A High-Potential Phenoxazine Sulfonate Posolyte for Aqueous Zinc–Organic Flow Batteries

Aqueous redox flow batteries (ARFBs) are a promising solution for large-scale energy storage; however, the development of organic posolytes that combine high redox potential with long-term stability remains a significant hurdle. This study introduces sodium 3-(10H-phenoxazin-10-yl)propane-1-sulfonate (POZS), a novel sulfonate-functionalized phenoxazine derivative designed to overcome these limitations. By incorporating hydrophilic anionic sulfonic groups, this molecular engineering strategy enhances the structural stability of redox-active phenoxazine materials. Although POZS shows limited solubility in pure water, its solubility increases to 0.98 M (equivalent to a charge capacity of 26.3 Ah L−1) upon the addition of 1.5 M tetraethylammonium chloride (TEAC). This enhancement suggests that the supporting electrolyte optimizes the ionic environment and mitigates intermolecular aggregation, thereby facilitating higher active species concentration. Electrochemical characterization of POZS reveals a highly positive redox potential of 1.51 V (vs. Zn/Zn2+) and rapid electron transfer kinetics (2.02 × 10−2 cm s−1). When tested in a zinc-based hybrid flow cell, the POZS posolyte demonstrates excellent rate capability (up to 50 mA cm−2) and a temporal capacity fade rate of 0.335% per hour over 500 cycles—a nearly five-fold improvement over previously reported quaternized phenoxazines. Post-cycling analyses indicate that while the phenoxazine core remains susceptible to nucleophilic ring substitution, the pendant sulfonate groups ensure that any resulting byproducts remain soluble, preventing the catastrophic depletion typically caused by the precipitation of degraded active species. These findings establish a robust molecular framework for the design of high-potential, durable organic posolytes for sustainable energy storage systems.