Seung Geol Lee

A molecular dynamics simulation study of hydrated sulfonated poly(ether ether ketone) for application to polymer electrolyte membrane fuel cells: Effect of water content

Sulfonated poly(ether ether ketone) (S-PEEK) with 40% of degree of sulfonation was studied using full atomistic molecular dynamics simulation in order to investigate the nanophase-segregated structures, focusing on the sulfonate group and water phase at various water contents such as 10, 13, and 20 wt %. By analyzing the pair correlation function, it is found that as the water solvation of sulfonate groups proceeds more with increasing water content, the distance between sulfonate groups is increased from 4.4 A (10 wt %) to 4.8 A (13 wt %) to 5.4 A (20 wt %), and the hydronium ions (H3O+) become farther apart from the sulfonate groups. The water coordination number for water and the water diffusion are enhanced with increasing water content because the internal structure of the water phase in S-PEEK approaches that of bulk water. Compared to the Nafion and Dendrion membranes, the S-PEEK membrane shows less internal structure in the water phase and smaller water diffusion, indicating that the S-PEEK has less nanophase segregation than the Nafion and Dendrion membranes.Sulfonated poly(ether ether ketone) (S-PEEK) with 40% of degree of sulfonation was studied using full atomistic molecular dynamics simulation in order to investigate the nanophase-segregated structures, focusing on the sulfonate group and water phase at various water contents such as 10, 13, and 20 wt %. By analyzing the pair correlation function, it is found that as the water solvation of sulfonate groups proceeds more with increasing water content, the distance between sulfonate groups is increased from 4.4 A (10 wt %) to 4.8 A (13 wt %) to 5.4 A (20 wt %), and the hydronium ions (H3O+) become farther apart from the sulfonate groups. The water coordination number for water and the water diffusion are enhanced with increasing water content because the internal structure of the water phase in S-PEEK approaches that of bulk water. Compared to the Nafion and Dendrion membranes, the S-PEEK membrane shows less internal structure in the water phase and smaller water diffusion, indicating that the S-PEEK has le...

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.

Large Scale Synthesis and Light Emitting Fibers of Tailor-Made Graphene Quantum Dots

Graphene oxide (GO), which is an oxidized form of graphene, has a mixed structure consisting of graphitic crystallites of sp2 hybridized carbon and amorphous regions. In this work, we present a straightforward route for preparing graphene-based quantum dots (GQDs) by extraction of the crystallites from the amorphous matrix of the GO sheets. GQDs with controlled functionality are readily prepared by varying the reaction temperature, which results in precise tunability of their optical properties. Here, it was concluded that the tunable optical properties of GQDs are a result of the different fraction of chemical functionalities present. The synthesis approach presented in this paper provides an efficient strategy for achieving large-scale production and long-time optical stability of the GQDs, and the hybrid assembly of GQD and polymer has potential applications as photoluminescent fibers or films.

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...

Synthesis of B-doped graphene quantum dots as a metal-free electrocatalyst for the oxygen reduction reaction

Boron-doped graphene quantum dots (BGQDs) have been synthesized by a one-step, facile and low temperature method through the hydrothermal treatment of glucose as the precursor in the presence of boric acid. The as-obtained BGQDs possess a high B-doping content up to 4.25% of uniform nm-size. Interestingly, the effect of different types of B–C bond species on the ORR catalytic activity has been investigated to clarify the origin of the electrochemical reduction of O2. Further, the composite of the reduced graphene oxide (rGO) and BGQD (G-BGQDs) was also prepared as a metal-free electrocatalyst for the oxygen reduction reaction (ORR). The G-BGQD composites exhibit a significantly enhanced electrocatalytic activity, including a positive onset potential and a high current density with a one step, four-electron pathway toward the ORR, comparable to the commercial Pt/C catalyst. Among various B–C bond structures in BGQDs, the graphite-like BC3 structure is considered to be an important site for the ORR by improving the electric conductivity and electrocatalytic activity of BGQDs, which is also confirmed by a DFT study. In addition, the G-BGQD composites show an outstanding long-term operational stability and high tolerance to the methanol crossover effect, which are comparable to the commercial Pt/C catalyst. These results demonstrate that the synthesized BGQD, as metal-free catalyst materials, may be inexpensive and efficient electrocatalysts for the replacement of Pt-based catalysts toward the ORR and other electrochemical applications.

Effect of Monomeric Sequence on Mechanical Properties of P(VP-co-HEMA) Hydrogels at Low Hydration

We have used molecular modeling of both random and blocky hydrogel networks of poly (N-vinyl-2-pyrrolidone-co-2-hydroxyethyl methacrylate) with VP:HEMA=37:13 composition to investigate the effect of the monomeric sequence on the mechanical properties. The degrees of monomer sequence randomness for the random and the blocky copolymers were 1.170 and 0.104, respectively, and the degree of polymerization was set as 50. The equilibrated density of the dry gel network was 0.968+/-0.007 and 0.911+/-0.007 g/cm3 for the random and the blocky sequences, respectively. In the partially hydrated state with 10 wt % water content, the effect of the monomeric sequence causes more distinct differences in density of 1.004+/-0.007 and 0.916+/-0.009 g/cm3 for the random and the blocky copolymer network, respectively. We observed that in such networks, the water molecules are associated more closely with the N-vinyl-2-pyrrolidone than with the hydroxyethyl methacrylate moieties, which is consistent with results from quantum mechanical solvation free energy calculations. By simulating a compressive deformation of the dry gels up to 80% strain, we found that the random sequence network develops higher stress levels than the blocky network. We also found that stress reduction occurs in the random sequence network due to the hydration, which is not evident in the blocky sequence network. This difference in stress reduction between the random and the blocky sequence networks is due to the difference in the structural rearrangement of monomers in the presence of water during deformation. The random sequence network is able to undergo much more efficient rearrangement of HEMA units than in the blocky sequence network.

A First-Principles Study of Lithium Adsorption on a Graphene–Fullerene Nanohybrid System

The mechanism of Li adsorption on a graphene-fullerene (graphene-C60 ) hybrid system has been investigated using density functional theory (DFT). The adsorption energy for Li atoms on the graphene-C60 hybrid system (-2.285 eV) is found to be higher than that on bare graphene (-1.375 eV), indicating that the Li adsorption on the former system is more stable than on the latter. This is attributed to the high affinity of Li atoms to C60 and the charge redistribution that occurs after graphene is mixed with C60 . The electronic properties of the graphene-C60 system such as band structure, density of states, and charge distribution have been characterized as a function of the number of Li atoms adsorbed in comparison to those of the pure graphene and C60 . Li adsorption is found to preferentially occur on the C60 side due to the high adsorption energy of Li on C60 , which imparts a metallic character to the C60 in the graphene-C60 hybrid system.

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.

Distribution and Diffusion of Water in Model Epoxy Molding Compound: Molecular Dynamics Simulation Approach

The distribution and diffusion of water with various water content in a fully crosslinked epoxy molding compound was simulated using a parallel full-atomistic molecular dynamics simulation method. We found that the free volume is 5.1%, 4.4%, and 4.0% of the total system volume at 0 wt%, 4 wt%, and 7 wt% of water content, respectively, accommodating the absorbed water molecules, where the molecules are distributed throughout the system. The hydrophilic groups of the epoxy molding compound (such as tertiary amine groups and hydroxyl groups) are uniformly distributed through the system: the average distance between the amine groups is ~9.5 ¿ and that between the hydroxyl groups is 3.8-7.2 ¿. The water molecules are distributed in proximity to these hydrophilic groups. By counting the number of these water molecules nearby the functional groups, we found that on average, each amine group has 2.47 and 3.86 water molecules, and each hydroxyl group has 0.61 and 0.85 water molecules at 4 wt% and 7 wt% water content, respectively. The water diffusion proceeds via the hopping mechanism and is enhanced with increasing water content: 0.1690 × 10-6 cm 2/s for 4 wt% water content and 0.2065 × 10-6 cm2/s for 7 wt% water content.

An Ultrasensitive, Visco‐Poroelastic Artificial Mechanotransducer Skin Inspired by Piezo2 Protein in Mammalian Merkel Cells

An artificial ionic mechanotransducer skin with an unprecedented sensitivity over a wide spectrum of pressure by fabricating visco-poroelastic nanochannels and microstructured features, directly mimicking the physiological tactile sensing mechanism of Piezo2 protein is demonstrated. This capability enables voice identification, health monitoring, daily pressure measurements, and even measurements of a heavy weight beyond capabilities of human skin.