A gradient pore gas electrode for the electrochemical reduction of carbon dioxide and its preparation method

Abstract

This invention provides a gradient porosity electrode for the electrochemical reduction of carbon dioxide, its preparation method, and its application. The gradient porosity electrode comprises at least two porous layers stacked sequentially along the thickness direction, with a porosity difference of ≥5% between adjacent layers, and the porosity exhibiting a gradient change along the electrode thickness direction. This invention also discloses a method for preparing the gradient porosity electrode: preparing electrode slurries with at least two different pore-forming agent contents, sequentially coating the slurries onto a substrate surface using a layer-by-layer coating method, and obtaining a porous electrode with a gradient porosity distribution after drying and sintering. The gradient porosity electrode of this invention maintains high porosity on the gas diffusion side to facilitate rapid carbon dioxide diffusion, and low porosity on the catalytic reaction side to increase the density of active sites, reducing the difficulty of controlling the three-phase interface position, achieving a match between mass transfer and reaction rate, and improving the Faraday efficiency and long-term operational stability of the electrochemical reduction of carbon dioxide.

Description

Technical Field

[0001] This invention relates to the field of electrochemical catalysis technology, specifically to a gradient porous electrode for the electrochemical reduction of carbon dioxide, its preparation method and application, and particularly to a gradient porous electrode with nickel, titanium and copper as metal substrates. Background Technology

[0002] Electrochemical reduction of carbon dioxide is an important pathway to convert carbon dioxide into high-value-added fuels and chemicals (such as formic acid, carbon monoxide, methane, ethylene, ethanol, etc.), which is of great significance for mitigating the greenhouse effect and realizing the recycling of carbon resources.

[0003] The gas diffusion electrode is a core component of the carbon dioxide electroreduction reaction, serving to simultaneously provide a gas transport channel, an electron conduction pathway, and reactive sites. In the gas diffusion electrode, carbon dioxide diffuses from the gas side to the catalytic reaction interface, where it undergoes an electrochemical reduction reaction on the catalyst surface. Simultaneously, reaction products and electrolyte need to be promptly removed.

[0004] However, traditional gas diffusion electrodes with uniform pore structures face the following technical challenges: First, there is a contradiction between mass transfer and reaction. High porosity is beneficial for rapid CO2 diffusion and mass transfer, but excessively high porosity will reduce the mechanical strength and electronic conductivity of the electrode; while low porosity is beneficial for electronic conduction and the increase of catalytic active sites, it will limit the CO2 diffusion rate, leading to insufficient reactant supply. A uniform pore structure cannot simultaneously meet the different porosity requirements of the gas diffusion layer and the catalytic reaction layer.

[0005] Secondly, it is pH sensitive. Under acidic conditions, hydrogen evolution is severe during the electrochemical reduction of carbon dioxide at high current densities; under alkaline conditions, it is prone to crystallization, which not only blocks the catalytic sites but also makes the electrode hydrophilic.

[0006] Third, the problem of water flooding, in addition to the hydrophilicity caused by crystallization, also includes the damage to the hydrophobic layer caused by the short lifespan of traditional carbon (carbon paper or carbon cloth) substrates under high current density (around 12 hours), which leads to the destruction of the three-phase interface and the formation of water flooding.

[0007] To address these issues, researchers have attempted to optimize electrode performance by constructing hierarchical or layered porous structures. However, existing technologies still suffer from the following shortcomings: There is a lack of systematic research on the quantitative relationship between porosity gradient and catalytic performance; the correlation between gradient parameters (such as the number of layers, thickness of each layer, porosity difference, pore size, etc.) and electrochemical performance is unclear; and there is a lack of a simple fabrication process that provides well-bonded interlayer interfaces, controllable pore gradients, and applicability to various metal substrates for gradient porous electrodes. Summary of the Invention

[0008] To address the shortcomings of existing technologies, this invention provides a gradient porous electrode for the electrochemical reduction of carbon dioxide, its preparation method, and its application, particularly relating to a gradient porous electrode with nickel, titanium, or copper as a metal substrate.

[0009] One objective of this invention is to provide a multilayer porous metal electrode structure with a gradient in interlayer porosity. By designing the porosity gradient along the electrode thickness direction, a high porosity is maintained on the side near the metal substrate (gas diffusion side) to facilitate the rapid diffusion of CO2, while a low porosity is maintained on the side away from the metal substrate (catalytic reaction side) to increase the density of active sites and electronic conductivity, thereby achieving a match between mass transfer and reaction rate.

[0010] Another objective of this invention is to provide a method for preparing gradient porous electrodes with a simple preparation process, controllable gradient parameters, and applicability to various metal substrates such as nickel, titanium, and copper. This method achieves an interlayer gradient distribution of porosity by coating metal slurries with different pore-forming agent contents layer by layer and sintering them.

[0011] Another object of the present invention is to provide the application of the gradient pore electrode in the electrochemical reduction reaction of carbon dioxide. To achieve the above-mentioned objectives, the present invention provides the following technical solution: A gradient pore electrode for the electrochemical reduction of carbon dioxide, comprising: The metal substrate is selected from any one of nickel substrate, titanium substrate, and copper substrate; and at least two porous metal layers attached to the surface of the metal substrate; The at least two porous metal layers are stacked sequentially in a direction away from the metal substrate, the porosity difference between adjacent layers is ≥5%, and the porosity of the porous metal layers varies in a gradient in a direction away from the metal substrate.

[0012] Preferably, the porosity gradient decreases layer by layer; the porosity of the first porous metal layer closest to the metal substrate is 60% to 90%; and the porosity of the Nth porous metal layer furthest from the metal substrate is 20% to 50%.

[0013] Preferably, the average pore size of the porous metal layer varies in a gradient along the direction away from the metal substrate, and the pore size range is 50 nm to 100 μm.

[0014] More preferably, the average pore size of the porous metal layer near the metal substrate is 10 μm to 100 μm, and the average pore size of the porous metal layer away from the metal substrate is 50 nm to 500 nm.

[0015] Preferably, the material of the porous metal layer is the same as or different from the material of the metal substrate, and is independently selected from any one or more metallic elements or alloys selected from nickel, titanium, and copper.

[0016] Preferably, the metal substrate is selected from any one of nickel foam, nickel mesh, titanium foam, titanium mesh, copper foam, and copper mesh.

[0017] Preferably, the total thickness of the gradient porous electrode is 100μm to 1000μm, and the thickness of the single-layer porous metal layer is 10μm to 300μm.

[0018] The present invention also provides a method for preparing the above-mentioned gradient pore electrode, comprising the following steps: (1) Provide a metal substrate, wherein the metal substrate is selected from any one of a nickel substrate, a titanium substrate, and a copper substrate; (2) Prepare at least two metal slurries with different contents of pore-forming agents; (3) The metal slurry is coated onto the surface of the metal substrate in order of pore-forming agent content from high to low or from low to high by using a layer-by-layer coating method to form a multilayer wet film; (4) The multilayer wet film is dried; (5) The dried multilayer film is sintered to decompose and volatilize the pore-forming agent, forming a porous metal layer with a gradient distribution of porosity along the thickness direction. (6) Optionally, the sintered electrode is subjected to reduction treatment.

[0019] Preferably, the layer-by-layer coating method is selected from any one or a combination of scraping, spraying, casting, and screen printing.

[0020] Preferably, the pore-forming agent is selected from any one or more of ammonium bicarbonate, sodium chloride, polymethyl methacrylate microspheres, and starch; the amount of the pore-forming agent added is 5wt% to 50wt% of the solid content of the metal slurry.

[0021] Preferably, the sintering temperature is 400℃~1000℃, the sintering time is 1h~10h, and the sintering atmosphere is an inert gas or a reducing gas.

[0022] The present invention also provides a gas diffusion electrode comprising the above-described gradient pore electrode.

[0023] The present invention also provides a membrane electrode assembly comprising the above-described gradient pore electrode.

[0024] The present invention also provides the application of the above-described gradient pore electrode or the above-described preparation method in the electrochemical reduction reaction of carbon dioxide.

[0025] Compared with existing technologies, this technical solution has the following characteristics and beneficial effects: 1. Mass transfer and reaction rate matching. The gradient pore electrode of this invention, through a gradient design of porosity along the thickness direction, maintains a high porosity (60%–90%) near the metal substrate (gas diffusion side), which is beneficial for rapid CO2 diffusion and transport; and maintains a lower porosity (20%–50%) away from the metal substrate (catalytic reaction side), increasing the density of catalytic active sites and electronic conductivity. This design effectively solves the contradiction between mass transfer and reaction in traditional uniform pore electrodes.

[0026] 2. Effective suppression of hydrogen evolution side reaction. The gradient pore structure can regulate the local water concentration distribution inside the electrode. The low porosity layer near the catalytic reaction side can limit the excessive permeation of the electrolyte and maintain an appropriate water content, thereby effectively suppressing the hydrogen evolution side reaction (HER) and improving the Faraday efficiency of the target product.

[0027] 3. Mitigating flooding issues and improving stability. In the gradient pore structure, the macroporous layer near the metal substrate acts as a gas buffer, effectively preventing electrolyte infiltration and blockage of gas channels; the microporous layer away from the metal substrate maintains a stable three-phase reaction interface. This structural design significantly extends the stable operating life of the electrode.

[0028] 4. Wide range of metal substrate options and broad applicability. This invention is applicable to nickel, titanium, and copper substrates, including various forms such as nickel foam, nickel mesh, titanium foam, titanium mesh, copper foam, and copper mesh. Different metal substrates offer selective catalytic advantages for different CO2 reduction products—nickel-based substrates favor CO generation, copper-based substrates favor C2+ product generation, and titanium-based substrates exhibit good corrosion resistance and structural stability.

[0029] 5. Controllable gradient parameters and simple preparation process. This invention prepares gradient porosity electrodes by coating metal slurries with different pore-forming agent contents layer by layer and sintering. The process is simple and low-cost. Furthermore, the porosity gradient (including gradient direction, gradient amplitude, number of layers, etc.) can be precisely controlled by adjusting parameters such as the type and content of pore-forming agent in each layer of slurry, coating thickness, and sintering temperature.

[0030] 6. Excellent interlayer bonding. This invention employs a process of coating layer by layer followed by integral sintering. The layers are metallurgically bonded through sintering, and there are no obvious physical boundaries or structural defects at the interlayer interfaces, ensuring the integrity and stability of the electrode. Attached Figure Description

[0031] Figure 1The diagram shows a cross-sectional structure of the gradient porosity electrode of the present invention, wherein 1 is a metal substrate, 2 is a first porous metal layer (high porosity), 3 is a second porous metal layer (medium porosity), 4 is a third porous metal layer (low porosity), and 5 is an Nth porous metal layer (low porosity).

[0032] Figure 2 This is a curve showing the porosity distribution along the thickness direction of the gradient pore electrode of the present invention.

[0033] Figure 3 This is a comparison chart of the Faraday efficiency and stability of the products from the gradient pore electrode and the contrasting uniform pore electrode in Example 1. Detailed Implementation

[0034] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention are within the scope of protection of the present invention.

[0035] Those skilled in the art should understand that, in the disclosure of this invention, the terms "longitudinal," "lateral," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," and "outer," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, the above terms should not be construed as limiting this invention.

[0036] Example 1 (Nickel substrate - three-layer gradient porous nickel electrode): This embodiment provides a three-layer gradient porous nickel electrode based on nickel foam and its preparation method.

[0037] 1. Electrode Structure The gradient porosity electrode in this embodiment has a three-layer structure, using nickel foam (0.3 mm thick, 95% porosity) as the metal substrate, and the layers are arranged sequentially away from the substrate: First layer (gas diffusion layer): thickness 200μm, porosity 85%, average pore size 50μm; Second layer (transition layer): 150μm thickness, 60% porosity, and 5μm average pore size; The third layer (catalytic reaction layer): 100 μm thick, 45% porosity, and 2 μm average pore size.

[0038] The total thickness of the electrode (including the substrate) is approximately 0.75 mm.

[0039] 2. Preparation method: (1) Provide a metal substrate: Select a nickel foam substrate (1.0 mm thick), and clean it with acetone, ethanol and deionized water for 10 min each, and then dry it with nitrogen for later use.

[0040] (2) Preparation of slurry: Slurry A (for the first layer): Nickel powder (average particle size 10 μm) is mixed with ammonium bicarbonate pore-forming agent (added at 40 wt% of the nickel powder mass), binder (polyvinyl butyral) and solvent (terpineol) are added, and ball milling is performed for 12 h to obtain a uniform slurry; Slurry B (for the second layer): Nickel powder (average particle size 5μm) is mixed with ammonium bicarbonate pore-forming agent (added at 25wt% of the nickel powder mass), binder and solvent are added, and ball milling is performed for 12 hours to obtain a uniform slurry; Slurry C (for the third layer): Nickel powder (average particle size 2μm) is mixed with ammonium bicarbonate pore-forming agent (added at 15wt% of the nickel powder mass), binder and solvent are added, and ball milling is performed for 12 hours to obtain a uniform slurry.

[0041] (3) Layer-by-layer coating: The coating method is adopted, and the slurry A, slurry B and slurry C are coated on the surface of the nickel foam substrate in sequence. After each coating, the coating is dried at 60°C for 30 minutes to form a three-layer wet film.

[0042] (4) Sintering: The dried multilayer film is placed in a tube furnace and heated to 900°C at a heating rate of 5°C / min under argon protection. The temperature is held for 3 hours to allow the pore-forming agent to completely decompose and volatilize. The film is then cooled to room temperature with the furnace to obtain a three-layer gradient porous nickel electrode with nickel foam as the substrate.

[0043] (5) Reduction treatment: The sintered electrode was reduced at 400°C for 2 hours in a mixed atmosphere of hydrogen / argon (H2 volume fraction 5%) to remove surface oxides.

[0044] 3. Performance Testing The gradient pore nickel electrode prepared in this embodiment was used as the cathode, and CO2 electrochemical reduction tests were conducted in a flow cell. The electrolyte was a 1M KHCO3 aqueous solution, and the catalyst was commercially available nano-bismuth oxide, loaded using an immersion spraying technique. The test temperature was 25°C, and the carbon dioxide gas pressure was 3 kPa.

[0045] Test results show that: At a potential of -1.0 V (vs. RHE), formic acid achieves a Faradaic efficiency of over 95%; at 200 mA / cm², 2 After running continuously for 100 hours at a current density, the Faraday efficiency of formic acid remained above 90%.

[0046] Example 2 (Titanium substrate - two-layer gradient porous titanium / copper composite electrode): This embodiment provides a two-layer gradient porous titanium / copper composite electrode with titanium mesh as the substrate and its preparation method.

[0047] 1. Electrode structure: The gradient porosity electrode in this embodiment has a two-layer structure, with a titanium mesh (0.5 mm thick) as the metal substrate: First layer (gas diffusion layer): 250μm thick, 80% porosity, 30μm average pore size, made of titanium; The second layer (catalytic reaction layer) has a thickness of 150 μm, a porosity of 40%, an average pore size of 150 nm, and is made of copper.

[0048] The total thickness of the electrode (including the substrate) is approximately 0.9 mm.

[0049] 2. Preparation method: (1) Provide a metal substrate: Select a titanium mesh substrate (thickness 0.5 mm), and clean it with acetone, ethanol and deionized water for 10 min each in sequence, and then blow it dry with nitrogen for later use.

[0050] (2) Preparation of slurry: Slurry A: Mix titanium powder (average particle size 10μm) with PMMA microsphere pore-forming agent (addition amount is 35wt% of titanium powder mass, microsphere diameter 30μm), add binder and solvent, and stir evenly; Slurry B: Mix copper powder (average particle size 100nm) with PMMA microsphere pore-forming agent (addition amount is 15wt% of copper powder mass, microsphere diameter 150nm), add binder and solvent, and stir evenly.

[0051] (3) Layer-by-layer coating: Using the casting method, slurry A and slurry B are sequentially cast onto the surface of the foamed titanium substrate to form a double-layer wet film, which is then dried at 80°C for 1 hour.

[0052] (4) Sintering: The dried double-layer film is placed in a tube furnace and heated to 700°C at a heating rate of 3°C / min under argon protection. It is held for 2 hours to allow the PMMA microspheres to completely decompose and volatilize. Then, it is heated to 900°C and held for 1 hour for densification sintering. The furnace is cooled to obtain a two-layer gradient porous titanium / copper composite electrode with foamed titanium as the substrate.

[0053] 3. Application The gradient-pore titanium / copper composite electrode prepared in this embodiment was used in the electrochemical reduction reaction of CO2. The copper-based catalyst layer facilitated the formation of C2+ products, while the titanium substrate provided good structural stability and corrosion resistance. Copper nanosheets were used as a catalyst for testing, and the results showed that at 60 mA / cm², [the desired effect was achieved]. -2At current density, the Faraday efficiency of ethylene reaches about 75%.

[0054] Example 3 (Nickel substrate - four-layer gradient porous titanium / copper composite electrode): The preparation process is based on Example 1, with the addition of slurry D (for the fourth layer): nickel powder (average particle size 500 nm) is mixed with ammonium bicarbonate pore-forming agent (added at 10 wt% of the nickel powder mass), binder and solvent are added, and ball milling is performed for 12 hours to obtain a uniform slurry. Slurry D is applied after slurry C is applied; the fourth layer has a thickness of 100 μm, a porosity of 20%, and an average pore size of 1 μm.

[0055] Test results show that: At a potential of -1.0V (vs. RHE), formic acid achieves a Faraday efficiency of over 95%. After continuous operation at a current density of 200 mA / cm² for 100 h, the Faradaic efficiency of formic acid remained above 90%. When the current density was increased to 500 mA / cm² and operation continued for another 50 h, the Faradaic efficiency of formic acid remained above 90%.

[0056] To verify the technical effects of the present invention, the following comparative examples are provided: Comparative Example 1: Uniform Porous Nickel Electrode (Nickel Foam Substrate) Using the same nickel powder and preparation process as in Example 1, the difference lies in the same pore-forming agent content (25 wt%) in all three layers of slurry, resulting in a three-layer nickel electrode with uniform porosity (approximately 60%). Tests were conducted under the same conditions as in Example 1. The results showed that after continuous operation at a current density of 200 mA / cm² for 80 hours, the Faradaic efficiency of formic acid remained above 90%, but dropped sharply to around 40% in the following 20 hours.

[0057] Comparative Example 2: Commercial Gas Diffusion Electrode (Carbon Substrate) The test was conducted under the same test conditions as in Example 1. The test results showed that after running continuously for 70 hours at a current density of 200 mA / cm², the Faradaic efficiency of formic acid remained above 90%, but dropped sharply to about 40% in the following 30 hours.

[0058] Comparative Example 3: Inverse Gradient Porous Nickel Electrode (Nickel Foam Substrate): The same nickel powder and preparation process as in Example 1 were used, except that the three-layer coating order was modified: slurry C was applied as the first layer, slurry B as the second layer, and slurry A as the third layer. Tests were conducted under the same conditions as in Example 1. The results showed that after 20 hours of continuous operation at a current density of 200 mA / cm², the Faradaic efficiency of formic acid remained above 90%. However, after the following 80 hours, the Faradaic efficiency plummeted to around 40%, and electrolyte appeared in the gas chamber. The electrode three-phase interface with reverse gradient pores is easily damaged. Once the first layer with low porosity is wetted, liquid is more likely to leak from the gas chamber, compromising electrolytic performance.

[0059] Those skilled in the art should understand that the technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments have been described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0060] The above embodiments are merely illustrative of several implementation methods of this application, and their descriptions are relatively specific and detailed, but they should not be construed as limiting the scope of this application. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of this application, and these all fall within the protection scope of this application. Therefore, the protection scope of this application should be determined by the appended claims.

Claims

1. A gradient pore electrode for the electrochemical reduction of carbon dioxide, characterized in that, include: The metal substrate is selected from any one of nickel substrate, titanium substrate, and copper substrate; and at least two porous metal layers attached to the surface of the metal substrate; The at least two porous metal layers are stacked sequentially in a direction away from the metal substrate, the porosity difference between adjacent layers is ≥5%, and the porosity of the porous metal layers varies in a gradient in a direction away from the metal substrate.

2. The gradient pore electrode for the electrochemical reduction of carbon dioxide according to claim 1, characterized in that, The porosity gradient decreases layer by layer; the porosity of the first porous metal layer closest to the metal substrate is 60% to 90%; and the porosity of the Nth porous metal layer furthest from the metal substrate is 20% to 50%.

3. The gradient pore electrode for electrochemical reduction of carbon dioxide according to claim 1, characterized in that, The average pore size of the porous metal layer varies in a gradient along the direction away from the metal substrate, and the pore size ranges from 50 nm to 100 μm.

4. The gradient pore electrode for electrochemical reduction of carbon dioxide according to claim 3, characterized in that, The average pore size of the porous metal layer near the metal substrate is 10 μm to 100 μm, and the average pore size of the porous metal layer away from the metal substrate is 50 nm to 500 nm.

5. The gradient pore electrode for electrochemical reduction of carbon dioxide according to claim 1, characterized in that, The material of the porous metal layer may be the same as or different from the material of the metal substrate, and is independently selected from any one or more metallic elements or alloys of nickel, titanium, and copper.

6. The gradient pore electrode for the electrochemical reduction of carbon dioxide according to claim 1, characterized in that, The metal substrate is selected from any one of the following: nickel foam, nickel mesh, nickel foil, nickel plate, titanium foam, titanium mesh, titanium plate, copper foam, copper mesh, copper foil, and copper plate.

7. The gradient pore electrode for electrochemical reduction of carbon dioxide according to claim 1, characterized in that, The total thickness of the gradient porous electrode is 100μm to 1000μm, and the thickness of the single-layer porous metal layer is 10μm to 300μm.

8. A method for preparing a gradient pore electrode for electrochemical reduction of carbon dioxide as described in any one of claims 1-7, characterized in that, Includes the following steps: 1) Provide a metal substrate, wherein the metal substrate is selected from any one of a nickel substrate, a titanium substrate, and a copper substrate; 2) Prepare at least two metal slurries with different contents of pore-forming agents; 3) The metal slurry is applied to the surface of the metal substrate in a layer-by-layer coating manner, in order of pore-forming agent content from high to low or from low to high, to form a multi-layer wet film; 4) The multilayer wet film is dried; 5) The dried multilayer film is sintered to decompose and volatilize the pore-forming agent, forming a porous metal layer with a gradient porosity along the thickness direction.

9. The method for preparing a gradient pore electrode for the electrochemical reduction of carbon dioxide according to claim 8, characterized in that, Including step 6), the sintered electrode is subjected to reduction treatment.

10. An application of the gradient pore electrode for the electrochemical reduction of carbon dioxide according to any one of claims 1-7, characterized in that, It is used in gas diffusion electrodes or membrane electrode assemblies.

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