Investigating Crossover in Aqueous Organic Redox Flow Batteries Using NMR Spectroscopy

Abstract

There is an urgent need for low-cost long-duration energy storage, to facilitate the transition to an all-renewable electricity grid. Aqueous organic redox-flow batteries (AORFBs) are promising candidates for this application. These batteries are composed of two aqueous electrolytes with organic redox-active material dissolved in solution. The two energy-storing electrolytes are separated by an ion-selective membrane; however, in practice the membrane is not perfectly selective. The wide-scale deployment of these batteries is currently limited by unwanted electrolyte degradation reactions and crossover. “Crossover” describes the transport of redox-active material through the membrane, which leads to decay of the battery capacity. Standard membrane permeability measurements do not reflect all of the contributions to crossover in working batteries, including migration and changes in ion size and charge. Furthermore, research into crossover and electrolyte degradation are usually studied separately, likely explaining why the relationship between the two phenomena is poorly understood. Here, a new method has been developed which enables quantitative detection of crossover in operating AORFBs using online 1H NMR spectroscopy. The quantitative on-line NMR crossover detection method developed in this work has been applied to the 2,6-dihydroxyanthraquinone (2,6-DHAQ)/ferrocyanide battery equipped with a Nafion membrane. These studies reveal a that the charging protocol significantly influences crossover. A doubling of 2,6-DHAQ crossover was observed during battery charging, which is most likely due to migration effects. These results demonstrate the importance of understanding the dynamic nature of crossover within operating batteries. Another key outcome from these experiments was the detection of crossover-driven side reactions in alkaline ferrocyanide-based electrolytes. These side reactions were found to be exacerbated at high state-of-charge when ferricyanide (i.e., the charged form of ferrocyanide) concentrations are highest. Promisingly, these side reactions were found to be mitigated by using anolytes which do not possess hydroxyl groups. Concluding, the new crossover detection methodology developed herein has improved our understanding of how charging protocol and electrolyte composition can be optimised to minimise AORFB degradation due to crossover. Application of these methods and insights derived from them will help accelerate the development of longer-lasting AORFBs in the future.