Imidazolium-grafted nitroxyl radical is synthesized through a simple two-step reaction route, and studied as redox-active catholyte for redox flow batteries. A discharge cell voltage of 1.64 V against a Zn2+/Zn anodic couple in aqueous electrolyte has been achieved. Compared to the commercial 4-OH-TEMPO and previous work, the excellent chemical and electrochemical stability of the newly synthesized compound has been confirmed from the enhanced and steady Coulombic efficiency over charge/discharge cycling. Moreover, stable cycling efficiencies and good capacity retention over long-term cycling have been observed.