A metal-organic framework derived gradient composite electrode with high selectivity and a preparation method and application thereof

Abstract

This invention relates to the field of electrochemical energy storage materials technology, specifically disclosing a highly selective metal-organic framework-derived gradient composite electrode, its preparation method, and its applications. Using porous carbon material as a substrate, a stable gradient structure of "substrate-intermediate layer-active layer" is formed by sequentially constructing an intermediate binder layer on its surface, using a spatial confinement agent to guide the selective growth of the metal-organic framework material, and then performing programmed temperature pyrolysis and atmospheric post-treatment. This composite electrode can achieve size-selective enrichment and mass transfer of chromium ion hydrated clusters, and greatly improve intrinsic catalytic activity and interfacial stability. When used as the anode of an iron-chromium flow battery, it can simultaneously and significantly enhance... Cr2+ / Cr3+ The electrode reaction kinetics are improved and hydrogen evolution side reactions are effectively suppressed, thereby achieving high energy efficiency and ultra-long cycle life at high current densities. This solves the technical problems of non-selective mass transfer, easy aggregation of active sites, and unstable interface of conventional modified electrodes.

Description

Technical Field

[0001] This invention relates to the field of electrochemical energy storage materials technology, specifically to a highly selective metal-organic framework-derived gradient composite electrode, its preparation method, and its applications. Background Technology

[0002] As the global energy structure shifts towards cleaner and lower-carbon energy sources, the development and utilization of renewable energy sources such as wind and solar power continue to expand. However, the inherent intermittency and instability of these energy sources pose significant challenges to the stable operation of the power grid. Large-scale energy storage technology is considered key to solving this problem, enabling the spatial and temporal shifting of electricity, smoothing output, and peak and frequency regulation, thereby improving the grid's ability to absorb renewable energy.

[0003] Among numerous large-scale energy storage technologies, flow batteries are considered one of the most promising technological routes due to their outstanding advantages such as power-capacity decoupling, flexible design, long cycle life, high safety, and environmental friendliness. Among them, the iron-chromium flow battery, as the earliest proposed flow battery system, has its unique advantages: Abundant resources and low cost: Its active materials, iron and chromium, are abundant elements in the Earth's crust. The raw materials are widely available and the price is significantly lower than that of vanadium in vanadium redox flow batteries, giving it a huge cost advantage and potential for large-scale application.

[0004] Wide environmental adaptability: Iron-chromium redox flow batteries have a wide operating temperature range (typically -20~70℃), making them suitable for various climates and geographical environments.

[0005] Intrinsically safe: The electrolyte solution is an aqueous system, posing no risk of combustion or explosion.

[0006] No self-discharge: Electrical energy is stored in the electrolyte of the tank in the form of chemical energy, and theoretically there is no self-discharge.

[0007] Environmentally friendly and easy to recycle: Battery components such as electrodes, separators, and current collectors are mostly made of carbon materials, plastics, or metals, which are easy to process and recycle in an environmentally friendly manner.

[0008] Despite the advantages mentioned above, the commercialization of iron-chromium redox flow batteries still faces key technological bottlenecks. One of the core issues lies in the chromium couple reaction at the negative electrode half-cell. The reaction equation for the chromium couple is: Cr... 3+ + e - ⇌ Cr 2+ This reaction presents the following inherent challenges: Slow reaction kinetics: Cr 3+ The reduction and Cr 2+ The oxidation process involves the rearrangement of inner orbital electrons, resulting in a high reaction energy barrier, slow kinetics, and high electrochemical polarization.

[0009] Competition from hydrogen evolution side reactions: The potential range of the chromium couple reaction is very close to that of the hydrogen evolution reaction from water. The slow chromium reaction kinetics make it easy for the operating potential to shift negatively towards the hydrogen evolution potential, triggering severe hydrogen evolution side reactions. This not only reduces coulombic efficiency, consumes water and protons, leading to electrolyte composition imbalance, but may also generate gas in the electrode pores, hindering electrolyte transport and even causing safety issues such as uneven pressure distribution in the battery stack.

[0010] Insufficient active sites: Traditionally, the materials used as electrodes in iron-chromium flow batteries are mainly graphite felt or carbon felt. Although these materials have good conductivity and high chemical stability, their surfaces are chemically inert sp. 2 Carbon inherently exhibits low catalytic activity towards chromium ions. Untreated carbon felt surfaces are hydrophobic, resulting in poor wettability with aqueous electrolytes and a limited effective reaction area.

[0011] To overcome these challenges, various electrode modification strategies have been attempted in the prior art: Thermal activation treatment: High-temperature treatment of carbon felt in air or an oxidizing atmosphere was the most commonly used method in the early days. This method introduces oxygen-containing functional groups such as carboxyl, hydroxyl, and carbonyl groups onto the carbon fiber surface, improving the hydrophilicity and electrochemical active area of ​​the electrode, which has a certain effect on improving battery performance. However, the number of active sites introduced by this method is limited, and the functional groups may detach after long-term operation, so the performance stability needs to be improved.

[0012] Acid / oxidation treatment: Carbon felt is treated with strong oxidizing acids such as concentrated nitric acid and sulfuric acid, also to introduce oxygen-containing functional groups. This method is highly corrosive and may damage the carbon fiber structure, affecting mechanical strength and causing environmental pollution problems.

[0013] Heteroatom doping: By introducing heteroatoms such as nitrogen, sulfur, and phosphorus onto the carbon framework, the electron cloud distribution of carbon materials is altered, creating active sites conducive to charge transfer. For example, nitrogen doping has been shown to effectively enhance the electrocatalytic activity of carbon materials for vanadium and chromium ions. However, the doping process typically requires high temperatures and specific precursors, making the process relatively complex, and controlling the uniformity and density of doping sites is a challenge.

[0014] Metal nanoparticle modification: Loading metals or their oxides, such as Bi, Pb, and Ti, which have high overpotentials for hydrogen evolution, onto the electrode surface can effectively suppress hydrogen evolution side reactions. However, nanoparticles are prone to aggregation and detachment under long-term cycling and liquid flow scouring, leading to performance degradation and potentially introducing impurity ions to contaminate the electrolyte.

[0015] In summary, developing a novel electrode material that can simultaneously provide a large number of highly active sites, optimize the reaction interface, effectively suppress side reactions, and has a stable structure is an urgent need to promote the development of iron-chromium redox flow battery technology. Summary of the Invention

[0016] To address the problems existing in the prior art, this invention provides a highly selective metal-organic framework-derived gradient composite electrode, its preparation method, and its application. A multi-layer gradient structure of "conductive substrate-intermediate binder layer-MOF-derived active layer" is designed. This structure provides a continuous electronic conduction path and stable mechanical support, enabling simultaneous selective enrichment of chromium ions, ultra-high intrinsic catalytic activity, and excellent interfacial stability, thus solving the problems mentioned in the background art.

[0017] To achieve the above objectives, the present invention provides the following technical solution: a highly selective metal-organic framework-derived gradient composite electrode, the composite electrode comprising: A conductive porous carbon substrate, an intermediate binder layer grown on the substrate surface, and a metal-organic framework-derived active layer grown on the surface of the intermediate binder layer. The multi-layered gradient structure of "conductive substrate-intermediate binder-MOF-derived active layer" provides a continuous electron conduction path and robust mechanical support.

[0018] Preferably, the intermediate binder layer is a nitrogen-doped carbon layer or a reduced graphene oxide layer with a thickness of 10-200 nm. By introducing an intermediate binder layer (such as nitrogen-doped carbon or reduced graphene oxide), a strong chemical bond and a three-dimensional continuous conductive network are established between the active layer and the carbon substrate, which greatly enhances the interlayer bonding force and the mechanical integrity of the overall structure, enabling it to withstand long-term liquid flow erosion.

[0019] Preferably, the active layer is composed of a nitrogen-doped carbon skeleton with a microporous-mesoporous hierarchical structure and M-N4 catalytic sites atomically dispersed in the carbon skeleton, wherein M is selected from at least one of Co, Fe or Zn; the micropore size of the active layer is distributed in the range of 0.7-1.2 nm, and the micropore volume accounts for more than 60% of the total pore volume.

[0020] The pore size range of 0.7-1.2 nm corresponds to the chromium ion hydrated clusters ([Cr(H2O)6]). 3+ With a kinetic diameter of approximately 0.96 nm, the size is highly matched, enabling a "size sieving" effect that preferentially adsorbs and transports chromium ions. Simultaneously, the steric hindrance effectively confines smaller hydrated protons (H₃O₂). + Excessive proximity and reaction between the two significantly increases the hydrogen evolution overpotential from a physical mechanism perspective.

[0021] In the active layer, the catalytic metal (M) is dispersed at the atomic level within a nitrogen-doped carbon framework, forming a well-defined M-N4 coordination structure (M can be Co, Fe, Zn, etc.). This single-atom catalytic center exhibits the highest atomic utilization, consistent intrinsic high activity, and extremely strong structural stability, avoiding the problems of nanoparticle aggregation and loss.

[0022] Preferably, the specific surface area of ​​the composite electrode is 800-1800 m². 2 / g, in 0.5 M Cr 3+ The hydrogen evolution overpotential in the electrolyte is increased by more than 150 mV compared to the original carbon substrate.

[0023] On the other hand, to achieve the above objectives, the present invention also provides the following technical solution: a method for preparing a highly selective metal-organic framework-derived gradient composite electrode, comprising the following steps: S1. Plasma treatment is performed on the conductive porous carbon substrate.

[0024] S2. An intermediate adhesive layer is constructed on the substrate surface after step S1 by chemical vapor deposition or impregnation-pyrolysis.

[0025] S3. Immerse the substrate loaded with the intermediate binder layer into a precursor solution containing a metal source, organic ligand, and a spatial confinement agent, and react at 40-80°C for 6-24 hours. A metal-organic framework-derived active layer is loaded onto the intermediate binder layer using a confinement growth method. In the precursor solution containing a spatial confinement agent (such as PVP), MOF crystal growth is guided on the intermediate binder layer. The spatial confinement agent can regulate the MOF grain size and orientation, promoting the formation of a richer and more uniform small-sized microporous structure, creating a favorable precursor for subsequent pyrolysis.

[0026] S4. Perform programmed temperature-controlled pyrolysis on the material loaded with the metal-organic framework-derived active layer, and obtain a highly selective metal-organic framework-derived gradient composite electrode after cooling.

[0027] Preferably, in step S3, the molar ratio of the metal source to the organic ligand is 1:2-8; the metal source is one of cobalt salt, iron salt, or zinc salt; and the organic ligand is 2-methylimidazole.

[0028] Preferably, the space confinement agent is polyethylene glycol-1000 or polyvinylpyrrolidone.

[0029] Preferably, in step S4, the programmed temperature-controlled pyrolysis specifically involves: heating to 550-700°C at a rate of 0.5-2°C / min under an inert atmosphere, holding at that temperature for 1-4 hours, and then treating at 500-600°C for 0.5-2 hours in an ammonia-containing atmosphere. Using a slow heating rate (0.5-2°C / min) to the medium temperature range (550-700°C) for pyrolysis is beneficial for the orderly carbonization of the MOF framework and maximizes the retention of nitrogen species. The subsequent secondary heat treatment in an ammonia-containing atmosphere allows the ammonia to further etch the carbon layer, create more defects, and act as a nitrogen source to help stabilize metal atoms, ultimately forming atomically dispersed M-N4 sites.

[0030] Preferably, the inert atmosphere is argon or nitrogen; the ammonia-containing atmosphere is a mixture of ammonia and an inert gas, wherein the volume percentage of ammonia is 5%-20%.

[0031] On the other hand, to achieve the above objectives, the present invention also provides the following technical solution: an application of a highly selective metal-organic framework-derived gradient composite electrode, using the highly selective metal-organic framework-derived gradient composite electrode as a negative electrode material in an iron-chromium flow battery, at 120 mA / cm². 2 After 1000 cycles at high current density, the performance degradation rate is less than 3%.

[0032] The beneficial effects of this invention are: 1) High ion transport selectivity and outstanding hydrogen suppression effect: The specific 0.7-1.2 nm micropores sieve and enrich chromium ions, making the electrode highly selective for chromium ions. 3+ Its adsorption capacity can reach H + It has more than three times the adsorption capacity. The physical confinement effect, combined with the modulation of the surface electronic structure, increases the hydrogen evolution overpotential of the electrode by more than 150 mV compared with the original carbon-based electrode, fundamentally alleviating the hydrogen evolution competition.

[0033] 2) Extreme catalytic activity and stability: Atomically dispersed M-N4 sites provide extremely high intrinsic catalytic activity while eliminating the inherent instability of nanoparticle catalysts. The electrode operates at 120 mA / cm². 2 After 1000 cycles at high current density, the performance degradation rate can be less than 3%, which is far superior to conventional nanoparticle modified electrodes (typically degradation >7%).

[0034] 3) Stable structure and low interface resistance: The introduction of gradient structure and intermediate bonding layer ensures the mechanical and interface stability of the electrode in harsh electrochemical environment, avoids active layer peeling, and ensures low and stable interface charge transfer impedance during long-term cycling.

[0035] 4) Excellent overall performance: The synergistic effect of the above advantages enables the iron-chromium flow battery using the electrode of this invention to achieve 120 mA / cm² performance. 2 The energy efficiency can be stabilized at over 83%, achieving a balance between high power output and long-life operation, and providing a key material solution for the industrialization of iron-chromium redox flow batteries. Attached Figure Description

[0036] Figure 1 This is a schematic diagram of the fabrication process of the highly selective metal-organic framework-derived gradient composite electrode of the present invention. Figure 2 Here is a scanning electron microscope image of the original carbon cloth electrode in Comparative Example 2; Figure 3 These are scanning electron microscope images of the gradient composite electrodes prepared in Example 1, Comparative Example 1, and Comparative Example 2 of the present invention. Figure 4 This is a comparison diagram of nitrogen adsorption-desorption isotherms between Example 1 of the present invention and Comparative Examples 1 and 2. Figure 5 This is a comparison diagram of the polarization curves of the electrodes prepared in Example 1 and Comparative Examples 1 and 2 as negative electrodes in a single iron-chromium redox battery. Figure 6 The iron-chromium redox flow battery assembled with the electrodes prepared in Example 1 operates at 80 mA / cm². 2 Comparison of energy efficiency and coulombic efficiency at current density; Figure 7 The iron-chromium redox flow battery assembled with electrodes prepared in Example 1 operates at 120 mA / cm². 2 Long-cycle performance test graph under current density. Detailed Implementation

[0037] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0038] Metal-organic frameworks (MOFs) are a class of crystalline porous materials formed by the self-assembly of metal ions or clusters with organic ligands through coordination bonds. MOF materials possess the following unique properties: Extremely high specific surface area and porosity: can provide huge interfaces and abundant mass transport channels for electrochemical reactions.

[0039] Regulated pore structure: Pore size and pore environment can be precisely designed and controlled at the molecular level.

[0040] Unsaturated metal sites: potential highly efficient catalytic centers.

[0041] Functionalized organic frameworks: Specific functional groups can be introduced by selecting or modifying organic ligands.

[0042] These properties give MOF materials great potential in catalysis, gas adsorption, separation, and sensing. However, MOF materials themselves typically have poor conductivity, limiting their direct use as electrode materials. Combining them with highly conductive carbon substrates to construct composite electrodes promises to leverage their strengths while mitigating their weaknesses, creating high-performance electrochemical interfaces. Through subsequent heat treatment, MOFs can be transformed into porous carbon materials with highly dispersed metal species and heteroatom doping, further optimizing their conductivity and catalytic stability.

[0043] However, directly applying conventional MOF materials to the anode of iron-chromium flow batteries still faces three major challenges: First, the pore structure of ordinary MOFs and their derivatives is broad and lacks selectivity for ions of specific sizes (such as chromium ion hydrated clusters), failing to effectively block competitive hydrogen ion transport, resulting in a still serious hydrogen evolution side reaction; second, the metal active species generated after high-temperature pyrolysis mostly exist in the form of nanoparticles, which are prone to aggregation and detachment under long-term electrochemical cycling and liquid flow scouring, leading to catalytic activity decay; third, the simple physical adhesion or weak bonding between the active layer and the carbon substrate results in insufficient interfacial stability in the harsh battery operating environment, affecting long-life cycling. Therefore, developing a composite electrode that can achieve selective ion mass transfer, possesses highly stable atomic-level active centers, and has a robust interfacial structure is the key to overcoming the current technological bottlenecks.

[0044] To overcome the shortcomings of existing MOF-modified electrodes in iron-chromium flow batteries, such as non-selectivity of ion mass transfer, poor stability of catalytic sites, and weak interlayer bonding, this invention provides a highly selective metal-organic framework-derived gradient composite electrode, comprising a conductive porous carbon substrate, an intermediate binder layer grown on the substrate surface, and a metal-organic framework-derived active layer grown on the surface of the intermediate binder layer. The electrode preparation method is as follows: Figure 1 As shown, it includes: S1. Conductive porous carbon substrate is subjected to plasma treatment to introduce oxygen-containing functional groups, such as carboxyl and hydroxyl groups, onto its surface.

[0045] S2. An intermediate adhesive layer is constructed on the substrate surface after step S1 by chemical vapor deposition or impregnation-pyrolysis.

[0046] S3. Immerse the substrate loaded with the intermediate binder layer into a precursor solution containing a metal source, organic ligand, and a spatial confinement agent, and react at 40-80°C for 6-24 hours. A metal-organic framework-derived active layer is loaded onto the intermediate binder layer using a confinement growth method. In the precursor solution containing a spatial confinement agent (such as PVP), MOF crystal growth is guided on the intermediate binder layer. The spatial confinement agent can regulate the MOF grain size and orientation, promoting the formation of a richer and more uniform small-sized microporous structure, creating a favorable precursor for subsequent pyrolysis.

[0047] S4. Perform programmed temperature-controlled pyrolysis on the material loaded with the metal-organic framework-derived active layer, and obtain a highly selective metal-organic framework-derived gradient composite electrode after cooling.

[0048] The following are specific examples: Example 1: Composite electrode with gradient structure and single-atom Co site Carbon cloth pretreatment: After cleaning, the carbon cloth is placed in an oxygen plasma treatment instrument (power 100W) for 10 minutes to introduce oxygen-containing functional groups.

[0049] Constructing the intermediate bonding layer: Immerse the above carbon cloth in 0.5 wt% graphene oxide aqueous dispersion for 10 minutes, take it out and heat treat it at 400℃ for 1 hour under argon to obtain carbon cloth (rGO / CC) with a reduced graphene oxide (rGO) layer on the surface.

[0050] ZIF-67 confined growth: Solution A: Dissolve 1.746 g of cobalt nitrate hexahydrate in 40 mL of methanol.

[0051] Solution B: Dissolve 3.944 g of 2-methylimidazole in 40 mL of methanol.

[0052] Solution C: Dissolve 2.0 g of polyvinylpyrrolidone (PVP K30) in 20 mL of methanol.

[0053] Mix solutions B and C, then pour the mixture into solution A and stir for 5 minutes. Immerse rGO / CC in this mixture and allow it to react at 60°C for 12 hours. After immersion, wash with methanol and dry under vacuum at 60°C to obtain ZIF-67@rGO / CC.

[0054] Programmable temperature-controlled pyrolysis: ZIF-67@rGO / CC was placed in a tube furnace and heated to 650℃ at 1℃ / min under argon atmosphere, and held for 2 hours. Then, an argon / ammonia mixture (volume ratio 9:1) was used, and the mixture was treated at 550℃ for 1 hour. After natural cooling, the target electrode was obtained, denoted as SA-CoNC / rGO / CC.

[0055] Comparison Example 1: Conventional MOF-derived electrode (gradient-free structure and confined growth) Except for omitting step 2 (no rGO layer), not adding PVP in step 3, and only heat-treating at 800°C for 2 hours under argon atmosphere in step 4, the rest is the same as in Example 5. The resulting electrode is denoted as C-CoNC / CC.

[0056] Comparative Example 2: Commercial thermally activated electrode The graphite felt was heat-treated at 400°C for 3 hours in air.

[0057] Characterization and performance testing were performed on the examples and control examples. Morphology and structural characterization: The surface morphology of each electrode was observed using scanning electron microscopy, such as... Figure 2 As shown, the original carbon cloth has a smooth surface and few activation sites, resulting in a weak ability to promote electrochemical behavior and failing to effectively promote electrochemical reactions in the flow battery. Figure 3 As shown, the electrode surface of Example 1 is uniformly covered with typical rhombic dodecahedral ZIF-67 crystals. After heat treatment at 700℃, the surface morphology of the ZIF-67 crystals becomes rough, forming a porous structure, which is a typical characteristic of MOF carbonization. This material structure has dense electrochemical active sites, which can effectively promote electrochemical reactions.

[0058] Specific surface area and pore size analysis: such as Figure 4 As shown, the specific surface area and pore size distribution of the material were analyzed using nitrogen adsorption-desorption tests. The figure shows that the original carbon cloth had a very low specific surface area (< 10 μm). 2 / g). The modified electrode of Example 1 has a significantly increased specific surface area and obvious microporous and mesoporous features. The higher specific surface area can significantly increase the contact area between the motor and the electrolyte, allowing the electrode to better exchange electrons with the electrolyte.

[0059] Electrochemical performance testing: The above electrodes were used as negative electrodes and assembled into a single iron-chromium redox flow battery with a commercial carbon felt positive electrode and a Nafion 115 separator. The electrolyte was 1.5 M FeCl2 + 2.0 M CrCl3 + 3.0 M HCl.

[0060] Polarization curve: such as Figure 5 As shown, under the same current density, the battery voltage polarization using the modified electrode of the present invention is significantly smaller than that of the original carbon cloth electrode of Comparative Example 2, indicating that its polarization resistance is smaller and the reaction kinetics are faster.

[0061] Single-cell performance test: at 120 mA / cm 2 Below, such as Figure 6 As shown, the battery with the high-selectivity gradient composite electrode in Example 1 has an initial energy efficiency of 83.2%, which remains high even after long-term cycling. The electrochemical performance is stable, and the coulombic performance is maintained above 95%, indicating that this electrode can work stably in the battery for a long time without significant structural changes.

[0062] Long-cycle testing shows that the electrode in Example 1 has good cycling stability during long-cycle testing (120 mA / cm). 2 After that, the result is as follows: Figure 7 As shown, after 500 cycles, the battery's energy efficiency decay rate is less than 5%, demonstrating excellent cycle stability.

[0063] This invention successfully prepared various MOF and its derivative-modified carbon-based composite electrodes and applied them to the anode of iron-chromium flow batteries. Systematic testing results show that this series of electrodes can significantly improve the reaction kinetics of the chromium couple by providing high specific surface area, hierarchical pores, and a large number of highly active sites, and effectively suppress hydrogen evolution side reactions, thereby comprehensively improving the coulombic efficiency, voltage efficiency, energy efficiency, and cycle stability of iron-chromium flow batteries. This preparation method provides a new technical path for developing high-performance, long-life iron-chromium flow batteries.

[0064] Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A highly selective metal-organic framework-derived gradient composite electrode, characterized in that, The composite electrode includes: The active layer comprises a conductive porous carbon substrate, an intermediate binder layer grown on the substrate surface, and a metal-organic framework-derived active layer grown on the surface of the intermediate binder layer. The active layer consists of a nitrogen-doped carbon framework with a microporous-mesoporous hierarchical structure and M-N4 catalytic sites atomically dispersed within the carbon framework, where M is selected from at least one of Co, Fe, or Zn. The micropore size of the active layer is distributed in the range of 0.7-1.2 nm, and the micropore volume accounts for more than 60% of the total pore volume. The intermediate binder layer is a nitrogen-doped carbon layer or a reduced graphene oxide layer with a thickness of 10-200 nm. The method for preparing the highly selective metal-organic framework-derived gradient composite electrode includes the following steps: S1. Plasma treatment of conductive porous carbon substrate; S2. An intermediate adhesive layer is constructed on the surface of the substrate treated in step S1 by impregnation-pyrolysis method. S3. Immerse the substrate loaded with the intermediate binder layer into a precursor solution containing a metal source, an organic ligand and a spatial confinement agent, and react at 40-80°C for 6-24 hours. Load a metal-organic framework-derived active layer onto the intermediate binder layer using a confinement growth method. The spatial confinement agent is polyethylene glycol-1000 or polyvinylpyrrolidone. S4. The material loaded with the metal-organic framework-derived active layer is subjected to temperature-controlled pyrolysis, and after cooling, a highly selective metal-organic framework-derived gradient composite electrode is obtained. The temperature-controlled pyrolysis specifically involves: heating to 550-700℃ at a rate of 0.5-2℃ / min under an inert atmosphere, holding at that temperature for 1-4 hours, and then treating at 500-600℃ for 0.5-2 hours in an ammonia-containing atmosphere; the ammonia-containing atmosphere is a mixture of ammonia and an inert gas, wherein the volume percentage of ammonia is 5%-20%.

2. The highly selective metal-organic framework-derived gradient composite electrode according to claim 1, characterized in that: The specific surface area of ​​the composite electrode is 800-1800 m². 2 / g, in 0.5 M Cr 3+ The hydrogen evolution overpotential in the electrolyte is increased by more than 150 mV compared to the original carbon substrate.

3. The highly selective metal-organic framework-derived gradient composite electrode according to claim 1, characterized in that: In step S3, the molar ratio of the metal source to the organic ligand is 1:2-8; the metal source is one of cobalt salt, iron salt or zinc salt; and the organic ligand is 2-methylimidazole.

4. The highly selective metal-organic framework-derived gradient composite electrode according to claim 1, characterized in that: The inert atmosphere is argon or nitrogen.

5. An application of a highly selective metal-organic framework-derived gradient composite electrode according to any one of claims 1-4, characterized in that: The application of highly selective metal-organic framework-derived gradient composite electrodes as anode materials in iron-chromium flow batteries, at 120 mA / cm², was demonstrated. 2 After 1000 cycles at high current density, the performance degradation rate is less than 3%.

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