Electrospinning fiberization of carbon nanotube hybrid sulfonated poly (ether ether ketone) ion conductive membranes for a vanadium redox flow battery

It is challenging for ion conductive membranes to achieve both high proton conduction and low vanadium ion permeation because both ions transport mainly through the hydrophilic domains in membranes. A novel strategy of electrospinning fiberization of the carbon nanotube hybrid sulfonated poly (ether ether ketone) is proposed. The improved conductivity is achieved through interconnective proton pathways induced by the electrospun nanofiber. Multiwall carbon nanotubes contain carboxyl groups, align and disperse well in the nanofibers under a strong electrostatic field of 1.3 kV cm−1 as evidenced by TEM and SAXS, therefore result in enhanced hydrogen bond networks for proton hopping. In contrast, vanadium ions cannot transport through hopping and, thus, are blocked by the carbon nanotubes in hydrophilic domains. Consequently, with the carbon nanotube content increasing from 0 to 0.5 wt %, the area resistance of the electrospun membrane remains unchanged, but vanadium ion permeability dramatically decreases by approximately 67.6%. A vanadium redox flow battery assembled with an electrospun membrane exhibits high chemical and capacity stability as well as an energy efficiency of 83.4%, even after 100 cycles at 100 mA cm−2. These parameters are substantially superior to those of a battery assembled with Nafion 211 with a similar membrane thickness.