Discovery of new materials provides moments of inspiration and shifts in understanding, shaping the dynamic field of materials science. Following the graphene breakthrough, many other 2D materials emerged. Although many of them remain subjects of purely academic interest, others have jumped into the limelight due to their attractive properties, which have led to practical applications. Among the latter are 2D carbides and nitrides of early transition metals known as MXenes . The family of MXenes has been expanding rapidly since the discovery of Ti3C2 in 2011. More than 30 different stoichiometric MXenes have been reported, and the structure and properties of numerous other MXenes have been predicted. Moreover, the availability of solid solutions on M and X sites, multi-element high-entropy MXenes, control of surface terminations, and the discovery of out-of-plane ordered double-M o-MXenes (e.g., Mo2TiC2), as well as in-plane i-MXenes, offer a potential for producing up to a thousand of new distinct structures and an infinite number of solid solutions. This presentation will describe the state of the art in the synthesis of MXenes, their assembly into films, fibers and 3D structures, as well as their electrochemical properties. The versatile chemistry of the MXene family renders their properties tunable for a large variety of applications, including active and passive materials in various kinds of batteries and supercapacitors . In particular, MXenes can act as host structures for cations and molecules. Chemical and electrochemical insertion of ions and molecules between the MXene layers allows modification of their properties, as well as electrochemical charge storage and harvesting, which use both, double-layer and redox mechanisms. We have developed an optical technique for monitoring the charge transfer during charge/discharge in-situ, which allows identification and quantitative analysis of changes in the oxidation state of transition metals without using expensive in-situ techniques, such as XAS or TEM/EELS.
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