Tuning the ion selectivity of porous poly(2,5-benzimidazole) membranes by phase separation for all vanadium redox flow batteries

Porous polybenzimidazole membranes have been suggested by far as the most promising candidate to replace perfluorinated ionomer membranes in all vanadium redox flow batteries (VRFB). These porous membranes can simply be prepared from a phase separation process induced by water vapor. In this work, the influence of the material properties of poly(2,5-benzimidazole) (ABPBI) on the membrane structure formation and ion transport characteristics is systematically investigated. Two batches of ABPBI polymers (intrinsic viscosity 2.0 and 1.4 dL g−1) are used; concentrations of polymer solution, the water vapor activity (temperature and relative humidity), and the exposure time of polymer solution to the water vapor are varied. It is found that high viscosity of ABPBI solution, among other factors, is essential in obtaining membranes with very low interconnectivity between macropores in the membrane bulk, therefore very low vanadium ion permeability suitable for VRFB applications. The porous ABPBI membranes in dry state have a low amount of pores ranging from 1 to ~ 200 nm. Due to the benzimidazole moieties in ABPBI polymers, the acid in vanadium electrolytes has two positive effects on ABPBI membranes: fixed positive charges, and absorption of acid in the membranes. The ion exchange capacity of ABPBI membranes increases with the increase in solution acid concentration, and reaches striking values as high as 8 mequ g−1 in 4.0 M H2SO4. This makes the Donnan exclusion towards positively charged vanadium ions very profound. The macropores in the porous membranes increase the absorption amount of free acid, therefore significantly increase the membrane ionic conductivity. These properties make porous ABPBI membranes very efficient separators for VRFB applications.