In the present review, available MXene surface-terminating species are explored in view of current synthesis protocols. Their chemical properties are also considered in view of stoichiometry and coordination, which govern the stability of both terminations and MXene sheets. Furthermore, available post-processing methods are discussed in relation to how the termination chemistry can be further tuned, enabling bare MXene sheets as well as terminations that are not native to the MXene synthesis. Finally, this review explores the properties enabled by the MXene surface chemistry and the emerging applications they facilitate. In the conversion of three-dimensional (3D) MAX phases to two-dimensional (2D) MXene sheets, the freshly exposed and highly reactive surfaces are terminated by species that originate from the ambient environment. Accordingly, these are known as surface terminations. The MXene sheets inherit properties such as composition and structure from the parent MAX phase; however, given the reduced dimensionality of MXenes, the surface terminations decisively influence their chemistry, which ultimately governs the MXene properties.

High-entropy (HE) MAX phases represent an emerging family of multi-constituent solid solutions that provide large compositional variations and, therefore, a wide variety of properties. Here, we report the experimental realization of M2AlC (M = Ti/Ta/V/Nb/Cr) MAX phase by an energy-efficient self-propagating high-temperature synthesis, which enables facile scalability to an environmentally friendly industrial production. The HE-MAX phase was developed according to crystal size, electronegativity, and valence electron concentration of corresponding metals required to form a substitutional single-phase material. Variations in initial mixture composition, inert gas pressure, additive amount and sample diameter played a decisive role in HE-MAX formation. The combustion of the stoichiometric mixture favors the formation of the HE-carbide. Deviation from the stoichiometry has resulted in the formation of 211 and/or 413 type HE-MAX phases. Fine-tuning the aluminum and carbon content in the initial mixture, facilitated the formation of a layered structure, characteristic of MAX phases. DSC/TG analysis proved an enhanced oxidation resistance of HE-MAX phases, which outperforms conventional MAX phases and several previously studied HE-MAX phases.