Eui-Tak Hwang

Cubic and octahedral Cu2O nanostructures as anodes for lithium-ion batteries

Well-defined nanostructured electrodes are known to have improved lithium ion reaction properties for lithium-ion batteries. Herein, we prepared shape-controlled Cu2O nanostructures as an anode material using ascorbic acid as a reducing agent with and without polyvinylpyrrolidone (PVP) as a surfactant. Using scanning electron microscopy, transmission electron microscopy, and X-ray diffraction methods, we observed that the sample prepared in the absence of PVP exhibited cubes with dominant {100} facets, whereas octahedral Cu2O nanostructures with dominant {111} facets were formed in the presence of PVP. During the charge–discharge process, an octahedron-shaped Cu2O nanostructured electrode having {111} facets favourable for lithium ion transport revealed an enhanced conversion reaction mechanism with high reversible capacity and high rate cycling performance, due to its low charge transfer resistance and high lithium ion diffusion coefficient.

Preparation and characterization of PtIr alloy dendritic nanostructures with superior electrochemical activity and stability in oxygen reduction and ethanol oxidation reactions

Pt-based alloy dendritic nanostructures have been known to exhibit improved electrocatalytic properties due to their particularly modulated surface and electronic structures favorable for alcohol oxidation and oxygen reduction reactions. We prepared PtIr alloy nanoparticles (NPs) with a dendritic shape as three-dimensional structures for enhanced ethanol oxidation reaction (EOR) and oxygen reduction reaction (ORR) by thermal decomposition in the presence of cetyltrimethylammonium chloride (CTAC) as surfactant. The PtIr alloy dendritic nanoparticles show a well-defined three-dimensional alloy nanostructure analyzed using TEM, XPS, and XRD. In particular, the PtIr alloy nanostructures exhibit 2.74 times higher electrochemical active surface areas (EASAs) than commercial Pt/C. Also, in the EOR, the PtIr alloy dendritic electrocatalyst exhibits excellent electrochemical properties, including high If/Ib ratio and current density, high negative onset potential, and good electrochemical stability compared to the commercial Pt/C electrocatalyst. In addition, the PtIr alloy dendritic electrocatalyst exhibits enhanced electrochemical activity and stability, i.e., 3.19 times higher specific mass-kinetic activity than the commercial Pt/C electrocatalyst, and an 8 mV reduction of the half-wave potential in the ORR. The improved electrochemical activity and stability of the PtIr alloy dendritic electrocatalyst in the EOR and ORR are ascribed to the dendritic structures, the surface state of the electrocatalyst, and the controlled electronic structure due to the Ir atoms in the alloy phase.

Synthesis of Pt-Rich@Pt–Ni alloy core–shell nanoparticles using halides

We demonstrated the synthesis of Pt–Ni alloy core–shell nanoparticles (NPs) via a one-pot thermal decomposition method, optimized by variation of the concentration of cetyltrimethylammonium chloride (CTAC) and reaction time. The samples prepared without CTAC and in 30 mM CTAC at 250 °C for 180 min exhibited the formation of single Pt-rich phases between metallic phases. With increasing CTAC concentrations (60–120 mM) at a constant temperature and time (250 °C for 180 min), the products contained both Pt-rich and Pt–Ni alloy phases, consisting of a Pt-rich core with a Pt–Ni alloy shell (Pt-rich@Pt–Ni), in contrast to the single Pt-rich phases prepared at low concentrations or in the absence of CTAC. As the reaction time increased from 10 to 180 min in 60 mM CTAC at 250 °C, the Pt-rich NPs were observed to grow in the initial stage, i.e. until a critical reaction time of 60 min, with subsequent formation of the Pt–Ni alloy phase on top of the as-formed Pt-rich NPs. The morphology and structure of the as-prepared NPs were characterized using TEM, EDX and XRD.

TiO 2 Branched Nanostructure Anode Material Prepared by Seeding Method for High-performance Lithium Ion Batteries

We demonstrate rutile TiO2 branched nanostructure (TiO2-BN) electrodes synthesized by seeding method for enhanced lithium intercalation properties. The morphology and crystal- line nature of the TiO2-BN were clearly observed by field-emission transmission electron microscopy and fast Fourier transform pattern. The TiO2-BN electrodes showed excellent capac- ity and high rate performance. The improved lithium-ion intercalation properties of the TiO2- BN may be attributed to relatively large specific surface area and short transport distance of the branched nanostructure.