A high-capacity 1,2:3,4-dibenzophenazine anode integrated into carbon felt for an aqueous organic flow battery in alkaline media

Abstract

Aqueous flow batteries (AFBs) that utilize inorganic or organic redox-active species dissolved in aqueous electrolytes to store electrical energy attract a great deal of attention in large-scale energy-storage applications because of their high efficiency, long cycling life, high design flexibility, excellent safety and environment friendliness. However, unsatisfactory energy density is one main challenge for AFBs, which originates from the relatively low solubility of redox-active substances in aqueous electrolytes. Herein, we synthesized a water-insoluble fused phenazine derivative, 1,2:3,4-dibenzophenazine (DBPZ), by a facile Schiff-base condensation reaction, featuring a very negative redox potential and highly reversible two-electron transfer kinetics in aqueous alkaline electrolytes. A DBPZ anode with high mass loading is prepared by integrating DBPZ into carbon felt via a deep eutectic solvent (DES)-assisted preparation approach. Aqueous hybrid flow batteries (AHFBs) employing this high-loading DBPZ anode achieve a high current density, a large areal capacity and remarkable cycling stability. Furthermore, using the high-loading DBPZ anode as a capacity booster, an aqueous organic flow battery (AOFB) with a benzo[a]hydroxyphenazine-7/8-carboxylic acid (BHPC) anolyte and a K₄[Fe(CN)₆] catholyte shows a significantly enhanced capacity and excellent cycling durability due to the rapid redox kinetics, similar redox potential and high stability of both DBPZ and BHPC, as well as a redox-targeting reaction occurring between DBPZ and BHPC.