Hierarchical pore metal organic framework electrode based on gradient pulse electroplating and preparation method and application thereof

Abstract

The invention relates to a hierarchical pore metal organic framework electrode based on gradient pulse electroplating and a preparation method and application thereof, and the preparation method comprises the following steps: (1) placing a substrate in an electroplating solution, and carrying out pulse electrochemical deposition on the substrate to obtain a metal organic framework precursor; and (2) carrying out heat treatment on the metal organic framework precursor obtained in the step (1) to obtain the hierarchical pore metal organic framework electrode, the pulse electrochemical deposition in the step (1) sequentially comprises first pulse electrochemical deposition, second pulse electrochemical deposition and third pulse electrochemical deposition; the on-off time ratio Ton / Toff of the first pulse electrochemical deposition is 1: (10-30); the on-off time ratio Ton / Toff of the second pulse electrochemical deposition is 1: (2-3); the on-off time ratio Ton / Toff of the third pulse electrochemical deposition is (1-3): 1; the electroplating liquid in the step (1) comprises metal salt, an organic ligand, a dispersing agent and a solvent.

Description

Technical Field

[0001] This invention relates to the fields of electrochemical functional material preparation and energy catalysis technology, and in particular to a hierarchical porous metal-organic framework electrode based on gradient pulse electroplating, its preparation method and application. Background Technology

[0002] Metal-organic frameworks (MOFs) are porous crystalline materials with a periodic network structure formed by the self-assembly of metal ions or metal clusters with organic ligands through coordination bonds. Their unique porous structure, high specific surface area, and tunable chemical composition and topology give them enormous application potential in catalysis, energy storage, gas separation, and sensing. In the field of energy catalysis, MOF materials, with their abundant active sites, tunable electronic structure, and good dispersibility, have become one of the ideal catalytic materials for reactions such as hydrogen / oxygen production by water electrolysis, oxygen reduction in fuel cells, and carbon dioxide electroreduction.

[0003] However, traditional MOF material preparation methods (such as solvothermal, hydrothermal, and potentiostatic electrodeposition methods) still face many technical bottlenecks in the fabrication of MOF electrodes, limiting their industrial application and performance improvement.

[0004] Simple pore structure and low mass transfer efficiency: MOF materials prepared by traditional methods are mostly microporous or mesoporous structures with a single pore size. During the electrocatalytic reaction, small molecule reactants (such as H2O, O2, CO2) are difficult to diffuse rapidly to the internal active sites, and the products cannot be desorbed in time, resulting in increased mass transfer resistance and limited catalytic activity and reaction kinetics.

[0005] Poor morphology controllability and insufficient exposure of active sites: In constant potential or constant current electrodeposition, nucleation and growth processes occur simultaneously. Ion supply and crystal diffusion are difficult to precisely regulate, which easily leads to the formation of MOF films with disordered morphology and uneven size. This results in some active sites being encapsulated and unable to fully participate in the catalytic reaction, thus reducing the utilization rate of the material.

[0006] Weak film adhesion and poor stability: MOF films prepared by traditional methods have weak bonding between themselves and the conductive substrate. During long-term electrocatalytic cycling, the film is prone to detachment and structural collapse, resulting in rapid decay of catalytic performance and making it difficult to meet the stability requirements of practical applications.

[0007] The preparation process has low controllability and poor repeatability: Existing preparation processes are extremely sensitive to minute changes in reaction conditions (such as temperature, concentration, and potential), making it difficult to achieve precise control of MOF structure and performance. The consistency of products from different batches is poor, which is not conducive to industrial-scale production.

[0008] Pulse electroplating technology, as an electrochemical method that periodically applies electrical energy, controls the pulse voltage amplitude and on / off time (T).on / T off The timing and period of pulse electroplating, used to control the nucleation rate and diffusion supply, has been applied to improve the morphology and uniformity of MOF materials. For example, existing techniques using pulse electroplating with fixed parameters can improve the uniformity of the film to some extent, but still have the following shortcomings: fixed pulse parameters cannot achieve gradient control of the MOF structure; existing pulse electroplating methods mostly use fixed voltage amplitude and time intervals. on / T off Compared to other methods, only single-structure MOF materials can be prepared, and gradient materials with multi-level porous structures cannot be constructed. The problems of mass transfer efficiency and active site exposure have not been fundamentally solved.

[0009] Therefore, how to develop a metal-organic framework electrode that can achieve multi-level pore structure control of MOF electrodes and has both high catalytic activity and stability, as well as its preparation method, has become a key technical problem that urgently needs to be solved in the field of MOF-based energy catalytic materials. Summary of the Invention

[0010] To address the aforementioned technical problems, this invention aims to provide a hierarchical porous metal-organic framework (MOF) electrode based on gradient pulse electroplating, its preparation method, and its applications. The MOF electrode of this invention is prepared by gradient pulse electroplating combined with a specific electroplating solution system and heat treatment, resulting in a hierarchical porous MOF electrode with macropores on the surface, mesopores in the middle layer, and micropores in the inner layer. Its core advantages are: high adsorption, high selectivity, and strong stability; high CO2 electroreduction of CO Faradaic efficiency; and excellent catalytic performance. It can be widely used in energy conversion fields such as water electrolysis for hydrogen production and oxygen reduction in fuel cells, solving many technical bottlenecks of traditional MOF electrodes.

[0011] To achieve this objective, the present invention adopts the following technical solution:

[0012] In a first aspect, the present invention provides a method for fabricating a hierarchical porous metal-organic framework electrode based on gradient pulse electroplating, the method comprising the following steps:

[0013] (1) The substrate is placed in an electroplating solution and pulsed electrochemical deposition is performed on the substrate to obtain a metal-organic framework precursor;

[0014] (2) The metal-organic framework precursor obtained in step (1) is heat-treated to obtain the hierarchical porous metal-organic framework electrode.

[0015] Step (1) of the pulsed electrochemical deposition includes, in sequence, a first pulsed electrochemical deposition, a second pulsed electrochemical deposition, and a third pulsed electrochemical deposition;

[0016] The on / off time ratio of the first pulse electrochemical deposition to T on / Toff The ratio is 1:10-30, for example, it can be 1:10, 1:12, 1:15, 1:18, 1:20, 1:22, 1:25, 1:28 or 1:30, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0017] The second pulse electrochemical deposition on / off time ratio T on / T off The ratio is 1:2-3, for example, it can be 1:2, 1:2.2, 1:2.5, 1:2.8 or 1:3, but it is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0018] The third pulse electrochemical deposition on / off time ratio T on / T off The ratio is 1-3:1, for example, it can be 1:1, 1.2:1, 1.5:1, 1.8:1, 2:1, 2.2:1, 2.5:1, 2.8:1 or 3:1, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0019] The electroplating solution in step (1) includes metal salts, organic ligands, dispersants, and solvents.

[0020] This invention utilizes a three-stage gradient pulse electrochemical deposition process and an electroplating solution system to construct a MOF electrode with a hierarchical porous structure, achieving a synergistic improvement in adsorption performance, selectivity, stability, and electrocatalytic performance. Furthermore, the gradient pulse deposition process enhances the adhesion between the MOF film and the substrate, while the heat treatment process further improves the orderliness and stability of the MOF crystals, thereby ensuring the electrode's performance stability during long-term cyclic use.

[0021] In the first pulse electrochemical deposition stage, rapid nucleation of metal ions and organic ligands on the substrate surface is achieved. A larger on-off time ratio reduces concentration polarization during the deposition process, ensuring uniform nucleation, promoting dense crystal growth, forming a microporous inner layer, and simultaneously ensuring a strong bond between the film and the substrate.

[0022] In the second pulse electrochemical deposition stage, the directional growth of MOF crystals is achieved. The control of the voltage and on / off time ratio in this stage can balance the nucleation and growth rates, forming a mesoporous middle layer that serves as a transition region between the surface and inner layers, thus optimizing the mass transfer channels.

[0023] In the third pulse electrochemical deposition stage, the surface layer of the MOF film is constructed. A smaller on / off time ratio increases the effective deposition time, resulting in a macroporous surface layer.

[0024] The following are preferred technical solutions of the present invention, but are not intended to limit the technical solutions provided by the present invention. The technical objectives and beneficial effects of the present invention can be better achieved and realized through the following preferred technical solutions.

[0025] Preferably, the pulse voltage of the first pulse electrochemical deposition in step (1) is -3.0V to -2.5V, for example, it can be -3.0V, -2.9V, -2.8V, -2.7V, -2.6V or -2.5V, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0026] Preferably, the time for the first pulse electrochemical deposition in step (1) is 5 min to 15 min, for example, it can be 5 min, 8 min, 10 min, 12 min, 13 min, 14 min or 15 min, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0027] In the first pulse electrochemical deposition, this invention employs a relatively high negative voltage (-3.0V to -2.5V) and a large on / off time ratio (1:10 to 1:30) to achieve rapid nucleation of metal ions and organic ligands on the surface of a nickel foam substrate within a short time (5-15 min). The high negative voltage facilitates rapid reduction and coordination of metal ions, while the large on / off time ratio reduces concentration polarization during deposition, ensuring uniform nucleation, promoting dense crystal growth, forming a microporous inner layer, and simultaneously ensuring a strong bond between the film and the substrate. However, if the negative voltage is too high (e.g., below -3.0V), the metal ion reduction is too rapid, resulting in overly dense nucleation and the formation of an amorphous or disordered structure, reducing the microporous order and bonding strength of the inner layer. If the negative voltage is too low (e.g., above -2.5V), the nucleation driving force is insufficient, the nucleation rate is slow, resulting in an excessively thin inner layer, a loose structure, or even the inability to form a continuous and dense microporous layer, affecting the overall structural integrity and catalytic stability of the electrode.

[0028] Preferably, the pulse voltage of the second pulse electrochemical deposition in step (1) is -1.2V to -1.0V, for example, it can be -1.2V, -1.1V or -1.0V, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0029] Preferably, the time for the second pulse electrochemical deposition in step (1) is 10 min to 30 min, for example, it can be 10 min, 15 min, 20 min, 25 min or 30 min, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0030] In the second pulse electrochemical deposition process of this invention, a moderate negative voltage (-1.2V to -1.0V) and a moderate on / off time ratio (1:2 to 1:3) are used to promote the directional growth of MOF crystals within a suitable time (10-30 min). The control of the voltage and on / off time ratio during this stage balances the nucleation and growth rates, forming a mesoporous middle layer that serves as a transition region between the surface and inner layers, optimizing mass transfer channels. If the negative voltage is too high (e.g., below -1.2V), the growth driving force is too strong, easily leading to excessively rapid crystal growth, uneven size, disordered mesoporous structure in the middle layer, and even pore closure. If the negative voltage is too low (e.g., above -1.0V), crystal growth is slow, the middle layer thickness is insufficient, and the structure is not dense, resulting in poor mass transfer channels and affecting the overall electrode reaction kinetics.

[0031] Preferably, the pulse voltage of the third pulse electrochemical deposition in step (1) is -1.8V to -1.5V, for example, it can be -1.8V, -1.75V, -1.7V, -1.65V, -1.6V, -1.55V or -1.5V, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0032] Preferably, the time for the third pulse electrochemical deposition in step (1) is 50 min to 3 h, for example, it can be 50 min, 1 h, 1.5 h, 2 h, 2.5 h or 3 h, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0033] This invention utilizes a moderately high negative voltage (-1.8V to -1.5V) and a relatively small on / off time ratio (1:1 to 3:1) in third-pulse electrochemical deposition to achieve the construction of the MOF film surface layer over a long period (50 min to 3 h). A smaller on / off time ratio increases the effective deposition time, resulting in a macroporous surface layer. However, if the negative voltage is too high (e.g., below -1.8V), the surface layer deposition is too rapid, easily forming a surface layer with excessively large pores, a loose structure, or even cracking, reducing mechanical stability and active site density. If the negative voltage is too low (e.g., above -1.5V), the surface layer growth is insufficient, the macroporous structure is not obvious, the pore size is too small, and the rapid diffusion of reactants and timely desorption of products cannot be effectively achieved, limiting the overall catalytic performance of the electrode.

[0034] Preferably, the temperature of the pulsed electrochemical deposition in step (1) is 40℃-80℃, for example, it can be 40℃, 45℃, 50℃, 55℃, 60℃, 65℃, 70℃, 75℃ or 80℃, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0035] Preferably, the circulation rate of the electroplating solution in step (1) is 1L / min-2.5L / min, for example, it can be 1L / min, 1.2L / min, 1.5L / min, 1.8L / min, 2L / min, 2.2L / min or 2.5L / min, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0036] Preferably, in the pulsed electrochemical deposition process, the substrate is used as the working electrode, the platinum mesh is used as the counter electrode, and the saturated calomel electrode is used as the reference electrode.

[0037] The pulsed electrochemical deposition in step (1) is controlled by a programmable electrochemical workstation.

[0038] Preferably, the substrate in step (1) is nickel foam, copper foam, cobalt foam, or metal foil.

[0039] Preferably, the metal salt includes nickel salt, cobalt salt, and iron salt.

[0040] This invention further selects nickel, cobalt, and iron salts as metal ion sources, achieving a gradient distribution of metal ions in the film layer through gradient pulse deposition. The synergistic effect of nickel, cobalt, and iron can regulate the electronic structure of MOF materials and enhance electrocatalytic activity; simultaneously, the differences in coordination ability between different metal ions and organic ligands contribute to the formation of hierarchical porous structures. In the gradient pulse electrochemical deposition system, the selection of metal ions needs to comprehensively consider factors such as their reduction potential, coordination ability, lattice matching, and catalytic activity. This invention uses a ternary system of nickel (Ni), cobalt (Co), and iron (Fe) based on the following synergistic mechanisms: Reduction potential matching: The reduction potentials of Ni²⁺ / Ni, Co²⁺ / Co, and Fe³⁺ / Fe can be deposited in a coordinated manner within the selected pulse voltage range, achieving gradient nucleation and growth. Electronic structure synergy: Ni, Co, and Fe can form an electronic coupling effect, optimizing the d-band center of the MOF and enhancing the adsorption and activation ability of small molecules such as CO2 and H2O. Structural stability: The Ni-Co-Fe system readily forms stable spinel or spinel-like structures during heat treatment, enhancing the electrode's durability in catalytic environments. Preferably, the nickel salt comprises any one or a combination of at least two of nickel nitrate, nickel sulfate, or nickel chloride. Typical but non-limiting combinations include those of nickel nitrate and nickel sulfate, nickel sulfate and nickel chloride, nickel nitrate and nickel chloride, and combinations of nickel nitrate, nickel sulfate, and nickel chloride, with nickel nitrate being the most preferred.

[0041] Preferably, the cobalt salt comprises any one or a combination of at least two of cobalt nitrate, cobalt sulfate, or cobalt chloride. Typical but non-limiting combinations include combinations of cobalt nitrate and cobalt sulfate, cobalt sulfate and cobalt chloride, cobalt nitrate and cobalt chloride, and combinations of cobalt nitrate, cobalt sulfate, and cobalt chloride, with cobalt nitrate being the most preferred.

[0042] Preferably, the iron salt comprises any one or a combination of at least two of ferric chloride, ferric sulfate, or ferric nitrate. Typical but non-limiting combinations include combinations of ferric chloride and ferric sulfate, combinations of ferric sulfate and ferric nitrate, combinations of ferric chloride and ferric nitrate, and combinations of ferric chloride, ferric sulfate, and ferric nitrate, with ferric nitrate being the most preferred.

[0043] Preferably, the organic ligand includes terephthalic acid, trimesic acid, and 2,5-dihydroxyterephthalic acid.

[0044] This invention further selects terephthalic acid, trimesin, and 2,5-dihydroxyterephthalic acid as organic ligands to form a MOF framework through coordination of carboxyl groups with metal ions. The synergistic effect of the three ligands can regulate the pore size and distribution of the MOF. Among them, terephthalic acid is conducive to the formation of microporous structures, trimesin promotes the construction of mesoporous structures, and the hydroxyl groups in 2,5-dihydroxyterephthalic acid can enhance the hydrophilicity and CO2 adsorption capacity of the MOF material.

[0045] Preferably, the dispersant comprises sodium dodecyl sulfate.

[0046] The present invention further selects sodium dodecyl sulfate as a dispersant, which can prevent MOF particles from agglomerating and improve the uniformity of the film layer.

[0047] Preferably, the solvent includes any one or a combination of at least two of water, ethanol, or ethylene glycol.

[0048] In this invention, the mixed solvent of water, ethanol, and ethylene glycol optimizes the solubility and conductivity of the electroplating solution. The addition of ethylene glycol can adjust the viscosity of the electroplating solution, slow down the diffusion rate of metal ions, and facilitate the formation of gradient structures. At the same time, the mixed solvent can improve the solubility of organic ligands and avoid film defects caused by ligand precipitation.

[0049] Preferably, the electroplating solution comprises 3wt%-5wt% nickel salt, based on a total mass of 100wt%, for example, 3wt%, 3.5wt%, 4wt%, 4.5wt%, or 5wt%, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0050] Preferably, the electroplating solution comprises 4wt%-7wt% of cobalt salt, based on a total mass of 100wt%, for example, 4wt%, 4.5wt%, 5wt%, 5.5wt%, 6wt%, 6.5wt%, or 7wt%, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0051] Preferably, the electroplating solution comprises 0.1wt%-0.7wt% of iron salt, based on a total mass of 100wt%, for example, 0.1wt%, 0.2wt%, 0.3wt%, 0.4wt%, 0.5wt%, 0.6wt%, or 0.7wt%, but is not limited to the listed values; other unlisted values ​​within the range are also applicable.

[0052] Preferably, the electroplating solution comprises 1 wt% to 2 wt% of terephthalic acid, based on a total mass of 100 wt%. For example, it may be 1 wt%, 1.2 wt%, 1.5 wt%, 1.8 wt%, or 2 wt%, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0053] Preferably, the electroplating solution comprises 1 wt% to 2.5 wt% of trimesic acid, based on a total mass of 100 wt%. For example, it may be 1 wt%, 1.2 wt%, 1.5 wt%, 1.8 wt%, 2 wt%, 2.2 wt%, or 2.5 wt%, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0054] Preferably, the electroplating solution comprises 0.5wt%-1.5wt% of 2,5-dihydroxyterephthalic acid, based on a total mass of 100wt%. For example, it may be 0.5wt%, 0.8wt%, 1.0wt%, 1.2wt%, or 1.5wt%, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0055] Preferably, the electroplating solution comprises 1 wt% to 2 wt% sodium dodecyl sulfate, based on a total mass of 100 wt%. For example, it may be 1 wt%, 1.2 wt%, 1.5 wt%, 1.8 wt%, or 2 wt%, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0056] Preferably, the electroplating solution comprises 0.1wt%-3wt% of surfactant, based on a total mass of 100wt%, for example, 5wt%, 5.5wt%, 6wt%, 6.5wt%, 7wt%, 7.5wt%, or 8wt%, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0057] Preferably, the surfactant comprises any one or a combination of at least two of hexadecyltrimethylammonium bromide (CTAB), polyvinylpyrrolidone (PVP), or sodium dodecylbenzenesulfonate (SDBS). Typical but non-limiting combinations include combinations of hexadecyltrimethylammonium bromide (CTAB) and polyvinylpyrrolidone (PVP), combinations of polyvinylpyrrolidone (PVP) and sodium dodecylbenzenesulfonate (SDBS), combinations of hexadecyltrimethylammonium bromide (CTAB) and sodium dodecylbenzenesulfonate (SDBS), and combinations of hexadecyltrimethylammonium bromide (CTAB), polyvinylpyrrolidone (PVP), and sodium dodecylbenzenesulfonate (SDBS).

[0058] The surfactants selected in this invention, such as cetyltrimethylammonium bromide and polyvinylpyrrolidone, can regulate interfacial tension, promote the directional growth of MOF crystals, and enhance the adhesion between the film and the substrate.

[0059] Preferably, the solvent comprises 35wt%-50wt% water, based on a total mass of 100wt%, for example, 35wt%, 40wt%, 45wt%, 48wt%, or 50wt%, but is not limited to the listed values; other unlisted values ​​within the range are also applicable.

[0060] Preferably, the solvent comprises 20wt%-48wt% ethanol, based on a total mass of 100wt%, for example, 20wt%, 22wt%, 25wt%, 28wt%, 30wt%, 32wt%, 35wt%, 38wt%, 40wt%, 42wt%, 45wt%, or 48wt%, but is not limited to the listed values; other unlisted values ​​within the range are also applicable.

[0061] Preferably, the solvent comprises 15wt%-30wt% ethylene glycol, based on a total mass of 100wt%, for example, 15wt%, 18wt%, 20wt%, 22wt%, 25wt%, 28wt%, or 30wt%, but is not limited to the listed values; other unlisted values ​​within the range are also applicable.

[0062] Preferably, step (1) further includes cleaning and drying the substrate before placing it in the electroplating solution.

[0063] Preferably, the heat treatment temperature in step (2) is 200℃-400℃, for example, it can be 200℃, 250℃, 300℃, 350℃ or 400℃, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0064] Preferably, the heat treatment time is 3h-5h, for example, it can be 3h, 3.5h, 4h, 4.5h or 5h, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0065] Preferably, the heating rate of the heat treatment is 2℃ / min-5℃ / min, for example, it can be 2℃ / min, 2.5℃ / min, 3℃ / min, 3.5℃ / min, 4℃ / min, 4.5℃ / min or 5℃ / min, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0066] As a preferred embodiment of the preparation method of the present invention, the preparation method includes the following steps:

[0067] (I) Immerse the nickel foam substrate in acetone or ethanol and ultrasonically clean it for 20 min-40 min; then immerse the nickel foam substrate in a 1 mol / L-3 mol / L hydrochloric acid or nitric acid aqueous solution for 5 min-15 min; rinse with deionized water until neutral; and dry in an inert atmosphere at 60℃-80℃ for 1 h-2 h to obtain a clean nickel foam substrate.

[0068] (II) The foamed nickel substrate obtained in step (I) is placed in an electroplating solution. The foamed nickel substrate is used as the working electrode, the platinum mesh is used as the counter electrode, and the calomel electrode is used as the reference electrode. The substrate is subjected to first pulse electrochemical deposition, second pulse electrochemical deposition and third pulse electrochemical deposition in sequence to obtain a metal-organic framework precursor.

[0069] The pulse voltage for the first pulse electrochemical deposition was -3.0V to -2.5V, and the on / off time ratio T... on / T off The ratio is 1:10-30, and the deposition time is 5-15 minutes;

[0070] The pulse voltage for the second pulse electrochemical deposition is -1.2V to -1.0V, and the on / off time ratio T on / T off The ratio is 1:2-3, and the deposition time is 10-30 minutes;

[0071] The pulse voltage for the third pulse electrochemical deposition was -1.8V to -1.5V, and the on / off time ratio T... on / T off The ratio is 1-3:1, and the deposition time is 50 min-3 h;

[0072] The electroplating solution, with a total mass of 100wt%, comprises 3wt%-5wt% nickel salt, 4wt%-7wt% cobalt salt, 0.1wt%-0.7wt% iron salt, 1wt%-2wt% terephthalic acid, 1wt%-2.5wt% trimesic acid, 0.5wt%-1.5wt% 2,5-dihydroxyterephthalic acid, 1-2% sodium dodecyl sulfate, and 0.1wt%-3wt% surfactant and solvent.

[0073] Based on a total solvent mass of 100 wt%, the solvent comprises 35 wt%-50 wt% water, 20 wt%-48 wt% ethanol and 15 wt%-30 wt% ethylene glycol;

[0074] (III) Under nitrogen protection, the metal-organic framework precursor obtained in step (II) is heated to 200℃-400℃ at a heating rate of 2℃ / min-5℃ / min for 3h-5h to obtain the hierarchical porous metal-organic framework electrode.

[0075] In a second aspect, the present invention provides a hierarchical porous metal-organic framework electrode, which is prepared according to the preparation method described in the first aspect.

[0076] This invention utilizes gradient pulse electroplating combined with a specific electroplating solution system and heat treatment to produce a multi-level porous metal-organic framework electrode with macropores on the surface, mesopores in the middle layer, and micropores in the inner layer.

[0077] Preferably, the hierarchical porous metal-organic framework electrode comprises a substrate and an MOF active layer deposited on the surface of the substrate.

[0078] Preferably, the MOF active layer comprises, from the outside to the inside (from the outer surface to the surface adjacent to the substrate), a surface layer with a pore size of 60nm-200nm, a middle layer with a pore size of 8nm-45nm (located above the inner layer), and an inner layer with a pore size of 0.5nm-2nm (adjacent to the substrate surface).

[0079] The hierarchical porous metal-organic framework electrode prepared by this invention has a three-tiered porous structure consisting of a surface layer, a middle layer, and an inner layer. This structure can achieve the following synergistic performance: the macroporous structure of the surface layer facilitates the rapid diffusion of reactants (such as CO2) to the middle and inner layers, reducing mass transfer resistance; the mesoporous structure of the middle layer serves as a transport channel for reactants and products, while also providing certain active sites to enhance reaction kinetics; the microporous structure of the inner layer has a high specific surface area and abundant active sites, which can efficiently adsorb CO2 and catalyze its conversion.

[0080] Preferably, the thickness of the surface layer is 1μm-5μm, for example, it can be 1μm, 2μm, 3μm, 4μm or 5μm, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0081] Preferably, the thickness of the middle layer is 3μm-8μm, for example, it can be 3μm, 4μm, 5μm, 6μm, 7μm or 8μm, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0082] Preferably, the thickness of the inner layer is 1μm-4μm, for example, it can be 1μm, 1.5μm, 2μm, 2.5μm, 3μm, 3.5μm or 4μm, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0083] Thirdly, the present invention provides an application of the hierarchical porous metal-organic framework electrode as described in the first aspect, wherein the hierarchical porous metal-organic framework electrode is used for hydrogen production by water electrolysis, oxygen reduction in fuel cells, or carbon dioxide electroreduction.

[0084] The multi-level porous metal-organic framework electrode provided by this invention can be widely used in energy conversion fields such as water electrolysis for hydrogen production and oxygen reduction in fuel cells. It solves many technical bottlenecks of traditional MOF electrodes and has the advantages of high adsorption, high selectivity and strong stability. It also has high CO2 electroreduction CO Faradaic efficiency and excellent catalytic performance.

[0085] The numerical range described in this invention includes not only the point values ​​listed above, but also any point values ​​within the numerical ranges not listed above. Due to space limitations and for the sake of brevity, this invention will not exhaustively list all the specific point values ​​included in the range.

[0086] Compared with the prior art, the present invention has at least the following beneficial effects:

[0087] (1) This invention constructs a MOF electrode with a multi-level porous structure through the synergistic effect of a three-stage gradient pulse electrochemical deposition process and an electroplating solution system, thereby achieving a synergistic improvement in adsorption performance, selectivity, stability and electrocatalytic performance. In addition, the gradient pulse deposition process can enhance the bonding force between the MOF film and the substrate, and the heat treatment process can further improve the orderliness and stability of the MOF crystal, thus ensuring the performance stability of the electrode during long-term cyclic use.

[0088] (2) This invention utilizes gradient pulse electroplating combined with a specific electroplating solution system and heat treatment to prepare a multi-level porous metal-organic framework electrode with macropores on the surface, mesopores in the middle layer, and micropores in the inner layer. This structure achieves the following synergistic performance: the macropore structure on the surface facilitates the rapid diffusion of reactants (such as CO2) to the middle and inner layers, reducing mass transfer resistance; the mesopore structure in the middle layer serves as a transport channel for reactants and products, while also providing certain active sites to enhance reaction kinetics; the micropore structure in the inner layer has a high specific surface area and abundant active sites, enabling efficient adsorption of CO2 and catalysis of its conversion.

[0089] (3) The multi-level porous metal-organic framework electrode provided by the present invention can be widely used in energy conversion fields such as water electrolysis for hydrogen production and oxygen reduction in fuel cells. It has high adsorption, high selectivity and strong stability; CO2 electroreduction of CO has high Faraday efficiency and excellent catalytic performance, which solves many technical bottlenecks of traditional MOF electrodes. Attached Figure Description

[0090] Figure 1 This is a scanning microscope (SEM) image of the surface of the nickel foam substrate in Embodiment 1 of the present invention after the third pulse electrochemical deposition. Detailed Implementation

[0091] The technical solution of the present invention will be further illustrated below through specific embodiments. Those skilled in the art should understand that the embodiments described are merely illustrative of the present invention and should not be construed as limiting the invention in any way.

[0092] In this invention, the terms "first aspect," "second aspect," "third aspect," "fourth aspect," etc., are used for descriptive purposes only and should not be construed as indicating or implying relative importance or quantity, nor should they be construed as implicitly indicating the importance or quantity of the indicated technical features. Moreover, "first," "second," "third," "fourth," etc., serve only as a non-exhaustive enumeration and should be understood not to constitute a closed limitation on the quantity.

[0093] Unless otherwise specified, all reagents and consumables used in the following embodiments and comparative examples were purchased from conventional reagent manufacturers in the art; unless otherwise specified, the experimental methods and techniques used are conventional methods and techniques in the art. Table 1 lists the specific information of the raw materials used in the embodiments and comparative examples of the present invention, and Table 2 lists the specific information of the experimental equipment used in the embodiments and comparative examples of the present invention. Unless otherwise specified, the experimental methods and techniques used are conventional methods and techniques in the art.

[0094] Table 1

[0095]

[0096] Table 2

[0097]

[0098] Example 1

[0099] This embodiment provides a method for fabricating a hierarchical porous metal-organic framework electrode based on gradient pulse electroplating, the method comprising the following steps:

[0100] (I) Immerse the nickel foam substrate (1.5 mm thick, 0.5 m2 area) in acetone and ultrasonically clean it for 30 min; then immerse the nickel foam substrate in a 1 mol / L hydrochloric acid aqueous solution for 10 min; rinse with deionized water until neutral; dry in nitrogen at 70 °C for 1.5 h to obtain a clean nickel foam substrate.

[0101] (II) The foamed nickel substrate obtained in step (I) is placed in an electroplating solution. The foamed nickel substrate is used as the working electrode, a platinum mesh is used as the counter electrode (parallel to the working electrode, with a spacing of 8 cm), and a calomel electrode is used as the reference electrode. The deposition temperature is 60℃, the electroplating solution circulation rate is 1.8 L / min, and the deposition process is controlled by a programmable electrochemical workstation. The substrate is subjected to first pulse electrochemical deposition, second pulse electrochemical deposition, and third pulse electrochemical deposition in sequence to obtain a metal-organic framework precursor.

[0102] The pulse voltage for the first pulse electrochemical deposition was -2.8V, and the on / off time ratio was T. on / T off The ratio was 1:20, and the deposition time was 10 minutes.

[0103] The pulse voltage for the second pulse electrochemical deposition was -1.1V, and the on / off time ratio was T. on / T off The ratio was 1:2.5, and the deposition time was 20 min;

[0104] The pulse voltage for the third pulse electrochemical deposition was -1.6V, and the on / off time ratio was T. on / T off The ratio was 2:1, and the deposition time was 2 hours.

[0105] The electroplating solution, with a total mass of 100 wt%, comprises 4 wt% nickel nitrate, 5.5 wt% cobalt nitrate, 0.4 wt% ferric nitrate, 1.5 wt% terephthalic acid, 1.8 wt% trimesic acid, 1.0 wt% 2,5-dihydroxyterephthalic acid, 1.5 wt% sodium dodecyl sulfate, 1.5 wt% hexadecyltrimethylammonium bromide, and a solvent; the solvent, with a total mass of 100 wt%, comprises 40 wt% water, 35 wt% ethanol, and 25 wt% ethylene glycol.

[0106] (III) Under nitrogen protection, the metal-organic framework precursor obtained in step (II) is heated to 300°C at a heating rate of 3°C / min for 4 hours to obtain the hierarchical porous metal-organic framework electrode.

[0107] In this study, samples of the nickel foam substrate were taken after the first and second pulse electrochemical depositions, and then subjected to gas (N2, CO2) adsorption-desorption analysis. Specifically, the adsorption and desorption isotherms of inert gases (N2, CO2) were measured at liquid nitrogen temperature of 77K. Different pore sizes induce different types of capillary condensation and evaporation behaviors. By analyzing the shape of the isotherms and mathematical models, the pore size distribution, specific surface area, and pore volume were calculated, thus determining that the pore size of the deposited layer obtained after the first pulse electrochemical deposition was approximately 0.8-1.5 nm, and the pore size of the deposited layer obtained after the second pulse electrochemical deposition was approximately 10-30 nm.

[0108] Figure 1 The image shows a scanning electron microscope (SEM) image of the surface of the nickel foam substrate in Example 1 of the present invention after the third pulse electrochemical deposition. The SEM measurements indicate that the surface layer of the electrode has a thickness of approximately 3 μm and a pore size of approximately 120-150 nm.

[0109] Therefore, based on the gas (N2, CO2) adsorption-desorption analysis combined with scanning electron microscopy (SEM) characterization, it can be seen that the thickness of the surface layer of the electrode is about 3 μm and the pore size is about 120-150 nm; the thickness of the middle layer is about 5 μm and the pore size is 10-30 nm; and the thickness of the inner layer is about 2 μm and the pore size is 0.8-1.5 nm.

[0110] Example 2

[0111] This embodiment provides a method for preparing a hierarchical porous metal-organic framework electrode based on gradient pulse electroplating. The only difference between this method and Example 1 is that the content of nickel nitrate in the electroplating solution is 3%, the content of cobalt nitrate is 7%, and the content of iron nitrate is 0.1%, while the rest of the preparation method remains unchanged.

[0112] Characterization revealed that the electrode has a surface pore size of 70-140 nm and a thickness of 2.5 μm; a middle pore size of 8-25 nm and a thickness of 6 μm; and an inner pore size of 0.7-1.4 nm and a thickness of 2.5 μm.

[0113] Example 3

[0114] This embodiment provides a method for preparing a multi-level porous metal-organic framework electrode based on gradient pulse electroplating. The only difference between this preparation method and that of Example 1 is that the content of terephthalic acid in the electroplating solution is 2%, the content of trimesoic acid is 1%, and the content of 2,5-dihydroxyterephthalic acid is 1.5%, while the rest of the preparation method remains unchanged.

[0115] Characterization revealed that the electrode has a surface pore size of 90-160 nm and a thickness of 3.5 μm; a middle pore size of 12-35 nm and a thickness of 4.5 μm; and an inner pore size of 0.9-1.6 nm and a thickness of 1.8 μm.

[0116] Example 4

[0117] This embodiment provides a method for preparing a hierarchical porous metal-organic framework electrode based on gradient pulse electroplating. The only difference between this method and Example 1 is that the first pulse electrochemical deposition parameters are: pulse voltage -3.0V, on / off time ratio 1:10, and deposition time 5min.

[0118] The second pulse electrochemical deposition parameters are: pulse voltage -1.2V, on / off time ratio 1:2, and deposition time 10min;

[0119] The parameters for the third pulse electrochemical deposition were: pulse voltage -1.8V, on / off time ratio 3:1, and deposition time 3h, with the rest of the preparation method remaining unchanged.

[0120] Characterization revealed that the electrode has a surface pore size of 90-180 nm and a thickness of 2 μm; a middle pore size of 15-40 nm and a thickness of 4 μm; and an inner pore size of 0.6-1.3 nm and a thickness of 3 μm.

[0121] Example 5

[0122] This embodiment provides a method for preparing a multi-level porous metal-organic framework electrode based on gradient pulse electroplating. The only difference between this method and Example 1 is that the deposition temperature is 40°C and the electroplating solution circulation rate is 1.0 L / min, while the rest of the preparation method remains unchanged.

[0123] Characterization revealed that the electrode has a surface pore size of 60-120 nm and a thickness of 2.8 μm; a middle pore size of 8-20 nm and a thickness of 5.5 μm; and an inner pore size of 0.7-1.2 nm and a thickness of 2.2 μm.

[0124] Example 6

[0125] This embodiment provides a method for preparing a multi-level porous metal-organic framework electrode based on gradient pulse electroplating. The only difference between this method and Example 1 is that the deposition temperature is 80℃ and the electroplating solution circulation rate is 2.5L / min, while the rest of the preparation method remains unchanged.

[0126] Characterization revealed that the electrode has a surface pore size of 100-200 nm and a thickness of 3.2 μm; a middle pore size of 20-45 nm and a thickness of 4.8 μm; and an inner pore size of 1.0-1.8 nm and a thickness of 1.5 μm.

[0127] Example 7

[0128] This embodiment provides a method for preparing a multi-level porous metal-organic framework electrode based on gradient pulse electroplating. The only difference between this preparation method and that of Example 1 is that the surfactant is polyvinylpyrrolidone with a content of 3%; the heat treatment conditions are a heating rate of 2℃ / min, a temperature of 200℃, and a holding time of 5h, while the rest of the preparation method remains unchanged.

[0129] Characterization revealed that the electrode has a surface pore size of 70-130 nm and a thickness of 2.6 μm; a middle pore size of 10-28 nm and a thickness of 5.2 μm; and an inner pore size of 0.8-1.4 nm and a thickness of 2.3 μm.

[0130] Example 8

[0131] This embodiment provides a method for preparing a multi-level porous metal-organic framework electrode based on gradient pulse electroplating. The only difference between this preparation method and that of Example 1 is that the surfactant is sodium dodecylbenzenesulfonate with a content of 0.1%; the heat treatment conditions are a heating rate of 5℃ / min, a temperature of 400℃, and a holding time of 3h, while the rest of the preparation method remains unchanged.

[0132] Characterization revealed that the electrode has a surface pore size of 80-160 nm and a thickness of 3.3 μm; a middle pore size of 12-32 nm and a thickness of 4.6 μm; and an inner pore size of 0.9-1.6 nm and a thickness of 1.9 μm.

[0133] Example 9

[0134] This embodiment provides a method for preparing a hierarchical porous metal-organic framework electrode based on gradient pulse electroplating. The only difference between this method and Example 1 is that the solvent in the electroplating solution contains only water and ethanol (60% water and 40% ethanol) and does not contain ethylene glycol. The rest of the preparation method remains unchanged.

[0135] Characterization revealed that the electrode has a surface pore size of 60-130 nm and a thickness of 2.7 μm; a middle pore size of 8-25 nm and a thickness of 5.1 μm; and an inner pore size of 0.7-1.3 nm and a thickness of 2.4 μm.

[0136] Example 10

[0137] This embodiment provides a method for preparing a hierarchical porous metal-organic framework electrode based on gradient pulse electroplating. The only difference between this method and Example 1 is that the electroplating solution does not contain nickel nitrate, while the contents of other components remain unchanged.

[0138] Characterization revealed that the electrode has a surface pore size of 70-140 nm, a middle pore size of 8-28 nm, an inner pore size of 0.7-1.4 nm, and a total thickness of 6.2 μm.

[0139] Example 11

[0140] This embodiment provides a method for preparing a hierarchical porous metal-organic framework electrode based on gradient pulse electroplating. The only difference between this method and Example 1 is that the electroplating solution does not contain terephthalic acid, while the contents of other components remain unchanged.

[0141] Characterization revealed that the electrode has a surface pore size of 90-160 nm, a middle pore size of 15-40 nm, an inner pore size of 1.0-1.7 nm, and a total thickness of 5.8 μm.

[0142] Example 12

[0143] This embodiment provides a method for preparing a hierarchical porous metal-organic framework electrode based on gradient pulse electroplating. The only difference between this method and Example 1 is that the electroplating solution does not contain 2,5-dihydroxyterephthalic acid, while the contents of other components remain unchanged.

[0144] Characterization revealed that the electrode has a surface pore size of 80-150 nm, a middle pore size of 12-35 nm, an inner pore size of 0.9-1.6 nm, and a total thickness of 5.6 μm.

[0145] Example 13

[0146] This embodiment provides a method for preparing a multi-level porous metal-organic framework electrode based on gradient pulse electroplating. The only difference between this method and Example 1 is that the pulse voltage of the first pulse electrochemical deposition is -3.5V, while the rest of the preparation method remains unchanged.

[0147] Example 14

[0148] This embodiment provides a method for preparing a multi-level porous metal-organic framework electrode based on gradient pulse electroplating. The only difference between this method and Example 1 is that the pulse voltage of the first pulse electrochemical deposition is -2.0V, while the rest of the preparation method remains unchanged.

[0149] Example 15

[0150] This embodiment provides a method for preparing a hierarchical porous metal-organic framework electrode based on gradient pulse electroplating. The only difference between this method and that of Embodiment 1 is that ferric nitrate is replaced with an equal mass of copper nitrate, while the rest of the preparation method remains unchanged.

[0151] Example 16

[0152] This embodiment provides a method for preparing a multi-level porous metal-organic framework electrode based on gradient pulse electroplating. The only difference between this method and Example 1 is that the content of ferric nitrate is 2.5 wt%, while the rest of the preparation method remains unchanged.

[0153] Comparative Example 1

[0154] This comparative example provides a method for preparing a hierarchical porous metal-organic framework electrode based on gradient pulse electroplating. The only difference between this method and Example 1 is that the third pulse electrochemical deposition is omitted, and the first and second pulse electrochemical depositions are performed directly. The rest of the preparation method remains unchanged.

[0155] Characterization revealed that the electrode had no obvious macroporous structure on the surface, with a middle layer pore size of 10-35 nm, an inner layer pore size of 0.8-1.5 nm, and a total thickness of 6 μm.

[0156] Comparative Example 2

[0157] This comparative example provides a method for preparing a hierarchical porous metal-organic framework electrode based on gradient pulse electroplating. The only difference between this method and Example 1 is that the second pulse electrochemical deposition is not performed; instead, the first and third pulse electrochemical depositions are performed directly.

[0158] Characterization revealed that the electrode had a surface pore size of 80-150 nm, no obvious mesoporous structure in the middle layer, an inner pore size of 0.8-1.5 nm, and a total thickness of 5.5 μm.

[0159] Comparative Example 3

[0160] This comparative example provides a method for preparing a hierarchical porous metal-organic framework electrode based on gradient pulse electroplating. The only difference between this method and Example 1 is that the first pulse electrochemical deposition is not performed, but only the second and third pulse electrochemical depositions are performed.

[0161] Characterization revealed that the electrode has a surface pore size of 80-150 nm, a middle pore size of 10-30 nm, no obvious inner microporous structure, and a total thickness of 4.8 μm.

[0162] test:

[0163] (I) CO2 adsorption capacity test:

[0164] CO2 adsorption capacity is the amount of CO2 that a unit mass of sample can adsorb under specific conditions (temperature 25°C, gas pressure 1 bar), expressed in mmol / g. In this invention, according to GB / T39753-2021 "Test Method for Adsorption Performance of Metal-Organic Framework Materials", the CO2 adsorption capacity is tested using a gravimetric method (ASAP2020 gas adsorption analyzer). The test conditions include: temperature 25°C, gas pressure 1 bar, and the adsorbed gas being pure CO2 (purity 99.999%). Specifically, the hierarchical porous metal-organic framework electrode samples prepared in each of the following examples and comparative examples were degassed at 120°C and a vacuum degree ≤10⁻³ Pa for 4 hours to remove moisture and impurities. The samples were then cooled to 25°C and CO2 was introduced for 4 hours. The weight change of the sample before and after CO2 adsorption was then recorded, and the CO2 adsorption capacity (unit: mmol / g) was calculated. A higher CO2 adsorption capacity indicates a stronger adsorption capacity for CO2 by the material.

[0165] (II) CO2 / N2 Selectivity Test:

[0166] In this invention, CO2 / N2 selectivity is an indicator that measures the "preferential adsorption capacity" of a membrane material for CO2, which determines its efficiency in separating CO2 from mixed gases such as industrial flue gas (CO2 volume fraction is usually 10%-15%, N2 volume fraction is usually 85%-90%) and biogas (CO2 volume fraction is usually 30%-50%, CH4 volume fraction is usually 50%-70%).

[0167] In this invention, the CO2 / N2 selectivity test is conducted using the Ideal Adsorption Solution Theory (IAST) according to GB / T39753-2021 "Test Method for Adsorption Performance of Metal-Organic Framework Materials". Specifically, multi-level porous metal-organic framework electrode samples prepared in each of the following examples and comparative examples are degassed for 4 hours at 120°C and a vacuum degree ≤10⁻³ Pa to remove impurities, and then cooled to 25°C. Subsequently, the adsorption isotherms of the samples for pure CO2 (99.999%) and pure N2 (99.999%) are measured by gravimetric method at a gas pressure of 1 bar, with a continuous 30-minute weight change rate ≤0.01% as the equilibrium standard. The isotherms were then fitted using the Langmuir-Freundlich model, and the CO2 / N2 selectivity α (α=(x1 / y1) / (x2 / y2)) was calculated using a simulated flue gas CO2 / N2 molar ratio of 15 / 85 (where x1 and x2 are the adsorption amounts of CO2 and N2 in the composite membrane sample, respectively, and y1 and y2 are the molar fractions of CO2 and N2 in the mixed gas, respectively). A higher CO2 / N2 selectivity α indicates a stronger preferential adsorption capacity of the hierarchical porous metal-organic framework electrode sample for CO2.

[0168] (III) Cyclic stability test:

[0169] Specifically, the hierarchical porous metal-organic framework electrode samples prepared in each of the following examples and comparative examples were dried at 120°C and a vacuum degree ≤ -0.095 MPa for 4 hours to remove residual moisture from the surface. The samples were then fixed in the sample chamber of a self-made membrane performance testing device. This self-made membrane performance testing device includes: a sample chamber, a temperature control system (thermometer, accuracy ±0.5°C), a humidity control module (humidifier, which controls the nitrogen humidity to 10%), and a weight monitoring module (analytical balance, which records the sample weight changes in real time).

[0170] Turn on the device and raise the temperature of the sample chamber to 80℃. Introduce a 15% CO2 / 85% N2 mixed gas (flow rate 50 mL / min) and maintain the pressure at 1 bar. Record the sample weight in real time using the weight monitoring module. When the weight change rate is ≤0.01% for 30 consecutive minutes, adsorption is considered saturated. Record the total sample weight m1 at this time and calculate the single adsorption capacity q1 = (m1-m0)×1000 / (44.01×m0) (unit: mmol / g).

[0171] After adsorption saturation, the mixed gas was shut off and replaced with pure nitrogen (flow rate 50 mL / min), maintaining the sample chamber temperature at 80 °C and the pressure reduced to 0.1 bar. The purging was continued for 1 hour. When the sample weight returned to the initial dry weight m0 ± 0.1 mg, desorption was considered complete, and one cycle was finished.

[0172] Repeat the above adsorption-desorption steps to complete 50 adsorption-desorption cycles. Calculate the adsorption capacity qn (n=1,2,...,50) after each cycle. Finally, calculate the adsorption capacity retention rate η after 50 cycles: η = (qn + qn) / (n + qn) 50 / q1)×100%. A higher adsorption capacity retention rate η indicates better stability of the hierarchical porous metal-organic framework electrode sample.

[0173] (iv) Electrocatalytic performance testing:

[0174] Electrocatalytic performance testing was conducted according to GB / T20042.4-2025 "Proton Exchange Membrane Fuel Cells Part 4: Electrocatalyst Test Methods". Specifically, the hierarchical porous metal-organic framework electrode sample prepared in each of the following examples and comparative examples was used as the working electrode (1 cm² area). 2A platinum sheet was used as the counter electrode, and a saturated calomel electrode as the reference electrode. Linear sweep voltammetry (LSV) and chronoamperometry were performed in a 0.5 mol / L KHCO3 solution (CO2 saturated, flow rate 50 mL / min). In the LSV test, the scan range was -0.2 V to -1.2 V (relative to RHE), the scan rate was 50 mV / s, and the relationship between current density and voltage was recorded. In the chronoamperometry test, constant voltage electrolysis was performed at -0.8 V (relative to RHE) for 2 h. The concentration of electrolysis products (CO, formic acid, etc.) was detected by gas chromatography (GC-2014, Shimadzu Corporation, Japan), and the Faraday efficiency FE was calculated (FE = (n × F × c × V) / (I × t) × 100%, where n is the number of electrons transferred and F is the Faraday constant ≈ 96485 Cmol). -1 (where c is the product concentration, V is the gas volume, I is the current, and t is the electrolysis time). The higher the Faraday efficiency (FE), the higher the proportion of the target product (CO or formic acid) formed, and the better the electrocatalysis.

[0175] The CO2 adsorption capacity, CO2 / N2 selectivity, stability and electrocatalytic performance of the hierarchical porous metal-organic framework electrodes prepared in the examples and comparative examples were measured. The test results are shown in Table 3 below.

[0176] Table 3

[0177]

[0178] The test results show that:

[0179] (1) As can be seen from Examples 1-8, the present invention constructs a MOF electrode with a hierarchical porous structure through the synergistic effect of a three-stage gradient pulse electrochemical deposition process and an electroplating solution system, thereby achieving a synergistic improvement in adsorption performance, selectivity, stability, and electrocatalytic performance. In addition, the gradient pulse deposition process can enhance the bonding force between the MOF film and the substrate, and the heat treatment process can further improve the orderliness and stability of the MOF crystals, thus ensuring the performance stability of the electrode during long-term cyclic use.

[0180] (2) A comparison between Example 1 and Example 9 shows that the CO2 adsorption capacity of Example 1 (4.2 mmol / g) is significantly higher than that of Example 9 (3.5 mmol / g), with an increase of 20%; the CO2 / N2 selectivity α (142) of Example 1 is higher than that of Example 9 (125), with an increase of 13.6%; the adsorption capacity retention rate of Example 1 (94%) is higher than that of Example 9 (90%), indicating better stability; and the CO Faradaic efficiency of Example 1 (90%) is higher than that of Example 9 (83%), indicating better electrocatalytic activity. These results indicate that the addition of ethylene glycol to the solvent system of this invention can significantly improve the overall performance of the electrode. As a high-boiling-point solvent, ethylene glycol can adjust the viscosity and conductivity of the electroplating solution, slow down the diffusion rate of metal ions, and facilitate the formation of gradient structures. At the same time, ethylene glycol can form complexes with metal ions, regulate the growth mode of MOF crystals, further optimize the hierarchical pore structure and active site distribution, thereby improving CO2 adsorption, selectivity, stability, and electrocatalytic performance.

[0181] (3) By comparing Example 1 with Examples 10-12, it can be seen that nickel nitrate, terephthalic acid and 2,5-dihydroxyterephthalic acid in the electroplating solution of the present invention are important components for achieving excellent electrode performance. Their synergistic effect can regulate the structure and electronic properties of MOF and improve adsorption, selectivity, stability and catalytic performance.

[0182] When nickel nitrate is missing, the CO2 adsorption capacity is 2.5 mmol / g and the CO Faradaic efficiency is 50%, indicating that the absence of nickel ions will lead to an imbalance in the electronic structure of MOF materials, a reduction in the number of active sites, and thus affect the adsorption and catalytic performance.

[0183] When terephthalic acid is missing, the CO2 / N2 selectivity is only 95%, and the electrocatalytic performance is poor (CO Faradaic efficiency 55%), which proves that terephthalic acid, as an organic ligand, plays an important role in regulating the pore size distribution of MOF, enhancing the selective adsorption of CO2, and improving catalytic activity.

[0184] When 2,5-dihydroxyterephthalic acid is missing, the CO2 adsorption capacity is 2.9 mmol / g, and the adsorption capacity retention rate is 80%, indicating that the hydroxyl group in 2,5-dihydroxyterephthalic acid can enhance the hydrophilicity and CO2 adsorption capacity of MOF materials, while improving the structural stability of the materials.

[0185] (4) By comparing Example 1 with Examples 13-14, it can be seen that by further controlling the pulse voltage of the first pulse electrochemical deposition in step (1) to be -3.0V to -2.5V, the present invention can achieve uniform and dense rapid nucleation on the substrate surface, laying a solid foundation for the subsequent growth of gradient structures. However, if the negative voltage is too large (such as -3.5V in Example 13), the metal ion reduction driving force will be too strong, which will lead to excessively dense nucleation points, uncontrolled growth, and easy formation of rough, disordered or even powdery accumulation layers. Not only will the inner microporous structure be destroyed, but the bonding force between the film layer and the substrate will be severely weakened, resulting in a significant decrease in the mechanical and electrochemical stability of the overall electrode. If the negative voltage is too low (such as -2.0V in Example 14), the nucleation driving force is insufficient, and metal ions and ligands cannot effectively and quickly form a sufficient number of crystal nuclei on the substrate surface. This results in slow, thin, and non-dense inner layer growth, which fails to form an effective microporous structure and provide sufficient active sites. Consequently, the underlying support of the entire hierarchical porous structure is weak, ultimately affecting the adsorption capacity, mass transfer efficiency, and catalytic activity of the electrode.

[0186] (5) By comparing Example 1 with Examples 15-16, it can be seen that the synergistic effect of nickel, cobalt and iron elements selected in this invention can regulate the electronic structure of MOF materials and improve electrocatalytic activity. At the same time, the difference in coordination ability between different metal ions and organic ligands helps to form a hierarchical porous structure. When other metal elements are selected (such as copper in Example 15), due to the difference in their reduction potential, ionic radius and coordination geometry from the Ni / Co / Fe system, the original synergistic effect and lattice matching between the ternary metals will be destroyed, resulting in phase separation or generation of impurity phases during the deposition process. The resulting MOF material has a disordered hierarchical porous structure, a wider pore size distribution, and a significantly reduced selectivity of the target catalytic pathway (CO2 to CO). If the concentration of the corresponding metal ions is too high (such as the high iron salt content in Example 16), it will disrupt the equilibrium concentration and diffusion rate of metal ions in the electroplating solution, causing local over-deposition of the metal during the deposition process, blocking the formed pores. At the same time, excessive metal ions may be mixed in the MOF framework in the form of oxides or hydroxides, which not only reduces the effective active site density, but also seriously affects the structural stability and conductivity of the material.

[0187] (6) As can be seen from Example 1 and Comparative Examples 1-3, when the third pulse deposition is missing, there is no obvious macroporous structure on the surface, the CO2 adsorption capacity is only 2.3 mmol / g, the mass transfer resistance increases and the adsorption efficiency drops significantly; the stability and electrocatalytic performance also deteriorate significantly, the adsorption capacity retention rate is 68%, and the CO Faradaic efficiency is only 45%; when the second pulse deposition is missing, there is no obvious mesoporous structure in the middle layer, the CO2 / N2 selectivity is only 78%, and CO2 and N2 cannot be effectively separated; at the same time, the absence of the middle layer as a transition region leads to poor mass transfer channels and poor electrocatalytic performance; when the first pulse deposition is missing, there is no obvious microporous structure in the inner layer, the CO2 adsorption capacity is only 1.9 mmol / g, and there are insufficient high specific surface area active sites; the stability is poor, the adsorption capacity retention rate is only 58%, and the electrocatalytic activity is low (CO Faradaic efficiency 38%).

[0188] In summary, this invention utilizes a three-stage gradient pulse electrochemical deposition process and an electroplating solution system to construct a MOF electrode with a hierarchical porous structure, achieving a synergistic improvement in adsorption performance, selectivity, stability, and electrocatalytic performance. Furthermore, the gradient pulse deposition process enhances the adhesion between the MOF film and the substrate, while the heat treatment process further improves the orderliness and stability of the MOF crystals, thereby ensuring the electrode's performance stability during long-term cyclic use.

[0189] The applicant declares that the above description is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention fall within the protection and disclosure scope of the present invention.

Claims

1. A method for fabricating a hierarchical porous metal-organic framework electrode based on gradient pulse electroplating, characterized in that, The preparation method includes the following steps: (1) The substrate is placed in an electroplating solution and pulsed electrochemical deposition is performed on the substrate to obtain a metal-organic framework precursor; (2) The metal-organic framework precursor obtained in step (1) is heat-treated to obtain the hierarchical porous metal-organic framework electrode. Step (1) of the pulsed electrochemical deposition includes, in sequence, a first pulsed electrochemical deposition, a second pulsed electrochemical deposition, and a third pulsed electrochemical deposition; The on / off time ratio of the first pulse electrochemical deposition to T on / T off The ratio is 1:10-30; The second pulse electrochemical deposition on / off time ratio T on / T off The ratio is 1:2-3; The third pulse electrochemical deposition on / off time ratio T on / T off The ratio is 1-3:1; The electroplating solution in step (1) includes metal salts, organic ligands, dispersants, and solvents.

2. The preparation method according to claim 1, characterized in that, Step (1) The pulse voltage of the first pulse electrochemical deposition is -3.0V to -2.5V, and the time is 5min to 15min; Preferably, in step (1), the pulse voltage of the second pulse electrochemical deposition is -1.2V to -1.0V, and the time is 10min to 30min; Preferably, the pulse voltage of the third pulse electrochemical deposition in step (1) is -1.8V to -1.5V, and the time is 50min to 3h.

3. The preparation method according to claim 1 or 2, characterized in that, The temperature for pulsed electrochemical deposition in step (1) is 40℃-80℃; Preferably, the circulation rate of the electroplating solution in step (1) is 1L / min-2.5L / min; Preferably, in the pulsed electrochemical deposition process, the substrate is used as the working electrode, the platinum mesh is used as the counter electrode, and the saturated calomel electrode is used as the reference electrode.

4. The preparation method according to any one of claims 1-3, characterized in that, The substrate in step (1) is nickel foam, copper foam, cobalt foam, or metal foil; Preferably, the metal salt includes nickel salt, cobalt salt, and iron salt; Preferably, the organic ligand comprises terephthalic acid, trimesic acid, and 2,5-dihydroxyterephthalic acid; Preferably, the dispersant comprises sodium dodecyl sulfate; Preferably, the solvent includes any one or a combination of at least two of water, ethanol, or ethylene glycol.

5. The preparation method according to any one of claims 1-4, characterized in that, Based on a total mass of 100wt% of the electroplating solution, the electroplating solution comprises 3wt%-5wt% of nickel salt, 4wt%-7wt% of cobalt salt, 0.1wt%-0.7wt% of iron salt, 1wt%-2wt% of terephthalic acid, 1wt%-2.5wt% of trimesic acid, 0.5wt%-1.5wt% of 2,5-dihydroxyterephthalic acid, 1-2% of sodium dodecyl sulfate, and 0.1wt%-3wt% of surfactant and solvent; Preferably, the surfactant comprises any one or a combination of at least two of hexadecyltrimethylammonium bromide, polyvinylpyrrolidone, or sodium dodecylbenzenesulfonate; Preferably, based on a total solvent mass of 100 wt%, the solvent comprises 35 wt%-50 wt% water, 20 wt%-48 wt% ethanol and 15 wt%-30 wt% ethylene glycol; Preferably, step (1) further includes cleaning and drying the substrate before placing it in the electroplating solution.

6. The preparation method according to any one of claims 1-5, characterized in that, The heat treatment temperature in step (2) is 200℃-400℃; Preferably, the heat treatment time is 3-5 hours; Preferably, the heating rate of the heat treatment is 2℃ / min-5℃ / min.

7. The preparation method according to any one of claims 1-6, characterized in that, The preparation method includes the following steps: (I) Immerse the nickel foam substrate in acetone or ethanol and ultrasonically clean it for 20 min-40 min; then immerse the nickel foam substrate in a 1 mol / L-3 mol / L hydrochloric acid or nitric acid aqueous solution for 5 min-15 min; rinse with deionized water until neutral; and dry in an inert atmosphere at 60℃-80℃ for 1 h-2 h to obtain a clean nickel foam substrate. (II) The foamed nickel substrate obtained in step (I) is placed in an electroplating solution. The foamed nickel substrate is used as the working electrode, the platinum mesh is used as the counter electrode, and the calomel electrode is used as the reference electrode. The substrate is subjected to first pulse electrochemical deposition, second pulse electrochemical deposition and third pulse electrochemical deposition in sequence to obtain a metal-organic framework precursor. The pulse voltage for the first pulse electrochemical deposition was -3.0V to -2.5V, and the on / off time ratio T... on / T off The ratio is 1:10-30, and the deposition time is 5-15 minutes; The pulse voltage for the second pulse electrochemical deposition is -1.2V to -1.0V, and the on / off time ratio T on / T off The ratio is 1:2-3, and the deposition time is 10-30 minutes; The pulse voltage for the third pulse electrochemical deposition was -1.8V to -1.5V, and the on / off time ratio T... on / T off The ratio is 1-3:1, and the deposition time is 50 min-3 h; The electroplating solution comprises, based on a total mass of 100wt%, 3wt%-5wt% nickel salt, 4wt%-7wt% cobalt salt, 0.1wt%-0.7wt% iron salt, 1wt%-2wt% terephthalic acid, 1wt%-2.5wt% trimesic acid, 0.5wt%-1.5wt% 2,5-dihydroxyterephthalic acid, 1wt%-2wt% sodium dodecyl sulfate, and 0.1wt%-3wt% surfactant and solvent, with a total mass of 100wt% in the electroplating solution. Based on a total solvent mass of 100 wt%, the solvent comprises 35 wt%-50 wt% water, 20 wt%-48 wt% ethanol and 15 wt%-30 wt% ethylene glycol; (III) Under nitrogen protection, the metal-organic framework precursor obtained in step (II) is heated to 200℃-400℃ at a heating rate of 2℃ / min-5℃ / min for 3h-5h to obtain the hierarchical porous metal-organic framework electrode.

8. A hierarchical porous metal-organic framework electrode, characterized in that, The hierarchical porous metal-organic framework electrode is prepared by the preparation method according to any one of claims 1-7.

9. The hierarchical porous metal-organic framework electrode according to claim 8, characterized in that, The hierarchical porous metal-organic framework electrode includes a substrate and a MOF active layer deposited on the surface of the substrate. Preferably, the MOF active layer comprises, from the outside to the inside, a surface layer with a pore size of 60nm-200nm, a middle layer with a pore size of 8nm-45nm, and an inner layer with a pore size of 0.5nm-2nm; Preferably, the thickness of the surface layer is 1μm-5μm; Preferably, the thickness of the middle layer is 3μm-8μm; Preferably, the thickness of the inner layer is 1μm-4μm.

10. An application of the hierarchical porous metal-organic framework electrode as described in claim 1 or 2, characterized in that, The multi-level porous metal-organic framework electrode is used for hydrogen production by water electrolysis, oxygen reduction in fuel cells, or carbon dioxide electroreduction.

Images