Effect of lithium-trapping on nitrogen-doped graphene as an anchoring material for lithium-sulfur batteries: a density functional theory study.

N-Doped graphene (NG) has been widely used as a cathode material for lithium-sulfur (Li-S) batteries due to its strong interaction with lithium polysulfide (LiPS) species. However, strong interaction between the NG substrate and the LiPS molecules induces undesirable molecular structure decomposition of LiPS. Due to the strong interaction between Li and NG, Li-trapping occurs during battery operation. Therefore, in this study, Li-trapped NG (LiNG) is introduced as a possible structure of NG, and the structural stability of LiNG under applied electric potential is examined. The effect of Li-trapping on the properties of NG as an anchoring material for Li-S batteries is investigated using density functional theory calculations. Li-trapping relieves the strong interaction between NG and LiPS, thereby avoiding decomposition of the LiPS molecule. Although the interaction between the LiPS molecule and the substrate is weakened, additionally formed interaction after Li-trapping, which is between Li in the substrate and S in the molecule, enables LiNG to suppress the shuttle effect. LiNG shows advanced anchoring behavior that suppresses the shuttle effect without any molecular decomposition of LiPS. This finding provides a further understanding of the effect of Li-trapping on the anchoring properties of NG for Li-S batteries.

Functionalization effect on a Pt/carbon nanotube composite catalyst: a first-principles study.

Chemical interactions between Pt and both pristine and defective carbon nanotubes (CNTs) that were functionalized with various surface functional groups, including atomic oxygen (-O), atomic nitrogen (-N), hydroxyl (-OH) and amine (-NH2) groups, were investigated through first-principles calculations. Our calculations suggest that the oxygen or nitrogen of the surface functional group can promote better structural stability of a Pt/CNT complex in terms of the binding energy enhancement between Pt and CNTs. Enhanced binding of the Pt/CNT complex would improve the long-term durability of the complex and thus enhance the catalytic activity of Pt catalysts supported on CNTs. Among the functional groups investigated, atomic nitrogen resulted in the most consistent increase in the Pt binding energies on pristine or defective CNTs. Moreover, atomic nitrogen decoration on the surface of CNTs rather than substitution into the CNTs appears to be more desirable. A d-band centre analysis and H2 adsorption calculations also revealed that the catalytic activity of Pt can be improved via efficient functionalization of the CNT support.

Electronic properties of transition-metal-decorated silicene.

The electronic properties of 3d transition metal (TM)-decorated silicene were investigated by using density functional calculations in an attempt to replace graphene in electronic applications, owing to its better compatibility with Si-based technology. Among the ten types of TM-doped silicene (TM-silicene) studied, Ti-, Ni-, and Zn-doped silicene became semiconductors, whereas Co and Cu doping changed the substrate to a half-metallic material. Interestingly, in cases of Ti- and Cu-doped silicene, the measured band gaps turned out to be significantly larger than the previously reported band gap in silicene. The observed band-gap openings at the Fermi level were induced by breaking the sublattice symmetry caused by two structural changes, that is, the Jahn-Teller distortion and protrusion of the TM atom. The present calculation of the band gap in TM-silicene suggests useful guidance for future experiments to fabricate various silicene-based applications such as a field-effect transistor, single-spin electron source, and nonvolatile magnetic random-access memory.

Achieving a direct band gap in oxygen functionalized-monolayer scandium carbide by applying an electric field.

In the present paper, the band gap characteristics of oxygen functionalized-monolayer scandium carbide (monolayer Sc2CO2) under a perpendicular external electric field (E-field) were studied using DFT calculations for the potential application of MXene in optoelectronic and optical nanodevices. In contrast to general pristine single-layer materials under an external E-field, monolayer Sc2CO2 undergoes an indirect to direct band gap transition under a positive E-field, and the band gap value changes sharply after the band gap transition. Remarkable variations of the band gap properties are induced by the distinct sensitivity between the Γ and K points in the lowest conduction band to the perpendicular E-field, and different types of orbital lead to the dissimilar response of each point. The present work clearly suggests an effective direction to obtain attractive band gap properties in monolayer MXene using an external E-field for next generation optoelectronic and optical devices.

Defect-induced semiconductor to metal transition in graphene monoxide.

This study investigates the influence of point defects on the geometric and electronic structure of graphene monoxide (GMO) via density functional theory calculations. In aspects of defect formation energy, GMOs with oxygen vacancies and bridge interstitial defects are more likely to form when compared to GMOs with defects such as carbon vacancies and hollow interstitial defects. It was also found that the oxygen vacancy or the hollow interstitial defect induces local tensile strain around the defective site and this strain increases the band gap energy of the defective GMO. In addition, the band gaps of GMO with carbon vacancies or bridge interstitial defects decreased mainly due to the dangling bonds, not due to the strain effect. It is noted that the dangling bond derived from the defects forms the defect-level in the band gap of GMO. The semiconductor to metal transition by the band gap change (0-0.7 eV) implies the possibility for band gap engineering of GMO by vacancies and interstitial defects.

Electric field effect on the magnetic property of Mn adatom on graphene: Ab-initio calculations.

Changes in the magnetic property of the Mn transition metal on graphene were observed using density functional calculations (DFT) where the Mn/graphene system was enforced by an external electric field. The magnetic moment of the Mn adatom on graphene showed continuous changes as a result of the external field. Analysis of the charge redistribution of the system revealed that the electrons are partially transferred between graphene and the Mn adatom from the effect of the external electric field. According to the density of states (DOS) data, the transferred charge originates from the electrons in 3d spin down states. In this study, it was found that the external electric field affected the changes in the electronic structure of the outermost shell of the Mn adatom, and this change resulted in change in the magnetic moment.

Enhanced hydrogen storage properties under external electric fields of N-doped graphene with Li decoration.

In this article, the imposition of an external electric field is proposed as an effective means to improve the hydrogen storage properties of a promising medium. To demonstrate the feasibility of this concept, the geometric stability and hydrogen capacity of Li functionalized N-doped graphene were investigated in the presence of an electric field using density functional theory (DFT) calculations. For Li decorated pristine and graphitic structures, the binding energy of the Li atom on the surface sheets exceeded the cohesive energy of the Li metal bulk under a positive electric field. From these results, Li adatom dispersion with atomic accuracy is expected for these two unstable structures. Furthermore, the hydrogen adsorption behavior of the pyridinic and pyrrolic structures was changed by the applied electric field in the range of 0.14-0.27 eV. It is therefore anticipated that the adsorption and desorption processes can be easily controlled using suitable field strength and direction.

Strain effects on hydrogen storage in Ti decorated pyridinic N-doped graphene.

Strain-engineered adsorption of Ti on pyridinic nitrogen-doped graphene (PNG) and the hydrogen storage characteristics of Ti-decorated PNG are examined by using a first-principles approach using density functional theory (DFT) calculations. Under the strain from -5% to 5%, binding energy (Eb) of Ti on PNG was higher than cohesive energy of the Ti bulk. Thus, it is expected that Ti atoms prefer atomic dispersion in PNG to clustering in the applied strain range. For this Ti-PNG system, the Eb variation of the second and third adsorbed H2 molecule according to the strain was a large value of 0.217 and 0.254 eV, respectively. Therefore, strain-engineered Ti-decorated PNG is adaptable to diverse operation conditions of hydrogen storage systems for mobile applications. In addition, by applying compressive strain, this system can adsorb the fourth H2 molecule, suggesting that the compressive strain can be used to improve hydrogen storage capacity. Thus, it can be expected that strain-engineered Ti-decorated PNG can be considered to be a promising potential hydrogen storage medium.

Stable surface structures and magnetic properties of L1(0)-ordered CoPt thin films according to thin film layer thickness.

Magnetic thin films are expected to be a promising substitute for existing semiconductor materials in the memory device industry due to their stable non-volatility, fast reading/writing speed, and durability. Because of the potential of thin film magnetic materials, Co-Pt alloys have attracted attention for application in magnetic memory devices due to their great magnetic anisotropy energy. In this paper, the stable surface structures of L1(0)-ordered CoPt alloys on a Pt (001) surface according to the thickness of the CoPt thin films was investigated using density functional calculations. The surface phase diagram of the Co-Pt alloys was first obtained to find the most stable surface phases of Co-Pt, and finally the perpendicular A, Pt-rich, perpendicular C and Co-rich B phases were found to be the most stable Co-Pt surface phases considering a thin film thickness from 1 to 4 MLs. Through calculation of the magnetic properties and the analysis of the spin-polarized 3d-electron density of the states of these stable surface phases, the changes in the magnetic properties were found to originate from the change in the relative electron filling in the 3d(x2-y2) and 3d(z2) orbitals of the Co atoms.

Effects of in-plane magnetization orientation on magnetic and electronic properties in a Bcc Co (001)/rock salt MgO (001)/Bcc Co (001) magnetic tunnel junction system: ab initio calculations.

Ab initio calculations were performed on a fully epitaxial bcc Co (001)/rock salt MgO (001)/bcc Co (001) magnetic tunnel junction system for two cases where the magnetization is parallel to bcc Co [100] and to bcc Co [110]. Structural optimization reveals that the two cases are equivalent systems and that the Co electrodes contract in the z-direction whereas the MgO insulating barrier expands. The magnetic moments of each monolayer vary slightly in each case; furthermore, only the magnetic moment at the surface of the Co atom shows any enhancement (12%). The layer decomposed density of states profiles reveals that the bonding character of the junction interface is derived mainly from the 2p-3d hybridization of the MgO and Co interfacial atoms.

Magnetic anisotropy variation of Fe single atom on Ti/Al(001) surface by the change of Ti-Al surface phase.

Using density functional theory based ab initio calculations, we investigated the effects of Ti/Al(001) surface phase variation on the Fe adatom magnetism. The symmetry of the in-plane magnetic anisotropy of the Fe adatomcorresponded to the symmetry of the Ti and Al atomic configurations on the top surface. When B2 or L1(2) structures of Ti and Al atoms were formed on the surface, the energy barriersfor the Fe in-plane magnetization rotations were smaller than the case of the bare Al(001) surface. The out-of-plane magnetization of Fe adatoms were induced only on the Al-terminated substrates while the Fe on the Ti-appearing surface had its magnetic easy axis in the in-plane directions. The magnetic anisotropy energy magnitude was, on the other hand, largely determined by the underlayer composition of Ti-Al alloy. The decomposed 3d-electron density of states showed that the 3d(xy) and 3d(z2) orbitals of Fe adatoms provide the main contribution to the variation of the magnetic anisotropy energy.

Stress-induced wurtzite to hexagonal phase transformation in zinc oxide nanowires.

The stress-induced wurtzite to hexagonal phase transformation in [0110] oriented zinc oxide nanowires were investigated using a molecular dynamics simulation and reactive force field potentials. The yield strength of the 2.13 x 1.93 nm wurtzite nanowires is 12 GPa at 50 K. The wurtzite to hexagonal phase transformation was successfully observed at stress plateaus (5-5.5 GPa at 50 K) located after the yield point of the wurtzite phase. The wurtzite to hexagonal phase transformation was a result of the propagation of {0111} twinning boundaries. During the phase transformation, the wurtzite and hexagonal phases were clearly separated by the {0111} twinning boundaries. To analyze the difference between ceramic and metallic systems, all the calculation data of wurtzite to hexagonal transformation were compared with stress-induced phase transformation in metallic nanowires such as CuZr and NiA1. As the result of the [0110] tensile loading of the ZnO nanowires, the hexagonal phase was obtained.

Metal (Li, Al, Ca and Ti) absorbed graphene with defects for hydrogen storage: first-principles calculations.

The characteristics of Li, Al, Ca and Ti metal adsorption on graphene with boron substitution and various vacancy defects are investigated using density functional theory calculation. Hydrogen adsorption characteristics and electronic structure of H2/metal adsorbed graphene were also calculated. It was found that Li, Al, Ca and Ti metal atoms are well dispersed on the graphene and can form a (2 x 2) pattern because clustering of metal atoms is hindered by the repulsive Coulomb interaction between the metal adatoms and the strong bonding force between the dispersed metal atom and graphene. Ca and Ti metal adatom show strong binding energy with the graphene in the cases of B substitution and specific vacancy, respectively. Ca and Ti were also found to be able to 8H2 molecules on the double side of the boron substituted graphene. This allows for a storage capacity of a 8.5 and 7.9 wt% hydrogen for Ca and Ti adatom, respectively.

Theoretical investigation of Ti-adsorbed graphene for hydrogen storage using the ab-initio method.

Based on first-principles plane wave calculations, it was shown that boron substituted graphene with Ti metal atom adsorption can be used as a high capacity hydrogen storage material. Boron substitution in graphene enhances the Ti metal adsorption energy, which is much larger than that in the case of pure graphene, and than the Ti cohesive energy. The Ti metal atom can be well dispersed on boron-substituted graphene and can form a 2 x 2 pattern because the clustering of the Ti atoms is hindered by the repulsive Coulomb interaction between them. The H2 adsorption behavior on Ti metal atoms was investigated, along with the H2 bonding characteristics and the open-metal states of Ti. It was found that one Ti adatom dispersed on the double sides of graphene can absorb up to eight H2 molecules, corresponding to a 7.9% hydrogen storage capacity. In addition, the adsorption behaviors of non-H2 atoms like C and B were calculated to determine if Ti atoms can remain in an open-metal state in boron-substituted graphene.

A comparative study of catalytic partial oxidation of methane over CeO2 supported metallic catalysts.

In the present study, the catalytic partial oxidation of methane (CPOM) over various active metals supported on CeO2 (M/CeO2, M = Ir, Ni, Pd, Pt, Rh and Ru) has been investigated. The catalysts were characterized by X-ray diffraction (XRD), BET surface area, H2-temperature programmed reduction (H2-TPR), CO chemisorption and transmission electron microscope (TEM) analysis. Ir/CeO2 catalysts showed higher BET surface area, higher metal dispersion, small active metal nano-particles (approximately 3 nm) than compared to other M/CeO2 catalysts. The catalytic tests were carried out in a fixed R(mix) ratio of 2 (CH4/O2) in a fixed-bed reactor, operating isothermally at atmospheric pressure. From time-on-stream analysis at 700 degrees C for 12 h, a high and stable catalytic activity has been observed for Ir/CeO2 catalysts. TEM analysis of the spent catalysts showed that the decrease in the catalytic activity of Ni/CeO2 and Pd/CeO2 catalysts is due to carbon formation whereas no carbon formation has been observed for Ir/CeO2 catalysts.

Theoretical Approach toward Optimum Anion-Doping on MXene Catalysts for Hydrogen Evolution Reaction: an Ab Initio Thermodynamics Study.

Developing highly active catalysts for hydrogen evolution reaction based on earth-abundant materials is challenging. Nitrogen doping has recently been reported to improve catalytic properties by modifying the electrochemical properties of titanium carbide MXene. However, systematic doping engineering, such as optimization of doping concentration, doping site, and thermodynamic phase stabilization have not been systematically controlled, which retards the reliable production of high-activity MXene catalysts. In this study, the optimum doping concentration of nitrogen and doping process conditions on O-functionalized Ti2C MXene for hydrogen evolution reaction were investigated using density functional theory with thermodynamics. To confirm the optimum nitrogen concentration, the catalytic properties are examined considering the Gibbs free energy of hydrogen adsorption and conductivity for 2.2-11.0 at % nitrogen concentration. It was confirmed that 8.8 at % nitrogen-doped Ti2CO2 had optimum catalytic properties under standard conditions. Moreover, when the doping concentration was higher, the decrease in the adsorption energies of hydrogen and the transition in the energy dispersion of the conduction band led to deterioration of the catalytic properties. Through theoretical results, the feasible process conditions for optimum nitrogen concentration while maintaining the structure of MXene are presented using a thermodynamics model taking into account chemical reactions with various nitrogen sources. This study provides further understanding of the nitrogen-doping mechanism of Ti2CO2 for hydrogen evolution reactions.

Poly(fluorenyl aryl piperidinium) membranes and ionomers for anion exchange membrane fuel cells.

Low-cost anion exchange membrane fuel cells have been investigated as a promising alternative to proton exchange membrane fuel cells for the last decade. The major barriers to the viability of anion exchange membrane fuel cells are their unsatisfactory key components-anion exchange ionomers and membranes. Here, we present a series of durable poly(fluorenyl aryl piperidinium) ionomers and membranes where the membranes possess high OH- conductivity of 208 mS cm-1 at 80 °C, low H2 permeability, excellent mechanical properties (84.5 MPa TS), and 2000 h ex-situ durability in 1 M NaOH at 80 °C, while the ionomers have high water vapor permeability and low phenyl adsorption. Based on our rational design of poly(fluorenyl aryl piperidinium) membranes and ionomers, we demonstrate alkaline fuel cell performances of 2.34 W cm-2 in H2-O2 and 1.25 W cm-2 in H2-air (CO2-free) at 80 °C. The present cells can be operated stably under a 0.2 A cm-2 current density for ~200 h.

Functionalized Sulfide Solid Electrolyte with Air-Stable and Chemical-Resistant Oxysulfide Nanolayer for All-Solid-State Batteries.

Sulfide solid electrolytes (SEs) with high Li-ion conductivities (σion) and soft mechanical properties have limited applications in wet casting processes for commercial all-solid-state batteries (ASSBs) because of their inherent atmospheric and chemical instabilities. In this study, we fabricated sulfide SEs with a novel core-shell structure via environmental mechanical alloying, while providing sufficient control of the partial pressure of oxygen. This powder possesses notable atmospheric stability and chemical resistance because it is covered with a stable oxysulfide nanolayer that prevents deterioration of the bulk region. The core-shell SEs showed a σion of more than 2.50 mS cm-1 after air exposure (for 30 min) and reaction with slurry chemicals (mixing and drying for 31 min), which was approximately 82.8% of the initial σion. The ASSB cell fabricated through wet casting provided an initial discharge capacity of 125.6 mAh g-1. The core-shell SEs thus exhibited improved powder stability and reliability in the presence of chemicals used in various wet casting processes for commercial ASSBs.

Interfacial spin polarization and magnetic structure of Co/MgO/Co magnetic tunnel junction: ab initio calculation.

Using ab initio method based on the density functional theory, the equilibrium bcc-Co(001)/rocksalt-MgO(001)/bcc-Co(001) magnetic tunnel junction structure was investigated. Spin polarization and magnetic moment were calculated for each atomic slab in the equilibrium structure by spin dependent density of states analysis. Interfacial Co atoms showed significantly larger spin polarization of -88.3%, compared to the value of inner Co slabs, -82.3%, and bulk bcc Co, -82.1%. Interestingly, Mg and O atoms also showed induced spin polarizability ranged from -45.0% to -66.0%, except for O atoms in the centered slab of barrier layer, which showed relatively small polarization, -14.9%. Magnetic moments for the electrode Co atoms were calculated to be approximately 1.74 microB with no significant variation across the electrode.

Band engineering in a van der Waals heterostructure using a 2D polar material and a capping layer.

Van der Waals (vdW) heterostructures are expected to play a key role in next-generation electronic and optoelectronic devices. In this study, the band alignment of a vdW heterostructure with 2D polar materials was studied using first-principles calculations. As a model case study, single-sided fluorographene (a 2D polar material) on insulating (h-BN) and metallic (graphite) substrates was investigated to understand the band alignment behavior of polar materials. Single-sided fluorographene was found to have a potential difference along the out-of-plane direction. This potential difference provided as built-in potential at the interface, which shift the band alignment between h-BN and graphite. The interface characteristics were highly dependent on the interface terminations because of this built-in potential. Interestingly, this band alignment can be modified with a capping layer of graphene or BN because the capping layer triggered electronic reconstruction near the interface. This is because the bonding nature is not covalent, but van der Waals, which made it possible to avoid Fermi-level pinning at the interface. The results of this study showed that diverse types of band alignment can be achieved using polar materials and an appropriate capping layer.