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We report viscous flow properties of a redox-active organic molecule,N-(2-(2-methoxyethoxy)ethyl)phenothiazine (MEEPT), a candidate for non-aqueousredox flow batteries, and two of its radical cation salts. A microfluidicviscometer enabled the use of small sample volumes in determining viscosity asa function of shear rate and concentration in the non-aqueous solvent,acetonitrile, both with and without supporting salts. All solutions tested showNewtonian behavior over shear rates of up to 30,000 1/s, which is rationalizedby scaling arguments for the diffusion-based relaxation time of a single MEEPTmolecule without aggregation. Neat MEEPT is flowable but with a large viscosity(412 mPa s) at room temperature), which is approximately 1,000 times largerthan acetonitrile. When dissolved in acetonitrile, MEEPT solutions have lowviscosities; at concentrations up to 0.5 M, the viscosity increases by lessthan a factor of two. From concentration-dependent viscosity measurements,molecular information is inferred from intrinsic viscosity (hydrodynamicdiameter) and the Huggins coefficient (interactions). Model fit credibility isassessed using the Bayesian Information Criterion (BIC). It is found that theMEEPT and its charged cation are "flowable" and do not flocculate atconcentrations up to 0.5 M. MEEPT has a hydrodynamic diameter of around 0.85nm, which is largely insensitive to supporting salt and state of charge. Thissize is comparable to molecular dimensions of single molecules obtained fromoptimized structures using density function theory calculations. The resultssuggest that MEEPT is a promising candidate for redox flow batteries in termsof its viscous flow properties.

Viscous flow properties and hydrodynamic diameter of phenothiazine-based redox-active molecules in different supporting salt environments