Poly(vinylidene fluoride) porous membranes precipitated in water/ethanol dual-coagulation bath: The relationship between morphology and performance in vanadium flow battery

Poly(vinylidene fluoride) (PVDF) porous membranes with tunable morphology are facilely prepared via dual-coagulation bath by phase inversion method and investigated in vanadium flow battery (VFB). Water/ethanol solutions with different compositions are selected as the coagulation baths and the effect of water/ethanol including content ratio and immersion time on membrane morphology is studied in detail. SEM results indicate the increased degree of crystallization and surface roughness of prepared membranes with higher ethanol content in coagulation bath and longer immersion time. In addition, beta phase is a dominated crystalline form in these membranes. The upper surface pore size variety and overall crystalline structure lead to a non-monotonous change of ions permeability and area resistance. As a result, the coulombic efficiency decreases on the whole with volume ratio of water/ethanol ranging from 10:0 to 3:7, whereas there is a raised peak at the ratio of 5:5. Meanwhile, the voltage efficiency of these membranes exhibits an opposite tendency. Therefore, the morphology and property of PVDF porous membranes can be adjusted via changing the composition of coagulation bath. This research presents an effective and feasible method to optimize the fabrication parameters of PVDF porous membranes for VFB application. (C) 2013 Elsevier B.V. All rights reserved.