Asymmetric porous polybenzimidazole membrane with tunable morphology for vanadium flow battery (VFB)

Polybenzimidazole (PBI) membranes have attracted much attention for flow batteries due to their good chemical stability. However, the high area resistance (AR) of dense PBI limits its further development in VFBs. Herein, we design a sintering method to remove the phthalates with different molecular structures in the ternary system (PBI as polymer, N-methylpyrrolidone as solvent, and phthalates as non-solvent). Asymmetric PBI membranes with adjustable and disconnected pores, as well as ultrathin-dense layers, are prepared. This series of porous membranes shows excellent overall performance in VFBs. Among them, the porous NPBI-DBP-200 membrane (0.139 Ω·cm2) with DBP as the non-solvent exhibits a lower AR compared to the dense NPBI (0.489 Ω·cm2). At a current density of 200 mA·cm-2, the energy efficiency (EE) of a single cell assembled with the NPBI-DBP-200 membrane (EE=78.26%) has improved by 26.37% compared to the dense NPBI membrane (EE=61.93%). In addition, the porous NPBI-DBP-200 membrane has excellent in-situ stability, and the single cell can operate stably for 1800 cycles at the current density of 120 mA·cm-2. This work presents a novel, easy-to-use technique for preparing high-performance porous PBI membranes with adjustable pore morphology.