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.

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.

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.

Graphene Monoxide Bilayer As a High-Performance on/off Switching Media for Nanoelectronics.

The geometries and electronic characteristics of the graphene monoxide (GMO) bilayer are predicted via density functional theory (DFT) calculations. All the possible sequences of the GMO bilayer show the typical interlayer bonding characteristics of two-dimensional bilayer systems with a weak van der Waals interaction. The band gap energies of the GMO bilayers are predicted to be adequate for electronic device application, indicating slightly smaller energy gaps (0.418-0.448 eV) compared to the energy gap of the monolayer (0.536 eV). Above all, in light of the band gap engineering, the band gap of the GMO bilayer responds to the external electric field sensitively. As a result, a semiconductor-metal transition occurs at a small critical electric field (EC = 0.22-0.30 V/Å). It is therefore confirmed that the GMO bilayer is a strong candidate for nanoelectronics.

Achieving type I, II, and III heterojunctions using functionalized MXene.

In the present work, type I, II, and III heterostructures are constructed with the same base material using three representative functionalized monolayer scandium carbides (Sc2CF2, Sc2C(OH)2, and Sc2CO2) by first-principles calculations based on density functional theory. In contrast to general bilayer heterosystems composed of two-dimensional semiconductors, type I and III heterojunctions are obtained in one Sc2CF2/Sc2CO2 system and the remains of the functionalized Sc2C heterostructures, respectively. It is noteworthy that the same monolayer Sc2CF2 and Sc2CO2 constituents lead to dissimilar heterostructure types in the two Sc2CF2/Sc2CO2 systems by modifying the stacking interface. In addition, in the two Sc2CF2/Sc2CO2 systems, remarkable changes in the heterojunction type are induced by a strain, and two distinct type-II heterostructures are generated where one layer with the conduction band minimum state and the other layer including the valence band maximum level are different. The present work suggests an attractive direction to obtain all heterostructure types with the same base material for novel nanodevices in various fields such as photonics, electronics, and optoelectronics using only the two monolayer components Sc2CF2 and Sc2CO2.

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.

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.

Tunable indirect to direct band gap transition of monolayer Sc2CO2 by the strain effect.

MXene has not yet been investigated in optical applications because it is a newly suggested two-dimensional material. In the present work, the first investigation of the prospects of MXene as a novel optical nanodevice was done by applying strain to monolayer Sc2CO2 using first-principles density-functional theory. This single-layer material experiences an indirect to direct band gap transition with variation of the band gap size at a relatively small critical strain of about 2%. The present work emphasizes that monolayer MXene can become a promising material for an optical nanodevice by modulating the band gap properties using strain engineering.

Detecting gas molecules via atomic magnetization.

Adsorptions of gas molecules were found to alter the directions and magnitudes of magnetic moments of transition metal (Co, Fe) atoms adsorbed on graphene. Using first-principles calculations, we demonstrated that magnetism of surface atoms can be used to identify the kind of existing gas molecules via spin-reorientation and/or demagnetizations caused by the reconfigurations of 3d electron energy levels of Co and Fe. We suggest for the first time that magnetic properties of transition metal-embedded nanostructures can be used in highly selective gas-sensing applications.

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 bioactivity and osteoconductivity of hydroxyapatite through chloride substitution.

The effect of chloride-substitution on bioactivity and osteoconductivity of hydroxyapatite (OHAp) was newly investigated. Chloride-substituted hydroxyapatites (ClAp) with low and high chloride concentrations were synthesized by reacting Ca(OH)2 and H3 PO4 with NH4 Cl of low and high concentrations, with subsequent sintering. As a control, pure OHAp was prepared under the same conditions but without addition of NH4 Cl. The ClAp showed markedly enhanced bioactivity in simulated body fluid (SBF) as the chloride substitution was increased. In contrast, OHAp did not show any bioactivity at all within the testing period. The solubility tests in deionized water also showed that the higher the chloride-substituting amount, the higher the dissolution amounts of the constituent elements of apatite, which directly affect bioactivity by increasing the degree of supersaturation of apatite in SBF. In addition, ClAp also showed noticeably higher osteoconductivity within the 4 weeks of implantation in calvarial defects of New Zealand white rabbits, compared with that of OHAp. The total system energy of the apatite calculated by the ab initio method showed that the higher the chloride-substituting amount, the higher the total system energy, which suggests that the ClAp was energetically less stable compared with OHAp. This result demonstrates the higher solubility of ClAp over that of OHAp in SBF and deionized water. The improved solubility of the OHAp enhances its bioactivity and consequent osteoconductivity. Taken together, it can be concluded that ClAp has encouraging potential for use as a bone grafting material due to its highly enhanced bioactivity and osteoconductivity compared with pure OHAp.

Enhanced osteoconductivity of sodium-substituted hydroxyapatite by system instability.

The effect of substituting sodium for calcium on enhanced osteoconductivity of hydroxyapatite was newly investigated. Sodium-substituted hydroxyapatite was synthesized by reacting calcium hydroxide and phosphoric acid with sodium nitrate followed by sintering. As a control, pure hydroxyapatite was prepared under identical conditions, but without the addition of sodium nitrate. Substitution of calcium with sodium in hydroxyapatite produced the structural vacancies for carbonate ion from phosphate site and hydrogen ion from hydroxide site of hydroxyapatite after sintering. The total system energy of sodium-substituted hydroxyapatite with structural defects calculated by ab initio methods based on quantum mechanics was much higher than that of hydroxyapatite, suggesting that the sodium-substituted hydroxyapatite was energetically less stable compared with hydroxyapatite. Indeed, sodium-substituted hydroxyapatite exhibited higher dissolution behavior of constituent elements of hydroxyapatite in simulated body fluid (SBF) and Tris-buffered deionized water compared with hydroxyapatite, which directly affected low-crystalline hydroxyl-carbonate apatite forming capacity by increasing the degree of apatite supersaturation in SBF. Actually, sodium-substituted hydroxyapatite exhibited markedly improved low-crystalline hydroxyl-carbonate apatite forming capacity in SBF and noticeably higher osteoconductivity 4 weeks after implantation in calvarial defects of New Zealand white rabbits compared with hydroxyapatite. In addition, there were no statistically significant differences between hydroxyapatite and sodium-substituted hydroxyapatite on cytotoxicity as determined by BCA assay. Taken together, these results indicate that sodium-substituted hydroxyapatite with structural defects has promising potential for use as a bone grafting material due to its enhanced osteoconductivity compared with hydroxyapatite.

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.

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.

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.

Dominant factors governing the rate capability of a TiO2 nanotube anode for high power lithium ion batteries.

Titanium dioxide (TiO(2)) is one of the most promising anode materials for lithium ion batteries due to low cost and structural stability during Li insertion/extraction. However, its poor rate capability limits its practical use. Although various approaches have been explored to overcome this problem, previous reports have mainly focused on the enhancement of both the electronic conductivity and the kinetic associated with lithium in the composite film of active material/conducting agent/binder. Here, we systematically explore the effect of the contact resistance between a current collector and a composite film of active material/conducting agent/binder on the rate capability of a TiO(2)-based electrode. The vertically aligned TiO(2) nanotubes arrays, directly grown on the current collector, with sealed cap and unsealed cap, and conventional randomly oriented TiO(2) nanotubes electrodes were prepared for this study. The vertically aligned TiO(2) nanotubes array electrode with unsealed cap showed superior performance with six times higher capacity at 10 C rate compared to conventional randomly oriented TiO(2) nanotubes electrode with 10 wt % conducting agent. On the basis of the detailed experimental results and associated theoretical analysis, we demonstrate that the reduction of the contact resistance between electrode and current collector plays an important role in improving the electronic conductivity of the overall electrode system.