Reversible Reduction of the TEMPO Radical: One Step Closer to an All-Organic Redox Flow Battery

Nitroxide radicals are considered as ideal redox species in all-organic redox flow batteries due to their redox potential of ∼2 V. These radicals are predominantly used in their polymerized form as cathode materials to a high efficiency. Attempts to use poly(nitroxide)s as anode materials have been unsuccessful due to irreversibility of the reduction process as the reduced form of a nitroxide undergoes a fast, irreversible proton transfer with an electrolyte. In this study, reduction of the nitroxide radical, TEMPO, was shown to become reversible in an ionic liquid. A redox potential of 2.5 V was achieved, with the reduction reversibility being maintained after 200 cycles. A fabricated symmetric electrochemical cell demonstrated a high coulombic efficiency of 60% over an extended period of time. This is the first report demonstrating a high degree of reversibility of nitroxide reduction, thus leading to a paradigm shift in the future design of redox flow batteries.