Gas diffusion layer and preparation method thereof

Abstract

The invention relates to the technical field of proton exchange membrane fuel cells, and discloses a gas diffusion layer and a preparation method thereof.The gas diffusion layer comprises a first microporous layer located on a supporting layer and a second microporous layer located on the first microporous layer, the thickness of the first microporous layer is 25-30 microns, the thickness of the second microporous layer is 15-20 microns, and the thickness of the second microporous layer is 20-30 microns. Cracks are formed in the surface of the second microporous layer, and the unit crack area of the cracks is 0.0038-0.0046 cm < 2 >. The number of the cracks on the surface of the gas diffusion layer is small, the unit area of the cracks is small, cross cracks cannot be generated, the contact resistance between the gas diffusion layer and a catalyst layer is small, and the proton diffusion membrane cannot be damaged when the gas diffusion layer is in extrusion contact with the proton diffusion membrane.

Description

Technical Field

[0001] The present invention relates to the technical field of proton exchange membrane fuel cells, and in particular to a gas diffusion layer and a preparation method thereof. Background Art

[0002] The gas diffusion layer is an important component structure of a fuel cell. It is located between the catalytic layer and the current collector plate, and is an important structure for supporting the catalytic layer and collecting electrons. At the same time, it provides multi-phase channels for gas, protons, electrons, water, etc. for the electrode reaction. It is composed of a support layer and a microporous layer. The commonly used materials for the support layer are carbon fiber paper, carbon fiber woven cloth, non-woven fabric and carbon black paper, with a thickness of 100 - 300 μm and a pore diameter greater than 20 μm. The microporous layer is composed of conductive carbon black and a hydrophobic agent, with a thickness of about 10 - 100 μm. The pore diameter is smaller than that of the support layer, about dozens of nanometers to several micrometers. The microporous layer can improve the pore structure and porosity of the support layer, reduce the contact resistance between the catalytic layer and the support layer, improve the redistribution of gas and water, and prevent "water flooding" of the electrode catalyst.

[0003] The pore size structure and composition of the microporous layer are of great significance for water and gas management. The traditional gas diffusion layer is composed of a support layer and a microporous layer provided on the support layer. However, the pore size structure and composition of a single microporous layer are relatively single. Therefore, in order to improve the ability of the microporous layer to manage water and gas, a double-layer microporous layer is adopted in the prior art. For example, a preparation method of a double-layer microporous layer type gas diffusion layer disclosed in CN111009666A. This technical solution adopts a double-layer microporous layer structure. The first microporous layer uses a pore-forming agent to form pores, so that the double-layer microporous layer has a gradient pore size. Another example is a preparation method of a gas diffusion layer with a gradient pore size microporous layer disclosed in CN111146467A. This technical solution uses two different carbon blacks to form two microporous layers, forming a microporous layer with a gradient pore size. By forming a gradient pore size structure, the water management ability is significantly improved, and the battery performance of the fuel cell is significantly enhanced. When the present invention studies the above technical solutions, it is found that different conductive carbon blacks will undergo homolytic cleavage during high-temperature drying, forming cracks, and the cracks will reduce the contact with the catalytic layer, thereby increasing the contact resistance and reducing the performance of the fuel cell. Summary of the Invention

[0004] Aiming at the problem that cracks are generated during high-temperature drying in the microporous layer in the prior art, the present invention provides a gas diffusion layer and a preparation method thereof. The gas diffusion layer adopts a double-layer microporous layer structure, composed of a support layer, a first microporous layer and a second microporous layer. The first microporous layer is made of Vulcan XC-72, and the second microporous layer is made of acetylene black. The gas diffusion layer has few surface cracks, small crack area per unit, no cross-shaped cracks, small contact resistance with the catalytic layer, and will not cause damage to the proton diffusion membrane when extruded and contacted with the proton diffusion membrane.

[0005] The specific technical solution of the present invention is as follows: A gas diffusion layer includes a first microporous layer on a support layer and a second microporous layer on the first microporous layer. The thickness of the first microporous layer is 25 - 30 μm, the thickness of the second microporous layer is 15 - 20 μm, and cracks are provided on the surface of the second microporous layer.

[0006] Preferably, cracks are provided on the surface, and the cracks include longitudinal cracks and transverse cracks.

[0007] Preferably, the number of the longitudinal cracks is 49 - 57 per cm 2 , the average length of the longitudinal cracks is 412.95 - 438.16 μm, and the average width of the longitudinal cracks is 17.58 - 17.92 μm.

[0008] Preferably, the number of the transverse cracks is 18 - 19 per cm 2 , the average length of the transverse cracks is 71.26 - 71.93 μm, and the average width of the transverse cracks is 10.92 - 12.11 μm.

[0009] Preferably, the unit crack area of the cracks is 0.0038 - 0.0046 cm 2 .

[0010] Preferably, the raw materials of the gas diffusion layer include conductive carbon black, dispersant, hydrophobic agent, solvent and thickening agent.

[0011] Preferably, the dispersant is Triton.

[0012] Preferably, the conductive carbon black is Vulcan XC - 72 and acetylene black.

[0013] Preferably, the hydrophobic agent is one or more of polytetrafluoroethylene, polyvinylidene fluoride and polychlorotrifluoroethylene.

[0014] Preferably, the solvent is one or more of water, n - propanol, isopropanol and ethanol.

[0015] Preferably, the thickening agent is one or more of carboxymethyl cellulose, polyethylene glycol, polyvinylpyrrolidone and hydroxyethyl cellulose.

[0016] The present invention provides a gas diffusion layer, which is composed of a support layer, a first microporous layer and a second microporous layer. The first microporous layer is made of Vulcan XC-72, and the second microporous layer is made of acetylene black. The present invention finds that when using Vulcan XC-72 to prepare the microporous layer, a large number of cracks will appear in the microporous layer after drying and sintering. These cracks are numerous and large in length and width, so the contact resistance between the microporous layer and the catalytic layer will increase significantly, and the generated cracks will appear as cross cracks. Such cross cracks will lead to a reduction in the strength of the microporous layer. At the same time, when a certain pressure is applied to the proton exchange membrane and it is extruded towards the microporous layer, too many cracks will cause the proton exchange membrane to rupture.

[0017] However, the present invention finds that compared with the microporous layer made of Vulcan XC-72, the number of cracks, crack length and crack width of the microporous layer made of acetylene black are significantly reduced, and the generated cracks do not cross, and the strength of the microporous layer is higher. Therefore, in order to solve the problem of excessive cracks in the Vulcan XC-72 microporous layer, the present invention coats a layer of acetylene black on the microporous layer made of Vulcan XC-72 to form a second microporous layer, and fills the cracks generated in the first microporous layer with the acetylene black slurry, significantly reducing the generation of cracks. Thus, the contact resistance between the microporous layer and the catalytic layer is significantly reduced.

[0018] In addition, in order to ensure the water vapor regulation ability of the microporous layer, the thickness of the microporous layer is usually limited to 40-50 μm. The present invention finds that when acetylene black is used as the raw material of the second microporous layer, the thickness of the second microporous layer made thereof has an important influence on the number, length and width of the cracks generated on the surface of the second microporous layer. The thicker the acetylene black thickness, the more cracks are generated, and the greater the crack length and width. Therefore, by adjusting the thickness of the first microporous layer and the second microporous layer, the number, length and width of the cracks on the surface of the gas diffusion membrane can be controlled, thereby reducing the contact resistance between the microporous layer and the catalytic layer, and improving the strength of the microporous layer and the support layer, and also ensuring that the proton exchange membrane will not be damaged when extruded towards the microporous layer.

[0019] A preparation method of the above gas diffusion layer includes the following steps: performing a hydrophobic treatment on carbon paper with a hydrophobic agent to make a support layer; making a first microporous layer slurry and a second microporous layer slurry from conductive carbon black, a dispersant, a hydrophobic agent, a solvent and a thickening agent; coating the first microporous layer slurry on the surface of the support layer and drying it to make a first microporous layer / support layer, and coating the second microporous layer slurry on the surface of the first microporous layer and drying and sintering it to make a gas diffusion layer.

[0020] Preferably, the drying temperature is 80-100 °C.

[0021] Preferably, the sintering temperature is 300-400 °C.

[0022] The present invention also provides a method for preparing the above gas diffusion layer. In this method, conductive carbon black, a dispersant, a hydrophobic agent, a solvent, and a thickening agent are made into a first microporous layer slurry and a second microporous layer slurry. Then, the first microporous layer slurry is coated on a support layer and dried to form a first microporous layer. Finally, the second microporous layer slurry is coated on the support layer and dried and sintered to form a second microporous layer. The present invention uses a relatively low drying temperature. In the prior art, drying is usually carried out at a high temperature of 250°C. The present invention has found that using too high a drying temperature will cause an increase in the number, length, and width of cracks. Therefore, the present invention selects a lower drying temperature for drying, so as to reduce the number, length, and width of cracks on the surface of the microporous layer.

[0023] Compared with the prior art, the present application has the following technical effects: The gas diffusion layer adopts a double-layer microporous layer structure, which is composed of a support layer, a first microporous layer, and a second microporous layer. The first microporous layer is made of Vulcan XC-72, and the second microporous layer is made of acetylene black. The gas diffusion layer has few surface cracks, a small unit area of cracks, does not generate cross cracks, has a small contact resistance with the catalyst layer, and does not cause damage to the proton diffusion membrane when in extrusion contact with the proton diffusion membrane. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 Optical microscope images of the gas diffusion layers of Example 1, Comparative Example 1, and Comparative Example 2.

[0025] Figure 2 Polarization curves of fuel cells made from Example 1, Comparative Example 1, and Comparative Example 2. DETAILED DESCRIPTION OF THE INVENTION

[0026] The present invention will be further described below in conjunction with examples.

[0027] Example 1: A method for preparing a gas diffusion layer, comprising the following steps: Hydrophobic treatment of carbon paper: Polytetrafluoroethylene emulsion (60 wt%) is added to ultrapure water to prepare an emulsion (3 wt%). The emulsion is stirred and dispersed evenly for standby. The carbon paper (Toray 060) is cut into the required size, clamped with tweezers, and immersed in the above emulsion. After immersion, it is dried at 80°C for 30 min and then sintered at 350°C to form hydrophobic carbon paper (5 wt% polytetrafluoroethylene); Preparation of the first microporous layer slurry: Place 0.15 g of carboxymethyl cellulose in 90 g of ultrapure water and stir until the carboxymethyl cellulose is completely dissolved. Then add 18 g of Triton and 27 g of conductive carbon black (Vulcan XC-72), and continue stirring for 30 min. Then sonicate for 30 min to uniformly disperse the conductive carbon black. Then add 19.29 g of polytetrafluoroethylene emulsion (60 wt%) after sonication for 30 min, then carry out high-speed stirring for 30 min, and then sonicate for 30 min. Repeat 2 times to prepare the first microporous layer slurry; Prepare the second microporous layer slurry: Place 0.17 g of carboxymethyl cellulose in 108 g of ultrapure water and stir until the carboxymethyl cellulose is completely dissolved. Then add 7.5 g of Triton and 7.5 g of conductive carbon black (acetylene black), and continue stirring for 30 min. Then sonicate for 30 min to uniformly disperse the conductive carbon black. Then add 5.37 g of polytetrafluoroethylene emulsion (60 wt%) after sonication for 30 min, then carry out high-speed stirring for 30 min, and then sonicate for 30 min. Repeat 2 times to prepare the second microporous layer slurry; Prepare the gas diffusion layer: Use a scraper to coat the first microporous layer slurry on the surface of the support layer. After coating, place it in an 80°C oven and dry for 30 min. After drying, make the first microporous layer / support layer composite. Then use a scraper to coat the second microporous layer slurry on the surface of the first microporous layer. After coating, place it in an 80°C oven and dry for 10 min, and then put it in a 350°C oven and sinter for 30 min to make the gas diffusion layer; The thickness of the first microporous layer in the gas diffusion layer is 27 μm, the thickness of the second microporous layer is 18 μm, and the total thickness of the first microporous layer and the second microporous layer is 45 μm.

[0028] Example 2: A method for preparing a gas diffusion layer, comprising the following steps: Hydrophobic treatment of carbon paper: Add polytetrafluoroethylene emulsion (60 wt%) to ultrapure water to prepare an emulsion (3 wt%). Stir and disperse the emulsion evenly for standby. Cut the carbon paper (Toray 060) into the required size, clamp the carbon paper with tweezers and immerse it in the above emulsion. After immersion, place it in an 80°C condition and dry for 30 min, and then place it in a 350°C condition and sinter to make hydrophobic carbon paper (5 wt% polytetrafluoroethylene); Prepare the first microporous layer slurry: Place 0.15 g of carboxymethyl cellulose in 90 g of ultrapure water and stir until the carboxymethyl cellulose is completely dissolved. Then add 18 g of Triton and 27 g of conductive carbon black (Vulcan XC-72), and continue stirring for 30 min. Then sonicate for 30 min to uniformly disperse the conductive carbon black. Then add 19.29 g of polytetrafluoroethylene emulsion (60 wt%) after sonication for 30 min, and then carry out high-speed stirring for 30 min. Then sonicate for 30 min again. Repeat this process 2 times to prepare the first microporous layer slurry. Prepare the second microporous layer slurry: Place 0.17 g of carboxymethyl cellulose in 108 g of ultrapure water and stir until the carboxymethyl cellulose is completely dissolved. Then add 7.5 g of Triton and 7.5 g of conductive carbon black (acetylene black), and continue stirring for 30 min. Then sonicate for 30 min to uniformly disperse the conductive carbon black. Then add 5.37 g of polytetrafluoroethylene emulsion (60 wt%) after sonication for 30 min, and then carry out high-speed stirring for 30 min. Then sonicate for 30 min again. Repeat this process 2 times to prepare the second microporous layer slurry. Prepare the gas diffusion layer: Use a scraper to coat the first microporous layer slurry on the surface of the support layer. After coating, place it in an 80 °C oven and dry for 30 min. After drying, make the first microporous layer / support layer composite. Then use a scraper to coat the second microporous layer slurry on the surface of the first microporous layer. After coating, place it in an 80 °C oven and dry for 10 min. Then put it in a 350 °C oven and sinter for 30 min to make the gas diffusion layer. The thickness of the first microporous layer in the gas diffusion layer is 24 μm, the thickness of the second microporous layer is 16 μm, and the total thickness of the first microporous layer and the second microporous layer is 40 μm.

[0029] Example 3: A method for preparing a gas diffusion layer, comprising the following steps: Hydrophobic treatment of carbon paper: Add polytetrafluoroethylene emulsion (60 wt%) to ultrapure water to prepare an emulsion (3 wt%). Stir and disperse the emulsion evenly for standby. Cut the carbon paper (Toray 060) into the required size, clamp the carbon paper with tweezers and immerse it in the above emulsion. After immersion, place it in an 80 °C condition and dry for 30 min. Then place it in a 350 °C condition and sinter to make hydrophobic carbon paper (5 wt% of polytetrafluoroethylene). Prepare the first microporous layer slurry: Place 0.15 g of carboxymethyl cellulose in 90 g of ultrapure water and stir until the carboxymethyl cellulose is completely dissolved. Then add 18 g of Triton and 27 g of conductive carbon black (Vulcan XC-72), and continue stirring for 30 min. Then sonicate for 30 min to uniformly disperse the conductive carbon black. Then add 19.29 g of polytetrafluoroethylene emulsion (60 wt%) after 30 min of sonication, and then carry out high-speed stirring for 30 min, and then sonicate for 30 min. Repeat 2 times to prepare the first microporous layer slurry; Prepare the second microporous layer slurry: Place 0.17 g of carboxymethyl cellulose in 108 g of ultrapure water and stir until the carboxymethyl cellulose is completely dissolved. Then add 7.5 g of Triton and 7.5 g of conductive carbon black (acetylene black), and continue stirring for 30 min. Then sonicate for 30 min to uniformly disperse the conductive carbon black. Then add 5.37 g of polytetrafluoroethylene emulsion (60 wt%) after 30 min of sonication, and then carry out high-speed stirring for 30 min, and then sonicate for 30 min. Repeat 2 times to prepare the second microporous layer slurry; Prepare the gas diffusion layer: Use a scraper to coat the first microporous layer slurry on the surface of the support layer. After coating, place it in an 80 °C oven and dry for 30 min. After drying, make the first microporous layer / support layer composite. Then use a scraper to coat the second microporous layer slurry on the surface of the first microporous layer. After coating, place it in an 80 °C oven and dry for 10 min, and then put it in a 350 °C oven and dry for 30 min to make the gas diffusion layer; The thickness of the first microporous layer in the gas diffusion layer is 30 μm, the thickness of the second microporous layer is 20 μm, and the total thickness of the first microporous layer and the second microporous layer is 50 μm.

[0030] Comparative Example 1: Compared with Example 1, in Comparative Example 1, only conductive carbon black (Vulcan XC-72) is used to prepare the microporous layer, including the following steps: Hydrophobic treatment of carbon paper: Add polytetrafluoroethylene emulsion (60 wt%) to ultrapure water to prepare an emulsion (3 wt%). Stir and disperse the emulsion evenly for standby. Cut the carbon paper (Toray 060) into the required size, hold the carbon paper with tweezers and immerse it in the above emulsion. After immersion, place it in an 80 °C condition and dry for 30 min, and then place it in a 350 °C condition and sinter to make a hydrophobic carbon paper (5 wt% of polytetrafluoroethylene); Prepare the microporous layer slurry: Put 0.15 g of carboxymethyl cellulose into 90 g of ultrapure water and stir until the carboxymethyl cellulose is completely dissolved. Then add 18 g of Triton and 27 g of conductive carbon black (Vulcan XC-72), and continue to stir for 30 min. Then sonicate for 30 min to uniformly disperse the conductive carbon black. Then add 19.29 g of polytetrafluoroethylene emulsion (60 wt%) after sonication for 30 min, and then carry out high-speed stirring for 30 min and sonication for 30 min. Repeat 2 times to prepare the microporous layer slurry. Preparation of gas diffusion layer: Use a spatula to coat the microporous layer slurry on the surface of the support layer. After coating, place it in an oven at 80 °C and dry for 30 min. After drying, place it in an oven at 350 °C and sinter for 30 min to make the gas diffusion layer; the thickness of the microporous layer in the gas diffusion layer is 45 μm.

[0031] Comparative Example 2: Compared with Example 2, in Comparative Example 2, only conductive carbon black (acetylene black) was used to prepare the microporous layer, including the following steps: Hydrophobic treatment of carbon paper: Add polytetrafluoroethylene emulsion (60 wt%) to ultrapure water to prepare an emulsion (3 wt%). Stir and disperse the emulsion evenly for standby. Cut the carbon paper (Toray 060) into the required size, hold the carbon paper with tweezers and immerse it in the above emulsion. After immersion, place it in an oven at 80 °C and dry for 30 min, and then place it in an oven at 350 °C and sinter to make hydrophobic carbon paper (5 wt% of polytetrafluoroethylene). Preparation of microporous layer slurry: Put 0.17 g of carboxymethyl cellulose into 108 g of ultrapure water and stir until the carboxymethyl cellulose is completely dissolved. Then add 7.5 g of Triton and 7.5 g of conductive carbon black (acetylene black), and continue to stir for 30 min. Then sonicate for 30 min to uniformly disperse the conductive carbon black. Then add 5.37 g of polytetrafluoroethylene emulsion (60 wt%) after sonication for 30 min, and then carry out high-speed stirring for 30 min and sonication for 30 min. Repeat 2 times to prepare the microporous layer slurry. Preparation of gas diffusion layer: Use a spatula to coat the microporous layer slurry on the surface of the support layer. After coating, place it in an oven at 80 °C and dry for 30 min. After drying, place it in an oven at 350 °C and sinter for 30 min to make the gas diffusion layer; the thickness of the microporous layer in the gas diffusion layer is 45 μm.

[0032] Comparative Example 3: Compared with Example 1, in Comparative Example 3, the thickness of the first microporous layer is 40 μm and the thickness of the second microporous layer is 5 μm, including the following steps: Hydrophobic treatment of carbon paper: Add polytetrafluoroethylene emulsion (60 wt%) to ultrapure water to prepare an emulsion (3 wt%). Stir and disperse the emulsion evenly for later use. Cut the carbon paper (Toray 060) into the required size, clamp the carbon paper with tweezers and immerse it in the above emulsion. After immersion, dry it at 80 °C for 30 min, and then sinter it at 350 °C to make hydrophobic carbon paper (5 wt% polytetrafluoroethylene); Preparation of the first microporous layer slurry: Put 0.15 g of carboxymethyl cellulose into 90 g of ultrapure water and stir until the carboxymethyl cellulose is completely dissolved. Then add 18 g of Triton and 27 g of conductive carbon black (Vulcan XC-72) and continue to stir for 30 min. Then ultrasonicate for 30 min to uniformly disperse the conductive carbon black; Then add 19.29 g of polytetrafluoroethylene emulsion (60 wt%) after 30 min of ultrasonication, and then carry out high-speed stirring for 30 min and ultrasonication for 30 min. Repeat 2 times to make the first microporous layer slurry; Preparation of the second microporous layer slurry: Put 0.17 g of carboxymethyl cellulose into 108 g of ultrapure water and stir until the carboxymethyl cellulose is completely dissolved. Then add 7.5 g of Triton and 7.5 g of conductive carbon black (acetylene black) and continue to stir for 30 min. Then ultrasonicate for 30 min to uniformly disperse the conductive carbon black; Then add 5.37 g of polytetrafluoroethylene emulsion (60 wt%) after 30 min of ultrasonication, and then carry out high-speed stirring for 30 min and ultrasonication for 30 min. Repeat 2 times to make the second microporous layer slurry; Preparation of the gas diffusion layer: Use a scraper to coat the first microporous layer slurry on the surface of the support layer. After coating, dry it in an 80 °C oven for 30 min. After drying, make the first microporous layer / support layer composite. Then use a scraper to coat the second microporous layer slurry on the surface of the first microporous layer. After coating, dry it in an 80 °C oven for 10 min, and then sinter it in a 350 °C oven for 30 min to make the gas diffusion layer; The thickness of the first microporous layer in the gas diffusion layer is 40 μm, the thickness of the second microporous layer is 5 μm, and the total thickness of the first microporous layer and the second microporous layer is 45 μm.

[0033] Comparative Example 4: Compared with Example 1, in Comparative Example 4, the thickness of the first microporous layer is 5 μm and the thickness of the second microporous layer is 40 μm, including the following steps: Hydrophobic treatment of carbon paper: Add polytetrafluoroethylene emulsion (60 wt%) to ultrapure water to prepare an emulsion (3 wt%). Stir and disperse the emulsion evenly for later use. Cut the carbon paper (Toray 060) into the required size, clamp the carbon paper with tweezers and immerse it in the above emulsion. After immersion, dry it at 80 °C for 30 min, and then sinter it at 350 °C to make hydrophobic carbon paper (5 wt% polytetrafluoroethylene); Prepare the first microporous layer slurry: Put 0.15 g of carboxymethyl cellulose into 90 g of ultrapure water and stir until the carboxymethyl cellulose is completely dissolved. Then add 18 g of Triton and 27 g of conductive carbon black (Vulcan XC-72) and continue to stir for 30 min. Then sonicate for 30 min to uniformly disperse the conductive carbon black; Then add 19.29 g of polytetrafluoroethylene emulsion (60 wt%) after 30 min of sonication, and then carry out high-speed stirring for 30 min and sonication for 30 min. Repeat 2 times to make the first microporous layer slurry; Prepare the second microporous layer slurry: Put 0.17 g of carboxymethyl cellulose into 108 g of ultrapure water and stir until the carboxymethyl cellulose is completely dissolved. Then add 7.5 g of Triton and 7.5 g of conductive carbon black (acetylene black) and continue to stir for 30 min. Then sonicate for 30 min to uniformly disperse the conductive carbon black; Then add 5.37 g of polytetrafluoroethylene emulsion (60 wt%) after 30 min of sonication, and then carry out high-speed stirring for 30 min and sonication for 30 min. Repeat 2 times to make the second microporous layer slurry; Prepare the gas diffusion layer: Use a scraper to coat the first microporous layer slurry on the surface of the support layer. After coating, place it in an 80 °C oven and dry for 30 min. After drying, place it in a 350 °C oven and dry for 30 min to make the first microporous layer / support layer composite. Then use a scraper to coat the second microporous layer slurry on the surface of the first microporous layer. After coating, place it in an 80 °C oven and dry for 10 min, and then put it in a 350 °C oven and dry for 30 min to make the gas diffusion layer; The thickness of the first microporous layer in the gas diffusion layer is 5 μm, the thickness of the second microporous layer is 40 μm, and the total thickness of the first microporous layer and the second microporous layer is 45 μm.

[0034] Comparative Example 5: Compared with Example 1, the drying temperature in Comparative Example 5 is 250 °C, and it includes the following steps: A method for preparing a gas diffusion layer, including the following steps: Hydrophobic treatment of carbon paper: Add polytetrafluoroethylene emulsion (60 wt%) to ultrapure water to prepare an emulsion (3 wt%). Stir and disperse the emulsion evenly for later use. Cut the carbon paper (Toray 060) into the required size, clamp the carbon paper with forceps and immerse it in the above emulsion. After immersion, dry it at 80 °C for 30 min, and then sinter it at 350 °C to make hydrophobic carbon paper (5 wt% polytetrafluoroethylene). Prepare the first microporous layer slurry: Place 0.15 g of carboxymethyl cellulose in 90 g of ultrapure water and stir until the carboxymethyl cellulose is completely dissolved. Then add 18 g of Triton and 27 g of conductive carbon black (Vulcan XC-72) and continue to stir for 30 min. Then ultrasonicate for 30 min to uniformly disperse the conductive carbon black. Then add 19.29 g of polytetrafluoroethylene emulsion (60 wt%) after 30 min of ultrasonication, and then carry out high-speed stirring for 30 min and ultrasonication for 30 min. Repeat 2 times to make the first microporous layer slurry. Prepare the second microporous layer slurry: Place 0.17 g of carboxymethyl cellulose in 108 g of ultrapure water and stir until the carboxymethyl cellulose is completely dissolved. Then add 7.5 g of Triton and 7.5 g of conductive carbon black (acetylene black) and continue to stir for 30 min. Then ultrasonicate for 30 min to uniformly disperse the conductive carbon black. Then add 5.37 g of polytetrafluoroethylene emulsion (60 wt%) after 30 min of ultrasonication, and then carry out high-speed stirring for 30 min and ultrasonication for 30 min. Repeat 2 times to make the second microporous layer slurry. Prepare the gas diffusion layer: Use a spatula to coat the first microporous layer slurry on the surface of the support layer. After coating, dry it in an oven at 250 °C for 30 min. After drying, make the first microporous layer / support layer composite. Then use a spatula to coat the second microporous layer slurry on the surface of the first microporous layer. After coating, dry it in an oven at 250 °C for 10 min, and then sinter it in an oven at 350 °C for 30 min to make the gas diffusion layer. The thickness of the first microporous layer in the gas diffusion layer is 27 μm, the thickness of the second microporous layer is 18 μm, and the total thickness of the first microporous layer and the second microporous layer is 45 μm.

[0035] Test the appearance of the gas diffusion layers prepared in Examples 1 to 3 and Comparative Examples 1 to 5. The test method includes the following steps: The above samples were detected by scanning electron microscopy. Quantitative analysis was carried out on the number of cracks and the area of cracks generated in 10 different regions within an area of 1*1 cm, and their average values were calculated as the data of the number of unit cracks, crack length, crack width, and unit crack area. Since there are cracks in two directions in the sample, the longer direction of the crack is defined as the long direction, and the shorter direction is defined as the short direction. The number of cracks includes all the cracks in both directions, and the length and width are represented separately. The crack area is the total crack area in both directions. Specifically, the number of cracks is counted with a complete crack as one, and the crack length is also calculated with a continuous and unbroken crack as its length. The length is calculated with an angle less than 45° between one direction and the other direction. If the angle between the two directions is greater than 45°, it is calculated within the length range of the other direction. The crack width is taken as the average of the middle width and the widths on both sides of a certain crack segment, and the unit crack area is the product of the average width and the average length; the test results are shown in Table 1.

[0036] Table 1 Test Results As shown in Table 1, the number of long cracks in Examples 1 to 3 is 49 - 57 cracks / cm 2 , the average length of the long cracks is 412.95 - 438.16 μm, the average width of the long cracks is 17.58 - 17.92 μm, the number of short cracks is 18 - 19 cracks / cm 2 , the average length of the short cracks is 71.26 - 71.93 μm, the average width of the short cracks is 10.92 - 12.11 μm, and the unit crack area is 0.0038 - 0.0046 cm 2 , The above results show that the gas diffusion layer prepared by the present invention has few surface cracks, a small unit area of cracks, a small contact resistance with the catalyst layer, and will not cause damage to the proton diffusion membrane when in extrusion contact with the proton diffusion membrane.

[0037] Comparative Example 1 is a technical solution for preparing a microporous layer using only conductive carbon black Vulcan XC-72. The unit crack area, average length of longitudinal cracks, average width of longitudinal cracks, number of transverse cracks, average length of transverse cracks, and average width of transverse cracks in Comparative Example 1 are all significantly higher than those in Example 1. Comparative Example 2 is a technical solution for preparing a microporous layer using only conductive carbon black acetylene black. Compared with Example 1, the unit crack area, average length of longitudinal cracks, average width of longitudinal cracks, number of transverse cracks, average length of transverse cracks, and average width of transverse cracks in Comparative Example 2 are all significantly increased. Compared with Comparative Example 1, the unit crack area, average length of longitudinal cracks, average width of longitudinal cracks, number of transverse cracks, average length of transverse cracks, and average width of transverse cracks in Comparative Example 2 are all significantly decreased. The above results show that there are significant differences in the shrinkage stress of the materials after drying for different conductive carbon blacks, which leads to significant differences in the crack morphology generated by different conductive carbon blacks. The shrinkage stress of Vulcan XC-72 is significantly higher than that of acetylene black. Therefore, the microporous layer made of Vulcan XC-72 will produce cracks that are longer, wider, and larger in area. At the same time, when multiple cracks cross, cross-shaped cracks will be formed. In addition, it is found that after assembling with the proton diffusion membrane, the cross-shaped cracks are more likely to cause damage to the proton diffusion membrane, resulting in a significant reduction in battery performance. However, when the microporous layer without cross-shaped cracks contacts the proton diffusion membrane, it will not damage the proton diffusion membrane.

[0038] In Comparative Examples 3 and 4, the thicknesses of the second microporous layer and the first microporous layer were tested respectively. The results of Comparative Examples 3 and 4 show that when the second microporous layer is too thin, it cannot inhibit the stress shrinkage effect of the underlying first microporous layer, and there will still be many wide cracks on the surface of the second microporous layer. When the thickness of the second microporous layer is too thick, although the number of cracks is significantly reduced, due to the lower mass transfer ability of acetylene black than that of Vulcan XC-72, the performance of the gas diffusion layer is reduced. Therefore, it is necessary to limit the thicknesses of the first microporous layer and the second microporous layer to ensure the performance of the gas diffusion layer while significantly reducing the number and area of cracks on the surface of the second microporous layer.

[0039] In Comparative Example 5, the drying temperature was also studied. Compared with Example 1, the unit crack area, average length of longitudinal cracks, average width of longitudinal cracks, number of transverse cracks, average length of transverse cracks, and average width of transverse cracks are all increased. This result shows that the drying temperature also has an important impact on the generation of cracks. Too high a drying temperature will cause more intense gasification of the solvent in the slurry, which will have a strong impact on the material surface, increasing the number and area of cracks on the surface of the microporous layer.

[0040] Detection Example 2: The gas diffusion layers prepared in Example 1, Comparative Example 2, and Comparative Example 3 were assembled into a fuel cell. The fuel cell testing method includes the following steps: The prepared gas diffusion layer was cut into samples of a certain size, and then the gas diffusion layer, the catalytic layer, and the proton exchange membrane were assembled into a fuel cell. The fuel cell was tested under the following conditions: the temperature was 70 °C, the relative humidity was 100%, the hydrogen flow rate was 0.2 L / min, the hydrogen back pressure was 150 Kpa, the oxygen flow rate was 0.8 L / min, and the oxygen back pressure was 150 KPa. The test results are shown in Figure 2 .

[0041] As Figure 2 shown, as the current increases, the voltage differences among Comparative Example 1, Comparative Example 2, and Example 1 also increase significantly. When the current increases to a certain value, the voltage of Comparative Example 1 decreases significantly. The above results indicate that the gas diffusion layer prepared by the present invention has more stable and excellent battery performance under high-current conditions.

[0042] The above are only the preferred embodiments of the present invention and do not limit the present invention in any way. Any simple modifications, changes, and equivalent transformations made to the above embodiments based on the technical essence of the present invention still fall within the protection scope of the technical solution of the present invention.

Claims

1. A gas diffusion layer, characterized in that, It includes a first microporous layer located on a support layer and a second microporous layer located on the first microporous layer. The thickness of the first microporous layer is 25 - 30 μm, the thickness of the second microporous layer is 15 - 20 μm, and cracks are provided on the surface of the second microporous layer.

2. The gas diffusion layer according to claim 1, wherein The cracks include longitudinal cracks and transverse cracks.

3. The gas diffusion layer according to claim 3, characterized in that, The number of the longitudinal cracks is 49 to 57 cracks / cm 2 , the average length of the longitudinal cracks is 412.95 to 438.16 μm, and the average width of the longitudinal cracks is 17.58 to 17.92 μm.

4. The gas diffusion layer according to claim 3, wherein, The number of the short cracks is 18 - 19 cracks / cm 2 , the average length of the short cracks is 71.26 - 71.93 μm, and the average width of the short cracks is 10.92 - 12.11 μm.

5. The gas diffusion layer according to claim 1 or 2, characterized in that, The unit crack area of the crack is 0.0038~0.0046 cm 2 .

6. The gas diffusion layer according to claim 1, characterized in that, The raw materials of the gas diffusion layer include conductive carbon black, a dispersant, a hydrophobic agent, a solvent, and a thickening agent.

7. The gas diffusion layer according to claim 6, characterized in that, The conductive carbon black is Vulcan XC - 72 and acetylene black; the dispersant is Triton; the hydrophobic agent is one or several of polytetrafluoroethylene, polyvinylidene fluoride, and polychlorotrifluoroethylene; the solvent is one or several of water, n - propanol, isopropanol, and ethanol; the thickening agent is one or more of carboxymethyl cellulose, polyethylene glycol, polyvinylpyrrolidone, and hydroxyethyl cellulose.

8. A method for preparing a gas diffusion layer according to any one of claims 1 to 7, characterized in that, It includes the following steps: Performing hydrophobic treatment on carbon paper with a hydrophobic agent to make a support layer; preparing a first microporous layer slurry and a second microporous layer slurry from conductive carbon black, a dispersant, a hydrophobic agent, a solvent, and a thickening agent; coating the first microporous layer slurry on the surface of the support layer and drying it to make a first microporous layer / support layer, and coating the second microporous layer slurry on the surface of the first microporous layer and drying and sintering it to make a gas diffusion layer.

9. The preparation method according to claim 8, characterized in that, The temperature of the drying is 80 - 100 °C.

10. The preparation method according to claim 8, characterized in that, The temperature of the sintering is 300 - 400 °C.

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