2D MXenes Nanosheets for Advanced Energy Conversion and Storage Devices: Recent Advances and Future Prospects

Since the initial MXenes were discovered in 2011, several MXene compositions constructed using combinations of various transition metals have been developed. MXenes are ideal candidates for different applications in energy conversion and storage, because of their unique and interesting characteristics, which included good electrical conductivity, hydrophilicity, and simplicity of large-scale synthesis. Herein, we study the current developments in two-dimensional (2D) MXene nanosheets for energy storage and conversion technologies. First, we discuss the introduction to energy storage and conversion devices. Later, we emphasized on 2D MXenes and some specific properties of MXenes. Subsequently, research advances in MXene-based electrode materials for energy storage such as supercapacitors and rechargeable batteries is summarized. We provide the relevant energy storage processes, common challenges, and potential approaches to an acceptable solution for 2D MXene-based energy storage. In addition, recent advances for MXenes used in energy conversion devices like solar cells, fuel cells and catalysis is also summarized. Finally, the future prospective of growing MXene-based energy conversion and storage are highlighted.     

MXenes复合材料的发展:界面调控及结构设计

MXenes作为一类新兴的二维材料, 因具有独特的亲水性、 优异的力学性能、 丰富的表面官能团、 高导电率、 光热以及光电效应等性能而成为研究热点, 广泛应用在电磁屏蔽、 电化学储能、 生物医药、 分离、 传感器和海水淡化等领域. 虽然MXenes具有这些优异的性能, 但是其存在的与疏水性高分子相容性差、 带负电的官能团阻碍电解质运输及易氧化等问题限制了其实际应用. 近年来, 通过对MXenes进行界面调控以解决其固有缺陷, 并在此基础上对MXenes进行针对性的结构设计以提升界面稳定性, 其性能得到了进一步提升. 本文对MXenes复合材料发展过程中在界面调控和结构设计方面的研究进展进行总结, 并重点介绍了MXenes的结构和性质及MXenes在复合材料中的界面调控手段, 对MXenes复合材料的结构设计进行了阐述, 并对MXenes复合材料的发展前景进行了展望.     

Role of MXene surface terminations in electrochemical energy storage: A review

MXenes are a group of recently discovered 2D materials and have attracted extensive attention since their first report in 2011; they have shown excellent prospects for energy storage applications owing to their unique layered microstructure and tunable electrical properties. One major feature of MXenes is their tailorable surface terminations (e.g., −F, −O, −OH). Numerous studies have indicated that the composition of the surface terminations can significantly impact the electrochemical properties of MXenes. Nonetheless, the underlying mechanisms are still poorly understood, mainly because of the difficulties in quantitative analysis and characterization. This review summarizes the latest research progress on MXene terminations. First, a systematic introduction to the approaches for preparing MXenes is presented, which generally dominates the surface terminations. Then, theoretical and experimental efforts regarding the surface terminations are discussed, and the influence of surface terminations on the electronic and electrochemical properties of MXenes are generalized. Finally, we present the significance and research prospects of MXene terminations. We expect this review to encourage research on MXenes and provide guidance for usingthese materials for batteries and supercapacitors.     

Chemistry of two-dimensional MXene nanosheets in theranostic nanomedicine

Transition metal carbide,carbonitride and nitride MXenes,as the emerging two-dimensional(2D)nanomaterials,have aroused burgeoning research interest in a broad range of applications ranging from energy conversion to biomedicines attributing to their distinctive planar nanostructure,physiochemical properties and biological effects.They are featured with fascinating electronic,optical,magnetic,mechanical and thermal properties,which exert significant roles in biomedical applications of 2D MXenes.In this review,we briefly summarize the recent research progress of 2D MXenes and highlight their intrinsic chemistry in theranostic nanomedicines,focusing on the synthetic chemistry for MXenes construction,surface chemistry for surface engineering,physiochemical property for theranostic application and biological chemistry for biosafety evaluation.Furthermore,based on the current achieve ments on MXenes,their potential research directio n,critical challenges and future development in biomedicine are also discussed.It is highly expected that 2D MXene-based nanosystems would have a broad application prospect in theranostic biomedicine provided the current facing critical issues and challenges are adequately solved.     

Recent progress in MXenes incorporated into electrospun nanofibers for biomedical application: Study focusing from 2017 to 2022

After discovering a new class of two-dimensional (2D) material, i.e., MXene, a further new scope, came into existence for researchers. Due to their remarkable physical, chemical, and biological properties, MXenes find their role in almost every research discipline. They have been used in biosensors, bioimaging, tissue engineering, drug delivery systems, and other areas. The MXenes can be functionalized with a wide range of atoms/molecules, making them diverse materials. Therefore, the potential of using MXenes in nanofibers can be much more than expected. In this review, we will understand the structure, synthesis, and general properties of MXenes. We will explain using MXenes while encasing them into nanofibers, providing their specific properties. For instance, MXenes-incorporated nanofibers are used in biomedical applications, including soft and hard-tissue engineering and delivery of antimicrobials. Furthermore, MXenes, when incorporated into nanofibers, are used in promoting cellular differentiation, wound healing, and neural tissue restoration, which are briefly discussed in this communication.     

The potential of MXene materials as a component in the catalyst layer for the Oxygen Evolution Reaction

MXenes are a class of 2D/layered materials which are highly conductive, hydrophilic, have a large electrochemical surface area and are easily processible into electrodes for energy applications. Since the discovery of MXenes over ten years ago, these materials have been mainly used in the preparation of electrodes for batteries and supercapacitors. However, due to their aforementioned properties, MXenes could potentially be utilised as a component in the catalyst layer for the Oxygen Evolution Reaction (OER). This opinion piece will discuss some of the recent literature in the area of hybrid catalysts consisting of various Transition Metal Oxides (TMOs) and MXenes for the OER. We will also discuss current drawbacks and future outlook in this new area of research.     

Current progresses in two-dimensional MXene-based framework: prospects from superficial synthesis to energy conversion and storage applications

MXenes are regarded as a type of two-dimensional (2D) inorganic material, mainly comprising a number of transition metal carbides, nitrides, or carbonitrides atomic planes. Nevertheless, the scientific community is continuously interested in exploring and structuring the engineered-based multifunctional material for numerous applications. The MXenes-based materials in this context, have emerged as highly active compounds owing to their superior surface area, substantial interlayer spacing, highly reactive surface-active sites and surface functional group, even though, recent studies have shown significant scientific and theoretical progress related to enormous prospects in MXenes, chemical nature, robust electrochemistry and high hydrophilicity of MXenes. The role of MXenes in all kinds of strategies is still in an upgrading phase for their further improvement, and is not sufficiently summarized in the literature now. To begin with this, herein, present review article is intended to critically discuss the diversity of MXenes with respect to different composition, formulation, plasmonic, complexation, and numerous geometric and morphological aspects, along with novel construction strategies to improve their surface characteristics in all aforesaid multidimensional applications. Following that, in terms of broadening the application, this review article is envisaged to endorse the use of MXenes and their hybrid configuration in a series of emerging environmental decontamination via adsorption, photodegradation, photocatalytic fuel production via hydrogen evolution, CO₂ reduction, electrocatalytic sensing, along with membrane distillation and energy storage. In addition, comprehensive information about existing obstacles and future perspectives have been addressed. Finally, an overview is succinctly summarized and discussed regarding the emerging prospects of MXenes for their potential uses in numerous research fields. At the end, it is anticipated that this review article will pave the way for the effective use of MXenes in different fields of environmental remediation, energy conversion, storage and biomedical applications as an innovative, reliable, and multifunctional material.     

Adsorption and Activation of CO2 on Nitride MXenes: Composition,Temperature, and Pressure effects

The interaction of CO₂ with nitride MXenes of different thickness is investigated using periodic density functional theory-based calculations and kinetic simulations carried out in the framework of transition state theory, the ultimate goal being predicting their possible use in Carbon Capture and Storage (CCS). We consider the basal (0001) surface plane of nitride MXenes with M_n+1N_n (n=1–3; M=Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, and W) stoichiometry and also compare to equivalent results for extended (001) and (111) surfaces of the bulk rock-salt transition metal nitride compounds. The present results show that the composition of MXenes has a marked influence on the CO₂-philicity of these substrates, whereas the thickness effect is, in general, small, but not negligible. The largest exothermic activation is predicted for Ti-, Hf-, and Zr-derived MXenes, making them feasible substrates for CO₂ trapping. From an applied point of view, Cr-, Mo-, and W-derived MXenes are especially well suited for CCS as the interaction with CO₂ is strong enough but molecular dissociation is not favored. Newly developed kinetic phase diagrams are introduced supporting that Cr-, Mo-, and W-derived MXenes are appropriate CCS substrates as they are predicted to exhibit easy capture at mild conditions and easy release by heating below 500 K.     

非贵金属助催化剂MXene在光催化领域应用的研究进展

环境友好型半导体光催化是当前最具前景的光催化技术之一,它不仅能够将太阳能转化为化学能以解决能源危机,还可以将污染物降解矿化从而解决环境问题.但是,传统的半导体光催化剂受限于光利用率低、光生载流子复合率高、稳定性较差等几个方面,无法达到理想的光催化效果.在半导体光催化剂上负载助催化剂是提升光催化效率的有效策略之一.负载助催化剂能够增强光生电荷在半导体与助催化剂界面间的传输,提供额外的催化活性位点,增强光捕获能力,因而被广泛应用于光催化剂的改性.目前广泛使用的贵金属助催化剂包括Au,Ag,Pt,Ru等,虽然这些贵金属助催化剂性能优异,但是它们存在储量少和成本高的问题,严重影响其规模化应用.因此,开展高效且成本低廉的非贵金属助催化剂的研究非常必要.近来,一种新型二维过渡金属材料(MXene)因其具有独特的二维层状结构、优异的导电性能、出色的光学和热力学性质而成为催化领域的研究热点.本文综述了有关非贵金属助催化剂MXene在光催化领域的最新研究进展,内容包括:(1)MXene材料的体相与表面结构特性;(2)薄层MXene的制备方法,例如氢氟酸刻蚀法、氢氟酸替代物刻蚀法以及熔融氟盐刻蚀法;(3)MXene基复合光催化剂的合成及改性策略,包括机械混合、自组装、原位氧化等;(4)MXene辅助增强光催化活性机理.论文还重点介绍了MXene作为助催化剂在光催化领域中的应用,包括光催化分解水产氢、光催化CO2还原、光催化固氮以及有机污染物的光催化降解.最后,论文分析了MXene基异质结光催化剂存在的问题与面临的挑战,并对MXene助催化剂的未来发展进行了展望.主要观点包括:(1)关于光催化分解水、空气净化、合成氨领域的研究较少,需要进一步开展;(2)MXene基异质结光催化剂的反应机理仍存在争议,需采用现代化仪器设备(包括原位表征技术)对其进行更为深入的探究;(3)目前,大多数MXene材料的制备都是通过强腐蚀性的氢氟酸或氢氟酸替代物刻蚀,开发环境友好且高效的MXene制备方法迫在眉睫;(4)阐明MXene表面终端基团的作用有助于提升MXene基复合光催化剂的性能;(5)引入新的改性策略如局域表面等离子体共振效应(LSPR)、缺陷调控、单原子催化(SAC)等来提高MXene基光催化剂的催化性能,是未来MXene基复合催化剂的发展方向.     

Environmental significance of wearable sensors based on MXene and graphene

Conductive layered materials such as MXenes (e.g., transition metal carbides, nitrides, and carbonitrides), graphene and their derivatives have attracted tremendous research interests in diverse fields of research for their unique structured merits and outstanding physical and chemical properties. Benefitting from their unique layered structures and fascinating multifunctional characteristic, MXenes and graphene serve as vital components in a variety of wearable devices. Especially, due to their large surface area and high electrocatalytic activity, these materials have also demonstrated great promise in biophysical and biochemical sensing systems. Following an introduction into the field, we summarize the recent progress in wearable sensors that can be accomplished by using layered materials, with a specific focus on kinematic, mechanical, thermal, pressure and strain sensors. A further large section underscores the recent progress in MXenes and graphene based wearable biochemical sensors including electrolyte monitoring, glucose monitoring, micro/mcromolecular organics metabolite, volatile gases monitoring and humidity sensors. The next section covers the sensing of small biomolecules serving as biomarkers, which are of great significance for early diagnosis and treatment of a spectrum of diseases. This review underscores the recent progress in wearable sensors to be used in different physiological and environmental signals. Finally, the review concludes with a debate on current challenges being faced and future perspectives.     

基于MXenes的功能纤维的制备及其在智能可穿戴领域的应用

在电子信息和物联网技术的推动下,人类对可穿戴电子器件和智能织物的需求愈发突出,功能纤维作为智能可穿戴设备的重要载体,近年来获得快速发展。功能纤维的性能很大程度上取决于纤维的基础构筑单元。过渡金属碳/氮化物(MXenes)作为一种新兴的二维材料,凭借其高电导率、优异的可加工性能、可调节的表面特性以及出色的机械强度等优点,受到了极大的关注,也逐渐成为构筑功能纤维的重要单元。本文将主要综述MXenes的湿化学、熔融盐、无氟试剂刻蚀等方法和力学、电学、光学和化学稳定性等性能,阐述基于该材料制备的功能纤维在传感、储能以及其他智能领域的应用,最后讨论了基于MXenes材料的功能纤维的未来应用前景和技术挑战。     

Unraveling the Oxidation Behaviors of MXenes in Aqueous Systems by Active-Learning-Potential Molecular-Dynamics Simulation

MXenes are 2D materials with great potential in various applications. However, the degradation of MXenes in humid environments has become a main obstacle in their practical use. Here we combine deep neural networks and an active learning scheme to develop a neural network potential (NNP) for aqueous MXene systems with ab initio precision but low cost. The oxidation behaviors of super large aqueous MXene systems are investigated systematically at nanosecond timescales for the first time. The oxidation process of MXenes is clearly displayed at the atomic level. Free protons and oxides greatly inhibit subsequent oxidation reactions, leading to the degree of oxidation of MXenes to exponentially decay with time, which is consistent with the oxidation rate of MXenes measured experimentally. Importantly, this computational study represents the first exploration of the kinetic process of oxidation of super-sized aqueous MXene systems. It opens a promising avenue for the future development of effective protection strategies aimed at controlling the stability of MXenes.     

A mini-review on MXenes as versatile substrate for advanced sensors

MXenes have emerged as versatile 2D materials that are already gaining paramount attention in the areas of energy,catalyst,electromagnetic shielding,and sensors.The unique surface chemistry,graphene-like mo rphology,high hydrophilicity,metal-like conductivity with redox capability identifies MXenes,as an ideal material for surface-related applications.This short review summarizes the most recent reports that discuss the potential application of MXenes and their hybrids as a transducer material for advanced sensors.Based on the nature of transducing signals,the discussion is categorized into three sections,which include electrochemical(bio) sensors,gas sensors,and finally,electro-chemiluminescence fluorescent sensors.The review provides a concise summary of all the analytical merits obtained subsequent to the use of MXenes,followed by endeavors that have been made to accentuate the future perspective of MXenes in sensor devices.     

MXenes在水系锌离子电池中的应用研究进展

水系锌离子电池(AZIBs)作为一种低成本、高安全的新兴且前景广阔的储能技术近年来备受关注。新型MXenes材料由于其独特的结构特征和物理化学性质,如易调节的二维结构、优异的导电性、化学组成多样和可控的表面化学特性,在AZIBs中表现出独特的应用优势。本文全面综述近年来MXenes在AZIBs中应用的研究进展,探讨MXenes应用于AZIBs正负极的结构设计及性能优化策略：在正极方面,MXenes可直接作为活性物质或活性物质前驱体、基体材料,以获得高活性、优异的循环寿命和倍率性能;在负极方面,MXenes可作为锌沉积的二维/三维载体、亲锌基体及锌金属界面保护层,以减缓电化学反应过程中锌金属的腐蚀和枝晶生长。此外,本文也对MXenes基材料在AZIBs中应用的发展方向进行展望。     

Interlayer Space Engineering of MXenes for Electrochemical Energy Storage Applications

The increasing demand for high-performance rechargeable energy storage systems has stimulated the exploration of advanced electrode materials. MXenes are a class of two-dimensional (2D) inorganic transition metal carbides/nitrides, which are promising candidates in electrodes. The layered structure facilitates ion insertion/extraction, which offers promising electrochemical characteristics for electrochemical energy storage. However, the low capacity accompanied by sluggish electrochemical kinetics of electrodes as well as interlayer restacking and collapse significantly impede their practical applications. Recently, interlayer space engineering of MXenes by different chemical strategies have been widely investigated in designing functional materials for various applications. In this review, an overview of the most recent progress of 2D MXenes engineering by intercalation, surface modification as well as heterostructures design is provided. Moreover, some critical challenges in future research on MXene-based electrodes have been also proposed.     

MXene fibers for electronic textiles: Progress and perspectives

The rapid evolution of portable and wearable electronic devices has fueled the development of smart functional textiles that are able to conduct electricity, sense body movements, or store energy. One main challenge inhibiting the further development of functional textile-based electronics is the lack of robust functional fibers with suitable electrical, electrochemical and sensing functionalities. MXenes, an emerging family of two-dimensional(2D) materials, have shown to be promising candidates...     

MXene-Based Nanocomposites for Energy Conversion and Storage Applications

Novel nanomaterials and advanced nanotechnology continuously push forward the rapid development of sustainable energy conversion and storage equipment. An emerging family of two-dimensional transition-metal carbides, nitrides and carbonitrides, also known as MXenes, have attracted increasing attention and in depth investigation. Benefitting from their unique intrinsic properties, MXenes have attracted significant attention and they have been considered as promising candidate materials for the development of environmentally friendly energy resources. A large number of studies show that MXenes have great potential in energy conversion and storage fields. Despite of their exceptional properties, MXenes also have some inherent characteristics, such as low capacities and unstable retention performances, which severely hinder their prospect applications in energy conversion and storage fields. In this Minireview, the latest progress on MXenes and their hybrid composites with small molecules, polymers, carbon or metal ions, and their applications in energy conversion and storage fields is highlighted, including their use in different types of batteries, supercapacitors, hydrogen/oxygen evolution reactions, electromagnetic interference absorption/shielding and solar steam generation. In addition, the critical challenges and further development prospects of MXene-based materials are also introduced.     

Atomic defects,functional groups and properties in Mxenes

MXenes,a new family of functional two-dimensional(2 D) materials,have shown great potential for an extensive variety of applications within the last decade.Atomic defects and functional groups in MXenes are known to have a tremendous influence on the functional properties.In this review,we focus on recent progress in the characterization of atomic defects and functional group chemistry in MXenes,and how to control them to directly influence various properties(e.g.,electron transport,Li⁺ adsorption,hydrogen evolution reaction(HER) activity,and magnetism) of 2 D MXenes materials.Dynamic structural transformations such as oxidation and growth induced by atomic defects in MXenes are also discussed.The review thus provides perspectives on property optimization through atomic defect engineering,and bottom-up synthesis methods based on defect-assisted homoepitaxial growth of MXenes.     

Preparation and characterization of poly(lipid)-coated, fluorophore-doped silica nanoparticles for biolabeling and cellular imaging

The fabrication, characterization, and implementation of poly(lipid)-coated, highly luminescent silica nanoparticles as fluorescent probes for labeling of cultured cells are described. The core of the probe is a sol-gel-derived silica nanoparticle, 65-100 nm in diameter, in which up to several thousand dye molecules are encapsulated (Lian, W.; et al. Anal. Biochem. 2004, 334, 135-144). The core is coated with a membrane composed of bis-sorbylphosphatidylcholine, a synthetic polymerizable lipid that is chemically cross-linked to enhance the environmental and chemical stability of the membrane relative to a fluid lipid membrane. The poly(lipid) coating has two major functions: (i) to reduce nonspecific interactions, based on the inherently biocompatible properties of the phosphorylcholine headgroup, and (ii) to permit functionalization of the particle, by doping the coating with lipids bearing chemically reactive or bioactive headgroups. Both functions are demonstrated: (i) Nonspecific adsorption of dissolved proteins to bare silica nanoparticles and of bare nanoparticles to cultured cells is significantly reduced by application of the poly(lipid) coating. (ii) Functionalization of poly(lipid)-coated nanoparticles with a biotin-conjugated lipid creates a probe that can be used to target both dissolved protein receptors as well as receptors on the membranes of cultured cells. Measurements performed on single nanoparticles bound to planar supported lipid bilayers verify that the emission intensity of these probes is significantly greater than that of single protein molecules labeled with several fluorophores.     

Cation-intercalated engineering and X-ray absorption spectroscopic characterizations of two dimensional MXenes

The geometrically multiplied development of 2D MXenes has already promoted the prosperity of various fields of scientific researches especially but not limited in energy storage and conversion.Notably,cation intercalation can improve the interlayer spacing of MXenes resulting in tunable physical and chemical properties.Moreover,the synchrotron radiation X-ray characterizations have also shown high potential on exploring the property and structu re of cation intercalated MXe nes.This review is mainly focused on the recent achievements of cation intercalated MXenes through different methods on energy storage systems.Synchrotron-based X-ray absorption spectroscopic characterizations are emphasized to probe the local coordination and electronic structure in intercalated MXenes.The outlook of cation intercalation on MXenes and their applications are also discus sed.