Electrochemical Impedance Spectroscopy Investigation of Organic Redox Reactions (p-Benzoquinone/Hydroquinone Couple)─Implications to Organic Redox Flow Batteries

Abstract

Redox reactions of organic redox couples are investigated on Nafion-free and Nafion-containing carbon (Vulcan XC-72)-modified glassy carbon disk (GCD) electrodes. The model system chosen is the p-benzoquinone (BQ)/hydroquinone (BQH 2 ) couple because BQ derivatives are common to most of the organic redox couples. The electrochemical impedance spectroscopy patterns show an unexpected rise in real resistance with time and with rotation speed (rpm). This increase in resistance is remarkable with Nafion-containing GCDs. It seems that the sulfonate group on the polymer chain attacks the redox couple to bind the species to the electrode surface. Moreover, the internal and external mass transfer features are better resolved than their nonbinding inorganic counterparts (V 4+ /V 5+ couple). These findings have enormous implications to the development of organic redox flow batteries, organic synthesis, supercapacitors, sensors, and so on.

What contributes to the internal mass-transport resistance of redox species through porous thin-film electrodes?

“…After the mass-transport correction, both the corrected LSVs obtained from Nafion-free and Nafion-containing electrodes overlap, suggesting that the major contribution to the increase in the impedance observed with the Nafion-containing electrode is from the internal mass-transport process. 29 With the BQ/BQH 2 system, a shift in the peak positions is observed in the voltammograms of the Nafion-free and Nafion-containing electrodes, in contrast to the V 5+ /V 4+ system. It is apparent that the ionomer blocks the active sites of the electrodes, and it imparts extra resistance towards the transport of the redox species through the porous medium.…”

“…The undesirable interaction of ionomers with the redox species was recently established by Joy et al using an array of characterizations including NMR and Raman IR spectra and its implications and deleterious effect on organic redox flow batteries was illustrated. 29 Here, both voltammograms are corrected for the total mass-transport loss, obtained from the fitting parameters of their corresponding EIS patterns.…”

“…9 Recently, Joy et al established the interaction of redox species with the ionomer and reported its effect on organic redox flow battery performance. 29…”

Molecular Engineering of Organic Species for Aqueous Redox Flow Batteries

“…Quinones are a series of carbonyl compounds that contain two carbonyl groups (−C=O) in one aromatic ring [33] . Its derivatives are one of the most frequently studied organic compounds in AORFBs, such as benzoquinone (BQ), [34–36] naphthoquinone (NQ), [37–40] and anthraquinone (AQ), [41–45] as shown in Figure 2. Both of the carbonyl groups store charges by an “ion‐coordination” mechanism in which the C=O groups are reduced to form oxygen anions that can pair with different cations (e. g., H + , Na + , Zn 2+ ) [46]…”

“…Quinones are a series of carbonyl compounds that contain two carbonyl groups (À C=O) in one aromatic ring. [33] Its derivatives are one of the most frequently studied organic compounds in AORFBs, such as benzoquinone (BQ), [34][35][36] naphthoquinone (NQ), [37][38][39][40] and anthraquinone (AQ), [41][42][43][44][45] as shown in Figure 2.…”