2, King Abdullah University of Science and Technology, Saudia Arabia (KAUST), Thuwal, , Saudi Arabia
A new family of two-dimensional transition metal carbides, nitrides, and carbonitrides – collectively referred to as MXenes – have shown great promise in a wide range of applications starting from electrochemical energy storage to electromagnetic shielding. The hydrophilicity of MXenes combined with their metallic conductivity and surface redox reactions are the key for high-rate pseudocapacitive energy storage in MXene electrodes. For instance, the most studied MXene, Titanium carbide (Ti3C2) has shown high specific volumetric capacitances in the range of 1000-1500 F/cm3 at ultra-high rates. These striking features including high conductivity and capacitance make MXenes great candidate materials for microscale on-chip and flexible energy storage devices. The ease with which MXenes can be dispersed in aqueous and organic solvents to make colloidal, functional inks allows for printing, spray coating, drop casting, and spin coating of MXenes in any pattern that can be imagined for microscale devices. Thin film MXene coatings for flexible on-chip energy storage applications can easily be produced by spray coating and direct laser patterning technique. Simple doctor blade technique has produced thick film (>100 µm) MXene microsupercapacitors for flexible, paper supported energy storage systems. Asymmetric devices were fabricated by combining negative MXene electrodes with positive carbon and metal oxide electrodes in extending the voltage window and further improving the output energy and power densities of the devices. MXene based energy storage devices produced thus far have higher energy and power densities than comparable state-of-the-art microsupercapacitors currently being researched, which opens up new avenues for further exploring MXene architectures for development of the next generation of microscale energy storage devices.