Daejeon, South Korea
Daejeon, South Korea

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Kim H.S.,Energy Material Group | Yu S.-H.,Research Center for Energy Conversion And Storage | Sung Y.-E.,Research Center for Energy Conversion And Storage | Kang S.H.,Chonnam National University
Journal of Alloys and Compounds | Year: 2014

Vertically aligned TiO2 nanotube (TONT) arrays on titanium substrate developed by facile electrochemical anodization in an aqueous solution of 0.5 M Na2SO4, 0.5 M H3PO4, 0.2 M sodium citrate, and 0.5 wt% NaF were prepared having a pore diameter and thickness of 100 nm and 1.2 μm, respectively. The undoped (u-doped) TONT arrays possessing an anatase phase were again annealed at 500 C under a mixed gas flux of nitrogen (N2) and acetylene (C2H2), to induce the enhancement of electrical conductivity. It was designated as carbon-doped (c-doped) TONT arrays. Undoped and c-doped TONT arrays were compared using various characterization tools, including X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM), and X-ray photoelectron spectroscopy (XPS). Furthermore, based on several electrochemical tests (galvanostatic charge/discharge tests, cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS)), it was observed that c-doped TONT arrays revealed improved charge/discharge capacity, cycle stability, and rate capability, due to the enhanced electrical conductivity of c-doped TONT arrays. © 2014 Elsevier B.V. All rights reserved.

Kim H.S.,Energy Material Group | Kwon S.G.,Seoul National University | Kang S.H.,Chonnam National University | Piao Y.,Korea Advanced Institute of Science and Technology | And 2 more authors.
Electrochimica Acta | Year: 2014

In this paper, we present a simple method for scalable synthesis of uniform carbon nanoshell coated monodispersed iron oxide nanocrystals and report on the electrochemical performances of such nanocomposites. The structure and morphology of the resulting nanocomposites are investigated with X-ray powder diffraction (XRD) and transmission electron microscopy (TEM). Cyclic voltammetry (CV) and cyclic test of the nanocomposites as an anode material for lithium ion batteries are also studied. The magnetite-C nanostructured material have a very high specific capacity of 859 mAh g-1 in the initial cycle and high capacity retention out to 50 cycles at a constant current density of 100 mA/g, showing its potential as an anode material in lithium-ion batteries. The superior electrochemical characteristics are a result of the uniform distribution of magnetite nanoparticles, proper nanostructure, and good conductivity as well as the prevention of aggregation by the carbon nanoshell during cycling. © 2014 Elsevier Ltd.

Kim H.S.,Energy Material Group | Yu S.-H.,Research Center for Energy Conversion And Storage | Cho Y.-H.,Kookmin University | Kang S.H.,Chonnam National University | Sung Y.-E.,Research Center for Energy Conversion And Storage
Electrochimica Acta | Year: 2014

A TiO2 nanotube (TONT) with an average pore diameter, length, and wall thickness of about 120 nm, 400 nm, and 20 nm, respectively was grown anodically on a Ti substrate. Sn nanophase is then coated through the entire area of the TONT by radio-frequency (RF) magnetron sputtering technique. The pore diameter of Sn coated TONT (denoted as Sn/TONT) arrays were decreased to 50 nm, while the wall thickness was increased to about 50 nm. Further, to make a conformal coating of the Sn layer through TONT arrays, while considering that the melting temperature of Sn is below 232 °C, the post-thermal treatment under different ambients (pure Ar gas vs. 5% H2 added Ar gas (designated as H2/Ar gas)) was performed at 350°C for 3 h. Compared with pure Ar annealing, H2/Ar annealing suppresses the formation of the SnOx phase, as confirmed by X-ray photoelectron spectroscopy (XPS) analysis, accelerating the formation of the pure Sn phase. The electrochemical properties of bare TONT and Sn/TONT arrays were compared using a cyclic voltammogram, showing that the TONT arrays with the pure Sn material participate in electrochemical lithiation and delithiation during the cycling process from the advent of the oxidation and reduction peaks in a certain potential range. Based on the active functions of the two materials, the capacity (0.042 mAh/cm2) of Sn/TONT arrays is increased relative to that (0.025 mAh/cm2) of bare TONT arrays and the capacity retention of Sn/TONT was maintained to be 75% at 5 C. © 2014 Elsevier Ltd.

Kim H.S.,Energy Material Group | Park S.-S.,Energy Material Group | Kang S.H.,Chonnam National University | Sung Y.-E.,Research Center for Energy Conversion And Storage
Journal of Applied Electrochemistry | Year: 2014

Cubic and star-shaped CaSnO3 particles with a perovskite structure were synthesized successfully using a simple hydrothermal method at a low temperature of 140°C. The structure and morphology of the CaSnO 3 powders were characterized using X-ray diffraction, X-ray photoelectron spectroscopy, and scanning electron microscopy. The electrochemical properties of the CaSnO3 as anode materials for lithium-ion batteries were tested by constant current discharge/charge and cyclic voltammetry. The large irreversible capacity in the initial cycle was similar to that of tin oxide, due to the decomposition of tin oxide into metallic tin and Li2O, followed by a reversible Li-Sn formation. The reversible capacity of the cubic CaSnO3 was 382 mAh g-1 in the first cycle and was maintained at 365 mAh g-1 in the following cycles. The cubic CaSnO3 particles had a higher reversible capacity than the star-shaped CaSnO3 particles and retained a capacity of about 365 mAh g-1 after 60 cycles as well as good cycle stability, showing potential as attractive anode materials for lithium-ion batteries. It is found that the particle shape had a marked effect on electrochemical performance. © 2014 Springer Science+Business Media Dordrecht.

Kim H.S.,Energy Material Group | Ahn K.-S.,Yeungnam University | Kang S.H.,Chonnam National University
Electronic Materials Letters | Year: 2014

Anodic TiO2 nanotube (TONT) with various morphological features was developed by simple electrochemical anodization in the variation of applied voltage where the morphological properties such as pore diameter, wall thickness, and inter-tube spacing can be adjusted to survey the influence on the photoelectrochemical (PEC) activity. Thus, in the applied voltage from 20 V to 60 V at different reaction times, the identical thickness of 3.0 ± 0.2 μm was maintained. From the field-emission scanning electron microscopy, the geometric factors were compared and the porosity of TONT films was calculated. The results demonstrated that the porosity of TONT film grown at 20 V (denoted as 20 V) is the highest and that the porosity steadily reduced with increasing applied voltage. On the basis of these geometric factors, the PEC performance of TONT film (20 V) shows the highest photocurrent of 1.64 mA/cm2 at 0 V vs. Ag/AgCl. This was attributed to the lowest series resistance due to the high porosity for the hole or ions pathway and to the high crystallite size for the beneficial charge transport. On the other hand, TONT films (above 40 V) exhibit a similar photocurrent with high series resistance for opposite reasons. © 2014 The Korean Institute of Metals and Materials and Springer Science+Business Media Dordrecht.

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