Improving the Volumetric Capacity of Gallocyanine Flow Battery by Adding a Molecular Spectator

Gallocyanine (<b>GAL</b>) was recently introduced as a promising aqueous-soluble electroactive molecule for preparing two-electron storage alkaline flow battery (FB) negolytes. The development of a cost-effective <b>GAL</b> FB electrolyte is limited by the unexpectedly low solubility of <b>GAL</b>. In this work, the compound 7-amino-4-hydroxy-2-naphthalenesulfonic acid was introduced as a molecular spectator to modulate the solubility of <b>GAL</b> in KOH; this formulation allowed the preparation of a negolyte with a theoretical volumetric capacity of 32.00 Ah L^-1. The cycling stability of an improved <b>GAL</b> electrolyte was demonstrated by operating a FB cell outside of the glovebox (bubbling N₂ in the tanks). The cell exhibited a Coulombic efficiency close to 98% and began operating with 72.1% of its theoretical capacity (16.06 Ah L^-1), retaining 88% of it after 110 cell cycles. This work demonstrates that significant improvement in electrolyte performance can be obtained with suitable additives and electrolyte engineering.