Azoniafluorenones: A New Family of Two-electron Storage Electrolytes for Sustainable Near-Neutral pH Aqueous Organic Flow Battery

Fluorenones are suitable candidate for negolytes in flow batteries, as they demonstrate ability to store 2 electrons, and can achieve reversibility, solubility, and stability with appropriate molecular design. However, limitations persist such as the use of alkaline media, high redox potentials, and a limited scope for optimization. Herein, we report azoniafluorenones as a novel class of negolytes. They can be readily accessed in a highly modular fashion from inexpensive commercially available materials (e.g., boronic acids). Variations in the substitution patterns reveals the 3-substituted N-alkylated AZON3, which demonstrates excellent solubility at neutral pH (1.64 M) with two low reversible redox potentials (–0.31 V and –0.58 V vs Ag/AgCl). AZON3 exhibits high stability when evaluated at high concentration in a neutral supporting electrolyte (1 M in 3 M KCl), paired with BTMAP-Fc on the positive side. Capacity retentions of 99.95% and 99.91% per cycle (99.35% and 99.21% per day) are achieved when cycling with 1 and 2 electrons, respectively, coupled with high volumetric capacity of 46.4 Ah L-1 (87% of capacity utilization).