A gas diffusion layer with gradient structure for fuel cell and preparation method thereof

Abstract

This invention relates to a gradient-structured gas diffusion layer for fuel cells and its preparation method. The gas diffusion layer is a composite layer of 2 to 4 layers, with the pore size of each layer exhibiting a gradient distribution, the average pore size gradually decreasing from bottom to top. The average pore size of the bottommost layer is 20–80 μm, and the average pore size of the topmost layer is 50–1500 nm. Each layer of the gas diffusion layer contains chopped carbon fibers, carbon microfibers, and resin carbon. The preparation method involves stacking 2 to 4 sheets of carbon fiber base paper with different pore sizes sequentially from bottom to top in descending order of pore size, hot-pressing them, and then sequentially performing impregnation curing, heat treatment, and hydrophobic treatment to obtain the gas diffusion layer. The method of this invention is simple to prepare, improves the matching between the gas diffusion layer and the catalyst layer of the fuel cell, and the resulting gradient-structured gas diffusion layer for fuel cells exhibits better water conductivity, better mechanical properties, and better electrical conductivity.

Description

Technical Field

[0001] This invention belongs to the field of fuel cell technology and relates to a gas diffusion layer with a gradient structure for fuel cells and its preparation method. Background Technology

[0002] The gas diffusion layer (GDL) is a crucial component of a proton exchange membrane fuel cell (PEMFC). Within the fuel cell, it supports the catalyst layer, collects current, and provides multiple transport channels for fuel gases, protons, and electrons. Therefore, the gas diffusion layer needs to exhibit functional characteristics such as high conductivity, high stability, high water management capability, and high permeability. In early research on gas diffusion layers, materials with macroporous matrices, such as carbon fiber paper, could be directly used as the gas diffusion layer.

[0003] Since the pore size of carbon fiber paper is generally on the micrometer scale, while the pore size of the catalyst layer is on the nanometer scale, the direct contact between the two affects the transport of electrons, gases, and water at the interface. To achieve a smooth transition and better matching between the pores of the gas diffusion layer and the catalyst layer, a microporous layer (MPL) was successfully introduced between the macroporous matrix (MPS, i.e., carbon fiber paper) and the catalyst layer (CL). Generally, the gas diffusion layer consists of two parts: a macroporous matrix and a microporous layer. The macroporous matrix is ​​made of carbon fiber paper, while the microporous layer is made by mixing nano-carbon particles with polytetrafluoroethylene binder to form a slurry, which is then coated onto the surface of the carbon fiber paper by spraying or scraping.

[0004] Chinese Patent Application No. 200610047931.2 discloses a method for preparing a gas diffusion layer for fuel cells. This gas diffusion layer consists of a macroporous matrix and a microporous layer. The microporous layer slurry is made by mixing a hydrophobic agent and a conductive carbon material and is coated on one side of the macroporous matrix to obtain the gas diffusion layer. The assembled fuel cell exhibits good output performance. Although good battery performance is achieved, the preparation process of the gas diffusion layer is relatively complex, requiring the preparation of the macroporous matrix material first, followed by the coating of the microporous layer. The entire preparation process is cumbersome and involves many controllable factors. Therefore, developing a gas diffusion layer that omits the microporous layer coating step in the preparation process can not only simplify the preparation process but also improve the stability of battery operation.

[0005] To better match the catalyst layer and achieve high battery efficiency, Chinese patent application 202211066247.4 discloses a method for preparing a microporous layer with a dual gradient of hydrophobicity and permeability. This method involves preparing two slurries containing different dosages of hydrophobic agents, and adding a pore-forming agent to the slurry with a higher hydrophobic agent content. This results in a hydrophobic gradient structure where the hydrophobicity of the carbon paper substrate, intermediate layer, and outer layer decreases sequentially. The pore size and porosity of the hydrophobic carbon paper substrate, intermediate layer, and outer layer also decrease sequentially. The microporous layer possesses a dual gradient of hydrophobicity and permeability, improving the water management and gas transport capabilities of the gas diffusion layer. However, despite endowing the microporous layer with both hydrophobic and permeable properties, improving the water conductivity of the gas diffusion layer, the pore size gradient between the microporous layer and the macroporous matrix is ​​too large. When water diffuses from the microporous layer into the macroporous matrix layer, it still accumulates there, causing electrode flooding and hindering mass transfer between fuel gas and water. Therefore, the water management capability of fuel cells needs further improvement.

[0006] To achieve higher output performance in fuel cells, the requirements for the thickness, pore structure control, permeability, normal resistance, and mechanical strength of the gas diffusion layer must be continuously increased. Controlling the thickness and pore structure of the gas diffusion layer during fabrication is extremely difficult. Improving permeability requires reducing the resin carbon content in the carbon fiber paper; however, reducing the resin carbon content leads to fewer bonding points between chopped carbon fibers, reducing the mechanical properties of the carbon fiber paper. Balancing permeability and mechanical strength is a challenging problem. Furthermore, currently, domestic fuel cell stack manufacturers primarily purchase gas diffusion layers from overseas manufacturers. Companies like Toray Industries of Japan and SGL of Germany have mature technologies for producing carbon fiber paper with or without microporous layers, holding a monopoly in the market, thus increasing the cost of domestic gas diffusion layer fabrication.

[0007] Therefore, it is of great significance to study a gas diffusion layer for fuel cells and its preparation method to solve the above problems. Summary of the Invention

[0008] The purpose of this invention is to solve the above-mentioned problems existing in the prior art and to provide a gas diffusion layer with a gradient structure for fuel cells and its preparation method. In view of the above problems, the main concept of this invention is as follows: (1) This invention constructs a gas diffusion layer with a gradient structure, which has the feature of integrated function and structure. Gas diffusion layers with different pore size gradient structures can optimize the water management capability of the battery, and can drain water quickly without affecting the entry of gas, thereby enhancing the performance and operational stability of the battery. (2) The constructed gradient structure is a multi-layer gradient structure, wherein the uppermost layer of the gradient structure can be added with nano-carbon particles as an intrinsic microporous layer. The pore size structure is adjusted by using nano-carbon particles to make the pore size structure conform to the characteristics of the microporous layer pore size structure, which can be well matched with the catalyst layer, and realize the integrated preparation of macroporous matrix and microporous layer, simplifying the preparation process, improving the uniformity of the gas diffusion layer, and reducing the preparation cost. (3) In the construction of this gradient structure, carbon microfibers are used to adjust the relationship between permeability and mechanical strength. The selection of materials generally chooses carbon microfiber precursors with high residual carbon content. Furthermore, during the preparation of the base paper, the carbon microfiber precursor plays a role in entanglement and bonding of chopped carbon fibers, resulting in better contact between the chopped carbon fibers and improving the strength of the carbon fiber base paper. After hot pressing, the carbon microfiber precursor structure is reorganized, allowing it to penetrate between layers and improving the normal bond strength. After heat treatment (carbonization and graphitization), the above structure is preserved through the morphology of the carbon microfiber, thereby improving the in-plane and normal conductivity of the gas diffusion layer. At the same time, it can reduce the amount of resin binder used in the impregnation process, reduce the resin carbon content in the final gas diffusion layer, and achieve improved air permeability without sacrificing mechanical strength. In addition, the carbon microfiber precursor also facilitates the dispersion of chopped carbon fibers during the slurry preparation process.

[0009] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0010] A gas diffusion layer with a gradient structure for a fuel cell is a composite layer of 2 to 4 layers, with the pore size of each layer distributed in a gradient structure.

[0011] The uppermost layer of the gas diffusion layer with a gradient structure used in the fuel cell is bonded to the catalyst layer in the fuel cell, and the lowermost layer is bonded to the bipolar plate of the fuel cell.

[0012] The gas diffusion layer with a gradient structure for the fuel cell has an average pore size of 20-80 μm and a roughness of ≤15 μm in the lowest layer; from bottom to top, the average pore size of each layer in the gas diffusion layer with a gradient structure for the fuel cell gradually decreases, with the average pore size of the uppermost layer being 50-1500 nm and a roughness of ≤7 μm.

[0013] Each layer of the gas diffusion layer with a gradient structure for the fuel cell contains chopped carbon fibers, carbon microfibers, and resin carbon. In each layer, carbon microfibers are wrapped around chopped carbon fibers, and resin carbon connects the carbon microfibers and chopped carbon fibers together. In the vertical direction, chopped carbon fibers also run through different layers and are wrapped by carbon microfibers and connected by resin carbon.

[0014] During wet vacuum deposition, most chopped carbon fibers stack in the planar direction, while a few chopped carbon fibers exhibit vertical penetration. Simultaneously, during hot pressing, the carbon microfiber precursor undergoes structural reorganization, which also influences the vertical arrangement of the chopped carbon fibers.

[0015] As a preferred technical solution:

[0016] As described above, a gas diffusion layer with a gradient structure for a fuel cell has a thickness of 80–280 μm, a porosity of 60–80%, and a permeability of 1600–2200 (mL·mm) / (cm). 2 The resistivity is 3.5–7.0 mΩ·cm, the normal resistivity is 40–80 mΩ·cm, and the tensile strength is 15–50 MPa. In this invention, the tensile strength, normal resistivity, in-plane resistivity and air permeability are all tested in accordance with Part VII of GB / T20042.7-2014, which describes the test methods for carbon paper characteristics.

[0017] As described above, a gas diffusion layer with a gradient structure for a fuel cell has chopped carbon fibers with a length of 3–10 mm and an average fiber diameter of 4–10 μm; and carbon microfibers with an average diameter of 10–500 nm.

[0018] As described above, the gas diffusion layer for a fuel cell with a gradient structure has carbon microfiber precursors that are one or more of plant pulp, chopped ultrafine organic fibers, and chemical fiber pulp.

[0019] Plant pulp includes cotton pulp, paper pulp, wood pulp, bamboo pulp, or grass pulp; chopped ultrafine organic fiber is ultrafine polyacrylonitrile fiber or ultrafine mesophase pitch-based fiber obtained by electrospinning; chemical fiber pulp includes para-aramid pulp or meta-aramid pulp, aramid sulfone pulp, or polyimide pulp.

[0020] As described above, a gas diffusion layer with a gradient structure for a fuel cell is a four-layer composite layer, consisting of a first layer of fiber sheets, a second layer of fiber sheets, a third layer of fiber sheets, and a fourth layer of intrinsic microporous layers, from bottom to top.

[0021] The average pore size of the first fiber sheet is 20–80 μm, the average pore size of the second fiber sheet is 10–46 μm, the average pore size of the third fiber sheet is 1–15 μm, and the average pore size of the fourth intrinsic microporous layer is 50–1500 nm.

[0022] The gas diffusion layer with a pore size gradient structure constructed in this invention is preferably a gradually changing gradient structure composed of four layers of carbon fiber paper with different pore sizes. Pore size gradient structures with three or fewer layers are disadvantaged compared to four-layer structures in that they cannot generate sufficient capillary pressure difference to smoothly expel water from the battery, and the support provided to the catalyst layer is weakened due to the absence of a single fiber layer. Five- or six-layer (i.e., more than four-layer) pore size gradient structures are disadvantaged compared to four-layer structures in that the overall gas diffusion layer becomes thicker, increasing the transport paths for protons, electrons, water, and gas, increasing mass transfer resistance, and affecting fuel cell performance; furthermore, they also increase manufacturing costs.

[0023] As described above, a gas diffusion layer with a gradient structure for a fuel cell, the fourth intrinsic microporous layer also contains nano-carbon particles.

[0024] The nano-carbon particles are one or more of carbon black, graphene, and carbon nanotubes;

[0025] The particle size of carbon black is 10–60 nm; the number of graphene layers is ≤10; the diameter of carbon nanotubes is 3–30 nm and the length is 1–50 nm.

[0026] The present invention also provides a method for preparing a gas diffusion layer with a gradient structure for fuel cells as described in any of the above claims, wherein 2 to 4 carbon fiber base papers with different pore sizes are stacked in order from bottom to top according to the pore size from large to small, hot-pressed and then impregnated, cured, heat-treated and hydrophobic treated in sequence to obtain a gas diffusion layer with a gradient structure for fuel cells.

[0027] Carbon fiber base paper is made by preparing pulp using chopped carbon fibers and carbon microfiber precursors as the main raw materials, and then producing it through wet papermaking.

[0028] As a preferred technical solution:

[0029] The method for preparing a gradient-structured gas diffusion layer for a fuel cell, as described above, comprises a four-layer composite layer, consisting of a first fibrous sheet layer, a second fibrous sheet layer, a third fibrous sheet layer, and a fourth intrinsically microporous layer, arranged sequentially from bottom to top. The first, second, and third fibrous sheets do not contain carbon nanoparticles, while the fourth intrinsically microporous layer may or may not contain carbon nanoparticles. The average pore size of the first fibrous sheet layer is 20–80 μm, the second fibrous sheet layer is 10–46 μm, the third fibrous sheet layer is 1–15 μm, and the fourth intrinsically microporous layer is 50–1500 nm. The specific steps for preparing the gradient-structured gas diffusion layer for a fuel cell are as follows:

[0030] (1) Preparation of carbon fiber base paper: Using short-cut carbon fiber and carbon microfiber precursor as the main raw materials, single-layer carbon fiber base paper with different pore sizes can be obtained by changing the mass ratio of the two and using wet papermaking process, namely the first layer of carbon fiber base paper, the second layer of carbon fiber base paper, the third layer of carbon fiber base paper and the fourth layer of carbon fiber base paper.

[0031] (2) Composite of four layers of carbon fiber base paper: The first layer of carbon fiber base paper, the second layer of carbon fiber base paper, the third layer of carbon fiber base paper and the fourth layer of carbon fiber base paper are stacked from bottom to top;

[0032] In the raw materials for preparing the first layer of carbon fiber base paper, the mass percentage of chopped carbon fiber is 80-95%, and the mass percentage of carbon microfiber precursor is 5-20%.

[0033] In the raw materials for preparing the second layer of carbon fiber base paper, the mass percentage of chopped carbon fiber is 55-75%, and the mass percentage of carbon microfiber precursor is 25-45%.

[0034] In the raw materials for preparing the third layer of carbon fiber base paper, the mass percentage of chopped carbon fiber is 30-50%, and the mass percentage of carbon microfiber precursor is 50-70%.

[0035] In the preparation of the fourth layer of carbon fiber base paper, based on the total mass of chopped carbon fibers and carbon microfiber precursors, the mass percentage of chopped carbon fibers is 10-30%, the mass percentage of carbon microfiber precursors is 70-90%, and the nano carbon particles account for less than 35% of the total oven-dry mass of the four-layer carbon fiber composite base paper.

[0036] (3) Preparation of precursor paper with pore size gradient structure: The four-layer carbon fiber base paper after composite in step (2) is subjected to hot pressing treatment (i.e., one hot pressing) to obtain precursor paper with pore size gradient structure;

[0037] (4) Preparation of gas diffusion layer with gradient structure for fuel cell: The precursor paper with pore size gradient structure obtained in step (3) is impregnated in binder solution and dried, and then hot-pressed (i.e., secondary hot pressing), heat treatment and hydrophobic treatment are performed to obtain gas diffusion layer for fuel cell.

[0038] The method for preparing a gas diffusion layer with a gradient structure for a fuel cell as described above, specifically step (1) is as follows:

[0039] (1.1) Short carbon fibers and carbon microfiber precursors are uniformly dispersed in polyethylene oxide solution with a concentration of 0.1-0.2 wt.% according to different proportions to obtain slurry for the first layer of carbon fiber base paper, slurry for the second layer of carbon fiber base paper, and slurry for the third layer of carbon fiber base paper, respectively; short carbon fibers, carbon microfiber precursors, and nano carbon particles are uniformly dispersed in polyethylene oxide solution with a concentration of 0.1-0.2 wt.% to obtain slurry for the fourth layer of carbon fiber base paper;

[0040] (1.2) The pulp obtained in step (1.1) is formed by wet papermaking machine to prepare the first layer of carbon fiber base paper, the second layer of carbon fiber base paper, the third layer of carbon fiber base paper and the fourth layer of carbon fiber base paper;

[0041] The pore size in each layer can be controlled by adjusting the ratio of chopped carbon fibers, carbon microfiber precursors, and nano-carbon particles, thus constructing a gas diffusion layer with a pore size gradient structure. Specifically, this invention provides a method for effectively controlling the ratio of chopped carbon fibers to carbon microfiber precursors to effectively adjust the pore size of the gas diffusion layer and construct a gas diffusion layer with a pore size gradient functionalization in the thickness direction.

[0042] The process parameters for hot pressing in step (3) of the above-mentioned method for preparing a gas diffusion layer with a gradient structure for a fuel cell are: temperature 150~320℃, pressure 5~40MPa;

[0043] In step (4), the adhesive solution is a solution with a concentration of 3 to 30 wt.% prepared by dissolving phenolic resin in anhydrous ethanol;

[0044] The process parameters for hot pressing curing in step (4) are: temperature 140~220℃, pressure 5~30MPa;

[0045] The heat treatment conditions for step (4) are as follows: under the protection of inert gas, the heating rate is 5℃ / min, and carbonization is carried out at a temperature of 1050~1200℃ for 1~1.5h; then the temperature is further increased to 1600~2800℃ for graphitization for 30~60min.

[0046] After graphitization, phenolic resin becomes resin carbon, carbon microfiber precursor becomes carbon microfiber, carbon microfiber and resin carbon are connected at the intersection of chopped carbon fibers. Carbon microfiber is distributed at the overlapping, splicing and intersection of chopped carbon fibers, while resin carbon is concentrated at the intersection of chopped carbon fibers. The interconnected network structure formed by carbon fiber sheets entangled and bonded by carbon microfiber and resin carbon can generate more electrical conduction paths.

[0047] The hydrophobic treatment conditions for step (4) are as follows: first, soak in a polytetrafluoroethylene solution with a concentration of 5-20 wt.% for 5-45 min, then dry at a temperature of 60-80℃ for 15-60 min, and finally treat at a temperature of 250-350℃ for 30-60 min.

[0048] Beneficial effects:

[0049] (1) The present invention constructs a gas diffusion layer with a gradient structure, which is a composite layer of 2 to 4 layers. The pore size of each layer is distributed in a gradient structure, thereby generating a capillary pressure gradient, giving the gas diffusion layer higher unidirectional hygroscopic performance, effectively solving the problem of water flooding in fuel cells in the prior art. While being able to quickly conduct water, it does not affect the gas transmission, thereby improving the output performance of fuel cells.

[0050] (2) In constructing a gas diffusion layer with a gradient structure, the present invention introduces carbon microfiber as a component. In the carbon fiber sheet, carbon microfiber can effectively wrap and bond the short carbon fiber. With the reduction of resin carbon content, not only will the mechanical strength of the gas diffusion layer not be reduced, but the pore structure can also be effectively adjusted to increase the air permeability. Thus, the air permeability can be improved without losing the mechanical strength of the gas diffusion layer.

[0051] (3) In the gas diffusion layer of the gradient structure constructed in this invention, the fourth layer is doped with nano carbon particles, and the ratio of nano carbon particles to short-cut carbon fibers and carbon microfibers is effectively controlled to successfully obtain a gas diffusion layer with intrinsic microporous layer. The preparation method of this microporous layer greatly simplifies the preparation process of the gas diffusion layer, improves the uniformity of the gas diffusion layer, and reduces the preparation cost. Attached Figure Description

[0052] Figure 1 This is a schematic diagram of a gas diffusion layer with a multilayer composite structure for a fuel cell, prepared in Example 2, along the thickness direction. Detailed Implementation

[0053] The present invention will be further described below with reference to specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the invention. Furthermore, it should be understood that after reading the teachings of this invention, those skilled in the art can make various alterations or modifications to the invention, and these equivalent forms also fall within the scope defined by the appended claims.

[0054]

[0055]

[0056] The porosity testing method is as follows:

[0057] (1) The gas diffusion layer with a multilayer composite structure for the fuel cell prepared in the example was fabricated to a size of 25 cm. 2 The test sample is 5cm x 5cm.

[0058] (2) Soak the sample in acetone solution for 0.5 h to remove oil and ash from its surface and interior. Then place it in an oven and dry it at 120 °C for at least 2 h. Then weigh the sample mass M using a precision electronic balance.

[0059] (3) Prepare a mixture of n-heptane and dibromoethane with a certain volume fraction and inject it into a stoppered graduated cylinder;

[0060] (4) Cut the sample fiber into small pieces and crush it into powder with an agate mortar until the length is less than 2 mm. Put it into the mixture in the stoppered graduated cylinder and stir with a glass rod to disperse the fiber in the mixture. Cover it with a ground glass stopper and put it into a constant temperature water bath at 25℃±1℃. The stopper and neck of the stoppered graduated cylinder should be exposed above the water surface.

[0061] (5) Observe the mixture. If the fibers float or sink in the mixture, n-heptane or dibromoethane needs to be added accordingly to adjust the density of the mixture until the fibers are evenly suspended in the mixture.

[0062] (6) After the mixture has stood for 4 hours, if the fibers are still evenly distributed in the mixture, measure the density of the mixture at that temperature using a densitometer. This density is the fiber density value (ρ). CF );

[0063] (7) The formula for calculating porosity is as follows:

[0064]

[0065] ε ——— Porosity of the sample, %;

[0066] M — Mass of the sample, in grams (g);

[0067] ρ CF — The density of carbon fiber, expressed in grams per cubic centimeter (g / cm³) 3 );

[0068] Lcp — the length of the sample, in centimeters (cm);

[0069] Wcp — The width of the sample, in centimeters (cm);

[0070] d — Sample thickness, in centimeters (cm).

[0071] Example 1

[0072] A method for preparing a gas diffusion layer with a gradient structure for a fuel cell, comprising the following specific steps:

[0073] (1) Preparation of raw materials:

[0074] Short-cut carbon fibers;

[0075] The precursor for carbon microfiber is para-aramid pulp;

[0076] (2) Preparation of carbon fiber base paper;

[0077] (2.1) Short carbon fibers and carbon microfiber precursors were uniformly dispersed in a 0.1 wt.% aqueous solution of polyethylene oxide according to different proportions to obtain the slurry of the first layer of carbon fiber base paper, the slurry of the second layer of carbon fiber base paper and the slurry of the third layer of carbon fiber base paper respectively; the short carbon fibers and carbon microfiber precursors were uniformly dispersed in a 0.1 wt.% aqueous solution of polyethylene oxide to obtain the slurry of the fourth layer of intrinsic microporous layer.

[0078] (2.2) The pulp obtained in step (2.1) is formed by wet papermaking machine to prepare the first layer of carbon fiber base paper, the second layer of carbon fiber base paper, the third layer of carbon fiber base paper and the fourth layer of carbon fiber base paper;

[0079] In the raw materials for preparing the first layer of carbon fiber base paper, the mass percentage of chopped carbon fiber is 95%, and the mass percentage of carbon microfiber precursor is 5%.

[0080] In the raw materials for preparing the second layer of carbon fiber base paper, the mass percentage of chopped carbon fiber is 75%, and the mass percentage of carbon microfiber precursor is 25%.

[0081] In the raw materials for preparing the third layer of carbon fiber base paper, the mass percentage of chopped carbon fiber is 40%, and the mass percentage of carbon microfiber precursor is 60%.

[0082] In the preparation of the fourth layer of carbon fiber base paper, based on the total mass of chopped carbon fiber and carbon microfiber precursor, the mass percentage of chopped carbon fiber is 25% and the mass percentage of carbon microfiber precursor is 75%.

[0083] (3) Composite of four layers of carbon fiber base paper: The first layer of carbon fiber base paper, the second layer of carbon fiber base paper, the third layer of carbon fiber base paper and the fourth layer of carbon fiber base paper are stacked from bottom to top;

[0084] (4) Preparation of precursor paper with pore size gradient structure: The four-layer carbon fiber base paper after composite in step (3) is hot-pressed at a temperature of 240℃ and a pressure of 40MPa to obtain precursor paper with pore size gradient structure.

[0085] (5) Preparation of a gas diffusion layer with a gradient structure for fuel cells: The precursor paper with a pore size gradient structure obtained in step (4) is impregnated in an adhesive solution and dried. Then, it is hot-pressed and cured at a temperature of 140℃ and a pressure of 30MPa. Then, under argon protection, the temperature is raised to 1050℃ at a rate of 5℃ / min and carbonized at 1050℃ for 1.5h. The temperature is then raised to 1600℃ for graphitization for 60min. The graphitized product is immersed in a 5wt.% polytetrafluoroethylene hydrophobic agent solution for 5min, dried at 60℃ for 60min, and finally treated at 350℃ for 30min to obtain a gas diffusion layer with a multilayer composite structure for fuel cells. The adhesive solution is a 3wt.% solution of phenolic resin dissolved in anhydrous ethanol. The phenolic resin is composed of thermosetting phenolic resin and thermoplastic phenolic resin in a mass ratio of 1:1.

[0086] like Figure 1 As shown, the gas diffusion layer with a gradient structure for fuel cells is a four-layer composite layer, consisting of a first layer of fiber sheets, a second layer of fiber sheets, a third layer of fiber sheets, and a fourth layer of intrinsic microporous layers, from bottom to top. The first layer of fiber sheets has an average pore size of 80 μm and a roughness of 15 μm; the second layer of fiber sheets has an average pore size of 46 μm; the third layer of fiber sheets has an average pore size of 11 μm; and the fourth layer of intrinsic microporous layers has an average pore size of 330 nm and a roughness of 5.3 μm. The uppermost layer (the fourth intrinsic microporous layer) of the gas diffusion layer with a gradient structure for fuel cells is bonded to the catalyst layer in the fuel cell, and the lowermost layer (the first layer of fiber sheets) is bonded to the bipolar plate of the fuel cell. Each layer of the gas diffusion layer with a gradient structure for fuel cells contains chopped carbon fibers, carbon microfibers, and resin carbon, with the carbon microfibers having an average diameter of 12 nm.

[0087] Example 2

[0088] A method for preparing a gas diffusion layer with a gradient structure for a fuel cell, comprising the following specific steps:

[0089] (1) Preparation of raw materials:

[0090] Short-cut carbon fibers;

[0091] The precursor for carbon microfiber is meta-aramid pulp;

[0092] The carbon nanoparticles are graphene; the average number of graphene layers is 5.

[0093] (2) Preparation of carbon fiber base paper;

[0094] (2.1) Short carbon fibers and carbon microfiber precursors were uniformly dispersed in a 0.15 wt.% aqueous solution of polyethylene oxide according to different proportions to obtain the slurry of the first layer of carbon fiber base paper, the slurry of the second layer of carbon fiber base paper, and the slurry of the third layer of carbon fiber base paper, respectively; Short carbon fibers, carbon microfiber precursors, and nano carbon particles were uniformly dispersed in a 0.15 wt.% aqueous solution of polyethylene oxide to obtain the slurry of the fourth intrinsic microporous layer.

[0095] (2.2) The pulp obtained in step (2.1) is formed by wet papermaking machine to prepare the first layer of carbon fiber base paper, the second layer of carbon fiber base paper, the third layer of carbon fiber base paper and the fourth layer of carbon fiber base paper;

[0096] In the raw materials for preparing the first layer of carbon fiber base paper, the mass percentage of chopped carbon fiber is 92%, and the mass percentage of carbon microfiber precursor is 8%.

[0097] In the raw materials for preparing the second layer of carbon fiber base paper, the mass percentage of chopped carbon fiber is 75%, and the mass percentage of carbon microfiber precursor is 25%.

[0098] In the raw materials for preparing the third layer of carbon fiber base paper, the mass percentage of chopped carbon fiber is 45%, and the mass percentage of carbon microfiber precursor is 55%.

[0099] In the preparation of the fourth layer of carbon fiber base paper, based on the total mass of chopped carbon fibers and carbon microfiber precursors, the mass percentage of chopped carbon fibers is 20%, the mass percentage of carbon microfiber precursors is 80%, and nano carbon particles account for 25% of the total oven-dry mass of the four-layer carbon fiber composite base paper.

[0100] (3) Composite of four layers of carbon fiber base paper: The first layer of carbon fiber base paper, the second layer of carbon fiber base paper, the third layer of carbon fiber base paper and the fourth layer of carbon fiber base paper are stacked from bottom to top;

[0101] (4) Preparation of precursor paper with pore size gradient structure: The four-layer carbon fiber base paper after composite in step (3) is hot-pressed at a temperature of 250℃ and a pressure of 35MPa to obtain precursor paper with pore size gradient structure.

[0102] (5) Preparation of a gas diffusion layer with a gradient structure for fuel cells: The precursor paper with a pore size gradient structure obtained in step (4) is impregnated in an adhesive solution and dried. Then, it is hot-pressed and cured at a temperature of 150°C and a pressure of 25 MPa. Then, under argon protection, the temperature is increased to 1070°C at a rate of 5°C / min and carbonized at 1070°C for 1.4 h. The temperature is further increased to 1750°C for graphitization for 55 min. The graphitized product is immersed in a 5 wt.% polytetrafluoroethylene hydrophobic agent solution for 10 min, dried at a temperature of 70°C for 50 min, and finally treated at a temperature of 320°C for 35 min to obtain a gas diffusion layer with a gradient structure for fuel cells. The adhesive solution is a 10 wt.% solution of phenolic resin dissolved in anhydrous ethanol. The phenolic resin is composed of thermosetting phenolic resin and thermoplastic phenolic resin in a mass ratio of 1:1.

[0103] The prepared gas diffusion layer for fuel cells with a gradient structure is a four-layer composite layer, consisting of, from bottom to top, a first layer of fiber sheets, a second layer of fiber sheets, a third layer of fiber sheets, and a fourth layer of intrinsically microporous material. The first layer of fiber sheets has an average pore size of 75 μm and a roughness of 13.6 μm; the second layer of fiber sheets has an average pore size of 46 μm; the third layer of fiber sheets has an average pore size of 9 μm; and the fourth layer of intrinsically microporous material has an average pore size of 132 nm and a roughness of 4.1 μm. The top layer of the gas diffusion layer with a gradient structure (i.e., the fourth intrinsically microporous layer) is adjacent to the catalyst layer in the fuel cell. The bottommost layer (i.e., the first fiber sheet layer) is bonded to the bipolar plate of the fuel cell. Each layer of the gradient-structured gas diffusion layer for the fuel cell contains chopped carbon fibers, carbon microfibers, and resin carbon. The average diameter of the carbon microfibers is 10 nm. In each layer, the carbon microfibers are entangled around the chopped carbon fibers, and the resin carbon connects the carbon microfibers and chopped carbon fibers together. Chopped carbon fibers also penetrate between different layers in the vertical direction, and are connected by carbon microfibers and resin carbon. The gradient-structured gas diffusion layer for the fuel cell has a thickness of 110 μm, a porosity of 78%, and a permeability of 2065 (mL·mm) / (cm). 2 The resistivity is 3.9 mΩ·cm (·h·mmAq), the in-plane resistivity is 47 mΩ·cm, and the tensile strength is 18 MPa.

[0104] Example 3

[0105] A method for preparing a gas diffusion layer with a gradient structure for a fuel cell, comprising the following specific steps:

[0106] (1) Preparation of raw materials:

[0107] Short-cut carbon fibers;

[0108] The carbon microfiber precursor is ultrafine polyacrylonitrile fiber obtained by electrospinning;

[0109] The carbon nanoparticles are carbon nanotubes; the average diameter of the carbon nanotubes is 3 nm and the average length is 1 nm.

[0110] (2) Preparation of carbon fiber base paper;

[0111] (2.1) Short carbon fibers and carbon microfiber precursors were uniformly dispersed in a 0.2 wt.% aqueous solution of polyethylene oxide according to different proportions to obtain the slurry of the first layer of carbon fiber base paper, the slurry of the second layer of carbon fiber base paper, and the slurry of the third layer of carbon fiber base paper, respectively; Short carbon fibers, carbon microfiber precursors, and nano carbon particles were uniformly dispersed in a 0.2 wt.% aqueous solution of polyethylene oxide to obtain the slurry of the fourth intrinsic microporous layer.

[0112] (2.2) The pulp obtained in step (2.1) is formed by wet papermaking machine to prepare the first layer of carbon fiber base paper, the second layer of carbon fiber base paper, the third layer of carbon fiber base paper and the fourth layer of carbon fiber base paper;

[0113] In the raw materials for preparing the first layer of carbon fiber base paper, the mass percentage of chopped carbon fiber is 90%, and the mass percentage of carbon microfiber precursor is 10%.

[0114] In the raw materials for preparing the second layer of carbon fiber base paper, the mass percentage of chopped carbon fiber is 75%, and the mass percentage of carbon microfiber precursor is 25%.

[0115] In the raw materials for preparing the third layer of carbon fiber base paper, the mass percentage of chopped carbon fiber is 50%, and the mass percentage of carbon microfiber precursor is 50%.

[0116] In the preparation of the fourth layer of carbon fiber base paper, based on the total mass of chopped carbon fibers and carbon microfiber precursors, the mass percentage of chopped carbon fibers is 30%, the mass percentage of carbon microfiber precursors is 70%, and nano carbon particles account for 35% of the total oven-dry mass of the four-layer carbon fiber composite base paper.

[0117] (3) Composite of four layers of carbon fiber base paper: The first layer of carbon fiber base paper, the second layer of carbon fiber base paper, the third layer of carbon fiber base paper and the fourth layer of carbon fiber base paper are stacked from bottom to top;

[0118] (4) Preparation of precursor paper with pore size gradient structure: The four-layer carbon fiber base paper after composite in step (3) is hot-pressed at a temperature of 150℃ and a pressure of 30MPa to obtain precursor paper with pore size gradient structure.

[0119] (5) Preparation of a gas diffusion layer with a gradient structure for fuel cells: The precursor paper with a pore size gradient structure obtained in step (4) is impregnated in an adhesive solution and dried. Then, it is hot-pressed and cured at a temperature of 160°C and a pressure of 20 MPa. Then, under argon protection, the temperature is raised to 1100°C at a rate of 5°C / min and carbonized at 1100°C for 1.3 h. The temperature is then raised to 1900°C for graphitization for 50 min. The graphitized product is immersed in a polytetrafluoroethylene hydrophobic agent solution with a concentration of 8 wt.% for 15 min, dried at a temperature of 80°C for 40 min, and finally treated at a temperature of 300°C for 40 min to obtain a gas diffusion layer with a gradient structure for fuel cells. The adhesive solution is a 15 wt.% solution of phenolic resin dissolved in anhydrous ethanol. The phenolic resin is composed of thermosetting phenolic resin and thermoplastic phenolic resin in a mass ratio of 1:1.

[0120] The prepared gas diffusion layer for fuel cells with a gradient structure is a four-layer composite layer, consisting of, from bottom to top, a first layer of fiber sheets, a second layer of fiber sheets, a third layer of fiber sheets, and a fourth layer of intrinsically microporous material. The first layer of fiber sheets has an average pore size of 61 μm and a roughness of 12.8 μm; the second layer has an average pore size of 46 μm; the third layer has an average pore size of 15 μm; and the fourth layer of intrinsically microporous material has an average pore size of 50 nm and a roughness of 4.6 μm. The topmost layer of the gradient gas diffusion layer (i.e., the fourth intrinsically microporous layer) is attached to the catalyst layer in the fuel cell. The bottommost layer (i.e., the first fiber sheet layer) is bonded to the bipolar plate of the fuel cell. Each layer of the gradient-structured gas diffusion layer for the fuel cell contains chopped carbon fibers, carbon microfibers, and resin carbon. The average diameter of the carbon microfibers is 380 nm. In each layer, the carbon microfibers are entangled around the chopped carbon fibers, and the resin carbon connects the carbon microfibers and chopped carbon fibers together. Chopped carbon fibers also penetrate between different layers in the vertical direction, and are connected by carbon microfibers and resin carbon. The gradient-structured gas diffusion layer for the fuel cell has a thickness of 150 μm, a porosity of 76%, and a permeability of 1990 (mL·mm) / (cm). 2 The resistivity is 4.7 mΩ·cm (·h·mmAq), the in-plane resistivity is 56 mΩ·cm, and the tensile strength is 21 MPa.

[0121] Example 4

[0122] A method for preparing a gas diffusion layer with a gradient structure for a fuel cell, comprising the following specific steps:

[0123] (1) Preparation of raw materials:

[0124] Short-cut carbon fibers;

[0125] The carbon microfiber precursor is an ultrafine mesophase pitch-based fiber;

[0126] The nano-carbon particles are carbon black; the average particle size of the carbon black is 40 nm.

[0127] (2) Preparation of carbon fiber base paper;

[0128] (2.1) Short carbon fibers and carbon microfiber precursors were uniformly dispersed in a 0.1 wt.% aqueous solution of polyethylene oxide according to different proportions to obtain the slurry of the first layer of carbon fiber base paper, the slurry of the second layer of carbon fiber base paper, and the slurry of the third layer of carbon fiber base paper, respectively; Short carbon fibers, carbon microfiber precursors, and nano carbon particles were uniformly dispersed in a 0.1 wt.% aqueous solution of polyethylene oxide to obtain the slurry of the fourth intrinsic microporous layer;

[0129] (2.2) The pulp obtained in step (2.1) is formed by wet papermaking machine to prepare the first layer of carbon fiber base paper, the second layer of carbon fiber base paper, the third layer of carbon fiber base paper and the fourth layer of carbon fiber base paper;

[0130] In the raw materials for preparing the first layer of carbon fiber base paper, the mass percentage of chopped carbon fiber is 87%, and the mass percentage of carbon microfiber precursor is 13%.

[0131] In the raw materials for preparing the second layer of carbon fiber base paper, the mass percentage of chopped carbon fiber is 70%, and the mass percentage of carbon microfiber precursor is 30%.

[0132] In the raw materials for preparing the third layer of carbon fiber base paper, the mass percentage of chopped carbon fiber is 45%, and the mass percentage of carbon microfiber precursor is 55%.

[0133] In the preparation of the fourth layer of carbon fiber base paper, based on the total mass of chopped carbon fibers and carbon microfiber precursors, the mass percentage of chopped carbon fibers is 25%, the mass percentage of carbon microfiber precursors is 75%, and nano carbon particles account for 30% of the total oven-dry mass of the four-layer carbon fiber composite base paper.

[0134] (3) Composite of four layers of carbon fiber base paper: The first layer of carbon fiber base paper, the second layer of carbon fiber base paper, the third layer of carbon fiber base paper and the fourth layer of carbon fiber base paper are stacked from bottom to top;

[0135] (4) Preparation of precursor paper with pore size gradient structure: The four-layer carbon fiber base paper after composite in step (3) is hot-pressed at a temperature of 150℃ and a pressure of 25MPa to obtain precursor paper with pore size gradient structure.

[0136] (5) Preparation of a gas diffusion layer with a gradient structure for fuel cells: The precursor paper with a pore size gradient structure obtained in step (4) is impregnated in an adhesive solution and dried. Then, it is hot-pressed and cured at a temperature of 170°C and a pressure of 18MPa. Then, under argon protection, the temperature is raised to 1120°C at a rate of 5°C / min and carbonized at 1120°C for 1.2h. The temperature is then raised to 2100°C for graphitization for 45min. The graphitized product is immersed in a polytetrafluoroethylene hydrophobic agent solution with a concentration of 11wt.% for 25min, dried at a temperature of 80°C for 30min, and finally treated at a temperature of 290°C for 45min to obtain a gas diffusion layer with a gradient structure for fuel cells. The adhesive solution is a solution with a concentration of 18wt.% prepared by dissolving phenolic resin in anhydrous ethanol. The phenolic resin is composed of thermosetting phenolic resin and thermoplastic phenolic resin in a mass ratio of 1:1.

[0137] The prepared gas diffusion layer for fuel cells with a gradient structure is a four-layer composite layer, consisting of, from bottom to top, a first layer of fiber sheets, a second layer of fiber sheets, a third layer of fiber sheets, and a fourth layer of intrinsically microporous material. The first layer of fiber sheets has an average pore size of 52 μm and a roughness of 11.6 μm; the second layer of fiber sheets has an average pore size of 39 μm; the third layer of fiber sheets has an average pore size of 12 μm; and the fourth layer of intrinsically microporous material has an average pore size of 960 nm and a roughness of 5.9 μm. The top layer of the gas diffusion layer with a gradient structure (i.e., the fourth intrinsically microporous layer) is adjacent to the catalyst layer in the fuel cell. The bottommost layer (i.e., the first fiber sheet layer) is bonded to the bipolar plate of the fuel cell. Each layer of the gradient-structured gas diffusion layer for the fuel cell contains chopped carbon fibers, carbon microfibers, and resin carbon. The average diameter of the carbon microfibers is 500 nm. In each layer, the carbon microfibers are entangled around the chopped carbon fibers, and the resin carbon connects the carbon microfibers and chopped carbon fibers together. Chopped carbon fibers also penetrate between different layers in the vertical direction, and are connected by carbon microfibers and resin carbon. The gradient-structured gas diffusion layer for the fuel cell has a thickness of 190 μm, a porosity of 73%, and a permeability of 1980 (mL·mm) / (cm). 2 The resistivity is 5.1 mΩ·cm (·h·mmAq), the in-plane resistivity is 68 mΩ·cm, and the tensile strength is 23 MPa.

[0138] Example 5

[0139] A method for preparing a gas diffusion layer with a gradient structure for a fuel cell, comprising the following specific steps:

[0140] (1) Preparation of raw materials:

[0141] Short-cut carbon fibers;

[0142] The carbon microfiber precursor is a mixture of aramid pulp and polyimide pulp in a mass ratio of 1:1;

[0143] The carbon nanoparticles are graphene; the average number of graphene layers is 10.

[0144] (2) Preparation of carbon fiber base paper;

[0145] (2.1) Short carbon fibers and carbon microfiber precursors were uniformly dispersed in a 0.15 wt.% aqueous solution of polyethylene oxide according to different proportions to obtain the slurry of the first layer of carbon fiber base paper, the slurry of the second layer of carbon fiber base paper, and the slurry of the third layer of carbon fiber base paper, respectively; Short carbon fibers, carbon microfiber precursors, and nano carbon particles were uniformly dispersed in a 0.15 wt.% aqueous solution of polyethylene oxide to obtain the slurry of the fourth intrinsic microporous layer.

[0146] (2.2) The pulp obtained in step (2.1) is formed by wet papermaking machine to prepare the first layer of carbon fiber base paper, the second layer of carbon fiber base paper, the third layer of carbon fiber base paper and the fourth layer of carbon fiber base paper;

[0147] In the raw materials for preparing the first layer of carbon fiber base paper, the mass percentage of chopped carbon fiber is 85%, and the mass percentage of carbon microfiber precursor is 15%.

[0148] In the raw materials for preparing the second layer of carbon fiber base paper, the mass percentage of chopped carbon fiber is 65%, and the mass percentage of carbon microfiber precursor is 35%.

[0149] In the raw materials for preparing the third layer of carbon fiber base paper, the mass percentage of chopped carbon fiber is 40%, and the mass percentage of carbon microfiber precursor is 60%.

[0150] In the preparation of the fourth layer of carbon fiber base paper, based on the total mass of chopped carbon fibers and carbon microfiber precursors, the mass percentage of chopped carbon fibers is 20%, the mass percentage of carbon microfiber precursors is 80%, and nano carbon particles account for 15% of the total oven-dry mass of the four-layer carbon fiber composite base paper.

[0151] (3) Composite of four layers of carbon fiber base paper: The first layer of carbon fiber base paper, the second layer of carbon fiber base paper, the third layer of carbon fiber base paper and the fourth layer of carbon fiber base paper are stacked from bottom to top;

[0152] (4) Preparation of precursor paper with pore size gradient structure: The four-layer carbon fiber base paper after composite in step (3) is hot-pressed at a temperature of 290℃ and a pressure of 20MPa to obtain precursor paper with pore size gradient structure.

[0153] (5) Preparation of a gas diffusion layer with a gradient structure for fuel cells: The precursor paper with a pore size gradient structure obtained in step (4) is impregnated in a binder solution and dried. Then, it is hot-pressed and cured at a temperature of 180℃ and a pressure of 14MPa. Then, under argon protection, the temperature is raised to 1140℃ at a rate of 5℃ / min and carbonized at 1140℃ for 1.1h. The temperature is then raised to 2300℃ for graphitization for 40min. The graphitized product is immersed in a 14wt.% polytetrafluoroethylene hydrophobic agent solution for 30min, dried at a temperature of 60℃ for 25min, and finally treated at a temperature of 280℃ for 50min to obtain a gas diffusion layer with a gradient structure for fuel cells. The binder solution is a 22wt.% solution prepared by dissolving phenolic resin in anhydrous ethanol. The phenolic resin is composed of thermosetting phenolic resin and thermoplastic phenolic resin in a mass ratio of 1:1.

[0154] The prepared gas diffusion layer for fuel cells with a gradient structure is a four-layer composite layer, consisting of, from bottom to top, a first layer of fiber sheets, a second layer of fiber sheets, a third layer of fiber sheets, and a fourth layer of intrinsic microporous material. The first layer of fiber sheets has an average pore size of 48 μm and a roughness of 10.7 μm; the second layer of fiber sheets has an average pore size of 33 μm; the third layer of fiber sheets has an average pore size of 13 μm; and the fourth layer of intrinsic microporous material has an average pore size of 865 nm and a roughness of 6.2 μm. The top layer of the gas diffusion layer with a gradient structure (i.e., the fourth intrinsic microporous layer) is adjacent to the catalyst layer in the fuel cell. The bottommost layer (i.e., the first fiber sheet layer) is bonded to the bipolar plate of the fuel cell. Each layer of the gradient-structured gas diffusion layer for the fuel cell contains chopped carbon fibers, carbon microfibers, and resin carbon. The average diameter of the carbon microfibers is 410 nm. In each layer, the carbon microfibers are entangled around the chopped carbon fibers, and the resin carbon connects the carbon microfibers and chopped carbon fibers together. Chopped carbon fibers also penetrate between different layers in the vertical direction, and are connected by carbon microfibers and resin carbon. The gradient-structured gas diffusion layer for the fuel cell has a thickness of 220 μm, a porosity of 71%, and a permeability of 1860 (mL·mm) / (cm). 2 The resistivity is 5.8 mΩ·cm (·h·mmAq), the in-plane resistivity is 71 mΩ·cm, and the tensile strength is 37 MPa.

[0155] Example 6

[0156] A method for preparing a gas diffusion layer with a gradient structure for a fuel cell, comprising the following specific steps:

[0157] (1) Preparation of raw materials:

[0158] Short-cut carbon fibers;

[0159] The precursor for carbon microfiber is bamboo pulp;

[0160] The carbon nanoparticles are carbon nanotubes; the average diameter of the carbon nanotubes is 10 nm and the average length is 20 nm.

[0161] (2) Preparation of carbon fiber base paper;

[0162] (2.1) Short carbon fibers and carbon microfiber precursors were uniformly dispersed in a 0.2 wt.% aqueous solution of polyethylene oxide according to different proportions to obtain the slurry of the first layer of carbon fiber base paper, the slurry of the second layer of carbon fiber base paper, and the slurry of the third layer of carbon fiber base paper, respectively; Short carbon fibers, carbon microfiber precursors, and nano carbon particles were uniformly dispersed in a 0.2 wt.% aqueous solution of polyethylene oxide to obtain the slurry of the fourth intrinsic microporous layer.

[0163] (2.2) The pulp obtained in step (2.1) is formed by wet papermaking machine to prepare the first layer of carbon fiber base paper, the second layer of carbon fiber base paper, the third layer of carbon fiber base paper and the fourth layer of carbon fiber base paper;

[0164] In the raw materials for preparing the first layer of carbon fiber base paper, the mass percentage of chopped carbon fiber is 82%, and the mass percentage of carbon microfiber precursor is 18%.

[0165] In the raw materials for preparing the second layer of carbon fiber base paper, the mass percentage of chopped carbon fiber is 60%, and the mass percentage of carbon microfiber precursor is 40%.

[0166] In the raw materials for preparing the third layer of carbon fiber base paper, the mass percentage of chopped carbon fiber is 35%, and the mass percentage of carbon microfiber precursor is 65%.

[0167] In the preparation of the fourth layer of carbon fiber base paper, based on the total mass of chopped carbon fibers and carbon microfiber precursors, the mass percentage of chopped carbon fibers is 15%, the mass percentage of carbon microfiber precursors is 85%, and nano carbon particles account for 10% of the total oven-dry mass of the four-layer carbon fiber composite base paper.

[0168] (3) Composite of four layers of carbon fiber base paper: The first layer of carbon fiber base paper, the second layer of carbon fiber base paper, the third layer of carbon fiber base paper and the fourth layer of carbon fiber base paper are stacked from bottom to top;

[0169] (4) Preparation of precursor paper with pore size gradient structure: The four-layer carbon fiber base paper after composite in step (3) is hot-pressed at a temperature of 300℃ and a pressure of 10MPa to obtain precursor paper with pore size gradient structure.

[0170] (5) Preparation of a gas diffusion layer with a gradient structure for fuel cells: The precursor paper with a pore size gradient structure obtained in step (4) is impregnated in an adhesive solution and dried. Then, it is hot-pressed and cured at a temperature of 200℃ and a pressure of 10MPa. Then, under argon protection, the temperature is raised to 1180℃ at a rate of 5℃ / min and carbonized at 1180℃ for 1h. The temperature is then raised to 2500℃ for graphitization for 35min. The graphitized product is immersed in a 17wt.% polytetrafluoroethylene hydrophobic agent solution for 35min, dried at 70℃ for 20min, and finally treated at 270℃ for 55min to obtain a gas diffusion layer with a gradient structure for fuel cells. The adhesive solution is a 26wt.% solution of phenolic resin dissolved in anhydrous ethanol. The phenolic resin is composed of thermosetting phenolic resin and thermoplastic phenolic resin in a mass ratio of 1:1.

[0171] The prepared gas diffusion layer for fuel cells with a gradient structure is a four-layer composite layer, consisting of, from bottom to top, a first layer of fiber sheets, a second layer of fiber sheets, a third layer of fiber sheets, and a fourth layer of intrinsic microporous material. The first layer of fiber sheets has an average pore size of 37 μm and a roughness of 15 μm; the second layer has an average pore size of 27 μm; the third layer has an average pore size of 4 μm; and the fourth layer of intrinsic microporous material has an average pore size of 1210 nm and a roughness of 6.5 μm. The topmost layer of the gradient gas diffusion layer (i.e., the fourth intrinsic microporous layer) is attached to the catalyst layer in the fuel cell. The bottommost layer (i.e., the first fiber sheet) is bonded to the bipolar plate of the fuel cell. Each layer of the gradient-structured gas diffusion layer for the fuel cell contains chopped carbon fibers, carbon microfibers, and resin carbon. The average diameter of the carbon microfibers is 20 nm. In each layer, the carbon microfibers are entangled around the chopped carbon fibers, and the resin carbon connects the carbon microfibers and chopped carbon fibers together. Chopped carbon fibers also penetrate between different layers in the vertical direction, and are connected by carbon microfibers and resin carbon. The gradient-structured gas diffusion layer for the fuel cell has a thickness of 260 μm, a porosity of 68%, and a permeability of 1720 (mL·mm) / (cm). 2 The resistivity is 6.8 mΩ·cm (·h·mmAq), the in-plane resistivity is 73 mΩ·cm, and the tensile strength is 43 MPa.

[0172] Example 7

[0173] A method for preparing a gas diffusion layer with a gradient structure for a fuel cell, comprising the following specific steps:

[0174] (1) Preparation of raw materials:

[0175] Short-cut carbon fibers;

[0176] The precursor for carbon microfiber is cotton pulp;

[0177] The nano-carbon particles are a mixture of carbon black and graphene in a mass ratio of 1:1; the average particle size of the carbon black is 60 nm, and the average number of graphene layers is 7.

[0178] (2) Preparation of carbon fiber base paper;

[0179] (2.1) Short carbon fibers and carbon microfiber precursors were uniformly dispersed in a 0.1 wt.% aqueous solution of polyethylene oxide according to different proportions to obtain the slurry of the first layer of carbon fiber base paper, the slurry of the second layer of carbon fiber base paper, and the slurry of the third layer of carbon fiber base paper, respectively; Short carbon fibers, carbon microfiber precursors, and nano carbon particles were uniformly dispersed in a 0.1 wt.% aqueous solution of polyethylene oxide to obtain the slurry of the fourth intrinsic microporous layer;

[0180] (2.2) The pulp obtained in step (2.1) is formed by wet papermaking machine to prepare the first layer of carbon fiber base paper, the second layer of carbon fiber base paper, the third layer of carbon fiber base paper and the fourth layer of carbon fiber base paper;

[0181] In the raw materials for preparing the first layer of carbon fiber base paper, the mass percentage of chopped carbon fiber is 80%, and the mass percentage of carbon microfiber precursor is 20%.

[0182] In the raw materials for preparing the second layer of carbon fiber base paper, the mass percentage of chopped carbon fiber is 55%, and the mass percentage of carbon microfiber precursor is 45%.

[0183] In the raw materials for preparing the third layer of carbon fiber base paper, the mass percentage of chopped carbon fiber is 30%, and the mass percentage of carbon microfiber precursor is 70%.

[0184] In the preparation of the fourth layer of carbon fiber base paper, based on the total mass of chopped carbon fibers and carbon microfiber precursors, the mass percentage of chopped carbon fibers is 10%, the mass percentage of carbon microfiber precursors is 90%, and nano carbon particles account for 5% of the total oven-dry mass of the four-layer carbon fiber composite base paper.

[0185] (3) Composite of four layers of carbon fiber base paper: The first layer of carbon fiber base paper, the second layer of carbon fiber base paper, the third layer of carbon fiber base paper and the fourth layer of carbon fiber base paper are stacked from bottom to top;

[0186] (4) Preparation of precursor paper with pore size gradient structure: The four-layer carbon fiber base paper after composite in step (3) is hot-pressed at a temperature of 320℃ and a pressure of 5MPa to obtain precursor paper with pore size gradient structure.

[0187] (5) Preparation of a gas diffusion layer with a gradient structure for fuel cells: The precursor paper with a pore size gradient structure obtained in step (4) is impregnated in an adhesive solution and dried. Then, it is hot-pressed and cured at a temperature of 220°C and a pressure of 5 MPa. Then, under argon protection, the temperature is raised to 1200°C at a rate of 5°C / min and carbonized at 1200°C for 1 h. The temperature is then raised to 2800°C for graphitization for 30 min. The graphitized product is immersed in a 20 wt.% polytetrafluoroethylene hydrophobic agent solution for 45 min, dried at 80°C for 15 min, and finally treated at 250°C for 60 min to obtain a gas diffusion layer with a gradient structure for fuel cells. The adhesive solution is a 30 wt.% solution of phenolic resin dissolved in anhydrous ethanol. The phenolic resin is composed of thermosetting phenolic resin and thermoplastic phenolic resin in a mass ratio of 1:1.

[0188] The prepared gas diffusion layer for fuel cells with a gradient structure is a four-layer composite layer, consisting of, from bottom to top, a first layer of fiber sheets, a second layer of fiber sheets, a third layer of fiber sheets, and a fourth layer of intrinsic microporous material. The first layer of fiber sheets has an average pore size of 20 μm and a roughness of 15 μm; the second layer of fiber sheets has an average pore size of 10 μm; the third layer of fiber sheets has an average pore size of 1 μm; and the fourth layer of intrinsic microporous material has an average pore size of 1500 nm and a roughness of 7 μm. The top layer of the gas diffusion layer with a gradient structure (i.e., the fourth intrinsic microporous layer) is bonded to the catalyst layer in the fuel cell. The bottommost layer (i.e., the first fiber sheet) is bonded to the bipolar plate of the fuel cell. Each layer of the gradient-structured gas diffusion layer for the fuel cell contains chopped carbon fibers, carbon microfibers, and resin carbon. The average diameter of the carbon microfibers is 60 nm. In each layer, the carbon microfibers are entangled around the chopped carbon fibers, and the resin carbon connects the carbon microfibers and chopped carbon fibers together. Chopped carbon fibers also penetrate between different layers in the vertical direction, and are connected by carbon microfibers and resin carbon. The gradient-structured gas diffusion layer for the fuel cell has a thickness of 280 μm, a porosity of 65%, and a permeability of 1640 (mL·mm) / (cm). 2 The resistivity is 7 mΩ·cm (·h·mmAq), the in-plane resistivity is 80 mΩ·cm, and the tensile strength is 50 MPa.

[0189] Example 8

[0190] A method for preparing a gas diffusion layer with a gradient structure for a fuel cell is basically the same as in Example 2, except that in Example 8, step (2) does not involve the preparation of a second layer of carbon fiber paper.

[0191] The fabricated gas diffusion layer for fuel cells, with a gradient structure, is a three-layer composite layer. From bottom to top, it consists of a first fiber sheet with an average pore size of 75 μm and a roughness of 13.6 μm; a second fiber sheet (formerly the third fiber sheet) with an average pore size of 9 μm; and a third intrinsic microporous layer (formerly the fourth intrinsic microporous layer) with an average pore size of 132 nm and a roughness of 4.1 μm. The uppermost layer of the gas diffusion layer is bonded to the catalyst layer in the fuel cell, and the lowermost layer is bonded to the fuel cell's... Bipolar plates are bonded together; each layer of the gas diffusion layer for fuel cells contains chopped carbon fibers, carbon microfibers, and resin carbon, with an average diameter of 10 nm for the carbon microfibers; in each layer, the carbon microfibers are wound around the chopped carbon fibers, and the resin carbon connects the carbon microfibers and chopped carbon fibers together; chopped carbon fibers also penetrate between different layers in the vertical direction, and are connected by carbon microfibers and resin carbon; the thickness of the gas diffusion layer for fuel cells is 105 μm, the porosity is 69%, and the air permeability is 2045 (mL·mm) / (cm). 2 The resistivity is 3.8 mΩ·cm (·h·mmAq), the in-plane resistivity is 52 mΩ·cm, and the tensile strength is 16 MPa.

Claims

1. A method for preparing a gas diffusion layer with a gradient structure for a fuel cell, characterized in that: Four carbon fiber base papers with different pore sizes are stacked from bottom to top in order of decreasing pore size, hot-pressed and then impregnated, cured, heat-treated and hydrophobically treated in sequence to obtain a gas diffusion layer with a gradient structure for fuel cells. Carbon fiber base paper is prepared by using short-cut carbon fibers and carbon microfiber precursors as the main raw materials to form a pulp, which is then obtained through wet papermaking. The length of chopped carbon fibers is 3–10 mm, and the average fiber diameter is 4–10 μm; the average diameter of carbon microfibers is 10–500 nm. The carbon microfiber precursor is one or more of plant pulp, chopped ultrafine organic fiber and chemical fiber pulp; Plant pulp includes cotton pulp, paper pulp, wood pulp, bamboo pulp, or grass pulp; chopped ultrafine organic fiber includes ultrafine polyacrylonitrile fiber or ultrafine mesophase pitch-based fiber obtained by electrospinning; chemical fiber pulp includes para-aramid pulp, meta-aramid pulp, aramid sulfone pulp, or polyimide pulp. The gas diffusion layer with a gradient structure used in the fuel cell is a four-layer composite layer, and the specific preparation steps are as follows: (1) Preparation of carbon fiber base paper: Single-layer carbon fiber base paper with different pore sizes is prepared by wet papermaking using short-cut carbon fibers and carbon microfiber precursors as the main raw materials, namely the first layer of carbon fiber base paper, the second layer of carbon fiber base paper, the third layer of carbon fiber base paper and the fourth layer of carbon fiber base paper. (2) Composite of four layers of carbon fiber base paper: The first layer of carbon fiber base paper, the second layer of carbon fiber base paper, the third layer of carbon fiber base paper and the fourth layer of carbon fiber base paper are stacked from bottom to top; In the raw materials for preparing the first layer of carbon fiber base paper, the mass percentage of chopped carbon fiber is 80-95%, and the mass percentage of carbon microfiber precursor is 5-20%. In the raw materials for preparing the second layer of carbon fiber base paper, the mass percentage of chopped carbon fiber is 55-75%, and the mass percentage of carbon microfiber precursor is 25-45%. In the raw materials for preparing the third layer of carbon fiber base paper, the mass percentage of chopped carbon fiber is 30-50%, and the mass percentage of carbon microfiber precursor is 50-70%. In the preparation of the fourth layer of carbon fiber base paper, based on the total mass of chopped carbon fibers and carbon microfiber precursors, the mass percentage of chopped carbon fibers is 10-30%, the mass percentage of carbon microfiber precursors is 70-90%, and the nano carbon particles account for less than 35% of the total oven-dry mass of the four-layer carbon fiber composite base paper. (3) Preparation of precursor paper with pore size gradient structure: The four-layer carbon fiber composite base paper after the composite in step (2) is subjected to hot pressing to obtain precursor paper with pore size gradient structure; (4) Preparation of gas diffusion layer with gradient structure for fuel cell: The precursor paper with pore size gradient structure obtained in step (3) is impregnated in an adhesive solution and dried, and then hot-pressed, heat-treated and hydrophobic treated to obtain gas diffusion layer with gradient structure for fuel cell. Each layer of the gas diffusion layer with a gradient structure for the fuel cell contains chopped carbon fibers, carbon microfibers, and resin carbon. In each layer, carbon microfibers are wrapped around chopped carbon fibers, and resin carbon connects the carbon microfibers and chopped carbon fibers together. In the vertical direction, chopped carbon fibers also run through different layers and are wrapped by carbon microfibers and connected by resin carbon.

2. The method for preparing a gas diffusion layer with a gradient structure for a fuel cell according to claim 1, characterized in that: In the gas diffusion layer with a gradient structure used in the fuel cell, the pore size of each layer is distributed in a gradient structure. The uppermost layer of the gas diffusion layer with a gradient structure used in the fuel cell is bonded to the catalyst layer in the fuel cell, and the lowermost layer is bonded to the bipolar plate of the fuel cell. The gas diffusion layer with a gradient structure for the fuel cell has an average pore size of 20–80 μm and a roughness of ≤15 μm in the lowest layer. From bottom to top, the average pore size of each layer in the gas diffusion layer with a gradient structure for the fuel cell gradually decreases, with the average pore size of the uppermost layer being 50–1500 nm and a roughness of ≤7 μm.

3. The method for preparing a gas diffusion layer with a gradient structure for a fuel cell according to claim 2, characterized in that, The gas diffusion layer with a gradient structure used in the fuel cell has a thickness of 80–280 μm, a porosity of 60–80%, and a permeability of 1600–2200 (mL·mm) / (cm). 2 The resistivity is 3.5–7.0 mΩ·cm in plane, the resistivity is 40–80 mΩ·cm in normal direction, and the tensile strength is 15–50 MPa.

4. The method for preparing a gas diffusion layer with a gradient structure for a fuel cell according to claim 2, characterized in that, In the four-layer composite layer of the gas diffusion layer with gradient structure for fuel cells, from bottom to top, there are a first layer of fiber sheet, a second layer of fiber sheet, a third layer of fiber sheet and a fourth layer of intrinsic microporous layer. The average pore size of the first fiber sheet is 20–80 μm, the average pore size of the second fiber sheet is 10–46 μm, the average pore size of the third fiber sheet is 1–15 μm, and the average pore size of the fourth intrinsic microporous layer is 50–1500 nm.

5. The method for preparing a gas diffusion layer with a gradient structure for a fuel cell according to claim 4, characterized in that, The fourth intrinsic microporous layer also contains nano-carbon particles; The nano-carbon particles are one or more of carbon black, graphene, and carbon nanotubes; The particle size of carbon black is 10–60 nm; the number of graphene layers is ≤10; the diameter of carbon nanotubes is 3–30 nm and the length is 1–50 nm.

6. The method for preparing a gas diffusion layer with a gradient structure for a fuel cell according to claim 1, characterized in that, Step (1) is as follows: (1.1) Short carbon fibers and carbon microfiber precursors are uniformly dispersed in a polyethylene oxide aqueous solution with a concentration of 0.1-0.2 wt.% according to different proportions to obtain the slurry of the first layer of carbon fiber base paper, the slurry of the second layer of carbon fiber base paper, and the slurry of the third layer of carbon fiber base paper, respectively; short carbon fibers, carbon microfiber precursors, and nano carbon particles are uniformly dispersed in a polyethylene oxide aqueous solution with a concentration of 0.1-0.2 wt.% to obtain the slurry of the fourth layer of carbon fiber base paper; (1.2) The pulp obtained in step (1.1) is formed by wet papermaking machine to prepare the first layer of carbon fiber base paper, the second layer of carbon fiber base paper, the third layer of carbon fiber base paper and the fourth layer of carbon fiber base paper.

7. The method for preparing a gas diffusion layer with a gradient structure for a fuel cell according to claim 1, characterized in that, The process parameters for hot pressing in step (3) are: temperature 150~320℃, pressure 5~40MPa; In step (4), the adhesive solution is a solution with a concentration of 3 to 30 wt.% prepared by dissolving phenolic resin in anhydrous ethanol; The process parameters for hot pressing curing in step (4) are: temperature 140~220℃, pressure 5~30MPa; The heat treatment conditions for step (4) are as follows: under the protection of inert gas, the heating rate is 5℃ / min, and carbonization is carried out at a temperature of 1050~1200℃ for 1~1.5h; then the temperature is further increased to 1600~2800℃ for graphitization for 30~60min. The hydrophobic treatment conditions for step (4) are as follows: first, soak in a polytetrafluoroethylene solution with a concentration of 5-20 wt.% for 5-45 min, then dry at a temperature of 60-80℃ for 15-60 min, and finally treat at a temperature of 250-350℃ for 30-60 min.

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