Electrochemical Characterization of Aromatic Molecules with 1,4-Diaza Groups for Flow Battery Applications

Abstract

The aqueous redox flow battery is a promising technology for large-scale low cost energy storage. The rich possibilities for the tailoring of organic molecules and the possibility to discover active materials of lower cost and decreased environmental impact continue to drive research and development of organic compounds suitable for redox flow battery applications. In this work, we focus on the characterization of aromatic molecules with 1,4-diaza groups for flow battery applications. We examine the influence of electron-withdrawing and electron-donating substituents and the effect of the relative position of the substituent(s) on the molecule. We found that electron-withdrawing substituents increased the potential, while electron-donating decreased it, in agreement with expectations. The number of carboxy-groups on the pyrazinic ring was found to have a strong impact on the heterogeneous electron transfer kinetics, with the slowest kinetics observed for pyrazine-2,3,5,6-tetracarboxylic acid. The stability of quinoxaline was investigated by cyclic voltammetry and in a flow cell configuration. Substitution at the 2,3-positions in quinoxaline was found to decrease the capacity fade rate significantly. Furthermore, we demonstrated how molecular aggregation reduces the effective number of electrons involved in the redox process for quinoxalines. This translates to a significant reduction of the achievable volumetric capacity at higher concentrations, yielding values significantly lower than the theoretical capacity. Finally, we demonstrate that such capacity-limiting molecular aggregation may be reduced by introducing flexible side chains with bulky charged groups in order to increase electrostatic repulsion and steric hindrance.

Voltammetric Determination of Favipiravir Used as an Antiviral Drug for the Treatment of Covid‐19 at Pencil Graphite Electrode

“…Pristine pyrazine itself belongs to the first group in which it is reversibly reduced to protonated 1,4‐dihydropyrazine by accepting two electrons and three protons. Its reversibility was decreased by the substitution of electron ‐withdrawing or ‐donating groups, and the reversibility can change based on electrode type and supporting electrolyte used [ 50 , 51 ]. The CVs show that the reduction of FAV is included in the third category because the pyrazine ring in the FAV molecule was substituted with three different groups, which led to an irreversible reduction of FAV.…”

Multielectron Organic Redoxmers for Energy-Dense Redox Flow Batteries

“…At pH < 11, rapid decomposition is observed even at the CV time scale. Hjelm et al confirmed a degradation mechanism via protonation of the N atoms followed by tautomerization resulting in loss of aromaticity and redox activity . Substitution at the 2- and 3-positions (ortho to N) in quinoxaline can mitigate such a decomposition and improve chemical stability.…”

Family Tree for Aqueous Organic Redox Couples for Redox Flow Battery Electrolytes: A Conceptual Review

“…After this theoretical study, the same group published an article in which they focused on single molecule pyrazines and quinoxalines in a voltametric study. Next to the different quinoxalines, they focused on six methylated as well as carboxylated pyrazine molecules [124] published in this issue. Unfortunately, they analyzed the first redox transition in slightly alkaline solutions (pH = 13) but not in the highly acidic region.…”

“…Increased overpotential on the negative side and gassing were observed during cycling of the cell. Another study was conducted by Hjelms' group on substituted pyrazines and quinoxalines, already cited above [124]. In this study, they substituted quinolines in the sixth and seventh position with methyl or carboxylic groups and looked at the influences in the voltammogram in slightly alkaline conditions (pH = 13) with sodium chloride as conducting salt.…”