A facile strategy for disentangling the conductivity and selectivity dilemma enables advanced composite membrane for vanadium flow batteries

Abstract

To resolve the long-standing conductivity-selectivity dilemma in ion exchange membranes (IEMs), a facile strategy for the preparation of the composite membrane based on perfluorosulfonic acid (PFSA) membrane with thin polybenzimidazole (PBI) selective layer is presented for vanadium redox flow batteries (VFBs). By simple impregnation method, the dense thin PBI selective layer with tunable thickness was coated on the highly conductive commercial PFSA proton exchange membrane (DMR). The thin PBI selective layer not only provided the membrane with high selectivity and proton conductivity, but the strong acid-base interaction between basic PBI and acidic PFSA membrane was also responsible for the formation of a stable layer on the PFSA supporting. With this design, the optimized composite membrane T-DMR-100s exhibited a low area resistance of 0.14 Ω cm2 in 3 M H2SO4 as well as low vanadium permeability of 3.6 × 10−8 cm2/min, which surpass the benchmark Nafion® 212 membrane under the same testing conditions (area resistance of 0.15 Ω cm2 and vanadium permeability of 1.2 × 10−7 cm2/min). Consequently, both high coulombic efficiency (CE) of 98.9% and energy efficiency (EE) of 86.1% were achieved for the VFB with T-DMR-100s composite membrane at a current density of 140 mA/cm2, much higher than the cell with the Nafion® 212 membrane (CE of 92.0% and EE of 80.2%). The chemical stability of the composite membrane was confirmed by stable operating for 960 cycles (700 h) at a current density of 140 mA/cm2 and ex situ oxidation stability tests. Moreover, the failure mechanism of the composite membrane in the operating VFBs was also investigated in the extended cycling testing, and the post-cell sample analysis of the aged membranes by XPS and permeability measurement indicated that the degradation of the NPBI selective layer may be the main reason for the continuous decay in EE after 960 cycles. This work provided a new insight into the design of the composite membranes with both high conductivity and selectivity for VFB application.