(226d) Flexible Ceramic Membranes for High Performance Redox Flow Batteries

Ion exchange membranes (IEM) are often one of the most expensive and performance limiting components of an end-use application. In commercial redox flow batteries (RFBs) they can account for 8 – 27% of the total battery cost. High IEM production costs are largely the result of intrinsically expensive raw materials and lengthy manufacturing processes that require tight environmental controls. In addition to high costs, many IEMs have low ion conductivity, poor ion selectivity and inadequate chemical stability that prevent RFBs from reaching their full potential. In this talk, we discuss the design and scale-up of a novel, nanoporous ceramic RFB membrane that is chemically robust, more mechanically durable, ion selective and 10x less expensive to produce than traditional IEMs.

In this work, we investigated the use of a primarily inorganic membrane created using sol-gel processing of silicates without calcination or sintering. Silica is an advantageous material for membranes 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 membrane pore structure. Model fitting can be utilized to extract the porosity as well as pore shape, size and size distribution. By varying the gelation conditions, average pore radii were found to be in the range of 0.4 – 2 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). Nanostructure was shown to be correlated to key membrane performance attributes such as area specific resistance and optimized to be as low as industry standard perfluoro sulfonic acid (PFSA) membranes that are only 50 µm thick.

Nanoporous silica membranes were also found to have improved mechanical properties when compared to PFSA membranes. In this work, we found nearly 30x less swelling but 20% higher water sorption. These improved mechanical and handling features are critical if RFBs are to achieve fully-automated stack assembly. Furthermore, 18-month accelerated chemical oxidative stability tests show no degradation for silica-based membranes which is important for all-vanadium RFB lifetime. Finally, we demonstrate that it is possible to reach the same energy efficiency as PFSA membranes in an all-vanadium RFB system under common industry cycling conditions. These promising results highlight the commercial potential for a new RFB membrane that is inexpensive, chemically robust, mechanically durable and ion selective.