Electrochemical Performance of Vanadium Containing MXenes in Aqueous Electrolytes

Two-dimensional (2D) transition metal carbides and nitrides, known as MXenes, have attracted attention due to their unique electrochemical performance in a variety of energy storage systems. MXenes have the general formula of M_n+1X_nT<i>_x</i>, where M is the transition metal, X is carbon, and T represents the surface terminations (F, O, OH).¹ Since their discovery, the primary candidate of study has been Ti₃C₂T_x, but many MXenes with different compositions and M_n+1X_nT<i>_x</i> structures have been synthesized. Vanadium containing MXenes are particularly attractive for redox energy storage because of the presence of V transition metal with multiple oxidation states. Herein, we study V₂CT<i>_x</i> and V₄C₃T<i>_x</i>, as electrodes in supercapacitors with various aqueous electrolytes. The vanadium containing MXenes, specifically V₂CT<i>_x</i>, show high charge storage capability in 6M KOH electrolyte (&gt;250 F g^-1) with distinct redox couples in their cyclic voltammograms. In the acidic electrolytes, both MXenes show pseudocapacitive behavior with V₄C₃T<i>_x</i> showing multiple redox couples in a wide potential window (~ 1V). Through comparing these two MXenes we demonstrate that change in the number of layers of MXenes affects charge storage mechanism and electrochemical behavior of these materials. Our results, provide new insights into structure- chemistry-electrochemical performance relations in MXenes and, therefore, may guide design of advanced electrodes based on these materials for pseudocapacitive energy storage.