Aramid Nanofiber Aerogels: Versatile High Complexity Components for Multifunctional Composites

Abstract

Aramid nanofibers (ANFs) have emerged as versatile and readily accessible building blocks for multifunctional, complex nanostructures. A key motivation for studying and utilizing ANFs lies in their potential to directly recycle commercial macro‐aramid plastics, such as poly(p‐phenylene terephthalamide), for diverse applications. Retaining the advantageous properties of parent aramid fibers and fabrics, ANFs enable the creation of a variety of nanostructured solids, both independently and in combination with polymeric and nanoscale components. A wide spectrum of ANF‐based aerogels is developed, many of which exhibit property sets previously unattainable in other materials—properties critical for advancing sustainable development. These biomimetic composites replicating complex organization of cartilage, combine high toughness, thermal resilience, nanoscale porosity, and low density with the capability for roll‐to‐roll manufacturing. The transformative advancements in materials for energy storage, electromagnetic interference shielding, thermal management, biomedical implants, water purification, and other applications are spurred by ANF‐based aerogels and related composites. This review summarizes recent progress in the engineering, fabrication, characterization, modification, and implementation of ANF aerogels. It also highlights future research directions, potential applications, and key challenges including the development of structural descriptors for ANF solids, that must be addressed to fully realize the potential of ANF‐based technologies.

Different forms of aramid nanofibers (ANFs) and especially aerogels from them, offer a sustainable route to high‐performance biomimetic nanocomposites. Due to the cartilage‐like architecture, ANF‐based materials enable breakthroughs in energy, electromagnetic, biomedical, and water purification technologies. This review outlines key advancements, applications, and future directions for ANF‐based structures, including their engineering using graph theory capable of capturing the complex organization characteristic for a variety of high‐performance biomaterials and nanostructured gels.

1. Introduction

Low density solids characterized by a high volume fraction of nanoscale pores are typically referred to as aerogels.^{[ 1} , ^{2 ]} As emblematic complex materials, aerogels combine intricate percolation patterns with non‐random disorder,^{[ 3} , ^{4 ]} exhibiting an exceptional combination of properties. These properties arise from the large specific surface area (SSA), exceptionally high volumetric density of interfaces,^{[ 5 ]} confinement‐induced physical properties,^{[ 6} , ^{7 ]} and rapid mass/stress transport.^{[ 8 ]} Since Kistler's successful fabrication of the first silica‐based aerogels with density as low as 0.02 g cm⁻³ in 1931,^{[ 9 ]} numerous inorganic aerogels^{[ 10} , ¹¹ , ^{12 ]}—including those from metals,^{[ 13} , ¹⁴ , ^{15 ]} semiconductors,^{[ 16} , ¹⁷ , ^{18 ]} and metal oxides^{[ 19} , ²⁰ , ^{21 ]}—have been extensively investigated. However, the brittleness of these materials limits their applicability in many areas. To address this limitation, aerogels derived from carbon‐based materials, such as carbon nanotubes (CNTs), nanofibers, nanoribbons, and various forms of graphene and graphene oxide, have emerged as promising alternatives to silica aerogels. These carbon‐based aerogels offer enhanced toughness and electrical conductivity,^{[ 22} , ²³ , ²⁴ , ²⁵ , ²⁶ , ^{27 ]} broadening their potential applications. Despite these advantages, challenges remain, including multistep fabrication processes, structural reproducibility of the nanocarbons, and environmental stability, particularly in the presence of water.

Compared to traditional inorganic aerogels, aerogels from nanoscale polymeric nanofibers should offer increased toughness, a wider range of chemical compositions, lower production costs, and improved environmental robustness compared to aerogels made from ceramics, metals, or nanocarbons. With respect to aerogels made from solutions of polymeric macromolecules, they should offer increased stiffness, often‐needed nanoscale porosity, and chemical versatility. The abundant surface functional groups, combined with the high aspect ratio of polymer nanofibers, allow these aerogels to form 3D networks that integrate chemical cross‐linking and extensive physical entanglement an advantage not achievable in particle‐based gels. This complex organization, which balances order and disorder, imparts enhanced mechanical properties,^{[ 3 ]} including markedly increased toughness,^{[ 28 ]} addressing the brittleness that limits ceramic aerogels. Furthermore, the network structure of polymer aerogels introduces new opportunities for their design using graph theory (GT)^{[ 29} , ^{30 ]} and the powerful analytical tools of discrete mathematics, including network science.^{[ 31 ]} These attributes make polymer aerogels highly versatile and directly applicable across a broad spectrum of fields, such as chemical separation,^{[ 32 ]} thermal management,^{[ 33} , ^{34 ]} catalytic support,^{[ 35 ]} energy storage,^{[ 36 ]} sensing,^{[ 37 ]} chemical adsorption,^{[ 38 ]} infrared (IR) stealth,^{[ 39 ]} and biomedical implants.^{[ 40 ]}

Aramid nanofibers (ANFs), first introduced in 2011, are convenient nanoscale building block for aerogels and other materials. They have garnered significant attention in various parts of academic community and industrial engineers.^{[ 41 ]} ANFs are typically produced by the deprotonation of macroscopic poly(p‐phenylene terephthalamide) (PPTA) fibers, commonly known as Kevlar (Figure 1a), which are composed of alternating benzene rings and amide bonds.^{[ 42 ]} This conjugated structure enhances the stability of the molecular chains and facilitates the hybridization of electronic states between adjacent macromolecules, strengthening intermolecular interactions. The amide groups within ANF molecular chains form cooperatively bound bundles stabilized by labile but numerous hydrogen bonds, further improving the cohesion and stability of the nanofibers. These fibers organize into imperfect 1D crystallites, characterized by extensive branching and entanglement (Figure 1b–d). The hierarchical organization of macroscale aramid fibers that gives rise to ANFs can be compared to the structures of wood and silk, which are composed of nanoscale fibrils of cellulose^{[ 43 ]} and fibroin peptide,^{[ 44 ]} respectively. Contrary to simplified depictions of materials such as cellulose fibers, the individual rod‐like crystallites in aramid macrofibers are neither perfectly aligned nor uniformly sized. Instead, they form interwoven bundles of partially aligned, interconnected crystallites.

Figure 1.

a) Molecular structures of PPTA. b–d) AFM, TEM, and SEM images of ANFs. Reproduced with permission.^{[ 42 ]} Copyright 2017, John Wiley and Sons. The similarity in the hierarchical organization of ANFs and cellulose macroscale fibers composed of nanoscale fibrils is unmistakable.^{[ 43} , ^{44 ]} e–g) Images of ANF dispersions and aerogels obtained under UV light with λ_max = 365 nm (aka ‘black light’); e) dispersion of deprotonated ANFs in KOH/DMSO media, no fluorescence; f) ANFs and ANF hydrogels in water; g) free‐standing ANF aerogel films.

ANFs can be readily dispersed in polar organic solvents, such as dimethylsulfoxide (DMSO) (Figure 1e) with strong proton acceptors, such as KOH. The ability to form indefinitely stable viscous dispersions results from their nanoscale diameter, filament flexibility, and relatively high surface charge, which arises from the deprotonation of amide groups on the nanofiber surface. The easily accessible fluid state of ANFs enables the preparation of aerogels in multiple forms,^{[ 45 ]} including microspheres,^{[ 46 ]} nanofibers,^{[ 47 ]} films and membranes,^{[ 48 ]} monoliths,^{[ 49 ]} and even 3D printed^{[ 50 ]} or molded objects.^{[ 51 ]} The dispersibility of ANFs also facilitates scalability, and supports advanced manufacturing techniques, such as roll‐to‐roll (R2R) processing and additive manufacturing, including 3D printing, making them highly adaptable for a wide range of applications.

While the original aramid‐based bulk polymers, microfibers, and fabrics are not known for strong fluorescence, ANFs exhibit strong blue‐green emission (Figure 1f,g). This fluorescence arises from the hybridization of aromatic groups, which leads to delocalized electronic states that lower their energy and increase emissivity within the visible spectrum. The energy of the photons is indicative of the formation of excitonic states, which results from the π–π stacking of adjacent aromatic groups. The nature of these emissive states is similar to those found in some molecular rotors,^{[ 52 ]} supramolecular polymers,^{[ 53 ]} and agglomerates of aromatic molecules.^{[ 54} , ^{55 ]} However, deprotonation quenches this fluorescence (Figure 1e) due to the altered conjugation state induced by the p‐orbitals of the deprotonated amide bonds. Additionally, the charge‐induced reconfiguration disrupts the coplanarity of the benzene rings, further diminishing emissivity.

Due to their unique combination of imperfections and crystallinity, individual ANFs and their networks, i.e., aerogels, are both stiff and tough, while maintaining exceptional strength. The local extensibility provided by misaligned chains and the entanglement of nanofibers drastically improves the distribution of applied axial loads, effectively preventing stress concentration. This remarkable combination of superior mechanical properties and a porous structure makes ANF aerogels promising for multiple technologies (Figure 2 ), particularly those essential for sustainable development. Moreover, additional functionalities can be integrated into ANF aerogels by incorporating other nanoscale components, resulting in composite materials and further expanding the scope of ANF applications.^{[ 56 ]}

Figure 2.

An overview of ANF aerogels, related composites, and current applications.

The same chemical properties that enable the preparation of ANF aerogels, organogels, and hydrogels also make it possible to produce them by recycling used materials, such as bulletproof vests, braking pads, camping tents, and truck tarps, which are currently often disposed of by burning.^{[ 57 ]} The combustion of these materials releases harmful gases and particulate matter, adversely affecting air quality. Additionally, due to the thermal stability of PPTA, coupled with the abundance of aromatic rings in its chemical structure, complete degradation during combustion is challenging. This process results in the formation of persistent organic and inorganic pollutants, such as air‐born nanocarbons and NOx (NO, NO₂) which pose significant risks to ecosystems and human health. In contrast, ANF‐based materials, including composites, are fully recyclable and can be remade into new products via solution‐based processes. They can be converted back into the original nanofibers by dissolution in the same KOH solutions in DMSO. Surprisingly, the ANFs derived from waste aramid exhibit outstanding tensile strength (149.2 MPa) and toughness (10.43 MJ m⁻³), excellent thermal stability (T _d of 542 °C), a high SSA (65.2 m² g⁻¹), and significantly reduced manufacturing costs by seven times, positioning it as a promising building block for functional composites.^{[ 58 ]} Furthermore, by introducing the molecular intercalation method, the recycling time of waste aramid to ANFs can be shortened to 4 min, facilitating scale‐up recycling (1000 kg in 30 min) (Figure 3a).^{[ 59 ]} The average molecular weight of the recycled ANFs via this method was maintained at 98.9% of the original aramid, indicating negligible performance degradation (Figure 3b). In addition to the solvent‐based method, waste PPTA can be depolymerized into monomers with high conversion (96%) and purity (>99%) via straightforward microwave‐assisted alkaline hydrolysis, followed by repolymerization to induce self‐assembly into virgin‐quality ANFs, thus achieving low environmental pollution, resource recycling, and economically feasible closed‐loop chemical recycling (Figure 3c).^{[ 60 ]} Beyond its significance for sustainability, the recyclability of widely used high‐performance aramid materials drastically improves access to ANF‐based materials, including economically disadvantaged regions, while also simplifying the transition to greener technologies.

Figure 3.

a) Recycled ANF aerogels, hydrogels and films from waste PPTA and b) the inherent viscosities and the average molecular weights of the aramid raw material and the recycled ANFs. Reproduced with permission.^{[ 59 ]} Copyright 2025, American Chemical Society. c) Microwave‐assisted alkaline hydrolysis reaction of PPTA into the monomers. Reproduced with permission.^{[ 60 ]} Copyright 2025, American Chemical Society. CF, carbon fiber.

This review focuses on recent developments in the field of ANF aerogels, related composites, and their wide‐ranging applications. Following the introductory overview in Section 1, Section 2 outlines the synthetic protocols used for preparation of ANF materials. Section 3 examines their physical and chemical properties in detail. Section 4 provides a comprehensive review of ANF composites, highlighting modifications and the integration of ANFs with various materials, including ceramics, metals, nanocarbons, polymers, and nanoparticles. Section 5 delves into the diverse applications of ANF‐based materials, while Section 6 elaborates on the challenges and prospects associated with ANFs.

2. Preparation of Aramid Nanofiber Gels and Aerogels

The synthesis of ANF aerogels typically involves three key stages: i) dispersing macroscale precursors, ii) gelation, and iii) conversion to aerogels by solvent removal.

2.1. Dispersions of Aramid Nanofibers

Since the initial studies on the dispersion of ANFs in KOH/DMSO mixtures,^{[ 41 ]} numerous strategies have been developed, including bottom‐up polymerization,^{[ 61} , ^{62 ]} top‐down electrospinning,^{[ 63} , ^{64 ]} mechanical disintegration,^{[ 65 ]} and deprotonation.^{[ 42} , ^{66 ]} These methods enable the production of ANFs with nanoscale characteristics as demonstrated by the Tyndall effect (Figure 4a). A comprehensive comparison of performance characteristics of ANF‐related materials produced by these various techniques, along with their relative advantages and disadvantages, is provided in Table 1 .

Figure 4.

a) General protocol for preparing ANFs by polymerization‐induced self‐assembly. The examples of Tyndall effect, TEM, and AFM images of ANFs. Reproduced with permission.^{[ 74 ]} Copyright 2016, Royal Society of Chemistry. b) Preparation procedures of the ANF dispersions by using mPEG. Reproduced with permission.^{[ 62 ]} Copyright 2017, Elsevier. c) Schematic processes of ANFs involving copolymerization with an additional monomer. Reproduced with permission.^{[ 75 ]} Copyright 2020, John Wiley and Sons.

Table 1.

Comparison of ANFs synthetic protocols.

Method	Time [h]	Diameter of ANFs [nm]	Strength of ANF films [MPa]	Advantages	Disadvantages
Polymerization induced self‐assembly[ 62 ]	>2	20‐60	66.0	Fast Easy to commercial production	Low stability Side reactions and minor products
Electrospinning[ 67 , 68 ]	>24	275‐15000	83.56	Continuous spinning Tunable parameters	Wide diameter distribution Equipment corrosion Complex steps
Mechanical disintegration[ 65 ]	>3	10‐200	26.8	Fast Easy to commercial production	Unstable fiber diameter High energy consumption Low mechanical strength
Deprotonation[ 41 , 69 ]	168	5‐30	153	Good thermal stability High mechanical strength	Time‐consuming Low efficiency
Proton donor assisted deprotonation[ 70 ]	4	10‐12	165	High efficiency Fast Excellent mechanical strength	Difficult to be commercialized High solvent consumption
Mechanically‐assisted deprotonation[ 71 ]	0.5	1‐4	271.7	Fast Simple steps High efficiency	Equipment requirements
Microfluidic deprotonation[ 72 ]	0.12	6‐11	0.57	Fast Small and uniform diameter	Poor mechanical strength Equipment requirements

2.1.1. Polymerization‐Induced Self‐Assembly

Polymerization‐induced self‐assembly (PISA) of ANFs represents an innovative approach for the large‐scale synthesis of polymer‐based nanomaterials by integrating both polymerization and assembly processes.^{[ 73 ]} Two monomers, such as p‐phenylenediamine (PPD) and terephthaloyl chloride (TPC) are commonly used for the synthesis of aramid polymers, which subsequently assemble into nanofibers (Figure 4a).^{[ 74 ]} However, achieving uniform ANFs using conventional polymerization methods is often challenging due to the aggregation and precipitation of PPTA as it forms liquid crystalline phases. Surface modifiers, such as methoxy polyethylene glycol (mPEG), are introduced in the polymerization process, to reduce the aggregation of molecules and control the size distribution of the ANFs (Figure 4b).^{[ 62 ]} mPEG also promotes the self‐assembly of molecular chains into uniform nanofibers with diameters ranging from 20 to 60 nm, which can be easily dispersed in water and various organic solvents.^{[ 62 ]} However, the terminal hydroxyl groups of mPEG form hydrogen bonds with PPTA molecules, disrupting inter‐chain hydrogen bonds within PPTA. This interaction diminishes the mechanical performance of the resulting nanofibers. To overcome the limitations of mPEG, Tuo et al.^{[ 49 ]} replaced it with polyethylene glycol dimethyl ether (PEGDME), a less reactive dispersant. PEGDME effectively reduces competition for hydrogen bonding with PPTA, preserving the integrity of the inter‐chain interactions. Additionally, PEGDME has minimal impact on the polymer's composition and crystallinity, maintaining the structural stability and thermal properties of the nanofibers.

ANF morphology can be tuned by incorporating co‐monomers during polymerization. Tuo et al.^{[ 75 ]} also demonstrated that the introduction of a third monomer can enhance the tensile strength and elongation at the break of the nanofibers (Figure 4c). For instance, the addition of 2‐(4‐aminophenyl)‐1H‐benzimidazol‐5‐amine (APBIA)^{[ 34 ]} resulted in the formation of a heterocyclic aramid containing a benzimidazole group, which improved the thermal stability and mechanical performance of the nanofibers. Wu et al.^{[ 76 ]} further discovered that APBIA transforms the conventional linear configuration of PPTA chains into a curved, elongated, and intertwining fiber bundle. This structural modification enhances the porosity of the nanofibers and improves their optical transparency in the visible range to 94%. Although promising, PISA as a synthetic pathway to ANFs still faces several challenges. These include side reactions, difficulties in controlling the molecular weight of the polymer, and inconsistencies in nanofiber quality, all of which need to be addressed for the broader application of this technique.

2.1.2. Nanofiber Spinning

Electrospinning is a widely used technique for the fabrication of polymer fibers, wherein polymer solutions or melts are transformed into fibers through electrohydrodynamic jetting induced by high electric fields.^{[ 77 ]} This method is particularly advantageous for producing fibers with uniform diameters, customizable structures, and tunable compositions. However, compared to traditional ANF preparations, electrospun fibers typically have larger diameters, often ranging up to hundreds of nanometers. Electrospinning of aramids is typically carried out using their dispersion in strong acids, following processes similar to the original fiber extrusion methods developed by Dr. Stephanie Kwolek at Dupont. For example, Yao et al.^{[ 67 ]} fabricated ANF‐like fibrous networks via electrospinning a solution of PPTA in concentrated H₂SO₄, extruded at 85 °C to mitigate high viscosity. The resulting aramid fibers revealed a broad diameter distribution, ranging from 275 nm to 15 µm, due to the challenges in controlling the electrospinning process at high aramid concentration (15–20 wt%) and the required high voltage (electric field strength of 4 kV cm⁻¹). Moreover, the use of strong acids damages equipment, increases costs, and poses additional challenges, particularly at elevated temperatures.

To overcome these challenges, salts such as LiCl or CaCl₂ have been added to meta‐aramid dispersions to disrupt intermolecular interactions between polymer chains, facilitating their dispersibility.^{[ 64} , ⁷⁸ , ^{79 ]} However, the use of salts and non‐protic solvents often compromises the mechanical strength and chemical stability of the fibers. Yao et al.^{[ 80 ]} conducted a comprehensive study on the effects of various solvents, including N,N‐dimethylacetamide (DMAc), N,N‐dimethylformamide (DMF), N‐methylpyrrolidone (NMP), and DMSO to optimize ANFs’ performance. Among these, the LiCl/DMAc solvent system yielded the best results due to the formation of [LiDMAc]⁺ cationic complexes. In this system, the release of Cl⁻ ions disrupt hydrogen bonding within the meta‐aramid chains, allowing for new interactions between the ions and the amide hydrogen atoms. These interactions enhance solution viscosity, stabilize the spinning process, and improve the quality of the electrospun nanofibers. By adjusting the polymer concentration (9–13 wt%) and Li salt content (1–8 wt%), ANFs with diameters ranging from 100 to 500 nm were fabricated (Figure 5a). To further enhance the mechanical properties of these nanofibers, He et al.^{[ 79 ]} employed thermal stretching treatments to electrospun films. This process resulted in a significant improvement in tensile strength (approximately 50%) and modulus (≈196%). However, the high ionic content of the spinning solution can hinder the formation of a stable Taylor cone, limiting traction forces and promoting fiber aggregation into clusters, which is undesirable for many applications.

Figure 5.

a) Fabrication process of electrospun ANFs. Reproduced with permission.^{[ 80 ]} Copyright 2010, Springer Nature. b) Schematic diagram of the preparation process of blow‐spinning ANFs and ANF nanosheets and c) comparison of fiber diameter before (19 µm) and after (98 nm) spinning. Reproduced with permission.^{[ 82 ]} Copyright 2022, Springer Nature.

To address challenges such as high energy consumption and equipment corrosion, alternative methods to electrospinning have been explored. One such technique is infiltrated rotational jet spinning (iRJS), as investigated by Parker et al.^{[ 81 ]} Unlike traditional electrospinning, iRJS uses centrifugal force to solidify nanofibers through rotational jetting into a precipitation bath. This technique produces aligned nanofibers with adjustable diameters (500–1000 nm) by varying parameters such as polymer concentration (0–10 wt%) and rotational speed (4500–6500 rpm). iRJS eliminates the need for high‐pressure systems and provides more precise control over fiber morphology. Additionally, the ionic solution blow‑spinning strategy (Figure 5b) has been introduced to fabricate ANFs with average diameters of ≈100 nm. This technique employs high‐speed airflow to produce nanofibers, with control over parameters such as feed rate, airflow rate, and solution concentration (Figure 5c).^{[ 82 ]} The method minimizes instability and safety risks associated with the high conductivity of solutions in traditional electrospinning, while enabling large‐scale production of nanofibers from various polymers.

While these alternative methods offer promising advantages, challenges remain. Equipment complexity and reduced nanofiber strength during high‐pressure mechanical exfoliation often lower the performance of ANFs compared to electrospun materials. Continued optimization of processing conditions will be critical to improve the mechanical and thermal properties of nanofibers and address the limitations of these techniques.

2.1.3. Mechanical Disintegration

A brute force approach via physical delamination of the macroscopic PPTA fibers into nanoscale fibers is also possible. Macrofibers of PPTA and similar polymers are composed of thin fibrils connected by intermolecular forces (Figure 6a),^{[ 70 ]} which makes it possible for their delamination in the axial direction when subjected to mechanical stress. This method has many advantages, such as high efficiency, simplicity, and high yield, all without the need for organic solvents or highly corrosive acids. The downside of this technique is the diameters of the produced fibers are typically larger than desired, making them unsuitable for multiple applications, and strictly speaking, exceed the typical dimensions of linear segments in nanostructured aerogels. Nevertheless, it is significant to include this method in this review for two key reasons: i) to provide a comprehensive comparison of different fabrication strategies and ii) to contribute to a deeper understanding of the physics and chemistry of ANFs.

Figure 6.

a) Simplified schematics of the nanoscale structure of a macroscale PPTA fiber consisting of the aramid fibrils. Reproduced with permission.^{[ 70 ]} Copyright 2019, American Chemical Society. b) SEM images with smooth surface changes to a large number of exfoliated microfibers after fibrillation. Reproduced with permission.^{[ 83 ]} Copyright 2015, John Wiley and Sons. c) Schematic diagram of ANFs prepared by mechanical disintegration and the nanofiber sheet fabricated by filtration, and d) FE‐SEM images of supernatant (up) and precipitated (down) fraction of ANFs. Reproduced with permission.^{[ 65 ]} Copyright 2014, Royal Society of Chemistry.

For example, Yang et al.^{[ 83 ]} employed a mechanical fibrillation method to axially peel microfibers, reducing their diameter to ≈500 nm (Figure 6b). To achieve large‐scale exfoliation of nanofibers from microfibers, Ifuku et al.^{[ 65 ]} combined mechanical disintegration with alkali treatment to weaken the intermolecular interactions between the para‐aramid chains (Figure 6c). The product was a phase‐separated liquid system where ANFs were present in both the supernatant (40 wt%) and the precipitate (60 wt%). The average diameter of the ANFs in the supernatant was ≈20 nm, with a relatively narrow diameter distribution of 14–33 nm. The average diameter of ANFs in the precipitate was large, with a distribution of diameters from 10 nm to 200 nm (Figure 6d). Lei et al.^{[ 84 ]} achieved large‐scale production of ANFs by combining alkaline hydrolysis with strong shear forces at 80 °C. Obtained nanofibers were 250–350 nm in diameter after 15 cycles of mechanical disintegration, then decreased to 50–60 nm after 30 cycles of mechanical disintegration. The final diameters are comparable to those obtained from KOH/DMSO dissolution^{[ 41 ]}  and reflect the characteristic dimensions of polymer crystals within fibrils. High energy consumption and the suboptimal mechanical properties of the produced nanofibers due to the shortening of the polymer chains are some obvious drawbacks that need to be considered.

2.1.4. Deprotonation

Deprotonation is the most widely used method for the preparation of ANFs. The technique is based on the protonation‐deprotonation equilibrium explored in 1965 by Kwolek—the original inventor of para‐aramid polymers at DuPont.^{[ 85 ]} In 1990, Burch et al.^{[ 86 ]} showed that aramids can react with strong bases to form polyanions that are soluble in DMSO. At that time, none of these teams analyzed the nanoscale morphology or structure of the resulting products, in part because an adequate nanotechnology toolbox was not yet common. In 2011, Kotov, Yang, and colleagues^{[ 41 ]} characterized ANFs obtained by dissolution PPTA fibers dissolved in a KOH/DMSO solution and stirred at room temperature for 7 days. This process yielded nanofibers with diameters ranging from 3 to 30 nm and lengths of ≈10 µm, exhibiting physical properties similar to PPTA macroscale fibers.

The key advantage of the deprotonation method is its simplicity, requiring only stirring at room temperature without the need for expensive equipment or complex procedures, making it an attractive approach for nanofiber fabrication. Since 2011, deprotonation‐based methods for ANF production have advanced considerably. For example, Koo et al.^{[ 87 ]} reduced the ANF preparation time from 7 days to just 15 h by lowering the molecular weight of PPTA and using cosolvents to dilute DMSO. Further developments were made by Zhang et al.,^{[ 70 ]} who explored additional strategies to reduce preparation time and improve the uniformity of the nanofibers by employing fibrillation, ultrasonication, or chemically‐assisted deprotonation (Figure 7a). In this case, the process was shortened to just 4 h. A more dramatic reduction in preparation time was achieved by Chen et al.,^{[ 57 ]} who employed an aqueous KOH/DMSO solution, wherein the in situ formation of small KOH particles enhanced the deprotonation efficiency. This method reduced the preparation time to only 26 min (Figure 7b).

Figure 7.

a) Proton‐assisted deprotonation process to fabricate ANFs. Reproduced with permission.^{[ 70 ]} Copyright 2019, American Chemical Society. b) Illustration of the ANF preparation methods using KOH(aq)/DMSO media. Reproduced with permission.^{[ 57 ]} Copyright 2021, Royal Society of Chemistry. c) Illustration of the preparation of ultrafine ANFs through the BMAD approach. Reproduced with permission.^{[ 71 ]} Copyright 2023, Royal Society of Chemistry. d) SEM images of the aramid particle evolution during the whole microfluidic deprotonation process. Reproduced with permission.^{[ 72 ]} Copyright 2024, John Wiley and Sons. e) Image of the assembled scale‐up prototype.^{[ 88 ]}

Another notable advancement came from the development of a wet ball milling‐assisted deprotonation (BMAD) strategy (Figure 7c).^{[ 71 ]} This approach utilized zirconia balls to apply intense shear and collision forces to break down macroscopic PPTA fibers into microfibers, which accelerated the deprotonation process and refined the diameter of the resulting nanofibers. The BMAD method successfully produced ultrafine ANFs with diameters of ≈2.0 nm and a concentration of up to 1 wt% within just 30 min. Additionally, Ding et al.^{[ 72 ]} introduced a microfluidic deprotonation technique for the continuous, scalable, and efficient (7 min for 2 wt%) preparation of ANFs. This method allows for real‐time monitoring and precise control over the nanofiber diameter distribution (6–11 nm) by adjusting experimental parameters such as flow rate and temperature. As illustrated in Figure 7d, the initially dense aramid particle layer is gradually disintegrated and falls off as the microfluidic deprotonation proceeds, exfoliating into small‐diameter, high SSA nanofibers with excellent dispersibility and processing characteristics.

For a scaled‐up of roll‐to‐roll production of ANF films using the deprotonation method, Tung et al.^{[ 88 ]} developed a synthetic protocol with continuous sol–gel film production (Figure 7e). The latter is composed of an injection system, die, conveyor belt, water bath, and hot air‐drying section. A pre‐made ANFs/DMSO dispersion is initially fed onto a Teflon‐coated conveyor belt via a syringe pump at a flow rate that forms a smooth ANFs/DMSO sol layer. The conveyor belt then moves through a water bath for solvent exchange, during which KOH and DMSO are removed, which concomitantly induces the formation of ANF hydrogels. Finally, the continuous ANF film is obtained after drying with hot compression. To ensure complete solvent exchange and the formation of a stable film, precise control of the syringe pump and conveyor belt speeds is essential to provide adequate residence time.

The high viscosity of ANF dispersions, compatibility with other solvents, and highly corrosive nature of KOH/DMSO media present practical challenges, albeit manageable. Although the cost of PPTA raw materials can be minimal due to recycling old bulletproof vests, the cost of DMSO solvent remains an issue that needs to be addressed for industrial production.

2.2. Molding and Gelation of ANFs

ANF dispersions exhibit viscoelastic‐dominated rheological behavior, with the storage modulus (G') exceeding the loss modulus (G'') within the shear stress range of 0.1–10 Pa.^{[ 50 ]} The high viscosity of ANF dispersions (22.3 Pa s at a shear rate of 1 s⁻¹) is due to entanglement of nanofibers in the liquid state. This property enables a variety of processing techniques—such as wet‐spinning, blade coating, mold casting, and 3D printing—that can be employed to yield diverse morphologies, including 0D spheres, 1D fibers, 2D films, and 3D bulks.^{[ 45 ]} This section aims to summarize how different molding techniques influence the structure of ANF‐based aerogels and to explore the relationship between their structure and performance.

2.2.1. ANF Aerogel Microspheres

ANF aerogels can be fabricated into microspheres through a wet spinning process, which has emerged as an effective method for producing complex particles with intricate internal architectures. Specifically, droplets of ANF dispersions were introduced into a coagulation bath, where the solvent composition (water and tertiary butyl alcohol, TBA) is controlled to maintain their colloidal stability (Figure 8a).^{[ 46 ]} When ANF/DMSO droplets are immersed in water, the ANFs, forming a dense outer shell layer that helps preserve the spherical shape. As the process continues, the solvent progressively permeates the droplets, replacing the DMSO and promoting the intertwining of the fibers at cross‐link sites, ultimately leading to the formation of a 3D network. This results in the creation of microspheres with a distinct core‐shell structure after thermally induced crosslinking (Figure 8b).^{[ 46 ]} More importantly, the ANF microspheres retain the characteristic porous architecture of traditional aerogels, and simultaneously their dense outer shell imparts enhanced mechanical strength and functional properties, which make them particularly suitable for applications such as adsorption^{[ 46 ]} and electromagnetic waves (EMWs) absorption.^{[ 89} , ^{90 ]}

Figure 8.

a) Fabrication of ANF aerogel spheres and b) schematic illustration of crosslinking ANF spheres. Reproduced with permission.^{[ 46 ]} Copyright 2024, Elsevier. c) The feasibility of scaling up the preparation of ANF spheres, d) SEM images of ANF microspheres with different sizes, and e) surface, cross‐section, and inner SEM images of ANF microspheres. Reproduced with permission.^{[ 89 ]} Copyright 2024, John Wiley and Sons.

In a study by Shao et al.,^{[ 89 ]} a combination of wet spinning and carbonization was utilized for processing ANF aerogels. By adjusting the concentration of the ANF solutions (4.0–8.0 mg mL⁻¹) and the composition of the coagulation solvent bath, the researchers facilitated the self‐assembly of ANFs into core‐shell particles, achieving scalable production of ANF spheres (Figure 8c). The diameter of these microspheres could be controlled from 1 mm to 2.8 mm by varying the nozzle diameter, and they maintained a diameter range of 0.8–2.3 mm even after carbonization, demonstrating minimal shrinkage (Figure 8d). More importantly, the microspheres exhibited a uniformly spherical morphology with a dense, wrinkled surfaces and highly porous, nanofibrous interiors (Figure 8e). The dense outer shell enhanced mechanical strength, while the hyperbranched internal structure improved porosity and surface area. Shao et al.^{[ 90 ]} found that the high porosity and large SSA of these microspheres optimized impedance matching while promoting multiple reflections and scattering of EMWs. The increased number of heterogeneous interfaces within the material further contributed to enhanced energy loss mechanisms, such as interface polarization and dipole polarization, which improved the material's EMWs absorption performance. These findings demonstrated the versatility of ANF aerogel microspheres, which combine mechanical strength, high surface area, and advanced energy absorption properties suitable for a wide range of functional applications.

2.2.2. ANF Aerogel Fibers

The complex nanostructured morphology of the ANF‐based spheres shown in Figure 8 can be transformed into macroscale fibers due to the spinnability of viscous ANF aerogels. This process produces “wet” fibers, which can be solidified in a coagulation bath and subsequently dried to form aerogels with a fibrous morphology.^{[ 91 ]} The distinct characteristics of complex nanostructured architecture typical of aerogels expand the potential applications of nanofiber‐based aerogels, particularly in fields that require the ability to conform to intricate shapes and surfaces, such as wearable devices, thermal insulation, and sensor technologies.^{[ 92 ]}

Currently, most methods for preparing nanogel fibers utilize wet spinning processes (Figure 9a) where a dispersion of ANFs (0‐2 wt%) is extruded into a coagulation bath using a syringe.^{[ 91 ]} The externally applied drawing force helps prevent the stacking and adhesion of the dispersion stream and retains the fibrous form of the ANFs. Due to variations in the rate of solvent replacement between the fiber's fine stream, a dense surface layer forms on the fiber's outer surface, ensuring high strength, while a porous internal structure develops, similar to the formation mechanism of 0D microspheres (Figure 9a).^{[ 91 ]} For example, Liu et al.^{[ 91 ]} prepared aerogel fibers using wet spinning on a large scale (Figure 9b). These fibers had a diameter of 300 µm and exhibited excellent formability, capable of being twisted, woven, and processed into various shapes. The high porosity of these fibers ensures outstanding thermal insulation over long‐term use, providing innovative thermal protection for wearable materials. Based on this, Bao et al.^{[ 93 ]} enhanced the functionality of these fibers by introducing phase change materials (PCMs) and waterproof coatings, imparting them with temperature control, shape memory, and self‐cleaning functions, demonstrating their great potential in dynamic response applications. Coaxial wet spinning technology also afforded the fabrication of aerogel fibers with a core‐shell structure (Figure 9c,d),^{[ 94 ]} in which a conductive core is wrapped by a durable aerogel fiber shell. This core–shell structure improves the mechanical properties of the fibers and enhances their resistance to chemical corrosion, extreme temperatures, and bending fatigue, making them highly promising for applications in advanced sensing technologies.

Figure 9.

a) Schematic of the wet spinning process and the SEM image of fabricated ANF aerogel fibers and b) scale‐up production of ANF aerogel fibers by wet spinning. Reproduced with permission.^{[ 91 ]} Copyright 2019, American Chemical Society. c) Schematic of the coaxial wet‐spinning and d) the core‐shell structure of ANF‐based aerogel fibers. Reproduced with permission.^{[ 94 ]} Copyright 2023, Springer Nature. e) Schematic of the liquid crystal spinning and f) in situ polarized optical microscopy images of ANF aerogel fibers at varying concentrations. Reproduced with permission.^{[ 95 ]} Copyright 2022, American Chemical Society. g) Schematic of sol‐gel centrifugal spinning and h) the resulting aerogel fibers. Reproduced with permission.^{[ 96 ]} Copyright 2024, John Wiley and Sons.

One would typically expect the viscosity of the ANF dispersions to increase as the concentration of the solid component in the gel increases. Liu et al.^{[ 95 ]} found that when the concentration of nanofibers reaches a critical threshold (8–10 wt%), the ANF dispersion begins to orient and transition into a liquid crystalline state, resulting in a sharp decrease in viscosity at high shear rates. At this stage, the liquid crystalline ANFs can be spun more easily, simplifying the spinning process due to their reduced viscosity (Figure 9e). Under low stretching ratios, a high orientation degree of the nanostructural units can be achieved. Notably, aerogel fibers composed of highly concentrated (10 wt%) liquid crystalline ANFs exhibit enhanced brightness under polarized light, indicating their potential for applications in information encryption (Figure 9f).

The traditional wet spinning techniques have relatively low spinning rates of about 0.5 mm min⁻¹, resulting in low production efficiency and making large‐scale preparation difficult. The key to increasing the yield of ANF aerogel fibers lies in improving the spinning speed. To address this, Wen et al.^{[ 96 ]} introduced an innovative sol‐gel centrifugal spinning method and designed the corresponding apparatus, as shown in Figure 9g. The setup consists of a custom cylindrical spinneret and a rotating coagulation bath. The spinneret speed is adjustable within the range of 200–3000 rpm, while the rotating coagulation bath facilitates the sol‐gel transition and the collection of gel fibers. The collected gel fibers are then subjected to solvent exchange and freeze‐drying, ultimately yielding aerogel fibers (Figure 9h). Unlike traditional wet spinning, this innovative method achieves a significantly higher spinning rate—up to 700 m min⁻¹—resulting in a speed increase of ≈140 000 times and enabling more efficient and scalable production. More importantly, the obtained ANF aerogel produced by this process also exhibits SSA of 313 m² g⁻¹ and a tensile strength of 12.48 MPa.

2.2.3. ANF Aerogel Films

The fabrication of ANF‐based films and membranes with aerogel morphology typically involves the deposition of ANF dispersions onto a mold, followed by a solvent exchange process to facilitate aerogel formation. The diffusion‐controlled permeation of a solvent into the ANF dispersions often leads to the layered architecture of films and membranes that is comparable to the core‐shell architecture of spheres (Figure 8) and fibers (Figure 9). Common techniques for processing ANF dispersions into aerogel films and membranes include doctor blading (Figure 10a), vacuum‐assisted filtration (Figure 10b), and spin coating to ensure uniformity, stability, and a smooth surface morphology across the material.^{[ 97} , ⁹⁸ , ⁹⁹ , ^{100 ]} As expected, these 2D aerogels are characterized by low density, high SSA, and high mechanical properties. Compared to ceramic, metallic, and other inorganic aerogels, the utilization of nanofibers as building blocks eliminates their brittleness, and drastically enhances their toughness and resistance to bending, which consequently broadens their applications in nearly all technological fields as exemplified by water purification,^{[ 101 ]} battery separators,^{[ 102} , ^{103 ]} and electromagnetic management.^{[ 56 ]}

Figure 10.

a) Continuous fabrication of the ANF aerogel membranes by doctor blading combined with roll‐to‐roll processing. Reproduced with permission.^{[ 97 ]} Copyright 2022, American Chemical Society. b) Fabrication of the ANF aerogel films by vacuum‐assisted filtration. Reproduced with permission.^{[ 98 ]} Copyright 2024, John Wiley and Sons.

Gan et al.^{[ 101 ]} utilized discarded PPTA fibers as a precursor to produce highly porous aerogel films. These films exhibited superhydrophilicity and underwater superoleophobicity, which improved their efficiency in separating surfactant‐stabilized oil‐water emulsions. Hu et al.^{[ 97 ]} constructed an asymmetric ANF aerogel film with a dense skin layer and a high‐porous nanofibrous body using a proton donor‐regulated self‐assembly process (Figure 10a). The resulting aerogel film exhibited excellent overall performance, including low thermal conductivity (0.031 W m⁻¹ K⁻¹), low density (19.2 mg cm⁻³), high porosity (99.53%), high tensile strength (11.8 MPa), high heat resistance (>500 °C), and high flame retardancy. These properties make it highly suitable for use as thermal insulation materials. Tung et al.^{[ 102} , ^{103 ]} and Wang et al.^{[ 30 ]} incorporated a polymer electrolyte into an ANF aerogel to prepare a battery separator in which the ANFs serve as a skeleton to enhance the overall structure, preventing dendrite penetration. Meanwhile, the polymer electrolyte provides ion transport channels to ensure rapid ion transport. The strong interactions between the components ensure consistent contact throughout the charge–discharge cycles, enhancing the long‐cycle reliability of batteries even in a deformable state. Furthermore, the surface of ANF films is rich in functional groups, which makes them highly amenable to surface modification by attaching additional functional groups or molecules. Xu et al.^{[ 98 ]} created a Janus‐layered ANFs/MXene aerogel film via a vacuum‐assisted filtration (Figure 10b), the Janus architecture comprising interconnected arch‐shaped substructures combined with MXene provides the aerogel films with anisotropic electromagnetic interference (EMI) capabilities coupled with robust structural and performance stability. Other conductive materials, such as metal nanoparticles, carbon‐based nanomaterials, and PCMs were integrated enabling a wide array of applications, particularly in EMI shielding, energy storage, thermal management, and sensor technologies.^{[ 56} , ⁹⁹ , ¹⁰⁴ , ¹⁰⁵ , ^{106 ]}

2.2.4. ANF Aerogels

ANF aerogels are characterized by a cartilage‐like biomimetic network structure, consisting of entangled nanofibers. This architecture forms a continuous, open‐pored network with extremely low density and an exceptionally high SSA. As materials in which a gas serves as the dispersed phase and a solid matrix functions as the continuous phase, aerogels typically exhibit very low thermal conductivity, outstanding absorption properties, and good electrical insulation. The fabrication of ANF aerogels generally relies on techniques such as mold casting^{[ 51 ]} and 3D printing.^{[ 50} , ¹⁰⁷ , ^{108 ]}

Mold casting is a straightforward approach for producing macroscale structures from ANF aerogels (Figure 11a).^{[ 51 ]} This method involves pouring an ANF dispersion into a mold, followed by solvent exchange and freeze‐drying to remove the solvent and form the aerogel structure. The advantages of mold casting are its simplicity and versatility, allowing for the customization of aerogel shapes for various applications (Figure 11b). However, its scalability is limited by the need for multiple molds to accommodate different shapes, which may hinder large‐scale production.^{[ 109} , ^{110 ]}

Figure 11.

Fabrication of the 3D ANF aerogel monoliths by a,b) mold casting. Reproduced with permission.^{[ 51 ]} Copyright 2021, Elsevier. Reproduced with permission.^{[ 109 ]} Copyright 2022, American Chemical Society. Reproduced with permission.^{[ 110 ]} Copyright 2023, John Wiley and Sons. c,d) 3D printing. Reproduced with permission.^{[ 107 ]} Copyright 2022, American Chemical Society.

3D printing has emerged as an advanced technique for fabricating ANF aerogels,^{[ 107 ]} drawing significant interest due to its precision and design flexibility. This method involves the layer‐by‐layer (LBL) deposition^{[ 5 ]} of an ANF dispersion followed by post‐processing steps to stabilize and solidify the printed structure. The primary advantage of 3D printing is that it enables the rapid production of aerogels with customized complex geometries (Figure 11c,d).^{[ 107 ]} As a result, this technique is increasingly utilized across various industries, particularly where precise control over the shape and structure of the aerogel is crucial.

2.3. Gel Drying

The transformation of ANF “wet” gels into aerogels requires the replacement of solvent with air to achieve the desired aerogel properties, such as high nanoscale porosity and low density. However, the drying process is more complex than it appears and can present significant challenges. While not as pronounced as for inorganic aerogels, drying the 3D network of polymer nanofibers can lead to shrinkage and cracking. These issues are attributed to capillary forces and the rapid evaporation of the solvent.^{[ 111 ]} Factors such as temperature, pressure, humidity, and choice of solvent during ANF gelation all influence the final morphology and properties of the aerogels. This section evaluates different drying techniques designed to mitigate these challenges and preserve the aerogel's intended structural and functional characteristics.

2.3.1. Ambient Drying

Spontaneous solvent evaporation at room temperature and atmospheric pressure is a widely used method for producing aerogels due to its simplicity. As mentioned above, the capillary forces generated during solvent evaporation may cause structural damage, particularly when high‐surface‐tension solvents like water are used in ANF hydrogel fabrication.^{[ 112 ]} Moreover, ambient drying is highly sensitive to environmental temperature and humidity, which can limit control over the drying process.

To improve drying efficiency and mitigate aerogel shrinkage, non‐aqueous solvents with lower surface tension have been explored.^{[ 113 ]} These solvents can reduce the negative capillary effects associated with high‐surface‐tension solvents and minimize structural distortion. For example, Liu et al.^{[ 112 ]} replaced water with ethanol (EtOH) and isopropanol (IPA)—both of which have lower surface tensions—in ANF hydrogels (Figure 12a). This substitution effectively reduced the plasticizing effect of solvent evaporation on the ANF framework and helped preserve the integrity of the aerogel's porous structure. Wu et al.^{[ 110 ]} discovered that an ethanol/acetic acid mixture at 90 °C resulted in minimal volumetric shrinkage, comparable to that observed in freeze‐drying (Figure 12b).

Figure 12.

a) Schematic illustration of the fabrication process for ANF aerogels by ambient drying. Reproduced with permission.^{[ 112 ]} Copyright 2023, American Chemical Society. b) Optical images of ANF aerogels by using different coagulation baths and drying method with SEM images of fracture areas. Reproduced with permission.^{[ 110 ]} Copyright 2023, John Wiley and Sons. c) Preparation of ANF aerogels by modified ambient drying. Reproduced with permission.^{[ 114 ]} Copyright 2021, American Chemical Society.

Xie et al.^{[ 114 ]} employed a pre‐freezing method at −18 °C to induce cross‐linking within the ANFs and stabilize their 3D structure. These pre‐frozen hydrogels were then dried under ambient conditions, producing aerogels with well‐maintained structures (Figure 12c). In another study, Zhang et al.^{[ 115 ]} utilized a post‐freezing heating method, which resulted in only 13% shrinkage—slightly less than the 13.9% shrinkage observed with traditional freeze‐drying techniques. The obtained aerogels were tough and flexible, allowing for compression, bending, and twisting without compromising structural integrity.

2.3.2. Supercritical Drying

Supercritical drying is a widely used technique for aerogel production, as it effectively preserves the material's architecture during solvent removal. When the temperature and pressure of a solvent exceed its critical point, its liquid and gas phases become indistinguishable,^{[ 116 ]} resulting in a drastic reduction in surface tension to zero. The rupture and deformation of the nanoscale structure of gels driven by surface tension effects during conventional drying is therefore prevented. Additionally, the enhanced diffusivity of supercritical solvents facilitates the removal of residual liquid from the aerogel, minimizing potential structural damage and ensuring the preservation of the aerogel's delicate structure.^{[ 117 ]}

For example, Li et al.^{[ 45 ]} demonstrated that ANF aerogels processed with supercritical CO₂ exhibited a density of 33 mg cm⁻³ and a SSA of 360 m² g⁻¹, outperforming aerogels dried by other methods. Similarly, Cheng et al.^{[ 50 ]} reported that ANF aerogels treated with supercritical drying exhibited superior SSA (350 m² g⁻¹) and pore volume (0.959 cm³ g⁻¹) compared to those prepared by freeze‐drying, which showed values of 215 m² g⁻¹ and 0.549 cm³ g⁻¹, respectively. Moreover, supercritical drying has been shown to effectively maintain the original structure of aerogels, as evidenced by Cheng's^{[ 107 ]} work on 3D‐printed ANF hydrogels, which retained their shape and dimensions almost identically after drying. Although crucial for maintaining microstructure and performance stability, this process has high operational costs, complexity, and inherent safety risks, as it requires maintaining the solvent under extreme pressure and temperature conditions.

2.3.3. Freeze‐Drying

Freeze‐drying is a commonly adopted technique for solvent removal in hydrogels, valued for its efficiency, environmental sustainability, and ability to produce composite aerogels, all while preserving the internal architecture of the material.^{[ 118 ]} This process relies on sublimation, a two‐step procedure that begins with rapid freezing to form ice crystals, followed by the sublimation of these crystals under vacuum conditions, ultimately producing an aerogel with a stable, porous structure.^{[ 119 ]} Similar to supercritical drying, sublimation minimizes capillary forces, mitigating the risk of structural collapse and thus preserving the integrity of the porous network. In the preparation of aerogels from ANF precursor dispersions, freezing is typically initiated by immersing the sample in liquid nitrogen or exposing it to other cryogenic conditions. Then, ice crystals are sublimated under vacuum to form the final aerogel. It should be noted that the freezing temperature and rate are critical factors in determining the structural characteristics of the resulting aerogels. For instance, moderate freezing temperatures (e.g., −18 °C) promote slower ice crystal formation, leading to larger crystals and consequently, aerogels with larger pores. Conversely, extremely low temperatures such as those achieved with liquid nitrogen (−196 °C) induce rapid freezing, resulting in smaller, more numerous ice crystals, which in turn yield aerogels with finer pores and a higher SSA.^{[ 114} , ^{115 ]}

In addition to conventional freeze‐drying, directional freezing has provided new opportunities to tailor the properties of aerogels by controlling the orientation of ice crystal growth.^{[ 120 ]} This approach allows precise modulation of key attributes such as mechanical strength, thermal conductivity, and anisotropy. During directional freezing, a temperature gradient is applied to direct the growth of ice crystals in a specific direction prior to sublimation. For example, Du et al.^{[ 121 ]} employed unidirectional freezing to create aerogels with a hexagonal honeycomb structure and aligned channels, which imparted anisotropic properties (Figure 13a–c). Xu et al.^{[ 122 ]} utilized bidirectional freezing with both horizontal and vertical temperature gradients applied to a wedge‐shaped resin, allowing them to control the direction of ice crystal growth (Figure 13d). At a wedge angle of 15°, they achieved a long‐range ordered porous structure, enhancing the mechanical strength of the aerogels. Obviously, this versatile and adaptable directional freeze‐drying technique offers a powerful tool for tailoring aerogel structures, with significant potential for applications in energy storage,^{[ 123 ]} tissue engineering,^{[ 124 ]} and thermal switches.^{[ 125 ]}

Figure 13.

a) Preparation of ANF‐based aerogels by unidirectional freezing and b,c) SEM images of surface and cross section of unidirectional ANF aerogels. Reproduced with permission.^{[ 121 ]} Copyright 2022, Royal Society of Chemistry. d) Fabrication of PI/ANF aerogels by bidirectional freezing. Reproduced with permission.^{[ 122 ]} Copyright 2020, Elsevier.

3. Properties of ANF Aerogels

The original ANF aerogels offer notable advantages across a range of characteristics, including low density, high porosity, large SSA, and efficient thermal insulation.^{[ 49 ]} These properties are largely governed by a variety of processing factors, such as the concentration of ANF dispersions, drying techniques, drying temperatures, and other related parameters.^{[ 114 ]} This section provides a comprehensive review of how processing methods influence the structural properties of ANF aerogels and how these structural variations, in turn, affect their overall performance. A summary of the properties is presented in Table 2 .

Table 2.

Materials properties of ANF aerogels.

Materials	Shape	Content	Density [mg cm−3]	Porosity [%]	Specific surface area [m2 g−1]	Drying	Stress (strain)	Thermal conductivity [mW m−1 K−1]	Refs.
ANF	3D	0.5 wt%	4.7	99.75	300.5	FD	C 17.5 kPa (80%)	29	[125]
ANF	3D	0.2 mg mL−1	302	79	≈100	AD	T 22.2 MPa (10%)	/	[112]
ANF	3D	55 wt%	8.1	99.4	542	SD	C 825 kPa (80%)	26	[132]
ANF	3D	2 wt%	36.5	/	230.2	SD	/	27.9	[107]
ANF	3D	0.10%	25±2	98.2±0.1	62.88±0.50	FD	C 165 ± 5 kPa (70%)	37.2 ± 0.4	[49]
ANF	3D	0.1 wt%	1.42	90	/	FD (‐30°C)	C 29 kPa (90%)	23.7	[84]
ANF	3D	2 wt%	61	95.7	0.7	AD	C 2.76 MPa (90%)	15.8	[110]
ANF	3D	0.5 wt% 0.8 wt% 1.0 wt%	/	/	134.32 126.95 118.34	FD (‐35°C)	/	31.37 31.46 32.52	[140]
ANF	3D	0.5 wt% 1.0 wt% 2.0 wt%	33 43 100	97.7 97.0 93.0	360 389 404	SD	/	27 31 50	[45]
ANF	3D	1% 2% 3% 4% 6% 8% 3% 3% 0.7%	40±2 56±3 73±3 90±3 136±4 185±5 87±3 62±3 20±2	97.2 96.1 94.9 93.8 90.6 87.2 94.0 95.7 98.6	11.8 10.9 9.8 7.9 5.7 4.3 8.7 10.3 12.0	AD (50°C) AD (50°C) AD (50°C) AD (50°C) AD (50°C) AD (50°C) AD (20°C) AD (100°C) AD (150°C)	/ / / / / C 5.1 MPa (70%) / / /	38.5 41.8 45.5 54.3 63.9 71.4 / / 33.9	[114]
ANF	2D	25 mg mL−1	19.2	99.53	344	FD	T 11.8 MPa (15%)	31	[97]
ANF	2D	25 mg mL−1	/	98.5	/	FD (‐50°C)	/	/	[101]
Cabonized ANF	2D	15 mg mL−1	54.4	96.8	173.03	FD	/	/	[126]
ANF	2D	5.2 wt %	62	95.6	137	FD	T 0.7 MPa (2.5%)	39	[34]
ANF	1D	5 wt%	114	> 92	239	FD (‐80 °C)	T 8.1 MPa (19.2%)	34	[141]
ANF	1D	4 wt%	52	96.4	142.85	FD	T 1.05 MPa (8%)	37	[142]
ANF	1D	25 mg mL−1	/	>96	286.8	FD	T 83.2 MPa (31.85%)	32	[143]
ANF	1D	2 wt%	23	98	240	FD (‐50°C)	/	36	[91]
Liquid crystal ANF	1D	/	/	/	204	FD	/	37	[95]
ANF	1D	2 wt%	/	/	180	FD	T 58.7±3.9 MPa	35	[144]
ANF	1D 2D 3D	2 wt%	26.7 / /	/	261 267 269	FD	T 2.5 MPa (28.2%) T 0.57 MPa (15.9%) /	/	[72]

AD: Ambient drying; SD: Supercritical drying; FD: Freeze drying; C: Compressive; T: Tensile; /: not mentioned in the original text.

3.1. Density and Porosity

The density (ρ₀) of lightweight, porous ANF aerogels, typically measured as mass (m, mg) per unit volume (v, cm³), usually falls within ranges from 1 to 100 mg cm⁻³. The porosity (P) of the aerogel is calculated using the following equation, where ρ denotes the density of aramid fiber (1.44 g cm⁻³).^{[ 34 ]}

$$\mathrm{P}\left(\%\right)=\left(1-\frac{{\rho }_0}{\rho }\right)\times 100$$ (1)

During the drying process, the removal of solvent induces the formation of numerous voids within the aerogel matrix, which is a key factor contributing to its low density and high porosity. Generally, an increase in precursor concentration leads to a higher density, as the formation of a more compact network reduces the available space for pore formation. For instance, when the ANF content was increased from 1 wt% to 4 wt%, the aerogel density rose from 40 mg cm⁻³ to 90 mg cm⁻³.^{[ 114 ]} The reduction of the overall porosity is due to the increased precursor content that occupied the available pore spaces in the aerogel. This inverse relationship between density and porosity is clearly reflected in Equation (1).

In addition to precursor concentration, other processing techniques such as carbonization also can influence the density and porosity of ANF aerogels.^{[ 126 ]} Carbonization involves both weight loss and volume shrinkage, which collectively alters the structural properties of the aerogel. During this process, gas release contributes to an increase in porosity, while the final density change depends on whether weight loss or volume shrinkage dominates. Specifically, if weight loss is more significant, the material's density will decrease, whereas if shrinkage prevails, the density may increase.^{[ 126 ]}

The rate and method of drying also play critical roles in determining the final porosity of the aerogel. Liu et al.^{[ 112 ]} observed that at lower drying rates, porosity increases with the drying rate, reaching a peak value of 79%. However, when the drying rate is excessively high, porosity decreases. This reduction in porosity at higher drying rates is likely due to the non‐uniform distribution of stress during rapid solvent evaporation, which leads to deformation and collapse of the nanostructure. Additionally, different drying methods exert varying degrees of surface tension and shrinkage forces during solvent volatilization, which also impact the final structural integrity of the aerogel. As such, the choice of drying method is a critical parameter in controlling the microstructure and performance of ANF aerogels.

3.2. Specific Surface Area

The high porosity of ANF aerogels is typically associated with a large SSA, which is a critical factor underlying their broad applicability in areas such as adsorption, thermal insulation, and catalysis.^{[ 96} , ¹²⁷ , ^{128 ]} It is well‐established that the concentration of ANFs has a considerable impact on the SSA of these aerogels. As the ANF content increases, there is a general decrease in SSA, due to the higher occupation of available pore space by the ANFs.^{[ 114 ]} In addition to precursor concentration, solvents with high surface tension, such as water, tend to promote the formation of larger, more unevenly distributed pores, which often causes subsequent shrinkage of structure. In contrast, solvents with lower surface tension, such as tert‐butanol or n‐hexane, facilitate the formation of finer, more uniformly distributed pore networks, which in turn enhances the SSA.^{[ 112 ]}

Furthermore, under the same ANF content, the SSA of aerogels is closely influenced by the drying conditions. For instance, a study by Lyu et al.^{[ 39 ]} demonstrated that supercritical drying by mitigating the capillary forces induced by the liquid‐gas phase transition, helps maintain a finer and more continuous pore structure in aerogels. In contrast, freeze‐drying can lead to partial damage or enlargement of pore structures due to the formation of ice crystals and associated physical forces. The SSA of aerogels produced through supercritical drying was measured at 365.99 m² g⁻¹, higher than the 272.5 m² g⁻¹ of aerogels produced by freeze‐drying. Meanwhile, various post‐treatment processes can also be employed to improve the SSA of ANF aerogels. Fu et al.^{[ 129 ]} demonstrated that carbonization could generate a multitude of micropores and mesopores within the aerogel, resulting in a significant increase in SSA, from 223.6 m² g⁻¹ for non‐carbonized aerogels to 341.9 m² g⁻¹ for carbonized samples.

3.3. Morphology and Structure

The morphological architecture of ANF aerogels is a key factor influencing their functional performance, as their lightweight and highly porous structure provides substantial advantages across various applications.^{[ 130 ]} Typically, these aerogels feature a broad open‐cell network, with pore size and distribution being critical determinants of key properties such as surface area, adsorption capacity, and thermal conductivity.

During the sol‐gel transition process, hydrogen bonding facilitates the entanglement and interconnection of nanofibers, facilitating the formation of a 3D network, rather than a mere physical stacking of particles. As illustrated in Figure 14a, at lower ANF concentrations (e.g., 0.5 wt%), the network structure remains relatively loose with larger pore sizes. As the concentration of ANFs increases, the network becomes denser and more compact, driven by enhanced nanofiber aggregation and entanglement. However, when the concentration reaches a critical threshold, solid components begin to occupy internal pore spaces, causing a gradual collapse of the porous structure and a decrease in overall porosity.^{[ 39 ]}

Figure 14.

a) SEM images of surface and cross‐section morphology of ANF aerogels using different content of ANF dispersions: a₁ 0.5 wt%, a₂ 1.0 wt%, a₃ 1.5 wt%. Reproduced with permission.^{[ 39 ]} Copyright 2019, American Chemical Society. b) SEM images of ANF aerogels at different freezing temperatures (−20, −80, and −196 °C). Reproduced with permission.^{[ 115 ]} Copyright 2023, John Wiley and Sons. c) SEM images of ANF aerogels formed by using different exchange solvents: c₁ deionized water, c₂ DMSO, c₃ ethanol, and c₄ n‐heptane. Reproduced with permission.^{[ 132 ]} Copyright 2021, John Wiley and Sons.

The distribution of pore sizes within ANF aerogels is also influenced by the freezing temperature used during gelation. A decrease in freezing temperature leads to a reduction in pore size, as illustrated by the shift in pore dimensions from 250 ± 50 µm at −20 °C to 140 ± 30 µm at −80 °C, and further to 70 ± 20 µm at −196 °C (Figure 14b).^{[ 115 ]} This behavior is attributed to the accelerated formation of ice crystals at lower temperatures, which compels the surrounding nanofibers to coalesce more tightly, resulting in stronger pore walls. At −20 °C, the slower rate of ice crystal formation allows for the largest ice crystals to form, producing aerogels with the largest pores, thickest pore walls, and enhanced structural integrity. In contrast, as the freezing temperature decreases, faster ice crystal growth leads to smaller ice crystals and consequently smaller, more compact pores.^{[ 131 ]}

In addition to freezing conditions, the choice of drying method also exerts a significant influence on the morphological and structural properties of ANF aerogels. Liu et al.^{[ 91} , ^{95 ]} investigated the effects of different drying techniques on aerogel morphology, finding that supercritical drying produces aerogels with smooth surfaces and a highly uniform, interconnected nanonetwork. In contrast, freeze‐dried aerogels tend to exhibit larger pores and inevitable structural shrinkage, primarily due to the damage caused by ice crystal formation. The surface tension of the exchanged solvent also significantly influences the aerogel's final structure. Solvents with lower surface tensions promote the formation of smaller pores and a more uniform porous network, while solvents with higher surface tensions can induce gel collapse or lead to the formation of larger pores during solvent removal, resulting in a less uniform pore distribution. Li et al.^{[ 132 ]} further investigated the effects of solvent surface tension on the properties of ANF aerogels, showing that a decrease in surface tension correlates with an increase in both pore uniformity and density, as illustrated in Figure 14c.

The concentration of ANFs, freezing conditions, drying methods, and solvent properties all play critical roles in determining the morphological and structural properties of ANF aerogels. Each of these factors directly impact the aerogel's final performance characteristics. Therefore, careful optimization of the synthesis and processing parameters is essential to achieving the desired material properties for specific applications.

3.4. Mechanical Properties

The intrinsic properties of PPTA fibers and the 3D network structure of aerogels are essential for enhancing their mechanical performance, especially in terms of compressive strength and durability.^{[ 84 ]} The highly porous architecture of these aerogels is crucial for energy absorption and stress distribution, while the interconnected ANF network contributes to the overall structural integrity of the material.^{[ 133} , ^{134 ]} Generally, mechanical properties, such as compressive and tensile strength, generally improve with increased precursor content, as this results in enhanced stability and a denser, more cohesive structure (Table 2). However, pure ANF aerogels often do not reach optimal mechanical strength through the hydrogen bond interaction between nanofibers, which has led to the development of various strategies aimed at improving their toughness.

One effective approach to enhance the mechanical strength and toughness of ANF aerogels is densification.^{[ 129 ]} For example, compression densification has been shown to significantly increase the tensile strength of ANF aerogel films from 1.3 MPa to 8.6 MPa, while also raising their Young's modulus from 12 to 178 kPa.^{[ 135 ]} Another successful method involves coating ANF aerogel films with an aramid solution, as demonstrated by Yang et al.^{[ 34 ]} who applied a dense, continuous aramid layer to the aerogels. This layer integrates deeply with the ANF network, creating a robust interlocking interface that dramatically improves mechanical properties, resulting in a 15‐fold increase in tensile strength, a 33‐fold increase in fracture toughness, and enhanced flexibility and foldability (Figure 15a).

Figure 15.

a) Fabrication of HA/ANF aerogel composite film from PANF aerogel film. HA: heterocyclic aramid. Reproduced with permission.^{[ 34 ]} Copyright 2022, American Chemical Society. b) Preparation of ANF aerogels by acid‐assisted gelation. Reproduced with permission.^{[ 110 ]} Copyright 2023, John Wiley and Sons. c) Schematic diagram of heat‐induced crosslinking of ANF aerogels and d) relative Young's modulus values of selected aerogels with low densities. Reproduced with permission.^{[ 125 ]} Copyright 2022, John Wiley and Sons.

In addition to densification and coating, another widely used strategy for enhancing the mechanical properties of aerogels is the fabrication of anisotropic directional structures. In directional freeze‐casting, the alignment of pore walls along the axial direction creates a robust framework capable of withstanding external forces. Xie et al.^{[ 49 ]} demonstrated that ANF aerogels produced by directional freeze‐casting exhibited higher compressive strengths in both the axial and radial directions compared to those fabricated via non‐directional casting. This is attributed to the presence of an ordered, layered structure that allows the material to better absorb deformation, with energy dissipating through the continuous fracture of connection points. However, constructing directional structures only concentrates on visible microscale properties and fails to thoroughly address the inherent brittleness of aerogel skeletons.^{[ 110 ]}

Crosslinking and welding of nanofibers are highly effective and universal methods for further enhancing mechanical performance in the nanoscale.^{[ 136 ]} For instance, Shi et al.^{[ 84 ]} used closed isocyanates as chemical crosslinking agents in combination with directional freeze‐drying to align nanofibers, resulting in aerogels capable of withstanding compression strains up to 90%. Similarly, Wu et al.^{[ 110 ]} combined directional freeze‐casting with acetic acid‐assisted crosslinking, producing a durable ANF aerogel with remarkable specific tensile strength (89 MPa cm³ g⁻¹), high toughness (1.3 MJ m⁻³), and significant fracture energy (7.36 kJ m⁻²) through multi‐scale synergistic toughening (Figure 15b). He et al.^{[ 137 ]} took advantage of the abundant hydrogen bonding between ANFs and PVA to construct a highly interconnected 3D network with strong crosslinking, endowing aerogels with superior stiffness and strength while maintaining structural integrity. This improvement is attributed to the continuous fracture of nanofiber connections, coupled with their reorientation and relative sliding during deformation. Similarly, Hu et al.^{[ 125 ]} employed thermal‐induced crosslinking to form a chemically crosslinked network, enabling the aerogel to exhibit 80% super‐elasticity under compression and retain significant elasticity even after compression cycles in liquid nitrogen (Figure 15c). In comparison to conventional lightweight aerogels (e.g., CNT, graphene aerogels), ANF aerogels demonstrate superior stiffness at lower density (fewer building blocks), which is attributed to the inherent rigidity of the PPTA molecular chains, the intricate physical entanglement of the fibers, and the 3D chemical crosslinking network (Figure 15d). Additionally, vacuum‐assisted filtration has been shown to enhance the mechanical strength and modulus of ANF aerogels by creating a compact, layered structure that improves load‐bearing capacity.^{[ 98 ]}

Together, these material engineering methods have advanced the mechanical performance of ANF aerogels, particularly in terms of strength, toughness, and durability. These efforts have broadened the potential applications of ANF aerogels across various fields, including engineering and materials science.

3.5. Thermal Insulation Properties

ANF aerogels, owing to their structural characteristics inherited from the original PPTA, exhibit exceptional thermal resistance. In a nitrogen atmosphere, pure ANF aerogels demonstrate an initial decomposition temperature exceeding 500 °C.^{[ 125 ]} Even after exposure to high temperature (300 °C) for more than 30 min, the aerogels maintain their shape and layered internal structure, with only minimal changes to their overall morphology.^{[ 112 ]}

In addition to their thermal stability, ANF aerogels demonstrate remarkable thermal insulation properties, due to their unique structural features. These properties arise from three primary mechanisms.^{[ 138 ]} i) Non‐convective effect: According to Knudsen's law, when the pore diameter of an ANF aerogel is smaller than 70 nm (below the mean free path of air molecules at standard pressure), gas molecules are effectively confined within the pores. This confinement disrupts convective heat transfer, leading to a reduction in thermal conductivity. ii) Infinite thermal shielding effect: The high surface area, porous structure, and tortuous fiber network of ANF aerogels create numerous solid‐gas interfaces, which influence heat transfer during thermal radiation. At these interfaces, heat is absorbed, reflected, and re‐emitted multiple times, which reduces the amount of heat transferred by radiation and enhances thermal insulation. iii) Infinite path length effect: The multitude of nanoscale pores within ANF aerogels obstructs continuous thermal conduction, causing heat to be conducted only along the walls of the pores. An extended thermal conduction path slows down the flow of heat and decreases thermal conductivity. The thermal conductivity of aerogels displays a pronounced correlation with material density, reflecting a characteristic U‐shaped trend.^{[ 139 ]} Generally, ANF aerogels exhibiting low density and high porosity demonstrate reduced thermal conductivity, primarily attributed to the dominance of gas‐phase heat transfer mechanisms.^{[ 45} , ¹¹⁴ , ^{140 ]} Nevertheless, at very low aerogel densities, air convection becomes a significant factor. Currently, the thermal conductivity of the aerogel increases with the decrease of density.

The synergy of these three effects—interruption of convective heat transfer, reduction of radiative heat transfer, and extension of the thermal conduction path—results in significantly reduced thermal conductivity, with values approaching those of air (≈25 mW m⁻¹ K⁻¹,^{[ 84} , ¹⁰⁷ , ¹¹⁰ , ¹²⁵ , ^{132 ]} positioning ANF aerogels as highly promising materials for thermal insulation applications. The combination of the properties of ANFs and porous aerogels preserves the intrinsic thermal stability of the fibers and optimizes the aerogel's insulation properties. A detailed analysis of the thermal insulation performance of ANF aerogels will be provided in Section 5.2 .

4. Composite Fabrication from ANF Aerogels

4.1. Chemical Modification

To expand the functional scope of ANF aerogels, structural modifications are often used to enhance specific properties, such as hydrophobicity, electrical conductivity, and thermal stability.^{[ 56 ]} These modifications typically involve a range of techniques, including structural alteration,^{[ 145} , ^{146 ]} cross‐linking,^{[ 141 ]} thermal treatment,^{[ 125 ]} and evaporation deposition,^{[ 147 ]} which allow the aerogel's architecture to introduce new functionalities, and facilitates the development of emerging applications in various fields.

For obtaining hydrophobicity, one prominent modification strategy is the incorporation of hydrophobic agents, which impart both hydrophobic and lipophilic characteristics to ANF aerogels (Figure 16a,b).^{[ 50} , ^{148 ]} Cheng et al.^{[ 50 ]} achieved a superhydrophobic surface with a contact angle of approximately 149° by applying a nanometer‐thick fluorocarbon resin coating. To achieve the overall hydrophobic properties, Liu et al.^{[ 95 ]} employed a cold plasma hydrophobization technique, where hydrophobic agents, such as octamethylcyclotetrasiloxane (also known as D4), are decomposed under an electric field and reassembled within the aerogel's fibrous structure. This approach enhances hydrophobic performance and improves the aerogel's self‐cleaning capabilities (Figure 16c). Alternatively, Bao et al.^{[ 93 ]} directly grafted hydrophobic alkanes onto the molecular chains of ANF aerogels through competitive reactions, which markedly improved their hydrophobicity. Similarly, Ma et al.^{[ 147 ]} enhanced the stability of superhydrophobicity in ANF/multi‐walled carbon nanotubes (MWCNT)/Fe₃O₄ composite aerogels by grafting methyltrimethoxysilane (MTMS) onto their surfaces, maintaining this property even under acidic and alkaline conditions (Figure 16d).

Figure 16.

a) Schematic for SiO₂ coating for ANF‐based aerogels. Reproduced with permission.^{[ 148 ]} Copyright 2022, American Chemical Society. b) Constructing hydrophobic ANF aerogels. Reproduced with permission.^{[ 50 ]} Copyright 2020, Royal Society of Chemistry. c) Schematic diagram of the hydrophobic functionalization of ANF aerogel fibers via cold plasma treatment. Reproduced with permission.^{[ 95 ]} Copyright 2022, American Chemical Society. d) Schematic diagram of vapor deposition hydrophobic treatment. Reproduced with permission.^{[ 147 ]} Copyright 2022, Elsevier.

In addition to hydrophobicity, structural modifications such as carbonization can substantially improve the electrical conductivity of ANF aerogels. Zhou et al.^{[ 126 ]} demonstrated that pure ANF aerogels exhibit remarkable electrical conductivity upon carbonization in a high‐temperature inert gas environment. As the carbonization temperature increased, conductivity improved correspondingly, reaching 1029.5 S m⁻¹ at 1500 °C—surpassing the EMI shielding target of 1.0 S m⁻¹. This enhancement in conductivity also boosts the aerogels' light absorption capacity, and promotes more efficient solar‐to‐thermal energy conversion.^{[ 126 ]} Furthermore, ANF aerogels' SSA and pore volume are advantageous for generating substantial capillary forces, which enable the encapsulation of various functional fluids within the aerogel matrix. Such fluids, including shear‐stiffening gels (SSGs)^{[ 149} , ^{150 ]} and phase change materials (PCMs),^{[ 93} , ¹⁵¹ , ^{152 ]} can be integrated into the aerogel structure to facilitate a wide range of applications in temperature regulation,^{[ 91} , ¹⁰⁶ , ^{153 ]} thermal storage,^{[ 50} , ¹⁵⁴ , ¹⁵⁵ , ^{156 ]} IR stealth,^{[ 39} , ¹⁵⁷ , ^{158 ]} and thermal diodes,^{[ 135 ]} as well as other heat‐related technologies.

4.2. ANF Composites

ANF aerogels also provide simple pathways to complex composites that combine two or more distinct substances—such as metals, polymers, or inorganic compounds—to optimize overall performance through synergistic interactions between their components and achieve desired functionalities.^{[ 159} , ^{160 ]} These materials maintain the unique properties of their constituent components at the macroscopic level, while their interfaces facilitate complex, often multifunctional architectures. Such structures harness the distinct advantages of each individual element, optimizing performance and functionality. The design and fabrication of ANF composite aerogels typically follow two primary approaches: i) direct mixing of functional materials with ANF precursor solutions to form composite aerogels and ii) the deposition of functional materials onto the surface of pre‐formed ANF aerogels without altering the bulk structure. These strategies enable the tuning of material properties, enhancing their functionality for a wide range of applications.^{[ 161 ]}

4.2.1. ANF Composites with Metals or Metal Compounds

Metals and their compounds are widely recognized for their outstanding electrical, thermal, and magnetic properties. However, limitations such as high density and cost hinder their application across various industries.^{[ 162 ]} In contrast, aerogels present notable advantages such as low density and high SSA. By integrating metal‐based materials as functional additives into high‐surface‐area ANF aerogels, the inherent benefits of both components can be synergistically harnessed to enhance their multifunctionality and broaden their potential applications across a diverse range of fields.^{[ 163} , ¹⁶⁴ , ¹⁶⁵ , ^{166 ]}

Highly conductive metallic nanomaterials, such as gold nanoparticles (AuNPs) and silver nanowires (AgNWs), can efficiently disperse and prevent aggregation within the ANF aerogel matrixs, and are employed in photothermal,^{[ 159} , ^{167 ]} electrothermal,^{[ 168 ]} electromagnetic management,^{[ 105 ]} and various other applications. For instance, Shi et al.^{[ 167 ]} directly mixed a HAuCl₄ aqueous solution with an ANF dispersion, to facilitate the precise immobilization of AuNPs on the ANF surface through electrostatic interactions between Au³⁺ ions and the negatively charged ANFs, creating a photothermal layer with an approximate thickness of 250 µm. Similarly, AgNWs were introduced as a photothermal agent to improve light absorption ability,^{[ 159 ]} and its distinctive antimicrobial characteristics expand the product's application domains. To avoid entanglement between fibrous AgNWs and ANFs, a dip‐coating technique is utilized instead of a directly mixing method to ensure uniform deposition of AgNWs on the surface of the ANF aerogels,^{[ 115 ]} and create a continuous conductive network within the aerogel. In addition, the highly adaptable AgNWs/ANF aerogel can be customized into arbitrary shapes, sizes, and structures to meet the specific demands of diverse applications. Furthermore, electromagnetic metals such as iron, cobalt, nickel, and their oxides can also be integrated into an ANF aerogel matrix for applications in electromagnetic management^{[ 160} , ^{169 ]} and energy storage.^{[ 123} , ^{127 ]} Zhu et al.^{[ 109 ]} proposed a magnetite (Fe₃O₄)‐decorated ANF‐based aerogel, demonstrating excellent EMI shielding performance and fatigue resistance, while retaining over 90% of its maximum compressive strength after 1000 compression cycles. In addition, CoNi nanoparticles were incorporated into an ANF‐based aerogel by using a catalytic chemical vapor deposition and reduction process, which provides the material enhanced magnetic dissipation properties.^{[ 169 ]} More importantly, magnetic metals like nickel can facilitate the precise modulation of the aerogel architecture. By applying an external magnetic field, magnetic fillers can be oriented and aligned within the aerogel matrix, achieving an anisotropy in the composite aerogel that aligns with the magnetic field direction by up to 98%.^{[ 164 ]} This magnetic alignment strategy enhances the overall performance of the composite aerogel and opens new possibilities for the multifunctional design of the material.

NiCoO₂, as a pseudocapacitive material, has been widely used in the fabrication of supercapacitors due to its superior theoretical specific capacitance and electrical conductivity. Gong et al.^{[ 123 ]} grew NiCoO₂ nanosheets on the surface of an ANF‐based aerogel by co‐deposition and annealing techniques to form a robust and stable NiCoO₂ shell. This multi‐shell structure enhanced the electrode's specific capacitance, cycling stability, conductivity, and ion diffusion efficiency. Moreover, iron single‐atom catalysts, known for their efficient oxygen redox reaction activity, unique electronic structures, and coordination environments, have been incorporated into ANF‐based aerogels to serve as catalysts for Zn–air batteries.^{[ 127 ]} Shen et al.^{[ 128 ]} chose low boiling point Cd as a porogen, and utilized the Fe atoms generated from the decomposition of ferrocene at high temperatures to interact with the N atoms in the ANF matrix. Through a chemical vapor deposition strategy, this process resulted in the formation of highly dispersed and stable single‐atom Fe‐N₄ sites. Additionally, temperature‐sensitive metal materials such as VO₂ ^{[ 170 ]} and Fe₃O₄ ^{[ 171 ]} can be integrated into ANF‐based aerogels to achieve dynamic responses under temperature change.

Beyond energy‐related applications, metal compounds also exhibit remarkable efficacy in adsorption processes. Zhao et al.^{[ 172 ]} employed a strategy of in situ polymerization and redox reactions to load MnO₂ onto 3D ANF aerogels. The high adsorption capacity of MnO₂ for heavy metal contaminants improved the adsorption of Pb²⁺, while the increase in active sites within the composite aerogels enhanced the overall adsorption efficiency (Figure 17a,b). Zhang et al.^{[ 173 ]} also enhanced Pb²⁺ adsorption by developing ANF/WS₂ composite aerogels. Metal‐organic frameworks (MOFs), which are known for their high surface area and porous structures, can also be integrated into ANF aerogels to prevent MOF aggregation. This integration improves the composite aerogels' porosity, tunable pore sizes, and chemical stability, making MOF/ANF composites ideal for applications such as gas separation,^{[ 174 ]} energy management,^{[ 165 ]} and adsorption.^{[ 175} , ¹⁷⁶ , ¹⁷⁷ , ^{178 ]} Zhao et al.^{[ 179 ]} used ANF aerogels as a mechanical scaffold for the in situ assembly of zeolitic imidazolate framework‐67 (ZIF‐67) on the surface of the aerogels, to form a uniform distribution of ZIF‐67 crystals and enhanced adsorption performance. In a similar approach, Zhao et al.^{[ 180 ]} immersed ANF aerogels in a Cu(NO₃)₂ ethanol solution, followed by a reaction with 1,3,5‐benzenetricarboxylate (H₃BTC), to prepare composite aerogels (HKUST/ANF) that enhanced CO₂ transport and interaction with active sites. These composites demonstrated remarkable CO₂ adsorption capacity and selectivity (Figure 17c).

Figure 17.

a) Fabrication process of ANF/MnO₂ composite aerogel and b) SEM images of ANF/MnO₂ composite aerogel with different magnifications. PPy: Polypyrrole. Reproduced with permission.^{[ 172 ]} Copyright 2021, Elsevier. c) Fabrication process of ANF/MOF composite aerogel. Reproduced with permission.^{[ 180 ]} Copyright 2023, Elsevier.

Therefore, the integration of metal‐based materials, including nanoparticles, magnetic compounds, and MOFs, into ANF aerogels represents a highly promising strategy for the development of advanced composite materials with enhanced performance across a broad spectrum of applications. By leveraging the complementary properties of both ANF structures and metal‐based additives, such composite aerogels offer significant improvements in functionality, ranging from environmental remediation to energy storage and management.

4.2.2. ANF Composites with Carbon‐Based Nanomaterials

Fullerenes, CNTs, graphene, and other nanocarbons represent a broad class of nanostructures that have garnered significant attention due to their outstanding chemical, thermal, electrical, and mechanical properties. These characteristics make them particularly valuable for applications in electronics, energy storage, and biomedical sciences.^{[ 181} , ¹⁸² , ¹⁸³ , ^{184 ]} A prominent area of research has focused on incorporating these carbon nanomaterials into flexible ANF aerogels, aiming to enhance the aerogel's performance by exploiting the unique properties of nanocarbons.

Hu et al.^{[ 56 ]} synthesized CNT/ANF composite aerogel films by integrating CNTs with ANFs using a freeze‐drying technique (Figure 18a). Given their comparable dimensions (approximately 30 nm), CNTs and ANFs were able to self‐assemble and interweave into a continuous 3D interconnected network. This self‐assembly effectively prevented CNT aggregation during the mixing process, resulting in a large SSA (232.8 m² g⁻¹), high electrical conductivity (230 S m⁻¹), and excellent hydrophobicity (contact angle of up to 137.0°) with exceptional Joule heating performance and supreme EMI shielding efficiency. Furthermore, the CNT/ANT aerogel film exhibited outstanding mechanical performance; there was no significant reduction in mechanical performance, even after 100 stretching–releasing cycles, indicating excellent mechanical stability for long‐term use. However, the random porous structure of the aerogel often hinders the formation of an efficient conductive CNT network, which typically requires substantial fillers to establish conductive pathways. To address this limitation, Fu et al.^{[ 129 ]} reconstructed the 3D network of 1D nanostructures in the ANF/CNT hybrid aerogel film to a laminated porous structure with preferential orientation and consecutively conductive pathways using densification and carbonization processes, resulting in a large SSA (341.9 m² g⁻¹) and high electrical conductivity (8540 S m⁻¹) (Figure 18b). Moreover, the carbonization process generated a microporous structure, and the combined effects of CNT toughening and structural reorganization imparted flexibility to the aerogel, enabling it to bend or fold without compromising its integrity.

Figure 18.

a) Fabrication of ANF/CNT aerogel film. FC: fluorocarbon resin. Reproduced with permission.^{[ 56 ]} Copyright 2020, American Chemical Society. b) Fabrication of densified ANF/CNT‐derived carbon aerogel. Reproduced with permission.^{[ 129 ]} Copyright 2022, American Chemical Society.

Graphene‐based aerogels have also been widely studied, although they often suffer from brittleness, compromising their structural integrity. To mitigate this issue, ANFs have been introduced as a reinforcing agent to enhance the mechanical properties of these aerogels. Liu et al.^{[ 185 ]} blended ANFs with graphene oxide and fabricated composite aerogels via freeze‐drying, followed by an annealing process. The addition of ANFs facilitated the dispersion of graphene sheets within the ANF matrix, preventing excessive stacking and promoting enhanced connectivity between the cell walls of the aerogel, which in turn improves the compressive strength of the aerogels. The annealing process promoted hydrogen bonding interactions between the graphene and ANFs, further enhancing the mechanical performance of the composite aerogels. These aerogels demonstrated a maximum compressive stress at 70% strain, which was eight times greater than that of pure graphene aerogels and exhibited superior wave and acoustic absorption performance.

Overall, the integration of carbon‐based nanomaterials such as CNTs and graphene with ANF aerogels effectively capitalizes on the unique advantages of each component. This synergistic combination significantly improves the mechanical, electrical, and structural performance of the resulting composite materials.

4.2.3. ANF Composites with Polymers

Recent advancements in materials science have increasingly focused on integrating ANF aerogels with multifunctional polymers, including polyvinyl alcohol (PVA), cellulose nanofibers (CNF), polyimide (PI), and chitosan (CHi), to fabricate composite aerogels with tailored characteristics.^{[ 124} , ¹⁸⁶ , ¹⁸⁷ , ¹⁸⁸ , ¹⁸⁹ , ^{190 ]} The abundance of functional groups present on ANFs facilitate robust interactions with various polymers, particularly through hydrogen bonding, which in turn enhances the mechanical strength, stability, and overall performance of the resulting composite materials. For example, ANF composite aerogels have led to notable improvements in application‐specific performance when combined with polymers such as PVA, CNF, PI, polyamidoamine dendrimers (PAMAM), poly (N‐isopropylacrylamide) (PNIPAM), and polyethylene oxide (PEO). He et al.^{[ 137 ]} developed an ANF/PVA composite aerogel with enhanced strength and toughness, wherein the extensive hydrogen bonding between ANFs and PVA facilitated the alignment and entanglement of the ANFs with the PVA matrix. The highly interconnected nodes in the 3D network structure, and the reversible nature of the hydrogen bonds, allowed for the self‐assembly of nanofibers in a directional manner, ultimately yielding aerogels with anisotropic characteristics without the need for additional freezing steps (Figure 19a). Li et al.^{[ 186 ]} introduced a novel “twice‐coagulate” strategy, which combined the flexibility of PVA with the rigidity of ANFs to create aerogels with superior extensibility (Figure 19b).

Figure 19.

a) Schematics of the assembly process of ANF/PVA aerogel. Reproduced with permission.^{[ 137 ]} Copyright 2022, Springer Nature. b) Schematic diagram of the evolution of the ANF‐PVA double gel network during the twice coagulation process. Reproduced with permission.^{[ 186 ]} Copyright 2023, Springer Nature.

The compatibility between ANFs and the chosen polymer is a critical consideration when preparing composite aerogels. When polymers display insufficient compatibility with ANFs, crosslinking agents are frequently incorporated to improve interfacial adhesion and promote the development of a stable, resilient, porous network. For instance, Wu et al.^{[ 124 ]} treated ANFs with phosphoric acid to introduce amino (─NH₂) and carboxyl (─COOH) groups, which were subsequently crosslinked with glutaraldehyde (GA) to connect ANFs with CHi, forming a stable network structure through covalent crosslinking. In addition to enhancing mechanical and structural properties, ANF aerogels can also function as resilient substrates or templates for polymer coatings. Yang et al.^{[ 106 ]} demonstrated the use of ANF hydrogels as templates for creating polydopamine (PDA)/ANF composite aerogels, wherein immersion of the hydrogels in a dopamine (DA) solution followed by polymerization and freeze‐drying resulted in aerogels with notable flame‐retardant and self‐extinguishing properties. Similarly, the incorporation of polypyrrole (PPy) into ANF aerogels via in situ polymerization resulted in a composite aerogel with enhanced electrical conductivity.^{[ 51} , ^{191 ]} The capillary action inherent in ANF aerogels also makes them ideal candidates for the development of composite materials with thermal management capabilities. Lyu et al.^{[ 135 ]} encapsulated PCMs (PNIPAM and C20) with varying thermal conductivities and surface wettabilities within ANF aerogel films to create thermal diodes that exhibited thermal rectification functionality, showcasing the potential of these composites for advanced thermal management applications.

The above analysis indicates that the integration of ANF aerogels with diverse polymer matrices presents a strategic method for optimizing the performance characteristics of aerogel materials for advanced applications. This synergistic approach enhances various properties, including mechanical strength, electrical conductivity, thermal insulation, and flame resistance. These advancements underscore a significant pathway for future research and the development of innovative technologies in the aerogel field.

4.2.4. ANF Composites with Inorganic Materials

Ceramics‐based nanomaterials of different dimensionalities, such as montmorillonite MXenes, SiO₂, and boron nitride (BN), are renowned for their excellent high‐temperature resistance, low thermal conductivity, and flame retardancy, making them promising candidates for a variety of advanced applications.^{[ 192} , ¹⁹³ , ^{194 ]} Since the advent of silica aerogels, there has been a growing interest in the development of various inorganic aerogels.^{[ 118 ]} However, their practical use has been hindered by challenges such as mechanical brittleness and high production costs.^{[ 195 ]} Recent research has shown that incorporating flexible ANF aerogels as supporting substrates can significantly improve the performance of inorganic materials owing to the synergistic interactions between the inorganic phase and the ANF matrix.^{[ 151} , ¹⁹⁶ , ¹⁹⁷ , ¹⁹⁸ , ^{199 ]}

Generally, ANF aerogels are highly effective in dispersing inorganic materials that are prone to aggregation, thus addressing one of the major limitations of conventional inorganic aerogels.^{[ 200 ]} Zhang et al.^{[ 201 ]} introduced 3‐aminopropyltriethoxysilane (APTES) as a surface modifier to improve the interaction between ANFs and SiO₂. This modification facilitated a more uniform distribution of SiO₂ within the ANF matrix, leading to the formation of a more compact and stable aerogel structure (Figure 20a). The increased SiO₂ content enhanced the SSA of the aerogel and also contributed to a reduction in thermal conductivity by impeding the movement of air molecules. Li et al.^{[ 200 ]} developed a thermally stable, interpenetrating double‐network SiO₂/ANF aerogel, where the silica network integrates with the ANF network to form a hierarchical pore structure, featuring pore size distributions of ≈4, 13, and 40 nm. The synergistic effects of this organic/inorganic network endow the composite aerogel fibers with high strength, elevated modulus, and good elongation properties, ensuring outstanding mechanical performance and functional characteristics even under extreme temperatures of −196 or 1100 °C. Furthermore, Yang et al.^{[ 202 ]} used fluoridated glass microspheres to enhance the interfacial compatibility with the ANF matrix. The incorporation of these microspheres compressed the pore size of the aerogel and endowed it with hydrophobicity and extremely low thermal conductivity and dielectric constant. Additionally, the incorporation of BN nanoribbons (BNNR) into the ANF network can enhance both the material's deformability and thermal stability. The strong hydrogen bonding between BNNR and ANFs tightly integrated the two phases, preventing breakage or slippage between the nanofibers^{[ 203 ]} and thus improving the aerogel's overall mechanical and thermal properties (Figure 20b).

Figure 20.

a) The synthesis of ANF/SiO₂ aerogel. Reproduced with permission.^{[ 201 ]} Copyright 2024, MDPI. b) Hydrogen bonding between ANF and BNNR and thermal stability of BNNR/ANF aerogels. Reproduced with permission.^{[ 203 ]} Copyright 2023, Elsevier. c) Fabrication of MXene/ANF hybrid aerogel. Reproduced with permission.^{[ 205 ]} Copyright 2021, Elsevier.

MXenes, a class of 2D materials known for their chemical stability, high electrical conductivity, and photothermal conversion efficiency, have also been successfully integrated into ANF aerogel composites.^{[ 121} , ¹⁴⁸ , ^{204 ]} The interaction between MXenes and ANFs is primarily governed by hydrogen bonding, which promotes adhesion between the two phases. The negatively charged surface of MXenes prevents aggregation through electrostatic repulsion, ensuring uniform dispersion within the ANF/MXene solution. Lu et al.^{[ 205 ]} demonstrated that the inclusion of MXenes enhanced the electrical conductivity of the composite aerogels, rendering them suitable for applications such as EMI shielding (Figure 20c). To further enhance the electrostatic self‐assembly and improve the material's properties, Ma et al.^{[ 206 ]} reported the use of amino‐functionalized MXenes (MXene‐NH₂) with reduced electronegativity to improve the interactions with ANFs, by reducing electrostatic repulsion and the interlayer spacing. Beyond their potential applications in EMI shielding and thermal management, MXene/ANF aerogels have also shown considerable promise in strain sensors,^{[ 207} , ^{208 ]} evaporators,^{[ 159} , ^{209 ]} and other functional devices.^{[ 210 ]}

5. Applications of ANF Aerogels

ANF‐based aerogels have been extensively developed for a wide range of applications, including EMI shielding,^{[ 104} , ²¹¹ , ^{212 ]} thermal insulation,^{[ 134} , ¹⁴² , ^{143 ]} adsorption,^{[ 173} , ¹⁷⁵ , ^{213 ]} and sensors.^{[ 51} , ^{208 ]} These materials exhibit remarkable versatility, underpinned by their unique structural and physical properties, positioning them as promising candidates in diverse fields. To enhance their performance and tailor them for specific applications, various modifications can be applied to optimize or impart corresponding characteristics. Moreover, the incorporation of functional additives into ANF‐based aerogels represents a compelling strategy for expanding their functionality, thereby enhancing their adaptability and efficacy in advanced technological domains.^{[ 130 ]}

5.1. Electromagnetic Management Applications

5.1.1. Electromagnetic Interference Shielding

The rapid development of mobile electronics and wireless communication technologies, particularly with the advent of 5G, has amplified the challenges posed by electromagnetic radiation.^{[ 214 ]} These challenges undermine the performance of electronic devices and the quality of communication networks, and also raise concerns regarding their potential adverse impacts on human health and the environment. Hence, there is a pressing need for the development of advanced materials that can safely provide effective EMI shielding.

Recent developments in ANF‐based aerogels have highlighted their promising potential, particularly when reinforced with electrically conductive fillers such as AgNWs,^{[ 115 ]} CNTs,^{[ 56 ]} 2D graphene,^{[ 215 ]} and MXenes.^{[ 205 ]} A primary factor driving the superior performance of these materials is their ability to form continuous conductive pathways, which enhance both the absorption and reflection of electromagnetic radiation. The inherent low density and high porosity of ANF‐based aerogels also alleviate impedance mismatches at the interfaces between the aerogel and air, allowing for more efficient dissipation of electromagnetic energy through multiple internal reflections. Compared to conventional EMI shielding materials, such as metals and alloys, ANF‐based aerogels offer distinct advantages due to their unique material properties.

These aerogels derive synergistic benefits from the conductive fillers, the unique structural characteristics of ANFs, and the inherent features of aerogels themselves, including: i) enhanced thermal stability, fatigue resistance, and chemical durability, ii) exceptional electrical conductivity coupled with favorable dielectric loss properties, and iii) an ultralight, highly porous, open‐cellular structure that enhances EMI shielding efficiency. Together, these properties position ANF‐based aerogels as highly effective candidates for EMI shielding, offering performance advantages over traditional shielding materials.

In particular, ANF aerogels transform into carbon aerogels through high‐temperature carbonization, a process that enhances their conductivity, and in turn improves their EMI shielding capabilities.^{[ 126 ]} This transformation generally results in the formation of a carbon skeleton rich in conjugated aromatic structures, which improves the electrical conductivity. Zhou et al.^{[ 126 ]} reported a low‐density, ultra‐thin carbon aerogel film derived from only ANFs, characterized by a unique skin‐core structure (Figure 21a) consisting of a dense outer layer (Figure 21b) and a 3D porous network core (Figure 21c). The compact skin structure at the surface and bottom of the aerogel film can repeatedly reflect the residual EMWs until they are completely adsorbed. As a result, the aerogel displays an impressive EMI shielding effectiveness (SE) of 41.4 dB, along with a high specific shielding effectiveness (SSE/t) of 47122.6 dB cm² g⁻¹. Additionally, the incorporation of metal nanomaterials, particularly AgNWs, has also enhanced EMI shielding efficiency due to their excellent conductivity and corrosion resistance.^{[ 105} , ^{166 ]} Zhang et al.^{[ 115 ]} developed a simple dip‐coating method to fabricate AgNWs/ANF composite aerogels. The porous structure aerogel facilitates uniform distribution of AgNWs, which in turn creates continuous conductive pathways that enhance the overall conductivity of the composite. This approach required only 7.5 wt.% of AgNWs to achieve a remarkable average EMI SE of 75.4 dB, a significant improvement compared to traditional direct mixing methods, which typically require much higher loadings of AgNWs (66.7 wt.%) for similar performance.

Figure 21.

a,b) Cross‐sectional SEM images of skin‐core ANF aerogels with different magnifications, and c) EMI shielding mechanism of ANF aerogels. Reproduced with permission.^{[ 126 ]} Copyright 2021, Elsevier. d) EMI shielding mechanism of ANF/CNT aerogel film. Reproduced with permission.^{[ 56 ]} Copyright 2020, American Chemical Society. e) EMI shielding mechanism diagram of FMAP including i) polarization loss, ii) multiple reflection, iii) ohmic loss, and iv) magnetic coupling loss. Reproduced with permission.^{[ 109 ]} Copyright 2022, American Chemical Society.

In addition to metal nanomaterials, the incorporation of carbon‐based materials such as CNTs and graphene has also been proven effective in further enhancing the EMI shielding performance. These materials, known for their excellent mechanical strength and electrical conductivity, are key components for improving the conductivity and EMWs absorption properties. Hu et al.^{[ 56 ]} demonstrated the fabrication of ANF/CNT composite aerogels, where the integration of CNTs boosted the material's conductivity and EMWs absorption and reflection capability (Figure 21d). Fu et al.^{[ 129 ]} further enhanced the electrical properties of ANF/CNT composite aerogel films through the oriented densification and high‐temperature carbonization techniques. Luo et al.^{[ 211 ]} employed a synergistic ice‐template formation and freeze‐drying method to ensure uniform distribution of graphene within the ANF matrix. The obtained composite aerogel exhibited exceptional EMI shielding performance, along with enhanced compressive strength and thermal stability. Drawing inspiration from natural structures such as pearls, Li et al.^{[ 99 ]} created a layered graphene/ANF carbonized film, where the graphene nanosheets were oriented to form continuous conductive pathways, simultaneously enhancing both the electrical conductivity and EMI shielding efficiency.

MXenes are known for their excellent conductivity and superior dielectric loss. Many studies have been conducted to enhance EMI shielding performance in composite aerogels by incorporating MXenes. For instance, Lu et al.^{[ 205 ]} developed MXene/ANF composite aerogels, utilizing a micro‐porous structure constructed by an ice crystal growth extrusion molding strategy. This structure is specifically designed to enhance the scattering of EMWs, while the MXene flakes were encapsulated in a skeleton for electromagnetic dissipation. The resulting aerogel demonstrated a high EMI SE of 56.8 dB and a SSE/t of 3645.7 dB cm² g⁻¹ at a minimal thickness of 1.9 mm in the X‐band. Similarly, Du et al.^{[ 121 ]} fabricated an anisotropic MXene/ANF aerogel with a rigid “cell‐wall” structure, achieving a remarkable EMI SE of 65.5 dB, along with excellent compressive performance. Yang et al.^{[ 216 ]} developed an anisotropic ANF/MXene aerogel using a vacuum‐assisted layer‐by‐layer method, achieving stable EMI shielding performance across a temperature range from −196 °C to 200 °C. Additionally, the encapsulation of MXenes within the ANF matrix was found to delay the oxidation of MXenes, maintaining the aerogel's thermal stability and flame retardancy. After exposure to 250 °C for 2 h, the EMI SE remained stable at 59.1 dB, suggesting its suitability for extreme conditions.

Meanwhile, many biomimetic, Janus, or layered structures of aerogel were also developed, for instance, Yao et al.^{[ 204 ]} created an exoskeleton‐like ANF/PI/MXene (APM) aerogel. The ANF component improved elasticity, while MXenes enhanced strength and rigidity, creating a conductive network that reduced IR emissivity and boosted EMI shielding performance. Yao et al.^{[ 217 ]} designed an asymmetric MXene/ANF/PI (AMAP) aerogel with a unique structure that enabled multiple shielding mechanisms, such as conduction loss, polarization loss, and additional attenuation during reflection, leading to low‐reflection EMI shielding. Zhang et al.^{[ 218 ]} employed directional freezing to fabricate an ANF/PVA aerogel, which was then immersed in a MXene dispersion to create a flexible MXene/ANF/PVA multifunctional film with excellent EMI shielding properties (70 dB and 13790 dB cm² g⁻¹ at 80 wt% MXenes). To meet evolving demands, Xu et al.^{[ 98 ]} also proposed a co‐assembly strategy to create a Janus‐layered ANF/MXene aerogel film, with gradient conductivity and minimal reflection, achieving an EMI SE of 60.49 dB and a low reflection coefficient of 0.0039 in the terahertz range, surpassing other previously reported shielding devices of similar thickness.

Yan et al.^{[ 219 ]} demonstrated a straight methodology for fabricating ultra‐light conductive aerogels composed of MXenes, CNTs, and ANFs, in a unique “sandwich” configuration. The ANF aerogel acts as the supporting framework to enhance mechanical strength, while MXenes and CNTs contribute to improved EMI shielding by absorbing and reflecting EMWs. The sandwich structure notably enhances the propagation path of EMWs and facilitates a highly efficient “absorb‐reflect‐reabsorb” mechanism. Moreover, the successful integration of CNTs simultaneously generates numerous defects and induces dipole polarization, thus enhancing the multiple reflection efficiency of EMWs.

However, it is essential to recognize that conductivity alone is not a sufficient indicator of EMI SE, as an excessive emphasis on conductivity will cause secondary wave reflection, potentially diminishing the shielding efficiency. To address this limitation, the incorporation of magnetic materials has proven to be an effective solution. For instance, Zhu et al.^{[ 109 ]} synthesized 3D‐ordered hierarchical porous Fe₃O₄‐decorated CNT (Fe₃O₄@CNT)/MXene/ANF/PI (FMAP) aerogels using a unidirectional freezing methodology (Figure 21e). The synergetic interaction between different fillers of ANFs, Fe₃O₄@CNT, and MXene/PI enabled FMAP composite aerogels to have ultra‐low density, ultra‐high elasticity, low microwave reflectivity, and anisotropic EMI shielding performance. The aligned layered structure effectively impedes the transmission of microwaves by promoting nearly infinite internal reflection and scattering phenomena, providing space for the electromagnetic synergistic network to play an effective attenuation role (Figure 21e, i–iv). Similarly, the combination of cobalt ferrite (CoFe₂O₄, CFO) nanoparticles on CNT (CNT@CFO) within ANF (CCA) aerogels has been demonstrated to improve EMI shielding by exploiting the electromagnetic coupling between the magnetic material.^{[ 220 ]} The CCA composite film with a thickness of 30 µm demonstrates an impressive EMI SE of 35 dB in X‐band. Moreover, the absorption coefficient of CCA is greater than 0.9, which significantly mitigates secondary reflection interference in comparison to CNT/ANF aerogel.

Overall, significant advancements have been made in the design and application of multifunctional ANF aerogels, particularly for EMI shielding. The incorporation of metallic nanoparticles, carbon‐based materials, and MXenes, combined with the synergistic effects of multiple components, has substantially improved the shielding performance of these composite aerogels (Table 3 ). Despite recent advancements, challenges related to conductivity, stability, and overall performance remain critical obstacles that must be confronted and overcome. Future research is expected to prioritize the advancement of efficient conductive pathways and the optimization of stable, oriented porous structures in ANF aerogels. These factors are crucial for improving the performance of EMI shielding materials and expanding their potential applications in various fields.

Table 3.

Comparison of EMI shielding performance of various ANF aerogels.

Materials	Processing method	Composite method	Physical properties			Shielding performances			Refs.
			Thickness [mm]	Density [mg cm−3]	Porosity [%]	Conductivity [S m−1]	EMI SE [dB]	SSE/t [dB cm2 g−1]
ANF	Blade‐coating and carbonization	/	0.162	54.4	96.8	1029.5	41.4	47122.6	[126]
AgNWs/ANF	Molding	Dip‐coating	5.2	8.1	/	991.1	75.4	/	[115]
GN/ANF	Molding	Mixing	2.0	41	> 97.5	/	31.55	3878.78	[211]
C‐GNS/ANF	Blade‐coating and carbonization	Mixing	0.017	754	/	80050	56.30	43923	[99]
GNS/ANF	Molding and hot pressing	Mixing	0.057	960	19.82	1083	31	5288	[215]
FC‐CNT/ANF	Blade‐coating	Mixing	0.568	40.3	/	230	54.4	33528.3	[56]
CNT/ANF	wet spinning	Mixing	0.92	12	97	345.2	> 30	/	[212]
CNT/ANF	Blade‐coating and carbonization	Mixing	0.024	170 150 120 87 58	/	1408.5 3449.4 8540.5 6561.6 2490.9	25.97 31.33 35.95 34.72 27.87	63652.0 87027.8 124826.4 166283.5 200215.5	[129]
MXene/ANF	Molding	Mixing	1.9	84.06	/	4832	56.8	3645.7	[205]
MXene/ANF	Molding	Mixing	2.5	23.3	/	854.9	65.5	11391	[121]
MXene/PI∖ANF	Molding	Impregnating	2.0	51	> 97	1853	55.315	5423	[204]
MXene/PI∖ANF	Molding	Mixing then dip‐coating	15.0	18.96	> 98	220.5	47.08	2483	[217]
MXene/ChNF/ANF	Molding	Impregnating	6.0	124	/	～350	75.0	1008	[208]
MXene/ANF/PVA	Molding and hot pressing	Impregnating	0.12	423	/	357.1	70.0	13790	[218]
MXene/CNT/ANF	Molding	Mix with CNT Spray MXene	2.0	42.8	/	654.2	63.5	7418.2	[219]
Fe3O4@CNT/ MXene/c‐ANF/PI	Molding and annealing	Mixing	3.0	6.27 4.87 2.47 1.62	/	/	67.42 53.66 41.78 30.45	35843 36728 56383 62654	[109]

GN, graphene; GNS, graphene nanosheets; FC, fluorocarbon resin.

5.1.2. . Absorption of Electromagnetic Waves

Mobile communication technology based on electromagnetic waves (EMWs) has enabled real‐time global connectivity, positioning, and remote control. However, the corresponding resulting electromagnetic pollution and radiation inevitably pose potential threats to human health and information security. To mitigate these hazards, traditional EMWs absorption materials such as carbon‐based materials (graphene, CNT) and magnetic metals, commonly used due to their capability of effectively converting electromagnetic energy into thermal energy through their excellent electrical conductivity or magnetic permeability.^{[ 221 ]} However, high density, processing difficulties, poor thermal stability, and susceptibility to oxidation generally limited their application. Additionally, effective EMWs‐absorbing materials should also have excellent impedance matching as well as strong energy dissipation capabilities. Aerogels, with their unique 3D porous structures, are particularly well‐suited for this purpose. The ability to match the impedance of air, coupled with the capacity to promote multiple scattering and reflection within the pore structure, enhances energy dissipation. Therefore, combining magnetic/conductive materials with aerogels, taking advantage of the strengths of both components, the performance of EMWs absorption can be significantly enhanced, offering promising prospects for practical applications.

Currently, many studies have explored the potential of ANF composite aerogels for EMWs absorption due to their good structural stability and large surface area. For instance, Wang et al.^{[ 222 ]} incorporated carboxylic MWCNT (c‐MWCNT) into ANF aerogels to fabricate a unidirectional ordered c‐MWCNT/ANF composite aerogel as shown in Figure 22b–d. The unique components and structure bring many advantages: i) the carboxyl groups and defects within MWCNT generate dipoles under high‐frequency radiation, benefitting energy dissipation; ii) the heterogeneous structure, along with abundant hydrogen bonds, enhances polarization; iii) the electron migration contributes to conduction losses; and iv) the unique 3D porous structures promote multiple scattering and reflection within the pore structure (Figure 22a). As a result, Wang's composite demonstrated good EMWs absorption, with a reflection loss of −60.43 dB and an absorption bandwidth of 5.34 GHz (Figure 22e). Similarly, Liu et al.^{[ 185 ]} used reduced graphene oxide (rGO) as an absorption filler. The 2D flake‐like structure of rGO promotes multiple reflections and scattering of EMWs within the material, while defects and dangling bonds in the rGO serve as polarization sites, improving wave attenuation. The addition of ANFs to this composite also prevents excessive stacking of rGO nanosheets, enhancing the interconnectivity and stability of the material in complex environments. To further improve the thermal stability and EMWs absorption capabilities of the composite, Zhou et al.^{[ 223 ]} introduced PI into the rGO/ANF aerogel. The strong hydrogen bonding and interfacial interactions between PI and the other components enhanced the thermal properties and EMWs absorption capabilities, allowing the composite to maintain stable performance up to 450 °C. In another study, Yang et al.^{[ 224 ]} investigated the impact of different freeze casting methods on the EMWs absorption performance of aerogels. They found that random and oriented structures exhibit distinct loss mechanisms: polarization loss dominates in random structures, while conductive loss becomes more significant in network‐oriented structures at higher temperatures.

Figure 22.

a) EMWs absorption mechanism of MWCNT/ANF aerogel, b) digital photo of MWCNT/ANF aerogel, c,d) SEM photographs of MWCNT/ANF aerogel in parallel and vertical directions, and e) EMWs absorption performance of the aerogel. Reproduced with permission.^{[ 222 ]} Copyright 2024, Elsevier. f) Schematic diagram of CNT/VO₂/ANF aerogel dynamic modulation of EMWs absorption and g) dynamic regulation mechanism under temperature change of the aerogel. Reproduced with permission.^{[ 170 ]} Copyright 2024, American Chemical Society.

In response to the demands for flexible electronic integration and miniaturization, Li et al.^{[ 99 ]} fabricated ultra‐thin and lightweight graphene nanosheets/ANF (GNS/ANF) carbonized aerogel films. The carbonization process significantly improves the conjugated carbon architecture, leading to an enhanced conjugation that facilitates electron transfer within the GNS/ANF composite film. Furthermore, the GNS/ANF composite films exhibit significant potential as effective fillers for lightweight microwave absorbers, driven by the improved impedance matching contributed by the GNS within the ANF matrix. The obtained composite microwave absorber demonstrates exceptional EMWs absorption performance with a minimum reflection loss of −56.07 dB at a thickness of only 1.5 mm and a maximum effective absorption bandwidth of 5.28 GHz with only 5 wt% inclusion of GNS/ANF. To achieve dynamic and intelligent EMWs absorption, Wang et al.^{[ 170 ]} proposed an intelligent EMWs‐modulated CNT/VO₂/ANF composite aerogel consisting of ANFs, CNTs, and vanadium dioxide (VO₂). The ANFs function as the 3D skeletal structure, CNTs serve as EMWs absorbers with dielectric loss, and VO₂ provides a temperature‐dependent mechanism for intelligent EMWs modulation. This allowed for dynamic adjustment of the material's conductivity and EMWs absorption properties (Figure 22f,g). As the temperature increases from 25 to 200 °C, the peak of resonance frequency aerogel shifts from 12.24 GHz to 8.56 GHz within the X‐band range, and the absorption intensity remains stable throughout this temperature span.

However, the challenge of impedance mismatch due to excessively high conductivity has prompted the incorporation of magnetic fillers to enhance magnetic losses and improve impedance matching.^{[ 169} , ²²⁵ , ^{226 ]} For instance, Wei et al.^{[ 178 ]} reported the growth of cobalt ferrite (CFO) nanoparticles on CNT, forming numerous heterogeneous interfaces. These interfaces, along with defects in the CNT, promote polarization loss, while the inherent magnetism of CFO introduces magnetic loss characteristics to the aerogel. In a similar approach, Ma et al.^{[ 147 ]} prepared lightweight ANF/MWCNT/Fe₃O₄ (AMF) composites using freeze‐drying and immersion methods. MTMS was deposited onto the surface of the framework in aerogels to achieve superhydrophobic properties through a chemical vapor deposition process. The AMF aerogel composite, as a microwave absorber, demonstrates a minimum reflection loss of up to −45.83 dB at a microwave frequency of 5.46 GHz and achieves a maximum effective absorption bandwidth of 4.0 GHz. Zhang et al.^{[ 160 ]} introduced PPy and Ni into ANF aerogels, enhancing both dielectric and magnetic losses. By adjusting the concentrations of PPy and Ni, they could fine‐tune the intensity and bandwidth of EMWs absorption, achieving a minimum reflection loss of −48.7 dB and a maximum effective absorption bandwidth of 8.42 GHz.

The porous polyhedral structure of MOFs and their derivatives also shows great promise for EMWs absorption. MOFs facilitate multiple reflections and elongated dissipation paths, making them highly effective for EMWs attenuation. He et al.^{[ 165 ]} assembled CoNi‐MOFs within ANF and CNF matrices, followed by carbonization to create a CoNi/ANF/CNF carbon aerogel. They found that, as the carbonization temperature increased, both dielectric and magnetic losses enhanced; however, temperatures above 900 °C caused the aerogel's pore structure to collapse. Hao et al.^{[ 227 ]} achieved the successful fabrication of Co@C/ANF aerogel via a controlled ammonia annealing process, using ZIF‐67/ANF composite material as the precursor. Incorporating MOF derivative particles results in enhanced resistance within the Co@C/ANF aerogel. This enhancement allows superior impedance matching, which in turn increases the penetration of microwaves into the material. As a result, the EMWs’ absorption efficiency is significantly enhanced. In addition to these composite aerogels, aerogel spheres, especially those with core‐shell structures, also show great potential for EMWs’ absorption. Shao et al.^{[ 89 ]} developed ANF‐derived hard carbon nanofiber aerogel microspheres (CNFAMs) with a distinct hierarchical skin‐core architecture using a wet‐spinning technique combined with re‐protonation‐mediated self‐assembly and carbonization processes. The substantial voids between adjacent microspheres, along with the microscale porosity inherent within the microspheres, enhance impedance matching and foster microwave reflection and scattering. The distinct graphitic domains and defects play crucial roles in both conduction and polarization losses, significantly impacting microwave attenuation. In a related study, Shao et al.^{[ 90 ]} used these spheres as substrates for growing ZIF‐67, further improving EMWs’ penetration and internal energy dissipation.

In summary, the integration of diverse fillers, structural modifications, and advanced processing techniques into ANF‐based aerogels has driven substantial progress in the development of EMWs absorption materials. These innovations significantly improve the performance of the materials while broadening their potential applications in next‐generation EMI shielding technologies. To address the growing demand for materials that are thin, lightweight, multifunctional, and exhibit high absorption capacity over a wide bandwidth, it is essential to conduct a thorough analysis of the aerogel's structural design, the mechanisms responsible for shielding and absorption, and the selection of appropriate fillers. These factors are critical in enhancing the material's potential applications and ensuring its effectiveness in various contexts.

5.2. Thermal Management

5.2.1. Thermal Insulation

The global energy crisis has intensified the demand for high‐performance thermal insulators, critical for enhancing energy efficiency, conserving resources, and mitigating CO₂ emissions. Among the diverse range of materials explored for these purposes, ANF aerogels have attracted considerable attention due to their exceptionally low thermal conductivity, positioning them as promising candidates for advanced thermal insulation applications.^{[ 130} , ^{202 ]} Compared with commercial insulating materials, ANF aerogels display superior thermal insulation properties, including a lower thermal conductivity and wider insulation range (−196 to 400 °C),^{[ 113 ]} and also exhibit an array of additional benefits, such as lightweight nature, flexibility, flame retardancy, and heat resistance. Such attributes make ANF‐based aerogels suitable for use in high‐demand sectors, including construction, aerospace, and battery insulation, where efficient thermal management is paramount.^{[ 228 ]}

Recent advancements in ANF‐based aerogels have demonstrated that these materials can achieve thermal conductivities in the range of 26–37 mW m⁻¹ K (Table 4 ), underscoring their potential to address the challenges posed by the growing need for effective thermal insulation in various industries. In a pioneering study by Hu et al.,^{[ 125 ]} an ultra‐elastic, highly porous ANF aerogel (with 99.75% porosity and a density of 4.7 mg cm⁻³) was fabricated using a method involving slow proton release and heat‐induced crosslinking. Following annealing, chemical crosslinking occurred between the ANFs, resulting in the formation of interconnected 3D porous networks. Benefiting from the complex thermal conduction pathways facilitated by the crosslinked structure, the resultant aerogel exhibited a low thermal conductivity of 29 mW m⁻¹ K⁻¹. Moreover, after being kept on a 350 °C heating plate for 1 minute, the temperature of the 20 mm thick aerogel remained at approximately 51.1 °C. After 15 minutes, the temperature slightly increased to 59.1 °C and remained unchanged after 30 min, further demonstrating the aerogel's excellent thermal insulating properties.

Table 4.

Thermal conductivity of different ANF aerogels.

Materials	Porosity [%]	Pore size	Thermal conductivity [mW m−1 K−1]	Refs.
ANF	>92	2–10 nm	34	[141]
ANF	/	25.31 nm	30±3	[201]
ANF	98	12.8 nm	37	[91]
ANF	99.53	/	31	[97]
ANF	90	30 nm	23.7	[84]
ANF	99.75	20 nm	29.2	[125]
ANF	/	≈20 nm	37	[95]
ANF	97.4	≈10 nm	27.9	[107]
ANF	97.9	13.51 nm	36	[39]
ANF	97.2	/	33.9	[114]
ANF	97.7	/	27	[45]
ANF	99.4	21.5 nm	26	[132]
ANF	95.2	<60 nm	28.87	[232]
Carbonized ANF	/	<50 nm	19.93	[230]
BNNR/ANF	99	/	35.7	[203]
SiO2/ANF	/	5.202 nm	30±3	[201]
ANF/PVA	>98.5	18.8 µm	25.78	[213]
ANF/PVA	91	140 nm	14	[137]
ANF/PI	/	<10 µm	28.6±0.53	[189]
ANF/PVA/CNF	>98.5	<20 µm	26.83	[188]
ANF/ChNF/ MXene	/	1 nm	10	[208]
ANF/PI/ MXene	98.5	/	29±3	[224]
CNT/ANF	98.4	40.4 µm	30.7	[222]

The insulating behavior of aerogels is strongly influenced by the pore size and spatial arrangement; to optimize the thermal properties of ANF‐based aerogels, significant attention has been directed toward the fine‐tuning of pore size and structural design. For instance, coarse‐grained molecular dynamics (MD) simulations have validated that gradient all‐nanostructured fibers and skin‐core structured fibers display distinct thermal distribution characteristics (Figure 23a).^{[ 229 ]} In contrast to the skin‐core structured fiber with low interfacial thermal resistance, the nanoporous outer layer of the gradient nanostructure fiber notably contributes to improving heat storage capacity, increasing intricate heat transfer path, and establishing significant interfacial thermal resistance at the gradient interface. Such a unique configuration effectively impedes radial heat transfer and results in a 40% reduction in thermal conductivity (Figure 23b). Based on this, Fu et al.^{[ 229 ]} proposed a novel microfluidic spinning strategy to fabricate a gradient, all‐nanostructured ANF aerogel fiber with dense external (average pore size of 150 nm) and loose internal (average pore size of 600 nm) structure. The synergistic effects of increased thermal resistance, extended conduction path, and limited air convection remarkably reduced the thermal conductivity of the gradient ANF aerogel fiber to 22.8 mW m⁻¹ K⁻¹, which is much lower than that of traditional wet‐spun aerogel fiber (32.7 mW m⁻¹ K⁻¹). Moreover, Shi et al.^{[ 84 ]} developed a layered ANF aerogel with dual microporous and mesoporous structures, with an average pore size of approximately 30 nm. This design effectively reduced thermal convection by trapping air molecules within the material, resulting in a low thermal conductivity of 23.7 mW m⁻¹ K⁻¹. Surprisingly, this aerogel demonstrated remarkable thermal stability, maintaining a surface temperature of 50 °C after 30 min of exposure at 200 °C. In another innovative approach, Hu et al.^{[ 97 ]} engineered an asymmetric ANF aerogel film with a dense outer layer and a 3D nanofiber scaffold, significantly improving thermal insulation by mitigating thermal radiation and convection. Wu et al.^{[ 110 ]} employed a unidirectional freeze‐casting method to fabricate a honeycomb‐shaped aerogel. The resulting oriented skeleton featured a greater alignment of pores and fewer radial solid channels compared to the random skeleton. Therefore, both the gas convection and thermal conduction were greatly isolated by this anisotropic structure. Moreover, the insufficient connectivity of the oriented aerogel in the radial direction further reduced thermal conduction in solid channels. Heat transfer simulations demonstrated that the oriented skeleton required a longer time (605 s) to reach thermal equilibrium when heated to 100 °C, compared to the random skeleton, which only took 487 s (Figure 23c). Moreover, the oriented aerogel exhibited anisotropic thermal conductivity, with radial conductivity of 15.8 mW m⁻¹ K⁻¹ and axial conductivity of 143 mW m⁻¹ K⁻¹, facilitating efficient heat dissipation along the aligned layers (Figure 23d). These developments emphasize the importance of tailored architectures of ANF aerogels in achieving optimal thermal performance, offering constructive insight into the microstructural design of thermal insulating aerogels.

Figure 23.

a) Coarse‐grained molecular dynamics simulations of thermal insulation for skin‐core and gradient nanostructured models and b) simulated thermal conductivity of skin‐core and gradient‐structured models. Reproduced with permission.^{[ 229 ]} Copyright 2025, Springer Nature. c) Heat transfer mechanisms in oriented and random aerogels and d) thermal conductivity of ANF aerogels in axial and radial directions. Reproduced with permission.^{[ 110 ]} Copyright 2023, John Wiley and Sons. e) Thermal conductivity of ANF and ANF‐based aerogel films and f) comparison of thermal conductivity between ANF‐based aerogels and polymer‐based aerogels. Reproduced with permission.^{[ 202 ]} Copyright 2025, Royal Society of Chemistry.

Beyond structural modifications, the incorporation of thermally stable inorganic nanomaterials, polymers, and other additives into ANF aerogels also has emerged as a promising strategy for enhancing their thermal insulation capabilities. Such additives can create more intricate thermal conduction pathways, improve interfacial scattering, and ultimately lower the overall thermal conductivity, while simultaneously introducing additional functional properties. For example, hollow glass spheres were introduced into ANF aerogels to compress the nanopores near the aerogel's interface of 75–150 nm to satisfy the Knudsen effect, and the unique structure of the hollow microspheres effectively blocks heat convection.^{[ 202 ]} In this work, the incorporation of fluorinated hollow glass microspheres significantly improved the interfacial compatibility with ANF aerogels, facilitating the formation of a honeycomb‐like pore structure instead of an open‐pore structure, thereby effectively suppressing air convection. As the content of fluorinated glass microspheres increased, the ANF aerogel containing 3 wt% of these microspheres exhibited the lowest thermal conductivity of 21.6 mW m⁻¹ K⁻¹ (Figure 23e). However, when the doping rate was further increased, the glass microspheres collided and fractured, leading to the collapse of the hollow structure. The fractured microspheres then acted as a heat transfer medium, which hindered the improvement of thermal insulation properties. As shown in Figure 23f, the thermal conductivity of the ANF‐based aerogel is markedly lower than those of both air and most polymer‐based aerogels, which provides valuable insights into the design of advanced superthermal insulation materials through structure optimization and composition regulation. He et al.^{[ 137 ]} developed a series of ANF/PVA aerogels, achieving thermal conductivities ranging from 14 to 28 mW m⁻¹ K⁻¹, demonstrating that increased porosity and reduced pore size contribute to lower thermal conductivity. Zhou et al.^{[ 230 ]} combined sodium alginate (SA) and ANF, constructed an interconnected nanofibrous cellular carbon aerogels by directional freezing and carbonization, resulting in the transition from ANF to highly graphitized carbon nanofibers. This highly crystalline structure effectively scatters phonons, reduces heat transfer, and absorbs thermal radiation, achieving a relatively low thermal conductivity of 19.93 mW m⁻¹ K⁻¹. Zhang et al.^{[ 208 ]} prepared a chitin nanofiber (ChNF)/ANF/MXene aerogel, achieving an exceptionally low thermal conductivity of 10 mW m⁻¹ K⁻¹, attributed to the enhanced dispersion effects of the nanocomposite. Additionally, Fan et al.^{[ 231 ]} developed a hydrophobic ZIF‐8/ANF aerogel with outstanding thermal insulation and sound absorption properties, which maintained stability under humid conditions. This aerogel, with a low density of 12.52 mg cm⁻³ and nanopores (≈1 nm), demonstrated exceptional performance even under challenging environmental conditions.

In summary, ANF‐based aerogels have shown great promise as advanced thermal insulators, with ongoing research focusing on optimizing their structural design and incorporating functional additives. These efforts improve thermal performance and expand the potential applications of ANF aerogels in a wide range of industries requiring efficient thermal management solutions.

5.2.2. Interfacial Solar‐Driven Evaporation

Interfacial solar‐driven evaporation (ISDE) has emerged as a highly promising and sustainable technology for water treatment and harnessing solar energy for both evaporation and purification (Figure 24a). The core mechanism of ISDE systems relies on photothermal materials, which convert solar energy into thermal energy, driving the evaporation process. ISDE has attracted much attention mainly due to its potential applications in desalination,^{[ 233} , ²³⁴ , ^{235 ]} wastewater treatment,^{[ 236} , ²³⁷ , ^{238 ]} and steam generation,^{[ 239} , ²⁴⁰ , ^{241 ]} owing to its energy efficiency, low operational costs, and ability to address the increasing global demand for freshwater. These benefits position ISDE as an innovative technology with substantial potential to mitigate the global water crisis.^{[ 242 ]} To optimize the performance of ISDE systems, the materials must meet several critical criteria. These include: i) high light absorption and photothermal efficiency for effective evaporation; ii) suitable hydrophilicity and porous structure for efficient water transport; iii) low thermal conductivity to concentrate heat at the water‐air interface and minimize energy losses; iv) sufficient buoyancy to allow for easy deployment and maintenance; and v) inherent flame‐retardant properties to mitigate fire risks associated with heat accumulation.^{[ 243 ]}

Figure 24.

a) Mechanistic diagram of IDSE of AgNWs/MXene/ANF aerogels. Reproduced with permission.^{[ 159 ]} Copyright 2023, Elsevier. b) Schematic and SEM images of the AuNPs@ANF/CNT aerogel. Reproduced with permission.^{[ 167 ]} Copyright 2022, Elsevier. c) Different energy sources of ANF/CNT/PPy aerogel evaporator, and d) IDSE performance of the aerogel under 1 sun radiation within 10 min and e) color change between the initial dye solution and the purified water. Reproduced with permission.^{[ 247 ]} Copyright 2023, John Wiley and Sons.

Among the materials explored for ISDE applications, aerogel‐based ANF composites have emerged as promising candidates due to their lightweight, highly porous, hydrophilic nature, and low thermal conductivity. Moreover, their inherent flame‐retardant properties enhance the safety of materials. However, a significant challenge with ANF‐based aerogels is their relatively low photothermal conversion efficiency, which limits their standalone performance in solar‐driven evaporation applications. In response to this limitation, researchers have investigated the incorporation of additional photothermal agents, such as metal nanoparticles, MXenes, carbon‐based materials, and conductive polymers to enhance their photothermal properties and evaporation efficiency.^{[ 244 ]} For example, Singh et al.^{[ 209 ]} demonstrated that the integration of MXene nanosheets into ANF aerogels substantially improved the photothermal performance of the composite material. These MXene@ANF aerogels demonstrate remarkable performance at the air‐water interface, achieving an evaporation rate of 1.48 kg m⁻² h⁻¹ and steam conversion efficiency of 93.8% under 1 sun irradiation (1 kW m⁻²). Similarly, Jiang et al.^{[ 245 ]} improved the photothermal efficiency of ANF aerogels by incorporating MoS₂ nanosheets, promoting the photocatalytic degradation of dye‐contaminated wastewater. Gao et al.^{[ 246 ]} developed a graphene oxide/ANF composite aerogel that demonstrated excellent resistance to salt and metal ions, maintaining high evaporation rates even in seawater, dye wastewater, and other corrosive liquids.

Despite these promising developments, a notable challenge with 2D materials such as MXenes and MoS₂ is their tendency to aggregate, which impedes water transport and limits the photothermal conversion efficiency. To address this issue, Tao et al.^{[ 159 ]} introduced 1D AgNWs into the MXene/ANF composite structure, effectively preventing the stacking of 2D materials. This modification improved the photothermal efficiency and enhanced water transport, significantly increasing the evaporation rate to 2.21 kg m⁻² h⁻¹ with an energy efficiency of 92%. In addition, the incorporation of AgNWs imparted antibacterial properties to the composite, further improving its safety (Figure 24a). Shi et al.^{[ 167 ]} fabricated an all‐in‐one solar‐driven interfacial evaporator via a hybrid nanofibrous aerogel of ANFs, CNTs, and gold nanoparticles (AuNPs). AuNPs@ANF/CNT aerogels exhibit remarkable porosity and an open‐cell cellular structure, which contributes to their lightweight and enables self‐floating capabilities (Figure 24b). Their low thermal conductivity minimizes heat loss, while high capillary action facilitates water wicking and retention within the aerogel structure. In addition to material composition, the structural design of the aerogel also plays a crucial role in optimizing evaporation efficiency. Traditional sol‐gel aerogels often suffer from closed pores, which limit the release of steam and reduce evaporation performance. To overcome this limitation, Chen et al.^{[ 247 ]} developed a cylindrical CNF/ANF/PPy aerogel with an open‐pore structure using a Pickering emulsion‐template method (Figure 24c). The resulting micropores facilitated the release of water vapor, while nanopores increased the evaporative surface area, and the cold surface of the aerogel absorbed heat from the surrounding environment to enhance evaporation (Figure 24d). When the solar evaporation system was placed in cold water under light illumination, the initial transparent lid became blurred due to the accumulation of condensed water on its inner surface. Moreover, the as‐prepared evaporator was also applied to treat model dye wastewater including negatively charged methyl orange (MO) solution (0.01 m) and positively charged methylene blue (MB) solution (0.01 m). After solar evaporation, the initial orange‐red or blue color was changed to colorless (Figure 24e), indicating the complete removal of MO and MB. These purification results illustrate that the evaporator is adaptable for generating freshwater from various dye‐containing wastewater.

However, relying only on photothermal conversion is less effective under cloudy or low‐light conditions, thus, some researchers have explored the incorporation of electrothermal conversion mechanisms to improve the system's overall performance. Wang et al.^{[ 248 ]} incorporated both photothermal and electrothermal conversion mechanisms into an ANF/CNT/PPy composite aerogel. This multifunctional aerogel enabled efficient water evaporation under solar radiation and operated effectively under low‐light (0.5 sun radiation) and dark (0 V) conditions. Under 1.0 sun radiation and 5 V input, the aerogel achieved an evaporation rate of 4.71 kg m⁻² h⁻¹, 1.7 times higher than that of photothermal‐only aerogels. Even under low‐light and dark conditions, the aerogel maintains evaporation rates of 2.28 kg m⁻² h⁻¹ and 0.99 kg m⁻² h⁻¹, respectively, offering a promising solution for all‐weather water purification.

In summary, while ANF composites show significant potential for ISDE applications, enhancement of their photothermal efficiency and structural design is critical for maximizing their performance. The integration of photothermal agents, innovative material design, and multifunctional approaches hold promise for developing high‐performance ISDE systems capable of addressing the global water scarcity issue under various environmental conditions.

5.2.3. Joule Heating

Joule heating refers to the process by which electrical energy is converted into heat due to the resistance of a material. This process occurs when an electric current flows through a resistive material, causing the electrical energy to be dissipated as heat.^{[ 249 ]} Efficient Joule heating requires the optimization of several characteristics, including high resistivity to maximize heat generation and excellent thermal conductivity to ensure effective heat dissipation. Additionally, mechanical strength, heat, oxidation, and corrosion resistance are critical to the stability and longevity of materials subjected to high temperatures.^{[ 56} , ¹⁶⁶ , ^{217 ]}

ANFs present distinct advantages among the potential materials due to their lightweight structure, high surface area, mechanical strength, and remarkable high‐temperature resistance. These properties make ANF aerogels suitable for portable heating devices and thermal management systems, requiring both insulation and efficient heat generation. Furthermore, their superior thermal insulation properties render them ideal candidates for use in smart wearables and personal thermal management. However, the relatively low electrical conductivity of ANFs remains a significant limitation, impeding their efficiency in converting electrical energy into heat. To address this challenge, incorporating conductive fillers into ANF‐based aerogels has become a common strategy to enhance their electrical conductivity.^{[ 148} , ²¹⁰ , ^{219 ]} For example, Hu et al.^{[ 56 ]} demonstrated that by combining ANF aerogels with CNTs, the electrical conductivity of the aerogel was increased to 230 S m⁻¹, while the original ANF gels are insulating. The modified aerogel exhibited rapid and stable heat generation upon the application of an electric current, along with the ability to return to ambient temperature once the current was stopped quickly. This suggests that the electrical and thermal properties of the aerogels can be finely tuned by adjusting the type, concentration, or structure of the conductive fillers. Fu et al.^{[ 129 ]} improved the performance of composite aerogels by employing a carbonization strategy, which significantly increased the conductivity of the aerogel from 74 S m⁻¹ to 8540 S m⁻¹. The obtained aerogel also demonstrated stable heating performance (Figure 25a,b). Additionally, the surface temperature of the aerogel film can be controlled by increasing the step voltage through the fast response of Joule heating (Figure 25c). In a different approach, He et al.^{[ 165 ]} developed a CoNi@C/ANF/CNF structure that combines low density and excellent Joule heating properties. The obtained aerogel has a linear voltage‐temperature relationship with a temperature range from 40 to 103 °C under varying input voltages (3–7 V) and achieved rapid heating at a rate of 5 °C s⁻¹ and a high cooling rate of 1.4 °C s⁻¹.

Figure 25.

a) Flexibility of ANF/CNT aerogel film, b) Joule heating performance of bending ANF/CNT aerogel film, and c) surface temperatures of the ANF/CNT aerogel film under step increased voltage. Reproduced with permission.^{[ 129 ]} Copyright 2022, American Chemical Society. d,e) Working mechanism diagram of AgNWs/PEG/ANF aerogels, f) photograph and schematic diagram of aerogel‐based waist support, and g,h) thermal infrared images of waist support when power on and off. Reproduced with permission.^{[ 168 ]} Copyright 2024, American Chemical Society.

Moreover, the incorporation of AgNWs into ANF/CNT aerogels can further enhance their Joule heating capabilities. Zheng et al.^{[ 166 ]} demonstrated that the addition of just 1.5 wt% AgNWs increased the aerogel's surface temperature. Under a low voltage of 1.6 V, the aerogel reached 111 °C within 20 s, maintaining this temperature for over 8 h. The aerogel also exhibited rapid heating and cooling response times, taking only 20 and 24 s, respectively. In addition to enhancing the stability of heaters, the combination of conductive fillers with PCMs offers a dual advantage of active heating and passive thermal insulation. Wu et al.^{[ 168 ]} developed a Janus AgNWs/PEG/ANF aerogel with high conductivity (960 S m⁻¹) and an asymmetric heat transfer structure, designed for continuous passive heating in human thermal management. When an external voltage was applied, Joule heat generated by AgNWs was efficiently absorbed and stored by PEG (Figure 25d,e), maintaining a comfortable temperature range of 37–39 °C for up to 5–8 min without requiring a power supply (Figure 25f–h). This innovative composite provides an effective, energy‐efficient, and durable solution for thermal regulation in cold environments.

Overall, ANF‐based conductive aerogels show significant promise due to their outstanding Joule heating properties, lightweight structure, high‐temperature resistance, and insulating capabilities. These materials have driven innovations in fields such as electronics, wearable technologies, and sustainable energy solutions. However, challenges remain in optimizing efficiency, enhancing temperature regulation, ensuring safety, extending heat storage time, and automating the Joule heating processes in devices that use ANF aerogels. Further research is required to improve conductivity, stability, and control, as well as to lower production costs, to unlock their full potential in practical applications.

5.3. Energy Storage and Conversion

5.3.1. Batteries

The ANF‐based aerogel is particularly well‐suited for use as a battery separator due to its excellent thermal insulation, high mechanical strength, and remarkable thermal stability, where it can enhance both safety and longevity.^{[ 184} , ^{250 ]} For instance, Sheng et al.^{[ 251 ]} developed a new type of ANF self‐standing separator with a high SSA (255 m² g⁻¹) and porosity (91%), which outperforms conventional glass fiber separators. The obtained ANF aerogel effectively reduces side reactions between the electrolyte and lithium anode, promoting uniform lithium deposition and preventing dendrite formation. Additionally, its outstanding thermal stability allows it to retain its original shape without any shrinkage at temperatures up to 200 °C, thereby reducing the risk of battery short‐circuiting. Similarly, Lee et al.^{[ 252 ]} introduced a poly(ether imide)/aramid nanofibrillar PEI/ANF composite aerogel separator where the ether (C−O−C) and carbonyl (C═O) groups in PEI enhance the separator's affinity for both electrolytes and lithium ions, achieving an ionic conductivity of 3.30 mS cm⁻¹ and a Li⁺ transference number of 0.84. These improvements facilitate the formation of a dense solid electrolyte interphase (SEI) layer, inhibiting dendrite growth under high charge‐discharge conditions. Lee et al.^{[ 253 ]} developed a PVA/ANF aerogel film, with enhanced mechanical strength and electrochemical stability due to the strong hydrogen bonding interactions between PVA and ANFs, effectively suppressing dendrite formation and ensuring stable performance under harsh operating conditions.

Moreover, the ANF framework is also compatible with polymer electrolytes, such as poly(ethylene oxide) (PEO) and PVA, in all‐solid‐state batteries. Unlike traditional separators that require liquid electrolytes, ANF‐based solid electrolytes provide both ionic conductivity and effective separation, which in turn improves cycle life, enhanced safety, and higher energy density.^{[ 254 ]} To enhance the performance of a battery, a CO₂‐assisted self‐assembly method was employed to create a dual‐layer ANF‐based aerogel solid electrolyte.^{[ 250 ]} The highly cross‐linked surface layer of this material prevents dendrite growth, while the inner layer, rich in amide groups, promotes better complexation with the polymer electrolyte, and enhances the cycle life, safety, and energy density of the battery. Tung et al.^{[ 102 ]} constructed a dendrite‐suppressing ANF‐based PEO composite separator via a LBL deposition method (Figure 26a). The dense ANF network film has a narrow pore diameter (20 nm), which is smaller than the growth area of the lithium dendrites (50–100 nm), to eliminate “weak links” where the dendrites pierce the separator. The narrow diameter also suppresses PEO crystallization, yielding a composite that exhibits high modulus, ionic conductivity, flexibility, ion flux rates, and thermal stability. Wang et al.^{[ 30 ]} also incorporated a PVA polymer electrolyte into an ANF film aerogel, which improved the separator's stability and flexibility, making it ideal for deformable batteries (Figure 26b). The strong interaction ensured consistent contact throughout the charge–discharge cycles, enhancing the long‐cycle reliability of Zn–air batteries. Meanwhile, taking advantage of the strong deformations of the ANF‐based composite electrolyte, several types of mini‐bots were successfully operated without stand‐alone batteries such as caterpillar, scorpion, spider, and ant (Figure 26c). The lightness and small volume of these deformable batteries eliminate design constraints related to bulky and heavy charge storage, which was essential in the realization of these miniaturized devices.

Figure 26.

a) Photograph and SEM image of PEO/ANF film and SEM image of lithium dendrite. Reproduced with permission.^{[ 102 ]} Copyright 2015, Springer Nature. b) SEM images of ANF aerogel film and ANF/PVA film and c) structural biomorphic batteries integrated into “battery‐less” miniaturized biomorphic robots (mini‐bots). Reproduced with permission.^{[ 30 ]} Copyright 2020, The American Association for the Advancement of Science. d) Schematic illustration of the fabrication procedure for Fe‐SA@PNC catalysts and e) schematic representation of the Zn–air battery. Reproduced with permission.^{[ 127 ]} Copyright 2023, Elsevier.

In addition to its application as a separator, ANF‐based aerogel can also be used as cathode materials for batteries. Upon carbonization, the ANF aerogel will transform into a porous carbon aerogel with a large SSA, providing abundant transport channels and active sites. This makes it an ideal candidate for use as a carrier in air battery catalysts. Shen et al.^{[ 127 ]} employed a chemical vapor deposition method to produce a nitrogen‐doped ANF‐derived carbon aerogel with an FeN₄ catalyst (Fe‐SA@PNC), which improved mass transport and catalysis in Zn‐air batteries (Figure 26d,e). The catalyst achieved an impressive power density of 149 mW cm⁻² and an energy density of 2936 Wh kg⁻¹. Recent advancements in catalyst design, such as nitrogen self‐doping and single‐atom engineering, have further enhanced its performance, resulting in a remarkably high SSA of 1173 m² g⁻¹, and substantially improved the kinetics of the oxygen reduction reaction.^{[ 128 ]}

Overall, the unique properties of ANF aerogels make them versatile and valuable components in the development of advanced energy storage devices, including both separators and cathode materials. Their high SSA, thermal stability, and ability to enhance ion transport and dendrite inhibition position them as key materials for next‐generation batteries with improved performance, safety, and longevity.

5.3.2. Supercapacitors

Supercapacitors have emerged as critical components in contemporary energy storage systems, owing to their superior power density, extended cycle durability, rapid charge‐discharge capabilities, broad operational temperature range, environmental compatibility, and minimal maintenance requirements.^{[ 255 ]} Porous carbon materials—characterized by low density, high SSA, good chemical stability, well‐controlled nanomorphology, multilevel porosity, and abundant active sites—offer additional adsorption sites for electrolyte ions and enhance energy storage performance. As a result, they are extensively employed as electrode materials in supercapacitors.^{[ 256 ]} However, the environmental impact and mechanical brittleness of porous carbon materials limit their utilization in flexible devices. As an alternative, ANF‐derived carbon aerogels provide efficient ion and electron transport pathways and superior flexibility, positioning them as promising candidates for supercapacitor electrode materials.

Gao et al.^{[ 257 ]} fabricated a nitrogen‐doped, ANF‐derived carbon aerogel electrode with a high SSA of 496 m² g⁻¹ and large pore volume of 1.24 cm³ g⁻¹ through carbonization. The resulting material exhibited enhanced electrical conductivity and excellent wettability with the electrolyte, both of which support pseudocapacitance and contribute to improved overall electrochemical performance. Benefits from the aerogel film's high porosity and SSA and the resulting numerous active sites facilitate the adsorption and storage of electrolyte ions. And the 3D continuous network supports rapid electron transport and ion diffusion, thereby minimizing the electrode material's resistance and enhancing the supercapacitor's power density. The flexible solid‐state supercapacitor fabricated from the ANF‐based carbon aerogel film exhibits an area‐specific capacitance of approximately 15.2 mF cm⁻² at a current density of 20 µA cm⁻². Upon a 15‐fold increase in current density, the area‐specific capacitance retains about 59.3% of its initial value. Furthermore, the capacitance retention rate remains at roughly 72% after 1200 folding cycles, indicating a high area‐specific capacitance, excellent rate performance, and exceptional cycling stability.

Furthermore, the incorporation of electroactive carbon nanomaterials, conductive polymers, and transition metal oxides or hydroxides into carbon aerogels has demonstrated efficacy in fabricating supercapacitor electrodes, leading to substantial enhancements in charge storage capacity and cycling stability. Combining ANFs, reduced holey graphene oxide (rHGO), and polyaniline (PANI), yields a composite aerogel film that demonstrates exceptional flexibility, mechanical robustness, and retains structural integrity under deformation, thereby rendering it suitable for flexible supercapacitor applications.^{[ 258 ]} And the ANF/rHGO/PANI aerogel film exhibits enhanced electrochemical stability and durability, effectively resisting electrolyte corrosion and oxidation, thereby ensuring the long‐term operational stability of the supercapacitor. Following 2500 mechanical bending cycles, the capacitance retention rate is 98.9%. Even under tensile stress, the capacitance retention remains near 100%. Gong et al.^{[ 123 ]} designed an ANF/PPy/NiCoO₂ aerogel with coaxial cable‐like structure to achieve high‐performance energy storage. The 3D porous architecture established by the internal carbonized ANFs offers an ample electrolyte reservoir, minimizes ion diffusion pathways, and facilitates rapid charge transport. The PPy interlayer enhances the interfacial contact between ANFs and NiCoO₂, thereby mitigating interfacial resistance and improving electrode conductivity, and provides an increased density of active sites. Furthermore, the PPy layer facilitates the uniform deposition of NiCoO₂. The outer layer of NiCoO₂ exhibits favorable pseudocapacitive characteristics, enabling charge storage via Faradaic reactions, thus enhancing the specific capacitance of the supercapacitor. In this work, the ANF/PPy/NiCoO₂ composite exhibited a specific capacitance of 1037 F g⁻¹ at a current density of 1 A g⁻¹, and retained 89.2% of its initial capacitance after 7000 charge–discharge cycles.

Despite the potential of ANF‐based aerogels in supercapacitor applications, their development is still in its nascent stages. Further investigation necessitates specific enhancements: i) Enhancing conductivity: by optimizing carbonization parameters, such as temperature and duration, to promote graphitization within the ANF structure. ii) Pore structure optimization: precise control over carbonization parameters is crucial for tailoring the aerogel's porous architecture, specifically the formation of a higher density of micropores and mesopores. These pores facilitate increased electrolyte ion accommodation and augment the SSA, thereby boosting the energy storage capacity. Furthermore, surface modifications of the carbonized ANF aerogels, such as oxidation or nitridation, can introduce surface functional groups to enhance electrolyte interactions and improve electrochemical performance.

5.3.3. Generators

The rapid advancement of wearable flexible electronics has catalyzed the development of compact, efficient, and sustainable self‐powered systems, aimed at improving device reliability, stability, and functionality, particularly in dynamic and diverse environments. Among the emerging technologies for autonomous power generation, thermoelectric generators (TEGs) and triboelectric nanogenerators (TENGs) have gained significant attention due to their potential to convert thermal and mechanical energy into electrical power. In contrast to conventional battery‐powered systems, which are often bulky, have limited lifespans, and require frequent replacements, TEGs and TENGs have distinct advantages, including reduced maintenance needs and smaller form factors, which make them ideal for applications such as fire monitoring, remote sensing, and energy harvesting.

• i)Thermoelectric Generators (TEGs): TEGs harness the Seebeck effect to convert heat into electricity. This process involves the generation of an electromotive force when a temperature gradient is applied across a thermoelectric material, causing charge carriers (electrons or holes) to move from the hot side to the cooler side, producing a voltage (Figure 27a).^{[ 94 ]} To enhance TEGs’ performance, many attempts have been directed toward engineering materials that exhibit low thermal conductivity, such as ANF‐based aerogels. These aerogels serve as effective thermal insulators, reducing thermal losses and maintaining a larger temperature differential between the thermoelectric materials, which ultimately improves the efficiency of power generation.

Figure 27.

a) TEG mechanism of p–n segmented thermoelectric fibers and b) optical image of TEG‐based fabrics. Reproduced with permission.^{[ 94 ]} Copyright 2023, Springer Nature. c) Temperatures, output voltages, and output currents of the TEG when cold side in air (top) and ice water (down). Reproduced with permission.^{[ 154 ]} Copyright 2024, American Chemical Society. d) Electricity generation mechanism of the TENG by a vertical contact‐separation mode. Reproduced with permission.^{[ 200 ]} Copyright 2024, John Wiley and Sons. e) A photograph of driving 23 LED lights and f) durability test of AgNWs/CNF/ANF aerogel‐based TENGs. Reproduced with permission.^{[ 166 ]} Copyright 2023, Elsevier.

Promising examples of such materials include MXenes and CNTs, both renowned for their high electrical conductivity and strong Seebeck effects. An alternating wet‐spinning method was employed to continuously and largely fabricate p/n‐type thermoelectric fibers, incorporating alternating n‐type MXenes (with a power factor of 0.098 µW m⁻¹ K⁻²) and p‐type MXene/SWCNT‐COOH (demonstrating a power factor of 0.179 µW m⁻¹ K⁻²) as the core materials. The addition of robust ANFs as a protective shell enhances both the flexibility and the efficiency of thermoelectric power generation.^{[ 94 ]} This thermoelectric fiber could generate a voltage signal when exposed to a high temperature to activate a fire warning system without requiring an additional power supply (Figure 27b). As the temperature difference increases, the output voltage and power density increase correspondingly, reaching 9.54 mV and 185.82 nW cm⁻², respectively. Furthermore, this composite aerogel fiber demonstrates remarkable mechanical stability and flexibility, with minimal resistance and voltage output changes after multiple cycles of heating, cooling, and bending. In addition to material improvements, the integration of PCMs into TEGs has emerged as a promising strategy for enhancing performance. PCMs are known for their high latent heat, which enables better thermal management and contributes to the stabilization of temperature gradients throughout the system. This enhanced thermal regulation improves the power output of TEGs.^{[ 151 ]} For example, Shi et al.^{[ 154 ]} demonstrated that incorporating PEG as a PCM effectively stabilized the temperature on the hot side, extending the duration of thermoelectric generation to approximately 10 minutes. Additionally, immersing the cold side of the TEGs in water further amplified the temperature differential, optimizing the overall energy conversion efficiency (Figure 27c). Under intense solar irradiation (4.0 kW m⁻²), their TEG system achieved a peak output voltage of 405.9 mV and a current of 36.9 mA, illustrating the potential for solar‐driven thermoelectric generation.

• ii)Triboelectric Nanogenerators (TENGs): TENGs are promising devices that convert mechanical energy into electrical energy by harnessing the triboelectric effect and electrostatic induction. The working principle of TENGs involves the generation of an electrical current through friction and contact electrification. When two materials with differing electronegativities come into contact and are rubbed together, electrons are transferred from one material to the other, resulting in charge accumulation and the development of a potential difference (Figure 27d).^{[ 200 ]} Upon separation of the materials, this potential difference drives the flow of electrons, generating an electrical current.

The material's ability to accumulate electrostatic charge is a critical factor for enhancing TENGs’ performance. The porous structure of ANF aerogels, which exhibit large SSA, electrical insulation, and thermal stability, makes them highly advantageous for electrostatic charge storage. Chi et al.^{[ 164 ]} pioneered the creation of anisotropic structures in ANF‐based composites using an in‐situ magnetic alignment technique. This method allowed for precise control of material orientation, which mitigated electronic thermal emission and enabled the composite aerogel to retain a high surface charge density of 75 µC m⁻², even at temperatures as high as 300 °C. As per Gauss's theorem, the efficiency of TENGs is positively correlated with the dielectric constant of the materials used, emphasizing the importance of material design in optimizing performance. In related research, Li et al.^{[ 200 ]} developed a dual‐network ANF composite aerogel fiber, which exhibited remarkable mechanical and thermal properties, including high tensile strength, low thermal conductivity, a large SSA, and superior flame resistance. Incorporation of BaTiO₃ into the aerogel enhanced its triboelectric performance. The resulting TENGs achieved an open circuit voltage (VOC) of 15.8 V, a short circuit current (ISC) of 0.18 µA, and a short circuit charge (QSC) of 6.0 nC (Figure 27d). Notably, even after exposure to a flame at 1100 °C for 5 seconds, the triboelectric performance remained stable above 94%, highlighting the durability and stability of the material under extreme conditions.

Meanwhile, due to their lightweight flexibility, comfort, and high‐temperature resistance, ANF‐based TENGs also show great promise for integration into self‐powered wearable devices, such as firefighting gear. He et al.^{[ 259 ]} developed MXene/AgNWs/ANF‐based TENGs designed for temperature sensing. The output voltage of these devices increased in proportion to the conductor content and the applied force. Following the completion of 2000 contact‐separation cycles and subsequent washing, the output voltage remained stable, indicating the TENG's robustness for applications in safety monitoring and rescue positioning in firefighting scenarios. Further improvements in the performance and durability of ANF‐based TENGs have been achieved by incorporating conductive materials. For instance, Chen et al.^{[ 260 ]} introduced PPy and CNTs into ANF composites, enhancing the material's network structure and interconnectivity via plasma treatment. This modification improved the wear resistance of the TENGs while enhancing their triboelectric output, as PPy acted as a strong electron donor. The hierarchical porous architecture of the aerogel enhanced charge storage capacity, boosting the output voltage. In these enhanced TENGs, the open‐circuit voltage (VOC) reached 110 V, the ISC was 15 µA, and the QSC was 0.09 µC. Even at elevated temperatures of 120 °C, the devices retained 40% of their room temperature voltage, demonstrating excellent thermal stability for energy harvesting and monitoring in high‐temperature environments.

However, while flexible electronics offer versatility and convenience, they often generate substantial EMI, which can degrade device performance and potentially harm human health. To address this challenge, researchers have been focusing on the integration of electromagnetic management capabilities within self‐powered flexible electronic devices. Chen and colleagues^{[ 163 ]} introduced a variety of conductive materials into ANF‐based TENGs, imparting EMWs absorption properties. These TENGs demonstrated stable ISC values of 81.2 nA, with the VOC increasing in response to both applied force and the content of AgNWs. The devices exhibited excellent charging performance, charging a 1 µF capacitor to 1.1 V within 60 seconds, with a peak output power of 1.16 µW. The mechanical stability of these TENGs was also impressive, as minimal performance degradation was observed following compression, twisting, and folding cycles. Further advancements were made by incorporating conductive AgNWs into ANF‐based aerogels.^{[ 166 ]} which combined triboelectric and EMI shielding properties. The addition of AgNWs enhanced the conductivity of the aerogel, while applied force facilitated charge transmission and accumulation, leading to improved output power. Under optimal conditions, these TENGs exhibited superior energy harvesting performance, with a VOC of 104 V, ISC of 1.52 µA, and a maximum power density of 694 mW m⁻². They were also capable of powering 23 LED lights. More importantly, there was no fluctuation or weakening of the electrical output in the operation of 17,480 cycles (Figure 27e,f). These results demonstrate the potential for ANF‐based TENGs to serve as durable and robust energy harvesters for practical applications in EMI management and energy generation.

In summary, TEGs and TENGs, particularly when integrated with advanced materials like ANF‐based aerogels and PCMs, offer significant potential for self‐powered, flexible electronics. These systems provide a sustainable alternative to traditional battery‐powered devices, with applications in fire monitoring, temperature sensing, and energy harvesting. Further advances in material design, including enhanced electromagnetic management and triboelectric performance, are likely to drive the next generation of self‐powered, multifunctional electronic systems.

5.4. Adsorption and Environmental Protection

Aerogels derived from ANFs exhibit exceptional porosity and high SSA, which provides numerous adsorption sites for various substances. The capillary forces within their micropores and mesopores can enhance their adsorption rate, making ANF aerogels and their composite counterparts highly promising for applications in different areas such as gas separation and storage,^{[ 174 ]} oil‐water separation,^{[ 101 ]} and environmental protection.^{[ 147 ]} Depending on the target adsorbate, the adsorption performance of ANF‐based aerogels can be broadly categorized into three main types: i) gas adsorption; ii) heavy metal ion adsorption; and iii) adsorption and filtration of oils, dyes, and organic solvents.

• i)Gas Adsorption: In gas separation and storage applications, ANF aerogels generally lack strong surface adsorption sites, limiting their adsorption capacity. To address this issue, various nanomaterials, such as MOFs, are often incorporated to enhance the performance of ANF aerogels. Combining ANFs and MOFs is particularly beneficial, as both materials offer abundant adsorption sites and transport channels.^{[ 261 ]} The aerogel matrix improves the loading capacity and stability of the MOFs, while the MOFs, in turn, enhance the overall adsorption performance by selectively adsorbing molecules through interactions like hydrogen bonding, π–π stacking, and electrostatic forces.

For instance, Zhao et al.^{[ 180 ]} developed a composite aerogel that combines ANFs with MOFs, achieved a low density of 5.86 mg cm⁻³, a high SSA of 636.62 m² g⁻¹, and a porosity of 99.33% created by an in‐situ loading and freeze‐drying method. The aerogel's mesoporous and macroporous structure facilitates efficient transport of CO₂ to increase adsorption efficiency and overall adsorption capacity. Additionally, the uncoordinated ─COOH groups and active metal sites of MOFs strongly interact with CO₂ molecules, significantly enhancing the affinity and adsorption performance. The resulting composite aerogel demonstrated a CO₂ adsorption capacity of 7.29 mmol g⁻¹ and exhibited high selectivity, with CO₂/N₂ and CO₂/O₂ selectivities of 39 and 42, respectively. Further research by Zhao and colleagues^{[ 174 ]} introduced UiO‐67‐SO₂, a sulfonated MOF, into ANF aerogels for CO₂ capture. The sulfonic groups, zirconium metal sites, and aromatic rings in the MOFs interacted with CO₂ molecules, ensuring stable adsorption performance even at elevated temperatures. Similarly, Duan et al.^{[ 177 ]} used coordination and π‐π stacking interactions between formaldehyde (HCHO) vapor and ZIF‐67 to create an ANF‐based composite aerogel with a high HCHO adsorption capacity of 5.31 mg g⁻¹, with the adsorption mechanism conforming to the pseudo‐second‐order kinetic model (Figure 28a). The ZIF‐67/ANF aerogels are also being explored for applications in personal protective equipment, specifically in the development of masks designed to adsorb and eliminate harmful gaseous contaminants from the environment (Figure 28b). The HCHO vapor, with a concentration ranging from 0.28 to 1.03 mg m⁻³, can be rapidly absorbed by the hybrid aerogels. In a related study, Ma et al.^{[ 175 ]} developed a high‐performance ANF‐ZIF‐8‐CNF aerogel with a N₂ adsorption capacity of 280.83 cm³ g⁻¹, and Wen et al.^{[ 96 ]} fabricated an ANF aerogel fiber designed for volatile organic compounds adsorption. The compelling findings from these studies highlight the remarkable versatility of ANF aerogels in diverse gas adsorption applications, indicating their significant potential to influence advancements in the field.

• ii)Heavy Metal Ion Adsorption: The adsorption and removal of heavy metal ions such as Cd²⁺, Hg²⁺, and Pb²⁺ from aqueous environments are critical for ecological protection and public health. Composite ANF aerogels have shown significant promise in this regard, mainly due to the incorporation of functional groups that interact with metal ions through complexation, ion exchange, and electrostatic interactions. Inspired by the natural structure of water hyacinth leaves, Zhang et al.^{[ 262 ]} synthesized a superhydrophobic ANF/PI composite aerogel, using an in‐situ silane polycondensation method. The aerogel's thiol groups are effectively complexed with Cu²⁺ ions (Figure 28c), achieving nearly 100% adsorption. In another study, Li et al.^{[ 263 ]} introduced polyamidoxime (PAO) into ANF aerogels, enabling efficient adsorption of uranium ions (UO₂ ²⁺), with a maximum adsorption capacity of 262.5 mg g⁻¹ at pH 6. The synergistic effects of electrostatic attraction and complexation are responsible for the enhanced adsorption performance. Zhang et al.^{[ 173 ]} developed a WS₂/ANF composite aerogel for Pb²⁺ removal, which simultaneously exhibited a stable micro‐crosslinked structure even at high temperatures (up to 300 °C for 7 days) and in aggressive solvents like ethanol, acetone, and HCl. Zhao et al.^{[ 172 ]} demonstrated the high performance of MnO₂‐loaded ANF aerogels in the adsorption of Pb²⁺ ions, achieving a maximum adsorption capacity of 554.36 mg g⁻¹ for Pb²⁺. Moreover, MOFs, combined with ANF aerogels, have demonstrated selective adsorption capabilities for various metal ions, including Cr⁶⁺, Cu²⁺, and Pb²⁺, making them highly effective for environmental decontamination.^{[ 177 ]}
• iii)Adsorption and filtration of oils, dyes, and organic solvents: ANF aerogels have attracted significant attention due to their uniquely tunable hydrophilic‐oleophilic balance and high porosity. The unique characteristics of ANF aerogels render them exceptionally proficient in the effective separation of oil‐water emulsions and mixtures. For instance, Gan et al.^{[ 101 ]} demonstrated that ANF aerogel membranes could effectively separate surfactant‐stabilized oil‐in‐water emulsions, achieving a high separation flux of 1940 Lm⁻² h⁻¹ and impressive separation efficiency of 98.1%. Notably, the aerogel membrane demonstrates a remarkable separation efficiency exceeding 98.0% even after ten cycles, highlighting its exceptional recyclability.

Figure 28.

a) Adsorption kinetics and b) schematic illustration of ZIF‐67/ANF aerogel textiles as protective masks. Reproduced with permission.^{[ 177 ]} Copyright 2024, American Chemical Society. c) Mechanism of Cu²⁺ adsorption of the aerogel. Reproduced with permission.^{[ 262 ]} Copyright 2021, John Wiley and Sons. d) Removal efficiency in mixed solution of MB and MO. Reproduced with permission.^{[ 264 ]} Copyright 2021, Elsevier. e) Selective adsorption of heavy oil (chloroform) under the water layer and light oil (n‐hexane) at top the water and f) adsorption capacities of the aerogel for different types of oils and organic solvents and g) adsorption recyclability of the aerogel for n‐hexane using mechanical squeezing and distillation for desorption. Reproduced with permission.^{[ 262 ]} Copyright 2021, John Wiley and Sons.

In addition to their separation capabilities, ANF aerogels also exhibit notable adsorption properties, particularly in dye removal. The negative charge of ANF aerogels, resulting from the formation of poly‐anions during dispersion, facilitates electrostatic interactions with cationic dyes, enabling selective and efficient adsorption. Yi et al.^{[ 264 ]} developed ANF/bacterial cellulose (BC) aerogels with a 3D network structure formed through hydrogen bonding and physical entanglement. These aerogels demonstrated a high adsorption capacity for the cationic dye methylene blue, removing 93.46% of it from mixed dye solutions, while only removing 5.42% of the anionic dye methyl orange (Figure 28d). This selective adsorption underscores the importance of electrostatic interactions in dye removal. Similarly, ANF/ZIF‐67 composite aerogels^{[ 179 ]} exhibited superior adsorption for the cationic dye crystal violet over methyl orange, with enhanced adsorption at higher pH levels, which was attributed to an increase in negatively charged sites and stronger electrostatic interactions between the aerogel and the dye. The structural design of ANF aerogels also plays a crucial role in their adsorption performance. Xu et al.^{[ 46 ]} prepared macroscopic aerogel spheres with core‐shell structures through a methodical approach that involved wet spinning, freeze‐drying, and thermal treatment. The heat‐induced crosslinking process converted simple physical entanglements between the nanofibers into a robust chemical network, enhancing the aerogel's compression strength and fatigue resistance. This reinforced structure allowed the aerogel to achieve a high adsorption capacity of 282.5 mg g⁻¹ for malachite green and demonstrated excellent cycling stability even after 500 compress‐release cycles at 50% strain. Additionally, these aerogels could adsorb various organic solvents.

For anionic dye adsorption, π–π stacking and hydrogen bonding interactions serve as the primary adsorption mechanisms. He et al.^{[ 38 ]} developed a PAMAM/ANF hybrid aerogel for adsorbing congo red, showing a much higher equilibrium adsorption capacity (1842.52 mg g⁻¹) as compared to pure ANF aerogel (537.72 mg g⁻¹). This improvement was attributed to the conversion of weaker π–π stacking interactions into stronger hydrogen bond interactions upon the incorporation of PAMAM. Despite their inherent advantages, ANF aerogels exhibit a significant limitation due to their hydrophilicity, which hinders their capacity to adsorb oils and organic solvents effectively. To address this issue, it is often essential to implement hydrophobic modifications to enhance their performance in these applications. Ma et al.^{[ 147 ]} employed MTMS using a vapor deposition process to modify ANF aerogels, rendering them hydrophobic. The resulting hydrophobic aerogels demonstrated rapid and complete removal of oil contaminants, including chloroform, toluene, and pump oil. Similarly, Zhang et al.^{[ 262 ]} fabricated hydrophobic ANF aerogels with unique hierarchical micro‐nanostructures. By soaking the aerogel in tetraethoxysilane (TEOS), followed by treatment with low surface energy siloxane, they created a Salvinia leaf‐like surface with superhydrophobic properties, which significantly enhanced the aerogel's efficiency in adsorbing various organic solvents (Figure 28e,f). Even after 50 cycles under mechanical compression or heating, the aerogel still retained over 80% of its adsorption capacity (Figure 28g).

In summary, due to their remarkable porosity, high SSA, and versatile surface chemistry, ANF aerogels represent an exciting material platform for a wide range of adsorption applications. Whether for gas separation, heavy metal ion removal, or the filtration of oils and organic solvents, these aerogels offer significant potential for addressing pressing environmental and industrial challenges. To maximize the commercial viability of ANF aerogels, it is essential to address critical challenges such as high production costs, restricted adsorption capacities, and the complexities associated with scaling up for mass production. Effectively addressing these issues will facilitate the broader adsorption of aerogel technology across various industries.

5.5. Sensors

Sensors are devices capable of detecting and responding to various environmental inputs, such as light, heat, motion, humidity, and pressure, typically by converting these inputs into electrical signals or other visual indicators for information transmission. The ideal sensor should possess high sensitivity, good selectivity and stability, rapid response and recovery capabilities, as well as strong resistance to interference.^{[ 265 ]} Aerogels, with their porous structure and high SSA, effectively prevent the aggregation of sensing materials, ensure the full exposure of active sites, and provide sufficient conductive pathways to promote faster response times and higher sensitivity. In particular, aerogels made from ANFs are particularly notable for their flexibility, chemical, thermal stability, and biocompatibility, making them a focus in sensor research.^{[ 94} , ¹⁶⁴ , ^{207 ]}

• i)Strain‐Pressure Sensors: Strain‐pressure sensors, which convert external forces into resistance changes, are widely used in wearable devices, health diagnostics, therapies, and human‐machine interactions. For instance, Wang et al.^{[ 207 ]} developed MXene/ANF composite aerogels with high conductivity network paths using controlled vacuum filtration and freeze‐drying techniques. These aerogels demonstrated remarkable elasticity (100% recovery after 1000 cycles), high sensitivity (128 kPa⁻¹), and ultra‐low detection limits (as low as 100 Pa) across a wide compression strain range (2.0–80.0%). Even after exposure to significant shock (≈623 kPa), the sensors retained stable performance, making them suitable for real‐time human dynamic monitoring and consistent electrical feedback. Yang et al.^{[ 148 ]} further improved these aerogels by applying a SiO₂ hydrophobic coating, enhancing their environmental adaptability. Song et al.^{[ 266 ]} engineered a 3D aerogel with highly ordered 1D pore arrays using rGO and ANFs. This design not only improved pressure transmission and conductivity but also maintained excellent piezoelectric effects across a wide temperature range (−196 to 200 °C). The multi‐dimensional hydrogen bonds between ANFs and rGO contributed significantly to the mechanical and electrical stability of the aerogel. Zhang et al.^{[ 208 ]} constructed a dual‐nanofiber crosslinked network of ChNF and ANFs, which exhibited a remarkable increase in maximum compressive stress, doubling that of pure ANF aerogels at 80% compressive strain. Due to the significant increase in stress, this aerogel was well‐suited for applications in human‐machine interaction sensors, such as throat vibration and handwriting recognition (Figure 29a,b).

Figure 29.

a) Relative resistive response from the vibration of the throat by saying the word "seven” and b) relative resistance response of the English letters “ABCD” written on the CAl‐Ms sensor. Reproduced with permission.^{[ 208 ]} Copyright 2023, Springer Nature. c,d) Schematics of the sensing principle of the sensor under stretching and bending deformations, and e–g) sensitive detection of H⁺ by ANF/CNT/PPy aerogel fiber. Reproduced with permission.^{[ 268 ]} Copyright 2022, American Chemical Society.

Conductive polymers, due to their excellent flexibility, low cost, and ease of processing, have also gained substantial attention as sensing materials. For example, Wang et al.^{[ 51 ]} conducted in situ polymerization of conductive PPy in anisotropic CNF/ANF aerogels, enabling the sensor to respond quickly and exhibit a good linear relationship with pressure changes. Additionally, the sensor demonstrated excellent compressibility, maintaining a stable response after 1000 cycles under 10% pressure. Similarly, Zhang et al.^{[ 160 ]} developed ANF/PPy/Ni aerogel materials with outstanding piezoelectric effects, high sensitivity, and mechanical properties. Zou et al.^{[ 267 ]} prepared rGO/ANF/PANI aerogels through freeze drying and low‐temperature annealing. This process formed a stable 3D crosslinked rGO, ANFs, and PANI network, enhancing the aerogel's mechanical strength, elasticity, and electrical conductivity. As a result, the aerogels exhibited improved pressure‐sensing capabilities, making them suitable for applications such as smart packaging. Huang et al.^{[ 268 ]} synthesized aerogel fibers composed of ANFs and CNTs, coated with PPy layers, using a wet spinning technique. During loading and unloading, the resistance changed synchronously with the pressure, allowing real‐time information transmission through pressure changes. When the ANF/CNT/PPy aerogel fiber sensor is stretched, the resistance increases as the tension disrupts the conductive network (Figure 29c). Conversely, when the sensor is bent, its thickness decreases, bringing the CNTs and PPy layers closer, resulting in an increased current (Figure 29d). The aerogel sensor exhibits a high sensitivity of 21 and 46 under low and high tensile loads, respectively, and maintains stability following 1000 deformation/recovery cycles, demonstrating rapid response and durability for signal transmission. Meanwhile, the sensor was highly sensitive to H⁺, with even small concentrations leading to a significant increase in electrical signals (Figure 29e,g). Therefore, by monitoring the concentration of H⁺ in a solution, its composition can be effectively determined, offering promising prospects for biomedical and health diagnostic applications.

• ii)Temperature Sensors: Temperature sensors are capable of detecting and measuring the temperature of an object or environment, and convert temperature changes into electrical signals, which can be read and processed by electronic devices or control systems. Generally, utilizing the Seebeck effect, thermoelectric materials such as MXenes^{[ 94} , ^{259 ]} and CNTs^{[ 260 ]} are introduced to achieve rapid high‐temperature sensing and warning. He et al.^{[ 259 ]} demonstrated the successful integration of MXenes with AgNWs to form ANF‐based aerogels. The prepared hydrogel fibers were subsequently freeze‐dried to obtain MXene/AgNWs/ANF aerogel fibers (MAA‐AF). Finally, the synthesized MAA‐AF was woven into the electronic textile (e‐textile) using a hand‐knitting machine. The resulting 3D interconnected conductive network, facilitated by hydrogen bonding between MXene nanosheets and AgNWs, enhanced the textiles' conductivity and electron transport properties. These MAA‐AF e‐textiles could sense temperatures ranging from 100 °C to 400 °C with a rapid response time of 1.6 s. Moreover, thermally sensitive materials can be also applied to gauge temperature due to their temperature‐dependent resistance and temperature‐tunable response. Yu et al.^{[ 171 ]} developed ANF/PEG/Fe₃O₄/PANI (APFP) aerogels, which combined fast response times (1.3 s when being burned) with effective thermal insulation properties. The Fe₃O₄ nanowires in these aerogels transition from an insulating to a conductive state at high temperatures, enhancing the conductivity of the aerogel for quicker temperature response. Additionally, the PEG, a PCM with its high latent heat of fusion, effectively regulated temperature, offering excellent buffering capabilities in extreme conditions. The APFP aerogels demonstrated significant thermal insulation, with surface temperatures rising by only 2.8 °C even when exposed to 580 °C in high humidity. These properties, combined with their fast response and flame‐retardant characteristics, make them highly promising for applications in protective firefighting apparel.
• iii)Tactile Sensors: Tactile sensors are devices designed to detect physical signals of external objects, including touch, pressure, vibration, and temperature. By simulating human tactile perception, these sensors transform physical stimuli into digital or analog signals suitable for processing by electronic systems, which are critical components in numerous applications, particularly in robotics, smart gloves, intelligent devices, medical instruments, and virtual reality systems. Chi et al.^{[ 164 ]} proposed an ANF/CNT@Ni triboelectric aerogel for wearable tactile sensing device in high temperature conditions. Due to the triboelectric effect, charges were immediately generated upon contact and separation, and subsequently harvested by surface electrodes and transduced into electrical signals, enabling tactile perception. High‐temperature performance tests reveal a decline in VOC and transferred charge with increasing temperature; however, the aerogel still maintains high sensitivity and real‐time response capabilities. This characteristic confers unique advantages to aerogels for tactile sensing applications within high‐temperature environments, especially in military and aerospace fields.

Current sensor technologies primarily depend on external battery power, which limits their operational longevity and poses challenges for continuous monitoring applications. Consequently, the advancement of self‐powered sensors is essential for facilitating independent, convenient, and low‐maintenance monitoring solutions. Self‐powered sensors typically operate by harnessing environmental energy for self‐sustenance. These sensors are particularly valuable for continuous monitoring in diverse fields, as they can provide a sustainable and cost‐effective alternative to conventional battery‐powered devices. Recent developments in the field have demonstrated significant progress in creating sensors with enhanced functionality and stability. For instance, Chen et al.^{[ 163 ]} developed a self‐powered sensor combining AgNWs and CoNi@C as sensing materials, with an ANF matrix as the reinforcing material and CNF as a crosslinker (Figure 30a). This combination created conductive and magnetic aerogels, which were then embedded in polydimethylsiloxane (PDMS) to form compressible elastomers. More notably, when integrated with TENGs, the sensor could operate independently without external power sources. Despite being exposed to extensive mechanical stresses such as compression, twisting, and folding, the sensor's output performance remained remarkably stable (Figure 30b). Chi et al.^{[ 164 ]} introduced ANF/CNT@Ni triboelectric aerogels, fabricated by in situ coupling magnetic alignment and proton reduction techniques. The resulting aerogels possessed a precisely controlled anisotropic structure that effectively mitigated electron neutralization and emission effects at elevated temperatures (up to 300 °C), enhancing both electro‐storage performance and thermal stability. When incorporated into wearable gloves, these aerogels could quickly detect temperature changes and gestures on object surfaces, offering significant potential for human‐machine interaction and environmental monitoring.

Figure 30.

a) The structure diagram of the assembled piezoresistive sensor and b) stability of aerogel elastomer‐based TENGs. Reproduced with permission.^{[ 163 ]} Copyright 2024, Elsevier. c) Fire warning test for sensor‐based fire alarm device when exposed to alcohol lamp fame and d) schematic of early fire warning response mechanism and e) stability of sensor in fire warning test. Reproduced with permission.^{[ 94 ]} Copyright 2023, Springer Nature.

For temperature sensors, the self‐power ability holds considerable implications for fire rescue applications. This ability enhances sensor robustness and operational safety, facilitating continuous functionality within the challenging and dynamic conditions of fire environments, thereby mitigating power supply‐related failures. By minimizing reliance on external power sources, these self‐powered temperature sensors can better achieve fire monitoring, rapid warning, and rescuing. This, in turn, improves fire response efficacy, reduces risks, and helps ensure the safety of personnel. The integration of temperature sensors with TENGs represents a promising approach for achieving self‐powered operation.^{[ 259} , ^{260 ]} Chen et al.^{[ 260 ]} designed an ANF/MWCNT/PPy aerogel‐based TENG that demonstrated robust performance at high temperatures and a remarkable output capacity (80 V, 20 µA at 120 °C). Furthermore, it can accurately transmit vital signs and escape routes of survivors in a fire scene, thereby enhancing search and rescue efficiency. A later study by He et al.^{[ 94 ]} utilized both n‐type MXene and p‐type MXene/SWCNT‐COOH as sensing materials to fabricate p‐n segment aerogel fibers through coaxial wet spinning. Different from the principle of TENG, the Seebeck effect enables voltage generation and self‐powered operation by applying a temperature gradient across n‐type and p‐type thermoelectric materials. The sensor serves as a reliable fire warning e‐textile, capable of detecting surface temperature changes in firefighting garments due to its linear temperature‐voltage relationship. It triggers an alarm within 2 seconds upon flame exposure and can detect temperature differences, activating the fire warning system (Figure 30c). Moreover, the sensor can reactivate the fire warning system after it is exposed to flames due to the reversible response characteristics of the sensing material (Figure 30d). Meanwhile, these sensors displayed impressive stability, with voltage fluctuations below 9.6% during repeated fire alarm tests (Figure 30e).

Overall, this section has explored the recent advancements in strain and temperature sensors, with a particular focus on the development of self‐powered, flexible, and porous aerogels. The distinctive properties of ANF aerogels, characterized by their flexibility and high porosity, position them as optimal materials for applications in wearable electronics and physiological signal sensors. However, several challenges currently impede the optimization of sensor performance. These challenges include the need to balance sensitivity with operational range, effectively decouple signals under multiple stimuli, and integrate multifunctional sensing capabilities within the devices. Future research should focus on refining material compositions and sensor architectures to further enhance detection range, improve performance, achieve ultra‐low detection limits, and enable distinct responses to various environmental stimuli, ensuring the next generation of sensors can meet the demands of increasingly complex and dynamic monitoring applications.

5.6. Infrared Stealth

Recent military science and technology advancements have significantly enhanced infrared (IR) reconnaissance and targeting capabilities, driving the need for effective IR stealth materials. These materials are critical for improving military operational efficiency, aircraft survivability, and protecting individual privacy. The primary objective of IR stealth technology is to minimize or alter the IR radiation emitted by a target, making it less detectable by IR sensors.^{[ 269 ]} Achieving IR stealth requires a reduction in the object's surface temperature and IR emissivity. Among the materials studied for this purpose, ANF‐based lightweight aerogels have gained attention due to their exceptional thermal insulation properties, including low thermal conductivity and high‐temperature resistance.^{[ 270 ]} These characteristics make ANF aerogels promising candidates for IR stealth applications.

One noteworthy study by Yao et al.^{[ 217 ]} introduced an innovative asymmetric aerogel structure composed of layered MXene/ANF and 3D porous MXene/ANF/PI (MAP). Compared to the relatively rough surface of conventional aerogels, the MXene layer plays a crucial role in reducing IR radiation. The Yao team found that the MAP aerogel exhibited IR emissivity values of 0.32 in the 3–5 µm range and 0.28 in the 8–14 µm range, much lower than the 0.68 average emissivity of traditional aerogels. Later, Yao and colleagues^{[ 204 ]} enhanced the IR stealth performance by developing a biomimetic exoskeleton structure using MXene/PI/ANF aerogels, which not only improved the mechanical properties of the aerogels but also enabled efficient IR stealth at temperatures up to 200 °C. At these elevated temperatures, the IR radiation temperature of the aerogel was only 91.3 °C. Theoretical simulations revealed that the ordered arrangement of MXene/ANF surfaces, combined with field confinement and localization effects, reduced local high electric fields, effectively lowering IR emissivity and enhancing stealth capabilities. Additionally, Dang et al.^{[ 210 ]} employed a secondary re‐protonation strategy to create a bilayer asymmetric MXene/ANF aerogel film (Figure 31a). This composite structure combined a dense MXene layer with low IR emissivity and a 3D porous aerogel layer offering high thermal insulation. Further development in their subsequent research^{[ 271 ]} demonstrated that a moderately protonic environment, such as HCOOH, strengthened the hydrogen bonding between ANF and MXene, which optimized the film's structure and enhanced its IR stealth performance.

Figure 31.

a) Fabrication of asymmetric ANF/MXene aerogel film and its IR stealth mechanism. Reproduced with permission.^{[ 210 ]} Copyright 2023, Springer Nature. b) Preparation of ANF/PCM aerogel and IR stealth mechanism of ANF and ANF/PCM films to the hot target. Reproduced with permission.^{[ 39 ]} Copyright 2019, American Chemical Society.

However, the effectiveness of IR stealth materials is limited by the challenge of maintaining consistency between the radiation intensity of a target object, coated with low thermal IR radiation materials, and its background environment. The object's coating may only match the environmental radiation at certain temperatures, and significant variations in surrounding temperatures could cause the coating to fail, reducing its stealth capabilities. As such, the development of intelligent dynamic IR stealth systems has become a critical area of research. To address this challenging issue, PCMs have been explored because their high latent heat of phase transition enables them to absorb and release significant amounts of heat during phase changes. This characteristic makes PCMs particularly valuable for temperature regulation and thermal radiation control. Zhang et al.^{[ 157 ]} developed PEG/ANF aerogel films via freeze‐drying, which exhibited good thermal stability and mechanical properties. The PEG component effectively absorbed IR radiation, suppressing the target's thermal radiation. A 2 mm thick aerogel film could prevent detection by thermal imaging cameras. However, if the target continued to emit heat, the surface of the ANF/PCM film would eventually heat up, causing its thermal radiation to exceed that of the surrounding background. Hence, the researchers proposed using insulation layers to prevent heat transfer. Lyv et al.^{[ 39 ]} introduced a composite structure that combined an ANF film (acting as a thermal insulation layer) with an ANF/PCM aerogel film (acting as a low IR transmission layer), to effectively conceal the target from IR detection (Figure 31b). Additionally, the incorporation of FC resin provided hydrophobicity, ensuring the aerogel's IR stealth capabilities remained stable, even in humid environments. The synergy between low IR emissivity coatings and high‐enthalpy PCMs enabled long‐term, efficient IR stealth. Furthermore, Liu et al.^{[ 158 ]} developed a multi‐layered tandem stealth cloak based on ANF aerogels. The top layer consisted of Ag/ANF films with high thermal resistance, low IR emissivity, and high optical reflectivity. The middle layer comprised PCM/ANF films, offering thermal management and hydrophobic properties. The bottom layer featured an ANF/Ag film, providing low IR transmission, high thermal reflectivity and resistance. This aerogel effectively suppressed IR radiation, regulated target temperature, and provided thermal insulation, resulting in long‐term (up to 12 hours) and efficient IR stealth, with a temperature difference of less than 1 °C.

In summary, the integration of advanced materials such as ANF aerogels, MXenes, and PCMs holds great promise in advancing IR stealth technologies. These materials enhance IR emissivity control and enable dynamic, adaptive systems that maintain stealth capabilities under varying environmental conditions. Future research will likely focus on optimizing these material's performance, to provide robust, long‐term IR concealment for military and privacy protection applications.

6. Closing remarks

As a novel class of biomimetic nanomaterials, ANF aerogels and their composites display an exceptional combination of properties–low density, mechanical strength, high‐temperature stability, nanoscale porosity, and water resistance. The synergy of properties conducive to high performance with recyclability significantly broadens their potential applications across various fields. Moreover, breakthroughs have been made in the production of ANF‐based aerogels and composites, owing to their mix and match versatility, well understood synthesis, and suitability for multiple technologies, which have greatly enhanced attention to ANF from academic and industrial communities. For instance, the application domains of aerogels have expanded from thermal insulation to electromagnetic shielding, energy technologies, and beyond. Researchers have also developed a variety of innovative methods to fabricate ANF aerogels in various forms (e.g., particle, fiber, sheet), catering to diverse performance requirements and extending their range of applications. With the rapid advancements in manufacturing technologies, 3D‐printed ANF aerogels are also becoming increasingly feasible. Despite the potential technical challenges ahead, we are confident that the application of new technologies will drive the development of a new generation of low‐cost and tunable ANF aerogels, which have the potential to incorporate novel functionalities and expand current application areas, particularly in energy, sensing, and biomedical fields.

6.1. Challenges

Future studies of ANF‐based materials also need to address several challenges. Some of them that were not highlighted in the text above include structural description, solvent optimization, and morphological control.

6.1.1. Structural Description

Unlike crystalline materials with distinct ordering, all aerogels including ANF‐based ones, display a seemingly random organizational pattern that is not possible to describe using traditional chemical approaches optimized for and derived from crystalline materials. Aerogels have more complex organization than crystals, combining order and disorder, which leads to multiscale organizational patterns. The incorporation of additional components leading to multifunctional composites as depicted in Figures 17, 18, 19, 20 makes it even more difficult to apply the current methodologies of structural description. The necessity to have a comprehensive and quantitative representation of the structural organization of these structurally and functionally sophisticated materials is difficult to overestimate because the make‐it‐and‐test‐it empirical approach to their design is no longer viable even with the statistical tools of experimental design and machine learning (ML). The urgency of the development of sustainable technologies augments this challenge.

6.1.2. Solvent Optimization

KOH/DMSO or strong acids used in the synthesis of ANF aerogels and related composites are corrosive and wasteful. Similar to the current large‐scale manufacturing of Kevlar, these problems can certainly be mitigated, and the solvents can be recycled. Nevertheless, finding solvent mixtures that can reduce the amount of KOH while controlling the nanofiber morphology will facilitate the technological adoption and large‐scale production of ANFs.

6.2. Opportunities

The challenges also define research opportunities in the fields of ANF aerogels, and composites. In addition to further development of the specific applications described, for instance in Figure 2, the following should be pursued with special attention.

6.2.1. Recycling Process

The total production of aramid‐based materials including Kevlar, Nomex, and Twaron exceeds 55 000 ton per year, and is increasing by 5–10% annually.^{[ 272 ]} Unlike polyethylene, polypropylene, and polyethylene terephthalate, much of the aramid products have a separate waste stream from the large‐scale commercial plastics and are easier to carry out, which makes the opportunity to produce ANFs from re‐using the bullet‐proof vests, truck tarps, and brake pads particularly attractive. Nevertheless, it does involve addressing difficult challenges such as variability of the feedstocks, added impurities, and polymer chain degradation. This research effort is worth pursuing in conjunction with solvent optimization and structural standardization. To provide quantitative justification for the process of Kevlar recycling, we add Life Cycle Analysis (LCA) in the Supporting Information. The LCA calculations clearly indicate the viability of aramid recycling into ANFs.

6.2.2. Advancing the Knowledge of Complex Materials

The research on ANF aerogels can also advance the basic knowledge across multiple fields connected to the complex structural patterns that they display at nanometer and other scales. The combination of order and disorder requires a fundamentally different toolbox for structural description of ANF gels and their composites than what is currently used for amorphous solids, crystals, and quasicrystals.^{[ 29} , ²⁷³ , ^{274 ]} The structural description based on Graph Theory (GT) may offer an opportunity to address this challenge (Figure 32 ). In this case, the linear segments of the nanofibers can be described as graph edges while their intersections are described by nodes.^{[ 29 ]} This results in structural graphs that are applicable to a wide range of biological and human‐made materials, exemplified by collagen gels, ANF gels, and networks of metal nanowires (Figure 32a–f). The comparison of GT parameters extracted from the SEM images of these materials shows clear similarities and differences in their organization. The close similarity between short‐, medium‐, and long‐range organization of collagen and ANF gels described by GT parameters, such as nodal degree, clustering coefficient, and betweenness centrality, respectively (Figure 32g–i), points to the cartilage‐like organization of ANF aerogels. Similarly, the difference between structural motifs governing the organization of rigid gold nanowires and flexible polymeric nanofibers becomes obvious when comparing graph density, closeness centrality, and betweenness centrality—all long‐range GT descriptors.^{[ 275 ]}

Figure 32.

Identification of structural patterns of ANF aerogels and other complex biomimetic materials using Graph Theory (GT) descriptors. SEM images (a,c,e) and corresponding GT model (b,d,f) of nanofibers in the ANF aerogel (a,b); nanofibers in collagen aerogel (c,d); gold nanowires in the metal nanoscale network (e,f). g) The list of the GT parameters used for the structural description of the cartilag‐elike nanofiber networks. h) Spider plot with the comparison of GT parameters for the nanofiber and nanowire networks in (a–f). Reproduced with permission.^{[ 275 ]} Copyright 2021, Springer Nature. i) Dependence of toughness and Young's modulus on Average Clustering Coefficient of ANF networks made with different concentrations of ANFs.^{[ 30 ]}

The correlation between the mechanical properties and GT parameters, such as the average clustering coefficient describing the prevalence of triangular structural motif in nodes and edges, points to the possibility of their utilization for the design of mechanical properties. The latest data and detailed GT study for nanowires also indicate the possibility of simultaneous optimization of mechanical, electrical, and electromagnetic properties.^{[ 29 ]}

6.2.3. ANF Aerogels, Complex Materials, Artificial Intelligence, and Machine Learning

Materials design using ML and artificial intelligence (AI) is developing very rapidly. However, the AI/ ML models are non‐transparent and require a lot of data for training. Among emergent AI tools for materials design, Large Language Models (LLMs) can be particularly powerful and can potentially serve as a universally applicable model to predict nanostructured phases and non‐equilibrium assemblies with complex morphologies, which will represent a paradigm shift in the design of these essential materials. However, LLMs are known to create physically unreal or inaccessible materials that are often described as “hallucinations”.

Due to the direct ground truth verification by imaging, ANF and similar aerogels can provide nearly ideal testing grounds for the development of AI/ML tools. GT models directly extracted from electron microscopy images are expected to provide a pathway for the physics‐, network‐ and GT‐based verification of AI predictions. The transitions from spectrum and pixel‐based data input to GT structural descriptors will drastically reduce the need for exorbitant data loads for training traditional AI algorithms, and specifically LLMs. Adaptation of graph neural networks (GNN)^{[ 276 ]} to the typical graph patterns encountered in the GT models extracted from ANF and similar aerogels will further accelerate materials design on their bases.^{[ 5 ]}

Conflict of Interest

N.A.K is a founder of a start‐up company working on ANF applications in energy technologies.

Supporting information

Supporting Information

Biographies

Mingqiang Wang received his Ph.D. degree from Harbin Institute of Technology (HIT) in 2019 under the supervision of Professor Yudong Huang. During his doctoral studies, he worked as a visiting scholar at the University of Michigan, conducting research under the guidance of Professor Nicholas N. Kotov. From 2019 to 2022, he completed his postdoctoral research at Harbin Institute of Technology. Currently, Dr. Wang serves as an Associate Professor at Harbin Institute of Technology. His research primarily focuses on aramid nanofiber composites, fiber‐reinforced polymer matrix composites, structural design using graph theory, synthesis of high‐performance organic fibers and functional nanofiber aerogels.

Nicholas A. Kotov pioneered the theoretical foundations and practical implementations of biomimetic nanostructures and nanocomposites. He demonstrated that the ability to self‐assemble is a universal property of nanostructures. They can be synthesized de novo with structural complexity comparable to or exceeding their biological prototypes. Examples of biomimetic nanocomposites that he developed include nacre‐like layered materials from graphite oxide, montmorillonite clay, and cellulose nanofibers. Chiral nanostructures, complex assemblies, and graph theoretical (GT) representations are the focal points of his current work. Nicholas founded seven start‐ups commercializing complex self‐assembled nanocomposites and chiral nanoparticles for sustainability and health technologies. Nicholas is a Fellow of the American Academy of Engineering, the American Association for Advancement of Science, the American Academy of Inventors, and the American Academy of Arts and Sciences. He is also a recipient of over 60 national and international awards that include the 2024 Chirality Medal, 2024 Centenary Prize, 2020 Newton Award and 2016 Stephanie Kwolek Award. Nicholas is an advocate for scientists with disabilities.

Wang M., Hu S., Bae S., et al. “Aramid Nanofiber Aerogels: Versatile High Complexity Components for Multifunctional Composites.” Adv. Mater. 37, no. 38 (2025): 37, 2502508. 10.1002/adma.202502508