Phase transition and electrochemical properties of S-functionalized MXene anodes for Li-ion batteries: a first-principles investigation.

The advancement of anode materials for achieving high energy storage is a crucial topic for high-performance Li-ion batteries (LIBs). Here, first-principles calculations were used to conduct a thorough and systematic investigation into lithium storage properties of MXenes with new S functional groups as LIB anode materials. Density of states, diffusion energy barriers, open circuit voltages and storage capacities were calculated to comprehensively evaluate the lithium storage properties of S-functionalized MXenes. Based on the computational results, Ti2CS2 and V2CS2 were selected as excellent candidates from ten M2CS2 MXenes. The diffusion energy barriers of M2CS2 within the range of 0.26-0.32 eV are lower than those of M2CO2 and M2CF2, indicating that M2CS2 anodes exhibit faster charge/discharge rates. By examining the stable crystal structures and comparing atomic positions before and after Li adsorptions, structural phase transitions during Li-ion adsorptions could happen for nearly all M2CS2 MXenes. The phase transitions predicted were directly observed using ab initio molecular dynamic simulations. The cycle stability, storage capacity and other lithium storage properties were enhanced by the reversible structural phase transition.

First-principles study of hydrogen sulfide decomposition on Sc-Ti3C2O2 single-atom catalyst.

CONTEXT: Hydrogen sulfide gas poses significant risks to both human health and the environment, with the potential to induce respiratory and neurological effects, and a heightened fatality risk at elevated concentrations. This article investigates the catalytic decomposition of H2S on a Sc-Ti3C2O2 single-atom catalyst(SAC) using the density functional theory-based first-principles calculation approach. Initially, the adsorption behavior of H2S on Ti3C2O2-MXene was examined, revealing weak physical adsorption between them. Subsequently, the transition metal atom Sc was introduced to the Ti3C2O2 surface, and its stability was studied, demonstrating high stability. Further exploration of H2S adsorption on Sc-Ti3C2O2 revealed direct dissociation of H2S gas molecules into HS* and H*, with HS* binding to Sc and H* binding to O on the Ti3C2O2 surface, resulting in OH groups. Using the transition state search method, the dissociation of H2S molecules on the SAC's surface was investigated, revealing a potential barrier of 2.45 eV for HS* dissociation. This indicates that the H2S molecule can be dissociated into H2 and S with the action of the Sc-Ti3C2O2 SAC. Moreover, the S atom left on the catalyst surface can aggregate to produce elemental S8, desorbing on the catalyst surface, completing the catalytic cycle. Consequently, the Sc-Ti3C2O2 SAC is poised to be an efficient catalyst for the catalytic decomposition of H2S. METHODS: The Dmol3 module in Materials Studio software based on density functional theory is used in this study. The generalized gradient approximation method GGA-PBE is used for the exchange-correlation function. The complete LST/QST and the NEB methods in the Dmol3 module were used to study the minimum energy path of the dissociation of hydrogen sulfide molecules on the catalyst surface.

2D MoB MBene: An Efficient Co-Catalyst for Photocatalytic Hydrogen Production under Visible Light.

Highly active and low-cost co-catalysts have a positive effect on the enhancement of solar H2 production. Here, we employ two-dimensional (2D) MBene as a noble-metal-free co-catalyst to boost semiconductor for photocatalytic H2 production. MoB MBene is a 2D nanoboride, which is directly made from MoAlB by a facile hydrothermal etching and manual scraping off process. The as-synthesized MoB MBene with purity >95 wt % is treated by ultrasonic cell pulverization to obtain ultrathin 2D MoB MBene nanosheets (∼0.61 nm) and integrated with CdS via an electrostatic interaction strategy. The CdS/MoB composites exhibit an ultrahigh photocatalytic H2 production activity of 16,892 µmol g-1 h-1 under visible light, surpassing that of pure CdS by an exciting factor of ≈1135%. Theoretical calculations and various measurements account for the high performance in terms of Gibbs free energy, work functions, and photoelectrochemical properties. This work discovers the huge potential of these promising 2D MBene family materials as high-efficiency and low-cost co-catalysts for photocatalytic H2 production.

Functionalized two-dimensional iron boride compounds as novel electrode materials in Li-ion batteries.

MBenes, a class of two-dimensional metal borides, have emerged as a cutting-edge research frontier and a hotspot for electrode materials in ion batteries. This work presents a systematic investigation of the performance of two-dimensional iron boride (FeB) as an electrode material for lithium-ion batteries (LIBs), utilizing first-principles calculations. The results indicate that FeB exhibits remarkable structural stability and excellent conductivity, making it an extremely promising electrode material for LIBs. FeB has the capability to adsorb a monolayer of Li atoms, and exhibits a maximum theoretical capacity of 364 mA h g-1, a high average open circuit voltage (OCV) of 1.08 V, and a low diffusion barrier energy of 0.24 eV. Through the investigation of electrochemical properties of functionalized FeB, it has been discovered that surface functionalization exerts a positive impact on lithium storage. Theoretical lithium storage capacities of FeBT (T = F, O and S) are 538 mA h g-1, 555 mA h g-1 and 476 mA h g-1, respectively. However, the introduction of F and O functional groups significantly reduces diffusion barriers to 0.081 eV and 0.036 eV, respectively, while the introduction of the S functional group markedly decreases the average OCV to approximately 0.25 V. These interesting findings suggest that FeB has great potential in the future development of LIBs.

Lithium storage performance enhanced by lithiation-induced structural phase transitions of fluorinated MXenes.

Structural phase transitions in electrode materials of Li-ion batteries (LIBs) often occur along with Li-ion extraction/intercalation during charge and discharge processes. Lithiation-induced phase transition behaviors of two-dimensional fluorinated MXenes were investigated systematically by first-principles density functional calculations. The calculated results show that fluorine atoms in the nine MXenes studied moved from the FCC site (or HCP site for Ta2CF2) to the TOP site during Li adsorption. Further all the predicted phase transitions were confirmed by ab initio molecular dynamic simulations. The band structure, density of state, diffusion energy barrier, average voltage and storage capacity were calculated to evaluate the lithium storage properties of fluorinated MXenes, which revealed that V2CF2 and Ti2CF2 are the optimal candidates for LIB electrode materials. The structural phase transition led to improvements in the cycle stability, storage capacity, average voltage, and other lithium storage properties of the fluorinated MXenes.

A systematic computational investigation of lithiation-induced structural phase transitions of O-functionalized MXenes.

Along with Li-ion extraction/intercalation during charge and discharge processes, structural phase transitions often occur in the electrode materials of Li-ion batteries (LIBs). By determining atomic positions before and after Li adsorptions, structural phase transitions of two-dimensional MXenes were investigated systematically using first-principles density functional calculations. The lithiation-induced phase transitions of ten M2C MXenes with oxygen groups can be divided into three types. No phase transitions occur for Ti-type MXenes including Ti2CO2, Zr2CO2 and Hf2CO2. The oxygens in Ta-type MXenes (Sc2CO2, Y2CO2, Nb2CO2 and Ta2CO2) move from one type of octahedral void to another type of octahedral void. However, for Mo-type MXenes including V2CO2, Cr2CO2 and Mo2CO2, the oxygens move from octahedral voids to tetrahedral voids. The mechanisms whether phase transitions happen or not are dependent on the sizes of M ions. Furthermore, all the predicted phase transitions were confirmed by ab initio molecular dynamics simulations. The calculated results of electron localization functions and Bader charge illustrate that there exist strong Coulomb interactions (ionic bonds) between Li and MXene surfaces. The band structure, diffusion energy barrier, open circuit voltage and storage capacity were calculated to evaluate the lithium storage properties of different MXenes, which reveals that V2CO2 and Cr2CO2 should be optimal candidates as electrode materials for LIBs.

High-Performance Wearable Strain Sensor Based on MXene@Cotton Fabric with Network Structure.

Flexible and comfortable wearable electronics are as a second skin for humans as they can collect the physiology of humans and show great application in health and fitness monitoring. MXene Ti3C2Tx have been used in flexible electronic devices for their unique properties such as high conductivity, excellent mechanical performance, flexibility, and good hydrophilicity, but less research has focused on MXene-based cotton fabric strain sensors. In this work, a high-performance wearable strain sensor composed of two-dimensional (2D) MXene d-Ti3C2Tx nanomaterials and cotton fabric is reported. Cotton fabrics were selected as substrate as they are comfortable textiles. As the active material in the sensor, MXene d-Ti3C2Tx exhibited an excellent conductivity and hydrophilicity and adhered well to the fabric fibers by electrostatic adsorption. The gauge factor of the MXene@cotton fabric strain sensor reached up to 4.11 within the strain range of 15%. Meanwhile, the sensor possessed high durability (>500 cycles) and a low strain detection limit of 0.3%. Finally, the encapsulated strain sensor was used to detect subtle or large body movements and exhibited a rapid response. This study shows that the MXene@cotton fabric strain sensor reported here have great potential for use in flexible, comfortable, and wearable devices for health monitoring and motion detection.

Ti3C2 MXene-Based Sensors with High Selectivity for NH3 Detection at Room Temperature.

In this study, from experiments and theoretical calculation, we reported that Ti3C2 MXene can be applied as sensors for NH3 detection at room temperature with high selectivity. Ti3C2 MXene, a novel two-dimensional carbide, was prepared by etching off Al atoms from Ti3AlC2. The as-prepared multilayer Ti3C2 MXene powders were delaminated to a single layer by intercalation and ultrasonic dispersion. The colloidal suspension of single-layer Ti3C2-MXene was coated on the surface of ceramic tubes to construct sensors for gas detection. Thereafter, the sensors were used to detect various gases (CH4, H2S, H2O, NH3, NO, ethanol, methanol, and acetone) with a concentration of 500 ppm at room temperature. Ti3C2 MXene-based sensors have high selectivity to NH3 compared with other gases. The response to NH3 was 6.13%, which was four times the second highest response (1.5% to ethanol gas). To understand the high selectivity, first-principles calculations were conducted to explore adsorption behaviors. From adsorption energy, adsorbed geometry, and charge transfer, it was confirmed that Ti3C2 MXene theoretically has a high selectivity to NH3, compared with other gases in this experiment. Moreover, the response of the sensor to NH3 increased almost linearly with NH3 concentration from 10 to 700 ppm. The humidity tests and cycle tests of NH3 showed that the Ti3C2 MXene-based gas sensor has excellent performances for NH3 detection at room temperature.

Comparison of Effects of Sodium Bicarbonate and Sodium Carbonate on the Hydration and Properties of Portland Cement Paste.

Carbonates and bicarbonates are two groups of accelerators which can be used in sprayed concrete. In this study, the effects of the two accelerators sodium carbonate (Na2CO3) and sodium bicarbonate (NaHCO3) (0%, 1%, 2%, 3%, and 4% by weight of ordinary Portland cement OPC) on the properties of OPC paste were compared. The results show that both of them could accelerate the initial and final setting time of OPC paste, but the effect of the two accelerators on the compressive strength were different. After 1 day, sodium bicarbonate at 3% had the highest strength while sodium carbonate at 1% had the highest strength. After 7 days, both of the two accelerators at 1% had the highest compressive strength. After 28 days, the compressive strength decreased with the increase of the two. The improved strength at 1 and 7 days was caused by the accelerated formation of ettringite and the formation of CaCO3 through the reactions between the two with portlandite. The decrease of strength was caused by the Na⁺ could reduce the adhesion between C-S-H gel by replacing the Ca2+. NaHCO3 was found be a better accelerator than Na2CO3.

The Synthesis Process and Thermal Stability of V2C MXene.

The effect of etching solution on the synthesis process of two-dimensional vanadium carbide (V2C MXene) was researched. Three etching solutions were used to etch ternary carbide V2AlC at 90 °C. The three solutions were: lithium fluoride + hydrochloric acid (LiF + HCl), sodium fluoride + hydrochloric acid (LiF + HCl), and potassium fluoride + hydrochloric acid (KF + HCl). It was found that only NaF + HCl solution was effective for synthesizing highly pure V2C MXene. The existence of sodium (Na⁺) and chloridion (Cl-) in etching solution was essential for the synthesis. The thermal stability of the as-prepared V2C MXene in argon or air was studied. From thermogravimetry and differential thermal analysis, V2C MXene was found to be stable in argon atmosphere at a temperature of up to 375 °C. As the temperature increased, V2C MXene was gradually oxidized to form nanoparticles composed of vanadium trioxide (V2O3) and a part of V2C MXene was broken and transformed to vanadium carbide (V8C7) at 1000 °C. In air atmosphere, V2C MXene was stable at 150 °C. At 1000 °C, V2C MXene was oxidized to form vanadium pentoxide (V2O5).

Ground-state structures, physical properties and phase diagram of carbon-rich nitride C5N.

Using the ab initio evolutionary algorithm, four thermodynamically stable C5N phases ([Formula: see text], [Formula: see text]-1, [Formula: see text]-2 and C2/m) are uncovered. The structures of the [Formula: see text], [Formula: see text]-1 and [Formula: see text]-2 phases possess layered features. The dense C2/m phase with 3D strong covalent bond network is calculated to be superhard with hardness of 75.4 and 83.9 GPa by Chen and Gao models. The electronic properties of the C5N is diverse including conductor ([Formula: see text]), indirect semiconductor ([Formula: see text]-1 and [Formula: see text]-2) and direct semiconductor (C2/m). Pressure-induced phase transition sequences and critical pressure points are calculated to be [Formula: see text] → [Formula: see text]-1 → [Formula: see text]-2 at 2.7 and 34 GPa, respectively. Though the C2/m phase is metastable at zero temperature, our established pressure-temperature phase diagram of C5N shows that the C2/m becomes stable under high temperatures and pressures. The pressure-temperature phase diagram will give theoretical guidance for further experimental synthesis of different C5N phases.

Synthesis and Electrochemical Properties of Two-Dimensional RGO/Ti3C2Tx Nanocomposites.

MXene is a new type of two-dimensional layered material. Herein, a GO/Ti3C2Tx nanocomposite was prepared by a simple liquid phase method, and the obtained GO/Ti3C2Tx was transformed into RGO/Ti3C2Tx under high temperature with Ar/H2. The prepared samples were characterized using X-ray diffraction (XRD), Raman measurement, scanning electron microscopy (SEM), energy disperse spectroscopy (EDS), and X-ray photoelectron spectroscopy (XPS). As an electrode material in lithium-ion batteries, the RGO/Ti3C2Tx nanocomposite exhibited an excellent electrochemical performance and an excellent rate performance. Compared to pure Ti3C2Tx, the nanocomposite had a better reversible capacity at different current densities and had no attenuation after 200 cycles, which is one time higher than pure Ti3C2Tx. The improvement in the specific capacity was due to the excellent electrical conductivity and the unique structure of RGO, in which a charge transfer bridge was built among the Ti3C2Tx flakes. Such a bridge shortened the transmission distance of the electrons and ions and effectively controlled the restacking of the laminated materials.

Structural Transformation of MXene (V2C, Cr2C, and Ta2C) with O Groups during Lithiation: A First-Principles Investigation.

For high capacities and extremely fast charging rates, two-dimensional (2D) crystals exhibit a significant promising application on lithium-ion batteries. With density functional calculations, this paper systematically investigated the Li storage properties of eight 2D M2CO2 (M = V, Cr, Ta, Sc, Ti, Zr, Nb, and Hf), which are the recently synthesized transition-metal carbides (called MXenes) with O groups. According to whether the structural transformation occurs or not during the adsorption of the first Li layer, the adsorption of Li can be grouped into two types: V-type (V2CO2, Cr2CO2, and Ta2CO2) and Sc-type (Sc2CO2, Ti2CO2, Zr2CO2, Nb2CO2, and Hf2CO2). The structural transformation behaviors of V-type are reversible during lithiation/delithiation and are confirmed by ab initio molecular dynamic simulations. Except for Nb-MXene, the V-type prefers the sandwich H2H1T-M2CO2Li4 structure and the Sc-type prefers the TH1H2-M2CO2Li4 structure during the adsorption of the second Li layer. The H2H1T-M2CO2Li4 structure of O layer sandwiched by two Li layers preferred by V-type can prevent forming Li dendrite and therefore stabilize the lithiated system. The tendency of O bonding to Li rather than M in V-type is bigger than that in Sc-type, which causes that the sandwich structure of H2H1T-M2CO2Li4 is more suitable for V-type than Sc-type.

MXene: a new family of promising hydrogen storage medium.

Searching for reversible hydrogen storage materials operated under ambient conditions is a big challenge for material scientists and chemists. In this work, using density functional calculations, we systematically investigated the hydrogen storage properties of the two-dimensional (2D) Ti2C phase, which is a representative of the recently synthesized MXene materials ( ACS Nano 2012 , 6 , 1322 ). As a constituent element of 2D Ti2C phase, the Ti atoms are fastened tightly by the strong Ti-C covalent bonds, and thus the long-standing clustering problem of transition metal does not exist. Combining with the calculated binding energy of 0.272 eV, ab initio molecular dynamic simulations confirmed the hydrogen molecules (3.4 wt % hydrogen storage capacity) bound by Kubas-type interaction can be adsorbed and released reversibly under ambient conditions. Meanwhile, the hydrogen storage properties of the other two MXene phases (Sc2C and V2C) were also evaluated, and the results were similar to those of Ti2C. Therefore, the MXene family including more than 20 members was expected to be a good candidate for reversible hydrogen storage materials under ambient conditions.