A decade after the first report, the family of two-dimensional (2D) carbides and nitrides (MXenes) includes structures with three, five, seven, or nine layers of atoms in an ordered or solid solution form. Dozens of MXene compositions have been produced, resulting in MXenes with mixed surface terminations. MXenes have shown useful and tunable electronic, optical, mechanical, and electrochemical properties, leading to applications ranging from optoelectronics, electromagnetic interference shielding, and wireless antennas to energy storage, catalysis, sensing, and medicine. Here we present a forward-looking review of the field of MXenes. We discuss the challenges to be addressed and outline research directions that will deepen the fundamental understanding of the properties of MXenes and enable their hybridization with other 2D materials in various emerging technologies.

Wearable gas sensors have received lots of attention for diagnostic and monitoring applications, and two-dimensional (2D) materials can provide a promising platform for fabricating gas sensors that can operate at room temperature. In the present study, the room temperature gas-sensing performance of TiCT nanosheets was investigated. 2D TiCT (MXene) sheets were synthesized by removal of Al atoms from TiAlC (MAX phases) and were integrated on flexible polyimide platforms with a simple solution casting method. The TiCT sensors successfully measured ethanol, methanol, acetone, and ammonia gas at room temperature and showed a p-type sensing behavior. The fabricated sensors showed their highest and lowest response toward ammonia and acetone gas, respectively. The limit of detection of acetone gas was theoretically calculated to be about 9.27 ppm, presenting better performance compared to other 2D material-based sensors. The sensing mechanism was proposed in terms of the interactions between the majority charge carriers of TiCT and gas species.

Rechargeable aluminum batteries (Al batteries) can potentially be safer, cheaper, and deliver higher energy densities than those of commercial Li-ion batteries (LIBs). However, due to the very high charge density of Al cations and their strong interactions with the host lattice, very few cathode materials are known to be able to reversibly intercalate these ions. Herein, a rechargeable Al battery based on a two-dimensional (2D) vanadium carbide (VCT) MXene cathode is reported. The reversible intercalation of Al cations between the MXene layers is suggested to be the mechanism for charge storage. It was found that the electrochemical performance could be significantly improved by converting multilayered VCT particles to few-layer sheets. With specific capacities of more than 300 mAh g at high discharge rates and relatively high discharge potentials, VCT MXene electrodes show one of the best performances among the reported cathode materials for Al batteries. This study can lead to foundations for the development of high-capacity and high energy density rechargeable Al batteries by showcasing the potential of a large family of intercalation-type cathode materials based on MXenes.

Electrochemical capacitors (ECs) that store charge based on the pseudocapacitive mechanism combine high energy densities with high power densities and rate capabilities. 2D transition metal carbides (MXenes) have been recently introduced as high‐rate pseudocapacitive materials with ultrahigh areal and volumetric capacitances. So far, 20 different MXene compositions have been synthesized and many more are theoretically predicted. However, since most MXenes are chemically unstable in their 2D forms, to date only one MXene composition, Ti3C2Tx, has shown stable pseudocapacitive charge storage. Here, a cation‐driven assembly process is demonstrated to fabricate highly stable and flexible multilayered films of V2CTx and Ti2CTx MXenes from their chemically unstable delaminated single‐layer flakes. The electrochemical performance of electrodes fabricated using assembled V2CTx flakes surpasses Ti3C2Tx in various aqueous electrolytes. These electrodes show specific capacitances as high as 1315 F cm−3 and retain ≈77% of their initial capacitance after one million charge/discharge cycles, an unprecedented performance for pseudocapacitive materials. This work opens a new venue for future development of high‐performance supercapacitor electrodes using a variety of 2D materials as building blocks.

Structural electrode materials that integrate high mechanical strength and high electrochemical performances are attractive as they are indispensable for building lightweight, flexible electronics. [1][2][3] These materials should be able to withstand extreme mechanical stress and deformations while maintaining high charge storage properties, and thereby decrease the electrochemically inactive weight and volume for packaging of devices, especially in limited spaces. [1] Most conventional electrode materials, however, fail to meet both requirements. [4] Some of reported strategies involved using carbon fiber-reinforced composites [5,6] or graphene-based materials [1] as structural electrodes to deliver mechanical strength. These materials, however, fall short on the electrochemi cal energy storage capacitance. Alternatively, metal oxides [7] or conducting polymers [8] can be incorporated to boost the capacitance of the graphene-based materials. The problem is the weak interactions between different components, which results in low mecha nical stability of the final composites. [7,9] Therefore, there is a crucial need for the development of new-generation structural energy storage nanocomposites, which monolithically integrate excellent mechanical properties, high electronic and ionic conductivities, and high charge storage capabilities. A balance should also exist between these properties without substantially sacrificing one property over the other. [1] The family of two-dimensional (2D) metal carbides and nitrides, collectively known as MXene, are interesting materials for building high-performance supercapacitors. [10][11][12][13][14][15][16] MXenes have a general formula of M n+1 X n T x , where M is an early transition metal such as Ti, X is carbon or nitrogen, and T x indicates the presence of different functional groups (O, OH, and F) on the surface of metal layers, a result of aqueous exfoliation synthesis of MXenes. [10,17,18] Ti 3 C 2 T x MXene has been widely reported as a high-performance electrode material either in its pristine form or in hybrids with other guest materials such as poly(vinyl alcohol) (PVA), [18] polypyrrole, [19,20] and polyaniline, [21] as well as in hybridization with other carbon materials such as graphene, [22] carbon nanotubes, [23][24][25] and carbon nanofibers. [26] Most of the MXene hybrid nanocomposites, however, have only shown improvement in either capacitance or mechanical properties while sacrificing one property over the other, and they lack the required mechanical integrityThe family of two-dimensional (2D) metal carbides and nitrides, known as MXenes, are among the most promising electrode materials for supercapacitors thanks to their high metal-like electrical conductivity and surface-functional-group-enabled pseudocapacitance. A major drawback of these materials is, however, the low mechanical strength, which prevents their applications in lightweight, flexible electronics. A strategy of assembling freestanding and mechanically robust MXene (Ti 3 C 2 T x ) nanoco...

In situdeposition of PANI on the surface of MXenes leads to the fabrication of high performance supercapacitor electrodes.

The sensitive detection of explosive and flammable gases is an extremely important safety consideration in today's industry. Identification of trace amounts of nonpolar analytes at ambient temperatures, however, is still a challenge because of their weak adsorption, and very few studies have been able to achieve it via a chemiresistive mechanism. Herein, we demonstrate the high performance of 2D vanadium carbide MXene (V 2 CT x ) gas sensors with ultrahigh sensitivity toward nonpolar gases. The fabricated 2D V 2 CT x sensor devices consisting of single-/few-layer 2D V 2 CT x on polyimide film were able to detect both polar and nonpolar chemical species including hydrogen and methane with a very low limit of detection of 2 and 25 ppm, respectively, at room temperature (23 °C). The performance of the fabricated V 2 CT x gas sensors in detection of nonpolar gases surpasses that of previously reported state-of-the-art gas sensors based on other 2D materials.

We report Layer-by-Layer (LbL) self-assembly of pillared two-dimensional (2D) multilayers, from water, onto a wide range of substrates. This LbL method uses a small molecule, tris(2-aminoethyl) amine (TAEA), and a colloidal dispersion of Ti 3 C 2 T x MXene to LbL self-assemble (MXene/TAEA) n multilayers, where n denotes the number of bilayers. Assembly with TAEA results in highly ordered (MXene/TAEA) n multilayers where the TAEA expands the interlayer spacing of MXene flakes by only ~ 1 Å and reinforces the interconnection between them. The TAEA-pillared MXene multilayers show the highest electronic conductivity of 7.3 × 10 4 S m −1 compared with all reported MXene multilayers fabricated by LbL technique. The (MXene/TAEA) n multilayers could be used as electrodes for flexible all-solid-state supercapacitors delivering a high volumetric capacitance of 583 F cm −3 and high energy and power densities of 3.0 Wh L −1 and 4400 W L −1 , respectively. This strategy enables large-scale fabrication of highly conductive pillared MXene multilayers, and potentially fabrication of other 2D heterostructures.