Tetrabutylammonium hexafluorophosphate and 1-ethyl-3-methyl imidazolium hexafluorophosphate ionic liquids as supporting electrolytes for non-aqueous vanadium redox flow batteries

Tetrabutylammonium hexafluorophosphate (TEAPF6) and 1-ethyl-3-methyl imidazolium hexafluorophosphate (EMIPF6) are synthesized and used as the supporting electrolytes of a non-aqueous redox flow battery using vanadium acetylacetonate (V(acac)3) as the active species. The conductivity and cyclic voltammograms of the electrolytes of the two ionic liquids are measured. The cyclic voltammograms show that both of them are stable in an operating potential range (−2.5–1.5 V). The diffusion coefficients of V(acac)3 in the electrolytes are determined as 0.92–1.47 × 10−6 cm2 s−1 in 0.5 mol l−1 TEAPF6 and 2.35–3.79 × 10−6 cm2 s−1 in 0.5 mol EMIPF6, respectively. The charge–discharge performances of the two non-aqueous V(acac)3 containing electrolytes with TEAPF6 and EMIPF6 as the supporting electrolytes are evaluated in an H-type glass cell. The coulombic efficiencies are measured as in the range of 53.31–57.44% at 50% state of charge with an electrolyte containing 0.2 mol l−1 TEAPF6, and as in the range of 46.04–43.46% for the electrolyte containing EMIPF6 with the same concentrations of the components.

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► Two ionic liquids are used in the V(acac)3 redox flow battery.
► Cyclic voltammograms and kinetics of reactions are tested.
► Charge–discharge performance shows a reasonable coulombic efficiency.