Ru‐Doping‐Engineered Oxygen Vacancies in MoO _3‐ <i> _x </i> Nanostructured Films With Ultrahigh Capacitance for Flexible Asymmetric Supercapacitors

ABSTRACT Molybdenum trioxide (MoO 3 ) 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 MoO 3‐ x (Ru‐MoO 3‐ 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‐MoO 3‐ x electrode delivers an ultrahigh specific capacitance of 2047.5 F g −1 at 1 A g −1 , approaching the theoretical capacitance limit of MoO 3 (∼2700 F g −1 at 1 V), with excellent cycling stability. Asymmetric supercapacitors assembled with Ru‐MoO 3‐ 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.