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Kim J.,Sungkyunkwan University | Kim J.,Sungkyun Advanced Institute of Nano Technology SAINT | Hong S.-A.,Korea Institute of Science and Technology | Yoo J.,Sungkyun Advanced Institute of Nano Technology SAINT | Yoo J.,Sungkyunkwan University
Chemical Engineering Journal | Year: 2015

Carbon-coated, hierarchical porous ZnO microspheres are synthesised continuously in supercritical methanol using oleic acid as the surface modifier and subsequent carbon coating, using sucrose as the carbon source. The physicochemical properties and the electrochemical performance of the ZnO microspheres are compared with those of commercially available ZnO and rod-type ZnO synthesised in supercritical water. The addition of oleic acid effectively inhibits the particle growth, resulting in nanosized primary ZnO particles with sizes of 10-50nm that are loosely agglomerated and form secondary microspheres with sizes of 100-800nm with a high porosity of 42.2%. After the carbon coating, the porous hierarchical ZnO microspheres with 6.8wt% carbon exhibit a much higher reversible capacity of 546.5mAhg-1 compared to the rod-type ZnO (361.2mAhg-1) and commercial ZnO (151.1mAhg-1) at a current density of 97.8mAg-1 (0.1C) after 30 cycles. In particular, at a high rate of 1.0C, a reversible capacity of 428.5mAhg-1 can be obtained after 100 cycles. The enhanced discharge capacity of the carbon-coated ZnO may be attributed to the combined beneficial effects of nanosized primary particles, hierarchical porous morphology and carbon-coating on Li+ storage. © 2014 . Source

Brand S.,Korea Institute of Science and Technology | Kim J.,Sungkyunkwan University | Kim J.,Sungkyun Advanced Institute of Nano Technology SAINT
Energy | Year: 2015

Herein, α-cellulose, d-xylose, and lignin, which are major lignocellulosic biomass constituents, are subject to being liquefied in scEtOH (supercritical ethanol). Biomass conversion, biocrude yield, gas composition and energy content resulting from the liquefaction are analyzed. When cellulose is used, the biocrude yield increased significantly from 1.6 to 48.4wt% with an increase in temperature from 265 to 350°C, while the yields of biocrudes obtained from lignin (25.2-28.8wt%) and xylose (32.1-42.0wt%) do not change significantly with varying temperature. The chemical composition of the biocrudes obtained from the cellulose and xylose liquefaction in scEtOH significantly differ when compared to those of the biocrudes obtained from fast pyrolysis and hydrothermal liquefaction. The unique chemical species in the scEtOH-liquefied biocrude include long-chain esters/ethers (C5-C10), long-chain acids (C4-C9), and tetrahydrofurans, which could be due to the enhanced esterification and hydrogenation reactions in the scEtOH medium. Plausible reaction pathways of xylose liquefaction in scEtOH are proposed. © 2014. Source

Hong S.-A.,Korea Institute of Science and Technology | Kim S.J.,Korea Institute of Science and Technology | Chung K.Y.,Korea Institute of Science and Technology | Lee Y.-W.,Seoul National University | And 3 more authors.
Chemical Engineering Journal | Year: 2013

This study investigates the effect of various process parameters during continuous synthesis in supercritical water on the physicochemical and electrochemical properties of lithium iron phosphate (LiFePO4) for use in large-scale lithium 2nd battery applications. The process parameters include reaction temperature (300-400°C), precursor solution concentration (0.01-0.18M), precursor solution flow rate (1.5-3.0g/min), water flow rate (9.0-36.0g/min), and residence time (9-72s). Under subcritical water conditions, mixed Fe3(PO4)2{dot operator}8H2O, Fe2O3, and Fe3O4 particles formed; in contrast, under supercritical fluid conditions, well-crystallized LiFePO4 particles with some Fe3+ impurities (i.e., Fe2O3 and Fe3O4) were obtained. In supercritical water, an increase in the precursor concentration leads to an increase in the Fe3+ impurity content. At a high water flow rate, a significant decrease in crystallinity and the formation of Fe3(PO4)2{dot operator}8H2O and Li6P6O18{dot operator}9H2O phases rather than LiFePO4 were observed. Highly crystalline LiFePO4 with good discharge capacity was obtained with high temperature, low precursor concentration, and low flow rate conditions. Depending on the synthetic conditions, bare LiFePO4 particles exhibit discharge capacities of 55-85mAh/g at 0.1 C-rate after 30 cycles. After carbon coating, a marginal capacity decay from 141 to 135mAh/g was observed during the 30 charge-discharge cycles. © 2013. Source

Yoon D.,Sungkyunkwan University | Chung K.Y.,Korea Institute of Science and Technology | Chang W.,Korea Institute of Science and Technology | Kim S.M.,Korea Institute of Science and Technology | And 4 more authors.
Chemistry of Materials | Year: 2015

Hydrogen-enriched reduced graphene oxide (RGO) was achieved using double-oxidized graphene oxide (GO2) as an anode in high-performance lithium batteries is reported. GO2 exhibited a much lower carbon-to-oxygen ratio, lower crystallinity, higher Brunauer-Emmett-Teller surface area, higher pore volume, and higher porosity as compared to graphene oxides produced using the typical modified Hummers method (GO1). The two forms of GO were reduced using two different reduction methods: supercritical isopropanol (scIPA) and heat treatment. The four types of RGOs synthesized using GO1/GO2 and scIPA/heat treatment exhibited significantly different chemical, morphological, and textural properties. The galvanostatic charge-discharge properties were highly dependent on the physicochemical properties of the RGOs. The scIPA-reduced GO2 exhibited superior electrochemical performance as compared to the thermally reduced GO1/GO2 and scIPA-reduced GO1. Highly reversible capacity (1331 mAh g-1 at 50 mA g-1 after 100 cycles), excellent rate-performance (328 mAh g-1 at 5 A g-1), and good cycling stability up to 1000 cycles even at a current density of 10 A g-1 were observed with the scIPA-reduced GO2 electrode. The characterization results suggested that a large amount of hydrogen-terminated groups, numerous defect sites, and large interlayer spacing have beneficial effects on the electrochemical performance of scIPA-reduced GO2. © 2014 American Chemical Society. Source

Brand S.,Korea Institute of Science and Technology | Brand S.,Korean University of Science and Technology | Susanti R.F.,Korea Institute of Science and Technology | Susanti R.F.,Korean University of Science and Technology | And 6 more authors.
Energy | Year: 2013

In this study, the influence of various physical process parameters on the liquefaction of lignocellulosic biomass (pine wood) in supercritical ethanol was investigated. The parameters include reaction temperature (280-400°C), initial nitrogen pressure (0.4-7.5MPa), reaction time (0-240min), and biomass-to-solvent ratio (0.06-0.25g/g). The reaction temperature and residence time were found to have a more significant effect on biomass conversion and product yield than pressure and biomass-to-solvent ratio had; conversion in the range 34.0-98.1% and biocrude yield in the range 15.8-59.9wt% were observed depending on the process parameters. Despite the absence of catalysts and external hydrogen source, solid biomass to liquid and gaseous products conversion of 98.1%, and a high biocrude yield of approximately 65.8wt% were achieved at 400°C, 120min, and a biomass-to-solvent ratio of 0.06g/g. Moreover, the biocrude contained considerably lower amounts of oxygen and higher amounts of carbon and hydrogen, resulting in a substantially higher heating value (>30MJ/kg) as compared to raw feedstock (20.4MJ/kg). A comparison with sub- or supercritical water-based liquefaction revealed that supercritical ethanol produced biocrude with a lower molecular weight and much better yield. Finally, a new biomass liquefaction reaction mechanism associated with supercritical ethanol is proposed. © 2013. Source

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