Influence of Hydrotropes on the Solubilities and Diffusivities of Redox-Active Organic Compounds for Aqueous Flow Batteries

Abstract

In this study, we explored the extent to which hydrotropes can be used to increase the aqueous solubilities of redox-active compounds previously used in flow batteries. We measured how five hydrotropes influenced the solubilities of five redox-active compounds already soluble in aqueous electrolytes (≥0.5 M). The solubilities of the compounds varied as a function of hydrotrope type and concentration, with larger solubility changes observed at higher hydrotrope concentrations. 4-OH-TEMPO underwent the largest solubility increase (1.18 ± 0.04 to 1.99 ± 0.12 M) in 20 weight percent sodium xylene sulfonate. The presence of a hydrotrope in solution decreased the diffusion coefficients of 4-OH-TEMPO and 4,5-dihydroxy-1,3-benzenedisulfonate, which was likely due to the increased solution viscosity as opposed to a specific hydrotrope–solute interaction because the hydrotropes did not alter their molecules’ hydraulic radii. The standard rate constants and formal potentials of both 4-OH-TEMPO and 4,5-dihydroxy-1,3-benzenedisulfonate remained largely unchanged in the presence of a hydrotrope. The results suggest that using hydrotropes may be a feasible strategy for increasing the solubilities of redox-active compounds in aqueous flow batteries without substantially altering their electrochemical properties.

Development of organic redox‐active materials in aqueous flow batteries: Current strategies and future perspectives

“…84 However, the solubility data of flavin mononucleotide seem to be controversial because the solubility was measured to be as high as 1.3 mol/L in 1.0 mol/L KOH without adding the nicotinamide hydrotrope, as reported by another group. 85 Briefly, using hydrotropic agents may increase the solubility of redox-active materials, but may also introduce some new factors, such as the chemical/ electrochemical stability variation of hydrotropes, the possible change of electrochemical properties, and the compatibility change with the membrane, which should be taken into consideration.…”

How anthraquinones can enable aqueous organic redox flow batteries to meet the needs of industrialization

“…Hydrotropes are known to solubilize organic compounds via molecular interactions such as ππ stacking. They allow the improvement of the solubility of anthraquinones in aqueous medium, also their efficiency is highly dependent on the redox active species, the pH and the electrolyte medium [32]. Thus, proline increased 2,7-AQDS solubility from 0.83 ± 0.03 M to 1.05 ± 0.23M in acidic medium, whereas sodium xylenesulfonate (NaXS) decreased it to 0.39 ± 0.06 M. Ethylene glycol has also been used to increase the solubility of 2,7-AQDS in 1M KCl from 0.3M to 0.81M [33].…”

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

“…Hydrotropy is a unique solubilization phenomenon, in which hydrotropes can be used to induce a multifold increase in the aqueous solubility of sparingly soluble solutes. Hydrotropes are a class of amphiphilic molecules in which the volume of the hydrophobic component is relatively small compared to that of the surfactant. − Hydrotropes have been researched in various fields, including but not limited to pharmaceutical research, where the aqueous solubility of drugs is a critical parameter. − By inducing the hydrotropic effect, which mediates the interaction between water and hydrophobic solutes, the solubility of hydrophobic molecules can be increased. − However, the exact mechanisms by which hydrotropes solubilize target molecules remain unclear …”

“…This difference suggests that the p-TsOH molecule is closer to the AQ core than the substituent (triethanol ammonium bromide) due to an attractive π−π interaction between the hydrophobic segment of p-TsOH and the aromatic ring of AQM-Br. 41,57 The coherent 1D-DPFGSE NOE buildup profile has been reported primarily in intramolecular NOE experiments, in which the geometric distance between two nuclear spins (source and interesting spin) is maintained during crossrelaxation. 58−60 We hypothesized that the coherent 1D-DPFGSE NOE buildup profile obtained for the intermolecular AQM-Br/p-TsOH system would originate from the formation of stable complex structures.…”

“…The protons on the hydrophobic part of AQM-Br exhibited cross-relaxation rates of 0.9 and 1.7, whereas the protons in the hydrophilic substituent exhibited relatively lower values of 0.4 and 0.5 (Figure c). This difference suggests that the p -TsOH molecule is closer to the AQ core than the substituent (triethanol ammonium bromide) due to an attractive π–π interaction between the hydrophobic segment of p -TsOH and the aromatic ring of AQM-Br. , …”