Stable sulfonated poly(oxindole biphenylene) as ion-solvating membranes toward durable alkaline zinc-iron redox flow battery

Ion-solvating membranes (ISMs) are one of the promising separators for alkaline zinciron flow battery (AZIFB) owing to its inherent ability to transport hydroxide ions in alkali media. However, insufficient alkaline stability and low conductivity at low concertation of alkali electrolytes prohibit its successful application in AZIFBs. In this work, we present a strategy to precisely attach sulfonic groups on poly(oxindole biphenylene) (POBP) by introducing a fluorinated comonomer, affording sulfonated POBP with different degrees of sulfonation as ISMs. The hydrophilic sulfonic groups are proven to not only increase the absorption of water and alkali in membranes and in turn boost the ionic conductivity, surpassing that of Perfluorosulfonic cation-exchange membranes (Nafion) and pristine POBP membranes but also to suppress the zinc dendrite/accumulation at anode by the charge repulsion between negatively charged SPOBP membrane and Zincate (Zn(OH) 4 2− ) ions. Ex situ alkaline stability tests in 8 M Sodium hydroxide (NaOH) at 80 °C reveal a conductivity loss of <5 % and no chemical decomposition over 3600 h of immersion for the SPOBP membrane. Thus, the AZIFB with optimal SPOBP-50 membrane demonstrates an energy efficiency (EE) of 90.49 %, a coulomb efficiency (CE) of 99.73 %, and stable cycling at 100 mA cm −2 for 2150 cycles (33 days), representing one of the highest efficiency and cycle-life reported in the literature for AZIFBs.