Composite electrode for all-vanadium redox flow battery and preparation method of composite electrode

Abstract

The invention belongs to the related technical field of batteries, and discloses a composite electrode for an all-vanadium redox flow battery and a preparation method of the composite electrode. The preparation method comprises the following steps: carrying out surface modification on a carbon felt substrate subjected to acid treatment, sequentially constructing a porous high-entropy oxide (HEO) core and a Bi monatomic shell group (Bi-N4 at NC), and carrying out hydrogen evolution inhibition group (Cr2O3 nano island modification) modification, and carrying out multi-element catalysis of the porous high-entropy oxide core, electronic regulation and control of a bismuth monatomic shell and local passivation of a Cr2O3 nano island site shielding group, so that the hydrogen evolution inhibition group (Cr2O3 nano island modification) is modified. Precise element division and space function zoning are jointly achieved, the technical problems of high-density operation, side reaction radical treatment, low cost and long service life of a composite electrode are solved, the contradiction between hydrogen evolution inhibition and reaction activity is balanced, coordination of three functions of charging catalysis, discharging catalysis and hydrogen evolution inhibition is synchronously achieved on a single electrode, and the hydrogen evolution inhibition effect is improved. And a core solution is provided for the electrode design of the high-power flow battery.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field related to batteries, and particularly relates to a composite electrode for a full vanadium redox flow battery and a preparation method thereof. BACKGROUND

[0002] The surge in global energy demand and the intensifying environmental pressure are driving the transformation of the power system to renewable energy. However, the inherent intermittency and volatility of renewable energy such as wind and solar energy pose challenges to the stability of the power grid, and efficient energy storage technology is urgently needed to improve energy utilization efficiency. The vanadium redox flow battery (VRFB) is considered to be a promising energy storage solution due to its high safety, long cycle life, good environmental compatibility, and significant cost-effectiveness throughout the life cycle. As the core component that affects the electrochemical performance of VRFB, the characteristics of electrode materials are crucial. Currently, widely used carbon-based electrode materials (such as graphite, carbon paper, carbon felt, and carbon cloth) are often limited by the limited effective electrochemical active area, insufficient intrinsic catalytic activity, structural aging and degradation during long-term cycling, and hydrogen evolution side reactions (HER), so there is an urgent need to optimize the performance of carbon-based electrode materials.

[0003] Currently, common modification strategies for carbon-based electrode materials include catalyst loading, surface etching, heteroatom doping, and composite material construction. For the VRFB system, an ideal modified electrode needs to synergistically improve the vanadium ion redox reaction (charging / discharging) catalytic activity and effectively suppress the hydrogen evolution side reaction. However, there is no composite optimization scheme that can simultaneously achieve the above three functions in the prior art. For example, Chinese Patent Publication No. CN104319409 A discloses a high-activity asymmetric electrode based on bismuth-based catalyst-deposited nanocarbon fibers. Although this electrode can to some extent suppress HER and improve capacity retention, it does not optimize the ion desolvation process, resulting in low charging efficiency and poor cycle stability. Chinese Patent Publication No. CN118472289A discloses a high-entropy alloy composite electrode for a full vanadium redox flow battery. This electrode mainly relies on the conductivity of high-entropy alloys, lacks targeted optimization of key electrode reaction kinetics, and does not integrate a hydrogen evolution suppression mechanism. SUMMARY

[0004] The present application aims to at least solve one of the technical problems existing in the prior art. To this end, the present application proposes a composite electrode for a full vanadium redox flow battery and a preparation method thereof. The composite electrode not only can simultaneously improve the charging / discharging catalytic activity of the full vanadium redox flow battery, but also can effectively suppress the hydrogen evolution side reaction, while realizing three-function complementarity and life guarantee.

[0005] To solve the above technical problems, the first aspect of the present application provides a preparation method of a composite electrode, comprising the following steps:

[0006] (1) acid treatment is performed on a carbon felt substrate 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 to perform a hydrothermal reaction; then a pore-forming agent is added to the reaction product, and calcination is performed in 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 metal compounds and a surfactant, and the metal compounds include equimolar Bi(NO3)3, (NH4)6Mo7O 24 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 containing a bismuth salt and an organic ligand; after standing, a ZIP-8 type MOF base is formed; then pyrolysis is performed on the ZIP-8 type MOF base to form a nitrogen-carbon-coated Bi monatomic shell base on the surface of the porous high-entropy oxide core, denoted as HEO / Bi-N4@NC, to obtain an HEO / Bi-N4@NC / carbon felt composite;

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

[0011] In the preparation of the composite electrode, acid treatment is first performed on the carbon felt substrate to remove the surface inert layer, form a micron-level groove structure, increase the specific surface area, and introduce oxygen-containing functional groups. Then a high-entropy oxide (HEO) core is constructed, and a pore-forming agent is added to form porous high-entropy oxide nanoparticles, thereby improving the reaction kinetics, exposing high-activity crystal faces, and enhancing the structural stability of the porous HEO, to form an HEO / carbon felt composite. A Bi monatomic shell base (Bi-N4@NC) is then constructed to anchor Bi monatomic atoms by using the MOF confinement effect, to realize atomic-level catalytic oxidation and reduction and to enhance the interface bonding effect. Finally, a hydrogen evolution inhibition base (Cr2O3 nano-island) is modified to precisely shield the active sites and inhibit the hydrogen evolution activity.

[0012] ​Specifically, the application adopts a porous BiMoWCeO2 high-entropy oxide, which has at least the following more beneficial technical effects relative to a conventional porous high-entropy alloy (such as ZrSnMnBiCo): 1) corrosion resistance and structural stability. The Ce-O bond in BiMoWCeO2 has a high energy of up to 795 kJ / mol, which is conducive to the formation of a protective oxide layer, significantly reduces the corrosion rate of the electrode surface, and the Ce 3 + / Ce 4+ Redox pairs drive the formation and filling of oxygen vacancies through reversible valence switching, realize the "release-supplement" closed loop of lattice oxygen, relieve structural degradation, and endow the material with self-healing ability. However, porous high-entropy alloys accelerate the penetration of electrolyte in the cycle, dissolve active metals, and easily cause electrode pulverization. 2) Electrochemical activity and reaction dynamics. BiMoWCeO2 contains a high concentration of oxygen vacancies, forms a local electron-rich region, enhances electron conductivity, and the dangling bonds of oxygen vacancies strongly adsorb H + , forming surface hydroxyl groups, locally increasing the solution pH value, improving the affinity of the electrolyte, inhibiting the hydrogen evolution side reaction, and reducing the V 3+ desolvation energy barrier. High-entropy alloy materials only rely on metal conductivity, lack controllable oxygen defects, cannot optimize the proton transfer path, and have a high desolvation energy barrier. 3) Ion selectivity and interface characteristics. The fluorite structure lattice distortion of BiMoWCeO2 forms a channel with a pore size of about 0.38 nm, allowing H + to rapidly diffuse, but blocking [VO] 2+ . And the high-valence cation W 6+ repels positively charged vanadium ions, further inhibiting cross-contamination. While high-entropy alloy materials lack ion sieving on the surface, vanadium ions and H + compete for adsorption, easily causing side reactions.

[0013] The application adopts the alpha-Cr2O3 crystal obtained by pulse deposition and annealing, and at least has the following more beneficial technical effects relative to the traditional electrodeposited alloy (such as lead-bismuth alloy): 1) guarantee material structure and stability. The amorphous Cr(OH)3 crystal is deposited by pulse, and ion migration to the graphite defect site is allowed in the pulse-off period to form a stable C-O-Cr covalent bond. The alpha-Cr2O3 formed after annealing forms a strong interface bond with the carbon felt substrate through epitaxial growth, and the alpha-Cr2O3 has a high melting point and a thermal expansion coefficient matching the graphite, and there is no risk of structure peeling in the cycle. The alloy formed by electrodeposition is physically adsorbed or weakly bonded to the surface of the graphite felt, and is easy to dissolve in a strong acid electrolyte. Moreover, the alloy generally has a low melting point, and atomic diffusion is intensified under the battery operating conditions, and the alloy particles are easy to agglomerate and coarsen, exposing more carbon felt substrate to initiate the hydrogen evolution side reaction. 2) Further inhibit hydrogen evolution. The alpha-Cr2O3 crystal nanometer island size of the application matches the surface defect of the carbon felt fiber, and low load can selectively cover the hydrogen evolution active site. Moreover, the alpha-Cr2O3 crystal is a p-type semiconductor, forms an electronic potential barrier to inhibit H + Reduction, Cr 3+ 3d 3 electronic structure of Cr + inhibit H + adsorption. The alloy needs to rely on high load to cover the active site, and there is an electric field distortion at the edge of the alloy particle, which induces H 3+ enrichment at the particle-graphite interface, which increases the local hydrogen evolution rate. 3) Improve the catalytic-inhibiting synergistic performance. The reaction site is exposed in the nanometer island gap, the pulse deposition optimizes the pore connectivity, and the ion diffusion coefficient is improved. In long cycle, the alpha-Cr2O3 crystal forms a passivation layer in the acidic environment, and repairs the lattice distortion in the cycle. Lead will inhibit hydrogen evolution and delay catalytic activity, and high lead content will easily lead to the decrease of Cr 3+ reduction kinetics.

[0014] Moreover, the above three functional modifications of the present application have relevance and specificity. First, the porous BiMoWCeO2 high-entropy oxide is formed by hydrothermal synthesis and instantaneous high-temperature reduction, which provides oxygen vacancies and mesoporous structures induced by lattice distortion, and lays a thermodynamic adsorption site and space carrier for single-atom anchoring. If the HEO core is missing, the surface of the carbon felt is inert and the conductivity is uneven, and subsequent MOF confined growth is difficult to achieve uniform single-atom dispersion. The Bi-N4@NC shell is formed on the surface of the HEO core by constructing ZIF-8, which forms a high-efficiency electron channel through MOF confinement and interface covalent bond, and realizes the dual functions of catalysis and conduction. Finally, the alpha-Cr2O3 crystal is generated by pulse deposition and annealing, which completely blocks the hydrogen evolution path through geometric shielding and semiconductor band design. At the same time, from the gradient temperature matching requirement, 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 reduced, and the annealing of Cr2O3 is even lower. If the reverse operation is performed, that is, the HEO high-temperature step is performed last, although Cr2O3 does not melt, lattice expansion will cause interface stress cracks. From the chemical bonding sequence requirement, oxygen vacancies need to be formed first, then MOF-N coordination is formed, then N-O-C bond is formed by pyrolysis, and finally Cr2O3 epitaxial growth is realized. If the order is changed, N-O-C bond cannot be formed due to the lack of oxygen vacancies.

[0015] Therefore, the three functional modifications of the present application need to strictly follow the logic of "substrate activation-catalytic site construction-side reaction inhibition", and any change in order will destroy the interface bonding sequence or cause high-temperature structure collapse. If Cr2O3 is deposited first and then the HEO core is constructed, the alpha-Cr2O3 nano-island will block the surface of the carbon felt, so that the HEO precursor cannot contact the substrate, and the hydrothermal reaction only generates free particles instead of core-shell structures. If the Bi-N4@NC shell is constructed first and then the HEO core is deposited, high-temperature reduction will destroy the Bi-N4 coordination structure, causing Bi atoms to agglomerate into nanoparticles, and causing the MOF carbon skeleton to graphitize, losing the confinement effect.

[0016] In some embodiments of the present application, 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.

[0017] In some embodiments of the present application, in step (1), the acid treatment process is as follows: the carbon felt substrate is immersed in a mixed solution of H2SO4 and H2O2, and is subjected to heat reflux, ultrasonic cleaning and drying to obtain the acid-treated carbon felt substrate.

[0018] In some embodiments of the present application, in step (1), the heat reflux is carried out at a temperature of 60-100℃ for 2-5 hours.

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

[0020] In some embodiments of the present application, in the chen 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.

[0021] In some embodiments of the present application, 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 application, in step (2), the hydrothermal reaction is performed at a temperature of 160-200℃ for 10-30 hours.

[0023] In some embodiments of the present application, in step (2), the calcination temperature is 800-1200℃, and the holding time is 2-5 seconds, i.e., the instant synthesis is performed for 2-5 seconds at a high temperature of 800-1200℃.

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

[0025] In some embodiments of the present application, 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 application, in step (3), the standing time is 20-28 hours.

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

[0028] ​​​Specifically, the application adopts a three-stage pyrolysis process for pyrolysis of a ZIP-8 type MOF base, wherein: in the first stage (200-400 DEG C), the organic ligand (2-methyl imidazole) is decomposed, the porous framework formed by volatilization of the pore-forming agent (zinc powder) is retained, and Bi 3+ is preliminarily coordinated with N in the ligand to form a precursor; in the second stage (600-850 DEG C), the carbon framework reacts with NH3 to form a nitride intermediate, and active single atoms generated by cracking of NH3 are doped into the carbon grid to form a nitrogen-rich carbon base; and in the third stage (900-1000 DEG C), short-time high-temperature pyrolysis promotes decomposition of the nitride intermediate, Bi atoms migrate to nitrogen vacancies in the carbon grid to form Bi-N4@NC, and stable single-atom sites are formed through thermodynamic reconstruction.

[0029] Meanwhile, the three-stage pyrolysis process adopted by the application has at least the following more beneficial technical effects relative to the traditional one-step high-temperature carbonization process: 1) single-atom anchoring is achieved. The application realizes precise anchoring of Bi single atoms through temperature gradient control, and the atomically dispersed Bi-N4 sites optimize the adsorption energy of vanadium ions, thereby improving catalytic activity. One-step high-temperature carbonization causes metal ions to be reduced and agglomerated into nanoparticles, which are dispersed on the surface of the carbon framework and are easily oxidized to form an insulating layer, thereby blocking active sites and reducing atomic utilization. 2) interface bonding is improved. ZIP-8 micropores constrain Bi atom migration and prevent high-temperature agglomeration. N atoms in the carbon framework form covalent bonds with oxygen vacancies on the high-entropy oxide surface, thereby constructing an electronic fast channel and reducing interface resistance. The metal nanoparticles of one-step high-temperature carbonization are only physically adsorbed on the carbon felt, and the particles are easily detached during the cycle. + 3) pore structure regulation. The three-stage pyrolysis process of the application retains the original micropores of ZIF-8 in the low-temperature stage, sieves H

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

[0031] In some embodiments of the application, in step (4), the electrolyte comprises 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 application, in step (4), the pH value of the electrolyte is 1.5-2.5.

[0033] In some embodiments of the present application, in step (4), the parameters of the pulse deposition are: pulse on for 5-20 milliseconds, off for 40-80 milliseconds, current density for 10-30 mA / cm 2 , and deposition amount for 0.3-1.0 mg / cm 2 .

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

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

[0036] The second aspect of the present application provides a composite electrode prepared by the above-mentioned method for preparing a composite electrode, 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 monatomic shell and Cr2O3 nanometer islands.

[0037] Specifically, the composite electrode of the present application simultaneously contains a porous high-entropy oxide core, a monatomic shell and a site barrier base, which breaks through the dilemma that the performance and stability of traditional electrodes cannot be considered together, and realizes the complementation of three functions and life guarantee through element precise division of labor and spatial functional zoning.

[0038] The third aspect of the present application provides a vanadium redox flow battery comprising the above-mentioned composite electrode.

[0039] The above technical solutions of the present application have at least the following technical effects or advantages compared with the prior art:

[0040] (1) The present application modifies the surface of the acid-treated carbon felt substrate, sequentially constructs a porous high-entropy oxide (HEO) core and a Bi monatomic shell base (Bi-N4@NC), and a hydrogen evolution inhibition base (Cr2O3 nanometer island modification), realizes element precise division of labor and spatial functional zoning through the multi-element catalysis of the porous high-entropy oxide core, the electronic regulation of the bismuth monatomic shell and the local passivation of the Cr2O3 nanometer island site shielding base, solves the technical problems of high-density operation, radical treatment of side reactions and low-cost long life of the composite electrode, balances the contradiction between hydrogen evolution inhibition and reaction activity, and simultaneously realizes the functions of charging catalysis, discharging catalysis and hydrogen evolution inhibition on a single electrode, thereby providing a core solution for electrode design of high-power flow batteries.

[0041] (2) The porous BiMoWCeO2 high-entropy oxide of the present application not only realizes element precise division (Bi catalyzes discharge / Mo catalyzes charging / W inhibits hydrogen evolution / Ce stabilizes interface) and high-entropy synergistic effect, has the effects of electronic structure complementation and site synergy; and improves the corrosion resistance and structural stability of the material, the electrochemical activity and reaction dynamics, and the ion selectivity and interface characteristics. At the same time, low carbon thermal shock temperature and MOF derived coupling are adopted to realize interface coupling, and pulse electrodeposition is adopted to improve efficiency, ensure precise coverage of Cr2O3 nano-islands, and the structure and stability of the material, and further inhibit hydrogen evolution.

[0042] (3) The composite material of the present application has a "HEO core-Bi monatomic shell-Cr2O3 nano-island" three-level configuration, realizes multi-element catalysis of the porous high-entropy core, electronic regulation and site shielding of the monatomic shell, and local passivation of the base. When applied to a vanadium redox flow battery, it has high voltage efficiency, coulomb efficiency and energy efficiency, and low hydrogen evolution performance and cycle stability; the coulomb efficiency is 99.2-99.5%, the voltage efficiency is 93.9-94.7%, the energy efficiency is 91.9-92.5%, the hydrogen evolution rate is 1.1-1.3 mA / cm 2 , and the capacity retention rate after 100 cycles is 98.9-99.2%. BRIEF DESCRIPTION OF DRAWINGS

[0043] Figure 1 is a preparation process schematic diagram of the composite electrode of the present application;

[0044] Figure 2 is a flow diagram of assembling an electric pile. DETAILED DESCRIPTION

[0045] The present application will be described in detail below with reference to examples in order to facilitate the understanding of the present application by those skilled in the art. It is necessary to point out here that the examples are only used to further illustrate the present application and cannot be understood as limiting the protection scope of the present application. Non-essential improvements and adjustments of the present application made by those skilled in the art according to the above content of the present application should still fall within the protection scope of the present application. At the same time, the raw materials mentioned below which are not described in detail are all commercially available products; the process steps or preparation methods which are not mentioned in detail are all process steps or preparation methods known to those skilled in the art.

[0046] The preparation process schematic diagram of the composite electrode of the following examples is shown in Figure 1 .

[0047] Example 1

[0048] A preparation method of a composite electrode, comprising the following steps:

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

[0050] (2) The acid-treated carbon felt substrate was immersed in a high-entropy oxide precursor solution and was hydrothermally reacted at 180℃ for 12 hours; then, zinc powder was added as a pore-forming agent (mass ratio of zinc powder to Bi(NO3)3 was 20:1) in the reaction product, and the reaction product was transferred into a H2 / Ar (volume ratio of H2 to Ar was 5:95) atmosphere furnace and was heated to 1000℃ for 2 seconds to obtain a porous high-entropy oxide core / carbon felt composite, which was denoted as HEO / carbon felt composite.

[0051] The preparation process of the high-entropy oxide precursor solution was as follows: equimolar Bi(NO3)3, (NH4)6Mo7O 24 , (NH4) 10 W 12 O 41 and Ce(NO3)3 were dissolved in ethylene glycol, and cetyltrimethylammonium bromide was added to obtain the solution. In 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 was left to stand for 24 hours to form a ZIP-8 type MOF base; then, the ZIP-8 type MOF base was subjected to three-stage pyrolysis under an NH3 / Ar atmosphere (volume ratio of NH3 to Ar was 1:4) to form a nitrogen-carbon coated Bi monatomic shell base on the surface of the porous high-entropy oxide core, which was denoted as HEO / Bi-N4@NC, and an HEO / Bi-N4@NC / carbon felt composite was obtained.

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

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

[0055] (4) The HEO / Bi-N4@NC / carbon felt composite was subjected to Cr 3+The electrolyte is transferred to an Ar atmosphere furnace and annealed at 400℃ for 2 hours to convert the amorphous Cr(OH)3 into stable α-Cr2O3 crystals, thereby preparing a Cr2O3 nano-island modified HEO / Bi-N4@NC / carbon felt composite, i.e., the composite electrode of the embodiment.

[0056] The electrolyte containing Cr 3+ is a mixed solution of 0.1 mol / L Cr(NO3)3 and 0.05 mol / L NH4Cl, and the pH is adjusted to 2.0 by 0.1 mol / L HNO3.

[0057] The process parameters of pulse deposition are as follows: pulse on for 10 milliseconds, pulse off for 50 milliseconds, current density of 15 mA / cm 2 , and deposition amount of 0.5 mg / cm 2 .

[0058] Embodiment 2

[0059] A preparation method of a composite electrode comprises the following steps:

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

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

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

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

[0064] The metal organic framework precursor solution is a methanol solution containing BiCl3 and 2-methyl imidazole, the concentration of BiCl3 is 0.03 mol / L, and the concentration of 2-methyl imidazole is 0.15 mol / L.

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

[0066] (4) The HEO / Bi-N4@NC / carbon felt composite is subjected to pulse deposition using an electrolyte containing Cr 3+ , and then is transferred to an Ar atmosphere furnace for annealing at 400℃ for 3 hours to convert amorphous Cr(OH)3 into stable α-Cr2O3 crystals, thereby obtaining a Cr2O3 nano-island modified HEO / Bi-N4@NC / carbon felt composite, i.e., the composite electrode of the present embodiment.

[0067] The electrolyte containing Cr 3+ is a mixed solution of Cr(NO3)3 with a concentration of 0.2 mol / L and NH4Cl with a concentration of 0.1 mol / L, and 0.1 mol / L HNO3 is used to adjust the pH to 2.0.

[0068] The process parameters of the pulse deposition are: pulse on for 15 milliseconds and pulse off for 60 milliseconds, the current density is 20 mA / cm 2 , and the deposition amount is 0.3 mg / cm 2 .

[0069] Example 3

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

[0071] (1) A carbon felt substrate is immersed in a mixed solution of H2SO4 and H2O2 (the volume ratio of H2SO4 to H2O2 is 3:1) and is subjected to reflux at 100℃ for 2 hours, followed by ultrasonic cleaning and drying, thereby obtaining an acid-treated carbon felt substrate.

[0072] (2) The acid-treated carbon felt substrate is immersed in a high-entropy oxide precursor solution, and a hydrothermal reaction is performed at 200 DEG C for 10 hours; then zinc powder is added as a pore-forming agent (the mass ratio of zinc powder to Bi(NO3)3 is 20:1) in the reaction product, and the reaction product is transferred to a H2 / Ar (H2 and Ar have a volume ratio of 5:95) atmosphere furnace, and heated to 800 DEG C for 5 seconds to prepare 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: equal molar amounts of Bi(NO3)3, (NH4)6Mo7O 24 , (NH4) 10 W 12 O 41 , and Ce(NO3)3 are dissolved in ethylene glycol, and cetyltrimethylammonium bromide is added to prepare the solution. In the high-entropy oxide precursor solution, the concentration of Bi(NO3)3 is 0.025 mol / L, and the concentration of the surfactant is 0.2 mol / L.

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

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

[0076] The temperature schedule of the three-stage pyrolysis is as follows: first, pyrolysis is performed at 400 DEG C for 1 hour, then pyrolysis is performed at 850 DEG C for 2 hours, and finally pyrolysis is performed at 1000 DEG C for 10 minutes, and the heating rate is 5 DEG C / min.

[0077] (4) The HEO / Bi-N4@NC / carbon felt composite is subjected to pulse deposition using an electrolyte containing Cr 3+ , and then is transferred to an Ar atmosphere furnace, and is annealed at 420 DEG C for 1.5 hours to convert amorphous Cr(OH)3 into stable alpha-Cr2O3 crystals, and a Cr2O3 nano-island modified HEO / Bi-N4@NC / carbon felt composite, that is, the composite electrode of the present embodiment, is prepared.

[0078] The electrolyte containing Cr 3+The electrolyte is a mixture of 0.05 mol / L Cr(NO3)3 and 0.01 mol / L NH4Cl, and 0.1 mol / L HNO3 is used to adjust the pH to 2.0.

[0079] The process parameters of pulse deposition are as follows: pulse on for 20 ms, pulse off for 80 ms, current density of 30 mA / cm 2 , and deposition amount of 1.0 mg / cm 2 .

[0080] Comparative Example 1

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

[0082] Comparative Example 2

[0083] The difference between Comparative Example 2 and Example 1 is only that the composite electrode of Comparative Example 2 does not contain step (4) in the preparation process, that is, the hydrogen evolution inhibition base (Cr2O3 nanometer island) modification is not constructed.

[0084] Comparative Example 3

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

[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 system of pyrolysis adopts a 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, a traditional Pb-Bi alloy electrodeposition process is used instead of the pulse deposition process of Example 1, specifically as follows:

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

[0091] Then the HEO / Bi-N4@NC / carbon felt composite is soaked in the lead-bismuth ion electrodeposition solution and ultrasonic treatment is performed for 5 minutes, and then electrodeposition modification is performed using a three-electrode system at a constant voltage of -0.8 V (deposition amount of 0.5 mg / cm 2), washed, dried to obtain the composite electrode of Comparative Example 5.

[0092] Comparative Example 6

[0093] The difference between Comparative Example 6 and Example 1 is that step (4) is exchanged with step (2) in Comparative Example 6, that is, the hydrogen evolution inhibition base modification is performed first, and then the high-entropy oxide HEO core is constructed.

[0094] Performance test

[0095] The composite electrodes prepared in Examples 1-3 and Comparative Examples 1-6 above were respectively assembled into stacks for charge-discharge tests under uniform test conditions, and 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] The assembly process is as shown in Figure 2 : electrode-separator pre-assembly (precise positioning and sealing reinforcement), liquid flow frame integration and sealing (flow channel matching and sealing glue application), modular stacking and pressing (layered pressing and anti-torsion tooling assistance), liquid system connection (symmetric shunt design), liquid injection and activation (vacuum injection and step activation).

[0097] The coulombic efficiency was tested by a battery test system at 200 mA / cm2, 25°C, and electrolyte 1.6 mol / L V 3+ / V 4+ , 3 mol / L H2SO4, and the coulombic efficiency was the ratio of the constant-current discharge capacity to the charge capacity. The voltage efficiency data was obtained from a high-precision data acquisition instrument, and the voltage efficiency was the ratio of the average discharge voltage to the average charge voltage. The energy efficiency was the product of the coulombic efficiency and the voltage efficiency. The hydrogen evolution rate was tested by a gas-tight test box and a gas chromatograph at the end of the stable stage of discharge, and the H2 volume was collected by the drainage gas collection method. The 100-cycle capacity retention rate was tested by a long-cycle test system for 100 charge-discharge cycles, and the capacity was calibrated every 10 weeks to obtain the ratio of the discharge capacity after cycling to the initial capacity.

[0098] Table 1:

[0099]

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

[0101] Meanwhile, Example 1 has higher voltage efficiency and coulomb efficiency than Comparative Example 1, because the d-orbital electrons of Mo / V elements and V 3+ Orbital hybridization occurs, weakening the V-O bonding force, significantly reducing the desolvation activation energy, significantly reducing the charging polarization, and the Ce element changing the valence produces oxygen vacancies, promoting H + jump to, maintain local pH stability.

[0102] Example 1 reduces the hydrogen evolution rate and improves the capacity retention rate compared to Comparative Example 2, because the presence of pulse electrodeposited Cr2O3 nanometer islands precisely shields the jagged edges of the carbon fibers, shifts the hydrogen evolution potential negatively, stabilizes the pH, and inhibits V 3+ precipitation.

[0103] Examples 1-3 have more excellent performance than Comparative Example 3, indicating that the all-vanadium redox flow battery three-function composite electrode prepared by the present application has a "porous high-entropy core", a "single-atom shell", and a "site barrier base". Through precise division of elements and spatial function partitioning, the traditional electrode performance and stability dilemma is broken through, realizing three-function complementation and life guarantee.

[0104] Comparative Example 4 relative to Example 1, using traditional one-step high-temperature carbon MOF carbonization, makes the Bi nanoparticle catalytic path long, increases the vanadium ion reaction energy barrier, and reduces the voltage efficiency.

[0105] Comparative Example 5 relative to Example 1, uses electrodeposited Pb-Bi alloy, where Bi ion dissolution leads to active site loss, and Pb deposition causes graphite corrosion, and after 100 weeks, the electrode porosity decreases. The α-Cr2O3 passivation layer of the example can inhibit graphite oxidation, and the high-entropy effect of the HEO core inhibits element segregation, with low capacity decay rate.

[0106] Comparative Example 6 relative to Example 1, the order of the preparation steps is changed, which increases the contact resistance due to the presence of interface cracks, and increases the ohmic polarization loss rate.

[0107] For those skilled in the art to which the present application belongs, without departing from the concept of the present application, a number of simple deductions or substitutions can be made without having to undergo creative labor. Therefore, the simple improvements made by those skilled in the art to the present application based on the disclosure of the present application should be within the protection scope of the present application. The above examples are preferred embodiments of the present application, and any similar processes and equivalent changes made should be within the protection scope of the present application.

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 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. 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 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. (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 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 (2), the hydrothermal reaction is carried out at a temperature of 160-200℃ for 10-30 hours; and / or, the calcination temperature is 800-1200℃ and the holding time is 2-5 seconds.

5. The method for preparing the composite electrode according to claim 1, characterized in that, In step (3), 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.

6. 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 1-3 hours, then heat to 600-850℃ for 2-4 hours, and finally heat to 900-1000℃ for 10-60 minutes; the heating rate is 2-8℃ / min.

7. 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.

8. 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.

9. A composite electrode, characterized in that, The composite electrode is prepared by the method of any one of claims 1-8, 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.

10. A vanadium redox flow battery, characterized in that, Includes the composite electrode as described in claim 9.

Images