Author to whom correspondence should be addressed. The ion exchange membrane of the Nafion series widely used in vanadium flow batteries (VFBs) is characterized by its high cost and high vanadium permeability, which limit the further commercialization of VFBs.
Innovative membranes are needed for vanadium redox flow batteries, in order to achieve the required criteria; i) cost reduction, ii) long cycle life, iii) high discharge rates and iv) high current densities. To achieve this, variety of materials were tested and reported in literature. 7.1. Zeolite membranes
The vanadium ion permeability was as low as 1/20 to 1/40 than that from Nafion likely due to the Donnan effect. The application of polybenzimidazole (PBI) anion membranes has attracted high attention for their application as a membrane in VRB's [82, 83].
Exposure of the polymeric membrane to the highly oxidative and acidic environment of the vanadium electrolyte can result in membrane deterioration. Furthermore, poor membrane selectivity towards vanadium permeability can lead to faster discharge times of the battery. These areas seek room for improvement to increase battery lifetime.
The hybrid membrane exhibited low vanadium ion permeation and high ion selectivity, owing to the nanotube blocking or prolonging the permeation path of vanadium ions. Meanwhile, the interaction between the ions on the surface of the nanotubes and the sulfonic groups in the Nafion matrix promoted proton transport along the surface of the nanotubes.
The use of low-cost hydrocarbon membranes in vanadium flow batteries (VFBs) still remains a great challenge because of the strong oxidation of VO 2+ catholyte and rapid capacity fading.