Gradient cathode catalyst layer, method for preparing same, and proton exchange membrane fuel cell

Abstract

This invention relates to a gradient cathode catalyst layer, its preparation method, and a proton exchange membrane fuel cell, belonging to the field of cathode catalyst layer preparation technology. The gradient cathode catalyst layer provided by this invention comprises a first functional layer and a second functional layer from the inside out. In the first functional layer, the mass ratio of hydrophilic modified carbon powder to platinum-carbon catalyst is greater than that in the second functional layer. By designing the cathode catalyst layer to have a gradient composite structure along its thickness direction, and with a larger mass ratio of hydrophilic modified carbon powder to platinum-carbon catalyst in the first functional layer near the proton exchange membrane and a smaller mass ratio in the second functional layer near the gas diffusion layer, an optimal balance between water retention and mass transfer is achieved under low humidity conditions, while significantly improving the electrochemical performance and long-term operational stability of the battery.

Description

Technical Field

[0001] This invention belongs to the field of cathode catalyst preparation technology, specifically relating to a gradient cathode catalyst, its preparation method, and a proton exchange membrane fuel cell. Background Technology

[0002] Proton exchange membrane fuel cells (PEMFCs) are considered one of the most promising power sources in the fields of new energy vehicles and distributed power generation due to their advantages such as high energy conversion efficiency, low operating temperature, fast start-up speed, and zero emissions. The cathode catalyst layer, as one of the core components of a PEMFC, is the site of the oxygen reduction reaction (ORR), and its microstructure and material composition directly determine the cell's electrochemical performance, hydrothermal management capabilities, and long-term operational stability.

[0003] During PEMFC operation, an electrochemical reaction occurs on the cathode side to generate water. Appropriate moisture content is crucial for maintaining the proton conductivity of the proton exchange membrane. Especially under low humidity or even no external humidification conditions, the cathode catalyst layer needs sufficient water retention capacity to utilize the water generated in the reaction for in-situ humidification of the proton exchange membrane, preventing a sharp increase in proton transport impedance due to membrane drying. However, excessive moisture accumulation in the catalyst layer pores hinders the transport of reactant gas (oxygen) to the active sites, causing a "flooding" phenomenon, leading to increased concentration polarization and decreased battery performance. Therefore, balancing "water retention" and "mass transfer" under low humidity conditions is a major technical challenge in cathode catalyst layer design.

[0004] In existing technologies, cathode catalyst layers typically employ a uniformly mixed structure, meaning that hydrophilic materials and catalysts are evenly distributed throughout the entire thickness of the catalyst layer. While this uniform structure simplifies the fabrication process to some extent, it presents significant limitations when addressing water management requirements under low humidity conditions. 1. Inability to balance water retention and mass transfer requirements: If the proportion of overall hydrophilic material is increased to improve water retention, although it is beneficial to the wetting of the membrane, it will cause the pores of the catalyst layer to be filled with water, significantly increasing the gas mass transfer resistance and causing flooding. Conversely, if the proportion of hydrophilic material is reduced to optimize gas mass transfer, it will be difficult to maintain the required wet environment at the membrane interface under low humidity, resulting in a decrease in proton conductivity and an increase in battery internal resistance.

[0005] 2. Inappropriate spatial functional allocation: The uniform mixing structure results in the catalyst layer having the same material composition on the side near the proton exchange membrane (which requires high humidity to reduce interfacial impedance) and the side near the gas diffusion layer (which requires high porosity to facilitate gas diffusion), leading to low hydrothermal management efficiency.

[0006] 3. Insufficient long-term stability: In traditional uniform structures, uneven distribution or agglomeration of hydrophilic materials may lead to excessively high local water content, which in turn causes damage to the catalyst layer structure or migration and agglomeration of platinum particles, affecting the battery's durability.

[0007] In summary, the uniformly mixed cathode catalyst layer structure in the existing technology cannot solve the contradiction between water retention and mass transfer under low humidity conditions, which limits the performance and application of PEMFC under conditions without external humidification.

[0008] Therefore, there is an urgent need to develop a novel cathode catalyst layer and its preparation method to achieve spatial optimization of water management within the catalyst layer, thereby effectively maintaining membrane wettability while ensuring gas mass transfer and improving the overall performance of the battery under low humidity conditions. Summary of the Invention

[0009] To address the shortcomings of existing technologies, the present invention aims to provide a gradient cathode catalyst layer, its preparation method, and a proton exchange membrane fuel cell, thereby solving the technical problem that existing cathode catalyst layers cannot simultaneously achieve water retention and mass transfer under low humidity (especially without external humidification) conditions, thus making it difficult to balance battery performance and stability.

[0010] To achieve the above objectives, the first aspect of the present invention provides a gradient cathode catalyst layer, which includes a first functional layer and a second functional layer from the inside to the outside, wherein the mass ratio of hydrophilic modified carbon powder to platinum-carbon catalyst in the first functional layer is greater than the mass ratio of hydrophilic modified carbon powder to platinum-carbon catalyst in the second functional layer.

[0011] Preferably, the mass ratio of hydrophilic modified carbon powder to platinum-carbon catalyst in the first functional layer is 3:7 to 4:6, and the mass ratio of hydrophilic modified carbon powder to platinum-carbon catalyst in the second functional layer is 1:9 to 2:8.

[0012] Preferably, the hydrophilic modified toner is a modified toner with hydrophilic sulfonic acid groups uniformly loaded on its surface.

[0013] Preferably, the method for preparing the hydrophilic modified toner includes: The toner powder is mixed and ground with a solid sulfonating agent, then heat-treated under inert gas protection, and subsequently cooled, washed, and dried to obtain the hydrophilic modified toner powder.

[0014] Preferably, the mass ratio of the carbon powder to the solid sulfonating agent is 1:(2~5).

[0015] Preferably, the heat treatment temperature is 300~400℃, and the heat treatment time is 2~4h.

[0016] A second aspect of the present invention provides a method for preparing a gradient cathode catalyst layer, comprising the following steps: A first functional layer slurry and a second functional layer slurry are coated from the inside to the outside on the surface of a base film, and then hot-pressed and transferred to form the film; wherein, the mass ratio of hydrophilic modified carbon powder to platinum-carbon catalyst in the first functional layer slurry is greater than the mass ratio of hydrophilic modified carbon powder to platinum-carbon catalyst in the second functional layer slurry.

[0017] Preferably, the mass ratio of hydrophilic modified carbon powder to platinum-carbon catalyst in the first functional layer is 3:7 to 4:6, and the mass ratio of hydrophilic modified carbon powder to platinum-carbon catalyst in the second functional layer is 1:9 to 2:8.

[0018] Preferably, the first functional layer slurry and the second functional layer slurry further include perfluorosulfonic acid resin and solvent; The mass ratio of the perfluorosulfonic acid resin to the carbon in the platinum-carbon catalyst is (0.8~1.1):1, and the sum of the mass of the perfluorosulfonic acid resin and the mass of the platinum-carbon catalyst accounts for 8%~11% of the mass of the solvent. The solvent is a mixed solution of alcohol and water in a mass ratio of (2~4):1.

[0019] A third aspect of the present invention provides a proton exchange membrane fuel cell, comprising a gradient cathode catalyst layer as described in the first aspect of the present invention or a gradient cathode catalyst layer prepared by the method described in the second aspect of the present invention.

[0020] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. Effectively balances water retention and mass transfer requirements under low humidity conditions, resolving the contradiction in water management. This invention designs a gradient composite structure along the thickness direction of the cathode catalyst layer, dividing the catalyst layer into a first (inner) functional region near the proton exchange membrane and a second (outer) functional region near the gas diffusion layer. The first functional region uses a higher proportion of hydrophilic modified carbon powder, which can efficiently adsorb water generated by the cathode reaction, achieving in-situ humidification of the proton exchange membrane and preventing the increase in proton impedance caused by membrane drying. The second functional region uses a lower proportion of hydrophilic modified carbon powder, retaining more porosity, which is conducive to the smooth transport of oxygen into the interior of the catalyst layer and avoids water accumulation blocking gas channels. This spatial functional decoupling design fundamentally solves the technical bottleneck that traditional uniform mixing structures cannot simultaneously meet the requirements of "water retention" and "mass transfer".

[0021] 2. Significantly improves battery electrochemical performance under conditions without external humidification. Thanks to the optimized water management of the gradient structure described above, the cathode catalyst layer provided by this invention can maintain a suitable wettability at the membrane electrode interface under low humidity or even without external humidification conditions, effectively reducing the battery's internal resistance and greatly improving the battery's electrochemical performance.

[0022] 3. Improve the long-term operational stability of the battery. On the one hand, the hydrophilic modified carbon powder used in this invention undergoes solid-state grinding and heat treatment processes, resulting in a uniformly loaded surface with hydrophilic groups and maintaining a complete pore structure. This has an anchoring effect on platinum particles, effectively inhibiting their migration and aggregation during operation and maintaining catalytic activity. On the other hand, the gradient structure avoids severe flooding caused by excessive local hydrophilic material within the catalyst layer, reducing the scouring and damage of liquid water to the catalyst layer structure, thereby improving the long-term operational stability of the battery. Detailed Implementation

[0023] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0024] In existing technologies, the cathode catalyst layer typically employs a uniformly mixed structure, meaning that the hydrophilic material and catalyst are evenly distributed throughout the thickness of the catalyst layer. While this uniform structure simplifies the preparation process to some extent, it has significant limitations when facing the water management requirements under low humidity conditions: firstly, it cannot simultaneously meet the requirements for water retention and mass transfer; secondly, it leads to a decline in the electrochemical performance of the battery; and thirdly, it results in insufficient long-term stability of the battery.

[0025] In view of this, the present invention provides a gradient cathode catalyst layer, its preparation method, and a proton exchange membrane fuel cell to solve the technical problem that existing cathode catalyst layers cannot simultaneously achieve water retention and mass transfer under low humidity (especially without external humidification) conditions, thus leading to a decrease in the electrochemical performance and stability of the battery.

[0026] In a first aspect, embodiments of the present invention provide a gradient cathode catalyst layer, which includes a first functional layer and a second functional layer from the inside out, wherein the mass ratio of hydrophilic modified carbon powder to platinum-carbon catalyst in the first functional layer is greater than the mass ratio of hydrophilic modified carbon powder to platinum-carbon catalyst in the second functional layer.

[0027] In this invention, the cathode catalyst layer is designed with a gradient composite structure along its thickness direction, corresponding to different mixing ratios of hydrophilic modified carbon powder and platinum-carbon catalyst. In the first functional layer near the proton exchange membrane, the hydrophilic modified carbon powder and platinum-carbon catalyst have a relatively large mass ratio, enabling efficient adsorption of water generated during the cathode reaction. This achieves in-situ humidification of the proton exchange membrane, maintaining the wetted state of the membrane and the three-phase boundary, reducing proton transport impedance, and effectively avoiding the problems of proton exchange membrane drying and increased proton transport impedance under low humidity. In the second functional layer near the gas diffusion layer (GDL), the hydrophilic modified carbon powder and platinum-carbon catalyst have a relatively small mass ratio, reducing water accumulation. Simultaneously, the hydrophobicity of the platinum-carbon catalyst ensures smooth oxygen transport from the GDL to the interior of the catalyst layer, preventing gas blockage and alleviating flooding. Therefore, a synergistic function of "inner water retention and outer air permeability" is achieved along the thickness direction, realizing the optimal balance between water retention and mass transfer under low humidity conditions, while significantly improving the electrochemical performance and long-term operational stability of the battery.

[0028] In some preferred embodiments, the mass ratio of hydrophilic modified carbon powder to platinum-carbon catalyst in the first functional layer is 3:7 to 4:6, and the mass ratio of hydrophilic modified carbon powder to platinum-carbon catalyst in the second functional layer is 1:9 to 2:8.

[0029] In some preferred embodiments, the hydrophilic modified carbon powder is a carbon powder with hydrophilic sulfonic acid groups uniformly loaded on its surface. Since hydrophilic sulfonic acid groups have a strong proton-conducting ability, using modified carbon powder loaded with hydrophilic sulfonic acid groups can improve the electrochemical performance of the battery.

[0030] In some preferred embodiments, the method for preparing the hydrophilic modified toner includes: The toner powder is mixed and ground with a solid sulfonating agent, then heat-treated under inert gas protection, and subsequently cooled, washed, and dried to obtain the hydrophilic modified toner powder.

[0031] In some preferred embodiments, the mass ratio of the toner to the solid sulfonating agent is 1:(2~5). If the mass ratio of the toner to the solid sulfonating agent is greater than 1:2 (insufficient sulfonating agent), the amount of sulfonic acid grafted onto the carbon surface will be low, affecting the performance of the modified toner; if the mass ratio of the toner to the solid sulfonating agent is less than 1:6 (excessive sulfonating agent), it will lead to excessive sulfonating agent residue, resulting in a high subsequent washing load and high cost.

[0032] In some preferred embodiments, the heat treatment temperature is 300~400℃ and the heat treatment time is 2~4h.

[0033] In this invention, the temperature and time of the heat treatment need to be strictly controlled because: The thermal decomposition of solid sulfonating agents (sodium pyrosulfate, sodium bisulfate) to generate active SO3 sulfonation active groups begins at approximately 280°C. Below 300°C, the decomposition rate of the sulfonating agent is extremely slow, making it almost impossible for it to react with the surface of the toner. 2 Covalent grafting reaction occurs at carbon sites; only physical adsorption occurs, and all sulfonic acid groups are removed after washing with water, making permanent hydrophilic modification impossible; for samples heat-treated below 300℃, the sulfonic acid loading is <0.3 mmol / g carbon, the carbon powder is highly hydrophobic, the proton conduction resistance of the catalyst layer is large, the single cell output power density is increased by less than 5%, and the modification has no practical value.

[0034] At high temperatures, the covalent bonds of sulfonic acid undergo thermal decomposition and breakage. The thermal stability limit of the C-SO3H covalent bonds on the carbon surface is about 410℃. Temperatures above 400℃ will cause desulfonation reaction, resulting in the decomposition and volatilization of a large number of sulfonic acid groups, and the loss of hydrophilic and proton conduction properties.

[0035] Sulfonation grafting is a solid-phase interfacial reaction, where carbon powder and solid sulfonating agent are in solid-solid contact, resulting in a low mass transfer rate. If the heat treatment time is less than 2 hours, the sulfonation reaction is incomplete, leading to uneven sulfonic acid loading on the surface of the carbon particles and almost no modification to the internal carbon particles. The low sulfonic acid loading also causes local proton conduction disruptions in the catalyst layer, increasing battery polarization.

[0036] Prolonged high-temperature heat treatment can lead to irreversible losses. If the heat treatment time exceeds 4 hours, the continuous heat action will cause a small amount of sulfonic acid to desorb from the surface, the micropores of the carbon powder to collapse, and the specific surface area to decrease. Energy consumption will increase significantly, and the economic efficiency of the process will deteriorate.

[0037] In some specific embodiments, the preparation method of the hydrophilic modified toner includes the following steps: 1. Mixing and grinding: Pour carbon powder and solid sulfonating agent into a planetary ball mill jar according to the weighing ratio; add zirconia grinding balls and mix by planetary ball milling; wherein, the ball milling speed is 300~500 r / min, the grinding time is 30~90 min, and the ball-to-material mass ratio is 5:1~10:1; 2. Heat treatment under inert gas protection: Spread the uniformly ground powder evenly on a quartz boat, with a powder layer thickness ≤5mm to ensure gas penetration and uniform heating; push the quartz boat into a tube furnace and seal the furnace tube; introduce inert protective gas (high-purity N2 / Ar, purity ≥99.999%), first ventilate for 30 minutes to purge air from the tube to prevent oxidation and burn-off of high-temperature carbon powder; the inert gas flow rate is 60~120mL / min, and the gas is continuously ventilated throughout the process; heat to the heat treatment temperature of 300~400℃ at a heating rate of 2~5℃ / min (slow heating to prevent local overheating and powder splashing), and hold for 2~4 hours; 3. Cooling process: After the heat preservation is completed, the heating is turned off, and inert gas is continuously introduced to allow the tubular furnace to cool naturally to room temperature (25℃). 4. Washing and impurity removal: After cooling, the powder is transferred to a beaker and repeatedly soaked, stirred, and filtered with deionized water to remove unreacted free solid sulfonating agent and sulfate byproducts. The washing solvent is room temperature deionized water, the solid-liquid ratio is 1g:(30~50 mL) per wash, and the stirring and washing time is 15~30min per wash. Washing continues until the pH of the filtrate is close to neutral (pH=6~7) and there are no sulfate ions (no white precipitate is detected by barium chloride).

[0038] 5. Drying to obtain the finished hydrophilic modified carbon powder: Transfer the washed filter cake to a vacuum drying oven to dry completely to remove the adsorbed moisture; wherein, the drying temperature is 60~80℃ (low temperature to avoid high temperature to remove surface sulfonic acid groups), the vacuum degree is ≤-0.08MPa; the drying time is 8~12h.

[0039] Secondly, embodiments of the present invention also provide a method for preparing a gradient cathode catalyst layer, comprising the following steps: A first functional layer slurry and a second functional layer slurry are coated from the inside to the outside on the surface of a base film, and then hot-pressed and transferred to form the film; wherein, the mass ratio of hydrophilic modified carbon powder to platinum-carbon catalyst in the first functional layer slurry is greater than the mass ratio of hydrophilic modified carbon powder to platinum-carbon catalyst in the second functional layer slurry.

[0040] In some preferred embodiments, the mass ratio of hydrophilic modified carbon powder to platinum-carbon catalyst in the first functional layer is 3:7 to 4:6, and the mass ratio of hydrophilic modified carbon powder to platinum-carbon catalyst in the second functional layer is 1:9 to 2:8.

[0041] In some preferred embodiments, the first functional layer slurry and the second functional layer slurry further include perfluorosulfonic acid resin and solvent; The mass ratio of the perfluorosulfonic acid resin to the carbon in the platinum-carbon catalyst is (0.8~1.1):1, and the sum of the mass of the perfluorosulfonic acid resin and the mass of the platinum-carbon catalyst accounts for 8%~11% of the mass of the solvent. The solvent is a mixed solution of alcohol and water in a mass ratio of (2~4):1.

[0042] In some preferred embodiments, the base film comprises a polytetrafluoroethylene (PTFT) film.

[0043] In this invention, hydrophilic modified toner is prepared by mixing and grinding toner with a solid sulfonating agent, followed by heat treatment under an inert gas. This ensures that the surface of the toner is uniformly loaded with hydrophilic sulfonic acid groups, while maintaining the original specific surface area and complete pore structure of the toner and preventing platinum particle agglomeration.

[0044] In this invention, a layered coating method is used to prepare the gradient cathode catalyst layer. First, a first functional layer slurry is coated onto the surface of the base film and then dried. Then, a second functional layer slurry is coated and dried. The drying temperature is 50-80°C, and the drying time is 20 min-1 h.

[0045] In this invention, the prepared cathode catalyst layer has a gradient composite structure, corresponding to different mixing ratios of hydrophilic modified carbon powder and platinum-carbon catalyst. In the first functional layer near the proton exchange membrane, the mass ratio of hydrophilic modified carbon powder and platinum-carbon catalyst is relatively large, which can efficiently adsorb water generated by the cathode reaction, realize in-situ humidification of the proton exchange membrane, maintain the wet state of the membrane and the three-phase boundary, reduce proton transport impedance, and effectively avoid the problems of proton exchange membrane drying and increased proton transport impedance under low humidity. In the second functional layer near the gas diffusion layer (GDL), the mass ratio of hydrophilic modified carbon powder and platinum-carbon catalyst is relatively small, which can reduce water accumulation. At the same time, the hydrophobicity of platinum-carbon catalyst ensures the smooth transport of oxygen from GDL to the interior of the catalyst layer, avoids gas blockage of channels, and alleviates the problem of water flooding. Therefore, the synergistic function of "water retention on the inside and air permeability on the outside" is achieved in the thickness direction, realizing the optimal balance between water retention and mass transfer under low humidity conditions, and greatly improving the electrochemical performance and long-term operational stability of the battery.

[0046] Thirdly, embodiments of the present invention also provide a proton exchange membrane fuel cell, including a gradient cathode catalyst layer as described in the first aspect of the present invention or a gradient cathode catalyst layer prepared by the method described in the second aspect of the present invention.

[0047] In this invention, because the cathode catalyst layer has a gradient composite structure, the hydrophilic modified carbon powder and platinum-carbon catalyst in the first functional layer near the proton exchange membrane have a relatively large mass, while the hydrophilic modified carbon powder and platinum-carbon catalyst in the second functional layer near the gas diffusion layer (GDL) have a relatively small mass. This gives the cathode catalyst layer a synergistic function of "water retention on the inside and gas permeability on the outside". Therefore, the proton exchange membrane fuel cell containing this cathode catalyst layer has excellent electrochemical performance and long-term operational stability.

[0048] Unless otherwise specified, all raw materials used in the embodiments of the present invention are commercially available, all instruments and equipment used are conventional instruments and equipment in the art, and all operating methods used are conventional methods in the art.

[0049] Example The basic raw materials used in the following examples and comparative examples are as follows: Platinum-carbon catalyst: Commercially available catalyst specifically for 50% Pt / C fuel cells; Carbon powder: Conductive carbon black XC-72; Solid sulfonating agent: sodium pyrosulfate; Perfluorosulfonic acid resin: Nafion solution (solid content 5.2%); Base film: Polytetrafluoroethylene (PTFE) transfer film; Inert gas: High-purity nitrogen (purity ≥ 99.999%).

[0050] General slurry mixing ratio rules: perfluorosulfonic acid resin mass: carbon mass in platinum-carbon catalyst = 1:1; total mass of platinum-carbon catalyst and perfluorosulfonic acid resin: solvent mass = 10%; the solvent is a mixed solution of n-propanol and deionized water in a mass ratio of 3.5:1.

[0051] Example 1 The method for preparing the gradient cathode catalyst layer provided in this embodiment includes the following steps: (1) Preparation of hydrophilic modified toner ① Weigh the materials according to the mass ratio of carbon powder to solid sulfonating agent = 1:3, put them into a planetary ball mill jar, add zirconia grinding balls, and mix them by planetary ball milling; wherein, the mass ratio of ball to material is 8:1, the ball milling speed is 400 r / min, and the grinding is carried out at room temperature for 60 min to mix evenly; ② Spread the evenly ground powder evenly on a quartz boat (layer thickness ≤ 5 mm), place it in a tube furnace, seal the furnace tube, and introduce nitrogen (purity ≥ 99.999%) at a flow rate of 80 mL / min. Pre-ventilate for 30 min to purge the air inside the tube; heat to 350℃ at a heating rate of 3℃ / min and hold for 3 h. ③ After the heat preservation is completed, turn off the heating, continue to introduce nitrogen, and allow the tubular furnace to cool naturally to room temperature; ④ After heating, the powder is transferred to a beaker and repeatedly soaked, stirred, filtered and washed in room temperature deionized water until the pH of the filtrate is 6~7 and no sulfate precipitate is detected by barium chloride test. ⑤ Transfer the separated filter cake to a vacuum drying oven and vacuum dry it for 10 hours at 70℃ and a vacuum degree of -0.09MPa to obtain hydrophilic sulfonic acid modified carbon powder for later use.

[0052] (2) Preparation of two-functional layer slurry Weigh out the hydrophilic modified carbon powder, platinum carbon catalyst, and perfluorosulfonic acid resin in proportion, add n-propanol-deionized water mixed solvent, stir magnetically at room temperature for 1 hour, and disperse by high-speed shearing for 1 hour to obtain the first functional layer (inner side, attached to the proton exchange membrane) slurry; wherein, in the first functional layer slurry, the mass ratio of hydrophilic modified carbon powder to platinum carbon catalyst is 3:7. The preparation method of the second functional layer (outer side, attached gas diffusion layer) slurry is the same as that of the first functional layer slurry. In the second functional layer slurry, the mass ratio of hydrophilic modified carbon powder to platinum-carbon catalyst is 1:9.

[0053] (3) Preparation of cathode catalyst layer First, a first functional layer slurry with a thickness of 80 μm is coated on the surface of the PTFE base film and dried in an oven at 60 °C for 30 min. Then, a second functional layer slurry with a thickness of 120 μm is coated on its surface and dried in an oven at 60 °C for 30 min to obtain the cathode catalyst layer.

[0054] (4) Fabrication of membrane electrodes The dried cathode catalyst layer was aligned and bonded to the proton exchange membrane, and then formed using a flatbed hot press. The hot pressing temperature was 155℃, the pressure was 3MPa, and the holding time was 2min. After cooling, the PTFE base membrane was peeled off to obtain the membrane electrode.

[0055] (5) Preparation of proton exchange membrane fuel cells Both the anode and cathode use 22BB carbon paper, and single-cell assembly is carried out in accordance with GB / T 20042.5-2024 proton exchange membrane fuel cells.

[0056] Example 2 The method for preparing the gradient cathode catalyst layer provided in this embodiment includes the following steps: (1) Preparation of hydrophilic modified toner ① Weigh the materials according to the mass ratio of carbon powder to solid sulfonating agent = 1:3, put them into a planetary ball mill jar, add zirconia grinding balls, and mix them by planetary ball milling; wherein, the mass ratio of ball to material is 8:1, the ball milling speed is 400 r / min, and the grinding is carried out at room temperature for 60 min to mix evenly; ② Spread the evenly ground powder evenly on a quartz boat (layer thickness ≤ 5 mm), place it in a tube furnace, seal the furnace tube, and introduce nitrogen (purity ≥ 99.999%) at a flow rate of 80 mL / min. Pre-ventilate for 30 min to purge the air inside the tube; heat to 350℃ at a heating rate of 3℃ / min and hold for 3 h. ③ After the heat preservation is completed, turn off the heating, continue to introduce nitrogen, and allow the tubular furnace to cool naturally to room temperature; ④ After heating, the powder is transferred to a beaker and repeatedly soaked, stirred, filtered and washed in room temperature deionized water until the pH of the filtrate is 6~7 and no sulfate precipitate is detected by barium chloride test. ⑤ Transfer the separated filter cake to a vacuum drying oven and vacuum dry it for 10 h at 70℃ and a vacuum degree of -0.09MPa to obtain hydrophilic sulfonic acid modified carbon powder for later use.

[0057] (2) Preparation of two-functional layer slurry Weigh out the hydrophilic modified carbon powder, platinum carbon catalyst, and perfluorosulfonic acid resin according to the proportion, add n-propanol-deionized water mixed solvent, stir magnetically at room temperature for 1 hour, and disperse by high-speed shearing for 1 hour to obtain the first functional layer (inner side, attached to the proton exchange membrane) slurry; wherein, in the first functional layer slurry, the mass ratio of hydrophilic modified carbon powder to platinum carbon catalyst is 4:6. The preparation method of the second functional layer (outer side, attached gas diffusion layer) slurry is the same as that of the first functional layer slurry. In the second functional layer slurry, the mass ratio of hydrophilic modified carbon powder to platinum-carbon catalyst is 1:9.

[0058] (3) Preparation of cathode catalyst layer First, a first functional layer slurry with a thickness of 80 μm is coated on the surface of the PTFE base film and dried in an oven at 60 °C for 30 min. Then, a second functional layer slurry with a thickness of 120 μm is coated on its surface and dried in an oven at 60 °C for 30 min to obtain the cathode catalyst layer.

[0059] (4) Fabrication of membrane electrodes The dried cathode catalyst layer was aligned and bonded to the proton exchange membrane, and then formed using a flatbed hot press. The hot pressing temperature was 155℃, the pressure was 3MPa, and the holding time was 2min. After cooling, the PTFE base membrane was peeled off to obtain the membrane electrode.

[0060] (5) Preparation of proton exchange membrane fuel cells Both the anode and cathode use 22BB carbon paper, and single-cell assembly is carried out in accordance with GB / T 20042.5-2024 proton exchange membrane fuel cells.

[0061] Example 3 The method for preparing the gradient cathode catalyst layer provided in this embodiment includes the following steps: (1) Preparation of hydrophilic modified toner ① Weigh the materials according to the mass ratio of carbon powder to solid sulfonating agent = 1:3, put them into a planetary ball mill jar, add zirconia grinding balls, and mix them by planetary ball milling; wherein, the mass ratio of ball to material is 8:1, the ball milling speed is 400 r / min, and the grinding is carried out at room temperature for 60 min to mix evenly; ② Spread the evenly ground powder evenly on a quartz boat (layer thickness ≤ 5 mm), place it in a tube furnace, seal the furnace tube, and introduce nitrogen (purity ≥ 99.999%) at a flow rate of 80 mL / min. Pre-ventilate for 30 min to purge the air inside the tube; heat to 350℃ at a heating rate of 3℃ / min and hold for 3 h. ③ After the heat preservation is completed, turn off the heating, continue to introduce nitrogen, and allow the tubular furnace to cool naturally to room temperature; ④ After heating, the powder is transferred to a beaker and repeatedly soaked, stirred, filtered and washed in room temperature deionized water until the pH of the filtrate is 6~7 and no sulfate precipitate is detected by barium chloride test. ⑤ Transfer the separated filter cake to a vacuum drying oven and vacuum dry it for 10 h at 70℃ and a vacuum degree of -0.09MPa to obtain hydrophilic sulfonic acid modified carbon powder for later use.

[0062] (2) Preparation of two-functional layer slurry Weigh out the hydrophilic modified carbon powder, platinum carbon catalyst, and perfluorosulfonic acid resin in proportion, add n-propanol-deionized water mixed solvent, stir magnetically at room temperature for 1 hour, and disperse by high-speed shearing for 1 hour to obtain the first functional layer (inner side, attached to the proton exchange membrane) slurry; wherein, in the first functional layer slurry, the mass ratio of hydrophilic modified carbon powder to platinum carbon catalyst is 3:7. The preparation method of the second functional layer (outer side, attached gas diffusion layer) slurry is the same as that of the first functional layer slurry. In the second functional layer slurry, the mass ratio of hydrophilic modified carbon powder to platinum carbon catalyst is 2:8.

[0063] (3) Preparation of cathode catalyst layer First, a first functional layer slurry with a thickness of 80 μm is coated on the surface of the PTFE base film and dried in an oven at 60 °C for 30 min. Then, a second functional layer slurry with a thickness of 120 μm is coated on its surface and dried in an oven at 60 °C for 30 min to obtain the cathode catalyst layer.

[0064] (4) Fabrication of membrane electrodes The dried cathode catalyst layer was aligned and bonded to the proton exchange membrane, and then formed using a flatbed hot press. The hot pressing temperature was 155℃, the pressure was 3MPa, and the holding time was 2min. After cooling, the PTFE base membrane was peeled off to obtain the membrane electrode.

[0065] (5) Preparation of proton exchange membrane fuel cells Both the anode and cathode use 22BB carbon paper, and single-cell assembly is carried out in accordance with GB / T 20042.5-2024 proton exchange membrane fuel cells.

[0066] Example 4 The method for preparing the gradient cathode catalyst layer provided in this embodiment includes the following steps: (1) Preparation of hydrophilic modified toner ① Weigh the materials according to the mass ratio of carbon powder to solid sulfonating agent = 1:2, put them into a planetary ball mill jar, add zirconia grinding balls, and mix them by planetary ball milling; wherein, the mass ratio of ball to material is 8:1, the ball milling speed is 400 r / min, and the grinding is carried out at room temperature for 60 min to mix evenly; ② Spread the evenly ground powder evenly on a quartz boat (layer thickness ≤ 5 mm), place it in a tube furnace, seal the furnace tube, and introduce nitrogen (purity ≥ 99.999%) at a flow rate of 80 mL / min. Pre-ventilate for 30 min to purge the air inside the tube; heat to 350℃ at a heating rate of 3℃ / min and hold for 3 h. ③ After the heat preservation is completed, turn off the heating, continue to introduce nitrogen, and allow the tubular furnace to cool naturally to room temperature; ④ After heating, the powder is transferred to a beaker and repeatedly soaked, stirred, filtered and washed in room temperature deionized water until the pH of the filtrate is 6~7 and no sulfate precipitate is detected by barium chloride test. ⑤ Transfer the separated filter cake to a vacuum drying oven and vacuum dry it for 10 h at 70℃ and a vacuum degree of -0.09MPa to obtain hydrophilic sulfonic acid modified carbon powder for later use.

[0067] (2) Preparation of two-functional layer slurry Weigh out the hydrophilic modified carbon powder, platinum carbon catalyst, and perfluorosulfonic acid resin in proportion, add n-propanol-deionized water mixed solvent, stir magnetically at room temperature for 1 hour, and disperse by high-speed shearing for 1 hour to obtain the first functional layer (inner side, attached to the proton exchange membrane) slurry; wherein, in the first functional layer slurry, the mass ratio of hydrophilic modified carbon powder to platinum carbon catalyst is 3:7. The preparation method of the second functional layer (outer side, attached gas diffusion layer) slurry is the same as that of the first functional layer slurry. In the second functional layer slurry, the mass ratio of hydrophilic modified carbon powder to platinum-carbon catalyst is 1:9.

[0068] (3) Preparation of cathode catalyst layer First, a first functional layer slurry with a thickness of 80 μm is coated on the surface of the PTFE base film and dried in an oven at 60 °C for 30 min. Then, a second functional layer slurry with a thickness of 120 μm is coated on its surface and dried in an oven at 60 °C for 30 min to obtain the cathode catalyst layer.

[0069] (4) Fabrication of membrane electrodes The dried cathode catalyst layer was aligned and bonded to the proton exchange membrane, and then formed using a flatbed hot press. The hot pressing temperature was 155℃, the pressure was 3MPa, and the holding time was 2min. After cooling, the PTFE base membrane was peeled off to obtain the membrane electrode.

[0070] (5) Preparation of proton exchange membrane fuel cells Both the anode and cathode use 22BB carbon paper, and single-cell assembly is carried out in accordance with GB / T 20042.5-2024 proton exchange membrane fuel cells.

[0071] Comparative Example 1 The method for preparing the cathode catalyst layer provided in this comparative example includes the following steps: (1) Preparation of catalyst layer slurry Weigh out platinum-carbon catalyst and perfluorosulfonic acid resin according to the proportion, add n-propanol-deionized water mixed solvent, stir magnetically at room temperature for 1 hour, and disperse by high-speed shearing for 1 hour to obtain catalyst slurry.

[0072] (2) Preparation of cathode catalyst layer A catalytic layer slurry with a thickness of 200 μm was coated on the surface of a PTFE base membrane and dried in an oven at 60 °C for 30 min to obtain the cathode catalytic layer.

[0073] (3) Fabrication of membrane electrodes The dried cathode catalyst layer was aligned and bonded to the proton exchange membrane, and then formed using a flatbed hot press. The hot pressing temperature was 155℃, the pressure was 3MPa, and the holding time was 2min. After cooling, the PTFE base membrane was peeled off to obtain the membrane electrode.

[0074] (4) Preparation of proton exchange membrane fuel cells Both the anode and cathode use 22BB carbon paper, and single-cell assembly is carried out in accordance with GB / T 20042.5-2024 proton exchange membrane fuel cells.

[0075] Comparative Example 2 The method for preparing the gradient cathode catalyst layer provided in this comparative example includes the following steps: (1) Preparation of hydrophilic modified toner ① Weigh the materials according to the mass ratio of carbon powder to solid sulfonating agent = 1:3, put them into a planetary ball mill jar, add zirconia grinding balls, and mix them by planetary ball milling; wherein, the mass ratio of ball to material is 8:1, the ball milling speed is 400 r / min, and the grinding is carried out at room temperature for 60 min to mix evenly; ② Spread the evenly ground powder evenly on a quartz boat (layer thickness ≤ 5 mm), place it in a tube furnace, seal the furnace tube, and introduce nitrogen (purity ≥ 99.999%) at a flow rate of 80 mL / min. Pre-ventilate for 30 min to purge the air inside the tube; heat to 350℃ at a heating rate of 3℃ / min and hold for 3 h. ③ After the heat preservation is completed, turn off the heating, continue to introduce nitrogen, and allow the tubular furnace to cool naturally to room temperature; ④ After heating, the powder is transferred to a beaker and repeatedly soaked, stirred, filtered and washed in room temperature deionized water until the pH of the filtrate is 6~7 and no sulfate precipitate is detected by barium chloride test. ⑤ Transfer the separated filter cake to a vacuum drying oven and vacuum dry it for 10 h at 70℃ and a vacuum degree of -0.09MPa to obtain hydrophilic sulfonic acid modified carbon powder for later use.

[0076] (2) Preparation of two-functional layer slurry Weigh out the hydrophilic modified carbon powder, platinum carbon catalyst, and perfluorosulfonic acid resin in proportion, add n-propanol-deionized water mixed solvent, stir magnetically at room temperature for 1 hour, and disperse by high-speed shearing for 1 hour to obtain the first functional layer (inner side, attached to the proton exchange membrane) slurry; wherein, in the first functional layer slurry, the mass ratio of hydrophilic modified carbon powder to platinum carbon catalyst is 1:9. The preparation method of the second functional layer (outer side, attached gas diffusion layer) slurry is the same as that of the first functional layer slurry. In the second functional layer slurry, the mass ratio of hydrophilic modified carbon powder to platinum-carbon catalyst is 1:9.

[0077] (3) Preparation of cathode catalyst layer First, a first functional layer slurry with a thickness of 80 μm is coated on the surface of the PTFE base film and dried in an oven at 60 °C for 30 min. Then, a second functional layer slurry with a thickness of 120 μm is coated on its surface and dried in an oven at 60 °C for 30 min to obtain the cathode catalyst layer.

[0078] (4) Fabrication of membrane electrodes The dried cathode catalyst layer was aligned and bonded to the proton exchange membrane, and then formed using a flatbed hot press. The hot pressing temperature was 155℃, the pressure was 3MPa, and the holding time was 2min. After cooling, the PTFE base membrane was peeled off to obtain the membrane electrode.

[0079] (5) Preparation of proton exchange membrane fuel cells Both the anode and cathode use 22BB carbon paper, and single-cell assembly is carried out in accordance with GB / T 20042.5-2024 proton exchange membrane fuel cells.

[0080] Comparative Example 3 The method for preparing the gradient cathode catalyst layer provided in this comparative example includes the following steps: (1) Preparation of hydrophilic modified toner ① Weigh the materials according to the mass ratio of carbon powder to solid sulfonating agent = 1:3, put them into a planetary ball mill jar, add zirconia grinding balls, and mix them by planetary ball milling; wherein, the mass ratio of ball to material is 8:1, the ball milling speed is 400 r / min, and the grinding is carried out at room temperature for 60 min to mix evenly; ② Spread the evenly ground powder evenly on a quartz boat (layer thickness ≤ 5 mm), place it in a tube furnace, seal the furnace tube, and introduce nitrogen (purity ≥ 99.999%) at a flow rate of 80 mL / min. Pre-ventilate for 30 min to purge the air inside the tube; heat to 350℃ at a heating rate of 3℃ / min and hold for 3 h. ③ After the heat preservation is completed, turn off the heating, continue to introduce nitrogen, and allow the tubular furnace to cool naturally to room temperature; ④ After heating, the powder is transferred to a beaker and repeatedly soaked, stirred, filtered and washed in room temperature deionized water until the pH of the filtrate is 6~7 and no sulfate precipitate is detected by barium chloride test. ⑤ Transfer the separated filter cake to a vacuum drying oven and vacuum dry it for 10 h at 70℃ and a vacuum degree of -0.09MPa to obtain hydrophilic sulfonic acid modified carbon powder for later use.

[0081] (2) Preparation of two-functional layer slurry Weigh out the hydrophilic modified carbon powder, platinum carbon catalyst, and perfluorosulfonic acid resin in proportion, add n-propanol-deionized water mixed solvent, stir magnetically at room temperature for 1 hour, and disperse by high-speed shearing for 1 hour to obtain the first functional layer (inner side, attached to the proton exchange membrane) slurry; wherein, in the first functional layer slurry, the mass ratio of hydrophilic modified carbon powder to platinum carbon catalyst is 3:7. The preparation method of the second functional layer (outer side, attached gas diffusion layer) slurry is the same as that of the first functional layer slurry. In the second functional layer slurry, the mass ratio of hydrophilic modified carbon powder to platinum carbon catalyst is 1:10.

[0082] (3) Preparation of cathode catalyst layer First, a first functional layer slurry with a thickness of 80 μm is coated on the surface of the PTFE base film and dried in an oven at 60 °C for 30 min. Then, a second functional layer slurry with a thickness of 120 μm is coated on its surface and dried in an oven at 60 °C for 30 min to obtain the cathode catalyst layer.

[0083] (4) Fabrication of membrane electrodes The dried cathode catalyst layer was aligned and bonded to the proton exchange membrane, and then formed using a flatbed hot press. The hot pressing temperature was 155℃, the pressure was 3MPa, and the holding time was 2min. After cooling, the PTFE base membrane was peeled off to obtain the membrane electrode.

[0084] (5) Preparation of proton exchange membrane fuel cells Both the anode and cathode use 22BB carbon paper, and single-cell assembly is carried out in accordance with GB / T 20042.5-2024 proton exchange membrane fuel cells.

[0085] The proton exchange membrane fuel cells prepared in Examples 1-4 and Comparative Examples 1-3 were subjected to performance testing. Specific test conditions are as follows: The battery temperature is 80℃, the anode is 50%RH, the anode gas is hydrogen, the anode flow rate is 210cc / min, the anode excess coefficient is 1.5, and the anode back pressure is 150kPa; the cathode is 0%RH, the cathode gas is air, the cathode flow rate is 500cc / min, the cathode excess coefficient is 3.0, and the cathode back pressure is 150kPa.

[0086] See Table 1 for specific performance test results.

[0087] Table 1

[0088] As can be seen from Table 1, compared with Comparative Example 1 (the cathode catalyst layer prepared using an existing uniformly mixed structure), the batteries using the cathode catalyst layers prepared in Examples 1-4 of this invention have higher output voltages, better battery performance, and lower internal resistance. Furthermore, compared with Comparative Examples 2-3, the batteries using the cathode catalyst layers prepared in Examples 1-4 of this invention have higher output voltages and lower internal resistance.

[0089] This demonstrates that the cathode catalyst layer designed in this invention has a gradient composite structure along the thickness direction. In the first functional layer near the proton exchange membrane, the mass ratio of hydrophilic modified carbon powder to platinum-carbon catalyst is relatively large, while in the second functional layer near the gas diffusion layer, the mass ratio of hydrophilic modified carbon powder to platinum-carbon catalyst is relatively small. This achieves an optimal balance between water retention and mass transfer under low humidity conditions, while greatly improving the electrochemical performance and long-term operational stability of the battery. This solves the technical problem that existing cathode catalyst layers cannot simultaneously achieve water retention and mass transfer under low humidity conditions (especially without external humidification), thus making it difficult to balance battery performance and stability.

[0090] In the description of this specification, the references to terms such as "one embodiment / mode," "some embodiments / modes," "example," "specific example," or "some examples," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment / mode or example is included in at least one embodiment / mode or example of this application. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment / mode or example. Moreover, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments / modes or examples. Furthermore, without contradiction, those skilled in the art can combine and integrate the different embodiments / modes or examples described in this specification, as well as the features of different embodiments / modes or examples.

[0091] It should be noted that in this application, relational terms such as "first" and "second" are used merely to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitations, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes said element. In this application, "a plurality of" means at least two, such as two, three, etc., unless otherwise expressly specified.

[0092] The above description is merely a specific embodiment of this application, enabling those skilled in the art to understand or implement this application. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of this application. Therefore, this application is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features claimed herein.

Claims

1. A gradient cathode catalytic layer, characterized in that, It comprises a first functional layer and a second functional layer from the inside out, wherein the mass ratio of hydrophilic modified carbon powder to platinum-carbon catalyst in the first functional layer is greater than the mass ratio of hydrophilic modified carbon powder to platinum-carbon catalyst in the second functional layer.

2. The gradient cathode catalytic layer of claim 1, wherein, The mass ratio of hydrophilic modified carbon powder to platinum-carbon catalyst in the first functional layer is 3:7 to 4:6, and the mass ratio of hydrophilic modified carbon powder to platinum-carbon catalyst in the second functional layer is 1:9 to 2:

8.

3. The graded cathode catalytic layer of claim 1, wherein, The hydrophilic modified toner is a modified toner with hydrophilic sulfonic acid groups uniformly loaded on its surface.

4. The gradient cathode catalytic layer of claim 3, wherein, The preparation method of the hydrophilic modified toner includes: The toner powder is mixed and ground with a solid sulfonating agent, then heat-treated under inert gas protection, and subsequently cooled, washed, and dried to obtain the hydrophilic modified toner powder.

5. The gradient cathode catalytic layer of claim 4, wherein, The mass ratio of the carbon powder to the solid sulfonating agent is 1:(2~5).

6. The graded cathode catalytic layer of claim 4, wherein, The heat treatment temperature is 300~400℃, and the heat treatment time is 2~4h.

7. A method for producing a gradient cathode catalytic layer, characterized by, Includes the following steps: A first functional layer slurry and a second functional layer slurry are coated from the inside to the outside on the surface of a base film, and then hot-pressed and transferred to form the film; wherein, the mass ratio of hydrophilic modified carbon powder to platinum-carbon catalyst in the first functional layer slurry is greater than the mass ratio of hydrophilic modified carbon powder to platinum-carbon catalyst in the second functional layer slurry.

8. The method of claim 7, wherein the method further comprises: The mass ratio of hydrophilic modified carbon powder to platinum-carbon catalyst in the first functional layer is 3:7 to 4:6, and the mass ratio of hydrophilic modified carbon powder to platinum-carbon catalyst in the second functional layer is 1:9 to 2:

8.

9. The method of claim 7, wherein the method further comprises: The first functional layer slurry and the second functional layer slurry also include perfluorosulfonic acid resin and solvent; The mass ratio of the perfluorosulfonic acid resin to the carbon in the platinum-carbon catalyst is (0.8~1.1):1, and the sum of the mass of the perfluorosulfonic acid resin and the mass of the platinum-carbon catalyst accounts for 8%~11% of the mass of the solvent. The solvent is a mixed solution of alcohol and water in a mass ratio of (2~4):

1.

10. A proton exchange membrane fuel cell, characterized by, It includes the gradient cathode catalyst layer as described in any one of claims 1 to 4 or the gradient cathode catalyst layer prepared by the method described in any one of claims 7 to 9.

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