Nonaqueous Redox Flow Batteries Incorporating Novel Pyridinium Anolytes

Abstract

While the use of nonaqueous solvents in redox flow batteries (RFBs) offers the promise of higher cell voltages than can typically be obtained in aqueous electrolytes, suitable compounds that are sufficiently soluble and stable to permit extended operation has proven challenging. Viologen anolytes have been successfully employed in aqueous systems, but their first reduction occurs at very modest potentials, thus limiting their advantage in nonaqueous systems. We have previously reported flow battery chemistry employing a series of extended bis(pyridinium) species with reduction potentials ca. 300-400 mV more negative than viologens and very long-lived, durable reduced states. However, these materials were difficult to access via common synthetic routes and often exhibited limited solubility. In this presentation, we describe an extension of those studies to new pyridinium compounds that provide still more negative reduction potentials (ca. -1.6 to -1.7 V vs Fc/Fc+). Moreover, these compounds are readily synthesized in good yield from inexpensive raw materials, are highly soluble, display excellent electrochemical kinetics, and are extremely persistent in the reduced state. The synthesis and electrochemical characterization of compounds incorporating these groups will be presented, along with preliminary RFB cycling results.