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Preparation and Application of Ultralight Nanofiber Aerogels

Fengqi Liu , Yonggang Jiang , Fei Peng , Junzong Feng , Liangjun Li , Jian Feng

Progress in Chemistry ›› 2022, Vol. 34 ›› Issue (6) : 1384 -1401.

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Progress in Chemistry ›› 2022, Vol. 34 ›› Issue (6) : 1384-1401 DOI: 10.7536/PC210805
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Preparation and Application of Ultralight Nanofiber Aerogels
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Abstract

Ultralight nanofiber aerogel is a new type of aerogel material with one-dimensional nanofibers as the basic building unit. Compared with traditional aerogels, it not only has a higher porosity and lower density but also with more excellent mechanical properties and physical and chemical properties thus the advanced preparation technology of this material and its innovative applications in emerging fields have become a research hotspot in the field of ultralight aerogels. Based on the research status, this paper systematically reviewed the preparation methods and structural characteristics of ultralight fiber aerogels and their important applications in the fields of thermal insulation, adsorption, electrode, sensing and biomedicine according to different materials systems. Moreover, some challenges faced by the material at this stage are put forward, and its future development direction has prospected.

Contents

1 Introduction

2 Preparation of ultralight nanofiber aerogels

2.1 Ultralight inorganic nanofiber aerogels

2.2 Ultralight carbon nanofiber aerogels

2.3 Ultralight organic nanofiber aerogels

3 Application of ultralight nanofiber aerogels

3.1 Thermal insulation materials

3.2 Adsorption materials

3.3 Electrode materials

3.4 Sensing materials

3.5 Biomedicine materials

4 Conclusion and outlook

Graphical abstract

Keywords

ultralight / nanofiber aerogel / freeze drying / electrospinning / superelastic

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Fengqi Liu, Yonggang Jiang, Fei Peng, Junzong Feng, Liangjun Li, Jian Feng. Preparation and Application of Ultralight Nanofiber Aerogels. Progress in Chemistry, 2022, 34(6): 1384-1401 DOI:10.7536/PC210805

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1 Introduction

Aerogel is a kind of highly dispersed solid material with nanoparticles as the skeleton and gas as the dispersion medium, which is formed by drying to replace the liquid phase in the gel. Its complex three-dimensional network structure, nano-scale pore size and extremely low solid volume ratio make it have high specific surface area, low thermal conductivity and low density[1,2]. Since the advent of SiO2 aerogels in 1931, in the following decades, the preparation technology of aerogels has been developed rapidly and expanded to form a rich system.It mainly includes inorganic oxide aerogels ( S i O 2 , Al2O3, Z r O 2 , etc.), organic aerogels (polyimide, polyurethane, cellulose and its derivatives, etc.), organic-inorganic hybrid aerogels [diisocyanate, styrene, alkyl trialkoxysilane and other polymer-reinforced silica aerogel], carbide aerogel (SiC, TiC, graphene, etc) and carbon aerogel-like[3][4][5][6][7][8][9][10][11][12][13][14][15][16].
However, with the rapid development of new industries, the traditional aerogel (ρ>0.1 g/cm3) prepared by sol-gel method has been difficult to meet the stringent requirements for more lightweight materials in the future. According to statistics, the launch cost will be saved by about $20,000 for every 1 kg reduction in the weight of the aircraft. If the weight of the fighter is reduced by 15%, the taxiing distance of the aircraft can be shortened by 15%, the range can be increased by 29%, and the payload can be increased by 30%. The fuel efficiency can be increased by 6% ~ 8% when the weight of the fuel vehicle is reduced by 10%[17]. It can be seen that material lightweight can bring significant economic benefits and performance improvement. Compared with traditional aerogels, ultra-light aerogels not only show lower density, which is conducive to the lightweight of materials, but also have the advantages of high surface activity, high porosity, low dielectric constant and other properties, which have great prospects for development in many fields such as thermal insulation, catalysis, optics, electricity and so on[18,19]. The density of aerogel can be reduced by adjusting the ratio of raw materials and solvents in the sol, but the reduction of the density of aerogel materials by this method is limited, and the prepared low-density aerogel skeleton is fragile and easy to collapse, so researchers are committed to using new methods and new material systems to prepare new ultra-low density aerogels[20].
Generally speaking, materials with a density lower than the 50 mg/cm3 are defined as ultra-lightweight materials. In recent years, the lower density limit of ultra-light aerogels has been continuously broken through, such as SiO2 aerogel, graphene aerogel, boron nitride (BN) aerogel and so on, which can reach the lowest density of 0.1 mg/cm3, but the inefficient connection between spherical or flaky microstructures leads to poor mechanical properties[21][22][23,24]. Like most porous ceramic materials, ceramic aerogels are generally rigid or otherwise brittle, with little elastic deformation before fracture unless they are mixed with polymers or grown on a pre-existing carbon network[23]. Since the elasticity of ceramic materials is intrinsically inferior to that of polymers or carbon, it is still a great challenge to construct ultra-light flexible porous ceramic networks.
The properties of aerogels are usually determined by the inherent properties of the structural units, so enhancing the effective connection between the aerogel microstructures is an effective way to improve the mechanical properties. The study shows that brittle SiO 2 , SiC and ZrO 2 ceramics can be made into nanofibers with ultra-high aspect ratio, which can become soft and elastic[25][26][27]. Inspired by this, researchers have proposed a new idea of using flexible nanofibers with high strength and high modulus as the basic unit of three-dimensional aerogel, which is expected to overcome the shortcomings of poor mechanical properties of traditional aerogels and prepare new high-performance fiber aerogels with ultra-light and good mechanical properties. In this paper, based on the research progress of ultralight nanofibrous aerogels at home and abroad in recent years, the preparation methods and structural characteristics of fibrous aerogels are described in detail according to different material systems, and the application status in the fields of heat insulation, adsorption, electrochemistry, sensing and biomedicine is summarized, and the future development direction is prospected.

2 Preparation of ultralight fiber aerogel

2.1 Ultralight inorganic nanofiber aerogel

Inorganic nanofiber aerogel systems mainly include SiO2, SiC, ZrO2, Al2O3, etc. Compared with organic fiber aerogels, inorganic fiber aerogels have good chemical stability and thermal stability, which expands the application of fiber aeragels in the field of high temperature.

2.1.1 SiO2 fiber aerogel

Traditional SiO2 aerogel is a mature aerogel material at present, but the further low density of SiO2 aerogel is still a difficult problem, so SiO2 fiber aerogels with ultra-low density and good mechanical properties are expected to attract extensive research. Si et al. Firstly prepared ultralight nanofiber aerogel (0.12 mg/cm3) with hierarchical pore structure by using electrospun polyacrylonitrile (PAN) fiber and SiO2 fiber as raw materials and cyclooxazine as crosslinking agent after freeze-drying and thermal crosslinking (Fig. 1)[28]. The aerogel has superelasticity, excellent oil absorption (up to 15000 times of its own weight) and excellent thermal insulation (thermal conductivity at room temperature is 0.026 W/ (m · K)). In order to improve the temperature resistance of fiber aerogel, the team further used inorganic fibers as raw materials and aluminosilicate borate (AlBSi) as a high-temperature binder to obtain ultra-light flexible SiO2 nanofiber aerogel with a density of 0.15 mg/cm3 after directional freeze-drying and high-temperature calcination[29]. The multistage honeycomb structure and the strong bonding between fibers endow the aerogel with superelasticity, which can quickly recover to its original shape at 80% strain and maintain good elasticity at 1100 ℃. The high porosity and ultra-low density of fiber aerogel effectively suppress the solid thermal conduction, resulting in a low thermal conductivity of only 0.025 W/ (m · K) at room temperature.
Recently, Wang et al. Also prepared ultralight SiO2 nanofiber aerogel with a density as low as 0.25 mg/cm3 by introducing hydrolyzed silane sol into the dispersion system to construct an elastic Si-O-Si cross-linked network in situ during freeze-drying[30]. Unlike previous studies, this method does not require further calcination, thus avoiding the formation of brittle microcrystalline structure at high temperature and effectively improving the mechanical properties of aerogels. With a strong biomimetic skeleton and elastic bonding sites, the aerogel exhibits superelasticity over a wide temperature range (-196 ~ 1100 ℃) and excellent fatigue resistance, and does not fail after 1 million compression cycles. In addition, the aerogel exhibits low thermal conductivity (0.024 W/ (m · K)), high temperature resistance, and high porosity, which opens up more possibilities for a range of applications such as sound absorption, thermal insulation, and catalysis. However, because the SiO2 itself does not have infrared shielding performance and there are a large number of micron-sized through holes in the aerogel, it will theoretically lead to higher thermal conductivity at high temperature, so it has not been used in the field of high temperature insulation[31].

2.1.2 SiC fiber aerogel

Compared with SiO2 aerogels, the study of SiC aerogels started later, which were mainly prepared by shape memory synthesis (SMS), using the carbothermal reduction reaction between carbon source and SiO2 skeleton to obtain SiC aerogel with porous structure inherited from SiO2[32]. However, the SiC aerogel skeleton prepared by this method is still point contact between particles, which is difficult to overcome the inherent brittleness of aerogel.
SiC nanofiber aerogel with a thickness of 3.5 mm was grown on a graphene substrate by chemical vapor deposition (CVD) using gases such as SiO and CO produced by the decomposition of siloxane gel at a high temperature of 1450 ℃ (argon atmosphere) as Si and C sources (Fig. 2)[33]. The aerogel is composed of interwoven ultrafine SiC nanofibers with diameters of 20 – 50 nm and lengths of tens to hundreds of micrometers, with a density of only 5 mg/cm3 and a room temperature thermal conductivity of 0.026 W/ (m · K) in nitrogen. Benefiting from the ultra-high aspect ratio and flexibility of the nanofiber structural unit, it has excellent mechanical properties, with almost no loss of energy loss coefficient, Young's modulus and compressive strength after 1000 compression cycles (ε = 60%).
Similarly, Ren et al. Prepared SiC/SiOx nanofiber aerogel with core-shell structure by CVD method at 1400 ℃ using silicon powder and metallurgical coke as raw materials[34]. The SiOx shell not only acts as a binder to firmly bond the SiC fibers, but also prevents the further oxidation of the SiC fibers at high temperatures, so the aerogel has excellent mechanical energy in the temperature range of -196 ~ 1100 ℃, and does not undergo structural damage even if it undergoes large compression deformation (ε = 50%). However, SiC aerogels grown on planar substrates do not have a true three-dimensional structure (flaky), and it is difficult to achieve an effective increase in the thickness direction, which seriously limits their practical application.
In order to solve this problem, researchers used bulk carbon materials as growth substrates, and further combined with CVD method to prepare SiC aerogels with three-dimensional structure. For example, Li et al. Used polycarbosilane (PCS) as a SiC precursor and carbon foam loaded with Ni2+ as a skeleton to carry out SiC fiber self-growth at 1300 ° C, and obtained pure SiC nanofiber aerogel with a density of 30 mg/cm3 after low temperature oxidation to remove carbon[35]. The aerogel has excellent high-temperature stability and thermal insulation performance, and the thermal conductivity is only 0.03 W/ (m · K) at room temperature. Similarly, Cheng et al. Prepared SiC nanofiber/graphene composite aerogel with hydrophobic and electromagnetic wave absorption functions by using graphene nanosheets loaded with Ni(NO3)2 catalyst as the growth template and CH3SiCl3 vapor as the Si and C sources after heat treatment at 1100 ° C[36]. CVD is a common method to realize fiber self-assembly, but the production efficiency of CVD is low, so it is difficult to prepare fiber aerogel on a large scale.
Ferraro et al. Prepared SiC fiber aerogel with tunable acoustic impedance (0.03 – 0.3 mol/L Rayl), high strength (3 MPa) and high stiffness (0.3 GPa) by directional freeze-drying using electrospun SiC chopped fiber as raw material, chitosan as fiber stabilizer, and Al2O3 and Y2O3 as binder, and it also had electrical conductivity and low thermal conductivity[37]. Recently, Liang et al. Skillfully combined "cooking" with chemistry to develop a green and low-cost method for preparing large-size three-dimensional SiC fiber aerogel (Fig. 3)[38]. Firstly, the carbon foam template with high porosity was prepared by carbonization of bread doped with Si powder, and then the SiC fiber aerogel was prepared by in-situ growth and self-assembly of SiC nanowires at high temperature. The material shows low density (55 mg/cm3), excellent high-temperature stability (1100 ° C), and good thermal insulation properties (0.038 W/ (m · K) in air at 200 ° C).

2.1.3 Al2O3 fiber aerogel

SiC fiber aerogel has excellent mechanical properties and rich functional properties, but its grain will grow rapidly at high temperature above 1100 ℃.Show structural collapse and large volume shrinkage, advanced ceramic aerogel materials with both sufficient mechanical strength and excellent high temperature resistance are urgently needed as thermal insulation materials in harsh environments[33]. Al2O3 is a kind of oxide with good thermal stability at high temperature. As the most stable phase of Al2O3, α-Al2O3 can theoretically withstand high temperatures above 1800 ℃. Therefore, researchers usually use Al2O3 alone to prepare high temperature resistant fiber aerogels or introduce Al2O3 components into the system to improve the temperature resistance of fibers.
Based on the hydrothermal method, Zhou Liang et al. Of Harbin Institute of Technology first prepared boehmite short fibers with a length of about 2 μm by using aluminum sec-butoxide as an aluminum source, then added chitosan to make the fibers disperse evenly, used ammonia vapor to assist the gel, and finally prepared low-density Al2O3 fiber aerogel (ρ=30 mg/cm3) by freeze-drying[39]. The thermal conductivity of the aerogel at room temperature is 0. 04 W/ (m · K), and the mass retention rate at 1 000 ℃ is still 71. 3%. Nakanishi et al. Firstly prepared ultra-light Al2O3 fiber aerogel (ρ=1.2 mg/cm3) with optical transparency using boehmite fiber aqueous dispersion as raw material and hexamethylenetetramine (HMT) as alkaline catalyst after gelation and carbon dioxide supercritical drying[40]. However, after heat treatment at 1300 ℃, the volume of aerogel shrinks greatly, the density increases by 150 times, the specific surface area decreases from 382 m2/g to 4 m2/g, the microstructure shrinks from fibrous to spherical, and the optical transparency disappears. This is due to the fact that the boehmite phase is unstable at high temperature and will undergo dehydration and a series of phase transformations with the increase of temperature, and finally transforms into stable α-phase alumina, which is accompanied by volume shrinkage and specific surface area decline, resulting in the decline of performance and structural collapse of Al2O3 fiber aerogels, so the temperature resistance of pure Al2O3 fiber aerogels needs to be further improved[41].
Mullite is a series of minerals composed of aluminosilicate, Al2O3 and SiO2 in a certain range, and can also form composite materials with Al2O3 and SiO2.Among them, the SiO2 component has good thermal insulation performance, and the Al2O3 component can provide better thermal stability. In addition, the Si distributed in the lattice vacancy of Al2O3 can also effectively suppress the lattice vibration and α-phase transformation at high temperature, thereby improving the temperature resistance of the fiber. Therefore, mullite fiber aerogel is expected to be an ideal high temperature thermal insulation material. Dong et al. Prepared mullite fiber aerogel by molding method with mullite fiber as matrix and silicone resin as binder[42]. When the sintering temperature increases to 1500 ℃, the grains in the mullite fiber grow up and the strength of the fiber aerogel decreases, but the compressive strength and elastic strain can still reach 1.36 MPa and 3.2%, respectively, which means that the mullite fiber aerogel sintered at 1500 ℃ can be used in high temperature fields (Fig. 4). However, the density (0.425~0.441 g/cm3) and thermal conductivity (0.083 ~ 0.089 W/ (m · K)) of mullite fiber aerogel are much higher than those of traditional aerogel thermal insulation materials due to the large diameter of mullite fiber, which is 10 ~ 15 μm.
To overcome the above problems, Liu et al. Prepared mullite nanofibers by electrospinning method, combined with freeze-drying method, and obtained mullite fiber aerogels with low density (03274) and low thermal conductivity (0.03274 – 0.04317 W/ (m · K)), which still had an ultra-high porosity of 98.9% and a compressive strength of 18 kPa at a sintering temperature of 1400 ° C[43]. Furthermore, the team used glycerol as a cryoprotectant and agarose as a gel-forming agent to adjust the microstructure of mullite nanofiber aerogel, respectively, and increased the compressive strength of the aerogel to 252.41 kPa[44]. Compared with the electrospinning method, Zhang et al. Proposed a simple and scalable preparation method. After mixing the hydrothermally generated alumina nanorods and silica sol, freeze-drying and annealing at 1200 ℃, a low-density mullite aerogel material resistant to 1400 ℃ was developed, which has low thermal conductivity (0.026 W/ (m · K) at 25 ℃) and good mechanical properties (compressive strength of 1.5 MPa)[45].
Chopped fiber combined with freeze-drying is the main method to prepare ultra-light fiber aerogel at present, but because the short fiber fragments are assembled into a network in a point-to-point contact manner under the action of force,The effective action region is limited, resulting in aerogel being too fragile (compressive strength less than 300 kPa for ε = 80%) to resist strong mechanical shock and heat flow in practical applications. Zhang et al., inspired by the extraordinary mechanical properties of wood sponges, laid layers of electrospun flexible ZrO2-Al2O3 fiber felts, and then used Al(H2PO4)3 as interlayer binder to prepare anisotropic, layer-structured ultralight ZrO2-Al3O3 fiber aerogel by freeze-drying (Fig. 5)[46]. The arched microscale structure arranged in parallel enables the aerogel to exhibit compressive strength up to 1100 kPa at 90% strain and fatigue resistance of 1000 cycles of compression at 60% strain. In addition, the ultra-high porosity (> 98%), temperature resistance up to 1300 ℃, and excellent thermal insulation performance (thermal conductivity as low as 0.0322 W/ (m · K)) prove that it is a high-performance thermal insulation material that can be used in extreme environments.

2.2 Ultralight carbon fiber aerogel

The classical synthesis route of carbon aerogels mainly involves a sol-gel process, in which polymer molecular precursors are converted into highly cross-linked organic aerogels (e.g., phenol-formaldehyde, polyimide, etc.) by a drying technique[47,48]. After carbonization in an inert atmosphere, the organic aerogel is transformed into a carbon aerogel with a three-dimensional porous network structure. Because carbon is a low atomic number element, carbon aerogel is easier to achieve low density, but the preparation of ultra-low density carbon aerogel materials is still a challenge. In recent years, the development of fiber aerogels has made it possible to prepare ultra-light carbon aerogels on a large scale, at low cost, and sustainably. The preparation methods mainly include polymer fiber conversion, hydrothermal carbonization (HTC), and biosynthesis.
Polymer fiber conversion method refers to the preparation of carbon fiber aerogel by pyrolysis of polymer fibers with excellent mechanical strength, good spinnability and high carbon yield, such as PAN and polyimide (PI). Huang et al. First freeze-dried the pre-oxidized electrospun PAN fiber (oPAN) and graphene oxide (GO), and used the crosslinking effect between the abundant oxygen-containing functional groups of GO and the surface groups of PAN fiber.A hierarchical carbon composite aerogel (GCA) composed of PAN-derived one-dimensional carbon nanofibers and two-dimensional GO nanosheets was successfully prepared by a surface-induced co-assembly method and subsequent high-temperature carbonization (Fig. 6)[49]. After the modification of MnO2 nanoparticles, the interconnected three-dimensional conductive network formed by carbon nanofibers and carbon sheets can not only improve the surface adsorption capacity of MnO2 to electrolyte, but also increase the electron transfer rate in the electrode material, showing excellent capacitance performance as a supercapacitor electrode material.
Similarly, Lai et al. And Xu et al. Used PAN and PI as precursors to prepare carbon fiber composite aerogels with good electrochemical properties through pyrolysis, which can be used for efficient energy storage and conversion applications, further broadening the potential applications of new aerogels in the energy field[50][51].
HTC refers to the process in which carbohydrates or lignocellulose are used as raw materials and water is used as the reaction medium in a closed system, and finally converted into carbon materials at a certain temperature (130 ~ 250 ℃) and self-generated pressure. Its history can be traced back to 1910. At first, coal was oxidized to produce fuels by this method, and cellulose was hydrothermally converted into coal-like materials[52]. But only recently has this method been applied to the preparation of carbon fiber aerogels. White et al., Titirici et al., and Zhao et al. Worked on the conversion of cheap and renewable polysaccharide biomass into porous carbon fiber aerogel by the HTC method[53][54][55]. It was found that gelation of carbohydrate solutions can occur during high temperature hydrothermal to form well-defined nanowire-like structural networks when ultrathin inorganic nanowires (Te NWs) or additives (borax or ovalbumin) are used. The solvent was removed from the wet gel by supercritical drying (or freeze-drying), and the conductive ultra-light carbon nanofiber aerogel (3.3 mg/cm3) was obtained by further pyrolysis at high temperature (600 – 1450 ° C)[56].
Although the HTC method can be used to prepare carbon-containing nanomaterials based on chemical operations at the nanoscale, with the rise of green chemistry, the rational development and efficient utilization of natural renewable resources have attracted increasing attention from researchers. Biosynthesis has incomparable advantages in the green and sustainable preparation of ultralight carbon fiber aerogels. Cellulose is one of the most abundant polymers on the earth, which is produced by the decomposition of animals and plants in nature. It has an intrinsic one-dimensional nanostructure and is an ideal precursor for the preparation of carbon fiber aerogels. Yu et al. Used a morphology-preserving and scalable carbonization method to prepare ultralight carbon nanofiber aerogel (density of 3.0~18.3 mg/cm3) with honeycomb structure by directional freeze-drying and thermal conversion of bacterial cellulose (Fig. 7)[57]. After the carbon yield is greatly improved by adding NH4H2PO4, the prepared carbon aerogel perfectly inherits the hierarchical structure of bacterial cellulose from macro to micro, and after 2×104 compression cycles (ε = 80%), the carbon aerogel can still maintain superelasticity without plastic deformation, and has superelasticity and excellent fatigue resistance in a wide temperature range of -100 ~ 500 ℃ in N2. The mechanical properties of carbon fiber aerogel without honeycomb structure were further tested, and it was found that the aerogel could not be restored to its original state under large deformation, which proved the important role of honeycomb structure in elastic properties. Similarly, Meng et al. Prepared nanocellulose aerogel with a density of 25 mg/cm3 by freeze-drying method using 2 wt% cellulose solution with cross-linked resin[58]. Subsequently, to prevent the excessive shrinkage of aerogel, the cellulose aerogel was pyrolyzed by heating in stages to obtain the carbon nanofiber aerogel with porosity of 99% and density of 10 mg/cm3.
In addition, ultra-light carbon fiber aerogel with excellent properties can also be prepared by CVD and template methods. Recently, Xue et al. Prepared ultralight carbon fiber aerogels with linear and helical structures by CVD using copper tartrate as a catalyst[59]. The aerogel exhibits promising electrochemical properties such as high capacity, long cycle life, and high initial coulombic efficiency (87.4%), making it a promising storage material for sodium ions. Yu et al. First used benzene-diphenol-formaldehyde (RF) resin as carbon source and nanofibers as structural template to obtain superelastic carbon nanofiber aerogel with a density of only 5.8 mg/cm3 after annealing by organic components[60]. Benefiting from the complex network structure of carbon nanofibers and the perfect overlap between fibers, aerogels show strong and stable mechanical properties, including superelasticity, high strength, extremely fast recovery speed (860 mm/s), and low energy loss coefficient (< 0.16).Even after 1×104 cycles (ε = 50%), it still maintains 93% of the original stress without structural collapse, which is expected to be used in high-performance stress sensors and flexible conductors.

2.3 Ultralight organic fiber aerogel

With the diversity of monomers and the designability of molecules, the organic fiber aerogel family contains a rich variety, mainly including cellulose aerogels and polymer fiber aerogels represented by PI, PAN and polycaprolactone (PCL). Organic fiber aerogels have great prospects in many application fields because they can obtain the desired functional properties by grafting specific modified functional groups.

2.3.1 Ultralight cellulose aerogel

Cellulose is a one-dimensional linear polymer composed of D-glucose and 1,4-glycosidic bonds, and it is also the most widely distributed and abundant polysaccharide in nature, which is ubiquitous in plants, some marine backocysts and bacteria. Cellulose aerogel is considered to be the "third generation of aerogel materials" after silica and synthetic polymer aerogels, and is also one of the most reported organic fiber aerogels.Its specific surface area (10~975 m2/g) and porosity (80.0-99.9%) are equivalent to those of the traditional SiO2 aerogel, and it also has better mechanical properties, biodegradability and biocompatibility than the traditional SiO2 aerogel, and its raw materials are widely available and easy to process and shape.
Freeze-drying of nanocellulose aqueous suspension can avoid the tedious steps such as solvent replacement in the drying process, and keep the fine porous skeleton intact, which is a common method to prepare low-density cellulose aerogel. Sepahvand et al. Added phthalimide powder into cellulose aqueous solution, and the obtained mixed solution was freeze-dried to prepare phthalimide-modified low-density cellulose aerogel (Fig. 8), which has a density of 16 mg/cm3 and is a potential carbon dioxide adsorption material[61]. Generally speaking, nanocellulose aerogel has good compressibility, which can be compressed by more than 90% without fracture, but its poor compression resilience can not recover from the compressed state, which seriously limits its application field. Qin et al. Cleverly used the "double ice template assembly" (DITA) strategy to assemble 2,2,6,6-tetramethylpiperidine oxide (TEMPO) -oxidized CNF into submicron fibers with improved elasticity by freezing at -196 ℃, and then frozen again at -20 ℃.The hyperelastic cellulose aerogel with hierarchical layered fiber structure was constructed, which has ultra-low density (2~20 mg/cm3), low thermal conductivity (0. 023 W/ (m · K)), and excellent elasticity at both room temperature and extremely cold environmental conditions (-196 ℃)[62].
Because the surface of cellulose contains a large number of hydroxyl groups and organic groups, it is easy to occur hydrogen bond physical crosslinking between hydroxyl groups or chemical crosslinking between cellulose derivatives, so the sublimation process during freeze-drying will lead to significant aggregation. In order to reduce the aggregation of cellulose nanofibers and prepare cellulose aerogels with lower density, the aqueous suspension of cellulose nanofibers can be converted into hydrogels with more stable structure, and then the aerogels can be prepared by freeze-drying. Darabitabar et al. Mixed carboxymethyl cellulose with citric acid to form a high-viscosity gel at 70 ° C, and then freeze-dried to prepare an ultra-light cellulose aerogel with a density of only 4 mg/cm3[63].
Atmospheric pressure drying is a method in which the cellulose gel is heated under atmospheric pressure until the solvent is removed to obtain a dry aerogel. Compared with supercritical drying and freeze drying, the method has the advantages of simple operation, low cost and low equipment requirement, but the obtained product has lower strength. In order to overcome this problem, Ebrahimi et al. Used cellulose as raw material, polyamide epichlorohydrin as crosslinking agent, and naphthalene was innovatively added to the solvent (acetone/ethanol) to prepare low-density cellulose aerogel with a density of only 55 mg/cm3 by atmospheric pressure drying[64]. Naphthalene is used as a template to fill the pores between the cellulose, thereby avoiding the structural contraction and collapse caused by the surface tension in the solvent evaporation process, and finally the complete cellulose three-dimensional porous network structure can be obtained after the naphthalene sublimation.
Swelling is a phenomenon that the volume of crosslinked polymer expands in solvent, and the preparation of ultra-light cellulose aerogel by using this characteristic is also a new method developed in recent years. Zhang et al. Grafted sodium acrylate (PAANa) onto TEMPO-oxidized cellulose gel by in situ radical polymerization using N, N '-methylenebisacrylamide as a crosslinking agent, at which the physical crosslinking (hydrogen bonding) between celluloses was replaced by chemical crosslinking, resulting in volume expansion (degree of expansion was 27), and the porosity of freeze-dried aerogel was as high as 99.88% and the density was as low as 1.5 mg/cm3[65].

2.3.2 Ultralight polymer fiber aerogel

Compared with inorganic fiber aerogels, polymer nanofiber aerogels have more outstanding mechanical properties due to the higher molecular weight and chain length of polymer monomers. In addition, the flexibility and designability of molecules endow them with more abundant functional properties, which have been widely studied in recent years. However, the formation of three-dimensional networks of ultralight polymer fiber aerogels remains a major challenge, and the selection of appropriate crosslinkers has been shown to be critical for their structural properties.
Polyimide (PI) is a polymer with imide ring structure in its main chain structure. It has been found that polyimide aerogel materials have better thermal insulation, temperature resistance and dielectric properties than other polymer materials. Traditional PI aerogels can be divided into linear structure and crosslinked structure, but the initial reaction system contains at least 7 wt% organic matter to ensure the formation of gel network, which limits the further reduction of the density of polyimide aerogels[66,67]. In 2017, Jiang et al. First used electrospun PI fiber as the building unit and polyamic acid (PAA) precursor as the crosslinking agent, and after freeze-drying and imidization, the PI converted from PAA was firmly wrapped around and in the middle of the PI fiber in a "self-adhesive" manner, forming a superelastic PI fiber aerogel with a density as low as 7.6 mg/cm3 and a thermal conductivity of 0.026 W/ (m · K) (Fig. 9)[68]. Similarly, Shen et al. Used dichloromethane (DCM) vapor to weld adjacent nano/microfiber intersections in a controlled manner to achieve the preparation of ultra-light PI fiber aerogel[69]. The obtained aerogel showed ultra-low density (4.81 mg/cm3), high porosity (99.66%), and recoverable strain limit of 80%. In order to avoid the complication of the preparation process and the inhomogeneity of the aerogel components caused by the use of crosslinking agents, Qian et al. Heated the dried uncrosslinked PI fiber aerogel at 500 ° C for 15 min to cause spontaneous molecular condensation between fibers, and obtained an ultra-light PI fiber aerogel (ρ=4.6 mg/cm3) with 99% superelastic ultimate strain[70].
Polycaprolactone (PCL) is an organic polymer synthesized by ring-opening polymerization of ε-caprolactone monomer catalyzed by metal anion complex catalyst, which has good biocompatibility, organic polymer compatibility and biodegradability. The ultralight PCL fiber aerogel prepared by the method is soft and elastic, has an interconnected hierarchical pore structure, and is similar to a natural extracellular matrix (ECM) in morphology, so the ultralight PCL fiber aerogel is very suitable to be used as a biomimetic scaffold for cell and tissue growth. For the first time, Xu et al. Prepared PCL nanofiber elastic scaffolds by the combination of thermally induced nanofiber self-agglomeration (TISA) and freeze-drying and used them to induce chondrogenesis (Fig. 10)[71]. Its porosity is as high as 96.4%, which has high cell activity and can promote the differentiation of mouse stem cells induced by BMP2. However, its density is high (41~63 mg/cm3) due to the large volume contraction caused by thermal agglomeration. In addition, its Young's modulus is only 0.45 kPa, which is difficult to support the growth of a large number of cells and tissues. Furthermore, Li et al. Also prepared Gelatin-PCL composite fiber aerogel tissue scaffolds by electrospinning combined with freeze-drying[72]. The scaffold can be used for cartilage regeneration and repair, and shows more excellent mechanical properties, with compressive strength and modulus up to 200 and 80 kPa, respectively. However, the degradation rate of gelatin is too fast, resulting in structural collapse during a relatively long period of tissue regeneration. In order to overcome this problem and expand the application field of PCL fiber aerogel, Xu et al. Prepared PCL and cellulose acetate (CA) blend nanofiber aerogel (ρ = 66 mg) as a tissue engineering scaffold by using the characteristic that PCL itself can bond/fuse nanofibers at low temperature (55 ℃)[73]. In addition to CA, polyglycolic acid, polyglycerol sebacate, polyaniline, chitosan, hydroxyapatite and other natural polymers can be blended with PCL to prepare nanofiber aerogels, which greatly expands the application range of PCL fiber aerogels[74][75][76][77][78].
Poly (lactic acid) (PLA) is a polyester polymer obtained by polymerization of lactic acid as the main raw material, which is a new type of biodegradable material. Similar to PCL, the ultra-low density composite fibrous aerogel prepared by blending PLA with natural polymers such as gelatin and regenerated cellulose by thermal crosslinking method can also be used for medical tissue engineering (ρ=32~94 mg/cm3)[79,80][81]. The difference is that polylactic acid has a higher melting point (155 ~ 185 ℃) and needs a higher thermal crosslinking temperature.
Polyvinyl alcohol, the raw material for producing polyvinyl alcohol (PVA) fiber, is a water-soluble polymer, which leads to poor hot water resistance of PVA fiber. Therefore, PVA is usually condensed with other polymers through hydrogen bonding to prepare versatile low-density composite fiber aerogels. Deuber et al. Prepared polyvinyl alcohol/pullulan (PVA/Pul) composite fiber aerogels with densities as low as 23.2 mg/cm3 by freeze-drying and thermal crosslinking[82]. The results show that the porosity of the composite fiber aerogel is as high as 98.4%, and the filtration efficiency of the composite fiber aerogel for particles is as high as 99.999%. On this basis, the team transformed the amphiphilic aerogel framework into a hydrophobic framework (hydrophobic angle of 136 °) through chemical vapor deposition of silane to achieve selective adsorption of non-polar solvents[83]. Xu et al. Successively used freeze-drying, silanization reaction, and impregnation to successfully prepare magnetic hydrophobic polyvinyl alcohol/nanocellulose (PVA/CNF) aerogel (MHPCA) with ultra-low density (13.84 mg/cm3), high porosity (98%), and good mechanical properties (water contact angle over 138 °); its oil absorption is as high as 136 G/G and it can be magnetically driven, which is expected to be used in the field of sewage purification[84].
PAN is a polymer obtained by free radical polymerization of acrylonitrile monomer. As early as 1942, German Laing and American Latham successfully prepared PAN fiber, but PAN fiber aerogel is a new field that has been defined and studied in recent years. Xu et al. Used PVA as a binder and short electrospun PAN and PI nanofibers as a skeleton to provide robust mechanical elasticity and stability, and after loading halloysite nanotubes (HNTs), they prepared composite fiber aerogels with low density (43.4 mg/cm3), high porosity (97.9%), good mechanical elasticity and thermal stability, which can be used as new dye adsorbents and gold particle catalyst supports[85]. Duan et al. Synthesized an acrylate copolymer with a photocrosslinking unit (poly (methyl acrylate (MA) -co-methyl methacrylate (MMA) -co-4-methacryloyloxybenzophenone (MABP)) (poly (MA-co-MMA-co-MABP)) and blended with PAN for electrospinning[86]. Photocrosslinking and freeze-drying were further employed to obtain ultralight fiber aerogel (ρ=2.7 mg/cm3). The hierarchical pore structure makes the aerogel soft and elastic, and in addition, the aerogel has high porosity and is a good cell culture substrate. On this basis, the team loaded artemisinin (ART) and deposited parylene coating (PPX) on the composite polymer fiber aerogel, which was used as a carrier for the programmed release of antimalarial drug ART (ρ=3.5 mg/cm3)[87]. The aerogel has a high ART loading capacity (2697 ± 73mg/G) due to its ultra-high porosity (99.5%), and the release rate of ART can be regulated by controlling the layer thickness of the PPX coating.
With the rapid development of aerogels, traditional organic aerogels have formed a rich and huge system. However, the emerging organic fiber aerogels are often constrained by the fiber forming technology, and it is difficult to prepare organic fiber aerogel from polymer with excellent characteristics. The novel poly (bis (benzimidazolyl) benzophenanthroline dione) (BBB) is a ladder-type rigid rod-like polymer with many aromatic and heterocyclic rings on its skeleton, which has good heat and chemical resistance. These properties make BBB a strong candidate for the preparation of high-temperature stable, inherently flame-retardant fiber aerogels. However, due to the insolubility and infusibility of BBB, the research and application of BBB fibers have been slow. Recently, Zhu et al. Reported a new method for the preparation of BBB fibers by electrospinning the corresponding BBB monomer with a template polymer, followed by solid state polymerization at high temperature, while the template polymer was removed by pyrolysis[88]. With this fiber preparation technology, the researchers further dispersed the prepared BBB fibers in PVA solution, and obtained elastic BBB fiber aerogel with low density (2.9~16.4 mg/cm3), low thermal conductivity (0.028 ~ 0.038 W/ (m · K)), good thermal stability (resistant to 600 ℃ in N2) and inherent flame retardancy by freeze-drying and pyrolysis[89]. The strong mechanical elasticity of aerogel is derived from the water-soluble template polymer PVA, which can guide the self-assembly of BBB fibers into a highly robust 3D porous structure. After annealing and pyrolysis at 500 ℃, the elastic BBB fiber aerogel without template can be obtained. In recent years, with the continuous breakthrough of fiber preparation technology, it is believed that more fiber aerogels with "attractive" characteristics will be developed and applied in practice.

3 Application of ultralight fiber aerogel

Compared with traditional aerogels, ultralight nanofiber aerogels based on one-dimensional nanofibers are a new type of aerogel materials, which not only have higher porosity and lower density due to their unique three-dimensional network structure,But also has more excellent mechanical properties and physicochemical properties, so the material has great application prospects in the fields of heat insulation, adsorption, electrochemistry, sensing, biomedicine and the like.

3.1 Thermal insulation material

Aerogel material is the solid material with the best thermal insulation performance at present. Its fine and complex nanoporous network skeleton structure greatly reduces the density of aerogel, increases the solid heat conduction path, and effectively reduces the solid heat conduction. Its interior is divided into numerous small spaces by the tortuous solid skeleton, which greatly limits the gas heat conduction and convection heat transfer, which makes the aerogel have extremely low thermal conductivity. Low-density aerogel thermal insulation materials can achieve the equivalent thermal insulation effect of traditional thermal insulation materials with lighter mass and smaller volume, which is of great significance to energy saving and emission reduction in the industrial field and structural weight reduction in the aerospace field. The low-density (2 mg/cm3)SNF aerogel prepared by Hu et al. Using natural silk nanofiber (SNF) as raw material and polyvinyl alcohol (PVA) as dispersant has good thermal insulation performance with thermal conductivity of only 0.028 W/ (m · K), but the poor temperature resistance of silk (about 150 ° C) makes it only used in the field of low-temperature thermal insulation[90]. Furthermore, Jiang et al. Prepared PI fiber aerogel with good thermal stability by "self-bonding" method[68]. Where the aerogel with a density of 10.1 mg/cm3 shows a low thermal conductivity of 0.026 W/ (m · K) and a thermal diffusivity of 1.009 mm2/s (Fig. 11 a). The sponge is compressible at least 10000 times and still exhibits good thermal insulation properties under high compression. As shown in Fig. 11 B, with the help of PI fiber aerogel, the hot iron plate (about 376 ℃) can be held by hand, and when the PI sponge with a thickness of 3 cm is placed on the hot plate (about 400 ℃), the cold surface temperature of the fiber aerogel is observed to be only 49.7 ℃. The low density BBB fiber aerogel prepared by Zhu et al. Also has a very low thermal conductivity (0.028 W/ (m · K)), and the cold surface temperature of the 2 cm thick aerogel is only 56.5 ℃ after being heated on a heating plate at 330 ℃ for 30 min[89]. In addition, compared with PS and PI aerogels, BBB fiber aerogels also showed good flame retardancy, and there was no sustainable flame and smoke release after heating under candle flame for 60 s. Metal organic frameworks (MOF) with high microporosity and high thermal stability are potential thermal insulation and flame retardant materials. However, MOF is very difficult to process and form, which greatly limits its application in this field. Zhou et al. Prepared CNF @ MOF aerogel with a thermal conductivity of 0.041 W/ (m · K) by in-situ growth of MOF in CNF, which is slightly lower than that of pure CNF aerogel (0.043 W/ (m · K)), due to the large number of micropores and hybrid structures in MOF that can reflect and scatter phonons, thus reducing the thermal conductivity[91]. In addition, the hybrid aerogel has good flame retardancy, and does not burn under the flame of a butane burner (about 1300 ℃), and the peak heat release rate is only 19 kW/m2.
Due to the ultra-high activity of framework nanoparticles, the porous microstructure of traditional aerogels will sinter and collapse at high temperature, and the service temperature in air is usually lower than 1200 ℃[92,93]. Although researchers have tried to use raw materials with outstanding thermal stability to prepare other ceramic aerogels, such as ZrO2 aerogel and Al2O3 aerogel, the thermal stability of these aerogels has not been significantly improved due to the limitations of their inherent nanoparticles. Unlike the nanoparticle-based porous structure of conventional aerogels, the three-dimensional network structure of nanofiber aerogels is constructed by the overlapping and entanglement of many nanofibers. The rapid growth of nanocrystals in ceramic fibers at high temperatures is the main reason why nanofiber aerogels cannot withstand high temperatures. However, if high temperature resistant ceramic nanofibers are used, the resulting fiber aerogel will show low thermal conductivity and excellent thermal stability, which has great potential in the field of high temperature insulation. Dou et al. Developed a SiO2 nanofiber aerogel with a hierarchical structure using electrospun SiO2 nanofibers and SiO2 nanoparticles as building blocks[94]. The material has an ultra-low thermal conductivity (23.27 W/ (m · K)) and a temperature-invariant superelasticity in the range of -196 ~ 1100 ℃. The thermal conductivity of the ultra-light ZrO2-Al2O3 layered fiber aerogel prepared by Zhang et al. Is only 0.0322 W/ (m · K), which still has good elasticity in the flame of n-butane blowtorch at 1300 ° C (Fig. 12), and the crystal grains of the fiber do not grow up obviously even after heat treatment at 1300 ° C for 1 H[46]. The low-density mullite fiber aerogel (03274) prepared by Liu et al. Also has a low thermal conductivity at room temperature (0.03274 ~ 0.04317 W/ (m · K)), and the thermal conductivity at 1000 ℃ is only 0.073 W/ (m · K)[43]. In addition, the aerogel still has ultra-high porosity (98.9%) and compressive strength (18 kPa) after heat treatment at 1400 ℃, which is an ideal thermal insulation material for extreme environments.

3.2 Adsorption material

In order to solve the problems of oil spill and industrial organic solvent discharge, there is an urgent need for adsorption materials with the ability to selectively absorb organic compounds, such as oil, organic solvents and dyes[95]. The surface-modified fiber aerogel is a very promising new adsorption material due to its excellent hydrophobicity and low pressure drop when the liquid medium passes through the pores.
For example, after incorporating SiO2 nanoparticles (SiO2NPs) into the PAN/SiO2 nanofiber aerogel, the hybrid aerogel with a hierarchical pore structure showed superhydrophobic/superlipophilic characteristics with a hydrophobic angle up to 162 °[29]. Because the tortuosity of the aerogel (21 ~ 28) is about one order of magnitude higher than that of the traditional UF membrane (< 1.5), the permeation path of the emulsion is significantly longer than the thickness of the aerogel. Based on the coalescence separation theory, emulsified water droplets are prone to coalescence in these tortuous tortuous microchannels, so it is expected to achieve higher separation flux at low driving pressure. The study shows that the aerogel can effectively separate surfactant-stabilized water-in-oil emulsions by gravity alone, and has extremely high separation flux (8140±220 L/(m2·h)) and separation saturation (7612±480 kg/m2) (Fig. 13). Deuber et al. Developed ultra-light polyvinyl alcohol/pullulan (PVA/Pul) composite fiber aerogel with layered structure, which can selectively separate organic liquids according to different relative dielectric constants after converting hydrophilic aerogel to hydrophobic (hydrophobic angle of 136 °) by using silane coating[82]. The PI-based composite nanofiber prepared by Xu et al. Is a good dye adsorbent, and its adsorption capacity for methyl blue (MB) is as high as 45.59 mg/G, and the removal rate of MB can still maintain more than 90% after five adsorption-desorption cycles, which indicates that it has good reusability as a dye absorber[96].
Although many fibrous aerogels have good adsorption capacity, expensive modifiers, toxic organic solvents, non-renewable raw materials and complex processes are still the main bottlenecks limiting their large-scale application. Recently, Xu et al. Fabricated ultralight cellulose aerogels (UECS) from electrospun regenerated cellulose nanofibers[97]. Compared with chemical modification, which requires precise control of conditions and the use of expensive chemicals, the method of heat treatment at a temperature of 230 ° C to convert cellulose from hydrophilic to hydrophobic is more green and cost-effective. The as-prepared UECS exhibited excellent absorption capacity for a variety of organic compounds due to high porosity (99.57%), low density (6.45 mg/cm3), and superhydrophobicity (water contact angle of about 141.2 °). In addition, by absorbing organic compounds, morphologically stable UECS organogels can be obtained.
In addition to the adsorption capacity, whether the material can be reused economically and quickly is an important factor to be considered in the actual pollutant treatment project. Xu et al. Successively successfully prepared magnetic hydrophobic PVA/CNF fiber aerogel (MHPCA) with ultra-low density (13.84 mg/cm3), high porosity (98%), good magnetic responsivity and mechanical properties through vacuum freeze-drying, silanization reaction and impregnation processes (Fig. 14)[84]. The polysiloxane coated on the fiber surface gives MHPCA excellent hydrophobic/lipophilic oil/water selectivity (water contact angle over 138 °) and adsorption capacity up to 136 G/G for pump oil. In addition, the impregnated Fe3O4 nanoparticles endow MHPCA with good magnetic response, and the adsorption material can be easily moved and recycled by the external magnetic field. More importantly, after 30 compression-release cycles, MHPCA exhibited outstanding resilience and mechanical durability, which demonstrated the excellent reusability of the adsorbent.
Harmful gases and particulate matter (PM) pose a serious threat to the natural environment and human health, and their effective removal has become an urgent need for environmental protection. Fiber aerogel is expected to achieve efficient adsorption of particulate matter and specific gases due to its high porosity and flexible and easy modification of surface groups. Deuber et al. Explored the potential of low-density PVA-based composite fiber aerogels for particle/aerosol filtration[82]. The material shows high particle filtration efficiency in a wide range of particle sizes and surface velocities, and can still achieve a removal rate of up to 99.999% at the most penetrating particle size. When the particle surface velocity increases from 0. 75 cm/s to 6 cm/s, the filtration efficiency remains at a high level of 1. 1 ~ 10, which is much higher than that of commercial fiber membrane particulate filters, and it has broad application prospects in personal protection, industrial dust removal and emission control. In another study, Sepahvand et al. Prepared a phthalimide modified ultralight cellulose aerogel[61]. Because phthalimide has — C = O, — COO — and —NH2 generated after ring-opening polymerization, these groups will have a strong interaction with CO2 molecules through hydrogen bonds. The aerogel was tested to adsorb CO2 up to 5. 2 mmol/G, which is much higher than the previously reported 2. 3 mmol/G, and it is a potential CO2 adsorption material.

3.3 Electrode material

In recent years, with the increasingly serious global energy crisis and the increasing awareness of environmental protection, countries around the world have vigorously developed green and clean energy technologies. New energy storage devices, such as lithium-ion batteries, supercapacitors, lithium-ion capacitors and lithium-sulfur batteries, have attracted the attention of researchers because of their irreplaceable advantages. The morphology and structure of electrode materials always play a crucial role in energy storage and conversion applications. Conventional electrode materials face severe challenges due to high cost, high energy consumption, insufficient charge storage capacity and oxygen evolution, which make the energy density, power density and cycle life of most available devices far below the theoretical value. In addition, the higher polarization and lower ion diffusion rate at the electrode interface can also lead to the performance degradation of the device. With the advantages of large specific surface area, high porosity, low density, good electrical conductivity, high thermal/chemical stability, and excellent mechanical properties, carbon-containing three-dimensional nanostructures including carbon nanofiber aerogels, carbon nanotube networks, and graphene aerogels have been considered as electrode materials for high-performance energy storage devices[98][99][100]. This interconnected structure not only shortens the transport distance of ions, but also provides a continuous channel that allows the rapid transport of electrons.
Huang et al. Developed an effective surface-induced-co-assembly strategy to prepare graphene/carbon nanofiber composite aerogel (GCA) with a hierarchical structure by using graphene (GO) flakes as a cross-linking agent[49]. Due to the firm interconnection between the graphene sheets and the carbon nanofibers, the specific surface area and the charge transfer efficiency of the graphene sheets are greatly increased, and the prepared aerogel has excellent capacitance performance. The specific capacitance of GCA is as high as 180 F/G at 1 A/G (Fig. 15 a). It is worth noting that GCA still exhibits a high specific capacitance performance of 80 F/G at current densities up to 20 A/G. The cycling stability experiment further shows that the GCA electrode still has a high capacitance retention of 94.8% after 2000 cycles (Fig. 15 B), which is attributed to the synergistic effect between graphene and CNF, forming a continuous conductive network that promotes the rapid transfer of electrons and the efficient energy storage process of the electric double layer capacitor (EDLC).
In supercapacitors, there are two storage mechanisms: pseudocapacitance and double layer capacitance. In the case of the same electrode area, the pseudocapacitance is absolutely dominant, which is 10 to 100 times the specific capacity of the electric double layer capacitor. It is well known that carbon/graphite materials have excellent electrochemical double-layer capacitive behavior, while transition metal oxides, conducting polymers, and carbon materials doped with heteroatoms exhibit pseudocapacitive behavior[101]. Therefore, to further improve the performance of fibrous aerogels as electrode materials for supercapacitors, an effective approach is to combine transition metal oxide/sulfide nanofibers and conductive polymer nanofibers to develop a three-dimensional porous network structure. Lai et al. Designed carbon aerogel (oPP) with unique porous structure by using oxidized polyacrylonitrile (o-PAN) -derived one-dimensional carbon nanofibers and PI-derived two-dimensional carbon nanosheets as raw materials, and further obtained oPP@MnO2 hybrid carbon aerogel by in situ growth of MnO2 nanosheets[50]. The sample with 33. 3% oPP-33.3@MnO2-2h(PI and 2 H redox reaction has the best electrochemical performance, and the maximum specific capacitance is 1066 F/G, which is close to the theoretical value of 1370 F/G of MnO2 material, and still has a high capacitance of 113. 2 F/G at a scan rate of 200 mV/s. It showed a high retention of 92.4% after 5000 cycles, exhibiting excellent cycling stability. In addition, the assembled oPP@MnO2// activated oPP asymmetric supercapacitor has an energy density as high as 30.3 Wh/kg. Zhang et al. Developed elastic electrospun PAN/PVP composite fiber aerogel, which was further freeze-dried, carbonized, and activated by CO2 to obtain carbon fiber aerogel, which can be used as anode materials for supercapacitors[102]. The assembled capacitor exhibits a high specific capacitance of 300 F/G at 0.3 A/G. Moreover, the hybrid aerogel showed a high specific capacitance of 2376 F/G at 1 A/G after modifying the carbon fiber aerogel surface with carbon nanotubes and in situ loading with Co3O4 nanoparticles (Fig. 16). The successful preparation of fiber aerogels extends the traditional electrospun layered membranes to multi-dimensional aerogels, which provides a new strategy for the energy storage application of nanofiber-based materials.

3.4 Pressure sensing material

The development of lightweight, flexible wearable sensors for use in the field of artificial intelligence has attracted significant research interest. Pressure sensor is an important sensing element for mapping mechanical stimuli, which reflects the change of mechanical force/deformation by monitoring the changes of resistance, capacitance, voltage and other signals. This requires sensing materials not only to have good mechanical strength, but also to have good electrical conductivity, so low-density elastic conductive fiber aerogel is an ideal material for building high-sensitivity pressure sensors. The superelastic carbon nanofiber aerogel prepared by Yu et al. Can be used as a stretchable or bendable pressure-sensitive sensor (CA/PDMS) by compounding with polydimethylsiloxane (PDMS)[60]. As the strain increases from 0% to 70%, the electrical resistance of the composite rises sharply. The increase in tensile resistance is essentially attributed to the partial separation of interconnected carbon nanofibers in the aerogel (Figure 17 a). Further study shows that the resistance variation trend is almost the same as that of the initial state (Fig. 17 B) even if 100 tensile cycles are carried out at tensile strains of 30% and 50%, which is expected to be used in stress sensors with high stability and wide detection range, as well as flexible conductors. In another study, ultralight polypyrrole/Ag NW fiber aerogel was prepared by dispersing silver nanowires (Ag NWs) in a suspension of pyrrole monomer, gelation and drying, which can be fully recovered upon compression and has conductivity (ε = 90%), and is a potential pressure sensing material[103].
When used as a compressive strain sensor, the electrical conductivity of the material directly determines the magnitude of the electrical signal when the same voltage is applied. Therefore, precise regulation of aerogel conductivity is crucial in practical applications. Xu et al. Prepared fiber aerogels with tunable conductivity by doping carbon nanotubes (CNFs) into PAN and PI nanofibers[51]. By changing the amount of CNFs, the resistance of aerogel can be accurately adjusted in the range of 200 Ω ~ 260 kΩ. In addition, the sensor has high sensitivity, excellent durability and stability under large compressive strain.

3.5 Biomedical materials

Fibrous aerogels have been initially investigated as scaffolds for tissue engineering, drug delivery, antimicrobial materials, and separation applications. Fiber aerogels can promote cell growth and tissue formation by virtue of their high porosity (> 99%) and relatively large pore size (> 10 μm). However, for ultralight and macroporous materials, it is still a difficult problem to provide enough cell adhesion surface and suitable environment for tissue growth. In addition, according to the basic concept of tissue engineering, the sacrificial scaffold should be biocompatible and biodegradable, which also limits the choice of fibrous aerogel material system[104].
Tissue engineering opens up a new way to repair tissue defects and regenerate tissues. In natural tissues, the ECM, which is composed of organic collagen nanofibers interwoven with each other, provides structural support to cells and regulates a variety of cellular functions. Therefore, there is an urgent need to simulate the key characteristics of natural ECM, including chemical composition, structural organization, and mechanical properties[105]. The ultra-light fiber aerogel obtained from the chopped electrospun polyacrylate fiber dispersion is a good tissue scaffold, and the fiber surface is coated with PVA to obtain sufficient hydrophilicity, so the cells adhere well to the aerogel fiber surface (Fig. 18 a, B)[86]. After 13, 20, and 30 days of culture, confocal microscopy revealed viable cells within the sponges, indicating significant cell growth (fig. 18 C, d, e). Xu et al. Developed a method of thermally induced nanofiber self-agglomeration (TISA), which can convert electrospun PCL nanofiber membranes into PCL nanofiber scaffolds with interconnected macroporous structures[71]. BMP2-induced osteogenic differentiation and bone formation proceeded well on PCL scaffolds, however, only limited new bone was found in the peri-scaffold area after culture. Therefore, the team further improved the bioactivity of PCL scaffolds by introducing PLA. The mechanical properties and bioactivity of PCL/PLA scaffolds were greatly improved, which could significantly promote osteogenic differentiation in vitro and new bone formation in vivo, reaching the maximum rate of new bone formation after 20 days[106].
In the field of drug delivery, achieving high drug loading and controlled drug release are two great challenges. Due to the high pore volume (> 100 mL/G) and surface modifiability of ultralight macroporous fiber aerogels, drug release may be another important application area. It was found that the PPX-coated polyacrylate-based composite fiber aerogel has a high drug loading capacity (2697 ± 73 mg/G), which is a new drug carrier[87]. The PPX coating prepared by CVD method can not only retain the extremely high pore volume (285 cm3/g) of aerogel, but also immobilize the anti-malarial drug ART in the carrier framework. By changing the density of aerogel and the thickness of PPX coating, the release rate of ART in the medium can be controlled, which provides a new idea for the programmed drug release in the treatment of chronic diseases.

4 Conclusion and prospect

Based on the latest research progress at home and abroad in recent years, this paper systematically reviews the innovative strategies of ultra-light nanofiber aerogel assembly according to different material types, as well as their unique properties and potential applications (Table 1). With the rapid development of freeze-drying, electrospinning, chemical vapor deposition, self-assembly, template and other nanomaterials preparation technologies, the lower density limit and application fields of ultralight aerogels have been continuously expanded. Nevertheless, this emerging field is still in its infancy, and ultralight fiber aerogels are far from "perfect" and face many challenges in material preparation and application. (1) In terms of material preparation, it is still difficult to prepare nanofibers with uniform morphology on a large scale by electrospinning, chemical vapor deposition, freeze-drying and other technologies.The complexity of post-treatment/surface modification/functionalization of 3D nanofiber structures is also a huge obstacle to improve the production efficiency, and the higher production cost and longer preparation cycle still can not meet the commercial demand. In addition, the ultra-light fiber aerogels reported at present are all random and disordered porous structures, and it is difficult to precisely control the pore size and pore arrangement to obtain multifunctional ordered porous materials; (2) In terms of material application, ultra-light fiber aerogel has more micron pores and higher thermal conductivity at high temperature than traditional aerogel thermal insulation materials, so it is still a difficult problem to make it have both ultra-low density and lower thermal conductivity at high temperature. In addition, tissue scaffolds used in biomedicine are assembled from short nanofibers, which are prone to form fragments remaining in the body under external pressure, thus causing inflammation and increasing the risk of infection.
Considering the above problems, the possible development directions of ultra-light fiber aerogels in the future are summarized as follows: (1) Using new raw materials: raw materials with three-dimensional network structure (bamboo, shell, etc.) Can be obtained from nature, and then fiber aerogels with three-dimensional porous structure can be prepared on a large scale and at low cost; (2) a novel preparation method is developed: the aerogel skeleton is subjected to surface modification and structure enhancement, so that the aerogel skeleton can resist the damage of the surface tension of a solvent to the skeleton, and the ultra-light fiber aerogel is further obtained through normal-pressure drying, so that the use of supercritical and freeze-drying equipment is avoided, and the high-efficiency and low-cost preparation of the material is realized; Based on the preparation method of ordered porous materials, the template method, ionic liquid and bionics principle are used to assemble the fibers orderly from the nanoscale to achieve the precise control of the pore structure. (3) Exploring new properties and expanding new applications: Developing nanofibers with special structures, such as nanoporous nanofibers, hollow nanofibers and three-dimensional oriented nanofibers, is an effective way to improve properties; Through molecular grafting, surface coating, adding nanofillers and other modification and functionalization methods, some new properties are obtained, and the innovative applications of fiber aerogels in sound absorption, gas sensing, optics, seawater desalination, fuel cells and other fields are expanded. Fiber aerogels are facing challenges as well as opportunities. With the deepening of research, ultra-light fiber aerogels will present a broader application prospect.

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Funding

Natural Science Foundation of Hunan Province(2018JJ2469)

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