Oxygen Vacancy-Optimized MoO _{3– <i>x</i>} Nanobelts Enabling Supercapacitors with Ultrafast Charge–Discharge and Long Cycle Life

Although asymmetric supercapacitors can simultaneously deliver high power density and high energy density, their widespread adoption is still limited by the low specific capacitance of conventional anode materials. Molybdenum trioxide (MoO 3 ) is an ideal anode material due to its high theoretical capacity and wide negative potential window, but its low electrical conductivity and limited active sites limit its application in energy storage. In this study, oxygen vacancy-rich molybdenum trioxide (MoO 3– x ) nanobelt anode materials were prepared through a proton predoping and thermal reduction strategy. Subangstrom protons (∼0.001 Å) can effectively insert into MoO 3 and induce interlayer expansion. Oxygen vacancies are precisely constructed through a reduction–dehydration synergistic mechanism, which includes PH 3 reduction and dehydration at temperatures above 200 °C. The expanded interlayer spacing facilitates reversible intercalation/deintercalation of electrolyte ions, enhances electrochemical reaction kinetics, and significantly improves cycling stability. Concurrently, oxygen vacancies effectively reconstruct the local electronic structure on nanobelt surfaces, providing additional H + storage sites that substantially boost the charge storage capacity. The optimized MoO 3– x exhibits a high specific capacitance of 1117 F g –1 at 1 A g –1, along with excellent rate performance (capacitance retention rate of 83.56% at 15 A g –1 ) and high cycling stability (capacitance retention rate of 95% after 10 000 cycles at 100 mV s –1 ). The asymmetric supercapacitor constructed with a MoO 3– x anode and activated carbon (AC) cathode delivers an outstanding energy density of 30 Wh kg –1 at 799 W kg –1 and still maintains 79% of the initial specific capacitance after 10 000 cycles at 5 A g –1, demonstrating exceptional rate capability and cycling stability.