Thin skinned asymmetric polybenzimidazole membranes with readily tunable morphologies for high-performance vanadium flow batteries

A series of thin skinned asymmetric polybenzimidazole (PBI) membranes with readily tunable morphologies are fabricated by leaching out the porogen dibutyl phthalate (DBP), for vanadium flow batteries (VFBs). The ultrathin defect-free skin layer fully guarantees high ion selectivity of the membrane. Meanwhile, the area resistance (AR) of the asymmetric PBI membrane is dramatically reduced compared to that of the dense one because of interconnected macro-pores in the sublayer. The membrane morphologies and properties are readily adjusted by varying the porogen content (0–300 wt%), thus managing well the trade-off between AR and ion selectivity. The membrane prepared by adding 200 wt% porogen has a high porosity of 74.9 vol% and an appropriately dense skin thickness of 4.9 μm, and yields the best balance between AR and ion selectivity, assembled with which the flow battery achieves excellent cell performances (coulombic efficiency, CE: 99.0%; energy efficiency, EE: 82.3%) as well as a moderate capacity decay rate (CDR, 0.4% per cycle) at 80 mA cm−2 over cycling. The thin skinned asymmetric PBI membranes prepared here surpass the commercial Nafion 211 membrane (CE: 84.6%; EE: 68.1%; CDR: 1.3% per cycle) in terms of cell performances and cost, becoming a promising candidate for VFBs.