Polymer electrolyte membranes (PEM) for vanadium redox flow batteries

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The amount of energy from renewable sources will increase in Germany in the upcoming years. Thus, storage systems for this fluctuating energy are of high importance. One possibility to store energy is the use of redox-flow-batteries (RFB), which are easy to scale. They allow for storage of electric energy in a liquid rechargeable electrolyte. Vanadium based systems (VRFB) are particularly interesting, since vanadium ions are present on both sides of the membrane in the electrochemical cell. While cross-over of vanadium ions in the other electrolyte half-cell will lead to a lowering of the energy density irreversible damage of the system due to contamination does not occur. Frequently, polymer electrolyte membranes (PEM) are used as separator in VRFBs. Particularly important is Nafion, which is a sulfonated fluoropolymer. Despite the fact that Nafion is used in a number of commercial applications, there are some shortcomings associated with Nafion and the development of new PEM materials with superior properties is of high importance. Within the interdisciplinary project novel PEM materials will be developed on the basis of commercially available fluoropolymer films and fabrics serving as scaffold. The radiation-induced activation of the fluoropolymers is followed by a graft copolymerization to incorporate functional groups allowing for sulfonation into the material. The influence of the irradiation conditions on radical type and concentration and subsequently on the polymerization is studied in detail. After sulfonation the PEMs are electrochemically characterized and application tests in VRFBs are performed. Important aspects to be addressed are high conductivity and prevention of cross-over. The experimental results constitute the basis for establishing a correlation of process, polymer, structure and the PEM properties. Moreover, the experiments are accompanied by modelling, e.g., with respect to optimization of the polymerization process and description of mass transfer through the membrane.