Recent advances in molecular engineering of redox active organic molecules for nonaqueous flow batteries

Cost-effective electrochemical energy storage will play a critical role as society transitions to a sustainable energy economy. Nonaqueous flow batteries employing redox active organic molecules are an emerging energy storage concept. A key advantage of this device over the more established aqueous flow battery is the promise for higher cell potentials (>3 V), enabled by the larger electrochemical stability windows of nonaqueous electrolytes. Additionally, nonaqueous flow batteries could leverage new redox couples that are incompatible with aqueous electrolytes due to low solubility, chemical reactivity, and redox potentials outside of the aqueous stability window. Taking advantage of these characteristics may lead to higher energy densities, smaller system footprints, and lower costs. This mini review summarizes recent developments in all-organic nonaqueous chemistries with a focus on tailoring organic molecules for improved physical and electrochemical properties. Key opportunities and challenges in the science and engineering of these devices are presented with a goal of meeting stringent grid cost targets.