Molecular Simulations and Experimental Insights for Redox Flow Batteries: Unraveling the Aggregation Behavior of TEMPOL Molecules in the Electrolyte

Flow batteries are promising for grid-scale energy storage since their energy and power can be scaled independently. Organic redox-active molecules offer several advantages over traditional metal-based electrolytes, including structural tunability, resource sustainability, and potentially lower costs. To build high-energy-density flow batteries, a high active material concentration is crucial. However, using high concentrations can be limited, e.g., by molecular aggregation, which can adversely affect electron-transfer kinetics and diffusion. Here, we investigate the aggregation behavior of 4-hydroxy-TEMPO in aqueous electrolytes as a model system using a combined approach of molecular dynamics simulations and electrochemical experiments under varying concentrations of active material, supporting electrolytes, and temperatures. In the experimental part, the diffusion coefficient, the reaction rate constant, and the electrochemically active electrode surface area are determined using cyclic voltammetry and chronoamperometry. In molecular dynamics simulations, the structural organization of the electrolyte systems is analyzed in detail. Aggregation is quantified by the relative solvent-accessible surface area. To gain detailed insights into the structural organization, intermolecular interactions within the clusters and the temporal stability, as well as cluster sizes are analyzed, and combined radial–angular distribution functions and the aggregation lifetimes are determined. We show that the mass transport properties of TEMPOL electrolytes are highly influenced by the electrolyte composition. The diffusion coefficient dictates the diffusion-limited maximum current of flow batteries and, therefore, limits the maximum receivable power output of an applied redox flow system. Despite this detrimental effect on overall battery performance, high concentrations of active material are necessary to achieve a high energy density in the battery system. Our insights underscore the importance of a molecular-level understanding of mass transport and aggregation behavior in high-concentration electrolytes and inform the rational design of redox-active species. This work contributes to the broader theme of material characterization and demonstrates the powerful combination of simulation and experiment.