Ru-Doping-Engineered Oxygen Vacancies in MoO3- x Nanostructured Films With Ultrahigh Capacitance for Flexible Asymmetric Supercapacitors.

Molybdenum trioxide (MoO3) is a promising pseudocapacitive material owing to its high theoretical charge-storage capacity, but its practical application is limited by low electrical conductivity and sluggish ion transport. Here, we report a synergistic defect-engineering strategy that combines in situ Ru doping with simultaneous oxygen-vacancy generation to synthesize Ru-doped MoO3-x (Ru-MoO3-x)via a one-step solvothermal reaction, followed by assembly into freestanding, flexible film electrode by vacuum filtration. Ru incorporation induces a dual nanobelt-nanowire architecture and enriches oxygen vacancies as electrochemically active sites, enabling efficient redox reactions and charge transport. Density functional theory calculations and ultraviolet photoelectron spectroscopy reveal that Ru-induced defect engineering narrows the bandgap, lowers the work function, and shifts the Fermi level upward, thereby enhancing intrinsic conductivity. The optimized Ru-MoO3- x electrode delivers an ultrahigh specific capacitance of 2047.5 F g-1 at 1 A g-1, approaching the theoretical capacitance limit of MoO3 (∼2700 F g-1 at 1 V), with excellent cycling stability. Asymmetric supercapacitors assembled with Ru-MoO3- x achieve a high energy density of 40.9 Wh kg-1 in acidic electrolyte and a wide operating voltage window of 2.4 V in organic ionic-liquid electrolyte, while maintaining mechanical robustness under severe bending and powering commercial electronics. This work provides a general strategy for defect-regulated, high-performance flexible energy-storage devices.