Electrochemical Properties of Mo₄VC₄T_<i>x</i> MXene in Aqueous Electrolytes

M₅C₄T_<i>x</i> 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 Mo₄VC₄T_<i>x</i> offer multiple oxidation states, making this MXene potentially attractive for electrochemical energy storage applications. Herein, we evaluated the electrochemical properties of Mo₄VC₄T_<i>x</i> 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 H₂SO₄. Further, we investigated the intercalation states of four different cations (H⁺, Li⁺, Na⁺, and K⁺) in MXenes through <i>ab initio</i> 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 Mo₄VC₄T_<i>x</i> MXene for energy storage in aqueous electrolytes.