Micellar Solubilization for High-Energy-Density and Long-Cycle-Life Aqueous Organic Redox-Flow Batteries

Abstract

Functional group control has been the most widely used strategy to enhance the solubility of redox-active organic materials (ROMs) in aqueous media for aqueous organic redox-flow batteries. However, this strategy inevitably requires complicated organic synthesis, typically involving 2–4 steps with high cost and under harsh conditions, reducing the full potential of ROMs. In this study, to achieve higher ROM solubility, we introduced surfactant molecules to provide a nonpolar microenvironment via micellization. The hexadecyltrimethylammonium chloride surfactant enabled micellar solubilization, resulting in an order-of-magnitude enhancement of the solubility of (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO). In addition, the micellar solubilization elevated the redox potential of TEMPO, increasing the energy density by more than 10 times compared with that in the bare electrolyte. Furthermore, the micellar solubilization improved the cycle stability due to not only mitigated crossover but also enhanced chemical stability of TEMPO+. This strategy can be applied to various ROMs and is expected to assist in achieving practical application of organic redox-flow batteries.