Rational Evaluation and Cycle Life Improvement of Quinone-Based Aqueous Flow Batteries Guided by In-Line Optical Spectrophotometry

We measure electrolyte concentration and state-of-charge in real time in an operating aqueous anthraquinone-based flow battery using in-line optical spectrophotometry. By capitalizing on the linear transition in absorption spectra between oxidized and reduced states, we show how discontinuous capacity fade rates in redox flow batteries can be caused by a transition from one capacity-limiting electrolyte to the other, which is caused by reactant crossover. With this insight, we demonstrate an electrolyte-balancing scheme, which can be broadly applicable to flow battery systems with asymmetric electrolyte crossover rates.