The preparation of wood aerogels is a top-down synthetic method that can be summarized in three steps (Fig. 2a). The first step involves treating the wood with chemical reagents to remove lignin and obtain delignified wood; the second step removes hemicellulose to create porous wood; and the third step employs a suitable drying method to eliminate the solvent within the wood, resulting in a wood aerogel with a porous structure.

Wood aerogels are a new type of aerogel material prepared through processes such as cell wall delignification, partial dissolution and regeneration of cellulose, and drying, while retaining the anisotropic structure of natural wood (lightweight and porous). Wood cell wall delignification refers to the process of removing matrix components such as lignin and hemicellulose from natural wood using chemical or physical methods, increasing the distance between cellulose microfibrils, and forming a nanoporous structure within the wood cell walls^[10]. Commonly used chemical agents for delignification include NaOH/Na₂SO₃, NaClO₂/CH₃COOH, deep eutectic solvents (DES), and ionic liquids (ILs). Due to the removal of the matrix, new porous structures appear in the cell walls and middle lamella, leading to increased specific surface area, enhanced porosity, and reduced density.

In the preparation of wood aerogels, Garemark et al. ^[15] proposed a versatile method to fabricate porous, robust, and anisotropic aerogels by utilizing the natural hierarchy and anisotropic structure of wood. The process involves immersing balsa wood in a solution containing 1 wt% sodium chlorite for delignification in an acetate buffer at pH 4.6. After a certain treatment period, the wood samples undergo solvent exchange with absolute ethanol and dimethylacetamide (DMAc), respectively. Subsequently, the samples are immersed in a DMAc/LiCl solvent containing 8 wt% LiCl, followed by immersion in a regenerated acetone bath. Finally, freeze-drying is performed to produce the wood aerogel. It has been reported that solvent systems such as DMAc/LiCl, dimethyl sulfoxide (DMSO), and ionic liquids (ILs) are capable of extracting cell wall components ^[16-17], primarily through dissolving cellulose, after which the cellulose diffuses into the solution within the lumina. During this extraction process, the intermolecular interactions in natural cellulose are disrupted. When an antisolvent (acetone) is introduced, the intermolecular hydrogen bonds reform, causing the dissolved cellulose chains to coagulate and in situ form a nanofibrillar structure within the lumina. Wang ^[18] immersed wood in 500 mL of a 2.5 wt% NaClO₂ aqueous solution at pH 4.6. The wood was then soaked at 80 degrees Celsius for 10 hours. Next, the resulting wood blocks were transferred into a mixed solution of NaOH and Na₂SO₃ and kept at 100 degrees Celsius for 12 hours. After cooling to room temperature, the obtained samples were thoroughly rinsed with deionized water. Finally, freeze-drying was carried out for 12 hours to obtain the delignified and hemicellulose-free wood aerogel.

The preparation of nanocellulose aerogels is a bottom-up process, mainly divided into three steps (Fig. 2b). The first step involves preparing a dispersed form of cellulose/nanocellulose in a liquid (water or organic medium). The second step is shaping the dispersion, and the third step involves forming a cellulose gel through a sol-gel process. A special drying method is then used to remove the solvent and preserve the three-dimensional porous structure.

Nanocellulose fibers have a diameter smaller than 100 nm and can be isolated from pure cellulose through mechanical or chemical methods. Based on the isolation method, nanocellulose can be classified into two categories: (1) cellulose nanocrystals (CNC) or cellulose whiskers; (2) cellulose nanofibers (CNF), also known as nanofibrillated cellulose (NFC). Nanocellulose aerogels refer to materials based on nanocellulose with diameters of 2–100 nm and lengths of hundreds of nanometers or micrometers. These materials are dissolved and dispersed via solvent or mechanical dispersion methods, cross-linked to form hydrogels, and then dried to obtain nanocellulose aerogels. Additionally, nanocellulose solid particles can act as emulsifiers/stabilizers to prepare nanocellulose Pickering emulsions through methods such as ultrasonication or high-pressure homogenization, followed by drying to obtain nanocellulose aerogel foams. Cellulose dispersion is a key technology in the formation of cellulose gels. During the preparation of cellulose dispersions, due to the poor solubility of cellulose in water, the reaction between cellulose and oxidizing agents typically occurs as a heterogeneous reaction, mainly taking place in the amorphous regions or on the surface of the crystalline regions of cellulose, where hydroxyl groups at the C2, C3, or C6 positions of the glucose units are oxidized into aldehyde groups or further oxidized into carboxyl groups. When hydroxyl groups in cellulose are oxidized to produce a large number of carboxyl groups, ionization of the carboxyl groups enhances the electrostatic repulsion between molecular chains, thus promoting cellulose dispersion. Among the commonly used oxidation systems, the 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) mediator system can selectively oxidize the hydroxyl groups at the C6 position of cellulose, exhibiting advantages such as mild reaction conditions, simple operation, and high oxidation efficiency. The TEMPO/NaBr/NaClO system is a commonly used TEMPO mediator system and is currently one of the most suitable approaches. Ye et al.^[19] combined the TEMPO oxidation method with ultrasonic treatment, utilizing the sonochemical effects of ultrasound to generate localized high temperature and pressure within an extremely short time, effectively enhancing the penetration of chemical reagents into cellulose fibers to form dispersions. Furthermore, to obtain a stable Pickering system, a prerequisite must be met: the surface of the nanocellulose particles should be partially wetted by both water and oil phases, enabling interfacial assembly as an emulsifier and forming a stable Pickering emulsion^[20]. Miao et al.^[21] transformed wood pulp fibers into small fragments and cellulose filaments (cellulose fibers, CF) through mechanical treatment. CF can stabilize oil-in-water and high internal phase Pickering emulsions, where the presence of fiber fragments prevents CF protofibrils from forming highly entangled structures, forming a dense three-dimensional network structure between droplets, playing a crucial role in emulsion stabilization. There are various methods currently utilized for shaping cellulose dispersions, including mold shaping and 3D printing. Using specific methods, gels of various shapes can be fabricated. As a highly diluted cross-linked system, gels are conventionally prepared via the sol-gel method, in which raw materials are uniformly mixed in liquid phase and undergo hydrolysis and condensation reactions to form a stable transparent sol system. After aging, the sol particles slowly polymerize to form a gel with a three-dimensional network structure, with the gel network filled with solvent that has lost its fluidity. After drying, the gel forms materials with molecular or even nano-substructures.

In the preparation of nanocellulose aerogels, Sivaraman et al.^[22] obtained a nanocellulose dispersion by combining cellulose fibers at different length scales through TEMPO oxidation and homogenization treatment. After shaping via 3D printing, the hydrogel was subjected to gelation induction with CaCl₂ solution, followed by solvent exchange and supercritical CO₂ drying to obtain pure cellulose aerogels. Cai et al.^[23] isolated cellulose nanocrystals (CNCs) from wood to prepare a CNC suspension. The CNC suspension was treated under ultrasound for 30 min, followed by further gelation with NH₄OH before being poured into a custom mold. Subsequently, the gel was unidirectionally frozen using liquid nitrogen and dried to prepare nanocellulose aerogels. Additionally, Li et al.^[24] first prepared a dichloroethane (DCE) solution containing a polymer as the oil phase at a certain concentration. Then, a CNF aqueous dispersion at a specific concentration was used as the aqueous phase and mixed with the oil phase. The mixture was emulsified using an ultrasonic disruptor to obtain a CNF-stabilized oil/water Pickering emulsion. After standing for 24 h, the CNF-stabilized emulsion formed a gel. Subsequently, the Pickering emulsion gel was freeze-dried under vacuum to obtain CNF/polymer composite aerogel foams.

The preparation of both regenerated lignocellulose aerogels and nanocellulose aerogels follows a bottom-up process, with the main difference being the regeneration of cellulose after dissolution, which transforms cellulose type I into cellulose type II. The process mainly involves four steps: first, dissolving lignocellulose and regenerating it in a regeneration bath; second, shaping and gelation; third, solvent exchange; and finally, drying the hydrogel to obtain the aerogel^[25].

Cellulose dissolution is a prerequisite for the preparation of regenerated cellulose aerogels. Due to the presence of highly ordered crystalline regions in cellulose, the hydrogen bond network connecting molecular chains makes cellulose insoluble in common polar/non-polar solvents. Therefore, cellulose solvents must disrupt both the hydrogen bond network and hydrophobic interactions. Regenerated cellulose aerogels have developed rapidly in recent years, and with in-depth research on cellulose solvents, selecting preparation conditions (such as cellulose solvents, regenerated cellulose solutions, and temperature) to control their porous structure remains an ongoing challenge. A variety of cellulose solvent systems have already been developed, including alkali solutions (NaOH, LiOH), lithium chloride (LiCl)/DMSO or DMAc, ionic liquids (ILs), deep eutectic solvents (DES), and aqueous organic base solutions. Zhang et al.^[26-27] developed a series of novel cellulose solvents, including three-component NaOH/thiourea/water, NaOH/urea/water, and four-component NaOH/thiourea/urea/water solvent systems. This method significantly improved cellulose dissolution and enabled the successful preparation of regenerated cellulose films. Additionally, as early as 2010, researchers summarized the effective dissolution of cellulose using ionic liquids, indicating that ionic liquids act as both electron donors and acceptors, forming complex structures with cellulose that enhance molecular reactivity and reduce the strong hydrogen bonding between cellulose molecules, thereby achieving efficient dissolution^[28]. The development of cellulose solvents has promoted the preparation and research of regenerated lignocellulosic aerogels.

Preparation conditions and techniques can affect the morphology and pore structure of regenerated cellulose aerogels. Lu et al.^[29] prepared a regenerated cellulose aerogel with high specific surface area (80 m²·g^-1) and homogeneous mesopores from hardwood using 1-allyl-3-methylimidazolium chloride (AMImCl) as an ionic liquid through a cyclic liquid nitrogen freeze-thaw (NFT) treatment process. The pore size distribution of the aerogel could also be adjusted from 30 nm to 10 nm by controlling the number of NFT treatment cycles. Li et al.^[30] successfully prepared a novel aerogel composed of filamentous natural cellulose microfibrils and layered regenerated cellulose with low density (8-10 mg·cm^-3) and high specific surface area, using a novel cosolvent system composed of DES and N-methylmorpholine (NMMO·H₂O) through dissolution of pulp followed by regeneration and freeze-drying. Ebrahimi et al.^[31] dissolved cellulose in an aqueous solution of NaOH/urea, preserving the solvent at -18 °C overnight and achieving complete dissolution of cellulose through vigorous mechanical stirring. The resulting solution was poured into a sulfuric acid aqueous solution as a regeneration coagulation bath for cellulose fiber regeneration, and finally super-light cellulose aerogels were prepared using ambient pressure drying. The regenerated cellulose aerogels exhibited excellent flexibility and lightweight characteristics, as well as larger average pore sizes, with pore properties closely related to their preparation parameters^[32]. On the other hand, the production techniques of regenerated cellulose aerogels are simple and low-cost, thus receiving extensive research and application.

The natural pores in wood can be categorized into macro-, meso-, and micro-pores. As observed from the SEM image in Fig. 3a, the porous structure of natural wood is clearly visible. The hierarchically porous structure of wood with various morphologies provides a naturally occurring template for the biomimetic preparation of novel materials without requiring additional processing or modification. Wood aerogels not only inherit vessels and perforations in the macro-pores of the original wood, but also possess nanoscale pores (1-100 nm) within the cell walls. As shown in the SEM image of Fig. 3b, after lignin removal, partial delamination of the cell wall layers occurs, generating nanoscale pores in the previously lignin-rich middle lamella. Additionally, the complete dissolution of the middle lamella in delignified wood leads to cell separation. Due to the presence of wood rays perpendicular to the fiber direction, the hierarchical structure of the wood aerogel remains protected during chemical treatments. Partially dissolved cell walls generate regenerated cellulose within the lumens under the action of an antisolvent, forming a uniform fibrous network that fills the lumen space (Fig. 3c). As shown in the X-ray image of Fig. 3d, natural wood and delignified wood exhibit long fibers and hollow cavities, while the wood aerogel displays intact long fibers along with clear evidence of an internal fibrous network within the lumens. Unlike wood aerogels, cellulose aerogels fully inherit the micro-pores at the molecular chain cross-section level, such as pores formed by the cross-sectional dimensions of cellulose molecular chains. The weight (m, mg) of lightweight porous cellulose aerogels divided by the volume density (ρ) typically ranges from 1 to 100 (mm^-3). The porosity of the aerogel, denoted as P (ρ_n represents the cellulose density), can be calculated using Equation (1).

Pore structure regulation of wood-based aerogels is critically important and can be achieved by controlling precursor solution concentrations and freeze-drying conditions, as well as by introducing crosslinkers or other types of nanoparticles and polymers to create composite aerogels. This not only allows regulation of the pore size, morphology, and distribution but also enhances material performance, potentially generating new pore characteristics such as ordered or disordered pore structures. Gong et al.^[35] added a crosslinker (styrene acrylic acid, SAE) into TEMPO-oxidized CNF and investigated the regulation of the cellulose aerogel pore structure by controlling the concentrations of the precursors (CNF, CNC) and the crosslinker, along with strict freeze-drying conditions, thus simply preparing cellulose aerogels. The resulting cellulose aerogels achieved a high specific surface area of 184 m²/g and a porosity as high as 99%. Yang et al.^[36] utilized modified polyvinyl alcohol (PVA) prepolymers to crosslink with the abundant hydroxyl groups of nanocellulose, forming a stable 3D network by establishing various hydrogen bonds between the nanocellulose and PVA molecules. The obtained nanocellulose aerogels exhibited a high porosity of 99%. Moreover, the multiscale pore structures of wood-based aerogels are crucial for determining their performance in thermal insulation, sound absorption, adsorption, and energy storage. These structures also depend on the form of the aerogel skeleton. Chen et al.^[37] categorized biomass aerogel pore structures into two main types: 1) disordered pore structures, primarily including random pore structures and hierarchical pore structures; and 2) ordered pore structures, mainly comprising honeycomb structures, layered structures, radially and vertically aligned structures, and naturally oriented channel structures. They also emphasized that the microstructure of aerogels serves as an important bridge connecting molecular information with macroscopic properties, playing a role in inheriting and even amplifying functionalities and properties at the molecular scale. By arranging their components into ordered or disordered structures, aerogels exhibit specific functionalities, further enabling functional material applications. Therefore, enhancing the performance of wood-based aerogels and expanding their application range largely depend on the regulation and design of the material's chemical composition and multidimensional pore structures.

Natural wood possesses certain thermal insulation properties due to its inherent porous structure, but its pores are mostly macropores, with only a small amount of mesopores, and a low porosity, which cannot fully suppress gas flow. By chemically treating the wood, such as through delignification, the internal pore structure can be regulated to prepare wood aerogels (cellulose-based aerogels with low bulk density and a nanofibrous network structure) containing more mesopores and micropores. The thermal conductivity of wood aerogel can be as low as 0.033 W·m^-1·K^-1, which is an order of magnitude lower than that of commercial insulation materials such as polystyrene foam and rockwool (0.12 W·m^-1·K^-1), indicating excellent thermal insulation performance. Sun et al.^[34] prepared lightweight and anisotropic wood aerogels with aligned cellulose fibers through a simple chemical treatment. The removal of lignin and hemicellulose optimized the structure of natural wood, including the disappearance of the middle lamella and the degradation of the amorphous regions of cellulose and hemicellulose. The synergistic effect of the multilayered structure and micro-pores reduced the thermal conductivity of the wood aerogel from 0.113 W·m^-1·K^-1 for natural wood to 0.033 W·m^-1·K^-1 for the wood aerogel. Additionally, the thermal conductivity of aerogels depends on the sum of thermal conduction through the solid material, the gas phase, and thermal radiation^[38]. When the pore size approaches the mean free path of gas molecules, the thermal conductivity of the gas decreases due to the Knudsen effect. This is because the pores restrict gas movement and prevent convection (Fig. 4a, b). When the pore size is smaller than 70 nm, gas conduction can be suppressed, and radiative heat transfer is limited, as air cannot circulate through such small pores. Therefore, the porous structure and nanoscale pore size of the wood aerogel endow the material with excellent thermal insulation properties (Fig. 4c).

Moreover, due to the presence of lignin in wood materials, lignin emits near-infrared light after absorbing heat. This near-infrared light is easily absorbed by molecules in the surrounding air, causing the air to warm and increase the ambient temperature. Therefore, after removing the vast majority of lignin, cellulose in wood materials contains a large amount of ether bonds (C—O—C), whose absorption vibration peak (1040 cm^-1) falls precisely within the atmospheric window (8–13 µm) range. The stretching vibrations of functional groups exhibit strong absorption and high emissivity in the fingerprint region. Additionally, the inherent lightweight white color and porous structure of cellulose aerogels facilitate total internal reflection and scattering of sunlight. Ultimately, cellulose acts as a photonic solar thermal emitter and reflector, showing significant potential for radiative cooling. Tian et al.^[39] combined the radiative cooling properties with the intrinsic pore structure of the material to prepare cellulose composites with low thermal conductivity and excellent thermal insulation. Under a solar radiation intensity of 834 W·m^-2, these composites achieved a temperature reduction of 5 °C; at 671 W·m^-2, the daytime net cooling power reached 104 W·m^-2.

The sound absorption performance of materials mainly depends on the characteristics of their surface structure and internal pores. Although natural wood is rich in pores, its sound absorption performance is not ideal due to its hard and dense surface and fewer continuous pores. Wood aerogels, however, have higher porosity and a more abundant surface pore structure based on wood, significantly enhancing the material's sound absorption capability. As shown in Fig. 5a, when sound propagates through wood and wood aerogel, the aerogel exhibits more significant power dissipation. This is because when sound waves propagate, the pores between the wood cell walls generated by removing lignin and hemicellulose provide numerous pathways, increasing the path length of sound waves within the material and causing gradual attenuation of sound energy. In addition, the sound waves undergo repeated reflections and friction between pore walls (Fig. 5b), ultimately dissipating the incident wave energy into heat and disappearing. Compared to natural wood, wood aerogels demonstrate a higher sound absorption coefficient, indicating superior sound absorption performance (Fig. 5c). Compared to wood aerogels, cellulose-based wood aerogels exhibit even higher porosity and consequently better sound absorption performance. Li et al.^[41] incorporated MIL-53(Al) nanoparticles into a cellulose nanofiber aerogel via carboxymethylated nanocellulose bridging. The nanoparticles provided abundant micropores and, in a certain manner, prevented the formation of intermolecular hydrogen bonds between fibers, thereby increasing the complexity of the pore structure. The aerogel exhibited excellent sound absorption coefficients (~0.9) in the high-frequency range of 1600~6300 Hz.

In addition, due to the better processability exhibited by nanocellulose, the internal pore structure of aerogels can be more effectively controlled, allowing deeper exploration of the material's sound absorption and wave absorption capabilities. Zong et al.^[42] successfully constructed a composite aerogel with a gradient structure featuring a cellulose nanofiber network through a stepwise directional freeze-casting technique. This aerogel exhibited a "macropore-mesopore-micropore" gradient structure along the sound wave propagation direction and a dual nanofiber network perpendicular to this direction. The combination of the gradient pore structure and the dual nanofiber network significantly increased the contact area and propagation path of the sound waves within the aerogel, thereby greatly enhancing the dissipation of sound energy. The synergistic effect of these two dissipation mechanisms endows the aerogel with lightweight properties and excellent sound absorption performance. The resulting lightweight gradient aerogel achieved a noise reduction coefficient of 0.58, capable of reducing low-frequency noise from an air compressor by 23.1 dB.

Wood aerogels possess a porous structure and high specific surface area, exhibiting high adsorption capacity for water, oil, organic solvents, and ions. Their adsorption capacity is an order of magnitude higher than that of natural adsorbents (such as activated carbon) and several times greater than that of commercial polypropylene adsorbents. A key factor in the pore structure of wood aerogels is their unique openness and interconnectivity; in this open system, gases or liquids can easily flow from one pore to another and eventually fill the entire material. This property depends on capillary effects, van der Waals forces, hydrophobic interactions, and the density and morphological characteristics of the aerogel, such as surface wettability, total pore volume, and pore structure. Therefore, the porous structure and high specific surface area of wood aerogels contribute to their high adsorption capacity for water, organic solvents, and other substances^[43]. Wood aerogels can also form wood composite aerogels with higher specific surface areas by incorporating organic or inorganic materials or using specific crosslinkers, thereby demonstrating improved adsorption performance. Zhao et al.^[44] prepared a nanofiber composite aerogel using a dual-crosslinking method for the removal of bisphenol A (BPA), an organic compound in water. The aerogel exhibited a high specific surface area of 2203 m²·g^-1 and demonstrated highly efficient adsorption capacity, removing 77.7% of the initially present BPA molecules in water within 10 seconds, with an adsorption rate as high as 532.2 mg·g^-1·min^-1, achieving a removal efficiency of 97.0%. Jing et al.^[45] developed a highly compressible anisotropic cellulose/graphene aerogel (MCGA) that exhibited superhydrophobicity (WCA=150.3°). MCGA showed remarkable oil absorption capacity for various oils and chemical solvents, ranging from 80 to 197 times its weight, surpassing most carbon-based aerogels. Moreover, the excellent recovery performance of MCGA enabled rapid and efficient oil recovery after absorption; approximately 85% of the absorbed oil could be recovered through simple mechanical compression. Additionally, after ten cycles, distillation resulted in less than a 3% reduction in oil absorption capacity, while mechanical compression caused less than a 10% reduction. Furthermore, in research on ion adsorption, Cao^[46] prepared wood aerogels with abundant pore structures and high specific surface areas by delignifying wood followed by TEMPO oxidation treatment. These aerogels exhibited an adsorption capacity for copper ions reaching 115 mg/g, and even after multiple reuses, the adsorption efficiency decreased by only about 10%.

Traditional inorganic aerogels have been significantly limited in their production scale and practical applications due to inherent brittleness and low compressive yield strength. Wood aerogels, among lignocellulosic aerogels, exhibit better mechanical properties because they inherit the unidirectionally distributed tubular structure of the original wood fibers. Previously reported wood aerogels achieved a yield strength of up to 5 MPa, with a Young's modulus reaching approximately 154 MPa^[17]. Lignocellulosic aerogels can also demonstrate excellent mechanical performance. Wang et al.^[47] constructed a nanocellulose triboelectric aerogel with a multiscale structure based on the Hofmeister effect. The aerogel exhibited ultra-high stiffness, with a Young's modulus as high as 142.9 MPa. Moreover, the mechanical strength of nanocellulose aerogels increases within a certain range as the nanocellulose content increases, because higher concentrations lead to enhanced mechanical strength of the cell walls, resulting in a more stable and dense structure. At the same mass fraction, fiber orientation of the aerogel is another critical parameter influencing its mechanical performance; higher orientation factors result in improved tensile strength and toughness. Liu et al.^[48] confirmed that both the tensile strength and strain of nanoporous aerogel fibers significantly increase with rising cellulose content. Additionally, under the same mass fraction, aerogel fibers obtained through supercritical drying exhibited the best fiber orientation and consequently the best mechanical performance. Wood aerogels can be combined with inorganic and organic materials to form wood composites that demonstrate improved mechanical properties. Pan et al.^[49-50] prepared silica/wood aerogels with excellent elasticity and fatigue resistance using a hybrid approach incorporating organic and inorganic components. The resulting wood aerogel exhibited a compressive stress of 200 kPa under 40% strain and maintained a height retention rate of 92.8% after 300 fatigue cycles, demonstrating good fatigue resistance. Duchemin et al.^[51] dissolved microcrystalline cellulose (MCC) in a LiCl/DMAc solution containing 8% LiCl. During MCC dissolution, large MCC crystals and fiber fragments gradually decomposed into smaller crystals and fragments, with the degree of decomposition affecting the mechanical properties of the aerogel—higher cellulose crystallinity resulted in better flexural strength and rigidity. By controlling the cellulose mass fraction between 5% and 20%, the resulting aerogels achieved densities ranging from 0.16 to 0.35 g/cm³, with flexural strength and stiffness reaching up to 8.1 MPa and 280 MPa, respectively.

In addition to the aforementioned properties, wood-based aerogels also require flame retardancy and hydrophobicity in practical application scenarios. To meet stringent safety requirements and practical application needs without compromising the material's thermal stability and mechanical properties, functional modification can be performed through specific treatment methods, endowing the material with more pronounced flame retardant and hydrophobic characteristics. This makes wood-based aerogels competitive candidates for effective thermal insulation and environmental purification in domestic buildings.

In recent years, new methods of polymer flame retardancy have been developed, including organic-inorganic hybrid flame retardancy, nano-synergistic flame retardancy, in-situ enhanced flame retardancy, and controlled carbonization flame retardancy, as well as additive flame retardant approaches represented by bio-based flame retardants, and surface treatment flame retardant methods represented by plasma/UV-assisted treatments, sol-gel techniques, and biomimetic coatings^[52]. Gan et al.^[53] combined wood densification with an anisotropic thermal conductive flame-retardant coating of hexagonal boron nitride (h-BN) nanosheets to develop a robust fire-resistant wood structural material. The thermal management properties of the BN coating enabled rapid in-plane heat diffusion, slowing down the heat transfer through the dense wood, thereby improving the ignition performance of the material. The material's ignition temperature (T_ign) increased by 41 °C, the ignition delay time (t_ign) tripled, and the peak heat release rate (HRR) of the BN-dense wood decreased by 25%. Farooq et al.^[54] successfully prepared flame-retardant and thermally insulating CNF aerogels using sodium bicarbonate as a flame retardant, effectively enhancing fire resistance. The minimum burning rate of the flame-retardant aerogel was 0.20 cm·s^-1, significantly lower than that of pure CNF aerogels. Additionally, He et al.^[55] prepared polyvinyl alcohol/cellulose nanofiber hybrid aerogels via freeze-drying. Using 0.8 wt% microencapsulated ammonium polyphosphate (MCAPP) as a flame retardant, the prepared aerogels exhibited excellent thermal insulation capabilities and superior mechanical properties. After flame-retardant modification, the lignocellulosic aerogels evolved into safe and energy-efficient structural materials for buildings while maintaining their original characteristics.

In addition, methods to improve the hydrophobic properties of wood aerogels mainly involve chemical reactions such as esterification, etherification, or graft copolymerization, which convert hydrophilic hydroxyl groups into ester groups, ether groups, or other hydrophobic groups. By modifying the surface structure and morphology of cellulose, its selectivity for oil-water separation can be enhanced. Alternatively, methods such as chemical vapor deposition, solution immersion coating, or acid hydrolysis silanization can be employed to prepare hydrophobic surfaces with rough structures^[56-57]. Nguyen et al.^[58] functionalized the material using methyltrimethoxysilane (MTMS) to enhance its hydrophobicity and oleophilicity. The water contact angles of the silanized modified cellulose aerogels were 143° and 145°, respectively. Benito-González et al.^[59] prepared hydrophobic aerogel sorbent pads through a simple freeze-drying and dip-coating method using cellulose and polylactic acid. The incorporation of polylactic acid rendered the aerogels hydrophobic, with contact angles ranging from 95° to 130°, and they exhibited selective oil absorption (5.9–9.2 g/g), significantly higher than their water absorption capacity (2.8–6.7 g/g). Korhonen et al.^[60] successfully obtained a hydrophobic lignocellulosic aerogel material with a core-shell structure by uniformly depositing a 7 nm thick hydrophobic and oleophilic titanium dioxide layer onto a nanocellulose aerogel framework through vacuum freeze-drying. Hydrophobically modified aerogels can effectively separate oil-water mixtures and adsorb water pollutants, making them promising adsorbent and separation materials for environmental remediation.

Wooden aerogels have demonstrated efficient cooling effects in the emerging passive daytime radiative cooling (PDRC) technology by simultaneously reflecting sunlight and radiating significant amounts of heat into the cold outer space through the atmospheric transparent window, achieving sub-ambient cooling under direct sunlight and providing a favorable indoor atmosphere without any energy consumption, thus reducing the energy consumption required for air conditioning to improve the indoor environment. Li et al.^[68] developed a structural material with daytime sub-ambient cooling effects by fully removing lignin and subsequently densifying the wood through mechanical pressing. When this "radiative cooling material" is used as roofs and walls for buildings (as shown in Fig. 6a), it can reduce indoor temperatures by approximately 10 °C and achieve a calculated daily net cooling power of 16 W·m^-2. Furthermore, to improve the efficient reflection of sunlight and outdoor stability of cellulose-based radiative cooling aerogels, the micro/nanostructures on the surface of the aerogels can be designed to synergistically achieve total internal reflection and hydrophobicity by combining with their inherent porous structure. Cai et al.^[69] constructed a radiative cooling aerogel film material with micro/nanostructures using directional freeze-drying technology and hot-pressing forming processes, optimizing its solar reflectance, infrared emissivity, and surface wettability, ultimately achieving a daytime radiative cooling performance of 6.9 °C. Simultaneously, the multi-dimensional micro/nanostructures on the material surface and the chemically cross-linked bonds endow the material with excellent hydrophobic properties. Additionally, Cai et al.^[70] developed a sustainable cellulose nanocrystal aerogel in which the designed supersurface containing regular tubes and irregular grooves functions as a grating to realize diffraction and broadband reflection of sunlight, thereby effectively achieving efficient radiative cooling. This aerogel exhibits an ultra-high solar reflectance (97.4%), high infrared emissivity (94%), and resistance to environmental aging. Under direct sunlight, the sub-ambient temperature drop reaches 10.5 °C and remains at about 9.4 °C even after six months, potentially saving up to 47% of global cooling energy consumption annually.

In recent years, although thermal insulation materials used in construction have achieved significant progress in effective insulation, they still face many challenges. Among them, the non-biodegradability of materials causes notable negative impacts on the environment, and their durability also needs further improvement. Currently, wood-based thermal insulation aerogel materials have been widely used in the production of insulation boards and play an important role in reducing energy consumption and maintaining optimal indoor conditions. Zhao et al.^[40] removed lignin from Paulownia using NaOH and obtained an insulating wood product through ambient temperature drying. By sealing and evacuating the air within the insulating wood, highly insulating vacuum insulation panels were formed, achieving a thermal conductivity as low as 0.01 W/(m·K). This work demonstrated a low-cost, sustainable, and scalable insulation material that can reduce heat and sound transmission to enhance residential comfort and provide significant potential energy benefits. Additionally, its ambient drying method laid the foundation for the industrialization of wood-based aerogel materials. Chen et al.^[61] fabricated a wood/polyimide composite aerogel by removing most of the hemicellulose and lignin from natural wood and subsequently rapidly impregnating polyimide into the wood using an "in situ gelation" process. After introducing polyimide into the delignified wood, its mechanical properties were significantly improved, with compressive strength increasing by more than fivefold. Notably, the developed composite exhibited a thermal conductivity of 0.034 W/(m·K). Moreover, the composite demonstrated excellent flame retardancy, hydrophobicity, and mechanical properties, making it suitable for application in commercial sustainable insulation boards (Fig. 6b).

In the construction industry, glass is widely used due to its excellent transparency, pollution-free properties, fashionable appearance, broad application, and low cost. However, it is difficult to achieve a comfortable indoor environment while effectively reducing the energy consumption required for indoor heating and cooling using traditional glass materials. Currently, people reduce the thermal conductivity of the material by creating a vacuum condition within the inner layers of the glass, thereby achieving a comfortable indoor temperature. However, this method incurs high costs and is therefore difficult to apply widely. To address the above challenges, Abraham et al.^[71] prepared a highly transparent silica-reinforced cellulose aerogel for use in glass components. This aerogel exhibits a visible light transmittance of 97%–99%, a haze of approximately 1%, and a thermal conductivity lower than that of still air (0.020 W/(m·K)). Designing glass with this cellulose aerogel as an interlayer and filler can effectively address the previously mentioned issues. Additionally, wood can also be treated via impregnation to realize "aerogel-type wood glass," achieving effects similar to those of glass while offering good thermal insulation properties. Zhang et al.^[72] successfully fabricated a highly porous, transparent, and mechanically flexible transparent wood aerogel (DAF) through a filtration-induced layer-by-layer gelation and ambient drying strategy. It exhibits a high visible light transmittance of 91.0% and a high selective emissivity of 90.1% within the atmospheric transparency window, along with significant hydrophobicity and durability. As a solar-thermal regulating energy-saving window, DAF not only provides effective indoor illumination but also contributes to indoor radiative temperature regulation. Additionally, wood can also be treated via impregnation to realize "aerogel-type wood glass," achieving effects similar to those of glass while offering excellent thermal insulation properties. Compared with traditional glass, wood aerogel-based glass offers better thermal insulation performance and lower costs, making it an ideal alternative for window applications (Fig. 6c), and a promising choice for future building materials markets.

With scientific advancements, the living environment people are in has gradually presented some interferences, such as noise pollution and electromagnetic pollution. Wood-based aerogel materials can achieve excellent thermal insulation while also possessing other characteristics, such as good sound absorption, noise isolation, and electromagnetic shielding ((Fig. 6d)). Zhao et al.^[40] prepared a wood aerogel only 10 mm thick that exhibited a large sound absorption coefficient within the frequency range of 250–1250 Hz. As the frequency increased to 2500 Hz, this coefficient rapidly increased to approximately 0.75. Zong et al.^[42] developed a cellulose composite aerogel with a cellulose nanonetwork, achieving a noise reduction coefficient (NRC) as high as 0.58. This aerogel not only reduced low-frequency noise from air compressors by 23.1 dB but also demonstrated excellent mechanical properties and long-term stability. Huang et al.^[73] controlled the concentration of cellulose in a sodium hydroxide/urea solution, designing the scaffold structure from a nanofiber network to a nanosheet network. The obtained conductive carbon nanotube/cellulose aerogel was first reported as an electromagnetic interference (EMI) shielding material, exhibiting an EMI shielding efficiency of 20.8 dB and a corresponding specific EMI shielding efficiency as high as 219 dB cm³·g^-1 within the microwave frequency range of 8.2–12.4 GHz. Shen et al.^[74] fabricated a MoS₂@Gd₂O₃/MXene porous composite carbon aerogel with magnetic-dielectric synergy and a multilayered structure, showing excellent electromagnetic wave absorption performance. At an ultrathin thickness of only 1.9 mm, the minimum reflection loss reached 57.5 dB, with an effective absorption bandwidth of 4.35 GHz. Due to their easy processing, excellent performance, and sustainability, wood-based aerogels are expected to replace commercial acoustic materials used in construction and serve as lightweight shielding materials against electromagnetic radiation, offering significant application potential in transportation and industrial noise reduction, as well as in aerospace applications.

In recent years, frequent oil leakage accidents and industrial discharges of heavy metal-contaminated wastewater pose serious threats to ecological systems and human health. Traditional adsorbent materials exhibit disadvantages such as poor reusability, insufficient selective adsorption capacity, and lack of biodegradability. Due to their porous structure, high specific surface area, and low density, cellulose-based aerogels can be surface-modified via silylation and the use of specific crosslinking agents to impart hydrophobicity and excellent adsorption properties, demonstrating high selective adsorption capacity for water, oil, and organic solvents. The separation of oil and water is achieved by creating a hydrophilic or oleophilic rough surface on the aerogel (Fig. 7a). Huang et al.^[33] prepared a superhydrophilic wood aerogel coated with a protonated nanocomposite chitosan layer to enhance water affinity and prevent adhesion of viscous oil. The prepared aerogel achieved a separation efficiency of 99.90% for high-viscosity crude oil and water. It exhibited an underwater oil contact angle exceeding 160°, demonstrating oil-repellent and self-cleaning properties. Peng et al.^[77] developed a superhydrophilic cellulose aerogel by mixing cellulose and chitosan solutions. During preparation, chitosan self-assembled into particles with diameters of several micrometers on the aerogel surface, and the disruption of the hydrogen bond network in cellulose exposed more hydrophilic groups (–OH). After immersing the aerogel in water, its rough surface formed a thin water film, endowing it with superoleophobic properties underwater. Additionally, Gao et al.^[78] utilized polydopamine (PDA) as an anchoring point between nanofibrillated cellulose (NFC) scaffolds and octadecylamine, leveraging its adhesive properties to coat the NFC scaffolds with PDA. The resulting cellulose composite aerogel exhibited a high contact angle of 152.5° and could collect oil and various organic solvents from aqueous phases, achieving a maximum absorption capacity of up to 176 g/g, showing promise as a separation material for treating oil and solvent spills.

In addition, the scarcity of freshwater resources has always been a major limiting factor constraining sustainable development in today's world. Wood-derived aerogels, inheriting the structure of naturally aligned cellulose fibers in wood, can be developed as interfacial solar evaporators for harvesting clean water from seawater or wastewater (Figure 7b). Chao et al.^[79] prepared a wood-derived aerogel material capable of efficiently collecting seawater in a directional manner and being bent or rolled. After surface modification with a photothermal coating, this material can efficiently trigger seawater evaporation through photothermal conversion. The material was designed into a "non-direct contact" suspended solar desalination device, forming a "bridge-like" structure that allows the flexible aerogel to achieve an evaporation rate of 1.351 kg·m^-2·h^-1 under natural sunlight intensity (1 kW·m^-2) with a high photothermal conversion efficiency of up to 90.89%. This represents a significant improvement in solar energy utilization compared to traditional "direct contact" solar evaporation. Furthermore, Wu et al.^[80] developed a wood-derived aerogel solar evaporator with an island-like structure, achieving a maximum evaporation rate of 3.14 kg·m^-2·h^-1. The concentrations of K⁺, Ca²⁺, Na⁺, and Mg²⁺ ions in seawater were significantly reduced before and after purification, and the system also effectively removed dyes and organic pollutants from water. These studies collectively highlight the promising applications of wood-derived aerogel materials in environmental remediation, such as oil-water separation and seawater desalination.

The rapid development of next-generation wearable electronic devices in biomedical and personal health monitoring fields has further promoted the application of aerogel materials. Cellulose aerogels, owing to their nanoscale and connected porous network structure, provide numerous accessible sites through their high specific surface area; in addition, their excellent biocompatibility, ultra-light weight, and tunable functionality make them ideal candidates for wearable triboelectric materials and sensors (Figure 8a)^[81-82]. Liu et al.^[47] prepared cellulose aerogel fibers with excellent flexibility via ionic liquid-assisted spinning, which demonstrated extraordinary application potential in the field of smart wearable devices. Zhao et al.^[83] developed a triboelectric aerogel film with ultra-high strength and multiscale structures. The "unfavorable" self-acceleration effect facilitated the formation of multiple hydrogen bonds among polymer molecular chains, resulting in rapid gelation. The rapidly matched dual hydrogen bonds induced self-assembly of cellulose nanofibers and polyaniline into a gel structure with multiscale entanglement within a nanoscale space. Combined with a leaf-inspired reinforcement mechanism, the tensile strength of this aerogel film was enhanced to an impressive 104 MPa. The wearable triboelectric sensor based on this aerogel film exhibited excellent robustness and ultrafast responsiveness (48 ms). Additionally, Nie et al.^[84] reported a multifunctional cellulose-based triboelectric material capable of achieving stable self-powered sensing under high humidity conditions. By employing this cellulose-based triboelectric material, they constructed a hydrophobic and moisture-resistant triboelectric nanogenerator that functions as a self-powered sensor for harvesting biomechanical energy in high-humidity environments.

In recent years, there has been significant interest in using renewable biomass resources such as wood and lignocellulosic materials as raw materials for preparing electrode materials ( Figure 8b ). Wood-based aerogels can achieve functionalization by incorporating conductive materials. Lyu et al.^[85] inserted cellulose nanofibrils (CNF) and carboxylated single-walled carbon nanotubes (SWCNT) into transition metal carbonitrides (MXene), resulting in a cellulose composite aerogel via supercritical drying. The CNF provided the aerogel with a high specific surface area (301.03 m²·g^-1), good electrolyte permeability, and moderate mechanical flexibility, achieving a specific capacitance of 746.68 mF·cm^-2. Xu et al.^[86] designed and fabricated a multifunctional conductive CNF/CNT/MXene composite aerogel with ultra-light weight and excellent mechanical strength through a simple bidirectional freezing method. The abundant oriented pore structure effectively transmitted stress and facilitated electron and ion transport. As a strain sensor electrode, the composite aerogel exhibited a high linear sensitivity of up to 817.3 kPa⁻¹, showing broad application prospects in human motion and physiological monitoring. Furthermore, the composite aerogel can be applied to solid-state compressible supercapacitors and demonstrated remarkable electrochemical performance, including high specific capacitance (849.2 mF·cm^-2), excellent cycle stability (88% capacitance retention after 10,000 cycles), and favorable mechanical flexibility. Wood-based aerogels can be carbonized in high-temperature environments under nitrogen or argon to obtain carbon aerogels. Pang et al.^[87] proposed a convenient delignification method to convert natural balsa wood into layered porous carbon materials, loading FeCo alloy onto N, S-doped wood-derived carbon aerogels (FeCo@NS-CA) as cathodes for rechargeable ZABs. The obtained aerogel with a porous layered structure exhibited excellent bifunctional electrocatalytic performance, including outstanding electrochemical activity and stability.

Moreover, owing to their unique material properties, wood aerogels can serve as lightweight and energy-saving components for aerospace materials, making them excellent candidate materials for spacecraft with significant application prospects in the military and aerospace fields ($\text{Fig.}~8\text{c}$).