Redox Flow Battery Membranes with Improved Vanadium-Ion Barrier Properties

The crossover of active redox species in flow batteries leads to a decrease in the faradaic efficiency and, in case of cells with different species on the positive and negative side, to cross-contamination of the electrolyte solutions. In the all-vanadium redox flow battery (VRB) typically vanadium sulfate in a background electrolyte of sulfuric acid is used as electrolyte. In case of commonly used proton exchange membranes, vanadium species can pass through the electrolyte more or less easily.

In this contribution, an approach to improve the vanadium-ion barrier properties of a membrane is presented, while largely maintaining its area resistance [1]. Ethylene tetrafluoroethylene (ETFE) films of 25 µm thickness were grafted with styrene and acrylonitrile (AN) using the radiation-induced grafting technique. Subsequently, the grafted AN units were amidoximated using hydroxylamine. Amidoxime groups are metal-ion complexing agents. The styrene units were sulfonated to introduce cation exchange sites. The composition of the obtained membranes was analyzed by FT-IR spectroscopy and the elemental distribution by energy-dispersive x-ray spectroscopy (EDX) coupled with scanning electron microscopy (SEM). The key properties of the membranes for the application in the redox flow cell, i.e., vanadium ion permeability and area resistance, were investigated. It was found that the permeability of vanadium ions was effectively reduced by increasing the amount of amidoxime groups in the membrane, while the area resistance, measured in a redox flow cell setup, did not considerably increase.

In this contribution, the key polymer design aspects to tune the ohmic resistance and vanadium barrier properties of the membrane are highlighted, relevant ex situproperties of membranes discussed, and redox cell test data presented to demonstrate the features of the grafted amidoxime membranes compared to Nafion 212.

[1] L. Gubler, O. Nibel, L. Bonorand, European Patent Application EP15154151, Paul Scherrer Institut, 2015