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

Viologens are a class of organic molecules with promising properties for redox flow battery energy storage. 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. Notably, the low radical concentration observed during battery cycling suggests that π-dimers occurs even at relatively low concentrations of 10 mM. Our findings highlight the strength of our 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.