Da-Hee Kwak

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.

Synergistic incorporation of hybrid heterobimetal–nitrogen atoms into carbon structures for superior oxygen electroreduction performance

Although Pt-based catalytic technology has led to significant advances in the development of electrocatalysts in fuel cells, Pt replacement with efficient and stable non-precious metal catalysts has a great technological significance for successful large-scale implementation of fuel cells. Here, we present the development of hybrid functional 1-dimensional carbon structures incorporated homogeneously with high contents of non-precious metal multi-dopants, consisting of iron, cobalt and nitrogen, as a promising alternative to Pt-based catalysts for the cathodic oxygen reduction reaction (ORR) through a modified electrospinning technique. These hybrid heterobimetal–nitrogen-incorporated carbon structures exhibit superior ORR electrocatalytic properties i.e., more positive reduction potential, high electroreduction current density, high electron transfer value (∼3.87) close to the perfect ORR and improved electrochemical stability with a very small decrease of ∼8 mV in half-wave potential. The observed enhancement in electrochemical performance can be ascribed to the increased amount of catalytically active sites with relatively high contents of heterometallic iron and cobalt atoms surrounded by nitrogen species and their homogeneous distribution on the catalyst surface.

Sulfur-Doped Porphyrinic Carbon Nanostructures Synthesized with Amorphous MoS2 for the Oxygen Reduction Reaction in an Acidic Medium

To develop doped carbon nanostructures as non-precious metal cathode catalysts, nanocomposites were synthesized by using SBA-15 and 5,10,15,20-tetrakis(4-methoxyphenyl)porphyrin-iron(III) chloride with different ratios of amorphous MoS2 precursor. From various analyses, it was found that, during pyrolysis at 900 °C under an N2 atmosphere, the amorphous MoS2 precursor decomposed into Mo and S, facilitating the formation of graphene sheet-like carbon with MoC and doping of sulfur in the carbon. In the nanocomposite formed from 10 wt % MoS2 precursor (denoted as Mo/S/PC-10), most of the MoS2 was decomposed, thus forming S-doped carbon, which was grown on the MoC phase without crystalline MoS2 . Furthermore, Mo/S/PC-10 exhibited better performance in the oxygen reduction reaction (specific activity of 1.23 mA cm-2 at 0.9 V and half-wave potential of 0.864 V) than a commercial Pt catalyst, owing to a heteroatom-doped carbon nanostructure with a fairly high specific surface area. In the polarization curve of the unit-cell performance measured at 80 °C under ambient pressure, Mo/S/PC-10 as a cathode catalyst exhibited an optimal power density of 314 mW cm-2 and a current density of 280 mA cm-2 at 0.6 V.

In situ formation of MoS2/C nanocomposite as an anode for high-performance lithium-ion batteries

Anode materials with excellent electrochemical properties as an alternative to carbon-based structures are suggested for advanced high-performance lithium-ion batteries. Here, composites containing MoS2 and carbon (MoS2/C) were in situ synthesized via heat treatment at 700 °C under a CH4 atmosphere with varying reaction times. XRD, Raman, XPS, and TEM data show that the MoS2/C composites consist of crystalline MoS2 and an amorphous carbon phase and show a homogeneous distribution of curved and bent MoS2 particles with a carbon matrix. In particular, the MoS2/C composite with an optimal content of the amorphous carbon phase exhibits relatively an excellent performance in lithium-ion batteries, facilitating the lithiation/delithiation process in MoS2 as an electroactive material.

Synthesis of Ge/C composites as anodes using glucose as a reductant and carbon source for lithium-ion batteries

Ge-based materials as anodes in lithium ion batteries (LIBs) having a large theoretical reversible capacity are needed to overcome the unstable structural and electrochemical properties and pulverization of the electrodes for high-performance LIBs. Here, we synthesized Ge/C composites as anodes for use in LIBs via heating a mixture of GeO2 powder and glucose as both a reductant and carbon source at 900 °C under a nitrogen atmosphere. The data from X-ray diffraction (XRD), Raman spectroscopy, and transmission electron microscopy (TEM) shows that the as-prepared samples consist of crystalline Ge particles and an amorphous carbon phase. Compared to pure Ge, the Ge/C samples exhibit discharge capacities of ∼627.1 mA h g−1, improved cyclability, and excellent rate properties at a current of 3200 mA g−1.

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.

Nature inspired cathodes using high-density carbon papers with an eddy current effect for high-rate performance lithium–air batteries

Phenomena observed in nature can be considered to be the result of optimized outcomes that have been obtained through trial and error for many years. Researchers have thus attempted to apply the lessons provided by nature to real scientific problems. In particular, in nature, the fluidic behavior of eddy currents is frequently observed, such as in a river, whereby the eddy develops into a whirlpool with an increased flow velocity, vorticity, and inner pressure. However, in this study on the design of a cathode for Li–air batteries (LABs), the eddy current of oxygen gas as a fluid can also be found in the gas diffusion layer (GDL) in the cathode fabricated using an imprinting process with micro-scale patterned metal molds. Herein, the cathode with the patterned GDL containing an increased density of carbon paper exhibits increased velocity, vorticity, and inner pressure of O2 flow due to the increased eddy current effect (the inner structuring). In addition, the patterned cathode increases the catalytic active sites for the oxygen reduction reaction due to the outer structuring effect such as an increased roughness factor. According to the simulated data from COMSOL, an average O2 gas velocity of the patterned cathode prepared through inner/outer structuring is increased by 45.8%, compared to that of the bare electrode. In the LAB test, the micro-scale patterned cathode shows an increased energy density and decreased IR drop by 79.6% and 23.2%, respectively. Consequently, the inner and outer structured GDL in the cathode for the ORR could be predominantly responsible for the superior LAB performance.

Effect of Pt coverage in Pt-deposited Pd nanostructure electrodes on electrochemical properties

We have fabricated Pt-deposited Pd electrodes via a two-gun sputtering deposition system by separately operating Pd and Pt target as a function of sputtering time of Pt target. For Pt-deposited Pd electrodes (Pd/Pt-X), Pd were first deposited on the substrates at 20 W for 5min, followed by depositing Pt on the Pd-only electrodes as a function of sputtering time (X=1, 3, 5, 7, and 10min) at 20W on the Pt target. As the sputtering time of Pt target increased, the portion of Pt on the Pd electrodes increased, representing an increased coverage of Pt on the Pd electrodes. The Pd/Pt-7 electrode having an optimized Pt coverage exhibits an excellent electrocatalytic activity for methanol oxidation reaction.

Micro-patterned 3D Si electrodes fabricated using an imprinting process for high-performance lithium-ion batteries

To overcome the volumetric expansion of Si used as an anode in lithium-ion batteries (LIBs), we propose 3D Si electrode structures formed on patterned Cu current collectors designed from metal molds fabricated using wire electrical discharge machining (WEDM). The line- and check-patterned Cu current collectors with microscale periods for LIBs are prepared using an imprinting technique with patterned metal molds fabricated using the WEDM process. The line- and check-patterned Si thin-film and powder-type electrodes as anodes are fabricated using radio frequency magnetron sputtering deposition method and conventional slurry casting process, respectively. The morphology of the Si electrodes before and after the cycling process is characterized using optical microscopy and scanning electron microscopy. The electrochemical properties of the Si electrodes are evaluated using a multi-channel battery tester and electrochemical impedance analyzer. In particular, the check-patterned Si electrodes exhibit relatively high-capacity and enhanced cycling performance due to the stress relief of the Si anode.Graphical Abstract