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.

The redox flow battery (RFB) is a promising technology for the long-duration storage of energy from domestic- to utility-scale. A RFB stores charge in a liquid electrolyte that is charged and discharged in an electrochemical cell reactor. Since storage and reaction site are separated, energy and power of the system can be scaled independently via electrolyte tank and cell stack dimensions. Iron as one of earth’s most abundant elements is an attractive active material, especially in symmetric RFBs using the same electrolyte on both negative and positive side of the battery. One way to realize such a system is by combining iron with redox-active ligands in non-aqueous solvents.[1],[2] We study the iron tris(2,2’-bipyridine) complex which shows a metal-centred reversible oxidation and three ligand-based reductions that are solvent-dependent.[3] The influence of electrolyte composition on charge and discharge reactions as well as on viscosity and conductivity is further investigated. Charge-discharge cycling experiments are carried out in 3-electrode H-cell and flow cell setups. We determine failure modes for the positive side due to electrode kinetics and for the negative side due to solubility changes. We propose improvements to this non-aqueous RFB system based on the selection of electrode materials and cycling conditions. [1] J. Mun et al., J. Electrochem. Soc. 2018, 165, 215-219. [2] C. X. Cammack et al., DaltonTrans . 2021, 50, 858. [3] D. M. Cabral, P. C. Howlett, D. R. MacFarlane, Electrochim. Acta 2016, 220, 347-353.

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