(Invited) MXenes: From Fundamental Science to Emerging Applications

Two-dimensional (2D) nanomaterials have been attractive candidates for a plethora of applications since the successful isolation and demonstration of intriguing electronic properties of graphene in 2004 by the Manchester group. The ‘beyond graphene’ march led to the discovery of new types of 2D materials, expanding the flatland research. MXenes, a relatively new and large family of 2D transition metal carbides, nitrides, and carbonitrides, which were discovered at Drexel in 2011 and have become popular due to their diverse chemistries and unique physicochemical properties. These materials are produced by top-down wet chemical or electrochemical extraction of interleaved ‘ A ’ atomic layers from the layered ternary carbide precursors (MAX phases). The resulting MXenes with surface terminations (-O, -F, -OH) impart negative surface charges, key for stable MXene dispersions in water and organic solvents which enable solution processing. More than 30 different MXenes have been experimentally synthesized, with dozens of new MXenes and their properties predicted computationally. The compositional versatility, metallic conductivity, control of surface chemistry, ordered structures versus solid solutions, etc. make MXenes diverse and suitable for a wide spectrum of applications including but not limited to energy storage, photo- and electro-catalysis, water desalination, optoelectronics, electromagnetic interference shielding, antennas, and smart textiles. The available 2D gallery spaces, redox chemistry at the transition metal oxide-like surface with metallic conductivity, enable MXenes as potential candidate materials for high rate capacitive energy storage applications. The ability of MXenes to spontaneously intercalate a variety of cations while promoting fast charge transfer rates, endorses them for hybrid metal-ion capacitors and battery applications. The electrocatalytic property of MXenes can be tuned based on type of transition metal, amount of defects and surface functionality. Furthermore, combination of slit-like pores with electronic conductivity, qualifies MXenes as capacitive deionization (CDI) electrodes for efficient and reversible removal of ionic species/organic molecules from brackish water. Unlike most of the 2D materials produced by bottom-up routes in small quantities, pilot scale synthesis of MXenes is a promising aspect for immediate future commercialization.