Three-dimensional carbon nanotube sponge-array architectures with high energy dissipation

Bioinspired Mechanically Robust and Recyclable Hydrogel Microfibers Based on Hydrogen-Bond Nanoclusters

Mechanically robust hydrogel fibers have demonstrated great potential in energy dissipation and shock-absorbing applications. However, developing such materials that are recyclable, energy-efficient, and environmentally friendly remains an enormous challenge. Herein, inspired by spider silk, a continuous and scalable method is introduced for spinning a polyacrylamide hydrogel microfiber with a hierarchical sheath-core structure under ambient conditions. Applying pre-stretch and twist in the as-spun hydrogel microfibers results in a tensile strength of 525 MPa, a toughness of 385 MJ m-3, and a damping capacity of 99%, which is attributed to the reinforcement of hydrogen-bond nanoclusters within the microfiber matrix. Moreover, it maintains both structural and mechanical stability for several days, and can be directly dissolved in water, providing a sustainable spinning dope for re-spinning into new microfibers. This work provides a new strategy for the spinning of robust and recyclable hydrogel-based fibrous materials.

Molecular Carbons: How Far Can We Go?

The creation and development of carbon nanomaterials promoted material science significantly. Bottom-up synthesis has emerged as an efficient strategy to synthesize atomically precise carbon nanomaterials, namely, molecular carbons, with various sizes and topologies. Different from the properties of the feasibly obtained mixture of carbon nanomaterials, numerous properties of single-component molecular carbons have been discovered owing to their well-defined structures as well as potential applications in various fields. This Perspective introduces recent advances in molecular carbons derived from fullerene, graphene, carbon nanotube, carbyne, graphyne, and Schwarzite carbon acquired with different synthesis strategies. By selecting a variety of representative examples, we elaborate on the relationship between molecular carbons and carbon nanomaterials. We hope these multiple points of view presented may facilitate further advancement in this field.

Processable Conjugated Microporous Polymer Gels and Monoliths: Fundamentals and Versatile Applications

Conjugated microporous polymers (CMPs) as a new type of conjugated polymers have attracted extensive attention in academia and industry because of the combination of microporous structure and π-electron conjugated structure. The construction and application of gels and monoliths based on CMPs constitute a fertile area of research, promising to provide solutions to complex environmental and energy issues. This review summarizes and objectively analyzes the latest advances in the construction and application of processable CMP gels and monoliths, linking the basic and enhanced properties to widespread applications. In this review, we open with a summary of the construction methods used to build CMP gels and monoliths and assess the feasibility of different preparation techniques and the advantages of the products. The CMP gels and monoliths with enhanced properties involving various special applications are then deliberated by highlighting relevant scientific literature and discussions. Finally, we present the issues and future of openness in the field, as well as come up with the major challenges hindering further development, to guide researchers in this field.

A Protein-Like Nanogel for Spinning Hierarchically Structured Artificial Spider Silk

Spider dragline silk is draw-spun from soluble, β-sheet-crosslinked spidroin in aqueous solution. This spider silk has an excellent combination of strength and toughness, which originates from the hierarchical structure containing β-sheet crosslinking points, spiral nanoassemblies, a rigid sheath, and a soft core. Inspired by the spidroin structure and spider spinning process, a soluble and crosslinked nanogel is prepared and crosslinked fibers are drew spun with spider-silk-like hierarchical structures containing cross-links, aligned nanoassemblies, and sheath-core structures. Introducing nucleation seeds in the nanogel solution, and applying prestretch and a spiral architecture in the nanogel fiber, further tunes the alignment and assembly of the polymer chains, and enhances the breaking strength (1.27 GPa) and toughness (383 MJ m^-3 ) to approach those of the best dragline silk. Theoretical modeling provides understanding for the dependence of the fiber's spinning capacity on the nanogel size. This work provides a new strategy for the direct spinning of tough fiber materials.

Laminated Structural Engineering Strategy toward Carbon Nanotube-Based Aerogel Films

Aerogel films with a low density are ideal candidates to meet lightweight application and have already been used in a myriad of fields; however, their structural design for performance enhancement remains elusive. Herein, we put forward a laminated structural engineering strategy to prepare a free-standing carbon nanotube (CNT)-based aerogel film with a densified laminated porous structure. By directional densification and carbonization, the three-dimensional network of one-dimensional nanostructures in the aramid nanofiber/carbon nanotube (ANF/CNT) hybrid aerogel film can be reconstructed to a laminated porous structure with preferential orientation and consecutively conductive pathways, resulting in a large specific surface area (341.9 m²/g) and high electrical conductivity (8540 S/m). Benefiting from the laminated porous structure and high electrical conductivity, the absolute specific shielding effectiveness (SSE/t) of a CNT-based aerogel film can reach 200647.9 dB cm²/g, which shows the highest value among the reported aerogel-based materials. The laminated CNT-based aerogel films with an adjustable wetting property also exhibit exceptional Joule heating performance. This work provides a structural engineering strategy for aerogel films with enhanced electric conductivity for lightweight applications, such as EMI shielding and wearable heating.

Ru@Carbon Nanotube Composite Microsponge: Fabrication in Supercritical CO2 for Hydrogenation of p-Chloronitrobenzene

Novel heterogeneous catalysts are needed to selectively anchor metal nanoparticles (NPs) into the internal space of carbon nanotubes (CNTs). Here, supercritical CO₂ (SC-CO₂) was used to fabricate the Ru@CNT composite microsponge via impregnation. Under SC-CO₂ conditions, the highly dispersive Ru NPs, with a uniform diameter of 3 nm, were anchored exclusively into the internal space of CNTs. The CNTs are assembled into a microsponge composite. The supercritical temperature for catalyst preparation, catalytic hydrogenation temperature, and time all have a significant impact on the catalytic activity of Ru@CNTs. The best catalytic activity was obtained at 100 °C and 8.0 MPa: this gave excellent selectivity in the hydrogenation of p-chloronitrobenzene at 100 °C. This assembly strategy assisted by SC-CO₂ will be promising for the fabrication of advanced carbon composite powder materials.

Highly Flexible and Durable Thermoelectric Power Generator Using CNT/PDMS Foam by Rapid Solvent Evaporation

Using a simple, rapid solvent evaporation process, the authors produced 3D carbon nanotube (CNT)/polydimethylsiloxane (PDMS) foams with both high thermoelectric (TE) and good mechanical performance and used them to fabricate highly flexible and durable TE generators. The numerous pores and interfaces in the CNT/PDMS foams, which have porosities exceeding 87%, afford very low thermal conductivity of 0.13 W m^-1 K^-1 . The foam has a zT value of 6.6 × 10^-3 , which is twice as high as that of pristine CNT foam. Importantly, the CNT/PDMS foam exhibits good tensile strength of 0.78 MPa, elongation greater than 20%, and excellent resilience even at compressive strain of 80%. This foam is used to fabricate a highly flexible TE foam generator that exhibits a moderate output power of 1.9 µW generated from the large temperature gradient of 18.1 K produced by applied heat. The authors also demonstrate a practical TE foam generator that produces sustainable output power of 3.1 µW under a compressive strain of 80% and 38.2 nW under the continuous vibrational stress produced by a car engine. The TE foam generator also exhibits excellent stability and durability under cyclic bending and harsh vibrational stress.

Highly Elastic Hydrated Cellulosic Materials with Durable Compressibility and Tunable Conductivity

Anisotropic cellular materials with direction-dependent structure and durable mechanical properties enable various applications (e.g., nanofluidics, biomedical devices, tissue engineering, and water purification), but their widespread use is often hindered by complex and scale-limited fabrication and unsatisfactory mechanical performance. Here, inspired by the anisotropic and hierarchical material structure of tendons, we demonstrate a facile, scalable top-down approach for fabricating a highly elastic, ionically conductive, anisotropic cellulosic material (named elastic wood) directly from natural wood via chemical treatment. The resulting elastic wood demonstrates good elasticity and durable compressibility, showing no sign of fatigue after 10 000 compression cycles. The chemical treatment not only softens the wood cell walls by partially removing lignin and hemicellulose but introduces an interconnected cellulose fibril network into the wood channels. Atomistic and continuum modeling further reveals that the absorbed water can freely and reversibly move inside the elastic wood and therefore helps the elastic wood accommodate large compressive deformation and recover to its original shape upon compression release. In addition, the elastic wood showed a high ionic conductivity of up to 0.5 mS cm^-1 at a low KCl concentration of 10^-4 M, which can be tuned by changing the compression ratio of the material. The demonstrated elastic, mechanically robust, and ionically conductive cellulosic material combining inherited anisotropic cellular structure from natural wood and a self-formed internal gel may find a variety of potential applications in ionic nanofluidics, sensors, soft robots, artificial muscle, environmental remediation, and energy storage.

Highly Compressible Polymer Composite Foams with Thermal Heating-Boosted Electromagnetic Wave Absorption Abilities

Polymer composite foams are desirable materials for electromagnetic (EM) energy attenuation. However, a number of challenges limit improvement in the EM energy attenuation properties of foams. In this study, a simple microcellular injection molding method was used to fabricate highly compressible thermoplastic urethane (TPU)/carbon nanotube (CNTs) composite foams, which also had increased conductivity with an increase in CNT content. Compared to unfoamed composites, foamed composites exhibited higher conductivity and EM attenuation properties because of the presence of a microcellular structure. Moreover, the TPU/CNT foam with 4 wt % CNTs (F(4)) demonstrated strong EM dissipation and an optimal reflection loss (RL) value of -30.4 dB. Furthermore, stimulated by thermal heating and cyclic compression, EM attenuation was observed to increase because of the higher conductivity. Note that F(4) foam having a small thickness of 1.3 mm when treated at 333 K had the highest EM dissipation and the lowest RL value of -51.8 dB. Enhanced polarization and ohmic losses and multiscattering were responsible for the increased EM absorption. This behavior is attributed to the movement of CNTs within the TPU elastomer walls via thermal or compression stimulation. For designing stimulation-dependent multifunctional materials, composite foams with response to thermal heating were proved to be an alternative approach.

Ultrathin Densified Carbon Nanotube Film with "Metal-like" Conductivity, Superior Mechanical Strength, and Ultrahigh Electromagnetic Interference Shielding Effectiveness

Flexible and lightweight high-performance electromagnetic interference shielding materials with minimal thickness, excellent mechanical properties, and outstanding reliability are highly desired in the field of fifth-generation (5G) communication, yet remain extremely challenging to manufacture. Herein, we prepared an ultrathin densified carbon nanotube (CNT) film with superior mechanical properties and ultrahigh shielding effectiveness. Upon complete removal of impurities in pristine CNT film, charge separation in individual CNTs induced by polar molecules leads to strong CNT-CNT attraction and film densification, which significantly improve the electrical conductivity, shielding performance, and mechanical strength. The tensile strength is up to 822 ± 21 MPa, meanwhile the electrical conductivity is as high as 902,712 S/m, and the density is only 1.39 g cm^-3. Notably, the shielding effectiveness is over 51 dB with a thickness of merely 1.85 μm in the broad frequency range of 4-18 GHz, and it reaches to ∼82 dB at 6.36 μm and ∼101 dB at 14.7 μm, respectively. Further, such CNT film exhibits excellent reliability after an extended period in strong acid/alkali, high temperature, and high humidity. It demonstrates the best overall performance among representative shielding materials by far, representing a critical breakthrough in the preparation of shielding film toward applications in wearable electronics and 5G communication.

A Highly Elastic and Fatigue-Resistant Natural Protein-Reinforced Hydrogel Electrolyte for Reversible-Compressible Quasi-Solid-State Supercapacitors

Compressible solid-state supercapacitors are emerging as promising power sources for next-generation flexible electronics with enhanced safety and mechanical integrity. Highly elastic and compressible solid electrolytes are in great demand to achieve reversible compressibility and excellent capacitive stability of these supercapacitor devices. Here, a lithium ion-conducting hydrogel electrolyte by integrating natural protein nanoparticles into polyacrylamide network is reported. Due to the synergistic effect of natural protein nanoparticles and polyacrylamide chains, the obtained hydrogel shows remarkable elasticity, high compressibility, and fatigue resistance properties. More significantly, the supercapacitor device based on this hydrogel electrolyte exhibits reversible compressibility under multiple cyclic compressions, working well under 80% strain for 1000 compression cycles without sacrificing its capacitive performance. This work offers a promising approach for compressible supercapacitors.

Effect of CNTs Additives on the Energy Balance of Carbon/Epoxy Nanocomposites during Dynamic Compression Test

Previous research has shown that nanocomposites show not only enhancements in mechanical properties (stiffness, fracture toughness) but also possess remarkable energy absorption characteristics. However, the potential of carbon nanotubes (CNTs) as nanofiller in reinforced epoxy composites like glass fiber-reinforced polymers (GFRP) or carbon fiber-reinforced polymers (CFRP) under dynamic testing is still underdeveloped. The goal of this study is to investigate the effect of integrating nanofillers such as CNTs into the epoxy matrix of carbon fiber reinforced polymer composites (CFRP) on their dynamic energy absorption potential under impact. An out-of-plane compressive test at high strain rates was performed using a Split Hopkinson Pressure Bar (SHPB), and the results were analyzed to study the effect of changing the concentration of CNTs on the energy absorption properties of the nanocomposites. A strong correlation between strain rates and CNT mass fractions was found out, showing that an increase in percentage of CNTs could enhance the dynamic properties and energy absorption capabilities of fiber-reinforced composites.

Temperature-Invariant Superelastic and Fatigue Resistant Carbon Nanofiber Aerogels

Superelastic and fatigue-resistant materials that can work over a wide temperature range are highly desired for diverse applications. A morphology-retained and scalable carbonization method is reported to thermally convert a structural biological material (i.e., bacterial cellulose) into graphitic carbon nanofiber aerogel by engineering the pyrolysis chemistry. The prepared carbon aerogel perfectly inherits the hierarchical structures of bacterial cellulose from macroscopic to microscopic scales, resulting in remarkable thermomechanical properties. In particular, it maintains superelasticity without plastic deformation even after 2 × 10⁶ compressive cycles and exhibits exceptional temperature-invariant superelasticity and fatigue resistance over a wide temperature range at least from -100 to 500 °C. This aerogel shows unique advantages over polymeric foams, metallic foams, and ceramic foams in terms of thermomechanical stability and fatigue resistance, with the realization of scalable synthesis and the economic advantage of biological materials.

Artificial spider silk from ion-doped and twisted core-sheath hydrogel fibres

Spider silks show unique combinations of strength, toughness, extensibility, and energy absorption. To date, it has been difficult to obtain spider silk-like mechanical properties using non-protein approaches. Here, we report on an artificial spider silk produced by the water-evaporation-induced self-assembly of hydrogel fibre made from polyacrylic acid and silica nanoparticles. The artificial spider silk consists of hierarchical core-sheath structured hydrogel fibres, which are reinforced by ion doping and twist insertion. The fibre exhibits a tensile strength of 895 MPa and a stretchability of 44.3%, achieving mechanical properties comparable to spider silk. The material also presents a high toughness of 370 MJ m-3 and a damping capacity of 95%. The hydrogel fibre shows only ~1/9 of the impact force of cotton yarn with negligible rebound when used for impact reduction applications. This work opens an avenue towards the fabrication of artificial spider silk with applications in kinetic energy buffering and shock-absorbing.

Oxygenated Surface of Carbon Nanotube Sponges: Electroactivity and Magnetic Studies

We report the synthesis of nitrogen-doped carbon nanotube sponges (N-CNSs) by pyrolysis of solutions of benzylamine, ferrocene, thiophene, and isopropanol-based mixture at 1020 °C for 4 h using an aerosol-assisted chemical vapor deposition system. The precursors were transported through a quartz tube using a dynamic flow of H₂/Ar. We characterized the N-CNSs by scanning electron microscopy, transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, Raman spectroscopy, and thermogravimetric analysis. We found that isopropanol, isopropanol-ethanol, and isopropanol-acetone as precursors promote the formation of complex-entangled carbon fibers making knots and junctions. The N-CNSs displayed an outstanding oxygen concentration reaching a value of 9.2% for those synthesized with only isopropanol. We identified oxygen and nitrogen functional groups; in particular, the carbon fibers produced using only isopropanol exhibited a high concentration of ether groups (C-O bonds). This fact suggests the presence of phenols, carboxyl, methoxy, ethoxy, epoxy, and more complex functional groups. Usually, the functionalization of graphitic materials is carried out through aggressive acid treatments; here, we offer an alternative route to produce a superoxygenated surface. The understanding of the chemical surface of these novel materials represents a huge challenge and offers an opportunity to study complex oxygen functional groups different from the conventional quinone, carboxyl, phenols, carbonyl, methoxy, ethoxy, among others. The cyclic voltammetry measurements confirmed the importance of oxygen in N-CNSs, showing that with high oxygen concentration, the highest anodic and cathodic currents are displayed. N-CNSs displayed ferromagnetic behavior with an outstanding saturation magnetization. We envisage that our sponges are promising for anodes in lithium-ion batteries and magnetic sensor devices.

"Sliding Crystals" on Low-Dimensional Carbonaceous Nanofillers as Distributed Nanopistons for Highly Damping Materials

Improving energy dissipation in lightweight polymer nanocomposites to achieve environmentally friendly and mechanically stable structures has reached a limit because of the low-density electrostatic interactions that can be harnessed through the stick-slip mechanism between carbonaceous nanofillers and polymeric chains wrapped around them. In this paper, the atomic friction between the two nanocomposite components is greatly amplified by locally increasing the density of the energetically higher noncovalent bonds. This gives rise to a new material design concept in which crystallite structures, nucleated around the carbonaceous nanofillers, become the source of enhanced energy dissipation. The rheological concept is a nanopiston unit consisting of a carbon nanotube (CNT) as a nanofiller coated with crystallite structures which, upon unconventionally and reversibly overcoming the interfacial interaction forces, monolithically roto-translate from an energetically stable state to the adjacent states. The efficiency of this novel "sliding crystals" mechanism is proven by its higher dissipation capability that turns out to be at least twice as much as that of the conventional CNT/polymer stick-slip within a larger strain range.

Recent Advances in Carbon-Based Metal-Free Electrocatalysts

Precious noble metals (such as Pt, Ir) and nonprecious transition metals (e.g., Fe, Co), including their compounds (e.g., oxides, nitrides), have been widely investigated as efficient catalysts for energy conversion, energy storage, important chemical productions, and many industrial processes. However, they often suffer from high cost, low selectivity, poor durability, and susceptibility to gas poisoning with adverse environmental issues. As a low-cost alternative, the first carbon-based metal-free catalyst (C-MFC based on N-doped carbon nanotubes) was discovered in 2009. Since then, various C-MFCs have been demonstrated to show similar or even better catalytic performance than their metal-based counterparts, attractive energy conversion and storage (e.g., fuel cells, metal-air batteries, water splitting), environmental remediation, and chemical production. Enormous progress has been achieved while the number of publications still rapidly increases every year. Herein, a critical overview of the very recent advances in this rapidly developing field during the last couple of years is presented.

Aggregate-driven reconfigurations of carbon nanotubes in thin networks under strain: in-situ characterization

This work focuses on the in-situ characterization of multi-walled carbon nanotube (CNT) motions in thin random networks under strain. Many fine-grain models have been devised to account for CNT motions in carbon nanotube networks (CNN). However, the validation of these models relies on mesoscopic or macroscopic data with very little experimental validation of the physical mechanisms actually arising at the CNT scale. In the present paper, we use in-situ scanning electron microscopy imaging and high-resolution digital image correlation to uncover prominent mechanisms of CNT motions in CNNs under strain. Results show that thin and sparse CNNs feature stronger strain heterogeneities than thicker and denser ones. It is attributed to the complex motions of individual CNTs connected to aggregates within thin and sparse CNNs. While the aggregates exhibit a collective homogeneous deformation, individual CNTs connecting them are observed to fold, unwind or buckle, seemingly to accommodate the motion of these aggregates. In addition, looser aggregates feature internal reconfigurations via cell closing, similar to foam materials. Overall, this suggests that models describing thin and sparse CNN deformation should integrate multiphase behaviour (with various densities of aggregates in addition to individual CNTs), heterogeneity across surface, as well as imperfect substrate adhesion.

3D Heteroatom-Doped Carbon Nanomaterials as Multifunctional Metal-Free Catalysts for Integrated Energy Devices

Sustainable and cost-effective energy generation has become crucial for fulfilling present energy requirements. For this purpose, the development of cheap, scalable, efficient, and reliable catalysts is essential. Carbon-based heteroatom-doped, 3D, and mesoporous electrodes are very promising as catalysts for electrochemical energy conversion and storage. Various carbon allotropes doped with a variety of heteroatoms can be utilized for cost-effective mass production of electrode materials. 3D porous carbon electrodes provide multiple advantages, such as large surface area, maximized exposure to active sites, 3D conductive pathways for efficient electron transport, and porous channels to facilitate electrolyte diffusion. However, it is challenging to synthesize and functionalize isotropic 3D carbon structures. Here, various synthesis processes of 3D porous carbon materials are summarized to understand how their physical and chemical properties together with heteroatom doping dictate the electrochemical catalytic performance. Prospects of attractive 3D carbon structural materials for energy conversion and efficient integrated energy systems are also discussed.

Modifying the morphology and properties of aligned CNT foams through secondary CNT growth

In this work, we report for the first time, growth of secondary carbon nanotubes (CNTs) throughout a three-dimensional assembly of CNTs. The assembly of nanotubes was in the form of aligned CNT/carbon (ACNT/C) foams. These low-density CNT foams were conformally coated with an alumina buffer layer using atomic layer deposition. Chemical vapor deposition was further used to grow new CNTs. The CNT foam's extremely high porosity allowed for growth of secondary CNTs inside the bulk of the foams. Due to the heavy growth of new nanotubes, density of the foams increased more than 2.5 times. Secondary nanotubes had the same graphitic quality as the primary CNTs. Microscopy and chemical analysis revealed that the thickness of the buffer layer affected the diameter, nucleation density as well as growth uniformity across the thickness of the foams. The effects of secondary nanotubes on the compressive mechanical properties of the foams was also investigated.

Supercompressible Coaxial Carbon Nanotube@Graphene Arrays with Invariant Viscoelasticity over -100 to 500 °C in Ambient Air

Vertically aligned carbon nanotube (CNT) arrays have been recognized as promising cushion materials because of their superior thermal stability, remarkable compressibility, and viscoelastic characteristics. However, most of the previously reported CNT arrays still suffer from permanent shape deformation at only moderate compressive strains, which considerably restricts their practical applications. Here, we demonstrate a facile strategy of fabricating supercompressible coaxial CNT@graphene (CNT@Gr) arrays by using a two-step route involving encapsulating polymer layers onto plastic CNT arrays and subsequent annealing processes. Notably, the resulting CNT@Gr arrays are able to almost completely recover from compression at a strain of up to 80% and retain ∼80% recovery even after 1000 compression cycles at a 60% strain, demonstrating their excellent compressibility. Furthermore, they possess outstanding strain- and frequency-dependent viscoelastic responses, with storage modulus and damping ratio of up to ∼6.5 MPa and ∼0.19, respectively, which are nearly constant over an exceptionally broad temperature range of -100 to 500 °C in ambient air. These supercompressibility and temperature-invariant viscoelasticity together with facile fabrication process of the CNT@Gr arrays enable their promising multifunctional applications such as energy absorbers, mechanical sensors, and heat exchangers, even in extreme environments.

True Low-Power Self-Locking Soft Actuators

Natural double-layered structures observed in living organisms are known to exhibit asymmetric volume changes with environmental triggers. Typical examples are natural roots of plants, which show unique self-organized bending behavior in response to environmental stimuli. Herein, light- and electro-active polymer (LEAP) based actuators with a double-layered structure are reported. The LEAP actuators exhibit an improvement of 250% in displacement and hold an object three times heavier as compared to that in the case of conventional electro-active polymer actuators. Most interestingly, the bending motion of the LEAP actuators can be effectively locked for a few tens of minutes even in the absence of a power supply. Further, the self-locking LEAP actuators show a large and reversible bending strain of more than 2.0% and require only 6.2 mW h cm^-2 of energy to hold an object for 15 min at an operating voltage of 3 V. These novel self-locking soft actuators should find wide applicability in artificial muscles, biomedical microdevices, and various innovative soft robot technologies.

Aligned N-doped carbon nanotube bundles with interconnected hierarchical structure as an efficient bi-functional oxygen electrocatalyst

The fabrication of cost effective and efficient electrocatalysts with functional building blocks to replace noble metal ones is of great importance for energy related applications yet remains a great challenge. Herein, we report the fabrication of a hierarchical structure containing CNTs/graphene/transition-metal hybrids (h-NCNTs/Gr/TM) with excellent bifunctional oxygen electrocatalytic activity. The synthesis was rationally designed by the growth of shorter nitrogen-doped CNTs (S-NCNTs) on longer NCNTs arrays (L-NCNTs), while graphene layers were in situ generated at their interconnecting sites. The hybrid material shows excellent OER and ORR performance, and was also demonstrated to be a highly active bifunctional catalyst for Zn–air batteries, which could be due to rapid electron transport and full exposure of active sites in the hierarchical structure.

A hierarchical structure containing aligned CNTs/graphene/transition-metal was fabricated and worked as a highly active bifunctional catalyst for Zn–air batteries.

Recent progress in layered double hydroxide based materials for electrochemical capacitors: design, synthesis and performance

As representative two-dimensional (2D) materials, layered double hydroxides (LDHs) have received increasing attention in electrochemical energy storage and conversion because of the facile tunability between their composition and morphology. The high dispersion of active species in layered arrays, the simple exfoliation into monolayer nanosheets and chemical modification offer the LDHs an opportunity as active electrode materials in electrochemical capacitors (ECs). LDHs are favourable in providing large specific surface areas, good transport features as well as attractive physicochemical properties. In this review, our purpose is to provide a detailed summary of recent developments in the synthesis and electrochemical performance of the LDHs. Their composites with carbon (carbon quantum dots, carbon black, carbon nanotubes/nanofibers, graphene/graphene oxides), metals (nickel, platinum, silver), metal oxides (TiO₂, Co₃O₄, CuO, MnO₂, Fe₃O₄), metal sulfides/phosphides (CoS, NiCo₂S₄, NiP), MOFs (MOF derivatives) and polymers (PEDOT:PSS, PPy (polypyrrole), P(NIPAM-co-SPMA) and PET) are also discussed in this review. The relationship between structures and electrochemical properties as well as the associated charge-storage mechanisms is discussed. Moreover, challenges and prospects of the LDHs for high-performance ECs are presented. This review sheds light on the sustainable development of ECs with LDH based electrode materials.

Ultralight, Thermally Insulating, Compressible Polyimide Fiber Assembled Sponges

Tunable density, thermally and mechanically stable, elastic, and thermally insulating sponges are required for demanding applications. Hierarchically structured sponges with bimodal interconnected pores, porosity more than 99%, and tunable densities (between 7.6 and 10.1 mg/cm³) are reported using polyimide (PI) as high temperature stable polymer. The sponges are made by freeze-drying a dispersion of short PI fibers and precursor polymer, poly(amic acid) (PAA). The concept of "self-gluing" the fibrous network skeleton of PI during sponge formation was applied to achieve mechanical stability without sacrificing the thermal properties. The sponges showed initial degradation above 400 and 500 °C in air and nitrogen, respectively. They have low thermal conductivity of 0.026 W/mK and thermal diffusivity of 1.009 mm²/s for a density of 10.1 mg/cm³. The sponges are compressible for at least 10 000 cycles and good thermal insulators even at high compressions. These fibrous PI sponges are promising candidates for potential applications in thermal insulation, lightweight construction, high-temperature filtration, sensors, and catalyst carrier for high-temperature reactions.

Surface-restrained growth of vertically aligned carbon nanotube arrays with excellent thermal transport performance

A vertically aligned carbon nanotube (VACNT) array is a promising candidate for a high-performance thermal interface material in high-power microprocessors due to its excellent thermal transport property. However, its rough and entangled free tips always cause poor interfacial contact, which results in serious contact resistance dominating the total thermal resistance. Here, we employed a thin carbon cover to restrain the disorderly growth of the free tips of a VACNT array. As a result, all the free tips are seamlessly connected by this thin carbon cover and the top surface of the array is smoothed. This unique structure guarantees the participation of all the carbon nanotubes in the array in the heat transport. Consequently the VACNT array grown on a Cu substrate shows a record low thermal resistance of 0.8 mm² K W^-1 including the two-sided contact resistances, which is 4 times lower than the best result previously reported. Remarkably, the VACNT array can be easily peeled away from the Cu substrate and act as a thermal pad with excellent flexibility, adhesive ability and heat transport capability. As a result the CNT array with a thin carbon cover shows great potential for use as a high-performance flexible thermal interface material.

Naturally Dried Graphene Aerogels with Superelasticity and Tunable Poisson's Ratio

A novel natural drying (ND) strategy for low-cost and simple fabrication of graphene aerogels (GAs) is highlighted. The as-formed NDGAs exhibit ultralarge reversible compressibility (99%) and tunable Poisson's ratio behaviors (-0.30 < ν < 0.46), which suggests promising applications in soft actuators, soft robots, sensors, deformable electronic devices, drug release, thermal insulator, and protective materials.

Super-elastic and fatigue resistant carbon material with lamellar multi-arch microstructure

Low-density compressible materials enable various applications but are often hindered by structure-derived fatigue failure, weak elasticity with slow recovery speed and large energy dissipation. Here we demonstrate a carbon material with microstructure-derived super-elasticity and high fatigue resistance achieved by designing a hierarchical lamellar architecture composed of thousands of microscale arches that serve as elastic units. The obtained monolithic carbon material can rebound a steel ball in spring-like fashion with fast recovery speed (∼580 mm s⁻¹), and demonstrates complete recovery and small energy dissipation (∼0.2) in each compress-release cycle, even under 90% strain. Particularly, the material can maintain structural integrity after more than 10⁶ cycles at 20% strain and 2.5 × 10⁵ cycles at 50% strain. This structural material, although constructed using an intrinsically brittle carbon constituent, is simultaneously super-elastic, highly compressible and fatigue resistant to a degree even greater than that of previously reported compressible foams mainly made from more robust constituents.

Low-density compressible materials often suffer from fatigue-induced failure or limited elasticity. Here, the authors create a hierarchical multi-arch carbon material that achieves high compressibility, superior elasticity and fatigue resistance simultaneously, inspired by properties of arches in daily life.

Multifunctional Polymer-Based Graphene Foams with Buckled Structure and Negative Poisson's Ratio

In this study, we report the polymer-based graphene foams through combination of bottom-up assembly and simple triaxially buckled structure design. The resulting polymer-based graphene foams not only effectively transfer the functional properties of graphene, but also exhibit novel negative Poisson’s ratio (NPR) behaviors due to the presence of buckled structure. Our results show that after the introduction of buckled structure, improvement in stretchability, toughness, flexibility, energy absorbing ability, hydrophobicity, conductivity, piezoresistive sensitivity and crack resistance could be achieved simultaneously. The combination of mechanical properties, multifunctional performance and unusual deformation behavior would lead to the use of our polymer-based graphene foams for a variety of novel applications in future such as stretchable capacitors or conductors, sensors and oil/water separators and so on.

A multi-functional nanoplatform for tumor synergistic phototherapy

Phototherapy, which mainly includes photothermal treatment (PTT) and photodynamic treatment (PDT), is a photo-initiated, noninvasive and effective approach for cancer treatment. The high accumulation of photosensitizers (PSs) in a targeted tumor is still a major challenge for efficient light conversion, to generate reactive oxygen species (ROS) and local hyperthermia. In this study, a simple and efficient hyaluronic acid (HA)-modified nanoplatform (HA-TiO2@MWCNTs) with high tumor-targeting ability, excellent phototherapy efficiency, low light-associated side effects and good water solubility was developed. It could be an effective carrier to load hematoporphyrin monomethyl ether (HMME), owing to the tubular conjugate structure. Apart from this, the as-prepared TiO2@MWCNTs nanocomposites could also be used as PSs for tumor PTT and PDT. Those results in vitro and in vivo showed that the anti-tumor effect of this system-mediated PTT/PDT were significantly better than those of single treatment manner. In addition, this drug delivery system could realize high ratio of drug loading, sustained drug release, prolonged circulation in vivo and active targeted accumulation in tumor. These results suggest that HA-TiO2@MWCNTs/HMME has high potential for tumor synergistic phototherapy as a smart theranostic nanoplatform.

Three-dimensional Sponges with Super Mechanical Stability: Harnessing True Elasticity of Individual Carbon Nanotubes in Macroscopic Architectures

Efficient assembly of carbon nanotube (CNT) based cellular solids with appropriate structure is the key to fully realize the potential of individual nanotubes in macroscopic architecture. In this work, the macroscopic CNT sponge consisting of randomly interconnected individual carbon nanotubes was grown by CVD, exhibiting a combination of super-elasticity, high strength to weight ratio, fatigue resistance, thermo-mechanical stability and electro-mechanical stability. To deeply understand such extraordinary mechanical performance compared to that of conventional cellular materials and other nanostructured cellular architectures, a thorough study on the response of this CNT-based spongy structure to compression is conducted based on classic elastic theory. The strong inter-tube bonding between neighboring nanotubes is examined, believed to play a critical role in the reversible deformation such as bending and buckling without structural collapse under compression. Based on in-situ scanning electron microscopy observation and nanotube deformation analysis, structural evolution (completely elastic bending-buckling transition) of the carbon nanotubes sponges to deformation is proposed to clarify their mechanical properties and nonlinear electromechanical coupling behavior.

Ultralight anisotropic foams from layered aligned carbon nanotube sheets

In this work, we present large scale, ultralight aligned carbon nanotube (CNT) structures which have densities an order of magnitude lower than CNT arrays, have tunable properties and exhibit resiliency after compression. By stacking aligned sheets of carbon nanotubes and then infiltrating with a pyrolytic carbon (PyC), resilient foam-like materials were produced that exhibited complete recovery from 90% compressive strain. With density as low as 3.8 mg cm(-3), the foam structure is over 500 times less dense than bulk graphite. Microscopy revealed that PyC coated the junctions among CNTs, and also increased CNT surface roughness. These changes in the morphology explain the transition from inelastic behavior to foam-like recovery of the layered CNT sheet structure. Mechanical and thermal properties of the foams were tuned for different applications through variation of PyC deposition duration while dynamic mechanical analysis showed no change in mechanical properties over a large temperature range. Observation of a large and linear electrical resistance change during compression of the aligned CNT/carbon (ACNT/C) foams makes strain/pressure sensors a relevant application. The foams have high oil absorption capacities, up to 275 times their own weight, which suggests they may be useful in water treatment and oil spill cleanup. Finally, the ACNT/C foam's high porosity, surface area and stability allow for demonstration of the foams as catalyst support structures.

Fabrication of Nanocarbon Composites Using In Situ Chemical Vapor Deposition and Their Applications

Nanocarbon (carbon nanotubes (CNTs) and graphene (GN)) composites attract considerable research interest due to their fascinating applications in many fields. Here, recent developments in the field of in situ chemical vapor deposition (CVD) for the design and controlled preparation of advanced nanocarbon composites are highlighted, specifically, CNT-reinforced bulk structural composites, as well as CNT, GN, and CNT/GN functional composites, together with their practical and potential applications. In situ CVD is a very attractive approach for the fabrication of composites because of its engaging features, such as its simplicity, low-cost, versatility, and tunability. The morphologies, structures, dispersion, and interface of the resulting nanocarbon composites can be easily modulated by varying the experimental parameters (such as temperature, catalysts, carbon sources, templates or template catalysts, etc.), which enables a great potential for the in situ synthesis of high-quality nanocarbons with tailored size and dimension for constructing high-performance composites, which has not yet been achieved by conventional methods. In addition, new trends of the in situ CVD toward nanocarbon composites are discussed.

Molecular dynamics study of a CNT-buckyball-enabled energy absorption system

An energy absorption system (EAS) composed of a carbon nanotube (CNT) with nested buckyballs is put forward for energy dissipation during impact owing to the outstanding mechanical properties of both CNTs and buckyballs. Here we implement a series of molecular dynamics (MD) simulations to investigate the energy absorption capabilities of several different EASs based on a variety of design parameters. For example, the effects of impact energy, the number of nested buckyballs, and of the size of the buckyballs are analyzed to optimize the energy absorption capability of the EASs by tuning the relevant design parameters. Simulation results indicate that the energy absorption capability of the EAS is closely associated with the deformation characteristics of the confined buckyballs. A low impact energy leads to recoverable deformation of the buckyballs and the dissipated energy is mainly converted to thermal energy. However, a high impact energy yields non-recoverable deformation of buckyballs and thus the energy dissipation is dominated by the strain energy of the EAS. The simulation results also reveal that there exists an optimal value of the number of buckyballs for an EAS under a certain impact energy. Larger buckyballs are able to deform to a larger degree yet also need less impact energy to induce plastic deformation, therefore performing with a better overall energy absorption ability. Overall, the EAS in this study shows a remarkably high energy absorption density of 2 kJ g(-1), it is a promising candidate for mitigating impact energy and sheds light on the research of buckyball-filled CNTs for other applications.

Determination of material constants of vertically aligned carbon nanotube structures in compressions

Different chemical vapour deposition (CVD) fabrication conditions lead to a wide range of variation in the microstructure and morphologies of carbon nanotubes (CNTs), which actually determine the compressive mechanical properties of CNTs. However, the underlying relationship between the structure/morphology and mechanical properties of CNTs is not fully understood. In this study, we characterized and compared the structural and morphological properties of three kinds of vertically aligned carbon nanotube (VACNT) arrays from different CVD fabrication methods and performed monotonic compressive tests for each VACNT array. The compressive stress-strain responses and plastic deformation were first compared and analyzed with nanotube buckling behaviours. To quantify the compressive properties of the VACNT arrays, a strain density energy function was used to determine their intrinsic material constants. Then, the structural and morphological effects on the quantified material constants of the VACNTs were statistically investigated and analogized to cellular materials with an open-cell model. The statistical analysis shows that density, defect degree, and the moment of inertia of the CNTs are key factors in the improvement of the compressive mechanical properties of VACNT arrays. This approach could allow a model-driven CNT synthesis for engineering their mechanical behaviours.

Self-Sensing, Ultralight, and Conductive 3D Graphene/Iron Oxide Aerogel Elastomer Deformable in a Magnetic Field

Three-dimensional (3D) graphene aerogels (GA) show promise for applications in supercapacitors, electrode materials, gas sensors, and oil absorption due to their high porosity, mechanical strength, and electrical conductivity. However, the control, actuation, and response properties of graphene aerogels have not been well studied. In this paper, we synthesized 3D graphene aerogels decorated with Fe3O4 nanoparticles (Fe3O4/GA) by self-assembly of graphene with simultaneous decoration by Fe3O4 nanoparticles using a modified hydrothermal reduction process. The aerogels exhibit up to 52% reversible magnetic field-induced strain and strain-dependent electrical resistance that can be used to monitor the degree of compression/stretching of the material. The density of Fe3O4/GA is only about 5.8 mg cm(-3), making it an ultralight magnetic elastomer with potential applications in self-sensing soft actuators, microsensors, microswitches, and environmental remediation.

Ultracompressible, high-rate supercapacitors from graphene-coated carbon nanotube aerogels

Emerging applications for electrochemical energy storage require devices that not only possess high power and energy, but also are capable of withstanding mechanical deformation without degradation of performance. To this end, we have constructed electric double layer capacitors (EDLCs), also referred to as supercapacitors, using thick, ultracompressible graphene-coated carbon nanotube aerogels as electrodes. These electrodes showed a high capacitance in both aqueous and room-temperature ionic liquid (RTIL) electrolytes, achieving between 60 and100 F/g, respectively, with the performance stable over hundreds of charge/discharge cycles and at high rates exceeding 1 V/s. This performance was retained fully under 90% compression of the systems, allowing us to construct cells with high volumetric capacitances of ∼5-18 F/cm(3) in aqueous and RTIL electrolytes, respectively, which are 50-100 times higher than comparable compressible EDLCs (∼0.1 F/cm(3)). Further, the volumetric capacitances approach values reported for compressible pseudocapacitors (∼15-30 F/cm(3)) but without the degraded lifetime and reversibility that typically plague compressible pseudocapacitors. The electrodes demonstrated largely strain-invariant ion transport with no change in capacitance and high-rate performance even at 90% compressive strain. This material serves as an excellent platform for exploring the possibility for use of extremely compressible EDLCs with negligible degradation in capacitance in applications such as electric vehicles and wearable electronics.

Near-infrared-triggered in situ hybrid hydrogel system for synergistic cancer therapy

The photo-polymerization of PEGDA hydrogel and its synergetic anti-tumor effect triggered by a single NIR laser.

Liquid-type cathode enabled by 3D sponge-like carbon nanotubes for high energy density and long cycling life of Li-S batteries

High energy density and long-term stability of Li-S batteries are achieved by employing a 3D sponge-like carbon nanotube cathode and a liquid-type polysulfide catholyte. Carbon nanotubes not only provide excellent electron pathways and polysulfide reservoirs, but they can also be used as a standalone cathode without current collectors, which greatly alleviates problems arising from insulating sulfur and polysulfide shuttles as well as remarkably increasing the energy density.

Nitrogen-doped aligned carbon nanotube/graphene sandwiches: facile catalytic growth on bifunctional natural catalysts and their applications as scaffolds for high-rate lithium-sulfur batteries

Nitrogen-doped aligned CNT/graphene sandwiches are rationally designed and in-situ fabricated by a facile catalytic growth on bifunctional natural catalysts that exhibit high-rate performances as scaffolds for lithium-sulfur batteries, with a high initial capacity of 1152 mA h g(-1) at 1.0 C. A remarkable capacity of 770 mA h g(-1) can be achieved at 5.0 C. Such a design strategy for materials opens up new perspectives to novel advanced functional composites, especially interface-modified hierarchical nanocarbons for broad applications.