High Energy Density Asymmetric Aqueous Supercapacitor Based on a 2D Manganese Carbide as a Positive Electrode

ABSTRACT Maximizing the energy density of aqueous supercapacitors based on MXenes remains a critical challenge due to narrow voltage windows, sluggish or irreversible redox reactions, particularly at extreme and positive potentials. Here, we report a previously unexplored etched manganese–aluminum carbide (EMAX) synthesized via a circular route from waste surgical masks, delivering sustainability alongside performance. EMAX features improved porosity, low work function (2 eV), and a layered architecture with rough surface, while encoding Mn 4+ /Mn 2+ redox sites that underpin fast and reversible pseudocapacitance. In a three–electrode configuration, EMAX achieves superior gravimetric (975 F g −1 ) and volumetric (1657 F cm −3 ) capacitances. Its intrinsically low work function enables favorable electrode potential alignment (avoiding irreversible anodic oxidation) and effective operation in acidic aqueous electrolytes at extended positive potentials. Consequently, when paired with high‐work‐function nitrogen‐doped graphene, the asymmetric device operates particularly efficiently at 1.6 V, reaching a specific energy of 125.5 Wh kg −1 and 213 Wh L −1 and at a specific power of 863 W L −1 , previously considered unachievable in aqueous electrolyte devices. Together, these results highlight work function engineering, coupled with sustainable carbide synthesis, fast and reversible Mn redox sites, and etching‐induced nano‐architecture, leading to a substantial performance advancement in aqueous asymmetric supercapacitors.