Composite gas diffusion layer for fuel cells, membrane electrode and method for producing the same

Abstract

The application provides a composite gas diffusion layer, belongs to the technical field of new energy and fuel cells, and is used for a fuel cell membrane electrode, comprising a porous substrate layer and a composite microporous layer which are arranged in layers, and the porous substrate layer serves as a support body; the composite microporous layer comprises a conductive support layer and a selective screening layer, the conductive support layer is arranged on the surface of the porous substrate layer and is formed of a conductive framework of carbon nanotube networks; the selective screening layer is located on the surface of the conductive support layer away from the porous substrate layer, is formed by paving two-dimensional sheet materials and is attached to the conductive support layer, the two-dimensional sheet materials are distributed with an array of nano-pores of the order of Angstrom, and the aperture of the nano-pores is 0.35-0.8 nm. The application further provides a plurality of preparation methods of the composite gas diffusion layer and a preparation method of the fuel cell membrane electrode. The application can solve the technical problems of poor barrier effect of the membrane electrode on fuel penetration and low proton conductivity in proton exchange membrane fuel cells and direct methanol fuel cells.

Description

Technical Field

[0001] This invention relates to the field of new energy and fuel cell technology, specifically to a composite gas diffusion layer, membrane electrode assembly, and preparation method thereof for fuel cells. Background Technology

[0002] Proton exchange membrane fuel cells (PEMFCs), as efficient and clean energy conversion devices, have broad application prospects in portable power supplies, electric vehicles, and aerospace. The membrane electrode assembly (MEA) is the core component of a fuel cell, typically composed of a proton exchange membrane, a catalyst layer, and a gas diffusion layer. However, the fuel permeation problem in MEAs, particularly methanol permeation in direct methanol fuel cells, is a key bottleneck limiting their commercial application. When methanol molecules pass through the proton exchange membrane from the anode to the cathode, it not only wastes fuel but also reacts with oxygen at the cathode in a mixed potential reaction, leading to a significant decrease in cell voltage and efficiency.

[0003] To address the aforementioned technical issues, the current common solution is to modify the proton exchange membrane (PEM) itself: for example, by increasing the membrane thickness, or by doping the PEM with inorganic fillers to extend the fuel permeation path using the labyrinth effect of the fillers. However, these methods can lead to blockage or elongation of the proton transport channels, significantly reducing proton conductivity and increasing the battery's ohmic resistance. At the same time, bulk doping often results in a decrease in the mechanical properties of the PEM, making it prone to phase separation or swelling and peeling during wet and dry cycling, thus affecting battery life. Therefore, based on the modification of the proton exchange membrane body, the micropores of the gas diffusion layer can also be optimized. The traditional microporous layer is usually formed by coating the surface of carbon paper with a mixture of conductive carbon black particles and hydrophobic agents. Its main function is to improve water management and provide conductive pathways. Therefore, sheet materials can be added to the gas diffusion layer to block fuel permeation. However, at present, most of the sheet materials are added by simple slurry mixing. During the coating process of the mixed slurry, the sheet materials are prone to agglomeration or random orientation, or even perpendicular to the membrane surface, which cannot form a continuous and dense planar barrier layer, resulting in poor barrier effect. Moreover, it will cause poor electrical contact between the sheet material and the conductive carbon paper matrix, increasing the interfacial contact resistance, which is not conducive to the rapid collection and conduction of electrons. Summary of the Invention

[0004] To address the shortcomings of existing technologies, this invention proposes a composite gas diffusion layer, a membrane electrode assembly (MEA) for fuel cells, and a method for preparing the MEA, thereby solving the technical problems of poor fuel permeation barrier effect and low proton conductivity in proton exchange membrane fuel cells and direct methanol fuel cells. The technical solution adopted by this invention is as follows: In a first aspect, a composite gas diffusion layer is provided for a fuel cell membrane electrode, comprising a porous substrate layer and a composite microporous layer stacked together, wherein the porous substrate layer serves as a support. The composite microporous layer includes a conductive support layer and a selective sieving layer. The conductive support layer is disposed on the surface of the porous substrate layer and forms a conductive framework with three-dimensional pores for the carbon nanotube network. The selective sieve layer is located on the side of the conductive support layer away from the porous substrate layer. It is composed of two-dimensional sheet material laid flat and attached to the conductive support layer. The two-dimensional sheet material has multiple nanopores with a pore size of 0.35-0.8 nm distributed on it.

[0005] Furthermore, the two-dimensional sheet material includes graphene or graphene oxide derivatives, and the fuel cell includes a direct methanol fuel cell.

[0006] In a second aspect, a method for preparing a composite gas diffusion layer is provided, for preparing the composite gas diffusion layer described in the first aspect, comprising the following steps: A carbon nanotube dispersion is sprayed onto a substrate and then sintered to obtain a carbon nanotube network framework as a conductive support layer; the thickness of the carbon nanotube network framework is 20-30 μm. A graphene oxide film was prepared by vacuum filtration as a selective sieve layer and transferred onto a nanotube network framework; the thickness of the graphene oxide film was 10-50 nm. In-situ pore formation of graphene oxide films using ultraviolet radiation; A composite gas diffusion layer is obtained by thermal reduction in an inert gas atmosphere.

[0007] Furthermore, when performing in-situ pore formation using ultraviolet radiation, the wavelengths of the ultraviolet light are 185nm and 254nm, and the light intensity density is 20-30mW / cm². 2 The irradiation time is 15 to 30 minutes; When performing thermal reduction in an inert gas atmosphere, thermal reduction is carried out for 2 hours in an argon atmosphere at 200°C.

[0008] Thirdly, another method for preparing a composite gas diffusion layer is provided, for preparing the composite gas diffusion layer described in the first aspect, comprising the following steps: A carbon nanotube network framework was grown in situ on a substrate using chemical vapor deposition as a conductive support layer; the thickness of the carbon nanotube network framework was 20-30 μm. A graphene oxide coating is sprayed onto a carbon nanotube network framework as a selective sieve layer, wherein the thickness of the graphene oxide film is 10-30 nm. Microscopic pores were created in the graphene oxide coating using an oxygen plasma etching process. Annealing and reduction were performed in an inert gas atmosphere to obtain a composite gas diffusion layer.

[0009] Furthermore, when spraying a graphene oxide coating onto the carbon nanotube network framework as a selective sieving layer, an ultrasonic atomization spraying technique is used to uniformly spray the graphene oxide dispersion onto the surface of the carbon nanotube network framework. When using oxygen plasma etching to create micro-pores in graphene oxide coatings, the radio frequency power of the plasma treatment is 40-60W, and the treatment time is 10 to 40 seconds. When performing annealing and reduction in an inert gas atmosphere, the temperature is increased to 300°C at a rate of 2-5°C / min, held at that temperature for 2 hours, and then allowed to cool naturally to room temperature.

[0010] Fourthly, another method for preparing a composite gas diffusion layer is provided, for preparing the composite gas diffusion layer described in the first aspect, comprising the following steps: A carbon nanotube dispersion is sprayed onto a substrate and then sintered to obtain a carbon nanotube network framework as a conductive support layer; the thickness of the carbon nanotube network framework is 20-30 μm. A functionalized graphene composite slurry was prepared and uniformly coated onto the surface of a carbon nanotube network framework using a scraping method. The carbon nanotube network skeleton coated with the composite slurry is placed in a heating device for thermal crosslinking curing and partial reduction treatment, forming a graphene oxide film with a thickness of 10-50 nm on the surface of the carbon nanotube network skeleton, thus obtaining a composite gas diffusion layer.

[0011] Furthermore, the functionalized graphene composite slurry is prepared as follows: graphene oxide dispersion and p-phenylenediamine crosslinking agent are mixed at a mass ratio of 5:1-20:1 and stirred at 60°C for 4 hours to allow diamine molecules to be covalently grafted between graphene oxide sheets. When the composite slurry is uniformly coated onto the surface of the carbon nanotube network skeleton by a scraping method, the scraper gap is set to 10 μm and the coating speed is 5 mm / s. The heating temperature for the thermal crosslinking and curing is 120-180℃, and the heating time is 4-24 hours.

[0012] Fifthly, a method for preparing a fuel cell membrane electrode assembly is provided, wherein the fuel cell membrane electrode assembly is prepared based on the composite gas diffusion layer described in the first aspect, comprising the following steps: The composite gas diffusion layer is placed on the anode side, with the selective sieve layer in the composite gas diffusion layer facing the proton exchange membrane or catalyst layer, and the porous substrate layer in the composite gas diffusion layer facing the bipolar plate flow field.

[0013] In a sixth aspect, a fuel cell membrane electrode is provided, which is prepared by the preparation method described in the fifth aspect. The fuel cell membrane electrode can be used in proton exchange membrane fuel cells and direct methanol fuel cells.

[0014] As can be seen from the above technical solution, the beneficial technical effects of the present invention are as follows: 1. This invention utilizes the size sieving effect to significantly suppress fuel permeation by constructing a graphene sieve layer with pore sizes of 0.35-0.8 nm on a carbon nanotube framework. Experiments show that in direct methanol fuel cell applications, this structure can reduce the methanol permeation current density by more than 80%, effectively eliminating the mixing potential effect on the cathode side, thereby significantly improving the open-circuit voltage and power density of the battery.

[0015] 2. This invention effectively solves the stability problem of two-dimensional materials in engineering applications. The "nano-steel" framework formed by carbon nanotubes supports the ultrathin graphene layer, greatly enhancing the interfacial bonding force and enabling it to withstand fluid erosion and assembly pressure during battery operation. After long-term wet and dry cycle testing, the composite structure showed no significant peeling or damage, significantly extending the device's lifespan.

[0016] 3. The all-carbon-based high-conductivity network constructed in this invention significantly reduces interfacial resistance. By eliminating the excessive reliance on insulating binders in traditional microporous layers and utilizing the van der Waals forces and π-π conjugation between carbon nanotubes and graphene for tight bonding, this invention reduces the surface resistance of the gas diffusion layer by approximately 40%-60%, effectively improving the polarization performance of the battery under high current density.

[0017] 4. The preparation process of this invention is highly compatible with existing gas diffusion layer production lines. The resulting product can be stored and transported as an independent standard component without requiring complex adjustments to the existing membrane electrode hot pressing assembly process. It has high potential for industrial application and commercial value. Attached Figure Description

[0018] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the accompanying drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. In all the drawings, similar elements or parts are generally identified by similar reference numerals. In the drawings, the elements or parts are not necessarily drawn to scale.

[0019] Figure 1 This is a schematic flowchart of the method for preparing a composite gas diffusion layer according to Embodiment 1 in this invention. Figure 2 This is a schematic flowchart of the method for preparing the composite gas diffusion layer according to Embodiment 2 of the present invention; Figure 3 This is a schematic flowchart of the method for preparing the composite gas diffusion layer according to Embodiment 3 of the present invention. Detailed Implementation

[0020] The embodiments of the technical solution of the present invention will now be described in detail with reference to the accompanying drawings. These embodiments are merely illustrative of the technical solution of the present invention and are therefore intended to limit the scope of protection of the present invention.

[0021] It should be noted that, unless otherwise stated, the technical or scientific terms used in this application should have the ordinary meaning as understood by one of ordinary skill in the art to which this invention pertains.

[0022] This embodiment provides a composite gas diffusion layer for a fuel cell membrane electrode assembly, comprising a porous substrate layer and a composite microporous layer stacked together. The porous substrate layer serves as a macroscopic support, and its specific implementation is not limited; for example, carbon paper or carbon cloth can be used.

[0023] The composite microporous layer includes a conductive support layer and a selective sieving layer.

[0024] The conductive support layer is located on the surface of the porous substrate and is composed of an interwoven network of carbon nanotubes forming a conductive framework with three-dimensional pores.

[0025] The selective sieving layer is located on the side of the conductive support layer away from the porous substrate layer. It is composed of two-dimensional sheet material laid flat and attached to the conductive support layer. The two-dimensional sheet material has a high density of nanopores (or nano-sieving channels). The arrangement of the nanopores on the surface of the two-dimensional sheet material is not limited, but is preferably randomly and uniformly distributed or in a disordered array; for example, square, hexagonal, or equilateral triangular arrays. The two-dimensional sheet material is preferably graphene or a graphene oxide derivative, with a nanopore size of 0.35-0.8 nm and a surface porosity of 1%-5%.

[0026] The aforementioned composite gas diffusion layer utilizes multi-scale pore sieving and electron-proton separation and transport mechanisms to effectively block fuel permeation while improving proton conductivity. Its working principle is as follows: Liquid fuels (such as methanol) pass through the macropores of the porous substrate layer and the conductive support layer (carbon nanotube network framework) and are then blocked by the selective sieving layer (flat graphene) on the top layer. The nanopores on the graphene are smaller than the dynamic diameter of the fuel molecules (e.g., the diameter of a methanol molecule is 0.38 nm), thus achieving physical interception and effectively preventing fuel permeation.

[0027] Protons (in the form of hydrated hydrogen ions) are extremely small and can pass smoothly through the nanopores of the selective sieve layer into the proton exchange membrane. The selective sieve layer collects electrons generated by the catalytic reaction and rapidly transmits them to the conductive support layer below through its high in-plane conductivity, and then to the external circuit, effectively reducing contact resistance and improving proton conductivity.

[0028] The composite microporous layer used in this embodiment has a different composite structure of carbon nanotubes and graphene compared to existing technologies. Existing technologies often employ disordered mixing / blending of carbon nanotubes and graphene, resulting in uncontrollable porosity in the resulting composite structure and a tendency for graphene to aggregate. This embodiment, however, uses a step-by-step stacking process—first the framework, then the skin—to ensure that graphene is smoothly spread across the carbon nanotube network. Simultaneously, a microporous fabrication process is used to precisely control the distribution of angstrom-scale nanopore arrays on the graphene, with pore sizes of 0.35-0.8 nm and surface porosity of 1%-5%.

[0029] To achieve the unique composite structure of carbon nanotubes and graphene described above in this invention, this embodiment provides a method for preparing the composite gas diffusion layer, which is illustrated by the following multiple embodiments: The main raw materials for the following embodiments include: Porous substrate: hydrophobically treated carbon paper (Toray TGP-H-060), commercially available.

[0030] Carbon nanotubes: Multi-walled carbon nanotubes, diameter 10-20 nm, length 5-15 μm, purity >95%, commercially available.

[0031] Graphene oxide dispersion: monolayer ratio >90%, sheet diameter 0.5-5μm, concentration 0.5-2mg / mL, commercially available.

[0032] Implementation Method 1: Preparation Process Based on Spraying and Vacuum Filtration Transfer Method After spraying a carbon nanotube dispersion onto a substrate, a carbon nanotube network framework was obtained as a conductive support layer through sintering: 0.5 g of multi-walled carbon nanotubes and 0.1 g of polytetrafluoroethylene (PTFE) emulsion (60 wt%) were mixed and added to 100 mL of a mixed solvent of isopropanol and water (volume ratio 1:1). 10-50 mg of surfactant (Triton X-100) was added, and the mixture was ultrasonically dispersed at 400 W for 60 minutes to obtain a uniform dispersion. The dispersion was then coated onto the surface of hydrophobic carbon paper using an automated spraying device. Subsequently, the paper was placed in a tube furnace and sintered at 350 °C for 30 minutes under nitrogen protection to remove the solvent and allow the PTFE to melt and bond, forming a carbon nanotube network framework with a thickness of approximately 20-30 μm.

[0033] A graphene oxide film was prepared by vacuum filtration as a selective sieve layer and transferred onto a nanotube network framework: a graphene oxide dispersion was taken and deposited on a filter membrane with a pore size of 0.22 μm to form a dense graphene oxide laminate film with a thickness of 10-50 nm by vacuum filtration. The graphene oxide film was then completely bonded to the surface of the aforementioned carbon nanotube conductive framework by a wet transfer process to obtain a composite structure of carbon nanotube network framework / graphene oxide film.

[0034] In-situ pore formation of the carbon nanotube network framework / graphene oxide film composite structure was achieved using ultraviolet radiation: the composite structure was first vacuum-dried at 80℃ for 2 hours to remove interlayer adsorbed water. Then, the dried composite structure was placed on the sample stage of an ultraviolet-ozone cleaner, with the surface of the composite structure adjusted to 5-10 mm from the ultraviolet lamp. A low-pressure mercury lamp light source was then turned on (main wavelengths of 185 nm and 254 nm, light intensity density of 20-30 mW / cm²). 2 The graphene is irradiated in an ambient air atmosphere (or with a continuous flow of oxygen at a velocity of 50-100 sccm). During this process, 185 nm ultraviolet light excites oxygen in the air to produce ozone, and 254 nm ultraviolet light further decomposes the ozone into highly reactive atomic oxygen. This atomic oxygen preferentially reacts with carbon atoms at sp3 hybridization defects or grain boundaries in the graphene lattice, generating CO or CO2 gas which escapes, thus etching angstrom-scale pores in situ. By controlling the irradiation time to 15 to 30 minutes, the pore size on the graphene surface is precisely controlled within the range of 0.35-0.8 nm.

[0035] A composite gas diffusion layer is obtained by thermal reduction under an inert gas atmosphere: After etching, the composite structure is placed in a tube furnace and thermally reduced at 200°C under an argon atmosphere for 2 hours to remove oxygen-containing functional groups (such as hydroxyl and epoxy groups) introduced during the oxidation process, restore the conductivity of the graphene lattice, and obtain a conductive composite microporous layer with high-density sieve pores, which is the composite gas diffusion layer.

[0036] Implementation Method 2: In-situ growth by chemical vapor deposition (CVD) and plasma etching This embodiment shows another preparation scheme for the composite gas diffusion layer. The main difference between this embodiment and Embodiment 1 is the construction method of the conductive support layer and the micro-pore-forming process.

[0037] A carbon nanotube network framework was grown in situ on a carbon paper substrate using chemical vapor deposition (CVD) as a conductive support layer. The carbon paper, impregnated with a nickel nitrate catalyst precursor, was placed in a CVD furnace and reduced at 500°C for 2 hours in a hydrogen atmosphere. Acetylene was then introduced as a carbon source, and the reaction was carried out at 700°C for 30 minutes, allowing the carbon nanotubes to grow vertically directly from the carbon fiber surface, forming a highly cohesive network framework with a thickness of 20-30 μm.

[0038] A graphene oxide coating was sprayed onto the carbon nanotube network framework as a selective sieving layer: Using ultrasonic atomization spraying technology, a graphene oxide dispersion with a concentration of 0.5-1.0 mg / mL was uniformly sprayed onto the surface of the in-situ grown carbon nanotube network framework. Considering the requirements of subsequent plasma etching for film penetration, the spraying amount needed to be strictly controlled, ensuring the thickness of the graphene oxide layer was controlled within the range of 10-30 nm (approximately 10-30 layers of graphene stacked). The film was then dried at 80°C to obtain a composite structure of carbon nanotube network framework / graphene oxide film. The 10-30 nm thickness range ensured both effective coverage of the carbon nanotube framework by the graphene oxide layer and the formation of effective interconnected sieving channels during subsequent etching.

[0039] Microscopic pore formation in graphene oxide coatings was achieved using an oxygen plasma etching process. The composite structure was placed in a plasma treatment machine with a radio frequency power of 40-60 W, an oxygen flow rate of 20 sccm, and a treatment time of 10-40 seconds. High-energy plasma bombardment created nanoscale pores within the graphene lattice. By strictly controlling the synergistic effect of radio frequency power and treatment time, excessive pore enlargement or merging was prevented, thus precisely controlling the pore size within the range of 0.35-0.8 nm. Subsequently, the sample was placed in a tube furnace for annealing and reduction treatment in a hydrogen-argon mixed atmosphere (hydrogen volume ratio 5%-10%). The specific annealing procedure involved heating to 300 °C at a rate of 2-5 °C / min, holding at that temperature for 2 hours, and finally allowing the furnace to cool naturally to room temperature. This step effectively removed excess oxygen-containing functional groups introduced during the etching process and repaired the lattice, ultimately resulting in the preparation of a composite gas diffusion layer with high conductivity and high selectivity.

[0040] Unlike the stepwise decoupling process of inert gas nucleation and reactive gas pore expansion, this embodiment employs a single-step reactive ion etching mechanism. Utilizing the combined physical bombardment kinetic energy and chemical oxidation activity of oxygen plasma, it simultaneously introduces and expands lattice defects in graphene on the rough carbon nanotube framework surface. After bombardment, nanosieve-like pores with numerous oxygen-containing functional groups at their edges are formed on the graphene surface. To eliminate amorphization damage caused by plasma impact and restore lattice conductivity, a subsequent thermal annealing reduction treatment at 300°C in a hydrogen-argon mixture is performed. This process not only simplifies the fabrication procedure but also significantly reduces the interfacial contact resistance of the composite layer through thermal repair.

[0041] The composite gas diffusion layer prepared using this embodiment exhibits a lower interfacial contact resistance compared to Embodiment 1 (or the transfer method in the prior art) which employs a physical transfer process. This is due to the in-situ construction and subsequent high-temperature thermal annealing process, which creates a tight atomic-level contact between the graphene layer and the carbon nanotube framework, significantly reducing the interfacial contact resistance and facilitating efficient electron collection on the anode side. Furthermore, it eliminates the need for complex transfer steps, avoiding the challenging and low-yield wet transfer process, making it more suitable for large-scale continuous production.

[0042] Implementation Method 3: Chemical Crosslinking Intercalation Method This embodiment illustrates a third method for preparing a composite gas diffusion layer, the core of which lies in the integrated design of conductive framework support and chemical cross-linking sieving.

[0043] First, following the same method as in Embodiment 1, a carbon nanotube dispersion is sprayed onto a substrate, and then sintered to obtain a carbon nanotube network framework as a conductive support layer. The thickness of the carbon nanotube network framework is 20-30 μm.

[0044] Secondly, prepare functionalized graphene composite slurry: mix graphene oxide dispersion with p-phenylenediamine crosslinking agent at a mass ratio of 5:1-20:1 (preferably 10:1), and stir and react at 60°C for 4 hours, so that diamine molecules are grafted between graphene oxide sheets through covalent bonds to act as "molecular pillars".

[0045] Subsequently, the composite slurry was uniformly coated onto the surface of the carbon nanotube network framework using a blade coating method, with a blade gap of 10 μm and a coating speed of 5 mm / s. The thickness of the dried graphene oxide crosslinked layer was controlled to be 10-50 nm.

[0046] Finally, the carbon nanotube network framework coated with the composite slurry is placed in an oven and subjected to thermal crosslinking curing and partial reduction treatment at 120-180℃ (preferably 150℃) for 4-24 hours (preferably 12 hours). During this process, the crosslinking agent molecules complete the interlayer anchoring of the graphene oxide sheets, while removing unstable oxygen-containing groups and restoring lattice conductivity, thereby directly obtaining a composite gas diffusion layer with high structural stability.

[0047] This embodiment does not directly deposit the cross-linked graphene oxide layer onto an insulating or macroporous substrate, but rather constructs it in situ on the surface of a highly conductive carbon nanotube microporous framework. This design cleverly utilizes the dual functions of p-phenylenediamine: 1. It precisely locks the graphene interlayer spacing at 0.35-0.8 nm (superior to the stability of physical stacking), using steric hindrance to block methanol molecules (kinetic diameter > 0.38 nm), solving the fuel permeation problem unique to fuel cells; 2. Combined with a 150°C heat treatment process, the introduction of diamine molecules not only repairs part of the conjugated network but also helps establish an electron transport channel between the graphene sheets and the underlying carbon nanotube framework. In contrast, traditional dialysis membranes only focus on ion selectivity, without requiring and lacking the electronic conductivity required for electrodes. This scheme, through the synergy of the carbon nanotube network framework and cross-linked graphene oxide, simultaneously achieves the triple functions of proton conduction, electron collection, and methanol blocking. The composite gas diffusion layer prepared by this invention is mainly used in membrane electrode assemblies (MEAs) of proton exchange membrane fuel cells and direct methanol fuel cells. During MEA assembly, the composite gas diffusion layer of this invention should be placed on the anode side, with its selective sieve layer (i.e., the graphene-modified surface) directly facing the proton exchange membrane or catalyst layer, while the porous substrate layer faces the bipolar plate flow field. The catalyst slurry can be directly sprayed onto the surface of the graphene sieve layer of this invention, or the product of this invention can be used as an anode backing layer and hot-pressed with a proton exchange membrane already coated with catalyst. During battery operation, the anode fuel passes through the substrate layer and carbon nanotube framework to reach the graphene sieve layer. Larger fuel molecules are blocked from flowing back, while protons generated by the oxidation reaction pass through the angstrom-scale pores of the graphene into the proton membrane, and electrons are conducted to the external circuit through the conductive framework.

[0048] The differences between this invention and the prior art lie in three aspects: the orderliness of the microstructure, the spatial layout of the functional layers, and the electronic transport mechanism.

[0049] 1. In terms of microstructure, existing technologies generally employ physical blending to introduce two-dimensional materials, simply mixing and coating graphene, carbon powder, and binders. This process inevitably leads to random orientation and even aggregation of graphene sheets within the microporous layer, failing to form a continuous, dense planar barrier network, and the sheets are easily encapsulated by insulating binders. In contrast, this invention constructs a hierarchical stacked structure. By first constructing a continuous three-dimensional carbon nanotube conductive framework, and then laying two-dimensional graphene sieves on its surface, the parallel orientation of the graphene sheets and the gas diffusion layer plane is ensured, thereby maximizing the in-plane barrier advantages of two-dimensional materials. Furthermore, the carbon nanotube framework achieves atomic-level flat support for the ultrathin graphene.

[0050] 2. In terms of functional layout, existing technologies mostly focus on doping and modifying the proton exchange membrane itself. This often leads to blockage of proton transport channels or a decline in membrane mechanical properties, resulting in a dilemma of "barrier performance versus conductivity." This invention moves the barrier function forward to the interface between the anode diffusion layer and the proton exchange membrane. Without changing the chemical composition of the proton exchange membrane, it achieves physical interception before fuel enters the membrane by constructing an angstrom-scale sieve interface on the diffusion layer surface, successfully realizing the spatial decoupling of fuel barrier function and proton conduction function.

[0051] 3. In terms of electron transport pathways, traditional microporous layers rely on point contacts between carbon black particles, resulting in high resistivity. This invention, however, utilizes carbon nanotubes to construct a long-range, continuous electron transport network, with the graphene sieve layer directly and tightly bonded to this conductive framework. This design not only avoids disrupting the electron pathway by introducing a barrier layer, but also leverages graphene's excellent in-plane conductivity to collect electrons generated in the catalytic layer and rapidly conduct them to the substrate through the carbon nanotube network, forming a low-impedance electron pathway from the microscopic catalytic sites to the macroscopic substrate.

[0052] The beneficial technical effects of the present invention are as follows: 1. This invention constructs graphene sieve layers with pore sizes of 0.35-0.8 nm on a carbon nanotube framework (the pore forms include, but are not limited to: ① physical perforations formed on the graphene lattice plane by etching (corresponding to embodiments 1 and 2); ② or two-dimensional interlayer channels between graphene sheets controlled by intercalation / crosslinking (corresponding to embodiment 3). As long as the equivalent sieve size is within the range of 0.35-0.8 nm, it falls within the protection scope of this invention). The size sieving effect significantly suppresses fuel permeation. Experiments show that in direct methanol fuel cell applications, this structure can reduce the methanol permeation current density by more than 80%, effectively eliminating the mixing potential effect on the cathode side, thereby significantly improving the open-circuit voltage and power density of the battery.

[0053] 2. This invention effectively solves the stability problem of two-dimensional materials in engineering applications. The "nano-steel" framework formed by carbon nanotubes supports the ultrathin graphene layer, greatly enhancing the interfacial bonding force and enabling it to withstand fluid erosion and assembly pressure during battery operation. After long-term wet and dry cycle testing, the composite structure showed no significant peeling or damage, significantly extending the device's lifespan.

[0054] 3. The all-carbon-based high-conductivity network constructed in this invention significantly reduces interfacial resistance. By eliminating the excessive reliance on insulating binders in traditional microporous layers and utilizing the van der Waals forces and π-π conjugation between carbon nanotubes and graphene for tight bonding, this invention reduces the surface resistance of the gas diffusion layer by approximately 40%-60%, effectively improving the polarization performance of the battery under high current density.

[0055] 4. The preparation process of this invention is highly compatible with existing gas diffusion layer production lines. The resulting product can be stored and transported as an independent standard component without requiring complex adjustments to the existing membrane electrode hot pressing assembly process. It has high potential for industrial application and commercial value.

[0056] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention, and they should all be covered within the scope of the claims and specification of the present invention.

Claims

1. A composite gas diffusion layer, characterized in that, A membrane electrode assembly for a fuel cell includes a porous substrate layer and a composite microporous layer stacked together, wherein the porous substrate layer serves as a support. The composite microporous layer includes a conductive support layer and a selective sieving layer. The conductive support layer is disposed on the surface of the porous substrate layer and forms a conductive framework with three-dimensional pores for the carbon nanotube network. The selective sieve layer is located on the side of the conductive support layer away from the porous substrate layer. It is composed of two-dimensional sheet material laid flat and attached to the conductive support layer. The two-dimensional sheet material has multiple nanopores with a pore size of 0.35-0.8 nm distributed on it.

2. The composite gas diffusion layer according to claim 1, characterized in that, The two-dimensional sheet material includes graphene or graphene oxide derivatives, and the fuel cell includes a direct methanol fuel cell.

3. A method for preparing a composite gas diffusion layer, characterized in that, The method for preparing a composite gas diffusion layer according to claim 1 or 2 includes the following steps: A carbon nanotube dispersion is sprayed onto a substrate and then sintered to obtain a carbon nanotube network framework as a conductive support layer; the thickness of the carbon nanotube network framework is 20-30 μm. A graphene oxide film was prepared by vacuum filtration as a selective sieve layer and transferred onto a nanotube network framework; the thickness of the graphene oxide film was 10-50 nm. In-situ pore formation of graphene oxide films using ultraviolet radiation; A composite gas diffusion layer is obtained by thermal reduction in an inert gas atmosphere.

4. The method for preparing the composite gas diffusion layer according to claim 3, characterized in that, When performing in-situ pore formation using ultraviolet radiation, the wavelengths of the ultraviolet light are 185nm and 254nm, and the light intensity density is 20-30mW / cm². 2 The irradiation time is 15 to 30 minutes; When performing thermal reduction in an inert gas atmosphere, thermal reduction is carried out for 2 hours in an argon atmosphere at 200°C.

5. A method for preparing a composite gas diffusion layer, characterized in that, The method for preparing a composite gas diffusion layer according to claim 1 or 2 includes the following steps: A carbon nanotube network framework was grown in situ on a substrate using chemical vapor deposition as a conductive support layer; the thickness of the carbon nanotube network framework was 20-30 μm. A graphene oxide coating is sprayed onto a carbon nanotube network framework as a selective sieve layer, wherein the thickness of the graphene oxide film is 10-30 nm. Microscopic pores were created in the graphene oxide coating using an oxygen plasma etching process. Annealing and reduction were performed in an inert gas atmosphere to obtain a composite gas diffusion layer.

6. The method for preparing the composite gas diffusion layer according to claim 5, characterized in that, When a graphene oxide coating is sprayed onto a carbon nanotube network framework as a selective sieving layer, an ultrasonic atomization spraying technique is used to uniformly spray the graphene oxide dispersion onto the surface of the carbon nanotube network framework. When using oxygen plasma etching to create micro-pores in graphene oxide coatings, the radio frequency power of the plasma treatment is 40-60W, and the treatment time is 10 to 40 seconds. When performing annealing and reduction in an inert gas atmosphere, the temperature is increased to 300°C at a rate of 2-5°C / min, held at that temperature for 2 hours, and then allowed to cool naturally to room temperature.

7. A method for preparing a composite gas diffusion layer, characterized in that, The method for preparing a composite gas diffusion layer according to claim 1 or 2 includes the following steps: A carbon nanotube dispersion is sprayed onto a substrate and then sintered to obtain a carbon nanotube network framework as a conductive support layer; the thickness of the carbon nanotube network framework is 20-30 μm. A functionalized graphene composite slurry was prepared and uniformly coated onto the surface of a carbon nanotube network framework using a scraping method. The carbon nanotube network skeleton coated with the composite slurry is placed in a heating device for thermal crosslinking curing and partial reduction treatment, forming a graphene oxide film with a thickness of 10-50 nm on the surface of the carbon nanotube network skeleton, thus obtaining a composite gas diffusion layer.

8. The method for preparing the composite gas diffusion layer according to claim 7, characterized in that, The functionalized graphene composite slurry is prepared as follows: graphene oxide dispersion and p-phenylenediamine crosslinking agent are mixed at a mass ratio of 5:1-20:1 and stirred at 60°C for 4 hours to allow diamine molecules to be covalently grafted between graphene oxide sheets. When the composite slurry is uniformly coated onto the surface of the carbon nanotube network skeleton by a scraping method, the scraper gap is set to 10 μm and the coating speed is 5 mm / s. The heating temperature for the thermal crosslinking and curing is 120-180℃, and the heating time is 4-24 hours.

9. A method for preparing a fuel cell membrane electrode, characterized in that, The fuel cell membrane electrode is prepared based on the composite gas diffusion layer as described in claim 1 or 2, and includes the following steps: The composite gas diffusion layer is placed on the anode side, with the selective sieve layer in the composite gas diffusion layer facing the proton exchange membrane or catalyst layer, and the porous substrate layer in the composite gas diffusion layer facing the bipolar plate flow field.

10. A fuel cell membrane electrode assembly, characterized in that, The fuel cell membrane electrode assembly prepared using the preparation method described in claim 9 can be used in proton exchange membrane fuel cells and direct methanol fuel cells.

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