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