A Stable Ultrathin Porous Membrane based on Rigid Polymer by Phase Separation for Flow Batteries

Abstract The membrane, one of the key components of flow batteries, ideally has high selectivity, conductivity, and stability. However, porous membranes prepared by conventional non-solvent-induced phase separation (NIPS) commonly suffer from low selectivity and poor mechanical stability. Here, we used rigid naphthalene-containing polybenzimidazole (NPBI) to prepare a porous membrane with unique egg-shaped pores by adjusting solvent/non-solvent exchange in NIPS. The dense pores with a size of 3.6 Å arranged dispersedly between egg-shaped pores. The rigid NPBI and 3.6-Å small pores enabled the membrane high mechanical strength. The thickness was thus decreased to 1.4 μm, which exhibited an ultrahigh tensile strength of 463.54 MPa. The dense pores were also smaller than hydrated vanadium ions, achieving a low permeability of 2.28 × 10 ‒7 cm 2 /h, indicating high selectivity. This is the first time to prepare such a highly selective and mechanically stable ultrathin porous membrane by NIPS. Importantly, the ion-transport pathways in the 1.4 μm membrane were shortened, decreasing the area resistance to as low as 0.015 Ω cm 2 . Demonstrated in a vanadium flow battery, its coulombic efficiency was 98.57% and energy efficiency reached 81.72% at 200 mA/cm 2 . This study proposes an effective strategy to prepare high-performance ultrathin porous membranes for flow batteries.