Synthesis of nanoporous PVDF membranes by controllable crystallization for selective proton permeation

Abstract

Controllable crystallization of crystalline polymers was proposed to fabricate nanoporous membranes for proton selective permeation. To prove this concept, a membrane was cast from poly(vinylidene fluoride) and sodium allyl sulfonate dissolved in dimethyl sulfoxide. The membrane was characterized by Fourier transform infrared spectroscopy, scanning electron microscopy, X-ray diffraction, differential scanning calorimetry and permeability of protons (H+) and tetravalent vanadium ions (VO2+). It was found that the membrane configuration was determined by crystal growth and that the pore size and distribution can be tuned by controlling the temperature and time of polymer crystallization. Changes in membrane configuration and permeation selectivity were characterized using H+ and VO2+ as probes. All membranes exhibited much higher diffusivities for H+ than for VO2+, and the selectivity for H+ relative to VO2+ was up to 75, which is sufficient for vanadium flow battery applications. Moreover, ion permeation and selectivity were found to occur by size exclusion and spatial hindrance, rather than ion exchange and static repulsion effect. Using a 6kW stack of vanadium flow battery to perform 650 charging-discharging cycles, the membrane is demonstrated being good resistance to chemical and electrochemical erosion. This membrane formation method shows promising applications for electrochemical devices.