Synthesis of (Nb 1‐ y V y ) 4 C 3 T x Solid‐Solution MXenes and Their Supercapacitive Properties

ABSTRACT MXene has attracted considerable attention as a highly promising electrode material owing to its exceptional electrochemical properties, mechanical flexibility, and tunable interfacial composition. Although M 4 C 3 ‐type MXenes are intrinsically more stable than their M 3 C 2 or M 2 C counterparts of identical M‐site chemistry, research to date has centered on Ti 3 C 2 T x and a handful of monometallic systems, leaving solid‐solution variants, especially M 4 C 3 configurations, scarcely investigated. Herein, we successfully fabricated a series of (Nb 1‐y V y ) 4 C 3 T x solid‐solution MXenes via controlled etching of corresponding MAX precursors. The incorporation of vanadium precisely regulates the surface terminal groups by reducing oxygen functional groups and increasing fluorine termination, which effectively modulates the ion transport and interfacial reaction kinetics. Benefiting from the optimized surface chemistry and solid‐solution synergy, the optimized (Nb 0.2 V 0.8 ) 4 C 3 T x electrode exhibits greatly improved rate capability and outstanding cycling stability, while the (Nb 0.4 V 0.6 ) 4 C 3 T x delivers the optimal specific capacitance. This work reveals the correlations among composition, surface chemistry, and electrochemical properties in M 4 C 3 ‐based solid‐solution MXenes, demonstrating that M‐site bimetallic solid‐solution engineering is an effective strategy to develop high‐performance MXene electrode materials.