A fuel cell gas diffusion layer containing a microporous layer and a preparation method thereof

Abstract

The present invention belongs to the field of fuel cells and specifically discloses a fuel cell gas diffusion layer containing a microporous layer and a preparation method thereof. (1) Carbon nanotubes and carbon fibers are uniformly stirred and mixed to form a turbid solution; (2) The turbid solution is filtered on a suction filtration bottle to form a thin film, namely a carbon membrane; (3) The film is then impregnated with a phenolic resin solution, hot-pressed and carbonized at high temperature; (4) A mixture slurry of carbon black and PTFE is uniformly sprayed onto the carbon membrane, and then treated at 350°C in an air atmosphere for a period of time to obtain a uniformly distributed fuel cell microporous layer carbon membrane. The battery gas diffusion layer prepared by the present invention has its physical and chemical properties modified by the production of the microporous layer, thereby improving its gas diffusion capacity and hydrophobicity. At the same time, the addition of the microporous layer also improves the toughness of the carbon membrane. The preparation process is simple and easy to scale and continuously produce.

Description

Technical Field

[0001] The invention belongs to the field of fuel cells and specifically discloses a fuel cell gas diffusion layer containing a microporous layer and a preparation method thereof. Background Art

[0002] The proton exchange membrane fuel cell (PEMFC) is a battery system developed in the last century that allows protons to pass through it. This battery has a relatively simple structure and features high power density, high energy conversion efficiency, low-temperature startup, zero pollution, and light weight. The main components of a PEMFC include bipolar plates, gas diffusion layers, catalyst layers, and proton exchange membranes. Bipolar plates are a key component of a PEMFC. Made from materials such as graphite, metal, and composite bipolar plates, they separate the oxidant and reductant and collect the conductive current. The diffusion layer not only serves as a support layer but also diffuses gas, current, water, and conducts heat. In the catalyst layer, the electrochemical reaction between hydrogen and oxygen generates electrons, water, protons, and heat. The proton exchange membrane is a key component in a PEMFC. It not only isolates the fuel and oxidant but also serves as an electrolyte. It is a good hydrogen ion conductor and a selective and permeable polymer membrane. The proton exchange membrane is crucial to fuel cell production costs and cell efficiency, and can be said to directly impact the performance and lifespan of a PEMFC.

[0003] As a low-temperature fuel cell, PEMFC operates at a temperature of 60-80°C. Its working principle is as follows: H2 and O2 enter the cathode and anode of the fuel cell respectively through the guide channels on the bipolar plate, then pass through the gas diffusion layer and the catalyst layer, and then enter the proton exchange membrane. At the other end of the anode, H2 is decomposed into H + 、e - In the form of water and protons, it migrates in the film and eventually reaches the cathode, thereby achieving proton conduction; at the same time, under the action of the cathode catalyst, O2 in the cathode reacts with H from the anode. + Combined, water is generated, and a voltage is formed between the positive and negative electrodes. When connected by an external circuit, the electrons in the circuit will generate electrical energy. The electronic reactions occurring at the anode and cathode are as follows:

[0004] Anode reaction: 2H2→4H + +4e －

[0005] Cathode reaction: O2+4H + +4e － →2H2O

[0006] Battery reaction: 2H2+O2→2H2O

[0007] Moisture migration and gas diffusion in PEMFCs are the main factors affecting cell performance, and the corresponding component is the gas diffusion layer (GDL). Conventional GDLs are mainly composed of a microporous layer (MPL) and carbon paper. The MPL is mainly composed of small pores, while the carbon paper is mainly composed of large pores. When the carbon fiber paper is combined with the MPL, the sudden change in porosity leads to a certain impact on the water transfer efficiency.

[0008] Requirements that the gas diffusion layer needs to meet:

[0009] (1) High conductivity. Since the diffusion layer is needed as a channel for electrons to propagate between the bipolar plate and the catalyst layer, and the current needs to be collected, the resistivity requirement for the GDL is very low.

[0010] (2) High strength. In order to ensure the stability of the electrode, the overall structure of the electrode must be stable, thereby extending the service life of the electrode.

[0011] (3) High porosity and pore size distribution within a specific range. Since the diffusion layer is the main channel for the production of hydrogen, oxygen, and raw water, it must have a high porosity and a certain pore distribution to ensure uniform distribution of gas and smooth discharge of product water to avoid "flooding".

[0012] (4) Good corrosion resistance. Because it has to work under oxidation and reduction conditions for a long time, it has high requirements for its corrosion resistance and corrosion resistance.

[0013] (5) With dense structure and flat surface. The compact and smooth GDL can reduce contact resistance and improve its conductivity. Summary of the Invention

[0014] The present invention provides a fuel cell gas diffusion layer containing a microporous layer and a preparation method thereof, so as to solve the defects of poor toughness and easy breakage and fragmentation of the fully carbonized carbon fiber paper gas diffusion layer of the currently widely used fuel cell membrane electrode.

[0015] In order to solve the above problems, the present invention adopts the following technical solutions:

[0016] A fuel cell gas diffusion layer containing a microporous layer comprises a surface-coated polytetrafluoroethylene and carbon black mixture as the microporous layer and a carbon nanotube / carbon fiber membrane composed of multi-walled carbon nanotubes and chopped carbon fibers as the carbon membrane layer.

[0017] The specific process of preparing the following method is to obtain:

[0018] (1) Multi-walled carbon nanotubes (M-GRADE MWNTs) and short-cut carbon fibers were mixed at a ratio of 1:3, a small amount of deionized water was added, and the mixture was stirred evenly with a stirrer. A defoaming agent was then used for defoaming. A small amount of polyacrylamide was then added to the slurry, and the mixture was stirred evenly at a slow speed. The mixture was then filtered and dried to obtain a carbon nanotube / carbon fiber membrane. The surface density of the carbon nanotube / carbon fiber membrane was controlled to be 3 mg cm -2 ;

[0019] (2) placing the carbon nanotube / carbon fiber membrane obtained in step (1) into a phenolic resin solution with a mass fraction of 5% and soaking it evenly until the carbon nanotube / carbon fiber membrane gains 50-60% weight, drying it, and hot pressing it after drying to a thickness of 140 μm. Then placing it in a muffle furnace, heating it to 1600° C. at a heating rate of 5° C. per minute in an inert gas atmosphere for carbonization, and removing the carbon membrane after cooling it to room temperature;

[0020] (3) Mix carbon black and polytetrafluoroethylene dispersion at a ratio of 1:300, stir evenly, and then apply the mixture to the surface of the carbon film obtained in step (2), and control the surface density of carbon black and polytetrafluoroethylene dry matter on the carbon film surface to be 1-2 mg cm -2 ; Use hot pressing to form it to a thickness of 20μm, then put it into a muffle furnace, heat it at 200℃-400℃ in an air atmosphere for 0.1-1h, take it out after cooling to room temperature, and obtain the fuel cell gas diffusion layer containing the microporous layer.

[0021] Furthermore, the carbon fiber in step (1) is chopped carbon fiber with a diameter of 7-10 μm and a length of 5 mm.

[0022] Furthermore, the polyacrylamide in step (1) is a polyacrylamide aqueous solution with a mass fraction of 0.5%.

[0023] The fuel cell gas diffusion layer containing a microporous layer prepared by the above method has a surface-coated polytetrafluoroethylene and carbon black mixture as the microporous layer, and a carbon nanotube / carbon fiber membrane composed of multi-walled carbon nanotubes and chopped carbon fibers as the carbon membrane layer. The microporous layer has a thickness of 20 μm and an area density of 1 to 2 mg·cm -2 The mass ratio of polytetrafluoroethylene to carbon black in the microporous layer is 300:1, the thickness of the carbon film layer is 120 μm, and the surface density is 3 mg·cm -2 The mass ratio of the multi-walled carbon nanotubes to the chopped carbon fibers in the carbon film layer is 1:3, and the carbon film layer further contains 2.4 to 2.9 wt % of phenolic resin.

[0024] In the above scheme of the present invention, the polyacrylamide in step (1) is a dispersant. Polyacrylamide contains amide groups and is easy to form hydrogen bonds, which makes it have good water solubility and high chemical activity. It also has the ability to reduce the viscosity of the aqueous solution and is easy to disperse the insoluble phase in the mixed solution;

[0025] The main purpose of impregnating the dried carbon nanotube / carbon fiber membrane with 5% phenolic resin in step (2) is to increase its mechanical strength. The main reason for using a low concentration of 5% is that a high concentration will clog the pores of the carbon paper. The carbonization process in step (2) starts at 600°C. As the carbonization temperature increases, the functional groups and chemical bonds inside the carbon paper material can also be removed by carbonization, increasing the porosity and stabilizing the carbon structure. At 1800°C, the carbon structure will transform into a graphitized structure. After graphitization, its conductivity can be improved. However, the cost required will also increase significantly. If the temperature is too high, the frequency of equipment maintenance will also increase. Therefore, the carbonization temperature is selected to be 1600°C. Under the premise of ensuring high porosity in the carbon paper, the cost is also controlled;

[0026] The purpose of applying polytetrafluoroethylene in step (3) is to act as a binder and mix with carbon black to produce the MPL layer, and it also acts as a hydrophobic agent.

[0027] In the solution of the present invention, the carbon fiber used is carbon fiber soaked in PAN solution in order to make its mechanical strength better. The carbon nanotubes used are M-GRADE MWNTS with an outer diameter of 70-80nm. M-GRADE MWNTS is a long multi-walled carbon nanotube that is softer than other carbon materials, not easy to break, more conductive, has better film-forming ability, and is light in weight, which greatly reduces manufacturing costs.

[0028] Fuel cell electrode reactions show that for reactions to proceed on the electrocatalyst, electrons must be transferred to the reaction sites in the catalytic layer. The anode diffusion layer collects the current generated by electrochemical oxidation, while the cathode diffusion layer transfers electrons through electrochemical reduction reactions. Carbon fiber is a highly conductive carbon material with excellent overall properties. In addition to its high electrical conductivity, it also features corrosion resistance, wear resistance, high temperature resistance, high strength, and lightweight, making it a preferred material for gas diffusion layers. A conductive path is required within the diffusion layer, and carbon fiber paper must be a highly conductive material to smoothly conduct electrons from the electrochemical reaction. Generally, the carbon material in the diffusion layer fulfills this role, and the resistivity of the carbon fiber paper is a key performance indicator. The lower the resistivity, the higher the conductivity, the better the electrode performance, the lower the partial pressure in the fuel cell, and thus the impact on the overall power consumption of the entire cell. Furthermore, the excellent conductivity of the gas diffusion membrane improves fuel cell efficiency. Increased resistance increases internal electrode losses, resulting in reduced battery efficiency and increased cost. The use of a carbon fiber / carbon nanotube combination here perfectly addresses this issue. Multi-walled carbon nanotubes have good film-forming properties and excellent electrical conductivity. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 These are scanning electron microscope images of the fuel cell gas diffusion layer in step (2) of Example 3 of the present invention, wherein (a) the carbon fiber + carbon nanotube gas diffusion layer is magnified 3000 times, (b) the carbon fiber + carbon nanotube gas diffusion layer is magnified 5000 times, (c) the MPL layer is magnified 5000 times, and (d) the MPL layer is magnified 10000 times;

[0030] Figure 2 A data graph showing the relationship between power density and current of the fuel cell gas diffusion layer prepared in step (2) of Example 3 of the present invention;

[0031] Figure 3 and Figure 4 The data graphs are of the relationship between power density and current of the fuel cell gas diffusion layer with 80% and 100% phenolic resin content prepared in step (3) of Example 3, respectively. DETAILED DESCRIPTION

[0032] To make the objectives, technical solutions, and advantages of the embodiments of this application more clear, the technical solutions in the embodiments of this application are clearly and completely described. Obviously, the described embodiments are part of the embodiments of this application, not all of the embodiments. Based on the embodiments of this application, all other embodiments obtained by ordinary technicians in this field without making any creative efforts are within the scope of protection of this application.

[0033] The carbon fibers used in the following examples are chopped carbon fibers with a diameter of 7-10 μm and a length of 5 mm; the multi-walled carbon nanotubes used are M-GRADE MWNTs (purchased from Nano Tech Labs) with an outer diameter of 70-80 nm and a length of 1-2 mm.

[0034] Example 1 A fuel cell gas diffusion layer is prepared by the following method:

[0035] (1) Using anhydrous ethanol as a solvent, multi-walled carbon nanotubes and carbon fibers are mixed in a mass ratio of 1:3, a small amount of deionized water is added and the mixture is stirred in a blender for 4 minutes, and then a PAM dispersant accounting for 0.5% of the mass of the multi-walled carbon nanotubes is added. After stirring evenly, the mixture is filtered using a vacuum filtration device. The filtered filter cloth is placed in an 80°C forced air drying oven. After drying completely, the carbon nanotube / carbon fiber film is peeled off from the filter cloth.

[0036] In the above steps, for every square centimeter of filter cloth, a total mass of 507 mg of the mixture of multi-walled carbon nanotubes and carbon fibers is required to achieve a surface density of 3 mg cm -2 After the carbon nanotube / carbon fiber membrane is peeled from the filter cloth, it is rolled on a roller press. The distance between the rollers is controllable, and therefore the thickness is controllable. The thickness of the carbon nanotube / carbon fiber membrane can reach 110-120μm. Too thick will affect its air permeability and power density, while too thin will lack mechanical strength.

[0037] (2) Then, N,N-dimethylformamide (DMF) was used to prepare a 5% polyacrylonitrile (PAN) solution, and the carbon nanotube / carbon fiber membrane of step (1) was placed in the prepared PAN solution and evenly soaked, and then dried at 80°C. After drying, a hot press was used at 1000kg / cm 2 The carbon film was obtained by hot pressing under pressure for 20 minutes, then carbonizing at 1000°C for 2 hours in an argon atmosphere using a muffle furnace, and taking out after cooling to room temperature to obtain a carbon membrane. Carbon black and polytetrafluoroethylene were mixed into a dispersion in a mass ratio of 1:300, and after stirring evenly, the dispersion was evenly coated on the carbon membrane with a scraper. Finally, the dispersion was placed in a muffle furnace, heated at 350°C for 0.5 hours in an air atmosphere, and taken out after cooling to room temperature to prepare a gas diffusion layer containing a microporous layer.

[0038] In the above steps, the carbon film with the MPL layer is heated and dried, and then passed through a hot press (80°C) and a roller press to reduce its thickness. Because the MPL layer has a greater deformation capacity than the carbon film itself, the thickness of the MPL layer can be more easily controlled. The thickness of the microporous MPL layer can reach 20μm. The surface density of the MPL layer needs to be controlled to 2mg cm -2, because the size of the surface density is related to thickness, surface resistance, permeability, density, and tensile strength, the relationship is: (1) Thickness: the greater the surface density, the greater the thickness. (2) Surface resistance: MPL is enhanced by PTFE and carbon black. The more the amount, the lower the surface resistance. However, due to the nature of the material itself, the surface resistance may increase in a certain range. (3) Permeability: It decreases with the increase of MPL surface density. The decrease in permeability affects the power density. (4) Density: It increases with the increase of MPL surface density. (5) Tensile strength: It increases with the increase of MPL surface density. Surface density is easy to make thick, but the surface density is 1mg cm -2 Under existing conditions, it is difficult to achieve a very uniform appearance because white areas are always found after heat treatment at 350°C. The method for controlling the areal density in this embodiment is: given the area of the carbon film to be produced (the area of the mold used), the mass of the carbon black and polytetrafluoroethylene is weighed, and then the mass is divided by the area to obtain the areal density.

[0039] Example 2 A fuel cell gas diffusion layer is prepared by the following method:

[0040] (1) Using deionized water as a solvent, multi-walled carbon nanotubes and carbon fibers are mixed in a mass ratio of 1:3, a small amount of deionized water is added, and the mixture is stirred in a blender for 4 minutes. IPA is added for defoaming treatment, and then a PAM dispersant accounting for 0.5% of the mass of the multi-walled carbon nanotubes is added. After stirring evenly, the mixture is filtered using a suction filtration device. The filtered filter cloth is placed in an 80°C blast drying oven. After complete drying, the carbon nanotube / carbon fiber film is peeled off from the filter cloth.

[0041] In the above steps, for every square centimeter of filter cloth, a mixture of 3 mg of multi-walled carbon nanotubes and carbon fibers is required to achieve a surface density of 3 mg cm on the filter cloth. -2 After the carbon nanotube / carbon fiber membrane is peeled from the filter cloth, it is rolled on a roller press. The distance between the rollers is controllable, and therefore the thickness is controllable. The thickness of the carbon nanotube / carbon fiber membrane can reach 110-120μm. Too thick will affect its air permeability and power density, while too thin will lack mechanical strength.

[0042] (2) Then, N,N-dimethylformamide (DMF) was used to prepare a 5% polyacrylonitrile (PAN) solution, and the carbon nanotube / carbon fiber membrane of step (1) was placed in the prepared PAN solution and evenly soaked, and then dried at 80°C. After drying, a hot press was used at 1000kg / cm 2The carbon film was obtained by hot pressing under pressure for 20 minutes, then carbonizing at 1600°C for 2 hours in an argon atmosphere using a muffle furnace, and taking out after cooling to room temperature to obtain a carbon membrane. Carbon black and polytetrafluoroethylene were mixed into a dispersion in a mass ratio of 1:300, and after stirring evenly, the dispersion was evenly coated on the carbon membrane with a scraper. Finally, the dispersion was placed in a muffle furnace and heated at 350°C for 0.5 hours in an air atmosphere. The dispersion was cooled to room temperature and taken out to prepare a gas diffusion layer containing a microporous layer.

[0043] In the above steps, the carbon film coated with the MPL layer is heated and dried, and then passed through a hot press (80°C) and a roller press to control the thickness of the microporous layer MPL to 20 μm. The surface density of the MPL layer needs to be controlled to 2 mg cm -2 .

[0044] The parameters of the gas diffusion layer without the MPL layer in Example 1 and Example 2 are shown in Table 1. It can be found that the carbon nanotube / carbon fiber membrane increases in weight by 50-60% after being soaked in a solution prepared with PAN as the solute and DMF as the solvent. Its tensile strength is not as good as the carbon membrane soaked in phenolic resin in the following examples. At the same time, the tensile strength of the carbon membranes in Example 1 and Example 2 after being coated with the MPL layer still does not reach the ideal effect, and is only 9.88 and 11.23 N / cm, respectively.

[0045] Table 1

[0046]

[0047] Example 3

[0048] Take ordinary multi-walled carbon nanotubes purchased from BTR New Energy Materials Co., Ltd., add N-methylpyrrolidone at a concentration of 30-50wt%, ultrasonicate for 2h, and disperse at a speed of 20,000rpm to obtain solution A; take carbon fiber, add N-methylpyrrolidone at a concentration of 40wt%, ultrasonicate for 0.4h to obtain solution B; mix solutions A and B, with the ratio of multi-walled carbon nanotubes to carbon fiber being 3:1, add polytetrafluoroethylene solution at a concentration of 10wt%, ultrasonicate for 1h, disperse at a speed of 15,000rpm, filter to form a film, dry and shape it, and treat it at 300-350℃ for 3h to obtain a diffusion layer (GDL / CNTs+CF). The thickness of GDL / CNTs+CF is 100-120μm, and the surface resistance is 7mΩcm 2 , porosity 75%, density 0.36g / cm 3 , maximum curvature 180°, hydrophilicity / contact angle 145°.

[0049] The diameter of BTR multi-walled carbon nanotubes is 15-30nm and the length is 100-200μm.

[0050] The peak power density curve of the diffusion layer (GDL / CNTs+CF) prepared in Example 3 is no more than 200 mW cm as published in patent CN110600749B. -2 When the specific gravity of multi-walled carbon nanotubes and carbon fibers is reduced, the peak power density decreases. In Example 3, the multi-walled carbon nanotubes act as a microporous layer, forming larger pores between the carbon confinements, which is conducive to water transport, while the hydrophobic pores of the microporous layer maintain the free distribution of gas.

[0051] CN110600749B also discloses GDL / Toray-060H obtained by spraying a slurry of polytetrafluoroethylene + carbon powder on carbon paper. Although the cost is low, it cannot be bent more than 90 degrees. The cost of GDL / Toray-060H is lower.

[0052] Example 4 A fuel cell gas exchange layer containing a microporous layer is prepared by the following method:

[0053] (1) Using deionized water as a solvent, multi-walled carbon nanotubes and carbon fibers are mixed in a mass ratio of 1:3, a small amount of deionized water is added, and the mixture is stirred in a blender for 4 minutes. IPA is added for defoaming treatment, and then a PAM dispersant accounting for 0.5% of the mass of the multi-walled carbon nanotubes is added. After stirring evenly, a vacuum filtration device is used for filtration. The filtered filter cloth is placed in an 80°C blast drying oven. After complete drying, the carbon nanotube / carbon fiber film is peeled off from the filter cloth.

[0054] In the above steps, for every square centimeter of filter cloth, a total mass of 507 mg of the mixture of multi-walled carbon nanotubes and carbon fibers is required to achieve a surface density of 3 mg cm -2 The carbon nanotube / carbon fiber membrane reaches a thickness of 140μm. After being peeled from the filter cloth, the membrane is rolled on a roller press. The distance between the rollers is controllable, and thus the thickness is controllable. The membrane can be compressed to a thickness of 120μm. Too thick will affect its air permeability and power density, while too thin will lack mechanical strength.

[0055] (2) Subsequently, anhydrous ethanol was used to prepare a phenolic resin solution with a resin mass fraction of 5%, and the carbon nanotube / carbon fiber membrane of step (1) was placed in the prepared phenolic resin solution and immersed for 30 seconds to increase the weight by 50-60%, and then dried at 80°C. After drying, a hot press was used to press the membrane at 1000 kg / cm 2The carbon film was hot-pressed under pressure for 20 minutes, and then carbonized at 1600°C for 2 hours in an argon atmosphere using a muffle furnace. During the process, the muffle furnace was first heated to 1600°C at a heating rate of 5°C per minute, and then carbonized for 2 hours. After cooling to room temperature, it was taken out to obtain a carbon film (CNTs+CF). Carbon black and polytetrafluoroethylene were mixed into a dispersion at a mass ratio of 1:300. After stirring evenly, it was evenly sprayed onto the carbon film using a sprayer. Then it was placed in a muffle furnace and heated at 350°C for 0.5 hours in an air atmosphere. After cooling to room temperature, it was taken out to make a gas diffusion layer (CNTs+CF+MPL) containing a microporous layer.

[0056] In the above steps, the carbon film coated with the MPL layer is heated and dried, and then passed through a hot press (80°C) and a roller press to control the thickness of the microporous layer MPL to 20 μm. The surface density of the MPL layer needs to be controlled to 2 mg cm -2 In the above steps, if the carbon nanotube / carbon fiber membrane of step (1) is immersed in the prepared 3wt% phenolic resin solution for 20s, the carbon membrane increases in weight by 20-40%; if the carbon nanotube / carbon fiber membrane of step (1) is immersed in the prepared 10wt% phenolic resin solution for 2min, the carbon membrane increases in weight by 80-100wt%. A weight increase of 50-60wt% corresponds to a phenolic resin content of 2.4-2.9wt% in the carbon membrane after immersion, a weight increase of 80-100wt% corresponds to a phenolic resin content of 3.8-4.8wt% in the carbon membrane after immersion, and a weight increase of 20-40wt% corresponds to a phenolic resin content of 0.99-1.9wt% in the carbon membrane after immersion.

[0057] The total thickness of the microporous carbon film (CNTs+CF+MPL) in this embodiment is controlled to be 140μm. When the total thickness is increased, the air permeability is insufficient, reducing the power density value. When the total thickness is reduced, it is too thin, which reduces the mechanical strength. The thickness of the carbon nanotube / carbon fiber film is 120μm, the thickness of the microporous layer MPL is 20μm, and the surface resistance of CNTs+CF+MPL is 7.9mΩcm 2 , air permeability is 576.3ml·mm / cm 2 hr mmAq, density is 0.32 g / cm 3 , tensile strength of 20.80N / cm, hydrophilic / hydrophobic contact angle of 150°. Peak power density exceeds 700mW cm -2 . Figure 1 -c and Figure 1 -d are scanning electron microscope images of the MPL on the surface of the microporous carbon film at magnifications of 5000 and 10000 times, respectively.

[0058] The carbon membrane (CNTs+CF) obtained in Example 4 without spraying carbon black and polytetrafluoroethylene is a gas diffusion layer without microporous layer (no MPL). Table 2 shows the parameter data of the gas diffusion layer without microporous layer and the gas diffusion layer with microporous layer obtained in Example 4. Figure 2 As shown in the figure, the peak power density curve of the no MPL obtained in Example 4 without spraying carbon black and polytetrafluoroethylene does not exceed 600 mW cm -2 , while the peak power density of the microporous gas diffusion layer in Example 4 exceeded 700 mW cm -2 .

[0059] Table 2

[0060]

[0061] The fuel cell gas diffusion layer containing a microporous layer prepared in Example 4 has a 3% higher air permeability than the gas diffusion layers containing a microporous layer prepared in Example 1 and Example 2. This is mainly because after replacing polyacrylamide as a dispersant, the dispersion effect of the carbon nanotubes and carbon fiber membranes is better, so that the carbon nanotubes / carbon fiber membranes do not agglomerate and do not cause excessive local density. At the same time, the internal pores are greater, and the carbon membrane produced is more uniform.

[0062] Since the mechanical strength of the carbon film impregnated with polyacrylonitrile needs to be improved, in order to increase the strength, Example 4 impregnates the carbon nanotube / carbon fiber membrane in a low concentration of phenolic resin solution. If the concentration of the phenolic resin is too high, the porosity will decrease and its gas diffusion ability will be reduced. This example conducts a gradient experiment based on the phenolic resin concentration and the impregnation time to detect the weight gain of the carbon film, and finally determines the weight gain corresponding to the time the carbon film is immersed in a 5wt% phenolic resin solution.

[0063] After spraying MPL on the surface of the carbon nanotube / carbon fiber membrane, the tensile strength and hydrophobicity are improved. The peak power density of the CNTs+CF+MPL obtained in Example 4 is greatly improved compared with the GDL / CNTs+CF in Example 3 because the electrochemical performance of the carbon nanotubes M-GRADE MWNTs used is better than that of the BYD carbon nanotubes in Example 3, and the carbon nanotube / carbon fiber membrane in Example 4 can be bent 60°.

[0064] When the carbon membrane is prepared according to the method of Example 4, if the concentration of phenolic resin and the infiltration time of the carbon membrane are further increased, when the weight of the carbon membrane increases to 80% and 100% after infiltration, the power density and current relationship data of the obtained gas diffusion layer containing microporous layer are shown as follows: Figure 3 and Figure 4 As shown, the power density is only 427mW cm -2 and 362 mW cm -2, because its air permeability is significantly reduced, below 300ml·mm / cm 2 ·hr·mmAq, which directly affects its power density. This is significantly lower than the power density of a carbon membrane impregnated with 5wt% phenolic resin and then gaining 50-60%. Similarly, when the phenolic resin concentration is reduced and the impregnation time is shortened, resulting in a carbon membrane weight gain of only 20-40%, its tensile strength significantly decreases, falling below 5N / cm. After the MPL layer is applied, its tensile strength is also around 10N / cm, significantly lower than that of the microporous gas diffusion layer in Example 4. This reduces the bending angle of the microporous gas diffusion layer.

[0065] The carbon nanotubes used in CN110600749B are three times the mass of carbon fibers, which greatly increases the manufacturing cost, while the carbon nanotubes used in Example 3 are only one-third the mass of carbon fibers and also improve the peak power density.

[0066] Through the above improvements, this embodiment prepares a microporous gas diffusion layer with higher power density and lower manufacturing cost, and has sufficient tensile strength and hydrophobicity.

Claims

1. A method for preparing a fuel cell gas diffusion layer containing a microporous layer, characterized in that: The preparation method comprises the following steps: (1) Multi-walled carbon nanotubes (M-GRADE MWNTs) and short-cut carbon fibers were mixed at a ratio of 1:3, and a small amount of deionized water was added. The mixture was stirred evenly with a stirrer, and then defoamed with a defoaming agent. A small amount of polyacrylamide was then added to the slurry, and the mixture was stirred evenly at a slow speed. The mixture was then filtered and dried to obtain a carbon nanotube / carbon fiber membrane. The surface density of the carbon nanotube / carbon fiber membrane was controlled to be 3 mg cm -2 ; (2) The carbon nanotube / carbon fiber membrane obtained in step (1) is placed in a phenolic resin solution with a mass fraction of 5% and is evenly soaked until the carbon nanotube / carbon fiber membrane increases in weight by 50-60%. The membrane is dried and hot-pressed to a thickness of 140 μm. The membrane is then placed in a muffle furnace and heated to 1600°C at a heating rate of 5°C per minute in an inert gas atmosphere for carbonization. The carbon membrane is removed after cooling to room temperature. (3) Mix carbon black and polytetrafluoroethylene dispersion at a ratio of 1:300, stir evenly, and then apply the mixture to the surface of the carbon film obtained in step (2). The surface density of carbon black and polytetrafluoroethylene dry matter on the carbon film surface is controlled to be 1-2 mg cm -2 ; Use hot pressing to form it to a thickness of 20μm, then put it into a muffle furnace, heat it at 200℃-400℃ in an air atmosphere for 0.1-1h, take it out after cooling to room temperature, and obtain the fuel cell gas diffusion layer containing the microporous layer.

2. The method for preparing a fuel cell gas diffusion layer containing a microporous layer according to claim 1, characterized in that: The carbon fiber in step (1) is chopped carbon fiber with a diameter of 7-10 μm and a length of 5 mm.

3. The method for preparing a fuel cell gas diffusion layer containing a microporous layer according to claim 1, characterized in that: The polyacrylamide in step (1) is a polyacrylamide aqueous solution with a mass fraction of 0.5%.

4. A fuel cell gas diffusion layer comprising a microporous layer prepared according to the method of any one of claims 1 to 3, characterized in that: The microporous layer is a mixture of polytetrafluoroethylene and carbon black coated on the surface, and the carbon nanotube / carbon fiber membrane composed of multi-walled carbon nanotubes and chopped carbon fibers is used as the carbon membrane layer. The microporous layer has a thickness of 20 μm and an area density of 1-2 mg·cm -2 The mass ratio of polytetrafluoroethylene to carbon black in the microporous layer is 300:1, the thickness of the carbon film layer is 120 μm, and the surface density is 3 mg·cm -2 The mass ratio of the multi-walled carbon nanotubes to the chopped carbon fibers in the carbon film layer is 1:3, and the carbon film layer further contains 2.4-2.9 wt % of phenolic resin.

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