Anisotropic and Lightweight Carbon/Graphene Composite Aerogels for Efficient Thermal Insulation and Electromagnetic Interference Shielding

Foaming ink for 3D-printing of ultralight and hyperelastic graphene architectures: Multiscale design and ultra-efficient electromagnetic interference shielding

Extrusion-based printing of macroscopic architectures layer-by-layer offers new opportunities for constructing customized electromagnetic interference (EMI) shielding materials. However, current research primarily focuses on improving the printability of material inks by increasing contents and adding various modifiers, controllable construction of ultralight and robust macro-architectures with structural design at both macro- and micro-scales is still challenging. Herein, we develop a graphene oxide foaming ink enriched with air bubbles for direct-ink writing, enabling the creation of macroscopic graphene architectures with arbitrary geometries. Meanwhile, air bubbles guide the self-assembly of nanosheets into a unique closed-cellular structure, which plays a critical role in enhancing EMI shielding performance. The resulting bubble-derived graphene aerogels (BGAs), fabricated through lyophilization and reduction of the foaming inks, exhibit ultralow densities of 0.0033-0.0045 g·cm-3, superior resilience even at cryogenic temperatures (-196 °C in liquid nitrogen), high compressive strength, and a negative Poisson's ratio. Remarkably, these BGAs achieve exceptionally high EMI shielding effectiveness (SE), reaching 103.2 dB with a low SE reflection of merely 4.8 dB. The specific SE (SSE/t), an absolute measure considering density and thickness, reaches an impressive value of 52,252 dB·cm2·g-1, ranking among the highest reported for synthetic foams. The desirable nanosheets-wrapped closed bubble-shaped cells, well-connected porous and conductive networks, and abundant interfaces in the BGAs collectively contribute to the intense interference and multireflection of electromagnetic waves, driving their outstanding shielding performance. This study presents a straightforward and practical approach to construct ultralight and resilient graphene architectures with multiscale designs, offering a promising solution for advanced EMI shielding applications.

Advances in Ice-Templated Graphene Aerogels: Fabrication, Properties, and Applications

Graphene has been one of the most widely explored two-dimensional (2D) assemblies due to its outstanding mechanical, electrical, and thermal properties resulting from its unique characteristics of high anisotropy and strong carbon-carbon bonds. Aerogels, characterized by their ultralow density and ultrahigh specific surface area, stand out as leading porous materials. Therefore, the integration of graphene and aerogels would boost the development of multifunctional porous materials. Among the various methods for the fabrication of aerogels, ice-templating has received significant interest due to its ecofriendly nature as a physical process, its broad applicability across material systems, and its proficiency in constructing abundant structures for multifunctionalities. Consequently, ice-templating has become a prevalent technique for the efficient assembly of graphene nanosheets into aerogels with the inherited properties of graphene, the multifunctionality derived from diverse constituents, and the well-controlled architecture. In this review, we systematically summarize the development and progress of ice-templated graphene-based aerogels. Initially, we introduce the fabrication process of these aerogels, elaborating each step from precursor preparation to freezing, drying, and post-treatment. Subsequently, we demonstrate the multifunctional applications of ice-templated graphene aerogels with various macroarchitectures and microstructures. Finally, this review concludes with a straightforward summary, highlighting the challenges and opportunities associated with the ice-templated fabrication of graphene-based aerogels. This systematic review of graphene aerogels aims to offer new insights into the design and ice-templated fabrication of innovative aerogels with multiscale architecture and multifunctionalities, which are crucial for a variety of engineering applications.

Microwave-Assisted Preparation of Hierarchical Porous Carbon Aerogels Derived from Food Wastes for Supercapacitors

Preparing carbon aerogel in an eco-friendly and inexpensive manner remains a significant challenge. The carbon aerogels derived from food waste (FWCAs) with a three-dimensional connected network structure are successfully synthesized using microwave radiation. The as-prepared FWCA-4 (The KOH/C ratio is 4) has a large specific surface area (1470 m2/g), pore volume (0.634 m3/g), and a high degree of graphitization. Band-like lattice stripes with a spacing of 0.34 nm, corresponding to the graphite plane, are observed. A high specific capacitance of 314 F/g at 1.0 A/g and an excellent capacitance retention (>90% after 10,000 cycles) make the FWCA-4 suitable for high-performance supercapacitor electrode materials. Furthermore, the specific surface area and pore volume of FWCA-4 are larger and the degree of graphitization is higher than in ordinary porous carbon derived from food waste (FWPC). The assembled symmetrical solid capacitor from FWCA-4 exhibits a maximum energy density of approximately 179.9 W/kg in neutral ion electrolytes. Thus, food waste is successfully used to prepare carbon aerogels through a gelation process using microwave radiation. The recycling of waste biomass is achieved, and the results provide insights for the preparation of carbon aerogels using biomass.

Ultralight Carbon Tube Foam-Derived SiC Nanofibrous Aerogels with Arbitrary Shape, Excellent Rigidity, and Resilience for Thermal Insulation

Ceramic aerogels are promising high-temperature thermal insulation materials due to their outstanding thermal stability and oxidation resistance. However, restricted by nanoparticle-assembled network structures, conventional ceramic aerogels commonly suffer from inherent brittleness, volume shrinkage, and structural collapse at high temperatures. Here, to overcome such obstacles, 3D ultralight and highly porous carbon tube foams (CTFs) were designed and synthesized as the carbonaceous precursors, where melamine foams were used as the sacrificial templates to form the hollow and thin-wall network structures in the CTFs (density: ∼4.8 mg cm-3). Then, the arbitrary-shaped silicon carbide (SiC) nanofibrous aerogels (SNFAs) were obtained through a simple chemical vapor deposition (CVD) process on the CTFs. The resulting SNFAs exhibited excellent comprehensive performances: ultralow density (4.2 mg cm-3) and thermal conductivity (21.3 mW m-1 K-1 at room temperature), excellent rigidity (specific modulus: 13.2 kN·m kg-1), and resilient compressibility. The SNFAs could support over 2100 times their weight without visible deformation. Furthermore, the SNFAs also showed exceptional high-temperature thermal stability, which means that they could withstand 1100 °C in an air atmosphere and 1550 °C in an inert atmosphere. These superior comprehensive performances enable SNFAs to be suitable for thermal insulation in harsh environments.

Fire-Safe Aerogels and Foams for Thermal Insulation: From Materials to Properties

The ambition of human beings to create a comfortable environment for work and life in a sustainable way has triggered a great need for advanced thermal insulation materials in past decades. Aerogels and foams present great prospects as thermal insulators owing to their low density, good thermal insulation, mechanical robustness, and even high fire resistance. These merits make them suitable for many real-world applications, such as energy-saving building materials, thermally protective materials in aircrafts and battery, and warming fabrics. Despite great advances, to date there remains a lack of a comprehensive yet critical review on the thermal insulation materials. Herein, recent progresses in fire-safe thermal-insulating aerogels and foams are summarized, and pros/cons of three major categories of aerogels/foams (inorganic, organic and their hybrids) are discussed. Finally, key challenges associated with existing aerogels are discussed and some future opportunities are proposed. This review is expected to expedite the development of advanced aerogels and foams as fire-safe thermally insulating materials, and to help create a sustainable, safe, and energy-efficient society.

Hierarchical Incorporation of Reduced Graphene Oxide into Anisotropic Cellulose Nanofiber Foams Improves Their Thermal Insulation

Anisotropic cellulose nanofiber (CNF) foams represent the state-of-the-art in renewable insulation. These foams consist of large (diameter >10 μm) uniaxially aligned macropores with mesoporous pore-walls and aligned CNF. The foams show anisotropic thermal conduction, where heat transports more efficiently in the axial direction (along the aligned CNF and macropores) than in the radial direction (perpendicular to the aligned CNF and macropores). Here we explore the impact on axial and radial thermal conductivity upon depositing a thin film of reduced graphene oxide (rGO) on the macropore walls in anisotropic CNF foams. To obtain rGO films on the foam walls we developed liquid-phase self-assembly to deposit rGO in a layer-by-layer fashion. Using electron and ion microscopy, we thoroughly characterized the resulting rGO-CNF foams and confirmed the successful deposition of rGO. These hierarchical rGO-CNF foams show lower radial thermal conductivity (λr) across a wide range of relative humidity compared to CNF control foams. Our work therefore demonstrates a potential method for improved thermal insulation in anisotropic CNF foams and introduces versatile self-assembly for postmodification of such foams.

Recent advances on outstanding microwave absorption and electromagnetic interference shielding nanocomposites of ZnO semiconductor

The electromagnetic interference shielding and microwave attenuation capabilities of ZnO semiconductor nanocomposites have recently been improved using a variety of approaches by correctly modifying their permittivity. To improve microwave attenuation, ZnO semiconductor nanostructures have been combined with graphene, multi-wall carbon nanotubes, metal nanoparticles and their alloys, two-dimensional MXene, spinel ferrite magnetic nanoparticles, polymer systems, and textiles. This paper covers the opportunities and constraints that these cutting-edge nanocomposites in the field of electromagnetic wave absorption encounter as well as the research progress of ZnO semiconductor-based nanocomposite. The structure-function relationship of electromagnetic wave absorption nanocomposites, design strategies, synthesis techniques, and various types of advanced nanocomposites based on ZnO semiconductor are also covered. In order to design and prepare high efficiency ZnO semiconductor based electromagnetic wave absorbing materials for use in applications of next-generation electronics and aerospace, this article can offer some useful ideas.

Solid Wood Modification toward Anisotropic Elastic and Insulative Foam-Like Materials

The methods used to date to produce compressible wood foam by top-down approaches generally involve the removal of lignin and hemicelluloses. Herein, we introduce a route to convert solid wood into a super elastic and insulative foam-like material. The process uses sequential oxidation and reduction with partial removal of lignin but high hemicellulose retention (process yield of 72.8%), revealing fibril nanostructures from the wood's cell walls. The elasticity of the material is shown to result from a lamellar structure, which provides reversible shape recovery along the transverse direction at compression strains of up to 60% with no significant axial deformation. The compressibility is readily modulated by the oxidation degree, which changes the crystallinity and mobility of the solid phase around the lumina. The performance of the highly resilient foam-like material is also ascribed to the amorphization of cellulosic fibrils, confirmed by experimental and computational (molecular dynamics) methods that highlight the role of secondary interactions. The foam-like wood is optionally hydrophobized by chemical vapor deposition of short-chained organosilanes, which also provides flame retardancy. Overall, we introduce a foam-like material derived from wood based on multifunctional nanostructures (anisotropically compressible, thermally insulative, hydrophobic, and flame retardant) that are relevant to cushioning, protection, and packaging.

Silicon Hybrid EPDM Composite with High Thermal Protection Performance

The effects of octaphenylsilsesquioxane (OPS), fumed silica, and silica aerogel on the thermal insulation properties of ethylene propylene diene monomer (EPDM) rubber were studied. On this basis, two kinds of fillers with good performances were selected to study the thermal insulation of an EPDM full-formula system. The results show that the addition of fumed silica or silica aerogel had a positive effect on the thermal insulation performance of EPDM rubber and its composite. A 30 wt% silica aerogel can be well dispersed in the EPDM rubber system and with a lower thermal conductivity compared with fumed silica. EPDM composite with 23.4 wt% fumed silica can produce more char residues at 1000 °C than at 500 °C in a burn-through test and formed the compact and porous char at 1000 °C, which had a lowest thermal conductivity. EPDM composite with fumed silica cannot be burned through 1000 °C burning, and comparison with silica aerogel revealed that it achieved the lowest back temperature and had a temperature of 388 °C after 800 s.

Highly Aligned Graphene Aerogels for Multifunctional Composites

Stemming from the unique in-plane honeycomb lattice structure and the sp2 hybridized carbon atoms bonded by exceptionally strong carbon-carbon bonds, graphene exhibits remarkable anisotropic electrical, mechanical, and thermal properties. To maximize the utilization of graphene's in-plane properties, pre-constructed and aligned structures, such as oriented aerogels, films, and fibers, have been designed. The unique combination of aligned structure, high surface area, excellent electrical conductivity, mechanical stability, thermal conductivity, and porous nature of highly aligned graphene aerogels allows for tailored and enhanced performance in specific directions, enabling advancements in diverse fields. This review provides a comprehensive overview of recent advances in highly aligned graphene aerogels and their composites. It highlights the fabrication methods of aligned graphene aerogels and the optimization of alignment which can be estimated both qualitatively and quantitatively. The oriented scaffolds endow graphene aerogels and their composites with anisotropic properties, showing enhanced electrical, mechanical, and thermal properties along the alignment at the sacrifice of the perpendicular direction. This review showcases remarkable properties and applications of aligned graphene aerogels and their composites, such as their suitability for electronics, environmental applications, thermal management, and energy storage. Challenges and potential opportunities are proposed to offer new insights into prospects of this material.

Resorcinol-Formaldehyde-Derived Carbon Xerogels: Preparation, Functionalization, and Application Aspects

Carbon xerogels (CXs) are materials obtained via the pyrolysis of resins prepared via the sol-gel polycondensation of resorcinol and formaldehyde. These materials attract great attention as adsorbents, catalyst supports, and energy storage materials. One of the most interesting features of CXs is the possibility of fine-tuning their structures and textures by changing the synthesis conditions in the sol-gel stage. Thus, the first part of this review is devoted to the processes taking place in the polycondensation stage of organic precursors. The formation of hydroxymethyl derivatives of resorcinol and their polycondensation take place at this stage. Both of these processes are catalyzed by acids or bases. It is revealed that the sol-gel synthesis conditions, such as pH, the formaldehyde/resorcinol ratio, concentration, and the type of basic modifier, all affect the texture of the materials being prepared. The variation in these parameters allows one to obtain CXs with pore sizes ranging from 2-3 nm to 100-200 nm. The possibility of using other precursors for the preparation of organic aerogels is examined as well. For instance, if phenol is used instead of resorcinol, the capabilities of the sol-gel method become rather limited. At the same time, other phenolic compounds can be applied with great efficiency. The methods of gel drying and the pyrolysis conditions are also reviewed. Another important aspect analyzed within this review is the surface modification of CXs by introducing various functional groups and heteroatoms. It is shown that compounds containing nitrogen, sulfur, boron, or phosphorus can be introduced at the polycondensation stage to incorporate these elements into the gel structure. Thus, the highest surface amount of nitrogen (6-11 at%) was achieved in the case of the polycondensation of formaldehyde with melamine and hydroxyaniline. Finally, the methods of preparing metal-doped CXs are overviewed. Special attention is paid to the introduction of a metal precursor in the gelation step. The elements of the iron subgroup (Fe, Ni, Co) were found to catalyze carbon graphitization. Therefore, their introduction can be useful for enhancing the electrochemical properties of CXs. However, since the metal surface is often covered by carbon, such materials are poorly applicable to conventional catalytic processes. In summary, the applications of CXs and metal-doped CXs are briefly mentioned. Among the promising application areas, Li-ion batteries, supercapacitors, fuel cells, and adsorbents are of special interest.

A Review of Graphene-Based Materials/Polymer Composite Aerogels

The fabrication of composite materials is an effective way to improve the performance of a single material and expand its application range. In recent years, graphene-based materials/polymer composite aerogels have become a hot research field for preparing high-performance composites due to their special synergistic effects in mechanical and functional properties. In this paper, the preparation methods, structures, interactions, properties, and applications of graphene-based materials/polymer composite aerogels are discussed, and their development trend is projected. This paper aims to arouse extensive research interests in multidisciplinary fields and provide guidance for the rational design of advanced aerogel materials, which could then encourage efforts to use these new kinds of advanced materials in basic research and commercial applications.