Solubility-Enhancing Hydrotrope Electrolyte with Tailor-Made Organic Redox-Active Species for Redox-Enhanced Electrochemical Capacitors

The solubility of redox molecules is a critical factor that influences cell performance in redox-enhanced electrochemical capacitors (redox ECs). Unfortunately, commonly used organic redox molecules have low solubility in aqueous systems, limiting further performance enhancements. To solve this issue, a complex and costly synthesis is generally required. Here, we introduce the concept of a hydrotropic-supporting electrolyte (HSE) that can function as both a solubility-enhancing hydrotrope and an ion-conducting supporting electrolyte. Using the HSE, the solubility of hydroquinone (HQ) and anthraquinone-derivative (AQM-Br) increased up to 7- and 3-fold, respectively, compared to that in the aqueous H2SO4 electrolyte. We develop a high-performance redox EC by implementing a bipolar AQM-Br with p-toluenesulfonic acid as the hydrotrope. Our mechanistic analysis provides insight into correlating the solute–hydrotrope interactions and hydrotrope efficiency. This work serves as a guideline for designing energy-dense redox-active electrolytes and for optimal selection of the HSE and redox-active electrolyte pairs.