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

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 (Delta E-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.

Automated summary

Finding water-soluble organic negolytes that can store two electrons is crucial for increasing the charge storage capacity of next-generation flow batteries. In this study, the reduction process of the gallocyanine compound was investigated and found to exhibit a reversible process of adsorption and diffusion pathways. The reversible redox chemistry of the compound was further confirmed through testing a flow cell, which showed excellent battery performance.