Coupled Benchtop NMR and EPR Spectroscopy Reveals the Electronic Structure of Viologen Radicals in a Redox Flow Battery.

Abstract

Viologens are a class of organic molecules with promising properties for redox flow battery applications. However, their molecular-level mechanisms remain challenging to fully probe and understand. In particular, the role of π-dimerization of singly reduced radicals is still debated, with conflicting views on its impact on battery performance. The electronic structure of a viologen radical in a working redox flow battery has remained elusive. Magnetic resonance spectroscopies offer powerful methods for studying flow batteries in operando, but their high cost and maintenance requirements make them less accessible to many researchers in the community. In this study, we introduce a novel dual benchtop nuclear magnetic resonance and electron paramagnetic resonance methodology to investigate viologen-based redox flow batteries. We revealed the electron spin density of in situ generated radicals via the hyperfine coupling interactions measured by EPR, aided by DFT calculations. Notably, the relatively low radical concentration observed during battery cycling suggests that π-dimers form even at a low concentration of 10 mM. Stability of the viologen radicals was also monitored. This study highlights the strength of the dual benchtop approach in uncovering molecular-scale processes in a redox flow battery. Moreover, its flexible and accessible design makes this coupled benchtop technique a versatile tool for investigating a wide range of flow electrochemical systems.