High power density near-neutral pH aqueous redox flow batteries using zinc chloride and 4,5-dihydroxy-1,3-benzenedisulfonate as redox couple with polyethylene glycol additive

Redox flow batteries (RFBs) using zinc chloride (ZnCl₂) and 4,5-dihydroxy-1,3-benzenedisulfonate (Tiron) redox couple is introduced. ZnCl₂ and Tiron are used as negative and positive active materials, which are dissolved in near-neutral pH, ammonium chloride (NH₄Cl) supporting electrolyte. Cell voltage of RFB using ZnCl₂ and Tiron is extremely high as 1.8 V, while two cellulose spacers are introduced to prevent the growth of zinc dendrite on the membrane. When the performances of RFBs using ZnCl₂ and Tiron are measured, their charge efficiency (CE), voltage efficiency (VE), and energy efficiency (EE) are 95, 79, and 75% at 40 mA cm⁻², whereas their capacity fading rate is high as 0.12 Ah L⁻¹ per cycle. This is due to the zinc dendrite formed on the electrode of RFB that is attributed to the aggregation of zinc ions. To suppress the aggregation and to preserve active sites for the redox reaction of zinc ions during cycling, the polyethylene glycol (PEG) additive is suggested. Regarding the optimization of PEG, 5000 ppm PEG induces both high efficiencies (97, 81, and 79% of CE, VE, and EE at 40 mA cm⁻²) and low capacity loss rate (0.03 Ah L⁻¹ per cycle), which are more enhanced performances than those of RFBs operated without PEG. In terms of power density of RFB using ZnCl₂ and Tiron, high power density of 144 mW cm⁻² is achieved under 120 mA cm⁻², which is far better performance than that of other RFBs operated under near-neutral pH electrolyte.