In Situ Growth of Nanorod-Shaped Ni,Co-MOF on Mo2CTx MXene Surface to Realize Enhanced Energy Storage for Supercapacitors.

Mo2CTx MXene materials, known for their high conductivity and abundant surface functional groups, are widely utilized as electrode materials in supercapacitors. However, their tendency to stack during electrochemical energy storage hinders their performance. The in situ growth of nanorod-shaped Ni,Co bimetallic metal-organic frameworks (Ni,Co-MOF) on Mo2CTx MXene effectively mitigates this stacking. With their porous structure and high specific surface area, MOFs excel in energy storage, and bimetallic MOFs outperform monometallic ones. The synergy between Mo2CTx MXene and Ni,Co-MOF yields an outstanding performance. In a three-electrode system with 1 M KOH, the Mo2CTx/Ni,Co-MOF composite shows a specific capacitance of 58 mAh g-1 (56.26 mAh cm-3) at 1 A g-1. When used in a Mo2CTx/Ni,Co-MOF//AC asymmetric supercapacitor, it achieves an energy density of 22.7 Wh kg-1(0.022 Wh cm-3) at a power density of 293 W kg-1 (0.284 W cm-3). Future work will focus on enhancing synthesis methods, exploring different bimetallic combinations, and optimizing electrode designs for gas sensors, batteries, fuel cells, biological sensors, and so on, with outstanding performance and sustainability.

Facile preparation of MoO3@Mo2CTxnanocomposite with high lithium storage performance byin situoxidation.

In this work, a new MoO3@Mo2CTxnanocomposite was prepared from two-dimensional (2D) Mo2CTxMXene byin situoxidization in air, which exhibited wonderful lithium-storage performance as anodes of lithium-ion batteries (LIBs). The precursor Mo2CTxwas synthesized from Mo2Ga2C by selective etching of NH4F at 180 °C for 24 h. Thereafter, the Mo2CTxwas oxidized in air at 450 °C for 30 min to obtain MoO3@Mo2CTxnanocomposite. In the composite,in situgenerated MoO3nanocrystals pillar the layer structure of Mo2CTxMXene, which increases the interlayer space of Mo2CTxfor Li storage and enhances the structure stability of the composite. Mo2CTx2D sheets provide a conductive substrate for MoO3nanocrystals to enhance the Li+accessibility. As anodes of LIBs, the final discharge specific capacity of the MoO3@Mo2CTxcomposite was 511.1 mAh g-1at a current density of 500 mA g-1after 100 cycles, which is about 36.7 times that of pure Mo2CTxMXene (13.9 mAh g-1) and 3.2 times that of pure MoO3(159.9 mAh g-1). In the composites, both Mo2CTxand MoO3provide high lithium storage capacity and can enhance the performance of each other. Moreover, this composite can be made by a facile method ofin situoxidation. Therefore, the MoO3@Mo2CTxMXene nanocomposite is a promising anode of LIB with high performance.

In Situ Preparation of Three-Dimensional Porous Nickel Sulfide as a Battery-Type Supercapacitor.

A one-step sulfurization method to fabricate Ni3S2 nanowires (Ni3S2 NWs) directly on a Ni foam (NF) was developed as a simple, low-cost synthesis method for use as a supercapacitor (SC), aimed at optimizing energy storage. Ni3S2 NWs have high specific capacity and are considered a promising electrode material for SCs; however, their poor electrical conductivity and low chemical stability limit their applications. In this study, highly hierarchical three-dimensional porous Ni3S2 NWs were grown directly on NF by a hydrothermal method. The feasibility of the use of Ni3S2/NF as a binder-free electrode for achieving high-performance SCs was examined. Ni3S2/NF exhibited a high specific capacity (255.3 mAh g-1 at a current density of 3 A g-1), good rate capability (2.9 times higher than that of the NiO/NF electrode), and competitive cycling performance (capacity retention of specific capacity of 72.17% after 5000 cycles at current density of 20 A g-1). Owing to its simple synthesis process and excellent performance as an electrode material for SCs, the developed multipurpose Ni3S2 NWs electrode is expected to be a promising electrode for SC applications. Furthermore, the synthesis method of self-growing Ni3S2 NW electrodes on 3D NF via hydrothermal reactions could potentially be applied to the fabrication of SC electrodes using a variety of other transition metal compounds.

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.

MXene-based chemical gas sensors: Recent developments and challenges.

The long-running Covid-19 pandemic has forced researchers across the globe to develop novel sensors and sensor materials for detecting minute quantities of biogenic viruses with high accuracy in a short period. In this context, MXene galleries comprising carbon/nitride two-dimensional nanolayered materials have emerged as excellent host materials in chemical gas sensors owing to their multiple advantages, including high surface area, high electrical conductivity, good thermal/chemical conductivity and chemical stability, composition diversity, and layer-spacing tunability; furthermore, they are popular in clinical, medical, food production, and chemical industries. This review summarizes recent advances in the synthesis, structure, and gas-sensing properties of MXene materials. Current opportunities and future challenges for obtaining MXene-based chemical gas sensors with high sensitivity, selectivity, response/recovery time, and chemical durability are addressed. This review provides a rational and in-depth understanding of the relationship between the gas-sensing properties of MXenes and structure/components, which will promote the further development of two-dimensional MXene-based gas sensors for technical device fabrication and industrial processing applications.

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.