Supramolecular Viologen–Cyclodextrin Electrolytes for Aqueous Organic Redox Flow Batteries

Abstract

Along with a primary modification of redox active materials, an additional introduction of secondary noncovalent interactions can synergistically enhance bulk properties of electrolytes for redox flow batteries. Herein, we highlight the host–guest complex formation between tailored viologens and highly water soluble (2-hydroxypropyl)-β-cyclodextrin as a key electrolyte interaction to modulate relevant electrochemical properties of aqueous redox flow batteries (AORFBs). The cyclodextrin-modified AORFB anolytes demonstrated a complex interrelation of molecular structure and inherent binding activity as well as bulk electrochemical stability of the anolyte. The screening of different combinations of viologen substituents in the presence of cyclodextrin enabled an electrochemically stable AORFB performance for more than 500 cycles with a temporary capacity fade rate of 0.26%/day at high energy (>70%) and Coulombic (>99.7%) efficiencies. A selective interplay of supporting electrolytes and engineered redox active materials is a promising strategy for enhanced energy characteristics of AORFB electrolytes.

Hydrogen-Bonding Trends in a Bithiophene with 3- and/or 4-Pyridyl Substituents

“…Upon protonation of a pyridine-containing polyaromatic heterocycle, one would expect to see an overall downfield shift of pyridyl proton signals in conjugation with the pyridinium ring due to the enhanced electron-accepting power of the pyridinium cation . One would also expect that the protons on the pyridinium would experience a stronger downfield shift due to the proximity of the pyridinium N atom pulling electron density away from the rest of the ring’s π cloud . We dissolved 5 in d 6 -DMSO because of its low-ppm solvent resonance and enhanced solubility of 5 over other polar aprotic solvents and added either one or two molar equivalents of triflic acid to protonate one or two pyridine N atoms (Figure ).…”

Recent advances of stimuli-responsive viologen-based nanocomposites

“…143 In 2021, Cekic-Laskovic, Ravoo, Winter, and coworkers prepared viologen derivatives to construct host–guest complexes with highly water-soluble (2-hydroxypropyl)-β-cyclodextrin and employed them as electrolytes for aqueous redox flow AORFBs batteries. 144 The electrochemical performance of the AORFBs batteries was effectively improved by using the viologen moiety and cyclodextrin with excellent redox properties.…”

Redox-Modulated Host–Guest Complex Realizing Stable Two-Electron Storage Viologen for Flow Battery

“…In comparison, β-cyclodextrin (β-CD) features a larger cavity and can accommodate bigger hydrophobic molecules. − Xu applied the host–guest inclusion strategy to assemble the reduced form of ferrocene into (2-hydroxypropyl)-β-cyclodextrin to increase its aqueous solubility. Cekic-Laskovic , used the same host to assemble a viologen derivative for the battery’s application. It was found that the complex yielded a negative shift in the redox potential of the viologen and an increase in the radicals’ stability against the dimerization, benefiting the AFB’s operation.…”