Advanced Porous Membranes with Tunable Morphology Regulated by Ionic Strength of Nonsolvent for Flow Battery

A simple salt-induced phase separation method is presented to prepare porous polybenzimidazole (PBI) membranes with tunable morphology for vanadium flow batteries (VFBs). This method is based on the traditional nonsolvent-induced phase separation (NIPS) method where salt is introduced into the coagulation bath to change the ionic strength of the nonsolvent. The change of ionic strength will affect the phase separation rate, and finally, the morphology of porous membranes is well tuned from finger-like voids to sponge-like pores in site. The membrane with sponge-like pores created multiple barriers to the transfer of vanadium ions, offering the membrane with superhigh selectivity; meanwhile, spongy cells filled with sulfuric acid could provide the membrane with high proton conductivity. As a result, the membrane with sponge-like pores demonstrated a much better performance than that with finger-like voids. The resultant sponge-like porous PBI membrane exhibited a very impressive VFB performance with an energy efficiency of 89.9% at a current density of 80 mA cm^-2, which was close to the highest values ever reported. The battery kept very stable performance even after continuously running for more than 10000 cycles at 160 mA cm^-2, showing excellent stability. This paper provides an easy to scale up and environment-friendly method to fabricate high-performance porous membranes with tunable morphology.