conductivity, and the dangling bonds of oxygen vacancies strongly adsorb H₂. + The formation of surface hydroxyl groups locally increases the solution pH, enhances electrolyte affinity, inhibits hydrogen evolution side reactions, and reduces V. 3+ Desolvation energy barrier. High-entropy alloys rely solely on metallic conductivity, lack controllable oxygen defects, and cannot optimize proton transport pathways, resulting in a high desolvation energy barrier. 3) Ion selectivity and interface characteristics. The fluorite lattice distortion of BiMoWCeO2 forms channels with a pore size of approximately 0.38 nm, allowing H... + It spreads rapidly, but blocks [VO]. 2+ And the high-valence cation W 6+ It repels positively charged vanadium ions, further suppressing cross-contamination. However, high-entropy alloy materials lack ion sieving on their surface, allowing vanadium ions and H₂ to pass through. + Competitive adsorption can easily trigger side reactions.

[0013] This invention utilizes α-Cr2O3 crystals obtained through pulse deposition followed by annealing, which offers at least the following advantages over traditional electrodeposited alloys (such as lead-bismuth alloys): 1) Ensuring material structure and stability. Pulse-deposited amorphous Cr(OH)3 crystals allow ions to migrate to graphite defect sites during the pulse off-phase, forming stable CO-Cr covalent bonds. The α-Cr2O3 formed after annealing forms a strong interfacial bond with the carbon felt substrate through epitaxial growth. Furthermore, α-Cr2O3 has a high melting point and a thermal expansion coefficient matching that of graphite, eliminating the risk of structural stripping during cycling. In contrast, electrodeposited alloys adhere to the graphite felt surface through physical adsorption or weak bonding, making them prone to elemental dissolution in strongly acidic electrolytes. Moreover, alloys generally have low melting points, and atomic diffusion intensifies under battery operating conditions, leading to particle agglomeration and coarsening, exposing more carbon felt substrate and triggering hydrogen evolution side reactions. 2) Further suppressing hydrogen evolution. The nano-islands of α-Cr2O3 crystals in this invention match the surface defects of the carbon felt fibers, allowing for selective coverage of hydrogen evolution active sites with a low loading. Furthermore, α-Cr₂O₃ crystal is a p-type semiconductor, forming an electronic barrier that suppresses H₂. + Reduction, Cr 3+ 3D 3 Electronic construction suppresses H + Adsorption. Alloys, however, require a high loading to cover the active sites, and electric field distortion exists at the edges of alloy particles, inducing H... + Enrichment at the particle-graphite interface increases the local hydrogen evolution rate. 3) Enhanced catalytic-inhibition synergistic performance. The interstices between nanoislands expose reaction sites, and pulse deposition optimizes pore connectivity, improving the ion diffusion coefficient. During long-term cycling, α-Cr2O3 crystals form a passivation layer in an acidic environment, repairing lattice distortion during cycling. Lead, however, inhibits hydrogen evolution, delaying catalytic activity, and high lead content easily leads to Cr... 3+ The reduction kinetics decrease.

[0014] Furthermore, the three functional modification sequences described above in this invention are related and specific. First, a porous BiMoWCeO2 high-entropy oxide is formed through hydrothermal synthesis followed by instantaneous high-temperature reduction, providing oxygen vacancies induced by lattice distortion and a mesoporous structure, laying the foundation for thermodynamic adsorption sites and spatial carriers for single-atom anchoring. Without the HEO core, the carbon felt surface is inert and has uneven conductivity, making it difficult to achieve uniform single-atom dispersion during subsequent MOF confined growth. Constructing a ZIF-8 shell on the HEO core surface to form a Bi-N4@NC shell creates efficient electronic channels through MOF confinement and interfacial covalent bonds, achieving dual functions of catalysis and conduction. Finally, pulse deposition-annealing generates α-Cr2O3 crystals, completely blocking the hydrogen evolution path through geometric shielding and semiconductor bandgap design. Simultaneously, from the perspective of gradient temperature matching requirements, the HEO core needs to be synthesized at high temperature instantaneously to activate the high-entropy effect; the pyrolysis temperature of the Bi-N4@NC shell is lower, while the annealing temperature of Cr2O3 is even lower. If the operation is reversed, i.e., the HEO high-temperature step is performed last, although Cr2O3 will not melt, lattice expansion will induce interfacial stress cracks. From the perspective of chemical bonding sequence, HEO oxygen vacancies must first be formed, followed by MOF-N coordination, then pyrolysis to form NOC bonds, and finally, Cr2O3 epitaxial growth. If the order is reversed, the lack of oxygen vacancies will prevent the formation of NOC bonds.

[0015] Therefore, the three functional modifications of this invention must strictly follow the logic of "substrate activation - catalytic site construction - side reaction suppression." Any change in the order will disrupt the interfacial bonding sequence or cause high-temperature structural collapse. If Cr2O3 is deposited first and then the HEO core is constructed, the α-Cr2O3 nanoislands will block the carbon felt surface, preventing the HEO precursor from contacting the substrate, and the hydrothermal reaction will only generate free particles rather than a core-shell structure. If the Bi-N4@NC shell is constructed first and then the HEO core is deposited, the high-temperature reduction will destroy the Bi-N4 coordination structure, causing Bi atoms to aggregate into nanoparticles and resulting in graphitization of the MOF carbon framework, thus losing its confinement effect.

[0016] In some embodiments of the present invention, in step (1), the solution used for the acid treatment includes H2SO4 and H2O2, and the volume ratio of H2SO4 to H2O2 is (1-3):1.

[0017] In some embodiments of the present invention, in step (1), the acid treatment process is as follows: the carbon felt substrate is immersed in a mixture of H2SO4 and H2O2, subjected to hot reflux, ultrasonically cleaned and dried to obtain the acid-treated carbon felt substrate.

[0018] In some embodiments of the present invention, in step (1), the hot reflux is performed at a temperature of 60-100°C for 2-5 hours.

[0019] In some embodiments of the present invention, in step (2), the preparation process of the entropy oxide precursor solution is as follows: equimolar amounts of Bi(NO3)3 and (NH4)6Mo7O are added. 24 (NH4) 10 W 12 O 41 Ce(NO3)3 is dissolved in ethylene glycol, and hexadecyltrimethylammonium bromide is added and mixed to obtain the product.

[0020] In some embodiments of the present invention, the concentration of Bi(NO3)3 in the entropy oxide precursor solution is 0.02-0.03 mol / L, and the concentration of the surfactant is 0.1-0.2 mol / L.

[0021] In some embodiments of the present invention, in step (2), the pore-forming agent includes zinc powder, and the mass ratio of the pore-forming agent to Bi(NO3)3 is (15-25):1.

[0022] In some embodiments of the present invention, in step (2), the hydrothermal reaction is carried out at a temperature of 160-200°C for 10-30 hours.

[0023] In some embodiments of the present invention, in step (2), the calcination temperature is 2-5 seconds, that is, instantaneous synthesis at a high temperature of 800-1200℃ for 2-5 seconds.

[0024] In some embodiments of the present invention, in step (2), the reducing atmosphere is an H2 / Ar atmosphere, and the volume ratio of H2 to Ar is (5-10):(90-95).

[0025] In some embodiments of the present invention, in step (3), the metal-organic framework precursor solution is a methanol solution containing BiCl3 and 2-methylimidazole, wherein the concentration of BiCl3 is 0.01-0.03 mol / L and the concentration of 2-methylimidazole is 0.05-0.2 mol / L.

[0026] In some embodiments of the present invention, in step (3), the settling time is 20-28 hours.

[0027] In some embodiments of the present invention, in step (3), the temperature regime of the pyrolysis is as follows: first, the temperature is raised to 200-400℃ for pyrolysis for 1-3 hours, then the temperature is raised to 600-850℃ for pyrolysis for 2-4 hours, and finally the temperature is raised to 900-1000℃ for pyrolysis for 10-60 minutes; the heating rate is 2-8℃ / min.

[0028] Specifically, this invention employs a three-stage pyrolysis process to pyrolyze the ZIP-8 type MOF group, wherein: in the first stage (200-400℃), the organic ligand (2-methylimidazole) decomposes, while retaining the porous framework formed by the volatilization of the pore-forming agent (zinc powder), Bi 3+ In the first stage, the carbon skeleton initially coordinates with N in the ligand to form a precursor; in the second stage (600-850℃), the carbon skeleton reacts with NH3 to generate a nitride intermediate, and the active single atoms generated by the cracking of NH3 are doped into the carbon grid to form a nitrogen-rich carbon substrate; in the third stage (900-1000℃), short-term high-temperature pyrolysis promotes the decomposition of the nitride intermediate, and Bi atoms migrate to the nitrogen vacancies in the carbon grid to form Bi-N4@NC, and stabilize the single atom sites through thermodynamic reconstruction.

[0029] Meanwhile, compared with the traditional one-step high-temperature carbonization process, the three-stage pyrolysis process of this invention has at least the following more beneficial technical effects: 1) Achieving single-atom anchoring. This invention achieves precise anchoring of Bi single atoms through temperature gradient control. The atomically dispersed Bi-N4 sites optimize the adsorption energy for vanadium ions, enhancing catalytic activity. In contrast, one-step high-temperature carbonization reduces and agglomerates metal ions into nanoparticles, which are dispersed on the carbon skeleton surface, easily oxidized to form an insulating layer, blocking active sites, and resulting in low atom utilization. 2) Enhancing interfacial bonding. ZIP-8 micropores constrain Bi atom migration, preventing high-temperature agglomeration. Carbon skeleton N atoms form covalent bonds with oxygen vacancies on the surface of high-entropy oxides, constructing fast electron channels and reducing interfacial resistance. In contrast, the metal nanoparticles and carbon felt of one-step high-temperature carbonization are only physically adsorbed, and the particles are easily detached during cycling. 3) Controlling pore structure. This invention uses three-stage pyrolysis to retain the original ZIF-8 micropores in the low-temperature stage, sieving H + It also blocks vanadium ions, and at high temperatures, the carbon skeleton shrinks to form a mesoporous network, accelerating electrolyte mass transfer, alleviating concentration polarization, and the Bi-N4 sites are positively charged, repelling positively charged vanadium ions. In contrast, one-step high-temperature carbonization forms a wide-distributed pore network, lacking size selectivity, and the large pores allow vanadium ions to diffuse freely. 4) Improved electrochemical performance. The Bi-N4@NC of this invention has dual-function charge-discharge catalysis, improving energy efficiency and peak power density. In contrast, the material prepared by one-step high-temperature carbonization has a high reaction energy barrier, large polarization loss at high current densities, and lower energy efficiency.

[0030] In some embodiments of the present invention, in step (3), the pyrolysis is carried out under an atmosphere of NH3 and Ar, with a volume ratio of NH3 to Ar of 1:(3-5).

[0031] In some embodiments of the present invention, in step (4), the electrolyte includes Cr(NO3)3 and NH4Cl, wherein the concentration of Cr(NO3)3 is 0.05-0.2 mol / L and the concentration of NH4Cl is 0.01-0.1 mol / L.

[0032] In some embodiments of the present invention, in step (4), the pH value of the electrolyte is 1.5-2.5.

[0033] In some embodiments of the present invention, in step (4), the parameters of the pulse deposition are: pulse on-time of 5-20 milliseconds, pulse off-time of 40-80 milliseconds, and current density of 10-30 mA / cm². 2 The deposition rate was 0.3-1.0 mg / cm³. 2 .

[0034] In some embodiments of the present invention, in step (4), the annealing is performed at a temperature of 380-420°C for 1-3 hours under an inert atmosphere to convert amorphous Cr(OH)3 into stable α-Cr2O3.

[0035] In some embodiments of the present invention, the inert atmosphere is an argon atmosphere.

[0036] A second aspect of the present invention provides a composite electrode prepared by the above-described method, the composite electrode comprising a carbon felt substrate, wherein the surface of the carbon felt substrate is loaded with a porous high-entropy oxide core, a bismuth single-atom shell, and Cr2O3 nanoislands.

[0037] Specifically, the composite electrode of the present invention contains a porous high-entropy oxide core, a single-atom shell, and a site barrier group. Through precise division of labor of elements and spatial functional zoning, it breaks through the dilemma of traditional electrodes being unable to balance performance and stability, and may achieve complementary functions and lifespan guarantee.

[0038] A third aspect of the present invention provides an all-vanadium redox flow battery, including the above-described composite electrode.

[0039] Compared with the prior art, the above-described technical solution of the present invention has at least the following technical effects or advantages:

[0040] (1) This invention modifies the surface of an acid-treated carbon felt substrate to sequentially construct a porous high-entropy oxide (HEO) core, a Bi single-atom shell (Bi-N4@NC), and a hydrogen evolution inhibition group (Cr2O3 nano-island modification). Through the multi-element catalysis of the porous high-entropy oxide core, the electronic regulation of the bismuth single-atom shell, and the local passivation of the Cr2O3 nano-island site shielding group, the precise division of labor of elements and spatial functional partitioning are achieved. This solves the technical problems of high-density operation, side reaction eradication, low cost and long life of composite electrodes, balances the contradiction between hydrogen evolution inhibition and reaction activity, and simultaneously realizes the three functions of charging catalysis, discharging catalysis and hydrogen evolution inhibition on a single electrode, providing a core solution for the electrode design of high-power flow batteries.

[0041] (2) The porous BiMoWCeO2 high-entropy oxide of the present invention not only achieves precise elemental division of labor (Bi catalytic discharge / Mo catalytic charging / W inhibiting hydrogen evolution / Ce stabilizing the interface) and high-entropy synergistic effect, but also has the effects of complementary electronic structure and site synergy; moreover, it improves the corrosion resistance and structural stability, electrochemical activity and reaction kinetics, as well as ion selectivity and interface characteristics of the material. At the same time, the use of low carbon thermal shock temperature and MOF-derived coupling to achieve interface coupling, and pulse electrodeposition to improve efficiency, ensures precise coverage of Cr2O3 nano-islands, and improves the structure and stability of the material, further inhibiting hydrogen evolution.

[0042] (3) The composite material of this invention has a three-level configuration of "HEO core-Bi single-atom shell-Cr2O3 nano-island", realizing multi-element catalysis of porous high-entropy core, electronic regulation of single-atom shell, and local passivation of site shielding groups. When applied to an all-vanadium redox flow battery, it exhibits high voltage efficiency, coulombic efficiency, and energy efficiency, as well as low hydrogen evolution rate and cycle stability; achieving a coulombic efficiency of 99.2-99.5%, a voltage efficiency of 93.9-94.7%, an energy efficiency of 91.9-92.5%, and a hydrogen evolution rate of 1.1-1.3 mA / cm². 2 The capacity retention rate after 100 cycles is 98.9-99.2%. Attached Figure Description

[0043] Figure 1 This is a schematic diagram of the fabrication process of the composite electrode of the present invention;

[0044] Figure 2 This is a schematic diagram of the fuel cell stack assembly process. Detailed Implementation

[0045] The present invention will now be described in detail with reference to embodiments to facilitate understanding of the invention by those skilled in the art. It is particularly important to note that the embodiments are merely illustrative of the invention and should not be construed as limiting the scope of protection of the invention. Non-essential improvements and adjustments made to the invention by those skilled in the art based on the above description should still fall within the scope of protection of the invention. Furthermore, all raw materials mentioned below, unless otherwise specified, are commercially available products; all process steps or preparation methods not mentioned in detail are process steps or preparation methods known to those skilled in the art.

[0046] The following is a schematic diagram of the fabrication process of the composite electrode in the embodiments. Figure 1 As shown.

[0047] Example 1

[0048] A method for preparing a composite electrode includes the following steps:

[0049] (1) The carbon felt substrate is immersed in a mixed solution of H2SO4 and H2O2 (the volume ratio of H2SO4 and H2O2 is 2:1), refluxed at 80°C for 3 hours, and then ultrasonically cleaned and dried to obtain an acid-treated carbon felt substrate.

[0050] (2) The acid-treated carbon felt substrate is immersed in a high-entropy oxide precursor solution and subjected to a hydrothermal reaction at 180°C for 12 hours. Then, zinc powder is added to the reaction product as a pore-forming agent (the mass ratio of zinc powder to Bi(NO3)3 is 20:1), and the product is transferred to an H2 / Ar (volume ratio of H2 to Ar is 5:95) atmosphere furnace. The temperature is raised to 1000°C and held for 2 seconds to obtain a porous high-entropy oxide core and carbon felt composite, which is denoted as HEO / carbon felt composite.

[0051] The preparation process of the high-entropy oxide precursor solution is as follows: Equimolar amounts of Bi(NO3)3 and (NH4)6Mo7O are added. 24 (NH4) 10 W 12 O 41 Bi(NO3)3 was dissolved in ethylene glycol, and hexadecyltrimethylammonium bromide was added to prepare the high-entropy oxide precursor solution. The concentration of Bi(NO3)3 was 0.025 mol / L, and the concentration of the surfactant was 0.1 mol / L.

[0052] (3) The HEO / carbon felt composite was immersed in a metal-organic framework precursor solution and left to stand for 24 hours to form a ZIP-8 type MOF group; then the ZIP-8 type MOF group was subjected to a three-stage pyrolysis under an NH3 / Ar atmosphere (the volume ratio of NH3 and Ar was 1:4) to form a nitrogen-carbon coated Bi single-atom shell group on the surface of the porous high-entropy oxide core, denoted as HEO / Bi-N4@NC, thus obtaining the HEO / Bi-N4@NC / carbon felt composite.

[0053] The metal-organic framework precursor solution was a methanol solution containing BiCl3 and 2-methylimidazole, with BiCl3 concentration of 0.025 mol / L and 2-methylimidazole concentration of 0.1 mol / L.

[0054] The temperature regime for the three-stage pyrolysis is as follows: first, heat to 350℃ for 1 hour, then heat to 700℃ for 2 hours, and finally heat to 950℃ for 30 minutes, with a heating rate of 5℃ / min.

[0055] (4) The HEO / Bi-N4@NC / carbon felt composite was treated with Cr-containing materials. 3+The electrolyte was pulse-deposited and then transferred to an Ar atmosphere furnace for annealing at 400°C for 2 hours to convert amorphous Cr(OH)3 into stable α-Cr2O3 crystals, thus obtaining a Cr2O3 nanoisland modified HEO / Bi-N4@NC / carbon felt composite, which is the composite electrode of this embodiment.

[0056] Among them: containing Cr 3+ The electrolyte was a mixture of 0.1 mol / L Cr(NO3)3 and 0.05 mol / L NH4Cl, and the pH was adjusted to 2.0 using 0.1 mol / L HNO3.

[0057] The pulse deposition process parameters are: pulse on for 10 milliseconds, pulse off for 50 milliseconds, and current density of 15 mA / cm². 2 The deposition rate was 0.5 mg / cm³. 2 .

[0058] Example 2

[0059] A method for preparing a composite electrode includes the following steps:

[0060] (1) The carbon felt substrate is immersed in a mixed solution of H2SO4 and H2O2 (the volume ratio of H2SO4 and H2O2 is 1:1), refluxed at 70°C for 4 hours, and then ultrasonically cleaned and dried to obtain the acid-treated carbon felt substrate.

[0061] (2) The acid-treated carbon felt substrate is immersed in a high-entropy oxide precursor solution and subjected to a hydrothermal reaction at 160°C for 20 hours. Then, zinc powder is added to the reaction product as a pore-forming agent (the mass ratio of zinc powder to Bi(NO3)3 is 20:1), and the product is transferred to an H2 / Ar (volume ratio of H2 to Ar is 5:95) atmosphere furnace. The temperature is raised to 950°C and held for 4 seconds to obtain a porous high-entropy oxide core and carbon felt composite, which is denoted as HEO / carbon felt composite.

[0062] The preparation process of the high-entropy oxide precursor solution is as follows: Equimolar amounts of Bi(NO3)3 and (NH4)6Mo7O are added. 24 (NH4) 10 W 12 O 41 Bi(NO3)3 was dissolved in ethylene glycol, and hexadecyltrimethylammonium bromide was added to prepare the high-entropy oxide precursor solution. The concentration of Bi(NO3)3 was 0.025 mol / L, and the concentration of the surfactant was 0.1 mol / L.

[0063] (3) The HEO / carbon felt composite was immersed in a metal-organic framework precursor solution and left to stand for 24 hours to form a ZIP-8 type MOF group; then the ZIP-8 type MOF group was subjected to a three-stage pyrolysis under an NH3 / Ar atmosphere (the volume ratio of NH3 and Ar was 1:4) to form a nitrogen-carbon coated Bi single-atom shell group on the surface of the porous high-entropy oxide core, denoted as HEO / Bi-N4@NC, thus obtaining the HEO / Bi-N4@NC / carbon felt composite.

[0064] The metal-organic framework precursor solution was a methanol solution containing BiCl3 and 2-methylimidazole, with BiCl3 concentration of 0.03 mol / L and 2-methylimidazole concentration of 0.15 mol / L.

[0065] The temperature regime for the three-stage pyrolysis is as follows: first, heat to 200℃ for 3 hours, then heat to 600℃ for 4 hours, and finally heat to 900℃ for 60 minutes, with a heating rate of 5℃ / min.

[0066] (4) The HEO / Bi-N4@NC / carbon felt composite was treated with Cr-containing materials. 3+ The electrolyte was pulse-deposited and then transferred to an Ar atmosphere furnace for annealing at 400°C for 3 hours to convert amorphous Cr(OH)3 into stable α-Cr2O3 crystals, thus obtaining a Cr2O3 nanoisland modified HEO / Bi-N4@NC / carbon felt composite, which is the composite electrode of this embodiment.

[0067] Among them: containing Cr 3+ The electrolyte was a mixture of 0.2 mol / L Cr(NO3)3 and 0.1 mol / L NH4Cl, and the pH was adjusted to 2.0 using 0.1 mol / L HNO3.

[0068] The pulse deposition process parameters are: pulse on for 15 milliseconds, pulse off for 60 milliseconds, and current density of 20 mA / cm². 2 The deposition rate was 0.3 mg / cm³. 2 .

[0069] Example 3

[0070] A method for preparing a composite electrode includes the following steps:

[0071] (1) The carbon felt substrate is immersed in a mixed solution of H2SO4 and H2O2 (the volume ratio of H2SO4 and H2O2 is 3:1), refluxed at 100°C for 2 hours, and then ultrasonically cleaned and dried to obtain an acid-treated carbon felt substrate.

[0072] (2) The acid-treated carbon felt substrate is immersed in a high-entropy oxide precursor solution and subjected to a hydrothermal reaction at 200°C for 10 hours. Then, zinc powder is added to the reaction product as a pore-forming agent (the mass ratio of zinc powder to Bi(NO3)3 is 20:1), and the product is transferred to an H2 / Ar (volume ratio of H2 to Ar is 5:95) atmosphere furnace. The temperature is raised to 800°C and held for 5 seconds to obtain a porous high-entropy oxide core and carbon felt composite, which is denoted as HEO / carbon felt composite.

[0073] The preparation process of the high-entropy oxide precursor solution is as follows: Equimolar amounts of Bi(NO3)3 and (NH4)6Mo7O are added. 24 (NH4) 10 W 12 O 41 Bi(NO3)3 was dissolved in ethylene glycol, and hexadecyltrimethylammonium bromide was added to prepare the high-entropy oxide precursor solution. The concentration of Bi(NO3)3 in the high-entropy oxide precursor solution was 0.025 mol / L, and the concentration of the surfactant was 0.2 mol / L.

[0074] (3) The HEO / carbon felt composite was immersed in a metal-organic framework precursor solution and left to stand for 24 hours to form a ZIP-8 type MOF group; then the ZIP-8 type MOF group was subjected to a three-stage pyrolysis under an NH3 / Ar atmosphere (the volume ratio of NH3 and Ar was 1:4) to form a nitrogen-carbon coated Bi single-atom shell group on the surface of the porous high-entropy oxide core, denoted as HEO / Bi-N4@NC, thus obtaining the HEO / Bi-N4@NC / carbon felt composite.

[0075] The metal-organic framework precursor solution was a methanol solution containing BiCl3 and 2-methylimidazole, with BiCl3 concentration of 0.01 mol / L and 2-methylimidazole concentration of 0.15 mol / L.

[0076] The temperature regime for the three-stage pyrolysis is as follows: first, heat to 400℃ for 1 hour, then heat to 850℃ for 2 hours, and finally heat to 1000℃ for 10 minutes, with a heating rate of 5℃ / min.

[0077] (4) The HEO / Bi-N4@NC / carbon felt composite was treated with Cr-containing materials. 3+ The electrolyte was pulse-deposited and then transferred to an Ar atmosphere furnace for annealing at 420°C for 1.5 hours to convert amorphous Cr(OH)3 into stable α-Cr2O3 crystals, thus obtaining a Cr2O3 nanoisland-modified HEO / Bi-N4@NC / carbon felt composite, which is the composite electrode of this embodiment.

[0078] Among them: containing Cr 3+The electrolyte was a mixture of 0.05 mol / L Cr(NO3)3 and 0.01 mol / L NH4Cl, and the pH was adjusted to 2.0 using 0.1 mol / L HNO3.

[0079] The pulse deposition process parameters are: pulse on for 20 milliseconds, pulse off for 80 milliseconds, and current density of 30 mA / cm². 2 The deposition rate was 1.0 mg / cm³. 2 .

[0080] Comparative Example 1

[0081] The only difference between Comparative Example 1 and Example 1 is that the composite electrode of Comparative Example 1 does not include step (2) in the preparation process, that is, it does not construct a porous high-entropy oxide core.

[0082] Comparative Example 2

[0083] The only difference between Comparative Example 2 and Example 1 is that the composite electrode of Comparative Example 2 does not include step (4) in the preparation process, that is, it does not construct hydrogen evolution inhibitory groups (Cr2O3 nano islands) for modification.

[0084] Comparative Example 3

[0085] Comparative Example 3 was a blank control group, namely the carbon felt substrate in Example 1 that was not treated with acid.

[0086] Comparative Example 4

[0087] The difference between Comparative Example 4 and Example 1 is that in step (3) of Comparative Example 4, the temperature regime of pyrolysis adopts the traditional one-step high-temperature carbonization process, that is, pyrolysis at 950°C for 2 hours.

[0088] Comparative Example 5

[0089] The difference between Comparative Example 5 and Example 1 is that in step (4) of Comparative Example 5, the traditional Pb-Bi alloy electrodeposition process is used instead of the pulse deposition process of Example 1, specifically:

[0090] Add 3g of sodium pyrophosphate, 0.161g of L-tartaric acid, 0.662g of lead nitrate, and 2.910g of bismuth(III) nitrate pentahydrate to 50mL of 5mol / L nitric acid solution, and then sonicate for 10 minutes to fully dissolve the solid powder to obtain a lead-bismuth ion electrodeposition solution; wherein the molar ratio of lead ions to bismuth ions is 1:3.

[0091] The HEO / Bi-N4@NC / carbon felt composite was then immersed in a lead-bismuth ion electrodeposition solution and ultrasonically treated for 5 minutes. Following this, electrodeposition modification was performed using a three-electrode system at a constant voltage of -0.8V (deposition amount: 0.5 mg / cm³). 2After cleaning and drying, the composite electrode of Comparative Example 5 was obtained.

[0092] Comparative Example 6

[0093] The difference between Comparative Example 6 and Example 1 is that Comparative Example 6 swaps steps (4) and (2), that is, it first performs hydrogen evolution inhibitor modification and then constructs a high-entropy oxide HEO core.

[0094] Performance testing

[0095] The composite electrodes prepared in Examples 1-3 and Comparative Examples 1-6 were assembled into battery stacks and subjected to charge-discharge tests under uniform test conditions. The battery coulombic efficiency, voltage efficiency, energy efficiency, and capacity retention rate after 100 cycles were recorded. The test results are shown in Table 1.

[0096] Among them: the assembly process of the fuel cell stack is as follows Figure 2 As shown: in sequence, the steps are: electrode-diaphragm pre-assembly (precise positioning and sealing enhancement), fluid flow frame integration and sealing (flow channel matching and sealant application), modular stacking and press fitting (layered pressing and anti-torsion tooling assistance), fluid system connection (symmetrical flow splitting design), and liquid injection and activation (vacuum liquid injection and step activation).

[0097] Coulombic efficiency was measured using a battery testing system at 200 mA / cm², 25°C, and 1.6 mol / L electrolyte. 3+ / V 4+ The coulombic efficiency was measured using 3 mol / L H₂SO₄, and was calculated as the ratio of constant-current discharge capacity to charging capacity. Voltage efficiency data, obtained from a high-precision data acquisition instrument, was calculated as the ratio of average discharge voltage to average charging voltage. Energy efficiency was the product of coulombic efficiency and voltage efficiency. The hydrogen evolution rate was tested using a gas-tight test chamber and gas chromatograph during the stable phase at the end of discharge, with H₂ volume collected using the water displacement method. Capacity retention after 100 cycles was determined using a long-cycle testing system, performing 100 charge-discharge cycles, calibrating the capacity every 10 weeks, and obtaining the ratio of the discharged capacity after cycles to the initial capacity.

[0098] Table 1:

[0099]

[0100] As shown in Table 1, the composite electrodes prepared in Examples 1-3 all exhibit high voltage efficiency, coulombic efficiency, and energy efficiency, as well as low hydrogen evolution performance and cycle stability; achieving coulombic efficiency of 99.2-99.5%, voltage efficiency of 93.9-94.7%, energy efficiency of 91.9-92.5%, and hydrogen evolution rate of 1.1-1.3 mA / cm². 2 The capacity retention rate after 100 cycles is 98.9-99.2%.

[0101] Meanwhile, Example 1 exhibits higher voltage efficiency and coulombic efficiency compared to Comparative Example 1, because the d orbital electrons of Mo / V elements interact with V... 3+ Orbital hybridization occurs, weakening the VO bond force, significantly lowering the desolvation activation energy, significantly reducing charge polarization, and the Ce element's valence change generates oxygen vacancies, promoting H... + The signal is transferred via a jump, maintaining local pH stability.

[0102] Example 1 reduced the hydrogen evolution rate and improved the capacity retention compared to Comparative Example 2. This is because the presence of pulsed electrodeposited Cr2O3 nanoislands precisely shielded the serrated edges of the carbon fibers, causing a negative shift in the original hydrogen evolution potential, stabilizing pH, and inhibiting V. 3+ The precipitation process is intensified.

[0103] Examples 1-3 exhibit superior performance compared to Comparative Example 3, demonstrating that the three-functional composite electrode for vanadium redox flow batteries prepared in this invention possesses a "porous high-entropy core," a "single-atom shell," and a "site barrier base." Through precise elemental division and spatial functional zoning, it overcomes the dilemma of traditional electrodes being unable to simultaneously achieve performance and stability, thus realizing complementary three functions and lifespan assurance.

[0104] Compared to Example 1, Comparative Example 4 uses conventional one-step high-temperature carbon MOF carbonization, which makes the catalytic path of Bi nanoparticles longer, increases the energy barrier of vanadium ion reaction, and reduces voltage efficiency.

[0105] Compared to Example 1, Comparative Example 5 used electrodeposited Pb-Bi alloy, where Bi ion dissolution led to the loss of active sites, and Pb deposition induced graphite corrosion, resulting in a decrease in electrode porosity after 100 cycles. In contrast, the α-Cr2O3 passivation layer in Example 5 suppressed graphite oxidation, and the high-entropy effect of the HEO core suppressed elemental segregation, resulting in a low capacity decay rate.

[0106] Compared to Example 1, Comparative Example 6, by changing the order of the preparation steps, resulted in interface cracks that increased contact resistance and increased the proportion of ohmic polarization in the overall polarization loss rate.

[0107] For those skilled in the art, several simple deductions or substitutions can be made without departing from the inventive concept, without requiring creative effort. Therefore, any simple improvements made to this invention by those skilled in the art based on the disclosure of this invention should be within the scope of protection of this invention. The above embodiments are preferred embodiments of this invention, and all processes similar to this invention and equivalent changes should fall within the scope of protection of this invention.

Claims

1. A method for preparing a composite electrode, characterized in that, Includes the following steps: (1) The carbon felt substrate is acid-treated to obtain an acid-treated carbon felt substrate; (2) The acid-treated carbon felt substrate is immersed in a high-entropy oxide precursor solution for hydrothermal reaction; then a pore-forming agent is added to the reaction product and calcined under a reducing atmosphere to obtain a porous high-entropy oxide core and carbon felt composite, denoted as HEO / carbon felt composite; the hydrothermal reaction is carried out at a temperature of 160-200℃ for 10-30 hours, the calcination temperature is 800-1200℃, and the holding time is 2-5 seconds; The high-entropy oxide precursor solution contains a metal compound and a surfactant, wherein the metal compound includes equimolar amounts of Bi(NO3)3 and (NH4)6Mo7O. 24 (NH4) 10 W 12 O 41 and Ce(NO3)3; (3) The HEO / carbon felt composite is immersed in a metal-organic framework precursor solution, which includes bismuth salt and organic ligand; it is left to stand to form a ZIP-8 type MOF group; then the ZIP-8 type MOF group is pyrolyzed to form a nitrogen-carbon coated Bi single-atom shell group on the surface of the porous high-entropy oxide core, denoted as HEO / Bi-N4@NC, to obtain the HEO / Bi-N4@NC / carbon felt composite. The metal-organic framework precursor solution is a methanol solution containing BiCl3 and 2-methylimidazole; in the organic framework precursor solution, the concentration of BiCl3 is 0.01-0.03 mol / L, and the concentration of 2-methylimidazole is 0.05-0.2 mol / L. (4) The HEO / Bi-N4@NC / carbon felt composite is pulse-deposited in an electrolyte comprising chromium salt and ammonium salt; After annealing, Cr2O3 crystals are generated, and a Cr2O3 nanoisland-modified HEO / Bi-N4@NC / carbon felt composite is obtained, which is the composite electrode.

2. The method for preparing the composite electrode according to claim 1, characterized in that, In step (1), the solution used for acid treatment includes H2SO4 and H2O2, and the volume ratio of H2SO4 to H2O2 is (1-3):

1.

3. The method for preparing the composite electrode according to claim 1, characterized in that, In step (2), the surfactant includes hexadecyltrimethylammonium bromide, and the pore-forming agent includes zinc powder; And / or, in the high-entropy oxide precursor solution, the concentration of Bi(NO3)3 is 0.02-0.03 mol / L, and the concentration of the surfactant is 0.1-0.2 mol / L; the mass ratio of the pore-forming agent to Bi(NO3)3 is (15-25):

1.

4. The method for preparing the composite electrode according to claim 1, characterized in that, In step (3), the temperature regime for pyrolysis is as follows: first, heat to 200-400℃ for pyrolysis for 1-3 hours, then heat to 600-850℃ for pyrolysis for 2-4 hours, and finally heat to 900-1000℃ for pyrolysis for 10-60 minutes; the heating rate is 2-8℃ / min.

5. The method for preparing the composite electrode according to claim 1, characterized in that, In step (4), the electrolyte includes Cr(NO3)3 and NH4Cl, wherein the concentration of Cr(NO3)3 is 0.05-0.2 mol / L and the concentration of NH4Cl is 0.01-0.1 mol / L.

6. The method for preparing the composite electrode according to claim 1, characterized in that, In step (4), the parameters for pulse deposition are: pulse on-time 5-20 ms, pulse off-time 40-80 ms, and current density 10-30 mA / cm². 2 The deposition rate was 0.3-1.0 mg / cm³. 2 ; and / or, the annealing is performed under an inert atmosphere at a temperature of 380-420°C for 1-3 hours.

7. A composite electrode, characterized in that, The composite electrode is prepared by the method of any one of claims 1-6, wherein the composite electrode comprises a carbon felt substrate, and the surface of the carbon felt substrate is loaded with a porous high-entropy oxide core, a bismuth single-atom shell and Cr2O3 nano islands.

8. A vanadium redox flow battery, characterized in that, Includes the composite electrode as described in claim 7.

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