Nine watt – Level aqueous organic redox flow battery stack using anthraquinone and vanadium as redox couple

In this study, nine watt (9 W) aqueous organic redox flow battery (AORFB) stack using vanadium (IV) oxide sulfate hydrate (VOSO₄) and anthraquinone-2,7-disulfonic acid (2,7-AQDS) as redox couple is developed for the first time. This redox couple is dissolved in sulfuric acid with cell voltage of 0.9 V and this is higher than that of other acid based AORFBs. Although this couple has precipitation and crossover issues of vanadium ions, they are solved by the adoption of two additives, ammonium chloride (NH₄Cl) and magnesium sulfate (MgSO₄). NH₄Cl plays a role in preventing the precipitation problem by enhancing their dispersion capability, while MgSO₄ can reduce the crossover problem by keeping the balance of ionic strength between two electrolytes. With that, the effects of concentration of VOSO₄, flow rate, and current density on the performances of AORFB single cells using these two additives are evaluated. When the effects are considered, capacity retention is improved and the precipitation and crossover of vanadium ions are suppressed, and these induce a better cycle stability of AORFB. Eventually, the five AORFB single cells are stacked and this AORFB stack shows the excellent maximum power of 9 W at 120 mA·cm⁻², while its performance is well preserved for 200 cycles. From this study, we substantiate that the 9 W AORFB stack using organic and vanadium active materials is well achieved by using efficient additives and optimizing the operational factors.