Long Lifetime Mild pH-decoupling Aqueous Flow Battery with Practical in Situ pH Recovery

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.