Self-Assembled Solute Networks in Crowded Electrolyte Solutions and Nanoconfinement of Charged Redoxmer Molecules

Redoxmers
are electrochemically active organic molecules storing
charge and energy in electrolyte fluids circulating through redox
flow batteries (RFBs). Such molecules typically have solvent-repelling
cores and solvent-attracting pendant groups introduced to increase
solubility in liquid electrolytes. These two features can facilitate
nanoscale aggregation of the redoxmer molecules in crowded solutions.
In some cases, this aggregation leads to the emergence of continuous
networks of solute molecules in contact, and the solution becomes
microscopically heterogeneous. Here, we use small-angle X-ray scattering
(SAXS) and molecular dynamics modeling to demonstrate formation of
such networks and examine structural factors controlling this self-assembly.
We also show that salt ions become excluded from these solute aggregates
into small pockets of electrolytes, where these ions strongly associate.
This confinement by exclusion is also likely to occur to charged redoxmer
molecules in a “sea” of neutral precursors coexisting
in the same solution. Here, we demonstrate that the decay lifetime
of the confined charged molecules in such solutions can increase several
fold compared to dilute solutions. We attribute this behavior to a
“microreactor effect” on reverse reactions of the confined
species during their decomposition.