Ji Il Choi

Interaction of C60 with Water: First-Principles Modeling and Environmental Implications

The nature of fullerene-water interactions has been the subject of much research and debate. Specifically, the presence of a stabilizing, negative surface potential on colloidal aggregates of C60 in water is unexpected, given the neutral nature of pure carbon, and is not well understood. Previous simulation efforts have focused on the C60-water interaction using molecular dynamics simulations that lacked the ability to account for charge transfer and distribution interactions. In this study, first-principles density functional theory was used to analyze the fundamental electronic interactions to elucidate the polarization and charge transfer between water and C60. Simulations show that charge is inductively transferred to the C60 from water molecules, with subsequent polarization of the C60 molecule. In a case with two neighboring C60 molecules, the charge polarization induces a charge onto the second C60. Simulation suggests that this charge transfer and polarization may contribute at least partly to the observed negative surface potential of fullerene aggregates and, combined with hydrogen bonding network formation around C60, provides a fundamental driving force for aggregate formation in water.

Mechanism of Li Adsorption on Carbon Nanotube-Fullerene Hybrid System: A First-Principles Study

The lithium (Li) adsorption mechanism on the metallic (5,5) single wall carbon nanotube (SWCNT)-fullerene (C(60)) hybrid material system is investigated using first-principles method. It is found that the Li adsorption energy (-2.649 eV) on the CNT-C(60) hybrid system is lower than that on the peapod system (-1.837 eV) and the bare CNT (-1.720 eV), indicating that the Li adsorption on the CNT-C(60) hybrid system is more stable than on the peapod or bare CNT system. This is due to the C(60) of high electron affinity and the charge redistribution after mixing CNT with C(60). In order to estimate how efficiently Li can utilize the vast surface area of the hybrid system for increasing energy density, the Li adsorption energy is calculated as a function of the adsorption positions around the CNT-C(60) hybrid system. It turns out that Li preferably occupies the mid-space between C(60) and CNT and then wraps up the C(60) side and subsequently the CNT side. It is also found that the electronic properties of the CNT-C(60) system, such as band structure, molecular orbital, and charge distribution, are influenced by the Li adsorption as a function of the number of Li atoms. From the results, it is expected that the CNT-C(60) hybrid system has enhanced the charge transport properties in addition to the Li adsorption, compared to both CNT and C(60).

Edge-selenated graphene nanoplatelets as durable metal-free catalysts for iodine reduction reaction in dye-sensitized solar cells

The I3− reduction mechanism on carbon-based materials is clarified by electrochemical kinetics and DFT-NEGF calculations. Metal-free carbon-based electrocatalysts for dye-sensitized solar cells (DSSCs) are sufficiently active in Co(II)/Co(III) electrolytes but are not satisfactory in the most commonly used iodide/triiodide (I−/I3−) electrolytes. Thus, developing active and stable metal-free electrocatalysts in both electrolytes is one of the most important issues in DSSC research. We report the synthesis of edge-selenated graphene nanoplatelets (SeGnPs) prepared by a simple mechanochemical reaction between graphite and selenium (Se) powders, and their application to the counter electrode (CE) for DSSCs in both I−/I3− and Co(II)/Co(III) electrolytes. The edge-selective doping and the preservation of the pristine graphene basal plane in the SeGnPs were confirmed by various analytical techniques, including atomic-resolution transmission electron microscopy. Tested as the DSSC CE in both Co(bpy)32+/3+ (bpy = 2,2′-bipyridine) and I−/I3− electrolytes, the SeGnP-CEs exhibited outstanding electrocatalytic performance with ultimately high stability. The SeGnP-CE–based DSSCs displayed a higher photovoltaic performance than did the Pt-CE–based DSSCs in both SM315 sensitizer with Co(bpy)32+/3+ and N719 sensitizer with I−/I3− electrolytes. Furthermore, the I3− reduction mechanism, which has not been fully understood in carbon-based CE materials to date, was clarified by an electrochemical kinetics study combined with density functional theory and nonequilibrium Green’s function calculations.

Effect of temperature on structure and water transport of hydrated sulfonated poly(ether ether ketone): A molecular dynamics simulation approach

The effects of temperature on hydrated sulfonated poly(ether ether ketone) are studied using molecular dynamics. Three different temperature conditions (298 K.15 K, 323.15 K, and 353.15 K) with two different water contents (10 wt. % and 20 wt. %) are simulated. Analyzing the pair correlation functions, it is found that there is limited temperature effect on the distribution and solvation of the sulfonate groups. The structure factor analysis shows that the temperature dependence of the nanophase-segregated morphology is not significant in the simulated temperature range. On the contrary, the structure factors S(q) at ∼30 A (q = ∼0.2 A−1) and ∼13 A (q = ∼0.5 A−1) clearly increase with water content, indicating that the development of water channels is mostly affected by the water content. Within such water phase in the nanophase-segregated structure, the internal structure of water phase becomes more developed with decreasing temperature and increasing water content. By analyzing the mean square displaceme...

Li adsorption on a graphene–fullerene nanobud system: density functional theory approach

In this study, we investigated the mechanisms of Li adsorption on a graphene–C60 nanobud system using density functional theory. Li adsorption on the hybrid system was enhanced compared to those using pure graphene and C60. The Li adsorption energies ranged from −1.784 to −2.346 eV for the adsorption of a single Li atom, and from −1.905 to −2.229 eV for the adsorption of two Li atoms. Furthermore, adsorption energies were similar at most positions throughout the structure. The Li adsorption energy of an 18-Li adsorbed system was calculated to be −1.684 eV, which is significantly lower than Li–Li binding energy (−1.030 eV). These results suggest that Li atoms will be adsorbed preferentially (1) between C60 and C60, (2) between graphene and C60, (3) on graphene, or (4) on C60, rather than form Li clusters. As more Li atoms were adsorbed onto the graphene–C60 nanobud system because of its improved Li adsorption capability, the metallic character of the system was enhanced, which was confirmed via analysis of band structure and electronic density of states.

Interactions of Pt nanoparticles with molecular components in polymer electrolyte membrane fuel cells: multi-scale modeling approach

In this study, a three-phase interfacial system of a fuel cell is simulated using a multi-scale simulation approach consisting of quantum mechanical density functional theory and molecular dynamics simulations. Through these simulations, the structural and transport properties of the three-phase system are investigated. The molecular interactions among the components of the three-phase interfacial system are examined by density functional theory and parameterized for potential energy functions of force field. First, we investigate the interactions of the Pt clusters with various molecules as a function of distance using the density functional theory method with dispersion correction. Based on the results of these calculations, a non-bonded interaction curve is built for each Pt–molecule pair. Such non-bonded interaction curves are reproduced by potential energy functions with optimized parameters. Based on these investigations, we develop a force field to describe the structures and transport properties of the Nafion–Pt–carbon (graphite) three-phase interfacial system using molecular dynamics simulations.

Effect of Temperature on Water Molecules in a Model Epoxy Molding Compound: Molecular Dynamics Simulation Approach

The effect of temperature on the distribution and transport of water molecules in a model epoxy molding compound (EMC) system is investigated using atomistic molecular dynamics simulation with 4 and 7 wt% water content at various temperatures, such as 298, 323, 353, and 373 K. The thermal expansion of the hydrated model EMC was evaluated as 1-5% of its dried volume with increasing temperature. The spatial distributions of the amine groups and hydroxyl groups are not significantly affected by temperature due to the crosslinked topological constraint. The correlation of these functional groups with water molecules was not affected by temperature due to their hydrophilicity. In contrast, it is observed that the water phase is expanded with increasing temperature, which is more distinct as a function of water content. The temperature effect on the water diffusion was clearly observed: the diffusion coefficient became larger with increasing temperature. The activation energy for the water diffusion via a hopping mechanism was 21.9 kJ/mol (0.23 eV) and 21.2 kJ/mol (0.22 eV) for the 4 wt% and the 7 wt% water contents, respectively, which infers that the water transport is more facilitated with increasing water content because the water structure of the water phase in the model EMC is more developed.

Enhanced Selectivity for CO2 Adsorption on Mesoporous Silica with Alkali Metal Halide due to Electrostatic Field: A Molecular Simulation Approach

Since adsorption performances are dominantly determined by adsorbate-adsorbent interactions, accurate theoretical prediction of the thermodynamic characteristics of gas adsorption is critical for designing new sorbent materials as well as understanding the adsorption mechanisms. Here, through our molecular modeling approach using a newly developed quantum-mechanics-based force field, it is demonstrated that the CO2 adsorption selectivity of SBA-15 can be enhanced by incorporating crystalline potassium chloride particles. It is noted that the induced intensive electrostatic fields around potassium chloride clusters create gas-trapping sites with high selectivity for CO2 adsorption. The newly developed force field can provide a reliable theoretical tool for accurately evaluating the gas adsorption on given adsorbents, which can be utilized to identify good gas adsorbents.

Effect of Uniaxial Deformation on Structure and Transport in Hydrated Nafion 117: Molecular Dynamics Simulation Study

The deformation of hydrated Nafion 117 was implemented using full-atomistic molecular dynamics simulation method to elucidate how the mechanical deformation affects the structure and transport of hydrated Nafion membrane. First, Nafion 117 membrane was equilibrated with 20 wt. % water content through an annealing procedure. The simulated characteristic correlation length and the diffusion coefficient of water and hydronium ions were analyzed for comparison with those observed in experiments. Then, the equilibrated Nafion membrane was deformed uniaxially up to 300 % of strain with a constant strain rate. The change in nanophase-segregation of hydrated Nafion during the deformation process was characterized using a directional structure factor as well as the pair correlation function in order to achieve fundamental understanding of the relationship of such structural change as a function of strain with the proton transport. It was found from the pair correlation analysis that the sulfonate distribution and sulfonate-hydronium correlation became stronger through the deformation while the hydronium ion solvation and the internal structure of water phase were not dependent on the deformation. From the directional structure factor profile, it was found that the long range correlation was developed in the perpendicular direction to the extension. The diffusions of water and hydronium ions were enhanced by 30 and 2 %, respectively, after the deformation. From this study, we suggested that it is desirable to investigate the proton transport using simulation methods covering larger dimensions with a long time scale.