A flow battery electrode with hierarchical pore structure constructed by in-situ etching of copper-MOF and a preparation method thereof

Abstract

This invention discloses a flow battery electrode with a hierarchical pore structure constructed by in-situ etching of copper-MOF and its fabrication method. This invention achieves synergistic directional formation of pores and introduction of oxygen-containing functional groups on the surface by in-situ anchoring copper-MOF on the surface of carbon felt or graphite felt and performing high-temperature etching at a controlled temperature. This significantly improves the electrode's specific surface area, effective active site density, and wettability, reduces charge transport impedance, and optimizes mass transfer channels. Practical results show that this invention can achieve fine control of pore size distribution and surface chemistry through controllable process parameters, thereby improving the energy efficiency, power density, and cycle life of flow batteries under high current density conditions (300-400 mA·cm⁻¹). ‑ Significant energy efficiency improvements are achieved under these conditions. Compared with existing technologies, the preparation process of this invention is simple, safe, energy-efficient, and easily scaled up.

Description

Technical Field

[0001] This invention belongs to the field of electrochemical energy storage technology, and specifically relates to a flow battery electrode with a hierarchical pore structure constructed by in-situ etching of copper-MOF and its preparation method. Background Technology

[0002] Redox flow batteries are considered important candidates for large-scale electrochemical energy storage due to their advantages such as spatial separation of electrolyte and electrode, independent design of power and energy, high safety, long cycle life, and ease of scalability. All-vanadium, iron-chromium, zinc-cerium, and all-iron flow battery systems have all received widespread attention, with the all-vanadium system being particularly representative due to its avoidance of electrolyte cross-contamination. However, current flow batteries generally suffer from problems such as slow electrode reaction kinetics, significant side reactions such as hydrogen and oxygen evolution, and high costs of some key materials. These issues lead to severe polarization and limited energy efficiency at medium to high current densities, hindering their engineering and commercialization. Therefore, research focusing on electrode materials and electrode / electrolyte interface structures, and synergistically improving reaction kinetics and suppressing side reactions through electrode design and interface engineering, is a key direction for promoting the practical application of flow batteries.

[0003] In flow batteries, electrodes, as carriers of electrochemical reactions, directly determine redox kinetics and the battery's voltage and energy efficiency. Although polyacrylonitrile-based carbon felt, graphite felt, and other carbon-based felt materials offer advantages in terms of sourcing, chemical stability, conductivity, cost, and mechanical properties, their insufficient surface chemical inertness and intrinsic electrochemical activity result in low density of usable active sites and poor wettability. This makes it difficult for the electrolyte to fully wet the electrode, limiting the actual reaction interface and weakening the adsorption, diffusion, and reversible transformation capabilities of active species on the electrode surface. Consequently, it is difficult to meet the requirements for rapid and stable discharge / charge at high current densities. Therefore, modification of electrode materials is necessary.

[0004] Currently, methods for modifying electrode materials mainly include high-temperature heat treatment, acidic or oxidative wet activation, heteroatom doping, carbon nanotube / graphene modification, and loading with metals / oxides. Although these modification methods can increase surface functional groups or conductive phases and improve some interfacial properties and charge transport, they mostly focus on introducing surface defects or adding external conductive networks. They have limited ability to finely control the internal pore structure of the electrode and cannot simultaneously achieve synergistic optimization of specific surface area, mass transfer channels, and interfacial chemistry. Existing technologies also report other modification methods. For example, Chinese patent CN111785978B discloses a porous electrode for flow batteries and its preparation method. The preparation method includes the following steps: S1, adding polyacrylonitrile (PAN) and Zn / Co salt to N,N-dimethylformamide or N,N-dimethylacetamide, heating and mixing to obtain an electrospinning solution; S2, electrospinning with the electrospinning solution to obtain an electrospinned fiber membrane; S3, dissolving an organic ligand in a solvent to obtain an organic ligand solution, immersing the electrospinned fiber membrane in the organic ligand solution, and waiting for MOF particles to be generated in situ on the surface of the electrospinned fiber filaments; S4, pre-oxidizing the fiber filaments with MOF particles deposited on the surface, and then carbonizing them under an inert gas atmosphere to obtain a porous carbon fiber electrode for use in flow batteries.

[0005] Chinese patent CN119008990A discloses a flow battery electrode felt and its preparation method. The preparation method includes the following steps: a PAN-based carbon fiber felt substrate is sequentially subjected to low-temperature carbonization and water vapor etching, then immersed in an Au-CNTs@MOF precursor solution, followed by drying, activation, and in-situ reduction to obtain the Au-CNTs@MOF layer. However, the above method has disadvantages such as cumbersome experimental procedures, the need for a specific gas (ammonia) for etching, high etching temperatures, high raw material costs, and large investment in experimental equipment. Summary of the Invention

[0006] To address the problems existing in the prior art, the present invention aims to provide a flow battery electrode with a hierarchical pore structure constructed by in-situ etching of copper-MOF and its preparation method.

[0007] To achieve the above objectives, the present invention adopts the following technical solution: The first aspect of this invention provides a method for fabricating a flow battery electrode with a hierarchical pore structure using in-situ etching of a copper-MOF, comprising the following steps: S1. Add copper nitrate trihydrate and terephthalic acid to the solvent and stir to mix to obtain the precursor solution; S2. The precursor solution is transferred to a hydrothermal reactor and graphite felt GF is added to carry out a hydrothermal reaction to obtain copper MOF modified graphite felt. S3. The obtained copper MOF modified electrode is transferred to a muffle furnace, heated, and subjected to high-temperature thermal etching to obtain an electrode with a hierarchical hole structure constructed by copper MOF etching.

[0008] Preferably, in step S1, the solvent is one or a combination of two or more of deionized water, N,N-dimethylformamide and anhydrous ethanol, more preferably a mixture of water, N,N-dimethylformamide and anhydrous ethanol, and the volume ratio of deionized water, N,N-dimethylformamide and anhydrous ethanol in the mixture is 8:1:1.

[0009] In the hydrothermal in-situ growth of copper MOFs, a mixed solvent system consisting of deionized water, N,N-dimethylformamide (DMF), and anhydrous ethanol is preferred. This is because copper nitrate has high solubility in water, and deionized water provides a highly polar reaction environment, promoting the growth of Cu. 2+ The complete dissociation of the metal ions provides the necessary driving force for the coordination reaction between the metal ions and the organic ligands. However, carboxylic acid organic ligands have limited solubility in water. DMF, as a polar organic solvent, can effectively dissolve these ligands and regulate their deprotonation process under hydrothermal conditions, thus facilitating the construction of the MOF framework and crystal growth. In addition, the graphite or carbon felt surface has a certain degree of hydrophobicity. The introduction of anhydrous ethanol can reduce the surface tension of the system, improve the wettability of the reaction solution on the graphite or carbon felt fibers, and inhibit free nucleation in the bulk solution phase, thereby promoting the uniform in-situ growth and directional deposition of copper MOFs on the graphite or carbon felt surface.

[0010] Preferably, in step S1, the molar ratio of copper nitrate trihydrate to terephthalic acid is 0.95–1.25 mmol: 0.50–1.50 mmol.

[0011] Preferably, in step S2, the electrode is a graphite felt or carbon felt electrode with a thickness of 1.5 to 4.6 mm.

[0012] Preferably, in step S2, the hydrothermal reaction temperature is 100–150°C, and the reaction time is 12–24 h. During hydrothermal in-situ growth, the reaction temperature has a significant impact on the nucleation and growth behavior of Cu-MOF. If the hydrothermal temperature is too low, the system's reaction kinetics are insufficient, and Cu… 2+Coordination reactions with organic ligands are difficult to occur effectively, limiting the nucleation process and resulting in fewer heterogeneous nuclei on the graphite or carbon felt surface. This leads to lower Cu-MOF coverage on the graphite felt surface and more unreacted precursor residues. Simultaneously, insufficient crystal growth at low temperatures results in lower crystallinity of the obtained MOF. If the hydrothermal temperature is too high, the Cu-MOF crystal growth rate accelerates significantly, easily inducing rapid bulk nucleation and crystal coarsening, making the MOF bonding on the graphite felt surface primarily physical adhesion with weak interfacial bonding. Furthermore, excessively high reaction temperatures can cause distortion of the MOF framework structure, thus affecting its structural stability. The hydrothermal reaction time also significantly impacts the morphology and structural integrity of Cu-MOFs. If the reaction time is too short, the number of crystal nuclei is limited, and the crystals do not grow sufficiently, resulting in uneven distribution and discontinuous coverage of Cu-MOF on the graphite or carbon felt surface. Simultaneously, numerous internal defects make it difficult to form a complete and stable MOF framework structure. If the hydrothermal reaction time is too long, Cu-MOF crystals are prone to agglomeration, leading to increased grain size, reduced specific surface area, and potential blockage of the porous structure within the graphite or carbon felt. Furthermore, further crystal coarsening introduces significant interfacial stress, causing some Cu-MOF to detach from the graphite or carbon felt surface.

[0013] Preferably, in step S3, the high-temperature thermal etching is performed in an air atmosphere, with an etching temperature of 300–800°C, a heating rate of 2–6°C / min, and an etching time of 2–5 h. During the high-temperature thermal etching of Cu-MOF / GF composite materials in an air atmosphere, the etching temperature has a decisive influence on the etching behavior. If the etching temperature is too low, the Cu-MOF structure decomposes incompletely, making it difficult to form catalytically active Cu-based species, resulting in insufficient catalytic oxidation etching of the graphite felt, limited etching degree, and uneven distribution. Conversely, if the etching temperature is too high, the oxidation reaction of the carbon material is significantly aggravated, and the catalytic etching gradually transforms into non-selective oxidation, easily causing significant thinning or even local breakage of the graphite felt carbon fibers, destroying its skeletal structure. Furthermore, the heating rate also has a significant impact on etching uniformity. An excessively rapid heating rate can cause the Cu-MOF structure to collapse instantaneously, leading to rapid aggregation of Cu species, thereby triggering locally concentrated etching behavior and reducing the controllability of the etching process. The control of etching time is also crucial. When the etching time is too short, the catalytically active species formed after the decomposition of Cu-MOF have not yet fully acted on the carbon matrix, resulting in limited etching effect; while when the etching time is too long, it may cause over-etching, which will cause the microporous structure to evolve into macroporous structures, resulting in a decrease in specific surface area instead of an increase.

[0014] The second aspect of the present invention provides a flow battery electrode with a hierarchical pore structure constructed by in-situ etching of copper-MOF obtained by the above preparation method.

[0015] The third aspect of this invention provides the application of the flow battery electrode prepared by the above-described method in an all-vanadium redox flow battery.

[0016] Compared with the prior art, the present invention has the following beneficial effects: (1) This invention achieves synergistic directional formation of channels and introduction of oxygen-containing functional groups on the surface by in-situ anchoring of copper-MOF on the surface of carbon felt or graphite felt and performing high-temperature etching at a controlled temperature. This significantly improves the specific surface area, effective active site density, electrode surface wettability and electrolyte contact of the resulting electrode, reduces charge transport impedance, and optimizes the construction of shorter mass transfer channels. Compared with existing high-energy-consuming or special atmosphere-requiring etching / modification processes, the preparation process of this invention is simple, safe, low-energy-consuming, and easy to scale up in engineering.

[0017] (2) This invention enables precise control of pore size distribution and surface functional group content through controllable process parameters, thereby achieving synergistic optimization of structure-chemical-electrochemical performance, and thus improving the energy efficiency, power density and cycle life of flow batteries under high current density conditions (e.g., at 300-400 mA·cm). -2 (Achieve significant energy efficiency improvements under the given conditions).

[0018] (3) This invention can reduce electrode polarization, enabling the all-vanadium redox flow battery to operate at 300 mA cm⁻¹. 2 The energy efficiency can reach 80.02% at current density. Attached Figure Description

[0019] To more clearly illustrate the technical solutions in the embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0020] Figure 1 SEM images of a graphite felt electrode after conventional heat treatment (a), a graphite felt electrode modified with a copper MOF precursor (b), a copper MOF modified electrode after high-temperature carbonization (c), and a copper MOF electrode modified with a hierarchical pore structure etched and constructed in Example 1 (d). Figure 2 The results of charge-discharge performance tests of graphite felt electrode after conventional heat treatment (Comparative Example 1), graphite felt electrode modified with copper MOF precursor (Comparative Example 2), copper MOF modified electrode after high temperature carbonization (Comparative Example 3), and copper MOF etched and constructed hierarchical pore structure modified electrode prepared in Example 1 as negative electrodes applied to vanadium redox flow batteries are presented. Figure 3 These are the results of electrochemical impedance spectroscopy; Figure 4 The results show the specific surface area and pore size analysis of different electrodes; Figure 5 The results represent the content of oxygen-containing functional groups in different electrodes; Figure 6 The specific surface area and pore size of the electrodes under different etching times are analyzed. Figure 7 The results show the content of oxygen-containing functional groups in the electrode at different etching times. Detailed Implementation

[0021] In the following description, specific details such as particular system architectures and techniques are set forth for illustrative purposes and not for limitation, in order to provide a thorough understanding of the embodiments of the invention. However, those skilled in the art will understand that the invention may be implemented in other embodiments without these specific details.

[0022] In the modification of porous electrodes for flow batteries, catalytic / template modification not only enhances surface active sites by introducing a catalytic phase but also improves mass transfer and wettability by constructing a rational pore network, thereby systematically improving the electrochemical performance of the electrode. Therefore, this invention uses copper-MOF as an in-situ etching template / sacrificial agent: Cu-MOF is uniformly grown on the surface of carbon felt or graphite felt under hydrothermal conditions, followed by localized oxidation-carbonization and MOF decomposition / reduction-volatilization processes under controlled high temperature (air atmosphere). This process has two synergistic effects: (a) Physical etching and pore construction—Local ablation / carbonization of Cu-MOF generates pores of different scales (micro / meso / macro) in the electrode substrate, forming an interconnected hierarchical pore structure, which significantly increases the specific surface area and constructs shorter mass transfer channels. (b) Chemical modification and hydrophilization—High-temperature oxidation and MOF decomposition are accompanied by the introduction of oxygen-containing functional groups (such as C–O, C=O, COOH), which improve the wettability of the electrode surface and the contact with the electrolyte, thereby increasing the available reaction interface area and promoting the diffusion of ions / species in the pores.

[0023] These two types of effects work together to reduce charge transfer impedance, decrease electrode surface polarization, and homogenize local current distribution, enabling the electrode to maintain a smaller overpotential and higher coulomb / energy efficiency under high current density conditions.

[0024] Example 1 A method for fabricating a carbon felt electrode for a flow battery using in-situ etching of a copper-MOF to construct a hierarchical pore structure includes the following steps: (1) Copper MOF was loaded onto the electrode surface via a hydrothermal reaction to obtain a carbon felt electrode modified with copper MOF precursor: Copper nitrate trihydrate (1.00 mmol) and terephthalic acid (1.00 mmol) were added to 50 ml of a mixed solvent consisting of N,N-dimethylformamide, deionized water and anhydrous ethanol (wherein the volume ratio of N,N-dimethylformamide, deionized water and anhydrous ethanol was 8:1:1), and stirred until clear and transparent to obtain the precursor solution. The precursor solution was transferred to a hydrothermal reactor and a carbon felt with a thickness of 1.5 mm was placed in it. The hydrothermal reaction was carried out at 120 °C for 12 h to load copper MOF onto the electrode surface and obtain a carbon felt electrode modified with copper MOF precursor. (2) The carbon felt electrode modified with the obtained copper MOF precursor was transferred to a muffle furnace and heated to 500°C at a heating rate of 5°C / min for etching. The etching time was 5h. After etching, a copper MOF etched and constructed hierarchical hole structure modified electrode was obtained.

[0025] The copper MOF etched and constructed hierarchical pore structure modified electrode obtained in this embodiment was used as the negative electrode in a vanadium redox flow battery for charge-discharge performance testing.

[0026] Relevant tests revealed that the electrode etched with copper MOF exhibits two key characteristics. First, the localized ablation / carbonization of Cu-MOF creates micro / meso / macro-sized pores in the electrode substrate, forming an interconnected hierarchical pore structure. This structure significantly increases the specific surface area and constructs shorter mass transfer channels. Second, through chemical modification and hydrophilication, high-temperature oxidation and MOF decomposition introduce oxygen-containing functional groups (such as C–O, C=O, COOH), improving the wettability of the electrode surface and its contact with the electrolyte. This increases the available reaction interface area and promotes the diffusion of ions / species within the pores. The positive electrode used in the experiment was heat-treated graphite felt (heat-treated at 500℃ for 5h), the separator was Nafion 212, and the negative electrode electrolyte was 1 MV. 3+ + 3M H₂SO₄, the positive electrode electrolyte is 1M VO₂ + + 3M H2SO4. Test results show: at 300mA cm 2 At the current density, the energy efficiency of the all-vanadium redox flow battery composed of this modified electrode is 80.02%, and the coulombic efficiency is 96.55%. Example 2 A method for fabricating a carbon felt electrode for a flow battery using in-situ etching of a copper-MOF to construct a hierarchical pore structure includes the following steps: (1) Copper MOF was loaded onto the electrode surface via a hydrothermal reaction to obtain a graphite felt electrode modified with a copper MOF precursor: Copper nitrate trihydrate (1.00 mmol) and terephthalic acid (0.80 mmol) were added to 50 ml of a mixed solvent consisting of N,N-dimethylformamide, deionized water and anhydrous ethanol (wherein the volume ratio of N,N-dimethylformamide, deionized water and anhydrous ethanol was 8:1:1), and stirred until clear and transparent to obtain the precursor solution. The precursor solution was transferred to a hydrothermal reactor and a carbon felt with a thickness of 1.5 mm was placed in it. The hydrothermal reaction was carried out at 120 °C for 12 h to load copper MOF onto the electrode surface and obtain a graphite felt electrode modified with copper MOF precursor. (2) The graphite felt electrode modified with the obtained precursor was transferred to a muffle furnace and heated to 500°C at a heating rate of 5°C / min for etching. The etching time was 5 h. After etching, a copper MOF etched and constructed hierarchical pore structure modified electrode was obtained. It was then used as the negative electrode in a vanadium redox flow battery. The copper MOF electrode with a hierarchical porous structure, fabricated in this embodiment, was used as the negative electrode in a vanadium redox flow battery, and its charge-discharge performance was tested. The tests revealed that, compared to Example 1, while reducing the amount of the polycarboxylic acid organic ligand, specifically terephthalic acid, still allowed for MOF structure formation, the smaller amount of terephthalic acid resulted in incomplete formation of the porous framework, making it difficult to achieve a three-dimensional porous structure. During the etching stage, the defective MOF structure hindered the formation of Cu-MOF, resulting in a hierarchical porous structure with different micro / meso / macro scales. The positive electrode used in the experiment was heat-treated graphite felt (heat-treated at 500℃ for 5 hours), the separator was Nafion 212, and the negative electrode electrolyte was 1 MV. 3+ + 3M H₂SO₄, the positive electrode electrolyte is 1M VO₂ + + 3M H2SO4. Test results show: at 300mA cm 2 At the current density, the energy efficiency of the all-vanadium redox flow battery composed of this modified electrode is 79.37%, and the coulombic efficiency reaches 96.46%.

[0027] Example 3 A method for fabricating a carbon felt electrode for a flow battery using in-situ etching of a copper-MOF to construct a hierarchical pore structure includes the following steps: (1) Copper MOF was loaded onto the electrode surface via a hydrothermal reaction to obtain a carbon felt electrode modified with copper MOF precursor: Add copper nitrate trihydrate (1.00 mmol) and terephthalic acid (1.00 mmol) to 50 ml of a mixed solvent consisting of N,N-dimethylformamide and anhydrous ethanol (wherein the volume ratio of N,N-dimethylformamide and anhydrous ethanol is 4:1), and stir until clear and transparent to obtain a precursor solution. The precursor solution was transferred to a hydrothermal reactor and a carbon felt with a thickness of 1.5 mm was placed in it. The hydrothermal reaction was carried out at 120 °C for 12 h to load copper MOF onto the electrode surface and obtain a carbon felt electrode modified with copper MOF precursor. (2) The carbon felt electrode modified with the precursor was transferred to a muffle furnace and heated to 500°C at a heating rate of 5°C / min for etching. The etching time was 5 h. After etching, a copper MOF-etched hierarchical pore structure modified electrode was obtained. It was then used as the negative electrode in a vanadium redox flow battery. The copper MOF electrode with a hierarchical pore structure fabricated in this embodiment was used as the negative electrode in a vanadium redox flow battery, and its charge-discharge performance was tested. The tests revealed that, compared to Example 1, the absence of solvent water during the hydrothermal process in this embodiment resulted in uneven dispersion of copper ions. This prevented some copper ions from participating in the formation of the MOF structure, leading to defects in the morphology and structure of the MOF, and limiting the loading of the copper MOF on the graphite felt surface. During subsequent etching, the localized ablation / carbonization process of the Cu-MOF was hindered, preventing the formation of a suitable hierarchical pore structure. The positive electrode used in the experiment was heat-treated graphite felt (heat treatment conditions: 500℃, 5h), the separator was Nafion 212, and the negative electrode electrolyte was 1 MV. 3+ +3M H2SO4, positive electrode electrolyte is 1M VO2 + + 3M H2SO4. Test results show: at 300mA cm 2 At the current density, the energy efficiency of the all-vanadium redox flow battery composed of this modified electrode is 77.81%, and the coulombic efficiency reaches 96.75%.

[0028] Example 4 A method for fabricating a carbon felt electrode for a flow battery using in-situ etching of a copper-MOF to construct a hierarchical pore structure includes the following steps: (1) Copper MOF was loaded onto the electrode surface via a hydrothermal reaction to obtain a carbon felt electrode modified with copper MOF precursor: Copper nitrate trihydrate (1.00 mmol) and terephthalic acid (1.00 mmol) were added to 50 ml of a mixed solvent consisting of N,N-dimethylformamide, deionized water and anhydrous ethanol (wherein the volume ratio of N,N-dimethylformamide, deionized water and anhydrous ethanol was 8:1:1), and stirred until clear and transparent to obtain the precursor solution. The precursor solution was transferred to a hydrothermal reactor and a carbon felt with a thickness of 1.5 mm was placed in it. The hydrothermal reaction was carried out at 140 °C for 12 h to load copper MOF onto the electrode surface and obtain a carbon felt electrode modified with copper MOF precursor. (2) The carbon felt electrode modified with the precursor was transferred to a muffle furnace and heated to 500°C at a heating rate of 5°C / min for etching. The etching time was 5 h. After etching, a copper MOF-etched hierarchical pore structure modified electrode was obtained. It was then used as the negative electrode in a vanadium redox flow battery.

[0029] The copper MOF electrode with a hierarchical pore structure fabricated in this embodiment was used as the negative electrode in a vanadium redox flow battery, and its charge-discharge performance was tested. The tests revealed that, compared to Example 1, at a higher hydrothermal temperature (140°C), the three-dimensional framework structure of the copper MOF in this embodiment experienced localized collapse, leading to framework dimensional degradation and ligand loss, thus preventing the formation of a suitable MOF morphology. During the etching stage, the collapse of the Cu-MOF structure and the loss of ligands hindered the formation of the pore structure on the graphite felt surface and the introduction of oxygen-containing functional groups. The positive electrode used in the experiment was heat-treated graphite felt (heat-treated at 500°C for 5 hours), the separator was Nafion 212, and the negative electrode electrolyte was 1 MV. 3+ + 3M H2SO4, positive electrode electrolyte is 1M VO2 + + 3M H2SO4. Test results show: at 300mA cm 2 At the current density, the energy efficiency of the all-vanadium redox flow battery composed of this modified electrode is 77.56%, and the coulombic efficiency reaches 95.54%.

[0030] Example 5 A method for fabricating a carbon felt electrode for a flow battery using in-situ etching of a copper-MOF to construct a hierarchical pore structure includes the following steps: (1) Copper MOF was loaded onto the surface of a graphite felt electrode via a hydrothermal reaction to obtain a graphite felt electrode modified with a copper MOF precursor: Add copper nitrate trihydrate (1.00 mmol) and terephthalic acid (1.00 mmol) to 50 ml of N,N-dimethylformamide and stir until clear and transparent to obtain the precursor solution; The precursor solution was transferred to a hydrothermal reactor and a graphite felt with a thickness of 1.5 mm was placed in it. The hydrothermal reaction was carried out at 120 °C for 16 h to load copper MOF onto the electrode surface and obtain a graphite felt electrode modified with copper MOF precursor. (2) The graphite felt electrode modified with the obtained precursor was transferred to a muffle furnace and heated to 500°C at a heating rate of 5°C / min for etching. The etching time was 5 h. After etching, a copper MOF etched and constructed hierarchical pore structure modified electrode was obtained. It was then used as the negative electrode in a vanadium redox flow battery.

[0031] The copper MOF etched and constructed hierarchical porous structure modified electrode obtained in this embodiment was used as the negative electrode in a vanadium redox flow battery, and its charge-discharge performance was tested. The tests revealed that, compared to Example 1, the copper MOF grains formed in this embodiment, under a longer hydrothermal time (16 h), were too large and non-uniform. This made loading on the graphite felt difficult and prone to distortion, hindering the formation of a suitable three-dimensional framework structure for the copper MOF. Local collapse of the framework structure led to dimensional degradation and ligand loss, resulting in an inability to form a suitable MOF morphology. During the etching stage, the excessively large Cu-MOF structure prevented etching from proceeding on the graphite felt surface, hindering the formation of a hierarchical porous structure and the introduction of oxygen-containing tube groups. The positive electrode used in the experiment was heat-treated graphite felt (heat-treated at 500℃ for 5 h), the separator was Nafion 212, and the negative electrode electrolyte was 1 MV. 3+ + 3M H2SO4, positive electrode electrolyte is 1M VO2 + + 3M H2SO4. Test results show: at 300mA cm 2 At the current density, the energy efficiency of the all-vanadium redox flow battery composed of this modified electrode is 78.64%, and the coulombic efficiency reaches 97.94%. Comparative Example 1 A vanadium redox flow battery assembled with conventionally heat-treated electrodes, wherein both the negative and positive electrodes are conventionally heat-treated graphite felt electrodes, and the heat treatment conditions are: 500℃ for 5h. The separator is Nafion 212, and the negative electrode electrolyte is 20mL 1M V. 3+ + 3M H2SO4, positive electrode electrolyte is 1M VO2 + + 3M H2SO4. Experimental results show that at 300mA cm 2 At the current density, the energy efficiency of the vanadium redox flow battery assembled in this comparative example is 67.07%, and the coulombic efficiency is 96.32%.

[0032] Comparative Example 2 The steps are basically the same as in Example 1, except that high-temperature etching in a muffle furnace is not performed.

[0033] A vanadium redox flow battery was assembled using a graphite felt electrode modified with the copper MOF precursor obtained through hydrothermal reaction in Example 1 as the negative electrode and a graphite felt electrode treated with conventional heat treatment as the positive electrode. The conventional heat treatment conditions for the graphite felt were 500°C for 5 hours, the separator was Nafion 212, and the negative electrode electrolyte was 20 mL of 1 MV. 3+ +3M H2SO4, positive electrode electrolyte is 20mL 1M VO 2++ 3M H2SO4. Experimental results show that at 300mA cm 2 At the current density, the energy efficiency of the vanadium redox flow battery assembled in this comparative example is 72.83%, and the coulombic efficiency is 95.94%.

[0034] Comparative Example 3 The process is essentially the same as in Example 1, except that a high-temperature carbonization process in a tube furnace is used instead of a high-temperature etching process in a muffle furnace. Specifically, the graphite felt electrode modified with the obtained copper MOF precursor is placed in a tube furnace and heated to a carbonization temperature of 500°C at a heating rate of 5°C / min under a N2 atmosphere for 5 hours to obtain a high-temperature heat-treated copper MOF modified electrode.

[0035] A vanadium redox flow battery was assembled using a high-temperature heat-treated copper MOF-modified electrode prepared in this comparative example as the negative electrode and a conventionally heat-treated graphite felt as the positive electrode. The conventionally heat-treated graphite felt was treated at 500℃ for 5 hours. A Nafion 212 separator was used, and the negative electrode electrolyte was 20 mL of 1 MV electrolyte. 3+ +3M H2SO4, positive electrode electrolyte is 20mL 1M VO 2+ + 3MH2SO4. Experimental results show that at 300mA cm 2 At the current density, the energy efficiency of the vanadium redox flow battery assembled in this comparative example is 74.59%, and the coulombic efficiency is 97.38%.

[0036] Comparative Example 4 The process is basically the same as in Example 1, except that the etching time of the electrode in the muffle furnace is reduced. Specifically, the graphite felt electrode modified with the obtained copper MOF precursor is placed in a muffle furnace and heated to a carbonization temperature of 500°C at a heating rate of 5°C / min, with an etching time of 3 hours, to obtain an electrode treated with copper MOF etching.

[0037] A vanadium redox flow battery was assembled using a high-temperature heat-treated copper MOF-modified electrode prepared in this comparative example as the negative electrode and a conventionally heat-treated graphite felt as the positive electrode. The conventionally heat-treated graphite felt was treated at 500℃ for 5 hours. A Nafion 212 separator was used, and the negative electrode electrolyte was 20 mL of 1 MV electrolyte. 3+ +3M H2SO4, positive electrode electrolyte is 20mL 1M VO 2+ + 3MH2SO4. Experimental results show that at 300mA cm 2 At the current density, the energy efficiency of the vanadium redox flow battery assembled in this comparative example is 75.37%, and the coulombic efficiency is 96.89%.

[0038] Comparative Example 5 The process is basically the same as in Example 1, except that the etching time of the electrode in the muffle furnace is increased. Specifically, the graphite felt electrode modified with the obtained copper MOF precursor is placed in a muffle furnace and heated to a carbonization temperature of 500°C at a heating rate of 5°C / min, with an etching time of 7 hours, to obtain an electrode treated with copper MOF etching.

[0039] A vanadium redox flow battery was assembled using a high-temperature heat-treated copper MOF-modified electrode prepared in this comparative example as the negative electrode and a conventionally heat-treated graphite felt as the positive electrode. The conventionally heat-treated graphite felt was treated at 500℃ for 5 hours. A Nafion 212 separator was used, and the negative electrode electrolyte was 20 mL of 1 MV electrolyte. 3+ +3M H2SO4, positive electrode electrolyte is 20mL 1M VO 2+ + 3MH2SO4. Experimental results show that at 300mA cm 2 At the current density, the energy efficiency of the vanadium redox flow battery assembled in this comparative example is 75.49%, and the coulombic efficiency is 97.53%. I. Morphological Characteristics The graphite felt electrode after conventional heat treatment (Comparative Example 1), the graphite felt electrode modified with copper MOF precursor (Comparative Example 2), the copper MOF modified electrode after high-temperature carbonization (Comparative Example 3), and the copper MOF etching-constructed hierarchical pore structure modified electrode prepared in Example 1 were characterized by SEM. The results are shown in [Figure 1]. Figure 1 .

[0040] Depend on Figure 1 (a) It can be seen that the graphite felt electrode after traditional heat treatment has obvious heat treatment marks on its surface, but no loaded material. Depend on Figure 1 (b) It can be seen that in the graphite felt electrode modified with copper MOF precursor obtained after hydrothermal reaction, a uniform rod-shaped copper MOF structure can be clearly seen on the surface of the graphite felt electrode.

[0041] Depend on Figure 1 (c) It can be seen that the graphite felt electrode modified with the copper MOF precursor obtained after hydrothermal reaction was further carbonized at high temperature in a nitrogen atmosphere (Comparative Example 3). After high-temperature treatment, the original morphology of the copper MOF did not change significantly.

[0042] Depend on Figure 1As shown in Figure (d), the copper MOF etching structure obtained by hydrothermal reaction and high-temperature etching modifies the graphite felt electrode. The uniform distribution of the hierarchical pore structure on the graphite felt electrode surface provides abundant active sites and increases the specific surface area. At the same time, it can promote the redox reaction kinetics on the electrode surface and greatly improve the performance of the flow battery.

[0043] contrast Figure 1 (b) and Figure 1 (d) It is concluded that the present invention anchors the prepared copper MOF structure onto the electrode surface through hydrothermal reaction, and then uses copper MOF to etch the electrode surface in a muffle furnace. The electrode obtained after the above modification has the characteristics of large specific surface area, rich active sites and excellent performance, which enables the flow battery to operate at a higher current density and significantly improves its power density.

[0044] II. Performance Testing A conventionally heat-treated graphite felt electrode (Comparative Example 1), a graphite felt electrode modified with a copper MOF precursor (Comparative Example 2), a copper MOF-modified electrode after high-temperature carbonization (Comparative Example 3), and a copper MOF-etched electrode with a hierarchical pore structure prepared in Example 1 were used as negative electrodes in a vanadium redox flow battery, and their charge-discharge performance was tested. The positive electrode used was heat-treated graphite felt at 500°C for 5 hours; the separator was Nafion 212; and the negative electrode electrolyte was 1 MV. 3+ +3M H2SO4, positive electrode electrolyte is 1M VO 2+ + 3M H2SO4. Results are shown below. Figure 2 . Depend on Figure 2 The results show that at 300mA cm -2 At the specified current density, the energy efficiency of the all-vanadium redox flow battery composed of electrodes modified with a hierarchical pore structure constructed by etching copper MOF according to the present invention as the negative electrode is 80.02%, and the coulombic efficiency reaches 96.55%. Compared with the conventional heat-treated graphite felt electrode (67.07%), the energy efficiency is improved by 12.95%; at 400 mA·cm⁻¹, the energy efficiency is significantly higher. 2 At current density, the energy efficiency of the all-vanadium redox flow battery assembled with the copper MOF etched and constructed hierarchical hole structure modified electrode provided by the present invention reaches 75.28%, which is 13.58% higher than that of the traditional heat-treated graphite felt electrode.

[0045] By testing the electrochemical impedance spectroscopy of the full cell ( Figure 3Comparing the three types of electrodes, it was found that the copper MOF etching-constructed hierarchical porous structure modified electrode provided by this invention has lower charge transfer resistance and mass transfer impedance. Therefore, the hierarchical porous structure formed after copper MOF etching increases the active sites of the electrode, optimizes the coordination environment, accelerates the vanadium ion Redox reaction, and improves the ion diffusion rate, proving that the polarization of the battery is effectively reduced after modification by this scheme.

[0046] By comparing the electrodes prepared using conventional heat treatment (Comparative Example 1), the graphite felt electrode modified with copper MOF precursor (Comparative Example 2), the copper MOF modified electrode after high-temperature carbonization (Comparative Example 3), and the copper MOF electrode with a hierarchical pore structure modified by etching prepared in Example 1, all vanadium redox flow batteries assembled at 300 mA cm⁻¹ were tested. 2 Based on the coulombic efficiency and energy efficiency at current densities, the following conclusions were drawn: Compared with electrodes treated with conventional heat treatment, the all-vanadium redox flow battery composed of an electrode modified with a hierarchical pore structure constructed by copper MOF etching in this invention can operate at higher current densities and has higher energy efficiency. However, the electrode performance of the copper MOF-modified electrode after further high-temperature carbonization (Comparative Example 3) decreased instead of increasing, possibly because the copper MOF structure collapses and active sites are lost at high temperatures. Conversely, high-temperature etching of the graphite felt electrode modified with the copper MOF precursor in an air atmosphere can introduce oxygen-containing functional groups ( ) onto the electrode surface while forming a uniform hierarchical pore structure. Figure 4 Table 1 and Figure 5 This improves the wettability of the electrode surface and its contact with the electrolyte, thereby increasing the available reaction interface area and promoting the diffusion of ions / species within the pores. Table 1. Specific surface area results for different electrodes

[0047] To investigate the regulatory effect of etching time on the structure and surface chemical properties of copper MOF-modified electrodes, BET surface area analysis was conducted. Figure 6 ), pore structure parameter statistics (Table 2) and X-ray photoelectron spectroscopy (XPS), Figure 7 Characterization was performed, and the specific surface area and surface oxygen-containing functional group (CO, C=O, COOH, etc.) content of the electrodes at different etching times were compared. The results are shown in [Figure number missing]. Figure 6 Table 2 and Figure 7 .

[0048] Table 2

[0049] Figure 6 Table 2 and Figure 7The results show that when the etching time is 5 h, the electrode exhibits optimal structural and surface chemical properties—not only does it have the largest specific surface area, but its surface oxygen-containing functional group content also reaches its peak. This phenomenon is attributed to the moderate degree of local ablation / carbonization of the copper MOF at this etching time, which not only fully constructs hierarchical channels of micro / meso / macro interconnections, but also effectively introduces a large number of oxygen-containing active groups. Further analysis shows that too short an etching time leads to incomplete channel structure formation: the dense structure of the copper MOF modification layer is not fully etched, and only a few non-penetrating micropores are formed on the surface, making it impossible to construct efficient mass transfer channels; at the same time, the oxidation effect on the electrode surface is limited by short etching time, resulting in a small number of introduced oxygen-containing functional groups, which makes it difficult to improve the interfacial wettability between the electrode and the electrolyte. Conversely, excessive etching time can lead to irreversible damage to the electrode surface structure: excessive etching can disrupt the coordination framework balance of copper MOF, causing some MOF structures to collapse, loss of channel permeability, and even corrosion of carbon frameworks such as graphite felt; in addition, excessive etching time can also lead to excessive accumulation of surface defects, decomposition or shedding of some oxygen-containing functional groups, ultimately resulting in significant deterioration of electrode structural integrity and surface chemical stability.

[0050] This invention is not limited to the specific embodiments described above. Any modifications made by those skilled in the art based on the above concept without creative effort are within the scope of protection of this invention.

Claims

1. A method for fabricating a flow battery electrode with a hierarchical pore structure using in-situ etching of a copper-MOF, characterized in that, Includes the following steps: S1. Add copper nitrate trihydrate and terephthalic acid to the solvent and stir to mix to obtain the precursor solution; S2. The precursor solution is transferred to a hydrothermal reactor and an electrode is placed in it to carry out a hydrothermal reaction to obtain a copper MOF modified electrode. S3. The obtained copper MOF modified electrode is transferred to a muffle furnace, heated, and subjected to high-temperature thermal etching to obtain an electrode with a hierarchical hole structure constructed by copper MOF etching.

2. The method for fabricating a flow battery electrode with a hierarchical pore structure using in-situ etching of copper-MOF according to claim 1, characterized in that, In step S1, the solvent is one or a combination of two or more of deionized water, N,N-dimethylformamide, and ethanol.

3. The method for fabricating a flow battery electrode with a hierarchical pore structure using in-situ etching of copper-MOF according to claim 2, characterized in that, When the solvent is a combination of deionized water, N,N-dimethylformamide, and anhydrous ethanol, the volume ratio of the three is 8:1:

1.

4. The method for fabricating a flow battery electrode with a hierarchical pore structure using in-situ etching of copper-MOF according to claim 1, characterized in that, In step S1, the molar ratio of copper nitrate trihydrate to terephthalic acid is 0.95–1.25 mmol: 0.50–1.50 mmol.

5. The method for fabricating a flow battery electrode with a hierarchical pore structure using in-situ etching of a copper-MOF according to claim 1, characterized in that, In step S2, the electrode is a graphite felt or carbon felt electrode with a thickness of 1.5 to 4.6 mm.

6. The method for fabricating a flow battery electrode with a hierarchical pore structure using in-situ etching of copper-MOF according to claim 1, characterized in that, In step S2, the temperature of the hydrothermal reaction is 100-150℃, and the reaction time is 12-24h.

7. The method for fabricating a flow battery electrode with a hierarchical pore structure using in-situ etching of copper-MOF according to claim 1, characterized in that, In step S3, the high-temperature thermal etching is performed in an air atmosphere, with a thermal etching temperature of 300–800°C, a heating rate of 2–6°C / min, and an etching time of 2–5 h.

8. A flow battery electrode with a hierarchical pore structure constructed by in-situ etching of copper-MOF as described in any one of claims 1-7.

9. The application of the copper-MOF in-situ etched flow battery electrode with a hierarchical pore structure as described in claim 8 in an all-vanadium redox flow battery.

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