Anomalous Pseudocapacitance of Vanadium Carbide MXene in Water-in-Salt Electrolytes

Abstract

MXenes are known for intercalation-induced capacitive charge storage in dilute electrolytes, resulting in a low specific capacity with a limited anodic potential window (<0.2 V vs Ag/AgCl). In this work, we report on the anomalous pseudocapacitance of V2CTx MXene beyond 0.2 V vs Ag/AgCl by deployment of mono (Li + and Na + ) and divalent (Ca 2+ and Zn 2+ ) metal-ion based concentrated electrolytes. Typically, a four-fold enhancement in the reversible sodiation capacity (131 mAh g -1 ) and a 0.9 V enlargement of the anodic potential window of V2CTx is observed compared to the dilute counterpart. Three-dimensional Bode analysis confirms the pseudocapacitive charge storage characteristics of V2CTx in concentrated electrolyte in anodic potentials. Tafel analysis further confirms the sluggish oxygen evolution and suppressed electrode irreversible oxidation at the V2CTx electrode-concentrated electrolyte interfaces, as evidenced by an order of magnitude lower exchange current density (0.1 mA cm -2 ) relative to the dilute electrolyte interface (1.2 mA cm -2 ). Thus, suppressed water activity not only expands the anodic potential window of operation of V2CTx MXene but also enhances the charge storage capacities and electrochemical stability in concentrated electrolytes. This study opens new doors for exploring MXenes in concentrated electrolytes towards the design of high performance MXene based aqueous electrochemical energy storage devices.