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

Abstract

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.

Stable organic radicals and their untapped potential in ionic liquids

“…2.2 eV in aqueous media), [45] and improving reversibility in flow systems. [25] The choice of IL is crucial because not all ILs are capable of preventing environmental proton transfer. Optimisation of the cation-anion pair can reduce proton transfer to the aminoxy anion.…”

“…ILs with positive Gibbs free energy values are found to preferentially stabilise the anionic radical compared to hydroxy-TEMPO (TEMPOH). [25] Specifically, interactions between TEMPO and imidazolium ILs are predicted to be dominated through electrostatics. [46] The functionalisation of TEMPO influences both solubility and oxidation potential.…”

“…[16] It is well known that the reduced form of TEMPO (aminoxy anion) degrades via fast environmental proton transfer (Fig. 2) [25] and nucleophilic attack [26] with the decomposition leading to a loss in the reversibility. The inherent instability can be overcome by adding stabilising functional groups, such as trifluoromethyl (-CF 3 ), to the nitroxide structure.…”

Anthraquinone-Catalyzed TEMPO Reduction to Realize Two-Electron Energy Storage of Poly(TEMPO-methacrylate)

“…6 However, the TEMPO •/− redox couple suffers from sluggish electron transfer kinetics and the proton-coupled side reaction forming hydroxylamine (TEMPOH). 7 Meanwhile, the electrochemical oxidation of TEMPOH requires a much higher overpotential and strongly depends on the electrolyte pH (i.e., in aqueous media), which significantly impedes the reversibility of TEMPO •/− couple. 8 As such, only the TEMPO +/• redox couple is mostly adopted in TEMPO-based energy storage applications, which merely provides a theoretical capacity around 110 mAh g −1 for the corresponding radical polymers (i.e., poly(TEMPO-methacrylate), PTMA), far below inorganic materials used in commercialized batteries.…”

The development of organic-based systems for use in redox flow batteries

“…However, the aminoxyl anion demonstrates poor chemical stability due to fast proton transfer with the electrolytes. In a recent study, Izgorodina et al 66 Furthermore, at higher concentration, the charged species precipitated during the cycling test.…”