Solution Properties and Practical Limits of Concentrated Electrolytes for Nonaqueous Redox Flow Batteries

Nonaqueous redox flow batteries (NRFBs) use energized organic fluids that contain redox active organic molecules (ROMs) and supporting electrolyte. Such all-organic electrolytes have wider electrochemical stability windows than the more familiar aqueous electrolytes, potentially allowing a higher energy density in the solutions of charged ROMs. As this energy density increases linearly with the concentration of the charge carriers, physicochemical properties of concentrated ROM solutions in both states of charge present considerable practical interest. For NRFBs to become competitive with other types of flow cells, the current techno-economic analyses favor highly concentrated solutions (>1 M) with high ionic conductivity (>5 mS/cm). It is not presently clear that such solutions can have the required dynamic properties. In this study, we show that ion diffusivities and conductivities of ROM-containing electrolytes reach maxima around 0.5 M and decrease significantly at higher concentrations; realistic limits are established for variations of these parameters. Furthermore, using closed-shell analogues for open-shell charged ROMs, we show that reconstitution of highly concentrated fluids during electrochemical charging will have strong adverse effects on their properties, including an increase in viscosity and decrease in conductivity and ion diffusivity. Given our results, it appears that the target concentrations of NRFB fluids need to be reconsidered in terms of concentration-dependent conductivity and viscosity.