Shiting Wu

Colloidal antireflection coating improves graphene-silicon solar cells.

Carbon nanotube-Si and graphene-Si solar cells have attracted much interest recently owing to their potential in simplifying manufacturing process and lowering cost compared to Si cells. Until now, the power conversion efficiency of graphene-Si cells remains under 10% and well below that of the nanotube-Si counterpart. Here, we involved a colloidal antireflection coating onto a monolayer graphene-Si solar cell and enhanced the cell efficiency to 14.5% under standard illumination (air mass 1.5, 100 mW/cm(2)) with a stable antireflection effect over long time. The antireflection treatment was realized by a simple spin-coating process, which significantly increased the short-circuit current density and the incident photon-to-electron conversion efficiency to about 90% across the visible range. Our results demonstrate a great promise in developing high-efficiency graphene-Si solar cells in parallel to the more extensively studied carbon nanotube-Si structures.

Core-Double-Shell, Carbon Nanotube@Polypyrrole@MnO2 Sponge as Freestanding, Compressible Supercapacitor Electrode

Design and fabrication of structurally optimized electrode materials are important for many energy applications such as supercapacitors and batteries. Here, we report a three-component, hierarchical, bulk electrode with tailored microstructure and electrochemical properties. Our supercapacitor electrode consists of a three-dimensional carbon nanotube (CNT) network (also called sponge) as a flexible and conductive skeleton, an intermediate polymer layer (polypyrrole, PPy) with good interface, and a metal oxide layer outside providing more surface area. These three components form a well-defined core-double-shell configuration that is distinct from simple core-shell or hybrid structures, and the synergistic effect leads to enhanced supercapacitor performance including high specific capacitance (even under severe compression) and excellent cycling stability. The mechanism study reveals that the shell sequence is a key factor; in our system, the CNT-PPy-MnO2 structure shows higher capacitance than the CNT-MnO2-PPy sequence. Our porous core-double-shell sponges can serve as freestanding, compressible electrodes for various energy devices.

Carbon nanotube-polypyrrole core-shell sponge and its application as highly compressible supercapacitor electrode

A carbon nanotube (CNT) sponge contains a three-dimensional conductive nanotube network, and can be used as a porous electrode for various energy devices. We present here a rational strategy to fabricate a unique CNT@polypyrrole (PPy) core-shell sponge, and demonstrate its application as a highly compressible supercapacitor electrode with high performance. A PPy layer with optimal thickness was coated uniformly on individual CNTs and inter-CNT contact points by electrochemical deposition and crosslinking of pyrrole monomers, resulting in a core-shell configuration. The PPy coating significantly improves specific capacitance of the CNT sponge to above 300 F/g, and simultaneously reinforces the porous structure to achieve better strength and fully elastic structural recovery after compression. The CNT@PPy sponge can sustain 1,000 compression cycles at a strain of 50% while maintaining a stable capacitance (> 90% of initial value). Our CNT@PPy core-shell sponges with a highly porous network structure may serve as compressible, robust electrodes for supercapacitors and many other energy devices.

Highly Twisted Double-Helix Carbon Nanotube Yarns

The strength and flexibility of carbon nanotubes (CNTs) allow them to be constructed into a variety of innovated architectures with fascinating properties. Here, we show that CNTs can be made into a highly twisted yarn-derived double-helix structure by a conventional twist-spinning process. The double-helix is a stable and hierarchical configuration consisting of two single-helical yarn segments, with controlled pitch and unique mechanical properties. While one of the yarn components breaks early under tension due to the highly twisted state, the second yarn produces much larger tensile strain and significantly prolongs the process until ultimate fracture. In addition, these elastic and conductive double-helix yarns show simultaneous and reversible resistance change in response to a wide range of input sources (mechanical, photo, and thermal) such as applied strains or stresses, light illumination, and environmental temperature. Our results indicate that it is possible to create higher-level, more complex architectures from CNT yarns and fabricate multifunctional nanomaterials with potential applications in many areas.

Overtwisted, Resolvable Carbon Nanotube Yarn Entanglement as Strain Sensors and Rotational Actuators

Introducing twists into carbon nanotube yarns could produce hierarchical architectures and extend their application areas. Here, we utilized such twists to produce elastic strain sensors over large strain (up to 500%) and rotation actuators with high energy density. We show that a helical nanotube yarn can be overtwisted into highly entangled, macroscopically random but locally organized structures, consisting of mostly double-helix segments intertwined together. Pulling the yarn ends completely resolved the entanglement in an elastic and reversible way, yielding large tensile strains with linear change in electrical resistance. Resolving an entangled yarn and releasing its twists could simultaneously rotate a heavy object (30 000 times the yarn weight) for more than 1000 cycles at high speed. The rotational actuation generated from a single entangled yarn produced energy densities up to 8.3 kJ/kg, and maintained similar capacity during repeated use. Our entangled CNT yarns represent a complex self-assembled system with applications as large-range strain sensors and robust rotational actuators.

Perovskite-Type LaSrMnO Electrocatalyst with Uniform Porous Structure for an Efficient Li–O2 Battery Cathode

Perovskite is an excellent candidate as low cost catalyst for Li-O2 cells. However, the limited porosity, which impedes molecular transport, and the inherent low electronic conductivity are the main barriers toward production of high-performance electrodes. Here, we designed a hierarchical porous flexible architecture by coating thin mesoporous yet crystalline LaSrMnO layers throughout a graphene foam to form graphene/meso-LaSrMnO sandwich-like nanosheets. In this well-designed system, the macropore between nanosheets facilitates O2 and Li(+) diffusion, the mesopore provides large surface area for electrolyte immersion and discharge products deposition, the perovskite phase catalyst decreases reactive overpotential, and the graphene serves as conductive network for electrons transport. When used as a freestanding electrode of Li-O2 cell, it shows high specific capacity, superior rate capability, and cyclic stability. Combination of mesoporous perovskites with conductive graphene networks represents an effective strategy for developing efficient electrodes in various energy storage systems.

Graphene aerogel composites derived from recycled cigarette filters for electromagnetic wave absorption

Assembling graphene nanosheets into three dimensional aerogels has attracted considerable interest due to their unique properties and potential applications in many fields. Here, graphene aerogels constructed from interconnected graphene nanosheet-coated carbon fibers are fabricated by using cigarette filters as templates via a simple dip-coating method. The composite aerogels are ultralight (ρ = 7.6 mg cm−3) yet have high mechanical strength (0.07 MPa); when used as electromagnetic wave absorbers, they showed a minimum reflection loss value of −30.53 dB at 14.6 GHz and the bandwidth of reflection loss less than −10 dB (90% absorption) was 4.1 GHz. Furthermore, coating polypyrrole onto the composite aerogels can increase the minimum reflection loss value to −45.12 dB. Our results provide a promising approach to fabricate graphene-based composite aerogels with a strong electromagnetic wave absorption ability.

Multifunctional graphene sheet–nanoribbon hybrid aerogels

Graphene sheets and nanoribbons are graphene-based nanostructures with different dimensions. Here, we show that these two materials can be combined to form highly porous, ultra-low density, compressible yet elastic aerogels, which can be used as efficient adsorbents and supercapacitor electrodes. The pore walls consist of stacked graphene sheets embedded with uniformly distributed thick nanoribbons unzipped from multi-walled carbon nanotubes as effective reinforcing skeletons. Owing to the large pore-size, robust and stable structure, and the nanoribbon-adhered pore walls, these hybrid aerogels show very large adsorption capacity for a series of organic solvents and oils (100 to 350 times of aerogel weight), and a specific capacitance of 256 F g−1 tested in a three-electrode electrochemical configuration, which is further improved to 537 F g−1 by depositing controlled loading pseudo-polymers into the aerogels. Our multifunctional graphene sheet–nanoribbon hybrid aerogels may find potential applications in many fields such as environmental cleanup and as flexible electrodes for energy storage systems such as supercapacitors and batteries.

Hierarchically Designed Three-Dimensional Macro/Mesoporous Carbon Frameworks for Advanced Electrochemical Capacitance Storage

Mesoporous carbon (m-C) has potential applications as porous electrodes for electrochemical energy storage, but its applications have been severely limited by the inherent fragility and low electrical conductivity. A rational strategy is presented to construct m-C into hierarchical porous structures with high flexibility by using a carbon nanotube (CNT) sponge as a three-dimensional template, and grafting Pt nanoparticles at the m-C surface. This method involves several controllable steps including solution deposition of a mesoporous silica (m-SiO2 ) layer onto CNTs, chemical vapor deposition of acetylene, and etching of m-SiO2 , resulting in a CNT@m-C core-shell or a CNT@m-C@Pt core-shell hybrid structure after Pt adsorption. The underlying CNT network provides a robust yet flexible support and a high electrical conductivity, whereas the m-C provides large surface area, and the Pt nanoparticles improves interfacial electron and ion diffusion. Consequently, specific capacitances of 203 and 311 F g(-1) have been achieved in these CNT@m-C and CNT@m-C@Pt sponges as supercapacitor electrodes, respectively, which can retain 96 % of original capacitance under large degree compression.

Highly Porous Core–Shell Structured Graphene-Chitosan Beads

Graphene oxide (GO) sheets have been assembled into various three-dimensional porous structures and composites, with potential applications in energy and environmental areas. Here, we show the combination of GO and chitosan (CTS) into inorganic-organic heterocomposites as ∼3 mm diameter core-shell beads with controlled microstructure. The spherical GO-CTS beads, made by a two-step freeze-casting method, consist of a GO core wrapped by a CTS shell with abrupt interface; both parts have high porosities (94-96%) and mesopores volume (0.246 cm(3)/g) yet with different pore morphologies. Incorporation of a GO core into the CTS beads significantly improved the methyl orange adsorption capacity (353 mg/g at 318 K) compared with pure CTS beads. Key factors such as the pH value, adsorbent dosage, concentration, time, and temperature have been studied in detail, whereas adsorption isotherm and kinetic studies reveal a Langmuir model following the pseudo-second order.