Micellar Solubilization for High-Energy-Density Aqueous Organic Redox Flow Batteries

Abstract

High solubility of active materials is crucial for achieving a high-energy-density catholyte/anolyte in redox flow batteries. However, solubility largely depends on the compatibility with electrolyte, limiting the types of redox-active materials that can be used in aqueous electrolytes. Herein, a universal strategy is introduced to attain a high solubility of active materials regardless of the compatibility with aqueous electrolytes while preserving their intrinsic redox activity via micellar solubilization. Leveraging the amphiphilic nature of surfactant molecules, insoluble redox-active materials are encapsulated by surfactants to be dissolvable with significant solubility. As a demonstration, it is showed that an order-of-magnitude solubility enhancement can be achieved for (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) in aqueous catholyte (& AP;0.8 m). Consequently, the catholyte performance of TEMPO is fully harnessed, leading to an energy density enhancement of more than ten times compared to that in bare electrolyte. It is also observed that micellar solubilization unexpectedly improves the cycle stability, attributed to the mitigation of intermolecular side reactions and reduced crossover. Finally, the fundamental electrochemical reaction mechanism of micelle-encapsulated TEMPO is discussed. This strategy offers a new insight regarding the solubility and stability of the catholyte/anolyte, and is expected to be applicable to other redox-active molecules, opening up an unexplored micellar chemistry in redox flow batteries. Micellar solubilization enables the solubility enhancement of (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) from 0.08 to 0.8 m in aqueous solution. Despite the encapsulation in micelles, micellar-solubilized TEMPO is electrochemically active and fully delivers reversible capacity in the flow cell test. Micellar solubilization also improves the chemical stability and mitigates the crossover, leading to an enhancement in capacity retention.image