Long-term cycling and effect of electrolyte composition on capacity decay of an alkaline Fe(DIPSO)/ferrocyanide flow battery

This study investigates the influence of the negolyte composition on the capacity decay of an alkaline flow battery using redox active iron 3-[N,N-Bis(2-hydroxyethyl)amino]-2-hydroxy-propane-sulfonic acid (Fe(DIPSO)) and ferro/ferricyanide ( Fe ( CN ) 6 4 − / Fe ( CN ) 6 3 − ). Electrolyte features such as metal-to-ligand ratio and in particular potassium hydroxide (KOH) concentration are found to determine the capacity fade of the flow battery. The optimal electrolyte composition of 0.2 M Fe(III)(DIPSO) in 3.45 M KOH (1:2 metal-to-ligand ratio) paired with 0.25 M ferricyanide/0.25 M ferrocyanide in 0.86 M KOH is found, resulting in a remarkably low capacity fade rate of 0.007% per day. However, after extensive cycling over 41 days, the capacity fade rate increased sharply to 0.120% per day. Further studies of the open-circuit voltage (OCV), internal resistance, and polarization curves during long-term cycling reveal that the chemical reaction between ferricyanide and the excess DIPSO ligands that penetrated the membrane is the main cause of the capacity decay. This study opens an avenue for further stabilization of promising all-iron flow batteries.