Low viscosity, high concentration pyridinium anolytes for organic non-aqueous redox flow batteries

The ability to tune various physicochemical and electrochemical properties of redox-active organic molecules (ROMs) independently from one-another has been a long-standing goal of researchers attempting to design active materials for redox flow batteries (RFBs). While 2 increasing ROM solubility is essential for improving energy densities, power densities, and lowering costs of RFBs, the use of elevated ROM concentrations in an RFB often causes solution properties – such as viscosity and conductivity – to vary in unpredictable and impactful ways. At elevated concentration, strong electrostatic interactions between ROMs, solvent, and supporting electrolyte often result in high viscosity and low solution conductivity, both of which are deleterious to practical RFB operation. Recently, it has been demonstrated that a class of 2,6-dimethylpyridinium-derived ROMs can achieve a broad range of solubilities in acetonitrile by fine-tuning unique intermolecular CH-pi interactions, which disrupt electrostatic solute-solute interactions. Here, we evaluate the electrochemical characteristics for a library of 23 N-substituted 4-aryl-2,6-dimethylpyridinium ROMs and measure solution viscosities and conductivities at variable concentrations for three representative species in acetonitrile. We show that this class of 2,6-dimethylpyridinium ROMs demonstrate low reduction potentials and rapid diffusion coefficients at low concentrations, and we find that representative pyridinium ROMs exhibit low dynamic viscosities (~1 cP), and high conductivities (25.0 - 32.8 mS cm-1) at elevated concentrations in acetonitrile. Our results suggest that trends in solution viscosity may be a consequence of distinct intermolecular interactions prevalent among pyridinium molecules and these desirable qualities further establish pyridiniums as a promising anolyte for emerging grid-scale energy storage technologies.