Multifunctional Pyridinium Systems for Nonaqueous Redox Flow Batteries

Abstract

Viologens are among the most thoroughly studied redox active organic compounds, and many examples display two distinct reversible reductions under appropriate conditions. However, by introducing p-phenylene or p-biphenylene bridges between the pyridinium rings, substantially different electrochemical behavior is observed. Specifically, the stabilization afforded by the quinoidal form of the doubly reduced species can lead to coalescence of the pyridinium reduction peaks into a single, net 2e- wave. We have employed simple and straightforward synthetic routes to various flavors of multi-pyridinium materials. Two, three, and four electron systems have been demonstrated, and more complex systems capable of accepting six or more electrons are currently being developed. These materials are well characterized by NMR, ESI-MS, electrochemical, and spectroelectrochemical methods. Correlations between structure, redox potential, spectroscopic properties, and stability will be described, as will our efforts to develop and optimize such systems for applications in redox flow batteries.