Reversibility of a U^VO₂ ⁺/U^VIO₂ ²⁺ 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 U^VO₂ ⁺ and U^VIO₂ ²⁺ as tetrachloro complexes.

Abstract

We studied electrochemical behavior of U^VO₂ ⁺/U^VIO₂ ²⁺ 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 U^VO₂ ⁺/U^VIO₂ ²⁺ was successfully observed on a glassy carbon working electrode under presence of Cl⁻ in [emim]Tf₂N, where [U^VIO₂Cl₄]²⁻+e⁻=[U^VO₂Cl₄]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 (x _DMF) 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 [U^VIO₂Cl₄]²⁻+e⁻=[U^VO₂Cl₄]³⁻ in the [emim]Tf₂N-DMF (50 : 50 v/v, x _DMF=0.769) liquid electrolyte containing [Cl⁻]=0.519 M, where η=6.2 mPa ⋅ s.