Inorganic Separators towards Low-Cost Flow Batteries

G.M. Newbloom, L.D. Pozzo
Ionic Windows LLC,
United States

Keywords: membrane, separator, flow battery, silica

Summary:

Grid-scale power storage remains one of the largest challenges to wide-spread adoption of clean energy technologies with intermittent energy sources (e.g., solar, wind and tidal). Redox flow batteries (RFB) are an environmentally friendly and scalable technology with the potential to meet this need. However, ion exchange membranes that are crucial to battery performance are expensive, lack chemical stability in a strong oxidizing environment, and/or have poor ion selectivity which leads to electrolyte diffusion across the membrane and performance instability. These issues have limited commercial flow battery manufacturers to two options: 1) perfluorocarbon membranes - which are chemically robust and moderately ion selective but very expensive, or 2) separators - which are cheap but chemically degrade over time and have poor ion selectivity. In this presentation, we discuss the design and scale-up of a novel flow battery separator that is chemically robust, ion selective and inexpensive enough to reduce the total production cost of commercial flow batteries by > 10%. In this work, we investigate the use of a primarily inorganic separator created using sol-gel processing of silicates without calcination or sintering. Silica is an advantageous material for separators because of its excellent chemical stability and extremely low cost. Furthermore, the gelation process can be utilized to tune the pore size for effective size exclusion of electrolytes (i.e., good ion selectivity). In this study, small angle x-ray scattering (SAXS) was utilized to characterize the separator pore structure. By varying the gelation conditions, average pore radii were found to be in the range of 0.4 – 0.9 nm. This pore structure is nearly ideal for vanadium-based RFBs given that pores must be able to effectively transport protons (radius of 0.25 nm) but not vanadium ions (radius of 0.5 nm). Proton conductivity and vanadium permeability were also determined and compared to an industry standard perfluorocarbon membrane (i.e., Nafion 117). Silica-based membranes can be designed to have nearly identical performance to perfluorocarbon membranes for these key performance attributes. Furthermore, on-going accelerated chemical oxidative stability testing shows no degradation for silica based membranes. These promising results highlight the commercial potential for a new flow battery separator that is inexpensive, chemically robust and ion selective. Ionic Windows is in the process of commercializing this technology under an NSF SBIR Phase I Award.