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One of the most fascinating 2D nanomaterials (NMs) ever found is various members of MXene family. Among them, the titanium‐based MXenes, with more than 70% of publication‐related investigations, are comparatively well studied, producing fundamental foundation for the 2D MXene family members with flexible properties, familiar with a variety of advanced novel technological applications. Nonetheless, there are still more candidates among transitional metals (TMs) that can function as MXene NMs in ways that go well beyond those that are now recognized. Systematized details of the preparations, characteristics, limitations, significant discoveries, and uses of the novel M‐based MXenes (M‐MXenes), where M stands for non‐Ti TMs (M = Sc, V, Cr, Y, Zr, Nb, Mo, Hf, Ta, W, and Lu), are given. The exceptional qualities of the 2D non‐Ti MXene outperform standard Ti‐MXene in several applications. There is many advancement in top‐down as well as bottom‐up production of MXenes family members, which allows for exact control of the M‐characteristics MXene NMs to contain cutting‐edge applications. This study offers a systematic evaluation of existing research, covering everything in producing complex M‐MXenes from primary limitations to the characterization and selection of their applications in accordance with their novel features. The development of double metal combinations, extension of additional metal candidates beyond group‐(III–VI)B family, and subsequent development of the 2D TM carbide/TMs nitride/TM carbonitrides to 2D metal boride family are also included in this overview. The possibilities and further recommendations for the way of non‐Ti MXene NMs are in the synthesis of NMs will discuss in detail in this critical evaluation. Non‐Ti MXenes may be synthesized from more abundant and environmental friendly sources, making them an attractive alternative. These non‐Ti MXene NMs exhibit unique properties, such as high electrical conductivity, mechanical strength, thermal stability, and good electrochemical and thermal properties, making them promising candidates for a variety of applications such as energy storage, catalysis, thermal management, and wear‐resistant coatings.

Double transition metal (DTM) MXenes are a recently discovered class of two‐dimensional composite nanomaterials with excellent potential in energy storage applications. Since their emergence in 2015, DTM MXenes have expanded their composition boundary beyond traditional single‐metal carbide and nitride MXenes. DTM MXenes offer tunable structures and properties through variations in the constituent transition metals and positioning within the layered lattice. These MXenes can exist in two primary forms: ordered DTMs and solid solutions. The compositional versatility of DTM MXenes offers opportunities to enhance their performance in electrochemical energy storage applications. However, the quality, stability, and surface chemistry of DTM MXenes are influenced by several factors, including the etching process, etchant type, and synthesis route. Currently, limited literature is available on experimentally synthesized DTM MXenes, with most studies focusing on carbide‐based MXenes. Most of the articles have dedicated their efforts only to generalized synthesis strategies. Although extensive theoretical studies have explored the suitability of etchants, synthesis parameters, and methods for producing high‐quality MXene with selective terminal functional groups, their stability issues have not been thoroughly examined. This review addresses various types of DTM MXenes, their synthesis techniques, and the impact of these methods on their physicochemical properties and electrochemical performance. Additionally, it provides a critical analysis of the causes of instability in MXenes, particularly DTMs, from synthesis to application. The challenges associated with these materials are discussed, along with opportunities and prospects for enhancing synthesis, structural tuning, surface modification, and applications in electrochemical energy storage. This review presents the synthesis, physicochemical properties, and electrochemical performance of double transition metal (DTM) MXenes, emphasizing the challenges and instabilities in their production and application. The work highlights the potential of DTM MXenes in energy storage and offers insights for improving their stability and functionality.

Unsustainable fossil fuel energy usage and its environmental impacts are the most significant scientific challenges in the scientific community. Two-dimensional (2D) materials have received a lot of attention recently because of their great potential for application in addressing some of society’s most enduring issues with renewable energy. Transition metal-based nitrides, carbides, or carbonitrides, known as “MXenes”, are a relatively new and large family of 2D materials. Since the discovery of the first MXene, Ti 3 C 2 in 2011 has become one of the fastest-expanding families of 2D materials with unique physiochemical features. MXene surface terminations with hydroxyl, oxygen, fluorine, etc., are invariably present in the so far reported materials, imparting hydrophilicity to their surfaces. The current finding of multi-transition metal-layered MXenes with controlled surface termination capacity opens the door to fabricating unique structures for producing renewable energy. MXene NMs-based flexible chemistry allows them to be tuned for energy-producing/storage, electromagnetic interference shielding, gas/biosensors, water distillation, nanocomposite reinforcement, lubrication, and photo/electro/chemical catalysis. This review will first discuss the advancement of MXenes synthesis methods, their properties/stability, and renewable energy applications. Secondly, we will highlight the constraints and challenges that impede the scientific community from synthesizing functional MXene with controlled composition and properties. We will further reveal the high-tech implementations for renewable energy storage applications along with future challenges and their solutions. Graphical Abstract

Two-dimensional MXenes possessed exceptional physiochemical properties such as high electrical conductivity (20,000 Scm−1), flexibility, mechanical strength (570 MPa), and hydrophilic surface functionalities that have been widely explored for energy storage, sensing, and catalysis applications. Recently, the fabrication of MXenes thin films has attracted significant attention toward electronic devices and sensor applications. This review summarizes the exciting features of MXene thin film fabrication methods such as vacuum-assisted filtration (VAF), electrodeposition techniques, spin coating, spray coating, dip-coating methods, and other physical/chemical vapor deposition methods. Furthermore, a comparison between different methods available for synthesizing a variety of MXenes films was discussed in detail. This review further summarizes fundamental aspects and advances of MXenes thin films in solar cells, batteries, electromagnetic interference shielding, sensing, etc., to date. Finally, the challenges and opportunities in terms of future research, development, and applications of MXenes-based films are discussed. A comprehensive understanding of these competitive features and challenges shall provide guidelines and inspiration for further growth in MXenes-based functional thin films and contribute to the advances in MXenes technology.

HighlightsThe progression of MXene's oxidation stability, the techniques available to monitor the phenomenon as well as the variables that contribute to its oxidation rate are discussed.Comprehensive aspects of the oxidation process in various storage settings and the debated oxidation mechanism along with the most effective antioxidation strategies are addressed in conjunction with current challenges to the air stability of MXenes.MXenes are under the spotlight due to their versatile physicochemical characteristics. Since their discovery in 2011, significant advancements have been achieved in their synthesis and application sectors. However, the spontaneous oxidation of MXenes, which is critical to its processing and product lifespan, has gotten less attention due to its chemical complexity and poorly understood oxidation mechanism. This perspective focuses on the oxidation stability of MXenes and addresses the most recent advancements in understanding and the possible countermeasures to limit the spontaneous oxidation of MXenes. A section is dedicated to the presently accessible methods for monitoring oxidation, with a discussion on the debatable oxidation mechanism and coherently operating factors that contribute to the complexity of MXenes oxidation. The current potential solutions for mitigating MXenes oxidation and the existing challenges are also discussed with prospects to prolong MXene’s shelf-life storage and expand their application scope.

HighlightsNb2C and Ta2C MXenes exhibit remarkable SERS performance with the enhancement factors of 3.0 × 106 and 1.4 × 106, which is synergistically enabled by the PICT resonance enhancement and electromagnetic enhancement.The excellent SERS sensitivity endows Ta2C MXene with the capability to sensitively detect and accurately identify the SARS-CoV-2 spike protein, which is beneficial to achieve real-time monitoring and early warning of novel coronavirus.The outbreak of coronavirus disease 2019 has seriously threatened human health. Rapidly and sensitively detecting SARS-CoV-2 viruses can help control the spread of viruses. However, it is an arduous challenge to apply semiconductor-based substrates for virus SERS detection due to their poor sensitivity. Therefore, it is worthwhile to search novel semiconductor-based substrates with excellent SERS sensitivity. Herein we report, for the first time, Nb2C and Ta2C MXenes exhibit a remarkable SERS enhancement, which is synergistically enabled by the charge transfer resonance enhancement and electromagnetic enhancement. Their SERS sensitivity is optimized to 3.0 × 106 and 1.4 × 106 under the optimal resonance excitation wavelength of 532 nm. Additionally, remarkable SERS sensitivity endows Ta2C MXenes with capability to sensitively detect and accurately identify the SARS-CoV-2 spike protein. Moreover, its detection limit is as low as 5 × 10−9 M, which is beneficial to achieve real-time monitoring and early warning of novel coronavirus. This research not only provides helpful theoretical guidance for exploring other novel SERS-active semiconductor-based materials but also provides a potential candidate for the practical applications of SERS technology.

Due to their unique layered microstructure, the presence of various functional groups at the surface, earth abundance, and attractive electrical, optical, and thermal properties, MXenes are considered promising candidates for the solution of energy- and environmental-related problems. It is seen that the energy conversion and storage capacity of MXenes can be enhanced by changing the material dimensions, chemical composition, structure, and surface chemistry. Hence, it is also essential to understand how one can easily improve the structure–property relationship from an applied point of view. In the current review, we reviewed the fabrication, properties, and potential applications of MXenes. In addition, various properties of MXenes such as structural, optical, electrical, thermal, chemical, and mechanical have been discussed. Furthermore, the potential applications of MXenes in the areas of photocatalysis, electrocatalysis, nitrogen fixation, gas sensing, cancer therapy, and supercapacitors have also been outlooked. Based on the reported works, it could easily be observed that the properties and applications of MXenes can be further enhanced by applying various modification and functionalization approaches. This review also emphasizes the recent developments and future perspectives of MXenes-based composite materials, which will greatly help scientists working in the fields of academia and material science.

In 2011, Gogotsi et al. discovered a new type of two‐dimensional transition metal carbides and nitrides, called MXenes, which have become a dazzling new star in the energy storage industry. MXenes are endowed with a series of fascinating properties due to their unique structures and tunable surface chemical functional groups. The application of MXenes in electrochemical energy storage has attracted special attention, especially showing great potential in supercapacitor applications. Compared with other materials, MXenes have high mechanical flexibility, high energy density, and good electrochemical performance, so they are especially suitable as electrode materials for supercapacitors. However, similar to other 2D materials, due to the strong van der Waals forces, MXene layers inevitably undergo stacking agglomeration, resulting in severe loss of electrochemically active sites. If the self‐stacking of MXenes layers can be effectively suppressed, their electrochemical performance will be enhanced. Structural optimization of MXenes and composite doping of MXenes with other materials are two strategies with significant effects. This review summarizes recent advances in MXene synthesis, fundamental properties, and composite materials, focusing on the latest electrochemical performance of MXene‐based electrodes/devices, and puts forward the challenges and new opportunities that MXenes face in this emerging energy storage field. MXene is a bright new star in the field of energy storage, and the application of MXene in supercapacitors has received special attention. This study summarizes the latest progress in MXene synthesis, basic properties, and composite materials, focusing on MXene‐based electrodes/devices and the latest electrochemical performance. It presents challenges and new opportunities for MXene in this future energy storage field.

HighlightsThis review summarizes applications and developments of MXenes in solar cells by far. The issues needing to be addressed for performance improvement of the related solar cells are discussed.Suggestions are given for pushing exploration of MXenes’ application in solar cells.Application of two-dimensional MXene materials in photovoltaics has attracted increasing attention since the first report in 2018 due to their metallic electrical conductivity, high carrier mobility, excellent transparency, tunable work function and superior mechanical property. In this review, all developments and applications of the Ti3C2Tx MXene (here, it is noteworthy that there are still no reports on other MXenes’ application in photovoltaics by far) as additive, electrode and hole/electron transport layer in solar cells are detailedly summarized, and meanwhile, the problems existing in the related studies are also discussed. In view of these problems, some suggestions are given for pushing exploration of the MXenes’ application in solar cells. It is believed that this review can provide a comprehensive and deep understanding into the research status and, moreover, helps widen a new situation for the study of MXenes in photovoltaics.

Ti3C2Tx MXene@rGo self-supporting film was prepared by HF etching and LiF/HCl etching method, and film welding was realized for the first time. The results showed that Ti3C2Tx MXene with typical accordion-like morphology was obtained by HF etching. In the Ti3C2Tx MXene@rGo self-supporting film welding test, it was found that in the time-sensitive period, Ti3C2Tx MXene@rGo self-supporting film can be welded with water as the medium, effectively ensuring the stability of the material welding. If beyond the time-sensitive period, the self-supporting film will lose weldability. This study is a significant advancement for the large-scale Ti3C2Tx MXene@rGo composites.