Stability enhancement for all-iron aqueous redox flow battery using iron-3-[bis(2-hydroxyethyl)amino]-2-hydroxypropanesulfonic acid complex and ferrocyanide as redox couple

In this study, long-term stability of all-iron aqueous redox flow batteries (all-iron ARFBs) using iron-3-[bis(2-hydroxyethyl)amino]-2-hydroxypropanesulfonic acid complex (Fe[DIPSO]) and ferrocyanide as redox couple is evaluated. In this system, there are two problems to address. First, stability issue of catholyte including ferrocyanide that is worsened under alkali electrolyte and second, unbalanced pH of catholyte and anolyte occurring by the crossover of water molecules during cycling of ARFB, and these degrade the performance of ARFB steadily. To enhance the stability of the catholyte, the stability of ferrocyanide is investigated in various electrolyte conditions. Furthermore, to alleviate the unbalanced pH effects of both electrolytes (catholyte and anolyte), their electrolyte conditions are set differently to quantify the crossover of water molecules and optimize electrolytes. When electrolyte condition applied in both electrolytes is not appropriate, the capacity decay rate of ARFB is 50% for cycling of 150 hours. In contrast, when pH and concentration conditions of catholyte and anolyte are optimally designed, the capacity of ARFB is well preserved for the entire cycling. More specifically, in catholyte, optimal pH and concentration are 12 and 0.25 M, while they are 14 and 0.5 M in anolyte. Eventually, using the optimized electrolyte condition, its performances are well maintained for 23 days, guaranteeing its long-term stability. These prove that the stability of ARFB using Fe(DIPSO) and ferrocyanide can be enhanced by maneuvering the pH and concentration conditions of its electrolytes.