High conductivity membrane containing polyphosphazene derivatives for vanadium redox flow battery

Abstract

The primary task to improve the performance of vanadium redox flow battery (VRFB) is to develop the membranes with high proton conductivity. Herein, a string of sulfonated polyimide (SPI) blend with poly [bis (4,4′-diaminobenzidine-2,2′-disulfonic acid) phosphazene] (PDAP) membranes were designed and prepared. The introduction of PDAP containing amphoteric functional groups not only provides more proton transport groups (–SO3H), but also reach the purpose to prevent the migration of vanadium ions, which is caused by the Donnan repulsion to vanadium ions from N-protonation. Therefore, the prepared membrane exhibits high proton conductivity (up to 1.33 × 10−1 S cm−1 at room temperature) and low vanadium permeability (1.37 × 10−8 cm2 s−1). The performance of VRFB with SPI/0.5% PDAP membrane exhibits a voltage efficiency (VE) of 87.02% and an energy efficiency (EE) of 84.04% at 100 mA cm−2, which were better than that of Nafion 212 (VE: 82.04%, EE: 76.99%). In addition, the VRFB with SPI/0.5% PDAP membrane can be cycled continuously and perfectly for 500 times at 100 mA cm−2 with stable efficiencies. After cycling, the membrane was intact without any defects, and the FTIR spectra and morphology of the membrane did not change significantly. These results prove that the SPI/0.5% PDAP membrane is an ideal membrane in VRFB applications.