Redox flow batteries (RFBs) are leading contenders for grid-scale energy storage devices. ¹. RFBs can be rapidly deployed and demonstrate high durability (>10,000 cycles²), making them appealing for long-term energy storage. Despite these advantages, commercial RFBs are hampered by high capital costs. Ion-selective membranes in RFB cell stacks make up a significant fraction (up to ~37% ³) of the RFB cost – for both new RFBs and expanding existing RFBs. The need for a large membrane area in the cell stacks stems from the cell power capacity (maximum charge-discharge current) being directly dependent on the membrane size.

In this talk, we report our progress on a new RFB layout coined the “split biphasic architecture.” This is a development on membrane-less biphasic RFBs which use mutually immiscible phases for the anolyte and catholyte⁴. Self-discharge at the anolyte-catholyte interface is a rampant issue in these biphasic RFBs, and we discuss our approach to resolving this issue and increasing Coulombic efficiencies from ~70% to >99%. Further, we will discuss trends in interfacial ion transfer resistance vis-a-vis dependence on the solvents and electrolytes (Hoffmeister series). Building on this fundamental proof-of-concept, we hope to extend the scope of battery chemistry compatible with the split biphasic layout in the future.

(1) Skyllas-Kazacos et al., J. Electrochem. Soc. 2011, 158 (8), R55

(2) Janoschka et. al., Nature 2015, 527 (7576), 78–81

(3) Minke, C.; Kunz, U.; Turek, T., J. Power Sources 2017, 361, 105–114

(4) Molina-Osorio, et. al., Curr. Opin. Electrochem. 2020, 21, 100–108