Polyimide-based carbon aerogel for fuel cells and method for preparing the same

Abstract

The application belongs to the technical field of fuel cell electrode material preparation, and particularly relates to a polyimide-based carbon aerogel for fuel cells and a preparation method thereof. The polyimide-based carbon aerogel is prepared by using aramid nanofiber primary gel as a fiber filler, solving the technical problem of easy agglomeration of aramid nanofiber. The uniformly dispersed aramid nanofiber is used as a three-dimensional skeleton reinforcing phase, and polyimide is used as a matrix precursor. Branched polyethylene imine is introduced to construct a multi-level crosslinked network, and a polyimide-based carbon aerogel material is obtained through sol-gel-carbonization forming. The material has uniform internal pore distribution, sufficient electrochemical active site quantity, small charge and reaction medium mass transfer resistance, and can be used as a fuel cell electrode functional material. The preparation process route is simple and controllable, the raw material adaptation range is wide, the finished product performance stability is good, and the polyimide-based carbon aerogel has the value of large-scale production and practical industrialization application.

Description

Technical Field

[0001] This invention belongs to the field of fuel cell electrode material preparation technology, specifically relating to a polyimide-based carbon aerogel for fuel cells and its preparation method. Background Technology

[0002] As the global energy structure transformation continues to accelerate, clean energy replacing traditional fossil fuels has become a core development trend in the energy sector. Fuel cells, with their outstanding advantages such as high energy conversion efficiency, zero pollutant emissions during operation, convenient refueling, and wide adaptability to various operating conditions, are gradually achieving industrial application in areas such as vehicle power, stationary power generation, and portable energy storage and power supply. Unlike traditional thermal power generation devices, fuel cells do not involve combustion; they directly convert the chemical energy within the fuel into electrical energy. The theoretical energy conversion ceiling is far higher than that of internal combustion engines. Furthermore, they operate with low noise and flexible operating condition control, making them a next-generation clean energy conversion device with significant development potential.

[0003] Electrode materials are core components that determine the output power, service life, and reaction conversion efficiency of fuel cells. Electrodes not only need to support catalytically active components but also perform multiple functions such as gas transport, electron conduction, electrolyte wetting, and reaction product removal. During actual fuel cell operation, anode fuel gas and cathode oxidant need to continuously pass through the electrode pores to reach the catalytic reaction interface. Water and byproducts generated in the reaction also need to be discharged promptly through the pores. The electrode itself must also maintain a continuous conductive path to ensure rapid charge migration. Conventional commercial electrode substrates mostly use shaped carbon materials such as carbon paper, carbon cloth, and graphite sheets. These materials have mature manufacturing processes and stable basic conductivity, but their fixed pore structure offers poor adjustability. The pore hierarchy is singular, and an excessively high proportion of micropores easily leads to gas mass transfer blockage, while an excessively high proportion of macropores reduces the active loading area, making it difficult to simultaneously meet the dual requirements of mass transfer efficiency and the number of reaction sites. Furthermore, traditional carbon-based electrode materials are relatively weak in toughness. After long-term exposure to gas erosion, temperature cycling, and small pressure fluctuations, they are prone to structural cracking, pulverization, and detachment, directly causing performance degradation and shortened lifespan of the fuel cell.

[0004] Porous aerogel carbon materials, with their three-dimensional interconnected network structure, ultra-high specific surface area, designable pore structure, and lightweight properties, are gradually becoming a key research and development direction for fuel cell electrode materials. Carbon aerogels can construct multi-level pore composite structures. Macropores ensure macroscopic gas transport, while mesopores and micropores provide ample electrochemical reaction sites, adapting to the multiphase reaction mass transfer requirements of fuel cells. Simultaneously, the three-dimensional framework structure can effectively disperse external forces and thermal stress, improving the overall structural stability of the electrode. Currently, the preparation of carbon aerogels in laboratories and industry mainly uses polymeric precursors such as polyacrylamide, phenolic resin, chitosan, and cellulose. These general-purpose polymeric raw materials are inexpensive and easy to mold, but they generally suffer from low heat resistance limits and large framework shrinkage during high-temperature carbonization. After carbonization, the carbon aerogel framework has many defects, limiting the continuity of electronic conduction. Furthermore, conventional polymeric cross-linked networks have poor thermal stability, and the precursors are prone to pore collapse and structural densification during thermal decomposition, resulting in significant loss of specific surface area in the final product, making it difficult to fully utilize the electrochemical advantages of the porous structure.

[0005] Therefore, developing a carbon electrode material for fuel cells with controllable structure, high mass transfer efficiency, good conductivity, and simple preparation process, as well as its preparation method, is of great significance for promoting the advancement and application of fuel cell technology. Summary of the Invention

[0006] The purpose of this invention is to provide a polyimide-based carbon aerogel for fuel cells and its preparation method. The polyimide-based carbon aerogel provided by this invention has a large specific surface area, numerous active sites, low resistivity, and good conductivity. Furthermore, the preparation process is simple and suitable for industrial production.

[0007] To solve the above-mentioned technical problems, the present invention adopts the following technical solution: A method for preparing polyimide-based carbon aerogel for fuel cells includes the following steps: (1) A protic solvent was added dropwise to a non-protic solvent dispersion containing aramid fibers and alkali metal hydroxides, and the dispersion was stirred to obtain an aramid nanofiber dispersion. (2) Immerse the aramid nanofiber dispersion in deionized water and let it stand for 2-4 hours. Then add the primary gel formed to the aqueous solution of polyamic acid powder and organic base. After stirring evenly, add branched polyethyleneimine and obtain hydrogel by sol-gelation. The mass ratio of branched polyethyleneimine to polyamic acid powder is (0.05-0.2):1. (3) The hydrogel was freeze-dried and thermally imidized to obtain an aerogel; (4) After high-temperature carbonization, washing and drying, a polyimide-based carbon aerogel for fuel cells is obtained.

[0008] Polyimides are a class of aromatic heterocyclic polymers with high mechanical strength, chemical inertness, and structural rigidity. Their main molecular chain contains numerous rigid imide ring structures, making the backbone resistant to breakage and deformation at high temperatures. Furthermore, during high-temperature pyrolysis, these polymers can maximally inherit the original porous morphology of the precursor, avoiding the problem of large-area shrinkage and collapse of the backbone. This makes them ideal precursor matrix materials for preparing high-performance carbon aerogels. Polyimides are typically obtained by polycondensation of dianhydride and diamine monomers to form a polyamic acid intermediate, followed by imidization ring-closure. The molecular structure can be flexibly controlled by the type of monomer, allowing adjustment of the proportion of rigid groups, hydrophobic groups, and aromatic rings according to the material's application requirements. This, in turn, alters the pore distribution, electrical conductivity, and mechanical properties of the carbonized carbon aerogel.

[0009] Pure polyimide-based aerogels suffer from insufficient toughness and brittle cross-linked networks, making them prone to microcracks during the sol-gel molding stage and susceptible to fracture under stress. Current techniques using fillers to reinforce polyimide aerogels require in-situ monomer polymerization, resulting in concentrated exothermic reactions and difficulty in precisely controlling the polymerization rate. This easily leads to localized over-polymerization or incomplete polymerization, resulting in uneven network density and numerous inherent microscopic defects within the gel. Furthermore, the monomer polymerization process is susceptible to interference from ambient temperature, impurities, and oxygen, leading to poor system stability, low batch-to-batch repeatability, and hindering large-scale stable preparation. Moreover, the pure polymer network formed by in-situ polymerization relies solely on the entanglement of single molecular chains, lacking a rigid framework support. This results in weak mechanical strength, poor thermal stability, and high drying shrinkage. The framework is prone to collapse during carbonization, ultimately leading to a carbon aerogel with disordered pores and poor structural integrity, resulting in insufficient electrode reliability. Aramid nanofibers possess ultra-high aspect ratio, excellent tensile strength, and resistance to temperature and corrosion. These nanofibers can be interwoven within the polymer network, acting as a physical support framework, constraining polymer chain deformation, buffering internal stresses generated during molding and heat treatment, and improving the overall bending and impact resistance of the aerogel. Branched polyethyleneimine molecules contain numerous active amino groups, which can simultaneously form multiple hydrogen bonds and chemical cross-links with polar groups on the aramid fiber surface and polyamic acid salt molecular chains, further densifying the three-dimensional network structure, regulating the density of gel pores, and reducing carbonization defects.

[0010] To address the shortcomings of existing technologies, such as insufficient overall performance of precursors, numerous defects in aerogel structures, poor adaptability to electrode operating conditions, and weak material service stability, this invention employs an alkali-soluble exfoliation method to prepare aramid nanofibers. Polyamic acid, copolymerized with polyamide, serves as the polyimide precursor matrix, and branched polyethyleneimine is combined to construct a multi-component cross-linked composite hydrogel. Polyimide-based carbon aerogels are then prepared through freeze-drying, segmented gradient thermal imidization, and high-temperature inert carbonization. Leveraging the high structural stability of polyimide combined with the reinforcing effect of the aramid nanofiber framework, a carbon aerogel material with a multi-level porous structure, high conductivity, and strong mechanical stability is obtained. This effectively solves the problems of limited mass transfer, easily damaged structure, and poor cycle performance in traditional carbon electrodes, broadening the practical application scope of polyimide-based porous carbon materials in fuel cell electrodes.

[0011] In one embodiment, the specific process of step (1) is as follows: aramid fibers and alkali metal hydroxides are added to an aprotic solvent, followed by the addition of a protic solvent, and the mixture is stirred and dispersed to obtain an aramid nanofiber dispersion.

[0012] In one embodiment, the aprotic solvent in step (1) is one or more of dimethyl sulfoxide (DMSO), dimethylformamide (DMF), and dimethylacetamide (DMAC). These solvents are all common aprotic organic solvents in the art and can effectively promote fiber dispersion.

[0013] In one embodiment, the proton solvent in step (1) is one or more of deionized water, methanol, and ethanol.

[0014] In one embodiment, the alkali metal hydroxide in step (1) is one or more of NaOH and KOH.

[0015] In one embodiment, the stirring in step (1) can be carried out at room temperature, and the stirring and dispersion time is 24-48h.

[0016] In one embodiment, the ratio of aramid fiber, alkali metal hydroxide, protic solvent, and aprotic solvent in step (1) is 1g:(0.5-2)g:(1-3)mL:(50-150)mL. The concentration of the formed aramid nanofiber dispersion is not particularly limited, but can specifically be 0.1-5wt%. Further, it can be 0.1wt%, 0.5wt%, 1wt%, 1.5wt%, 2wt%, 2.5wt%, 3wt%, 3.5wt%, 4wt%, 4.5wt%, or 5wt%. Specifically, the concentration of the aramid nanofiber dispersion is 0.5-1.5wt%.

[0017] In one embodiment, in step (2), the primary hydrogel is formed by standing for 2-4 hours, and then the prepared primary hydrogel is mixed with an aqueous solution of polyamic acid salt. Specifically, to facilitate the preparation of the primary hydrogel, the aramid nanofiber dispersion can be placed in an open mold and then immersed in deionized water to facilitate the sol-gelation process and solvent replacement. Furthermore, the aramid nanofiber dispersion can also be placed in deionized water with intermittent replacement to facilitate solvent replacement and further washing of the primary gel. The mold size is not particularly limited and can be adjusted according to the production scale. Specifically, an open mold with a volume of 4cm (length) × 4cm (width) × 2cm (height) can be used (i.e., a mold with a bottom and opening area of ​​4cm × 4cm and a height of 2cm). Further, in a specific embodiment, multiple sets of molds can be set simultaneously to prepare a large amount of primary gel for reinforcing polyimide.

[0018] To address the issue of uneven dispersion of aramid nanofibers in highly polar polyimide polymers, this invention first prepares aramid nanofibers into a primary hydrogel, which is then blended with a polyimide precursor, thus resolving the problem of easy agglomeration of aramid nanofibers upon direct addition. First, deionized water is introduced into the aramid nanofiber dispersion and allowed to stand for 2-4 hours. This causes a gradual change in the polar environment within the system, allowing numerous reversible, weakly interacting intermolecular hydrogen bonds to spontaneously form between the exposed hydroxyl, carboxyl, and amide groups on the surface of the aramid nanofibers. This constructs a loose, reversible primary physical gel precursor with a weak network constraint effect. This primary hydrogen bond network does not form a high-strength cross-linked structure, but only plays a role in spatial confinement and dispersion. It can effectively fix the spatial distribution of exfoliated aramid nanofibers, inhibit the van der Waals aggregation, chain segment entanglement and local sedimentation and agglomeration of nanofibers during solvent environment changes, solve the problem of uneven dispersion and local agglomeration and stacking of aramid nanofibers in subsequent polyaminate blending, and provide a stable and monodisperse fiber matrix environment for the uniform composite of multi-component interfaces. Based on this, an aqueous solution of polyamic acid salt was added to the primary weak gel system and mechanically stirred. The shear force generated by stirring could directionally destroy the weak hydrogen bond primary network structure temporarily constructed in the early stage, release the spatial binding effect between fibers, and enable the originally fixed aramid nanofibers to regain flow compatibility. At the same time, the linear long-chain polyamic acid salt molecules fully expanded in the aqueous solution, interpenetrating, entangled and wetting with the aramid nanofiber segments, realizing the homogeneous mixing of the two phases at the molecular scale, forming a thermodynamically stable, phase-separated and particle-free homogeneous composite aqueous solution system, which greatly improved the interfacial adhesion and compatibility of the two phases, and effectively avoided the technical problem of agglomeration caused by the direct addition of aramid nanofibers.

[0019] It is worth noting that this step requires controlling the settling time of the aramid nanofiber mixture solution. Controlling the settling time to 2-4 hours is crucial to adapt to the slow, reversible association kinetics of hydrogen bonds on the aramid nanofiber surface, smoothly constructing a uniform, loose, and easily disrupted primary weak hydrogen bond network, achieving fiber monodispersion, and meeting the requirements of the subsequent homogeneous composite molding process. If the settling time is too short, the solvent replacement is insufficient, the activation level of the polar groups on the aramid nanofiber surface is inadequate, and the fibers cannot uniformly overlap to form a complete primary confined network. The fibers remain in a free and disordered state, and the spatial confinement effect does not meet the process requirements. When polyamic acid salt aqueous solution is added and stirred for blending, under the influence of solvent interfacial tension fluctuations and the superposition of intermolecular van der Waals forces, free aramid nanofibers rapidly undergo large-scale secondary entanglement and localized agglomeration, resulting in regional fiber enrichment and component segregation and stratification. The two phases cannot achieve microscale uniform compatibility, directly causing uneven density and increased primary defects in the internal structure of the composite hydrogel. This leads to overall shrinkage during the subsequent drying stage. After high-temperature carbonization, the carbon skeleton in the agglomerated areas becomes densely packed with no effective mass transfer channels, while the weakly dispersed areas collapse and pulverize. Ultimately, the conductive network of the carbon electrode breaks down, and the multi-level porous structure is destroyed. If the standing time is too long, the weak hydrogen bonds between the aramid nanofibers continue to superimpose and crosslink, continuously associating and densifying. The originally loose and reversible primary soft gel gradually solidifies into a highly crosslinked rigid pre-formed gel, with a significant increase in network bonding strength. Subsequent conventional stirring shear forces cannot completely break up the hardened gel clumps; they can only break the surface structure, leaving the internal dense gel core intact. This core cannot interpenetrate and dissolve with the polyamic acid salt molecular chains, forming permanent interfacial phase separation hard defects. The subsequent molding of composite hydrogels resulted in severe stress concentration, easy deformation during drying, loss of rigid fiber skeleton during carbonization, stress collapse of local cross-linked structures, unbalanced overall residual carbon distribution, and disordered and impermeable pore arrangement. The final carbon aerogel had poor mechanical stability and insufficient structural integrity, and could not meet the core usage requirements of fuel cell electrodes for long-term cycling, stable conductivity, and efficient mass transfer.

[0020] In one embodiment, the polyamic acid powder in step (2) is obtained by polymerizing dianhydride monomers and diamine monomers. The specific process is as follows: dianhydride monomers and diamine monomers are dissolved and dispersed in an organic solvent, and a copolymerization reaction is performed to form a polyamic acid solution. The polyamic acid solution is then added dropwise to deionized water, and after precipitation, polyamic acid powder is obtained. In particular, various diamines or dianhydride monomers can be used to copolymerize and prepare copolymerized polyimides to improve the processability of the polyimide polymer.

[0021] In one embodiment, the molar ratio of dianhydride monomer to diamine monomer in step (2) is (0.9-1.1):(0.9-1.1).

[0022] In one embodiment, the organic base in step (2) is one or more of triethylamine, tripropylamine, diethylenetriamine, triethylenetetramine, and triethylenediamine.

[0023] In one embodiment, the mass ratio of organic base to polyamic acid powder in step (2) is (0.1-1):1. The polyamic acid powder and organic base are dispersed in deionized water. After dispersion, a polyamic acid salt solution is prepared, which improves the dispersion performance of the polyimide precursor. The amount of water used is not particularly limited; it is sufficient to ensure uniform dispersion of all components.

[0024] In one embodiment, the organic solvent in step (2) is one or more of N,N-dimethylacetamide, dimethyl sulfoxide, N,N-dimethylformamide, and N-methylpyrrolidone.

[0025] In one embodiment, the dianhydride monomer in step (2) is not particularly limited, and any monomer commonly used in the art can be used. Specifically, it can be pyromellitic dianhydride, 3,3,4,4-diphenyl ether tetracarboxylic dianhydride, 3,3,4,4-biphenyl tetracarboxylic dianhydride, 2,2-bis(3,4-dicarboxylic acid)hexafluoropropane dianhydride, 4,4-hexafluoroisopropylphthalic anhydride, 2,3,3,4-benzophenone tetracarboxylic dianhydride, 3,3,4,4-benzophenone tetracarboxylic dianhydride, 3,3,4,4-biphenyl tetracarboxylic dianhydride, p-phenylene-bisphenylene One or more of the following: triterpenoid dianhydride, bis(3,4-dicarboxyphenyl)methane dianhydride, 2,2-bis(3,4-dicarboxyphenyl)propane dianhydride, 1,3-bis(3,4-dicarboxyphenoxy)phenyl dianhydride, 1,4-bis(3,4-dicarboxyphenoxy)phenyl dianhydride, 1,4-bis(3,4-dicarboxyphenoxy)biphenyl dianhydride, and 2,2-bis[(3,4-dicarboxyphenoxy)phenyl]propane dianhydride.

[0026] In one embodiment, the type of diamine monomer in step (2) is not particularly limited, and any commonly used monomer in the art can be used. Specifically, it can be p-phenylenediamine, m-phenylenediamine, 3,3-dimethylbenzidine, 2,2-dimethylbenzidine, 2,4-diaminotoluene, 2,6-diaminotoluene, 3,5-diaminobenzoic acid, 4,4-diaminodiphenyl ether, 3,4-diaminodiphenyl ether, 4,4-diaminodiphenylmethane (methylenediamine), 3,3-dimethyl-4,4-diaminobiphenyl, 2,2-dimethyl-4,4-diaminobiphenyl, 2,2-bis(trifluoromethyl)-4,4-diaminobiphenyl, 3,3,5-diaminodiphenylmethane (methylenediamine), 3,3-dimethyl-4,4-diaminobiphenyl, 2,2-dimethyl-4,4-diaminobiphenyl, 3,3,5-diaminodiphenylmethane (methylenediamine), 3,3-dimethyl-4,4-diaminobiphenyl, 2,2-bis(trifluoromethyl)-4,4-diaminobiphenyl, 3,3,5-diaminodiphenylmethane (methylenediamine), 3,3 ... One or more of the following: 5-tetramethyl-4,4-diaminodiphenylmethane, 4,4-diaminobenzoylaniline, 3,3-dimethoxybenzidine, 2,2-dimethoxybenzidine, 3,3-diaminodiphenyl ether, 3,3-diaminodiphenyl sulfide, 3,4-diaminodiphenyl sulfide, 4,4-diaminodiphenyl sulfide, 3,3-diaminodiphenyl sulfone, 3,4-diaminodiphenyl sulfone, 4,4-diaminodiphenyl sulfone, 3,3-diaminodibenzophenone, and 4,4-diaminodibenzophenone.

[0027] In one embodiment, the stirring rate in step (2) is 400-800 rpm. Specifically, ultrasound may be used to promote the dispersion of the components. The stirring rate is not particularly limited, but can be 400 rpm, 450 rpm, 500 rpm, 550 rpm, 600 rpm, 650 rpm, 700 rpm, 750 rpm, or 800 rpm. Furthermore, the stirring rate can be 500-700 rpm. After adding polyamic acid powder and an organic base to the polyamic acid salt aqueous solution, the primary weak hydrogen bond gel network formed in the early stage can be controlled to disintegrate by stirring, while ensuring uniform mixing and compatibility between the aramid nanofibers and the polyamic acid salt molecular chains.

[0028] In one embodiment, the type of branched polyethyleneimine in step (2) is not particularly limited. Specifically, branched polyethyleneimine with a weight-average molecular weight of 500-5000 can be selected. More specifically, at least one of the following weight-average molecular weights can be selected: 600 (PEI600), 800 (PEI 800), 1200 (PEI 1200), 1500 (PEI 1500), 1800 (PEI 1800), 2000 (PEI2000), 3000 (PEI 3000), 4000 (PEI 4000), and 5000 (PEI 5000). In particular, branched polyethyleneimine with a weight-average molecular weight of 1000-2000 can be selected. Branched polyethyleneimine with a suitable molecular weight can fully play the role of hydrogen bonding crosslinking, while avoiding the problems of excessive molecular weight, excessive steric hindrance, and difficulty in dispersion.

[0029] Branched polyethyleneimine is introduced into a homogeneous mixture. The surface of this polymer molecule is rich in high-density multi-branched active amino groups, which can easily form hydrogen bonds or undergo dehydration reactions with oxygen-containing functional groups. It can serve as a multifunctional crosslinking bridge, forming strong bonds with polar groups such as hydroxyl, carboxyl, and amide groups on the surface of aramid nanofibers at one end, and undergoing multi-site coordination crosslinking with high-density carbonyl functional groups on the polyamic acid acid chain at the other end. Through bidirectional synchronous bonding and the interaction between aramid nanofibers and polyamic acid acid, a three-dimensional continuous, high-strength composite hydrogen bond or covalent bond network with controllable crosslinking density and strong interfacial bonding is reconstructed within the system. The stepwise hydrogen bond-controlled molding method of primary gelation and supplementation with polyamic acid salts adopted in this invention not only ensures that the aramid nanofibers are monodisperse and do not agglomerate throughout the process, but also achieves deep interfacial composite between the rigid fiber phase and the flexible polymer phase. The final result is a composite hydrogel with uniform structure, mechanical stability and continuous three-dimensional channels, which can withstand the harsh conditions of subsequent drying and dehydration and high-temperature full-process pyrolysis and carbonization. Relying on the high residual carbon rigid skeleton of aramid and the in-situ pore-forming advantage of polyimide, a high-performance carbon aerogel with well-developed multi-level pores, complete and stable structure and rich nitrogen-doped active sites is finally prepared, which is suitable for the actual application requirements of electrode materials for fuel cells.

[0030] The reason this invention chooses branched polyethyleneimine instead of linear polyethyleneimine is that branched polyethyleneimine possesses a three-dimensional branched topology with high molecular spatial freedom, a dense and uniformly distributed number of terminal active amino groups, enabling multi-directional, multi-site synchronous hydrogen bond bridging and crosslinking in aqueous solution. On one hand, its numerous high-density terminal amino groups can simultaneously form omnidirectional, multi-node synergistic hydrogen bonds or covalent bonds with the oxygen-containing polar groups on the surface of aramid nanofibers and the numerous carbonyl groups on the polyamic acid acid molecular chains. This results in abundant crosslinking sites and high crosslinking efficiency, enabling the rapid construction of a uniformly crosslinked, dense, and mechanically strong three-dimensional integrated composite gel network, effectively strengthening the interfacial bonding between the two phases and ensuring the regularity and integrity of the composite gel framework. On the other hand, the dendritic three-dimensional spatial structure has good spatial adaptability, unlike linear straight-chain molecules which are prone to chain segment entanglement, stacking, and curling. It can be uniformly interwoven in the gaps between aramid nanofibers and polyamic acid acid molecules, avoiding defects such as excessive local crosslinking or blank local crosslinking, ensuring uniform crosslinking density throughout the gel, resulting in minimal subsequent drying shrinkage and structural deformation. In contrast, linear polyethyleneimine (PEI) has a one-dimensional linear molecular configuration, with active amino groups distributed along a single chain in limited numbers. It has few effective cross-linking sites and a single cross-linking direction, enabling only point-to-point linear overlap and failing to form a three-dimensional spatial cross-linking network. This results in low gel cross-linking strength and weak mechanical support. Furthermore, linear molecules are prone to self-entanglement and aggregation, leading to localized cross-linking accumulation, uneven interfacial bonding, and numerous microstructural defects. PII is unable to stably anchor aramid nanofibers and cannot effectively suppress skeletal slippage and pore collapse during subsequent carbonization, ultimately resulting in a loose carbon aerogel structure, poor pore connectivity, and insufficient mechanical stability, severely reducing the electrochemical performance of the electrode material.

[0031] In one embodiment, the mass ratio of polyamic acid powder to aramid nanofibers in step (2) is (8-14):1. By adjusting the polyimide content, the molecular chains are moderately stretched, the viscosity is moderate, and the carboxyl groups are evenly distributed. This allows for full wetting and compatibility with aramid nanofibers, as well as uniform hydrogen bond crosslinking with branched polyethyleneimine. The resulting gel structure is regular and stable, with well-developed pores and a complete skeleton after carbonization, ultimately yielding a carbon aerogel electrode material for fuel cells with excellent comprehensive performance.

[0032] In one embodiment, the mass ratio of polyamic acid powder to aramid nanofibers in step (2) can be 8:1, 9:1, 10:1, 11:1, 12:1, 13:1, or 14:1; further, it can be (10-12):1. A suitable amount of aramid nanofibers can achieve both rigid framework support and uniform two-phase composite effects, resulting in a composite carbon aerogel precursor with a regular structure, mechanical stability, and excellent mass transfer and electrical conductivity. If the amount of aramid nanofibers is too low, a continuous rigid support network cannot be constructed, leading to weak shrinkage resistance and easy collapse of the carbonized framework in the composite gel; if the amount is too high, it easily causes fiber entanglement and agglomeration, uneven component dispersion, stress concentration within the gel, pore blockage, and increased material brittleness, which is detrimental to improving product performance.

[0033] In one embodiment, the mass ratio of branched polyethyleneimine to polyamic acid powder in step (2) is (0.1-0.2):1. Specifically, it can be 0.10:1, 0.12:1, 0.14:1, 0.16:1, 0.18:1, or 0.20:1. Furthermore, the mass ratio of branched polyethyleneimine to polyamic acid powder is (0.15-0.20):1. By controlling the branched polyethyleneimine within a reasonable and optimal dosage range, the crosslinking density can be uniformly controlled, ensuring a strong bridging of the interface between aramid nanofibers and polyimide, constructing a complete and stable three-dimensional composite gel network, while avoiding the introduction of excess organic impurities and internal stress defects. This allows the precursor to maintain its structural integrity throughout the drying and carbonization process, ultimately obtaining a high-quality carbon aerogel with well-developed pores, a stable skeleton, excellent conductivity, and abundant active sites.

[0034] In one embodiment, the thermal imidization in step (3) employs a gradient heating process. Specifically, thermal imidization can be carried out at 130-150℃ / 1h, 150-170℃ / 3h, 180-200℃ / 3h, and 200-250℃ / 1h. The thermal imidization adopts a multi-stage gradient heating mode, where residual binding solvent is slowly removed in the low-temperature range, and the polyamic acid molecule ring-closure imidization reaction is gradually completed in the medium-temperature range. The stepwise heating effectively releases the shrinkage stress of the molecular chain, prevents the rapid volume change caused by a one-time high-temperature reaction from causing structural damage, and after sufficient reaction, it is transformed into a structurally stable polyimide aerogel.

[0035] In one embodiment, the high-temperature carbonization in step (4) is carried out under the protection of an inert gas; the inert gas can be one or more of nitrogen, helium, and argon.

[0036] In one embodiment, the high-temperature carbonization temperature in step (4) is 800-1300℃; the high-temperature carbonization time is 1-3h. Specifically, the high-temperature carbonization temperature can be 800℃, 850℃, 900℃, 950℃, 1000℃, or 1100℃. Furthermore, the high-temperature carbonization temperature can be 850℃-1000℃. When the carbonization temperature is too low, the overall pyrolysis energy supply is insufficient, the polymer pyrolysis is incomplete, it is difficult to form a through-hole multi-level pore structure, and the specific surface area of ​​the material is low; at the same time, the cross-linking carbonization degree of aramid nanofibers is insufficient, the macromolecular chains are not completely aromatized, the degree of graphitization of the carbon skeleton is low, and the intrinsic resistance is high. Under low temperature conditions, the heteroatom doping binding force is weak, the active sites are unstable, there are more organic residual components inside the structure, and it is impossible to construct efficient ion and electron transport channels. When the carbonization temperature is too high, the excessive thermal shock can cause the originally well-ordered three-dimensional porous framework to undergo excessive graphitization and shrinkage, resulting in the melting and closure of a large number of micropores and mesopores, collapse and densification of the hierarchical porous structure, and a decrease in the effective specific surface area. At the same time, the high temperature can cause excessive carbonization and embrittlement of the rigid framework of aramid nanofibers, increasing the number of internal carbon lattice defects, significantly increasing the brittleness of the material, and greatly reducing its mechanical toughness. By adjusting the carbonization temperature and carbonization time, it is possible to ensure the full pyrolysis of polyimide and promote the appropriate graphitization of aramid nanofibers, constructing a high-strength, highly conductive, continuous carbon framework, while precisely retaining an appropriate amount of stable nitrogen-doped active sites. This synergistically optimizes the hierarchical pore structure, conductive network, and catalytic active centers, ultimately producing a composite carbon aerogel with a complete structure, well-developed pores, excellent conductivity, and strong electrochemical stability.

[0037] In one embodiment, the heating rate of high-temperature carbonization in step (4) is 1-10℃ / min. Specifically, the heating rates of high-temperature carbonization are 1℃ / min, 2℃ / min, 3℃ / min, 4℃ / min, 5℃ / min, 6℃ / min, 7℃ / min, 8℃ / min, 9℃ / min, and 10℃ / min. A suitable heating rate can achieve carbonization quickly while avoiding structural collapse caused by excessive heating. The washing method is not particularly limited. Specifically, hydrochloric acid and deionized water can be used for washing. After high-temperature carbonization, the rigid aromatic ring skeleton of polyimide and aramid nanofibers is decomposed and reconstructed at high temperature to form a graphite-like conductive carbon network. Heteroatoms are appropriately doped into the carbon skeleton to further enhance the electrochemical reaction activity. The carbonization product is washed to remove surface decomposition impurities and dried to obtain the final polyimide-based carbon aerogel.

[0038] On the other hand, this invention also provides a polyimide-based carbon aerogel for fuel cells prepared using the above method. The main raw materials used in the preparation process of this invention—aramid fibers, polyimide monomers, branched polyethyleneimine, solvents, and alkaline hydroxides—are all common types in the art and can be prepared or purchased independently. For example, DuPont's Kevlar 29 aramid fibers can be selected, and Shanghai Aladdin series polyimide monomers and branched polyethyleneimine can be selected, etc. This carbon aerogel electrode material not only has a high specific surface area but also high conductivity, showing broad application prospects. By introducing aramid nanofibers as a three-dimensional rigid framework into the polyimide gel system, the high strength, high modulus, and high thermal stability of aramid nanofibers significantly improve the structural strength of the composite aerogel, effectively suppressing volume shrinkage and pore collapse during solvent exchange, drying, and high-temperature carbonization processes, while simultaneously improving the overall residual carbon rate of the system and ensuring the structural integrity of the carbon aerogel after carbonization. Aramid nanofibers can achieve in-situ nitrogen doping during carbonization, optimizing the surface active sites and conductivity of carbon materials. Combined with the hierarchical pore structure formed by in-situ pore creation through polyimide pyrolysis, carbon aerogels with high mechanical strength, well-developed pores, good stability, and excellent conductivity are finally obtained, which can meet the comprehensive requirements of fuel cell electrode materials.

[0039] Beneficial effects: (1) This invention utilizes short-time static setting to construct a primary weak hydrogen bond network, effectively suppressing the secondary entanglement and aggregation of aramid nanofibers in an aqueous system, achieving uniform dispersion and composite of the rigid reinforcing phase and the flexible carbon source phase at the molecular scale, eliminating microscopic defects such as component segregation and weak interfacial bonding in traditional composite gels from the source, significantly improving the overall structural uniformity and mechanical support stability of composite hydrogels and aerogel precursors, and greatly reducing the risk of process scrap such as drying shrinkage and molding cracking.

[0040] (2) The carbon aerogel prepared by this invention forms a multi-level porous structure with synergistic distribution inside, with a high specific surface area, which can load sufficient catalytic active materials. At the same time, the through-channels greatly reduce the mass transfer resistance of fuel, oxygen and electrolyte. After carbonization, aramid nanofibers form a carbon fiber support skeleton that runs through the entire material, significantly improving the mechanical strength and deformation resistance of the carbon aerogel and making it resistant to gas impact and temperature changes inside the fuel cell. The continuous carbon network formed by carbonization of polyimide endows the material with excellent electronic conduction efficiency, low charge migration loss and small electrode polarization.

[0041] (3) The reaction conditions of the preparation process of the present invention are mild, the sol-gel molding does not require complex equipment, the gradient heat treatment and carbonization process can be achieved with conventional equipment, the monomers, solvents and modifiers can be selected from a wide range of suitable raw materials, the raw material ratio and heat treatment parameters can be adjusted according to different fuel cell power requirements, and carbon aerogel electrode materials with different pores, strengths and conductivity levels can be customized. Attached Figure Description

[0042] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on the provided drawings without creative effort.

[0043] Figure 1 This is a scanning electron microscope image of the carbon aerogel prepared in Example 10 of the present invention.

[0044] Figure 2 This is a scanning electron microscope image of the carbon aerogel prepared in Comparative Example 1 of this invention.

[0045] Figure 3 This is a scanning electron microscope image of the carbon aerogel prepared in Comparative Example 2 of this invention.

[0046] Figure 4 This is a scanning electron microscope image of the carbon aerogel prepared in Comparative Example 3 of this invention. Detailed Implementation

[0047] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions 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 described embodiments of the present invention without creative effort are within the scope of protection of the present invention. Unless otherwise defined, the technical or scientific terms used herein should have the ordinary meaning understood by those skilled in the art to which this invention pertains.

[0048] In the following examples and comparative examples, the polyamic acid powder used was obtained by polymerization of diamine monomers and dianhydride monomers. The specific preparation process is as follows: dianhydride monomers 3,3,4,4-benzophenone tetracarboxylic dianhydride and 3,3,4,4-biphenyltetracarboxylic dianhydride in a molar ratio of 2:1 were dispersed in N,N-dimethylacetamide, and then diamine monomers 4,4-diaminodiphenyl sulfone and 4,4-diaminodiphenyl ether in a molar ratio of 1:2 (the molar ratio of diamine monomer to dianhydride monomer is 1:1.01) were added. The mixture was reacted at room temperature for 10 hours to obtain a polyamic acid mixed solution. The obtained polyamic acid mixed solution was added dropwise to deionized water for precipitation, and after filtration, polyamic acid powder was obtained.

[0049] Example 1

[0050] A method for preparing polyimide-based carbon aerogel for fuel cells includes the following steps: (1) Aramid fibers and KOH were dispersed in DMSO, and then deionized water was added dropwise. After stirring and dispersing for 30 h, an aramid nanofiber dispersion was obtained; the ratio of aramid fibers, KOH, deionized water and DMSO was 1 g: 1.2 g: 2.1 mL: 100 mL. (2) Place 10g of aramid nanofiber dispersion in a mold (volume 4cm×4cm×2cm), then immerse it in 2000mL of deionized water and let it stand for 2h. Add the formed primary gel to the aqueous solution of polyamic acid powder and triethylamine, stir evenly at 650rpm, then add branched polyethyleneimine PEI 800, stir evenly, and obtain hydrogel by sol-gelation; the mass ratio of polyamic acid powder to aramid nanofiber is 10:1; the mass ratio of triethylamine to polyamic acid powder is 0.5:1; the mass ratio of branched polyethyleneimine to polyamic acid powder is 0.05:1.

[0051] (3) The hydrogel was freeze-dried and then subjected to thermal imidization at 140℃ / 1h, 160℃ / 3h, 190℃ / 3h, and 220℃ / 1h to obtain an aerogel; (4) The aerogel was carbonized at 950℃ for 2.5h under nitrogen protection, with a carbonization temperature rise rate of 2℃ / min. After washing and drying, a polyimide-based carbon aerogel for fuel cells was obtained. The specific surface area and conductivity of the carbon aerogel were tested using the BET method and a conductivity meter. The specific surface area was 1146 m². 2 g -1 The conductivity is 16.6 Scm -1 .

[0052] Example 2

[0053] A method for preparing polyimide-based carbon aerogel for fuel cells includes the following steps: (1) Aramid fibers and KOH were dispersed in DMSO, and then deionized water was added dropwise. After stirring and dispersing for 35 h, an aramid nanofiber dispersion was obtained; the ratio of aramid fibers, KOH, deionized water and DMSO was 1 g: 0.9 g: 2.1 mL: 100 mL. (2) Place 10g of aramid nanofiber dispersion in a mold (volume 4cm×4cm×2cm), then immerse it in 2000mL of deionized water and let it stand for 4h. Add the formed primary gel to the aqueous solution of polyamic acid powder and triethylamine, stir evenly at 650rpm, then add branched polyethyleneimine PEI 1800 and stir evenly. The hydrogel is obtained by sol-gelation. The mass ratio of polyamic acid powder to aramid nanofiber is 14:1; the mass ratio of triethylamine to polyamic acid powder is 0.5:1; and the mass ratio of branched polyethyleneimine to polyamic acid powder is 0.2:1.

[0054] (3) The hydrogel was freeze-dried and then subjected to thermal imidization at 140℃ / 1h, 160℃ / 3h, 190℃ / 3h, and 220℃ / 1h to obtain an aerogel; (4) The aerogel was carbonized at 1000℃ for 2 hours under nitrogen protection, with a carbonization temperature rise rate of 5℃ / min. After washing and drying, a polyimide-based carbon aerogel for fuel cells was obtained. The specific surface area and conductivity of the carbon aerogel were tested using the BET method and a conductivity meter. The specific surface area was 986 m². 2 g -1 The conductivity is 18.1 Scm. -1 .

[0055] Example 3

[0056] A method for preparing polyimide-based carbon aerogel for fuel cells includes the following steps: (1) Aramid fibers and KOH were dispersed in DMSO, and then deionized water was added dropwise. After stirring and dispersing for 35 h, an aramid nanofiber dispersion was obtained; the ratio of aramid fibers, KOH, deionized water and DMSO was 1 g: 1 g: 2.1 mL: 100 mL. (2) Place 10g of aramid nanofiber dispersion in a mold (volume 4cm×4cm×2cm), then immerse it in 2000mL of deionized water and let it stand for 3h. Add the formed primary gel to the aqueous solution of polyamic acid powder and triethylamine, stir evenly at 650rpm, then add branched polyethyleneimine PEI 3000 and stir evenly. The hydrogel is obtained by sol-gelation. The mass ratio of polyamic acid powder to aramid nanofiber is 11:1; the mass ratio of triethylamine to polyamic acid powder is 0.5:1; and the mass ratio of branched polyethyleneimine to polyamic acid powder is 0.15:1.

[0057] (3) The hydrogel was freeze-dried and then subjected to thermal imidization at 140℃ / 1h, 160℃ / 3h, 190℃ / 3h, and 220℃ / 1h to obtain an aerogel; (4) The aerogel was carbonized at 980℃ for 2 hours under nitrogen protection, with a carbonization temperature rise rate of 4℃ / min. After washing and drying, a polyimide-based carbon aerogel for fuel cells was obtained. The specific surface area and conductivity of the carbon aerogel were tested using the BET method and a conductivity meter. The specific surface area was 1102 m². 2 g -1 The conductivity is 17.4 Scm. -1 .

[0058] Example 4

[0059] A method for preparing polyimide-based carbon aerogel for fuel cells includes the following steps: (1) Aramid fibers and KOH were dispersed in DMSO, and then deionized water was added dropwise. After stirring and dispersing for 30 h, an aramid nanofiber dispersion was obtained; the ratio of aramid fibers, KOH, deionized water and DMSO was 1 g: 0.9 g: 2.1 mL: 100 mL. (2) Place 10g of aramid nanofiber dispersion in a mold (volume 4cm×4cm×2cm), then immerse it in 2000mL of deionized water and let it stand for 2.2h. Add the formed primary gel to the aqueous solution of polyamic acid powder and triethylamine, stir evenly at 650rpm, then add branched polyethyleneimine PEI 1200 and stir evenly. The hydrogel is obtained by sol-gelation. The mass ratio of polyamic acid powder to aramid nanofiber is 12:1; the mass ratio of triethylamine to polyamic acid powder is 0.5:1; and the mass ratio of branched polyethyleneimine to polyamic acid powder is 0.1:1.

[0060] (3) The hydrogel was freeze-dried and then subjected to thermal imidization at 140℃ / 1h, 160℃ / 3h, 190℃ / 3h, and 220℃ / 1h to obtain an aerogel; (4) The aerogel was carbonized at 970℃ for 2.5h under nitrogen protection, with a carbonization temperature rise rate of 3℃ / min. After washing and drying, a polyimide-based carbon aerogel for fuel cells was obtained. The specific surface area and conductivity of the carbon aerogel were tested using the BET method and a conductivity meter. The specific surface area was 1036 m². 2 g -1 The conductivity is 15.7 Scm. -1 .

[0061] Example 5

[0062] A method for preparing polyimide-based carbon aerogel for fuel cells includes the following steps: (1) Aramid fibers and KOH were dispersed in DMSO, and then deionized water was added dropwise. After stirring and dispersing for 35 h, an aramid nanofiber dispersion was obtained; the ratio of aramid fibers, KOH, deionized water and DMSO was 1 g: 1 g: 2.1 mL: 100 mL. (2) Place 10g of aramid nanofiber dispersion in a mold (volume 4cm×4cm×2cm), then immerse it in 2000mL of deionized water and let it stand for 3h. Add the formed primary gel to the aqueous solution of polyamic acid powder and triethylamine, stir evenly at 650rpm, then add branched polyethyleneimine PEI 1200 and stir evenly. The hydrogel is obtained by sol-gelation. The mass ratio of polyamic acid powder to aramid nanofiber is 11:1; the mass ratio of triethylamine to polyamic acid powder is 0.5:1; and the mass ratio of branched polyethyleneimine to polyamic acid powder is 0.15:1.

[0063] (3) The hydrogel was freeze-dried and then subjected to thermal imidization at 140℃ / 1h, 160℃ / 3h, 190℃ / 3h, and 220℃ / 1h to obtain an aerogel; (4) The aerogel was carbonized at 980℃ for 2 hours under nitrogen protection, with a carbonization temperature rise rate of 4℃ / min. After washing and drying, a polyimide-based carbon aerogel for fuel cells was obtained. The specific surface area and conductivity of the carbon aerogel were tested using the BET method and a conductivity meter. The specific surface area was 1177 m². 2 g -1 The conductivity is 18.5 S / cm. -1 .

[0064] Example 6

[0065] A method for preparing polyimide-based carbon aerogel for fuel cells includes the following steps: (1) Aramid fibers and KOH were dispersed in DMSO, and then deionized water was added dropwise. After stirring and dispersing for 35 h, an aramid nanofiber dispersion was obtained; the ratio of aramid fibers, KOH, deionized water and DMSO was 1 g: 1.2 g: 2.1 mL: 100 mL. (2) Place 10g of aramid nanofiber dispersion in a mold (volume 4cm×4cm×2cm), then immerse it in 2000mL of deionized water and let it stand for 2.5h. Add the formed primary gel to the aqueous solution of polyamic acid powder and triethylamine, stir evenly at 650rpm, then add branched polyethyleneimine PEI 800 and stir evenly. The hydrogel is obtained by sol-gelation. The mass ratio of polyamic acid powder to aramid nanofiber is 11.5:1; the mass ratio of triethylamine to polyamic acid powder is 0.5:1; and the mass ratio of branched polyethyleneimine to polyamic acid powder is 0.12:1.

[0066] (3) The hydrogel was freeze-dried and then subjected to thermal imidization at 140℃ / 1h, 160℃ / 3h, 190℃ / 3h, and 220℃ / 1h to obtain an aerogel; (4) The aerogel was carbonized at 960℃ for 2 hours under nitrogen protection, with a carbonization temperature rise rate of 2℃ / min. After washing and drying, a polyimide-based carbon aerogel for fuel cells was obtained. The specific surface area and conductivity of the carbon aerogel were tested using the BET method and a conductivity meter. The specific surface area was 997 m². 2 g -1 The conductivity is 16.5 Scm -1 .

[0067] Example 7

[0068] A method for preparing polyimide-based carbon aerogel for fuel cells includes the following steps: (1) Aramid fibers and KOH were dispersed in DMSO, and then deionized water was added dropwise. After stirring and dispersing for 35 h, an aramid nanofiber dispersion was obtained; the ratio of aramid fibers, KOH, deionized water and DMSO was 1 g: 1 g: 2.1 mL: 100 mL. (2) Place 10g of aramid nanofiber dispersion in a mold (volume 4cm×4cm×2cm), then immerse it in 2000mL of deionized water and let it stand for 3h. Add the formed primary gel to the aqueous solution of polyamic acid powder and triethylamine, stir evenly at 650rpm, then add branched polyethyleneimine PEI 1200 and stir evenly. The hydrogel is obtained by sol-gelation. The mass ratio of polyamic acid powder to aramid nanofiber is 9:1; the mass ratio of triethylamine to polyamic acid powder is 0.5:1; and the mass ratio of branched polyethyleneimine to polyamic acid powder is 0.15:1.

[0069] (3) The hydrogel was freeze-dried and then subjected to thermal imidization at 140℃ / 1h, 160℃ / 3h, 190℃ / 3h, and 220℃ / 1h to obtain an aerogel; (4) The aerogel was carbonized at 980℃ for 2 hours under nitrogen protection, with a carbonization temperature rise rate of 4℃ / min. After washing and drying, a polyimide-based carbon aerogel for fuel cells was obtained. The specific surface area and conductivity of the carbon aerogel were tested using the BET method and a conductivity meter. The specific surface area was 1078 m². 2 g -1 The conductivity is 17.8 Scm. -1 .

[0070] Example 8

[0071] A method for preparing polyimide-based carbon aerogel for fuel cells includes the following steps: (1) Aramid fibers and KOH were dispersed in DMSO, and then deionized water was added dropwise. After stirring and dispersing for 35 h, an aramid nanofiber dispersion was obtained; the ratio of aramid fibers, KOH, deionized water and DMSO was 1 g: 1.1 g: 2.1 mL: 100 mL. (2) Place 10g of aramid nanofiber dispersion in a mold (volume 4cm×4cm×2cm), then immerse it in 2000mL of deionized water and let it stand for 3.5h. Add the formed primary gel to the aqueous solution of polyamic acid powder and triethylamine, stir evenly at 650rpm, then add branched polyethyleneimine PEI 1800 and stir evenly. The hydrogel is obtained by sol-gelation. The mass ratio of polyamic acid powder to aramid nanofiber is 13:1; the mass ratio of triethylamine to polyamic acid powder is 0.5:1; and the mass ratio of branched polyethyleneimine to polyamic acid powder is 0.14:1.

[0072] (3) The hydrogel was freeze-dried and then subjected to thermal imidization at 140℃ / 1h, 160℃ / 3h, 190℃ / 3h, and 220℃ / 1h to obtain an aerogel; (4) The aerogel was carbonized at 990℃ for 2 hours under nitrogen protection, with a carbonization temperature rise rate of 3℃ / min. After washing and drying, a polyimide-based carbon aerogel for fuel cells was obtained. The specific surface area and conductivity of the carbon aerogel were tested using the BET method and a conductivity meter. The specific surface area was 1093 m². 2 g -1 The conductivity is 15.9 S / cm. -1 .

[0073] Example 9

[0074] A method for preparing polyimide-based carbon aerogel for fuel cells includes the following steps: (1) Aramid fibers and KOH were dispersed in DMSO, and then deionized water was added dropwise. After stirring and dispersing for 30 h, an aramid nanofiber dispersion was obtained; the ratio of aramid fibers, KOH, deionized water and DMSO was 1 g: 1.1 g: 2.1 mL: 100 mL. (2) Place 10g of aramid nanofiber dispersion in a mold (volume 4cm×4cm×2cm), then immerse it in 2000mL of deionized water and let it stand for 3.2h. Add the formed primary gel to the aqueous solution of polyamic acid powder and triethylamine, stir evenly at 650rpm, then add branched polyethyleneimine PEI 600, stir evenly, and obtain hydrogel by sol-gelation; the mass ratio of polyamic acid powder to aramid nanofiber is 12:1; the mass ratio of triethylamine to polyamic acid powder is 0.5:1; the mass ratio of branched polyethyleneimine to polyamic acid powder is 0.18:1.

[0075] (3) The hydrogel was freeze-dried and then subjected to thermal imidization at 140℃ / 1h, 160℃ / 3h, 190℃ / 3h, and 220℃ / 1h to obtain an aerogel; (4) The aerogel was carbonized at 950℃ for 2.5h under nitrogen protection, with a carbonization temperature rise rate of 5℃ / min. After washing and drying, a polyimide-based carbon aerogel for fuel cells was obtained. The specific surface area and conductivity of the carbon aerogel were tested using the BET method and a conductivity meter. The specific surface area was 1058 m². 2 g -1 The conductivity is 16.3 Scm -1 .

[0076] Example 10

[0077] A method for preparing polyimide-based carbon aerogel for fuel cells includes the following steps: (1) Aramid fibers and KOH were dispersed in DMSO, and then deionized water was added dropwise. After stirring and dispersing for 35 h, an aramid nanofiber dispersion was obtained; the ratio of aramid fibers, KOH, deionized water and DMSO was 1 g: 1 g: 2.1 mL: 100 mL. (2) Place 10g of aramid nanofiber dispersion in a mold (volume 4cm×4cm×2cm), then immerse it in 2000mL of deionized water and let it stand for 3h. Add the formed primary gel to the aqueous solution of polyamic acid powder and triethylamine, stir evenly at 650rpm, then add branched polyethyleneimine PEI 600, stir evenly, and obtain hydrogel by sol-gelation; the mass ratio of polyamic acid powder to aramid nanofiber is 11:1; the mass ratio of triethylamine to polyamic acid powder is 0.5:1; the mass ratio of branched polyethyleneimine to polyamic acid powder is 0.15:1.

[0078] (3) The hydrogel was freeze-dried and then subjected to thermal imidization at 140℃ / 1h, 160℃ / 3h, 190℃ / 3h, and 220℃ / 1h to obtain an aerogel; (4) The aerogel was carbonized at 980℃ for 2 hours under nitrogen protection, with a carbonization temperature rise rate of 4℃ / min. After washing and drying, a polyimide-based carbon aerogel for fuel cells was obtained. The specific surface area and conductivity of the carbon aerogel were tested using the BET method and a conductivity meter. The specific surface area was 1086 m². 2 g -1 The conductivity is 17.5 Scm. -1 .

[0079] Comparative Example 1 A method for preparing polyimide-based carbon aerogel for fuel cells includes the following steps: (1) Aramid fibers and KOH were dispersed in DMSO, and then deionized water was added dropwise. After stirring and dispersing for 35 h, an aramid nanofiber dispersion was obtained; the ratio of aramid fibers, KOH, deionized water and DMSO was 1 g: 1 g: 2.1 mL: 100 mL. (2) Take 10g of aramid nanofiber dispersion, and obtain aramid nanofiber aqueous dispersion by filtration, washing and deionization. Add the aramid nanofiber aqueous dispersion directly to the aqueous solution formed by polyamic acid powder and triethylamine. Stir at 650rpm until uniform, then add branched polyethyleneimine PEI 600 and stir until uniform. Obtain hydrogel by sol-gelation. The mass ratio of polyamic acid powder to aramid nanofiber is 11:1; the mass ratio of triethylamine to polyamic acid powder is 0.5:1; and the mass ratio of branched polyethyleneimine to polyamic acid powder is 0.15:1.

[0080] (3) The hydrogel was freeze-dried and then subjected to thermal imidization at 140℃ / 1h, 160℃ / 3h, 190℃ / 3h, and 220℃ / 1h to obtain an aerogel; (4) The aerogel was carbonized at 980℃ for 2 hours under nitrogen protection, with a carbonization temperature rise rate of 4℃ / min. After washing and drying, a polyimide-based carbon aerogel for fuel cells was obtained. The specific surface area and conductivity of the carbon aerogel were tested using the BET method and a conductivity meter. The specific surface area was 682 m². 2 g -1 The conductivity is 2.3 Scm. -1 .

[0081] Comparative Example 2 A method for preparing polyimide-based carbon aerogel for fuel cells includes the following steps: (1) Aramid fibers and KOH were dispersed in DMSO, and then deionized water was added dropwise. After stirring and dispersing for 35 h, an aramid nanofiber dispersion was obtained; the ratio of aramid fibers, KOH, deionized water and DMSO was 1 g: 1 g: 2.1 mL: 100 mL. (2) Place 10g of aramid nanofiber dispersion in a mold (volume 4cm×4cm×2cm), then immerse it in 2000mL of deionized water and let it stand for 8h. Add the formed primary gel to the aqueous solution of polyamic acid powder and triethylamine, stir evenly at 650rpm, then add branched polyethyleneimine PEI 600 and stir evenly. The hydrogel is obtained by sol-gelation. The mass ratio of polyamic acid powder to aramid nanofiber is 11:1; the mass ratio of triethylamine to polyamic acid powder is 0.5:1; and the mass ratio of branched polyethyleneimine to polyamic acid powder is 0.15:1.

[0082] (3) The hydrogel was freeze-dried and then subjected to thermal imidization at 140℃ / 1h, 160℃ / 3h, 190℃ / 3h, and 220℃ / 1h to obtain an aerogel; (4) The aerogel was carbonized at 980℃ for 2 hours under nitrogen protection, with a carbonization temperature rise rate of 4℃ / min. After washing and drying, a polyimide-based carbon aerogel for fuel cells was obtained. The specific surface area and conductivity of the carbon aerogel were tested using the BET method and a conductivity meter. The specific surface area was 723 m² / min. 2 g -1 The conductivity is 5.1 Scm. -1 .

[0083] Comparative Example 3 A method for preparing polyimide-based carbon aerogel for fuel cells includes the following steps: (1) Aramid fibers and KOH were dispersed in DMSO, and then deionized water was added dropwise. After stirring and dispersing for 35 h, an aramid nanofiber dispersion was obtained; the ratio of aramid fibers, KOH, deionized water and DMSO was 1 g: 1 g: 2.1 mL: 100 mL. (2) Place 10g of aramid nanofiber dispersion in a mold (volume 4cm×4cm×2cm), then immerse it in 2000mL of deionized water and let it stand for 3h. Add the formed primary gel to the aqueous solution of polyamic acid powder and triethylamine, stir evenly at 650rpm, then add branched polyethyleneimine PEI 600, stir evenly, and obtain hydrogel by sol-gelation; the mass ratio of polyamic acid powder to aramid nanofiber is 11:1; the mass ratio of triethylamine to polyamic acid powder is 0.5:1; the mass ratio of branched polyethyleneimine to polyamic acid powder is 0.4:1.

[0084] (3) The hydrogel was freeze-dried and then subjected to thermal imidization at 140℃ / 1h, 160℃ / 3h, 190℃ / 3h, and 220℃ / 1h to obtain an aerogel; (4) The aerogel was carbonized at 980℃ for 2 hours under nitrogen protection, with a carbonization temperature rise rate of 4℃ / min. After washing and drying, a polyimide-based carbon aerogel for fuel cells was obtained. The specific surface area and conductivity of the carbon aerogel were tested using the BET method and a conductivity meter. The specific surface area was 804 m². 2 g -1 The conductivity is 4.6 Scm -1 .

[0085] As can be seen from the above embodiments and comparative examples, the present invention selects polyimide as the precursor material for carbon aerogel. The high proportion of rigid aromatic ring structures in the molecular backbone results in excellent thermal stability and minimal skeletal shrinkage and deformation during high-temperature carbonization. This maximizes the preservation of the original porous morphology of the precursor, reducing negative issues such as pore collapse and structural densification, and ensuring a high specific surface area and hierarchical porous structure in the carbonized material. Aramid nanofibers, as the reinforcing phase, possess excellent mechanical properties and intercalate within the polymer network to form a three-dimensional support structure, effectively improving the brittleness and cracking tendency of pure polyimide gels and enhancing the overall mechanical strength of the material. Through hydrogen bonding and chemical cross-linking reactions between the carbonyl groups of polyaminate, the oxygen-containing functional groups on the surface of aramid fibers, and the amino groups of branched polyethyleneimine, a multi-component composite cross-linked network is constructed. The cross-linking sites are evenly distributed, effectively binding the movement of molecular chain segments and releasing internal stress generated throughout the gel forming, solvent drying, and heat treatment processes, significantly reducing the probability of gel cracking, delamination, and localized shrinkage defects.

[0086] in, Figure 1The image shows a scanning electron microscope (SEM) image of the carbon aerogel prepared in Example 10. As can be seen from the image, the polyimide-based carbon aerogel prepared by this invention exhibits a continuous and complete three-dimensional porous framework without large-area collapse, fracture, or dense agglomeration. The pore layers are distinct, with micropores densely distributed on the pore wall surface. The pore size distribution range is concentrated, and the pore walls are flat and dense, without a large number of disordered cracks and closed pores. Figure 2 , Figure 3 , Figure 4 The images show scanning electron microscope (SEM) images of the carbon aerogels prepared in Comparative Examples 1-3. As can be seen from the images, the carbon aerogels prepared in Comparative Examples 1-3 have significantly lower structural regularity than the carbon aerogel prepared in Example 10, with multiple micro-cracks and small-scale collapse areas, uneven distribution of overall density, a large number of micro-defects, and lower overall structural continuity and stability.

[0087] Specific data also show that the embodiments of the present invention first prepare aramid nanofibers into a primary hydrogel, and then blend it with polyamic acid salt, thus solving the problem of easy agglomeration of aramid nanofibers. Compared with Example 10, Comparative Example 1 directly adds aramid nanofibers to the polyimide solution. The nanofibers are unevenly dispersed in the polyamic acid salt solution, easily agglomerate, and cannot interpenetrate and dissolve with the polyamic acid salt molecular chains, forming permanent interfacial phase separation hard defects. The subsequent molding of the composite hydrogel results in severe stress concentration, easy deformation during drying, loss of the rigid fiber skeleton during carbonization, stress collapse of local cross-linked structures, and an unbalanced overall residual carbon content distribution. The final carbon aerogel has poor mechanical stability, insufficient structural integrity, and significantly reduced specific surface area and electrical conductivity.

[0088] Meanwhile, as can be seen from Comparative Examples 2 and 3, although the specific surface area of ​​the prepared carbon aerogels was improved to some extent compared with Comparative Example 1, their conductivity was still low. Specifically, in Comparative Example 2, the standing time was too long during the preparation of the primary gel. The weak hydrogen bonds between the aramid nanofibers continued to superimpose and crosslink, constantly associating and densifying. The originally loose and reversible primary soft gel gradually solidified into a highly crosslinked rigid pre-gel. The subsequent stirring shear force could not completely break up the hardened gel clumps, only breaking the surface structure. The internal dense gel core remained intact and could not interpenetrate and dissolve with the polyimide molecular chains, forming permanent interfacial phase separation hard defects. The subsequent molding of the composite hydrogel resulted in severe stress concentration, easy deformation during drying, and damage to the rigid fiber skeleton and stress collapse of the local crosslinked structure during carbonization. The final prepared carbon aerogel had poor mechanical stability, insufficient structural integrity, and reduced conductivity. Compared with Example 10, Comparative Example 3 used too much branched polyethyleneimine, resulting in excessive accumulation of a large number of excess branched molecules in the system. This caused an increase in local crosslinking density, forming local high-density crosslinking agglomerates, while the crosslinking in other areas was relatively poor. This led to uneven crosslinking distribution in the entire gel system, generating a large number of residual internal stresses and micro-stress concentration points, which damaged the uniformity and compatibility of the composite system. As a result, the gel became too rigid and less tough, making it prone to brittleness during drying and carbonization. This also disrupted the orderly arrangement of the multi-level pore structure of the carbon aerogel, and its specific surface area and electrical conductivity decreased to a certain extent.

[0089] This demonstrates that the present invention, by using aramid nanofiber primary gel as a fiber filler, solves the technical problem of easy agglomeration of aramid nanofibers. Uniformly dispersed aramid nanofibers serve as a three-dimensional framework reinforcing phase, polyimide as a matrix precursor, and branched polyethyleneimine is introduced to construct a multi-level cross-linked network. A polyimide-based carbon aerogel material is obtained through sol-gel-carbonization molding. This material exhibits uniform internal pore distribution, a sufficient number of electrochemically active sites, and low resistance to charge and reaction medium mass transfer. It can be used as a functional material for fuel cell electrodes. The preparation process is simple and controllable, with a wide range of raw material compatibility, and the finished product demonstrates excellent performance stability, possessing value for large-scale production and practical industrial applications.

[0090] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing polyimide-based carbon aerogel for fuel cells, characterized in that, Includes the following steps: (1) A protic solvent was added dropwise to a non-protic solvent dispersion containing aramid fibers and alkali metal hydroxides, and the dispersion was stirred to obtain an aramid nanofiber dispersion. (2) Immerse the aramid nanofiber dispersion in deionized water and let it stand for 2-4 hours. Then add the primary gel formed to the aqueous solution of polyamic acid powder and organic base. After stirring evenly, add branched polyethyleneimine and obtain hydrogel by sol-gelation. The mass ratio of branched polyethyleneimine to polyamic acid powder is (0.05-0.2):

1. (3) The hydrogel was freeze-dried and thermally imidized to obtain an aerogel; (4) After high-temperature carbonization, washing and drying, a polyimide-based carbon aerogel for fuel cells is obtained.

2. The method for preparing polyimide-based carbon aerogel for fuel cells as described in claim 1, characterized in that, The stirring and dispersion time in step (1) is 24-48 hours.

3. The method for preparing polyimide-based carbon aerogel for fuel cells as described in claim 1, characterized in that, In step (2), the polyamic acid powder is obtained by polymerizing dianhydride monomer and diamine monomer.

4. The method for preparing polyimide-based carbon aerogel for fuel cells as described in claim 3, characterized in that, The molar ratio of dianhydride monomer to diamine monomer is (0.9-1.1):(0.9-1.1).

5. The method for preparing polyimide-based carbon aerogel for fuel cells as described in claim 1, characterized in that, In step (2), the organic base is one or more of triethylamine, tripropylamine, diethylenetriamine, triethylenetetramine, and triethylenediamine.

6. The method for preparing polyimide-based carbon aerogel for fuel cells as described in claim 1, characterized in that, In step (2), the mass ratio of organic base to polyamic acid powder is (0.1-1):

1.

7. The method for preparing polyimide-based carbon aerogel for fuel cells as described in claim 1, characterized in that, In step (2), the mass ratio of polyamic acid powder to aramid nanofibers is (8-14):

1.

8. The method for preparing polyimide-based carbon aerogel for fuel cells as described in claim 1, characterized in that, In step (3), thermal imidization is performed using a gradient temperature rise process.

9. The method for preparing polyimide-based carbon aerogel for fuel cells as described in claim 1, characterized in that, In step (4), the high-temperature carbonization is carried out under the protection of an inert gas.

10. A polyimide-based carbon aerogel for fuel cells, characterized in that, It is prepared by the method for preparing a polyimide-based carbon aerogel for fuel cells according to any one of claims 1-9.

Images