A gradient microporous layer gas diffusion layer for carbon dioxide reduction and a method of manufacturing the same

Abstract

The present application relates to a kind of gradient microporous layer gas diffusion layer for carbon dioxide reduction and its preparation method.The gas diffusion layer includes substrate layer and gradient microporous layer;The gradient microporous layer includes multiple microporous sublayers with different hydrophilic-hydrophobic along its thickness direction;Along the direction of away from substrate layer, the hydrophobicity of each microporous sublayer shows gradient decreasing trend.The gradient microporous layer gas diffusion layer presented in the present application is suitable for carbon dioxide reduction technical field, and the microporous layer with the gradient change of hydrophobicity is constructed using microporous sublayer with different hydrophobic agent content, and the small hole smaller than 0.1 μm is formed using the mutual filling of carbon material between different microporous sublayer, and the gradient microporous layer gas diffusion layer is simple and repeatable in preparation operation, and has very high application value.

Description

Technical Field

[0001] This invention relates to a gradient microporous gas diffusion layer for carbon dioxide reduction and its preparation method, and more particularly to a multilayer microporous gas diffusion layer with a hydrophobic gradient and its preparation method. Background Technology

[0002] Carbon dioxide reduction technology driven by renewable electricity opens up the possibility of converting carbon dioxide, a major component of greenhouse gases, into value-added chemical feedstocks or fuels. Furthermore, using carbon dioxide as a feedstock for chemical production can reduce dependence on fossil fuel resources, offering both environmental and economic benefits.

[0003] Compared to planar metal electrodes immersed in liquid electrolytes, the diffusion path of carbon dioxide through the gas diffusion layer to the catalyst surface (~50 nm) is approximately three orders of magnitude smaller than the diffusion path through the bulk electrolyte (~50 μm). This improved carbon dioxide transport enables electrolyzers with gas diffusion layers to achieve higher current densities, making it more advantageous to scale up carbon dioxide reduction to industrial-scale applications.

[0004] The gas diffusion electrode (GDE) consists of a substrate layer (CFS), a microporous layer (MPL), and a catalyst layer (CL). The gas diffusion layer is a complex porous structure that is electrically conductive, gas-conducting, thermally conductive, and water-conducting. More specifically, the microporous layer reduces the contact resistance between the substrate layer and the catalyst layer by forming a flat and robust filler layer, and is crucial for maintaining effective transport of the liquid and gas phases. An ideal microporous layer requires a suitable pore size distribution and hydrophilicity / hydrophobicity to prevent liquid electrolytes from permeating into the gas diffusion layer and blocking gas transport channels, thereby enhancing the effectiveness of carbon dioxide transport to the active catalyst sites and improving carbon dioxide reduction performance.

[0005] CN114481184A discloses a gas diffusion layer for the electrochemical reduction of carbon dioxide and its preparation method. A general hydrophobic agent regulates the base layer and microporous layer, forming a suitable hydrophobicity, porosity and pore size distribution in the gas diffusion layer, which promotes the mass transfer of carbon dioxide and exhibits a high operating current density.

[0006] CN113308707A discloses a gas diffusion electrode for the electrochemical reduction of carbon dioxide to produce hydrocarbon fuels. The substrate is hydrophobically treated, and the electrode's conductivity is enhanced by a spray gun coating method to prevent electrolyte overflow, thus forming an ideal gas diffusion electrode. liquid Solid-phase three-phase interfaces are used for carbon dioxide reduction reactions.

[0007] In existing carbon dioxide reduction technologies, most employ indiscriminate gas diffusion layer structures for water / gas management, and no reports have been found regarding gradient diffusion electrodes. Therefore, we constructed a gradient microporous gas diffusion layer. Two or three microporous sublayers with progressively decreasing hydrophobic agent concentrations are formed from the side adjacent to the substrate layer towards the side adjacent to the catalyst layer. This design provides the catalyst layer with a certain degree of moisture retention and allows for the timely removal of excess electrolyte, ensuring sufficient transport channels are not blocked by electrolytes for effective carbon dioxide transport. This invention provides a novel gradient microporous gas diffusion layer structure that can improve operating current density and water management capabilities, which is of significant importance. Summary of the Invention

[0008] This invention relates to a gradient microporous gas diffusion layer for carbon dioxide reduction and its preparation method. The invention prepares a gradient microporous layer with a decreasing concentration of hydrophobic agent from the side adjacent to the substrate layer to the side adjacent to the catalyst layer. This gradient microporous layer exhibits good water / gas management capabilities and can significantly improve the performance of the carbon dioxide reduction gas diffusion electrode.

[0009] The objective of this invention is achieved through the following technical solution:

[0010] The present invention provides a gradient microporous gas diffusion layer for carbon dioxide reduction, wherein the gradient microporous gas diffusion layer is composed of a base layer and a gradient microporous layer; the gradient microporous layer includes a plurality of microporous sublayers with different hydrophilicity and hydrophobicity along its thickness direction; the hydrophobicity of each microporous sublayer decreases in a gradient trend along the direction away from the base layer.

[0011] In one embodiment of the present invention, the gradient microporous gas diffusion layer has a hydrophobic gradient, with the hydrophobicity gradually decreasing from the substrate towards the catalyst layer. The capillary pressure difference created by this hydrophobic gradient drives liquid water to drain from the catalyst layer to the substrate layer, preventing the catalytic sites from being covered by the electrolyte. Simultaneously, small pores smaller than 0.1 μm are formed in the gas diffusion electrode, and the pore volume of the hydrophobic pores accounts for more than 95% of the total pore volume. Since the critical pressure for liquid water to enter larger pores is lower than that for smaller pores, liquid water will preferentially penetrate the larger hydrophobic pores. Therefore, the formed hydrophobic pores (less than 0.1 μm) are beneficial for maintaining the free transport of carbon dioxide.

[0012] Each of the microporous sublayers is composed of a conductive material and a hydrophobic agent.

[0013] As one embodiment of the present invention, the base layer is one of carbon cloth, carbon paper, nickel foam, titanium foam, and titanium fiber sintered felt.

[0014] As one embodiment of the present invention, the hydrophobic agent is a powder, solution or emulsion composed of one or more of polytetrafluoroethylene, vinylidene fluoroethylene, polyvinylidene fluoroethylene, polyvinylidene fluoroethylene, fluorinated ethylene propylene, and ethylene / tetrafluoroethylene copolymer in any proportion.

[0015] As one embodiment of the present invention, the conductive material is one or more of conductive carbon black, carbon powder, carbon fiber, carbon nanotube, carbon nanofiber, graphene, nano-graphite powder, etc., mixed in any proportion to form a mixture.

[0016] The number of microporous layers is two or three.

[0017] The present invention also provides a method for preparing the aforementioned gas diffusion layer, comprising the following steps:

[0018] S1. Immerse the substrate layer in a hydrophobic emulsion and dry it to constant weight to obtain the substrate layer;

[0019] S2. Mix emulsions containing different amounts of conductive materials, hydrophobic agents, and dispersants, and stir thoroughly to obtain emulsion I, emulsion II, and emulsion III;

[0020] S3. Deposit at least two of the emulsions I, II, and III described in S2 onto one side of the substrate layer obtained in S1 in the order of emulsion I, II, and III. After each slurry is deposited, dry it to constant weight at 50-80°C to obtain a two- or three-layer gradient microporous gas diffusion layer precursor.

[0021] S4. Heat-treat the gradient microporous gas diffusion layer precursor described in S3 to obtain the gradient microporous gas diffusion layer.

[0022] In one embodiment of the present invention, in step S2, the mass ratio of conductive material to hydrophobic agent in emulsion I is 1:0.66 to 1:1.5; the mass ratio of conductive material to hydrophobic agent in emulsion II is 1:0.12 to 1:0.66; and the mass ratio of conductive material to hydrophobic agent in emulsion III is 1:0.05 to 1:0.12. An excessively high proportion of hydrophobic agent in emulsion I will reduce the conductivity of the electrode, while an excessively low proportion will make the electrode easily submerged in liquid water, hindering gas transport.

[0023] As one embodiment of the present invention, when preparing a bilayer gradient microporous gas diffusion layer, any two of emulsions I, II, and III have the same amount of conductive agent loaded on the substrate layer; when preparing a trilayer gradient microporous gas diffusion layer, emulsions I, II, and III have the same amount of conductive agent loaded on the substrate layer.

[0024] In one embodiment of the present invention, the total loading of conductive material in the gradient microporous layer of the gas diffusion layer is 0.03~5 mg cm⁻¹.-2 .

[0025] As one embodiment of the present invention, the dispersant is one or more of anhydrous ethanol, isopropanol, and ultrapure water mixed in any proportion.

[0026] As one embodiment of the present invention, the deposition method is selected from any one of screen printing, air spraying, electrostatic spraying, ultrasonic spraying, dip coating, blade coating, and roll forming.

[0027] As one embodiment of the present invention, the heat treatment is as follows: heating from room temperature to 245°C for 30-60 minutes; holding at 245°C for 30-60 minutes; heating from 245°C to 345°C for 30-60 minutes; holding at 345°C for 30-60 minutes; and cooling to room temperature for 60-120 minutes.

[0028] Compared with the prior art, the present invention has the following beneficial effects:

[0029] (1) This invention proposes a gradient microporous gas diffusion layer for carbon dioxide reduction and its preparation method. By controlling the content of hydrophobic agent, the hydrophobicity of the microporous layer is gradient-distributed in the thickness direction of the gas diffusion layer, thereby constructing a transitional microporous layer with a hydrophobic gradient between the catalyst layer and the substrate layer. The capillary pressure difference formed by the hydrophobic gradient drives liquid water to drain from the catalyst layer to the substrate layer, preventing the catalytic sites from being covered by electrolyte and improving the water management capability of carbon dioxide reduction.

[0030] (2) The present invention proposes a gradient microporous gas diffusion layer for carbon dioxide reduction and its preparation method. Compared with the prior art, it can form more pores, especially including a certain number of hydrophobic micropores with a pore size of less than 0.1 μm, thereby ensuring free transport of gas under high current density and improving the gas transport capacity for carbon dioxide reduction.

[0031] (3) The present invention proposes a gradient microporous gas diffusion layer for carbon dioxide reduction. Its preparation method is simple, highly repeatable, and has low raw material cost, which is conducive to large-scale production. Attached Figure Description

[0032] Other features, objects, and advantages of the present invention will become more apparent from the following detailed description of non-limiting embodiments with reference to the accompanying drawings:

[0033] Figure 1 A schematic diagram of the three-layer gradient microporous gas diffusion layer used for carbon dioxide reduction in Example 2;

[0034] Figure 2Pore ​​size distribution diagrams of the gradient microporous gas diffusion layer for carbon dioxide reduction obtained in Comparative Example 1, Example 1, and Example 2;

[0035] Figure 3 The gradient microporous gas diffusion layer obtained in Comparative Example 1 and Example 1 was used in a carbon dioxide reduction electrolysis cell to convert carbon dioxide into carbon monoxide at different voltages.

[0036] Figure 4 The gradient microporous gas diffusion layer obtained in Comparative Example 1 and Example 2 was used in a carbon dioxide reduction electrolysis cell to convert carbon dioxide into carbon monoxide at different voltages.

[0037] Figure 5 The gradient microporous gas diffusion layer obtained in Comparative Example 2 and Example 1 was used in a carbon dioxide reduction electrolysis cell to convert carbon dioxide into carbon monoxide at different voltages. Detailed Implementation

[0038] The present invention will now be described in detail with reference to specific embodiments. These embodiments will help those skilled in the art to further understand the present invention, but do not limit the invention in any way. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention. These all fall within the scope of protection of the present invention.

[0039] The following embodiment provides a gradient microporous layer gas diffusion layer for carbon dioxide reduction, comprising a hydrophobic agent-treated substrate layer and a gradient microporous layer, wherein the gradient microporous layer includes multiple microporous sublayers with different hydrophilicity and hydrophobicity; the hydrophobicity of each microporous sublayer decreases in a gradient trend along the direction away from the substrate layer.

[0040] The microporous layer comprises a conductive material and a hydrophobic agent.

[0041] The base layer is one of carbon cloth, carbon paper, nickel foam, titanium foam, or titanium fiber sintered felt.

[0042] The hydrophobic agent is a powder, solution, or emulsion composed of one or more of polytetrafluoroethylene, vinylidene fluoroethylene, polyvinylidene fluoroethylene, polyvinylidene fluoroethylene, fluorinated ethylene propylene, ethylene / tetrafluoroethylene copolymer, etc., mixed in any proportion.

[0043] The conductive material is one or more of conductive carbon black, carbon powder, carbon fiber, carbon nanotubes, carbon nanofibers, graphene, nano-graphite powder, etc., mixed in any proportion.

[0044] The method for preparing a gradient microporous gas diffusion layer includes the following steps:

[0045] 1) The substrate layer is immersed in a hydrophobic emulsion and dried to constant weight to obtain the substrate layer;

[0046] 2) Mix emulsions containing different amounts of conductive materials, hydrophobic agents, and dispersants, and stir thoroughly to obtain microporous layer mixed emulsions I, II, and III; the mass ratio of conductive material to hydrophobic agent in emulsion I is 1:0.66 to 1:1.5; the mass ratio of conductive material to hydrophobic agent in emulsion II is 1:0.12 to 1:0.66; and the mass ratio of conductive material to hydrophobic agent in emulsion III is 1:0.05 to 1:0.12.

[0047] 3) Deposit at least two of the emulsions I, II, and III described in 2) sequentially onto one side of the substrate layer obtained in 1) in the order of emulsion I, II, and III; after each deposition of a slurry, dry it to constant weight at 50-80℃ to obtain a precursor for a bilayer or trilayer gradient microporous gas diffusion layer; the deposition method is selected from any one of screen printing, air spraying, electrostatic spraying, ultrasonic spraying, dip coating, blade coating, and roll forming; when preparing a bilayer gradient microporous gas diffusion layer, the conductive agent loading of any two of the emulsions I, II, and III deposited on the substrate layer is the same; when preparing a trilayer gradient microporous gas diffusion layer, the conductive agent loading of the emulsions I, II, and III deposited on the substrate layer is the same;

[0048] 4) Heat-treat the gradient diffusion electrode layer precursor described in step 3) to obtain a gradient microporous gas diffusion layer; the high-temperature sintering process is as follows: heat from room temperature to 245°C for 30-60 min; hold at 245°C for 30-60 min; heat from 245°C to 345°C for 30-60 min; hold at 345°C for 30-60 min; cool to room temperature for 60-120 min.

[0049] The conductive material loading in the microporous layer of the gas diffusion layer is 0.03~5 mg cm⁻¹. -2 .

[0050] The carbon dioxide reduction gas diffusion layer described in this invention can be prepared under all the above parameter conditions.

[0051] Example 1

[0052] This embodiment relates to a gas diffusion layer for carbon dioxide electrolytic reduction and its preparation method, the steps of which are as follows:

[0053] 1) Immerse the W0S1009 carbon cloth produced by CeTech Co., Ltd. of Taiwan in a 5 wt% polytetrafluoroethylene emulsion for 30 seconds, then remove it and vacuum dry it in a drying oven at 60°C until a constant weight is achieved. Repeat the above steps until the weight difference between the carbon cloth before immersion and after drying accounts for 30 wt% of the total weight of the carbon cloth after immersion and drying.

[0054] 2) Mix 0.25g of Vulcan XC-72 conductive carbon black, 3.33g of 5 wt% polytetrafluoroethylene emulsion, and 2.5g of ethanol, and stir thoroughly to obtain emulsion I; mix 0.25g of Vulcan XC-72 conductive carbon black, 0.56g of 5 wt% polytetrafluoroethylene emulsion, and 2.5g of anhydrous ethanol, and stir thoroughly to obtain emulsion III.

[0055] 3) Screen print the emulsion I obtained in step 2) onto the substrate layer obtained in step 1) to form a microporous layer 1, and dry it at 60°C for 30 min. Screen print the emulsion III obtained in step 2) onto the microporous layer 1 to form a microporous layer 2, and dry it at 60°C for 30 min. After 30 min, the temperature is increased from 60°C to 245°C, and held at 245°C for 30 min. After another 30 min, the temperature is increased to 345°C and held for 30 min. Finally, the temperature is cooled to room temperature.

[0056] 4) The total loading of conductive carbon black in the microporous layer of the gas diffusion layer obtained in step 3) is 0.7 mg / cm³. -2 The total thickness of the gas diffusion layer is 0.33 mm.

[0057] Example 2

[0058] A gradient microporous gas diffusion layer for carbon dioxide electrolytic reduction and its preparation method are as follows:

[0059] 1) Immerse the W0S1009 carbon cloth produced by CeTech Co., Ltd. of Taiwan in a 5 wt% polytetrafluoroethylene emulsion for 30 seconds, then remove and vacuum dry in a drying oven at 60°C until a constant weight is achieved. Repeat the above steps until the weight difference between the carbon cloth before immersion and after drying accounts for 30 wt% of the total weight of the carbon cloth after immersion and drying.

[0060] 2) Mix 0.17g Vulcan XC-72 conductive carbon black, 2.22g 5 wt% polytetrafluoroethylene emulsion, and 1.67g anhydrous ethanol, and stir thoroughly to obtain emulsion I; mix 0.17g Vulcan XC-72 conductive carbon black, 1.14g 5 wt% polytetrafluoroethylene emulsion, and 1.67g anhydrous ethanol, and stir thoroughly to obtain emulsion II; mix 0.17g Vulcan XC-72 conductive carbon black, 0.37g 5 wt% polytetrafluoroethylene emulsion, and 1.67g anhydrous ethanol, and stir thoroughly to obtain emulsion III.

[0061] 3) such as Figure 1 As shown, the emulsion I obtained in step 2) is screen-printed onto the substrate layer obtained in step 1) to form a microporous layer 1, and dried at 60°C for 30 min. The emulsion II obtained in step 2) is screen-printed onto the microporous layer 1 to form a microporous layer 2, and dried at 60°C for 30 min. The emulsion III obtained in step 2) is screen-printed onto the microporous layer 2 to form a microporous layer 3, and dried at 60°C for 30 min. After 30 min, the temperature is increased from 60°C to 245°C, held at 245°C for 30 min, then increased to 345°C for another 30 min and held for 30 min. Finally, the temperature is cooled to room temperature.

[0062] 4) The total loading of conductive carbon black in the microporous layer of the gas diffusion layer obtained in step 3) is 0.7 mg / cm³. -2 The total thickness of the gas diffusion layer is 0.33 mm.

[0063] Comparative Example 1

[0064] W1S1010, a conventional commercial carbon cloth with a microporous layer produced by CeTech Co., Ltd. of Taiwan, was used as a comparative example 1.

[0065] Comparative Example 2

[0066] Comparative Example 2 uses a carbon dioxide electrolysis reduction gradient microporous layer gas diffusion layer with a conductive material to hydrophobic agent mass ratio exceeding that of this technical solution. The steps are as follows:

[0067] 1) Immerse the W0S1009 carbon cloth produced by CeTech Co., Ltd. of Taiwan in a 5 wt% polytetrafluoroethylene emulsion for 30 seconds, then remove it and vacuum dry it in a drying oven at 60°C until a constant weight is achieved. Repeat the above steps until the weight difference between the carbon cloth before immersion and after drying accounts for 30 wt% of the total weight of the carbon cloth after immersion and drying.

[0068] 2) Mix 0.25g Vulcan XC-72 conductive carbon black, 9.3g 5 wt% polytetrafluoroethylene emulsion, and 2.5g ethanol, and stir thoroughly to obtain emulsion I (the mass ratio of conductive material to hydrophobic agent in emulsion I is 1:1.86); mix 0.25g Vulcan XC-72 conductive carbon black, 0.56g 5 wt% polytetrafluoroethylene emulsion, and 2.5g anhydrous ethanol, and stir thoroughly to obtain emulsion III.

[0069] 3) Screen print the emulsion I obtained in step 2) onto the substrate layer obtained in step 1) to form a microporous layer 1, and dry it at 60°C for 30 min. Screen print the emulsion III obtained in step 2) onto the microporous layer 1 to form a microporous layer 2, and dry it at 60°C for 30 min. After 30 min, the temperature is increased from 60°C to 245°C, and held at 245°C for 30 min. After another 30 min, the temperature is increased to 345°C and held for 30 min. Finally, the temperature is cooled to room temperature.

[0070] 4) The result of step 3) is as follows Figure 1 The total loading of conductive carbon black in the microporous layer of the gradient microporous gas diffusion layer shown is 0.7 mg cm⁻¹. -2 The total thickness of the gas diffusion layer is 0.33 mm.

[0071] Effect test

[0072] 1. Volume ratio and pore size distribution of hydrophobic pores

[0073] The hydrophobic pore volume ratio and pore size distribution of Examples 1, 2, and Comparative Example 1 are shown in Table 2. Figure 2 As shown, the gradient gas diffusion electrode can significantly increase the proportion of hydrophobic pores and significantly increase the number of pores smaller than 0.1 μm.

[0074]

[0075] 2. Electrochemical performance testing

[0076] In a zero-gap electrolytic cell, dry carbon dioxide at 80 sccm was introduced into the cathode, and ultrapure water at 60 °C was introduced into the anode. A Sustainance X37-50 RT membrane was used as the diaphragm. Cobalt phthalocyanine slurry was sprayed onto the gas diffusion layer of Comparative Example 1, as well as the gas diffusion layers prepared in Comparative Example 2, Examples 1, and Examples 2, to form the cathode. Iridium oxide slurry was sprayed onto a nickel mesh to form the anode. The desired result was obtained. Figure 3 , Figure 4 , Figure 5The electrochemical performance curves shown indicate that the gradient microporous gas diffusion layers prepared in Examples 1 and 2 have better gas transport and water management capabilities, and significantly improve the operating current density and conversion rate for the electrochemical reduction of carbon dioxide.

[0077] The above description of the embodiments is provided to enable those skilled in the art to understand and use the invention. It will be apparent to those skilled in the art that various modifications can be made to these embodiments, and the general principles described herein can be applied to other embodiments without inventive effort. Therefore, the present invention is not limited to the above embodiments, and any improvements and modifications made by those skilled in the art based on the disclosure of the present invention without departing from the scope of the invention should be within the protection scope of the present invention.

Claims

1. A gradient microporous gas diffusion layer for carbon dioxide reduction, characterized in that, The gas diffusion layer includes a base layer and a gradient microporous layer; the gradient microporous layer includes multiple microporous sublayers with different hydrophilicity and hydrophobicity along its thickness direction; the hydrophobicity of each microporous sublayer decreases in a gradient trend along the direction away from the base layer. The gradient microporous gas diffusion layer for carbon dioxide reduction is prepared by a method comprising the following steps: S1. Immerse the substrate layer in a hydrophobic emulsion and dry it to constant weight to obtain the substrate layer; S2. Mix emulsions containing different amounts of conductive materials, hydrophobic agents, and dispersants, and stir thoroughly to obtain emulsion I, emulsion II, and emulsion III; S3. Deposit at least two of the emulsions I, II, and III described in S2 onto one side of the substrate layer obtained in S1 in the order of emulsion I, II, and III. After each slurry is deposited, dry it to constant weight at 50-80°C to obtain a two- or three-layer gradient microporous gas diffusion layer precursor. S4. Heat-treat the gradient microporous gas diffusion layer precursor described in S3 to obtain the gradient microporous gas diffusion layer. The mass ratio of conductive material to hydrophobic agent in emulsion I is 1:0.66 to 1:1.5; the mass ratio of conductive material to hydrophobic agent in emulsion II is 1:0.12 to 1:0.66; and the mass ratio of conductive material to hydrophobic agent in emulsion III is 1:0.05 to 1:0.

12.

2. The gradient microporous gas diffusion layer for carbon dioxide reduction according to claim 1, characterized in that, The microporous layer comprises a conductive material and a hydrophobic agent.

3. The gradient microporous gas diffusion layer for carbon dioxide reduction according to claim 1, characterized in that, The base layer is one of carbon cloth, carbon paper, nickel foam, titanium foam, or titanium fiber sintered felt.

4. The gradient microporous gas diffusion layer for carbon dioxide reduction according to claim 2, characterized in that, The conductive material is one or more of conductive carbon black, carbon powder, carbon fiber, carbon nanotubes, carbon nanofibers, graphene, and nano-graphite powder, mixed in any proportion.

5. The gradient microporous gas diffusion layer for carbon dioxide reduction according to claim 2, characterized in that, The hydrophobic agent is a powder, solution, or emulsion composed of one or more of polytetrafluoroethylene, vinylidene fluoroethylene, polyvinylidene fluoroethylene, polyvinylidene fluoroethylene, fluorinated ethylene propylene, and ethylene / tetrafluoroethylene copolymer in any proportion.

6. The gradient microporous layer of the gas diffusion layer according to claim 1, characterized in that, The number of microporous layers is two or three.

7. The gradient microporous gas diffusion layer for carbon dioxide reduction according to claim 1, characterized in that, When preparing a bilayer gradient microporous gas diffusion layer, the conductive agent loading of any two of emulsions I, II, and III deposited on the substrate is the same; when preparing a trilayer gradient microporous gas diffusion layer, the conductive agent loading of emulsions I, II, and III deposited on the substrate is the same.

8. The gradient microporous gas diffusion layer for carbon dioxide reduction according to claim 1, characterized in that, The heat treatment is as follows: heating from room temperature to 245°C for 30-60 minutes; holding at 245°C for 30-60 minutes; heating from 245°C to 345°C for 30-60 minutes; holding at 345°C for 30-60 minutes; and cooling to room temperature for 60-120 minutes.

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