Nanoporous Aramid Nanofiber Separators for Non-Aqueous Redox Flow Batteries

Redox flow batteries (RFBs) possess a unique combination of attractive attributes including decoupled power and energy storage capacities, low cost, and high efficiencies, and are promising for large-scale energy storage. A significant challenge in the development of high energy density, non-aqueous systems is the lack of selective membrane/separator materials. This paper describes a novel nanoporous separator based on aramid nanofibers (ANF) produced using a spin-coating, layer-by-layer technique. The multilayer structure yields 5 nm pores, enabling nanofiltration and high selectivities, in terms of the transport of species with differing sizes. Vanadium acetylacetonate (V(acac)₃), a candidate for non-aqueous RFB electrolytes, was used as the model active species. It’s permeability through the ANF separator was an order of magnitude lower than that for Celgard 2325, a commercial separator while the support conductivities were comparable. The combined effect resulted in a doubling of the coulombic and energy efficiencies for cells using V(acac)₃ based electrolytes. For asymmetric cells with solutions of V(acac)₃ as the negative electrolyte and ferrocene as the positive electrolyte, a 60% improvement in coulombic efficiency was achieved using the ANF separator over Celgard. The results demonstrate the feasibility of using ANF separators for symmetric and asymmetric non-aqueous RFBs and encourages further development.