High-Performance Nanoparticle-Modified Membrane for Nonaqueous Redox Flow Batteries

For the effective implementation of environmentally friendly large scale renewable energy, low-cost grid-capable storage solutions must be implemented to accommodate the effective capture and load leveling of the typically periodic and variable production levels. One solution of particular interest is redox flow batteries (RFBs), which provide a high number of deep cycles and the ability to store large amounts of electrical energy cheaply and efficiently. However, aqueous RFBs typically suffer from low energy density, limited electrolyte stability, and capacity decay during cycling. Alternatively, switching RFBs to nonaqueous solvents and charge carrying redox complexes provides higher operating voltages allowing for higher energy density cells. However, to successfully implement these non-aqueous RFBs (NARFB) there remains the need to improve the durability of the cell membranes and reduce their susceptibility to crossover of active redox species.<br/><br/>To this end, Mainstream Engineering successfully demonstrated the preparation of a stable nanoparticle composite protective film that effectively reduces redox molecule crossover (&gt; 90 % reduction compared to the non-reinforced anion exchange membrane [AEM]) with no effect on the key membrane ionic conductivity and shows promise as a high-performance membrane for NARFBs. Mainstream Engineering's approach to optimizing the AEM for NARFBs builds on our previous work developing scalable selective ion exchange membranes and membrane coatings of a fuel cell, batteries, and highly selective liquid and gas separation applications. For this work, we screened various potential AEMs for their chemical and mechanical stability in non-aqueous solutions as well as demonstrating a wide electrochemical stability window. After down selecting an AEM base membrane with excellent chemical stability, we formulated a nanoparticle/ionic binder coating to allow the deposition of a thin-film protective coating and the formation of a composite membrane on the AEM base membrane. We examined both screen printing and knife coating approaches to provide a robust, uniform, and repeatable method of coating the slurry compositions to provide a stable, uniform thin-film coating. The surface characteristics of our composite films were evaluated with optical and scanning electron microscopy, to determine the base porosity and the coating uniformity. These composite membranes were demonstrated to be both chemically and mechanically stable in the non-aqueous solvents. The ionic conductivity and iron tris(2,2’-bipyridine) (FeBpy) permeability of the films were benchmarked against commercially available AEMs. We have further optimized our composite membranes for decreased FeBpy permeability and performed half- and full- cell testing of our membranes, both under stationary and flowing electrolyte conditions. The performance of the protective film was further optimized via surface charge modifications of the nanoparticles to allow a 75% reduction in the ionic resistance of the nanoparticle modified membranes, lowering the cell voltage drop and improving the power density. Our materials show increased performance, by lowering the redox molecule crossover without increasing the membrane thickness or overall ionic resistance. Mainstream Engineering’s novel composite AEMs provide a promising approach to solving outstanding membrane performance issues with current NARFB chemistries, providing a commercially viable path forward for high-energy density non-aqueous RFB storage technologies.