2D transition metal carbides, nitrides, and carbonitrides, also known as MXenes, are versatile materials due to their adjustable structure and rich surface chemistry. The physical and chemical diversity has recognized MXenes as a potential 2D material with a wide spectrum of application domains. Since the discovery of MXenes in 2011, a wide variety of synthetic routes has been proposed with advancement toward large-scale preparing methods for MXene nanosheets and derivative products. Herein, the critical synthesis aspects and the operating conditions that influence the physical and chemical characteristics of MXenes are discussed in detail. The emerging etching methods including HF etching methods, in situ HF-forming etching methods, electrochemical etching methods, alkali etching methods, and molten salt etching methods, as well as delamination strategies are discussed. Considering the future developments and practical applications, the large-scale synthesis routes and the antioxidation strategies of MXenes are also summarized. In summary, a generalized overview of MXenes synthesis protocols with an outlook for the current challenges and promising technologies for large-scale preparation and stable storage is provided.

Advances in the Synthesis of 2D MXenes.

The osmotic pressure difference between river water and seawater is a promising source of renewable energy. However, current osmotic energy conversion processes show limited power output, mainly owing to the low performance of commercial ion-exchange membranes. Nanofluidic channels with tailored ion transport dynamics enable high-performance reverse electrodialysis to efficiently harvest renewable osmotic energy. In this Review, we discuss ion diffusion through nanofluidic channels and investigate the rational design and optimization of advanced membrane architectures. We highlight how the structure and charge distribution can be tailored to minimize resistance and promote energy conversion, and examine the possibility of integrating nanofluidic osmotic energy conversion with other technologies, such as desalination and water splitting. Finally, we give an outlook to future applications and discuss challenges that need to be overcome to enable large-scale, real-world applications. Osmotic energy conversion is a promising renewable energy source. This Review discusses nanofluidics-based osmotic energy conversion systems, investigating the principles of ion diffusion in nanofluidic systems, optimization of membrane architectures to increase energy conversion and possible integration with other technologies, such as water splitting.

Nanofluidics for osmotic energy conversion

Electromagnetic (EM) absorbers drive the development of EM technology and advanced EM equipment. The utilization of EM energy conversion of the EM absorber to design a variety of devices is attractive and promising, especially in personal protection and healthcare. In this review article, wearable EM materials are reviewed, ranging from design strategies, EM response mechanism, EM performance improvement, to the construction of smart EM devices. For EM response mechanism, the relaxation and charge transport associated with radiation energy conversion are dissected. For wearable EM devices, two main functions are highlighted, including EM sensors to replace of human senses, as well as EM absorbers to block transmission radiation. Furthermore, the current issues and potential opportunities of the wearable EM devices are pointed out, and new directions for future prospects are proposed.

Electromagnetic absorber converting radiation for multifunction

Graphene aerogels (GAs) offer a distinctive combination of high porosity, low density, large specific surface area and high compressibility, which make it grab considerable attention in various applications, in particular for high performance electromagnetic wave attenuation. The internal porous structure and three-dimensional (3D) network of GAs solve the phenomenon of graphene sheet layer agglomeration, high conductivity and impedance mismatch in two-dimensional graphene, which is conducive to the improvement of microwave absorption performance. In addition, GAs incorporate other lossy materials as a framework have been widely studied to achieve more efficient microwave absorption. Herein, the latest advances in the synthetic strategies and structural characteristics of graphene-based materials are reviewed. Furthermore, we summarized recent advances in graphene-based aerogels as microwave absorbing materials, including pure GAs and hybrid aerogels with other lossy materials. In addition, we also highlighted the multifunctional microwave absorbing materials. On this basis, we summarized the research status of graphene-based microwave absorbing aerogels and put forward the challenges and outlook of graphene-based microwave absorbing aerogels.

A review of three-dimensional graphene-based aerogels: Synthesis, structure and application for microwave absorption

The revolutionary and pioneering advancements of flexible electronics provide the boundless potential to become one of the leading trends in the exploitation of wearable devices and electronic skin. Working as substantial intermediates for the collection of external mechanical signals, flexible strain sensors that get intensive attention are regarded as indispensable components in flexible integrated electronic systems. Compared with conventional preparation methods including complicated lithography and transfer printing, 3D printing technology is utilized to manufacture various flexible strain sensors owing to the low processing cost, superior fabrication accuracy, and satisfactory production efficiency. Herein, up-to-date flexible strain sensors fabricated via 3D printing are highlighted, focusing on different printing methods based on photocuring and materials extrusion, including Digital Light Processing (DLP), fused deposition modeling (FDM), and direct ink writing (DIW). Sensing mechanisms of 3D printed strain sensors are also discussed. Furthermore, the existing bottlenecks and future prospects are provided for further progressing research.

3D Printed Flexible Strain Sensors: From Printing to Devices and Signals

Ti3C2Tx MXene is an emerging class of two-dimensional nanomaterials with exceptional electroconductivity and electrochemical properties, and is promising in the manufacturing of multifunctional macroscopic materials and nanomaterials. Herein, we develop a straightforward, continuously controlled, additive/binder-free method to fabricate pure MXene fibers via a large-scale wet-spinning assembly. Our MXene sheets (with an average lateral size of 5.11 μm2) are highly concentrated in water and do not form aggregates or undergo phase separation. Introducing ammonium ions during the coagulation process successfully assembles MXene sheets into flexible, meter-long fibers with very high electrical conductivity (7,713 S cm−1). The fabricated MXene fibers are comprehensively integrated by using them in electrical wires to switch on a light-emitting diode light and transmit electrical signals to earphones to demonstrate their application in electrical devices. Our wet-spinning strategy provides an approach for continuous mass production of MXene fibers for high-performance, next-generation, and wearable electronic devices. Large-scale production of fibers from two dimensional materials opens a pathway to promising applications. Here the authors report meter-long MXene fibers with high electrical conductivity that are fabricated via continuous wet spinning and demonstrated in electrical wires.

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Large-scale wet-spinning of highly electroconductive MXene fibers.

Self-assembly of two-dimensional MXene sheets is used in various fields to create multiscale structures due to their electrical, mechanical, and chemical properties. In principle, MXene nanosheets are assembled by molecular interactions, including hydrogen bonds, electrostatic interactions, and van der Waals forces. This study describes how MXene colloid nanosheets can form self-supporting MXene hydrogels. Three-dimensional network structures of MXene gels are strengthened by reinforced electrostatic interactions between nanosheets. Stable gel networks are beneficial for fabricating highly aligned fibers because MXene gel can endure structural deformation. During wet spinning of highly concentrated MXene colloids in a coagulation bath, MXene sheets can be transformed into perfectly aligned fibers under a mechanical drawing force. Oriented MXene fibers exhibit a 1.5-fold increase in electrical conductivity (12 504 S cm-1) and Young's modulus (122 GPa) compared with other fibers. The oriented MXene fibers are expected to have widespread applications, including electrical wiring and signal transmission.

Highly electroconductive and mechanically strong Ti3C2Tx mxene fibers using a deformable mxene gel

Over the past few years, several nanofluidic channels have been constructed using ion-conductive materials. However, the design and fabrication of surface-charge-controllable nanochannels remain a scientific as well as a technological challenge. This study investigated the feasibility of graphene oxide (GO)-based fiber-type nanochannels for generating electrical energy by converting the salinity gradient. Owing to their large lateral size and the localized charged species on the edge, the low charge density of the GO fibers remains a critical bottleneck in their wider investigation. To address this critical issue, highly negatively charged and extremely small (2.42 ± 0.38 nm) graphene quantum dots (GQDs) were synthesized and intercalated through the interstitial network of GO sheets in fibers. With the application of GQDs, the charge density was significantly increased to 1.12 mC m−2 so that the ion conductance was enhanced to an average of 21 nS and the electrical energy generation was 0.25 W m−2. This study presents a facile and novel approach of enhancing ion selectivity and ion conductivity of graphene-fiber based miniaturized nanofluidic channels, proving their potential for osmotic energy generation and efficiency.

Graphene quantum dots/graphene fiber nanochannels for osmotic power generation

Holey graphene oxide membranes containing both nanopores and nanochannels for highly efficient harvesting of water evaporation energy

Inspired by the role of cellular structures, which give three-dimensional robustness to graphene structures, a new type of graphene cantilever with mechanical resilience is introduced. Here, NH4SCN...

Dong Jun Kang

Papers

Large-scale wet-spinning of highly electroconductive MXene fibers.

Highly electroconductive and mechanically strong Ti3C2Tx mxene fibers using a deformable mxene gel

Graphene quantum dots/graphene fiber nanochannels for osmotic power generation

Holey graphene oxide membranes containing both nanopores and nanochannels for highly efficient harvesting of water evaporation energy

Graphene Foam Cantilever Produced via Simultaneous Foaming and Doping Effect of an Organic Coagulant