Creat membership Creat membership
Sign in

Forgot password?

Confirm
  • Forgot password?
    Sign Up
  • Confirm
    Sign In
home > search

Now showing items 49 - 64 of 84

  • Disorder induced polymorphic transitions in the high hydrogen density compound Sr( BH4) 2( NH3BH3) 2+

    Jorgensen, Mathias   Lee, Young-Su   Bjerring, Morten   Jepsen, Lars H.   Akbey, Umit   Cho, Young Whan   Jensen, Torben R.  

    The new compound Sr(BH4)(2)(NH3BH3)(2) has been synthesized and characterized with in situ powder X-ray diffraction and fast (28 or 60 kHz) magic angle spinning H-1, B-11 and N-15 NMR and structurally optimized with density functional theory calculations. This investigation reveals complex structural rearrangements for this compound as a function of temperature. A room temperature orthorhombic polymorph, -Sr(BH4)(2)(NH3BH3)(2), with the space group symmetry Pbca, has been determined with a layered structure of alternating ammonia borane and Sr(BH4)(2), partially stabilized by dihydrogen bonding. Surprisingly the crystal symmetry is lowered upon heating, as evidenced both by in situ synchrotron powder X-ray diffraction and B-11 MAS NMR data, resulting in an intermediate polymorph, -Sr(BH4)(2)(NH3BH3)(2), present from approximate to 65 to 115 degrees C. -Sr(BH4)(2)(NH3BH3)(2), a sub structure of the -polymorph showing higher symmetry with the space group symmetry Aba2, forms upon further heating. Ab initio molecular dynamics simulations show that the ammonia borane molecule can dynamically alternate between a bidentate and a tridentate coordination to Sr at finite temperature. The dynamic properties of the ammonia borane molecule in the solid state are suggested to cause the observed structural complexity. Based on simultaneous thermogravimetric analysis, differential scanning calorimetry and mass spectrometry, the decomposition of the compound was investigated showing a stabilization of ammonia borane in the structure relative to other metal borohydride ammonia boranes and neat ammonia borane.
    Download Collect
  • Controlling the Dehydrogenation Reaction toward Reversibility of the LiBH4-Ca(BH4)(2) Eutectic System

    Yan, Yigang   Remhof, Arndt   Mauron, Philippe   Rentsch, Daniel   Lodziana, Zbigniew   Lee, Young-Su   Lee, Hyun-Sook   Cho, Young Whan   Zuettel, Andreas  

    Controlling the dehydrogenation process in a suitable reaction route by choosing the appropriate reaction conditions is crucial for a hydrogen storage system. The dehydrogenation process of the eutectic 0.68LiBH(4)-0.32Ca(BH4)(2) mixture was investigated by dynamic pressure composition isotherms, X-ray diffraction, and solid-state nuclear magnetic resonance in order to determine the optimal reaction route for the dehydrogenation and to improve the hydrogen absorption reaction. In a temperature range from 330 to 450 degrees C, the LiBH4-Ca(BH4)(2) mixture decomposes in two major steps. First, Ca(BH4)(2) decomposes into CaH2, CaB6, CaB12H12, H-2, and probably amorphous boron. Second, CaH2 reacts with LiBH4 to CaB6, LiH, and H-2. Li2B12H12 has been identified as a byproduct It was observed that the lower dehydrogenation temperature, the more CaB6 and the less [B12H12](2-) containing phases are present in the final dehydrogenation products, resulting in improved. absorption performance. The temperature dependence is discussed, providing instructions to improve reversibility for potential applications and new insights into the hydrogen sorption mechanism of metal borohydrides.
    Download Collect
  • Si/iron silicide nanocomposite anodes with furfuryl-alcohol-derived carbon coating for Li-ion batteries

    Jang, Juyoung   Kang, Inyeong   Kim, Moon-Soo   Kim, Jae-Hun   Lee, Young-Su   Yi, Kyung-Woo   Cho, Young Whan  

    A new type of carbon-coated Si/iron silicide nanocomposite anode material for lithium ion batteries is made employing furfuryl alcohol as a carbon precursor. A wet coating technique is applied to cover the surface of ball-milled ferrosilicon powders with polyfurfuryl alcohol resin derived from furfuryl alcohol. To optimize the electrochemical performance of this anode material, the carbonization heat treatment temperature is systematically varied between 600 and 1000 A degrees C. The effects of the carbonization temperature on the physical properties of the carbon-coated nanocomposites, such as the specific surface area and phase composition, and on the electrochemical performance characteristics, such as the initial discharge/charge capacity and capacity retention ratio of coin half-cells made with these nanocomposite anodes, are investigated. The electrochemical performance of the anode during cycling is found to depend strongly on the characteristics of the carbon coating layer, which is significantly affected by the carbonization temperature. An initial discharge capacity of 720 mAh g(-1) and a capacity retention of 75% after 300 cycles at 1 C are obtained from the coin half-cell made with the Si/iron silicide nanocomposite carbonized at 1000 A degrees C.
    Download Collect
  • On the Formation and the Structure of the First Bimetallic Borohydride Borate, LiCa(3)(BH(4))(BO(3))(2)

    Lee, Young-Su   Filinchuk, Yaroslav   Lee, Hyun-Sook   Suh, Jin-Yoo   Kim, Ji Woo   Yu, Jong-Sung   Cho, Young Whan  

    We present the first report on the formation and the crystal structure of a bimetallic borohydride borate, LiCa(3)(BH(4))(BO(3))(2). It is discovered during decomposition of LiBH(4)-Ca(BH(4))(2)-carbon composite studied for hydrogen storage. A small amount of oxygen impurity in the starting material or introduced during sample preparation gives rise to the formation of [BO(3)](3-). Its crystal structure was solved from synchrotron powder diffraction data as space group Fd (3) over barm and a = 143911(2) angstrom at 387 degrees C. The experimental structural parameters are closely reproduced and the orientational disorder in [BH(4)](-) is confirmed by first-principles calculations.
    Download Collect
  • Ammonia-assisted fast Li-ion conductivity in a new hemiammine lithium borohydride,LiBH4 center dot 1/2NH(3)

    Yan, Yigang   Grinderslev, Jakob B.   Lee, Young-Su   Jorgensen, Mathias   Cho, Young Whan   Cerny, Radovan   Jensen, Torben R.  

    Hemiammine lithium borohydride, LiBH4 center dot 1/2NH(3), is characterized and a new Li+ conductivity mechanism is identified. It exhibits a Li+ conductivity of 7 x 10(-4) S cm(-1) at 40 degrees C in the solid state and 3.0 x 10(-2) S cm(-1) at 55 degrees C after melting. The molten state of LiBH4 center dot 1/2NH(3) has a high viscosity and can be mechanically stabilized in nano-composites with inert metal oxides and other hydrides making it a promising battery electrolyte.
    Download Collect
  • Thermodynamics of the dehydrogenation of the LiBH(4)-YH(3) composite: Experimental and theoretical studies

    Shim, Jae-Hyeok   Lee, Young-Su   Suh, Jin-Yoo   Cho, Wonchul   Han, Sang Sup   Cho, Young Whan  

    Pressure-composition-temperature curves were constructed for the dehydrogenation of the LiBH(4)-YH(3) composite. The Van't Hoff plot of the plateau pressure values showed that the reaction enthalpy and entropy were 51 kJ/mol H(2) and 101 J/K mol H(2), respectively. Thermodynamic calculations were performed for the reaction in combination with first principles calculation. The calculated reaction enthalpy and entropy are in good agreement with the measured values. The equilibrium temperature for the reaction at 1 bar of hydrogen was estimated to be 232 degrees C. (C) 2011 Elsevier B.V. All rights reserved.
    Download Collect
  • First-principles study on the thermal expansion of Ni-X binary alloys based on the quasi-harmonic Debye model

    Shin, Yongjin   Jung, Woo-Sang   Lee, Young-Su  

    In this study, we use the quasi-harmonic Debye model to predict the coefficient of thermal expansion of Ni- X binary alloys. The method bridges between the macroscopic elastic behavior and thermodynamic properties of materials without an expensive calculation of the volume dependence of the phonon density of states. Furthermore, the Gruneisen parameter is derived from the volume dependence of the Debye temperature, which is calculated from the first-principles elastic stiffness constants. The experimental coefficient of thermal expansion (CTE) of pure nickel is well reproduced, especially in the low temperature region. Among the few alloying elements tested, Al is predicted to slightly decrease the CTE whereas Mo and W are more effective in reducing the CTE. For the cases of Ni-X binary alloy systems, where the variation in the CTE is relatively small, the method used here appears to perform better than certain other formulations that rely entirely on the energy vs. volume relationship.
    Download Collect
  • Synthesis, Crystal Structure, Thermal Decomposition, and B-11 MAS NMR Characterization of Mg(BH4)(2)(NH3BH3)(2)

    Jepsen, Lars H.   Ban, Voraksmy   Moller, Kasper T.   Lee, Young-Su   Cho, Young Whan   Besenbacher, Flemming   Filinchuk, Yaroslav   Skibsted, Jorgen   Jensen, Torben R.  

    A metal borohydride ammonia borane complex, Mg(BH4)(2)(NH3BH3)(2) was synthesized via a solid-state reaction between Mg(BH4)(2) and NH3BH3. Different mechanochemical reaction mechanisms are observed, since Mg(BH4)(2)(NH3BH3)(2) is obtained from alpha-Mg(BH4)(2), whereas a mixture of Mg(BH4)(2)(NH3BH3)(2), NH3BH3, and amorphous Mg(BH4)(2) is obtained from gamma-Mg(BH4)(2). The crystal structure of Mg(BH4)(2)(NH3BH3)(2) has been determined by powder Xray diffraction and optimized by first-principles calculations. The borohydride groups act as terminal ligands, and molecular complexes are linked via strong dihydrogen bonds (<2.0 angstrom), which may contribute to the high melting point of Mg(BH4)(2)(NH3BH3)(2) found to be similar to 48 degrees C in contrast to those for other molecular metal borohydrides. Precise values for the B-11 quadrupole coupling parameters and isotropic chemical shifts are reported for the two NH3BH3 sites and two BH4- sites in Mg(BH4)(2)(NH3BH3)(2) from B-11 MAS NMR spectra of the central and satellite transitions and MQMAS NMR. The B-11 quadrupole coupling parameters agree excellently with the electric field gradients for the B-11 sites from the DFT calculations and suggest that a more detailed structural model is obtained by DFT optimization, which allows evaluation of the dihydrogen bonding scheme.
    Download Collect
  • Low temperature formation of Mg2FeH6 by hydrogenation of ball-milled nano-crystalline powder mixture of Mg and Fe

    Fadonougbo, Julien O.   Jung, Jee-Yun   Suh, Jin-Yoo   Lee, Young-Su   Shim, Jae-Hyeok   Cho, Young Whan  

    Low temperature formation of Mg2FeH6 is demonstrated by hydrogenation of Mg-Fe elemental powder mixture at a temperature as low as 350 degrees C which is lower than the conventional process temperature, 500 degrees C. To enable the low temperature synthesis, the powder mixture of Mg and Fe has been prepared by high energy ball milling using different process control agents (PCAs). A systematic study on the ball milling and hydrogenation conditions has been carried out to maximize the yield of the ternary line compound. The hydrogenation conditions together with the particle size of the starting materials turn out to play a significant role in the hydrogenation kinetics of the system. An optimized condition has demonstrated a significant hydrogenation as well as a robust cycling ability at low temperature which suggests the strong potential of the process for practical applications. (C) 2017 Elsevier Ltd. All rights reserved.
    Download Collect
  • Mechanochemical synthesis of Ce3Al11 powder and its catalytic effect on the hydrogen sorption properties of NaAlH4

    Choi, Jongkwan   Ha, Taejun   Park, Jihye   Lee, Young-Su   Han, Heung Nam   Lee, Joonho   Shim, Jae-Hyeok  

    Mechanochemical synthesis of the Ce3Al11 powder was performed through a reaction among CeCl3, AlCl3 and NaH powders. alpha-Ce3Al11 particles with an average size of 250 nm were obtained. The catalytic effect of the synthesized Ce3Al11 on the hydrogen sorption properties of NaAlH4 was investigated. The hydrogen absorption kinetics of NaAlH4 with Ce3Al11 was significantly enhanced and the hydrogen capacity was improved compared to those properties of NaAlH4 with CeCl3. This can be attributed to the fine distribution of the Ce3Al11 catalyst in NaAlH4 during ball milling. Although the volume of the NaAlH4 pellets generally tends to increase during the hydrogen sorption cycles, the volume expansion of the pellet of NaAlH4 with Ce3Al11 was relatively low with a small pore fraction compared to that of the pellet of NaAlH4 with conventional chloride catalysts such as CeCl3, TiCl3 and TiCl4. The use of a nonchloride catalyst appears to provide an effective approach for suppressing the volume expansion of NaAlH4 pellets during the hydrogen sorption cycles. (C) 2019 Elsevier B.V. All rights reserved.
    Download Collect
  • Mechanochemical synthesis of Ce3Al11 powder and its catalytic effect on the hydrogen sorption properties of NaAlH4

    Choi, Jongkwan   Ha, Taejun   Park, Jihye   Lee, Young-Su   Han, Heung Nam   Lee, Joonho   Shim, Jae-Hyeok  

    Download Collect
  • Enhanced Desorption and Absorption Properties of Eutectic LiBH(4)-Ca(BH(4))(2) Infiltrated into Mesoporous Carbon

    Lee, Hyun-Sook   Lee, Young-Su   Suh, Jin-Yoo   Kim, Minwoo   Yu, Jong-Sung   Cho, Young Whan  

    We present enhanced hydrogen storage properties of the eutectic LiBH(4)-Ca(BH(4))(2) composite confined inside a mesoporous carbon scaffold via melt infiltration. The eutectic composition was examined by analyzing the enthalpy of fusion as a function of the LiBH(4)-to-Ca(BH(4))(2) ratio. Elemental analysis by energy dispersive X-ray spectroscopy and phase analysis by X-ray diffraction patterns provided both direct and indirect evidence for the infiltration of the LiBH(4)-Ca(BH(4))(2) composite into the mesoporous channels of the carbon. The major dehydrogenation event occurred at similar to 300 degrees C, which is lower compared with the same composite without carbon or previously reported pure LiBH(4) confined in mesoporous carbons. Without additional catalytic additives, almost half of the initial hydrogen capacity was recovered after one cycling. This improved performance is a synergistic effect of the nanoconfinement and the mutual destabilization between LiBH(4) and Ca(BH(4))(2), and it also indicates that the eutectic melting phenomenon can be exploited as an effective way of coinfiltration of reactive hydride composites.
    Download Collect
  • Cycloadditions to control bond breaking in naphthalenes, fullerenes, and carbon nanotubes: A first-principles study

    Lee, Young-Su   Marzari, Nicola  

    Covalent functionalizations represent a very promising avenue to engineer or manipulate carbon nanotubes. However, in metallic tubes the electrical conductance can drop by several orders of magnitude following functionalization, due to sp(3) rehybridization of the sidewall carbons that strongly disrupts the conjugated pi-network. First-principles calculations have predicted that some divalent functional groups, carbenes or nitrenes, can instead recover the original sp(2) hybridization and perfect metallic conductance of the pristine tubes. In these cycloaddition reactions, the extra bond added by the functional group with each of the bridgehead carbons is compensated by a breaking of the sidewall bond between them, restoring in the process the original sp(2) environment. We characterize this bond-breaking chemistry with extensive first-principles calculations and highlight its sensitivity to the orientation of the pi-electron system of the chosen addend. Using dinitrocarbene as a model case, we show that the bridgehead carbon atoms can reversibly rehybridize from sp(2) to sp(3) in response to the pi orientation of the addends. These results suggest a novel route to modulating the electronic properties of carbon nanotubes that is based on orbital rehybridization and that can be directed with optical or electrochemical means.
    Download Collect
  • A finite outlet volume correction to the time lag method:The case of hydrogen permeation through V-alloy and Pd membranes

    Lee, Young-Su   Shim, Jae-Hyeok   Suh, Jin-Yoo  

    The time-lag method is a technique to derive the permeability of a membrane from the time-variant outflux of a permeate. The original formulation assumes an invariant outlet pressure; however, for practical reasons, the permeate is often contained in a finite outlet volume, and the outflux is evaluated using the increase in the outlet pressure. The outlet pressure change inevitably introduces an error in the diffusivity and in the solubility, i.e., Sieverts' constant. We demonstrate that when a time lag is obtained from the tangent line at the maximum flux, the error in the diffusivity stays within ca. 2% under usual experimental conditions. On the other hand, Sieverts' constant is underestimated to a larger degree if the outlet pressure change is not considered. To solve this problem, we propose a finite outlet volume correction to the time-lag method for the case of hydrogen permeation. The proposed scheme is an approximate solution that is valid when a linear hydrogen concentration profile is developed across the membrane. When the correction method is applied to characterize V-alloy and Pd membranes, the simulated outlet pressure from the corrected Sieverts' constant closely approximates the experimental data. The proposed method does not require any additional measurements and greatly improves the accuracy of the permeability measurement.
    Download Collect
  • Discovery of Fluidic LiBH4 on Scaffold Surfaces and Its Application for Fast Co-confinement of LiBH4-Ca(BH4)(2) into Mesopores

    Lee, Hyun-Sook   Hwang, Son-Jong   To, Magnus   Lee, Young-Su   Cho, Young Whan  

    Generation of fluidic LiBH4 molecules, f-LiBH4, was demonstrated by NMR spectroscopy of LiBH4 bulk powder Mixed with silica scaffold surface materials under minor heat treatment. In the presence of the fumed scaffold mesoporous MCM-41 and SBA-15, LiBH4 shows increased translational mobility at relatively low temperature (ca. 95 degrees C) and becomes liquid-like by evidence from H-1-B-11 J-coupling in H-1 and B-11 MAS NMR or substantial line narrowing of Li-2 static NMR This high diffusional mobility of LiBH4 at the molecular level has never been seen for bulk LiBH4, and the property is attributed to the interfacial interaction with the mesoporous scaffold surfaces. While f-LiBH4 facilitates the confinement of LiBH4 itself into various scaffold materials, LiBH4 migrates along the SBA-15 surface to reach other metal borohydride particles, Ca(BH4)(2) in this case, and promotes the formation of similarly fluidic LiBH4-Ca(BH4)(2) composite (LC solid solution) for coconfinement into mesopores. In situ variable temperature (VT) NMR spectroscopy detects the co-infiltration process of eutectic LiBH4-Ca(BH4)(2) composite (LC) into mesopores of SBA-15. The infiltration rates measured for LiBH4 bulk powder or LC composite showed dependence on pore sizes (MCM-41 vs SBA-15) and heat treatment conditions (static vs MAS).
    Download Collect
  • Glucosylation of flavonol and flavanones by Bacillus cyclodextrin glucosyltransferase to enhance their solubility and stability

    Lee, Young-Su   Woo, Jang-Bin   Ryu, Soo-In   Moon, Seong-Kwon   Han, Nam Soo   Lee, Soo-Bok  

    Enzymatically modified isoquercitrin (EMIQ), oligoglucosyl naringenin-7-(glucose [G]), and oligoglucosyl hesperetin (H)-7-G were produced via oligoglucosylation of quercetin-3-glucose, naringenin-7-G (prunin), and H-7-G, respectively, by cyclodextrin glucosyltransferase from Bacillus macerans. The aim was to explore the oligoglucosylation and the resulting changes in physicochemical properties. Water solubility of EMIQ oligoglucosyl prunin, and oligoglucosyl H-7-G enormously increased in comparison with that of their aglycones. Glycosylation of an aglycone generally enhances its solubility. Resistance of the aglycones to oxidative degradation by the Cu2+ ion was strongly increased by the oligoglucosylation. This is probably because oligoglucosylation may protect sensitive parts of an aglycones molecule from the Cu2+ oxidation. Only EMIQ maintained its structure during thermal treatment much longer than quercetin did. Degradation of flavonoid aglycones by ultraviolet light C irradiation at 254 nm was not affected, and their antioxidant activities gradually decreased with the greater extent of oligoglucosylation. (C) 2017 Elsevier Ltd. All rights reserved.
    Download Collect
1 2 3 4 5 6

Contact

If you have any feedback, Please follow the official account to submit feedback.

Turn on your phone and scan

Submit Feedback