New Method for the Synthesis of 2D Vanadium Nitride (MXene) and Its Application as a Supercapacitor Electrode.

MXenes are the class of two-dimensional transition metal carbides and nitrides that exhibit unique properties and are used in a multitude of applications such as biosensors, water purification, electromagnetic interference shielding, electrocatalysis, supercapacitors, and so forth. Carbide-based MXenes are being widely explored, whereas investigations on nitride-based ones are seldom. Among the nitride-based MXenes obtained from their MAX phases, only TiN and TiN are reported so far. Herein, we report a novel synthesis of VNT (T is the surface termination) obtained by the selective removal of "Al" from VAlN by immersing powders of VAlN in the LiF-HCl mixture (salt-acid etching) followed by sonication to obtain VNT (T = -F, -O) MXene which is then delaminated using the dimethyl sulfoxide solvent. The VNT MXene is characterized by X-ray diffraction studies, field emission scanning electron microscope imaging, energy-dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, and high-resolution transmission electron microscope imaging. Supercapacitor electrodes are prepared using VNT MXenes and their electrochemical performances are examined by cyclic voltammetry, galvanostatic charge/discharge measurement, and electrochemical impedance spectroscopy. The VNT MXene electrode exhibits a specific capacitance of 112.8 F/g at a current density of 1.85 mA/cm with an energy and power density of 15.66 W h/kg and 3748.4 W/kg, respectively, in 3.5 M KOH aqueous electrolyte. The electrode exhibits an excellent capacitance retention of 96% even after 10,000 charge/discharge cycles. An asymmetric supercapacitor fabricated with VNT as a negative electrode and MnO nanowalls as a positive electrode helps obtain a cell voltage of 1.8 V in aqueous KOH electrolyte.