Bipolar Isoindoline Nitroxide for Nonaqueous Symmetrical Redox Flow Battery

Crossover in non-aqueous redox flow batteries remains a critical challenge to the cycle stability of these devices. The use of bipolar redox-active organic materials (ROM) is an emerging strategy for mitigating crossover. Herein we report the first example of a bipolar ROM derived from an isoindoline nitroxide, a ring class which gives a number of advantages over the more commonly employed piperidines, including greater stability and a 200 mV higher oxidation potential. Through facile synthetic transformation, the unsubstituted isoindoline nitroxide was nitrated to give a novel bipolar molecule, 5-nitro-1,1,3,3-tetramethylisoindoline-2-yloxyl (NTMIO). This material was investigated electrochemically, revealing a reversible oxidation and quasi-reversible reduction giving a cell potential of 2.07 V. NTMIO was then assessed as an active material in both a static and flow battery model, where cycling was observed for both oxidative and reductive redox couples for over 70 and 20 cycles respectively.

Automated summary

This study reports a novel bipolar redox-active organic material (ROM) derived from an isoindoline nitroxide, which exhibits higher stability and oxidation potential. The material shows reversible oxidation and quasi-reversible reduction reactions, with a cell potential of 2.07 V. It demonstrates excellent cycling performance in both static and flow battery models.