Enhancing Anionic Exchange Membrane Robustness in Vanadium Redox Flow Systems via Multication Side Chain Addition

A more robust energy storage system is needed to provide for increasing energy demands in the world. One highly robust and scalable technology is the vanadium redox flow battery (VRB). Limitations of the standard VRB cell membrane, Nafion, are many. Nafion is too expensive to scale up in numerous cell stacks. A highly acidic and oxidative environment has been shown to foul and degrade the polymer membrane. Also, a primary concern is the vanadium ion crossover through percolation channels which leads to self-discharge reducing the coulombic efficiency. Cheaper and more durable cell membranes have been investigated. Diels-Alder poly(phenylene) (DAPP) materials are very mechanically robust membranes that have the potential to be a cheaper alternative. DAPP with pendant methyl groups on the phenyl rings can be brominated and substituted with quaternary ammonium to create benzyltrimethylammonium groups (QDAPP). Specifically, these cationic groups have been shown to possess lower vanadium ion permeability than Nafion because of the electrostatic repulsion described by the Donna effect. However, higher water uptake at higher IEC values creates swelling and limits crossover prevention. Additionally, ex-situ degradation studies have shown that membranes are prone to oxidation earlier than Nafion. To combat oxidation of the membrane more sterically hindered polymers will be investigated in this work. By incorporating previously reported quaternary ammonium multication side chains with different alkyl spacer lengths it is thought that membrane durability of DAPP will be increased. Additionally, further improvement of the ion conductivity on the previously reported carbonate counter ion form is hoped to be gained with additional cationic groups. Planned studies for this project include cell diffusional studies to determine crossover, ex-situ degradation studies, in-plane and through-plane conductivity measurements, mechanical strength determination, toughness determination, and VRB performance factors such as coulombic and voltage efficiency as well as open current voltage decay and charge discharge behavior. Future studies involve blending QDAPP with varying degrees of IEC.