Accelerating Development of a Nonaqueous Flow Battery Membrane Separator Material

"To facilitate increased adoption of renewable energy, increasing amounts of stationary energy storage will be
needed to accommodate the intermittency of wind and solar power. One promising option for grid-scale
energy storage is flow batteries, which offer the flexibility of modular and scalable design while separating the
energy and storage components. Commercial flow batteries rely on aqueous electrolytes; however, the use of
aqueous electrolytes limits the operating voltage and energy density of these devices. Lower operating voltages
also means that more cells are required to reach a desired output voltage, increasing the size, cost, and power
stack complexity. An alternative is to develop flow batteries with nonaqueous electrolytes, where higher
voltages are possible due to the wider voltage stability window. The higher voltage per cell increases the energy
density and reduces the battery footprint, a consideration that can be important depending on the cost and
footprint required at the installation site. Nonaqueous organic redox flow batteries enable new chemistry for
the electrochemically active species that store energy, and energy density could be increased, for example,
through increases in solubility of these compounds relative to aqueous counterparts.
While nonaqueous flow batteries have the advantages described above, a critical limitation is that membrane
separators have not been developed for successful operation of this technology. New ion-selective membranes
are needed with 1) a high conductivity for the ion that facilitates charge/discharge, 2) selective exclusion of the
organic redox species that participate in electrochemical reactions to reduce crossover and capacity fade, and
3) dimensional, mechanical, and chemical stability. There are very limited materials and process-structureproperty
relationships available for these membranes, as development of nonaqueous flow batteries has not
received as substantive research intensity as its aqueous counterparts.
To meet this need, Luna Innovations and the University of Virginia (UVA) will continue development of a
membrane based on poly(phenylene oxide) functionalized with phenoxyaniline trisulfonate. This UVAdeveloped
material has among the best reported conductivity, selectivity, and stability properties in organic
electrolytes while possessing the commercial potential to be applied to nonaqueous flow batteries. In Phase I,
the membrane material will be improved with side chain modifications towards improved conductivity and
crosslinking to improve dimensional stability and toughness. Luna and UVA have partnered to accelerate
membrane testing, refinement and scale-up in Phase I and transition the technology to nonaqueous flow
battery collaborators in Phase II. Luna also has accelerated test capabilities and process scaling experience to
transition the membrane from the laboratory to pilot-scale (toll manufacturing) demonstrations in Phase II. It
is expected that the knowledge gained from this project will both impact the commercialization of this
promising membrane material and more broadly nonaqueous selective ion conducting polymers."