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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.

Redoxmers are redox-active molecules that can store energy in electrolytes for redox flow batteries (RFBs), and their electrochemical properties are significantly affected by the choice of supporting electrolytes. Herein, we use 2,1,3-benzothiadiazole (BzNSN) as a model system to scrutinize the supporting electrolyte impact. By systemically varying the components of supporting salts, BzNSN not only shows substantial redox potential shifts but also exhibits varying electrochemical stabilities. Specifically, changing the size of cations can effectively alter the coordination between the supporting salt and BzNSN species. From Li+, Na+, K+, to NEt4+, the redox potential of BzNSN shifts negatively, from −1.63 V to −1.82 V vs. Ag/Ag+. Molecular dynamics and density functional theory simulations revealed that smaller cations, like Li+, are closer to the charged BzNSN when coordinated, implying stronger coordination, while larger cations, like K+ and NEt4+, are farther away. Interestingly, the large cation electrolytes also lead to much improved electrochemical stability, evidenced by the extraordinarily enhanced kinetic lifetime from electron paramagnetic resonance measurement. This study demonstrates the first example of tuning an anolyte redoxmer toward a concurrent improvement of lowered redox potentials AND enhanced calendar lives via solvation means, which is usually constrained by the thermodynamic–kinetic relation.

Redox flow batteries (RFBs), in which electric charge is stored in redox-active materials (redoxmer) dissolved in liquid electrolytes, show significant potential for grid-scale energy storage applications. Because of the liquid nature, RFBs feature remarkable flexibilities, including decoupled energy and power that is highly desired for adapting various scales of energy storage requirements. Aprotic organic solvents are extensively studied as they can significantly extend the electrochemical window, leading to higher cell voltage and energy density of RFBs. To that end, developing redoxmers with high solubility and stability in non-aqueous solvents are actively pursed in order to realize the full potentials. Inorganic-based redoxmer materials showed good cyclability in aqueous redox flow batteries. Designing non-aqueous compatible redoxmers based on traditional inorganic-based materials encounter critical technical and economic limitations such as low solubility, inferior electrochemical activity, and high cost. In this presentation, a ferrate(III) based low potential redoxmer (anolyte) will be discussed. The synthesized redoxmer showed high solubility in acetonitrile (>2 M) with redox potential of ~-1.36 V (vs Ag/Ag+) and deliver very stable cycling performance evidenced by the symmetric H-cell cycling. The design approach for non-aqueous compatible redoxmers based on traditional inorganic-based materials may represent a promising strategy for constructing high energy dense and stable RFBs.