Huanwen Wang

III–Nitride UV Devices

The need for efficient, compact and robust solid-state UV optical sources and sensors had stimulated the development of optical devices based on III–nitride material system. Rapid progress in material growth, device fabrication and packaging enabled demonstration of high efficiency visible-blind and solar-blind photodetectors, deep-UV light-emitting diodes with emission from 400 to 250 nm, and UV laser diodes with operation wavelengths ranging from 340 to 350 nm. Applications of these UV optical devices include flame sensing; fluorescence-based biochemical sensing; covert communications; air, water and food purification and disinfection; and biomedical instrumentation. This paper provides a review of recent advances in the development of UV optical devices. Performance of state-of-the-art devices as well as future prospects and challenges are discussed.

One-step strategy to three-dimensional graphene/VO2 nanobelt composite hydrogels for high performance supercapacitors

A facile one-step strategy has been developed to prepare 3D graphene/VO2 nanobelt composite hydrogels, which can be readily scaled-up for mass production by using commercial V2O5 and graphene oxide as precursors. During the formation of the graphene/VO2 architecture, 1D VO2 nanobelts and 2D flexible graphene sheets are self-assembled to form interconnected porous microstructures through hydrogen bonding, which facilitates charge and ion transport in the electrode. Due to the hierarchical network framework and the pseudocapacitance contribution from VO2 nanobelts, the hybrid electrode demonstrates excellent capacitive performances. In the two-electrode configuration, the graphene/VO2 nanobelt composite hydrogel exhibits a specific capacitance of 426 F g−1 at 1 A g−1 in the potential range of −0.6 to 0.6 V, which greatly surpasses that of each individual counterpart (191 F g−1 and 243 F g−1 at 1 A g−1 for VO2 nanobelt and graphene hydrogel, respectively). The hybrid electrode also shows an improved rate capability and cycling stability, which is indicative of a positive synergistic effect of VO2 and graphene on the improvement of electrochemical performance. These findings reveal the importance and great potential of graphene composite hydrogels in the development of energy storage devices with high power and energy densities.

A High Energy and Power Li‐Ion Capacitor Based on a TiO2 Nanobelt Array Anode and a Graphene Hydrogel Cathode

A novel hybrid Li-ion capacitor (LIC) with high energy and power densities is constructed by combining an electrochemical double layer capacitor type cathode (graphene hydrogels) with a Li-ion battery type anode (TiO(2) nanobelt arrays). The high power source is provided by the graphene hydrogel cathode, which has a 3D porous network structure and high electrical conductivity, and the counter anode is made of free-standing TiO(2) nanobelt arrays (NBA) grown directly on Ti foil without any ancillary materials. Such a subtle designed hybrid Li-ion capacitor allows rapid electron and ion transport in the non-aqueous electrolyte. Within a voltage range of 0.0-3.8 V, a high energy of 82 Wh kg(-1) is achieved at a power density of 570 W kg(-1). Even at an 8.4 s charge/discharge rate, an energy density as high as 21 Wh kg(-1) can be retained. These results demonstrate that the TiO(2) NBA//graphene hydrogel LIC exhibits higher energy density than supercapacitors and better power density than Li-ion batteries, which makes it a promising electrochemical power source.

Asymmetric supercapacitors based on nano-architectured nickel oxide/graphene foam and hierarchical porous nitrogen-doped carbon nanotubes with ultrahigh-rate performance

A pulsed laser deposition process using ozone as an oxidant is developed to grow NiO nanoparticles on highly conductive three-dimensional (3D) graphene foam (GF). The excellent electrical conductivity and interconnected pore structure of the hybrid NiO/GF electrode facilitate fast electron and ion transportation. The NiO/GF electrode displays a high specific capacitance (1225 F g−1 at 2 A g−1) and a superb rate capability (68% capacity retention at 100 A g−1). A novel asymmetric supercapacitor with high power and energy densities is successfully fabricated using NiO/GF as the positive electrode and hierarchical porous nitrogen-doped carbon nanotubes (HPNCNTs) as the negative electrode in aqueous KOH solution. Because of the high individual capacitive performance of NiO/GF and HPNCNTs, as well as the synergistic effect between the two electrodes, the asymmetric capacitor exhibits an excellent energy storage performance. At a voltage range from 0.0 to 1.4 V, an energy density of 32 W h kg−1 is achieved at a power density of 700 W kg−1. Even at a 2.8 s charge–discharge rate (42 kW kg−1), an energy density as high as 17 W h kg−1 is retained. Additionally, the NiO/GF//HPNCNT asymmetric supercapacitor exhibits excellent cycling durability, with 94% specific capacitance retained after 2000 cycles.

Cutting and unzipping multiwalled carbon nanotubes into curved graphene nanosheets and their enhanced supercapacitor performance.

We report a remarkable transformation of multiwalled carbon nanotubes (MWCNTs) to curved graphene nanosheets (CGN) by the Hummers method. Through this simple process, MWCNTs can be cut and unzipped in the transverse and longitudinal directions, respectively. The as-obtained CGN possess the unique hybrid structure of 1D nanotube and 2D graphene. Such a particular structure together with the improved effective surface area affords high specific capacitance and good cycling stability during the charge-discharge process when used as supercapacitor electrodes. The electrochemical measurements show that CGN exhibit higher capacitive properties than pristine MWCNTs in three different types of aqueous electrolytes, 1 M KOH, 1 M H(2)SO(4), and 1 M Na(2)SO(4). A specific capacitance of as high as 256 F g(-1) at a current density of 0.3 A g(-1) is achieved over the CGN material. The improved capacitance may be attributed to high accessibility to electrolyte ions, extended defect density, and increased effective surface area. Meanwhile, this high-yield production of graphene from low cost MWCNTs is important for the scalable synthesis and industrial application of graphene. Furthermore, this novel CGN nanostructure could also be promisingly applied in many fields such as nanoelectronics, sensors, nanocomposites, batteries, and gas storage.

Polyethylenimine Facilitated Ethyl Cellulose for Hexavalent Chromium Removal with a Wide pH Range

Ethyl cellulose (EC) composites modified with 20.0 wt % polyethylenimine (PEI) (PEI/ECs) demonstrated effective hexavalent chromium, [Cr(VI)], removal from solutions with a wide pH range. For example, 4.0 mg/L Cr(VI) solution with a pH below 3.0 was completely purified by 3.0 g/L PEI/ECs within 5 min, much faster than the as-received EC (2 h) and activated carbon (several hours). These PEI/ECs adsorbents has overcome the low pH limitation of Cr(VI) removal; for example, 4.0 mg/L Cr(VI) solution with a pH of 11.0 was completely purified within 15 min. These adsorbents followed chemical adsorption as revealed from the pseudo-second-order kinetic study. These PEI/ECs following the isotherm Langmuir model have a maximum adsorption capacity of 36.8 mg/g, much higher than pure EC (12 mg/g), tetrabutylammonium-modified celluloses (16.67 mg/g), and magnetic carbon (16 mg/g). The reduction of Cr(VI) to Cr(III) by the oxidation of amine groups and hydroxyl groups of PEI/ECs was verified as the main mechanism for the Cr(VI) removal.

Advanced asymmetric supercapacitors based on CNT@Ni(OH)2 core–shell composites and 3D graphene networks

Asymmetric supercapacitors (ASCs) with carbon nanotube@nickel hydroxide nanosheet (CNT@Ni(OH)2) core–shell composites as positive electrodes and three-dimensional (3D) graphene networks (3DGNs) as negative electrodes were reported in aqueous KOH electrolyte. The CNT@Ni(OH)2 core–shell composites were prepared through a facile chemical bath deposition method, while 3DGNs were obtained by freeze-drying of graphene hydrogels. By virtue of their unique microstructures, superb electrochemical properties were achieved in a three-electrode system, e.g., 1136 F g−1 at 2 A g−1 for the CNT@Ni(OH)2 electrode within 0–0.5 V and 203 F g−1 at 1 A g−1 for the 3DGN electrode within −1–0 V. Benefiting from these merits, the as-fabricated CNT@Ni(OH)2//3DGN ASC showed a maximum energy density of 44.0 W h kg−1 at a power density of 800 W kg−1 and even retained 19.6 W h kg−1 at 16 000 W kg−1 in the voltage region of 0–1.6 V.

Nonaqueous Hybrid Lithium‐Ion and Sodium‐Ion Capacitors

Hybrid metal-ion capacitors (MICs) (M stands for Li or Na) are designed to deliver high energy density, rapid energy delivery, and long lifespan. The devices are composed of a battery anode and a supercapacitor cathode, and thus become a tradeoff between batteries and supercapacitors. In the past two decades, tremendous efforts have been put into the search for suitable electrode materials to overcome the kinetic imbalance between the battery-type anode and the capacitor-type cathode. Recently, some transition-metal compounds have been found to show pseudocapacitive characteristics in a nonaqueous electrolyte, which makes them interesting high-rate candidates for hybrid MIC anodes. Here, the material design strategies in Li-ion and Na-ion capacitors are summarized, with a focus on pseudocapacitive oxide anodes (Nb2 O5 , MoO3 , etc.), which provide a new opportunity to obtain a higher power density of the hybrid devices. The application of Mxene as an anode material of MICs is also discussed. A perspective to the future research of MICs toward practical applications is proposed to close.

Co9S8/MoS2 Yolk–Shell Spheres for Advanced Li/Na Storage

Uniform sized Co9 S8 /MoS2 yolk-shell spheres with an average diameter of about 500 nm have been synthesized by a facile route. When evaluated as anodes for lithium-ion and sodium-ion batteries, these Co9 S8 /MoS2 yolk-shell spheres show high specific capacities, excellent rate capabilities, and good cycling stability.

3D Hierarchical Porous Mo2C for Efficient Hydrogen Evolution

Porous electrocatalyst for hydrogen production. 3D hierarchical porous molybdenum carbide provides a low operating potential (97 mV at 10 mA cm(-2) ). These beneficial textures of large specific surface area (302 m(2) g(-1) ) and hierarchical porous architecture containing dominant pore size distribution peak at 11 Å in width can provide large surface active sites and facilitate proton mass transport.