Highly Selective Anion Exchange Membranes for Non-aqueous Redox Flow Batteries

There is a need for long duration energy storage at wind farms and solar photovoltaic facilities. Non- aqueous redox flow batteries would be superior to aqueous flow batteries because they can avoid the problems caused by the breakdown of water at high voltage. However, vanadium non-aqueous flow batteries require a selective membrane that allows BF4- (5.5 Angstroms) to cross back and forth, while preventing diffusion of the slightly larger vanadium active species (9.1 Angstroms). Very few membranes have nanopores in this size range (7-8 nanometers) that can provide this selectivity while being able to resist swelling by the organic solvent. This project is developing a solvent-stable, nanoporous polymer anion-exchange membrane that features highly uniform, well-aligned, nanostructured channels formed by the self-assembly of polymerizable surfactants. The membranes contain 7.5-Angstrom diameter pores, entirely composed of positively charged cations that act as anion diffusion channels. The material not only has a high conductivity, but also an excellent selectivity, while being resistant to swelling by organic solvents. In Phase I TDA prepared the nanoporous anion-exchange membranes and measured their performance in non-aqueous redox flow batteries. We varied the geometry of the surfactants and evaluated the structure-property relationship of the starting surfactant molecular geometry and the resulting nanoporous structure. Our best membranes outperformed Nafion®-115 (the current state- of-the-art-material) by not only having the required high ionic conductivity (15 mS/cm2), but also by having an extremely high selectivity, allowing BF4- to pass rapidly, while nearly eliminating all transport of vanadium. This selectivity resulted in a drastic decrease in capacity fade during charge- discharge cycling at a current density of 10 mA/cm2. In a potential follow-on Phase II project, we plan to further refine the production of the best membranes and fully evaluate them in a larger flow batteries. We will scale up production of the precursor monomers and the finished membranes and supply them for beta-testing. We will fully benchmark the performance of our membrane against existing commercial membranes for both non- aqueous and aqueous vanadium redox flow batteries. Commercial applications include redox flow batteries for long duration energy storage (e.g. wind farms, solar photovoltaic facilities and back-up power systems).