Yung-Eun Sung

Failure Modes of Silicon Powder Negative Electrode in Lithium Secondary Batteries

Si composite negative electrodes for lithium secondary batteries degrade in the dealloying period with an abrupt increase in internal resistance that is caused by a breakdown of conductive network made between Si and carbon particles. This results from a volume contraction of Si particles after expansion in the previous alloying process. Due to the large internal resistance, the dealloying reaction is not completed while Si remains as a lithiated state. The anodic performance is greatly improved either by applying a pressure on the cells or loading a larger amount of conductive carbon in the composite electrodes.

Galvanic Replacement Reactions in Metal Oxide Nanocrystals

Hollowing Out Metal Oxide Nanoparticles Corrosion is normally a problem, but it can be useful, for example, when you wish to create hollow metal nanoparticles, whereby the reduction of one metal species in solution drives the dissolution of the core of the particle. Oh et al. (p. 964; see the Perspective by Ibáñez and Cabot) adapted this approach to metal oxide nanoparticles by placing Mn3O4 nanocrystals in solution with Fe2+ ions, which replaces the nanocrystal exterior with γ-Fe2O3. At sufficiently high Fe2+ concentrations, hollow γ-Fe2O3 nanocages formed. These hollow structures could be used as anode materials for lithium ion batteries. Hollow mixed-metal oxide nanoparticles can be made by replacing the metal cations through redox reactions in solution. [Also see Perspective by Ibáñez and Cabot] Galvanic replacement reactions provide a simple and versatile route for producing hollow nanostructures with controllable pore structures and compositions. However, these reactions have previously been limited to the chemical transformation of metallic nanostructures. We demonstrated galvanic replacement reactions in metal oxide nanocrystals as well. When manganese oxide (Mn3O4) nanocrystals were reacted with iron(II) perchlorate, hollow box-shaped nanocrystals of Mn3O4/γ-Fe2O3 (“nanoboxes”) were produced. These nanoboxes ultimately transformed into hollow cagelike nanocrystals of γ-Fe2O3 (“nanocages”). Because of their nonequilibrium compositions and hollow structures, these nanoboxes and nanocages exhibited good performance as anode materials for lithium ion batteries. The generality of this approach was demonstrated with other metal pairs, including Co3O4/SnO2 and Mn3O4/SnO2.

Hyaluronic Acid−Quantum Dot Conjugates for In Vivo Lymphatic Vessel Imaging

A simple and novel electrostatic coupling method is reported, which provides a hyaluronic acid-quantum dot conjugate (HA-QD) that is colloidally stable and size-tunable from 50 to 120 nm. The HA-QDs show cancer targeting efficiency, which suggests diagnostic and imaging applications. The conjugates are also demonstrated for the fluorescence staining capability for lymphatic vessels in vitro and in vivo. Using the HA-QDs in a small animal model, lymphatic vessels are visualized real-time in vivo for days. Comprehensive cytotoxicity evaluations are made for the conjugates and the unconjugated counterpart. The HA-QDs showcase the potentials toward cancer imaging and real-time visualization of changes in lymphatic vessels such as lymphangiogenesis.

Conversion Reaction-Based Oxide Nanomaterials for Lithium Ion Battery Anodes.

Developing high-energy-density electrodes for lithium ion batteries (LIBs) is of primary importance to meet the challenges in electronics and automobile industries in the near future. Conversion reaction-based transition metal oxides are attractive candidates for LIB anodes because of their high theoretical capacities. This review summarizes recent advances on the development of nanostructured transition metal oxides for use in lithium ion battery anodes based on conversion reactions. The oxide materials covered in this review include oxides of iron, manganese, cobalt, copper, nickel, molybdenum, zinc, ruthenium, chromium, and tungsten, and mixed metal oxides. Various kinds of nanostructured materials including nanowires, nanosheets, hollow structures, porous structures, and oxide/carbon nanocomposites are discussed in terms of their LIB anode applications.

Role of Electronic Perturbation in Stability and Activity of Pt-Based Alloy Nanocatalysts for Oxygen Reduction

The design of electrocatalysts for polymer electrolyte membrane fuel cells must satsify two equally important fundamental principles: optimization of electrocatalytic activity and long-term stability in acid media (pH <1) at high potential (0.8 V). We report here a solution-based approach to the preparation of Pt-based alloy with early transition metals and realistic parameters for the stability and activity of Pt(3)M (M = Y, Zr, Ti, Ni, and Co) nanocatalysts for oxygen reduction reaction (ORR). The enhanced stability and activity of Pt-based alloy nanocatalysts in ORR and the relationship between electronic structure modification and stability were studied by experiment and DFT calculations. Stability correlates with the d-band fillings and the heat of alloy formation of Pt(3)M alloys, which in turn depends on the degree of the electronic perturbation due to alloying. This concept provides realistic parameters for rational catalyst design in Pt-based alloy systems.

Ordered macroporous platinum electrode and enhanced mass transfer in fuel cells using inverse opal structure

Three-dimensional, ordered macroporous materials such as inverse opal structures are attractive materials for various applications in electrochemical devices because of the benefits derived from their periodic structures: relatively large surface areas, large voidage, low tortuosity and interconnected macropores. However, a direct application of an inverse opal structure in membrane electrode assemblies has been considered impractical because of the limitations in fabrication routes including an unsuitable substrate. Here we report the demonstration of a single cell that maintains an inverse opal structure entirely within a membrane electrode assembly. Compared with the conventional catalyst slurry, an ink-based assembly, this modified assembly has a robust and integrated configuration of catalyst layers; therefore, the loss of catalyst particles can be minimized. Furthermore, the inverse-opal-structure electrode maintains an effective porosity, an enhanced performance, as well as an improved mass transfer and more effective water management, owing to its morphological advantages.

Facile and economical synthesis of hierarchical carbon-coated magnetite nanocomposite particles and their applications in lithium ion battery anodes

Hierarchical sea urchin-like structured carbon–Fe3O4 nanocomposite particles composed of a nanoporous interior and a carbon-coated surface have been prepared by a simple, economical and scalable synthetic process. When the nanocomposite particles were tested as lithium ion battery anodes, they exhibited high capacity, excellent cycle stability and rate performance due to their unique hierarchical nanoporous structure and carbon shell.

A facile hydrazine-assisted hydrothermal method for the deposition of monodisperse SnO2 nanoparticles onto graphene for lithium ion batteries

In this manuscript, we introduce a facile hydrothermal method for the controlled growth of SnO2 nanoparticles onto graphene oxide. Hydrazine plays a fundamental role in controlling the formation and crystallization of SnO2 nanoparticles, and the reduction of graphene oxide to graphene. The SnO2–graphene composite consists of 3–4 nm monodisperse SnO2 nanocrystals homogeneously dispersed at the surface of graphene. It is demonstrated that the composite can accommodate the large volume change of SnO2 which occurs during lithiation–delithiation cycles. When used as an anode material for lithium ion batteries, it exhibits a first discharge capacity of 1662 mA h g−1, which rapidly stabilizes and still remains at 626 mA h g−1 even after 50 cycles, when cycled at a current density of 100 mA g−1. Even at the very high current density of 3200 mA g−1, the composite displays a stable capacity of 383 mA h g−1 after 50 cycles.

Large-Scale Synthesis of Carbon-Shell-Coated FeP Nanoparticles for Robust Hydrogen Evolution Reaction Electrocatalyst

A highly active and stable non-Pt electrocatalyst for hydrogen production has been pursued for a long time as an inexpensive alternative to Pt-based catalysts. Herein, we report a simple and effective approach to prepare high-performance iron phosphide (FeP) nanoparticle electrocatalysts using iron oxide nanoparticles as a precursor. A single-step heating procedure of polydopamine-coated iron oxide nanoparticles leads to both carbonization of polydopamine coating to the carbon shell and phosphidation of iron oxide to FeP, simultaneously. Carbon-shell-coated FeP nanoparticles show a low overpotential of 71 mV at 10 mA cm-2, which is comparable to that of a commercial Pt catalyst, and remarkable long-term durability under acidic conditions for up to 10 000 cycles with negligible activity loss. The effect of carbon shell protection was investigated both theoretically and experimentally. A density functional theory reveals that deterioration of catalytic activity of FeP is caused by surface oxidation. Extended X-ray absorption fine structure analysis combined with electrochemical test shows that carbon shell coating prevents FeP nanoparticles from oxidation, making them highly stable under hydrogen evolution reaction operation conditions. Furthermore, we demonstrate that our synthetic method is suitable for mass production, which is highly desirable for large-scale hydrogen production.

Fast switchable electrochromic properties of tungsten oxide nanowire bundles

The authors prepared uniformly shaped WO2.72 nanowire bundles using the solvothermal synthesis method. They investigated the potential of the WO2.72 nanowire bundles to be used as a cathode electrode for electrochromic devices and the effect of the Li+ insertion (or extraction) kinetics and diffusion of Li+. An electrode consisting of arrays of WO2.72 nanowire bundles was formed and used in an experiment using the Langmuir-Blodgett technique. The one-dimensional nanostructure of WO2.72 has a high Li-ion diffusion coefficient (∼5.2×10−11cm2∕s) and low charge transfer resistance (∼28.6Ω), which result in its having a fast electrochromic response time (coloring time 55cm2∕C).