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

Abstract

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.

Utilizing Activated Carbon Nanopores for Electrochemical Oxidation of Perylene to Redox-Active 3,10-Perylenedione: Application in Electrochemical Capacitor Electrodes

“…The latter requires complex molecular design to ensure solubility in the electrolyte solution before charging, convertibility to solid forms on the electrode surfaces upon charging, fast electron transfer with the electrode surfaces, electrochemical stability, and appropriate redox potentials. 55 Conversely, the hybridization of redox-active materials within the AC pores through the simple gas-phase adsorption of organic compounds, followed by their subsequent electro-chemical oxidation, offers advantages in preparing highperformance electrochemical capacitor electrodes. This method avoids the use of redox-active electrolytes, eliminates multistep synthesis processes, reduces material waste, simplifies molecular design, and obviates the need for ion-exchange membranes.…”

Recent advances in hydrotropic solvent systems for lignocellulosic biomass utilization

“…48,49,58–61,63 The hydrophilic moieties of hydrotropes, such as hydroxyl and carboxyl, carbonyl, amino, and phosphate, facilitate the hydrotrope in an aqueous solution. 64 p -TsOH, having a hydrophobic component with an aromatic ring and non-polar methyl group. 65 The sulfonic acid group is the hydrophilic component of p -TsOH and functions as an electrolyte.…”

“…The sulfonic acid can be ionized or dissociated due to its electrical conductivity and dissolve biomass components in an aqueous solution. 64,66 Maleic acid is another acid hydrotrope for plant biomass fractionation. 49 It is a dicarboxylic acid with a polar group (hydrophilic) on one side and a non-polar CC bond on the opposite side.…”

Addressing Intrinsic Limitations of Aqueous Electric Double-Layer Capacitors with Designer Redox-Active Deep Eutectic Solvent Electrolytes

“…To increase the energy density of EDLCs, researchers have focused on the development of redox-enhanced electrochemical capacitors (redox ECs) by replacing the inert electrolytes in EDLCs with redox-active electrolytes. Redox ECs provide increased energy density via redox reactions of the redox-active species, in addition to electric double-layer charging. − Consequently, redox ECs exhibit both capacitive and Faradaic charge storage mechanisms in a single system. As redox ECs display well-defined redox peaks in the cyclic voltammetry (CV) as well as a nonlinear galvanostatic charge–discharge (GCD) voltage profile, they are defined as hybrid systems .…”

“…Ensuring the high solubility of redox-active molecules is a critical factor that significantly impacts the energy density of redox ECs. , Upon introducing EVBr 2 into Ethaline, the solubility of EVBr 2 was found to be less than ∼1 M. To ensure sufficient solubility of EVBr 2 in DES for optimal cell performance, we increased the molar ratio of EG, as the increased number of EG molecules facilitated solvation of EV 2+ via ion-dipole interactions. Specifically, the positively charged EV 2+ molecules attract the negatively polarized oxygen of ethylene glycol, enabling ethylene glycol to solvate EV 2+ by forming the closest shell around the ion .…”