Bipolar Isoindoline Nitroxide for Nonaqueous Symmetrical Redox Flow Battery

Organic bipolar material for nonaqueous redox flow batteries: An isoindoline nitroxide-based bipolar redox-active material was evaluated for nonaqueous flow batteries. The material showed a cell voltage of over 2 V, negligible loss of capacity over 77 cycles, and an electrolyte solubility of 0.42 M.

Abstract

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.

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