Nanoporous Aramid Nanofiber Separators for Non-Aqueous Redox Flow Batteries

Monday, October 12, 2015: 14:40
102-C (Phoenix Convention Center)
S. O. Tung (University of Michigan, Ann Arbor), L. T. Thompson (University of Michigan), S. Laramie (University of Michigan), R. Zhang (University of Michigan, Ann Arbor), and N. Kotov (University of Michigan, Ann Arbor)
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.