Huey Hoon Hng

Nanostructured metal oxide-based materials as advanced anodes for lithium-ion batteries.

The search for new electrode materials for lithium-ion batteries (LIBs) has been an important way to satisfy the ever-growing demands for better performance with higher energy/power densities, improved safety and longer cycle life. Nanostructured metal oxides exhibit good electrochemical properties, and they are regarded as promising anode materials for high-performance LIBs. In this feature article, we will focus on three different categories of metal oxides with distinct lithium storage mechanisms: tin dioxide (SnO(2)), which utilizes alloying/dealloying processes to reversibly store/release lithium ions during charge/discharge; titanium dioxide (TiO(2)), where lithium ions are inserted/deinserted into/out of the TiO(2) crystal framework; and transition metal oxides including iron oxide and cobalt oxide, which react with lithium ions via an unusual conversion reaction. For all three systems, we will emphasize that creating nanomaterials with unique structures could effectively improve the lithium storage properties of these metal oxides. We will also highlight that the lithium storage capability can be further enhanced through designing advanced nanocomposite materials containing metal oxides and other carbonaceous supports. By providing such a rather systematic survey, we aim to stress the importance of proper nanostructuring and advanced compositing that would result in improved physicochemical properties of metal oxides, thus making them promising negative electrodes for next-generation LIBs.

A Leavening Strategy to Prepare Reduced Graphene Oxide Foams

For instance, freestanding graphene macroscopic structures have shown unique catalytic, electrochemical, and mechanical properties together with potential applications in chemical fi lters and electrodes for energy storage devices. [ 6–8 , 11 , 30 ] However, in most cases, during the process of assembling nanometerscale building blocks into macroscopic paper-like structures, the large accessible surface area of 2D graphene sheets is lost. The reason for this is that the individual graphene sheets tend to irreversibly aggregate and restack owing to the strong π π stacking and van der Waals force between the planar basal planes of graphene sheets. This reduces the potential applications of graphene materials in electrochemical electrodes, composite materials, and so on. [ 20 ] Therefore, preventing aggregation of graphene sheets in the macroscopic structures, such that the properties of the individual graphene sheets are not compromised, is a critical challenge in constructing functional graphene-based macroscopic structures. Currently, a number of strategies for preventing aggregation have been developed, which include adding spacers (e.g., surfactants, nanoparticles, polymers), [ 27–36 ] template-assisted growth, [ 37 ] and crumpling the graphene sheets. [ 18 , 38 ] Alternatively, several groups have reported the formation of freestanding 3D graphene-based macroscopic structures without the assistance of any spacers or templates. [ 7 , 39,40 ] For instance, Li and coworkers reported the preparation of freestanding multilayered graphene fi lms by vacuum-assisted fi ltration based on the effective prevention of graphene intersheet restacking. [ 7 ] Shi and coworkers demonstrated the formation of a 3D graphene hydrogel by a hydrothermal method. [ 39 ] However, preparing freestanding and fl exible graphene fi lms with large accessible surface area but

An Effective Method for the Fabrication of Few-Layer-Thick Inorganic Nanosheets†

Intercalation and exfoliation of lithium: Few-layer-thick inorganic nanosheets (BN, NbSe(2), WSe(2), Sb(2)Se(3), and Bi(2)Te(3)) have been prepared from their layered bulk precursors by using a controllable electrochemical lithium intercalation process. The lithium intercalation conditions, such as cut-off voltage and discharge current, have been systematically studied and optimized to produce high-quality BN and NbSe(2) nanosheets.

High-power and high-energy-density flexible pseudocapacitor electrodes made from porous CuO nanobelts and single-walled carbon nanotubes.

We report a simple wet-chemical process to prepare porous CuO nanobelts (NBs) with high surface area and small crystal grains. These CuO NBs were mixed with carbon nanotubes in an appropriate ratio to fabricate pseudocapacitor electrodes with stable cycling performances, which showed a series of high energy densities at different power densities, for example, 130.2, 92, 44, 25, and 20.8 W h kg(-1) at power densities of 1.25, 6.25, 25, and 50 k Wh kg(-1), respectively. CuO-on-single-walled carbon nanotube (SWCNT) flexible hybrid electrodes were also fabricated using the SWCNT films as current collectors. These flexible electrodes showed much higher specific capacitance than that of electrodes made of pure SWCNTs and exhibited more stable cycling performance, for example, effective specific capacitances of >62 F g(-1) for the hybrid electrodes after 1000 cycles in 1 M LiPF6/EC:DEC at a current density of 5 A g(-1) and specific capacitance of only 23.6 F g(-1) for pure SWCNT electrodes under the same testing condition.

Embedding Sulfur in MOF‐Derived Microporous Carbon Polyhedrons for Lithium–Sulfur Batteries

As a promising rechargeable battery system, lithium– sulfur (Li–S) batteries can deliver an exceptionally high theoretical specific capacity of 1672 mAhg 1 and an energy density of 2500 Whkg 1 with the low-cost and environmentfriendly sulfur as the cathode material. Although the potential use of sulfur as a cathode material has long been discovered, several severe drawbacks have hindered the realization of Li–S batteries. One limitation is the insulating nature of sulfur with a very low conductivity of 5 10 30 Scm , which results in low utilization of sulfur. Another well-known problem is associated with the easy dissolution of polysulfides, the intermediate products formed during the electrochemical reaction, in organic electrolytes. The dissolved polysulfides “shuttle” between the electrodes, leading to the low Coulombic efficiency and deposition of a highly resistive layer on the surface of electrodes. These detrimental issues result in unsatisfactory electrochemical performance with rapid fading of capacity. Several approaches have been proposed to overcome the above-mentioned challenges in Li–S batteries, such as developing novel electrolytes and electrode materials. Among these efforts, using sulfur-containing composites instead of pure sulfur as the cathode materials has been demonstrated as an effective way towards high-performance Li–S batteries. Polymers and porous carbons are the common candidates to form composites with sulfur, which immobilize the loaded sulfur, and probably also the derived polysulfides via physical and/or chemical interactions. In addition, the electrical conductivity of composite materials is also better than that obtained with pristine sulfur. In particular, porous carbon materials have attracted intensive attention due to their good compatibility with sulfur, easy accessibility, and the abundance of candidates with diverse porosity and structures. Mesoporous carbon materials have been widely studied as the host materials to confine sulfur. For example, nanocomposites consisting of sulfur and ordered mesoporous carbon or mesoporous hollow carbon spheres have shown improved sulfur utilization and cycling stability. Nonetheless, continuous capacity fading upon prolonged cycling is still commonly observed, and the use of optimized ether-based electrolytes seems to be indispensable. Recent reports on carbon materials with rich micropores have revealed distinct characteristics. 25] Sulfur embedded in microporous carbon shows a pronounced discharge plateau at a lower potential of about 1.8 V versus Li/Li, which is different from the two plateaus of a typical sulfur cathode. More importantly, these microporous carbon/sulfur nanocomposites generally show outstanding capacity retention upon cycling and good compatibility with conventional carbonate-based electrolytes. However, the origins of the unusual characteristics of microporous carbon are not fully understood yet. In recently years, syntheses of porous carbon materials from metal-organic frameworks (MOFs) or porous coordination polymers (PCPs) have attracted growing attention due to the facile preparation procedures, high carbon yield, and unique porous structures. For example, carbonization of MOF-5 with furfuryl alcohol results in nanoporous carbon, which shows excellent supercapacitive performance. The carbon materials with fiber-like morphology prepared from Al-based PCPs exhibit remarkably high porosity. In particular, MOFs and PCPs are very attractive as both the template and the precursor for the fabrication of microporous carbon. Compared with many other highly porous carbon materials, such as those prepared by post-activation processes, the porous carbon derived from MOFs and PCPs exhibits highly uniform porosity, largely originating from the ordered crystalline structures of the MOFs and PCPs. However, the interesting application of these carbon materials derived from MOFs and PCPs for Li–S batteries needs to be further explored. Herein, we report the facile synthesis of microporous carbon polyhedrons (MPCPs) using unique MOF polyhedrons as both the template and precursor, and their use as carbon host to incorporate sulfur for Li–S batteries. The asprepared MPCPs with abundant and uniform micropores serve as an ideal model system for investigating the electrochemical behaviors of sulfur embedded in microporous [a] H. B. Wu, S. Wei, Dr. L. Zhang, Prof. R. Xu, Prof. X. W. Lou School of Chemical and Biomedical Engineering Nanyang Technological University 62 Nanyang Drive, Singapore 637459 (Singapore) E-mail : rxu@ntu.edu.sg xwlou@ntu.edu.sg Homepage: http://www.ntu.edu.sg/home/xwlou [b] H. B. Wu, Prof. H. H. Hng School of Materials Science and Engineering Nanyang Technological University 50 Nanyang Avenue, Singapore 639798 (Singapore) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201301689.

Direct Synthesis of Anatase TiO2 Nanowires with Enhanced Photocatalytic Activity

IO N Nanostructured materials with controlled shapes and structures have drawn considerable attention for both fundamental research and practical applications. [ 1–5 ] Particularly, onedimensional (1D) nanostructures such as nanowires, nanorods, and nanotubes are of great importance owing to their unique physicochemical properties and anisotropic structures. [ 6–9 ] 1D nanostructures of a wide variety of materials have been successfully synthesized, including carbons (e.g., carbon nanotubes [ 10 ]

Enhanced Thermopower of Graphene Films with Oxygen Plasma Treatment

In this work, we show that the maximum thermopower of few layers graphene (FLG) films could be greatly enhanced up to ∼700 μV/K after oxygen plasma treatment. The electrical conductivities of these plasma treated FLG films remain high, for example, ∼10(4) S/m, which results in power factors as high as ∼4.5 × 10(-3) W K(-2) m(-1). In comparison, the pristine FLG films show a maximum thermopower of ∼80 μV/K with an electrical conductivity of ∼5 × 10(4) S/m. The proposed mechanism is due to generation of local disordered carbon that opens the band gap. Measured thermopowers of single-layer graphene (SLG) films and reduced graphene oxide (rGO) films were in the range of -40 to 50 and -10 to 20 μV/K, respectively. However, such oxygen plasma treatment is not suitable for SLG and rGO films. The SLG films were easily destroyed during the treatment while the electrical conductivity of rGO films is too low.

Oxidation-Etching Preparation of MnO2 Tubular Nanostructures for High-Performance Supercapacitors

1D hierarchical tubular MnO(2) nanostructures have been prepared through a facile hydrothermal method using carbon nanofibres (CNFs) as sacrificial template. The morphology of MnO(2) nanostructures can be adjusted by changing the reaction time or annealing process. Polycrystalline MnO(2) nanotubes are formed with a short reaction time (e.g., 10 min) while hierarchical tubular MnO(2) nanostructures composed of assembled nanosheets are obtained at longer reaction times (>45 min). The polycrystalline MnO(2) nanotubes can be further converted to porous nanobelts and sponge-like nanowires by annealing in air. Among all the types of MnO(2) nanostructures prepared, tubular MnO(2) nanostructures composed of assembled nanosheets show optimized charge storage performance when tested as supercapacitor electrodes, for example, delivering an power density of 13.33 kW·kg(-1) and a energy density of 21.1 Wh·kg(-1) with a long cycling life over 3000 cycles, which is mainly related to their features of large specific surface area and optimized charge transfer pathway.

Reducing the symmetry of bimetallic Au@Ag nanoparticles by exploiting eccentric polymer shells.

We demonstrate a facile colloidal method for synthesizing Janus nanoparticles, whose eccentric polymer shells are exploited to fabricate eccentric bimetallic cores.

p-type Bi0.4Sb1.6Te3 nanocomposites with enhanced figure of merit

We report enhanced figure of merit, ZT, in p-type Bi0.4Sb1.6Te3 nanocomposites fabricated by a rapid and high throughput method of mixing nanostructured Bi0.4Sb1.6Te3 particles obtained through melt spinning with micronsized particles obtained via solid state reaction. Due to effective scattering of phonons over a wide wavelength spectrum, low thermal conductivity, and moderately good power factor were obtained in the nanocomposites to achieve ZT above 1.5 at room temperature. A maximum ZT of 1.80 was attained at 43u2009°C for the nanocomposite consisting 40 wtu2009% nanoinclusions. This was a 56% increment over the bulk sample, and the highest ZT reported for Bi2Te3-based materials.