Harnessing washing-driven oxidation to engineer Mo₂CT_x/MoO_x hybrid interfaces for energy storage

MXenes are emerging as promising electrode materials for energy storage applications owing to their high conductivity and redox-active surfaces. However, their inherent propensity to undergo oxidation is frequently regarded as a limitation. Here, we demonstrate that the routine washing step following molten-salt etching of Mo₂CT_x can be exploited as a controlled strategy to engineer Mo₂CT_x/MoO_x hybrid interfaces. By systematically varying the washing duration in HCl/CuCl₂ solution (30, 90, and 240 minutes), we precisely modulate the degree of Mo oxidation, with an additional H₂O₂ treatment serving as a reference for deliberate chemical oxidation. In-depth structural and spectroscopic analyses reveal two distinct oxidation mechanisms: the washing process induces selective Mo-centered oxidation that maintains the integrity of the carbide framework, resulting in conductive Mo₂CT_x/MoO_x domains, whereas H₂O₂ oxidation also involves both Mo and C sites, yielding a hydroxylated surface that compromises long-term stability. The electrochemical performance in acidic electrolyte highlights the advantages of the washing protocol: Mo₂C-90 delivers the highest capacitance (∼150 F g^-1 at 10 mV s^-1) with robust cycling stability (90% retention after 10 000 cycles). This study establishes washing-induced oxidation as a simple method for tailoring Mo₂CT_x MXenes in supercapacitor applications, transforming a standard synthesis into a deliberate tool for performance optimization.