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

Gallocyanine (GAL) 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 GAL FB electrolyte is limited by the unexpectedly low solubility of GAL. In this work, the compound 7-amino-4-hydroxy-2-naphthalenesulfonic acid was introduced as a molecular spectator to modulate the solubility of GAL 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 GAL 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.