MXenes have received increasing attention due to their two-dimensional layered structure, high conductivity, hydrophilicity, and large specific surface area. Because of these distinctive advantages, MXenes are considered as very competitive pressure-sensitive materials in applications of flexible piezoresistive sensors. This work reviews the preparation methods, basic properties, and assembly methods of MXenes and their recent developments in piezoresistive sensor applications. The recent developments of MXene-based flexible piezoresistive sensors can be categorized into one-dimensional fibrous, two-dimensional planar, and three-dimensional sensors according to their various structures. The trends of multifunctional integration of MXene-based pressure sensors are also summarized. Finally, we end this review by describing the opportunities and challenges for MXene-based pressure sensors and the great prospects of MXenes in the field of pressure sensor applications.

Ti3C2Tx MXene-Based Flexible Piezoresistive Physical Sensors.

2D interfacial heterostructures have found an unassailable status in energy storage systems, particularly in supercapacitors citing the intriguing structural and electrochemical characteristics. Exactly a decade ago, MXene, a promising 2D transition metal carbide/nitride/carbonitride was found to possess excellent conductivity, hydrophilicity, laudable charge storage opportunities, and enriched surface functionalities conducive for supercapacitors with inherent challenging shortcomings. To substantially improve, assembled 2D/2D MXene heterostructures exhibit commendable performance backed by the fact of swift increase in research interest. In this review, state‐of‐the‐art research progress in material design and electrochemical performance of 2D/2D MXene heterostructures for supercapacitors are investigated. Discussion is initially on MXene fundamentals including synthesis and energy storage governing properties. Particularly, different preparation including electrostatic assembly, in situ growth, hydrothermal treatment, and objective specific strategies and its implications are elaborated. Especially, the electrochemical interface science, electrode–electrolyte interaction and ion/electron dynamics and synergistic enhancement of MXene/rGO, MXene/LDH, MXene/metal sulfides and timely investigations on other 2D MXene architectures are provided for its compatibility from solid‐state to microsupercapacitors for commerciality. To conclude, a well‐comprehended outlook, key challenges, and prospective research guidelines stretching from fundamental mechanism investigations to material and electrolyte optimizations are presented to encourage advanced 2D MXene architectures for future generation supercapacitors.

Insights into 2D/2D MXene Heterostructures for Improved Synergy in Structure toward Next‐Generation Supercapacitors: A Review

Opportunities for organocatalysis in polymer synthesis via step-growth methods

Thermoplastic polyacetals: chemistry from the past for a sustainable future?

Manipulating the morphology of 3D flower-like CoMn2O4 bimetallic catalyst for enhancing the activation of peroxymonosulfate toward the degradation of selected persistent pharmaceuticals in water

Water-dispersible Ti3C2Tz MXene nanosheets by molten salt etching.

Titanium carbide/reduced graphene oxide (Ti3C2Tz/rGO) gels were prepared by a one-step hydrothermal process. The gels show a highly porous structure with a surface area of ∼224 m2 g−1 and average pore diameter of ∼3.6 nm. The content of GO and Ti3C2Tz nanosheets in the reaction precursor was varied to yield different microstructures. The supercapacitor performance of Ti3C2Tz/rGO gels varied significantly with composition. Specific capacitance initially increased with increasing Ti3C2Tz content, but at high Ti3C2Tz content gels cannot be formed. Also, the retention of capacitance decreased with increasing Ti3C2Tz content. Ti3C2Tz/rGO gel electrodes exhibit enhanced supercapacitor properties with high potential window (1.5 V) and large specific capacitance (920 F g−1) in comparison to pure rGO and Ti3C2Tz. The synergistic effect of EDLC from rGO and redox capacitance from Ti3C2Tz was the reason for the enhanced supercapacitor performance. A symmetric two-electrode supercapacitor cell was constructed with Ti3C2Tz/rGO, which showed very high areal capacitance (158 mF cm−2), large energy density (∼31.5 μW h cm−2 corresponding to a power density of ∼370 μW cm−2), and long stability (∼93% retention) after 10 000 cycles.

One-step hydrothermal synthesis of porous Ti3C2Tz MXene/rGO gels for supercapacitor applications.

Acetal metathesis copolymerization (AMCP) of renewable isohexide diacetals and aliphatic long-chain diacetals is reported and access to a small family of copolyacetals has been established. Crucial 1–2D NMR and MALDI-ToF-MS fi ndings unambiguously confi rm the existence of a copolymeric structure. In a stark contrast to the earlier reported isohexide-polyacetals, the current copolyacetals reveal very slow degradation. Hydrolytic degradation of copolyacetal pellets is extremely slow at pH 7, whereas only 30% degradation over a period of 15 d is observed in 9 M hydrochloric acid solution. GPC investigations reveal that with increasing chain-length the rate of degradation reduces, whereas copolyacetals with short-chain aliphatic segments display a faster degradation profi le. The reduced rate of degradation can be attributed to the hydrophobic nature of long-chain acetal segments. In situ NMR spectroscopy reveals the existence of formates, hemiacetals, and diols as degradation products. Thus, the rate of degradation can be tuned by the judicious choice of isohexide-diacetal and linear-diacetals in a copolyacetal.

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Synthesis of renewable copolyacetals with tunable degradation

MXene/polymer composites have gained widespread attention due to their high electrical conductivity and extensive applications, including electromagnetic interference (EMI) shielding, energy storage, and catalysis. However, due to the difficulty of dispersing MXenes in common polymers, the fabrication of MXene/polymer composites with high electrical conductivity and satisfactory EMI shielding properties is challenging, especially at low MXene loadings. Here, we report the fabrication of MXene-armored polymer particles using dispersion polymerization in Pickering emulsions and demonstrate that these composite powders can be used as feedstocks for MXene/polymer composite films with excellent EMI shielding performance. Ti3C2Tz nanosheets are used as the representative MXene, and three different monomers are used to prepare the armored particles. The presence of nanosheets on the particle surface was confirmed by X-ray photoelectron spectroscopy and scanning electron microscopy. Hot pressing the armored particles above Tg of the polymer produced Ti3C2Tz/polymer composite films; the films are electrically conductive because of the network of nanosheets templated by the particle feedstocks. For example, the particle-templated Ti3C2Tz/polystyrene film had an electrical conductivity of 0.011 S/cm with 1.2 wt % of Ti3C2Tz, which resulted in a high radio frequency heating rate of 13-15 °C/s in the range of 135-150 MHz and an EMI shielding effectiveness of ∼21 dB within the X band. This work provides a new approach to fabricate MXene/polymer composite films with a templated electrical network at low MXene loadings.

Kailash Arole

Papers

Water-dispersible Ti3C2Tz MXene nanosheets by molten salt etching.

One-step hydrothermal synthesis of porous Ti3C2Tz MXene/rGO gels for supercapacitor applications.

Synthesis of renewable copolyacetals with tunable degradation

Synthesis and Electronic Applications of Particle-Templated Ti3C2Tz MXene-Polymer Films via Pickering Emulsion Polymerization.

Interparticle interactions and rheological signatures of Ti3C2Tz MXene dispersions.