Ion conductive mechanisms and redox flow battery applications of polybenzimidazole-based membranes

Redox flow batteries (RFBs) have attracted immense attention as one of the most promising grid-scale energy storage technologies. However, designing cost-effective systems with high efficiency and long cycle life requires more advanced ion-conducting membranes. Polybenzimidazole (PBI), doped with acid or alkaline solutions, has been widely recognized as one of the most promising low-cost non-fluorinated ion-conducting membranes for RFBs, primarily because of their well-known excellent chemical stability and ultra-low cross-contamination, which have allowed RFBs to run steadily for an impressively long term. In addition, the membranes are free from the issue of acid or alkali leakage and keep stable conductivity since the working medium in RFBs are acid or alkali solutions in most cases. In this review, recent progress on PBI-based membranes for RFBs is summarized. First, the ever-proposed ion conductive mechanisms in acid- and alkali-doped PBI membranes are presented from the view of intermolecular interactions. Then, recent efforts in improving PBI-based membranes for RFBs, from chemistry to microstructure, are highlighted. Last, the challenges and future perspectives in this research field are elaborated.