Cellular graphene aerogel combines ultralow weight and high mechanical strength: A highly efficient reactor for catalytic hydrogenation

Morphology of Graphene Aerogel as the Key Factor: Mechanical Properties Under Tension and Compression

Graphene aerogels with high surface areas, ultra-low densities, and thermal conductivities have been attracted a lot of attention in recent years. However, considerable difference in their deformation behavior and mechanical properties lead to their poor performance. The problem can be solved by preparing graphene aerogel of given morphology and by control the properties through the special structure of graphene cells. In the present work, molecular dynamics simulation is used to overview the mechanical properties of four different morphologies of graphene aerogel: honeycomb, cellular, lamellar and randomly distributed graphene flakes. All the structures are considered under uniaxial compression and tension with the detailed analysis of the deformation behavior. It is found that cellular structures have much better compressibility and elasticity. During both compression and tension, cellular structures can be transformed from one to another by controlling the compression/tensile direction. The highest strength and fracture strain are found for the lamellar GA under tension along the direction perpendicular to the alignment of the graphene walls. This reveals that the mechanical properties of graphene aerogels can be controlled by enhancing the structural morphology. The obtained results is the contribution which provide the insights into recent developments concerning the design of carbon-based structures and their application.

Synthesis of Ni-Doped Graphene Aerogels for Electrochemical Applications

Carbonaceous materials used in most electrochemical applications require high specific surface area, adequate pore size distribution, and high electrical conductivity to ensure good interaction with the electrolyte and fast electron transport. The development of transition metal doped graphene aerogels is a possible solution, since their structure, morphology, and electrical properties can be controlled during the synthesis process. This work aims to synthesize Ni-doped graphene aerogels to study the role of different nickel salts in the sol-gel reaction and their final properties. The characterization data show that, regardless of the nature of the Ni salts, the surface area, volume of micropores, and enveloped density decrease, while the porosity and electrical conductivity increase. However, differences in morphology, mesopore size distribution, degree of order of the carbon structure, and electrical conductivity were observed depending on the type of Ni salt. It was found that nickel nitrate results in a material with a broader mesopore distribution, higher electrical conductivity, and hence, higher electrochemical surface area, demonstrating that graphene aerogels can be easily synthesized with tailored properties to fit the requirements of specific electrochemical applications.

Graphene-Based Aerogels for Biomedical Application

Aerogels are three-dimensional solid networks with incredibly low densities, high porosity, and large specific surface areas. These aerogels have both nanoscale and macroscopic interior structures. Combined with graphene, the aerogels show improved mechanical strength, electrical conductivity, surface area, and adsorption capacity, making them ideal for various biomedical applications. The graphene aerogel has a high drug-loading capacity due to its large surface area, and the porous structure enables controlled drug release over time. The presence of graphene makes it a suitable material for wound dressings, blood coagulation, and bilirubin adsorption. Additionally, graphene's conductivity can help in the electrical stimulation of cells for improved tissue regeneration, and it is also appropriate for biosensors. In this review, we discuss the preparation and advantages of graphene-based aerogels in wound dressings, drug delivery systems, bone regeneration, and biosensors.

Mechanical Properties of Graphene Networks under Compression: A Molecular Dynamics Simulation

Molecular dynamics simulation is used to study and compare the mechanical properties obtained from compression and tension numerical tests of multilayered graphene with an increased interlayer distance. The multilayer graphene with an interlayer distance two-times larger than in graphite is studied first under biaxial compression and then under uniaxial tension along three different axes. The mechanical properties, e.g., the tensile strength and ductility as well as the deformation characteristics due to graphene layer stacking, are studied. The results show that the mechanical properties along different directions are significantly distinguished. Two competitive mechanisms are found both for the compression and tension of multilayer graphene—the crumpling of graphene layers increases the stresses, while the sliding of graphene layers through the surface-to-surface connection lowers it. Multilayer graphene after biaxial compression can sustain high tensile stresses combined with high plasticity. The main outcome of the study of such complex architecture is an important step towards the design of advanced carbon nanomaterials with improved mechanical properties.

A Review of the Mechanical Properties of Graphene Aerogel Materials: Experimental Measurements and Computer Simulations

Graphene aerogels (GAs) combine the unique properties of two-dimensional graphene with the structural characteristics of microscale porous materials, exhibiting ultralight, ultra-strength, and ultra-tough properties. GAs are a type of promising carbon-based metamaterials suitable for harsh environments in aerospace, military, and energy-related fields. However, there are still some challenges in the application of graphene aerogel (GA) materials, which requires an in-depth understanding of the mechanical properties of GAs and the associated enhancement mechanisms. This review first presents experimental research works related to the mechanical properties of GAs in recent years and identifies the key parameters that dominate the mechanical properties of GAs in different situations. Then, simulation works on the mechanical properties of GAs are reviewed, the deformation mechanisms are discussed, and the advantages and limitations are summarized. Finally, an outlook on the potential directions and main challenges is provided for future studies in the mechanical properties of GA materials.

Application of Metal-Organic Framework-Based Composites for Gas Sensing and Effects of Synthesis Strategies on Gas-Sensitive Performance

Gas sensing materials, such as semiconducting metal oxides (SMOx), carbon-based materials, and polymers have been studied in recent years. Among of them, SMOx-based gas sensors have higher operating temperatures; sensors crafted from carbon-based materials have poor selectivity for gases and longer response times; and polymer gas sensors have poor stability and selectivity, so it is necessary to develop high-performance gas sensors. As a porous material constructed from inorganic nodes and multidentate organic bridging linkers, the metal-organic framework (MOF) shows viable applications in gas sensors due to its inherent large specific surface area and high porosity. Thus, compounding sensor materials with MOFs can create a synergistic effect. Many studies have been conducted on composite MOFs with three materials to control the synergistic effects to improve gas sensing performance. Therefore, this review summarizes the application of MOFs in sensor materials and emphasizes the synthesis progress of MOF composites. The challenges and development prospects of MOF-based composites are also discussed.

A facile and industrial method for synthesis of modified magnetic lipophilic graphene as a super oil additive

Friction and wear are the two major reasons for energy and material losses in mechanical processes. In this research, a simple, industrial and fast exfoliation technique for the production of graphene using sodium azide and graphite in a water solvent without the need for a specific device has been presented following by lipophilizing with octylamine and only with Fe (II). Magnetic nanoparticles were applied on graphene surface, and simultaneously the graphene surface was both lipophilic and magnetic. The method used for graphene production is unique up to now and also it does not oxidize in production procedure. Performed analyzes demonstrate non-destructive properties without any changes in surface functional groups.

From 2D Graphene Nanosheets to 3D Graphene-based Macrostructures

The combination of exceptional functionalities offered by 3D graphene-based macrostructures (GBMs) has attracted tremendous interest. 2D graphene nanosheets have a high chemical stability, high surface area and customizable porosity, which was extensively researched for a variety of applications including CO₂ adsorption, water treatment, batteries, sensors, catalysis, etc. Recently, 3D GBMs have been successfully achieved through few approaches, including direct and non-direct self-assembly methods. In this review, the possible routes used to prepare both 2D graphene and interconnected 3D GBMs are described and analyzed regarding the involved chemistry of each 2D/3D graphene system. Improvement of the accessible surface of 3D GBMs where the interface exchanges are occurring is of great importance. A better control of the chemical mechanisms involved in the self-assembly mechanism itself at the nanometer scale is certainly the key for a future research breakthrough regarding 3D GBMs.

Ultralight magnetic aerogels from Janus emulsions

Magnetite containing aerogels were synthesized by freeze-drying olive oil/silicone oil-based Janus emulsion gels containing gelatin and sodium carboxymethylcellulose (NaCMC). The magnetite nanoparticles dispersed in olive oil are processed into the gel and remain in the macroporous aerogel after removing the oil components. The coexistence of macropores from the Janus droplets and mesopores from freeze-drying of the hydrogels in combination with the magnetic properties offer a special hierarchical pore structure, which is of relevance for smart supercapacitors, biosensors, and spilled oil sorption and separation. The morphology of the final structure was investigated in dependence on initial compositions. More hydrophobic aerogels with magnetic responsiveness were synthesized by bisacrylamide-crosslinking of the hydrogel. The crosslinked aerogels can be successfully used in magnetically responsive clean up experiments of the cationic dye methylene blue.

Magnetite containing aerogels were synthesized by freeze-drying olive oil/silicone oil-based Janus emulsion gels containing gelatin and sodium carboxymethylcellulose (NaCMC).

3D Graphene-Based Macrostructures for Water Treatment

Recently, 3D graphene-based macrostructures (3D GBMs) have gained increased attention due to their immense application potential in water treatment. The unique structural features (e.g., large surface area and physically interconnected porous network) as well as fascinating properties (e.g., high electrical conductivity, excellent chemical/thermal stability, ultralightness, and high solar-to-thermal conversion efficiency) render 3D GBMs as promising materials for water purification through adsorption, capacitive deionization, and solar distillation. Moreover, 3D GBMs can serve as scaffolds to immobilize powder nanomaterials to build monolithic adsorbents and photo-/electrocatalysts, which significantly broadens their potential applications in water treatment. Here, recent advances in their synthesis and application toward water purification are highlighted. Remaining challenges and future perspectives are elaborated to highlight future research directions.

Promoting Electrocatalysis upon Aerogels

Electrocatalysis plays a prominent role in renewable energy conversion and storage, enabling a number of sustainable processes for future technologies. There are generally three strategies to improve the efficiency (or activity) of the electrocatalysts: i) increasing the intrinsic activity of the catalyst itself, ii) improving the exposure of active sites, and iii) accelerating mass transfer during catalysis (both reactants and products). These strategies are not mutually exclusive and can ideally be addressed simultaneously, leading to the largest improvements in activity. Aerogels, as featured by large surface area, high porosity, and self-supportability, provide a platform that matches all the aforementioned criteria for the design of efficient electrocatalysts. The field of aerogel synthesis has seen much progress in recent years, mainly thanks to the rapid development of nanotechnology. Employing precursors with different properties enables the resulting aerogel with targeted catalytic properties and improved performances. Here, the design strategies of aerogel catalysts are demonstrated, and their performance for several electrochemical reactions is reviewed. The common principles that govern electrocatalysis are further discussed for each category of reactions, thus serving as a guide to the development of future aerogel electrocatalysts.

Structurally Controlled Cellular Architectures for High-Performance Ultra-Lightweight Materials

The design and synthesis of cellular structured materials are of both scientific and technological importance since they can impart remarkably improved material properties such as low density, high mechanical strength, and adjustable surface functionality compared to their bulk counterparts. Although reducing the density of porous structures would generally result in reductions in mechanical properties, this challenge can be addressed by introducing a structural hierarchy and using mechanically reinforced constituent materials. Thus, precise control over several design factors in structuring, including the type of constituent, symmetry of architectures, and dimension of the unit cells, is extremely important for maximizing the targeted performance. The feasibility of lightweight materials for advanced applications is broadly explored due to recent advances in synthetic approaches for different types of cellular architectures. Here, an overview of the development of lightweight cellular materials according to the structural interconnectivity and randomness of the internal pores is provided. Starting from a fundamental study on how material density is associated with mechanical performance, the resulting structural and mechanical properties of cellular materials are investigated for potential applications such as energy/mass absorption and electrical and thermal management. Finally, current challenges and perspectives on high-performance ultra-lightweight materials potentially implementable by well-controlled cellular architectures are discussed.

A Plesiohedral Cellular Network of Graphene Bubbles for Ultralight, Strong, and Superelastic Materials

Advanced materials with low density and high strength impose transformative impacts in the construction, aerospace, and automobile industries. These materials can be realized by assembling well-designed modular building units (BUs) into interconnected structures. This study uses a hierarchical design strategy to demonstrate a new class of carbon-based, ultralight, strong, and even superelastic closed-cellular network structures. Here, the BUs are prepared by a multiscale design approach starting from the controlled synthesis of functionalized graphene oxide nanosheets at the molecular- and nanoscale, leading to the microfluidic fabrication of spherical solid-shelled bubbles at the microscale. Then, bubbles are strategically assembled into centimeter-scale 3D structures. Subsequently, these structures are transformed into self-interconnected and structurally reinforced closed-cellular network structures with plesiohedral cellular units through post-treatment, resulting in the generation of 3D graphene lattices with rhombic dodecahedral honeycomb structure at the centimeter-scale. The 3D graphene suprastructure concurrently exhibits the Young's modulus above 300 kPa while retaining a light density of 7.7 mg cm^-3 and sustaining the elasticity against up to 87% of the compressive strain benefiting from efficient stress dissipation through the complete space-filling closed-cellular network. The method of fabricating the 3D graphene closed-cellular structure opens a new pathway for designing lightweight, strong, and superelastic materials.

Room-Temperature Synthesis of Covalent Organic Framework (COF-LZU1) Nanobars in CO2 /Water Solvent

The development of facile, rapid, low-energy, environmentally benign routes for the synthesis of covalent organic frameworks (COFs) is of great interest. This study concerns the utilization of water containing dissolved CO₂ as a solvent for the room-temperature synthesis of COF. The as-synthesized particles, denoted COF-LZU1, combine advantages of good crystallinity, nanoscale size, and high surface area, which suggests promising application as a support for heterogeneous catalysts. Moreover, this versatile CO₂ -assisted method is also applicable for the room-temperature synthesis of Cu-COF-LZU1. This method gives rise to new opportunities for fabricating COFs and COF-based materials with different compositions and structures.

Directionally Antagonistic Graphene Oxide-Polyurethane Hybrid Aerogel as a Sound Absorber

Innovative sound absorbers, the design of which is based on carbon nanotubes and graphene derivatives, could be used to make more efficient sound absorbing materials because of their excellent intrinsic mechanical and chemical properties. However, controlling the directional alignments of low-dimensional carbon nanomaterials, such as restacking, alignment, and dispersion, has been a challenging problem when developing sound absorbing forms. Herein, we present the directionally antagonistic graphene oxide-polyurethane hybrid aerogel we developed as a sound absorber, the physical properties of which differ according to the alignment of the microscopic graphene oxide sheets. This porous graphene sound absorber has a microporous hierarchical cellular structure with adjustable stiffness and improved sound absorption performance, thereby overcoming the restrictions of both geometric and function-orientated functions. Furthermore, by controlling the inner cell size and aligned structure of graphene oxide layers in this study, we achieved remarkable improvement of the sound absorption performance at low frequency. This improvement is attributed to multiple scattering of incident and reflection waves on the aligned porous surfaces, and air-viscous resistance damping inside interconnected structures between the urethane foam and the graphene oxide network. Two anisotropic sound absorbers based on the directionally antagonistic graphene oxide-polyurethane hybrid aerogels were fabricated. They show remarkable differences owing to the opposite alignment of graphene oxide layers inside the polyurethane foam and are expected to be appropriate for the engineering design of sound absorbers in consideration of the wave direction.

Superelastic Graphene Aerogel/Poly(3,4-Ethylenedioxythiophene)/MnO2 Composite as Compression-Tolerant Electrode for Electrochemical Capacitors

Ultra-compressible electrodes with high electrochemical performance, reversible compressibility and extreme durability are in high demand in compression-tolerant energy storage devices. Herein, an ultra-compressible ternary composite was synthesized by successively electrodepositing poly(3,4-ethylenedioxythiophene) (PEDOT) and MnO2 into the superelastic graphene aerogel (SEGA). In SEGA/PEDOT/MnO2 ternary composite, SEGA provides the compressible backbone and conductive network; MnO2 is mainly responsible for pseudo reactions; the middle PEDOT not only reduces the interface resistance between MnO2 and graphene, but also further reinforces the strength of graphene cellar walls. The synergistic effect of the three components in the ternary composite electrode leads to high electrochemical performances and good compression-tolerant ability. The gravimetric capacitance of the compressible ternary composite electrodes reaches 343 F g−1 and can retain 97% even at 95% compressive strain. And a volumetric capacitance of 147.4 F cm−3 is achieved, which is much higher than that of other graphene-based compressible electrodes. This value of volumetric capacitance can be preserved by 80% after 3500 charge/discharge cycles under various compression strains, indicating an extreme durability.

Pickering emulsions stabilized by a metal-organic framework (MOF) and graphene oxide (GO) for producing MOF/GO composites

Herein we demonstrate the formation of a novel kind of Pickering emulsion that is stabilized by a Zr-based metal-organic framework (Zr-MOF) and graphene oxide (GO). It was found that the Zr-BDC-NO₂ and GO solids assembling at the oil/water interface can effectively stabilize the oil droplets that are dispersed in the water phase. Such a Pickering emulsion offers a facile route for fabricating Zr-MOF/GO composite materials. After removing water and oil by freeze drying from Pickering emulsions, the Zr-MOF/GO composites were obtained and their morphologies, structures and interaction properties were characterized by scanning electron microscopy, transmission electron microscopy, X-ray diffraction and Fourier transform infrared spectrometry, respectively. The influences of the concentration of GO and Zr-MOF on the emulsion microstructures and the properties of the MOF/GO composites were studied. Based on experimental results, the mechanisms for the emulsion formation by Zr-MOF and GO and the as-synthesized superstructures of the Zr-MOF/GO composite were proposed. It is expected that this facile and tunable route can be applied to the synthesis of different kinds of MOF-based or GO-based composite materials.

Highly reusable and superhydrophobic spongy graphene aerogels for efficient oil/water separation

Graphene aerogels (GAs) are three-dimensional (3D) graphene sponges with unique wettability and have demonstrated the potential for reducing contamination from oil spills and chemical accidents. Herein, we report new polyurethane (PU) sponge-reinforced GAs with low surface energy, high sorption capacity and excellent recyclability for use as efficient oil sorbents. Spongy graphene aerogels (SGAs) with a hierarchical porous morphology were produced by simply freeze-casting reduced graphene oxide (rGO) to form compacted macroscale sponges. This novel micro-structure benefits from the advantages of embedded graphene and presents reversible large-strain deformation (90%), high compressive strength (63 kpa) and viscoelastic stability. These superior properties, in addition to super-hydrophobicity, endow the aerogels with excellent recyclability without deteriorating the oil absorption performance. Furthermore, SGA has selective and high-volume absorbability (>100%) and can efficiently separate oil from water under continuous pumping action. The excellent absorption performance and robust mechanical properties make this graphene material promising for the large-scale recovery of spilled oil.