High Potential Isoindoline-Based Nitroxides Posolytes for Aqueous Organic Redox Flow Batteries

The growing transition from fossil fuels to renewable energy sources such as wind and solar requires efficient stationary energy storage systems to ensure grid stability. Among emerging technologies, redox flow batteries (RFBs) offer a promising solution due to their unique decoupling of energy and power capacities, allowing flexible system design. Recent advances in organic RFBs (ORFBs) have highlighted redox-active organic molecules as sustainable alternatives to conventional vanadium-based systems, addressing issues of cost and corrosivity. In particular, nitroxide radicals such as tetramethylpiperidinyloxyl (TEMPO) derivatives have demonstrated high reversibility and fast kinetics in aqueous systems, though the stability of their oxidized N-oxoammonium form remains a challenge for long-term storage. Isoindoline-based nitroxides offer potential for enhanced stability but have been limited by complex and low-yield synthetic routes. Herein, we present a convenient metal-catalyzed [2 + 2 + 2] intermolecular cycloaddition strategy for the synthesis of isoindoline-based nitroxides and their aza analogs, including two new candidates, TC-TMIO and PPO. Electrochemical characterization reveals that PPO, a cationic 2,3-dihydropyrrolo[3,4-c]pyridinium nitroxide, exhibits an oxidation potential 220 mV higher than the benchmark 4-TMA-TEMPO and achieves solubility exceeding 3 M in 1 M NaCl aqueous solution. Preliminary stability assessments of the PPO and RFB testing using a methyl viologen/PPO system demonstrate its potential as a high-performance, sustainable posolyte for aqueous ORFBs.