TEMPO microemulsion enabling extremely high capacity catholyte in aqueous organic redox flow batteries

The low aqueous solubility of 2,2,6,6-tetramethylpiperidinooxy (TEMPO) severely limits the capacity of aqueous organic redox flow batteries (AORFBs). Herein, a microemulsion solubilization strategy is developed to address this issue. By screening the co-solvent and the surfactant, a high aqueous solubility of 3.45 M is realized in the microemulsion consisting of acetonitrile and tetrabutylammonium chloride. Physicochemical characterizations reveal that the alkyl chains of tetrabutylammonium chloride are partitioned in the organic phase to realize the fusion and equilibrium of the organic and water phases, which realizes a superior stability, decent ionic conductivity and low viscosity. The AORFB fed with the 2.5 M electrolyte shows a capacity of 60.6 Ah L⁻¹, which is one of the highest practical capacities among TEMPO-based AORFBs. This work manifests a facile and universal strategy by simply regulating the electrolytic solution to realize a high capacity in AORFBs, which shows advantages over the complicated molecule engineering strategy.