Mitigating the Viscosity-Concentration Trade-Off in Imidazolium-Functionalized Viologens for Aqueous Organic Redox...

The practical deployment of aqueous organic redox flow batteries (AORFBs) is often constrained by the steep viscosity rise observed in many high-concentration electrolytes, which directly governs energy density and power performance. In this study, we report a methyl-imidazolium-functionalized viologen (DiMIm-Vi) that enables multimolar operation while maintaining exceptionally low viscosity (11.50 mPa·s at 2.5 M), thereby fulfilling the dual requirements for energy-dense, power-efficient electrolytes. The low viscosity is accompanied by a rapid electrochemical response, yielding a peak galvanic power density of 0.275 W cm^-2. Notably, the battery demonstrates an outstanding capacity utilization of 94.0% at 100 mA cm^-2 under 2.5 M operation, providing compelling evidence that mass-transport bottlenecks are effectively mitigated even under near-saturation conditions. As a result, the system achieves a volumetric capacity up to 64.0 Ah L^-1 and an energy density of 41.1 Wh L^-1, demonstrating superior performance compared to existing viologen-based AORFBs. Mechanistic insights from computational modeling and electrostatic potential mapping suggest that the incorporation of charged methyl-imidazolium moieties modulates intermolecular interactions, leading to a more dispersed molecular environment and weakened π-π stacking, which is consistent with the suppressed viscosity growth at high concentration. This molecular engineering strategy effectively retards the buildup of viscosity typically observed in concentrated electrolytes, offering a generalizable paradigm for developing high-energy, high-power organic flow batteries.