Two-dimensional metal carbides and nitrides from head to toe with energy applications: A topical review

An increase in population and demand for energy are the major challenges in today's world. As the Internet of Things (IoTs) grows, there is a pressing need for new kinds of novel materials. These issues greatly attract the scientific community. Therefore, the hottest topic now is energy conversion and energy storage by using two-dimensional (2D) materials beyond that of graphene. In the year 2011, Gogosti et al., and his research team discovered 2D transition metal carbides, nitrides and carbonitrides called MXenes Ti₃C₂T_x etching from Ti₃AlC₂ with the help of toxic hydrofluoric acid. Now, so many novel routes have been developed with mild reducing agents, etchants, and intercalants. Many new members of 2D materials have been added to the constellation. MXene based composites have drawn extensive attention from the scientific community owing to their high conductivity and other unique properties such as hydrophilic surfaces, large interlayer spacing, environmental flexibility, large surface area, safety, thermal conductivity, improved electrochemical activity, enhancing faradic capacitance (pseudocapacitance) and reducing the shortcomings in the normal capacitance etc. In this review, we systematically summarize the recent developments and various synthesis mechanisms of different multi-layered material, intercalations, and its different applications as electrocatalysts, Li-ion batteries (LIBs), supercapacitors, biosensors, and other electrochemical storage systems.     

Highly conductive and flexible bilayered MXene/cellulose paper sheet for efficient electromagnetic interference shielding applications

In this work, a robust and flexible bilayered MXene/cellulose paper sheet with superhigh electrical conductivity was prepared via vacuum-assisted filtration and a subsequent hot-pressing process for electromagnetic interference (EMI) shielding applications. By tightly assembling few-layered MXene (f-Ti₃C₂T_x) on the cellulose substrate via hydrogen bonds, an effective and interconnected conductive network was constructed in the paper sheet, resulting in a high electrical conductivity of 774.6–5935.4 S m^?1 at various f-Ti₃C₂T_x loadings. The highly conductive MXene layer can promptly reflect a great amount of incident EM waves, a process which preceded the transmission of EM waves in the cellulose matrix. Owing to the highly efficient reflection-dominated EMI shielding mechanism, the resultant bilayered MXene/cellulose paper sheets exhibit excellent EMI shielding effectiveness of 34.9–60.1 dB and specific EMI shielding efficiency of 290.6–600.7 dB mm^?1. Moreover, the MXene/cellulose paper sheets demonstrated improved mechanical strength (up to 25.7 MPa) and flexibility due to the mechanical frame effect acted by the cellulose substrate. Consequently, the robust and flexible bilayered MXene/cellulose paper sheet is a promising candidate for application in next-generation electric devices.     

Microwave-assisted efficient exfoliation of MXene and its composite for high-performance supercapacitors

MXene, as a promising electrode material, exhibits outstanding performance in supercapacitors because of its excellent chemical and physical properties. However, the conventional inefficient exfoliation methods and the blocked ion transmission channels caused by self-weight accumulation of MXene nanoflakes all severely limit its development. Here, this work reports an efficient microwave exfoliation method that requires only 90s to exfoliate multilayered MXene into few-layer MXene (2–3 nm) with large-size (4～6 μm). Additionally, to enhance its capacitance and cycling stability, the exfoliated MXene was composited with graphene quantum dots, which shows a larger specific surface area, and importantly performs ultrahigh capacitance (343 F/g at a current density of 1 A/g) and excellent capacitance retention (100% after 10,000 cycles at a current density of 5 A/g). When using composite material as an anode, the assembled supercapacitor exhibits an excellent energy density of 35 Wh/kg at a power density of 384 W/kg with 107% capacitance retention after 10,000 cycles. Not only does this work provide an efficient approach to exfoliate MXene, but it also prepares a highly promising material for energy storage materials.     

MXene supported rhodium nanocrystals for efficient electrocatalysts towards methanol oxidation

Since conventional Pt/carbon catalysts usually suffer from CO poisoning as well as carbon corrosion issues during the methanol oxidation reaction, it is essential to explore high-efficiency Pt-alternative electrocatalysts supported by a robust matrix in the direct methanol fuel cells. Herein, we report a convenient low-temperature approach to the controllable fabrication of well-dispersive Rh nanocrystals in situ grown on Ti₃C₂T_x MXene nanosheets. The ultrathin lamellar MXene structure reveals unique superiorities on the construction of advanced Rh-based hybrid catalysts, which can not only provide a large number of efficient anchoring sites for immobilizing small-sized Rh nanocrystals with abundant exposed catalytic crystal planes, but also enable direct electronic interaction with Rh for strong synergistic effects and facilitate the fast charge transportation during the catalytic process. As a consequence, the resulting Rh/Ti₃C₂T_x hybrid exhibits prominent electrocatalytic properties towards methanol oxidation reaction, such as a large electrochemical active surface area of 71.6 m² g^?1, a high mass activity of 600.2 mA mg^?1, and good long-term stability, all of which are much better than those of conventional carbon-supported Rh as well as Pt/C and Pd/C catalysts.     

Flexible and high-sensitivity piezoresistive sensor based on MXene composite with wrinkle structure

Due to the portability, good flexibility and excellent sensing performance, flexible piezoresistive sensors have received great attention in the field of transient electronic skin, intelligent robots and human-machine interaction. However, achieving both high sensitivity and wide sensing range by low-cost and large-scale method still remains a key challenge for developing high performance piezoresistive sensors. Here, a flexible and highly sensitive piezoresistive sensor was designed and realized by combining the 2D MXene material with wrinkle structure. The MXene composite based sensor with wrinkle structure was fabricated by spraying the active material onto the surface of a pre-stretched polyacrylate tape, which is facile, efficient and low-cost. The MXene composite based sensor demonstrates high sensitivity (148.26 kPa⁻¹), wide pressure range (up to 16 kPa), short response time (120 ms) and excellent durability (>13000 cycles). Moreover, benefiting from the extraordinary sensing performance and flexibility, the sensor can detect human physiological signals, monitor intelligent robot postures and map spatial pressure distributions, thus exhibiting great potential in physiological analysis systems, humanoid robotics and biomedical prostheses.     

Review MXenes as a new type of nanomaterial for environmental applications in the photocatalytic degradation of water pollutants

Two-dimensional titanium carbide (MXene) with an adjustable bandgap (0.92–1.75eV), excellent structural stability, high conductivity and hydrophilicity has always been a hotspot in the field of environmental photocatalysis. However, the rapid recombination of light-excited carriers of a single photocatalytic material decreases quantum efficiency and photocatalytic performance. The modification of MXene could overcome these problems to improve photocatalytic properties. Among various improvement strategies, the composition of MXene heterostructure and Schottky junction is an effective and straightforward strategy for adjusting electronic structure and accelerating photocatalytic performance. This review aims to design typical, cost-effective heterojunctions and Schottky junctions and their progress, mechanisms, and trends in environmental organic pollutants' degradation. This review detailed the heterogeneous catalytic mechanism of MXene-based photocatalysts for the degradation of organic pollutants. It is discussed the way to improve the photocatalytic performance of MXene by constructing heterojunction and Schottky junction. The surface properties, catalyst performance and pollution management of various MXene-based catalysts were compared, and then some dilemmas and application strategies of MXene development were analyzed in depth. This review can open up ideas for new approaches and provide valuable clues for designing MXene as a cocatalyst to develop more effective photocatalysts for practical application in environmental pollution management.     

Waste cotton Fabric/MXene composite aerogel with heat generation and insulation for efficient electromagnetic interference shielding

Electromagnetic interference (EMI) shielding materials have become more and more indispensable due to serious electromagnetic-radiation pollution. Herein, waste cotton cellulose aerogels were prepared by dissolving waste cotton fabrics (WCF) in NaOH/urea aqueous solution, and MXene nanosheets were subsequently deposited on the cellulose aerogels by a facile dip coating method to obtain WCF/MXene composite aerogels. The WCF/MXene composite aerogels with highly porous network structure show remarkable electrical conductivity (8.2 Ω/sq of surface resistance), high EMI shielding effectiveness (EMI SE) in the range of 2–18 GHz (39.3–48.1 dB). The WCF/MXene aerogel possesses high SSE and SSE/t of 677.94–829.74 dB cm³ g^?1 and 3512.62–4299.17 dB cm² g^?1, respectively (2–18 GHz). In addition, the heating temperature of WCF/MXene composite aerogels reaches 199 °C when 3 V positive voltage is applied on them. The WCF/MXene composite aerogels possess excellent electromagnetic shielding effectiveness, heat generation property and insulation, which can be potentially used as multifunctional materials for EMI shielding, electrical-heating and high temperature protection.     

Facile synthesis of cobalt modified 2D titanium carbide with enhanced hydrogen evolution performance in alkaline media

2D transition metal carbides, nitrides and carbonitrides, namely the MXenes, attract more and more attentions due to their unique properties. Here, we report a simple one-step molten salt etching method to prepare Co modified MXene hybrid (Ti₃C₂T_x:Co) by the reaction of Ti₃AlC₂ with Lewis acid CoCl₂ at 750 °C. Most of Co atoms aggregates in the interlayered space of Ti₃C₂T_x. Benefitting from the improved electron charge transfer efficiency and increased active sites, the sulfuric acid treated Ti₃C₂T_x:Co-12h hybrid exhibits excellent electrocatalytical activity for hydrogen evolution reaction in alkaline media, delivering a current density of 10 mA cm⁻² at an overpotential of 103.6 mV, which is lower than most noble metal free MXene based electrocatalysts. The results illustrate that the proposed method is very facile and useful to incorporate mid-to-late transition metals into the MXene phase to prepare MXene based HER electrocatalysts.     

2D MXene Nanofilms with Tunable Gas Transport Channels

2D materials' membranes with well‐defined nanochannels are promising for precise molecular separation. Herein, the design and engineering of atomically thin 2D MXene flacks into nanofilms with a thickness of 20 nm for gas separation are reported. Well‐stacked pristine MXene nanofilms are proven to show outstanding molecular sieving property for H₂ preferential transport. Chemical tuning of the MXene nanochannels is also rationally designed for selective permeating CO₂. Borate and polyethylenimine (PEI) molecules are well interlocked into MXene layers, realizing the delicate regulation of stacking behaviors and interlayer spacing of MXene nanosheets. The MXene nanofilms with either H₂‐ or CO₂‐selective transport channels exhibit excellent gas separation performance beyond the limits for state‐of‐the‐art membranes. The mechanisms within these nanoconfined MXene layers are discussed, revealing the transformation from “diffusion‐controlled” to “solution‐controlled” channels after chemical tuning. This work of precisely tailoring the 2D nanostructure may inspire the exploring of nanofluidics in 2D confined space with applications in many other fields like catalysis and energy conversion processes.     

Flexible Ti3C2Tx@Al electrodes with Ultrahigh Areal Capacitance: In Situ Regulation of Interlayer Conductivity and Spacing

Although Ti₃C₂ MXene has shown great potential in energy storage field, poor conductivity and restacking between MXene flakes seriously hinders the maximization of its capacitance. Herein, a new strategy to solve the problems is developed. Gallery Al atoms in Ti₃AlC₂ are partially removed by simple hydrothermal etching to get Ti₃C₂T_x reserving appropriate Al interlayers (Ti₃C₂T_x@Al). Ti₃C₂T_x@Al keeps stable layered structure rather than isolated Ti₃C₂T_x flakes, which avoids flake restacking. The removal of partial Al frees up space for easy electrolyte infiltration while the reserved Al as “electron bridges” ensures high interlayer conductivity. As a result, the areal capacitance reaches up to 1087 mF cm^?2 at 1 mA cm^?2 and over 95% capacitance is maintained after 6000 cycles. The all‐solid‐state supercapacitor (ASSS) based on Ti₃C₂T_x@Al delivers a high capacitance of 242.3 mF cm^?2 at 1 mV s^?1 and exhibits stable performance at different bending states. Two ASSSs in tandem can light up a light‐emitting diode under the planar or wrapping around an arm. The established strategy provides a new avenue to improve capacitance performances of MXenes.