Propylene carbonate-derived size modulation of water cluster in pore-filled nafion/polypropylene composite membrane for the use in vanadium redox flow batteries

Abstract

There is an increasing need to develop an alternative to conventional Nafion membrane in order to reduce the cost of vanadium redox flow batteries (VRFBs). Pore-filled composite membranes based on Nafion is one way to reduce the amount of costly material, however the chronic problem of this approach is low conductivity and low water uptake ability of the composite membrane. In this work, we suggest a novel approach to enhance the conductivity and water uptake by incorporating propylene carbonate (PC) in pore-filled Nafion phase to modulate morphologies of the water channel in Nafion directly. Small-angle X-ray scattering shows that the PC enlarges the water cluster in a Nafion membrane by 25% of its original size. Intermolecular interactions inside the water nanocluster between PC and sulfonic acid group, which is responsible for the expansion of the water cluster, are elucidated by molecular dynamics simulations. We present a pore-filled type Nafion composite membrane based on a porous polypropylene (PP) substrate to prove the effect of PC addition and demonstrate that the PC-derived Nafion/PP membrane has a similar membrane resistance and reduced vanadium crossover compared with a commercial Nafion membrane. We suggest that the PC-derived Nafion structure modulation could be a facile and effective approach to improve transport properties of Nafion-based membranes for VRFB applications.