Electrochemical Properties of Mo 4 VC 4 T x MXene in Aqueous Electrolytes.

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.