Je-Deok Kim

Large Reversible Li Storage of Graphene Nanosheet Families for Use in Rechargeable Lithium Ion Batteries

The lithium storage properties of graphene nanosheet (GNS) materials as high capacity anode materials for rechargeable lithium secondary batteries (LIB) were investigated. Graphite is a practical anode material used for LIB, because of its capability for reversible lithium ion intercalation in the layered crystals, and the structural similarities of GNS to graphite may provide another type of intercalation anode compound. While the accommodation of lithium in these layered compounds is influenced by the layer spacing between the graphene nanosheets, control of the intergraphene sheet distance through interacting molecules such as carbon nanotubes (CNT) or fullerenes (C60) might be crucial for enhancement of the storage capacity. The specific capacity of GNS was found to be 540 mAh/g, which is much larger than that of graphite, and this was increased up to 730 mAh/g and 784 mAh/g, respectively, by the incorporation of macromolecules of CNT and C60 to the GNS.

Fast Li-Ion Insertion into Nanosized LiMn2O4 without Domain Boundaries

The effect of crystallite size on Li-ion insertion in electrode materials is of great interest recently because of the need for nanoelectrodes in higher-power Li-ion rechargeable batteries. We present a systematic study of the effect of size on the electrochemical properties of LiMn(2)O(4). Accurate size control of nanocrystalline LiMn(2)O(4), which is realized by a hydrothermal method, significantly alters the phase diagram as well as Li-ion insertion voltage. Nanocrystalline LiMn(2)O(4) with extremely small crystallite size of 15 nm cannot accommodate domain boundaries between Li-rich and Li-poor phases due to interface energy, and therefore lithiation proceeds via solid solution state without domain boundaries, enabling fast Li-ion insertion during the entire discharge process.

Anhydrous Proton-Conducting Properties of Nafion–1,2,4-Triazole and Nafion–Benzimidazole Membranes for Polymer Electrolyte Fuel Cells

Recently, fuel cells have gathered a lot of interest due to their potential as future sources of clean energy. Polymer electrolyte fuel cells (PEFCs) require high operating temperatures (100-200°C) for high conversion efficiency. Anhydrous proton-conducting polymers employing Nafion-l,2,4-triazole- and Nafion-benzimidazole-blend membranes were studied for use in PEFCs above 100°C. The solvents 1,2,4-triazole and benzimidazole are thermally stable, organic reagents with low volatility and are used to replace water as the proton acceptor in the perfluorinated ionomer membrane. The Nafion-benzimidazole blend had high thermal stability above 200°C and proton conductivity of 8.64 X 10 -3 S/cm at 200°C under nonhumidified conditions. Fuel cell tests using a single membrane electrode assembly cell with the Nafion-blend membranes were successfully performed.

Organic–Inorganic Hybrid Membranes for a PEMFC Operation at Intermediate Temperatures

Recently, polymer electrolyte membrane fuel cells (PEMFCs) have been investigated extensively as a future energy source to solve a global energy and environmental problem due to their high energy conversion efficiency. As a membrane is recognized to be a key element for more efficient PEMFCs, new classes of polymer electrolytes have been investigated elsewhere. We have also studied new temperature-tolerant electrolytes using organic-inorganic materials such as polydimethylsiloxane, polytetramethylene oxide (PTMO), zirconia, and titania materials. 12-phosphotungstic (PWA) and phosphoric acids as a proton source were incorporated in the hydrophilic interface of organic-inorganic hybrid membranes. In this paper, they were synthetically displayed, and the cell performance for intermediate temperature PEFC using a single membrane electrolyte assembly cell was also investigated. The maximum power densities for zirconium phosphate-PTMO and titania-PTMO-PWA composite membranes were 13 and 30 mW/cm 2 , respectively. The cell performance of the membranes increased with increasing cell temperature up to 130°C under saturated humidity conditions. The organic-inorganic hybrid electrolytes synthesized from PTMO with metal alkoxides show promise for applications in intermediate temperature PEFCs.

Small-Angle X-Ray Scattering and Proton Conductivity of Anhydrous Nafion–Benzimidazole Blend Membranes

The small-angle X-ray scattering (SAXS) and proton conductivity of Nafion-benzimidazole (na―bz) blend membranes were investigated at various annealing temperatures. The SAXS data showed that the bz base as a proton source was successfully incorporated in the Nafion nanostructure, and the bz in the nanostructure was stable under both wet and dry conditions at room temperature. The nanostructural stability of the na―bz blend membrane was also investigated at various temperatures and was compared to the X-ray diffraction and proton conductivity data. In the na―bz blend membrane, both the nanostructure and the bz were stable up to 150°C under anhydrous (nonhumidified) conditions. The proton conductivity was also stable over this temperature range. The na―bz blend membrane may be useful as an anhydrous membrane for high temperature polymer electrolyte fuel cells.

SrTiO3 Thin Films with Visible-Light Band Gap Fabricated by Nitrogen Reactive Sputtering

Strontium titanate (SrTiO3; STO) films with extremely high nitrogen concentration, grown by an rf reactive magnetron sputtering system, were investigated by spectroscopic ellipsometry, X-ray diffraction, and X-ray photoemission spectroscopy measurements. A significant shift of the absorption edge to a visible-light region was observed, and surprisingly, the bandgap of nitrogen-doped STO containing up to 9.5 at. % of nitrogen shifted from 3.5 eV of pure STO to 1.9 eV. The result indicates the successful substitutional doping of nitrogen into the STO lattice, demonstrating the feasibility of band-gap narrowing through nitrogen reactive sputtering.

Intermediate Temperature Proton Electrolytes for Polymer Fuel Cells

Flexible and temperature tolerant organic-inorganic nano-hybrid membranes using metal oxides and organic monomers have been synthesized by sol-gel processes. The membranes showed enhanced thermal stability up to 300degC due to the presence of cross-linkable inorganic nano-phase in the hybrid macromolecular matrix. The membrane becomes proton conducting polymer electrolytes when doped with 12-phosphotungstic or phosphoric acids. The proton conducting properties of the hybrid membranes were measured in the temperature range from room temperature to 160degC under saturated humidity conditions. The modified organic-inorganic hybrid membranes could be applied to temperature tolerant proton conducting PEFCs.

Conjugated polymer-based carbonaceous films as binder-free carbon electrodes in supercapacitors

We present a facile preparation method for carbonaceous film electrodes using poly(3,4-ethylenedioxythiophene) (PEDOT) and polyacetylene (PA) films as precursors via a morphology-retaining carbonization process. Carbonization was performed on acceptor-doped conjugated polymer films in the temperature range of 600–1100 °C. The obtained carbonaceous films had similar surface morphologies to those of the original conjugated polymer films. The carbonaceous film prepared from the electrochemically synthesized PEDOT film and the carbon film prepared from the chemically synthesized PA film showed hierarchical porous structures consisting of granular and fibril morphologies, respectively. The PEDOT and PA films carbonized at 1100 °C exhibited average electrical conductivities of 2.1 × 100 S cm−1 and 9.9 × 101 S cm−1, respectively. The carbonaceous films could be used as binder-free carbon electrodes in supercapacitors, and the PEDOT-based carbonaceous film prepared in the range of 1000–1100 °C exhibited the most efficient performance on the basis of the electrochemical capacitance in neutral and alkaline aqueous solutions determined from cyclic voltammograms and galvanostatic charge/discharge curves. This approach requires no binders/additives and no further activation processes or additional treatments for the enhancement of the capacities of the carbon materials, enabling one-step fabrication and their direct use as carbon electrodes for energy-storage devices. This is the first report of PEDOT- and PA-based carbonaceous films being used as carbon electrodes in supercapacitors.

Enhanced cycle stability of hybrid Li–air batteries with carbon nanofiber grown on carbon black

The use of advanced carbon materials as an air electrode in hybrid Li–air batteries was thought to improve electrochemical performances such as cycle stability and a low voltage gap between discharge and charge. In this study, a carbon nanofiber grown on carbon black (CNF–CB) was prepared by a chemical vapor deposition (CVD) method at different temperatures (640–840 °C), and the electrochemical performance of the hybrid Li–air batteries based on the CNF–CB electrodes was investigated. The Li–air cell based on CNF–CB 740, with a cut-off voltage in the range of 2.5–4.2 V at 0.5 mA cm−2 showed good cycle stability and demonstrated about 75 cycles (about 300 h) without an obvious increase in charge voltage.

Development of high quality Pt-CeO 2 based anode materials for direct methanol fuel cell applications

Pt-pure Ceria (CeO2) supported by carbon black (CB) anode was synthesized for development of eco-electrode material for fuel cell application. The onset potential for methanol oxidation reaction on Pt-round shaped CeO2 shifted to a lower potential as compared with that on Pt-mixed shape (i.e. round shape + rod like shape) CeO2/CB. This onset potential on Pt-mixed shape CeO2 /CB anode at 60degC was equal to that on commercially available Pt-Ru/CB anode, although the on-set potential on Pt-mixed shape CeO2/CB was lower than that on Pt-Ru/CB at room temperature. This suggests that the anode performance of Pt-CeO2/CB anode is improved at operation temperature (80degC) of fuel cell by enhancement of diffusion and formation of oxygen species from nano-sized CeO2 particles. It is expected that the nano-size Pt-pure CeO 2/CB anode in the present study will be one of promising eco-anode materials for development of direct methanol fuel cells (DMFCs)