Acidic aqueous redox flow battery using 12-phosphotungstic acid and 2,4,5,6-tetrahydroxybenzene-1,3-disulfonic acid as redox couple

In this study, acidic aqueous redox flow battery (ARFB) using 12-phosphotungstic acid (PTA) and 1,2-benzoquinone-3,5-disulfonic acid (Tiron) as negative and positive active materials is suggested. As negative active material, PTA shows high solubility in an acidic state (0.4 M PTA is soluble in 1.0 M sulfuric acid [H₂SO₄]), negative reaction potential (−0.25 V vs Ag/AgCl), and is linked to four-electron reaction. Regarding positive active material, Tiron is chosen due to its high solubility in H₂SO₄ (1.5 M Tiron is soluble in 1 M H₂SO₄) and highly positive reaction potential (0.76 V vs Ag/AgCl) with two-electron related reactions. For preserving its redox reactivity, Tiron is initially transformed to its desirable 2,4,5,6-tetrahydroxybenzene-1,3-disulfonic acid (TironA). As a result, the cell potential of ARFB using PTA and TironA is 0.92 V, while their low viscosities reduce the possibility of precipitation. With this, its cycle stability is well preserved. Regarding the performance of ARFB using PTA and TironA, this maintains cycle stability for more than 170 cycles with capacity retention of 99% and high columbic and energy efficiencies of 99% and 70% at 60 mA cm⁻².