Wintersweet‐Flower‐Like CoFe ₂ O ₄ /MWCNTs Hybrid Material for High‐Capacity Reversible Lithium Storage

Wang, Ying,Park, Jinsoo,Sun, Bing,Ahn, Hyojun,Wang, Guoxiu Wiley (John WileySons) 2012 Chemistry - An Asian Journal Vol.7 No.8

<P>CoFe(2)O(4)/multiwalled carbon nanotubes (MWCNTs) hybrid materials were synthesized by a hydrothermal method. Field emission scanning electron microscopy and transmission electron microscopy analysis confirmed the morphology of the as-prepared hybrid material resembling wintersweet flower 'buds on branches', in which CoFe(2)O(4) nanoclusters, consisting of nanocrystals with a size of 5-10 nm, are anchored along carbon nanotubes. When applied as an anode material in lithium ion batteries, the CoFe(2)O(4)/MWCNTs hybrid material exhibited a high performance for reversible lithium storage. In particular, the hybrid anode material delivered reversible lithium storage capacities of 809, 765, 539, and 359 mA h g(-1) at current densities of 180, 450, 900, and 1800 mA g(-1), respectively. The superior performance of CoFe(2)O(4)/MWCNTs hybrid materials could be ascribed to the synergistic pinning effect of the wintersweet-flower-like nanoarchitecture. This strategy could also be applied to synthesize other metal oxide/CNTs hybrid materials as high-capacity anode materials for lithium ion batteries.</P>

Superior electrochemical performance of sulfur/graphene nanocomposite material for high-capacity lithium-sulfur batteries.

Wang, Bei,Li, Kefei,Su, Dawei,Ahn, Hyojun,Wang, Guoxiu Wiley-VCH 2012 Chemistry - An Asian Journal Vol.7 No.7

<P>Sulfur/graphene nanocomposite material has been prepared by incorporating sulfur into the graphene frameworks through a melting process. Field-emission scanning electron microscope analysis shows a homogeneous distribution of sulfur in the graphene nanosheet matrix. The sulfur/graphene nanocomposite exhibits a super-high lithium-storage capacity of 1580?mA h?g(-1) and a satisfactory cycling performance in lithium-sulfur cells. The enhancement of the reversible capacity and cycle life could be attributed to the flexible graphene nanosheet matrix, which acts as a conducting medium and a physical buffer to cushion the volume change of sulfur during the lithiation and delithiation process. Graphene-based nanocomposites can significantly improve the electrochemical performance of lithium-sulfur batteries.</P>

Mesoporous LiFePO₄/C Nanocomposite Cathode Materials for High Power Lithium Ion Batteries with Superior Performance

Wang, Guoxiu,Liu, Hao,Liu, Jian,Qiao, Shizhang,Lu, Gaoqing Max,Munroe, Paul,Ahn, Hyojun WILEY‐VCH Verlag 2010 Advanced Materials Vol.22 No.44

<P><B>Hexagonally ordered mesoporous LiFePO<SUB>4</SUB>/C nanocomposites</B> can be synthesised with LiFePO<SUB>4</SUB> nanoparticles embedded in an interconnected carbon framework. Mesoporous LiFePO<SUB>4</SUB>/C nanocomposites exhibit superior electrochemical performance and ultra‐high specific power density, which makes this architecture suitable for high power applications such as HEVs and stationary energy storage for smart grids. </P>

Highly Ordered Mesoporous Cobalt Oxide Nanostructures: Synthesis, Characterisation, Magnetic Properties, and Applications for Electrochemical Energy Devices

Wang, Guoxiu,Liu, Hao,Horvat, Josip,Wang, Bei,Qiao, Shizhang,Park, Jinsoo,Ahn, Hyojun WILEY-VCH Verlag 2010 Chemistry Vol.16 No.36

<P>Highly ordered mesoporous Co<SUB>3</SUB>O<SUB>4</SUB> nanostructures were prepared using KIT-6 and SBA-15 silica as hard templates. The structures were confirmed by small angle X-ray diffraction, high resolution transmission electron microscopy, and N<SUB>2</SUB> adsorption–desorption isotherm analysis. Both KIT-6 cubic and SBA-15 hexagonal mesoporous Co<SUB>3</SUB>O<SUB>4</SUB> samples exhibited a low Néel temperature and bulk antiferromagnetic coupling due to geometric confinement of antiferromagnetic order within the nanoparticles. Mesoporous Co<SUB>3</SUB>O<SUB>4</SUB> electrode materials have demonstrated the high lithium storage capacity of more than 1200 mAh g<SUP>−1</SUP> with an excellent cycle life. They also exhibited a high specific capacitance of 370 F g<SUP>−1</SUP> as electrodes in supercapacitors.</P> <B>Graphic Abstract</B> <P>Highly ordered mesoporous Co<SUB>3</SUB>O<SUB>4</SUB> nanostructures have been prepared by using a hard-template method. Mesoporous Co<SUB>3</SUB>O<SUB>4</SUB> (general TEM image shown) exhibits a low Néel temperature and bulk antiferromagnetic coupling. When used as electrode materials in lithium-ion cells and supercapacitors, mesoporous Co<SUB>3</SUB>O<SUB>4</SUB> demonstrates a high reversible lithium storage capacity and a high specific supercapacitance. <img src='wiley_img_2010/09476539-2010-16-36-CHEM201000562-content.gif' alt='wiley_img_2010/09476539-2010-16-36-CHEM201000562-content'> </P>

Graphene-supported SnO ₂ nanoparticles prepared by a solvothermal approach for an enhanced electrochemical performance in lithium-ion batteries

Wang, Bei,Su, Dawei,Park, Jinsoo,Ahn, Hyojun,Wang, Guoxiu Springer 2012 Nanoscale research letters Vol.7 No.1

<P>SnO<SUB>2</SUB> nanoparticles were dispersed on graphene nanosheets through a solvothermal approach using ethylene glycol as the solvent. The uniform distribution of SnO<SUB>2</SUB> nanoparticles on graphene nanosheets has been confirmed by scanning electron microscopy and transmission electron microscopy. The particle size of SnO<SUB>2</SUB> was determined to be around 5 nm. The as-synthesized SnO<SUB>2</SUB>/graphene nanocomposite exhibited an enhanced electrochemical performance in lithium-ion batteries, compared with bare graphene nanosheets and bare SnO<SUB>2</SUB> nanoparticles. The SnO<SUB>2</SUB>/graphene nanocomposite electrode delivered a reversible lithium storage capacity of 830 mAh g<SUP>−1</SUP> and a stable cyclability up to 100 cycles. The excellent electrochemical properties of this graphene-supported nanocomposite could be attributed to the insertion of nanoparticles between graphene nanolayers and the optimized nanoparticles distribution on graphene nanosheets.</P>

QSDB: An Encrypted Database Model for Privacy-Preserving in Cloud Computing

( Guoxiu Liu ),( Geng Yang ),( Haiwei Wang ),( Hua Dai ),( Qiang Zhou ) 한국인터넷정보학회 2018 KSII Transactions on Internet and Information Syst Vol.12 No.7

With the advent of database-as-a-service (DAAS) and cloud computing, more and more data owners are motivated to outsource their data to cloud database in consideration of convenience and cost. However, it has become a challenging work to provide security to database as service model in cloud computing, because adversaries may try to gain access to sensitive data, and curious or malicious administrators may capture and leak data. In order to realize privacy preservation, sensitive data should be encrypted before outsourcing. In this paper, we present a secure and practical system over encrypted cloud data, called QSDB (queryable and secure database), which simultaneously supports SQL query operations. The proposed system can store and process the floating point numbers without compromising the security of data. To balance tradeoff between data privacy protection and query processing efficiency, QSDB utilizes three different encryption models to encrypt data. Our strategy is to process as much queries as possible at the cloud server. Encryption of queries and decryption of encrypted queries results are performed at client. Experiments on the real-world data sets were conducted to demonstrate the efficiency and practicality of the proposed system.

Highly ordered mesoporous NiO anode material for lithium ion batteries with an excellent electrochemical performance

Liu, Hao,Wang, Guoxiu,Liu, Jian,Qiao, Shizhang,Ahn, Hyojun Royal Society of Chemistry 2011 Journal of materials chemistry Vol.21 No.9

<P>In this work, we have synthesized highly ordered mesoporous NiO materials by a nanocasting method using mesoporous silica KIT-6 as the hard templates. Mesoporous NiO particles were characterized by small angle X-ray diffraction (XRD), nitrogen adsorption/desorption, and transmission electron microscopy (TEM). The results demonstrated that the as-prepared mesoporous NiO had an ordered <I>Ia</I>3<I>d</I> symmetric mesostructure, with a high surface area of 96 m<SUP>2</SUP>/g. Mesoporous NiO materials were tested as an anode material for lithium ion batteries, exhibiting much lower activation energy (20.75 kJ mol<SUP>−1</SUP>) compared to the bulk NiO (45.02 kJ mol<SUP>−1</SUP>). We found that the mesoporous NiO electrode has higher lithium intercalation kinetics than its bulk counterpart. The specific capacity of mesoporous NiO after 50 cycles was maintained 680 mAh/g at 0.1 C, which was much higher than that of the commercial bulk NiO (188 mAh/g). Furthermore, at a high rate of 2C, the discharge capacity of mesoporous NiO was as high as 515 mAh/g, demonstrating the potential to be used for high power lithium ion batteries.</P> <P>Graphic Abstract</P><P>Ordered mesoporous NiO anode materials demonstrate high lithium intercalation kinetics and excellent electrochemical performance. <IMG SRC='http://pubs.rsc.org/services/images/RSCpubs.ePlatform.Service.FreeContent.ImageService.svc/ImageService/image/GA?id=c0jm03132a'> </P>

Octahedral tin dioxide nanocrystals as high capacity anode materials for Na-ion batteries

Su, Dawei,Wang, Chengyin,Ahn, Hyojun,Wang, Guoxiu The Royal Society of Chemistry 2013 Physical chemistry chemical physics Vol.15 No.30

<P>Single crystalline SnO<SUB>2</SUB> nanocrystals (∼60 nm in size) with a uniform octahedral shape were synthesised using a hydrothermal method. Their phase and morphology were characterized by XRD and FESEM observation. TEM and HRTEM analyses identified that SnO<SUB>2</SUB> octahedral nanocrystals grow along the [001] direction, consisting of dominantly exposed {221} high energy facets. When applied as anode materials for Na-ion batteries, SnO<SUB>2</SUB> nanocrystals exhibited high reversible sodium storage capacity and excellent cyclability (432 mA h g<SUP>−1</SUP> after 100 cycles). In particular, SnO<SUB>2</SUB> nanocrystals also demonstrated a good high rate performance. <I>Ex situ</I> TEM analysis revealed the reaction mechanism of SnO<SUB>2</SUB> nanocrystals for reversible Na ion storage. It was found that Na ions first insert into SnO<SUB>2</SUB> crystals at the high voltage plateau (from 3 V to ∼0.8 V), and that the exposed (1 × 1) tunnel-structure could facilitate the initial insertion of Na ions. Subsequently, Na ions react with SnO<SUB>2</SUB> to form Na<SUB><I>x</I></SUB>Sn alloys and Na<SUB>2</SUB>O in the low voltage range (from ∼0.8 V to 0.01 V). The superior cyclability of SnO<SUB>2</SUB> nanocrystals could be mainly ascribed to the reversible Na–Sn alloying and de-alloying reactions. Furthermore, the reduced Na<SUB>2</SUB>O “matrix” may help retard the aggregation of tin nanocrystals, leading to an enhanced electrochemical performance.</P> <P>Graphic Abstract</P><P><I>Ex situ</I> TEM analysis revealed the reaction mechanism of SnO<SUB>2</SUB> nanocrystals for reversible Na ion storage. <IMG SRC='http://pubs.rsc.org/services/images/RSCpubs.ePlatform.Service.FreeContent.ImageService.svc/ImageService/image/GA?id=c3cp52037d'> </P>

Multifunctional Free-Standing Gel Polymer Electrolyte with Carbon Nanofiber Interlayers for High-Performance Lithium-Sulfur Batteries

Choi, Sinho,Song, Jianjun,Wang, Chengyin,Park, Soojin,Wang, Guoxiu Wiley (John WileySons) 2017 Chemistry, an Asian journal Vol.12 No.13

<P>Free-standing trimethylolpropane ethoxylate triacrylate gel polymer electrolyte is synthesized by a chemical cross-linking process and used as an electrolyte and separator membrane in lithium-sulfur batteries. The cross linked gel polymer electrolyte also exhibited a stable geometric size retention of 95% at the high temperature of 130 degrees C. The as-prepared gel polymer electrolyte membrane with carbon nanofibers interlayer can effectively prevent polysulfide dissolution and shuttle effect, leading to significantly enhanced electrochemical properties, including high capacity and cycling stability, with an enhanced specific capacity of 790 mAhg(-1) after 100 cycles.</P>

Hydrothermally Processed Oxide Nanostructures and Their Lithium–ion Storage Properties

Ahn, Jung-Ho,Kim, Yong-Jin,Wang, Guoxiu Springer 2010 NANOSCALE RESEARCH LETTERS Vol.5 No.11

<P>Y- and Si-based oxide nanopowders were synthesized by a hydrothermal reaction of Y or Si powders with NaOH or LiOH aqueous solution. Nanoparticles with different morphology such as elongated nanospheres, flower-like nanoparticles and nanowires were produced by a control of processing parameters, in particular, the starting composition of solution. The preliminary result of electrochemical examination showed that the hydrothermally processed nanowires exhibit high initial capacities of Li-ion storage: 653 mAh/g for Y<SUB>2</SUB>O<SUB>3</SUB> nanowires as anode materials and 186 mAh/g for Li<SUB>2</SUB>Si<SUB>2</SUB>O<SUB>5</SUB> nanowires as cathode materials in a Li secondary cell. Compared to the powder with elongated sphere or flower-like shapes, the nanowires showed a higher Li-ion capacity and a better cycle property.</P>