Reversible Redox Chemistry in a Phenoxazine-Based Organic Compound: A Two-Electron Storage Negolyte for Alkaline Flow Batteries

Abstract

Finding two-electron storage aqueous organic negolytes is crucial for increasing the charge storage capacity in next-generation flow batteries. Inspired by the ability of gallocyanine compound (GAL) to act as an efficient two-electron transfer mediator in biology, we investigated its reduction process by cyclic voltammetry at alkaline conditions, detecting ion pairing (with Na+, K+, or Li+ ions) and H+ transfer pathways in electrogenerated species. The voltammetric responses obtained were dependent on the concentrations of analyte and supporting electrolyte and exhibited values of peak-to-peak potential distance (ΔE p) close to the limiting value established in flow battery protocols for screening irreversible systems (>200/n mV). The results presented in this work can not be explained by irreversible mechanisms. This atypical behavior was related to the evolution of a mixed and reversible process of adsorption and diffusion pathways, where GAL species near the electrode first attach to the electroactive surface without passivating it, so at high concentrations of GAL compound, the reactions can take place by a mainly diffusive process (at the modified electrode). The reversible redox chemistry in compound studied was corroborated by testing a flow cell of 0.83 V composed of a GAL-KOH system and the posolyte ferrocyanide-KOH solution. The cell exhibited a Coulombic efficiency close to 100% and retained 87% of its capacity after 200 charge–discharge cell cycles. This is the first report concerning the use of a phenoxazine-based derivative for storing two electrons per molecule in an aqueous flow cell.

A quaternized anthraquinone derivative for pH-neutral aqueous organic redox flow batteries

“…6d) is associated with a reformulation process of 1,4-DHAQ electroactive layers formed at the surface of carbon felt electrode during charge-discharge cycling. 30 Fig. 7d presents the 1 H NMR spectrum of the BDEAQCl 2 anolyte after cycling.…”

“…6d) is associated with a reformulation process of 1,4-DHAQ electroactive layers formed at the surface of carbon felt electrode during charge–discharge cycling. 30…”

Density functional theory and machine learning for electrochemical square-scheme prediction: an application to quinone-type molecules relevant to redox flow batteries

“…Proton-electron transfer (PET) is a fundamental reaction in electrochemistry, 1-3 biochemistry, 4,5 material science, 6,7 and in some other elds. 8,9 To give one example, we can consider the charging and discharging processes in aqueous redox ow batteries, [10][11][12][13][14] in which the two elementary steps, i.e., electron transfer (ET) and proton transfer (PT), are taken either competitively or jointly to interconvert electricity and chemical energy. Therefore, understanding the pathways from reactants to products through PET reactions is crucial to understand ow battery performance.…”

Benzidine Derivatives as Electroactive Materials for Aqueous Organic Redox Flow Batteries

“…These materials represent a novel option as they mainly involve multielectron-transfer reactions. Some publications report redox indicators and dyes involving bielectron-transfer processes in RFBs with acceptable efficiencies, such as methylene blue, 8,9 basic red 5, 10 indigo carmine, 11 gallocyanine, 12 and neutral red. 13 Although benzidine is not itself a redox indicator, certain Nsubstituted and 3,3′-substituted benzidines, naphthidine, 3,3′substituted naphthidines, and diverse derivatives of these compounds exhibit color changes in a potential range of 0.7− 1.1 V|SHE.…”

“…These materials represent a novel option as they mainly involve multielectron-transfer reactions. Some publications report redox indicators and dyes involving bielectron-transfer processes in RFBs with acceptable efficiencies, such as methylene blue, , basic red 5, indigo carmine, gallocyanine, and neutral red …”