Emerging 2D materials beyond mxenes and TMDs: Transition metal carbo-chalcogenides

The discovery of graphene sparked significant interest in 2D materials, which present an ultra-thin layered structure with high anisotropy and adjustable energy-band structure. Interestingly, it opens the door for the development of the 2D materials family, which includes different classes of 2D materials. Among them, transition metal dichalcogenides (TMDs) and transition metal carbide MXenes (TMCs) have emerged. TMDs have unique layered structures, low cost, and are composed of earth abundant elements, but their poor electronic conductivity, poor cyclic stability, their structural and morphological changes during electrochemical measurements hinder their practical use. Recently, TMC MXenes have garnered attention in the 2D material world, but the issue of restacking and aggregation limits their direct use in large-scale energy conversion and storage. To address these challenges, hetero structures based on conductive TMCs MXenes and electrochemically active TMDs have emerged as a promising solution. However, understanding the solid/solid interface in heterostructured materials remains a challenge. To tackle this, 2D single component crystals with high capacity, low diffusion barrier, and good electronic conductivity are highly sought. The emergence of transition metal carbo-chalcogenides (TMCCs) has provided a potential solution, as these 2D nanosheets consist of TM2X2C, where TM represents transition metal, X is either S or Se, and C atom. This new class of 2D materials serves as a remedy by avoiding the challenges related to solid/solid interfaces often encountered in heterostructures. This review focuses on the latest developments in TMCCs, including their synthetic strategies, surface/interface engineering, and potential application in batteries, water splitting, and other electro-catalytic processes. The challenges and future perspectives of the design of TMCCs for electrochemical energy conversion and storage are also discussed.