In order to broaden the working voltage (1.23 V) of aqueous supercapacitors, a high-performance asymmetric supercapacitor with a working voltage window reaching up to 2.1 V is assembled using a nanorod-shaped molybdenum trioxide (MoO₃) negative electrode and an activated carbon (AC) positive electrode, as well as a sodium sulfate–ethylene glycol ((Na₂SO₄-EG) electrolyte. MoO₃ electrode materials are fabricated by adjusting the hydrothermal temperature, hydrothermal time and solution’s pH value. The specific capacity of the optimal MoO₃ electrode material can reach as high as 244.35 F g⁻¹ at a current density of 0.5 A g⁻¹. For the assembled MoO₃//AC asymmetric supercapacitor with a voltage window of 2.1 V, its specific capacity, the energy density, and the power density are 13.52 F g⁻¹, 8.28 Wh kg⁻¹, and 382.15 W kg⁻¹ at 0.5 A g⁻¹, respectively. Moreover, after 5000 charge–discharge cycles, the capacity retention rate of the device still reaches 109.2%. This is mainly attributed to the small particle size of MoO₃ nanorods, which can expose more electrochemically active sites, thus greatly facilitating the transport of electrolyte ions, immersion at the electrolyte/electrolyte interface and the occurrence of electrochemical reactions.