Hydrotrope based supporting electrolytes enhance water solubility of quinones for high-performance redox-enhanced electrochemical capacitors

Abstract

Redox-enhanced electrochemical capacitors (redox ECs) increase the energy density of electric double-layer capacitors (EDLCs) by utilizing the faradaic reactions of redox-active molecules dissolved in the electrolyte. The solubility of redox-active molecules is an important factor for the development of high-performance redox ECs. Unfortunately, most commonly used redox molecules have low solubility in an aqueous electrolyte, and thus enhancement in energy performance has been limited. In order to solve this issue, a complex and costly synthetic process involving substitution of derivatives has been required and practiced. In the present work, we introduce the novel concept of hydrotropic supporting electrolyte (HSE) which can function as both a solubility-enhancing hydrotrope and a supporting electrolyte. By using the HSE, we report a 7-fold increase in the solubility of hydroquinone (HQ) compared with that of HQ in the conventional H2SO4 aqueous electrolyte. Based on this new insight, we synthesized tri-ethanol ammonium bromide substituted anthraquinone (AQM-Br) as advanced dual redox species which can concurrently function as both anolyte and catholyte. Through mechanistic investigation using dynamic light scattering (DLS), and Nuclear Overhauser Effect (NOE), we discovered that there are two distinct solubilization mechanisms (co-solubilizer vs quasi-micelle structure). We developed a dual-redox EC with the addition of p-TsOH as an HSE. This system provides a specific energy of ~ 58 Wh/kg at 0.5 A/g and maintains 81% specific energy over 10,000 cycles at 5 A/g with ~ 99.2% Coulombic efficiency.