(301d) 2D Transition Metal Carbides (MXenes): Synthesis and Applications in Electrochemical Energy Storage

Two-dimensional (2D) materials, such as graphene, metal oxides/hydroxides, dichalcogenides, hexagonal boron nitride, and others are garnering increasing attention due to their unique properties. About 4 years ago, our group discovered a new class of 2D transition metal carbides and nitrides we labeled MXenes, because they are obtained by selectively etching the A-layers from their three-dimensional (3D) layered, parent compounds the M_n+1AX_n, or MAX, phases. So far, synthesis of ~20 MXenes with different chemistries have been achieved. MXenes offer a unique combination of metallic conductivity or semiconducting properties and hydrophilicity, coupled with good mechanical properties. They have shown superior capacity for reversible intercalation of many metal cations (e.g. Li⁺, Na⁺, K⁺, Mg²⁺, Al³⁺, etc.), which, in turn, has led to their use as electrode materials in energy storage devices, such as supercapacitors, Li-ion and Na-ion batteries/capacitors, and Li-S batteries. Moreover, several methods, including introduction of interlayer spacers, porosity, and hybridization with metal oxides or conductive polymers, have been utilized to significantly improve the electrochemical performance of MXenes. As a new family of 2D materials, MXenes also show great potential in the application of fuel cells, transparent conductors, separators, and catalysis, etc.