Abstract
M(5)C(4)T(x) MXenes represent the most recently discovered and least studied subfamily of out-of-plane ordered double transition metal carbides with 11 atomic layers, probably the thickest of all 2D materials. Molybdenum (Mo) and vanadium (V) in Mo(4)VC(4)T(x) offer multiple oxidation states, making this MXene potentially attractive for electrochemical energy storage applications. Herein, we evaluated the electrochemical properties of Mo(4)VC(4)T(x) free-standing thin films in acidic, basic, and neutral aqueous electrolytes and observed the highest gravimetric capacitance of 219 F g(-1) at 2 mV s(-1) in a 3 M H(2)SO(4). Further, we investigated the intercalation states of four different cations (H(+), Li(+), Na(+), and K(+)) in MXenes through ab initio molecular dynamics (AIMD) simulation and used density functional theory (DFT) calculations to assess the charge storage mechanisms in different electrolytes. These studies show hydrated Li(+), Na(+), and K(+) ions forming an electric double layer (EDL) at the MXene surface as the primary charge storage mechanism. This work shows the promise of Mo(4)VC(4)T(x) MXene for energy storage in aqueous electrolytes.