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

Abstract

The present application belongs to the technical field of fuel cell electrode material preparation, and particularly relates to a polyacrylamide-based carbon aerogel for fuel cells and a preparation method thereof. The present application discards the traditional direct blending process and adopts a step-by-step dispersion strategy of "preliminary shaping first and then compounding", constructs a reversible weak hydrogen bond primary gel through water induction, and locks the aramid nanofiber monodisperse state by using the space confinement effect. The weak hydrogen bond network of the primary gel only plays a space fixing role and has no high-strength crosslinking constraint, so it can be easily dispersed by shear force to make the nanofiber restore the flow compatibility characteristics, realize the molecular scale homogeneous interpenetration compounding with the stretched polyacrylamide molecular chain, completely solve the core problems of nanofiber agglomeration, entanglement, sedimentation and phase separation in the traditional process, and guarantee the uniformity of the composite aerogel and the carbon aerogel after carbonization, thereby laying a structural foundation for the continuous and uniform conductive network and mass transfer pore.

Description

Technical Field

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

[0002] With the increasing prominence of traditional fossil fuel shortages and the continuous intensification of environmental pollution control efforts, clean energy replacement of traditional fossil fuels has become a core trend in the global energy structure transformation. Fuel cells, as a novel energy conversion device that can directly and efficiently convert the chemical energy of fuel into electrical energy, break through the efficiency limitations of the Carnot cycle in traditional thermal power generation. They possess numerous core advantages, including high energy conversion efficiency, low operating noise, zero emissions, wide availability of fuel sources, and rich adaptability to various scenarios, making them one of the most promising power generation devices in the new energy power field. Currently, fuel cells are widely used in several key areas such as onboard power for new energy vehicles, stationary energy storage and power generation in industrial parks, portable outdoor power supply, marine auxiliary power, and aerospace backup power. Market demand is growing rapidly year by year, placing higher demands on the pore structure, conductivity, stability, and mass production cost of core functional materials for fuel cells.

[0003] Carbon aerogel is a novel nanomaterial with a three-dimensional interconnected porous network structure. With its ultra-high specific surface area, tunable multi-level pore structure, excellent electronic conductivity, good chemical inertness, and structural designability, it has become a core functional material in fuel cell electrodes, electrocatalysis, and energy storage. Compared to traditional carbon paper, carbon cloth, and bulk carbon materials, carbon aerogel has a rich and controllable pore structure, exposing a large number of electrochemical active sites. This provides ample channels for charge transport, electrolyte wetting, and fuel gas diffusion. It also possesses advantages such as lightweight, corrosion resistance, and deformation resistance, perfectly adapting to the complex electrochemical conditions of fuel cells, making it a preferred substrate for next-generation high-performance fuel cell electrodes.

[0004] Pure carbon aerogels possess inherent performance defects, including high structural brittleness, weak mechanical resistance to deformation, a scarcity of electrochemical active sites, and a single pore size. Under long-term operating conditions, they are prone to problems such as framework collapse and breakage of conductive pathways. Furthermore, under the continuous gas scouring, charge impact, temperature alternation, and electrochemical cycling of fuel cells, they are highly susceptible to rapid performance degradation, failing to meet the requirements of high-power, long-cycle, and highly stable fuel cells. Therefore, modifying polymeric aerogel precursors with nano-functional fillers to control the microstructure and electrochemical properties of carbonized carbon aerogels is currently the core direction in the research and development of high-performance fuel cell carbon aerogel materials.

[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 polyacrylamide-based carbon aerogel for fuel cells and its preparation method. The non-ionic polyacrylamide-based carbon aerogel electrode material provided by this invention has a large specific surface area, numerous active sites, low resistivity, and good conductivity. Furthermore, its 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 polyacrylamide-based carbon aerogel for fuel cells includes the following steps: (1) Using protic solvent as a modifier, aramid nanofiber dispersion was prepared by alkali dissolution method; (2) Immerse the aramid nanofiber dispersion in deionized water and let it stand for 2-4 hours. Then add the formed primary gel to the nonionic polyacrylamide aqueous solution, stir evenly, add polyethylene glycol, let it stand again, and exchange solvent with deionized water to obtain hydrogel; the mass ratio of polyethylene glycol to nonionic polyacrylamide is (0.1-0.25):1. (3) The hydrogel is freeze-dried, then carbonized at high temperature, washed and dried to obtain a polyacrylamide-based carbon aerogel for fuel cells.

[0008] Aramid nanofibers, a novel one-dimensional nanopolymer fiber material, are prepared from macroscopic aramid fibers through chemical exfoliation and pyrolysis. They possess high tensile strength, a regular molecular chain structure, and abundant surface polar active groups, making them an ideal reinforcing and modifying phase for polymeric aerogels. Introducing aramid nanofibers into carbon aerogel precursor systems can build a three-dimensional supporting framework within the matrix, improving the mechanical stability of both the aerogel and the carbonized carbon aerogel. Simultaneously, it enriches interfacial active sites and optimizes electrical conductivity and mass transfer performance. However, existing traditional composite processes generally employ direct blending. Due to their large specific surface area, high surface energy, and strong intermolecular van der Waals forces, aramid nanofibers are prone to fiber entanglement, localized aggregation, sedimentation, and stratification. This results in uneven component distribution, loose interfacial bonding, and excessive structural defects in the composite aerogel, ultimately leading to discontinuous conductive pathways, disordered pores, and a significant decrease in electrochemical performance in the carbonized carbon aerogel.

[0009] In previous work, the inventors directly used polyacrylic acid as the polymer matrix to prepare gels and aerogel precursors by compositing it with aramid nanofibers. However, polyacrylic acid is an acidic polyelectrolyte, and its molecular chains are prone to coiling and agglomeration under acidic conditions. The hydrogen bonding with aramid nanofibers is of a single form, and the interfacial compatibility is limited. This results in severe stress concentration within the composite gels and aerogels, easily leading to cracking, shrinkage, and framework collapse during molding, drying, and carbonization. Consequently, the aerogels exhibit low yield and poor structural stability. Furthermore, the pyrolysis rate during the polyacrylic acid carbonization process is uncontrollable, easily disrupting the pre-designed multi-level pore structure, resulting in insufficient mass transfer capacity and severe polarization loss in the carbon aerogels, representing a significant technical bottleneck.

[0010] Nonionic polyacrylamide possesses unique advantages such as good molecular chain flexibility, abundant amide polar groups, wide hydrogen bond compatibility, and high carbonization structural regularity. It can form a multi-hydrogen bonded composite network with aramid nanofibers, exhibiting interfacial bonding strength and system uniformity far superior to the polyacrylic acid system. Simultaneously, combined with polyethylene glycol flexible modification, the toughness of the gel and aerogel, as well as the pore structure of the carbonized carbon aerogel, can be optimized in a synchronized manner. Based on this, this invention constructs a process system of "first preparing aramid nanofiber-reinforced polyacrylamide composite aerogel, then preparing carbon aerogel for fuel cells through high-temperature carbonization," which significantly improves the specific surface area and conductivity of the carbon aerogel.

[0011] The reason for adding aramid nanofiber gel precursor to polyacrylamide in this invention is that nonionic polyacrylamide is a hydrophilic polymer with flexible molecular chains. The resulting hydrogel relies solely on weak hydrogen bonds for cross-linking, resulting in extremely low mechanical strength. During post-processing such as solvent exchange and drying, it is prone to significant shrinkage, cracking, and overall structural collapse, making it impossible to maintain a continuous and stable three-dimensional porous network. Simultaneously, nonionic polyacrylamide has poor thermal stability and undergoes violent thermal decomposition during high-temperature carbonization, easily causing pore collapse, product powdering, and other problems. The resulting carbon aerogel is brittle, has poor structural integrity, and its conductivity is insufficient to meet the actual requirements of fuel cell electrodes. Although existing technologies have attempted to modify and optimize nonionic polyacrylamide-based carbon aerogels using cross-linking agents, inorganic nanoparticles, or other polymers to improve their structural stability, conventional modifying components are unable to construct a high-temperature resistant, high-strength, continuous rigid framework. Their effectiveness in suppressing structural collapse during drying and carbonization, improving overall residual carbon rate and pore structure retention is limited, making it difficult to achieve a synergistic improvement in high specific surface area, excellent mechanical properties, and good electrocatalytic performance. More importantly, existing technologies using filler-reinforced nonionic polyacrylamide aerogels require on-site initiation of acrylamide monomer polymerization. This process involves concentrated exothermic reactions and difficulty in precisely controlling the polymerization rate, easily leading to problems such as localized over-polymerization and incomplete polymerization. The resulting gel exhibits uneven network density and numerous inherent microscopic defects. Furthermore, the monomer polymerization process is susceptible to interference from ambient temperature, impurities, and oxygen, resulting in 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 leads to weak mechanical strength, poor thermal stability, and high drying shrinkage. The framework is prone to collapse during carbonization, resulting in a chaotic pore structure and poor structural integrity in the final carbon aerogel, leading to insufficient electrode reliability.

[0012] To address the problems of complex synthesis processes, low mechanical strength, poor thermal stability, and easy structural collapse and breakage after carbonization of nonionic polyacrylamide aerogels, this invention employs aramid nanofibers as a reinforcing phase, abandoning in-situ polymerization and directly strengthening and optimizing the skeleton of nonionic polyacrylamide aerogels. Aramid nanofibers possess high strength, high modulus, and excellent thermal stability, forming a continuous, interconnected three-dimensional rigid support network within the nonionic polyacrylamide gel system. This significantly improves the mechanical strength and structural stability of the composite system, effectively suppressing volume shrinkage and structural collapse during solvent exchange, drying, and carbonization. Aramid nanofibers exhibit high carbon residue under high-temperature carbonization conditions, enabling them to tightly bond with the carbon matrix formed by the pyrolysis of nonionic polyacrylamide, improving the overall carbon residue and structural integrity of the system. This maintains a stable hierarchical porous structure in the carbonized products, increasing their surface area. Through the combined action of aramid nanofibers and nonionic polyacrylamide, the resulting carbon aerogel possesses high specific surface area, good conductivity, and excellent structural stability, better meeting the application requirements of fuel cell electrode materials.

[0013] In one embodiment, step (1) is specifically 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.

[0014] 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 solvents in the art and can effectively promote fiber dispersion.

[0015] In one embodiment, the alkali metal hydroxide in step (1) is one or more of NaOH and KOH.

[0016] In one embodiment, the stirring in step (1) can be carried out at room temperature, and the stirring and dispersion time is 24-48h.

[0017] In one embodiment, the proton solvent in step (1) is one or more of deionized water, methanol, and ethanol.

[0018] 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%.

[0019] 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 nonionic polyacrylamide. 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 the nonionic polyacrylamide.

[0020] Aramid nanofibers exhibit poor dispersibility and are prone to agglomeration when directly blended. They exhibit significant nanoscale effects, possessing a large specific surface area and extremely high surface free energy. Strong van der Waals attraction between fibers leads to spontaneous entanglement and aggregation of individual fibers when directly mixed with polymer solutions. This results in phase separation phenomena characterized by localized fiber density and localized fiber absence, causing uneven distribution of components in composite gels and aerogels. Subsequent carbonization of the carbon aerogel further exacerbates this, as the internal conductive network varies in density and the pores become disordered, severely hindering charge transport and reactant mass transfer.

[0021] To address the issue of uneven dispersion of aramid nanofibers in nonionic polyacrylamide, a highly polar polymer, this invention first prepares aramid nanofibers into a primary hydrogel, which is then blended with nonionic polyacrylamide, 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 stabilization. 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 of aramid nanofibers in subsequent nonionic polyacrylamide polymer blending, and provide a stable and monodisperse fiber matrix environment for the uniform composite of multi-component interfaces. Based on this, a nonionic polyacrylamide aqueous solution was added to the primary weak gel system and mechanically stirred. The shear force generated by stirring can 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 nonionic polyacrylamide molecules fully expand in the aqueous solution, interpenetrate, entangle, and wet the aramid nanofiber segments, and achieve homogeneous mixing of the two phases at the molecular scale. This forms a thermodynamically stable, phase-separated, and particle-free homogeneous composite aqueous solution system, which greatly improves the interfacial adhesion and compatibility of the two phases and effectively avoids the technical problem of agglomeration caused by the direct addition of aramid nanofibers.

[0022] 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. This ensures the monodisperse stability of the fibers and meets 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 a nonionic polyacrylamide aqueous solution is subsequently added and stirred, the free aramid nanofibers rapidly undergo large-scale secondary entanglement and localized agglomeration under the influence of solvent interfacial tension fluctuations and intermolecular van der Waals forces. This results in regional fiber enrichment, component segregation, and stratification, making it impossible for the two phases to achieve microscale uniform compatibility. This directly leads to uneven density and increased primary defects in the internal structure of the composite hydrogel, making it prone 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 directly. 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 will 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 and shearing forces cannot completely break up the hardened gel clumps, only disrupting the surface structure. The dense internal gel core remains intact and cannot interpenetrate and dissolve with the nonionic polyacrylamide molecular chains, forming permanent interfacial phase separation 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, an unbalanced overall residual carbon distribution, and a disordered and impermeable pore arrangement. The final carbon aerogel exhibits poor mechanical stability and insufficient structural integrity, failing to meet the core requirements of long-term cycling, stable conductivity, and efficient mass transfer in fuel cell electrodes.

[0023] In one embodiment, the nonionic polyacrylamide in step (2) is a common type in the art, specifically obtained by polymerization of acrylamide as a monomer through an oxidant-reducing agent initiation system. The redox dual initiation system has mild reaction conditions, uniform exothermic reaction, and controllable polymerization rate, and can prepare nonionic polyacrylamide aqueous solutions with regular molecular chains, uniform molecular weight, and excellent gelling properties, without local crosslinking and agglomeration defects.

[0024] In one embodiment, the oxidant in step (2) is one or more of ammonium persulfate, potassium persulfate, potassium sulfite, azobisisobutyronitrile, and hydrogen peroxide.

[0025] In one embodiment, the reducing agent in step (2) is one or more of sodium sulfite, sodium bisulfite, and ferrous chloride.

[0026] Polyacrylamide generally includes nonionic polyacrylamide (without ionic functional groups), anionic polyacrylamide (such as with the introduction of acrylic acid reactive monomers), cationic polyacrylamide (such as with the introduction of ammonium salt monomers), and amphoteric polyacrylamide. This invention chooses nonionic polyacrylamide because ionic polyacrylamide exhibits electrostatic repulsion / adsorption between charged groups, causing the molecular chains to easily coil and twist in aqueous solution; while nonionic molecules are uncharged, allowing the chain segments to fully extend and uniformly penetrate the gaps between aramid nanofibers, significantly inhibiting fiber entanglement and aggregation, resulting in higher uniformity of the mixture. Simultaneously, the nonionic amide groups are stable and can form a multi-hydrogen bond crosslinking network with the hydroxyl groups of aramid fibers and the terminal hydroxyl groups of polyether diols; whereas the charged groups of ionic polyacrylamide tend to crowd out hydrogen bond sites, resulting in a single crosslinking form. The nonionic system exhibits a moderate gelation rate, a regular gel structure, and is less prone to localized uneven crosslinking. Furthermore, the nonionic polyacrylamide adheres tightly to the aramid nanofiber interface, with no obvious gaps at the composite interface. During high-temperature carbonization, the organic framework undergoes more uniform pyrolysis and recombination, making it less prone to structural breaks and enabling the formation of continuous carbon conductive pathways, thus ensuring the stable conductivity of carbon aerogels.

[0027] In one embodiment, the stirring rate in step (2) is 400-800 rpm. Specifically, ultrasound may also 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 a nonionic polyacrylamide aqueous solution to the system, the primary weak hydrogen-bonded 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 nonionic polyacrylamide molecular chains.

[0028] In one embodiment, the mass ratio of polyethylene glycol to nonionic polyacrylamide 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 polyethylene glycol to nonionic polyacrylamide is (0.15-0.20):1. By controlling the amount of polyethylene glycol within a reasonable range, the crosslinking density can be made uniform and controllable, ensuring a strong bridging between the aramid nanofibers and the nonionic polyacrylamide at the two-phase interface, 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.

[0029] Polyethylene glycol is a hydrophilic, flexible, long-chain polymer with a large number of ether bonds and terminal hydroxyl groups in its molecular chain. It can be adapted to the hydrogen-bonded composite system of polyacrylamide and aramid nanofibers without introducing impurity functional groups. At the same time, it has the functions of network toughening and internal stress relief. It solves the defects of pure aramid nanofiber / polyacrylamide hydrogels, such as high brittleness, easy cracking during molding, and secondary agglomeration of nanofibers, from the precursor stage. It lays the precursor structure foundation for the excellent performance of subsequent aerogels and carbon aerogels.

[0030] First, polyethylene glycol (PEG) can optimize the hydrogen-bonded crosslinking network, improve gel homogeneity, and inhibit the aggregation of aramid nanofibers. Pure aramid nanofiber / polyacrylamide hydrogels rely solely on the amide groups of polyacrylamide to form a single hydrogen-bonded crosslinking network with the hydroxyl and amide groups on the surface of aramid nanofibers. This results in concentrated crosslinking sites, strong network rigidity, and the high specific surface energy of aramid nanofibers makes them prone to chain entanglement and local aggregation during crosslinking, leading to uneven distribution of gel components and defects such as localized density and localized looseness. By introducing PEG, the hydroxyl groups at both ends of its molecular chain and the ether groups in the middle can simultaneously form secondary hydrogen-bonded crosslinking nodes with the amide groups in polyacrylamide and the polar groups on the aramid nanofibers, constructing a ternary multi-hydrogen-bonded composite network of "polyacrylamide-aramid nanofiber-PEG". This flexible secondary crosslinking network can uniformly disperse the crosslinking density of the system, solving the problem of excessive local crosslinking in the pure system, weakening the tendency of polyacrylamide molecular chains to coil and stack, and simultaneously utilizing the steric hindrance effect of long chains to physically isolate individual aramid nanofibers, inhibiting secondary agglomeration of nanofibers during freeze-drying, solvent exchange, and curing processes. This allows the aramid nanofibers to achieve monodisperse distribution in the hydrogel matrix, ensuring high uniformity of the overall gel composition and eliminating structural defects. Secondly, polyethylene glycol can alleviate molding internal stress, solving the problems of gel cracking, shrinkage, and delamination. The crosslinking network of pure polyacrylamide / aramid nanofiber hydrogel is rigid and has poor toughness. During long-term solvent exchange, ice crystal formation, and structural curing, the internal stress of the system continues to accumulate, easily leading to problems such as surface cracking, interlayer delamination, and structural collapse, significantly reducing the yield of aerogel precursor products. Polyethylene glycol, a flexible long-chain polymer, interspersed in the gaps between the rigid networks of polyacrylamide and aramid nanofibers, can act as a flexible buffer segment. During hydrogel molding, solvent replacement, and low-temperature pre-freezing, the long polyether chains can release internal stress through stretching, curling, and sliding, offsetting the volumetric deformation stress caused by temperature changes and solvent replacement. This significantly improves the flexibility, extensibility, and structural stability of the composite hydrogel, ensuring that the gel maintains a complete and flat macroscopic morphology without cracking or shrinkage, guaranteeing that the subsequent aerogel completely replicates the three-dimensional framework of the precursor. Furthermore, polyethylene glycol has lower thermal stability than polyacrylamide and aramid nanofibers, allowing for stepwise, gradual, and controllable pyrolysis and volatilization during gradient heating carbonization, avoiding the problem of violent gas generation impacting the framework. In the low-temperature range, the long polyether chains gradually decompose, removing small molecular fragments and forming uniform micropores within the carbon skeleton. In the medium-temperature range, the long chains are completely thermally decomposed, expanding to form numerous mesopores. The trace amounts of gas generated by pyrolysis escape uniformly, further expanding the pores and constructing interconnected macroporous channels. This allows the carbon aerogel to form a hierarchical, interconnected multi-level porous structure. Compared to the single-pore structure of the pure system, the pore utilization rate is significantly improved, greatly enriching the electrochemical active interface of the carbon aerogel and providing sufficient active sites for the electrochemical reaction of fuel cells.

[0031] In one embodiment, the type of polyethylene glycol in step (2) is not particularly limited. Specifically, polyethylene glycol with a number average molecular weight of 500-10000 can be selected. Furthermore, polyethylene glycol with good solubility can be selected. Specifically, it can be at least one of PEG500, PEG600, PEG800, PEG1000, PEG1500, PEG2000, PEG2500, PEG3000, PEG4000, PEG5000, PEG6000, PEG8000, and PEG10000. In particular, polyethylene glycol with a number average molecular weight of 2000-4000 can be selected. A polyethylene glycol with a suitable molecular weight can fully play the role of hydrogen bonding and crosslinking, while avoiding the problem of excessive molecular weight, excessive steric hindrance, and difficulty in dispersion.

[0032] Polyethylene glycol (PEG) is introduced into a homogeneous mixture. This polymer molecule is rich in ether bonds and hydroxyl groups, which can easily form hydrogen bonds or undergo dehydration reactions with oxygen-containing functional groups. It can serve as a multifunctional crosslinking bridge. One end forms strong bonds with polar groups such as hydroxyl, carboxyl, and amide groups on the surface of aramid nanofibers, while the other end undergoes multi-site hydrogen bond coordination crosslinking with high-density amino functional groups on the nonionic polyacrylamide molecular chain. Through bidirectional synchronous bonding and the interaction between aramid nanofibers and nonionic polyacrylamide, 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 employed in this invention, involving primary gelation and the addition of nonionic polyacrylamide, ensures that the aramid nanofibers remain monodisperse and do not agglomerate throughout the process. It also achieves deep interfacial composite between the rigid fiber phase and the flexible polymer phase, ultimately forming a composite hydrogel with a uniform structure, mechanical stability, and continuous three-dimensional channels. This hydrogel can stably withstand the harsh conditions of subsequent drying, dehydration, and high-temperature pyrolysis carbonization. Leveraging the high residual carbon rigid framework of aramid combined with the in-situ pore-forming advantage of nonionic polyacrylamide, a high-performance carbon aerogel with well-developed multi-level pores, a complete and stable structure, and abundant nitrogen-doped active sites is finally prepared, which is suitable for the practical application requirements of electrode materials for fuel cells.

[0033] In one embodiment, the mass ratio of nonionic polyacrylamide to aramid nanofibers in step (2) is (15-25):1. An appropriate amount of nonionic polyacrylamide can fully wet and be compatible with aramid nanofibers, and can also achieve uniform hydrogen bond crosslinking with polyethylene glycol, resulting in a regular and stable gel structure. After carbonization, the gel has well-developed pores and a complete skeleton, ultimately yielding a carbon aerogel electrode material for fuel cells with excellent comprehensive performance. The nonionic polyacrylamide is added in the form of an aqueous solution. The amount of water is not particularly limited; it is sufficient to ensure that all components are fully and uniformly dispersed.

[0034] In one embodiment, the mass ratio of nonionic polyacrylamide to aramid nanofibers in step (2) can be 15:1, 16:1, 17:1, 18:1, 19:1, 20:1, 21:1, 22:1, 23:1, 24:1, or 25:1; further, it can be (18-23):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, fiber entanglement and agglomeration and uneven component dispersion can easily occur, causing stress concentration and pore blockage within the gel, increasing material brittleness and hindering product performance improvement.

[0035] In one embodiment, step (2) involves a second settling period followed by solvent exchange with deionized water, lasting for 1-5 days. This settling and solvent exchange further enhances the stability of the gel network and reduces the impact of unwanted impurities.

[0036] In one embodiment, the high-temperature carbonization in step (3) is carried out under inert gas protection.

[0037] In one embodiment, the inert gas in step (3) is one or more of nitrogen, helium, and argon.

[0038] In one embodiment, the high-temperature carbonization temperature in step (3) is 800-1100℃; the high-temperature carbonization time is 1-3h. Specifically, the high-temperature carbonization temperature can be 800℃, 850℃, 900℃, 950℃, 1000℃, 1010℃, or 1100℃. Furthermore, the high-temperature carbonization temperature can be 900℃-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 complete pyrolysis of nonionic polyacrylamide and promote the appropriate graphitization of aramid nanofibers to construct a high-strength, highly conductive, continuous carbon framework. At the same time, an appropriate amount of stable nitrogen-doped active sites are precisely retained, and the hierarchical pore structure, conductive network, and catalytic active center are synergistically optimized. Finally, a composite carbon aerogel with complete structure, well-developed pores, excellent conductivity, and strong electrochemical stability is prepared.

[0039] In one embodiment, the heating rate for high-temperature carbonization in step (4) is 1-10℃ / min. Specifically, the heating rates for 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 excessively rapid heating. The washing method is not particularly limited. Specifically, hydrochloric acid and deionized water can be used for washing.

[0040] On the other hand, this invention also provides a nonionic polyacrylamide-based carbon aerogel for fuel cells prepared using the above method. The main raw materials used in the preparation process of this invention—aramid fiber, nonionic polyacrylamide, polyethylene glycol, solvent, and alkaline hydroxide—are all common types in the art and can be prepared or purchased independently. For example, DuPont Kevlar 29 aramid fiber can be selected, and Shanghai Aladdin series polyethylene glycol can be selected, etc. In particular, the nonionic polyacrylamide used in this invention can also be prepared using conventional processes in the art: acrylamide monomer is dispersed in deionized water, the pH is adjusted to 4, nitrogen gas is introduced, and then an ammonium persulfate-sodium bisulfite (mass ratio 2:1) initiator is added. The reaction is carried out at room temperature for 15 hours, followed by precipitation with anhydrous ethanol, washing, and drying to obtain nonionic polyacrylamide; wherein the mass ratio of initiator to acrylamide monomer is 0.25%.

[0041] The carbon aerogel electrode material prepared by this invention 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 a nonionic polyacrylamide 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. Simultaneously, it increases the overall residual carbon content of the system, 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 the carbon material. Combined with the hierarchical pore structure formed by in-situ pore creation through the pyrolysis of nonionic polyacrylamide, a carbon aerogel with high mechanical strength, well-developed pores, good stability, and excellent conductivity is finally obtained, meeting the comprehensive requirements of fuel cell electrode materials.

[0042] Beneficial effects: This invention employs a stepwise reinforcement strategy of aramid nanofibers to modify nonionic polyacrylamide, synergistically utilizing the stereolinking regulation effect of polyethylene glycol. Compared to traditional single nonionic polyacrylamide-based carbon aerogel materials and conventional linear polymer composite modification techniques, this invention offers several significant advantages.

[0043] (1) This invention abandons the traditional direct blending process and adopts a stepwise dispersion strategy of "first shaping, then compounding". It constructs a reversible weak hydrogen bond primary gel through water induction and uses the spatial confinement effect to lock the monodisperse state of aramid nanofibers. The weak hydrogen bond network of the primary gel only plays a spatial fixing role and has no high-strength cross-linking constraint. It can be easily dispersed by shear force, so that the nanofibers can regain their flow compatibility characteristics and achieve molecular-scale homogeneous interpenetration and compounding with the stretched polyacrylamide molecular chains. This completely solves the core problems of nanofiber aggregation, entanglement, sedimentation and phase separation in the traditional process, and ensures the uniformity of the composite aerogel and carbonized carbon aerogel components, laying the structural foundation for a continuous and uniform conductive network and mass transfer channels.

[0044] (2) In this invention, a composite aerogel is first prepared and then carbonized, preserving the original three-dimensional network and multi-level pore structure of the gel. The composite aerogel precursor has well-developed pores, a continuous skeleton, and uniform components, completely replicating the three-dimensional skeleton structure of the aerogel, forming a continuous and interconnected carbon-based conductive network and multi-level mass transfer channels. This solves the defects of traditional carbonization processes such as pore blockage, breakage of conductive pathways, and structural collapse, achieving dual optimization of the structure and performance of carbon aerogel.

[0045] (3) The nonionic polyacrylamide direct composite process is adopted, which eliminates the complex process of adding acrylamide monomers to the fiber to initiate the polymerization of hydrogels in situ, which is required in traditional processes. The overall preparation process of this invention is mild and highly controllable, without the need for additional high-temperature crosslinking, external dopants and complex post-processing. The raw materials are readily available, the process is simple, and the batch repeatability is good, which is suitable for the needs of large-scale batch preparation. It comprehensively solves the problems of easy collapse, insufficient strength, poor conductivity and low catalytic activity of traditional nonionic polyacrylamide-based carbon electrode structures. The carbon aerogel electrode material obtained in the end has excellent structural stability, high mass transfer capacity and good conductivity, and can be used as a fuel cell electrode material. Attached Figure Description

[0046] 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.

[0047] Figure 1 This is a scanning electron microscope image of the carbon aerogel prepared in Example 10 of the present invention.

[0048] Figure 2 This is a scanning electron microscope image of the carbon aerogel prepared in Comparative Example 1 of this invention.

[0049] Figure 3 This is a scanning electron microscope image of the carbon aerogel prepared in Comparative Example 2 of this invention.

[0050] Figure 4 This is a scanning electron microscope image of the carbon aerogel prepared in Comparative Example 3 of this invention. Detailed Implementation

[0051] 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.

[0052] Example 1

[0053] A method for preparing polyacrylamide-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 26 hours, an aramid nanofiber dispersion was obtained. The ratio of aramid fibers, KOH, deionized water and DMSO was 1g:1.2g:2mL:100mL. (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 a nonionic polyacrylamide aqueous solution, stir evenly at 600rpm, add polyethylene glycol PEG1500, disperse, and let it stand for 2 days. Then exchange solvent with deionized water to obtain hydrogel. The mass ratio of nonionic polyacrylamide to aramid nanofiber is 17:1; the mass ratio of polyethylene glycol to nonionic polyacrylamide is 0.1:1.

[0054] (3) The hydrogel was freeze-dried to obtain an aerogel; then the aerogel was carbonized at 930℃ for 2.5h under nitrogen protection, wherein the carbonization temperature rise rate was 2℃ / min. After washing and drying, a polyacrylamide-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 1038 m². 2 g -1 The conductivity is 14.7 Scm -1 .

[0055] Example 2

[0056] A method for preparing polyacrylamide-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 34 hours, an aramid nanofiber dispersion was obtained. The ratio of aramid fibers, KOH, deionized water and DMSO was 1g:0.9g:2mL:100mL. (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 a nonionic polyacrylamide aqueous solution, stir evenly at 600rpm, add polyethylene glycol PEG5000, disperse, and let it stand for 1.5 days to exchange solvent with deionized water to obtain hydrogel; the mass ratio of nonionic polyacrylamide to aramid nanofiber is 23:1; the mass ratio of polyethylene glycol to nonionic polyacrylamide is 0.25:1.

[0057] (3) The hydrogel was freeze-dried to obtain an aerogel; then the aerogel was carbonized at 980℃ for 1.5h under nitrogen protection, wherein the carbonization temperature rise rate was 5℃ / min. After washing and drying, a polyacrylamide-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 921 m². 2 g -1 The conductivity is 11.9 S / cm. -1 .

[0058] Example 3

[0059] A method for preparing polyacrylamide-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 34 hours, an aramid nanofiber dispersion was obtained. The ratio of aramid fibers, KOH, deionized water and DMSO was 1g:1g:2mL:100mL. (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 a nonionic polyacrylamide aqueous solution, stir evenly at 600rpm, add polyethylene glycol PEG4000, disperse, and let it stand for 2 days. Then exchange solvent with deionized water to obtain hydrogel. The mass ratio of nonionic polyacrylamide to aramid nanofiber is 20:1; the mass ratio of polyethylene glycol to nonionic polyacrylamide is 0.2:1.

[0060] (3) The hydrogel was freeze-dried to obtain an aerogel; then the aerogel was carbonized at 970℃ for 2 hours under nitrogen protection, with a carbonization temperature rise rate of 4℃ / min. After washing and drying, a polyacrylamide-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 942 m². 2 g -1 The conductivity is 12.9 Scm -1 .

[0061] Example 4

[0062] A method for preparing polyacrylamide-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 26 hours, an aramid nanofiber dispersion was obtained. The ratio of aramid fibers, KOH, deionized water and DMSO was 1g:0.9g:2mL:100mL. (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 a nonionic polyacrylamide aqueous solution, stir evenly at 600rpm, add polyethylene glycol PEG3000, disperse, and let it stand for 2 days. Then exchange solvent with deionized water to obtain hydrogel. The mass ratio of nonionic polyacrylamide to aramid nanofiber is 18:1; the mass ratio of polyethylene glycol to nonionic polyacrylamide is 0.12:1.

[0063] (3) The hydrogel was freeze-dried to obtain an aerogel; then the aerogel was carbonized at 940℃ for 2.5h under nitrogen protection, wherein the carbonization temperature rise rate was 3℃ / min. After washing and drying, a polyacrylamide-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 951 m². 2 g -1 The conductivity is 15.1 Scm. -1 .

[0064] Example 5

[0065] A method for preparing polyacrylamide-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 34 hours, an aramid nanofiber dispersion was obtained. The ratio of aramid fibers, KOH, deionized water and DMSO was 1g:1g:2mL:100mL. (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 a nonionic polyacrylamide aqueous solution, stir evenly at 600rpm, add polyethylene glycol PEG3000, disperse, and let it stand for 2 days. Then exchange solvent with deionized water to obtain hydrogel. The mass ratio of nonionic polyacrylamide to aramid nanofiber is 20:1; the mass ratio of polyethylene glycol to nonionic polyacrylamide is 0.2:1.

[0066] (3) The hydrogel was freeze-dried to obtain an aerogel; then the aerogel was carbonized at 970℃ for 2 hours under nitrogen protection, wherein the carbonization temperature rise rate was 4℃ / min. After washing and drying, a polyacrylamide-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 1042 m². 2 g -1 The conductivity is 16.2 Scm. -1 .

[0067] Example 6

[0068] A method for preparing polyacrylamide-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 34 hours, an aramid nanofiber dispersion was obtained; the ratio of aramid fibers, KOH, deionized water and DMSO was 1g:1.2g:2mL:100mL. (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 a nonionic polyacrylamide aqueous solution, stir evenly at 600rpm, add polyethylene glycol PEG1500, disperse, and let it stand for 2 days. Then exchange solvent with deionized water to obtain hydrogel. The mass ratio of nonionic polyacrylamide to aramid nanofiber is 19:1; the mass ratio of polyethylene glycol to nonionic polyacrylamide is 0.14:1.

[0069] (3) The hydrogel was freeze-dried to obtain an aerogel; then the aerogel was carbonized at 960℃ for 2h under nitrogen protection, wherein the carbonization temperature rise rate was 2℃ / min. After washing and drying, a polyacrylamide-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 963m². 2 g -1 The conductivity is 13.6 Scm -1 .

[0070] Example 7

[0071] A method for preparing polyacrylamide-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 34 hours, an aramid nanofiber dispersion was obtained. The ratio of aramid fibers, KOH, deionized water and DMSO was 1g:1g:2mL:100mL. (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 a nonionic polyacrylamide aqueous solution, stir evenly at 600rpm, add polyethylene glycol PEG3000, disperse, and let it stand for 2 days. Then exchange solvent with deionized water to obtain hydrogel. The mass ratio of nonionic polyacrylamide to aramid nanofiber is 15:1; the mass ratio of polyethylene glycol to nonionic polyacrylamide is 0.2:1.

[0072] (3) The hydrogel was freeze-dried to obtain an aerogel; then the aerogel was carbonized at 970℃ for 2 hours under nitrogen protection, wherein the carbonization temperature rise rate was 4℃ / min. After washing and drying, a polyacrylamide-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 10¹⁸ m². 2 g -1 The conductivity is 15.4 Scm. -1 .

[0073] Example 8

[0074] A method for preparing polyacrylamide-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 34 hours, an aramid nanofiber dispersion was obtained. The ratio of aramid fibers, KOH, deionized water and DMSO was 1g:1.1g:2mL:100mL. (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 a nonionic polyacrylamide aqueous solution, stir evenly at 600rpm, add polyethylene glycol PEG4000, disperse, and let it stand for 1.5 days. Then exchange solvent with deionized water to obtain hydrogel. The mass ratio of nonionic polyacrylamide to aramid nanofiber is 22:1; the mass ratio of polyethylene glycol to nonionic polyacrylamide is 0.18:1.

[0075] (3) The hydrogel was freeze-dried to obtain an aerogel; then the aerogel was carbonized at 930℃ for 1.5h under nitrogen protection, wherein the carbonization temperature rise rate was 3℃ / min. After washing and drying, a polyacrylamide-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 1105 m². 2 g -1 The conductivity is 14.5 Scm -1 .

[0076] Example 9

[0077] A method for preparing polyacrylamide-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 26 hours, an aramid nanofiber dispersion was obtained. The ratio of aramid fibers, KOH, deionized water and DMSO was 1g:1.1g:2mL:100mL. (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 a nonionic polyacrylamide aqueous solution, stir evenly at 600rpm, add polyethylene glycol PEG2000, disperse, and let it stand for 2 days. Then exchange solvent with deionized water to obtain hydrogel. The mass ratio of nonionic polyacrylamide to aramid nanofiber is 21:1; the mass ratio of polyethylene glycol to nonionic polyacrylamide is 0.23:1.

[0078] (3) The hydrogel was freeze-dried to obtain an aerogel; then the aerogel was carbonized at 950℃ for 2 hours under nitrogen protection, wherein the carbonization temperature rise rate was 5℃ / min. After washing and drying, a polyacrylamide-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 13.1 Scm. -1 .

[0079] Example 10

[0080] A method for preparing polyacrylamide-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 34 hours, an aramid nanofiber dispersion was obtained. The ratio of aramid fibers, KOH, deionized water and DMSO was 1g:1g:2mL:100mL. (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 a nonionic polyacrylamide aqueous solution, stir evenly at 600rpm, add polyethylene glycol PEG2000, disperse, and let it stand for 2 days. Then exchange solvent with deionized water to obtain hydrogel. The mass ratio of nonionic polyacrylamide to aramid nanofiber is 20:1; the mass ratio of polyethylene glycol to nonionic polyacrylamide is 0.2:1.

[0081] (3) The hydrogel was freeze-dried to obtain an aerogel; then the aerogel was carbonized at 970℃ for 2 hours under nitrogen protection, wherein the carbonization temperature rise rate was 4℃ / min. After washing and drying, a polyacrylamide-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 998 m². 2 g -1 The conductivity is 13.2 Scm. -1 .

[0082] Comparative Example 1 A method for preparing polyacrylamide-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 34 hours, an aramid nanofiber dispersion was obtained. The ratio of aramid fibers, KOH, deionized water and DMSO was 1g:1g:2mL:100mL. (2) Take 10g of aramid nanofiber dispersion, filter, wash and disperse in deionized water to obtain aramid nanofiber aqueous dispersion, add the aramid nanofiber aqueous dispersion directly to nonionic polyacrylamide aqueous solution, stir evenly at 600rpm and add polyethylene glycol PEG2000, disperse and stand for 2 days, and exchange solvent with deionized water to obtain hydrogel; the mass ratio of nonionic polyacrylamide to aramid nanofiber is 20:1; the mass ratio of polyethylene glycol to nonionic polyacrylamide is 0.2:1.

[0083] (3) The hydrogel was freeze-dried to obtain an aerogel; then the aerogel was carbonized at 970℃ for 2 hours under nitrogen protection, wherein the carbonization temperature rise rate was 4℃ / min. After washing and drying, a polyacrylamide-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 652 m² / h. 2 g -1 The conductivity is 2.1 Scm. -1 .

[0084] Comparative Example 2 A method for preparing polyacrylamide-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 34 hours, an aramid nanofiber dispersion was obtained. The ratio of aramid fibers, KOH, deionized water and DMSO was 1g:1g:2mL:100mL. (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 a nonionic polyacrylamide aqueous solution, stir evenly at 600rpm, add polyethylene glycol PEG2000, disperse, and let it stand for 2 days. Then exchange solvent with deionized water to obtain hydrogel. The mass ratio of nonionic polyacrylamide to aramid nanofiber is 20:1; the mass ratio of polyethylene glycol to nonionic polyacrylamide is 0.2:1.

[0085] (3) The hydrogel was freeze-dried to obtain an aerogel; then the aerogel was carbonized at 970℃ for 2 hours under nitrogen protection, wherein the carbonization temperature rise rate was 4℃ / min. After washing and drying, a polyacrylamide-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 707 m². 2 g -1 The conductivity is 4.2 Scm. -1 .

[0086] Comparative Example 3 A method for preparing polyacrylamide-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 34 hours, an aramid nanofiber dispersion was obtained. The ratio of aramid fibers, KOH, deionized water and DMSO was 1g:1g:2mL:100mL. (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 a nonionic polyacrylamide aqueous solution, stir evenly at 600rpm, add polyethylene glycol PEG2000, disperse, and let it stand for 2 days. Then exchange solvent with deionized water to obtain hydrogel. The mass ratio of nonionic polyacrylamide to aramid nanofiber is 20:1; the mass ratio of polyethylene glycol to nonionic polyacrylamide is 0.5:1.

[0087] (3) The hydrogel was freeze-dried to obtain an aerogel; then the aerogel was carbonized at 970℃ for 2 hours under nitrogen protection, wherein the carbonization temperature rise rate was 4℃ / min. After washing and drying, a polyacrylamide-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 751 m². 2 g -1 The conductivity is 4.9 Scm -1 .

[0088] As can be seen from the above examples and comparative examples, to address the problems of complex synthesis process, low mechanical strength, poor thermal stability, and easy structural collapse and breakage after carbonization of nonionic polyacrylamide aerogel, this invention uses aramid nanofibers as a reinforcing phase, abandoning the in-situ polymerization process and directly strengthening and optimizing the skeleton of nonionic polyacrylamide aerogel. Aramid nanofibers possess high strength, high modulus, and excellent thermal stability, and can form a continuous, interconnected three-dimensional rigid support network in the nonionic polyacrylamide gel system, significantly improving the mechanical strength and structural stability of the composite system, and effectively suppressing volume shrinkage and structural collapse during solvent exchange, drying, and carbonization. Aramid nanofibers have a high carbon residue rate under high-temperature carbonization conditions, and can tightly bond with the carbon matrix formed by the pyrolysis of nonionic polyacrylamide, improving the overall carbon residue rate and structural integrity of the system, and maintaining a stable hierarchical porous structure in the carbonization product. Under the combined action of aramid nanofibers and nonionic polyacrylamide, the final carbon aerogel obtained by carbonization possesses high specific surface area, good conductivity, and excellent structural stability, which can better meet the application requirements of fuel cell electrode materials.

[0089] in, Figure 1 The image shows a scanning electron microscope (SEM) image of the carbon aerogel prepared in Example 10. As can be seen from the image, the carbon aerogel material constructs a continuous and complete three-dimensional interconnected porous framework structure. The macroscopic framework exhibits no fractures, collapses, or large-area dense agglomerations, with a regular overall morphology and no obvious cracks or delamination defects. The framework demonstrates excellent extensibility and continuity. The pores are arranged in an orderly and staggered manner, with significantly improved pore distribution uniformity and good spatial framework stability, providing a sound microstructural foundation for efficient mass transfer, stable conductivity, and long-term service of the electrode.

[0090] Figure 2 The image shows a scanning electron microscope (SEM) image of the carbon aerogel prepared in Comparative Example 1. In Comparative Example 1, compared to Example 10, the aramid nanofibers were not prepared as a primary gel but were directly added to a nonionic polyacrylamide solution. Figure 2It can be seen that the carbon skeleton structure is damaged, the pore structure is not obvious, and large, dense carbon agglomerates appear in local areas, accompanied by continuous void defects, resulting in poor skeleton integrity. This is because when aramid nanofibers are directly added, they are unevenly dispersed in the nonionic polyacrylamide solution, easily agglomerating and unable to interpenetrate and dissolve with the nonionic polyacrylamide 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, rigid fiber skeleton defects and stress collapse of local cross-linked structures during carbonization, and an unbalanced overall residual carbon distribution. The final carbon aerogel has poor mechanical stability and insufficient structural integrity, and cannot meet the core application requirements of fuel cell electrodes for long-term cycling, stable conductivity, and efficient mass transfer. Figure 3 and Figure 4 The images show scanning electron microscope (SEM) images of carbon aerogels corresponding to Comparative Example 2 and Comparative Example 3, respectively. As can be seen from the images, the pore density is uneven, and the carbon skeleton collapses in some areas, resulting in an irregular porous structure.

[0091] The specific data also show that the embodiments of the present invention first prepare aramid nanofibers into a primary hydrogel, and then blend it with nonionic polyacrylamide, which solves the problem of easy agglomeration of aramid nanofibers. Compared with Example 10, Comparative Example 1 directly adds aramid nanofibers to the nonionic polyacrylamide solution, resulting in uneven dispersion, inability to effectively build a reinforcing network, poor gel structure stability, collapse during drying and carbonization, inability to build a conductive network, and a significant reduction in specific surface area and conductivity.

[0092] Meanwhile, as can be seen from Comparative Examples 2 and 3, the carbon aerogels prepared in these examples have low electrical conductivity. Specifically, in Comparative Example 2, the prolonged standing time during the preparation of the primary gel caused the weak hydrogen bonds between the aramid nanofibers to continuously overlap, crosslink, and associate, leading to densification. The originally loose and reversible primary soft gel gradually solidified into a highly crosslinked rigid pre-gel. Subsequent stirring and shearing forces were unable to completely break up the hardened gel clumps, resulting in severe stress concentration in the formed composite hydrogel. During the carbonization process, the rigid fiber skeleton was damaged, ultimately resulting in carbon aerogels with poor mechanical stability, insufficient structural integrity, and reduced electrical conductivity. Compared with Example 10, Comparative Example 3 used too much polyethylene glycol, which caused an increase in local crosslinking density and formed local high-density crosslinking agglomerates, while the crosslinking in other areas was relatively poor. This resulted in 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 its toughness deteriorated, making it prone to brittleness during drying and carbonization. It 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.

[0093] This invention abandons the traditional direct blending process and innovatively adopts a stepwise dispersion strategy of "preliminary shaping followed by compounding." It constructs a reversible, weakly hydrogen-bonded primary gel through water induction, utilizing the spatial confinement effect to lock the monodisperse state of aramid nanofibers. The weakly hydrogen-bonded network of the primary gel only serves a spatial fixing function, lacking high-strength cross-linking constraints. It can be easily dispersed by shear force, allowing the nanofibers to regain their flow compatibility properties and achieve homogeneous molecular-scale interpenetration and compounding with the stretched polyacrylamide molecular chains. This completely solves the core problems of nanofiber aggregation, entanglement, sedimentation, and phase separation in traditional processes, ensuring the uniformity of the composite aerogel and the carbonized carbon aerogel components, laying the structural foundation for a continuous and uniform conductive network and mass transfer channels.

[0094] 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 polyacrylamide-based carbon aerogel for fuel cells, characterized in that, Includes the following steps: (1) Add aramid fibers and alkali metal hydroxide to an aprotic solvent, then add a protic solvent dropwise, and stir to disperse to obtain an aramid nanofiber dispersion; the aprotic solvent is one or more of dimethyl sulfoxide, dimethylformamide, and dimethylacetamide; the protic solvent is one or more of deionized water, methanol, and ethanol. (2) Immerse the aramid nanofiber dispersion in deionized water and let it stand for 2-4 hours. Then add the formed primary gel to a nonionic polyacrylamide aqueous solution, stir evenly, add polyethylene glycol, let it stand again, and exchange solvent with deionized water to obtain a hydrogel. The mass ratio of polyethylene glycol to nonionic polyacrylamide is (0.1-0.25):1; the mass ratio of nonionic polyacrylamide to aramid nanofiber is (15-25):1; the number average molecular weight of polyethylene glycol is 500-10000. (3) The hydrogel is freeze-dried and then carbonized at high temperature. After washing and drying, a polyacrylamide-based carbon aerogel for fuel cells is obtained. The high temperature carbonization temperature is 800-1100℃ and the high temperature carbonization time is 1-3h.

2. The method for preparing polyacrylamide-based carbon aerogel for fuel cells as described in claim 1, characterized in that, In step (2), the nonionic polyacrylamide is obtained by polymerization of acrylamide monomer through an oxidant-reducing agent initiation system.

3. The method for preparing polyacrylamide-based carbon aerogel for fuel cells as described in claim 2, characterized in that, The oxidizing agent is one or more of ammonium persulfate, potassium persulfate, potassium sulfite, azobisisobutyronitrile, and hydrogen peroxide.

4. The method for preparing polyacrylamide-based carbon aerogel for fuel cells as described in claim 2, characterized in that, The reducing agent is one or more of sodium sulfite, sodium bisulfite, and ferrous chloride.

5. The method for preparing polyacrylamide-based carbon aerogel for fuel cells as described in claim 1, characterized in that, In step (2), the mass ratio of polyethylene glycol to nonionic polyacrylamide is (0.1-0.2):

1.

6. The method for preparing polyacrylamide-based carbon aerogel for fuel cells as described in claim 1, characterized in that, The heating rate during high-temperature carbonization in step (3) is 1-10℃ / min.

7. The method for preparing polyacrylamide-based carbon aerogel for fuel cells as described in claim 1, characterized in that, In step (3), the high-temperature carbonization is carried out under the protection of an inert gas.

8. A polyacrylamide-based carbon aerogel for fuel cells, characterized in that, It is prepared by the method for preparing a polyacrylamide-based carbon aerogel for fuel cells according to any one of claims 1-7.

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