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Metal–fluoride possesses a high potential as new high-performance water oxidation catalysts due to a highly polarized electronic configuration. However, low conductivity, related to high iconicity ...

MXenes have potential as a hydrogen evolution reaction (HER) catalyst based on their large surface area and excellent electrical properties. In this study, the change in HER catalytic properties according to the surface state of Ti2C-based MXene was analyzed theoretically based on first principles calculation. It was confirmed that the reactivity of the surrounding active sites is enhanced by the influence of isolated fluorine. The mechanism of this phenomenon was investigated, focusing on the electron migration and structural changes. In the MXene family, the Ti2C-based MXene best utilized the isolated fluorine effect. It was found that Ti2CO2 functionalized with a small amount of fluorine was superior as a HER catalyst to Ti2CO2 fully functionalized with oxygen, and the optimal proportion of fluorine was 1.5 to 3%.

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.

Transition metal layered double hydroxides (LDHs) have received much attention as high-performance oxygen evolution reaction (OER) catalysts due to their large number of active sites with favorable adsorption/desorption energies for intermittent reactants. However, the relatively sluggish charge transfer kinetics of transition metal LDHs due to their intrinsically low conductivity often hinders their use in practical applications as high-performance water oxidation catalysts. Here, we disclose a novel strategy of metalloid incorporation into transition metal LDHs, allowing us to simultaneously optimize surface electronic configuration and charge transfer between adsorbed reactants and catalyst surface. Importantly, incorporated metalloid can enhance the density of states (DOS) near the Fermi level and alter the nature of the chemical bonds in the catalytically active atoms, resulting in fast reaction kinetics. Thus, metalloid incorporation into transition metal LDHs can substantially improve the overall reaction kinetics and thermodynamics for water oxidation due to a large number of active sites and high conductivity, boosting OER performance of transition metal LDHs. The metalloid-incorporated transition metal LDHs far outperform their counterpart transition metal LDHs and even the noble metal catalyst RuO2.

Herein, as a binder (or catalyst ionomer) for AEMFCs, we investigated the effect of two different cationic copolymers based on poly(phenylene oxide) (PPO) with N-cyclic quaternary ammonium (QA) groups, including six-membered dimethyl piperidinium (DMP) and bis-six-membered azonia-spiro undecane (ASU). An earlier report on the same polymers for membranes in AEMFCs indicated the better electrochemical performance of PPO-ASU compared with PPO-DMP. Therefore, we would like to investigate these two polymers for catalyst ionomers. The outcome in this study using these two copolymers as catalyst ionomers indicates the opposite result; the electrochemical performance of the PPO-DMP ionomer is much better than the PPO-ASU ionomer. The commercial Fumion ionomer was used for the qualitative comparison. The density functional theory (DFT) calculation of the adsorption energy according to different orientations of the cationic groups on the catalyst surface shows that there is no difference between the adsorption energy of DMP and ASU cations, in compliance with the orientations of the cations. Although the PPO-ASU ionomer membrane has the highest hydroxide conductivity at 60 °C in liquid water, the hydrogen oxidation/reduction (HOR) activity of PPO-DMP and PPO-ASU showed similar values with the Fumion ionomer. While the PPO-DMP ionomer membrane shows relatively large fuel gas (hydrogen) permeability in dry and wet conditions, due to the chain flexibility and the presence of two methyl groups compared to the single methyl groups and lower flexibility of the PPO-ASU and Fumion ionomers. The electrochemical performance of a membrane electrode assembly (MEA) using the PPO-DMP ionomer exhibited an exceptional peak power density of 335 mW cm−2 compared to lower peak power densities the of PPO-ASU and Fumion ionomers under 60 °C and a fully humidified condition (H2/O2). The SEM images of MEAs after testing supports the conclusion that the PPO-DMP ionomer forms a uniform catalyst interface that is very well bound between the electrode and membrane, unlike the PPO-ASU and Fumion ionomers. The PPO-DMP ionomer membrane also showed better tensile strength and elongation at break than the PPO-ASU ionomer membrane. Therefore, we conclude that the well-prepared three-phase boundary structure played a critical role for the catalyst ionomer in each electrode, overcoming one of the critical performance-limiting factors.

Charge separation is the most important factor in determining the photocatalytic activity of a 2D/2D heterostructure. Despite the exclusive advantages of 2D/2D heterostructure semiconductor systems such as large surface/volume ratios, their use in photocatalysis is limited due to the low efficiency of charge separation and high recombination rates. As a remedy for the weak interlayer binding and low carrier transport efficiency in 2D/2D heterojunctioned semiconductors, we suggested an impurity intercalation method for the 2D/2D interface. PtS2/C3N4, as a prototype heterojunction material, was employed to investigate the effect of anion intercalation on the charge separation efficiency in a 2D/2D system using density functional theory. With oxygen intercalation at the PtS2/C3N4 interface, a reversed and stronger localized dipole moment and a built-in electric field were induced in the vertical direction of the PtS2/C3N4 interface. This theoretical work suggests that the anion intercalation method can be a way to control built-in electric fields and charge separation in designs of 2D/2D heterostructures that have high photocatalytic activity.

As a strategy to improve the catalytic performances of two-dimensional transition metal dichalcogenide materials, doping and defect engineering are widely used. However, it is extremely challenging to find proper new materials and doping/defect engineering conditions relying only on experimental trial-and-error. In this study, use of vanadium disulfide (VS2) was suggested as an effective cathode catalyst for Li-air batteries (LABs), under the condition of proper doping engineering to optimize electrochemical performances. To investigate the dopant screening and doping processing optimization of VS2, a theoretical concept, which combines first-principles calculations and thermodynamic modeling, was first derived and suggested. Through our approach, it is worth noting that F- and N-doping on H-VS2 shows superior overpotential properties (0.68 V and 0.76 V) than the carbon-based cathode and NH3 and O2 as reactant for T-VS2, and N2 and H2 for H-VS2 are necessary to control the N doping on VS2. Our theoretical work provides the guideline for application of VS2 to cathode of LABs and leads to further insights for designing a new cathode materials based on two-dimensional materials.

Transmission electron microscopy (TEM) and photocurrent (PC) measurements were carried out to investigate the microstructural properties and excitonic transitions in InxGa1−xAs/In0.52Al0.48As multiple quantum wells (MQWs) for x = 0.54, 0.57 and 0.60. TEM images showed that high-quality 11-period InxGa1−xAs/In0.52Al0.48As MQWs had high-quality heterointerfaces. The results for the PC spectra at 300 K showed that the peaks corresponding to the excitonic transitions from the ground state electronic sub-band to the ground state heavy-hole band (E1-HH1) and the ground state electronic sub-band to the ground state light-hole band (E1-LH1) became closer to each other with decreasing In mole fraction and that E1-HH1 and E1-LH1 excitonic peaks shifted to longer wavelength with increasing applied electric field. The calculated values of the E1-HH1 interband transition energies were in qualitative agreement with those obtained form the PC measurements with and without applied electric field. These results can be helpful in understanding potential applications of InxGa1−xAs/InyAl1−yAs MQWs dependent on In mole fraction and applied electric field in long-wavelength optoelectronic devices.

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.

The optical gains of the CdTe/ZnTe single quantum wells with various CdTe widths were calculated by using a non-interacting pair Green's function and by an energy space integrated function. The interband transition energies from the ground electronic subband to the ground heavy-hole subband calculated taking into account optical gains were in qualitatively reasonable agreement with those determined from the PL spectra.