Soonchul Kwon

Silicon carbide-free graphene growth on silicon for lithium-ion battery with high volumetric energy density

Silicon is receiving discernable attention as an active material for next generation lithium-ion battery anodes because of its unparalleled gravimetric capacity. However, the large volume change of silicon over charge–discharge cycles weakens its competitiveness in the volumetric energy density and cycle life. Here we report direct graphene growth over silicon nanoparticles without silicon carbide formation. The graphene layers anchored onto the silicon surface accommodate the volume expansion of silicon via a sliding process between adjacent graphene layers. When paired with a commercial lithium cobalt oxide cathode, the silicon carbide-free graphene coating allows the full cell to reach volumetric energy densities of 972 and 700 Wh l−1 at first and 200th cycle, respectively, 1.8 and 1.5 times higher than those of current commercial lithium-ion batteries. This observation suggests that two-dimensional layered structure of graphene and its silicon carbide-free integration with silicon can serve as a prototype in advancing silicon anodes to commercially viable technology.

Photocatalytic Applications of Micro- and Nano-TiO2 in Environmental Engineering

The photocatalytic activity of micro- and nano-titanium dioxide (TiO 2 ) has been utilized to significantly improve the degradation efficiencies of various contaminants in both water treatment and air pollution control. This article is a review of the literature covering current research on environmental applications of micro- and nano-TiO 2 . The mechanisms of contaminant degradation of nanoparticle TiO 2 are reviewed, and its special properties are compared to micro-sized TiO 2 in air purification and water treatment.

Mechanisms of Na adsorption on graphene and graphene oxide: density functional theory approach

We investigated the adsorption of Na on graphene and graphene oxide, which are used as anode materials in sodium ion batteries, using density functional theory. The adsorption energy for Na on graphene was -0.507 eV at the hollow sites, implying that adsorption was favorable. In the case of graphene oxide, Na atoms were separately adsorbed on the epoxide and hydroxyl functional groups. The adsorption of Na on graphene oxide-epoxide (adsorption energy of -1.024 eV) was found to be stronger than the adsorption of Na on pristine graphene. However, the adsorption of Na on graphene oxide-hydroxyl resulted in the generation of NaOH as a by-product. Using density of states (DOS) calculations, we found that the DOS of the Na-adsorbed graphene was shifted down more than that of the Na-adsorbed graphene oxide-epoxide. In addition, the intensity of the DOS around the Fermi level for the Na-adsorbed graphene was higher than that for the Na-adsorbed graphene oxide-epoxide.

Synthesis and characterization of various coordination modes of 1,1′-bis(diphenylphosphino)ferrocene in iron carbonyl complexes. X-Ray crystal structures of (η-BPPF)Fe(CO)4, (μ,η 2-BPPF)(Fe(CO)4)2, and (μ,η2-BPPF)Fe3(CO)10

Reactions of 1,1′-bis(diphenylphosphino)ferrocene (BPPF) with Fe2(CO)9 and Fe3(CO)12 produced a series of iron carbonyl derivatives: (η2-BPPF)Fe(CO)3 (1), (η2-BPPF)Fe(CO)4 (2), (μ,η2BPPF)(Fe(CO)4)2 (3), (η2-BPPF)Fe2(CO)7 (4), and (μ,η2BPPF)Fe3(CO)10 (5). The synthesis and characterization of 4 and 5 including X-ray crystal structures of 2, 3, and 5 confirmed various coordination modes of BPPF. Crystal data are as follows: (η1-BPPF)Fe(CO)4 (2): orthorhombic, space group Pbca, a = 26.27(1), b = 23.18(1), c = 10.938(8) A, V = 6661(7) A3, Z = 8; 3017 data with I > 3.0σ(I were refined to R = 0.057, Rw = 0.056; (μ,η2-BPPF)Fe3(CO)10 (5): monoclinic, space group P21 / n, a = 11.231(1) b = 21.043(4), c = 20.373(9) A, β = 97.373(9)° V = 4693(4) A3 V, Z = 4; 6082 data with I > 3.0σ(I) were refined to R = 0.054, Rw = 0.071; (μ,η2-BPPF)Fe2(CO)8 (3): monoclinic, space group C2/ c, a = 17.223(7), b = 14.97(2), c = 18.558(3) A, β = 108.39(3)°, V = 4541(6) A3, Z = 4; 3201 data with I > 3.0σ(I) were refined to R = 0.070, Rw = 0.139, as a result of difficulties with modeling electron density peaks associated with highly disordered solvent atoms.

Reaction Kinetics of CO2 Carbonation with Mg-Rich Minerals

Due to their low price, wide availability, and stability of the resulting carbonates, Mg-rich minerals are promising materials for carbonating CO(2). Direct carbonation of CO(2) with Mg-rich minerals reported in this research for the first time could be considerably superior to conventional liquid extraction processes from an energy consumption perspective due to its avoidance of the use of a large amount of water with high specific heat capacity and latent heat of vaporization. Kinetic models of the reactions of the direct CO(2) carbonation with Mg-rich minerals and within simulated flue gas environments are important to the scale-up of reactor designs. Unfortunately, such models have not been made available thus far. This research was initiated to fill that gap. Magnesium silicate (Mg(2)SiO(4)), a representative compound in Mg-rich minerals, was used to study CO(2) carbonation reaction kinetics under given simulated flue gas conditions. It was found that the chosen sorbent deactivation model fits well the experimental data collected under given conditions. A reaction order of 1 with respect to CO(2) is obtained from experimental data. The Arrhenius form of CO(2) carbonation with Mg(2)SiO(4) is established based on changes in the rate constants of the chosen deactivation model as a function of temperature.

Factors affecting the direct mineralization of CO2 with olivine

Olivine, one of the most abundant minerals existing in nature, is explored as a CO2 carbonation agent for direct carbonation of CO2 in flue gas. Olivine based CO2 capture is thermodynamically favorable and can form a stable carbonate for long-term storage. Experimental results have shown that water vapor plays an important role in improving CO2 carbonation rate and capacities. Other operation conditions including reaction temperature, initial CO2 concentration, residence time corresponding to the flow rate of CO2 gas stream, and water vapor concentration also considerably affect the performance of the technology.

CO2 Enhanced Chemical Vapor Deposition Growth of Few-Layer Graphene over NiOx

The use of mild oxidants in chemical vapor deposition (CVD) reactions has proven enormously useful. This was also true for the CVD growth of carbon nanotubes. As yet though, the use of mild oxidants in the CVD of graphene has remained unexplored. Here we explore the use of CO2 as a mild oxidant during the growth of graphene over Ni with CH4 as the feedstock. Both our experimental and theoretical findings provide in-depth insight into the growth mechanisms and point to the mild oxidants playing multiple roles. Mild oxidants lead to the formation of a suboxide in the Ni, which suppresses the bulk diffusion of C species suggesting a surface growth mechanism. Moreover, the formation of a suboxide leads to enhanced catalytic activity at the substrate surface, which allows reduced synthesis temperatures, even as low as 700 °C. Even at these low temperatures, the quality of the graphene is exceedingly high as indicated by a negligible D mode in the Raman spectra. These findings suggest the use of mild oxidants in the CVD fabrication as a whole could have a positive impact.

Enhanced Electrochemical Stability of Quasi-Solid-State Electrolyte Containing SiO2 Nanoparticles for Li-O2 Battery Applications

A stable electrolyte is required for use in the open-packing environment of a Li-O2 battery system. Herein, a gelled quasi-solid-state electrolyte containing SiO2 nanoparticles was designed, in order to obtain a solidified electrolyte with a high discharge capacity and long cyclability. We successfully fabricated an organic-inorganic hybrid matrix with a gelled structure, which exhibited high ionic conductivity, thereby enhancing the discharge capacity of the Li-O2 battery. In particular, the improved electrochemical stability of the gelled cathode led to long-term cyclability. The organic-inorganic hybrid matrix with the gelled structure played a beneficial role in improving the ionic conductivity and long-term cyclability and diminished electrolyte evaporation. The experimental and theoretical findings both suggest that the preferential binding between amorphous SiO2 and polyethylene glycol dimethyl ether (PEGDME) solvent led to the formation of the solidified gelled electrolyte and improved electrochemical stability during cycling, while enhancing the stability of the quasi-solid state Li-O2 battery.

Graphene Coating of Silicon Nanoparticles with CO2‐Enhanced Chemical Vapor Deposition

Understanding the growth of graphene over Si species is becoming ever more important as the huge potential for the combination of these two materials becomes more apparent, not only for device fabrication but also in energy applications, particularly in Li-ion batteries. Thus, the drive for the direct fabrication of graphene over Si is crucial because indirect approaches, by their very nature, require processing steps that, in general, contaminate, damage, and are costly. In this work, the direct chemical vapor deposition growth of few-layer graphene over Si nanoparticles is systematically explored through experiment and theory with the use of a reducer, H2 or the use of a mild oxidant, CO2 combined with CH4 . Unlike the case of CH4 , with the use of CO2 as a mild oxidant in the reaction, the graphene layers form neatly over the surface and encapsulate the Si particles. SiC formation is also prevented. These structures show exceptionally good electrochemical performance as high capacity anodes for lithium-ion batteries. Density functional theory studies show the presence of CO2 not only prevents SiC formation but helps enhance the catalytic activity of the particles by maintaining an SiOx surface. In addition, CO2 can enhance graphitization.

Transient color changes in oxidative-stable fluorinated polyimide film for flexible display substrates

Stable optical properties of high transmittance and low yellow index are required for a polyimide film as a flexible display substrate, but thermal processes could result in its color change by thermal imidization. To prevent the color change, anti-oxidants have been used, but as yet though, the effect of oxidation in polyimide has remained unexplored. We explore the yellow index and absorbance changes of fluorinated polyimide after thermal imidization to determine the color change kinetics. Furthermore, we investigated the effect of anti-oxidants on film color change, showing that anti-oxidants even accelerate the color change due to their decomposition. Both experimental findings and density functional theory calculations suggest that the oxidative stability for the thermal imidization could have a modest impact, and in turn yellow index is less dependent on the oxidation of fluorinated polyimide, attributed to mild interaction between fluorinated polyimide and oxygen insufficient to form strong oxidation of diamine groups.