Raman spectroelectrochemistry for operando characterization of redox flow batteries

Abstract

Despite the potential of Aqueous Organic Redox Flow Batteries (AORFBs) to address intermittent energy generation from renewable sources, they must improve some key performance indicators to become competitive, in particular cycle stability. Development of advanced in-situ and time-resolved techniques plays a critical role to improve performance of AORFBs by enabling elucidation of the sources for energy storage capacity fading. The development and implementation of operando Raman spectroscopy is herein reported for dihydroxyanthraquinone–ferrocyanide alkaline flow battery. Validation of the technique is carried out using symmetrical cells, confirming that Raman spectroscopy can monitor in-situ the state of charge. In a full battery, time-resolved Raman spectroscopy is used for investigating the Faradaic imbalance process, showing that presence of oxygen in the anolyte leads to the progressive loss of available ferrocyanide. Raman spectroscopy also shows that ferricyanide self-discharges when left at open circuit. Finally, this technique is used to track in-situ the crossover of 2,6-dihydroxyanthraquinone. Surprisingly, the rate is found to increase at full state of charge, supporting recent findings using NMR. This operando Raman spectroscopy is anticipated to provide unique insights into critical processes in emerging redox flow battery chemistries.