Two-dimensional MXene Material Research Articles

Revealing the Two-Stage Charging Process in Sulfuric Acid Electrolyte by Molecular Dynamics Simulation.

Two-dimensional MXene materials perform excellently in supercapacitor applications, but self-stacking and overlap limit their applications. Constructing a reasonable layered structure by combining MXene and graphene can effectively inhibit the restacking and overlap of MXene and improve the performance of supercapacitors. In this work, we studied the energy storage performance of a conventional MXene electrode and MXene/graphene composite electrode in sulfuric acid aqueous electrolyte by molecular dynamics (MD) simulation and analyzed their energy storage mechanisms. The simulation results reveal that the MXene/graphene composite electrode showed faster charge-discharge speed and larger capacity and had more obvious advantages as a cathode. The charging process of the composite cathode can be divided into two stages. In the first stage, SO42- and H3O+ enter the electrode as a whole in a nearly 1:2 ratio, and a unique three-layer structure is formed in the graphene area, while a large number of HSO4- leaves the electrode. In the second stage, SO42- with a part of H3O+ (ratio of 2:2 to 2:3) leave the electrode, and the three-layer structure is gradually destroyed. The cooperation of these two stages leads to a particular "concave" in the total energy change of the composite cathode. The introduction of graphene has brought about changes in ion distribution, migration mechanism, and energy change, making the MXene/graphene cathode show significant advantages in energy storage. This work is of great significance for understanding the microscopic energy storage mechanism of MXene/graphene-based electrodes.

Insight into the mechanism of nitrogen doping in MXenes with controllable surface chemistry

Nitrogen doping emerges as a potent strategy to enhance the electrochemical attributes of two-dimensional MXene materials, recognized for their utility in energy storage solutions. Although advancements have been made, the specific roles of varied doping sites in modifying electrochemical behavior are not fully delineated. This investigation explores nitrogen-doping mechanisms in MXenes with tailored surface chemistries, utilizing a blend of experimental approaches and density functional theory calculations. The distinct roles of nitrogen dopant sites are explored, and the evolution of the electronic structure and stability of the material before and after nitrogen modifications via the urea hydrothermal method are also investigated. The synthesized N-V2CTx-160 MXene electrodes exhibit favorable surface characteristics for energy storage, achieving impressive capacitance properties with a gravimetric capacitance of 592.9 F/g at 2 mV/s in 1 M H2SO4 electrolyte. This work provides a valuable insight for the development of high-performance MXene electrodes for energy storage applications.

Au/Ti3C2/g-C3N4 Ternary Composites Boost H2 Evolution Efficiently with Remarkable Long-Term Stability by Synergistic Strategies.

The use of novel two-dimensional MXene materials and conventional g-C3N4 photocatalysts to fabricate the composites for hydrogen evolution reaction (HER) has attracted much attention, for which there is plenty of room for the enhancement of hydrogen evolution rates particularly under visible light and photostability. Herein, g-C3N4 was modified by copolymerization of malonamide and melamine and used to fabricate the ternary composites of Au particles and Ti3C2 MXene, and based on the synergistic effect, the composites enhanced the hydrogen evolution rates by 2.1, 99.8, and ∞ times compared with the unmodified g-C3N4 under UV, simulated sunlight, and visible light illumination, respectively. Moreover, the composite exhibited a sustained hydrogen evolution capacity in the cycle test for up to 120 h. Theoretical calculations and experimental results indicated that the hot electrons of Au are injected into the modified g-C3N4 and transferred to Ti3C2 simultaneously along with the photogenerated electrons of the modified g-C3N4 and then further transferred to Au, forming a photogenerated electron transfer channel of Au and modified g-C3N4 → Ti3C2 → Au within the composite. Ti3C2 acts as a bridge for fast separation of photogenerated electrons and holes on Au and modified g-C3N4, playing a key role in the enhanced photocatalytic performance. In addition, the visible light absorption ability of Au also positively contributed to the enhancement of visible light photocatalytic performance by providing hot electrons. Therefore, the selection of suitable cocatalysts for the design of composites is a crucial research direction to improve the photocatalytic performance and photostability of photocatalysts.

Fully Transient 3D Origami Paper-Based Ammonia Gas Sensor Obtained by Facile MXene Spray Coating.

Developing high-performance chemiresistive gas sensors with mechanical compliance for environmental or health-related biomarker monitoring has recently drawn increasing research attention. Among them, two-dimensional MXene materials hold great potential for room-temperature hazardous gas (e.g., NH3) monitoring regardless of the complicated fabrication process, insufficient 2D/3D flexibilities, and poor environmental sustainability. Herein, a Ti3C2Tx MXene/gelatin ink was developed for patterning electrodes through a facile spray coating. Particularly, the patterned Ti3C2Tx-based coating exhibited good adhesion on the paper substrate against repeated peeling-off and excellent mechanical flexibility against 1000 cyclic stretching. The porous morphology of the coating facilitated the NH3 sensing ability. As a result, the 2D kirigami-shaped NH3 sensor exhibited a good response of 7% to 50 ppm of NH3 with detectable concentrations ranging from 5-500 ppm, decent selectivity over interferences, etc., which could be well-maintained even at 50% stretched state. In addition, with the help of mechanically guided compressive buckling, 3D mesostructured MXene origamis could be obtained, holding promise for detecting the coming direction and height distribution of hazardous gas, e.g., the NH3. More importantly, the as-fabricated MXene/gelatin origami paper could be fully degraded in PBS/H2O2/cellulase solution within 19 days, demonstrating its potential as a high-performance, shape morphable, and environmentally friendly wearable gas sensor.

High-entropy MXene Ti3(B0.25C0.25N0.25O0.25)2O2 as anode materials for lithium-ion batteries: Insight from first principles

Two-dimensional MXene materials had attracted extensive research interest due to their excellent energy storage properties. Meanwhile, high-entropy materials exhibited excellent properties due to their unique lattice structure and stability. This study investigated the potential of a high-entropy MXene, Ti3(B0.25C0.25N0.25O0.25)2O2, as an advanced anode material for lithium-ion batteries through first-principles calculations. The structural, electronic, and lithium adsorption properties were systematically examined. Notably, the material exhibited a metallic-like character with favorable adsorption energies for lithium ions at the C-site. Additionally, the diffusion energy barrier for Li-ions was found to be exceptionally low at 0.18 eV, indicating promising rate performance. The study explored the adsorption of multiple lithium layers, and the maximum theoretical specific capacity of 466.59 mAh/g and a minimum open circuit voltage of 0.48 V. The stability of the material after lithium adsorption was confirmed through analyses of formation energy and cohesive energy. Overall, this research highlighted the considerable potential of high-entropy MXene as a stable and high-performance anode material for lithium-ion batteries.

A polarization-sensitive, high on/off ratio and self-powered photodetector based on Nb2CT x and Nd2CT x @MoS2

MXene two-dimensional materials have been widely used in energy storage, catalysis, sensing and other fields, Nb2C as a typical two-dimensional MXene material, its exploration in the field of optoelectronics is still in its infancy, especially Nb2C-based photodetectors are still to be developed. This paper demonstrates that two-dimensional films based on few-layer Nb2C have a photoelectric response in the wavelength range from visible to near-infrared. We have found that the light response performance can be easily adjusted by controlling the thickness of the spin-coated film, and that Nb2C photodetectors show great advantages in terms of wide bandwidth, polarization response, high switching ratio, etc. By adjusting the material concentration and sample thickness, the photocurrent can reach up to 330 nA, the switching ratio can reach 410, and the responsivity can reach 8.3 × 10−4 A W−1. In the polarization characteristic test, an extinction ratio of 7.6 can be obtained. By adjusting the content of that doped MoS2 quantum dot, the dark current can reach 7.6 × 10−13 A, and the switching ratio can reach 3 × 105, which can be increased by 700 times. The above results show that the few-layer Nb2C nanosheets can be used as optoelectronic detectors in the visible to near-infrared bands, which further broadens the application prospects of two-dimensional MXene.

Ultra-high flux mesh membranes coated with tannic acid-ZIF-8@MXene composites for efficient oil-water separation

Oil/water separation has become a global concern due to the increasing discharge of multi-component harmful oily wastewater. Super wetting membranes have been shown to be an effective material for oil/water separation. Ultra-high flux stainless-steel meshes (SSM) with superhydrophilicity and underwater superoleophobicity were fabricated by tannic acid (TA) modified ZIF-8 nanoparticles (TZIF-8) and two-dimensional MXene materials for oil/water separation. The TZIF-8 increased the interlayer space of MXene, enhancing the flux permeation (69,093 L m−2h−1) and rejection of the composite membrane (TZIF-8@MXene/SSM). The TZIF-8@MXene/SSM membrane showed an underwater oil contact angle of 154.2°. The membrane maintained underwater superoleophobic after stability and durability tests, including various pH solutions, organic solvents, reusability, etc. In addition, the oil/water separation efficiency of TZIF-8@MXene/SSM membranes was higher than 99% after treatment in harsh conditions and recycling. The outstanding anti-fouling, stability, durability, and recyclability properties of TZIF-8@MXene/SSM membrane highlight the remarkable potential of membranes for complex oil/water separation process.

Interfacial regulation of BiOI@Bi2S3/MXene heterostructures for enhanced photothermal and photodynamic therapy in antibacterial applications.

Developing environmentally friendly, broad-spectrum, and long-lasting antibacterial materials remains challenging. Our ternary BiOI@Bi2S3/MXene composites, which exhibit both photothermal therapy (PTT) and photodynamic therapy (PDT) antibacterial properties, were synthesized through in-situ vulcanization of hollow flower-shaped BiOI on the surface of two-dimensional Ti3C2 MXene. The unique hollow flower-shaped BiOI structure with a high exposure of the (001) crystal plane amplifies light reflection and scattering, offering more active sites to improve light utilization. Under 808nm irradiation, these composites achieved a photothermal conversion efficiency of 57.8%, boosting the PTT antibacterial effect. The heterojunction between Bi2S3 and BiOI creates a built-in electric field at the interface, promoting hole and electron transfer. Significantly, the close-contact heterogeneous interface enhances charge transfer and suppresses electron-hole recombination, thereby boosting PDT bacteriostatic performance. EPR experiments confirmed that ∙O2- and •OH radicals play major roles in photocatalytic bacteriostatic reactions. The combined antibacterial action of PTT and PDT led to efficiencies of 99.7% and 99.8% against P. aeruginosa and S. aureus, respectively, under 808nm laser irradiation. This innovative strategy and thoughtful design open new avenues for heterojunction materials in PTT and PDT sterilization. STATEMENT OF SIGNIFICANCE: Photodynamic and photothermal therapy is a promising antibacterial treatment, but its efficiency still limits its application. To overcome this limitation, we prepared three-dimensional heterogeneous BiOI@Bi2S3/MXene nanocomposites through in-situ vulcanization of hollow flower-shaped BiOI with a high exposure of the (001) crystal plane onto the surface of two-dimensional MXene material. The resulting ternary material forms a close-contact heterogeneous interface, which improves charge transfer channels, reduces electron-hole pair recombination, and amplifies photodynamic bacteriostatic performance. These nanocomposites exhibit photothermal conversion efficiency of 57.8%, enhancing their photothermal bactericidal effects. They demonstrated antibacterial efficiencies of 99.7% against P. aeruginosa and 99.8% against S. aureus. Therefore, this study provides a promising method for the synthesis of environmentally friendly and efficient antibacterial materials.

Self-assembly of MXene V2CTx/Ag/rGO layered multiscale structure nanocomposite

Two-dimensional MXene materials exhibit excellent performance in various fields, especially in fuel cells and energy storage due to their two-dimensional atomic thickness topology. However, the MXene material is prone to aggregation, and its defects such as collapse seriously affect its electrochemical performance and severely limit its range of application. In this paper, V2CTx + Ag + rGO composite electrode materials were prepared with classical hydrothermal method. The doping of Ag+ and rGO consumes the negative charge on the surface of V2CTx, prevents the delamination from collapse and stacking overlap, and accelerates the penetration of electrolyte ions. The entry of Ag particles and reduced graphene oxide can provide support sites for the layered structure, widen the interlayer spacing and provide a more active contact area for the electrolyte ions, thus opening up a green channel for ion transport. The entry of rGO plays a bridging role in making the contact between Ag particles, rGO and V2CTx closer and the cross-border transport of carriers more convenient. With the incorporation of rGO and Ag particles, the capacitance contribution rate of V2CTx + Ag + rGO composite can reach 63% while the transfer charge resistance is only 17.5 Ω, and the specific capacitance decreases very little with the increase of scanning time. The reason for this is that the rGO and Ag particles transform the original two-dimensional structure of V2CTx into a sandwich structure, which gives the V2CTx + Ag + rGO composite excellent electrical conductivity. The rGO and Ag particles synergize with each other to build an excellent conductive three-dimensional network that accelerates the transfer of ions and charges, resulting in excellent electrochemical performance of the V2CTx + Ag + rGO composite electrode material.

Application of Two-Dimensional MXene materials in sensors

Transition metal carbides and nitrides/carbonitrides (MXenes) with satisfactory conductivity, high specific surface area and excellent hydrophilicity have attracted the attention of many researchers in the field of electrochemical sensors in recent years. Etched MXenes usually show an accordion-like morphology; moreover, monolayer MXenes exhibit better electrical conductivity and mechanical properties. The electronic and chemical properties of MXenes can be significantly changed by means of compounding, hybridization and modification. MXenes with large surface areas can be used in biosensors as carriers for recognition elements to achieve direct electron transfer to target analytes. In addition, MXenes exhibit excellent biological compatibility so that they can be incorporated into biological receptors without affecting biological activity. MXenes can be combined with other metal oxide nanoparticles to expand the electrode surface area while reducing the overpotential effect in the electroanalysis process to enhance catalytic activity. MXene-based sensors for detecting strain, pressure, temperature, gas, various biomarkers, and some environmental pollutants, and for use in other major research fields are summarized. The future challenges in MXenes research, development and applications of electrochemical sensing are discussed.

Flexible, humidity- and contamination-resistant superhydrophobic MXene-based electrospun triboelectric nanogenerators for distributed energy harvesting applications.

The low surface-charge density, poor stability and irreparable surface of triboelectric materials under harsh environments are still some obstacles for developing high-performance triboelectric nanogenerators (TENGs). In particular, a two-dimensional MXene material's surface is likely to be corroded by water molecules under high humidity conditions owing to its hydrophilic nature, limiting the output performance and stability of TENGs. Herein, an approach for fabricating a humidity- and contamination-resistant MXene-based TENG is established using the electrospinning technique. First, nanofibrous layers of MXene/MoS2 composites blended in a cellulosic polymer matrix were prepared, benefitting the high surface roughness and controlled air-trapping pores. Furthermore, the prepared nanofibrous layers were chemically modified with stearic acid (SA), which enhances the hydrophobicity and electronegativity of MXene/MoS2 composites. In a typical synthesis, four different compositions of MXene/MoS2/cellulose acetate nanofibers were prepared, which illustrates that an increasing concentration of MoS2 could effectively tune the surface oxidation, hydrophilic nature, and surface roughness of MXene as well as induce a piezoelectricity-enhanced triboelectric potential. On the other side, the SA modification ultimately generated a superhydrophobic surface with low surface energy and a high water contact angle of ∼154°. The integrated TENG displayed an enhanced output voltage of ∼140 V and an instantaneous power density of ∼2975 mW cm-2 with long-term stability under high humidity conditions. Additionally, the self-cleaning properties were demonstrated, ensuring the sustainability and reusability of the TENG in a contaminated environment. Moreover, the fabricated MXene-based superhydrophobic layer can harvest the energy on dripping water droplets based on the liquid-solid contact-electrification TENG mode. Overall, this work paves the way for the design and development of humidity- and contamination-resistant triboelectric materials and guides the study of harvesting of distributed environmental energy efficiently.

Growing CoNi nanoalloy@N-doped carbon nanotubes on MXene sheets for excellent microwave absorption

• 0D CoNi nanoalloy-encapsulated 1D NCNT were grown on 2D Ti 3 C 2 T x sheets. • High surface areas and excellent thermal oxidation stabilities were achieved for the composite. • The minimum RL reached −55.3 dB at 2.1 mm. • The EAB of 4.3 GHz at 1.5 mm could be attained. Recently, two-dimensional MXene materials have attracted numerous attention in electromagnetic wave shielding/absorption. Hybridizing magnetic materials and constructing multi-dimensional structures in MXene is highly beneficial to improve electromagnetic wave absorption properties. Herein, we demonstrate a strategy for in situ growing 0D CoNi nanoalloy-encapsulated 1D N-doped carbon nanotubes on a 2D Ti 3 C 2 T x MXene sheet through an electrostatic assembly process followed by a high-temperature pyrolysis process. The resultant 201-structured MXene-CoNi@N-doped carbon nanotube (MXene-CoNi@NCNT) composites displayed high surface areas (55.6–103.7 m 2 /g), moderate magnetism (19.8–24.6 emu/g), and excellent thermal oxidation stabilities (≥ 307 °C). In addition, the unique 2D/0D/1D architectures entrusted the composites with abundant interfaces, various defects, and numerous nitrogen dopants. Taking advantage of the special 201 structure and the existence of both magnetic and dielectric loss, the MXene-CoNi@NCNT composite showed great impedance matching and strong attenuation performance. A strong reflection loss of –55.3 dB was achieved when the coating thickness was 2.1 mm, and a wide effective absorption bandwidth of 4.3 GHz was achieved at a thickness of 1.5 mm, much superior to that of similar absorbers. This work demonstrates a novel strategy for designing electromagnetic wave absorbers with magnetic and dielectric losses accompanied by multiple dimensional structures.

Functional groups and vertical strain regulate the electronic properties of Nb2NT2/MoTe2 heterojunction

In the current effort, a novel attempt to construct heterojunctions using Nb2N two-dimensional MXene materials with fascinating property and MoTe2 transition metal disulfide compounds is proposed. The effect of vertical strain on the electronic structure and interfacial contact properties of the heterojunction is investigated by first principles calculations. The O, F, and OH functional groups prefer to be adsorbed on top of the N atoms of the Nb2N MXene material. The configuration III of heterojunction has the lowest energy and stable model, and the equilibrium layer spacing is within the range of 3.061 Å to 3.328 Å, indicating that they belong to typical vdW heterojunctions. The large work function difference between the two components induces the transfer and rearrangement of charge. 0.12 e per cell spontaneously transfer from MoTe2 to Nb2NO2 layer. In contrast, the Nb2NF2 and Nb2N(OH)2 layers transfer 0.01 e and 0.21 e to the MoTe2 layer, respectively. n-type Ohmic contacts are formed in O terminal system, while p-type and n-type Schottky contacts are formed in F and OH terminal systems, respectively. Additionally, for vertical strain, with the decrease of the layer spacing, the p-type Schottky barrier height (SBH) decreases from 0.270 eV to 0.095 eV in former system, meanwhile the n-type Schottky barrier height in the latter system also decreases from 0.260 eV to 0.156 eV. These regulations of interest lays a theoretical foundation for bandgap engineering and Schottky junction preparation and opens a window into MXene-based heterojunction electron device.

Sea-urchin-like NiCo2S4 modified MXene hybrids with enhanced microwave absorption performance

The development of electromagnetic wave absorbing materials with lightweight and high absorption capacity is an effective way to solve electromagnetic wave pollution. Although the two-dimensional MXene material has become one of the most attractive electromagnetic wave absorbing materials due to its unique structure, the poor impedance matching makes it difficult to obtain both lightweight and efficient absorption capacity. Herein, few-layer MXene sheets and sea-urchin-like NiCo2S4 (NCS) hybrids were successfully synthesized by a simple electrostatic self-assembly method. MXene/NCS hybrids achieve good absorption performances. The maximum absorption bandwidth is 5.6 GHz at a matching thickness of only 1.53 mm with a filling ratio of only 10 wt%. In addition, the excellent microwave absorption performance is attributed to the improved impedance matching of MXene and the synergistic effects of dielectric loss and interfacial polarization. This work provides new ideas for the design and synthesis of materials with lightweight and efficient electromagnetic wave absorption.

Pebax and CMC@MXene-Based Mixed Matrix Membrane with High Mechanical Strength for the Highly Efficient Capture of CO2

Sustainable and energy-efficient molecular separation requires membranes with high permeability and selectivity. Among various membranes, the mixed matrix membranes (MMMs) prepared from two-dimensional MXene materials with well-defined nanochannels are expected to be used for the precise separation of molecules. However, the stacking of a large number of 2D MXene nanosheets creates gas mass transfer barriers, resulting in a significant reduction in gas permeability. To solve this issue, herein, a high-performance self-supporting Pebax/CMC@MXene MMM was prepared by a combination of 2D MXene nanosheets with branched carboxymethyl cellulose (CMC) for CO2 gas separation from natural gas and flue gas. Gas separation experimental results show that the as-prepared self-supporting Pebax/CMC@MXene MMMs can significantly improve gas permeance and selectivity. When the loading of MXene nanosheets in the CMC@MXene filler was fixed at 1.5 mg/mL, the CO2 permeance of Pebax/CMC@MXene MMMs was found to be 521 GPU, and the CO2/N2 selectivity was calculated to be 40.1. Meanwhile, the CO2 permeance was measured to be 444 GPU, and the CO2/CH4 selectivity was calculated to be 40.4. In addition, the resulted as-prepared self-supporting Pebax/CMC@MXene MMM possesses excellent thermal stability and long-term durability; after a 60 h test, its separation performance has no obvious change. The work provides a novel strategy to construct MMMs with less mass transfer barrier for highly efficient capture of CO2 from natural gas and flue gas.

A in-plane biaxial strain tunable electronic structures and magnetic properties of Fe2C monolayer

This work intends to study the effect of a in-plane biaxial strain (ε) on the electronic structures and magnetic properties of monolayer Fe2C by first-principles calculation. The unstrained and strained Fe2C monolayer is thermally, dynamically, and mechanically stable, and unstrained Fe2C monolayer is ferromagnetic metal with the Curie temperature (TC) of 510 K. Introducing the tensile strain, the magnetic moments, spin polarization and magnetic anisotropy energy (MAE) can be significantly enhanced. Furthermore, a phase transition from ferromagnetic to antiferromagnetic ordering occurs at the ε of 12%, −10%. Especially, the result of Monte Carlo simulations demonstrates that the TC decreases with strain and Néel temperature TN increases with the tensile strain. A tensile strain applied on Fe2C monolayer with the value less than 8% facilitates the enhancement of MAE and spin polarization, and maintaining the TC above room temperature. This study provides a theoretical guide for the application of monolayer Fe2C in spintronic devices, and provides a basis for tuning the physical properties to two-dimensional MXene materials.

Mid-infrared all-optical wavelength converter based on highly nonlinear MXene-decorated microfiber

We propose an all-optical wavelength converter for the mid-infrared spectral range with enhanced conversion efficiency. A highly nonlinear MXene-decorated microfiber is fabricated by the optical deposition method as the conversion medium, and the fiber exhibits a remarkable nonlinear optical response. We combine the benefits of the two-dimensional MXene materials and tapered microfiber to promote conversion efficiency. Homebuilt holmium-doped fiber lasers emitting at 2.05 μm wavelength are used as pump and signal sources. The experimental conversion efficiency observed by the optical spectrum analyzer is measured at –27.22 dB. This optical device allows an arbitrary tuning range of 17 nm. The fluctuations of the conversion efficiency remain within 1 dB during 2 h. The results of this study may contribute to the realization of optical converter application in the all-optical network at 2 μm wavelength.

Highly Thermoelectric ZnO@MXene (Ti3C2Tx) Composite Films Grown by Atomic Layer Deposition.

Due to its unique high conductivity and flexibility, the two-dimensional MXene material (Ti3C2Tx) is expected to possess great potential in the thermoelectric field. However, the low thermoelectric performance from high thermal conductivity and a low Seebeck coefficient has limited its practical application. In this report, we demonstrate the uniform growth of ZnO layers on the laminar Ti3C2Tx membrane by atomic layer deposition (ALD). Benefiting from the low-temperature deposition characteristics of the ALD technique, the ZnO@Ti3C2Tx composite films maintain the basic apparent morphology of the original films after the deposition. We reveal that the Schottky barrier formed between ZnO and Ti3C2Tx exhibits an energy-filtering effect, significantly enhancing the Seebeck coefficient to result in more than a double increase in the power factor. Meanwhile, the strong phonon-interface scattering between ZnO and Ti3C2Tx is found to reduce the thermal conductivity of the composite films by a factor of four as compared to pure Ti3C2Tx ones, further improving the overall thermoelectric properties of the ZnO@Ti3C2Tx composite films. Our investigation provides an ALD-based strategy for growing wide band gap layers on the narrow band gap films to improve the thermoelectric performance of various MXene materials.

Two-dimensional vanadium carbide for simultaneously tailoring the hydrogen sorption thermodynamics and kinetics of magnesium hydride

Magnesium hydride (MgH 2 ) is a potential material for solid-state hydrogen storage. However, the thermodynamic and kinetic properties are far from practical application in the current stage. In this work, two-dimensional vanadium carbide (V 2 C) MXene with layer thickness of 50−100 nm was fist synthesized by selectively HF-etching the Al layers from V 2 AlC MAX phase and then introduced into MgH 2 to improve the hydrogen sorption performances of MgH 2 . The onset hydrogen desorption temperature of MgH 2 with V 2 C addition is significantly reduced from 318 °C for pure MgH 2 to 190 °C, with a 128 °C reduction of the onset temperature. The MgH 2 + 10 wt% V 2 C composite can release 6.4 wt% of H 2 within 10 min at 300 °C and does not loss any capacity for up to 10 cycles. The activation energy for the hydrogen desorption reaction of MgH 2 with V 2 C addition was calculated to be 112 kJ mol −1 H 2 by Arrhenius's equation and 87.6 kJ mol −1 H 2 by Kissinger's equation. The hydrogen desorption reaction enthalpy of MgH 2 + 10 wt% V 2 C was estimated by van't Hoff equation to be 73.6 kJ mol −1 H 2 , which is slightly lower than that of the pure MgH 2 (77.9 kJ mol −1 H 2 ). Microstructure studies by XPS, TEM, and SEM showed that V 2 C acts as an efficient catalyst for the hydrogen desorption reaction of MgH 2 . The first-principles density functional theory (DFT) calculations demonstrated that the bond length of Mg− H can be reduced from 1.71 Å for pure MgH 2 to 2.14 Å for MgH 2 with V 2 C addition, which contributes to the destabilization of MgH 2 . This work provides a method to significantly and simultaneously tailor the hydrogen sorption thermodynamics and kinetics of MgH 2 by two-dimensional MXene materials.

Characterization of phonon thermal transport of Ti3C2T x MXene thin film

Two-dimensional MXene materials with high electrotonic conductivity, good chemical stability, and unique laminar structure show great potential in the field of electrochemistry. In contrast to the widely concerned electrical properties, studies on the thermal properties of MXene materials are very limited. This paper presents a comprehensive analysis of the thermal properties of Ti3C2T x MXene thin film. Thermal diffusivity and thermal conductivity of Ti3C2T x films are characterized by the transient electro-thermal technique. The experimental results show a 16% enhancement in thermal conductivity when the temperature is increased from 307 K to 352 K. The phonon transport contributes substantially to thermal conductivity compared with electron transport. Molecular dynamic simulation is employed to further investigate the role of phonon thermal transport of Ti3C2 layer. It is found that the combined effect of specific heat capacity, stacking structure and internal stress states is responsible for the thermal transport performance of Ti3C2T x MXene thin film.