Securing Reversibility of UVO2+/UVIO22+ Redox Equilibrium in [emim]Tf2N‐Based Liquid Electrolytes towards Uranium Redox‐Flow Battery

We studied electrochemical behavior of UVO2+/UVIO22+ in non‐aqueous liquid electrolytes to clarify what is required to attain its reversibility for utilizing depleted U in a redox‐flow battery. To transfer knowledge from former pyrochemical systems in high temperature molten salts, 1‐ethyl‐3‐methylimidazolium bis(trifluoromethyl)sulfonylamide ([emim]Tf2N) ionic liquid was employed here. As a result, a reversible redox reaction of the UVO2+/UVIO22+ was successfully observed on a glassy carbon working electrode under presence of Cl− in [emim]Tf2N, where [UVIO2Cl4]2−+e−=[UVO2Cl4]3− occurs after stabilization of both U oxidation states by the Cl− coordination. The observed electrochemical responses are rather sensitive to an electrode material, so that cyclic voltammograms on a Pt working electrode were actually irreversible. To improve diffusivity of solutes, viscosity (η) of [emim]Tf2N diluted with an auxiliary molecular solvent, N,N‐dimethylformamide (DMF), was examined under absence and presence of Cl−. When the mole fraction of DMF (xDMF) is 0.769, η of the mixture becomes sufficiently low to be utilized as a liquid electrolyte. Finally, we have succeeded in demonstrating a reversible redox reaction of [UVIO2Cl4]2−+e−=[UVO2Cl4]3− in the [emim]Tf2N‐DMF (50 : 50 v/v, xDMF=0.769) liquid electrolyte containing [Cl−]=0.519 M, where η=6.2 mPa ⋅ s. Reversibility of a UVO2+/UVIO22+ redox equilibrium in a [emim]Tf2N‐based liquid electrolyte was successfully established after addition of DMF and Cl− appropriately. The former was employed to reduce viscosity of the system for improving diffusivity of the U‐based electrode active materials, while the latter is also essential to stabilize both UVO2+ and UVIO22+ as tetrachloro complexes.