1, Drexel University, Philadelphia, Pennsylvania, United States
Since their discovery in 2011, the family of two-dimensional (2D) transition metal carbides, nitrides, and carbonitrides known as MXenes have received considerable attention for energy storage applications due to their tunable electronic and physical properties. MXenes are ideal candidates for applications requiring materials that can store and deliver large amounts of energy at high rates. MXenes have high specific capacitances due to their redox-active transition metal surfaces and when MXenes are fabricated into binder free, free-standing films they have exceptional electronic conductivity (>8000 S/cm). Different MXenes are available with a large variety of surface chemistries, all of which have demonstrated high performance in energy storage applications. Titanium, vanadium, molybdenum, and niobium based MXenes are some of the more commonly researched members of this family of materials. More recently, a new family of double-ordered MXenes, such as Mo2TiC2, was discovered which have unique properties due to combinations of different inner and outer transition metal atoms.
Ion intercalation and redox reactions at the transition metal surfaces enable MXenes to store more charge than traditional double layer capacitors and due to their unique chemical composition and layered structure, MXenes can store charge in a wide variety of aqueous electrolytes with various charges (H+, K+, Na+, Mg2+, Al3+). Utilizing proton induced pseudocapacitance in acidic electrolytes leads to even higher energy densities, and the water confined within the MXene nanosheets enables ion transport rates that are far superior to any other known pseudocapacitive material. By using saturated aqueous electrolytes, redox reactions between alkali ions and MXenes over an extended voltage range is also possible, leading to enhanced energy densities and safer MXene based electrochemical systems. Furthermore, MXenes are not limited to operating in aqueous environments, and reversible intercalation and deintercalation of lithium and sodium ions from organic electrolytes has been demonstrated with competitive energy power densities and high charge-discharge rates. The same mechanism was also demonstrated in organic and ionic liquid electrolytes with large imidazolium and alkylammonium cations.
The ability MXenes have for storing and delivering large amounts of energy at high power densities in numerous electrolytes paired with the versatility with which they can be processed for use as active materials in energy storage devices puts MXenes in a promising position to lead the research and development of the next generation of devices for energy storage and delivery.