Preparation process of highly graphitized fuel cell microporous layer with hierarchical pore structure and gradient wetting performance

Abstract

This invention discloses a process for preparing a highly graphitized microporous layer for fuel cells with a hierarchical pore structure and gradient wetting properties. First, carbon black particles are uniformly dispersed in a polyvinylpyrrolidone ethanol solution to prepare a homogeneous viscous spinning solution. Then, a fiber membrane with a hierarchical pore structure in a planar structure is prepared by electrospinning, constructing excellent water and gas transport channels; partially entangled fibers are formed in the thickness direction. After drying, the membrane is stabilized and pre-oxidized under carbon paper support, resulting in uniform shrinkage and refinement of the fibers. The pre-oxidized fiber membrane undergoes a highly graphitized transformation under high-purity graphite sheet support. Then, it is horizontally immersed in a polytetrafluoroethylene (PTFE) suspension, and after drying and melting, the PTFE is distributed in a gradient in the vertical direction, achieving good water and gas management performance under all humidity conditions (0%–100% relative humidity), significantly improving the overall performance of the fuel cell.

Description

Technical Field

[0001] This invention belongs to the field of proton exchange membrane fuel cells, specifically relating to a process for preparing a highly graphitized microporous layer for fuel cells with a hierarchical pore structure and gradient wetting properties. Background Technology

[0002] Proton exchange membrane fuel cells (PEMFCs) have become an ideal choice under the global sustainable development strategy due to their high energy conversion efficiency, fast start-up speed, and low emissions. However, the presence of activation polarization, ohmic polarization, and concentration polarization leads to overall performance loss in PEMFCs during operation.

[0003] To improve the overall performance of PEMFCs, researchers have focused on reducing polarization losses under different operating conditions. Initially, the gas diffusion layer consisted only of carbon paper or carbon cloth, which provided elemental gas diffusion and structural support. However, these early designs suffered from insufficient water and gas management performance, limiting the scalability and wider application of PEMFCs. In the early 1990s, researchers at Ballard Power Systems made a breakthrough by coating the gas diffusion layer (GDL) closest to the catalyst layer with microporous carbon material. This innovative design significantly enhanced water and gas management performance, thereby improving the overall performance of the fuel cell and becoming a key component of PEMFC design. Since then, the microporous layer (MPL) has been a focus of extensive research, with various materials and structures developed to optimize mass transport, reduce polarization losses, and improve the efficiency and durability of PEMFCs.

[0004] Currently, the design of MPLs is mainly divided into hydrophilic, hydrophobic, and hydrophilic-hydrophobic composite types. For example, Wuxi Weifu High-Tech Group Co., Ltd. disclosed a hydrophilic microporous layer of proton exchange membrane fuel cell membrane electrode and its preparation method (CN118738429A). Specifically, a hydrophilic microporous layer slurry is prepared by mixing carbon materials and hydrophilic polymers, which is then coated on the microporous layer of the cathode catalyst layer or cathode gas diffusion layer. After drying, the hydrophilic microporous layer of the proton exchange membrane fuel cell membrane electrode is obtained, which improves the performance of PEMFC at a relative humidity of 20% to 50%.

[0005] Shanghai Electric Group Co., Ltd. disclosed a gas diffusion layer and its preparation method, membrane electrode and fuel cell (CN118367157A). Specifically, carbon, hydrophobic agent and dispersant are placed on a carbon paper substrate, and then calcined after standing at a specific temperature for a period of time to obtain a hydrophobic microporous layer, which improves the drainage performance of the fuel cell.

[0006] Dalian Meiniu New Energy Co., Ltd. disclosed a gradient pore structure for a gas diffusion layer, its construction method, and its application (CN118299591A). Specifically, conductive carbon powder material is mixed with polystyrene microspheres of different particle sizes to form slurries, which are then coated onto the surface of carbon paper according to the particle size from small to large. Finally, the coated hydrophobic carbon paper is subjected to high-temperature heat treatment to form a pore structure with different sizes and gradient changes, which can effectively improve the gas and liquid transport performance of the gas diffusion layer.

[0007] China Automotive Innovation Technology Co., Ltd. disclosed a gas diffusion layer and its preparation method and application (CN118841578A). Specifically, a hydrophilic reagent mainly composed of sodium dioctyl succinate sulfonate, ethyl acetate and / or polyethylene glycol and water is sprayed onto a hydrophobic microporous layer by atomization and calcination to prepare a microporous layer with hydrophilic and hydrophobic composite properties, thereby improving the membrane electrode performance under high electrical density conditions.

[0008] Their similarities are: both improve the water and gas management performance of PEMFCs under small relative humidity ranges through MPL wettability modification and structural design; their differences are: one gas diffusion layer and its preparation method, membrane electrode and fuel cell (CN118367157A) is a single-layer MPL, possessing only hydrophobic properties; while one proton exchange membrane fuel cell membrane electrode hydrophilic microporous layer and its preparation method (CN118738429A), one gas diffusion layer gradient pore structure and its construction method and application (CN118299591A), and one gas The diffusion layer, its preparation method, and application (CN118841578A) are all multilayer MPLs. Among them, a gas diffusion layer gradient pore structure, its construction method, and application (CN118299591A) constructs a gradient pore structure in the vertical direction. A hydrophilic microporous layer for a proton exchange membrane fuel cell membrane electrode and its preparation method (CN118738429A) and a gas diffusion layer and its preparation method and application (CN118841578A) prepare microporous layers with both hydrophilic and hydrophobic composite properties through multilayer composite structures. Single-layer MPLs with gradient pore structures and wettability suitable for 0%-100% humidity conditions are not covered. Summary of the Invention

[0009] Given the varying requirements of fuel cells for efficient water and gas management under different operating conditions, the purpose of this invention is to provide a process for preparing a highly graphitized microporous layer for fuel cells with a hierarchical pore structure and gradient wetting properties. This process enables the prepared microporous layer to have good water and gas management capabilities under 0%-100% humidity conditions, significantly improving the performance of the fuel cell.

[0010] 1. A process for preparing a highly graphitized microporous layer for fuel cells with a hierarchical pore structure and gradient wetting properties, characterized by comprising the following steps:

[0011] Step 1: Preparation of homogeneous viscous spinning solution. Carbon black particles and polyvinylpyrrolidone (hereinafter referred to as PVP) are uniformly dispersed in an ethanol solution to prepare a homogeneous spinning solution with a specific viscosity.

[0012] Step 2: Preparation of fiber membrane with hierarchical pore structure. The homogeneous viscous spinning solution is electrospun under specific spinning parameters. After spinning, the fiber membrane is removed and a specific pressure is applied to prepare a fiber membrane with hierarchical pore structure.

[0013] Step 3: High graphitization. After the fiber membrane is dried at a specific temperature, carbon paper is used as a clamp to stabilize and pre-oxidize the fiber membrane at a specific temperature to control uniform shrinkage. The uniformly shrunken fiber membrane is then clamped with a high-purity graphite plate as a clamp, and under specific temperature and atmosphere conditions, the fiber membrane is controlled to shrink uniformly again and graphitization is induced.

[0014] Step 4: Gradient wettability modification. The graphitized fiber membrane is immersed in a polytetrafluoroethylene (PTFE) dispersion of a certain concentration. The wettability is controlled by controlling the concentration of the dispersion and the immersion time. After immersion, the membrane is dried to remove moisture. Then, the PTFE is melted and hot-pressed at a specific temperature for a certain period of time, so that the PTFE is solidified together with the graphitized fiber membrane. The PTFE is distributed in a gradient in the vertical direction of the fiber membrane, thus constructing a gradient wettability.

[0015] 2. The preparation of the homogeneous viscous spinning solution in step 1 is characterized by:

[0016] The PVP has a molecular weight of 1,000,000 to 1,500,000 and a mass fraction of 6% to 12%; the carbon black particles are one or more of 0-dimensional carbon materials such as acetylene black, pigment carbon black, or Ketjen carbon black, and the mass fraction is 12% to 18% of the total mass of PVP and carbon black.

[0017] The key to the preparation process of the homogeneous spinning solution is to slowly add acetylene black to the ethanol solution while the magnetic stirrer speed is between 800 rpm and 1000 rpm, followed by sonication in an ice bath for 40 to 60 minutes, then adding PVP and stirring for 4 to 6 hours.

[0018] 3. The hierarchical porous structure fiber membrane preparation in step 2 is characterized by:

[0019] The electrospinning process is adopted, with the positive and negative electrode voltages being +8 to +10kV and -7 to -9kV, respectively. The receiving screen is a metal roller with silicone paper attached. The distance between the receiving roller and the spinning needle is 9 to 11 cm, the roller speed is 300 to 400 rppm, the inner diameter of the spinning needle is 0.4 to 0.6 mm, and the solution propulsion speed is 1.5 to 2.2 mL / h.

[0020] Under the action of electrostatic force, the fiber membrane forms a hierarchical pore structure with micropores, mesopores and macropores on the horizontal plane;

[0021] The spinning solution is a homogeneous viscous spinning solution containing dispersed carbon black particles prepared in step 1. During the spinning process, due to the local aggregation of carbon black particles at the microscale, the fibers bond together, the contact area increases, and the ohmic impedance in the vertical direction decreases.

[0022] After electrospinning is completed and before the ethanol has completely evaporated, a specific pressure of 1.5–2.0 N / cm is applied to the fiber membrane. 2 This makes the fibers of the fiber membrane overlap more tightly in the vertical direction, further reducing the ohmic impedance.

[0023] 4. In step 3, the highly graphitized characteristic is:

[0024] The fiber membrane is dried at 50-60°C for 3-4 hours in an air atmosphere to allow the ethanol to evaporate completely.

[0025] The stabilization process is characterized by removing the fiber membrane from the silicone paper, cutting the fiber membrane to a specific size, using carbon paper as a clamp to hold the fiber membrane in the center of the carbon paper, and using paper clips to clamp the four corners of the carbon paper respectively, so that the external air can evenly contact the fiber membrane through the carbon paper, controlling the uniform shrinkage of the fiber membrane; the stabilization temperature is 140-160℃, the atmosphere is air, the heating and cooling rates are both 1℃ / min, and the holding time is 12-20h, so that PVP undergoes dehydrogenation and cyclization, changing from a thermoplastic polymer to a thermosetting polymer, preventing PVP from melting and causing damage to the fiber morphology during subsequent heat treatment;

[0026] The pre-oxidation process is characterized by still being carried out under carbon paper clamping, so that the fiber membrane is uniformly contacted with oxygen in the external air to prevent curling and non-uniform shrinkage. The temperature is 350-370℃, in air atmosphere, the heating rate and cooling rate are both 2℃ / min, and the holding time is 4-5h, so that the side chains of PVP molecules are destroyed, the main chain of PVP molecules is oxidized and the fibers are refined.

[0027] The carbonization process is characterized by removing the pre-oxidized fiber membrane from the carbon paper, using a 1mm thick high-purity graphite plate as a clamp to carry the fiber membrane, inducing graphitization of the fiber membrane, carbonization temperature of 1100-1300℃, nitrogen or argon atmosphere, heating and cooling rate of 3-5℃ / min, and holding time of 4-5h, which refines the fibers, completely dehydrogenates PVP into an amorphous form, forms a disordered graphite structure, increases the conductivity of the nanofibers, and because carbon black particles do not have shrinkage properties, heterogeneous shrinkage occurs at the microscale, the fiber surface shows a wavy shape and a large number of grooves, while at the macroscale, due to the effect of the graphite sheet, it presents a relatively flat horizontal surface.

[0028] 5. The gradient wettability modification in step 4 is characterized by:

[0029] The PTFE dispersion contains 10% PTFE by mass, and the dispersant is distilled water. The soaking time is 30-40 min, the drying temperature is 60-70℃, and the holding time is 6-10 h, so that the water is completely evaporated. During the drying process, the fiber membrane is kept horizontal so that the PTFE is evenly distributed on the horizontal surface.

[0030] The melting temperature is 340–360°C, the heating rate is 2°C / min, and the holding time is 2–5 hours. During the melting process, high-purity graphite plates are still used as clamps. At the same time, the clamps provide a certain pressure to the fiber membrane between the graphite plates and keep the fiber membrane horizontal. This allows the PTFE melt to form a uniform gradient deposition in the vertical direction due to gravity and pressure, and to adhere to the corrugated surface and grooves of the fibers described in step 3 without affecting the overall electron transport performance. When used in fuel cells, this effectively avoids the performance degradation of the battery caused by PTFE loss.

[0031] The beneficial effects of this invention are as follows: The hierarchical pore structure of micropores, mesopores, and macropores in the planar structure of the microporous layer prepared by the above steps ensures good water and gas transport channels; the gradient PTFE content distribution in the vertical direction constructs gradient wetting performance, effectively avoiding proton membrane dehydration under low humidity and catalyst layer flooding under high humidity, achieving good water and gas management performance under all humidity conditions (0% to 100% relative humidity); the entangled fibers and the induced highly graphitized structure provide good electron transport channels; overall, it not only ensures excellent water and gas management performance of the fuel cell under all humidity conditions, but also effectively reduces ohmic impedance and installation difficulty, and improves overall performance. Attached image description:

[0032] Figure 1 A process flow for preparing a highly graphitized microporous layer for fuel cells with hierarchical pore structure and gradient wetting properties.

[0033] Figure 2 SEM of the prepared microporous layer planar hierarchical pore structure

[0034] Figure 3 SEM image of the vertical cross-section of the prepared microporous layer layered graphitized structure

[0035] Figure 4 SEM image of the fiber entanglement structure on the vertical surface of the prepared microporous layer

[0036] Figure 5 EDS diagram of F element distribution on the vertical cross section of the prepared microporous layer.

[0037] Figure 6 EDS plot of F element gradient distribution on the vertical cross section of the prepared microporous layer Detailed Implementation

[0038] Example 1:

[0039] 1.2g of acetylene black was slowly added to 69g of ethanol that was being stirred. After sonicating in an ice bath for 1 hour, 6g of polyvinylpyrrolidone (PVP) with a molecular weight of 1,300,000 was slowly added. The mixture was stirred at 900 rpm for 5 hours to obtain a homogeneous viscous spinning solution.

[0040] 18 mL of the spinning solution prepared in the above steps was used for electrospinning. A No. 21 spinning needle was used, the spinning distance was 10.5 cm, the spinning voltage was +9.5 kV and -8.5 kV, the feed rate was 2.0 mL / min, the receiving roller speed was 300 rpm, the roller length was 20 cm, and the roller diameter was 30 cm. This yielded a fiber membrane with a thickness of 180 μm and a hierarchical pore structure. Before the ethanol completely evaporated, the fiber membrane was sandwiched between glass plates and subjected to an electrospinning process of 1.75 N / cm². 2 Apply pressure, then remove the glass plate and place it in a 55℃ oven for 4 hours to allow the solvent to evaporate completely.

[0041] The above-mentioned fiber membrane was cut into 10cm×10cm pieces and sandwiched between two sheets of TGP-H-060 carbon paper (10cm×10cm) produced by Toray Industries. The membrane was placed in a muffle furnace and heated at 150℃ for 15 hours at a heating rate of 1℃ / min. Then, the temperature was increased to 360℃ and held for 4 hours at a heating rate of 2℃ / min. The temperature was then reduced to room temperature at a cooling rate of 2℃ / min to obtain a fiber membrane with a size of about 5cm×5cm and a thickness of about 140μm. The membrane was sandwiched between a high-purity graphite plate and placed in a muffle furnace. The membrane was held at 1100℃ for 4 hours under an argon atmosphere with a heating and cooling rate of 5℃ / min to obtain a highly graphitized fiber membrane with a thickness of about 120μm and a length and width of 4cm×4cm.

[0042] The graphitized fiber membrane was immersed in a 10% PTFE suspension (water as solvent) for 30 minutes. After being removed, it was placed horizontally on a graphite plate and kept warm in a 60°C oven for 10 hours. After the moisture had completely evaporated, it was sandwiched in a graphite plate and placed horizontally in a muffle furnace at 350°C for 2 hours. The heating and cooling rates were both 2°C / min. Under the action of gravity and clamp pressure, the molten PTFE was distributed in a gradient in the grooves formed by shrinkage during the fiber graphitization process, and finally a highly graphitized fuel cell microporous layer with hierarchical pore structure and gradient wetting properties was obtained.

[0043] The aforementioned microporous layer was installed on the cathode side of a 2cm × 2cm fuel cell (anode: YLS35T, CCM: Nafion 211 & 0.12 / 0.24mg). -2 Pt, cathode MPS: TGP-H-060@40% PTFE), under the conditions of H2 flow rate of 0.2 L / min, O2 flow rate of 0.7 L / min, and stack temperature of 80℃, and at relative humidity of 0%, 20%, 40%, 60%, and 100%, respectively, achieved a current of 1.80 A / cm. -2 1.86A.cm -2 2.05A.cm -2 2.09A.cm -2 1.45A.cm -2 1.36A.cm -2 The power density of the [specific component] was less than 10% after 200 durability cycles under different humidity levels. In comparison, the maximum power density of both GDL poles using YLS35T at 0%, 20%, 40%, 60%, and 100% relative humidity was 1.5 A / cm³. -2 1.3A.cm -2 0.8A.cm -2 0.6A.cm -2 (Severe flooding at 100% RH). The highly graphitized microporous layer for fuel cells, which possesses a hierarchical pore structure and gradient wetting properties, can effectively improve the overall performance of fuel cells.

[0044] Example 2:

[0045] 1.0g of pigment carbon black was slowly added to 69g of ethanol that was being stirred. After sonicating in an ice bath for 50min, 6g of polyvinylpyrrolidone (PVP) with a molecular weight of 1,300,000 was slowly added. The mixture was stirred at 900rppm for 6h to obtain a homogeneous viscous spinning solution.

[0046] 16 mL of the spinning solution prepared in the above steps was used for electrospinning. A No. 21 spinning needle was used, the spinning distance was 11 cm, the spinning voltage was +10.0 kV and -9.0 kV, the feed rate was 2.2 mL / min, the receiving roller speed was 300 rpm, the roller length was 20 cm, and the roller diameter was 30 cm, resulting in a 160 μm thick fiber membrane with a hierarchical pore structure. A glass plate was used to sandwich the fiber membrane, and a flux of 1.80 N / cm was applied. 2 Apply pressure, then remove the glass plate and place it in a 60℃ oven for 4 hours to allow the solvent to evaporate completely.

[0047] The above-mentioned fiber membrane was cut into 10cm×10cm pieces and sandwiched between two sheets of TGP-H-060 carbon paper (10cm×10cm) produced by Toray Industries. The membrane was placed in a muffle furnace and heated at 140℃ for 12 hours at a heating rate of 1℃ / min. Then, the temperature was increased to 360℃ and held for 4 hours at a heating rate of 2℃ / min. The temperature was then reduced to room temperature at a cooling rate of 2℃ / min to obtain a fiber membrane with a size of about 5cm×5cm and a thickness of about 120μm. The membrane was sandwiched between a high-purity graphite plate and placed in a muffle furnace. The membrane was held at 1300℃ for 4 hours under a nitrogen atmosphere with a heating and cooling rate of 5℃ / min to obtain a highly graphitized fiber membrane with a thickness of about 100μm and a size of about 4cm×4cm.

[0048] The graphitized fiber membrane was immersed in a 10% PTFE suspension (water as solvent) for 40 minutes. After being removed, it was placed horizontally on a graphite plate and kept in a 70°C oven for 6 hours. After the moisture had completely evaporated, it was sandwiched in a graphite plate and placed horizontally in a muffle furnace at 350°C for 2 hours. The heating and cooling rates were both 2°C / min. Under the action of gravity and clamp pressure, the molten PTFE was distributed in a gradient in the grooves formed by shrinkage during the fiber graphitization process, and finally a highly graphitized fuel cell microporous layer with hierarchical pore structure and gradient wetting properties was obtained.

[0049] The aforementioned microporous layer was installed on the cathode side of a 2cm × 2cm fuel cell (anode: YLS30T, CCM: Nafion 211 & 0.12 / 0.24mg). -2 Pt (cathode MPS: TGP-H-060@40% PTFE), under the conditions of H2 flow rate of 0.22 L / min, O2 flow rate of 0.8 L / min, and stack temperature of 80℃, and at relative humidity of 0%, 20%, 40%, 60%, and 100%, respectively, achieved a current of 1.87 A / cm². -2 1.94A.cm -2 2.18A.cm -2 2.22A.cm -2 2.20A.cm-2 2.05A.cm -2 The power density.

[0050] Matters not covered in this invention are common knowledge.

[0051] The above embodiments are only for illustrating the technical concept and features of the present invention, and are intended to enable those skilled in the art to understand the content of the present invention and implement it accordingly. They should not be construed as limiting the scope of protection of the present invention. All equivalent changes or modifications made in accordance with the spirit and essence of the present invention should be covered within the scope of protection of the present invention.

Claims

1. A process for preparing a highly graphitized microporous layer for fuel cells with a hierarchical pore structure and gradient wetting properties, characterized in that, Includes the following steps: Step 1: Preparation of homogeneous viscous spinning solution. Carbon black particles and polyvinylpyrrolidone (hereinafter referred to as PVP) are uniformly dispersed in an ethanol solution to prepare a homogeneous spinning solution with a specific viscosity. Step 2: Preparation of fiber membrane with hierarchical pore structure. The homogeneous viscous spinning solution is electrospun under specific spinning parameters. After spinning, the fiber membrane is removed and a specific pressure is applied to prepare a fiber membrane with hierarchical pore structure. Step 3: High graphitization. After the fiber membrane is dried at a specific temperature, carbon paper is used as a clamp to stabilize and pre-oxidize the fiber membrane at a specific temperature to control uniform shrinkage. The uniformly shrunken fiber membrane is then clamped with a high-purity graphite plate as a clamp, and under specific temperature and atmosphere conditions, the fiber membrane is controlled to shrink uniformly again and graphitization is induced. Step 4: Gradient wettability modification. The graphitized fiber membrane is immersed in a polytetrafluoroethylene (PTFE) dispersion of a certain concentration. The wettability is controlled by controlling the concentration of the dispersion and the immersion time. After immersion, the membrane is dried to remove moisture. Then, the PTFE is melted and hot-pressed at a specific temperature for a certain period of time, so that the PTFE is solidified together with the graphitized fiber membrane. The PTFE is distributed in a gradient in the vertical direction of the fiber membrane, thus constructing a gradient wettability.

2. The process for preparing a highly graphitized fuel cell microporous layer with hierarchical pore structure and gradient wetting properties as described in claim 1, characterized in that: The PVP has a molecular weight of 1,000,000 to 1,500,000 and a mass fraction of 6% to 12%; the carbon black particles are one or more of acetylene black, pigment carbon black, or Ketjen carbon black, and their mass fraction is 12% to 18% of the total mass of PVP and carbon black. The key to the preparation process of the homogeneous spinning solution is to slowly add acetylene black to the ethanol solution while the magnetic stirrer is rotating at a speed between 800 rpm and 1000 rpm, followed by sonication in an ice bath for 40 to 60 minutes, and then adding PVP and stirring for 4 to 6 hours.

3. The process for preparing a highly graphitized fuel cell microporous layer with hierarchical pore structure and gradient wetting properties as described in claim 1, characterized in that... : In the electrospinning process, the positive and negative electrode voltages are +8~+10kV and -7~-9kV, respectively. The receiving screen is a metal roller with silicone paper attached. The distance between the receiving roller and the spinning needle is 9~11cm. The roller rotation speed is 300~400rppm. The inner diameter of the spinning needle is 0.4~0.6mm. The solution propulsion speed is 1.5~2.2mL / h. Under the action of electrostatic force, the fiber membrane forms a hierarchical pore structure with micropores, mesopores and macropores on the horizontal plane; The spinning solution is a homogeneous viscous spinning solution containing dispersed carbon black particles prepared in step 1. During the spinning process, due to the local aggregation of carbon black particles at the microscale, the fibers bond together, the contact area increases, and the ohmic impedance in the vertical direction decreases. After the electrospinning is completed and before the ethanol has completely evaporated, a specific pressure of 1.5~2.0 N / cm2 is applied to the fiber membrane, which makes the fiber membrane more tightly overlapped in the vertical direction, further reducing the ohmic impedance.

4. The process for preparing a highly graphitized fuel cell microporous layer with hierarchical pore structure and gradient wetting properties as described in claim 1, characterized in that: The fiber membrane is dried at 50~60℃ for 3~4 hours in an air atmosphere to allow the ethanol to evaporate completely. During the stabilization process, the fiber membrane is removed from the silicone paper and cut to a specific size. Carbon paper is used as a clamp to hold the fiber membrane in the center of the carbon paper. Paper clips are used to hold the four corners of the carbon paper so that the external air can evenly contact the fiber membrane through the carbon paper, controlling the uniform shrinkage of the fiber membrane. The stabilization temperature is 140~160℃, in air atmosphere, with a heating and cooling rate of 1℃ / min and a holding time of 12~20h. This causes PVP to undergo dehydrogenation and cyclization, transforming it from a thermoplastic polymer into a thermosetting polymer, preventing PVP from melting and damaging the fiber morphology during subsequent heat treatment. The pre-oxidation process is still carried out under carbon paper clamping, so that the fiber membrane is in uniform contact with oxygen in the outside air, preventing curling and non-uniform shrinkage. The temperature is 350~370℃, air atmosphere, heating rate and cooling rate are both 2℃ / min, and the holding time is 4~5h, so that the side chains of PVP molecules are destroyed, the main chain of PVP molecules is oxidized and the fiber is refined. During the graphitization process, the pre-oxidized fiber membrane is taken out from the carbon paper, and a 1mm thick high-purity graphite plate is used as a clamp to carry the fiber membrane, inducing graphitization of the fiber membrane. The graphitization temperature is 1100~1300℃, in a nitrogen or argon atmosphere, with a heating and cooling rate of 3~5℃ / min and a holding time of 4~5h. This process refines the fibers, completely dehydrogenates the PVP and transforms it into an amorphous state, forming a disordered graphite structure, which increases the conductivity of the nanofibers. Since the carbon black particles do not have shrinkage properties, heterogeneous shrinkage occurs at the microscale, and the fiber surface exhibits a wavy shape and a large number of grooves, while at the macroscale, due to the effect of the graphite sheet, it presents a relatively flat horizontal surface.

5. The process for preparing a highly graphitized fuel cell microporous layer with hierarchical pore structure and gradient wetting properties as described in claim 1, characterized in that: The PTFE dispersion contains 10% PTFE by mass, and the dispersant is distilled water. The soaking time is 30-40 min, the drying temperature is 60-70℃, and the holding time is 6-10 h to ensure complete evaporation of moisture. During the drying process, the fiber membrane is kept horizontal to ensure that the PTFE is evenly distributed on the horizontal surface. The melting temperature is 340~360℃, the heating rate is 2℃ / min, and the holding time is 2~5h. During the melting process, high-purity graphite plates are still used as clamps. At the same time, the clamps provide a certain pressure to the fiber membrane between the graphite plates and keep the fiber membrane horizontal. This allows the PTFE melt to form a uniform gradient deposition in the vertical direction due to gravity and pressure, and to adhere to the corrugated surface and grooves of the fibers described in step 3, without affecting the overall electron transport performance. When used in fuel cells, this can effectively avoid the performance degradation of the battery caused by the loss of PTFE.

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