Planar Porous Graphene Woven Fabric/Epoxy Composites with Exceptional Electrical, Mechanical Properties, and Fracture Toughness

Planar interconnected graphene woven fabrics (GWFs) are prepared by template-based chemical vapor deposition and the GWFs are employed as multifunctional filler for epoxy-based composites. Apart from flexibility, transparency, lightweight, and high electrical conductivity, the GWFs have unique morphological features consisting of orthogonally interweaved, inherently percolated, hollow graphene tubes (GTs). The orthogonal GT structure means that the GWF/epoxy composites hold significant anisotropy in mechanical and fracture properties. The composites with 0.62 wt % graphene deliver a combination of excellent electrical and fracture properties: e.g., an electrical conductivity of ∼0.18 S/cm; and fracture toughness of 1.67 and 1.78 MPa·m^1/2 when loaded along the 0° and 45° directions relative to the GT direction, respectively, equivalent to notable 57% and 67% rises compared to the solid epoxy. Unique fracture processes in GWF/epoxy composites are identified by in situ examinations, revealing crack tip blunting that occurs when the crack impinges GTs, especially those at 45° to the crack growth direction, as well as longitudinal tearing of hollow GTs as the two major toughening mechanisms

Ultrafast-Charging and Long-Life Li-Ion Battery Anodes of TiO₂‑B and Anatase Dual-Phase Nanowires

Ideal lithium-ion batteries (LIBs) should possess a high power density, be charged extremely fast (e.g., 100C), and have a long service life. To achieve them all, all battery components, including anodes, cathodes, and electrolytes should have excellent structural and functional characteristics. The present work reports ultrafast-charging and long-life LIB anodes made from TiO₂-B/anatase dual-phase nanowires. The dual-phase nanowires are fabricated with anatase TiO₂ nanoparticles through a facile and cost-effective hydrothermal process, which can be easily scaled up for mass production. The anodes exhibit remarkable electrochemical performance with reversible capacities of ∼225, 172, and 140 mAh g^–1 at current rates of 1C, 10C, and 60C, respectively. They deliver exceptional capacity retention of not less than 126 and 93 mAh g^–1 after 1000 cycles at 60C and 100C, respectively, potentially worthwhile for high-power applications. These values are among the best when the high-rate capabilities are compared with the literature data for similar TiO₂-based anodes. The Ragone plot confirms both the exceptionally high energy and power densities of the devices prepared using the dual-phase nanowires. The electrochemical tests and operando Raman spectra present fast electrochemical kinetics for both Li⁺ and electron transports in the TiO₂ dual-phase nanowires than in anatase nanoparticles due to the excellent Li⁺ diffusion coefficient and electronic conductivity of nanowires

Multilayer Graphene Enables Higher Efficiency in Improving Thermal Conductivities of Graphene/Epoxy Composites

The effects of number of graphene layers (<i>n</i>) and size of multilayer graphene sheets on thermal conductivities (TCs) of their epoxy composites are investigated. Molecular dynamics simulations show that the in-plane TCs of graphene sheets and the TCs across the graphene/epoxy interface simultaneously increase with increasing <i>n</i>. However, such higher TCs of multilayer graphene sheets will not translate into higher TCs of bulk composites unless they have large lateral sizes to maintain their aspect ratios comparable to the monolayer counterparts. The benefits of using large, multilayer graphene sheets are confirmed by experiments, showing that the composites made from graphite nanoplatelets (<i>n</i> > 10) with over 30 μm in diameter deliver a TC of ∼1.5 W m^–1 K^–1 at only 2.8 vol %, consistently higher than those containing monolayer or few-layer graphene at the same graphene loading. Our findings offer a guideline to use cost-effective multilayer graphene as conductive fillers for various thermal management applications

Exceptional Electrical Conductivity and Fracture Resistance of 3D Interconnected Graphene Foam/Epoxy Composites

Cellular-structured graphene foam (GF)/epoxy composites are prepared based on a three-step fabrication process involving infiltration of epoxy into the porous GF. The three-dimensional (3D) GF is grown on a Ni foam template <i>via</i> chemical vapor deposition. The 3D interconnected graphene network serves as fast channels for charge carriers, giving rise to a remarkable electrical conductivity of the composite, 3 S/cm, with only 0.2 wt % GF. The corresponding flexural modulus and strength increase by 53 and 38%, respectively, whereas the glass transition temperature increases by a notable 31 °C, compared to the solid neat epoxy. The GF/epoxy composites with 0.1 wt % GF also deliver an excellent fracture toughness of 1.78 MPa·m^1/2, 34 and 70% enhancements against their “porous” epoxy and solid epoxy counterparts, respectively. These observations signify the unrivalled effectiveness of 3D GF relative to 1D carbon nanotubes or 2D functionalized graphene sheets as reinforcement for polymer composites without issues of nanofiller dispersion and functionalization prior to incorporation into the polymer

Ultralight Graphene Foam/Conductive Polymer Composites for Exceptional Electromagnetic Interference Shielding

Ultralight, high-performance electromagnetic interference (EMI) shielding graphene foam (GF)/poly­(3,4-ethylenedioxythiophene):poly­(styrenesulfonate) (PEDOT:PSS) composites are developed by drop coating of PEDOT:PSS on cellular-structured, freestanding GFs. To enhance the wettability and the interfacial bonds with PEDOT:PSS, GFs are functionalized with 4-dodecylbenzenesulfonic acid. The GF/PEDOT:PSS composites possess an ultralow density of 18.2 × 10^–3 g/cm³ and a high porosity of 98.8%, as well as an enhanced electrical conductivity by almost 4 folds from 11.8 to 43.2 S/cm after the incorporation of the conductive PEDOT:PSS. Benefiting from the excellent electrical conductivity, ultralight porous structure, and effective charge delocalization, the composites deliver remarkable EMI shielding performance with a shielding effectiveness (SE) of 91.9 dB and a specific SE (SSE) of 3124 dB·cm³/g, both of which are the highest among those reported in the literature for carbon-based polymer composites. The excellent electrical conductivities of composites arising from both the GFs with three-dimensionally interconnected conductive networks and the conductive polymer coating, as well as the left-handed composites with absolute permittivity and/or permeability larger than one give rise to significant microwave attenuation by absorption

Three-Dimensional Porous Graphene Aerogel Cathode with High Sulfur Loading and Embedded TiO₂ Nanoparticles for Advanced Lithium–Sulfur Batteries

Three-dimensional graphene aerogel/TiO₂/sulfur (GA/TiO₂/S) composites are synthesized through a facile, one-pot hydrothermal route as the cathode for lithium–sulfur batteries. With a high sulfur content of 75.1 wt %, the conductive, highly porous composite electrode delivers a high discharge capacity of 512 mA h/g after 250 cycles at a current rate of 1 C with a low capacity decay of 0.128% per cycle. The excellent capacities and cyclic stability arise from several unique functional features of the cathode. (i) The conductive graphene aerogel framework ameliorates ion/electron transfer while accommodating the volume expansion induced during discharge, and (ii) TiO₂ nanoparticles play an important role in restricting the dissolution of polysulfides by chemical bonds with sulfur

Graphene Aerogel/Epoxy Composites with Exceptional Anisotropic Structure and Properties

3D interconnected graphene aerogels (GAs) are prepared through one-step chemical reduction and rational assembly of graphene oxide (GO) sheets, so that the difficulties to uniformly disperse the individual graphene sheets in the polymer matrixes are avoided. Apart from ultralow density, high porosity, high electrical conductivity, and excellent compressibility, the resulting GAs possess a cellular architecture with a high degree of alignment when the graphene content is above a threshold, ∼0.5 wt %. The composites prepared by infiltrating GA with epoxy resin present excellent electrical conductivities, together with high mechanical properties and fracture toughness. The unusual anisotropic structure gives rise to ∼67% and ∼113% higher electrical conductivity and fracture toughness of the composites, respectively, in the alignment direction than that transverse to it

Graphene Size-Dependent Multifunctional Properties of Unidirectional Graphene Aerogel/Epoxy Nanocomposites

Unidirectional graphene aerogels (UGAs) with tunable densities, degrees of alignment, and electrical conductivities are prepared by varying the average size of precursor graphene oxide (GO) sheets between 1.1 and 1596 μm². UGAs prepared using ultralarge GO (UL-UGA) outperform those made from small GO in these properties. The UL-UGA/epoxy composites prepared by infiltrating liquid epoxy resin into the porous UGA structure exhibit an excellent electrical conductivity of 0.135 S/cm, along with an ultralow percolation threshold of 0.0066 vol %, which is one of the lowest values ever reported for all graphene-based composites. Owing to their three-dimensional interconnected network, a high degree of alignment, and effective reduction, UL-UGAs effectively enhance the fracture toughness of epoxy by 69% at 0.11 vol % graphene content through unique toughening mechanisms, such as crack pinning, crack deflection, interfacial debonding, and graphene rupture. These aerogels and composites can be mass-produced thanks to the facile, scalable, and cost-efficient fabrication process, which will find various multifunctional applications

Graphene/Boron Nitride–Polyurethane Microlaminates for Exceptional Dielectric Properties and High Energy Densities

Hexagonal boron nitride (h-BN) has tremendous potential for dielectric energy storage by rationally assembling with graphene. We report the fabrication of microlaminate composites consisting of alternating reduced graphene oxide (rGO) and h-BN nanosheets embedded in a polyurethane (PU) matrix using a novel, two-step bidirectional freeze casting process. Porous, highly-aligned rGO–PU aerogels having ultrahigh dielectric constants with relatively high dielectric losses and low dielectric strengths are fabricated by initial freeze casting. The losses are suppressed, whereas the dielectric strengths are restored by assembling the porous rGO–PU skeleton with electrically insulating BN–PU tunneling barrier layers in the second freeze casting routine. The ligaments bridging the conductive rGO–PU layers are effectively removed by the BN–PU barrier layers, eliminating the current leakage in the transverse direction. The resultant rGO–PU/BN–PU microlaminate composites deliver a remarkable dielectric constant of 1084 with a low dielectric loss of 0.091 at 1 kHz. By virtue of synergy arising from both the rGO–PU layers with a high dielectric constant and the BN–PU barrier layers with a high dielectric strength, the microlaminate composites present a maximum energy density of 22.7 J/cm³, 44 folds of the neat rGO–PU composite acting alone. The promising overall dielectric performance based on a microlaminate structure offers a new insight into the development of next-generation dielectric materials

Ultrafine TiO₂ Decorated Carbon Nanofibers as Multifunctional Interlayer for High-Performance Lithium–Sulfur Battery

Although lithium–sulfur (Li–S) batteries deliver high specific energy densities, lots of intrinsic and fatal obstacles still restrict their practical application. Electrospun carbon nanofibers (CNFs) decorated with ultrafine TiO₂ nanoparticles (CNF-T) were prepared and used as a multifunctional interlayer to suppress the volume expansion and shuttle effect of Li–S battery. With this strategy, the CNF network with abundant space and superior conductivity can accommodate and recycle the dissolved polysulfides for the bare sulfur cathode. Meanwhile, the ultrafine TiO₂ nanoparticles on CNFs work as anchoring points to capture the polysulfides with the strong interaction, making the battery perform with remarkable and stable electrochemical properties. As a result, the Li–S battery with the CNF-T interlayer delivers an initial reversible capacity of 935 mA h g^–1 at 1 C with a capacity retention of 74.2% after 500 cycles. It is believed that this simple, low-cost and scalable method will definitely bring a novel perspective on the practical utilization of Li–S batteries