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 of operation (<0.2 V vs Ag/AgCl). In this work, we report the anomalous pseudocapacitance of V₂CT_x MXene beyond 0.2 V vs Ag/AgCl by the deployment of mono- (Li⁺ and Na⁺) and divalent (Ca²⁺ and Zn²⁺) 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 V₂CT_x are observed compared to the dilute counterpart. Three-dimensional Bode analysis confirms the pseudocapacitive charge storage characteristics of V₂CT_x in concentrated electrolytes at anodic potentials. Tafel analysis further confirms the sluggish oxygen evolution and suppressed electrode irreversible anodic oxidation at the V₂CT_x electrode–concentrated electrolyte interface, 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 V₂CT_x MXene but also enhances the charge storage capacity and electrochemical stability in concentrated electrolytes. This study opens new doors for exploring MXenes in concentrated electrolytes leading towards the design of high-performance MXene-based aqueous electrochemical energy storage devices.