Person:

George, Thomas

Aqueous redox flow batteries (ARFBs) constitute a promising technology for grid-scale electricity storage, but it is challenging to implement cell voltages exceeding the 1.23 V thermodynamic water splitting window with high Coulombic efficiency and long lifetime. pH decoupling – the creation of a pH difference between the negolyte and posolyte – can broaden the operating voltage window and improve long-term operational stability. This penalizes the efficiency, however, due to acid-base crossover induced by the pH gradient. As the voltage of the water splitting window varies linearly with pH whereas crossover fluxes vary exponentially, we employed mildly acidic and mildly basic electrolytes to develop a cell with high round-trip energy efficiency at an open-circuit voltage > 1.7 V. Moreover, we implemented an in situ acid-base regeneration system to periodically restore the negolyte and posolyte pH to their initial values. The combined system exhibits a capacity fade rate of less than 0.07% per day, a roundtrip energy efficiency of over 85%, and a Coulombic efficiency of approximately 99%. This work demonstrates principles for addressing critical issues such as lifespan, rate capability, long-term practicability, and energy efficiency in pH-decoupling ARFBs, providing guidance for the design of the next generation of high-voltage ARFBs.

Aqueous organic redox flow batteries are promising candidates for large-scale energy storage. However, the design of stable and inexpensive electrolytes is challenging. Here, we report a highly stable, low redox potential, and potentially inexpensive negolyte species, sodium 3,3′,3′′,3′′′-((9,10-anthraquinone-2,6-diyl)bis(azanetriyl))tetrakis(propane-1-sulfonate) (2,6-N-TSAQ), which is synthesized in a single step from inexpensive precursors. Pairing 2,6-N-TSAQ with potassium ferrocyanide at pH=14 yielded a battery with the highest open-circuit voltage, 1.14 V, of any anthraquinone-based cell with a capacity fade rate <10 %/yr. When 2,6-N-TSAQ was cycled at neutral pH, it exhibited two orders of magnitude higher capacity fade rate. The great difference in anthraquinone cycling stability at different pH is interpreted in terms of the thermodynamics of the anthrone formation reaction. This work shows the great potential of organic synthetic chemistry for the development of viable flow battery electrolytes and demonstrates the remarkable performance improvements achievable with an understanding of decomposition mechanisms.

Volumetrically unbalanced compositionally symmetric cell cycling with potentiostatic (CV) or galvanostatic-with-potential-hold (CCCV) protocols is a rigorous technique for evaluating the calendar lifetime of reactants for redox flow batteries.