Exploring Redox Active and Electrochemically Stable Organic Molecules for > 3 V Non-Aqueous Redox Flow Batteries

Abstract

Non-Aqueous Redox flow batteries (NARFBs) are emerging grid-scale energy storage systems that have the potential to overcome issues associated with aqueous RFBs, such as a limited voltage window due to water electrolysis and low energy densities. However, most non-aqueous electrolytes consist of metal coordination complexes that have significantly lower solubility due to their large molecular size compared to their aqueous counterparts and their cell voltages are typically limited to < 3 V.1 Besides, widespread acceptance of metal coordination complexes as the non-aqueous electrolyte is also restricted by the higher cost of the metal. Many of these metal coordination complexes undergo disproportionation; therefore, resulting flow batteries do not require selective membranes. However, cost targets outlined by Darling et al. and the U.S. Department of Energy require redox active materials to have > 3 V cell voltage and 4-5 M solubility and ~$5/Kg material cost.2 Therefore, there have been substantial efforts made to develop organic molecules either as anolyte or catholyte that meet many of these criteria for application in NARFBs. Some reported redox active organic molecules have solubility up to 5 M in common organic solvents.3-4 The lower molecular weights of organic molecules provide more room and freedom for molecular engineering to improve solubility, stability, and increase the potential window. However, deployment of these molecules in asymmetrical NARFBs raises the inherent issue of RFBs, i.e., irreversible capacity fade due to the absence of selective membranes.4 In this presentation, we will report a new class of sulfur and oxygen-containing redox active organic molecules for high voltage (> 3 V) NARFBs. Specifically, we will highlight the screening procedure and electrochemical properties of various anolytes and catholytes with higher redox potentials. We will also discuss strategies to minimize the capacity fade due to crossover. As proof of concept, anolyte and catholyte will be combined in a full cell, and cycling performance will also be discussed. Acknowledgements: The authors would like to thank Dr. Imre Gyuk and the U.S. Department of Energy, Office of Electricity for supporting this work.